Lithium Ion Batteries Lecture WS 2016/2017

Transcription

1 Ulm, Lithium Ion Batteries Lecture WS 2016/2017 Margret Wohlfahrt-Mehrens Zentrum für Sonnenenergie- und Wasserstoff-Forschung (ZSW) Baden-Württemberg - 1 -

2 Major types of reaction: Insertion reaction Negative Positive Charge Discharge Change in composition of solid phase xli + + MX 2 + xe - = Li x MX 2 Source: M. Armand; J.-M. Tarascon, Building better batteries, Nature 2008, 451,

3 Lithium insertion materials for positive electrode - Requirements 1 - Key requirements for a material to be used as a positive electrode in a rechargeable lithium battery are as follows contains at least one reducible/oxidizable ion, for example a transition metal e.g. Co, Ni, Mn, Fe, reacts with lithium highly reversible, via an insertion-type reaction; the host structure does not change as lithium is added or removed reacts with lithium with a high free energy of reaction: High voltage, preferably around 4 V; given by the Gibbs free energy of the reaction: U 0 = - G/n F High capacity, given by the number of lithium ions which can be inserted reversibly; preferably at least one lithium per transition metal - 3 -

4 Lithium insertion materials for positive electrode - Requirements 2 - reacts with lithium very rapidly both on insertion and removal This leads to high power density, the kinetics is dependent on both lithium ion transport and electronic transport The material should be a good electronic conductor, preferably a metallic conductor This allows the easy addition or removal of electrons during the electrochemical reaction. This allows reaction at all contact points between the positive active material and the electrolyte If the electronic conductivity of the active material is poor, the reaction has to take place at ternary contact points between the positive active material, the electrolyte and the electronic conductor (such as carbon black). Good electronic conductivity of the active material minimizes the need for inactive conductive additives, such as carbon black The material should be a good ionic conductor, lithium diffusion should occur easy within the structure The material should be stable, i.e., not change the host structure, the lattice expansion or shrinkage should be minimized - 4 -

5 Lithium insertion reactions Advantages of insertion mechanism: Rather small volume changes during cycling Good mechanical stability, good cycle life (for graphite: >1000 cycles) No electrolyte excess necessary No lithium metal during cell production Limitations of insertion materials: Rate limitations, Li + diffusion in solid state materials Limited capacity (structural stability) Lithiated cathode materials necessary Parasitic reactions lead to irreversible losses no lithium excess - 5 -

6 Lithium insertion materials - chalcogenides - Layered dichalcogenides e.g. titanium disulfide (TiS 2 ) Layered material TiS 2 Lightest material among chalcogenides High electronic conductivity, semimetallic, no additional conductive additive needed Single phase reaction with lithium over the entire composition range of Li x TiS 2 for 0 x 1 No phase change during insertion and removal of lithium; highly reversible TiS 2 + Li + + e - LiTiS 2-6 -

7 Layered structure of LiTiS 2, LiVSe 2, LiCoO 2, LiNiO 2, showing the lithium ions between the transition-metal oxide/sulfide sheets. The actual stacking of the metal anion sheets depends on the transition metal and the anion - 7 -

8 Lithium insertion in TiS 2 charge discharge Discharge: TiS Li e - LiTiS 2 Charge: LiTiS 2 TiS Li e - Discharge/charge curve of Li/TiS 2 ; shows the typical single phase behavior, no nucleation of new phases Many chalcogenides show a similar single phase behavior - 8 -

9 2 to 3 V cathode materials Dichalcogenides are normally synthesized in the delithiated state; lithiation needs strong reducing lithiation compounds like n-buthyllithium; The lithiated phases are not stable in air and have to be handled under inert gas Therefore they have to be combined with a lithiated negative electrode like lithium metal Cells are assembled in the charged state and have to be discharged In these cells the negative electrode is the lithium source, the positive is the lithium sink - 9 -

10 Insertion materials for lithium metal batteries - assembled in the delithiated (charged) state - Delithiated cathode Positive Lithiated anode Negative

11 Lithium insertion materials - oxides - In 1981 Goodenough discovered that LiCoO 2 has a structure similar to the layered structures of the dichalcogenides; This material can be synthesized in the lithiated state and Goodenough showed that lithium can be removed electrochemically These results opened new directions for rechargeable lithium batteries using delithiated negative insertion materials like carbon and lithiated positive materials like LiCoO 2 The cell is assembled in the discharged state and has to be charged before use Lithium ion cells: positive material is the lithium source

