Atomic Layer Deposition of Stable LiAlF 4 Lithium Ion. Conductive Interfacial Layer for Stable Cathode Cycling

Transcription

1 Atomic Layer Deposition of Stable LiAlF 4 Lithium Ion Conductive Interfacial Layer for Stable Cathode Cycling Jin Xie 1, Austin D. Sendek 2, Ekin D. Cubuk 1, Xiaokun Zhang 1, Zhiyi Lu 1, Yongji Gong 1, Tong Wu 1, Feifei Shi 1, Wei Liu 1, Evan J. Reed 1, Yi Cui 1,3 1 Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA 2 Department of Applied Physics, Stanford University, CA 94305, USA 3 Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA

2 Table of contents: 1. Additional experimental details 2. Prediction of electrochemical stability window and lithiation potentials 3. XPS analysis of ALD LiF and AlF 3 films 4. GIXRD characterization of ALD LiF, AlF 3 and LiAlF 4 films 5. CV and XPS analysis of ALD LiAlF 4 film 6. EIS characterization of LiAlF 4 film from 80 C to 0 C 7. SEM, XRD and ICP-MS characterizations of NMC Cycle stability of NMC-811 at different electrochemical windows 9. SEM characterization of NMC-811 electrode and LiAlF 4 /NMC-811 electrode 10. Cycle stability of multiple NMC-811 and LiAlF 4 /NMC-811 samples 11. A brief discussion of failure mechanisms of NMC-811 (analysis of Coulombic efficiency, analysis of electrochemical impedance spectra data, SEM characterization and TEM characterization) 12. A brief summary of cycle stability of NMC-811 in this study and in the literatures

3 Additional Experimental Details Materials Characterizations: X-ray diffraction (XRD, PANalytical X Pert Diffractometer) was used for phase identification of NMC-811 using Cu Kα radiation of nm. To measure the Ni, Mn and Co contents, NMC- 811 powders were dissolved using aqua regia. The resulting solution was then diluted and tested using ICP-MS. Grazing incidence X-ray diffraction (GIXRD, Bruker D8 Venture) was used for phase identification of ALD films using Cu Kα radiation of nm. Cyclic voltammetry scans were recoreded using a Biologic VSP potentiostat at a scan rate of 1 mv/s. TEM characterization was performed at 300 kv using a FEI Titan.

4 Prediction of Electrochemical Stability Window and Lithiation Potentials Figure S1. Phase diagram of Li-Al-F and formation energies of various Li-Al-F compounds. The phase diagram of Li-Al-F and formation energies of LiF, AlF 3, Li 3 AlF 6, LiAl, Li 3 Al 2 and Li 9 Al 4 were acquired from the materials project (materialsproject.org). Our DFT calculation predicted a formation energy of ev/atom or kj/mol for LiAlF 4. It was more stable than LiAlCl 4 ( ev/atom) but less stable than LiYF 4 ( ev/atom). The calculated stability window was V vs. Li + /Li for LiAlF 4 based on the anodic reaction (E1) and cathodic reaction (E2) listed below. The V vs. Li + /Li electrochemical window is wider than the operation electrochemical windows of most cathodes being used today. (E1) LiAlF 4 Li + 0.5F 2 + AlF 3 (E2) LiAlF 4 + Li 2/3Li 3 AlF 6 + 1/3Al E = 5.72 V vs. Li + /Li E = 2.05 V vs. Li + /Li The electrochemical stability window of Li 3 N, Li 2 O, LiF, LiAlO 2 and Li 3 PO 4 were calculated based on the same strategy described above. Their decomposition reactions were listed as below: (E3) Li 3 N 3Li + 0.5N 2 (E4) Li 2 O 2Li + 0.5O 2 (E5) LiF Li + 0.5F 2 (E6) LiAlO 2 0.8Li + 0.2LiAl 5 O O 2 E= 0.61V vs. Li + /Li E= 3.11V vs. Li + /Li E= 6.36V vs. Li + /Li E= 3.70V vs. Li + /Li

