WO2023114294A1 - Localized saturated electrolytes and rechargeable batteries containing the same - Google Patents

Abstract

Described are electrolytes for use in lithium ion and lithium metal electrochemical cells, lithium ion and lithium metal electrochemical cells, and methods of making lithium ion and lithium metal electrochemical cells. The electrolytes described herein include localized saturated electrolytes that provide beneficial performance characteristics to the electrochemical cells incorporating the electrolytes, and particularly to electrochemical cells with high-nickel containing cathodes. The localized saturated electrolytes can use low-cost salts, like hexafluorophosphate (PF6 -) salts (e.g., LiPF6), diluents, such as a fluoralkyl ether diluents, and carbonate solvents. The electrolytes can provide protection against reduction of components of the cathode of the electrochemical cell (e.g., reduction of Ni3+ to Ni2+) and loss of oxygen from the cathode, and can form cathode electrolyte interphases and solid electrolyte interphases that are rich in inorganic components.

Description

LOCALIZED SATURATED ELECTROLYTESAND RECHARGEABLE BATTERIES CONTAINING THE SAME

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of and priority to U.S. Provisional Application NO. 63/289,357, filed on December 14, 2021, which is hereby incorporated by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with government support under Grant no. DE-EE0007762 awarded by the Department of Energy. The government has certain rights in the invention.

FIELD

[0003] This invention is in the field of lithium ion and lithium metal batteries. This invention relates generally to an improvement to lithium ion and lithium metal battery electrolytes and performance with use of low/no-cobalt containing cathode materials like LiNii-x-yMnxCoyCh with Ni content > 0.8 and Co content < 0.1, LiNii-xMxCh with Ni content > 0.8 and M content < 0.2 where M is one or more other metals, LiMm.s-xNio.s-yCL, LiFePCL etc.

BACKGROUND

[0004] Developing rechargeable batteries with high energy density, long cycle life, and reliable safety is becoming a crucial step to fulfill the ambitious goal of net-zero carbon emissions by mid-century. Among the various types of rechargeable batteries, lithium-ion batteries (LIBs) have dominated the battery markets in portable devices and are regarded as the only practical candidate for electric vehicles (EVs). Over the past decade, high-nickel layered oxides (LiNii-_x-yMn_xCoyO2 and LiNii-x-yCo_xAlyO2 with (1-x-y) > 0.8) have solidified their status as the cathode materials for passenger EV batteries due to the high practical capacity (> 200 mA h g^-1) and high average operating voltage (3.8 V versus Li/Li⁺). As the end member of the high-nickel cathode materials, LiNiO2 has been appealing over 30 years after it was introduced as a cathode active material, but a number of practical considerations limits its use. For example, conventional use of LiNiO2 suffers from poor cycling stability, thermal stability, and air storage stability. SUMMARY

[0005] The present disclosure provides electrolytes for use in electrochemical cells, electrochemical cells incorporating such electrolytes, and methods of making electrolytes and electrochemical cells. The electrolytes described herein include localized saturated electrolytes (LSE) that provide beneficial performance characteristics to the electrochemical cells incorporating the electrolytes.

[0006] In examples, a localized saturated electrolyte comprises a solvent, a metal hexafluorophosphate salt, such as where the metal hexafluorophosphate salt is present in the solvent at a saturated concentration or within about 20% of a saturated concentration; and a diluent. In some examples, the metal hexafluorophosphate salt is present in the solvent within 1% of a saturated concentration, within 5% of a saturated concentration, within 10% of a saturated concentration, or within 15% of a saturated concentration. Example metal hexafluorophosphate salts include, but are not limited to an alkali metal hexafluorophosphate salt or an alkaline earth hexafluorophosphate salt. For example, the metal hexafluorophosphate may comprises LiPFe, NaPFe, KPFe, Mg(PFe)2, Ca(PFe)2, Al(PFe)3, or Zn(PFe)2. Other metal hexaflurophosphate salts may be used. Optionally, the metal hexafluorophosphate salt has a concentration of from about 1 mole per liter of the solvent to about 8 moles per liter of the solvent, such as from 1.0 mole per liter of the solvent to 1.5 moles per liter, from 1.5 moles per liter of the solvent to 2.0 moles per liter of the solvent, from 2.0 moles per liter of the solvent to 2.5 moles per liter, from 2.5 moles per liter of the solvent to 3.0 moles per liter of the solvent, from 3.0 moles per liter of the solvent to 3.5 moles per liter, from 3.5 moles per liter of the solvent to 4.0 moles per liter of the solvent, from 4.0 moles per liter of the solvent to 4.5 moles per liter, from 4.5 moles per liter of the solvent to 5.0 moles per liter of the solvent, from 5.0 moles per liter of the solvent to 5.5 moles per liter, from 5.5 moles per liter of the solvent to 6.0 moles per liter of the solvent, from 6.0 moles per liter of the solvent to 6.5 moles per liter, from 6.5 moles per liter of the solvent to 7.0 moles per liter of the solvent, from 7.0 moles per liter of the solvent to 7.5 moles per liter, or from 7.5 moles per liter of the solvent to 8.0 moles per liter of the solvent. Optionally, the metal hexafluorophosphate salt has a concentration of from about 0.5 moles per liter of the solvent and diluent combined to about 4 moles per liter of the solvent and diluent combined, such as from 0.5 moles per liter of the solvent and diluent combined to 1.0 mole per liter of the solvent and diluent combined, from 1.0 mole per liter of the solvent and diluent combined to 1.5 moles per liter of the solvent and diluent combined, from 1.5 moles per liter of the solvent and diluent combined to 2.0 moles per liter of the solvent and diluent combined, from 2.0 moles per liter of the solvent and diluent combined to 2.5 moles per liter of the solvent and diluent combined, from 1.5 moles per liter of the solvent and diluent combined to 2.0 moles per liter of the solvent and diluent combined, from 2.0 moles per liter of the solvent and diluent combined to 2.5 moles per liter of the solvent and diluent combined, from 2.5 moles per liter of the solvent and diluent combined to 3.0 moles per liter of the solvent and diluent combined, from 3.0 moles per liter of the solvent and diluent combined to 3.5 moles per liter of the solvent and diluent combined, or from 3.5 moles per liter of the solvent and diluent combined to 4.0 moles per liter of the solvent and diluent combined.

[0007] Suitable diluents may be included in the localized saturated electrolyte, including diluents that do not significantly interact with the solvent or the hexafluorophosphate salt or ionic components thereof. Optionally, the diluent comprises a fluoroalkyl ether, a fluorinated carbonate, a fluorinated borate, or a fluorinated orthoformate. Some non-limiting example diluents include one or more of l,l,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, l,l,2,3,3,3-hexafluoropropyl-2,2,3,3-tetrafluoropropyl ether, 2,2,3,3,3-pentafluoropropyl- 1,1,2,2-tetrafluoroethyl ether, l,l,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, hexafluoroisopropyl methyl ether, 1,1,2,2-tetrafluoroethyl ethyl ether, bis(2,2,2-trifuoroethyl) ether, lH,lH,5H-octafluoropentyl 1,1,2,2-tetrafluoroethyl ether, bis(2,2,2-trifluoroethyl) carbonate, tris(2,2,2-trifluoroethyl) borate, fluorobenzene, 1,2-difluorobenzene, 1,3- difluorobenzene, 1,4-difluorobenzene, 1,3, 5 -trifluorobenzene, or tris(2,2,2-trifluoroethyl) orthoformate. The diluent may be present at any suitable concentration or amount in the electrolyte. Optionally, a volume ratio of the diluent to the solvent may be from about 1 : 1 to about 10:1, such as from 1:1 to 2:1, from 2:1 to 3:1, from 3:1 to 4:1, from 4:1 to 5: 1, from 5:1 to 6:1, from 6: 1 to 7:1 from 7:1 to 8:1, from 8: 1 to 9:1, or from 9: 1 to 10:1.

[0008] Suitable solvents may be included in the localized saturated electrolyte, including one or more carbonate solvents. Example, non-limiting solvents include one or more of propylene carbonate, ethylene carbonate, fluoroethylene carbonate, butylene carbonate, dimethyl carbonate, ethylmethyl carbonate, or diethyl carbonate.

[0009] Localized saturated electrolytes may optionally comprise one or more additives, such as, but not limited to vinylene carbonate, fluoroethylene carbonate, lithium nitrate, lithium difluoroborate, lithium bis(oxalato)borate, lithium tetrafluoroborate, lithium phosphorodifluoridate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, pyridine-boron trifluoride, pyridine phosphorus pentafluoride, or any combination of these. [0010] The localized saturated electrolyte may have a viscosity of from about 1 cP to about 10 cP, such as at 25 °C. Optionally, the localized saturated electrolyte may have a viscosity of from 1 cP to 2 cP, from 2 cP to 3 cP, from 3 cP to 4 cP, from 4 cP to 5 cP, from 5 cP to 6 cP, from 6 cP to 7 cP, from 7 cP to 8 cP, from 8 cP to 9 cP, or from 9 cP to 10 cP, at 25 °C. In some examples, the concentration of metal hexafluorophosphate salt may impact the viscosity of the localized saturated electrolyte. Optionally, the viscosity of the localized saturated electrolyte can be adjusted by controlling the amount of diluent included localized saturated electrolyte.

