Anode-free sodium battery cells

Abstract

Th present disclosure relates to the field of energy storage devices such as lithium-ion batteries, sodium-ion batteries, and sodium anode-free batteries. More specifically, the present disclosure provides room temperature sodium-based batteries with an anode-free configuration, where no anode active material is present upon cell assembly and in its discharged state.

Description

Attorney Ref: 41873-64438ANODE-FREE SODIUM BATTERY CELLSFIELD

[0001] This disclosure relates to the field of energy storage devices such as lithium-ion batteries, sodium-ion batteries, and sodium anode-free batteries. More specifically, the disclosure relates to the field of room temperature sodium-based batteries with an anode-free configuration, where no anode active material is present upon cell assembly and in its discharged state.BACKGROUND

[0002] Lithium-ion batteries are a ubiquitous, advanced technology for powering a large range of electronic devices, electric vehicles, and even for certain grid-storage applications. However, for many applications, including low-cost electric vehicles and widespread renewable grid storage, state-of-the-art Li-ion batteries remain too expensive to provide an economical solution. Sodium-based batteries provide an alternative that promises to further reduce costs by incorporating lower-cost materials in the cell. Despite this, today's best sodium batteries suffer from reduced energy density versus Li-ion counterparts, and are therefore larger and heavier than a Li-ion alternative. This is due to the larger size and higher mass of sodium vs. lithium, and the use of hard carbon as an anode in sodium batteries, which has a lower specific capacity than graphite anodes used in lithium batteries. This extra size and volume increases costs, preventing sodium batteries from reaching truly transformative reductions in cell-level cost, normalized in $ / kWh. The most cost-, energy-, volume-, and weight-efficient configuration for sodium batteries is an anode-free architecture, where no anode active material exists in the battery upon assembly in its discharged state. This is uniquely different from cell designs with hard carbon anodes used for state-of-the-art sodium-ion batteries. Instead, only the anodic current collector exists at cell assembly, which may be coated with a thin layer of a substrate material; however, there is minimal or no active material at the anode. Consequently, during cell charging sodium metal is plated onto the anodic current collector, with no excess capacity of sodium available in the system. Sodium metal is the highest energy density anode for sodium batteries. Thus, anode-freeAttorney Ref: 41873-64438 cells can achieve higher energy density than conventional cells with hard carbon anodes, while also reducing cell-level costs by removing the costs associated with the anode active material and anode manufacturing. Nonetheless, despite their promise, anode-free cells remain impractical due to low first-cycle Coulombic efficiency (FCE) that reduces the cell capacity, and poor cycle life. Anode-free sodium batteries typically reach end-of-life (<80% of initial capacity) in <100 cycles.

[0003] Anode-free batteries that include an anode substrate, which provides a suitable platform for reversible sodium stripping and plating, and an electrolyte, which is highly reductively stable and generates a thin and flexible solid electrolyte interface to accommodate the volume change of the sodium metal at the anode, are of great interest.SUMMARY

[0004] The present disclosure provides battery cell components and cell designs for improved anode-free sodium battery cells and batteries with higher FCE and cycle life, that include improved electrolytes and anode-free substrate designs. In some embodiments, a sodium battery design is described. The sodium battery can contain at least one positive electrode (i.e., cathode), at least one negative electrode (e.g., anode, anode-free substrate), and a liquid electrolyte comprising a sodium salt and a solvent. In some embodiments, the negative electrode (e.g., anode-free substrate) has no deposited active material prior to battery charging. In some embodiments, the N / P ratio (i.e., capacity ratio) of non-sodium anode active material to cathode active material is less than 1, and the battery cell is configured such that sodium metal may be deposited onto the anode during battery charging.

[0005] In some embodiments, the battery of this disclosure is therefore an anode-free sodium battery, as minimal or no anode active material is required during cell assembly in the discharged state. During charging, sodium metal can be plated onto the anode, such that in the charged state the battery contains sodium metal as the anode active material. The minimally-coated anode may be referred to as the anode-free substrate, which is intended to provide a substrate or scaffold for the reversible deposition and stripping of sodium metal. In exemplary, though nonAttorney Ref: 41873-64438 limiting, cases the negative electrode (e.g., anode-free substrate) therefore comprises sodium metal on at least part of its surface when the energy storage device is charged or fully charged. The thickness of this sodium metal will be dependent upon the capacity of the cathode material. In some embodiments, the average thickness of the sodium metal may be between 8 and 88 pm on the negative electrode surface when the battery cell is fully charged. A thickness in the range of 8 to 88 pm may indicate efficiently deposited sodium metal in an anode-free cell from a cathode having an areal capacity between 1 and 10 mAh / cm2.

[0006] In some embodiments, an electrolyte for an anode-free sodium battery is described. In some embodiments, the electrolyte composition can be a liquid electrolyte that includes at least one sodium borate salt and at least one solvent.

[0007] In some embodiments, the at least one sodium borate salt has the formula:NaB(R)4wherein: each R is independently selected from alkoxy, substituted alkoxy, carboxyl group, phenoxy, substituted phenoxy, silyl group, siloxy group, sulfonyl group, sulfate group, alkyl, substituted alkyl, a nitrile group, an amide group, an optionally substituted heterocyclyl (e.g., a 5-membered N-containing azole hetereocyclyl, such as a pyrrolyl, or substituted pyrrolyl), and a cyanate group.

[0008] In some embodiments, the at least one sodium borate salt has the formula NaB(R)4wherein each R is selected from an alkoxy group, a fluoro-alkoxy group, a cyano-alkoxy group, a functionalized alkoxy group, a carboxyl group, a fluoro-carboxyl group, a functionalized carboxyl group, a phenoxy group, a fluoro-phenoxy group, a cyano-phenoxy group, a functionalized phenoxy group, a silyl group, a siloxy group, a sulfonyl group, a sulfate group, an alkyl group, a fluoro-alkyl group, a functionalized alkyl group, a nitrile group, an amide group, a pyrrole group, a functionalized pyrrole group, an azole group, a functionalized azole group, and a cyanate group.

[0009] In some embodiments, the at least one sodium borate salt has the formula NaBFa(R) wherein each R is selected from an alkoxy group, a fluoro-alkoxy group, a cyano-alkoxy group, a functionalized alkoxy group, a carboxyl group, a fluoro-carboxyl group, a functionalized carboxyl group, a phenoxy group, a fluoro-phenoxy group, a cyano-phenoxy group, a functionalizedAttorney Ref: 41873-64438 phenoxy group, a silyl group, a siloxy group, a sulfonyl group, a sulfate group, an alkyl group, a fluoro-alkyl group, a functionalized alkyl group, a nitrile group, an amide group, a pyrrole group, a functionalized pyrrole group, an azole group, a functionalized azole group, and a cyanate group.

[0010] In some embodiments, the at least one sodium borate salt has the formula NaBfOCHshfR) wherein R is selected from an alkoxy group, a fluoro-alkoxy group, a cyano-alkoxy group, a functionalized alkoxy group, a carboxyl group, a fluoro-carboxyl group, a functionalized carboxyl group, a phenoxy group, a fluoro-phenoxy group, a cyano-phenoxy group, a functionalized phenoxy group, a silyl group, a siloxy group, a sulfonyl group, a sulfate group, an alkyl group, a fluoro-alkyl group, a functionalized alkyl group, a nitrile group, an amide group, a pyrrole group, a functionalized pyrrole group, an azole group, a functionalized azole group, and a cyanate group.

[0011] In some embodiments, the at least one sodium borate salt has the formula Na B(OSi[CH3]3)3(R) wherein R is selected from an alkoxy group, a fluoro-alkoxy group, a cyanoalkoxy group, a functionalized alkoxy group, a carboxyl group, a fluoro-carboxyl group, a functionalized carboxyl group, a phenoxy group, a fluoro-phenoxy group, a cyano-phenoxy group, a functionalized phenoxy group, a silyl group, a siloxy group, a sulfonyl group, a sulfate group, an alkyl group, a fluoro-alkyl group, a functionalized alkyl group, a nitrile group, an amide group, a pyrrole group, a functionalized pyrrole group, an azole group, a functionalized azole group, and a cyanate group.

[0012] In some embodiments, the at least one sodium borate salt is selected from sodium tetra(trifluoro acetoxyjborate, sodium tetra(l,l,l-trifluoro isopropoxy)borate, sodium tetra(2- trifluoromethyl isopropoxyjborate, sodium tetra(trifluoro ethoxy)borate, sodium tetra(difluoro ethoxy)borate, sodium bisfperfluoro pinacolatojborate, sodium tetra(pentafluoro phenoxyjborate, sodium tetrafperfluoro tertbutoxyjborate, sodium tetra(hexafluoro isopropoxyjborate, sodium tetra(3-methylphenoxy)borate, sodium tetra(4- methylphenoxy)borate, sodium tetra(2-fluoro-3-methylphenoxy)borate, sodium tetra(4-fluoro- 3-methylphenoxy)borate, sodium tetra(2-fluoro-5-methylphenoxy)borate, sodium tetra(4-cyano phenoxyjborate, sodium tetra(3-cyano phenoxyjborate, sodium tetra(3,4-dicyano phenoxyjborate, sodium tetra(3,5-dicyano phen oxy) bo rate, sodium tetra(3-cyano-5-fluoroAttorney Ref: 41873-64438 phenoxyjborate, sodium tetra(l-phenyl-2,2,2-trifluoro ethoxyjborate, sodium tetra(l-cyano- 2,2,2-trifluoro ethoxyjborate, sodium tetra(l,l,l-trifluoro-2-cyano isopropoxyjborate, sodium tetra(2-cyano isopropoxyjborate, sodium tetra(l-cyano isopropoxy)borate, sodium tetra(2- cyano ethoxyjborate, sodium tetra(l-cyano-l-methoxy methoxyjborate, sodium tetra(l-cyano- 1-ethoxy methoxyjborate, sodium tetra(l-cyano-l-phenyl methoxy)borate, sodium tetra(2- cyano-l-methoxy ethoxy)borate, sodium tetra(2-cyano-l-ethoxy ethoxy) borate, sodium tetra(methysulfonyl)borate, sodium tetra(trifluoromethysulfonyl)borate, sodium tetra(trimethylsilyl)borate, sodium tetra(methyl)borate, sodium (trifluoro methanesulfonyljborate, sodium (trifluoro hexafluoroisopropoxyjborate, sodium (trifluoro trifluoroethoxy)borate, sodium (trifluoro pentafluoropropoxy)borate, sodium (trifluoro trifluoromethyl)borate, sodium tetra(trifluoromethyl)borate, sodium (difluoro bis- trifluoromethyl)borate, sodium (trifluoro dicyanamide)borate, sodium (trifluoro phenoxy)borate, sodium (trifluoro pyrrole)borate, sodium (trifluoro pyrazole)borate, sodium (trifluoro imidazolejborate, sodium (trifluoro triazolejborate, sodium (trifluoro trimethylsilyl)borate, sodium (trifluoro trimethylsiloxy)borate, sodium (trifluoro bis[trimethylsilyl]amide)borate, sodium (trifluoro 2,3-dicyanoimidazole)borate, sodium (trifluoro cyanatejborate, sodium (trifluoro thiocyanatejborate, sodium (trimethyl cyanate)borate, sodium (trimethyl thiocyanatejborate, sodium (trimethyl methanesulfonyljborate, sodium (trimethyl hexafluoroisopropoxy)borate, sodium (trimethyl trifluoroethoxy)borate, sodium (trimethyl phenoxy)borate, sodium (trimethyl pyrrolejborate, sodium (trimethyl pyrazole)borate, sodium (trimethyl imidazole)borate, sodium (trimethyl triazole)borate, sodium (trimethyl 2,3-dicyanoimidazole)borate, sodium (trimethyl trimethylsilyl)borate, sodium (trimethyl trimethylsiloxyjborate, sodium (trimethyl bis[trimethylsilyl]amide)borate.

[0013] In some embodiments, the at least one sodium borate salt comprises 3% to 70% of the total electrolyte mass, such as 5% to 50% by weight, or 10% to 40% by weight.