5 (E7) LiAlO Li 0.5Li 5 AlO Li 3 Al 2 (E8) Li 3 PO 4 Li + 0.5Li 4 P 2 O O 2 (E9) Li 3 PO 4 + 8Li 4Li 2 O + Li 3 P E= 0.17V vs. Li + /Li E = 4.21V vs. Li + /Li E = 0.69V vs. Li + /Li According to these decomposition reactions, we conclude the electrochemical stability windows for the following compounds: Li 3 N (0-0.61V vs. Li + /Li), Li 2 O (0-3.11V vs. Li + /Li), LiF (0-6.36V vs. Li + /Li), LiAlO 2 ( V vs. Li + /Li) and Li 3 PO4 ( V vs. Li + /Li). Five degradation reactions are thermodynamically possible when AlF 3 was applied as a protection film in lithium ion batteries. (E10) AlF Li 0.5Li 3 AlF Al (E11) 0.5Li 3 AlF Al + 1.5Li 3LiF + Al (E12) 3LiF + Al + Li 3LiF + LiAl (E13) 3LiF + LiAl + 0.5Li 3LiF + 0.5Li 3 Al 2 (E14) 3LiF + 0.5Li 3 Al Li 3LiF Li 9 Al 4 E = 1.28V vs. Li + /Li E = 1.06V vs. Li + /Li E = 0.36V vs. Li + /Li E = 0.19V vs. Li + /Li E = 0.07V vs. Li + /Li Depending on the exact applied potential, it is thermodynamically possible for AlF 3 to convert to other phases (such as Li 3 AlF 6 and Li x Al y alloys). These phases may have high intrinsic lithium ion conductivity and porous structures due to volume change upon phase transformation, both will facilitate the lithium ion transport across the film. However, AlF 3 film remain intact on the cathode and it should not been lithiated according to our calculation.

6 XPS Analysis of ALD LiF and AlF 3 Films Figure S2. (a) XPS characterization of ALD LiF films grown at different temperatures; (b) refractive index (n) and extinction coefficient (k) of ALD LiF film; (c) growth rate per cycle of ALD LiF at different temperatures. ALD LiF films can be obtained at a temperature range from 200 to 300. The as-prepared films were characterized by XPS, which displayed distinct Li and F peaks (figure S2a). The thicknesses of LiF films were measured using an ellipsometer. The refractive index of ALD LiF was close to 1.4 and the extinction coefficient was close to 0 (figure S2b). The highest deposition rate of 0.5Å/cycle was achieved at a deposition temperature of 250.

7 Figure S3. (a) XPS characterization of ALD LiF film before and after sputtering; (b) chemical compositions of ALD LiF film analyzed by XPS; (c) XPS characterization of ALD AlF 3 before and after sputtering; (d) chemical compositions of ALD AlF 3 film analyzed by XPS. XPS characterization was carried out for both LiF and AlF 3 films. There were C and O impurities presumably absorbed on the surfaces of both films before sputtering. The C and O contents were higher on AlF 3 than those on LiF, which might be explained by the hygroscopic nature of AlF 3. XPS characterization was then carried out for both films after sputtering. The intensities of both C and O peaks were reduced after mild sputtering. While the Li: F atomic ratio in ALD LiF film obtained in this study was close to its stoichiometry value of 1 (Li: F = 1: 0.97), the ALD AlF 3 film showed excess of F with Al: F atomic ratio being 1: Different ALD processes may yield films with different Al: F ratios. For instance, Lee et al. reported Al: F = 1: 2.4 (characterized by XPS) for their ALD AlF 3 films using Al(CH 3 ) 3 and HF as precursors;

8 Mantymaki et al. reported Al: F ratio of 3.2 to 5 (characterized by time-of- flight elastic recoil detection analysis) for their ALD AlF 3 films using AlCl 3 and TiF 4 as precursors.