12 Insertion materials for lithium ion batteries - assembled in the lithiated (discharged) state - Positive Negative

13 Prof. John Goodenough a pioneer of Li-ion cathode materials LiNiO 2 - J.B. Goodenough, D.G. Wickham, J. Phys. Chem. Solids 5 (1958) LiCoO 2 - K. Mizushima, P.C. Jones, P.J. Wiseman, J.B. Goodenough, Mater. Res. Bull. 15 (1980) LiMn 2 O 4 - M.M. Thackeray, P.J. Johnson, L.A. de Piciotto, P.G. Bruce, J.B. Goodenough, Mater. Res. Bull. 19 (1984) SONY - First Li-ion battery (cobaltite) LiFePO 4 - A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, JECS 144, (1997)

14 Lithium-Ion-Batteries: Materials combinations Cyclic carbonates LMnP / LMnPO

15 Electrode materials and stability of inactive materials Cu Oxidation Al corrosion Dependent on electrolyte

16 Lithium insertion materials In general there are three main classes of lithium insertion materials for use as positive active materials in rechargeable lithium batteries 1. layered compounds with an anion close-packed or almost closepacked lattice in which alternate layers between the anion sheets are occupied by redox-active transition metal lithium inserts itself into the essentially empty remaining layers 2. spinel type structure can be considered as a special case where the transition-metal cations are ordered in all the layers 3. materials with more open structures, like many of the vanadium oxides, manganese oxides with tunnel structure and transition metal phosphates with olivine structure,

17 Positive electrode materials Layered structure LiCoO 2, Li(Ni 0,80 Co 0,15 Al 0,05 )O 2, LiCo 1/3 Ni 1/3 Mn 1/3 O 2 Spinel structure LiMn 2 O 4, LiMn 1.5 Ni 0.5 O 4 Olivine structure LiFePO 4, LiMePO 4 Dependent on structure: Differences in Li + diffusion Differences in reaction mechanisms (phase transformation) Differences in lattice stability in the delithiated state

18 Lithium insertion materials: LiCoO 2 easy synthesis via solid state process LiCoO 2 has the α-nafeo 2 structure with the oxygens in a cubic close-packed arrangement highly reversible, limited kinetics due to lower electronic conductivity compared to chalcogenides LiCoO 2 Li 1-x CoO 2 + x Li + + x e - (0 x 1) complete removal of the lithium leads to a rearrangement of oxygen layers to give hexagonal close packing of the oxygen in CoO 2 Potential gegen Li/Li+ in mv charge discharge spez. Kapazität in mah/g limited to removal of 0.5 Li by end of charge potential LiCoO 2 Li 0.5 CoO Li e

19 Lithium insertion materials: LiNiO 2 LiNiO 2 isostructural with LiCoO 2 Preparation of pure LiNiO 2 not successful, due to Li/Ni disorder within the structure, in contrast to cobalt nickel tends to occupy the lithium positions within the structure, which leads to effective blocking of the lithium diffusion within the material The fully delithiated state NiO 2 does not exist; Li x NiO 2 with low lithium content tends to a decomposition reaction and loss of oxygen Therefore pure LiNiO 2 cannot be used as positive material in lithium batteries

20 Layered materials LiCoO 2 LiNiO 2 LiNiO 2 Li(Ni,Co)O 2 LiCoO 2 Complete series of mixed oxides: LiNi 1-x Co x O 2 y Li + + Li 1-y Ni 1-x Co x O 2 + y e - 0 x 1; 0 y 1 single phase reaction

21 Problem: Li/Ni-disorder [Li] [Ni III ]O 2 O 2 Ni II Li 2 O [Li 1-x Ni II x/2] [Ni III 1-xNi II x/2]o 2-x + x/2 Li 2 O + x/4 O 2 Controlled by Synthesis parameters: po 2, pco 2, T

22 Effect and interpretation irreversible loss during 1.cycle ( x) high polarisation x NiO 2 -Schicht layer Li + Li + Li + Li + Li + Li + Li + NiO 2 -Schicht layer Li + NiO 2 -Schicht layer Li + Ni 3+ Li + Li + Li 0.94 Ni 1.06 O 2 NiO 2 -Schicht layer Delmas et al., J. Power Sources 1997, 68,

23 Reduction of Li/Ni disorder by Co content Whittingham, Chem. Rev. 2004,

24 Layered materials Mixed oxides III III IV IV charge LiNi 1-y Co y O 2 Ni 1-y Co y O 2 + Li + + e - discharge Completely delithiated Ni 1-y Co y O 2 not stable, limitation of removal of lithium by electronics or substitution with other dopands Partial substitution of Ni or Co by Al leads to limitation of removal of lithium ions from the host lattice charge LiNi 1-x-y Co x Al y O 2 Li y Ni 1-x-y Co x Al y O y Li y e - discharge 0 x 1; 0 y 1; 0 (x+y)

