High-performance sodium batteries at ultralow temperatures

Abstract

The present disclosure relates to a sodium-metal cell comprising (a) an anode; (b) an electrolyte comprising a base electrolyte containing diglyme and sodium hexafluorophosphate, dioxolane, and tris(trimethylsilyl)borate; and (c) a cathode comprising sodium or Na2 / 3Cu1 / 12Ni1 / 4Mn2 / 3O2, having a high mass loading of at least 15 mg cm-2, wherein the sodium-metal cell enables stable cycling at a temperature of -40 °C or lower.

Description

HIGH-PERFORMANCE SODIUM BATTERIES AT ULTRALOW TEMPERATURESCROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority to the Singapore patent application no. 10202401359R filed on 10 May 2024, the contents of which are hereby incorporated by reference in their entirety for all purposes.TECHNICAL FIELD

[0002] The present disclosure relates generally to batteries and components thereof. More particularly, the present disclosure relates to sodium-mctal cells including an ultralow- temperature electrolyte, and to high-performance sodium batteries including the sodium-metal cells.BACKGROUND

[0003] Lithium-ion batteries (LIBs) have been widely applied in electronic devices, particularly in electric vehicles. However, electric vehicles require excellent low-temperature performance to operate reliably in cold environments. Current commercial LIBs often suffer from significant capacity loss at temperatures below 0 °C. In addition, the cost of key raw materials for LIBs, such as lithium carbonate ( ITCOv). has sharply increased in recent years. Consequently, there is an urgent need to find alternatives to LIBs.

[0004] Sodium-based batteries have emerged as strong contenders due to lower costs and widespread geographical availability of sodium resources. Among these, sodium-metal batteries (SMBs) are particularly promising due to their high theoretical specific capacity (1166 mAh g-1) and a low electrochemical potential (-2.714 V vs. the standard hydrogen electrode). Furthermore, sodium possesses a lower first ionization energy compared to lithium (495.8 vs. 520.2 kJ mol1) and a lower desolvation barrier, which enhances its suitability for low- tcmpcraturc battery applications.

[0005] Despite these advantages, the low-temperature performance of SMBs remains limited by several factors, including sluggish ion conductivity in the bulk electrolyte, slow charge transfer kinetics, and the formation of sodium dendrites. Among these, the slow chargetransfer process, particularly the Na+desolvation process, is considered the rate-limiting factor in the operation of SMBs at low temperatures.

[0006] Previous research has proposed a binary-solvent electrolyte composed of diglyme and dioxolane to improve cycling performance between -20 °C and -60 °C by forming a stable solid electrolyte interface (SEI) at low temperatures. However, the use of NaOTf salts limits the electrolyte pairing with high-voltage cathodes (> 3.8 V) due to continuous corrosion of the current collector.

[0007] It is therefore desirable to provide a sodium-metal cell and battery that seeks to address at least one of the problems described hereinabove, or at least to provide an alternative solution.SUMMARY

[0008] In one aspect, there is provided a sodium-metal cell comprising (a) an anode; (b) an electrolyte comprising a base electrolyte containing diglyme and sodium hexafluorophosphate, dioxolane, and tris(trimethylsilyl)borate; and(c) a cathode having a high mass loading of at least 15 mg cm’2, wherein the sodium-metal cell enables stable cycling at a temperature of -40 °C or lower.

[0009] In various embodiments, the electrolyte is an ultralow -temperature electrolyte, operable at a temperature ranging from ambient temperature down to -86 °C.

[0010] In various embodiments, the sodium-metal cell is operable at a voltage of greater than 3.8V.

