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1 review Energy Storage Surface/Interfacial Structure and Chemistry of High-Energy Nickel-Rich Layered Oxide Cathodes: Advances and Perspectives Peiyu Hou, Jiangmei Yin, Meng Ding, Jinzhao Huang, and Xijin Xu* The urgent prerequisites of high energy-density and superior electrochemical properties have been the main inspiration for the advancement of cathode materials in lithium-ion batteries (LIBs) in the last two decades. Nickel-rich layered transition-metal oxides with large reversible capacity as well as high operating voltage are considered as the most promising candidate for next-generation LIBs. Nonetheless, the poor long-term cycle-life and inferior thermal stability have limited their broadly practical applications. In the research of LIBs, it is observed that surface/interfacial structure and chemistry play significant roles in the performance of cathode cycling. This is due to the fact that they are basically responsible for the reversibility of Li + intercalation/deintercalation chemistries while dictating the kinetics of the general cell reactions. In this Review, the surface/interfacial structure and chemistry of nickel-rich layered cathodes involving structural defects, redox mechanisms, structural evolutions, side-reactions among others are initially demonstrated. Recent advancements in stabilizing the surface/interfacial structure and chemistry of nickel-rich cathodes by surface modification, core shell/concentration-gradient structure, foreign-ion substitution, hybrid surface, and electrolyte additive are presented. Then lastly, the remaining challenges such as the fundamental studies and commercialized applications, as well as the future research directions are discussed. 1. Introduction Lithium-ion batteries (LIBs), first developed as power sources for portable electronic devices by Sony Corporation, are now extended to the plug-in hybrid electric vehicles (PHEVs) as well as pure electric vehicles (EVs). This is due to the energy crisis and environment concerns around the globe. [1] The practical LIBs used in EVs require higher energy density to ascertain a long driving range of above 300 miles per charge and a durable service life of around 10 years as per the U.S. Department of Energy. [2] Furthermore, the advanced LIBs as power sources applied into PHEVs and EVs ought to also shorten the charge Dr. P. Hou, Dr. J. Yin, Dr. M. Ding, Prof. J. Huang, Prof. X. Xu School of Physics and Technology University of Jinan Jinan, Shandong Province, China sps_xuxj@ujn.edu.cn DOI: /smll time further and enhance power density during acceleration process so as to perfect its practicality. [1b,3] Electrode materials (cathode and anode) play an important role in the electrochemical properties of LIBs, including energy-density, cycle-life, rate capability, and safety among others. Graphite materials with high capacity greater than 300 mah g 1, superior structural stability, as well as low cost are significantly applied as anode electrode in commercialized LIBs. [4] The other anode, graphite/silicon composites that can deliver higher capacity of around 1000 mah g 1 and that of pure graphite electrode are gradually commercialized to further enhance the energy density of LIBs. [5] The first commercial cathode material, layered oxide LiCoO 2 with O3 structure (in which oxygen anion have a cubic close packing arrangement in the form of ABCABC ), illustrates the unpleasant transformation from hexagonal phase to monoclinic phase, particularly when the Li + deintercalation ratio (x) exceeds 0.5 in Li 1 x CoO 2. [6] This is in contrast with graphite-based anode with preferred electrochemical performance and low cost. Also, this phase transition derived from the order/disorder transformation of Li + is anticipated to cause rapid decay of reversible capacity. [6d] Consequently, the LiCoO 2 cathode can only deliver around 150 mah g 1, which has limi ted the performance promotion of LIBs and the far-ranging applications of LIBs in PHEVs and EVs. Additionally, developing high-capacity (energy-density) cathode materials with desired electrochemical behaviors in place of the conventional LiCoO 2 has been the main motivation behind LIBs in the recent past. Three main groups of cathode materials, layered oxides including lithium-stoichiometric Li[Ni x Co y Mn 1 x y ]O 2 and lithium-rich Li 1+z M 1 z O 2 (M = Mn, Ni, Co, Ru, Sn, Ir, etc.), spinel LiM 2 O 4 (M = Ni, Mn), together with olive LiMXO 4 (M = Fe, Mn, Co; X = P, Si) as illustrated in Figure 1, are broadly examined as the next-generation cathode materials for LIBs. [3a,6e,7] Among these candidates lithiumrich and manganese-based layered oxides display the highest energy-density of approach 1000 Wh Kg 1 as a result of the large reversible capacity of 300 mah g 1 and high voltage of 3.5 V (vs. Li/Li + ). However, there are various significant (1 of 29)

2 challenges, low initial coulombic efficiency, capacity and voltage fade, as well as poor rate capability, which have seriously limited its commercialized process. [7c,8] Still, it is expected that practical applications of lithium-rich cathode materials might take quite long to be achieved. Presently, lithium transitionmetal oxides, particularly for the nickel-rich (Ni-rich) layered oxides Li[Ni 1 x M x ]O 2 (M = Co, Mn, Al, etc., x 0.5) with high reversible capacity of mah g 1, reasonable rate capability and low cost are considered to be the only available highenergy electrode materials as the next-generation cathode for LIBs. [7b,9] Notably, higher nickel fraction in nickel-rich layered cathodes, a higher specific capacity can commonly be delivered without regard for cycling and thermal stability. [10] Interestingly, the Al 3+ substituted binary Li[Ni 0.8 Co 0.15 Al 0.05 ]O 2 (NCA), which belongs to a family of nickel-rich cathodes, has been successfully applied into Tesla Model 3 EVs. Tesla Model 3 is regarded as a landmark event for EVs market since it offers affordable prize and above 200 miles of range per charge. Nickel-rich Li[Ni 1 x M x ]O 2, derived from the pure LiNiO 2 with different transition-metal-ion substitutions, have a typical rhombohedral crystal structure with R-3m space group. The Ni 2+/3+/4+ redox-active couples provide the majority of the reversible capacity for the nickel-rich cathodes. Nevertheless, the nickelrich electrodes exhibit undesired structural and thermal instability, particularly at the highly delithiated state: (1) the original R-3m layered structure typically change into Li[Ni 1 x M x ] 2 O 4 -like Fd-3m spinel phase and/or NiO-type Fm-3m rock-salt phase at the electrode surface since the Ni 2+ (octahedral 3a sites) with similar ionic radius to Li + is likely to migrate to the neighboring vacant Li + octahedral 3b sites; [9d,11] (2) the highly oxidizing Ni 4+ ions promptly react with the organic electrolyte on the electrode/electrolyte interface during continual charge/discharge cycles, which result in the Ni 2+ dissolution and electrolyte decomposition; [9c,12] (3) oxygen release from host lattice at the highly delithiated electrode surface preferentially takes place, probably being likely to react with the organic electrolyte and bring about safety problems. [13] Therefore, it is obvious that the surface/interfacial structure and chemistry contribute largely in the electrochemical properties of nickel-rich electrodes. In this review, the surface/interfacial structure and chemistry of nickel-rich layered lithium transition-metal oxides largely concerning the surface structural evolution, interfacial sidereaction between electrode and electrolyte, as well as the chemistry of solid-electrolyte interphase (SEI) are first demonstrated. Recent advances on enhancing electrochemical characteristics Peiyu Hou joined in the School of Physics and Technology at University of Jinan in September He received his PhD degree in Materials Science from School of Materials Science and Engineering at Nankai University in June His research focuses on the layered lithium-transition-metal oxides, especially for these with core shell and (full) concentration-gradient structure prepared via coprecipitation reaction, as cathode materials for advanced lithium-ion and sodium-ion batteries. Xijin Xu is a Professor in the School of Physics and Technology at University of Jinan since He completed his PhD degree in Institute of Solid State Physics, Chinese Academy of Sciences in 2007 followed by postdoctoral training at Nanyang Technological University in Singapore ( ), National Institute for Materials Science in Japan ( ), and Griffith University in in Australia ( ). His research focuses on functional micro/nanostructures materials and their applications in environmental remediation and energy storage. of nickel-rich layered cathode materials through modification of the surface/interfacial structure and chemistry are further summarized. Lastly, the remaining challenges such as fundamental researches and practical applications, together with the present potential solutions are also discussed. Through the new insights accomplished in this review, enhancing the electrochemical process of other layered, spinel or olive electrode materials, and promoting the commercialization of high-energy nickel-rich cathodes for advanced LIBs is also possible. Figure 1. Crystal structure of a) layered LiMO 2, b) spinel LiM 2 O 4, and c) olivine LiMPO 4 (blue: transition-metal ions; red: Li ions; yellow: P ions), corresponding to 2D (dimensionality), 3D, and 1D transport path of Li + ions during redox, respectively. Reproduced with permission. [6e] Copyright 2012, Elsevier (2 of 29)

3 2. Surface/Interfacial Structure and Chemistry Overall, surface structure refers to a few to tens of nanometers thickness at the surface of particle grains whereas interfacial structure shows tens of nanometers region between the electrode and the electrolyte. [10a,11,14a] However, there has been no clear definition of both surface structure and interfacial structure. A stable surface/interface is a basic requirement for an excellent cathode. Nonetheless, actual surfaces of nickel-rich cathode materials usually include defects like antisite defects, size effects, surface coating layers, and heterostructures among others. [14] The types and concentrations of the defects, which highly influence electrons transfer and Li + ions storage in bulk, surface and interface, are regulated by thermodynamics of material surface. Understanding the surface/interfacial structure and chemistry allows for regulation of the Li + /electron transport, Li + storage as well as structural evolutions of the nickel-rich electrodes Bulk and Surface Structure Li[Ni 1 x M x ]O 2, (M = Co, Mn, Al, etc.) derived from LiNiO 2 with different transition-metal-ion substitutions are isostructural to the α-nafeo 2 -type layered structure with the close-packed oxygen anions in a cubic arrangement. The transition-metal ions (TMs) and Li + ions that occupy the octahedral sites of alternating layers with an ABCABC stacking sequence known as O3-type structure are shown in Figure 2a. It is observable that Li + ions diffusion in the layered structure of nickel-rich cathodes occurs along a 2D interstitial space. This is regarded as a pathway for fast Li + migration. [9a,10] Structural defects, particularly for particle surfaces like antisites, size effects, surface coating layers, and heterostructures, are exhibited in the nickel-rich layered oxides. Antisite defects are the most common forms of disorders in the lithium and transition planes of layered cathode materials with 2D Li + diffusion channels. As the major antisite defect, Li + /Ni 2+ cation mixing involved cations disordering between TMs sites (octahedral 3a sites) and Li + sites (octahedral 3b sites), which is one of the main origins of poor reactivity for nickel-rich materials. Cation mixing promptly stimulates the undesired transformation from well-ordered layered phase to disordered spinel and/or rock-salt phases during long cycling as displayed in Figure 2b. It is also notable that the latter spinel and/or rock-salt ones possess higher activation energy barrier for Li + diffusion owing to the reduced distance between the slabs. Therefore, the nickel-rich electrodes with disordered spinel and/or rocksalt structured surface exhibit a lower Li + diffusion coefficient due to the TMs in the lithium layer. Ultimately, this results in sluggish Li + kinetics during redox reactions. [11a,c] Accordingly, the electrochemical behaviors such as rate capability, reversible capacity as well as voltage also decrease steadily as the cation mixing increases for nickel-rich cathodes. The surface size effect is also an important factor that regulates the crystal lattice stability and Li + diffusion kinetics. For solid-state diffusion of Li + in electrodes during redox reactions, the average diffusion time (τ eq ) is determined by the diffusivity (D) as well as the diffusion distance (L), as per the following Equation (1) [15] 2 τ = L /2D (1) eq Increasing D by foreign-ions doping can improve the Li + diffusion. However, the introduction of heteroatoms potentially induces thermodynamic metastable state of crystal structure. A perfect alternative approach; lowering of L, has been accomplished by building nanostructured electrode materials. It is also worth noting that nanostructured nickel-rich materials are favorable as a result of the improved kinetics. Their practical applications are limited by low thermodynamic stability, high reactive activity toward surface side reactions and poor processing problems. All of these attributed to the nanosize and the high surface area. [15d] In order to avoid these setbacks of nanostructured electrode materials, kinetically stabilized nanostructured materials developed by the use of micro/nanohierarchical structures and surface engineering science are recommended or proposed. [15b] The surface modifications as well as core shell/gradient heterostructures will be presented comprehensively Surface Structural Evolutions Figure 2. Schematic illustrations of the ordered and disordered phase in layered nickel-rich lithium transition-metal oxides and their structural transition. a) Well-ordered R-3m phase called O3-type structure; b) The cation disorder or cation mixing phase with Fm-3m structure transformed from R-3m layered phase. Reproduced with permission. [9a] Copyright 2015, Wiley-VCH. Overall, nickel element acts as valence states of +2 and +3 in the nickel-rich electrodes. The radius of Ni 2+ (0.069 nm) is similar to that of Li + (0.076 nm). The Ni 2+ (3a sites) is likely to migrate to the neighboring vacant (3 of 29)