[0014] In some embodiments, the at least one solvent is selected from 1,2-dimethoxyethane (monoglyme), diethylene glycol dimethyl ether (diglyme), triethylene glycol dimethyl ether (triglyme), tetraethylene glycol dimethyl ether (tetraglyme), dimethyl carbonate, diethylAttorney Ref: 41873-64438 carbonate, ethyl methyl carbonate, ethylene carbonate, propylene carbonate, triethyl phosphate, trimethyl phosphate, triethyl phosphite, trimethyl phosphite, fluoroethylene carbonate, ethyl butyrate, gamma-butyrolactone, gamma-valerolactone, triacetin, tetrahydrofuran, 1,3-dioxolane, 1,3-dioxane, 1,4-dioxane, acetic anhydride, N,N- dimethylformamide, dimethyl maleate, diethyl maleate, diallyl maleate, dimethyl malonate, diethyl malonate, dimethyl oxalate, diethyl oxalate, anisole, ethyl phenyl ether, methoxy perfluorobutane, methoxy perfluoropropane, ethoxy perfluorobutane, ethoxy perfluoropropane, diethyl ether, dipropyl ether, dibutyl ether, ethylene glycol diethyl ether, ethylene glycol dipropyl ether, ethylene glycol dibutyl ether, diethoxymethane (ethylal), 1,1- diethoxyethane, diethylene glycol divinyl ether, triethyl orthoformate, 1,1,1-triethoxyethane, 1,1,1-triethoxypropane, sulfolane, 3-methyl sulfolane, sulfolene, 1,3-propane sultone, 1,4- butane sultone, 1,3,2-dioxathiolane 2,2-dioxide, 1,3,2-dioxathiane 2,2-dioxide, tetramethylene sulfoxide, 2,2-dimethoxypropane, butyric anhydride, isobutyric anhydride, 1, 1,2,2- tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, perfluoro-15-crown-5-ether, 1, 1,3, 3,3- pentafluoro-2-trifluoromethylpropyl methyl ether, 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether, methyl(2,2,2-trifluoroethyl)carbonate, 2,2,3,3,3-pentafluoropropyl-l,l,2,2-tetrafluoro- ethyl ether, fluorinated triethylene glycol monobutyl ether, (trifluoromethoxy)benzene, 4- toluene trifluoromethyl ether, bis-(4-fluorophenyl) ether, bis-(4-trifluoromethylphenyl) ether, 1,1,2,3,3,3-hexafluoropropyl methyl ether, 1,1,2,3,3,3-hexafluoropropyl ethyl ether, 1, 1,2, 3,3,3- hexafluoropropyl propyl ether, 1,1,2,3,3,3-hexafluoropropyl butyl ether, hexafluoroisopropyl methyl ether, hexafluoroisopropyl ethyl ether, hexafluoroisopropyl propyl ether, hexafluoroisopropyl isopropyl ether, hexafluoroisopropyl butyl ether, trifluoroethyl methyl ether, trifluoroethyl ethyl ether, trifluoroethyl propyl ether, trifluoroethyl butyl ether, 2,2,2- trifluoroethyl acetate, ethyl trifluoroacetate, propyl trifluoroacetate, isopropyl trifluoroacetate, butyl trifluoroacetate, 2,2,2-trifluoroethyl difluoromethyl ether, methyl cyanoformate, ethyl cyanoformate, methoxyacetonitrile, ethoxyacetonitrile, 2-phenoxyacetonitrile, methylsulfonylacetonitrile, tris(2,2,2-trifluoroethyl) phosphite, tris(2,2,2-trifluoroethyl) phosphate, dimethyl sulfate, diethyl sulfate, dimethyl sulfone, diethyl sulfone, divinyl sulfone, dipropyl sulfone, dibutyl sulfone, or any combination thereof.Attorney Ref: 41873-64438

[0015] In some embodiments, the at least one solvent comprises 30% to 95% of the total electrolyte mass.

[0016] In some embodiments, an electrolyte for an anode-free sodium battery is described. The electrolyte is comprised of a sodium borate salt selected from sodium tetra(hexafluoro isopropoxyjborate and sodium tetra(trifluoro ethoxy)borate, and a solvent selected from dimethoxyethane (monoglyme), diethylene glycol dimethyl ether (diglyme), triethylene glycol dimethyl ether (triglyme), and tetraethylene glycol dimethyl ether (tetraglyme).

[0017] In some embodiments, an electrolyte for an anode-free sodium battery is described. The electrolyte composition may further comprise at least one supplemental salt.

[0018] In some embodiments, an electrolyte for an anode-free sodium battery is described. The electrolyte composition may further comprise at least one additive to improve performance.

[0019] In some embodiments, a sodium battery design is described. The sodium battery contains at least one sodium cathode, which comprises a cathode active material coated onto a current collector. Any sodium-containing cathode active material may be used.

[0020] In some embodiments, the sodium-containing cathode active material is a lower voltage cathode material. In some embodiments, the sodium-containing cathode active material is sodium ferric phosphate pyrophosphate (NFPP, Na FegfPO^zfPzO?)), NasVzPOzi, and sodium chromium oxide (NaCrOz) are preferable due to their stability. These cathodes may be further doped with supporting materials to improve performance, such as Ca, Mg, K, Ti, and Al. Sodium nickel-iron-manganese oxide (NFM, NaNixFeyMnzO2, where x+y+z = 1), sodium transition metal oxides including NaxTMOz (TM = a transition metal or combination thereof), NaaNii-x y- zMnxMgyTizOz, and NaxMO2 (M = Fe, Mn, Ni, Co, Cr, Ti, V, and their combinations), sodium copper- iron-manganese oxide (NaCuxFeyMnzO2, where x+y+z = 1), Prussian White (Na2FeMn(CN)e), or Prussian Blue (NazFezfCNje) cathodes may also be used.

[0021] In some embodiments, the cathode may have an areal capacity between 1.0 and 4.0 mAh / cm2, where a higher areal capacity may improve cell energy density.Attorney Ref: 41873-64438

[0022] In some embodiments, a sodium battery design is described. The battery may be operated under a pressure between 10 kPa and 2500 kPa.

[0023] In some embodiments, an anode substrate for an anode-free sodium battery is described. The anode substrate may include an aluminum foil current collector.

[0024] In some embodiments, the aluminum foil current collector may have a thickness that ranges from 1 to 50 pm, where a thinner foil reduces inactive material mass and volume, while a thicker foil is easier to handle during manufacturing.

[0025] In some embodiments, the aluminum foil current collector may have a surface roughness (Ra) ranging from 0.03 to 15 pm. The roughness of the substrate is an important parameter for determining the morphology of the deposited sodium metal during charging, which affects cell performance.

[0026] In some embodiments, the aluminum foil current collector may contain a thin surface film of aluminum oxide (AI2O3) having a thickness of 1 to 15 nm, to enable cell assembly under ambient conditions.

[0027] In some embodiments, the aluminum foil current collector may have an electrical conductivity between 1 x 107and 1 x 108S / m.

[0028] Material properties of the aluminum foil may be characterized using calipers, microscopy, spectroscopy, surface topography, profilometry, or other methods.

[0029] In some embodiments, the aluminum foil current collector may be further coated on one or two sides with a conductive coating. This coating may improve the deposition morphology of the sodium metal that is plated onto the substrate during battery charging.

[0030] In some embodiments, the conductive coating may have a thickness between 0.1 to 100 pm, where a thicker substrate may provide a scaffold for supporting z-axis expansion associated with sodium plating (i.e., deposition).Attorney Ref: 41873-64438

[0031] In some embodiments, the conductive coating may have a porosity between 1% and 99%, where a porous coating may support internal deposition of sodium metal that may improve morphology and performance.

[0032] In some embodiments, the conductive coating may be comprised of a material selected from amorphous carbon, graphitic carbon, single walled carbon nanotubes, multiwalled carbon nanotubes, carbonized polymer including polyacrylonitrile, glassy carbon, graphene, fullerene, or any combination thereof.

[0033] In some embodiments, the conductive coating may be further comprised of platinum, titanium, copper, lithium, zinc, silver, or any combination thereof.

[0034] Material properties of the conductive coating may be characterized using calipers, microscopy, spectroscopy, surface topography, profilometry, or other methods.

[0035] In some embodiments, an anode substrate for an anode-free sodium battery is described. The anode substrate may be freestanding (i.e., without a current collector), comprised of a conductive carbon material.

[0036] In some embodiments, the freestanding anode substrate may have a thickness between 1 and 50 pm.

[0037] In some embodiments, the freestanding anode substrate may have a porosity between 1% and 99%, where a porous coating may support internal deposition of sodium metal that may improve morphology and performance.

[0038] In some embodiments, the freestanding anode substrate may be comprised of a material selected from amorphous carbon, graphitic carbon, single walled carbon nanotubes, multiwalled carbon nanotubes, carbonized polymer including polyacrylonitrile, glassy carbon, graphene, fullerene, or any combination thereof.

[0039] In some embodiments, the freestanding anode substrate may be further comprised of a binder material.Attorney Ref: 41873-64438

[0040] Material properties of the freestanding conductive substrate may be characterized using calipers, microscopy, spectroscopy, surface topography, profilometry, or other methods.

[0041] The anode-free sodium battery cell design detailed herein overcomes the challenges set forth in the Background by improving first-cycle Coulombic efficiency, discharge capacity, and cycle life. Exemplary embodiments of the invention are described in the Detailed Description and Drawings. The technology described herein offers a range of compelling advantages, including, but not necessarily limited to:

[0042] Manufacturing cost reduction: The removal of the anode active material reduces material costs and manufacturing steps, decreasing cell manufacturing costs.

[0043] Increased energy density: The removal of the conventional hard carbon anode active material improves both gravimetric and volumetric energy density of the battery cell.

[0044] Increased cycle life: The materials and cell designs described may improve cycle life beyond the current state-of-the-art.

[0045] These and other aspects of the technology described herein will be apparent after consideration of the Detailed Description and Drawings herein.DESCRIPTION OF THE DRAWINGS

[0046] FIG. 1, panels A-B, show diagrams of two single-layer anode-free sodium battery cells, with Panel A) an anode comprising aluminum foil with a conductive film coated onto one side, and Panel B) an anode comprising a freestanding conductive carbon film. Key components are labeled: 100 is an aluminum foil current collector, 110 is an optional conductive coating on the aluminum foil, 120 is the cathode active material, 130 is a liquid electrolyte coupling the anode to the cathode that allows for the transfer of sodium ions, and 140 is a freestanding conductive carbon film. The diagrams are not drawn to scale, to improve readability.

[0047] FIG. 2, panels A-B, shows diagrams of two multi-layer anode-free sodium battery cells, with Panel A) an anode comprising aluminum foil with a conductive film coated onto two sides, and Panel B) an anode comprising a freestanding conductive carbon film. Both diagrams show aAttorney Ref: 41873-64438 smaller zoomed-out perspective of a six-layer anode-free sodium battery cell, with the larger diagrams being zoomed-in to show key components for two of the layers. Key components are labeled: 100 is an aluminum foil current collector, 110 is an optional conductive coating on the aluminum foil, 120 is the cathode active material, 130 is a liquid electrolyte coupling the anode to the cathode that allows for the transfer of sodium ions, and 140 is a freestanding conductive carbon film. The diagrams are not drawn to scale, to improve readability.

[0048] FIG. 3 shows the first-cycle charge and discharge curves for 1) a conventional sodium-ion battery comprising a hard carbon anode and transition metal cathode (red), and 2) an anode-free sodium battery comprising an 18 pm aluminum foil anode with a 1 pm film of conductive graphitic carbon and the same transition metal cathode (blue). The cathode is sodium chromium oxide (NaCrO2). Both batteries contain an electrolyte comprising a sodium borate salt. The anode- free sodium cell provides a superior first-cycle Coulombic efficiency (94.3% vs. 79.7%), a higher specific discharge capacity (113.6 vs. 95.9 mAh / g), a higher average discharge voltage (2.96V vs. 2.86V), and a higher energy efficiency (91.8% vs. 77.2%).

[0049] FIG. 4 shows the charge and discharge voltage profiles as a function of the areal capacity for the same cells shown in FIG. 3. The anode-free cell achieves a commercially-relevant area- normalized capacity of about 2.5 mAh / cm2. This indicates that in the charged state, approximately 2.5 mAh / cm2of sodium metal has been deposited onto the carbon-coated aluminum anode substrate, nearly 95% of which can be reversibly transported back to the cathode during discharging.