[0011] In another aspect, there is provided a high-performance sodium-metal battery comprising one or more sodium-metal cells of the present disclosure.BRIEF DESCRIPTION OF DRAWINGS

[0012] Various embodiments of the present disclosure are described hereinbelow in the detailed description with reference to the following drawings:FIG. 1 is a chart illustrating the rate capability for the DIG:DOL electrolyte and the DIG:DOLelectrolytes with different amounts of TMSB added to the electrolytes, at -40 °C.FIG. 2 is a chart illustrating the cycling performance for the DIG:DOL electrolyte and the DIG:DOL electrolytes with different amounts TMSB added to the electrolytes, at -40 °CFIG. 3A is a schematic illustration of the solvation sheath with anion-rich, easy desolvation characteristics at lower temperatures, induced by elevated ESPmax and ESPmin, and its counterp art.FIG. 3B shows the computed electrostatic potential profiles of commonly used weak and strong solvents within the system of the present disclosure which comprises diglyme (DIG).FIG. 3C shows the Nuclear Magnetic Resonance (NMR) results for electrolytes comprising (i) pure diglyme and dioxolane (pure DIG:DOL); (ii) base electrolyte comprising diglyme and sodium hexafluorophosphate, with dioxolane (DOL-based electrolyte); (iii) base electrolyte with tetrahydrofuran (THF-based electrolyte); and (iv) base electrolyte comprising diglyme and sodium hexafluorophosphate (baseline electrolyte), at -40 °C.FIG. 3D shows the NMR results for the electrolytes of FIG. 3C in an enlarged view. The circles highlight peaks representing the presence of weak solvent-anion interactions. The extent of the high field shift directly correlates with the strength of this interaction.FIG. 4A shows the atomic ratio of C Is, O Is, F Is, Na Is, and P 2p in solid electrolyte interphase (SEI) with different sputtering time in DOL-based electrolyte after cycling at -40 °C. The sputtering times are 0, 2.5, 5, 7.5, and 10 min, respectively.FIG. 4B shows the atomic ratio of C Is, O Is, F Is, Na Is, and P 2p in solid electrolyte interphase (SEI) with different sputtering time in THF-based electrolyte after cycling at -40 °C. The sputtering times are 0, 2.5, 5, 7.5, and 10 min, respectively.FIG. 4C shows the XPS spectra of O Is in the SEI for the DOL-based electrolyte.FIG. 4D shows the XPS spectra of O Is in the SEI for the THF-based electrolyte.FIG. 4E shows the XPS spectra of F Is in the SEI for the DOL-based electrolyte.FIG. 4F shows the XPS spectra of F Is in the SEI for the THF-based electrolyte.FIG. 4G is a schematic illustration of the inorganic-rich SEI composition in the DOL-based electrolyte.FIG. 4H is a schematic illustration of the organic-rich SET composition in the THF-based electrolyte.FIG. 5A shows the cycling stability of Na||Na symmetric battery at 0.5 mA cm'2for 1 hr at -40 °C.FIG. 5B shows the cycling stability of Na||Na symmetric battery at 1.0 mA cm'2for 1 hr at -40 °C.FIG. 5C is a SEM image showing the surface morphology of the sodium electrode after 10 cycles at 0.5 mA cm2and 0.5 mAh cm'2, at -40 °C using the DOL-based electrolyte.FIG. 5D is a SEM image showing the surface morphology of the sodium electrode after 10 cycles at 0.5 mA cm'2and 0.5 mAh cm'2, at -40 °C using the THF-based electrolyte.FIG. 6A shows the rate capabilities of N Na2 / 3Cu1 / 12Ni1 / 4Mn2 / 3O2 (NaNCM) full cells at -40 °C (1 C = 170 mAh g'1), using (i) baseline electrolyte with TMSB; (ii) DOL-based electrolyte with TMSB; and (iii) THF-based electrolyte with TMSB, respectively.FIG. 6B shows the long-term cycling performance of Na||NaNCM cells at charge / discharge rate of 0.2 C at -40 °C, with a high cathode loading of 15 mg cm'2, using (i) the baseline electrolyte with TMSB; (ii) DOL-based electrolyte with TMSB; and (iii) THF-based electrolyte with TMSB, respectively.FIG. 6C shows the galvanostatic charge / discharge curves at 200thcycle of the Na||NaNCM cells at charge / discharge rate of 0.2 C at -40 °C, with a high cathode loading of 15 mg cm'2, using (i) the baseline electrolyte with TMSB; (ii) DOL-based electrolyte with TMSB; and (iii) THF- based electrolyte with TMSB, respectively.FIG. 6D shows the cycling performance of Na||NaNCM cells at -78 °C and a rate of 0.05 C with a cathode loading of 15 mg cm'2, using (i) baseline electrolyte with TMSB; (ii) DOL- based electrolyte with TMSB; and (iii) THF-based electrolyte with TMSB, respectively.FIG. 6E shows the comparison of the cycling performance of sodium full cells at -40 °C withpreviously reported results.DETAILED DESCRIPTION

[0013] Reference throughout this specification to “one embodiment”, “another embodiment” or “an embodiment” (or the like) means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” or the like in various places throughout this specification arc not necessarily all referring to the same embodiment. Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided to give a thorough understanding of embodiments. One skilled in the relevant art will recognize that the various embodiments be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, some or all known structures, materials, or operations may not be shown or described in detail to avoid obfuscation.