9 GIXRD Characterization of ALD LiF, AlF 3 and LiAlF 4 Films Figure S4. Grazing incidence X-ray diffraction characterization of LiF, AlF 3 and LiAlF 4 thin films deposited at 250 ºC by atomic layer deposition. Among three types of films, ALD LiF film is crystalline (figure S4). In this study, ALD LiF film is prepared using lithium tert-butoxide and TiF 4 as precursors. AlF 3 film prepared by ALD is amorphous according to our GIXRD characterization (figure S4). The result agrees with literature report that AlF 3 films prepared by ALD are amorphous when the deposition temperature is below 280 ºC. 1 LiAlF 4 film prepared by ALD in this study is also amorphous according to GIXRD characterization (figure S4). The adoption of AlF 3 into LiF may affect the rearrangement of atomic order at thermal activation. For instance, the LiAlF 4 film prepared by evaporation remained amorphous even after high temperature annealing up to 600 ºC. 2,3 The amorphous LiAlF 4 film was reported to be a good lithium ion conductor with a lithium ion conductivity in the order of S/cm, 2,3 and it has shown great stability in electrochromic devices. 4,5

10 CV and XPS Analysis of ALD LiAlF 4 Film Figure S5. Cyclic voltammetry scans of LiAlF 4 /stainless steel and bare stainless steel in organic electrolyte. The counter electrode is Li foil and the scanning rate is 1 mv/s. In addition to the DFT calculation, cyclic voltammetry (CV) was performed to confirm the stability of LiAlF 4 in the electrochemical window ( V vs. Li + /Li) tested. ALD LiAlF 4 film with a thickness of 50 nm was deposited onto a stainless steel (SS) substrate as the working electrode. The counter electrode is Li foil. 1 M LiPF 6 in EC/DEC was added as the liquid electrolyte. When comparing working electrode with and without LiAlF 4 coating (figure S5), no additional peaks within the potential window of V vs. Li + /Li were found for working electrode with LiAlF 4 coating. The result supports our claim that the LiAlF 4 film is electrochemically stable within the potential window we tested. Although there is no lithiation or delithiation peaks found in the CV scans, there is anodic current at high applied potential for both working electrodes with and without LiAlF 4 coating, which might relate to the decomposition of organic liquid electrolyte.

11 Figure S6. (a) XPS characterization of LiAlF 4 /stainless steel after CV scans with and without surface Ar sputtering; (b) chemical compositions of LiAlF 4 /stainless steel after CV scans with and without surface Ar sputtering analyzed by XPS; (c-e) XPS fine scans of Li 1s, Al 2p and F 1s peaks of LiAlF 4 /stainless steel after CV scans with surface Ar sputtering. To further confirm the stability of LiAlF 4 film, the chemical composition of the working electrode was measured using XPS (figure S6) after repeated CV scans. The surface of the working electrode with LiAlF 4 coating contains Li, Al, F, C and O elements. The C and O elements could come from (1) contamination from ambient environment during sample transfer; (2) residue of organic electrolyte and LiPF 6 salt; and (3) decomposition of organic liquid electrolyte. After mild sputtering, C and O concentration decreased significantly. The Li: Al: F atomic ratio is 1.3: 1.0: 4.5 after sputtering, which is close to pristine LiAlF 4 film without any electrochemical cycling (main text, figure 3). The Li 1s, Al 2p and F 1s peaks are located at 55.4

12 ev, 75.2 ev and ev according to XPS fine scans. DFT, CV and XPS results confirmed the LiAlF 4 film prepared by ALD is stable in a wide electrochemical window we tested.

13 EIS Characterizations of LiAlF 4 Film From 80 C to 0 C Figure S7. (a, b) Lithium ion conductivity measurements using electrochemical impedance spectroscopy (EIS); (c) conductivity vs. temperature plot of LiAlF 4 film. EIS spectra were also recorded for LiAlF 4 film by decreasing the testing temperatures from 80 to 0. The calculated lithium ion conductivities and activation energy were similar compared to the numbers acquired by increasing temperatures from 20 to 90 (figure 4 of the main text).