[0050] FIG. 5 shows rate testing of an anode-free battery comprising a sodium chromium oxide cathode, sodium borate electrolyte, and an anode substrate comprising an 18 pm aluminum foil with a 1 pm film of conductive graphitic carbon. Charge and discharge curves are shown at C / 10, C / 3 and 1C, demonstrating the excellent rate performance of the anode-free battery design.

[0051] FIG. 6 shows images of several anode materials for anode-free sodium batteries. In order, these images show anodes with: 1) uncoated aluminum foil with a thickness of 20pm, a surface roughness of 0.3 pm, and an electrical conductivity (denoted E.C.) of 3.5xio7S / m, 2) uncoated aluminum foil with a thickness of 20pm, a surface roughness of 0.7 pm, and an electrical conductivity (denoted E.C.) of 3.5xio7S / m, 3) aluminum foil with a thickness of 18pm, coatedAttorney Ref: 41873-64438 with a lpm graphitic carbon surface film, 4) aluminum foil with a thickness of 10pm, coated with a 90pm carbon fiber surface film, and 5) a freestanding conductive carbon film with a thickness of 25pm, comprised of carbon nanotubes. The insets of the third and fifth images show zoomed- out images of the materials, which are folded to demonstrate that the third anode includes an aluminum foil current collector, and the fifth image is freestanding (i.e., without a current collector).

[0052] FIG. 7 shows the first-cycle charge and discharge curves at a rate of C / 10 for anode-free sodium batteries comprising a sodium chromium oxide cathode, a sodium borate electrolyte, and an anode substrate. Four anode substrates are shown, including an uncoated aluminum foil anode with a thickness of 20pm and a surface roughness of 0.3pm (pink), an uncoated aluminum foil anode with a thickness of 20pm and a surface roughness of 0.7pm (purple), an aluminum foil anode with a thickness of 18pm, coated with a 1pm graphitic carbon surface film (blue), and a freestanding conductive carbon film with a thickness of 25pm, comprised of carbon nanotubes (cyan). A) shows the full voltage profiles over the range of 2.0 to 3.5V, and B) shows a zoomed-in version of the curves, highlighting the first 10 mAh / g of capacity, where sodium metal first begins plating on the substrate. The different character of the four curves in this initial region highlights the importance of the anode substrate properties, which can ultimately affect cell properties such as first-cycle Coulombic efficiency and discharge capacity. Ra is the surface roughness. The Al foil with higher Ra shows more noise in the voltage curve during the early sodium plating.

[0053] FIG. 8 shows the charge and discharge curves of an anode-free battery comprising a sodium chromium oxide cathode, a sodium borate electrolyte, and an anode substrate comprising an 18 pm aluminum foil with a 1 pm film of conductive graphitic carbon. Three different internal pressures are shown in coin cell format, 10 kPa, 40 kPa, and 80 kPa. The low pressure cell does not cycle; however, the highest pressure cell performs worse than the intermediate pressure cell, indicating the importance of cell pressure on performance.

[0054] FIG. 9 shows the capacity retention of two anode-free sodium batteries comprising a sodium chromium oxide cathode, a sodium borate electrolyte, and an anode substrate comprising an 18 pm aluminum foil with a 1 pm film of conductive graphitic carbon. The cells areAttorney Ref: 41873-64438 cycled at a higher and lower applied pressure, further demonstrating the importance of applied pressure on cell performance.

[0055] FIG. 10 shows the charge and discharge curves of an anode-free battery comprising a sodium chromium oxide cathode, a sodium borate electrolyte, and an anode substrate comprising an 18 pm aluminum foil with a 1 pm film of conductive graphitic carbon. The curves indicate that the overpotential does not change throughout cycling, and that the anode-free cell design provides excellent capacity retention.

[0056] FIG. 11 shows the discharge capacity, normalized to the cathode surface area, of an anode-free sodium battery comprising a sodium chromium oxide cathode, a sodium borate electrolyte, and an anode substrate comprising an 18 pm aluminum foil with a 1 pm film of conductive graphitic carbon. The battery provides a high areal capacity above 2.0 mAh / cm2, with excellent capacity retention, maintaining 93% of the C / 3 capacity at cycle 80. The first cycle is at C / 10. The rate of capacity decay is affected by the C-rate.

[0057] FIG. 12 shows the Coulombic efficiency (CE) of an anode-free sodium battery comprising a sodium chromium oxide cathode, a sodium borate electrolyte, and an anode substrate comprising an 18 pm aluminum foil with a 1 pm film of conductive graphitic carbon. The first cycle is at C / 10. The battery provides a high first-cycle CE of 94.3% and stabilizes at a high CE that reaches 99.98% during C / 3 cycling. The 1C cycling achieves lower CE than the C / 3 cycles.

[0058] FIG. 13 shows the first-cycle voltage curves and Coulombic efficiency (CE) of an anode- free sodium battery pouch cell comprising a sodium chromium oxide cathode, a sodium borate electrolyte, and an anode substrate comprising an 18 pm aluminum foil with a 1 pm film of conductive graphitic carbon. The first cycle is at C / 10. The battery provides a high first-cycle CE of 96.5%. The battery is a pouch cell having a discharge capacity of 44 mAh.

[0059] FIG. 14 shows the specific discharge capacity of two anode-free sodium battery pouch cells comprising a sodium chromium oxide cathode, and an anode substrate comprising an 18 pm aluminum foil with a 1 pm film of conductive graphitic carbon. The electrolytes are IM NaPFe in diglyme, and IM sodium tetra(hexafluoro isopropoxy)borate (i.e., NaBhfip) in diglyme. The NaBhfip-based cell provides higher initial specific discharge capacity and superior capacity retention vs. the NaPFe cell.Attorney Ref: 41873-64438

[0060] FIG. 15 shows the total cell discharge capacity of two anode-free sodium battery pouch cells comprising a sodium chromium oxide cathode, and an anode substrate comprising an 18 pm aluminum foil with a 1 pm film of conductive graphitic carbon. The electrolytes are IM NaPFe in diglyme, and IM sodium tetra(hexafluoro isopropoxyjborate (i.e., NaBhfip) in diglyme. The NaBhfip-based cell provides higher initial specific discharge capacity and superior capacity retention vs. the NaPFe cell. An image of the pouch cells is shown in the inset.DETAILED DESCRIPTION

[0061] Embodiments are described more fully below with reference to the accompanying Figures, which form a part hereof and show, by way of illustration, specific exemplary embodiments. However, embodiments may be implemented in many different forms and should not be construed as being limited to the embodiments set forth herein. The following detailed description is, therefore, not to be taken in a limiting sense.

[0062] Described herein are electrolyte compositions having suitability for use in anode-free sodium batteries. As used herein, the term anode-free sodium battery is intended to describe a battery incorporating sodium as the active material in the electrodes, and further having minimal or no active material (e.g., hard carbon) at the anode. In exemplary anode-free batteries, most or all of the sodium is deposited onto the anode as sodium metal during battery charging. As used herein, the term fully charged (e.g., 100% state of charge) refers to a battery that has been charged to the upper limit of its operating voltage window, which may optionally be defined in the battery specification sheet or may optionally be the standard stopping point for charging a device comprising a battery. This upper voltage limit may be in the range of 2.5V to 4.8V. As used herein, the term fully discharged (e.g., 0% state of charge) refers to a battery that has been substantially discharged to the lower limit of its operating voltage window, which may optionally be defined in the battery specification sheet or may optionally be the point where the battery can no longer provide operating power to a device. This lower voltage limit may be in the range of O.OV to 3.0V.Attorney Ref: 41873-64438

[0063] Described herein are anode-free substrates having suitability for use in anode-free sodium batteries. As used herein, the term anode-free substrate is intended to describe the anode of a sodium battery where sodium metal may be deposited during cell charging. In exemplary cases, no, or substantially no, sodium metal or active material such as hard carbon exists at the anode-free substrate during cell assembly and while the cell remains in its fully discharged state.

[0064] In some embodiments, the electrolyte composition generally includes at least one sodium borate salt. The sodium borate salt may have the formula NaB(R)4, or NaB(M)3(R), wherein R in any of the preceding formula is selected from an alkoxy group, a fluoro-alkoxy group, a cyanoalkoxy group, a functionalized alkoxy group, a carboxyl group, a fluoro-carboxyl group, a functionalized carboxyl group, a phenoxy group, a fluoro-phenoxy group, a cyano-phenoxy group, a functionalized phenoxy group, a silyl group, a siloxy group, a sulfonyl group, a sulfate group, an alkyl group, a fluoro-alkyl group, a functionalized alkyl group, a nitrile group, an amide group, a pyrrole group, a functionalized pyrrole group, an azole group, a functionalized azole group, and a cyanate group, and M is selected from fluorine, a methoxy group, an alkoxy group, a phenoxy group, and a trimethylsiloxy group. While the composition may, in some embodiments, include one sodium borate salt and one solvent, other embodiments of the electrolyte composition may include multiple different types of sodium borate salts and / or solvents. For example, the electrolyte composition described herein may include two different sodium borate salts and one solvent, one sodium borate salt and two different solvents, two different sodium borate salts and two different solvents, etc.

[0065] With respect to the formula NaB(R)4, exemplary, though non-limiting, sodium borate salts having the formula NaB(R)4 that are suitable for use in the electrolyte compositions described herein include sodium tetra(hexafluoro isopropoxyjborate, sodium tetra(phenoxy)borate, sodium tetra(l,l,l-trifluoro isopropoxyjborate, sodium tetra(2-trifluoromethyl isopropoxy)borate, sodium tetra(trifluoro ethoxy) borate, sodium tetra(difluoro ethoxy)borate, sodium tetra(pentafluoro phenoxyjborate, sodium tetra(perfluoro tertbutoxy)borate, sodium tetra(trifluoro acetoxy)borate, sodium tetra(3-methylphenoxy)borate, sodium tetra(4- methylphenoxy)borate, sodium tetra(2-fluoro-3-methylphenoxy)borate, sodium tetra(4-fluoro-Attorney Ref: 41873-644383-methylphenoxy)borate, sodium tetra(2-fluoro-5-methylphenoxy)borate, sodium tetra(4-cyano phenoxyjborate, sodium tetra(3-cyano phenoxyjborate, sodium tetra(3,4-dicyano phenoxyjborate, sodium tetra(3,5-dicyano phen oxy) bo rate, sodium tetra(3-cyano-5-fluoro phenoxyjborate, sodium tetra(l-phenyl-2,2,2-trifluoro ethoxyjborate, sodium tetra(l-cyano- 2,2,2-trifluoro ethoxyjborate, sodium tetra(l,l,l-trifluoro-2-cyano isopropoxyjborate, sodium tetra(2-cyano isopropoxyjborate, sodium tetra(l-cyano isopropoxy)borate, sodium tetra(2- cyano ethoxyjborate, sodium tetra(l-cyano-l-methoxy methoxyjborate, sodium tetra(l-cyano- 1-ethoxy methoxyjborate, sodium tetra(l-cyano-l-phenyl methoxyjborate, sodium tetra(2- cyano-l-methoxy ethoxy)borate, sodium tetra(2-cyano-l-ethoxy ethoxy) borate, sodium tetra(methysulfonyl)borate, sodium tetra(trifluoromethysulfonyl)borate, sodium tetra(trimethylsilyl)borate, and sodium tetra(methyl)borate.

