WO2019226200A2 - High energy-density cathode materials for secondary lithium ion batteries - Google Patents

Abstract

Cathode materials for lithium ion batteries, lithium ion batteries incorporating the cathode materials, and methods of operating the lithium ion batteries are provided. The materials are composed of lithium metal oxides that include two different metals.

Description

HIGH ENERGY-DENSITY CATHODE MATE-REALS FOR SECONDARY LITHIUM

ION BATTERIES

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to U.S. provisional patent application number 62/609,620 that was filed December 22, 2017, the entire contents of which are hereby incorporated herein by reference.

REFERENCE TO GOVERNMENT RIGHTS

[0002] This invention was made with government support under DEAC02-06CH11357 awarded by the U.S. Department of Energy; under 70NANB14H012 awar ded by the Dept of Commerce and NIST; under N00014-13-P-1056 awarded by the DQD, Office of Na vy Research; and under DMR1309957 awarded by the NSF. The government has certain rights in the invention.

BACKGROUND

[0003] Lithium-ion batteries (LIBs) have become one of the most widely-used electrical energy' storage technologies and have enabled the wireless revolution of consumer electronics. Conventional cathode materials used in LIBs are typically lithium-containing transition metal (TM) oxides or phosphates that can store and release electrical energy via the insertion and extraction of LC ions, accompanied by redox reactions of the TM cation .

Recently, Freire et ah reported a new disordered rocksalt-type Li-exeess LriMmOs cathode material with partially occupied cation and anion sites that exhibits a high discharge capacity·'. (See, M. Freire et ah, A new active Li-Mn-O compound for high energy density Li-ion batteries. Nat. Mater. 15, 173-177 (2016).)

SUMMARY

[0004] Cathode materials for lithium ion batteries and lithium ion batteries incorporating the cathode materials are provided.

[0005] One embodiment of a lithium ion battery comprises; an anode; a cathode comprising a lithium mixed metal oxide compound in electrical communication with the anode, wherein the lithium mixed metal oxide compound has the formula LriiMix^Os, where Mix represents two or more metal cations, including a first metal cation, M, and one or more dopant metal cations, M\ the lithium mixed metal oxide compound being characterized in that it can be reversibly converted into a lithium mixed metal oxide having an energetically stable state in which the M' cations have a valence of 5+ or above during the redox cycle of the battery; and an electrolyte disposed between the anode and the cathode

[0006] One embodiment of a lithium ion battery comprises: an anode; a cathode comprising lithium metal oxides in electrical communication with the anode, wherein the lithium metal oxides have the formula LhMp-_xj AOs, where M and M’ represent different metal cations and 0 < x < 2; and an electrolyte disposed between the anode and the cathode.

[0007] Other principal features and advantages of the invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWI GS

[0008] Illustrative embodiments of file invention will hereafter be described with reference to the accompanying drawings, wherein like numerals denote like elements.

[0009] FIG. 1A - ID. Determining the rocksa!t type structure of L14M112O5. (FIG. 1A) A schematic illustration of the disordered rocksalt LuM^Os structure with Li/Mn randomly mixed on the cationic sites and oxygen/vacancies (O/Vac) randomly mixed on die anionic sites. (FIG. IB) Simulated disordered structure using the special quasi-random stmcture (SQS) method. (FIG 1C) Predicted ground-state structure of LuMnsOs, with all Mn

octahedrally coordinated by O atoms and Li ions square-planariy or square-pyramidally coordinated because of O/Vae neighboring (space group Cmmm). (FIG. ID) Total energy distribution of the 100 structures selected the SQS structure, and the NatMnzQs prototype structure from Density Functional Theory (DFT) calculations. The Cmmm structure exhibits the lowest total energy.

[0010] FIG. 2A - 2C. Thermodynamic and dynamic stabilities ofliiMr^Os. (FIG. 2A) Calculated Li-Mn-O (T=0 K) phase diagram. The LffMmGs phase is slightly higher in energy (13.6 meV/atom) relative to that of the ground state phases— a mixture of LhO and LiMhOi. (FIG. 2B) Phonon dispersion of ground-state LuMniOs and (FIG. 2C) calculated temperature-dependent free energy of LMMII_ZO_S, LisO, and LiMnCh_, as well as the stability

9 of U₄MH₂O₅ vs. temperature relative to L1₂O and L1M11O₂ phase mixtures. It was found that L MiteOs is dynamically stable and can be entropically stabilized at -1350 K.

[0011] FIG. 3A and 3B. Electrochemical delithiation process ofL Mn₂0s. (FIG. 3 A) LUMnzOs-MhaOs convex hull with calculated deiithiated structures generated from ordered and disordered (SQS) LLf nsOs phase. (FIG. 3B) Corresponding voltage profile during the delitMation process in Li^MnsGs, which showed excellent agreement with the experimentally obtained charging curve. (M. Freire etal, 2016.)

[0012] FIG. 4A - 4E. Cationic and anionic redox sequence dining the delithiation of LhiMifcOs. (FIG. 4A) Local atomistic environments for Mn and O ions in Lh-MnsOs (x = 4, 3, 2, L and 0). (FIG. 4B) Energies needed to oxidize octahedrally coordinated biir⁺(d⁴) and Mri^iVi³) and tetrahedrally coordinated Mn^4÷(d³) to the reference state £k(Li metal)

(indicated by the arrows). p-DQS of the O 2 p and Mn 3 d orbitals (%, r¾) of O² ions hi the ‘Li-O-LF configurations and the nearest Mn ion in (FIG. 4C) LhiMifcOs and (FIG. 4D) LfeMitaQs. (FIG. 4E) Energy difference between ordered and disordered M112G5 with the partial Mn migration from octahedral to tetrahedral sites. The redox reaction along with the LLMiiiOs delithiation proceeds in three steps: i) cationic Mn³7Mn⁴⁺ (4 > x > 2), if) anionic OTO¹ (2 > x > 1), and Hi) mixed cationic Mn⁴⁺/Mn⁵⁺ and anionic GPiO¹ (1 > x > 0). The oxidation ofMn⁴ to Mn⁵ necessitated the migration of the Mn ion from its octahedral site to a nearby unoccupied tetrahedral site and impaired the reaction reversibility.