14 SEM, XRD and ICP-MS Characterizations of NMC-811 Powder Figure S8. (a, b) SEM characterization of NMC-811 used in this study; (c) XRD characterization of pristine NMC-811; (d) ICP-MS characterization of pristine NMC-811. Figure S8a and S8b shown the morphology of NMC-811 powders used in this study. XRD characterization confirmed that NMC-811 particle is of single phase. The Ni: Mn: Co atomic ratio is 79 %: 10 %: 11 % according to ICP-MS.

15 Cycle Stability of NMC-811 at Different Electrochemical Windows Figure S9. (a) Cycle performance of pristine NMC-811 electrodes at room temperature with electrochemical windows of V and V vs. Li + /Li. High nickel content NMC is known for its instability when cycled to high cut-off potentials. 6,7 In figure S9, we tested long-term cycle stability of pristine NMC-811 with different electrochemical windows. The one cycled to 4.2V vs. Li + /Li has a lower initial specific capacity, but a higher capacity retention than the one cycled to 4.5V vs. Li + /Li.

16 SEM Characterization of NMC-811 Electrode and LiAlF4/NMC-811 Electrode Figure S10. SEM characterization of NMC-811 electrode (a-d) and ALD LiAlF4 coated NMC811 electrode (e-h). SEM images of pristine NMC-811 electrode and ALD LiAlF4 coated NMC-811 electrode at different magnifications were shown in figure S10. On the electrode level, the morphology of the electrode does not change after coating (figure S10a-b and S10e-f). No film cracking or film peeling off were observed. The result indicates the interconnectivity of NMC-811 particles is not compromised after ALD coating. The morphology of the NMC-811 particles also remained same after ALD LiAlF4 coating. First, the secondary particles still bond tightly together, which suggests NMC-811 particles did not crack during heating and cooling of the ALD process (figure S10c and S10g). Second, due to the thinness and uniformity of ALD coating, the surface of NMC-811 secondary particles remained smooth and flat after ALD LiAlF4 coating (figure S10d and S10h).

17 Cycle Stability of Multiple NMC-811 and LiAlF 4 /NMC-811 Samples Figure S11. (a) Cycle performance of 20c ALD LiAlF 4 coated NMC-811 electrodes with an electrochemical window of V vs. Li + /Li at room temperature. Three cells were tested at 50mA/g. (b) Cycle performance of pristine NMC-811 electrodes with an electrochemical window of V vs. Li + /Li at room temperature. Two cells were tested at 50mA/g. Multiple cells with and without ALD coating have been tested to confirm the effectiveness of coating layer in enhancing the cathode stability. Three cells were tested for LiAlF 4 coated NMC- 811 and shown similar stabilities in the range of 100 cycles. Two cells were tested for pristine NMC-811 and also shown similar stabilities in the range of 100 cycles.

18 A Brief Discussion of Failure Mechanisms of NMC-811 (Analysis of Coulombic Efficiency and Electrochemical Impedance Spectra Data) Among various cathode candidates, high Ni content layered lithium metal oxides (LiNi 1 x y Mn x Co y O 2 ) have drawn worldwide interests mainly because of their high specific capacity and high operating voltage. They are capable of delivering a ~200 mah/g capacity when 1 - x - y > 0.7; however, the capacity retention and thermal stability are compromised when Ni content is high. The reasons for the poor capacity retention have been discussed in the literature, which are associate with these two main factors: 1. Side reactions between electrode, in particular high oxidation state Ni 4+, and organic electrolyte. 8,9 The highly reactive Ni 4+ ions can accelerate electrolyte decomposition, especially at high applied potentials. The accumulation of side products would hinder lithium ion transportation at the electrode-electrolyte interface. 2. Phase transition from layered to rock salt structure at particle s surface. Such transition would also reduce the lithiation kinetics at the interface of electrode-electrolyte The reported failure mechanisms by literature highlighted the importance of our research, as stabilizing the surface is critical to achieve long-term cyclability for high Ni content layered lithium metal oxides. 6,7,13-18 The coating layer needs to be uniformly covering the entire surface, chemically inert and electrochemically stable against the applied voltage window, while offering high lithium ion conductivity. Such stable and uniform coating in principle could minimize the side reactions between the electrode (such as Ni 4+ ) and electrolyte. Average CE of NMC-811 Average CE of LiAlF 4 /NMC-811 Room temperature 99.3% 99.8%