[0066] With respect to the formula NaB(M)3(R), exemplary, though non-limiting, sodium borate salts having the formula NaB(M)3(R) that are suitable for use in the electrolyte compositions described herein include sodium (trifluoro methanesulfonyljborate, sodium (trifluoro hexafl uoroisopropoxy)borate, sodium (trifluoro trifluoroethoxyjborate, sodium (trifluoro pentafluoropropoxy)borate, sodium (trifluoro trifluoromethyljborate, sodium tetra(trifluoromethyl)borate, sodium (difluoro bis-trifluoromethyl)borate, sodium (trifluoro dicyanamidejborate, sodium (trifluoro phenoxyjborate, sodium (trifluoro pyrrolejborate, sodium (trifluoro pyrazole)borate, sodium (trifluoro imidazole)borate, sodium (trifluoro triazolejborate, sodium (trifluoro trimethylsilyljborate, sodium (trifluoro trimethylsiloxy)borate, sodium (trifluoro bis[trimethylsilyl]amide)borate, sodium (trifluoro 2,3-dicyanoimidazole)borate, sodium (trifluoro cyanatejborate, sodium (trifluoro thiocyanatejborate, sodium (trimethyl cyanate)borate, sodium (trimethyl thiocyanatejborate, sodium (trimethyl methanesulfonyljborate, sodium (trimethyl hexafluoroisopropoxy)borate, sodium (trimethyl trifluoroethoxyjborate, sodium (trimethyl phenoxy)borate, sodium (trimethyl pyrrolejborate, sodium (trimethyl pyrazolejborate, sodium (trimethyl imidazolejborate, sodium (trimethyl triazolejborate, sodium (trimethyl 2,3-dicyanoimidazole)borate, sodium (trimethyl trimethylsilyljborate, sodium (trimethyl trimethylsiloxyjborate, and sodium (trimethyl bis[trimethylsilyl]amide)borate.Attorney Ref: 41873-64438

[0067] The total amount of sodium borate salt(s) in the electrolyte composition, whether based on one type of sodium borate salt or multiple types of sodium borate salts that may be used in the composition, is generally in the range of from about 0.1 mol / L to 5 mol / L of the electrolyte composition, such as from 0.3 mol / L to 5 mol / L, or from 0.8 mol / L to 1.5 mol / L. In some embodiments, the electrolyte composition includes 3% to 70% by weight of the sodium borate salt(s), such as 5% to 50% by weight, or 10% to 40% by weight of the sodium borate salt(s).

[0068] In some embodiments, the liquid electrolyte includes the at least one sodium borate salt dissolved in the at least one solvent. In some embodiments, the solvent of the electrolyte composition is non-aqueous.

[0069] In some embodiments, a nonaqueous solvent is any suitable nonaqueous solvent that can dissolve the sodium borate salt. Examples of suitable solvents include high-dielectric-constant solvents such as cyclic esters (cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC)), y-butyrolactone, and sulfolane. Other suitable solvents include low-viscosity solvents such as chain ester (chain carbonate such as dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC)), acetates such as methyl acetate and ethyl acetate, and ethers such as diglyme, tetraglyme, and 1,3-dioxolane. In other cases, an ionic liquid may be used. Mixtures of solvents, such as mixtures of high dielectric constant solvents the low viscosity solvents can also be used.

[0070] In some embodiments, the electrolyte composition includes at least one solvent. The solvent may be selected from an ether solvent, an ester solvent, a carbonate solvent, a fluoroether solvent, a phosphate solvent, a phosphite solvent, a fluoro-ester solvent, an anhydride solvent, an amide solvent, a cyclic-ether solvent, a cyclic-ester solvent, a cyclic-sulfate solvent, a cyclic-carbonate solvent, a nitrile solvent, a cyclic-phosphate solvent, a sulfate solvent, a sulfone solvent, and a sultone solvent.

[0071] Exemplary, though non-limiting, solvents that are suitable for use in the electrolyte compositions described herein include 1,2-dimethoxyethane (monoglyme), diethylene glycol dimethyl ether (diglyme), triethylene glycol dimethyl ether (triglyme), tetraethylene glycol dimethyl ether (tetraglyme), dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate,Attorney Ref: 41873-64438 ethylene carbonate, propylene carbonate, triethyl phosphate, trimethyl phosphate, triethyl phosphite, trimethyl phosphite, fluoroethylene carbonate, ethyl butyrate, gammabutyrolactone, gamma-valerolactone, triacetin, tetrahydrofuran, 1,3-dioxolane, 1,3-dioxane, 1,4- dioxane, acetic anhydride, N,N-dimethylformamide, dimethyl maleate, diethyl maleate, diallyl maleate, dimethyl malonate, diethyl malonate, dimethyl oxalate, diethyl oxalate, anisole, ethyl phenyl ether, methoxy perfluorobutane, methoxy perfluoropropane, ethoxy perfluorobutane, ethoxy perfluoropropane, diethyl ether, dipropyl ether, dibutyl ether, ethylene glycol diethyl ether, ethylene glycol dipropyl ether, ethylene glycol dibutyl ether, diethoxymethane (ethylal), 1,1-diethoxyethane, diethylene glycol divinyl ether, triethyl orthoformate, 1,1,1- triethoxyethane, 1,1,1-triethoxypropane, sulfolane, 3-methyl sulfolane, sulfolene, 1,3-propane sultone, 1,4-butane sultone, 1,3,2-dioxathiolane 2,2-dioxide, 1,3,2-dioxathiane 2,2-dioxide, tetramethylene sulfoxide, 2,2-dimethoxypropane, butyric anhydride, isobutyric anhydride, l,l,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, perfluoro-15-crown-5-ether, 1, 1,3, 3,3- pentafluoro-2-trifluoromethylpropyl methyl ether, 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether, methyl(2,2,2-trifluoroethyl)carbonate, 2,2,3,3,3-pentafluoropropyl-l,l,2,2-tetrafluoro- ethyl ether, fluorinated triethylene glycol monobutyl ether, (trifluoromethoxy)benzene, 4- toluene trifluoromethyl ether, bis-(4-fluorophenyl) ether, bis-(4-trifluoromethylphenyl) ether, 1,1,2,3,3,3-hexafluoropropyl methyl ether, 1,1,2,3,3,3-hexafluoropropyl ethyl ether, 1, 1,2, 3,3,3- hexafluoropropyl propyl ether, 1,1,2,3,3,3-hexafluoropropyl butyl ether, hexafluoroisopropyl methyl ether, hexafluoroisopropyl ethyl ether, hexafluoroisopropyl propyl ether, hexafluoroisopropyl isopropyl ether, hexafluoroisopropyl butyl ether, trifluoroethyl methyl ether, trifluoroethyl ethyl ether, trifluoroethyl propyl ether, trifluoroethyl butyl ether, 2,2,2- trifluoroethyl acetate, ethyl trifluoroacetate, propyl trifluoroacetate, isopropyl trifluoroacetate, butyl trifluoroacetate, 2,2,2-trifluoroethyl difluoromethyl ether, methyl cyanoformate, ethyl cyanoformate, methoxyacetonitrile, ethoxyacetonitrile, 2-phenoxyacetonitrile, methylsulfonylacetonitrile, tris(2,2,2-trifluoroethyl) phosphite, tris(2,2,2-trifluoroethyl) phosphate, dimethyl sulfate, diethyl sulfate, dimethyl sulfone, diethyl sulfone, divinyl sulfone, dipropyl sulfone, and dibutyl sulfone.Attorney Ref: 41873-64438

[0072] The total amount of solvent in the electrolyte composition, whether based on one type or species of solvent or multiple types or species of solvents that may be used in the composition, is generally in the range of from about 1 wt% to about 99 wt% of the electrolyte composition. That is to say, when an electrolyte composition includes more than one type or species of solvent, the sum of the content of the multiple solvents present in the electrolyte composition is within the range of from 1 wt% to 99 wt%.

[0073] While the core components of the electrolyte composition are one or more sodium borate salts and one or more solvents, the electrolyte composition may include other materials as well. In some embodiments, the electrolyte composition may also include one or more supplemental salts and / or one or more additives. These components may be present in the electrolyte composition in any suitable amount, though in some embodiments, these materials may be present at a wt% that is less than the amount of sodium borate salts and primary solvents used in the electrolyte composition.

[0074] With respect to supplemental salts, the electrolyte composition can include as supplemental salts any salt that are non-sodium borate salts having the formula NaB(R)4 or NaB(M)3(R), and which do not prevent the electrolyte composition from functioning in the anode- free sodium battery. Exemplary, though non-limiting, supplemental salts that can be used in the electrolyte composition include sodium hexafluorophosphate, sodium tetrafluoroborate, sodium perchlorate, sodium bis(oxalato)borate, sodium difluoro(oxalato)borate, sodium (trifluoromethane sulfonyljimide, sodium bis(fluoro sulfonyljimide, sodium trifluoroacetate, sodium heptafluorobutyrate, sodium pentafluoropropionate, sodium trifluoromethanesulfonate, sodium fluorosulfate, sodium nitrate, sodium difluoro(bisoxalato)phosphate, sodium difluorophosphate, sodium methanesulfonate, sodium sulfamate, sodium naphthalene-2-sulfonate, sodium bis(trimethylsilyl)amide, sodium trimethylsilanolate, sodium bis(malato)borate, and sodium difluoro(malato)borate.

[0075] With respect to additives, any suitable additive can be used provided that the additives do not prevent the electrolyte composition from functioning in the anode-free sodium battery. Exemplary, though non-limiting, additives that can be used in the electrolyte composition includeAttorney Ref: 41873-64438 vinyl carbonate, vinylene carbonate, fluoroethylene carbonate, 1,3-propane sultone, 1,4-butane sultone, 1,3,2-dioxathiolane 2,2-dioxide, thiophene, sodium difluoro(bisoxalato) phosphate, sodium difluorophosphate, sodium bis(oxalato)borate, sodium difluoro(oxalato)borate, trimethyl phosphite, tris(trimethylsilyl)phosphite, trimethyl borate, sodium tetra(methyl)borate, triethyl borate, tris(trimethylsilyl)borate, biphenyl, boron trifluoride, maleic anhydride, succinic anhydride, itaconic anhydride, trifluoroacetic anhydride, trifluoromethanesulfonic anhydride, trimethylboroxine, trihydroxybenzene, succinimide, lithium nitrate, diethyl pyrocarbonate, vinyl acetate, trimethyl(trifluoromethyl)silane, aluminum ethoxide, titanium isopropoxide, lithium difluoro(bisoxalato)phosphate, lithium difluorophosphate, lithium bis(oxalato)borate, and lithium difluoro(oxalato)borate.

[0076] With respect to the electrolyte, an exemplary electrolyte for anode-free sodium batteries has a composition that is comprised of sodium tetra(hexafluoro isopropoxyjborate in a diglyme solvent, where the major components comprise approximately 40% and 60% of the total electrolyte mass, respectively.

[0077] In some embodiments, an electrolyte for an anode-free sodium battery may be fabricated by mixing the at least one sodium borate salt and at least one solvent to yield a liquid sodium borate electrolyte.

[0078] In exemplary, though non-limiting, cases, the positive electrode is comprised of cathode active material, wherein the cathode active material may be particles. In some embodiments, the cathode active material particles have an average length that is less than 10 pm. In some embodiments, the cathode active material particles have an average length that is between 0.05 pm and 5 pm. In some embodiments, the cathode active material particles have an average aspect ratio between 1 and 10.

[0079] In exemplary, though non-limiting, cases, the cathode active material contains sodium. In some embodiments, the sodium-containing cathode active material is selected from the set containing sodium nickel-iron-manganese oxide (NFM, e.g., NaNixFevMnzO2, where x+y+z = 1), sodium ferric phosphate pyrophosphate (NFPP, IS^FeafPO^zfPzO?)), sodium vanadium phosphate (NaaN^PC ), sodium chromium oxide (NaCrO2>, Prussian White (NaaFeMn(CN)e, whereAttorney Ref: 41873-64438 a > 0 and < 2), Prussian Blue (NaaFe2(CN)6, where a > 0 and < 2), potassium-doped sodium manganese oxide (NaaKbMnO2, where a+b < 1), a sodium transition metal oxide (e.g., NaxTM02, wherein TM = a transition metal, Fe, Mn, Cu, Ni, Co, Cr, Ti, V, or combination thereof; x > 0 and < 1), a sodium metal phosphate, pyrophosphate, or diphosphate, a metal sulfide (e.g., sodium sulfide, sodium ferric sulfide), and a combination thereof.

[0080] In exemplary, though non-limiting, cases, the battery cell further comprises a separator between the at least one positive electrode and the at least one negative electrode. In some embodiments, the separator may comprise a polymer having porosity. In some embodiments, the separator is typically wetted with the liquid electrolyte material. In some embodiments, the separator between the negative and positive electrodes is comprised of polyethylene, polypropylene, cellulose, polyethylene terephthalate, glass microfiber, or any combination thereof. In some embodiments, the separator has a porosity between 30% and 60%.