[0014] The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. As used herein, the singular ‘a’ and ‘an’ may be construed as including the plural “one or more” unless apparent from the context to be otherwise.

[0015] The terms "about" and "approximately" as applied to a stated numeric value encompass the exact value and a reasonable variance as will be understood by one of ordinary skill in the art, and the terms “generally” and “substantially” are to be understood in a similar manner, unless otherwise specified.

[0016] As used herein, "diglyme" (DIG) refers to diethylene glycol dimethyl ether, a linear ether compound having the chemical formula C6H14O3.

[0017] As used herein, "dioxolane" (DOL) refers to 1 ,3-dioxolane (DOL), a five-membered cyclic ether comprising two oxygen atoms positioned at the 1- and 3-positions, having the chemical formula C3H6O2.

[0018] In accordance with one aspect of the present disclosure, a sodium-metal cell is provided. The sodium-metal cell comprises (a) an anode; (b) an electrolyte comprising a base electrolyte containing diglyme (DIG) and sodium hexafluorophosphate (NaPFe), dioxolane (DOL) and tris(trimethylsilyl)borate (TMSB); and (c) a cathode comprising sodium (Na) or Na2 / 3Cui / i2Nii / 4Mn2 / 3O2(NaNCM), having a high mass loading of at least 15 mg cm-2; wherein the sodium-metal cell enables stable cycling at a temperature of -40 °C or lower.

[0019] In various embodiments, the electrolyte of the present disclosure is an ultralow- temperature electrolyte. As used herein, the term “ultralow temperature” refers to temperatures ranging from -40 °C to -60 °C, and in some embodiments, to temperatures ranging from -40 °C to -86 °C. The terms “ultralow temperature” and “low temperature” are used interchangeably herein.

[0020] In various embodiments, the electrolyte is operable across a wide temperature range, including from ambient temperature down to -40 °C, or in some embodiments, from ambient temperature down to -86 °C. The ability to function at such ultralow temperatures makes the sodium-metal cell suitable for applications in extremely cold environments.

[0021] The ability of the electrolyte to operate at ultralow temperatures is attributed to the specific components contained in the electrolyte. In particular, the selection of a suitable solvent combination is one of the factors. The optimal solvent combination was identified based on ESPmax and ESPmin calculations, the details of which are further described in Example 3. Using this approach, an efficient electrolyte composition comprising diglyme (DIG) and dioxolane (DOL) (DIG:DOL) was determined. This solvent combination, DIG:DOL (denoted as DOL-based electrolyte) enhances low-temperature ion conductivity and modifies the solvation structure of the sodium-metal cell. It was found that the DIG:DOL combination reduces activation energy barrier at low temperatures, thereby improving the cycling performance of the sodium-metal cell under ultralow-temperature conditions. The DOL-based electrolyte exhibits high Na+ionic conductivity and superior kinetic performance at ultralow temperatures. Such high ionic conductivity is attributed to the formation of a reasonable solvation structure.

[0022] As demonstrated by the experimental results and theoretical calculations described in the Examples section, the electrolyte of the present disclosure exhibits a solvation sheathstructure in which the weakly solvating solvent (DOL) coordinates preferentially with more PFfi’ due to the higher ESPmax, while interactions with Na+cations are reduced due to the higher ESPmm. This unique solvation environment promotes formation of an inorganic -rich solid electrolyte interphase (SEI) and facilitates fast ion diffusion and de-solvation kinetics at ultralow temperatures. The formation of a stable SEI and the enhanced ion transport kinetics result in outstanding reversibility of Na||Na cells, achieving cycling stability exceeding 10,000 hours at -40 °C.