[0013] FIG. 5. HT-DFT screening for doping into the Mn sublattice in the LUCMn ^Os cathode system. Computational screening of mixing on the Mn sites with metal cations (M) that produce energetically stable LriCMiiAffeGs mixtures and have stable 5+ oxidation states was performed by examining the mixing energy·' and L1₄M₂O₅ stability. The top candidates with -30 < E_mix < 30 meV/site and the lowest formation energies are presented. The top three TM dopant candidates in the LriCMnAffeQs system are located in the left center of the plot:

M = V. Fe, and Cr.

[0014] FIG. 6. The cationic ordering between Li and Mn in L14M112O5 and between Ga and Zr hi GaiZr. The cationic ordering between LisMfrzOs and GaiZr is identical.

[0015] FIG. 7. The phonon dispersion of the ground state LiMnCb.

[0016] FIG. 8. The phonon dispersion of the ground state Li 0. [0017] FIG . 9 A and 9B. The magnetization and oxidation state evolution of (FIG. 9 A) Mn and (FIG. 9B) O ions in intermediate phases LfrMmOs (x = 4, 3, 2, 1, and 0) dining delithiation. In nearly-delithiated intermediated phases (x = 2, 1, OSQS, Os_tdoed), oxidation states of Mn and O ions are not identical with the partition of certain oxidation state marked by fractions (i.e. I/5N). Mn and O magnetizations show wide distribution in the x = 0 phase as a result of the various local environments of Mn in the disordered SQS structure. The electronic configurations of Mn³*, Mn^4÷, Mn⁵⁺, O² , and O³ are presented. Mn and O magnetizations in ordered Mh 0; are shown as a reference.

DETAILED DESCRIPTION

[0018] Cathode materials for lithium ion batteries, lithium ion batteries incorporating the cathode materials and methods of operating the lithium ion batteries are provided. The materials are composed of a lithium mixed metal oxide compound in electrical

communication with the anode, wherein the lithium mixed metal oxide compound has the formula LriCMix^’hOs, where Mix represents two or more metal cations, including a first metal cation, M, and one or more dopant metal cations, Mb The lithium mixed metal oxide compounds are characterized in that they have an energetically stable state in which the valence of M’ is 5+ or above that is accessible during the Mn^4÷/Mn^5÷ redox process that takes place during the charging and discharging of the battery in some embodiments of the cathode materials the mixed metal oxide compound has the formula LUMe-_xjM’_xOs, where 0 < x < 2. In this formula, M can represent a single dopant metal cation or it can represent more than one (for example, two) dopant metal cation hi some embodiments of the compounds, 0 < x < 1.

[0019] A basic embodiment of a lithium ion battery includes: a cathode; an anode in electrical communication with tire cathode; an electrolyte disposed between tire anode and the cathode; and, optionally, a separator also disposed between the anode and the cathode.

[0020] The electrolytes are ionically conductive materials and may include solvents, ionic liquids, metal salts, ions such as metal ions or inorganic ions, polymers, ceramics, and oilier components. An electrolyte may be an organic or inorganic solid or a liquid, such as a solvent (e.g., a non-aqueous solvent) containing dissolved salts. No -aqueous electrolytes can include organic solvents, such as cyclic carbonates, linear carbonates, fhiorinated carbonates, benzomtrile, acetonitrile, tetrahydrofuran, 2-methyltetiahydrofuran, y- butyro!actone, dioxolane, 4 rnetliyldioxolane, N,N-dimethylfonnamide, N,N- dimethylacetamide, N,N-dimethylsulfoxide, dioxane, 1 ,2-dimethoxyethane, sulfolane, dichi oroethane, chlorobenzene, nitrobenzene, diethyleneglycol, dmiethylether, and mixtures thereof. Example salts that may be included in electrolytes include lithium salts, such as LiPFg, LiBF_4, LiSbFs, LiAsFe, IiCI0₄, LiCF₃SO_¾ Li(CF₃Si¾)₂N, Ii(FS02)₂N, LiC^SOs, LiAlCb, LiAlCU, LiNCCxFsc+iSCh) (C_yF2y-iS02), (where c and y are natural numbers), LiCl, Lil, and mixtures thereof. D ing battery operation, lithium ions can be inserted/extracted reversibly from/to the electrolyte of the battery to/from the lithium sites of the cathode materials during the dischar ge and charge cycles of the cell, as illustrated in the reactions shown in Table 2.

[0021] The separators are typically thin, porous or semi-permeable, insulating films with high ion permeabilities. The separators can be composed of polymers, such as olefin-based polymers (e.g., polyethylene, polypropylene, and/or polyvinyhdene fluoride). If a solid polymer electrolyte is used as the electrolyte, the solid polymer electrolyte may also act as the separator.

[0022] The anodes are composed of an active anode material that takes pari i an

electrochemical reaction during the operation of the battery. Example anode active materials include elemental materials, such as lithium; alloys including alloys of Si and Sn, or oilier lithium compounds; and intercalation host materials, such as graphite. By way of illustration only, the anode active material ma include a metal and/or a metalloid alloyable with lithium, an alloy thereof or an oxide thereof Metals and metalloids that can be alloyed with lithium include Si, Sn, Al, Ge, Pb. Bi. and Sb. For example, an oxide of the metal/metalloid alloyable with lithium may be lithium titanate, vanadium oxide, lithium vanadium oxide, Sn0₂, or SiO_x (0 < x < 2).

[0023] The cathodes include lithium metal oxides that take part in an electrochemical reaction during the operation of the battery. Some embodiments of the cathodes comprise lithium metal oxides that include only one other metal element hi addition to lithium and have a stoichiometry represented by the formula LI₄M₂O₅. In some embodiments of these lithium metal oxides, M is a metal other than Mn. hi some embodiments of these lithium metal oxides, M is V, Fe, Cr, Pd, or Rh. This formula can be considered nominal in that it may be virtually impossible to get rid of very low (e.g., trace) concentr ations of impurities in the synthesis of any chemical compound. These impurities, when present, are generally inert hut may result in very small deviations from the stoichiometry represented in the formula. Such inert metal impurities may he introduced into the compounds as the result of impurities present in the materials used to synthesize the compounds and/or due to impurities present in the synthesis environment. Typically, such inert metal impurities are present in very small amounts, for example, at concentrations of 1 ppm or lower, including 1 ppb or lower.