[0011] The localized saturated electrolyte may have an ion conductivity (e.g., a metal ion, such as an alkali metal ion or alkaline earth metal ion) of from about 0.1 mS/cm to about 50 mS/cm, such as at 25 °C. Optionally, the localized saturated electrolyte may have an ion conductivity of from 0.1 mS/cm to 0.5 mS/cm, from 0.5 mS/cm to 1.0 mS/cm, from 1.0 mS/cm to 5.0 mS/cm, from 5.0 mS/cm to 10 mS/cm, from 10 mS/cm to 20 mS/cm, from 20 mS/cm to 30 mS/cm, from 30 mS/cm to 40 mS/cm, or from 40 mS/cm to 50 mS/cm.

[0012] In some examples, electrochemical cells are provided herein, such as an electrochemical cell comprising a cathode; an anode; and an electrolyte between the cathode and the anode, and particularly where the electrolyte is a localized saturated electrolyte. In various examples, the localized saturated electrolyte may be any of the localized saturated electrolytes described above and herein. In some examples, the electrolyte comprises: a metal hexafluorophosphate salt; a solvent; and a diluent. Optionally the metal hexafluorophosphate salt is present in the electrolyte at a saturated concentration or within 20% of a saturated concentration. Optionally, the metal hexafluorophosphate salt is present in the electrolyte at a concentration of from 0.5 moles per liter of the solvent and diluent combined to 4 moles per liter of the solvent and diluent combined. Optionally, the metal hexafluorophosphate salt is present in the electrolyte at concentration of from 1 moles per liter of the solvent to 8 moles per liter of the solvent or from 2.5 moles per liter of the solvent to 3.5 moles per liter of the solvent. In some examples, the metal hexafluorophosphate salt comprises an alkali metal hexafluorophosphate salt or an alkaline earth hexafluorophosphate salt. Optionally, the metal hexafluorophosphate comprises LiPFe, NaPFe, KPFe, Mg(PFe)2, Ca(PF₆)₂, A1(PF₆)₃, or Zn(PF₆)2.

[0013] As with the localized saturated electrolytes described above, in the electrochemical cell, the diluent is present in the electrolyte at any suitable volume ratio with the solvent, such as from 1:1 to 10:1. Optionally, the diluent comprises a fluoroalkyl ether, a fluorinated carbonate, a fluorinated borate, or a fluorinated orthoformate. Optionally, the diluent comprises one or more of 1,1,2, 2-tetrafluoroethyl-2, 2,3, 3 -tetrafluoropropyl ether, 1, 1,2, 3,3,3- hexafluoropropyl-2,2,3,3-tetrafluoropropyl ether, 2, 2,3,3, 3 -pentafluoropropyl- 1, 1,2,2- tetrafluoroethyl ether, l,l,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, hexafluoroisopropyl methyl ether, 1,1,2,2-tetrafluoroethyl ethyl ether, bis(2,2,2-trifuoroethyl) ether, 1H,1H,5H- octafluoropentyl 1,1,2,2-tetrafluoroethyl ether, bis(2,2,2-trifluoroethyl) carbonate, tris(2,2,2- trifluoroethyl) borate, fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene, 1,4- difluorobenzene, 1,3,5-trifluorobenzene, or tris(2,2,2-trifluoroethyl) orthoformate.

[0014] In some examples, the solvent comprises a carbonate solvent, such as one or more of propylene carbonate, ethylene carbonate, fluoroethylene carbonate, butylene carbonate, dimethyl carbonate, ethylmethyl carbonate, or diethyl carbonate. Optionally, the electrolyte further comprises one or more additives selected from the group consisting of vinylene carbonate, fluoroethylene carbonate, lithium nitrate, lithium difluoroborate, lithium bis(oxalato)borate, lithium tetrafluoroborate, lithium phosphorodifluoridate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, pyridine-boron trifluoride, pyridine phosphorus pentafluoride, and any combination of these.

[0015] Any suitable anode for the electrochemical cell can be used. In some examples, the anode comprises graphite, an alkali metal in metallic or alloy form, silicon, or a silicon- graphite composite. The anode may be prepared using any suitable technique. Cathodes useful with the electrochemical cells described herein include, but are not limited to an alkali transition metal oxide cathode material, a layered transition metal oxide cathode material, a spinel cathode material, a poly anion cathode material, or a transition metal oxide cathode material. Optionally, the cathode comprises a high nickel alkali metal oxide. In some examples, the cathode comprises ANii-_aM_a02, where A is an alkali metal, a is from 0 to 0.5, and M is one or more metals, such as one or more of Mn, Al, Mg, Co, Fe, Cr, B, Ti, Zr, Ga, Zn, V, Cu, Yb, Li, Na, K, F, Ba, Ca, Lu, Y, Nb, Mo, Ru, Rh, Ta, Pr, W, Ir, In, Tl, Sn, Sr, S, P, Cl, Ge, Sb, Er, Te, La, Ce, Nd, Dy, Eu, Sc, Se, Si, Tc, Pd, Pm, Sm, Gd, Tb, Ho, or Tm. Optionally, the cathode comprises ANii-_aMl_aiM2_a2O2, where Ml and M2 are different metals, and al + a2 = a. Optionally, the cathode comprises LiNii-_aM_a02.

[0016] Electrochemical cells may optionally include other components. For example, an electrochemical cell may comprise a separator between the cathode and the anode, a cathode electrolyte interphase in contact with the cathode, a solid electrolyte interphase in contact with the anode, an anode current collector in contact with active material of the anode, or a cathode current collector in contact with active material of the cathode. Optionally, the cathode electrolyte interphase and/or the solid electrolyte interphase independently comprise an inorganic rich component, such as one or more of AF, A_xPF_yOz, or ANi_xF_yOz, where A is an alkali metal or alkaline earth metal, and x, y, and z are from 0 to 1.

[0017] Methods of making electrochemical cell are also disclosed. In some examples, such a method comprises providing a cathode; providing an anode; and positioning an electrolyte between the cathode and the anode, such as a localized saturated electrolyte. The electrolyte may be prepared by dissolving a metal hexafluorophosphate in a solvent to create a high concentration electrolyte, and then combining the high concentration electrolyte with a diluent.

[0018] Without wishing to be bound by any particular theory, there can be discussion herein of beliefs or understandings of underlying principles relating to the invention. It is recognized that regardless of the ultimate correctness of any mechanistic explanation or hypothesis, an embodiment of the invention can nonetheless be operative and useful.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG. 1 provides a schematic illustration of an example electrochemical cell.

[0020] FIG. 2 provides an overview and data relating to the structure of electrolytes with different salt concentrations. Electrolyte structure of the (Panel A) conventional LP57 electrolyte, (Panel B) LiPFe-based HCE, and (Panel C) LiPFe-based LSE. (Panel D) FTIR characterization of different solvents and the three electrolytes. (Panel E) FTIR spectra from 700 cm’¹ to 740 cm’¹ to study the EC coordination. (Panel F) FTIR spectra from 810 cm’¹ to 890 cm’¹ to study the PFg coordination. (Panel G) FTIR spectra from 1700 cm’¹ to 1850 cm’¹ to study the effect of TTE diluent on EC and EMC solvents.

[0021] FIG. 3 provides data relating to materials and electrochemical characterizations of LiNiCh in different electrolytes: (Panel A) SEM images of LiNiCh microspheres with an average diameter of 12 pm (secondary particles). The insert zooms into the surface of a secondary particle to show the primary particles. (Panel B) Ri etv eld refinement of X-ray diffraction data and schematic structure of the layered LiNiCh. (Panel C) Self-discharge performance of LiNiCh cathodes tested at 45 °C. (Panel D) Cycling performance of LiNiCh with the LP57 electrolyte and the LSE electrolyte. The charge rate was 0.5C and the discharge rate was 1C (C/2-1C). (Panel E) Rate capability of LiNiCh electrodes tested at different C-rates. The charge rate was C/5. (Panel F) Electrochemical charge and discharge curves of LiNiCh cathodes at C/10 after cycling. (Panel G and Panel H) The chemical diffusion coefficient of Li obtained from the GITT tests, correlated with the dQ dV¹ curves for LiNiCh electrodes after different cycles in the LP57 electrolyte (Panel G) and the LSE electrolyte (Panel H).