[0023] In various embodiments, the electrolyte comprises diglyme (DIG) and dioxolane (DOL) in a volume ratio ranging from 1:2 to 1:4. In some embodiments, the diglyme (DIG) and dioxolane (DOL) are present in the electrolyte in a volume ratio of 1 :2.

[0024] In various embodiments, tris(trimethylsilyl)borate (TMSB) is added to the DOL- based electrolyte as an additive to enhance the cycling performance of the sodium-metal cell at ultralow temperatures. FIG. 1 is a chart showing the rate capability for the DOL-based electrolyte and DOL-based electrolytes containing different amounts of TMSB at -40 °C. The rate capability test reveals that the addition of TMSB at 1% and 5% may adversely affect the rate performance. However, when TMSB is added at 3%, the rate capability remains comparable to that of the electrolyte without TMSB.

[0025] FIG. 2 is a chart illustrating the cycling performance of the DOL-based electrolyte and DOL-based electrolytes containing different amounts of TMSB at -40 °C. The results show that the addition of TMSB consistently improves cycling performance, regardless of the amount added, compared to the DOL-based electrolyte without TMSB. Notably, the electrolyte with 3% TMSB exhibits the highest initial capacity (88.7 mAh g1) and excellent capacity retention after 200 cycles (82.6%). Accordingly, in various embodiments, TMSB is present in an amount of about 3 wt.% based on the total weight of the electrolyte to achieve improved full-cell cycling performance.

[0026] Some conventional electrolytes are unsuitable for use with high-voltage cathodes, such as those operating above 3.8V, thereby limiting their applicability in high-energy systems. In contrast, the electrolyte of the present disclosure, which comprises the non-corrosive salt, sodium hexafluorophosphate (NaPFg), can be effectively utilized with high-voltage and high mass loading cathodes. The presence of the NaPFe salt broadens the chemical stability windowof the electrolyte, thereby enabling compatibility with high-voltage cathodes operating at 4.0V or higher. As a result, the cycling performance of the sodium-metal cell is enhanced, offering potential for the practical implementation of sodium-metal cells and batteries in advanced energy storage systems. In some embodiments, the sodium-metal cell comprising the electrolyte of the present disclosure is operable at a charging voltage greater than 3.8V, or in some embodiments, greater than 4.0V. In some embodiments, the sodium-metal cell is operable at a charging voltage ranging from 3.8 V to 4.0V, 3.8V to 4.1V or 4.0 to 4.1V.

[0027] In various embodiments, the sodium hexafluorophosphate (NaPF6) is present in the electrolyte at a concentration ranging from 0.2 to 1.2 M. In some embodiments, the sodium hexafluorophosphate (NaPF6) is present in the electrolyte at a concentration of 0.5M.

[0028] In various embodiments, the anode comprises sodium. In some embodiments, both the anode and the cathode comprise sodium (Na), forming a Na||Na cell configuration. In other embodiments, the anode comprises sodium (Na) and the cathode comprises Na2 / 3Cu1 / 12Ni1 / 4Mn2 / 3O2 (NaNCM), forming a Na||NaNCM cell configuration.

[0029] In various embodiments, the sodium-metal cell comprises a sodium (Na) anode and a Na2 / 3Cu1 / 12Ni1 / 4Mn2 / 3O2 (NaNCM) cathode having a high mass loading of 15 mg cm-2. The sodium-metal cell with the high mass loading cathode is capable of achieving a capacity retention of about 92.4% after 600 cycles, cycled between 2.0 and 4.0 V, at an ultralow temperature of -40 °C, with a charge and discharge rate of 0.2C (1C = 170 mAh g-1), and with a high initial specific capacity of 89.4 mAh g1.

[0030] In various embodiments, the sodium-metal cell comprises a sodium (Na) anode and a Na2 / 3Cu1 / 12Ni1 / 4Mn2 / 3O2 (NaNCM) cathode having a high mass loading of 15 mg cm2, capable of achieving a capacity retention of about 81.7% after 50 cycles, cycled between 2.0 and 4.0 V, at an ultralow temperature of -78 °C, with a charge and discharge rate of 0.05C, and an initial specific capacity of 75.3 mAh g1.