However, the electrochemically inert metal elements can also be present at higher

concentrations, provided that they do no materially affect tire operation of the cathode.

[0024] Other embodiments of the cathodes comprise mixed metal lithium metal oxides having the formula LrilMix Oi . Some embodiments of the mixed metal oxide compounds include electrochemically inert metal elements in addition to the M and M’ elements, where an electrochemically inert metal element is a metal element that does not alter the

electrochemical properties (performance) of the electrode. The lithium mixed metal oxide compounds include compounds having the formula L^M^-_s M’_xOs, where M and M represent different metal cations, and 0 < x < 2. This formula can be considered nominal in that it may be virtually impossible to get rid of very low (e.g., tr ace) concentrations of impurities in the synthesis of any chemical compound. These impurities, when present, are generally inert but may result in vei _? small deviations from the stoichiometry represented in the formula. Such inert metal impurities may be introduced into the compounds as the result of impurities present in the materials used to synthesize tire compounds and/or due to

impurities present in the synthesis environment. Typically, such inert metal impurities are present in very small amounts, for example, at concentrations of 1 ppm or lower, including 1 ppb or lower. However, the electrochemically inert metal elements can also be present at higher concentrations provided that they do no materially affect the operation of the cathode.

[0025] Jii some embodiments of the lithium metal oxides, M is Mn and/or M’ is CT, Fe, V, Rh, or Pd. hi the mixed metal oxide compounds in which M is Mn, the M metal elements (e.g., transition metal elements) partially substitute Mn in LiiMnzQs. As illustrated in the Example, the lithium mixed metal oxide compounds can be reversibly converted into an energetically stable state in which the M elements can access the oxidation state of 5+ or above during the MiriT'Miri redox process, thereby eliminating the need for oxidation of Mn to 5+. The lithium metal oxides may be free of noble metal elements in order to reduce raw material costs. [0026] Bateries incorporating the cathode material are able to provide a high specific capacity. For example, some embodiments of the batteries have a specific capacity of at least 300 mAh/g. As such, the batteries are useful for a variety of devices, including consumer electronics and power devices, electric vehicles, distributed energy storage for solar and wind, and advanced electric energy storage for smart grid applications.

[0027] The preparation of L14M2O5, IJ Me-sjM’_xOs, or LuCMix^Os compounds could proceed through a two-step route, using a mechanochemical activation. First, high- temperature (HT)-LiMO? compounds, HT-L1MO2, or HT-Li(MM’)0₂ are produced by a solid-state reaction method using a reagent mixture of LiOH and metal oxide compounds (e.g., MO, MO₂, M₂O_3, MΌ, MΌ₂, M’_IO , or mixtures of two or more thereof), taken in a corresponding molar ratio which is ground thoroughly. By way of illustration, the metal oxide compounds may be MnO+MnCfe, V₂O₃,€¾(¾, FezCh, RlteCfi, and/or

PdO+Pd( H3)2Cl2_. Different M or M’ containing oxide precursors also could he used.

Excess lithium compound can be added to compensate for lithium evaporation at Irigtr temperatures. Second, the homogeneous mixture is heat-treated at high tempera true (for example, around 1,000 °C) under inert gas (e.g., argon) flow. Then, HT-LiMOz, HT-LiMOs, or HT-Li{MM’)C>₂ is ground with L1₂O (2: 1 molar ratio) and 5 wt.% of carbon black to form L1₄M₂O₅ or L Mcz-_xiM’_xQs. (-See, M. Freire et a 2016 for guidance on the general steps described above.)

[0028] l Mess otherwise indicated, temperature and/or pressure dependent measured and calculated values recited herein refer to the values as measured or calculated at room

temperature (23 °C) and atmospheric pressure.

EXAMPLE

[0029] in this example, the disordered rocksali LriMmQs structure was simulated through the SQS method (with Li/Mn mixing on the cation subiattice of rocksalt and Q/Vac mixing on the anion subiattice). The ground state ordered LriMmQs structure was also determined via DFT-based calculations. The ordered structure as determined was predicted to have much lower energy (-119 meV/atom) compared to the disordered structure. Next, the structural evolution of phases dining the delithiaticn of Lh_tMmCb was investiga ted, and these phases were used to compute delithiation voltage profiles. The DFT-calculated voltages show excellent agreement with the experimentally-measured ones. (See, M. Freire et al, 2016.)

The TM and O redox sequences of Mii³¾fe⁴⁺¾ln⁵ and 0²70⁵ were further elucidated during the charging cycle and showed that the electrochemical deiithiation process of

L MiteOs occurred in the following three-step reaction pathway: 1) initial oxidation of Mir⁺ to Mh⁴⁺ for L MII₂G₅ (4 > x > 2), 2) followed by anionic redox ofO² to O¹ for Li_rMnzOs (2 > x > 1), and finally 5) further cation oxidation of Mn⁴⁺ to Mn^5÷ for LixMniOs (1 > x > 0), validating the observations ofFreire et «1.(2016). The calculations show that the oxidation of MB⁴* to Mnⁱ⁺ imposed a migration of the Mn ion from its octahedral site to a nearby, unoccupied tetrahedral site. Lastly, computational screening of mixing was performed on the Mn sites with metal cations (M) that produce energetically stable Li₄(Mn,M)₂0s mixtures, and also have stable 5+ oxidation states. This approach demonstrates that alloying this compound with the following elements produces new compounds with substantially improved electrochemical properties, particularly for embodiments in winch M = V and Cr in Li4(Mn,M) 05.

Result and Discussions

Determining the rocksalt type structure of LtiMmOs

[0030] The X-ray diffraction analysis of the LUM^Os samples led to several broad peaks, indicating a disordered rocksalt type structure with Li/Mn randomly mixed on the cation sites and O/Vac randomly mixed on the anion sites (FIG. I A). Here the SQS method was employed, using a rocksalt-based 108-site supercell with Li/Mn occupying the 54 cation sites in a 2:1 ratio, and O/Vac occupying the 54 anion sites in a 5:1 ratio. The SQS was generated (FIG. IB) using a Monte Carlo algorithm as implemented in the ATAT package with the pair and triplet correlation functions of the SQS constrained to be identical to those of the statistically random compound (Li/Mn occupying the cation sites, and O/Vac

occupying the anion sites) at least up to the third nearest neighbor. (See, E. Cockayne, et al. , Building Effective Models from Scarce but Accurate Data: Application to an Alloy Cluster Expansion Model. Phys. Rev. B. 81, 12104-12113 (2010); A. van de Walle, Multicomponent Multisublattice Alloys, Nonconfigurational Entropy and Ollier Additions to the Alloy Theoretic Automated Toolkit. Calphad. 33, 266-290 (2009); and A. van de Walk, Methods for First-Principles Alloy Thermodynamics. JOM - 1 Min. Met. Mat S. 65, 1523-1532 (2013).)