19 Elevated temperature 97.4% 99.7% Table S1. Average Coulombic efficiency of NMC-811 and LiAlF 4 /NMC-811 at both room temperature and elevated temperature. In order to prove this hypothesis, the magnitude of parasitic reactions can be monitored by measuring the Coulombic efficiency (CE) of the battery The Coulombic efficiency is defined as: CE = capacity of discharge/capacity of charge. The cycle life of lithium ion battery is not infinite because small amount of active materials (electrode and/or electrolyte) are consumed by parasitic reactions during each cycle. As Li is excess in the counter electrode in the NMC-811/Li half-cells, the detected CE lower than unity (100.0%) can be explained by the side reactions on the cathode side. When tested at room temperature, the average Coulombic efficiency of pristine NMC-811 was 99.3% (table S1), indicating a high degree of parasitic reactions, such as electrolyte oxidation by highly reactive Ni 4+ at high-applied potentials, took place. In contrast, the average Coulombic efficiency of LiAlF 4 /NMC-811 was 99.8% (stable S1). At elevated temperatures, side reactions would speed up. 14,17 Indeed, the poor cycle stability at elevated temperatures is the most considerable drawback for high Ni content layered lithium metal oxides. 6,7,15 Therefore, testing the long-term cycle stability at elevated temperatures presents a rigorous, meaningful, and real-world relevant test for high Ni content layered lithium metal oxides. The average CE dropped from 99.3% at room temperature to 97.4% at elevated temperature for pristine NMC-811. The decrease of CE suggests the magnitude of parasitic reactions increased at elevated temperature. With a stable and uniform ALD LiAlF 4 coating, the average CE improved significantly from 97.4% to 99.7% at elevated temperature. The difference

20 we observed in our CE measurement strongly supports our hypothesis that the improvement of cycle stability by LiAlF 4 coating is through reduced parasitic reactions. Figure S12. Equivalent circuit of NMC-811/Li half-cells. R ct at 1 st cycle R ct at 10 th cycle R ct at 25 th cycle R ct at 50 th cycle (Ω.g) (Ω.g) (Ω.g) (Ω.g) NMC LiAlF 4 /NMC Table S2. Calculated charge transfer resistance (R ct ) of NMC-811 and LiAlF 4 /NMC-811 after different numbers of battery cycles at room temperature. The parasitic reactions at the interface of electrode and electrolyte also cause the accumulation of side products on the surface of the cathode, and its accumulation can be detected using impedance spectroscopy. 8 As shown in figure 6 of the main text, the impedance spectra of the battery at different cycle numbers were recorded. The impedance spectra of NMC-811 electrodes with and without LiAlF 4 coating comprised two semicircles and a straight line angled close to 45. The small semicircle at high frequency was attributed to the solid electrolyte interface (R SEI and CPE SEI ), while the one at medium frequency corresponded to the charge transfer process at the cathode/electrolyte interface (R ct and CPE ct ). 8 The 45 inclined line at low frequency was due to the Warburg impedance (Z w ), which was related to the diffusion of lithium ions within the cathode. Upon cycling, the semicircle at medium frequency increased significantly for the pristine