[0031] In some embodiments, the sodium-metal cell is a Na||Na cell comprising sodium (Na) as the anode and the cathode, achieving a cycling stability exceeding 10,000 hours at -40

[0032] In accordance with another aspect of the present disclosure, a high-performancesodium-metal battery is provided. The high-performance sodium-metal battery comprises one or more of the sodium-metal cells of the present disclosure.

[0033] The present disclosure provides significant advantages over conventional electrolyte systems used for sodium-metal batteries. Conventional systems typically rely on simple binary solvent mixtures and are optimized for low to moderate-voltage cathodes (2.0 V to 3.8 V), limiting their applicability in high-energy storage applications. In contrast, the present disclosure offers an electrolyte composition that not only enables reliable operation at ultralow temperatures but also supports stable cycling performance with high-voltage cathodes operating above 3.8 V and cathodes having high mass loading. The inclusion of the dioxolane (DOL) and tris(trimethylsilyl)borate (TMSB), and the use of sodium hexafluorophosphate (NaPFe) salts, strategically enhances the solvation structure and stabilizes the solid electrolyte interphase (SEI), resulting in improved ion transport kinetics, reduced activation barriers at ultralow temperatures, and superior cycling stability under demanding conditions. Accordingly, the present disclosure extends the practical utility of sodium-metal batteries, making them well-suited for high-performance energy storage applications, including those operating in extremely cold environments.

[0034] To facilitate a better understanding of the present disclosure, the following examples of specific embodiments are given. In no way should the following examples be read to limit or define the entire scope of the disclosure. One skilled in the art will recognize that the examples set out below are not an exhaustive list of the embodiments of this disclosure.EXAMPLESExample 1

[0035] Chemicals and materials

[0036] Sodium hexafluorophosphate (NaPF6, >99.8%), diglyme (DIG, >99.8%), dioxolane (DOL, >99.9%), and tetrahydrofuran (THF, 99.9%) were purchased from DoDo Chcm. Tris(trimethylsilyl)borate (TMSB, 99%) and sodium cubes (99.9%) were purchased from Sigma- Aldrich. All solvents were purified using 4 A molecular sieve.

[0037] Preparation of electrolyte

[0038] The electrolyte was prepared by dissolving 168 mg NaPFf, in 1.76 mL of solvent comprising DIG:DOL in a volume ratio of 1:2 to make a 0.5 M solution. 3 wt.% of TMSB was added to enhance the full-cell performance.

[0039] Electrolytes comprising the solvent DIG, and mixed solvents of DIG and THF in a volume ratio of 1:2 were prepared in the same manner.

[0040] All sodium surfaces were wiped clean of oil, then the surface oxidation was removed. The sodium was then rolled and punched into sodium circles to serve as the electrodes.

[0041] Preparation of Na2 / 3Cu1 / 12Ni1 / 4Mn2 / 3O2 (NaNCM) cathode

[0042] The Na2 / 3Cu1 / 12Ni1 / 4Mn2 / 3O2 (NaNCM) cathode was synthesized using a previously reported method (L. Zheng, J. Li, M. N. Obrovac, Chem. Mater. 2017, 29, 1623-1631). A mixture of 0.706 g Na2CO3, 0.398 g NiO, 0.132 g CuO, and 1.16 g MnO2 was ball-milled for 60 minutes. The resulting powder was then pressed into pellets at 10 MPa, followed by placement in a crucible and calcination in a muffle furnace at 900 °C for 12 hours. NaNCM electrodes were prepared by blending 85 wt.% NaNCM, 5 wt.% ketjenblack, and 10 wt.% polyvinylidene fluoride (PVDF) binder in N-methylpyrrolidone (NMP). The resulting slurry was coated onto a carbon-coated Al foil and dried at 100 °C overnight in a vacuum oven. The electrodes were then cut into circles with a diameter of 12 mm.Example 2