4 NANO MICRO Li+ octahedral 3b sites, which results in the Li+/Ni2+ site exchanges. Consequently, nonstoichiometric structures, also known as cation mixing, are usually observed for nickel-rich Li[Ni1 xmx]o2 materials.[16] Prior investigations have shown that the structural deterioration mainly occurs at the surface of the nickel-rich layered oxides. It involves the phase transformation from layered structure (R-3m) to the disordered spinel phase (Fd-3m) and/ or the rock-salt phase (Fm-3m).[9d,11] First, the structural evolutions occur on the surface where the delithiated process causes decreased lithium content and/or increased lithium vacancies at the surface of the active particles. Kang and co-workers studied the degradation mechanism of nickel-rich Li[Ni0.5Co0.2Mn0.3]O2 electrode cycled under diverse cutoff voltages (4.5 V and 4.8 V vs. Li/Li+) by combined X-ray diffraction (XRD) and TEM (transmission electron microscopy) analyses.[11a] They realized that the surface crystal structures suffer from an irreversible transition from the pristine rhombohedral structure to a mixture of spinel as well as rock-salt phases. In the meantime, the type and degree of transformation is highly dependent on the upper cutoff voltages as shown in Figure 3a. Under a higher operating voltage (4.8 V), the highly oxidative circumstance possibly activates the oxygen release from the crystal surface, further accelerating the formation of the rock-salt phase. Chang and co-workers also exemplified that kinetic effects are likely to cause a greater deintercalation of Li+ from the surface of Li[Ni0.8Co0.15Al0.05]O2 (NCA) particles, hence resulting in structural instability.[11b] These instabilities might further stimulate the reduction of TMs, the release of oxygen to balance charge neutrality as well as the formation of new structures or phases at the surface. The electron energy loss spectroscopy (EELS) results denote that these phase transformations from the well-ordered layered phase (space group R-3m) to the disordered spinel phase (Fd-3m), and ultimately to the rock-salt phase (Fm-3m) are likely to become more extensive as the deep of charge increases (as illustrated in Figure 3b). The irreversible lattice distortions together with surface structural or chemical evolutions of nickel-rich cathode in the graphite/li[ni0.6co0.2mn0.2]o2 full cells were also extensively studied through aberration-corrected TEM by Lee and coworkers.[17a] From this investigation, the structural deterioration preferentially occurs at the surface region, and it becomes more serious toward the electrode/electrolyte interface. In particular, the growth of the newly formed disordering phase at Figure 3. Illustrations of the phase transformation from well-ordered layered structure (R-3m) to the disordered spinel structure (Fd-3m) and/or the rock-salt structure (Fm-3m). a) Degradation mechanisms of nickel-rich cathode Li[Ni0.5Co0.2Mn0.3]O2 and phase transformation under various upper cutoff voltages; Reproduced with permission.[11a] Copyright 2014, Wiley-VCH. b) Schematic illustrations of the crystallographic and electronic structure evolutions occurred in Lix[Ni0.8Co0.15Al0.05]O2 upon electrochemical cycling reactions;[11b] Copyright 2014, American Chemical Society. c) High-resolution STEM analysis of structural degradation of Li[Ni0.6Co0.2Mn0.2]O2 after 10 cycles. Reproduced with permission.[17a] Copyright 2016, Elsevier (4 of 29)

5 the surface occurs from the initial cycle, reaching up to 25 nm depth after only five cycles. Even though the disordered layer does not grow deeper within 10 cycles, a 5 nm rock-salt phase (Fm-3m) layer is noticeable on the grain surface (as indicated in Figure 3c). At the same time, the local microstructural changes yield the transformations of the chemical states of TMs. More specifically, Ni 2+ ions locally augment at the surface region, further inducing the instability of stoichiometricity and thereby compensate for it, and the valence states of manganese ranges from +4 to near to +2. [17] At the highly delithiated state (thermodynamic metastability), larger Li vacancies form at the surface of primary grains. This encourages movement of Ni ions, especially for Ni 2+ with similar ion radius with Li +, and occupies vacant Li sites so as to maintain the thermodynamic stability. [10,11] Consequently, as the cycle continues, nickel ions potentially improve at the surface region of the primary grains. Summarily, the actual surfaces of nickel-rich electrodes commonly suffer from oxygen loss, TMs migration as well as dissolution. The surface structural evolutions triggered by the migration of Ni 2+ ions accompanies the creation of hole states at the O 2 2p level, especially the high states-of-charge (SOC) which causes oxygen loss at the surface of active particles. Therefore, the phase transformation from pristine rhombohedral structure with R-3m group to spinel and/or rock-salt structures preferentially takes place at the electrode surface. The ionically insulating disordered spinel phase and/or rock-salt phase on the particle surface blocks the Li + migration, resulting in sluggish kinetics during redox reactions, and thus reducing capacity retention Redox Mechanisms The redox reactions and consequent reversible capacity are highly related to the electronic structure of active elements in the transition-metal layer. The energy diagrams of LiCoO 2, LiNiO 2, and LiMnO 2 are indicated in Figure 4a, [18] wherein cobalt and nickel are trivalent in the electronic configuration Figure 4. a) Comparison of the energy diagram of LiCoO 2, LiNiO 2, and LiMnO 2. Reproduced with permission. [18] Copyright 2008, Royal Society of Chemistry. b) The energy versus density of states indicating the relative Fermi level of the Ni 4+/3+ and Co 4+/3+ redox couples for Li x Ni y Mn y Co 1 2y O 2 during the various charge state determined by the Li concentration x(li). Reproduced with permission. [19] Copyright 2014, Multidisciplinary Digital Publishing Institute (5 of 29)

6 (t 2g ) 6 (e g ) 0, that is, in the low-spin state (S = 0). An excess overlaps of the top of O 2 2p band with the redox active Co 3+/4+ :t 2g band induces chemical instability when over 0.5 Li + is extracted from layered LiCoO 2 due to the electron loss of O 2 2p. Fortunately, the location of Ni 3+/4+ :e g band overlapped with the top of the O 2 2p band is below the Co 3+/4+ :t 2g band from the electronic structure viewpoint, which indicates less electron delocalization with Ni 3+/4+. [18,19] The Ni 3+/4+ redox couple with high Li + chemical potential offers a high voltage of 4 V like LiCoO 2, and high reversible capacity of above 200 mah g 1. Nevertheless, LiNiO 2 also suffers from some shortcomings: (1) difficulty to prepare stoichiometric pure LiNiO 2 phase without Li + /Ni 2+ mixing in the lithium/transition- metal layer; (2) irreversible H2/H3 phase transitions at high voltage stage during the redox process; And (3) safety hazards attributable to the exothermic release of oxygen, particularly at highly delithited stage and at elevated temperatures. [20] Accordingly, LiNiO 2 does not show potential as a cathode candidate for commercial LIBs. In theory, Julien et al. [19] established the improved structural stability of ternary LiNi y Mn y Co 1 2y O 2 oxides relative to the LiNiO 2 and LiCoO 2 ones. They achieved this by calculating the relative location of the Fermi level as well as the O 2 2p energy band with respect to the Ni 3+/4+ and Co 3+/4+ redox couples as a function of x(li) during the charge (as illustrated in Figure 4b). From the recent experimental results, it is notable that cobalt incorporation was confirmed as a promising means of enhancing its layered structural stability and suppressing Li + /Ni 2+ cation mixing for nickel-rich Li[Ni 1 x Co x ]O 2. [21] The partial substitution of nickel by inactive manganese and/or aluminum can enhance safety by increasing thermodynamical stability, especially at highly delithiated state. [22] Therefore, the ternary Li[Ni 1 x y Co x Mn y ]O 2 (x + y 0.5), the solid solution of LiNiO 2 LiCoO 2 LiMnO 2, and the Al 3+ substituted binary Li[Ni 1 x y Co x Al y ]O 2 (x + y 0.5) phases permit overcoming of the main shortcomings shown in both LiCoO 2 and LiNiO 2. [12c,23,24] 2.4. Surface/Interfacial Side Reactions Side Reactions between Residual Lithium Oxygen Species and Moisture Besides the capacity fading associated with the abovementioned phase transformations at high SOC, the sensitivity of nickel-rich materials to air and moisture exposure is also a major setback. Since excess lithium is needed for synthesizing well-ordered layered structure with lower cation mixing, residual active lithiated oxygen species (Li 2 O, Li 2 O 2 ) remain on the surface of nickel-rich cathodes. [20,25] It is prone to reacting with H 2 O and CO 2 once the active oxides are exposed in ambient air, forming LiOH and Li 2 CO 3, respectively, on the particle surface [26] as depicted in Figure 5. Likewise, the presence of insulating layers composed of lithium compounds (LiOH, Li 2 CO 3 ) delay Li + diffusion and cause large irreversible capacity during initial cycle. During successive charge discharge cycles, LiOH layer on the Li 2 O is notably transformed to Li 2 CO 3 layers along with the formation of H 2 O. In general, the organic electrolyte using LiPF 6 as lithium salt, which is unstable in the presence of H 2 O molecules during redox, is decomposed by the following equations: [27] LiPF6 LiF + PF (2) 5 PF5 + HO 2 POF3 + 2HF (3) 2POF 3Li O 6LiF PO or Li POF + + ( ) x y (4) The new formation of LiF deposited on the surface of active nickel-rich cathodes is greatly reliant on the amount of available H 2 O molecules in the electrolyte. These insulating lithium compound layers LiF, Li 2 CO 3, and LiOH, coexist. This inhibits Li + diffusion, especially within the long-term cycles, thus degrading the electrochemical properties. [26b] Side Reactions between Surface/Interface and Electrolyte Spontaneous side reactions also take place on the surface/interface of nickel-rich electrodes by contacting with electrolytes. [28] The nature of these side reactions is realized to be the same as that of the course formed in electrochemical cycles. [28e] The resultant mechanisms and the final products are established by both the surface chemistry of electrodes as well as the nature of electrolyte used in LIBs. Koyama et al. proposed that the surfaces of the layered transition-metal oxides were decreased at nanometer scale by immersion of the active materials into organic electrolyte. The tendency of the surface reduction was compatible with the Figure 5. Surface evolutions of nickel-rich Li[Ni 0.7 Mn 0.3 ]O 2 cathode materials after exposure in air and the subsequent effect of the residual lithium compounds on the particle surface. Reproduced with permission. [26b] Copyright 2014, ECS the Electrochemical Society (6 of 29)