[0081] With respect to FIG.3, exemplary, though non-limiting, anode-free sodium battery cell designs comprising a sodium borate electrolyte can achieve higher specific capacity and first- cycle Coulombic efficiency than a similar sodium-ion battery comprising the same cathode, the same electrolyte, and a hard carbon anode.

[0082] With respect to FIG.4, exemplary, though non-limiting, anode-free sodium battery cell designs comprising a sodium borate electrolyte are described herein with a commerciallyrelevant areal capacity >2 mAh / cm2.

[0083] With respect to FIG.5, exemplary, though non-limiting, anode-free sodium battery cell designs comprising a sodium borate electrolyte are described herein that can operate with high specific capacity at slower (i.e., C / 10 or 10-hour discharge) or higher (i.e., 1C or 1-hour discharge) rates.

[0084] In some embodiments, the anode-free sodium battery comprises an anode-free substrate as the anodic counter-electrode to the cathode. The anode-free substrate may be comprised of an aluminum current collector, which may optionally include a conductive coating. Alternatively, the anode-free substrate may not contain an aluminum current collector, and is therefore insteadAttorney Ref: 41873-64438 a freestanding substrate. A freestanding anode-free substrate may be comprised of a conductive carbon material.

[0085] Exemplary, though non-limiting, anode-free substrates that are suitable for use in the anode-free sodium battery described herein include aluminum foil, aluminum foil coated with a conductive material, aluminum foil coated on both sides with a conductive material, and a freestanding conductive carbon film. Exemplary, though non-limiting drawings of an anode-free sodium battery are described in FIGs. 1A and IB, having anode-free substrates comprising aluminum foil coated with a conductive material and a freestanding conductive carbon film, respectively. Exemplary, though non-limiting drawings of a multi-layer anode-free sodium battery are described in FIGs. 2A and 2B, having anode-free substrates comprising aluminum foil coated on both sides with a conductive material and a freestanding conductive carbon film, respectively.

[0086] With respect to the anode-free substrate, material properties including thickness, surface roughness, porosity, and electronic conductivity can affect the performance of the anode-free sodium battery described herein and are therefore of importance to characterize and optimize.

[0087] With respect to an anode-free substrate comprised of aluminum foil that is suitable for use in the anode-free sodium battery described herein, exemplary, though non-limiting, material properties of the aluminum foil include a thickness of 1 to 50 pm, an average surface roughness of 0.03 to 15 pm, a thin surface film of aluminum oxide (Al 2O3) having a thickness of 1 to 15 nm, and an electrical conductivity between 1 x 107and 1 x 108S / m.

[0088] With respect to an anode-free substrate comprised of aluminum foil that is coated with a conductive film on one or both sides that is suitable for use in the anode-free sodium battery described herein, exemplary, though non-limiting, material properties of the conductive coating include a thickness of 0.05 to 100 pm, an average surface roughness of 0.01 to 15 pm, and a porosity between 1% and 99%.

[0089] With respect to the conductive coating, any suitable coating can be used provided that the coating does not prevent the anode-free substrate from functioning in the anode-free sodium battery. Exemplary, though non-limiting, materials that can be used in the conductive coatingAttorney Ref: 41873-64438 composition include amorphous carbon, graphitic carbon, single walled carbon nanotubes, multiwalled carbon nanotubes, carbonized polymer including polyacrylonitrile, glassy carbon, graphene, fullerene, platinum, titanium, copper, lithium, aluminum, tin, zinc, and silver. While the conductive coating may, in some embodiments, include one conductive material, other embodiments of the conductive coating may include multiple different types of conductive materials.

[0090] In some embodiments, the conductive coating may further comprise a binder material. Exemplary, though non-limiting, binder materials for use in a conductive coating for an anode- free substrate include polyvinylidene fluoride, polytetrafluoroethylene, sodium carboxymethyl cellulose, styrene-butadiene rubber, sodium alginate, polyacrylic acid, polyaniline, polypyrrole, and polyacrylonitrile.

[0091] With respect to an anode-free substrate comprised of a freestanding conductive carbon film that is suitable for use in the anode-free sodium battery described herein, exemplary, though non-limiting, material properties of the freestanding film include a thickness of 1 to 50 pm, and an average surface roughness of 0.01 to 15 pm. Exemplary, though non-limiting, materials that can be used in the freestanding conductive film composition include amorphous carbon, graphitic carbon, single walled carbon nanotubes, multiwalled carbon nanotubes, carbonized polymer including polyacrylonitrile, glassy carbon, graphene, fullerene, platinum, titanium, copper, lithium, aluminum, tin, zinc, and silver. While the freestanding film may, in some embodiments, include one conductive material, other embodiments of the conductive coating may include multiple different types of conductive materials.

[0092] In some embodiments, the freestanding conductive carbon film may further comprise a binder material. Exemplary, though non-limiting, binder materials for use in a freestanding conductive carbon film for an anode-free substrate include polyvinylidene fluoride, polytetrafluoroethylene, sodium carboxymethyl cellulose, styrene-butadiene rubber, sodium alginate, polyacrylic acid, polyaniline, polypyrrole, and polyacrylonitrile.

[0093] With respect to an anode-free substrate, FIG.6 describes several exemplary, though nonlimiting, anode-free substrate materials and their properties. FIG.7 of the drawings describes theAttorney Ref: 41873-64438 voltage curves and performance of exemplary anode-free sodium batteries comprising the anode-free substrate materials shown in FIG.6.

[0094] In some embodiments, the anode-free sodium battery may be operated with pressure applied to the electrode materials. Exemplary, though non-limiting, methods for applying pressure to the electrodes include pressing a soft-shell battery (i.e., a pouch battery) between external plates, coiling the electrodes into a roll that is contained within a hard-shell container (i.e., a cylindrical battery), layering the electrodes into a stack that is contained within a hardshell container (i.e., a prismatic battery). In some embodiments, the applied pressure is between 10 and 2500 kPa.

[0095] With respect to the applied pressure, FIGs. 8 and 9 describe performance differences of anode-free sodium batteries cycled at multiple exemplary, though non-limiting, applied pressures.

[0096] Exemplary, though non-limiting, data for anode-free sodium batteries in coin cell and pouch cell format are described in FIGs. 10-11 and FIGs. 12-15, respectively. The data includes capacity retention, Coulombic efficiency, voltage profiles, and cycling performance comparisons between sodium borate electrolytes and a state-of-the-art conventional electrolyte comprising NaPFe in diglyme. These exemplary anode-free sodium batteries are fabricated by stacking an anode-free substrate, separator, and cathode, and then wetting the separator and electrodes with electrolyte. The exemplary, though non-limiting, sodium borate electrolyte is described herein.

[0097] This data indicates that the sodium borate electrolytes and anode-free substrates described herein can support highly reversible cycling for long cycle life, provide high Coulombic efficiency, and provide high specific capacity, all of which are optimal metrics for a sodium battery electrolyte.

[0098] From the foregoing, it will be appreciated that specific embodiments of this disclosure have been described herein for purposes of illustration, but that various modifications may be made without deviating from the scope of the disclosure.Attorney Ref: 41873-64438

[0099] Although the technology has been described in language that is specific to certain structures and materials, it is to be understood that the invention defined in the appended claims is not necessarily limited to the specific structures and materials described. Rather, the specific aspects are described as forms of implementing the claimed invention. Because many embodiments of the invention can be practiced without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.

[0100] Unless otherwise indicated, all number or expressions, such as those expressing dimensions, physical characteristics, etc., used in the specification (other than the claims) are understood as modified in all instances by the term "approximately". At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the claims, each numerical parameter recited in the specification or claims which is modified by the term "approximately" should at least be construed in light of the number of recited significant digits and by applying rounding techniques. Moreover, all ranges disclosed herein are to be understood to encompass and provide support for claims that recite any and all sub-ranges or any and all individual values subsumed therein. For example, a stated range of 1 to 10 should be considered to include and provide support for claims that recite any and all sub-ranges or individual values that are between and / or inclusive of the minimum value of 1 and the maximum value of 10; that is, all sub-ranges beginning with a minimum value of 1 or more and ending with a maximum value of 10 or less (e.g., 5.5 to 10, 2.34 to 3.56, and so forth) or any values from 1 to 10 (e.g., 3, 5.8, 9.9994, and so forth).ADDITIONAL EMBODIMENTS