[0031] In addition, ionic ordering in the LuMrirOs compound was studied. The lowest- ertergy, ground state structure of LUM^Os was determined by exploring a vast number of geometrically-distinct Li/Mn/O ordered configurations using DFT calculations. Starting from the cubic rocksalt primitive ceil, two sets of supercells were generated: 1) containing 6 cations and 6 anions with all symmetrically distinct supercell shapes; 2} containing 12 cations and 12 anions with two specific shapes, given by 3x2x2 and 2x3x2 multiples of the primitive rocksalt unit cell. The cation sites were then populated with Li and Mn atoms in the ratio 2:1, and Vac were introduced on the anion sites with a Vac.O ratio of 1:5. 616 geometrically different configurations were generated using the Enum code. (See, G. Hart, et al_s Algorithm for Generating Derivative Structures. Phys. Rev B. 77, 224115-224126 (2008); G. L. W. Hart, et al, Generating derivative structures from multilattices: Algorithm and application to hep alloys. Phys Rev . B. SO, 014120-014127 (2009); and G. L. W. Hart, et al, Generating Derivative Structures at A Fixed Concentration . Comput. Mater. Set. 59, 101-107 (2012).) The electrostatic total energy for all configurations were calculated using nominal charge states for the ions in the system as a quick energy' sampling step. (See, K. J. Michel, et al. , Fast Mass Transport Kinetics in B20H16 : A High-Capacity Hydrogen Storage Material. J Phys. Chem. C. 117, 19295-19301 (2013).) All structures were ranked by their normalized electrostatic energies. The 100 L MifcOs structures with the lowest electrostatic energies were fully relaxed, and their energies were calculated using DFT. Hie structure with the lowest DFT total energy, i.e., the ground state structure of L MDZOS, was found to have a space group of Cmmm with all Mn³⁺ ions octahedrally-coordinated by 6 oxygen atoms (FIG. 1C). Meanwhile, Li ions were square-planar ly or square-pyramidally coordinated by 4 or 5 oxygen atoms as a result of O/Vac neighboring. The cation ordering between Li and Mn in the LLtMiizOs ground state structure was the ordering of the Ga₂Zr compound ( see FIG. 6), with crystallographic information given in Table 1. (See, K. Schubert, et al , Zum Aufbaii einiger T4-B3 homologer und quasihomologer Systems. I. Die Systems Ti-Ga, Zr-G und Hf-Ga. Naturwissenschaften. 53, 474-488 (1962).) The fully-relaxed DFT energy of the SQS structure was found to be higher than the ordered ground state by 119 meV/atom (34 ordered structures having lower total energies (FIG. ID)). The thermodynamic and dynamical stability of the ordered Cmmm LHM^Os structure is discussed in detail hi the following section.

Table 1, Structure information of the LUM Os ground state. Space group name: Cmmm. Lattice parameters: a = 4.0390 A, b = 12.4312 A, e = 4.0268 A, a = b = y = 90.0000°.

Structure parameters:

and thermodynamic stability of ordered (Cmmm) LltMmOs [0032] Phase diagrams represent the thermodynamic phase equilibria of multicomponent systems and provide useful information on reactions of phases. While the experimental determination of a phase diagram for specific system is significantly time and labor consuming, the phase diagram constructions can be accelerated by calculating energies of all known compounds in a specific chemical system using DFT and using them to construct a T = OK convex hull. (See, A. K. Akbarzadeh, et al, First-Principles Determination of

Multicomponent Hydride Phase Diagrams: Application to the Li-Mg-N-H System. Adv. Mater. 19, 3233-3239 (2007); C. Wolverton, et al. Incorporating first-principles energetics in computational thermodynamics approaches. Acta Mater. 50, 2187-2197 (2002).) hi this study, ternary Li-M-O gr ound state convex hulls were constructed using the structures with the lowest energ for each composition for M = Mn and all metal elements with possible oxidation states of 5+ or above: i.e. , M = Bi, Cr, Fe, Ir, Mo, Nb, Os, Pd, Pi; Pt, Re, Rh, Ru, Sb, Ta, V, and W. (See, N. N. (Norman N. Greenwood, et al. , Chemistry of the elements (Buterworth-Heinemann, 1997).) All compounds within each Li-M-O ternary system were adopted from the Inorganic Crystal Structure Database (ICSD). (See, A. Belsky, et al.. New Developments in the Inorganic Crystal Structure Database (ICSD): Accessibility' in Support of Materials Research and Design. Acta Crystallogr. Sect. B Struct. Set 58, 364-369 (2002).) The elemental reference states (Li, M, non-solid 0₂) were obtained by fitting to experimental formation energies, mainly fro two major databases: the SGTE substance database (SSUB) and a database constructed by P. Nash et al (See, S. Grindy, et al, Approaching Chemical Accuracy with Density Functional Calculations: Diatomic Energy Corrections. Phys. Rev. B. 87, 075150-075157 (2013): L. Wang, et al. Oxidation Energies of Transition Metal oxides within the GGA+U framework. Phys. Rev. B. 73, 195107-1951 12 (2006); S. Kirkiin et al. The Open Quantum Materials Database (OQMD): Assessing the Accuracy of DFT Formation Energies npj Comput. Mater. 1, 15010-15024 (2015); V. Stevanovic, et al, Correcting density functional theory for accurate predictions of compound enthalpies of formation:

Fitted elemental-phase reference energies. Phys. Rev. B. 85, 115104—115115 (2012); SGTE, Thermodynamic Properties of Inorganic Materials (Berlin, Heidelberg, 1999); and P. Nash, Thermodynamic database (2013). .) Calculations to construct equilibrium Li-M-O phase diagrams were earned out within the Open Quantum Materials Database (OQMD)

framework. (See, S. Kirkiin et al. , The Open Quantum Materials Database (OQMD):