[0022] FIG. 4 provides data characterizing the surface of cycled LiNiCh. (Panels A-D) X- ray photoelectron spectroscopy (XPS) data of C Is (Panel A), O ls (Panel B), F Is (Panel C), and P 2p (Panel D) for the CEI from LiNiO2 surface after 200 cycles. Measurement data (dots) are fitted by several individual Lorentzian/Gaussian functions (colored regions) to separate different components. The fitted spectra are shown as an envelope that matches the experimental data points. (Panel E) Quantitative analysis of different components derived from the XPS data. The ratios of different types of atoms are labeled.

[0023] FIG. 5 provides data showing cycling performances of LiNiCh with different electrolytes. (Panel A) Cycling performance of LiNiCh at C/3 for both charging and discharging at 2.8 - 4.4 V. (Panel B) Cycling performance of LiNiCh with the HCE electrolyte tested at C/2 charge rate and 1C discharge rate. Three cells are shown to illustrate the poor compatibility between the HCE electrolyte and the LiNiCh cathode.

[0024] FIG. 6 provides data showing cycling performances of LiNiCh with different electrolytes. (Panel A) Cycling performance of LiNiCh at C/3 for both charging and discharging at 2.8 - 4.4 V. (Panel B) Cycling performance of LiNiCh with the HCE electrolyte tested at C/2 charge rate and 1C discharge rate. Three cells are shown to illustrate the poor compatibility between the HCE electrolyte and the LiNiCh cathode. (Panel C) Capacity retention.

[0025] FIG. 7 provides cyclic voltammetry (CV) measurements of LiNiCh electrode. (Panels A-C) CV measurement results with different scan rates for the LiNiCh electrode tested in the LP57 electrolyte (Panel A), the HCE (Panel B), and the LSE (Panel C). The scan rates are 0.05 mV s'¹, 0.1 mV s'¹, 0.2 mV s'¹, and 0.4 mV s'¹. (Panel D) Relationship between the peak current and the scan rate of the LiNiCh electrodes with three different electrolytes.

[0026] FIG. 8 provides electrochemical impedance spectroscopy (EIS) data of LiNiCh cells tested at 3.8 V (Panel A) EIS data of LiNiCh cells before and after 200 cycles in the LP57 electrolyte. (Panel B) EIS data of LiNiCh cells before and after 200 cycles in LSE.

[0027] FIG. 9 provides data from a galvanostatic intermittent titration technique (GITT) measurement of the diffusion coefficient of Li in LiNiCh at different voltages. The insert zooms into a charge and a discharge region to highlight the voltage profiles. [0028] FIG. 10 provides cycle data showing full cell results with LiNiCh as the cathode and graphite as the anode. 10% FEC is applied as the additive, and different diluents are used in LSE. FEC: fluoroethylene carbonate; TTE: l,l,2,2-tetrafluoroethyl-2, 2,2,3- tetrafluoropropyl ether; TME: 1,1,2,2-tetrafluoroethyl methyl ether; TFTFE: 1, 1,2,2, - tetrafluoroethyl-2,2,2- trifluoroethyl ether; and BTFE: bis(2,2,2-trifluoroethyl) ether.

[0029] FIG. 11 provides data characterizing the surface of cycled Li metal with different electrolytes. (Panels A-D) XPS data of C Is (Panel A), O Is (Panel B), F Is (Panel C), and P 2p (Panel D) for the surface of Li -metal anode after 200 cycles. Measurement data (dots) are fitted by several individual Lorentzian/Gaussian functions (colored regions) to separate different components. The fitted spectra are shown as an envelope that matches the experimental data points. (Panel E) Quantitative analysis of different components derived from the XPS data.

DETAILED DESCRIPTION

[0030] Described herein are electrolytes for use in alkali metal, alkali metal ion, alkaline- earth metal, and other metal electrochemical cells, and methods of making such electrochemical cells. The electrolytes described herein include localized saturated electrolytes (LSE) that provide beneficial performance characteristics to the electrochemical cells incorporating the electrolytes. The electrolytes can include characteristics that may normally be found in highly concentrated electrolytes (HCE), such as where some cations and anions form contact ion pairs that are solvated or partially solvated rather than completely solvated anions and cations. These contact ion pairs can be present in the electrolytes described herein because the salt may have a relatively low solubility in the solvent and also due to the presence of diluents that have limited interaction between both the solvent and the salt and can create regions within the electrolyte where the salt is present at relatively high concentrations despite being present at relatively lower concentrations in the electrolyte as a whole than in comparable HCEs. Advantageously, the localized saturated electrolytes can use low-cost salts, like hexafluorophosphate (PFg ) salts (e.g., LiPFe). Hexafluorophosphate salts can achieve saturation in commonly used solvents, like carbonates, at relatively low concentrations compared to more expensive salts, like lithium bis(fluorosulfony)imide (LiFSI), which are normally used to achieve high concentrations in ether solvents.

[0031] The electrolytes provide beneficial performance for a variety of electrochemical cells and particularly electrochemical cells in which the cathode comprises a layered transition metal oxide cathode material, a spinel cathode material, or a polyanion cathode material. In some examples, cathodes including high-nickel-containing material can achieve beneficial performance in an electrochemical cell in which the disclosed localized saturated electrolytes are used. For example, the electrolytes can provide protection against reduction of Ni³⁺ to Ni²⁺ and loss of oxygen from the cathode. The electrolytes can also provide protection against oxidation of the solvent. In some examples, the electrolytes can form a protective cathode electrolyte interphase (CEI), which can conduct alkali metal ions like Li⁺, through surface reactions of the electrolyte at the cathode. For example, the high concentration of hexafluorphophate ions can form a CEI that is rich in inorganic components, like LiF, Li_xPF_yOz, and LiNi_xF_yOz (in the case of the alkali metal being Li), and different in nature from organic CEIs commonly formed in conventional electrochemical cells through reactions with the organic solvent component of the electrolyte. In some examples, the electrolytes can form a protective solid electrolyte interphase (SEI), which can conduct alkali metal ions like Li⁺, through surface reactions of the electrolyte at the anode. For example, the high concentration of hexafluorphophate ions can form a SEI that is rich in inorganic components, like LiF, LixPFyOz, and LiNixFyOz (in the case of alkali metal being Li), and different in nature from organic SEIs commonly formed in conventional electrochemical cells through reactions with the organic solvent component of the electrolyte.

[0032] FIG. 1 provides a schematic illustration of an example electrochemical cell 100. Electrochemical cell 100 is described as a lithium-ion electrochemical cell, but it will be appreciated that other alkali metal systems, alkaline earth metal systems, or other metal systems, can be used in place of lithium, such as sodium ion systems, potassium ion systems, magnesium ion systems, aluminum ion systems, zinc ion systems, etc. Electrochemical cell 100 includes a cathode current collector 105 (e.g., aluminum foil), a cathode active material 110, a cathode electrolyte interphase (CEI) 115, an electrolyte 120, a separator 125, a solid electrolyte interphase (SEI) 130, an anode active material 135, and an anode current collector 140 (e.g., copper foil). In some cases, additional components beyond those illustrated may be included in electrochemical cell 100. In some cases, fewer components beyond those illustrated may be included in electrochemical cell 100. For example, although CEI 115 and SEI 130 are shown for purposes of discussion of operational and beneficial aspects herein, electrochemical cells incorporating aspects described herein need not contain a CEI and/or SEI, such as at the time of assembly of the electrochemical cell, but can form such a CEI and/or SEI during operation (e.g., cycling of the electrochemical cell). Separator 125 can comprise any suitable ion conducting and electrically insulating material (e.g., a porous polymeric layer). In some cases, separator 125 may not be present, such as if cathode active material 110 and anode active material 135 are separated from one another by some other means. [0033] Electrolyte 120 can comprise a localized saturated electrolyte comprising a salt, a solvent, and a diluent. The salt can be LiPFe, for example, which may be present in the electrolyte 120 at relatively high concentrations, such as at or near a saturated concentration of the salt in the solvent, e.g., within 20% of a saturated concentration. In some examples, the salt can be present at a concentration which would be at or near saturation in the solvent in the absence of the diluent. LiPFe can achieve saturation in some carbonate solvents at about 3 moles per liter, for example. In some examples, the salt can be present at a concentration of from about 1 moles per liter of the solvent to about 8 moles per liter of the solvent, such as from about 2.5 moles per liter of the solvent to about 3.5 moles per liter of the solvent. Optionally, the electrolyte may comprise one or more additives, including but not limited to, vinylene carbonate, fluoroethylene carbonate, lithium nitrate, lithium difluoroborate, lithium bis(oxalato)borate, lithium tetrafluoroborate, lithium phosphorodifluoridate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, pyridine-boron trifluoride, pyridine phosphorus pentafluoride, or combinations of these.