[0101] Aspects of the present disclosure are described by the following numbered clauses.1. A battery cell comprising: a cathode that comprises sodium in its discharged state; an anode-free substrate; and a liquid electrolyte coupling the anode to the cathode, wherein the electrolyte comprises: at least one sodium borate salt; and at least one solvent.Attorney Ref: 41873-64438 The battery of clause 1, wherein the anode-free substrate has a ratio by capacity (e.g., Ah) of anode active material to cathode active material is less than 1 (e.g., < 0.5, <0.4, <0.3, <0.2, <0.1, <0.05, or <0.01). The battery cell of clause 1, wherein the anode-free substrate has no anode active material prior to battery charging, and the battery cell is configured to deposit anode active material (e.g., sodium metal) onto the anode-free substrate during battery charging. The battery cell of any one of clauses 1-3, wherein the at least one sodium borate salt has the formula:NaB(R)4wherein: each R is independently selected from alkoxy, substituted alkoxy, carboxyl group, phenoxy, substituted phenoxy, silyl group, siloxy group, sulfonyl group, sulfate group, alkyl, substituted alkyl, a nitrile group, an amide group, an optionally substituted heterocyclyl (e.g., a 5-membered N-containing azole hetereocyclyl, such as a pyrrolyl, or substituted pyrrolyl), and a cyanate group. The battery cell of clause 4, wherein each R is independently selected from an alkoxy group, a fluoro-alkoxy group, a cyano-alkoxy group, a functionalized alkoxy group, a carboxyl group, a fluoro-carboxyl group, a functionalized carboxyl group, a phenoxy group, a fluoro-phenoxy group, a cyano-phenoxy group, a functionalized phenoxy group, a silyl group, a siloxy group, a sulfonyl group, a sulfate group, an alkyl group, a fluoro-alkyl group, a functionalized alkyl group, a nitrile group, an amide group, a pyrrole group, a functionalized pyrrole group, an azole group, a functionalized azole group, and a cyanate group. The battery cell of any one of clauses 1-3, wherein the at least one sodium borate salt has the formula:NaB(M)3(R)i wherein:R is selected from an alkoxy group, a fluoro-alkoxy group, a cyano-alkoxy group, a functionalized alkoxy group, a carboxyl group, a fluoro-carboxyl group, a functionalized carboxyl group, a phenoxy group, a fluoro-phenoxy group, a cyano-phenoxy group, aAttorney Ref: 41873-64438 functionalized phenoxy group, a silyl group, a siloxy group, a sulfonyl group, a sulfate group, an alkyl group, a fluoro-alkyl group, a functionalized alkyl group, a nitrile group, an amide group, a pyrrole group, a functionalized pyrrole group, an azole group, a functionalized azole group, and a cyanate group, and each M is independently selected from F, an alkoxy group (e.g., methoxy), a phenoxy group, and a trimethylsiloxy group. The battery cell of clause 6, wherein each M is F (i.e., fluorine). The battery cell of clause 6, wherein each M is OCH3. The battery cell of clause 6, wherein each M is OSi[CH3]3. The battery cell of any one of clauses 1-9, wherein the at least one sodium borate salt is selected from sodium tetra(hexafluoro isopropoxyjborate, sodium tetra(phenoxy)borate, sodium tetra(l,l,l-trifluoro isopropoxyjborate, sodium tetra(2-trifluoromethyl isopropoxyjborate, sodium tetra(trifluoro ethoxyjborate, sodium tetra(difluoro ethoxy)borate, sodium bis(perfluoro pinacolato)borate, sodium tetra(pentafluoro phenoxy)borate, sodium tetra(perfluoro tertbutoxy) borate, sodium tetra(trifluoro acetoxy) borate, sodium tetra(3-methylphenoxy)borate, sodium tetra(4- methylphenoxyjborate, sodium tetra(3,5-dimethylphenoxy)borate, sodium tetra(3,4- dimethylphenoxyjborate, sodium tetra(3,4,5-trimethylphenoxy)borate, sodium tetra(2- fluoro-3-methylphenoxy)borate, sodium tetra(4-fluoro-3-methylphenoxy)borate, sodium tetra(2-fluoro-5-methylphenoxy)borate, sodium tetra(4-cyano phenoxyjborate, sodium tetra(3-cyano phenoxyjborate, sodium tetra(3,4-dicyano phenoxyjborate, sodium tetra(3,5- dicyano phenoxyjborate, sodium tetra(3-cyano-5-fluoro phenoxyjborate, sodium tetrafl- phenyl-2,2,2-trifluoro ethoxyjborate, sodium tetra(l-cyano-2,2,2-trifluoro ethoxyjborate, sodium tetra(l,l,l-trifluoro-2-cyano isopropoxyjborate, sodium tetra(2-cyano isopropoxyjborate, sodium tetra(l-cyano isopropoxyjborate, sodium tetra(2-cyano ethoxyjborate, sodium tetra(l-cyano-l-methoxy methoxy) borate, sodium tetra(l-cyano-l- ethoxy methoxyjborate, sodium tetra(l-cyano-l-phenyl methoxyjborate, sodium tetra(2- cyano-l-methoxy ethoxyjborate, sodium tetra(2-cyano-l-ethoxy ethoxyjborate, sodium tetra(methysulfonyl)borate, sodium tetra(trifluoromethysulfonyl)borate, sodiumAttorney Ref: 41873-64438 tetra(trimethylsiloxy)borate, sodium tetra(methyl)borate, sodium (trifluoro methanesulfonyl)borate, sodium (trifluoro hexafluoroisopropoxyjborate, sodium (trifluoro trifluoroethoxy)borate, sodium (trifluoro pentafluoropropoxyjborate, sodium (trifluoro trifluoromethyljborate, sodium (difluoro bis-trifluoromethyl)borate, sodium (trifluoro dicyanamidejborate, sodium (trifluoro phenoxyjborate, sodium (trifluoro pyrrolejborate, sodium (trifluoro pyrazolejborate, sodium (trifluoro imidazolejborate, sodium (trifluoro triazolejborate, sodium (trifluoro trimethylsilyljborate, sodium (trifluoro trimethylsiloxyjborate, sodium (trifluoro bis[trimethylsilyl]amide)borate, sodium (trifluoro 2,3-dicyanoimidazole)borate, sodium (trifluoro cyanatejborate, sodium (trifluoro thiocyanate)borate, sodium (trimethyl cyanate) borate, sodium (trimethyl thiocyanate)borate, sodium (trimethyl methanesulfonyl)borate, sodium (trimethyl hexafl uoroisopropoxy)borate, sodium (trimethyl trifluoroethoxyjborate, sodium (trimethyl phenoxy)borate, sodium (trimethyl pyrrole)borate, sodium (trimethyl pyrazole)borate, sodium (trimethyl imidazolejborate, sodium (trimethyl triazolejborate, sodium (trimethyl 2,3-dicyanoimidazole)borate, sodium (trimethyl trimethylsilyljborate, sodium (trimethyl trimethylsiloxyjborate, sodium (trimethyl bis[trimethylsilyl]amide)borate. The battery cell of clause 10, wherein the at least one sodium borate salt is sodium tetra(hexafluoro isopropoxyjborate. The battery cell of any one of clauses 1-11, wherein the liquid electrolyte comprises the at least one sodium borate salt dissolved in the at least one solvent. The battery cell of any one of clauses 1-12, wherein the at least one solvent is non-aqueous. The battery cell of any one of clauses 1-12, wherein the at least one solvent is selected from an ether solvent, an ester solvent, a carbonate solvent, a fluoro-ether solvent, a phosphate solvent, a phosphite solvent, a fluoro-ester solvent, an anhydride solvent, an amide solvent, a cyclic-ether solvent, a cyclic-ester solvent, a cyclic-sulfate solvent, a cyclic-carbonate solvent, a nitrile solvent, a cyclic-phosphate solvent, a sulfate solvent, a sulfone solvent, and a sultone solvent. The battery cell of any one of clauses 1-14, wherein the at least one solvent is selected from 1,2-dimethoxyethane (monoglyme), diethylene glycol dimethyl ether (diglyme), triethyleneAttorney Ref: 41873-64438 glycol dimethyl ether (triglyme), tetraethylene glycol dimethyl ether (tetraglyme), dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethylene carbonate, propylene carbonate, triethyl phosphate, trimethyl phosphate, triethyl phosphite, trimethyl phosphite, fluoroethylene carbonate, ethyl butyrate, gamma-butyrolactone, gamma-valerolactone, triacetin, tetra hydrofuran, 1,3-dioxolane, 1,3-dioxane, 1,4-dioxane, acetic anhydride, N,N- dimethylformamide, dimethyl maleate, diethyl maleate, diallyl maleate, dimethyl malonate, diethyl malonate, dimethyl oxalate, diethyl oxalate, anisole, ethyl phenyl ether, methoxy perfluorobutane, methoxy perfluoropropane, ethoxy perfluorobutane, ethoxy perfluoropropane, diethyl ether, dipropyl ether, dibutyl ether, ethylene glycol diethyl ether, ethylene glycol dipropyl ether, ethylene glycol dibutyl ether, diethoxymethane (ethylal), 1,1- diethoxyethane, diethylene glycol divinyl ether, triethyl orthoformate, 1,1,1-triethoxyethane, 1,1,1-triethoxypropane, sulfolane, 3-methyl sulfolane, sulfolene, 1,3-propane sultone, 1,4- butane sultone, 1,3,2-dioxathiolane 2,2-dioxide, 1,3,2-dioxathiane 2,2-dioxide, tetramethylene sulfoxide, 2,2-dimethoxypropane, butyric anhydride, isobutyric anhydride,1.1.2.2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, perfluoro-15-crown-5-ether, l,l,3,3,3-pentafluoro-2-trifluoromethylpropyl methyl ether, 1,1,2,2-tetrafluoroethyl 2,2,2- trifluoroethyl ether, methyl(2,2,2-trifluoroethyl)carbonate, 2,2,3,3,3-pentafluoropropyl-1.1.2.2-tetrafluoro-ethyl ether, fluorinated triethylene glycol monobutyl ether, (trifluoromethoxy)benzene, 4-toluene trifluoromethyl ether, bis-(4-fluorophenyl) ether, bis- (4-trifluoromethylphenyl) ether, 1,1,2,3,3,3-hexafluoropropyl methyl ether, 1, 1,2, 3,3,3- hexafluoropropyl ethyl ether, 1,1,2,3,3,3-hexafluoropropyl propyl ether, 1,1,2,3,3,3- hexafluoropropyl butyl ether, hexafluoroisopropyl methyl ether, hexafluoroisopropyl ethyl ether, hexafluoroisopropyl propyl ether, hexafluoroisopropyl isopropyl ether, hexafluoroisopropyl butyl ether, trifluoroethyl methyl ether, trifluoroethyl ethyl ether, trifluoroethyl propyl ether, trifluoroethyl butyl ether, 2,2,2-trifluoroethyl acetate, ethyl trifluoroacetate, propyl trifluoroacetate, isopropyl trifluoroacetate, butyl trifluoroacetate,2.2.2-trifluoroethyl difluoromethyl ether, methyl cyanoformate, ethyl cyanoformate, methoxyacetonitrile, ethoxyacetonitrile, 2-phenoxyacetonitrile, methylsulfonylacetonitrile, tris(2,2,2-trifluoroethyl) phosphite, tris(2,2,2-trifluoroethyl) phosphate, dimethyl sulfate,Attorney Ref: 41873-64438 diethyl sulfate, dimethyl sulfone, diethyl sulfone, divinyl sulfone, dipropyl sulfone, dibutyl sulfone, or any combination thereof. The battery cell of any one of clauses 1-3, wherein: the at least one sodium borate salt is sodium tetra(hexafluoro isopropoxy)borate; and the at least one solvent is selected from 1,2-dimethoxyethane (monoglyme), diethylene glycol dimethyl ether (diglyme), triethylene glycol dimethyl ether (triglyme), and tetraethylene glycol dimethyl ether (tetraglyme). The battery cell of any one of clauses 1-16, wherein the at least one sodium borate salt comprises 3 to 70 wt% of the liquid electrolyte. The battery cell of any one of clauses 1-17, wherein the liquid electrolyte further comprises at least one supplemental sodium salt, or a combination of two or more supplemental sodium salts. The battery cell of clause 18, wherein the at least one supplemental salt is selected from sodium hexafluorophosphate, sodium tetrafluoroborate, sodium perchlorate, sodium bis(oxalato)borate, sodium difluoro(oxalato)borate, sodium (trifluoromethane sulfonyl)imide, sodium bis(fluoro sulfonyljimide, sodium trifluoroacetate, sodium heptafluorobutyrate, sodium pentafluoropropionate, sodium trifluoromethanesulfonate, sodium fluorosulfate, sodium nitrate, sodium difluoro(bisoxalato)phosphate, sodium difluorophosphate, sodium methanesulfonate, sodium sulfamate, sodium naphthalene-2- sulfonate, sodium bis(trimethylsilyl)amide, sodium trimethylsilanolate, sodium bis(malato)borate, and sodium difluoro(malato)borate. The battery cell of any one of clauses 1-19, wherein the liquid electrolyte further comprises at least one additive, or a combination of two or more additives. The battery cell of clause 20, wherein the at least one additive is selected from vinyl carbonate, vinylene carbonate, fluoroethylene carbonate, 1,3-propane sultone, 1,4-butane sultone, 1,3,2-dioxathiolane 2,2-dioxide, thiophene, sodium difluoro(bisoxalato) phosphate, sodium difluorophosphate, sodium bis(oxalato)borate, sodium difluoro(oxalato)borate, trimethyl phosphite, tris(trimethylsilyl)phosphite, trimethyl borate, sodium tetra(methyl)borate, triethyl borate, tris(trimethylsilyl)borate, biphenyl, boron trifluoride,Attorney Ref: 41873-64438 maleic anhydride, succinic anhydride, itaconic anhydride, trifluoroacetic anhydride, trifluoromethanesulfonic anhydride, trimethylboroxine, trihydroxybenzene, succinimide, lithium nitrate, diethyl pyrocarbonate, vinyl acetate, trimethyl(trifluoromethyl)silane, aluminum ethoxide, titanium isopropoxide, lithium difluoro(bisoxalato)phosphate, lithium difluorophosphate, lithium bis(oxalato)borate, and lithium difluoro(oxalato)borate. The battery cell of any one of clauses 1-21, wherein the cathode comprises cathode active material that is a sodium-containing cathode active material. The battery cell of clause 22, wherein the sodium-containing cathode active material is selected from: sodium nickel-iron-manganese oxide (NFM, e.g., NaNixFeyMnz02, where x+y+z = 1), sodium transition metal oxide (e.g., NaxTM02, wherein TM = a transition metal, Fe, Mn,Ni, Co, Cr, Ti, V, or combination thereof; x > 0 and < 1),NaaNii-x-y-zMnxMgyTizO2 (a > 0 and < 1; x+y+z = 1), sodium copper-iron-manganese oxide (NaaCuxFeyMnzO2, where a > 0 and < 1; x+y+z = 1), sodium ferric phosphate pyrophosphate (NFPP, NaAFesfPOziMPzO?)),Na3V2PO4, sodium chromium oxide (NaCrO2), potassium-doped sodium manganese oxide (NaaKbMnO2, where a+b < 1),Prussian White (NaaFeMn(CN)e, where a > 0 and < 2),Prussian Blue (NaaFe2(CN)e, where a > 0 and < 2), a metal phosphate, pyrophosphate, or diphosphate containing, but not limited to, Fe, Mn, V, Ti, or a combination thereof, a metal sulfide, e.g., sodium sulfide, and a combination thereof. The battery cell of any one of clauses 1-23, wherein the cathode is further doped with a supporting element. The battery cell of clause 24, wherein the supporting element is selected from Ca, Mg, K, Ti, Al, Zr, F, Sr, Nb, Mo, Y, and a combination thereof.Attorney Ref: 41873-64438 The battery cell of any one of clauses 1-25, wherein the anode-free substrate comprises aluminum foil having a thickness between 1 and 50 pm. The battery cell of clause 26, wherein the aluminum foil has an average surface roughness (Ra) between 0.03 and 15 pm. The battery cell of clause 26 or 27, wherein the aluminum foil contains a thin surface film of aluminum oxide (AI2O3) having a thickness of 1 to 15 nm. The battery cell of any one of clauses 26-28, wherein the aluminum foil has an electrical conductivity between 1 x 107and 1 x 108S / m. The battery cell of any one of clauses 26-29, wherein the aluminum foil is coated on one or both sides with a conductive coating. The battery cell of clause 30, wherein the conductive coating has a thickness between 0.05 to 100 pm. The battery cell of clause 30 or 31, wherein the conductive coating has a porosity between 1% and 99%. The battery cell of any one of clauses 30-32, wherein the conductive coating material is selected from amorphous carbon, graphitic carbon, single walled carbon nanotubes, multiwalled carbon nanotubes, carbonized polymer including polyacrylonitrile, glassy carbon, graphene, fullerene, or any combination thereof. The battery cell of any one of clauses 30-33, wherein the conductive coating comprises platinum, titanium, copper, lithium, aluminum, tin, zinc, silver, or any combination thereof. The battery cell of any one of clauses 1-34, wherein the anode-free substrate is carbon-coated aluminum foil, wherein the carbon coating has a thickness of 0.5 to 2 pm. The battery cell of any one of clauses 1-34, wherein the anode-free substrate is a freestanding conductive carbon film having a thickness between 1 and 50 pm, and not containing aluminum foil, wherein >95 wt% (e.g., >98 wt%, or >99wt%) of the anode-free substrate is composed of the aluminum-free conductive carbon film. The battery cell of clause 36, wherein the freestanding conductive carbon film is composed of material selected from amorphous carbon, graphitic carbon, single walled carbonAttorney Ref: 41873-64438 nanotubes, multiwalled carbon nanotubes, carbonized polymer including polyacrylonitrile, glassy carbon, graphene, fullerene, and any combination thereof. The battery cell of clause 36 or 37, wherein the freestanding conductive carbon film further comprises a metal additive selected from platinum, titanium, copper, lithium, aluminum, tin, zinc, silver, and any combination thereof. A battery comprising a battery cell of any one of clauses 1-38. The battery of clause 39, wherein the battery is configured to operate under an externally applied pressure between 10 kPa and 2500 kPa. The battery of clause 39, wherein the battery comprises an external shell that applies a pressure to the electrodes in the range of 10 kPa to 2500 kPa. A battery cell comprising: a cathode that contains sodium in its discharged state; an anode-free substrate comprising an aluminum foil current collector having a thickness between 1 and 50 pm; and a liquid electrolyte coupling the anode to the cathode. The battery of clause 42, wherein the anode-free substrate has a ratio by capacity (e.g., Ah) of anode active material to cathode active material is less than 1 (e.g., < 0.5, <0.4, <0.3, <0.2, <0.1, <0.05, or <0.01) and the battery cell is configured to deposit anode active material (e.g., sodium metal) onto the anode-free substrate during battery charging. The battery cell of clause 42 or 43, wherein the aluminum foil has an average surface roughness (Ra) between 0.03 and 15 pm. The battery cell of any one of clauses 42-44, wherein the aluminum foil contains a thin surface film of aluminum oxide (AI2O3) having a thickness of 1 to 15 nm. The battery cell of any one of clauses 42-45, wherein the aluminum foil has an electrical conductivity between 1 x 107and 1 x 108S / m. The battery cell of any one of clauses 42-46, wherein the aluminum foil is coated on one side with a conductive coating. The battery cell of any one of clauses 42-46, wherein the aluminum foil is coated on both sides with a conductive coating.Attorney Ref: 41873-64438 The battery of clause 47 or 48, wherein the conductive coating has a thickness between 0.05 to 100 pm. The battery cell of any one of clauses 47-49, wherein the conductive coating has a porosity between 1% and 99%. The battery cell of any one of clauses 47-50, wherein the conductive coating is composed of a material selected from amorphous carbon, graphitic carbon, single walled carbon nanotubes, multiwalled carbon nanotubes, carbonized polymer including polyacrylonitrile, glassy carbon, graphene, fullerene, and any combination thereof. The battery cell of any one of clauses 47-51, wherein the conductive coating comprises platinum, titanium, copper, lithium, aluminum, tin, zinc, silver, or any combination thereof. A battery cell comprising: a cathode that contains sodium in its discharged state; an anode-free substrate comprising a freestanding conductive carbon film having a thickness between 1 and 50 pm, and not containing aluminum foil, wherein >95 wt% of the anode- free substrate is the conductive carbon film; and a liquid electrolyte coupling the anode to the cathode. The battery of clause 53, wherein the anode-free substrate has a ratio by capacity (e.g., Ah) of anode active material to cathode active material is less than 1 (e.g., < 0.5, <0.4, <0.3, <0.2, <0.1, <0.05, or <0.01) and the battery cell is configured to deposit anode active material (e.g., sodium metal) onto the anode-free substrate during battery charging. The battery cell of clause 53 or 54, wherein the freestanding conductive carbon film material is selected from amorphous carbon, graphitic carbon, single walled carbon nanotubes, multiwalled carbon nanotubes, carbonized polymer including polyacrylonitrile, glassy carbon, graphene, fullerene, or any combination thereof. The battery cell of any one of clauses 53-55, wherein the freestanding conductive carbon film further comprises a metal additive selected from platinum, titanium, copper, lithium, aluminum, tin, zinc, silver, or any combination thereof. The battery cell of any one of clauses 53-56, wherein the freestanding conductive carbon film further comprises a binder material.Attorney Ref: 41873-6443858. The battery cell of clause 57, wherein the binder material is selected from polyvinylidene fluoride, polytetrafluoroethylene, sodium carboxymethyl cellulose, styrene-butadiene rubber, sodium alginate, polyacrylic acid, polyaniline, polypyrrole, and polyacrylonitrile.EQUIVALENTS AND INCORPORATION BY REFERENCE