Assessing the Accuracy of DFT Formation Energies npj Comput. Mater. 1, 15010-15024 (2015): and J. E. Saal, et al, Materials Design and Discovery with High-Throughput Density Functional Theory. The Open Quantum Materials Database (OQMD). JOM. 65, 1501-1509 (2013).) The convex hull of stable phases, i.e., the set of compounds that have an energy l ower than that of any other compound or linear combination of compounds at that

composition, was constructed for each ternary Li-M-O system. Using such convex hulls, or T = OK phase diagrams, the ground state stability of transition metal oxides, e.g., L14M2O5 and Ii4(Mn,M)205, could then be evaluated by using the GCLP technique. (See, S. Kirklhi ef a!.. The Open Quantum Materials Database (OQMD): Assessing the Accuracy of DFT Formation Energies npj Comput Mater. 1, 15010-15024 (2015); and A. Jsm etaL, Commentary; The Materials Project: A materials genome approach to accelerating materials innovation. APL Mater. 1, 11002 (2013).)

[0033] The Li-Mn-O phase diagram (T= 0 K) is shown in FIG. 2A with the ground state, stable compounds marked by filled circles (i.e. , having lower energy than the linear

combination of other structures) . L nzOs is shown as an empty circle, which is

compositionally located on the tie-line between LhO and LiMnOs. LhMmOs at T= 0 K was predicted to have an energy' only slightly higher (+13.6 meV/atom) than a two-phase mixture of LiiO and LiMriCb. The L^MnsOs compound is, therefore, not a ground state structure, but rather is unstable at T= 0 K, albeit by a very small energy. The compound is very close to the convex hull, and hence may be stabilized at elevated temperatures by entropic contributions, e.g., vibrational entropy. The DFT calculated phonon dispersion of the Cmmrn LriMmGs is provided in FIG. 2B. No imaginary phonons are shown in FIG. 2B, which Indicates the predicted LhMmGs compound was dynamically stable.

[0034] By computing the harmonic phonons and vibrational entropies of LhMmGs (FIG. 2B), LiMnG2 (FIG. 7), and LhO (FIG. 8), the temperature dependence of the free energies can be calculated between these three competing phases. It was found that Li^M Qs has a higher entropy than the combination of LhO and LiMnOi, suggesting that L M^Os should become stable at elevated temperatures. The temperature-dependent free energy curves (FIG. 2C), F(T) = E - TS, consist of the energy of static lattice and the harmonic vibrational fr ee energy at the same volume. Both LhO and liMnth were calculated to be dynamically stable, having only real phonon frequencies. Usin the free energies for each of the three

compounds, the stability (formation free energy) for LuMnzOs can he calculated as

Stabili OJ MfiiOs) = CLhMmOs) - i^LhO) - 2 (LiMn02). The stability' of Li^M ^Qs as the function of T is shown in FIG. 2C with a temperature at which LhjMnaOs is stabilized to be approximately -1350 K. The positive formation entropy is mainly because of the small entropy of LHO, stemming from the light Li atom and strong bonding interaction between Li and O as shown in FIG. 8. Due to the relatively small entropy differences between phases the uncertainty of this temperature (e.g., due to an uncertainty of ±1 meV/atom in free energy) results in a range of transition temperatures of 1240-1450 K. The calculations then suggest that the L14M112O5 compound is stable at high temperatures, implying the favored formation of this compound above the transition temperature. However, the elevated temperature would also favor ionic disorder because of the greater configurational entropy contribution. The possible stable decomposition phases are LiMnCb, LhO, LhMnCh, LigMnO^, as predicted by the Li-Mn-O phase diagr am, which might be observed as“impurity” phases during synthesis.

Electrochemical delithiation process of LitM Os and TM/O redox competition

[0035] Having explored the structural ordering and thermodynamic stability of the L MII_IO_S phase, the electrochemical delithiation process of this compound was explored next. To examine delithiation, the energies of disordered SQS-L MifcQs and the fir!ly- delithiated SQS-IioMniQs were calculated. Meanwhile, compositions of LfiMnsOs (x = 4, 3, 2, 1 , and 0) were considered, in which (4 - x) Li ion(s) were removed from the original ordered Cmmm LhsMr_fcOs structure using many geometrically-distinct configurations, and they were further relaxed using DFT. The energies for these structures were evaluated according to the following reaction: LLMnsQs M112O5 + xLi*. Tire energies of these ordered/ disordered delithiation products were plotted, and the delithiation convex hull of LLiMn Os-MmQs was then constructed, as shown in FIG. 3 A. hi FIG. 3 A, the delithiation convex hull of L M^Os-MtfeOs is shown, where the ordered Lfe oaOs (x = 3, 2, 1) and disordered SQS-LhMnzGs (x = 0) structures were found to be on the hull. Converting the energies along this delithiation pathway into voltages, it was found that the predicted voltage profile shows good general agreement with the experimental charging curve in FIG. 3B.

[0036] The calculations of the LiiMnzOs phase and its delithiation products were next used to interrogate in detail the TM/O redox sequence. Hie oxidation states of Mn and O ions were examined during the delithiation process, and the local atomistic environments for cations and anions were investigated. The oxidation states can be determined by comparing calculated magnetizations of Mn and O ions with the number of unpaired electrons of the corresponding ions with known oxidation states. The numbers of unpaired electrons for Mn³* (octahedrally-coordinated), Mn⁴* (octahedrally-coordinated), and Mn⁵* (tetrahedrally- coordinated) are 4, 3, and 2, respectively, as shown in FIG. 9 A. The numbers of unpaired electrons for O² and O¹ (octahedraliy-coordiiiated) are 0 and 1 respectively (FIG. 9B). It was found that the electrochemical delitMation of L &_fcOs can be categorized in three different reaction steps, where each step contains a dominant redox of either TM or O ions:

[0037] (i) Cationic redox Mn³~/Mn^4~r delithiation (Li_xMn&s, 4 > x > 2): During the delitMation process of LLMnrQs LLMriiOs L^Mn^s, it was found that the Mn magnetizations decrease from 3.94 LIB ® 3.56 IΪB ® 3.14 m_¾ (see FIG. 9A), indicating an overall oxidation of Mn ⁺ to Mn⁴⁺. Meanwhile, the O magnetizations retain a value between 0.01 1_B to 0.14 lis (see FIG. 9B), implying a constant anion oxidation state of O² . The initial energetic preference of TM redox over O redox was confirmed by examining the projected density of states (p-DOS) of O 2 p and Mn 3d orbitals (<¾ and ) of O² ions and Mn^3÷ ions in both ordered and disordered LuMnzOs. As shown in FIG. 4C, the contribution from Mn e_s to the valence band immediately below the Fermi level (Ey) was larger than that from O, which shows a preference for electron extraction from Mn (FIG. 4B) during the initial stages of the charging process as discussed above. As a result, the first delitMation step was dominated by cationic redox of Mn^3÷/Mn^4÷. It is interesting to connect the competition between cation and anion redox to the local ionic environments in the Li_xMifeOs structures. Recently. Seo et al. proposed that a specific local Li-excess environment around the oxygen atoms (/._<?., a Li-O-Li linear configuration) is a key structural signature indicating the feasibility of both cationic (TM) and anodic (oxygen) redox process in Li-rich cathode materials. (See, D.-H. Seo etal. The Structural and Chemical Origin of the Oxygen Redox Activity in Layered and Cation- disordered Li-excess Cathode Materials. Nat. Chern. 8, 692-697 (2016).) hi other words, the electrons from the oxygen atom in this local Li-O-Li configuration can more easily contribute to the redox process due to the overlapping TM states and O 2p states interestingly, the examination of the local environments of oxygen shows that many O ions were in this Li-O- Li configuration in the ordered (Cmmm) and disordered structures, as shown in FIG. 4A. However, it was found that Mh³⁺ to Mn⁴⁴ cation oxidation was still the main redox

contribution during the initial charging process. After 2 Li⁺ ions were removed (i.e...

LbMiiiOs), a large fraction of O ions (4/5) still remained in these linear Li-excess

environments (see FIG. 4A). The -DOS of O 2 p and Mn 3d orbitals (% and /¾) for O^2- ions in the Li-excess environment and nearest neighbor Mn⁴⁺ ions are shown in FIG. 4D. The contribution from oxygen in the valence band immediately below Ey was significantly larger than that coming from Mn

implying the possibility of oxygen redox participation in the second dslithiation step as described below. Taking extra electrons out from the orbital of Mn was significantly more difficult compared to the e_g orbital (FIG. 4B), as discussed above.

[0038] (ii) Anionic redox Cd/O¹ dominant delithiation (Li_xMmOs, 2 > x > 1): Upon further delithiation of LriMnrCb into IiMifcOs, it was found that the observed Mn

magnetizations were largely constant in the range 3.14 ps to 3.30 ps, indicative of Mn⁴⁺. Here, the Mn ions still were octahedrally-coordinated. Interestingly, it was found that 1/5 of the O ions exhibited magnetic moments around 0.69 p_¾, implying the partial oxidation of O² toward O⁵ . By examining the local atomistic environments of all O^i_ ions in LiiMiizOs and comparing to their previous local environments in LizMi_feOs, it was noticed that all O^5- ions participating in redox during this step were located in the Li-O-Li Li-excess environments (FIG. 4A). The calculations thus indicate the delithiation step from LLMmOs to LiMmOs was dominated by anionic redox processes (r.a, with O² being partially oxidized to O¹ ). For the LiMifcOs phase, as shown in FIG. 4E, the contribution from Mn orbitals (mainly ¾) to the valence band immediately below the Fenni level (E_F) was still slightly lower than that from O, implying a preference tor electron extraction from O (FIG. 4E) during the final stages of the charging process (L½Mn₂C>5, 1 > x > 0). However, in tire experimental studies, further oxidation ofMh⁴ to Mn³⁺ was observed during this stage. {See, M. Freire et ah, A new active Li-Mn-O compound for high energy density⁷ Li-ion batteries. Nat. Mater. 15, 173-177 (2016).) Therefore, it was suggested that some additional reaction mechanism must account for the Mn oxidation dining the final stage.

[0039] (in) Mixed cationic. Mn^{4^} /Mn^{5^} and anionic ( /O¹ redox delithiation (Li_xMnsOs,

I > x > 0): During the final delithiation step, i.e., LiMn₂Os to Mn₂Os(here, the disordered SQS-MmCL with the lowest DFT energy were examined), it was found that the Mn magnetizations were distributed from 3.3 p_¾ to 1.9 p¾ (see FIG. 9 A), indicating that Mh⁴⁺ ions were partially (1/6) oxidized to Mn⁵⁺. At tire same time, the magnetizations of 1/3 of the O ions were found to be 0.71 ps - 0.82 p¾ (FIG. 9B), implying a further oxidation of O² to O¹ (remember, it was found that 1/5 of anions were O¹ in LiM^Qs). Interestingly, as depicted in FIG. 4A, it was found that all the Mn⁵⁺ ions were tetrahedraliy-coordinated in this Mn/Li vacancy disordered configuration. The observation of tetrahedraliy-coordinated Mn^s+ confirms the discussion above that the oxidation of tetrahedraliy-coordinated Mn⁴* to Mn^s+ was energetically favored compared to the octahedrally-coordinated Mn⁴* because of the crystal field f¾/% effects (FIG. 4B). Therefore, it was suggested that the Mn ion migration to tetrahedral positions was necessary⁷ in the final delithiation process toward Mn Os, which corresponds to the oxidation of Mn from 4+ to 5+. At the same time, all the Mn ions in the ordered MifeQs (dashed rectangle in FIG. 4 A, 0.85eV Mn higher in energy compared to the disordered configuration), however, were still located in the octahedr al sites, where the oxidation states» were preserved at 4+, as shown in FIG. 9 A. As a result, the Mn migration not only enabled the Mn^4÷ oxidation to Mn⁵* but also lowered the energy of the system at this stoichiometry by 0.85e\7Mn. In order to achieve a reversible redox reaction, these Mn³⁺ ions would need to migrate back to their original octahedr al sites during tire following lithiation (i.e., the discharge process). The large reverse migration barrier (Le. at least 0.85 eV/Mii, the difference in DFT energetic stability between these two structures) will result in a significant kinetic barrier for the tetrahedral Mr to migrate back to its original octahedral position; therefore, this suggests that this metal migration will impair the reversibility of the reaction. After the extended cycling of LfiMii Gs cathodes, it is likely that more Mn ions will migrate into the in tetrahedral sites and get trapped. The phase transformation caused by the Mn ion migrations could be one of the most significant factors for the capacity fade observed experimentally after the first cycle. (See, M. Freire etal. A new active Li-Mn-O compound for high energy density Li-ion batteries. Nat. Mater. 15, 173-177 (2016).) Improved performance and reversibility could be achieved by limiting charging to avoid the formation of Mir and hence the migration of these metal cations.