[0034] The solvent can comprise one or more carbonate solvents, such as propylene carbonate, ethylene carbonate, fluoroethylene carbonate, butylene carbonate, dimethyl carbonate, ethylmethyl carbonate, diethyl carbonate, or any combination of these and including optional additives or the like.

[0035] The diluent may comprise a component that does not significantly interact with the salt or the solvent. Useful diluents include fluoroalkyl ethers, fluorinated carbonates, fluorinated borates, and fluorinated orthoformates. Example diluents include, but are not limited to, l,l,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 1, 1,2, 3,3,3- hexafluoropropyl-2,2,3,3-tetrafluoropropyl ether, 2, 2,3,3, 3 -pentafluoropropyl- 1, 1,2,2- tetrafluoroethyl ether, l,l,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, hexafluoroisopropyl methyl ether, 1,1,2,2-tetrafluoroethyl ethyl ether, bis(2,2,2-trifuoroethyl) ether, 1H,1H,5H- octafluoropentyl 1,1,2,2-tetrafluoroethyl ether, bis(2,2,2-trifluoroethyl) carbonate, tris(2,2,2- trifluoroethyl) borate, fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene, 1,4- difluorobenzene, 1,3, 5 -trifluorobenzene, tris(2,2,2-trifluoroethyl) orthoformate, or any combination of these. The diluent may be present in the electrolyte 120 in any suitable amount. In some examples, the amount of diluent may be defined relative to the volume of solvent. For example, the diluent may be present at a volume ratio with the solvent of from 1:1 (e.g., equal volumes of diluent and solvent) to 10:1 (e.g., diluent volume ten times solvent volume). [0036] With the diluent present in the electrolyte 120, the absolute concentration of the salt in the electrolyte 120 may be defined relative to the total volume of solvent plus diluent. For example, the concentration of the salt may be from about 0.5 moles per liter of solvent and diluent combined to about 2 moles per liter of solvent and diluent combined. Combining the solvent with a diluent in the electrolyte 120 can impact the viscosity. In some cases, when the salt concentration of hexafluorophosphate increases in an electrolyte 120, the viscosity can increase. In some cases, a high salt concentration can result in reduced ion conductivity of the electrolyte 120. The presence of the diluent can result in reducing the viscosity and increasing the ion conductivity, such as compared with an electrolyte 120 lacking the diluent. In some examples, the electrolyte 120 can have a viscosity of from about 1 cP to about 10 cP at about 25 °C. In some examples, electrolyte has a lithium ion conductivity of from about 0.1 mS/cm to about 50 mS/cm at about 25 °C.

[0037] Cathode current collector 105 can be any suitable material. In a non-limiting example, cathode current collector 105 comprises aluminum. Cathode active material 110 may comprise any suitable cathode material, but aspects described herein may provide benefits to certain cathode materials. For example, cathode active material 110 may comprise a lithium transition metal oxide cathode material, a layered transition metal oxide cathode material, a spinel cathode material, a polyanion cathode material, or a transition metal oxide cathode material. In some examples, cathode active material 110 comprises a nickel-rich cathode, such as LiNii-aMaCh, where a is from 0 to 0.5, and wherein M is one or more metals (e.g., one or more transition metals or one or more non-transition metals), such as Mn, Al, Mg, Co, Fe, Cr, B, Ti, Zr, Ga, Zn, V, Cu, Yb, Li, Na, K, F, Ba, Ca, Lu, Y, Nb, Mo, Ru, Rh, Ta, Pr, W, Ir, In, Tl, Sn, Sr, S, P, Cl, Ge, Sb, Er, Te, La, Ce, Nd, Dy, Eu, Sc, Se, Si, Tc, Pd, Pm, Sm, Gd, Tb, Ho, Tm. In an example, cathode active material 110 may comprise LiNiCh (e.g., where a is 0). In some cases, the cathode active material 110 may be lithium rich or lithium poor, meaning lithium may be present at amounts relative to Nii-_aM_a slightly above or below unity (e.g., from 0.9 to 1.3). In some cases, the cathode active material 110 may be oxygen rich or oxygen poor, meaning oxygen may be present at amounts relative to Nii-_aM_a slightly above or below two (e.g., from 1.90 to 2.10).

[0038] CEI 115 can comprise an inorganic-rich layer on cathode active material 110. In some examples, CEI 115 can comprise one or more of LiF, Li_xPF_yOz, or LiNi_xF_yOz. CEI 115 may advantageously be ionically conductive to lithium ions and provide a protective coating to cathode active material 110, limiting reduction of the active material, such as Ni³⁺ components or loss of oxygen. [0039] SEI 130 can comprise an inorganic-rich layer on anode active material 135. In some examples, SEI 130 can comprise one or more of LiF, Li_xPF_yOz, or LiNi_xF_yOz. SEI 130 may advantageously be ionically conductive to lithium ions and provide a protective coating to anion active material 135.

[0040] Anode active material 135 can comprise any suitable material. In some nonlimiting examples, anode active material 135 comprises lithium metal or a lithium metal alloy, silicon, graphite, a silicon-graphite composite, or the like. Anode current collector 140 can comprise any suitable material. In a non-limiting example, anode current collector 140 comprises copper.

[0041] The invention may be further understood by the following non-limiting examples.

EXAMPLE 1: LOCALIZED SATURATED ELECTROLYTES FOR LONG-LIFE RECHARGEABLE BATTERIES

[0042] High-nickel layered oxide cathode materials, in particular the end member Co-free LiNiO2, are the most promising candidates for developing lithium-ion batteries and lithium- metal batteries because of their high energy density and low cost. However, these materials suffer from poor cycling stability due to high surface reactivity and severe structural changes during cycling. This Example presents a LiPFe-based localized saturated electrolyte (LSE) that can stabilize the LiNiO2 cathode material and lithium metal anode during cycling by forming an inorganic-rich cathode-electrolyte interphase (CEI) layer and an inorganic-rich solid-electrolyte interphase (SEI). Compared to the conventional LP57 electrolyte consisting of 1.0 M LiPFe in ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (3 : 7 wt. ratio) along with 2 wt. % of vinylene carbonate (VC) that retains 74% of the LiNiCh initial capacity after 100 cycles, the LSE electrolyte maintains 84% of the initial capacity after 400 deep cycles at a high cut-off voltage of 4.4 V (versus Li/Li⁺). Moreover, the LSE electrolyte significantly reduces the rate of self-discharge when the LiNiCh cell is stored at 45 °C. The inorganic-rich CEI protects the surface of LiNiCh from degrading into spinel -like and rocksalt phases during cycling, reducing the overpotential that generally limits the state-of-charge and capacity of LiNiCh during cycling. No bulk fatigue phases are observed in the cycled LiNiCh, regardless of the electrolyte applied, indicating that protecting the surface can be useful for improving the cycling stability of LiNiCh. This Example highlights the utility of surface protection for high-Ni cathodes and provides a method to design robust interphase for cathodes in high-energy batteries. [0043] High-concentration electrolytes (HCE) and localized high-concentration electrolytes (LHCE) have been developed and investigated for their application in lithium-ion batteries and lithium-metal batteries (LMBs). However, they are based on salts with high solubility in a selected solvating solvent, like lithium bis(fluorosulfony)imide (LiFSI). This Example describes a new type of LSE based on LiPFe salt that has limited solubility in carbonate solvents. The applicability of the LiPFe-based LSE is demonstrated in LMBs with the cobalt- free LiNiCh cathode. By forming an inorganic-rich CEI on the cathode and an inorganic-rich SEI on the anode, the LiPFe-based LSE largely extends the cycle life and improves the storage stability of LMBs with the LiNiCh cathode. The developed LSE has the potential to be further applied to improve the performance of other batteries with transition-metal oxide cathodes.

[0044] The high interest in LiNiCh comes, at least in part, from the much lower cost and higher natural abundance of nickel compared to cobalt. In addition, LiNiCh can deliver a practical capacity of over 220 mA h g^-1 with an average voltage of over 3.8 V versus Li/Li⁺. However, the application of LiNiCh suffers from its poor cycling stability, thermal stability, and air storage stability. The poor performance of LiNiCh comes from multiple aspects. First, precisely controlling the composition of LiNiCh during synthesis is not straightforward because it is strongly prone to Li off-stoichiometry (Lii-zNii+zCh, z > 0) due to the similar size of Li⁺ and Ni²⁺. During charging, these Ni²⁺ cations in the Li-sites can be oxidized to smaller Ni³⁺ ions, which shrinks the local environment and hinders Li diffusion in subsequent cycles. Additionally, the conventional organic carbonate electrolytes easily react with LiNiCh surface at charged state, leading to a reduction of Ni⁴⁺ to Ni²⁺ and Ch release.