[0043] Characterization

[0044] Nuclear magnetic resonance (NMR) spectroscopy was performed using a Bruker Avance NEO 500 MHz NMR instrument. NMR tubes with coaxial inserts were employed, with the deuterated solvent placed in the insert tube and the electrolyte sample in the outer tube. All NMR data were collected at -40 °C using an internal temperature controller. For X-ray photoelectron spectroscopy (XPS) analysis, a sample was prepared by depositing 1 mAh cm2sodium (Na) onto a copper (Cu) foil at a current density of 0.5 mA cm'2. XPS spectra were acquired using an Omicron EA125 system equipped with an Al Ka (1486.7 eV) X-ray source. To prevent sample exposure to air, a home-built vacuum transfer chamber was employed for transferring samples from the glovebox to the XPS chamber. Sputtering was carried out usingan Argon ion sputtering gun operated at an energy level of 1.0 KeV under an argon pressure of 5. Ox 10"5mbar. Scanning electron microscope (SEM) images were captured using a JEOL JSM- 670 IF field emission scanning electron microscopy (FESEM).

[0045] Electrochemical Measurements

[0046] 2032 type coin cells were assembled in an argon-filled glovebox (Vigor Pte Ltd.) with moisture and oxygen concentrations maintained below 0.02 ppm and 0.5 ppm, respectively. For Na||Na batteries, two pieces of 3501 separators and 40 pL of electrolyte were used. For Na||NaNCM full cells, two pieces of 3501 separators, one piece of glass fiber type A (GF / A), and 120 pL of electrolyte were used. All galvanostatic charge / discharge cycling tests were conducted using Neware battery testers. The batteries were tested inside a low- temperature oven at -40 °C. When testing at -78 °C, a mixture of dry ice and ethanol was stored in a dewar, where all batteries were tested. Dry ice was replenished every 12 hours to ensure temperature stability.

[0047] Theoretical Calculations

[0048] All computational quantum chemistry calculations were carried out utilizing the Gaussian 16 software package. The M06-2X functional was selected due to its demonstrated accuracy in describing long-range van der Waals interactions in intermolecular interactions. Structure optimization and energy calculations employed the triple-zeta basis set (de£2- TZVP). Following geometry optimizations, all optimized structures were confirmed as potential minima, devoid of any frequency modes with imaginary eigenvalues, through frequency analyses. The implicit solvation model used was the integral equation formalism variant of the polarizable continuum model (IEFPCM), and parameters used were from DIG solvent (e = 7.3 and n2=1.982). Subsequently, the electrostatic potential (ESP) of each solvent was analyzed using the Multiwfn software.Example 3

[0049] Selection of solvent combination

[0050] In this example, sodium hexafluorophosphate (NaPFs) in diglyme (DIG) was selected as the base electrolyte at a concentration of 0.5 M to maintain reasonable ionconductivity at ultralow temperatures. To enhance the ultralow-temperature performance, a weak solvent was added. Traditionally, weak solvents are selected based on dielectric constant (s) and donor number (DN). Previous studies suggest that low-DN solvents with moderate dielectric constants promote modest ion-solvent binding and salt dissociation, favoring ultralow-temperature battery operation. However, dioxolanc (DOL) and tetrahydrofuran (THF) solvents, despite having similar e values (7.30 vs. 7.58) and DN values (21.2 vs. 20.0), exhibit differing performances at ultralow temperatures.

[0051] To investigate this phenomenon, a selection criteria using the minimum and maximum electrostatic potential (ESPmin and ESPmax) for low -temperature weak solvents is adopted. ESP represents the unbalanced charge distribution around a molecule and effectively describes the interaction between ions and solvents. ESPmin indicates the most negatively charged region, promoting stronger interactions with cations (Na+) compared to other sites around the molecule. A smaller ESPmin facilitates greater interaction between the solvent and Na+. Conversely, ESPmax represents the most positively charged site of the molecule, indicating stronger interactions with anions. A higher ESPmax results in increased interaction between the solvent and anions.