7 defect chemistries by first-principles calculations. [28a] The surface reduction tends to show the nature of nickel-rich materials, and is regarded to be accountable for the deterioration of electrodes. Cherkashinin et al. further suggested that the shift of E F of the cathode material was related to the highest occupied molecular orbitals (HOMO) of the electrolyte, and that their vicinity in energy led to the formation of a thin SEI film (usually less than 30 Å) already after contact of the lithium transition-metal oxides to the organic electrolyte. [28b] Aurbach also considered that the LiNiO 2 -based electrode materials were very nucleophilic and that the pristine electrodes were covered by Li 2 CO 3 as a result of the reaction between residual lithium compounds and CO 2 in the air. Once the battery is collected in the electrolyte solutions, these pristine surface species are replaced by surface films from direct reactions between the active mass as well as the electrolyte solvent species [28f] as shown in Figure 6. Later, Aurbach and co-workers further specified that the existence of nickel element in electrodes augmented the nucleophilicity of the surface oxygen. [28g] Therefore, Li[Ni Co Mn]O 2 electrodes commonly present passivation in LiPF 6 /alkyl carbonate-based electrolyte for the formation of surface species. Parasitic reactions between electrolytes and charged electrodes potentially form a SEI film at the surface of active materials. Dahn and co-workers supposed that the parasitic reactions essentially occurred during the entire life-span of LIBs, but rather slowed at a rate (dx/dt) following the general Equation (5) shown below: [28d,29] d /d /2 1/2 1/2 x t k t (5) = ( ) where k is a constant dependent on the electrolyte/electrode system, x denotes the hypothetic thickness of an ideal SEI layer. Notably, the influence of the said interphase reactions is rather obvious on graphite anode. [30] Nonetheless, this side reactions would control on the surface of cathodes when the upper cutoff potential exceeds 4.5 V (vs. Li/Li + ). [31] Cherkashinin et al. clarified that high oxidative Ni 4+ and Co 4+ at high SOC, which are dissolved into electrolyte during redox, are related to the formation of SEI film. [28b] A charging potential makes further lowering E F of the nickel-rich cathodes below the HOMO of the electrolyte. Considerably, this is accompanied by a hole transfer from TM 3d states to the HOMO of the electrolyte, thus results in the decomposition of active oxides and the reduction of TMs as indicated in Figure 7. Additionally, as referred above, the acidic impurity HF produced during hydrolysis of PF 6 with trace moisture further corrodes the surface of active materials by the following Equations (6) Li Ni1 x ycoxmn y O2() s + 2xHF 2xLiF + Li Ni Co Mn O + xho 1 2x 1 x y x y 2 x 2 These side reactions also make the dissolution of TMs in organic electrolyte. For nickel-rich Li[Ni 0.6 Co 0.2 Mn 0.2 ]O 2, Gao and co-workers measured the dissolution rates of TMs in 1 m LiPF 6 /ethylene carbonate (EC) as well as diethyl carbonate (DEC) (1:1 in volume) electrolyte within various cycles at 55 C (6) Figure 6. Illustration of surface film formation on these LiNiO 2 -based electrodes. Reproduced with permission. [28f] Copyright 2000, Elsevier (7 of 29)

8 Figure 7. Schematic view of the Li + delithiated reaction in the layered cathode material. a) Li + removing is accompanied by the electron release from the host lattice, leading to the oxidation of TMs. b) The lowering E F below the highest occupied molecular orbital (HOMO) of electrolyte would give rise to hole transfer from the TM3d states to the HOMO until the formation of SEI between the cathode/electrolyte interface. c) The SEI film generally consists of reduced TM oxides and the lithium compounds of decomposed electrolyte. Reproduced with permission. [28b] Copyright 2015, American Chemical Society. through the use of the ICP-AES (inductively coupled plasmaatomic emission spectrometry) measurement. [9c] Their findings showed abrupt increases in the dissolutions of TMs, particularly for Ni 2+ ions after 100 cycles. Accordingly, latest report by Gallus et al. affirm that use of LiBF 4 instead of LiPF 6 as lithium salt in the electrolyte could significantly bring down the dissolution of TMs. [32] Chemical crossover of these dissolved TMs onto the graphite anode in LIBs is also considered to be among the primary reasons for reversible capacity fade. In particular, deposition of the manganese ions on the graphite anode is considered to correlate with capacity decay by improving retention of Li + within the SEI layer. [33] Generally, the manganese ions initially dissolve from the positive electrode, transport to the graphite negative, and ultimately deposit onto the outer surface of the innermost SEI. In this location, the manganese ions make it possible for the reduction of the organic electrolyte and the subsequent formation of lithium-containing products on the graphite negative. This further eliminates Li + from the normal operation of the LIBs, thus, leading to the gradual capacity fade. Most recently, Manthiram and co-workers substantiated that lithium deposition on graphite negative is caused by some TMs (especially for manganese) dissolved from the positive in a disrupted SEI without relevant kinetic or stoichiometric limitations in the nickel-rich Li[Ni 0.61 Co 0.12 Mn 0.27 ]O 2 /graphite pouch cells. Furthermore, this insidious effect is found to set off at an extremely early stage of cell operation (<200 cycles). [33d] Previous studies support the fact that several types of compounds, lithium-containing oxides, fluorides, and carbonates formed on the surface of electrodes and the compositions are related to the electrolyte solvents. For lithium salt LiClO 4 dissolved in propylene carbonate (PC) electrolyte solvent, the major side reaction corresponds to the formation of lithium carbonate, whereas the formation of Li x PF y - and Li x PF y O z -type compounds controls when dimethyl carbonate (LiPF 6 -EC/DMC) electrolyte is used. [34] Andersson et al. further investigated the electrode surface from type lithium-ion full-batteries subjected to accelerated cycle-life measurement. [35] These results exhibit that the aged Li[Ni 0.8 Co 0.2 ]O 2 positive electrode laminate surface contains a mixture of organic/inorganic species such as polycarbonates, LiF, Li x PF y -type, and Li x PF y O z -type compounds, despite operating temperature, cycling duration as well as SOC. Generally, these deposits layers on the surface of active materials inhibit the diffusion of Li + ions from host structure to bulk electrolyte as a result of its insulating properties, and thus potentially degrading the electrochemical properties. The interreactions of bulk electrolyte solvents, such as the transesterification of carbonates, and similar nucleophilic reactions between pristine and cycled carbonates produce the formation of undesirable alkyldicarbonates that follow the general formula (Equation (7)): [36] The above-mentioned interreactions could be initiated by alkoxide and subsequently catalyzed by HF acidic impurities and Li 1 z [Ni 1 x y Co x Mn y ]O 2 transition-metal oxides with structural defects. Lastly, the alkyldicarbonates would possibly focus on the cathode/electrolyte interphase as electrochemical charge/discharge process increases Side Reactions Involving Conductive Carbon Additives In the recent past, there have been increasing evidences revealing the surface reactivity of conductive carbon additives that were once considered inactive with electrolyte in LIBs. [37] As for carbonaceous materials, there are various chemical functional groups at the surface, such as hydroxyl-, carboxyl-, carbonyl-, and aromatic groups, [38] which seem to react with (7) (8 of 29)

9 LiPF 6 /EC-based electrolyte upon contact before any electrochemical cycling. [37a,b] Spontaneous decomposition of electrolyte takes place at the surface region of carbon black particles after storing even without external potential and current. The surface deposits (hydrocarbons, carbonates, ethers, and LiF, Li x PO y F z among others) are also detected on cathodes, and in part decompose upon charging to V. The decomposition species reduce as cycled voltage ranges from 2.5 to 4.3 V, suggesting that they are oxidized or desorbed when electrochemical processes start. [39] Manthiram et al. [37c] considered the significant role of carbon blacks in passivating nickel-rich Li[Ni 0.7 Co 0.15 Mn 0.15 ]O 2 electrode in the range of V (vs. Li/Li + ). Their results showed that organic complexes impulsively generated on carbon during aging migrate to the active material surface, and to some extent repress these unwanted surface or interfacial reactions during cells working. Notably, this process varies from the conventional view of SEI generation on graphite anode electrode, even though the passivation effect appears rather weak in comparison. However, this finding might clarify the effectiveness of some electrolyte additives, as they possibly interact with the chemical functional groups of conductive carbon and/or the spontaneously produced surface species in organic electrolytes. In addition to the chemical reactivity of conductive carbon additives toward electrolyte components before battery operation, they are also dependent on electrochemical degradation at extreme potentials, more so for high operating voltage and temperature. For instance, the passivation effect by conductive carbon tends to lose stability under high upper cut-off voltages of above 4.5 V (vs. Li/Li + ). [40] Therefore, the interface stability of conductive carbon agents also seems to be crucial for cycling stability of advanced highvoltage LIBs. In summary, the actual surface of nickel-rich electrodes typically suffers from oxygen loss, TMs migration from the octahedral 3a sites to the neighboring Li + vacant octahedral 3b sites and dissolution into the electrolyte, especially at the deep SOC. These undesired side reactions on the surface/interface of electrodes further result in phase transformations from the layered to disordered spinel, and ultimately to rock-salt structure and the deposits SEI layers on the surface of active cathode materials. Thus, electrochemical properties, such as reversible capacity, rate capability, and thermal stability among others are decayed as the cycle increases. Comprehending the surface chemistry and controlling the surface structure permits regulations of the electron transport, Li + storage together with side reactions of the nickel-rich electrodes. 3. Recent Advances From these prior discussions, it is realizable that the surface/ interfacial structure and chemistry fundamentally contributes in the electrochemical performance of nickel-rich cathodes. In order to enhance the stability of surface/interfacial structure and chemistry, much effort has been put in surface modification, foreign-ions substitution, hybrid surface, core shell structure, as well as electrolyte additives Surface Modification Electrochemically Inactive Protective Coating As aforementioned, it is generally considered that the side reactions between the acidic HF and electrode surface, high oxidative Ni 4+ as well as organic electrolyte are responsible for TMs dissolution and structural evolutions of nickel-rich electrode surface. To protect the electrode from directly contacting organic electrolyte, suitable coating matrix layers are commonly applied for nickel-rich electrodes. Moreover, the modification matrix can act as an effective HF scavenger and neutralize the acidity near the electrode/electrolyte interface. As a result, the improved capacity retention of nickel-rich cathodes is logically anticipated by the incorporation of an HF scavenger. Up to now, many modification materials for this reason mostly contain oxides [41] (Al 2 O 3, MgO, CeO 2, ZnO, La 2 O 3, ZrO 2, TiO 2, SiO 2, Bi 2 O 3, In 2 O 3, Co 3 O 4, etc.), phosphates [42] (AlPO 4, Co 3 (PO 4 ) 2, Ni 3 (PO 4 ) 2, FePO 4, etc.), and fluorides [43] (AlF 3, CaF 2, etc.). Out of these coating matrixes, amphoteric metal oxides tend to readily react with HF and downgrade HF concentration in the electrolyte. For instance, ZnO, [41f] and Al 2 O 3 [41a,b] coating layers scavenge the acidic HF produced in the electrolyte as per the following respective reactions (Equations (8) and (9)) in which Gibbs formation energies of the metal fluorides are much lower than those of the metal oxides: ZnO+ 2HF ZnF2 + H2O (8) Al2O3 + 6HF 2AlF3 + 3H2O (9) The metal fluoride layer grows steadily with continuous cycles because of by-products of the said reactions and act as a protective coating layer for mitigating decay of the cathode surface. Nonetheless, once the metal oxide reacts with HF, water is produced as a by-product, facilitating acidic HF generation in the electrolyte. The regenerated HF corrodes the electrode surface. These series of processes repeat, which further deteriorates surface structure and chemistry. Therefore, metal fluoride and (transition-) metal phosphates are further generated as stable coating layers on nickel-rich cathode materials. Sun et al. coated Li[Ni 0.5 Mn 0.5 ]O 2 by 10 nm AlF 3 layer, which would suppress the TMs dissolution and maintain low electrochemical resistances as demonstrated in Figure 8. [43b] Time-of-flight secondary ion mass spectroscopic (TOF-SIMS) results revealing the deposition of insulating LiF, as a product of decomposed LiPF 6, is effectively inhibited by AlF 3 coating. Consequently, the AlF 3 -coated Li[Ni 0.5 Mn 0.5 ]O 2 delivers improved reversible capacity, cycling stability, as well as rate capability as compared with the pristine Li[Ni 0.5 Mn 0.5 ]O 2. Apart from the above electrochemically inactive protective coating, multifunctional electrochemically active coating, like Li-insertion host V 2 O 5, [44] was also introduced. Since V 2 O 5 serves as an intercalation host to hold the excess Li + ions after the initial charge process, it is capable of reducing initial reversible capacity (IRC) loss for nickel-rich cathodes. Notably, too much V 2 O 5 coating ratio is unfavorable for maintaining high reversible capacities. In addition, the formed Li x V 2 O 5 by Li (9 of 29)