[0102] While the invention has been particularly shown and described with reference to a preferred embodiment and various alternate embodiments, it will be understood by persons skilled in the relevant art that various changes in form and details can be made therein without departing from the spirit and scope of the invention.

[0103] All references, issued patents and patent applications cited within the body of the instant specification, and U.S. application no. 63 / 706,383, filed Oct. 11, 2024, are hereby incorporated by reference in their entirety, for all purposes.

Claims

Attorney Ref: 41873-64438CLAIMSWhat is claimed is:

1. A battery cell comprising: at least one positive electrode, wherein the positive electrode comprises: a cathode material that comprises sodium; and at least one negative electrode, wherein the negative electrode comprises: a negative current collector; and sodium metal on at least part of the negative electrode when the battery cell is fully charged; and a liquid electrolyte coupling the at least one negative electrode to the at least one positive electrode, wherein the electrolyte comprises: at least one sodium borate salt; and at least one solvent.

2. The battery cell of claim 1, wherein the sodium metal on at least part of the negative electrode has an average thickness between 8 pm and 88 pm (e.g., 8-65 pm, 8-55 pm, 8-45 pm, or 18-45 pm) when the energy storage device is fully charged.

3. The battery cell of claims 1 or 2, wherein the negative electrode further comprises anode active material that is not sodium metal on at least part of the negative electrode, wherein the ratio by capacity (e.g., Ah, mAh / cm2) of anode active material to cathode active material is less than 1 (e.g., < 0.5, <0.4, <0.3, <0.2, <0.1, <0.05, or <0.01).

4. The battery cell of any one of claims 1-3, wherein the at least one sodium borate salt has the formula:NaB(R)4wherein: each R is independently selected from an alkoxy group, a fluoro-alkoxy group, a cyanoalkoxy group, a functionalized alkoxy group, a carboxyl group, a fluoro-carboxyl group, aAttorney Ref: 41873-64438 functionalized carboxyl group, a phenoxy group, a fluoro-phenoxy group, a cyano-phenoxy group, a functionalized phenoxy group, a silyl group, a siloxy group, a sulfonyl group, a sulfate group, an alkyl group, a fluoro-alkyl group, a functionalized alkyl group, a nitrile group, an amide group, a pyrrole group, a functionalized pyrrole group, an azole group, a functionalized azole group, an optionally substituted heterocyclic group (e.g., a 5-membered N-containing azole hetereocyclyl), and a cyanate group.

5. The battery cell of any one of claims 1-3, wherein the at least one sodium borate salt has the formula:NaB(M)3(R)i wherein:R is selected from an alkoxy group, a fluoro-alkoxy group, a cyano-alkoxy group, a functionalized alkoxy group, a carboxyl group, a fluoro-carboxyl group, a functionalized carboxyl group, a phenoxy group, a fluoro-phenoxy group, a cyano-phenoxy group, a functionalized phenoxy group, a silyl group, a siloxy group, a sulfonyl group, a sulfate group, an alkyl group, a fluoro-alkyl group, a functionalized alkyl group, a nitrile group, an amide group, a pyrrole group, a functionalized pyrrole group, an azole group, a functionalized azole group, and a cyanate group; and each M is independently selected from fluorine (i.e., F), an alkoxy group (e.g., methoxy), a phenoxy group, and a trimethylsiloxy group.

6. The battery cell of claim 5, wherein each M is fluorine (i.e., F).

7. The battery cell of any one of claims 1-6, wherein the at least one sodium borate salt is selected from sodium tetra(trifluoro acetoxyjborate, sodium tetra(methysulfonyl)borate, sodium tetra(trifluoromethysulfonyl)borate, sodium tetra(trimethylsiloxy)borate, sodium tetra(l,l,l-trifluoro isopropoxyjborate, sodium tetra(2-trifluoromethyl isopropoxy)borate, sodium tetra(trifluoro ethoxy)borate, sodium tetra(pentafluoro propoxy) borate, sodium tetra(heptafluoro butoxyjborate, sodium tetra(difluoro ethoxy)borate, sodium bis(perfluoroAttorney Ref: 41873-64438 pinacolato)borate, sodium tetra(pentafluoro phenoxyjborate, sodium tetrafperfluoro tertbutoxyjborate, sodium tetra(hexafluoro isopropoxy)borate, sodium tetra(3- methylphenoxy)borate, sodium tetra(4-methylphenoxy)borate, sodium tetra(3,5- dimethylphenoxyjborate, sodium tetra(3,4-dimethylphenoxy)borate, sodium tetra(3,4,5- trimethylphenoxy)borate, sodium tetra(2-fluoro-3-methylphenoxy)borate, sodium tetra(4- fluoro-3-methylphenoxy)borate, sodium tetra(2-fluoro-5-methylphenoxy)borate, sodium tetra(4-cyano phenoxy)borate, sodium tetra(3-cyano phenoxy)borate, sodium tetra(3,4- dicyano phenoxy)borate, sodium tetra(3,5-dicyano phenoxyjborate, sodium tetra(3-cyano-5- fluoro phenoxyjborate, sodium tetra(l-phenyl-2,2,2-trifluoro ethoxyjborate, sodium tetra(l- cyano-2,2,2-trifluoro ethoxyjborate, sodium tetrafl, l,l-trifluoro-2-cya no isopropoxy)borate, sodium tetra(2-cyano isopropoxyjborate, sodium tetrafl-cyano isopropoxyjborate, sodium tetra(2-cyano ethoxyjborate, sodium tetrafl-cyano- 1-methoxy methoxyjborate, sodium tetra(l-cyano-l-ethoxy methoxyjborate, sodium tetrafl-cyano- 1-phenyl methoxy)borate, sodium tetra (2-cyano-l-methoxy ethoxy)borate, sodium tetra(2-cyano-l-ethoxy ethoxy)borate, sodium tetra(methyl)borate, sodium (trifluoro methanesulfonyljborate (i.e., NaBFafSOaCHa]), sodium (trifluoro hexafluoroisopropoxy)borate, sodium (trifluoro trifluoroethoxy)borate, sodium (trifluoro pentafluoropropoxy)borate, sodium (trifluoro heptafluorobutoxy)borate, sodium (trifluoro trifluoromethyl)borate, sodium (difluoro bis- trifluoromethyljborate, sodium (trifluoro dicyanamidejborate, sodium (trifluoro phenoxyjborate, sodium (trifluoro pyrrole)borate, sodium (trifluoro pyrazolejborate, sodium (trifluoro imidazole)borate, sodium (trifluoro triazole)borate, sodium (trifluoro trimethylsilyl)borate, sodium (trifluoro trimethylsiloxyjborate, sodium (trifluoro bis[trimethylsilyl]amide)borate, sodium (trifluoro 2,3-dicyanoimidazole)borate, sodium (trifluoro cyanatejborate, sodium (trifluoro thiocyanatejborate, sodium (trimethyl cyanatejborate, sodium (trimethyl thiocyanate)borate, sodium (trimethyl methanesulfonyl)borate, sodium (trimethyl hexafluoroisopropoxyjborate, sodium (trimethyl trifluoroethoxy)borate, sodium (trimethyl phenoxyjborate, sodium (trimethyl pyrrolejborate, sodium (trimethyl pyrazolejborate, sodium (trimethyl imidazolejborate, sodium (trimethyl triazolejborate, sodium (trimethyl 2,3-dicyanoimidazole)borate, sodium (trimethylAttorney Ref: 41873-64438 trimethylsilyl)borate, sodium (trimethyl trimethylsiloxy)borate, sodium (trimethyl bis[trimethylsilyl]amide)borate.