25 Layered materials - mixed oxides - insertion/removal of lithium within the host lattice requires reduction/oxidation of transition metal within the lattice partial substitution of Ni or Co by fixed trivalent or divalent elements e.g. Al or Mg leads to reduced number of lithium, which can be removed from the host lattice; but to higher structural stability in the delithiated state Lithiated state Delithiated state max. rev. lithium [Li] [Co III ]O 2 [Co IV ]O 2 1 [Li] [Ni III 1-xCo III x]o 2 [Ni IV IV 1-xCo x ]O 2 1 [Li] [Ni III 1-xAl III x]o 2 [Li] x [Ni IV 1-xAl III x]o 2 1-x [Li] [Ni III 1-x-yCo III xal III y]o 2 [Li] y [Ni IV 1-x-yCo IV xal y ]O 2 1-y [Li] [Ni III 1-2xNi IV xmg II x]o 2 [Li] 2x [Ni IV 1-xMg II x]o 2 1-2x

26 Layered materials - mixed oxides - Lithiated state Delithiated state max. rev. lithium 1 [Li] [Ni III 0.8Al III 0.2]O 2 [Li] 0.2 [Ni IV 0.8Al III 0.2]O [Li] [Ni III 1-x-yCo III xal III y]o 2 [Li] y [Ni IV 1-x-yCo IV xal y ]O 2 1-y [Li] [Ni III 1 2*0.5Ni IV 0.5Mg II 0.5]O [Li] 2*0.5x [Ni IV 1-0.5Mg II 0.5]O 2 1-2x =

27 Any composition of the materials class Li(Ni 1-x-y Mn x Co y )O 2 can be represented by one data point in the ternary diagram shown here

28 Layered materials: manganese based oxides Ion distribution of LiMO 2 LiCo III 1/3Mn IV 1/3Ni II 1/3O 2 LiMn IV Ni II O 2 Metal ratio M IV M II M III Mn Ni Co substitution LiCo III O 2 : Co III Co IV + e LiMn IV Ni II O 2 : Ni II Ni IV + 2e

29 Layered materials Mixed Nickel-Manganese Dioxide, LiNi 1-y Mn y O 2 - Multielectron Redox Systems LiMn 0.5 Ni 0.5 O 2 Mn 0.5 Ni 0.5 O 2 + Li + + e - Manganese shows no significant layer stabilization normally 10% Li/Ni disorder limited rate capability IV II III IV IV III charge LiMn y Ni y Co 1-2y O 2 Li 1-y Mn y Ni y Co 1-2y O 2 + y Li + +y e y 0.33 LiNi 1/3 Co 1/3 Mn 1/3 O 2 Can be described as solid solution between LiMn 0.5 Ni 0.5 O 2 and LiCoO IV II IV IV charge discharge discharge

30 Layered materials: Example of Synthesis homogenous distribution of transition metals!

31 Layered materials: manganese based oxides LiNi 0.33 Mn 0.33 Co 0.33 O 2 (NCM) 4800 Voltage vs. Li/Li + / mv Capacity / mah g -1 Development trends: reduction of Co content, surface modifications tuning of electrolyte interface in order to increase end of charge potential

32 Layered oxides State of the art Al doped Li(Ni,Co)O 2 with 15-20% Co Highest energy density High rate capability, excellent cycling stability demonstrated High raw materials costs Quality depends critically on synthesis process sensitive to hydrolysis increased gas evolution Strong exothermal reaction with oxygen evolution during overcharge NMC mostly used 1/3 material; development of materials with reduced Co content Lower rate capability compared to NCA Less sensitive to hydrolysis Lower tap density decreased mass loading of electrodes Optimization of particle size morphology and distribution Core shell materials with NCA core

33 Solid solution from LiMn 0.5 Ni 0.5 O 2 and LiCoO 2 - summary - (1) capacity about 170 mah/g under mild cycling conditions to 4.4 V (2) Cobalt reduces the number of nickel ions in the lithium layer. (3) Lithium/nickel disorder dependent on heat treatment; The final heating temperature needs to be higher than 700 C and not higher than 1000 C (4) ratio of Co/Ni needs to be greater than 1 to eliminate all Ni in the lithium layer (5) determination of acceptable level of nickel in the lithium layer necessary, it is not zero (6) single phase behavior for all lithium values from 0 to 1 in Li x (NiMnCo)O 2 not clear yet (7) The structure needs determining at low x values. (8) Nickel is the electrochemically active ion at low potentials. (9) Cobalt is only active at higher potentials. (10)The electronic conductivity has to be improved (11) optimum composition still under investigation determined for energy storage, power capability, lifetime, and cost considerations

34 Further readings Robert A. Huggins, Advanced batteries, Springer, New York (2009) M. S. Whittingham, Intercalation compounds, in Fast Ion Transport, ed. By. B. Scrosati er. al., Kluwer Academic, Dordrecht, (1993) p