10 Figure 8. TEM bright-field images of as-prepared a) pristine and b) AlF 3 -coated cathode materials Li[Ni 0.5 Mn 0.5 ]O 2. c) Cole Cole plots for pristine and AlF 3 -coated Li[Ni 0.5 Mn 0.5 ]O 2 electrodes at the 20th charge. d) Amount of Ni and Mn dissolved in electrolyte for the delithiated pristine and AlF 3 -coated Li[Ni 0.5 Mn 0.5 ]O 2 electrodes stored at 90 C for 7 d. Reproduced with permission. [43b] Copyright 2008, ECS the Electrochemical Society. inserting into V 2 O 5 also scavenge the acidic HF produced in the electrolyte by the following reaction Equation (10): LixV2O5 + 10HF V2F10 x + xlif+ 5H2O (10) Water is produced as a byproduct due to the side reaction between metal oxide matrix and the acidic HF decomposed by LiPF 6. Thus, metal oxide coating can probably mitigate the surface degeneration only. The structurally and thermally stable (transition-) metal fluorides as well as (transition-) metal phosphates will be promising matrixes for surface modification of nickel-rich electrodes Li + /Electron Conducting Coating Though the mentioned oxides as resistance layers decrease the harmful side reactions, they mostly demonstrate electrically and electrochemically inactive natures. Then, electron conducting and/or Li + conductive matrixes have been examined as the alternative coating layers. The electron conducting layers largely contain carbon, graphene oxide and conducting polymers, while the Li + conducting layers are LiTaO 3, Li 2 ZrO 3, Li 2 SiO 3, Li 2 TiF 6, Li 3 VO 4, and fluoroborate glass xli 2 O-yB 2 O 3 -zlif. [45] Yoon et al. utilized electron conducting graphene to wrap the nickel-rich Li[Ni 0.8 Co 0.15 Al 0.05 ]O 2 particles via a simple highenergy ball milling technique (illustrated in Figure 9a). [45a] The increase in R p proceeding from the continuous cycles, termed as R p, is plotted vs. the deep of discharge (DOD) (as shown in Figure 9b). It is obvious that R p of the Li[Ni 0.8 Co 0.15 Al 0.05 ]O 2 / graphene composite is much less as compared with that of pristine electrode. This indicates improved surface chemical stability and electronic conductivity. Likewise, Li[Ni 0.8 Co 0.15 Al 0.05 ] O 2 /graphene composite electrodes deliver large reversible capacities of around 152 and 112 mah g 1, even at high rates of 10 C and 20 C, respectively. This is almost twice as high as that of the bare Li[Ni 0.8 Co 0.15 Al 0.05 ]O 2 electrode (as displayed in Figure 9c). Lithium-ion conductors are also broadly applied as coating matrixes for nickel-rich cathode surfaces. Li 2 ZrO 3 as a superior Li + conductor with a high conductivity of S m 1 at 573 K was reported. [46] Most importantly, Li 2 ZrO 3 belongs to a family of ternary oxides that are thermodynamically stable against lithium metal. Therefore, Manthiram et al. prepared nickel-rich Li[Ni 0.7 Co 0.15 Mn 0.15 ]O 2 modified by a uniform surface of Li 2 ZrO 3. [45e] HRTEM (high resolution transmission electron microscopy) confirms the formation of Li 2 ZrO 3 islands with large areas and exceptional homogeneity on the primary grains (as depicted in Figure 9d,e), which would also inhibit the side reactions occurring on the electrode surface. Additionally, a single Li 2 ZrO 3 island is considered to be partly amorphous and partially crystalline, wherein the latter crystalline domain is surrounded by an amorphous layer. The amorphous domain of Li 2 ZrO 3 is regarded to make Li + transport possible, thus a pouch-type fullcells assembled by the Li 2 ZrO 3 - coated cathode and graphite anode shows long-term cycling (10 of 29)

11 Figure 9. a) SEM images of the Li[Ni 0.8 Co 0.15 Al 0.05 ]O 2 graphene composite. b) Variations of R p (increase in R p on going from the 2nd to 20th cycle) with DOD and c) normalized rate capability retention for Li[Ni 0.8 Co 0.15 Al 0.05 ]O 2 and the Li[Ni 0.8 Co 0.15 Al 0.05 ]O 2 graphene composite. Reproduced with permission. [45a] Copyright 2012, Elsevier. d f) TEM images of the 5 wt% Li 2 ZrO 3 -coated Li[Ni 0.7 Co 0.15 Mn 0.15 ]O 2 samples postannealed at 650 C. Reproduced with permission. [45e] Copyright 2017, American Chemical Society. stability, 73.3% capacity retention after 1500 cycles at a C/3 rate. Aside from the protecting effect for electrode surface during respective cycles, developing the new coating matrix with high Li + /electron conductivity along with the surface/interface will be a considerable direction in the near future. These newly found 2D materials, phosphorene, arsenene, antimonene, and other low-dimensional materials with exclusive properties possibly as very promising candidates ought to be verified in future studies Lithium-Reactive Coating The excess of lithium sources is needful in forming wellordered layered structure with lower Li + /Ni 2+ cation mixing for nickel-rich cathodes. Residual active lithiated oxygen species (Li 2 O, Li 2 O 2 ) would remain on its surface. [25] Therefore, nickel-rich materials seem to absorb moisture and present high concentration of residual lithium impurities (Li 2 CO 3 and LiOH) at the surface. [26] Unfortunately, these residual species on surface cause a gel state of cathode slurry and might lead to undesired gas evolution as well as insulating LiF deposition. In the recent past, lithium-reactive coating has been suggested for regulation of the surface chemistry, more so for the nickel-rich cathodes. Coating precursors, AlPO 4, Co 3 (PO 4 ) 2, an H 3 PO 4, among others are used to react directly with surface residual lithium species (Li 2 O, LiOH and Li 2 CO 3 ) to produce Li + insertion/delithiation compounds Li x AlPO 4, Li x CoPO 4, and Li 3 PO 4, which are also fast Li + ion conductors and are steady in the organic electrolyte. [47] Jo et al. [47d] provided a new lithium-reactive coating direction (as shown in Figure 10a). Nickel-rich Li[Ni 0.6 Co 0.2 Mn 0.2 ]O 2 cathode is first modified by H 3 PO 4. Then, the products are further heated to 500 C in air. The added H 3 PO 4 is changed to Li 3 PO 4 by reacting with residual lithium compounds, LiOH and Li 2 CO 3, on the surface. A uniform Li 3 PO 4 nanolayer (<10 nm) is noticed on the electrode surface. Besides the reduction of residual lithium compounds, the Li 3 PO 4 coating layers scavenge HF and water molecules available in the electrolyte. As a result, the Li 3 PO 4 -coated nickel-rich Li[Ni 0.6 Co 0.2 Mn 0.2 ]O 2 electrodes exhibit extremely enhanced cycling stability as well as rate capability. In addition, the NH 4 VO 3 is ionized to VO 3 in aqueous solution, which reacts with Li + ions obtained from the dissolution of residual lithium impurities on the nickelrich particle surface. This lithium-reactive coating route is suggested by Cho et al., [47e] (as indicated in Figure 10b). Prior reports have also established that phosphates AlPO 4 and Co 3 (PO 4 ) 2 could react with residual lithium oxygen species under ensuing annealing. [47a,48] Shao-Horn and co-workers have shown that the AlPO 4 coating matrix generated Al-rich LiAl y Co 1 y O 2 phase and P-rich Li 3 PO 4 regions on the surface of LiCoO 2. In reality, this lithium-reactive coating reduces residual lithium species. Besides, the combination Li 3 PO 4 as fast Li + conductor, and the Al-doped LiMO 2 contribute to an improved cycling stability as well as rate capability. [49] AlPO 4 coating was (11 of 29)

12 Figure 10. a) Schematic illustration of surface structure and chemistry of bare and the process of Li 3 PO 4 -coated Li[Ni 0.6 Co 0.2 Mn 0.2 ]O 2 via lithium-reactive coating. Reproduced with permission. [47d] Copyright 2015, Springer. b) Schematic view of the lithium-reactive coating process of Li[Ni 0.8 Co 0.15 Al 0.05 ]O 2 performed using NH 3 VO 3. Reproduced with permission. [47e] Copyright 2015, Royal Society of Chemistry. further expanded to nickel-rich cathodes, Li[Ni 0.8 Co 0.1 Mn 0.1 ]O 2 and Li[Ni 0.8 Co 0.2 ]O 2, by many researchers, [47a,48b,50] which was also significant for minimizing unfavorable side reactions through forming the above-mentioned protective coating layer on the electrode/electrolyte interface. Expectedly, enhanced storage performance, thermal stability together with electrochemical properties had been accomplished. Like AlPO 4, Co 3 (PO 4 ) 2 can also react with the residual lithium compounds on the particle surface during heat treatment, producing a uniform coating layer and lithium-deficient olivine Li x CoPO 4 on the electrode surface. The olivine Li x CoPO 4 phase covered on the high-capacity cathode Li[Ni 0.8 Co 0.16 Al 0.04 ]O 2 is favorable for limiting the transformation from layered phase into spinel phase as well as TMs dissolution. Lithium-reactive coating taking place between lithiated oxygen species and coating matrix on the electrode surface can reduce residual lithium impurities. It can also form multi functional surface with enhanced structural stability and kinetics. This simple and superficial coating is appropriate for large-scale production. It can, therefore, probably promote the extensive applications of nickel-rich cathode materials in LIBs Artificial SEI Fabricating artificial SEI film on the electrode surface is also an effectual technique that can help in controlling the surface structure and chemistry. Son et al. reported a simplistic chemical vapor deposition (CVD) process that involved mixed gases of CO 2 and CH 4 to produce thin artificial SEI layer on nickelrich Li[Ni 0.6 Co 0.1 Mn 0.3 ]O 2 active particles. [51] Interestingly, this modification process is self-terminated provided that the SEI thickness reaches 10 nm. This is due to the growing binding energy between the gas mixture and the surface products. In situ diffuse reflective infra-red Fourier transform spectra (in situ DRIFTs) results exhibit this artificial SEI layer largely containing alkyl lithium carbonate (LiCO 3 R) as well as lithium carbonate (Li 2 CO 3 ). The surface reaction mechanisms of the (12 of 29)