[0052] As the desolvation process is the rate-limiting step for low-temperature battery cycling, reducing the desolvation barrier emerges as the most effective method to enhance low- temperature performance. A complete ion desolvation process typically consists of two parts: ion desolvation in the electrolyte and ion adsorption on the SEI. To expedite the ion desolvation process, a relatively small ESPmin is required for weaker interaction between Na+and the solvent. Additionally, an anion-derived inorganic-rich SEI is preferable as it facilitates easier Na+adsorption on the SEI, thereby accelerating Na+desolvation. At low temperatures, weak solvents play a more significant role in the solvation sheath compared to strong solvents. If the interaction between the weak solvent and anion is strong, indicated by a higher ESPmax, more anions will participate in the solvation sheath, leading to the reduction and formation of an inorganic-rich SEI. Therefore, both higher ESPmax and ESPmin are necessary for low- tcmpcraturc batteries. FIG. 3A illustrates how two types of weak solvents with different ESPmax and ESPmin values exhibit distinct solvation sheaths.

[0053] ESPmax and ESPmin values were calculated for commonly used weak solvents (FIG. 3B), including 1 ,2-dibutoxyethane (DBE), methoxypropane (MP), methoxybutane (MB),diethyl ether (Et20), t-butylmethyl ether (TBME), 2- methyltetrahydrofuran (MeTHF), 2,5- dimethyltetrahydrofuran (diMeTHF), butyl butyrate (BB), ethyl butyrate (EB), propyl butyrate (PB), dimethoxymethane (DMM), diethoxymethane (DEM), ethyl propionate (EP), ethyl acetate (EA), and propyl acetate (PA), THF, DOL, fluorobenzene (FB), 1 ,2-difluorobenzene (2FB), and the strong solvent DIG.

[0054] In combination with the experimental results, the electrolyte containing dioxolane (DOL) with relatively higher ESPmax and decent ESPmin demonstrates the best low-temperature performance compared to other weak solvents. It is important to note that ESPmin cannot be too high. When ESPmax >> ESPmin, the solvent behaves as an anti-solvent, resulting in low ion conductivity at low temperatures as it does not participate in the Na+solvation sheath. This stronger interaction between the weak solvent and anion can be observed in the NMR spectra collected at -40 °C (FIG. 3C). The small peak between 4.4 to 3.9 ppm shown in FIG. 3D indicates the interaction between the anion and solvent, with a smaller chemical shift indicating a stronger anion-solvent interaction. Spectrum for pure DIG: DOL solvent is included to identify the small peak for the 0.5 M NaPFr, in DOL-based electrolyte. Evidently, the peak of the DOL-based electrolyte at 3.98 ppm exhibits the smallest chemical shift, indicating the strongest anion-solvent interaction compared to the 0.5 M NaPF6 in the THF-based electrolyte at 4.10 ppm and the 0.5 M NaPFr, in the baseline electrolyte at 4.27 ppm. These NMR results align with the calculation results, showing that the electrolyte containing dioxolane (DOL) can achieve anion-rich solvation sheath at ultralow temperatures.Example 4

[0055] Solid Electrolyte Interphase (SEI)

[0056] An inorganic -rich SEI can be developed during the chargc / dischargc process with the anion-rich solvation structure described in Example 3. The composition and distribution of the formed SEI after pre-cycling at -40 °C were evaluated in detail using XPS. Atomic ratios of selected elements were compared for the DOL-based electrolyte (FIG. 4A) and the THF- based electrolyte (FIG. 4B). The SEI formed with the THF-based electrolyte exhibited a higher carbon (C) content, indicating a greater presence of organic species at the surface compared to the DOL-based electrolyte. As sputtering time increased, the C atomic ratio for both electrolytes decreased gradually. However, even after 10 minutes of sputtering, the SEI of theTHF-based electrolyte still contains more organic compounds. Furthermore, the inorganic species in the SEI formed with the DOL-based electrolyte are greater than in the THF-based electrolyte at all sputtering times, as evidenced by the higher fluorine (F) content, since the inorganic compound is solely derived from NaPFt, in the system. Additionally, the atomic ratios for the DOL-based electrolyte remained stable after 10 minutes of sputtering, whereas the atomic ratios for the THF-based electrolyte changed slightly. These observations suggest that the SEI formed with the THF-based electrolyte is less uniform than that formed with the DOL- based electrolyte.