[0040] The above results illustrate a design strategy' to improve the extended cyclability of the rocksalt Li_tMniOs cathodes would be to avoid Mn migration to the tetrahedral sites during the Mn⁴⁺/Mn⁵⁺ redox process. The electrochemical cycling of LriMmGs could be confined to a smaller range: LfrM^Cty 4 > x > 1, without removing all Li from the system and oxidizing Mn to 5+. Thus, improved cyclability could be achieved by sacrificing a limited amount of capacity. An alternate str ategy to achieve this goal of improved

reversibility would be to partially substitute Mn in L^ iiOs with other TM elements that can access the oxidation state of 5+ or above, thereby eliminating the need for oxidation of Mi to 5+. hi the following section, a high-throughput DFT screening strategy is presented to determine stable metal dopants (M) in LtyCMmMtyOs compounds.

TM doping in LUMni-_{x t}Os with accessible 5+ oxidation state or above

[0041] All the metal elements (M) with possible oxidation states of 5+ or above were first started with: r.e., M = Bi, Cr, Fe, Ir, Mo, Nb, Os, Pd, Pr, Pt, Re, Rh, Ru, Sb, Ta, V, and W.

For each of these elements, the properties of mixed-metal LfiCMmMkOs compounds were computed, specifically focusing on stability and mixing energy. The mixing energies between L MniOs and LLjMiOs in Ii$(Mn,M)205 helped determine tire stability of metal mixing in this structure. When the mixing energy (E_mix) is found to be slightly negative or positive (near-zero, i.e. -30 to 30 meV/site), the mixing entropy at finite temperatures will overcome the mixing energy, and hence there will be a tendency for metal mixing in a solid-solution. A larger positive mixing energy (> 30 meV/site) or a larger (in magnitude) negative mixing energy (< -30 meV/site) would lead to phase separation in the former case, and a quaternary ordered compound in die latter. These cases may have undesired phase transformations or possible mass transport kinetic limitations. As a result, the list of candidates was narrowed down to those with near-zero mixing energies between -30 to 30 meV/site in this study (FIG. 5). Similar to I^MnXX all L14M2O5 compounds are unstable at T = OK with the potential to be entropically stabilized at finite temperatures. For cases where the LiiMsOs convex hull distance {i.e., stability) is significandy positive, it will lead to an instability of the

corresponding Li^MnjVffeQs compound. Here, the Li*(Mh, )₂0s compounds with related L14M2O5 convex hull distance larger than 50 meV/atcin were excluded. FIG. 5, indicates the top Li4(Mii,M)205 candidates with stability near the convex hull and a small mixing energy in the Mn sublattice (favoring solid solution formation). It was predicted that mixing with M = V, Cr, Fe, Rh, and Pd would be particularly useful additives. Predicted gravimetric capacities (theoretical) and average voltages of recommended candidates are listed in Table 2. The doping/substitution of these elements into LLsiMmM^Os cathodes was expected to lead to reduced phase transformation and controlled oxygen anodic chemistry for further improved electrochemical performance.

Table 2. Top L C^^M^Os cathode candidates from the HT-DFT screeni g with predicted gravimetric capacities (theoretical) and averaged voltages using L MifeOs as the benchmark.

Capacity C Volta °se E avg

R ti

Materials and Methods

Density functional theory calculations

[0042] All DFT calculations reported in this study were performed using the Vienna Ab- initio Simulation Package (VASP) with the projector augmented wave (PAW) potentials and the Perdew-Becke-Emzerhof (PBE) exchange-correlation. (See, G. Kresse, et al, Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B. 47, 558-561 (1993); G. Kresse, et al, Ab initio Molecular-dynamics Simulation of the Liquid-metal-amorphous-semiconductor Transition in Germanium. Phys. Rev. B. 49, 14251-14269 (1994); G. Kresse, et al.,

Efficiency of Ab-initio Total Energy Calculations for Metals and Semiconductors Using a Plane-wave Basis Set. Comput. Mater. Set. 6, 15-50 (1996); G. Kresse, Efficient Iterative Schemes for Ab Initio Total-energy Calculations Using a Plane- Wave Basis Set. Phys. Rev.

B. 54, 11169-11186 (1996); P. E. Blochl, Projector Augmented-wave Method. Phys. Rev. B. 50, 17053-17970 (1904); and J. P. Perdew, et al, Rationale for Mixing Exact Exchange with Density Functional Approximations. J Chem. Phys. 105, 9982-9985 (1996).) A plane wave basis with a cutoff energy of 520 eV and /’-centered ^-meshes with a density of 8000 points per reciprocal atom were used for all calculations. All calculations were spin-polarized, with Mn atoms initialized in a high-spin ferromagnetic configuration and relaxed to self- consistency. The DFT + U method introduced by Dudarev et al. was used to treat the localized 3 d electrons of Mn with a U of 3.8, obtained by fitting it to experimental and calculated formation enthalpies in a previous study. (See, S. L. Dudarev, et al, Electron- energy-loss Spectra and The Structural Stability ofNickel Oxide: An LSDA+U Study. Phys. Rev. B. 57, 1505-1509 (1998); and L. Wang, et al., Oxidation Energies of Transition Metal oxides within the GGA+U framework. Phys. Rev. B. 73, 195107-195112 (2006).) Phonon calculations were carried out with the frozen phonon approach as implemented in the

PHONOPY package, and phonon density of states was computed usin a dense 30x30x30 mesh in the irreducible Brillouin zone. (See, A. Togo, etal, First-principles calculations of the ferroeiastic transition between rutile-type and CaCfe-type SiO at high pressures. Phys. Rev. B. 78, 134106-134114 (2008).) Further, Heyd-Scuseria-Emzerhof screened hybrid functional (HSE06) was used to accurately determine the energies, magnetic and electronic states of Mn and O in the dehthiated phases with structures relaxed using DFT + U LU- _xMri Qs (: x = 0. 1, 2, 3, 4). (See, J. Heyd, et ah, Hybrid functionals based on a screened Coulomb potential. J. Chetn. Phys. 118, 8207-8215 (2003).)