13 sample including the mixed gases of CO 2 and CH 4 are summarized by the following formulas (Equation (11), (12), (13)): CO + CH CH O+ H + CO (11) 2 4 x 2 CHxO+ LixMeO LiOCHx ( = LiOR) (12) LiOCHx + CO2 LiCO3R (13) The artificial SEI coating layer can inhibit the particles from directly getting in contact with the electrolyte and slowing down the side-reactions during redox. The enhanced cycle life and coulombic efficiency are achieved as shown in Figure 11. To conclude, the surface modification layer can help in protecting the active compounds from direct contact with the organic electrolyte, suppressing the side reactions occurring at the electrode/electrolyte interface, and ultimately enhancing the structural stability, especially for the surface/ interfacial region. [52] In the meantime, other effects are also found. They include: (1) reduction of the dissolution of TMs; (2) higher electron/li + conductivity; and (3) HF scavenger in the electrolyte. [52a] Nonetheless, several surface/interface science issues still need to be illustrated. For instance, there is need to demonstrate mechanisms of the surface modification and the various roles of varied modification layers as well as the directions on controllable and homogeneous modification for electrode materials Core Shell and Concentration-Gradient Structure Markedly, in most cases where surface coating agents are applicable, to make sure that Li + transport through the electrolyte/ electrode interface, nanosized coating layer that is generally less than 10 nm on the surface of host materials is applied. Then, the isolated islands of the coated materials are formed instead of a uniform and full coverage. [52b] This leads to insufficient long-term cycling stability, more particularly at advanced upper cutoff voltage as well as operating temperatures. Different from these conventional coating, core shell structure, which is generally termed as thick coating where the scale of coating layer (shell) ranges from submicron to micron order, is rationally suggested in the electrode materials. [3b,53] Core shell structure are regarded as the double or multiple layers where the interior component (core) and the outer surface (shell) improve different chemical components, thus enabling the recombination and complementation of the benefits of core and shell compounds. As proposed by Hou et al., [3b] the design of core shell nickel-rich cathode materials follows these principles in general: (1) high-capacity materials such as Li[Ni 0.8 Co 0.1 Mn 0.1 ]O 2 and Li[Ni 0.8 Co 0.15 Al 0.05 ]O 2, among others Figure 11. SEM images of a) the pristine and b) artificial SEI layer coated Ni-rich layered oxide cathodes after 100 cycles between 2.5 to 4.5 V (vs. Li/Li + ). c) The cycling life of discharge capacity retentions of the prepared NCMs in the voltage range of the pristine and artificial SEI layer coated Ni-rich electrodes with an areal capacity of 3 ma cm 2 in the potential range of V at 25 C. Reproduced with permission. [51] Copyright 2015, American Chemical Society (13 of 29)

14 act as the inner core. The surface shell layer should have good structural and thermal stabilities. For example, high manganese content layered oxides Li[Ni x Co 1 2x Mn x ]O 2 (1/3 x 1/2); (2) the crystal structures of core and shell compositions should correspond; (3) to avoid serious cation diffusion between the interface of core and shell, a relatively low solid-state reaction temperature is suggested. Sun et al. [54] have introduced the aspect of core shell structure into the preparation of nickel-rich cathode Li[(Ni 0.8 Co 0.1 Mn 0.1 ) 0.8 (Ni 0.5 Mn 0.5 ) 0.2 ]O 2, namely the nickel-rich highcapacity Li[Ni 0.8 Co 0.1 Mn 0.1 ]O 2, and the structurally stable Li[Ni 0.5 Mn 0.5 ]O 2 acting as the core and shell, respectively. As shown in Figure 12a, core shell spherical micron-sized precursors [(Ni 0.8 Co 0.1 Mn 0.1 ) 0.8 (Ni 0.5 Mn 0.5 ) 0.2 ](OH) 2 are initially synthesized via a modified hydroxide co-precipitation route. Then, the as-prepared core shell precursors are mixed with the stoichiometric lithium sources. After that, the mixture is calcined to accomplish the intended core shell structure. The SEM image of the core shell powder depicts that the inner core is compactly summarized or encapsulated by the outer shell layer. The thickness of this shell exceeds 1 µm as illustrated in Figure 12b,c. In shell Li[Ni 0.5 Mn 0.5 ]O 2, the chemical valence of nickel and manganese is 2+ and 4+, respectively. Therefore, the electrochemical inactive Mn 4+ can support the layered structure, whereas the Ni 2+ provides electrochemical activity. Summarily, the shell offers a steady surface and makes it possible for both fast Li + insertion/delithiation and electron transfer. As a result, this core shell cathode presents a long-term cycling life with capacity retention of 98% after 500 cycles, whereas the core oxide cathode only exhibits capacity retention of only 81% over the same cycles. Furthermore, the thermal stability of the fully charged electrode is realized to be significantly enhanced for this core shell cathode. Following this concept of core shell structure, core/doubleshell and core/multishell-type nickel-rich cathodes were additionally suggested and prepared by Zhang and co-workers, [55] Manthiram and co-workers, [56] and other scholars, enriching the group of core shell structures. Particularly, Zhang and coworkers introduced the triangular and/or tetrahedral phase diagram into the design of the core shell structured layered lithium transition-metal oxide cathodes. [3b] The triangular phase diagram of LiNiO 2 LiCoO 2 LiMnO 2 and the matching compositions of Li[Ni Co Mn]O 2 situated in this triangular phase diagram are displayed in Figure 13. It is clearly observable that nickel-rich cathode Li[Ni 0.5 Co 0.2 Mn 0.3 ]O 2 is placed in the triangular field and surrounded by Li[Ni 0.8 Co 0.1 Mn 0.1 ]O 2, Li[Ni 0.4 Co 0.2 Mn 0.4 ]O 2 and Li[Ni 1/3 Co 1/3 Mn 1/3 ]O 2. Thus, Li[Ni 0.5 Co 0.2 Mn 0.3 ]O 2 is considered to have been redesigned into Li{[(Ni 0.8 Co 0.1 Mn 0.1 ) 2/7 ] core [(Ni 1/3 Co 1/3 Mn 1/3 ) 3/14 ] shell1 [(Ni 0.4 Co 0.2 Mn 0.4 ) 1/2 ] shell2 }O 2 with core/double-shell structure. Accordingly, the core/double-shelled cathode provides exceptional electrochemical properties as the improved thickness of shells. Besides the core and shell with same crystal structure (layered phase), the core shell heterostructures were developed by Cho et al. by use of different ratio manganese precursors to coat the bare [Ni 0.7 Co 0.15 Mn 0.15 ](OH) 2 precursors. [57] In this, the core is layered phase whereas the shell is spinel or rock-salt phase. As indicated in Figure 14, spinel shell Li 1+x [(CoNi) x Mn 2 x ] 2 O 4 forms on particle surface after calcination if 20 wt% manganese precursors are used to coat the pristine [Ni 0.7 Co 0.15 Mn 0.15 ] (OH) 2. [57a] However, when 10 wt% of manganese precursors is coated on the [Ni 0.7 Co 0.15 Mn 0.15 ](OH) 2, 10 nm rock-salt pillar layer is produced on the surface. [57b] The spinel phase with 3D structure makes it possible for Li + diffusion. Thus, the heterostructured Li[Ni 0.54 Co 0.12 Mn 0.34 ]O 2 cathode with layered-spinel core shell structure possesses both high energy density and safety. When the nanoscale rock-salt structure with Fm-3m Figure 12. a) Schematic view of formation process and b,c) SEM images with various magnifications of micro-sized core-shelled Ni-rich layered oxide cathode Li[(Ni 0.8 Co 0.1 Mn 0.1 ) 1 x (Ni 0.5 Mn 0.5 ) x ]O 2. Reproduced with permission. [54a] Copyright 2005, American Chemical Society (14 of 29)

15 Figure 13. Triangular phase diagram of LiNiO 2 LiCoO 2 LiMnO 2 and the corresponding compositions located in this triangular phase diagram. Reproduced with permission. [3b] Copyright 2017, Royal Society of Chemistry. space group is introduced into the surface of nickel-rich cathodes, the cycle-life at elevated cutoff voltage and testing temperature, as well as the thermal stability are remarkably improved. Subsequently, a structural disparity on the interface of core and shell, that is, void layer of tens nanometers between the two components, was noticed within repeated charge/discharge cycles. [56,58] The void layer severely blocks Li + migration and electron transfer during redox. This results in the fast deterioration of electrochemical properties. To resolve the shortage of core shell structure, Sun et al. suggested the (full) concentration gradient structure, [59] wherein the transition-metal cations increase and/or decrease steadily from the inner section to the outer surface of particles. The nickelrich concentration-gradient Li[Ni 0.64 Co 0.18 Mn 0.18 ]O 2, where the high-capacity core Li[Ni 0.8 Co 0.1 Mn 0.1 ]O 2 was surrounded by a concentration-gradient outer layer, as illustrated in Figure 15a, was detailed by Sun et al. [59a] Most recently, the full concentration-gradient layered oxide cathode Li[Ni 0.75 Co 0.10 Mn 0.15 ]O 2, as depicted in Figure 15b, wherein the nickel fraction decreases steadily whereas the manganese content increases linearly from the inner section to the outer surface for each secondary particle, was further described by Sun et al. [59c] By using this full-gradient approach, advantages of both the high capacity of the nickel-rich core and the structural stability of the manganese-rich surface layer can possibly bet integrated so as to obtain outstanding electrochemical behaviors as illustrated in Figure 15c e. These (full) concentration-gradient nickel-rich electrodes have high energy-density and superior electrochemical performance such as long cycle life, high rate capability, and exceptional thermal stability among others. This is due to the complete protection by the structurally stable and Li + conductive outer surface. Nevertheless, the exact coprecipitation parameters, notably for ph, concentration of ligand and adding ratio of transition-metal ion solution ought to be sensitively controlled during the entire preparation process of the target concentration-gradient precursors, which leads to the less-than-ideal product constancy as well as high cost in commercialization. [3b] In addition, the undesired diffusion of transition-metal cation simply takes place among the high-temperature solid-state reactions between expected precursors and lithium sources. This is anticipated to affect the maintenance of core shell and concentration-gradient structures. [60] Therefore, significant effort Figure 14. Schematic diagram of core shell heterostructured cathode materials, in which the outer shell layer consists of spinel phase (top) or rock-salt structure (bottom) depending on the synthetic conditions. Reproduced with permission. [57a] Copyright 2011, Wiley-VCH; Reproduced with permission. [57b] Copyright 2013, American Chemical Society (15 of 29)