Moreover, LiNiCh shows worse mechanical stability than other high-nickel layered oxides when cycled beyond 4.2 V, during which the material goes through H2

H3 phase transition that introduces a sudden decrease in the c lattice parameter and unit cell volume. The relative volume change is as large as 9.0% from the beginning to the end of charge. Such a large volume change indices high strain within particles, leading to crack formation during long-term cycling. The particle crack will not only cause poor electrical connection between particles but expose a fresh surface to the organic electrolytes. Therefore, improving the (electro)chemical and mechanical stabilities of LiNiCh and developing electrolytes compatible with both the cathode and anode are urgently needed.

[0045] Different methods have been developed to improve the cycling stability of LiNiCh, including optimizing synthesizing conditions, elemental substitution/doping, surface engineering/coating, core-shell structures, and gradient strategies. Despite these methods alleviating the degradation of LiNiCh, many of them increase the synthesis complexity and sacrifice the energy density of the cathode. Recently, advanced electrolytes have been developed and applied to enable long cycling stability of LiNiCh without scarifying its capacity and energy density. In one example, a high-fluorinated electrolyte with LiDFOB additive that maintains 80% of LiNiCh initial capacity after 400 cycles was developed by forming a robust fluoride and boron-rich CEI on the cathode. The result suggests that developing advanced electrolytes with low cost is a route to apply LiNiCh as a cathode for commercial batteries.

[0046] This Example presents the development of a lithium hexafluorophosphate (LiPFe)- based LSE for its application in LiNiCh lithium-metal batteries (LMBs). Due to its relatively low solubility, the LiPFe salt has not been explored for application in localized high- concentration electrolytes. For example, the saturation concentration of LiPFe in carbonate solvents (ethylene carbonate (EC) : ethyl methyl carbonate (EMC) = 3 : 7 by volume) is only around 3 M. The low saturation concentration reduces the amount of the salt to form the required solvation structure, and thus reduces the cost of the electrolyte. This Example demonstrates that the LiPFe-based LSE can stabilize LiNiCh cathode and Li metal anode during cycling by forming an inorganic-rich CEI and an inorganic-rich SEI. The LiNiCh electrode can be cycled over 500 cycles before its capacity decreases to 80% of its initial capacity. In sharp contrast, it can be cycled to less than 100 times with the same capacity retention in the conventional LP57 electrolyte. This Example highlights the importance of developing advanced electrolytes to protect Ni-rich cathodes and metal anodes for applications in high-energy batteries and provides one step further towards the development of practical batteries with the LiNiCh cathode.

[0047] Solvation Structure of the LiPFe-Based Localized Saturated Electrolyte. The design principle of a HCE and an LSE is to reduce the number of free solvent molecules by increasing the concentration of the salt. Generally, almost all the solvating-solvent molecules in the HCE and the LSE are coordinated to the cation of the salt (Li⁺). The resulting solvation structures for HCE and LSE are completely different from the conventional electrolytes with a 1 M salt concentration.

[0048] FIG. 2 panels A-C compare the solvation structure of the LP57 electrolyte, a LiPFe- based HCE, and a LiPFe-based LSE. As the concentration of the LiPFe is 1 M in the LP57 electrolyte, all Li⁺ ions are solvated by the solvents (EC and EMC), as depicted in FIG. 2 panel A. Additionally, there are sufficient free solvents to enable fast transport of Li⁺in the LP57 electrolyte. By comparison, the concentration of the LiPFe in the HCE is 3 M that is close to the saturation concentration of the LiPFe in the EC/EMC solvent. Almost all solvent molecules are involved in the solvation structure of Li⁺ ions (FIG. 2 panel B). The deficiency of the solvating-solvent molecules leads to the formation of contact ion pairs and cationanion aggregates. In the LSE electrolyte (FIG. 2 panel C), l,l,2,2-tetrafluoroethyl-2,2,3,3- tetrafluoropropyl ether (TTE) is added as a diluent to reduce the viscosity of the electrolyte. The non-solvating property of the diluent ensures a similar structure between the HCE and LSE.

[0049] Fourier-transform infrared spectroscopy (FTIR) experiments were applied to examine the change of the solvation structure in the HCE and LSE. FIG. 2 panel D compares the FTIR spectra of the two main solvents (EC and EMC), the TTE diluent, and the three electrolytes (LP57, HCE, and LSE). The coordination between solvents and Li salt (LiPFe) can be analyzed from the characteristic peaks. As the TTE has no characteristic peaks at the high wavenumber region (1700 cm'¹ to 1850 cm'¹), the interaction between TTE and the two main solvents can be studied at such a region with FTIR spectroscopy. For example, FIG. 2 panel G displays the C = O stretching band in EC and EMC. Compared to the LP57 electrolyte, the C = O stretching peaks in the EC and EMC show different shapes and positions in HCE due to the different coordination structures between the solvents and Li⁺. The similar shapes and positions of the C = O stretching peaks in HCE and LSE suggest that the TTE diluent interacts weakly or has no interaction with the EC and EMC solvents. Thus, LSE and HCE share a similar solvation structure.

[0050] The solvation structure of Li⁺ and PFg in electrolytes can be analyzed and derived by deconvoluting and designating the FTIR peaks as coordinated and uncoordinated based on reference data. FIG. 2 panel E shows the C = O breathing vibration band at 710 cm'¹ to 730 cm'¹, which can be used to distinguish the responses from different C = O binding states. The peaks at 728 cm'¹ and 715 cm'¹ can be identified, respectively, as coordinated EC and uncoordinated EC (free EC). Compared to the LP57 electrolyte, the HCE has much more coordinated EC and less free EC. A quantitative analysis of the peak area suggests that the free EC is reduced from 28% to 10% in the HCE. In addition, FIG. 2 panel F displays the P- F bond stretching band in PFg at 830 cm'¹ to 880 cm'¹. The peak at around 840 cm'¹ is from uncoordinated PFg , while the peak at around 870 cm'¹ arises through Li⁺ -coordinated PFg . Compared to the LP57 electrolyte, the ratio of the coordinated PFg is increased from 11% to 39% in the HCE. It will be appreciated that TTE shows strong characteristic peaks in these regions (700 cm'¹ to 750 cm'¹ (FIG. 2 panel E), 800 cm'¹ to 890 cm'¹ (FIG. 2 panel F)), making the quantitative analysis of the LSE hard from the FTIR spectrum. As the LSE shares similar solvation structure to the HCE (FIG. 2 panel G), these FTIR results support the generic structures of the three electrolytes shown in FIG. 2 panels A-C.

[0051] Characterization and Electrochemical Performance of LiNiCh in Different

Electrolytes. The LiNiCh cathode materials are agglomerates with a diameter of around 12 pm that are composed of primary particles with a size of around 100 nm, as shown in the scanning electron microscopy (SEM) images in FIG. 3 panel A. FIG. 3 panel B displays the powder X-ray diffraction (XRD) pattern with Cu ka radiation. A Rietveld refinement of the XRD pattern suggests the LiNiCh electrode material is a 3/?-type layered rhombohedral system with a hexagonal unit cell. The fitted lattice parameters are a = b = 2.8731 A and c = 14.1860 A. Additionally, there is around 2.47% of cation mixing, with Ni²⁺ located in the lithium layers. The cation mixing is caused by similar sizes of Li⁺ and Ni²⁺ that can hardly be eliminated during synthesis.

[0052] FIG. 3 panels C-H shows the electrochemical performance of the LiNiCh cathode in the three electrolytes, i.e. LP57 electrolyte, LiPFe-based HCE, and LiPFe-based LSE. The LSE largely improves the storage capability (FIG. 3 panel C) and cycling stability of LiNiCh (FIG. 3 panel D). FIG. 3 panel C compares the self-discharge performance of the LiNiCh electrode. The cell voltage decreases the fastest in the LP57 electrolyte and reaches 4.160 V after being stored at 45 °C for 100 h. By comparison, the cell voltage maintains at 4.214 V and 4.235 V, respectively, with the HCE and LSE after the storage. A full discharge at the C/10 rate was conducted after the storage to evaluate the capacity retention. The capacity remaining is ranked in the decreasing order as LSE (99.8%) > HCE (97.9%) > LP57 (85.0%), which follows the same trend as the open-circuit voltage retention.