Voltage profile calculations

[0043] The average iithiation/delithiation voltage (relative to Li/Li⁺) can be computed using the negative of the reaction free energy per Li added/removed, as shown in i¾r. (1):

V = 2L·

F N L i (1) where F is die Faraday constant, AN_hi is the amount of Li added/removed, and AG_{is the (molar) change in free energy of the reaction. (See, M. K. Aydinol, etal, Ab initio study of lithium intercalation in metal oxides and metal dichalcogenides. Phys. Rev. B. 56, 1354-1365 (1997).) Considering a two-phase reaction between Li_xMO and Li_vMO: LhMO + (y - x)L i ® Li MO. AG_;· can be approximated by the total internal energies from DFT calculations neglecting the entropic contributions (0 K),

where E (Li_;s;MO) and E (Li_vMO) are the DFT energies at the respective compositions. The neglect of entropic contributions means that the lithiation voltage profiles will follow the T=0K ground state convex hull and will consist of a series of constant voltage steps along the two-phase regions of the convex hull, separated by discontinuities which indicate the single- phase compounds on the hull it is worth mentioning here that, in practice,

lithiation/delithiation do not necessarily proceed through two-phase reactions. Thus, the calculated T = OK voltage profiles should be viewed as an approximation to the actual voltage profiles. (See, M. K. Y. Chan, et l , First principles simulations of the

electrochemical lithiation and delitkiation of faceted crystalline silicon. J Am. Chem. Sac. 134, 14362-14374 (2012).) At finite temperatures (e.g., room temperature), the voltage drops in the profile become more rounded, due to entropic effects. (See, C. o!verton, et at. , First- Principles Prediction of Vacancy Order-Disorder and intercalation Battery Voltages in Li_xCo0_2. Phys. Rev. Lett. 81, 606-609 (1998).)

Mixing energy

[0044] The tendency of two ordered rocksalt L14M2O5 and LL₄M₂O₅ (space gr oup Cmmm) materials to mix and form a mixed-metal rocksalt LLMM’Os structure can be evaluated by calculating the mixing energy as shown in Eg. (3): E_mix = EiL iV M')₂0_s) - 1/2(E(LUM₂0₅) + £^’(Li₄M^, ₃O_s}) (3) where £(Li₄(M, M')₂O_s), 5(Li₄M₂0s), and F (LL_tM^Gs) are the total energies of the Cmmm structure with two geometrically identical TM sites occupied by metal atoms M and M\ M alone, and M' alone, respectively.

[0045] The word "illustrative” is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as "illustrative" is not necessarily to be construed as preferred or advantageous over other aspects or designs. Further, for the purposes of this disclosure and unless otherwise specified, "a” or "an" means "one or more.”

[0046] The foregoing description of illustrative embodimen ts of the invention has been presented for purposes of illustration and of description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention.

The embodiments were chosen and described hi order to explain the principles of the invention and as practical applications of the invention to enable one skilled in the art to utilize the invention hi various embodiments and with various modifications as suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.

Claims

WHAT IS CLAIMED IS:

1. A lithium ion battery comprising:

an anode;

a cathode comprising a lithium mixed metal oxide compound in electrical

communication with the anode, wherein the lithium mixed metal oxide compound has the formula LfiCMixjbQs, where Mix represents two or more metal cations, including a first metal cation, M, and one or more dopant metal cations, M\ the lithium mixed metal oxide compound being characterized in that it can be reversibly converted into a lithium mixed metal oxide having an energetically stable state in which the M’ cations have a valence of 5+ or above during the redox cycle of the battery; and

an electrolyte disposed between the anode and the cathode.

2. The battery of claim 1, wherein Mix consists of a first metal cation, M, one or more dopant metal cations, My and, optionally one or more electrochemical!)? inert impurity metal cations.

3. The battery of claim 2, wherein M is Mn.

4. The battery of claim 1 , wherein M is Mn.

5. The battery of claim 4, wherein M’ is Cr.

6. The battery of claim 4, wherein M’ is V.

7. The batter of claim 4, wherein M is Fe.

8. The battery of claim 4, wherein M’ is Pd.

9. Tire batter of claim 4, wherein M is Rh.

10. Tire batter of claim 4, wherein M comprises two different dopant metal cations.

11. The batter of claim 10, wherein the two different dopant metal cations ar e selected from the group consisting of Cr, V, Fe, Pd, and Rh.

12. T!ie battery of claim 10, wherein the two different dopant metal cations are Cr and V.

13. The battery of claim 1, wherein the lithium mixed metal oxide compound has the formula

14. The battery of claim 12, wherein M is Mn.

15. The battery of claim 14, where 0 < x < 1.

16. The battery of claim 14, wherein the lithium mixed metal oxide compound has the formula LuMup-xjCixOs.

17. The battery of claim 14, wherein the lithium mixed metal oxide compound has the formula LuMhp-xjVxOs.

18. The battery of claim 14, wherein the lithium mixed metal oxide compound lias the formula LUMn_fi-xjFexOs.

19. The battery of claim 14, wherein the lithium mixed metal oxide compound has the formula LuMn^Pd_xOs.

20. The battery of claim 14, wherein the lithium mixed metal oxide compound has the formula Li4Mn(2-_x)Rh_xOs.

21. A lithium ion batery comprising:

an anode:

a cathode comprising L CteOs in electrical communication with the anode; and an electrolyte disposed between the anode and the cathode.

22. A lithium ion battery comprising:

an anode;

a cathode comprising L1₄V₂O₅ in electrical communication with the anode; and an electrolyte disposed between the anode and the cathode.

A lithium ion battery comprising:

an anode; a cathode comprising LuF^Os in electrical communication with the anode; and an electrolyte disposed between the anode and the cathode.