16 Figure 15. a) Schematic diagram of concentration-gradient cathode particle with Ni-rich core encapsulated by Ni-decreased and Mn-increased concentration-gradient outer layer. Reproduced with permission. [59a] Copyright 2009, Macmillan Publishers Limited. b) Schematic view of the full concentration-gradient cathode particle with the nickel content decreasing gradually and the manganese concentration increasing linearly from the centre toward the outer surface. c) Cycling life of half-cells that utilize the full concentration-gradient, inner compostion, and outer compostion as cathode materials between 2.7 and 4.5 V (vs. Li/Li + ) under a constant current of about 44 ma g 1. d) Rate capabilities of the full concentration-gradient, inner compostion and outer compostion cathode materials during upper cutoff voltage of 4.3 V (vs. Li/Li + ). e) Thermal stablity of the delithiated full concentration-gradient, inner compostion, and outer compostion cathodes after charged to 4.3 V (vs. Li/Li + ). Reproduced with permission. [59c] Copyright 2012, Macmillan Publishers Limited. ought to be further made in resolving the above-mentioned issues before the practical applications of core shell and concentration-gradient nickel-rich cathodes in LIBs Foreign-Ions Substitution At highly delithiated state, the Ni 2+ ions (3a sites) appear to be migrating to the neighboring Li + vacant (3b sites), while the structural instability causes oxygen release from host crystal lattice. [10,11a] These side reactions lead to a series of constant structural evolutions from R-3m layered to Fm-3m spinel and/or Fd-3m rock-salt phases for these nickel-rich electrodes. This TMs migration destroys active Li + sites whereas the structural evolutions make the rapid increase of electrochemical inhibition, thereby causing gradual capacity/rate decay as the cycles increase. [11] Hence, electrochemically inactive foreign-ions substitution is usually utilized in increasing the bonding energy between oxygen and TMs and suppressing Ni 2+ migration through stabilizing its chemical valence or forming electrostatic repulsion. These foreign ions are commonly divided into two main classes; cation and anion, as per the charge The Role of Cations Substitution The electrochemically active Co 3+ is established to be an effectual technique of decreasing the Li + /Ni 2+ cation mixing and improving the well-ordered layered properties. [21] Mn 4+ as an electrochemically inactive and low-cost element is capable of keeping the layered structural stability, more so at high SOC. As the results of Ni 2+/3+ substitution by Co 3+ and Mn 4+, three classes of solid-solution systems, LiNiO 2 LiCoO 2, LiNiO 2 LiMnO 2, LiNiO 2 LiCoO 2 LiMnO 2, are provided. [11c,22] These solid-solution materials present improved electrochemical as well as thermal performance compared to that of the pure LiNiO 2 (as illustrated in Figure 16a), even though the surface stability as well as the chemistry ought to be further improved. Electrochemically inactive ions, Na +, [61] Mg 2+, [62] Al 3+, [63] Ga 3+, [64] and Ti 4+, [65] among others, are suggested to enhance structural stability and reduce oxygen release as well as cation mixing. Na + and Mg 2+ can occupy Li 3a sites during solidstate reactions and serve as the stable pillar effect in nickelrich cathode materials (as shown in Figure 16b,c). Meanwhile, Mg 2+ (0.072 nm) with similar ionic radius to Li + (0.076 nm) is at first introduced into nickel-rich materials as pillar effect during charge/discharge cycles. However, the degree of Li-site (16 of 29)

17 Figure 16. a) A map of relationship between reversible capacity, thermal stability and capacity retention of solid-solution Li[Ni x Co y Mn z ]O 2 (x = 1/3, 0.5, 0.6, 0.7, 0.8, and 0.85). Reproduced with permission. [24b] Copyright 2013, Elsevier. b) Scheme of the inter-slab space of Li x Ni 1+z O 2 and Mg-doped Li x Ni 1 y Mg y O 2, in which Ni 2+ leads to local collapse of inters-lab during the initial cycle and thus blocks Li + diffusion and reintercalation, whereas electrochemically inactive Mg 2+ ions do not impede Li + diffusion in that its ion size is close to that of the Li +. Reproduced with permission. [62c] Copyright 2000, ECS the Electrochemical Society. c) Schematic representation of the interslab space in the Li[Ni 0.8 Co 0.15 Al 0.05 ]O 2. Reproduced with permission. [61] Copyright 2016, American Chemical Society. d) dx/dv incremental capacity curves versus voltage for pristine Li 0.98 Ni 1.02 O 2, Ti-doped Li 0.93 [Ni 1.02 Ti 0.05 ]O 2 and Li 0.86 [Ni 1.03 Ti 0.11 ]O 2. Reproduced with permission. [65a] Copyright 2000, American Chemical Society. substitution by Mg 2+ is still unclear. Taking into account the fact that the ion radius of Na + (rna + = nm) is larger than that of Li +, the Li + substitution by Na + as the stable pillar effect is also applied for nickel-rich electrodes by Hu and co-workers. [61] Therefore, the invariable valence (electrochemically inactive) of Na + and Mg 2+ substitution can deliver a greater stable pillar effect as compared with that of electrochemically active Ni 2+. Al 3+ and Ti 4+ are also usually used as promising substitution ions in stabilizing the layered structure. Generally, with the increasing dopant ratio, the reversible capacity decreases whereas the structural stability improves or increases. Then, Al 3+ substitution ratios in nickel-rich cathode materials are mostly concentrated on the low Al 3+ content (below 5 mol%) for optimizing stability and capacity. Notably, the 5 mol% Al 3+ doped binary compound Li[Ni 0.8 Co 0.15 Al 0.05 ]O 2 (NCA) with high capacity of above 200 mah g 1 has been successfully commercialized and applied into the Tesla EVs. Latest research by Cho and co-workers has increased the Al 3+ ratio to further improve the structural as well as thermal stability. [66] The prepared Li[Ni 0.81 Co 0.1 Al 0.09 ]O 2 electrode exemplifies better rate capability and thermal stability as compared with that of other Li[Ni 1 x 0.05 Co x Al 0.05 ]O 2 as well as a large reversible capacity of around 200 mah g 1 at high upper voltage cutoff of 4.5 V (vs. Li/Li + ). Ti 4+ substitution is established to be significant for improving the structural stability of nickel-rich materials by inhibiting the irreversible H2/H3 phase transition (as depicted in Figure 16d). Realizably, the positive role of Ti 4+ substitution initiated from the substitution of Co 3+ by high valence Ti 4+, as well as the valence discrepancy between Co 3+ and Ti 4+ was counteracted by reduction of Mn 4+ to Mn 3+. [65a] Furthermore, Ti 4+ substitution can effectually suppress the phase transformation from layered to spinel phases through prevention of the migration of Ni 2+ 3a sites to neighboring Li + 3b sites. Expectedly, Al 3+ and Ti 4+ substitution are considered significant for advancing the long-term cycle-life and thermal stability for nickel-rich electrode materials. Substitution with electrochemically active cations like Fe 2+ and Cr 3+ was studied as well. Unfortunately, it showed unsatisfied electrochemical behaviors as compared with the electrochemically inactive cations. [67] For example, Fe 2+ doped nickel-rich cathodes present a rapid decrease of reversible capacity as doping molar ratio increases wherein the Fe 2+ ions enhance the oxidation probability of nickel cations while hindering Li + diffusing during charge/discharge process. [67c] The Role of Anions Substitution Different from the cation substitution, Li + in 3b sites and Ni 2+ / Co 3+ in 3a sites replaced by Na +, Mg 2+, Al 3+, Ga 3+, and Ti 4+, and others, the anions substitution presented that the O 2 anions are substituted by other anions such as F, [68] Cl, [69] S 2, [70] among others. In overall, the intercalation or deintercalation potential of electrodes is highly dependent on the redox potential of TMs whose chemical valence differs during redox. Ceder et al. affirmed that O 2 certainly performed an essential role in the electron exchange. [71] It was also demonstrated that O 2 compensated for much of the reduced electron charge in nickel-rich materials by EELS experiments. Therefore, anion substitution is also regarded as an effective strategy that can solve the above issue. Among these anions, F with more electronegativity (3.98) than O 2 (3.44) is broadly utilized in strengthening the binding energy between the transition-metal cations and anions. [72] Sun and co-workers [68b] synthesized F -substituted nickelrich Li[Ni 0.8 Co 0.1 Mn 0.1 ]O 2 z F z (z = 0, 0.02, 0.04, and 0.06) by hydroxide coprecipitation and solid-state methods. This exhibited that F substitution enhances the lattice parameters as illustrated in Figure 17a. It is also pertinent for protecting the surface from HF attack and preventing formation of inhibition-raising interphases. Consequently, the O 2 substitution by F significantly enhances the electrochemical properties like capacity retention, rate capability, as well as thermal stability as shown in Figure 17b. Though the electrochemical properties are improved via F substitution, it does not favor in charge compensation since (17 of 29)

18 Figure 17. a) Lattice parameters a and c of Li[Ni 0.8 Co 0.1 Mn 0.1 ]O 2 z F z with various fluorine content of z = 0, 0.02, 0.04, and b) Comparison of reversible capacity of Li[Ni 0.8 Co 0.1 Mn 0.1 ]O 2 z F z (z = 0, 0.02, 0.04, 0.06) at a current density of 20 ma g 1. Reproduced with permission. [68b] Copyright 2007, ECS the Electrochemical Society. Cyclic voltammograms for c) LiNiO 2 and d) LiNiS 0.02 O 1.98 measured at 0.2 mv s 1. Reproduced with permission. [70a] Copyright 2002, Elsevier. the more electronegativity F is harder to lose electrons than O 2. Thus, the electronegativity of Cl (3.16) is significantly weaker as compared with O 2, while then the polarization effect of Cl is stronger than O 2. As a result, it is easier for Cl to lose electrons than O 2. Cl is initially selected to replace O 2 in Li[Ni 0.7 Co 0.3 ]O 2 by Kang and co-workers. [69] The results indicate that the oxidation state of Co 3+ and Ni 3+ ions decreases with the enhancement of Cl ratio. In the meantime, the discharged specific capacity and the cycle life are improved. The other low electronegativity anion S 2 substituted LiNiS y O 2 y was also prepared by Sun and co-workers, [70a] which would restrain the underside H2/H3 phase transition under high voltage of over 4.2 V (vs. Li/Li + ) as illustrated in Figure 17c,d, which reveals greatly enhanced capacity retention. In the recent past, by using density functional theory, Cho and co-workers [73] reported a systematic investigation on the effects of three common anion dopants; F, S 2, and Cl, on a wide range of properties for LiNiO 2 cathode, including redox potential, ionic conductivity, Li + /Ni 2+ exchange, lattice distortion, together with Ni 2+ migration upon charge/ discharge. The results show that the anion dopants are capable of enhancing some properties, just as they can worsen others as indicated in Table 1. Inactive-cations substitution can stabilize structure of nickel-rich electrodes. However, it is commonly achieved without regard for decreased specific capacity. Apart from Co 3+, substitution with other electrochemically active cations shows undesired effects for nickel-rich electrodes. Anions substitution is considered as an efficient means of improving the structure and properties of nickel-rich electrodes, while the acting mechanisms should be visibly illustrated further in the subsequent research Hybrid Surface Structure Hybrid structured surface with the benefits of surface coating as well as bulk substitution has also suggested to be capable of building better surface/interfacial structure and chemistry for nickel-rich electrodes. Cho and co-workers have recently formulated a hybrid structured surface (shown in Figure 18), wherein the complex surface species localized in various region Table 1. Summary of anion doping effects on the battery performance. Reproduced with permission. [73] Copyright 2016, American Chemical Society. Battery performance a) Materials properties Doping effects F Cl S Battery voltage Redox potential Positive Negative Negative Rate performance Ionic conductivity Positive Positive (L) Negative (H) Positive (L) Negative (H) Li/Ni antisite defect Negative Positive Positive Structural stability Lattice distortion Positive (L) Ni migration Positive Positive (H) Negative (L) a) L and H represent low doping concentration and high doping concentration, respectively. Positive (H) Negative (L) (18 of 29)