[0053] The LSE significantly improves the cycling stability of LiNiCh with a high cut-off voltage at 4.4 V. The capacity retention is increased from 54% to 89% after 100 cycles when tested at a C/3 rate (FIG. 5 panel A). However, cells with the HCE perform poorly with a nonlinear capacity drop (FIG. 5 panel B). Moreover, the cycling stability is even worse than the cells with the LP57 electrolyte (FIG. 5 panel A). Thus, the HCE was not included in the following investigation, unless specifically mentioned. FIG. 3 panel D shows that the LiNiCh cell with the LSE retains 84% of its initial capacity after 400 cycles. By comparison, the cell with the conventional LP57 electrolyte maintains only 60% of its initial capacity after 165 cycles. In addition, the cell capacity suddenly drops to a low value at around 170 cycles, which could be from the failure of the Li metal anode. Re-pairing the cycled LiNiCh electrode with a fresh Li-metal anode brings back the capacity to 135 mA h g'¹ at a 1C discharge rate. The Coulombic efficiency (CE) is also largely increased in the LSE. For example, the average CE in the first 200 cycles is increased from 97.82% (LP57) to 99.58% (LSE). These results indicate that the developed LSE can stabilize both the LiNiCh cathode and Li-metal anode during cycling. It will be appreciated that the additives play a useful role to extend cell cycle life. After screening through commonly used electrolyte additives (FIG. 6), 1% lithium phosphorodifluoridate (PFO) and 10% fluoroethylene carbonate (FEC) were added to the LSE to optimize its performance.

[0054] The LSE also improves the rate capability of LiNiCh from C/5 to 3C (FIG. 3 panel E). The discharge capacity at 3C is increased from 186 mA h g for LP57 to 193 mA h g for LSE. However, when the C-rate is increased to 5C, both HCE and LSE show poor rate capability due to their relatively low ionic conductivity compared to the LP57 electrolyte. The measured Li⁺ conductivities in the three electrolytes are LP57 (8.70 mS cm⁴) > LSE (3.55 mS cm⁴) > HCE (3.08 mS cm⁴). To further compare the kinetics of LiNiCh in the three electrolytes, FIG. 7 compares the cyclic voltammetry (CV) scanning of LiNiCh. The peak current and the scan rate can be described by the Randles-Sevcik equation for all the three types of cells (FIG. 7 panel D), indicating a diffusion control behavior. Compared to the LP57 electrolyte, the current peaks are much broader in the HCE, especially at high scan rates (0.2 mV s and 0.4 mV s⁴). In contrast, the current peaks maintain a similar shape and position in the LSE as that in the LP57 electrolyte, suggesting the necessity of adding a diluent (TTE) to promote the kinetics of the electrolyte.

[0055] FIG. 3 panel F compares the cycling curves of LiNiCh electrodes at a C/10 rate after 200 cycles and 400 cycles tested in the LP57 electrolyte and the LSE. Compared to the LP57 electrolyte, the LSE shows a minimal increase of cell overpotential and decrease of cell capacity after cycling. The relatively small increase of the overpotential matches well with the electrochemical impedance spectroscopy data shown in FIG. 8. Incremental capacity analysis (ICA) was applied to the charge curves at a C/10 rate to show the LiNiCh phase evolution during de-lithiation (FIG. 3 panels G and H). LiNiCh cathode undergoes multiple phase transitions during de-lithiation, as labeled in FIG. 3 panels G and H. Each dQ dV peak represents a two-phase coexistence region during charging. In the LP57 electrolyte (FIG. 3 panel G), all the dQ dV peaks shift to a higher voltage after 200 cycles, indicating that a larger overpotential is required to move the Li⁺ out of the cathode. Moreover, the intensity of the H2

H3 phase transition peak largely decreases, suggesting that most of the cathode materials do not go through the phase transition and, thus, stay as the M phase and H2 phase at the fully charged state. By comparison, FIG. 3 panel H shows that the dQ dV peaks only slightly shift to a higher voltage after 200 and 400 cycles for the LiNiCh electrode tested in LSE, indicating a much smaller increase in the overpotential. In addition, the intensity of the H2 ->H3 phase transition peak maintains well after 200 cycles and is slightly reduced after 400 cycles. These results suggest that the LiNiCh electrode is protected much better in LSE during cycling, leading to the ability to fully use the active materials after longterm cycling.

[0056] Galvanostatic intermittent titration technique (GITT) was employed to measure the Li diffusion coefficient at different voltages in LiNiCh. The voltage profile of a GITT measurement for a new LiNiCh electrode in the LP57 electrolyte is shown in FIG. 9. The derivation of the apparent diffusion coefficient for Li (Du) from the voltage profile is described below. FIG. 3 panels G and H display the Du of LiNiCh at the intial status and after cycling. The Du drops significantly at around 3.45 V, 3.65 V, 4.0 V, and 4.2 V. These voltage values match with the phase transition regions in the dQ dV¹ curves, indicating a much larger driving force is required to motivate the phase transition process than the singlephase process. In addition, FIG. 3 panel G shows that the Du was largely reduced after 200 cycles in the LP57 electrolyte, especially at the high voltage regions (> 3.9 V) that correspond to the M - H2 and H2

H3 phase transition. By comparison, the Du was maintained well with only a slight decrease after 200 cycles and 400 cycles in the LSE. Good maintenance of the Du could be one of the reasons for the much better capacity retention during cycling of LiNiCh electrodes with LSE (FIG. 3 panels D and F).

[0057] The LSE also improves the performance of LIBs with graphite as the anode. Results in FIG. 10 suggest that adding 10% FEC as the additive in HCE and LSE can stabilize the full cell performance during cycling. In addition, applying different diluents also affects the performance of the full cell. These results demonstrate the applicability of the LSE in practical LIBs.

[0058] Surface and Interphase Characterization for LiNiCh Electrodes. To further understand the largely improved cycling stability and less surface degradation of LiNiCh when tested in LSE, X-ray photoelectron spectroscopy (XPS) was applied to characterize the cycled LiNiCh cathodes and their surface layers, i.e., cathode-electrolyte interphase (CEI). FIG. 4 panels A-D compare the XPS of different species on the surface of aged LiNiCh cathodes, including C Is, O Is, F Is, and P 2p. FIG. 4 panel E further quantitatively compares the atomic ratios among different species derived from the XPS data.

[0059] FIG. 4 panel A suggests that the carbonate components on cycled LiNiCh cathodes are similar in the two electrolytes, including conductive carbon (C-C and C-H at -284.8 eV), carbonates (C-0 at -286.5 eV and C=O at -289.0 eV), and PVDF binder (CF2-CH2 at 287.5 eV and C-F at -291 eV). However, the relative amount of C atom among all the measured atoms is much higher in the CEI of LiNiCh with the LP57 electrolyte (54.6%) than that with the LSE (45.7%). Since all organic species have carbon as their backbone, a higher amount of C in the CEI indicates more organic components and fewer inorganic components in the CEI that are formed in the LP57 electrolyte. In FIG. 4 panel B, the O Is spectra show a stronger lattice oxygen signal in LSE compared to that in the LP57 electrolyte. Quantitative analysis of O in FIG. 4 panel E shows that the detectable lattice O is increased from 1.7% (LP57) to 2.2% (LSE). More detectable bulk O from LiNiCh indicates a thinner CEI layer on the surface of the electrode, and thus fewer side reactions between the electrolytes and the LiNiCh during cycling. Thus, compared to the LP57 electrolyte, the LSE helps form a thinner CEI layer with more inorganic components on the surface of the LiNiCh cathode.

[0060] In the FIs spectra (FIG. 4 panel C), the cycled LiNiCh electrode in the LSE shows a strong LiF, Li_xPF_yOz, and LiNi_xF_yOz signal at - 685.5 eV. The atomic ratio of these components increases from 4.8% (LP57) to 9.3% (LSE), as shown in FIG. 4 panel E. The existence of more LixPFyOz in the CEI with the LSE electrolyte can also be seen from the O Is spectra at - 534.5 eV (FIG. 4 panel B) and the P 2p spectra at - 136 eV (FIG. 4 panel D). It is believed that the in-situ formed LiF-rich CEI layer is an ideal shield to protect the surface of LiNiO2 from the electrolytes without reducing the ion-transport kinetics. In addition, the degraded LiPFe species, i.e. LixPFyOz, have been proved to be able to scavenge transition metals and suppress cross-talk to the anode. These components protect LiNiO2 from the attack of HF and other free radical groups in the electrolytes, reducing the loss of oxygen and surface reconstruction into disordered NiO-like rock salt phase. Similar XPS results are found on the Li-metal anode side, where an inorganic-rich surface protection layer (e.g., SEI) is observed in the anode with LSE (FIG. 11). Therefore, the LiNiCh cell shows much better cycling stability in LSE compared to the conventional LP57 electrolyte.