21 NMC-811, indicating the slowdown of charge transport across the cathode/electrolyte interface. By fitting the impedance spectra using the equivalent circuit (figure S12), the charge transfer resistance at the electrode and electrolyte interface for both pristine NMC-811 and ALD LiAlF4 coated NMC-811 after different numbers of cycles can be calculated (table S2). Although the initial charge transfer resistance was similar for both pristine NMC-811 and ALD LiAlF4 coated NMC-811 after the 1st cycle, it increased much faster for pristine NMC-811 compared to ALD LiAlF4 coated NMC-811 upon cycling. The fact that the LiAlF4 coating layer suppressed the fast impedance growth supports that LiAlF4 film could minimize the magnitude of parasitic reactions at the electrode and electrolyte interface. Figure S13. SEM characterization of cycled NMC-811 electrode (a-d) and cycled ALD LiAlF4 coated NMC-811 electrode (e-h). SEM images of cycled NMC-811 electrode and cycled ALD LiAlF4 coated NMC-811 electrode at different magnifications were shown in figure S13. On the electrode level, the morphology of the electrode does not change after battery cycling for both NMC-811 electrode (figure S10a-b

22 and figure S13a-b) and ALD LiAlF 4 coated NMC-811 electrode (figure S10e-f and S13e-f). The electrodes remained intact without cracking or peeling off. The morphology of the NMC-811 particles with and without ALD coating also remained same cycling. First, the secondary particle still bond tightly together (figure S13c and S13g). Second, no particle cracking has been observed for individual secondary particles (figure S13d and S13h). For similar NMC-811 electrodes cycled with a cut-off potential of 4.4 V vs. Li + /Li, no significant damage such as particle cracking or disconnection between particles, was observed in the literature as well. 14 The result suggests that unlike micrometer-scale primary LiCoO 2 particles, 22 particle cracking is not one of the major failure mechanisms for NMC-811 particles tested in this study. Figure S14. TEM characterization of 50 ALD cycles of LiAlF 4 coated NMC-811 electrode after cycling. TEM images of 50 ALD cycles of LiAlF 4 coated NMC-811 electrode after battery cycling were shown in figure S12. The coating layer is uniform on the surface of NMC-811 particle after battery cycling. The result agrees well with our XRD characterization that the LiAlF 4 film prepared by ALD is amorphous while the NMC-811 core is crystalline. The TEM characterization, along with DFT, CV and EIS characterization, highlighted the stability of

23 LiAlF 4 film prepared by ALD. Notably, several groups have conducted very detailed TEM characterization recently to investigate the failure mechanism. And it was reported that the phase transition from layered to rock salt may also contribute to the failure of high Ni content layered lithium metal oxides By designing a stable surface on the electrode 10 or improving the stability of the electrolyte, 13 such phase transition could be suppressed.

24 A Brief Summary of Cycle Stability of NMC-811 in This Study and in the Literatures Test conditions Capacity retention Additional comments References V, 1/2C 70.2%, 100 cycles Noh et al. 23 (99.65% per cycle) V, 1/5C 74%, 57 cycles, Li et al. 14 (99.47% per cycle) V, 1/5C 64%, 100 cycles, Zhang et al. 24 (99.55% per cycle) V, 1/2C 48%, 100 cycles Wu et al. 25 (99.27% per cycle) V, 1/4C 76%, 300 cycles ALD LiAlF 4 This study (99.91% per cycle) NMC-811 Table S3. Stability of NMC-811 in this study compared to literature reports. High Ni content NMC (NMC-811 etc.) is known for its instability compared to LCO or low Ni content NMC. 6,7 Depending on the cathode preparation methods and detailed cycle conditions, the capacity retentions per cycle were often in the range of 99.3%-99.6%. 14,23-25 For pristine

25 NMC-811, the capacity retention per cycle decreased significantly with increased cut-off potentials. With ALD LiAlF 4 coating, we have improved the capacity retention per cycle to higher than 99.9% at a wide electrochemical widow of V vs. Li + /Li. Future work in optimizing LiAlF 4 ALD (chemical composition, lithium ion conductivity and thickness) may yield higher stability.

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