19 Figure 18. Schematic diagram of the Ni-rich cathode material with hybrid surface consisting of nanoscale coating layers (V 2 O 5 and Li x V 2 O 5 ) and V-doped layer (Li δ [Ni 0.75 z Co 0.11 Mn 0.14 V z ]O 2 ). Reproduced with permission. [74] Copyright 2014, American Chemical Society. contain coating layer of oxides and substitution layer by foreign ions. [74] For instance V 2 O 5 as the first coating layer is concentrated on the outer surface, while Li x V 2 O 5 is produced at the intermediate second layer. Meanwhile, at the third layer in contact with the bulk particle, a V 4+/5+ ions substituted layer Liδ[Ni 0.75 z Co 0.11 Mn 0.14 V z ]O 2 is generated. The as-prepared hybrid surface assists in Li + /electron diffusion during redox. It also reveals faster charge/discharge capability for nickel-rich cathodes. As well, the stable hybrid surface prevents harmful side reactions with the electrolyte in the cells and moisture in the air. This ensures superior structural stability and rate capability. Multifunctional hybrid surface can integrate the merits of surface coating and foreign-ion substitution, providing a new insight into enhancing the performance of nickel-rich materials as advanced cathodes for the next-generation LIBs Electrolyte Additive Different from the effort spent on anode interphase, few additives were advanced for the intention of making cathode/electrolyte interface perfect until Despite the carbonate-based electrolytes generally being stable, they are also vulnerable to oxidative decomposition at increased upper cutoff and working temperature. Exploiting new electrolyte components is imperative, especially solvents and additives, to provide the prerequisites of interfacial structure and chemistry. Vinylene carbonate (VC) that is one of the most effective reducible additives for stabilizing the electrodes of LIBs is extensively applied in commercialization. [75] Dahn and coworkers systematically researched the roles of these selected electrolyte additives individually and/or in combination with Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 /graphite pouch cells. The results for over 36 additive sets were compared in detail. [76] A Figure of Merit approach (as depicted in Figure 19) was utilized to rank the effectiveness of the additives as well as their combinations. It is realized that the combination of VC and/or prop-1-ene-1,3 sultone (PES), a sulfur containing additive, like methylenemethane disulfonate (MMDS), together with either tris(-trimethlysilyl)-phosphate (TTSP) and/or tris(-trimethyl-silyl)-phosphite (TTSPi) as additives in the electrolyte provide cells with high coulombic efficiency, superior Li + storage behaviors, low electrochemical impedance as well as long-term cyclability. Notably, additive mixtures like 2% PES + 1% MMDS + 1% TTSPi depict extremely exceptional effects among all respects. This comprehensive investigation sets a benchmark for future studies and can be utilized as a reference for the comparison of other electrolyte additives. From the above study, the effects of electrolyte additives on these Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 /graphite, Li[Ni 0.5 Mn 0.3 Co 0.2 ]O 2 / graphite together with Li[Ni 0.6 Mn 0.2 Co 0.2 O 2 ]/graphite pouch cells have been measured by use of ultrahigh precision charger, EIS, gas evolution measurements as well as through the cyclestore tests. [77] VC, PES, pyridine-boron trifluoride (PBF), 2% PES + 1% MMDS + 1% TTSPi (indicated as PES211 ) and 0.5% pyrazine di-boron trifluoride (PRZ) + 1% MMDS are picked as the target electrolyte additives. The different properties such as charge end-point capacity slippage, capacity decay, coulombic efficiency, impedance change within cycling, gas evolution as well as voltage drop during cycle-store testing were also evaluated and compared as illustrated in Figure 20. PES211, PBF, and PRZ + MMDS additives are extremely effective in cells of all compositional grades. Even though Li[Ni 0.6 Mn 0.2 Co 0.2 ]O 2 with higher nickel content is capable of enhancing the energy density when charged to 4.4 V, it stimulates more gas production during initial formation and subsequent cycling, even under the best additives developed herein. EC as a cosolvent was extensively utilized in LIBs generated today. EC was formerly considered to be significant for organic electrolyte system. However, Dahn and co-workers have totally eliminated all EC from typical organic carbonate-based electrolyte recently and added amounts of electrolyte additives to assemble Li[Ni 0.4 Co 0.2 Mn 0.4 ]O 2 /graphite full cells. [78] Surprisingly, the cells are capable of delivering better electrochemical behaviors as compared with the cells containing EC. For instance, electrolyte consisting of 98% ethylmethyl carbonate (EMC) and only 2% VC, with optimized additives, can provide Li[Ni 0.4 Co 0.2 Mn 0.4 ]O 2 /graphite cells with low impedance, alleviated electrolyte oxidation, perfect graphite passivation, low gas production as well as suitable conductivity. Accordingly, the cells can provide excellent properties in cycled up to 4.4 V, which is capable of improving the energy density by at least 10%. This discovery of EC-free electrolyte solvent opens an utterly new space for electrolyte advancement. Recently, EC-free electrolytes, a solvent mixture containing >95% EMC as well as 2% and 5% of an enabler, were further studied in Li[Ni 0.42 Mn 0.42 Co 0.16 ]O 2 /graphite pouch cells. [79] The enablers, needed to passivate graphite during formation, which can be VC, methylene ethylene carbonate (MEC), fluoroethylene carbonate (FEC), or difluoro ethylene carbonate (DiFEC), among others. It is exhibited that the graphite negative electrode cannot be fully passivated when the proportion of enabler is too low. This is likely to result in gas production as well as capacity fade. Utilizing excess enabler is likely to cause large impedance as together with gas production in most cases. The selection of enabler also influences cell performance. A solvent blend of 5% FEC with 95% EMC (by weight) can provide the best combination of properties in the cells operated to 4.4 V. The sulfone functional group ( SO 2 ) that operates as significant chemical component within the additive structure has been broadly selected since it decrease irreversible decomposition of organic electrolyte at the electrolyte/electrode interface (19 of 29)

20 Figure 19. Figure of Merit (FOM) for the for the NMC111/graphite cells containing the electrolyte additives. The data has been arranged from best in the lower left to worst in the upper right. The dashed blue line indicates the FOM for cells with 2% VC. Also shown is the amount of gas generated during formation (where data were available) for cells containing the indicated additives. Note that the scales in the top two panels are different than the scales in the bottom two panels. The short notation 2VC + 1MMDS + 1TTSPi, for example, means 2% VC + 1% MMDS + 1% TTSPi. Reproduced with permission. [76] Copyright 2014, ECS the Electrochemical Society. during cycles. [80] The vinyl-based organic compounds can also expectedly have better mechanical properties since this group promptly plays a part in the formation of the 3D network. [81] Yu et al. selected divinyl sulfone (DVS) as an electrolyte additive for improving the interfacial stability (as displayed in Figure 21a). [81f] They realized that these functional groups certainly takes part in stabilizing sulfone-based SEI intermediates once the DVS is electrochemically oxidized. The oxidative electrochemical reaction of DVS is provided to demonstrate the mechanism of this additive on the electrode surface as shown in Figure 21b. The stable protective layer is equally formed on the surface. This can also reduce the decomposition of electrolyte solvents as well as the dissolution of TMs, thereby generating better interfacial stability as indicated in Figure 21c,d. Even if it was already stated as anodically unstable when working voltages exceeds 4.2 V (vs. Li/Li + ), an unanticipated electrolyte additive was LiBOB. [82] However, Täubert et al. clarified that the use of 2 wt% LiBOB as electrolyte additive could benefit nickel-rich Li[Ni 0.8 Co 0.15 Al 0.05 ]O 2 as well, besides its recognized ability of stabilizing graphitic anode. [83] The cathode/ electrolyte interphase formed in the presence of LiBOB surprisingly exhibits higher rate capability along with oxygen release temperature than its LiPF 6 counterpart. FTIR and XPS analyses show that surface deposition is obviously characterized by the chemical signature of BOB -anion. Moreover, inter-reactions between LiBOB with LiPF 6 can be triggered by alkoxide and can subsequently be catalyzed by acidic impurities during cycles. Lucht and co-workers elucidated on how LiBOB react with LiPF 6 upon thermal storage to generate tetrafluorooxalato-phosphate anion via a disproportionation pathway (Equation (14)): [84] (14) LiBF 4 with similar intentions was also utilized by Zuo et al. to improve capacity maintenance for Li[Ni 0.5 Co 0.2 Mn 0.3 ]O 2 electrodes at high charge cutoff of 4.5 V (vs. Li/Li + ). [85] Whereas capacity retention, coulombic efficiency, as well as cell impedance are enhanced, they noted that 1 wt% of this salt brings about a cathode interphase almost absent of LiF, which has been abundant in SEI generated by control electrolytes. As the hybridized salt of LiBF 4 and LiBOB, LiDFOB draws (20 of 29)

21 Figure 20. Radar plots summarizing the effects of selected electrolyte additives (combinations) on a) NMC111/graphite, b) NMC532/graphite, and c) NMC622/graphite pouch cells studied using UHPC and the cycle-store procedure to 4.4 V. The axes are normalized to the worst value being equal to 100% and they consist of the average coulombic inefficiency (CIE) (from 11 to 15 cycles), the average charge end-point capacity slippage (from 11 to 15 cycles), the impedance (R ct ) after UHPC cycling, the gas evolution during UHPC cycling, the capacity loss after 35 cycle-store cycles, the impedance after the whole cycle-store process, the voltage drop at 35 cycle-store cycle and the gas evolution during the whole cycle-store process. Values closest to the center of the radar plot are best. Reproduced with permission. [77] Copyright 2015, Elsevier. significant attention as a valuable additive for different cathode chemistries. [86] Ping et al. [87] studied the effects of trimethoxy boroxine (TMOBX) on the Li[Ni 0.8 Co 0.15 Al 0.05 ]O 2 cathode surface. They concluded that while there was no clear oxidation of TMOBX, the cell impedance was clearly reduced when its concentration was below 1 wt%. Though significant achievements are presented by the use of electrolyte additive in the electrolyte, the selective sensitivity of spectroscopic tools has been poor and unable to precisely establish reaction mechanism as well as chemical compositions on the surface/interface of electrodes. Thus, in situ, nondestructive, and quantitative techniques are urgently needed to permit us to demonstrate these complicated reaction pathways and chemical compositions on the surface/interphase. 4. Remain Challenges and Prospects 4.1. Moisture Absorption and Storage Property To get well-ordered layered structure with low Li + /Ni 2+ mixing for nickel-rich materials, there is need for excess lithium sources (Li/TMs > 1) during the solid-state reactions. [25a] Residual lithium-active oxide species, Li 2 O and Li 2 O 2, commonly form on the particle surface. The contact with H 2 O and CO 2 in ambient air makes the formation of LiOH and Li 2 CO 3 as indicated in Figure 22a. [25b,26,88a] During storage, the spontaneous reaction of Ni 3+ to Ni 2+ is also likely to take place, [88] which might cause a loss in structural ordering as shown in Figure 22b. In addition, these nickel-rich electrodes structural evolutions would increase electrochemical impedance and block the Li + diffusion during redox, which would further result in the fats decay of electrochemical properties after storage in air for a period of time as revealed in Figure 22c. Furthermore, these residual basic compounds might lead to the jelly state during the slurry preparation, making it difficult for coating onto the current collector (Al foil). The formed Li 2 CO 3 would decay in the activation process and produce CO, and CO 2 gases among others in the interior of full batteries as displayed in Figure 22d. This is likely to further result in pack swelling and invalidation of batteries. [89] The sensitivity of nickel-rich layered cathodes to air exposure is still a considerable challenge for practical applications in commercial LIBs. Coming up with new calcined technology (21 of 29)

22 Figure 21. a) 2D and 3D molecular structures of DVS. b) Reaction mechanism for the formation of a protective layer on the Li[Ni 0.7 Co 0.2 Mn 0.1 ]O 2 electrode surface by oxidative electrochemical reaction of DVS. SEM analysis results for Li[Ni 0.7 Co 0.2 Mn 0.1 ]O 2 cathode c) without additive and d) with DVS additive after 100 cycles at high temperature 60 C. Reproduced with permission. [81f] Copyright 2016, Elsevier. by use of stoichiometric lithium sources as well as facile surface modification means (including lithium-reactive coating technique) in order to reduce the residual lithium-containing compounds on surface of nickel-rich particles are the primary focuses that should be considered in future investigations Safety Concerns Exothermic reactions seem to take place between the surface of greatly delithiated nickel-rich cathodes and organic electrolyte. This further induces structural transitions along with oxygen loss from the host lattice. Particularly, the released oxygen active-species are so reactive with the carbonate-based electrolyte solvents. Thus, they will produce the thermal runaway which might result in severe safety hazards for the reliable applications of LIBs. [12] Therefore, to improve the safety features of lithium-ion batteries by use of nickel-rich layered oxide as cathodes, it is needful to decompose the onset temperature and reduce the proportion of heat production during the exothermic reactions along with O 2 gas release. In studying the principal correlation between thermally stimulated structural evolutions and oxygen release of nickel-rich electrodes, in situ time-resolved X-ray diffraction (TR-XRD) combined with mass spectroscopy (MS) were applied by Yang and co-workers. [90] For the delithiated Li 1 x [Ni 0.8 Co 0.15 Al 0.05 ]O 2 upon heating, a series of phase transitions from layered structure to disordered spinel phase and ultimately to rock-salt phase supplement the evolution of O 2 together with CO 2 as shown in Figure 23. The onset temperature of O 2 release is specifically believed to be dependent on the SOC, and is capable of being as low as 175 C for the delithiated Li 0.1 [Ni 0.8 Co 0.15 Al 0.05 ]O 2, wherein a phase transformation from the layered to disordered spinel structure takes place. Stach and co-workers [11b] further demonstrated that the deintercalation of Li + from the Li 1 x [Ni 0.8 Co 0.15 Al 0.05 ]O 2 surface could trigger the reduction of TMs as well as oxygen release in order to sustain charge neutrality. This can also further lead to structural reconstruction along with porosity formation. Regardless of the intensive efforts made to understand thermally stimulated structural transitions of nickel-rich cathodes, the basic correlation has remained uncertain or unclear between the surface/interfacial structure and chemistry. To better comprehend this phenomenon, future researches ought to focus on the state-of-the-art in situ or in operando microscopic analytical techniques at complementary length scales (22 of 29)