[0061] Materials Preparation. The baseline electrolyte was LP57 + 2% vinylene carbonate (VC, Gotion, purity 99.98%). The LP57 electrolyte consisted of 1 M lithium hexafluorophosphate (LiPFe) in a solvent mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a 3 : 7 ratio by weight. The high-concentration electrolyte (HCE) consisted of 3 M LiPFe (Gotion, purity 99.9%) dissolved in a mixture of EC (Gotion, purity 99.96%) and EMC (Gotion, purity 99.72%) in a 3 : 7 ratio by weight. The localized saturated electrolyte (LSE) was prepared by adding l,l,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE, SynQuest, purity 99%) into the HCE electrolyte with a volume ratio of 2 : 1. LiNiCh was prepared and used as the cathode material. The hydroxide precursor of the LiNiCh powder was obtained with an in-house coprecipitation process with a tank reactor. The precursor was mixed with LiOH FhO at a molar ratio of 1 : 1.03, followed by a heating treatment at 655 °C for 12 h. The calcinated cathode material was then mixed with conductive carbon black (Super P) and polyvinylidene fluoride binder in N-methyl-2- pyrrolidone with the weight ratio of 90 : 5 : 5. The resulting slurry was cast onto an Al foil with an active material loading of ~ 8 mg cm'². The cathode electrode was dried in a vacuum furnace at 110 °C overnight before making cells.

[0062] Materials Characterization. Fourier transform infrared (FTIR) spectra of different solvents and electrolytes were collected to study the solvation structure of the electrolytes with a Thermofisher FTIR Spectrometer equipped with an attenuated total reflection attachment and a germanium crystal. FTIR spectra were collected from 400 to 4000 cm'¹ at a 0.5 cm'¹ resolution and averaged over 35 scans. Scanning electron microscope (SEM, FEI Quanta 650) images of synthesized LiNiCh particles were collected to confirm the particle size. Laboratory Cu ka X-ray was utilized to characterize the crystal structure of LiNiCh powder from 10° to 80° with a 0.02° scan step (Rigaku Miniflex 600). X-ray photoelectron spectroscopy (XPS) measurements were conducted on cycled LiNiCh cathode electrodes with a Kratos Axis Ultra DLD spectrometer with Al Ka radiation (1486.6 eV) excitation source. Aged cells were disassembled inside an Argon-filled glovebox to harvest the electrolyte samples. These samples were loaded into an in-house transfer chamber (U.S. Patent No. 9,945,761) inside the glovebox to avoid air exposure during sample transfer. Regions scans were performed with a step size of 0.1 eV. CasaXPS software was utilized to deconvolute the peaks by fitting the experimental data with multiple Gaussian-Lorentzian functions after a Shirley background correction. The adventitious carbon peak at 248.8 eV was used for calibration.

[0063] Electrochemical Tests. CR2032-type coin cells were assembled inside an Argon- filled glovebox with an O2 and H2O level below 1 ppm. LiNiCh cathode, Celgard 2325 separator, and Li metal anode were sandwiched together with 100 pl electrolyte and crimped in the coin cell casings. Cyclic voltammetry (CV) scan was recorded with a Biologic VMP3 potentiostat from 2.8 to 4.4 V with scan rates of 0.05 mV s'¹, 0.1 mV s'¹, 0.2 mV s'¹, and 0.4 mV s'¹. Cycling tests and galvanostatic intermittent titration technique (GITT) measurements were carried out with an Arbin battery test station. The cells were cycled at C/10 (1C = 180 mA g'¹) rate from 2.8 V to 4.4 V three times before conducting the cycling tests. During the GITT measurement, the cells were charged/discharged at a pulse current of 18 mA g'¹ (C/10) for a duration of 15 min, followed by a relaxation of 1 h. The cut-off voltage for the GITT measurement was 4.4 V (charge) and 2.8 V (discharge). The electrochemical impedance spectroscopy (EIS) experiments were carried out by applying an AC voltage of 10 mV amplitude over the frequency range of 1 MHz to 1 mHz at an open-circuit voltage of 3.8 V with a Solartron SI 1287 potentiostat. The self-discharge test was carried out with an Arbin battery test station. Fully charged cells were stored in an ESPEC BTZ-133 environmental chamber at 45 °C for 100 h, followed by discharge at C/10 rate to 2.8 V.

[0064] Calculation of the apparent diffusion coefficient of lithium in LiNiCh. The apparent diffusion coefficient of lithium Du can be calculated by the following equation due to the small pulse current and short pulse period.

where r is the radius of the LiNiCh particle (12 pm), T is the current pulse time, A£) is the total transient voltage change during the current pulse period, E_S is the change of the steadystate voltage of the electrode for the corresponding step.

[0065] Conclusions for Example 1. The surface and interface stability of LiNiCh is one of the most crucial factors that limit its practical viability. This Example describes a LiPFe- based LSE that promotes the formation of an inorganic-rich interphase layer on the surface of both the LiNiCh cathode and the lithium anode. Such a layer protects the LiNiCh electrode surface from degrading into the spinel-like and rock-salt phases. The LiNiCh cathode in LSE delivers a discharge capacity of -220 mA h g (~ 840 W h kg ) at a 1C rate. The electrode could be cycled over 400 cycles at 1C discharge rate with a capacity retention of 84% in LSE, which is a significant improvement from a 74% capacity retention after only 100 cycles in the LP57 electrolyte. This is the best cycling performance ever reported for a LiNiCh cathode. The concept of the LiPFe-based LSE provides a new route and large unexplored candidates for designing novel electrolytes by tailoring the solvents, diluent, and the additives. These electrolytes will enable high-energy cathodes for developing the next-generation LIBs and LMBs.

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STATEMENTS REGARDING INCORPORATION BY REFERENCE AND VARIATIONS [0090] All references throughout this application, for example patent documents, including issued or granted patents or equivalents and patent application publications, and non-patent literature documents or other source material are hereby incorporated by reference herein in their entireties, as though individually incorporated by reference.

[0091] All patents and publications mentioned in the specification are indicative of the levels of skill of those skilled in the art to which the invention pertains. References cited herein are incorporated by reference herein in their entirety to indicate the state of the art, in some cases as of their filing date, and it is intended that this information can be employed herein, if needed, to exclude (for example, to disclaim) specific embodiments that are in the prior art.

[0092] When a group of substituents is disclosed herein, it is understood that all individual members of those groups and all subgroups and classes that can be formed using the substituents are disclosed separately. When a Markush group or other grouping is used herein, all individual members of the group and all combinations and subcombinations possible of the group are intended to be individually included in the disclosure. As used herein, “and/or” means that one, all, or any combination of items in a list separated by “and/or” are included in the list; for example, “1, 2 and/or 3” is equivalent to “1, 2, 3, 1 and 2, 1 and 3, 2 and 3, or 1, 2, and 3”.

[0093] Every formulation or combination of components described or exemplified can be used to practice the invention, unless otherwise stated. Specific names of materials are intended to be exemplary, as it is known that one of ordinary skill in the art can name the same material differently. It will be appreciated that methods, device elements, starting materials, and synthetic methods other than those specifically exemplified can be employed in the practice of the invention without resort to undue experimentation. All art-known functional equivalents, of any such methods, device elements, starting materials, and synthetic methods are intended to be included in this invention. Whenever a range is given in the specification, for example, a temperature range, a time range, or a composition range, all intermediate ranges and subranges, as well as all individual values included in the ranges given are intended to be included in the disclosure. [0094] As used herein, “comprising” is synonymous with “including,” “containing,” or “characterized by,” and is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. As used herein, “consisting of’ excludes any element, step, or ingredient not specified in the claim element. As used herein, “consisting essentially of’ does not exclude materials or steps that do not materially affect the basic and novel characteristics of the claim. Any recitation herein of the term “comprising”, particularly in a description of components of a composition, in a description of a method, or in a description of elements of a device, is understood to encompass those compositions, methods, or devices consisting essentially of and consisting of the recited components or elements, optionally in addition to other components or elements. The invention illustratively described herein suitably may be practiced in the absence of any element, elements, limitation, or limitations which is not specifically disclosed herein.

[0095] The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the concepts herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention as defined by the appended claims.

Claims

Atorney Docket No.: 093331-1363840 (7871 MAN) WHAT IS CLAIMED IS:

1. An electrochemical cell comprising: a cathode; an anode; and an electrolyte between the cathode and the anode, wherein the electrolyte is a localized saturated electrolyte and wherein the electrolyte comprises: a metal hexafluorophosphate salt; a solvent; and a diluent.

2. The electrochemical cell of claim 1, wherein the metal hexafluorophosphate salt comprises an alkali metal hexafluorophosphate salt or an alkaline earth hexafluorophosphate salt.

3. The electrochemical cell of claim 1, wherein the metal hexafluorophosphate comprises LiPFe, NaPFe, KPFe, Mg(PFe)2, Ca(PFe)2, Al(PFe)3, or Zn(PF₆)2.

4. The electrochemical cell of claim 1, wherein the metal hexafluorophosphate salt is present in the electrolyte at a saturated concentration or within 20% of a saturated concentration.

5. The electrochemical cell of claim 1, wherein the metal hexafluorophosphate salt is present in the electrolyte at concentration of from 1 moles per liter of the solvent to 8 moles per liter of the solvent or from 2.5 moles per liter of the solvent to 3.5 moles per liter of the solvent.