8. The battery cell of any one of claims 1-7, wherein the at least one solvent is selected from an ether solvent, an ester solvent, a carbonate solvent, a fluoro-ether solvent, a phosphate solvent, a phosphite solvent, a fluoro-ester solvent, an anhydride solvent, an amide solvent, a cyclic-ether solvent, a cyclic-ester solvent, a cyclic-sulfate solvent, a cyclic-carbonate solvent, a nitrile solvent, a cyclic-phosphate solvent, a sulfate solvent, a sulfone solvent, and a sultone solvent.

9. The battery cell of any one of claims 1-8, wherein the at least one solvent is selected from1.2-dimethoxyethane (monoglyme), diethylene glycol dimethyl ether (diglyme), triethylene glycol dimethyl ether (triglyme), tetraethylene glycol dimethyl ether (tetraglyme), dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethylene carbonate, propylene carbonate, triethyl phosphate, trimethyl phosphate, triethyl phosphite, trimethyl phosphite, fluoroethylene carbonate, ethyl butyrate, gamma-butyrolactone, gamma-valerolactone, triacetin, tetrahydrofuran, functionalized tetrahydrofuran, 2-methyl tetrahydrofuran, 18- crown-6-ether, 12-crown-4-ether, 12-crown-4-ether, 1,3-dioxolane, 1,3-dioxane, 1,4- dioxane, acetic anhydride, N,N-dimethylformamide, dimethyl maleate, diethyl maleate, diallyl maleate, dimethyl malonate, diethyl malonate, dimethyl oxalate, diethyl oxalate, anisole, ethyl phenyl ether, methoxy perfluorobutane, methoxy perfluoropropane, ethoxy perfluorobutane, ethoxy perfluoropropane, diethyl ether, dipropyl ether, dibutyl ether, ethylene glycol diethyl ether, ethylene glycol dipropyl ether, ethylene glycol dibutyl ether, diethoxymethane (ethylal), 1,1-diethoxyethane, diethylene glycol divinyl ether, triethyl orthoformate, 1,1,1-triethoxyethane, 1,1,1-triethoxypropane, sulfolane, 3-methyl sulfolane, sulfolene, 1,3-propane sultone, 1,4-butane sultone, 1,3,2-dioxathiolane 2,2-dioxide, 1,3,2- dioxathiane 2,2-dioxide, tetra methylene sulfoxide, 2,2-dimethoxypropane, butyric anhydride, isobutyric anhydride, l,l,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, perfluoro-15-crown-5-ether, l,l,3,3,3-pentafluoro-2-trifluoromethylpropyl methyl ether,1.1.2.2-tetrafluoroethyl 2,2,2-trifluoroethyl ether, methyl(2,2,2-trifluoroethyl)carbonate,Attorney Ref: 41873-644382,2,3,3,3-pentafluoropropyl-l,l,2,2-tetrafluoro-ethyl ether, fluorinated triethylene glycol monobutyl ether, (trifluoromethoxy)benzene, 4-toluene trifluoromethyl ether, bis-(4- fluorophenyl) ether, bis-(4-trifluoromethylphenyl) ether, 1,1,2,3,3,3-hexafluoropropyl methyl ether, 1,1,2,3,3,3-hexafluoropropyl ethyl ether, 1,1,2,3,3,3-hexafluoropropyl propyl ether, 1,1,2,3,3,3-hexafluoropropyl butyl ether, hexafluoroisopropyl methyl ether, hexafluoroisopropyl ethyl ether, hexafluoroisopropyl propyl ether, hexafluoroisopropyl isopropyl ether, hexafluoroisopropyl butyl ether, trifluoroethyl methyl ether, trifluoroethyl ethyl ether, trifluoroethyl propyl ether, trifluoroethyl butyl ether, 2,2,2-trifluoroethyl acetate, ethyl trifluoroacetate, propyl trifluoroacetate, isopropyl trifluoroacetate, butyl trifluoroacetate, 2,2,2-trifluoroethyl difluoromethyl ether, methyl cyanoformate, ethyl cyanoformate, methoxyacetonitrile, ethoxyacetonitrile, 2-phenoxyacetonitrile, methylsulfonylacetonitrile, tris(2,2,2-trifluoroethyl) phosphite, tris(2,2,2-trifluoroethyl) phosphate, dimethyl sulfate, diethyl sulfate, dimethyl sulfone, diethyl sulfone, divinyl sulfone, dipropyl sulfone, dibutyl sulfone, or any combination thereof.

10. The battery cell of any one of claims 1-9, wherein: the at least one sodium borate salt is selected from sodium tetra(l,l,l-trifluoro isopropoxyjborate, sodium tetra(perfluoro tertbutoxyjborate, sodium tetra(hexafluoro isopropoxyjborate and sodium tetra(trifluoro ethoxyjborate; and the at least one solvent is selected from 1,2-dimethoxyethane (monoglyme), diethylene glycol dimethyl ether (diglyme), triethylene glycol dimethyl ether (triglyme), and tetraethylene glycol dimethyl ether (tetraglyme), or any combination thereof.

11. The battery cell of any one of claims 1-10, wherein the at least one sodium borate salt comprises 3 to 70 wt% of the liquid electrolyte.

12. The battery cell of any one of claims 1-11, wherein the liquid electrolyte further comprises at least one supplemental sodium salt, or a combination of two or more supplemental sodium salts.Attorney Ref: 41873-6443813. The battery cell of claim 12, wherein the at least one supplemental salt is selected from sodium hexafluorophosphate, sodium tetrafluoroborate, sodium perchlorate, sodium bis(oxalato)borate, sodium difluoro(oxalato)borate, sodium (trifluoromethane sulfonyl)imide, sodium bis(fluoro sulfonyljimide, sodium trifluoroacetate, sodium heptafluorobutyrate, sodium pentafluoropropionate, sodium trifluoromethanesulfonate, sodium fluorosulfate, sodium nitrate, sodium difluoro(bisoxalato)phosphate, sodium difluorophosphate, sodium methanesulfonate, sodium sulfamate, sodium naphthalene-2- sulfonate, sodium bis(trimethylsilyl)amide, sodium trimethylsilanolate, sodium bis(malato)borate, and sodium difluoro(malato)borate.

14. The battery cell of any one of claims 1-13, wherein the liquid electrolyte further comprises at least one additive, or a combination of two or more additives.

15. A battery cell comprising: at least one positive electrode, wherein the positive electrode comprises: a cathode material that comprises sodium; and at least one negative electrode, wherein the negative electrode comprises: a negative current collector having a thickness between 1 pm and 50 pm (e.g., 1-30 pm, 10-20 pm, 10-15 pm, or 15-25 pm) and an average surface roughness (Ra) between 0.03 and 15 pm (e.g., 0.03-5 pm, 0.03-2 pm, 0.03-0.75 pm, 0.03-0.25 pm, 0.1-0.25 pm, or 1-5 pm); and sodium metal on at least part of the negative electrode when the battery cell is fully charged; and a liquid electrolyte coupling the at least one negative electrode to the at least one positive electrode.

16. The battery of claim 15, wherein the negative current collector is aluminum foil.Attorney Ref: 41873-6443817. The battery cell of claim 16, wherein the aluminum foil contains a surface film of aluminum oxide (AI2O3) having a thickness of 1 to 15 nm (e.g., 1-10 nm, 1-5 nm, 2-10 nm, 10-15 nm).

18. The battery of claims 16 or 17, wherein the aluminum foil is coated on at least part of its surface by a conductive coating having a thickness between 0.05 to 100 pm (e.g., 0.05-30 pm, 0.05-10 pm, 0.05-2 pm, 0.5-2 pm, or 1-5 pm).

19. The battery cell of claim 18, wherein the conductive coating on at least part of the aluminum foil has an average surface roughness (Ra) between 0.01 and 15 pm (e.g., 0.01-5 pm, 0.01-2 pm, 0.01-0.5 pm, 0.1-0.5 pm, 0.1-2 pm, 0.1-5 pm, 0.5-5 pm, or 1-4 pm).

20. The battery cell of any one of claims 18-19, wherein the conductive coating is composed of a material selected from amorphous carbon, graphitic carbon, single walled carbon nanotubes, multiwalled carbon nanotubes, carbonized polymer including polyacrylonitrile, glassy carbon, graphene, fullerene, hard carbon, soft carbon, and any combination thereof.

21. The battery cell of any one of claims 18-20, wherein the conductive coating further comprises a binder material.

22. The battery cell of claim 21, wherein the binder material is selected from polyvinylidene fluoride, polytetrafluoroethylene, sodium carboxymethyl cellulose, styrene-butadiene rubber, sodium alginate, polyacrylic acid, polyaniline, polypyrrole, and polyacrylonitrile.

23. The battery of claim 15, wherein the negative current collector is a freestanding conductive carbon film not containing aluminum foil, wherein >95 wt% (e.g., >98 wt%, >99 wt%, 100 wt%) of the negative current collector is the conductive carbon film.Attorney Ref: 41873-6443824. The battery cell of claim 23, wherein the freestanding conductive carbon film has an average surface roughness (Ra) between 0.01 and 15 pm (e.g., 0.01-5 pm, 0.01-2 pm, 0.01-0.5 pm, 0.1-0.5 pm, 0.1-2 pm, 0.1-5 pm, 0.5-5 pm, or 1-4 pm).

25. The battery cell of claim 23 or 24, wherein the freestanding conductive carbon film material is selected from amorphous carbon, graphitic carbon, single walled carbon nanotubes, multiwalled carbon nanotubes, carbonized polymer including polyacrylonitrile, glassy carbon, graphene, fullerene, hard carbon, soft carbon, or any combination thereof.

26. The battery cell of any one of claims 23-25, wherein the freestanding conductive carbon film further comprises a binder material.

27. The battery cell of claim 26, wherein the binder material is selected from polyvinylidene fluoride, polytetrafluoroethylene, sodium carboxymethyl cellulose, styrene-butadiene rubber, sodium alginate, polyacrylic acid, polyaniline, polypyrrole, and polyacrylonitrile.

28. The battery cell of any one of claims 15-27, wherein the sodium metal on at least part of the negative electrode has an average thickness between 8 pm and 88 pm (e.g., 8-65 pm, 8-55 pm, 8-45 pm, or 18-45 pm) when the energy storage device is fully charged.

29. The battery cell of any one of claims 15-28, wherein the negative electrode further comprises anode active material that is not sodium metal on at least part of the negative electrode, wherein the ratio by capacity (e.g., Ah, mAh / cm2) of anode active material to cathode active material is less than 1 (e.g., < 0.5, <0.4, <0.3, <0.2, <0.1, <0.05, or <0.01).

30. A battery pack comprising a battery cell of any one of claims 1-29.

31. An electronic device comprising the battery pack of claim 30.

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