23 Figure 22. a) Variations of the amount of residual lithium compounds in Li[Ni 0.6 Mn 0.2 Co 0.2 ]O 2 (C622) as a function of the storage time. b) Relative intensities I(003)/I(104) (=R w ) and I(006) + I(102)/I(101) (=R f ) of C622-C-x and C622-G-x. c) Capacity retention rates of C622-C-x and C622-G-x at various current densities. The C622 materials stored in the humidity chamber and in the glovebox for x numbers of days are denoted as C622-C-x and C622-G-x, respectively. Reproduced with permission. [88a] Copyright 2016, KCS Publications. d) Gassing during a typical formation cycle. The first shaded area in green shows the initial open circuit voltage (OCV) period, which is followed by a constant current step until the cell potential of 3.7 V (vs. Li/Li + ) is reached. The OCV period thereafter lasts for 14, 12 h at 45 C (shaded area in red), and 2 h at 25 C (shaded area in green). The cell was charged to 4.3 V and discharged to 3 V (vs. Li/Li + ) at C/10. Reproduced with permission. [89a] Copyright 2016, Springer Microcracks in Secondary Particles Electrolyte is dependent on infiltration into the pores of secondary particles. The dissolution of TMs near the grain boundary causes isolation of primary grains. The crystallographic plane arrangement of primary grains is disordered in the secondary particles, thereby making the nickel-rich cathodes to undergo continual anisotropic expansion as well as contraction during cycles. [17] Accordingly, the significant strain induces volume shrinkage of secondary particles along with pore formation between the primary grains. In the recent past, some reports have pointed out that the reversible capacity loss was brought about by masses of microcracks produced in the nickel-rich secondary particles. Miller et al. have formulated a microbattery for in situ SEM detection of a single nickel-rich Li[Ni 0.8 Co 0.15 Al 0.05 ]O 2 secondary particle during cycles, by which it could directly investigate the relationship between microstructure changes and electrochemical processes. [91] As shown in Figure 24a,b, the microcrack developed among primary intergrains become intensified as the cycle increases. These cracks and separations make the loss of intergrains connectivity and possibly give rise to increased polarization. They further result in the degradation of electrochemical properties. Wantanabe et al. have further established that the micro-crack production is highly dependent on the conditions of DOD. [92] Only after a few cycles in the DOD of 0 100%, clear microcracks produced in the intersurface among primary grains, and NiO-like surface formed on the primary grains. In contrast, there are almost no cracks generated in the test condition of DOD of 0 60% even after 5000 cycles. Only NiO-like phase formed on the surface of the cycled secondary particles are evident. The origination of microcracks together with new resistance layer at the aged secondary particles created lack of connection among the primary grains and the fast rise of electrochemical impedance, respectively. Lately, Sun and co-workers [93a,b] have designed and organized a series of compositionally graded nickel-rich layered oxides with concentration-gradient structure. In this structure, a single rod-shaped primary grain with decreased nickel fraction was introduced for concentrationgradient layer as demonstrated in Figure 24d. The distinctive morphology assembled by radially aligned primary grains is anticipated to restrain microcracks in secondary particles by reducing anisotropic expansion and contraction during the continuous cycles. Liang and co-workers [93c,d] noted that the aligned architectures had the following superiorities: (1) highly organized charger transfer and Li + migration pathways; and (2) tunable interspaces between architecture units. Additionally, as illustrated in Figure 24c, Cho and co-workers [94] also demonstrated the feasibility of utilizing a simplistic coating to develop a glue-nanofiller layer (G-layer), middle-temperature Li x CoO 2 (x < 1) with spinel-like phase, between the primary (23 of 29)

24 Figure 23. TR-XRD patterns and simultaneously measured mass spectra (MS) for O 2 and CO 2, released from Li 0.5 [Ni 0.8 Co 0.15 Al 0.05 ]O 2 during heating to 500 C. The left panel shows the models of ideal crystals with rhombohedral, spinel, and rock-salt structures. Reproduced with permission. [90a] Copyright 2013, American Chemical Society. grains of the nickel-rich Li[Ni 0.8 Co 0.15 Al 0.05 ]O 2 secondary particles in mitigating the microcracks, specifically at high operating temperatures. Although even though above-mentioned reports have established that the propagation of microcracks was derived from primary grain boundaries, latest investigations by Wang and co-workers have contended that intragranular particle cracking is a critical impediment for high-voltage usage of layer-structured cathodes for LIBs. [95] They revealed that the formation of the intragranular cracks was directly related to high-voltage cycling, which is an electrochemically driven and diffusioncontrolled process. The intragranular cracks were seen to be characteristically instigated from the primary grain interior, which is an outcome of a dislocation-based crack incubation mechanism. It is also worth noting that this finding sharply contrasts the general theoretical models, which predict the initiation of intragranular cracks from primary grain boundaries or particle surfaces. This study exemplifies that stabilizing structural stability by adjusting the chemistry as well as structure such that it can improve the internal grain strain, and retain a stable lattice is a crucial step in pushing the NCM-layered cathode materials for high-voltage applications. Significant progress has been made in restraining the undesired microcracks in nickel-rich secondary particles. However, Figure 24. SEM images of Li[Ni 0.8 Co 0.15 Al 0.05 ]O 2 secondary particle fracture and fragmentation as a function of cycle using the snapshot approach. These images show the same particle in the a) as-prepared pristine condition and after b) three full charge/discharge cycles. Reproduced with permission. [91] Copyright 2013, Wiley-VCH. c) The schematic diagram presents a formation of glue layer (purple) in a Li[Ni 0.8 Co 0.15 Al 0.05 ]O 2 secondary particle (gray) during the coating process. Reproduced with permission. [94] Copyright 2016, Wiley-VCH. d) Scheme of the concentration gradient core shell spherical particle comprising of nanoparticle shell and nanorod shell. Reproduced with permission. [93a] Copyright 2014, Wiley-VCH (24 of 29)

25 there is need to make more efforts in illustrating the underlying origin of this phenomenon, and then seeking simplistic and more effective strategies of solving this intractable problem Elusive Cathode/Electrolyte Interphase In the LIBs, the electrode/electrolyte interface is no longer 2D entities owing to the great possibilities where cathode and anode operate. Instead, the interface became 3D independent interphases, which resulted from the decay of electrode surface as well as electrolyte components. Different from its counterpart on anode, restrained effort has been made in understanding the surface/interface of cathode. As noted by Xu, the selective sensitivity of most spectroscopic strategies to certain chemical species and simultaneous blindness to others commonly lowered our understanding of the interphases to a blindmen-elephant interaction. [96] Few studies were available in term of their formation mechanisms and chemical compositions. According to Winter, [97] cathode/electrolyte interphase is the most significant, though least understood component. The interphase is crucial and essentially dependable for reversibility of Li + -intercalation chemistries while dictating the kinetics of the general cell redox reactions. In other words, they are difficult to characterize owing to their sensitive chemical nature, elusive manner of formation, as well as the lack of reliable in situ measurement tools. [98] In the recent past, some developments have been made in the use of various new models and tools. Particularly, the high precision coulometry technology that was suggested by Dahn and co-workers, [99] despite its phenomenological nature, leads to a breakthrough in interphasial characterizations. However, there have been debates on many foundational issues, including the basic chemical compositions of electrode/electrolyte interphase. As a much needed in situ, nondestructive and quantitative techniques (as summarized in Figure 25) [13a] have permitted achieving of insights into these complex reaction Figure 25. Space-resolution schemes of experimental techniques for the measurement of surface/interfacial chemistries in LIBs. Reproduced with permission. [14a] Copyright 2016, the Chinese Physical Society. mechanisms as well as evolutions taking place on the electrolyte/cathode surface/interphase. 5. Summary and Outlook Nickel-rich layered lithium transition-metal oxides generate large reversible capacities of above 200 mah g 1, high operating voltage of 3.8 V, good rate capability along with low cost, hence it is perceived as one of the most promising cathodes for the next-generation LIBs. Interestingly, the type LIBs which utilized the high-capacity nickel-rich Li[Ni 0.8 Co 0.15 Al 0.05 ]O 2 as cathode have been successfully applied as power sources for Tesla EVs. Moreover, whereas nickel-rich positive electrodes are paired with high-capacity Si/C anode electrodes, the energy density of full battery can reach up to 300 Wh Kg 1. Even if these above-mentioned superior properties performed in nickel-rich cathodes, some complex problems are still experienced, which restrict their particle applications in LIBs. The structure evolutions from pristine layered to spinel and/or rock-salt phases, side reactions between highly oxidizing TMs with organic electrolyte, as well as oxygen release from host lattice preferentially take place at surface/interface of the highly delithiated nickel-rich cathodes. These are likely result in capacity fade together with safety hazard. Thus, it is clear that the features of surface/interfacial structure as well as chemistry are at the core of making nickel-rich cathodes perfect for the next-generation LIBs. In the recent past, five main strategies, including surface modification, core shell/concentration-gradient structure, foreign-ion substitution, hybrid surface, together with electrolyte additive have rationally been suggested in order to make the surface/interfacial structure and chemistry better. The uniform and thickness-controllable coating layer performed by clear surface modification strategies, like atomic layer deposition (ALD), and pulsed laser deposition (PLD), among others ought to draw much attention. Prepared repeatability as well as consistency of core shell nickel-rich cathode materials is a considerable challenge for their practical applications in LIBs. The most intriguing direction that is yet to be realized is on how to effectively stabilize the surface structure and chemistry under low inactive foreign-ion substitution ratio (probably below 3 wt%). The concentration-gradient doped tactics, wherein doped foreign-ions enrich at the particle and/or grain surface, opens up a new insight into accomplishing the mentioned target. The surface doping together with coating coexisted hybrid structure is regarded as an effective process to formulate perfect electrode/ electrolyte interphase. Nonetheless, coming up with the simple and superficial means of realizing this new design need needs more foundational studies. It is also significantly important to advance and use state-of-the-art noninvasive, surface-sensitive characterization strategies to understand in operando behaviors of electrolyte additives. In contracts with the nonaqueous (organic electrolyte) lithium-ion batteries previously discussed, the aqueous systems exhibits exceptional safety properties, especially for the highcapacity battery pack for EVs. [100a] Recently, layered lithium transition-metal oxide Li[Ni 1/3 Co 1/3 Mn 1/3 ]O 2 was introduced as a beneficial cathode for aqueous rechargeable lithium-ion (25 of 29)