6. The electrochemical cell of claim 1, wherein the diluent is present in the electrolyte at a volume ratio with the solvent of from 1 : 1 to 10: 1.

7. The electrochemical cell of claim 1, wherein the metal hexafluorophosphate salt is present in the electrolyte at a concentration of from 0.5 moles per liter of the solvent and diluent combined to 4 moles per liter of the solvent and diluent combined.

8. The electrochemical cell of claim 1, wherein the diluent comprises a fluoroalkyl ether, a fluorinated carbonate, a fluorinated borate, or a fluorinated orthoformate.

26

9. The electrochemical cell of claim 8, wherein the diluent comprises one or more of l,l,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 1, 1,2, 3,3,3- hexafluoropropyl-2,2,3,3-tetrafluoropropyl ether, 2, 2,3,3, 3 -pentafluoropropyl- 1, 1,2,2- tetrafluoroethyl ether, l,l,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, hexafluoroisopropyl methyl ether, 1,1,2,2-tetrafluoroethyl ethyl ether, bis(2,2,2-trifuoroethyl) ether, 1H,1H,5H- octafluoropentyl 1,1,2,2-tetrafluoroethyl ether, bis(2,2,2-trifluoroethyl) carbonate, tris(2,2,2- trifluoroethyl) borate, fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene, 1,4- difluorobenzene, 1,3,5-trifluorobenzene, or tris(2,2,2-trifluoroethyl) orthoformate.

10. The electrochemical cell of claim 1, wherein the solvent comprises a carbonate solvent.

11. The electrochemical cell of claim 10, wherein the carbonate solvent comprises one or more of propylene carbonate, ethylene carbonate, fluoroethylene carbonate, butylene carbonate, dimethyl carbonate, ethylmethyl carbonate, or diethyl carbonate.

12. The electrochemical cell of claim 1, wherein the electrolyte further comprises one or more additives selected from the group consisting of vinylene carbonate, fluoroethylene carbonate, lithium nitrate, lithium difluoroborate, lithium bis(oxalato)borate, lithium tetrafluoroborate, lithium phosphorodifluoridate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, pyridine-boron trifluoride, and pyridine phosphorus pentafluoride.

13. The electrochemical cell of claim 1, wherein the anode comprises graphite, an alkali metal in metallic or alloy form, silicon, or a silicon-graphite composite.

14. The electrochemical cell of claim 1, wherein the cathode comprises an alkali transition metal oxide cathode material, a layered transition metal oxide cathode material, a spinel cathode material, a polyanion cathode material, or a transition metal oxide cathode material.

15. The electrochemical cell of claim 1, wherein the cathode comprises a high nickel alkali metal oxide.

16. The electrochemical cell of claim 15, wherein the cathode comprises ANii-aMaCh, wherein A is an alkali metal, wherein a is from 0 to 0.5, and wherein M is one or metals.

17. The electrochemical cell of claim 16, wherein M is one or more of Mn, Al, Mg, Co, Fe, Cr, B, Ti, Zr, Ga, Zn, V, Cu, Yb, Li, Na, K, F, Ba, Ca, Lu, Y, Nb, Mo, Ru, Rh, Ta, Pr, W, Ir, In, Tl, Sn, Sr, S, P, Cl, Ge, Sb, Er, Te, La, Ce, Nd, Dy, Eu, Sc, Se, Si, Tc, Pd, Pm, Sm, Gd, Tb, Ho, Tm.

18. The electrochemical cell of claim 16, wherein the cathode comprises ANii-_aMlaiM2a2C)2, wherein Ml and M2 are different metals, and wherein al + a2 = a.

19. The electrochemical cell of claim 16, wherein the cathode comprises LiNii-aMaCh.

20. The electrochemical cell of claim 1, wherein the electrolyte has a viscosity of from 1 cP to 10 cP at 25 °C.

21. The electrochemical cell of any of claim 1, wherein the electrolyte has an alkali metal ion conductivity of from 0.1 mS/cm to 50 mS/cm at 25 °C.

22. The electrochemical cell of claim 1, further comprising a cathode electrolyte interphase in contact with the cathode, wherein the cathode electrolyte interphase comprises an inorganic rich component.

23. The electrochemical cell of claim 22, wherein the cathode electrolyte interphase comprises one or more of AF, AxPFyOz, or ANixFyOz, wherein A is an alkali metal or alkaline earth metal, and x, y, and z are from 0 to 1.

24. The electrochemical cell of claim 1, further comprising a solid electrolyte interphase in contact with the anode, wherein the solid electrolyte interphase comprises an inorganic rich component.

25. The electrochemical cell of claim 24, wherein the solid electrolyte interphase comprises one or more of AF, AxPFyOz, or ANixFyOz, wherein A is an alkali metal or alkaline earth metal, and x, y, and z are from 0 to 1.

26. A localized saturated electrolyte for an electrochemical cell, the localized saturated electrolyte comprising: a metal hexafluorophosphate salt; a solvent, wherein the metal hexafluorophosphate salt is present in the solvent at a saturated concentration or within 20% of a saturated concentration; and a diluent.

27. The localized saturated electrolyte of claim 26, wherein the metal hexafluorophosphate salt comprises an alkali metal hexafluorophosphate salt or an alkaline earth hexafluorophosphate salt.

28. The localized saturated electrolyte of claim 26, wherein the metal hexafluorophosphate comprises LiPFe, NaPFe, KPFe, Mg(PFe)2, Ca(PFe)2, Al(PFe)3, or Zn(PF₆)2.

29. The localized saturated electrolyte of claim 26, wherein the metal hexafluorophosphate salt has a concentration of from 1 moles per liter of the solvent to 8 moles per liter of the solvent or from 2.5 moles per liter of the solvent to 3.5 moles per liter of the solvent.

30. The localized saturated electrolyte of claim 26, wherein a volume ratio of the diluent to the solvent is from 1:1 to 10:1.

31. The localized saturated electrolyte of claim 26, wherein the metal hexafluorophosphate salt has a concentration of from 0.5 moles per liter of the solvent and diluent combined to 4 moles per liter of the solvent and diluent combined.

32. The localized saturated electrolyte of claim 26, wherein the diluent comprises a fluoroalkyl ether, a fluorinated carbonate, a fluorinated borate, or a fluorinated orthoformate.

33. The localized saturated electrolyte of claim 32, wherein the diluent comprises one or more of 1,1,2, 2-tetrafluoroethyl-2, 2,3, 3 -tetrafluoropropyl ether, 1, 1,2, 3,3,3- hexafluoropropyl-2,2,3,3-tetrafluoropropyl ether, 2, 2,3,3, 3 -pentafluoropropyl- 1, 1,2,2- tetrafluoroethyl ether, l,l,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, hexafluoroisopropyl methyl ether, 1,1,2,2-tetrafluoroethyl ethyl ether, bis(2,2,2-trifuoroethyl) ether, 1H,1H,5H- octafluoropentyl 1,1,2,2-tetrafluoroethyl ether, bis(2,2,2-trifluoroethyl) carbonate, tris(2,2,2- trifluoroethyl) borate, fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene, 1,4- difluorobenzene, 1,3,5-trifluorobenzene, or tris(2,2,2-trifluoroethyl) orthoformate.

34. The localized saturated electrolyte of claim 26, wherein the solvent comprises a carbonate solvent.

35. The localized saturated electrolyte of claim 34, wherein the carbonate solvent comprises one or more of propylene carbonate, ethylene carbonate, fluoroethylene

29 carbonate, butylene carbonate, dimethyl carbonate, ethylmethyl carbonate, or diethyl carbonate.

36. The localized saturated electrolyte of claim 26, wherein the electrolyte further comprises one or more additives selected from the group consisting of vinylene carbonate, fluoroethylene carbonate, lithium nitrate, lithium difluoroborate, lithium bis(oxalato)borate, lithium tetrafluoroborate, lithium phosphorodifluoridate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, pyridine-boron trifluoride, and pyridine phosphorus pentafluoride.

37. The localized saturated electrolyte of claim 26, having a viscosity of from 1 cP to 10 cP at 25 °C.

38. The localized saturated electrolyte of claim 26, having an alkali metal ion or alkaline earth metal ion conductivity of from 0.1 mS/cm to 50 mS/cm at 25 °C.

39. A method of making an electrochemical cell, the method comprising: providing a cathode; providing an anode; positioning an electrolyte between the cathode and the anode, wherein the electrolyte is a localized saturated electrolyte and wherein the electrolyte comprises: a metal hexafluorophosphate salt; a solvent; and a diluent.

40. The method of claim 39, wherein the electrochemical cell is the electrochemical cell of any of claims 1-25.

41. The method of claim 39, wherein the localized saturated electrolyte is the localized saturated electrolyte of any of claims 26-38.

30