[0057] Peak fittings for XPS results were conducted to compare the SEI component differences between the two electrolytes. FIG. 4C shows the XPS spectra of O 1 s in the SEI for the DOL-based electrolyte. The spectra shows that the DOL-based electrolyte contains organic species on the surface of the SEI, including C-O-Na (531.8 eV) and Na2CO3 (533.5 eV and 534.5 eV for C=O and C-O, respectively). The peak between 537.0 to 538.0 eV corresponds to the Na KLL Auger line. As the sputtering time increases from 5 minutes, Na2O emerges and the intensity of organic species decreases gradually, indicating a less organic SEI interior. For the THF-based electrolyte, the species are similar to those in the DOL-based electrolyte, but the distribution is different, as shown in FIG. 4D. An organic surface is present on the SEI. Although the inorganic species Na2O appears early, from 2.5 minutes of sputtering time, such an uneven SEI cannot ensure uniform Na+flux to diffuse through.

[0058] Additionally, in the F Is spectra as shown in FIG. 4E, the surface of the SEI formed with the DOL-based electrolyte is dominated by organic fluorine species, including C-F (689.0 eV) and CF2-CF2 (590.0 eV), while inorganic NaF (685.5 eV) and partial decomposition species NaxPFyOz (687.0 eV) from NaPFr, are less abundant. However, after sputtering, the intensity of the organic fluorine compounds decreases dramatically, and the SEI becomes dominated by the inorganic fluorine species, suggesting the formation of an inorganic fluorine- rich SEI interior. This indicates that a higher proportion of PFe" anions are present in the solvation sheath, leading to enhanced decomposition of PF<> and formation of a fluorine-rich SEI. In comparison, for the THF-based electrolyte as shown in FIG. 4F, the fluorine species are similar, but the overall intensity of each peak is smaller, and the distribution among species is more uneven, suggesting less PF6" decomposition, resulting in a fluorine-lean SEI.

[0059] Combining the element ratio analysis with the peak fitting results, it can be deducedthat the DOL-based electrolyte promotes the formation of an inorganic-rich, uniform SEI 401 during battery cycling (FIG. 4G). This SEI enhances rapid charge transfer, desolvation processes, and Na+kinetics through the SEI. Moreover, the derived inorganic -rich SEI layer exhibits high interfacial energy with sodium metal, facilitating the two-dimensional diffusion of Na+along the sodium metal surface. In contrast, the THF-bascd electrolyte only forms an organic -rich, non-uniform SEI 402 (FIG. 4H), which is associated with sluggish charge transfer and Na+migration kinetics and may lead to rough sodium plating and growth of sodium dendrites. These differences in SEI composition will lead to differences in battery performance at ultralow temperatures.Example 5

[0060] Ultralow-temperature reversible battery performance

[0061] Owing to its ability to lower the activation barrier and form an inorganic-rich SEI at low temperatures, the DOL-based electrolyte is expected to achieve remarkable ultralow- temperature reversible battery performance. In Na||Na symmetric cells, stable cycling was demonstrated for over 10,000 hours with a small overpotential of 8 mV, under a current density of 0.5 mA cm'2and a capacity of 0.5 mAh cm'2at -40 °C (FIG. 5A). In contrast, the baseline electrolyte failed rapidly due to its low ionic conductivity at the onset of cycling. For the THF- based electrolyte, the initial voltage profile exhibited instability, likely due to the formation of the SEI. The overpotential decreased to 15 mV after 50 hours of activation but subsequently after 2,000 hours of cycling, the overpotential gradually increased and increased to 80 mV by 4,600 hours. This aligns with the XPS analysis, indicating that the SEI formed with the THF- based electrolyte is thin and non-uniform, unable to effectively block the side reactions between the sodium metal anode and the electrolyte, thereby leading to electrolyte consumption. Therefore, under harsher conditions of -40 °C, 1 .0 mA cm'2, and 1 .0 mAh cm'2, the Na||Na cell with the THF-based electrolyte still failed at the onset of cycling, even with a longer activation time due to the larger activation barrier (FIG. 5B). In contrast, the DOL-based electrolyte continues to exhibit superior stability for over 7,000 hours, with a low overpotential of 22 mV.

[0062] The morphology of the sodium metal surface after 10 cycles of sodium plating / stripping at -40 °C, 0.5 mA cm'2, and 0.5 mAh cm2was characterized by scanning electron microscopy (SEM). For the DOL-based electrolyte, smooth surfaces and bulk sodiummetal deposition with a diameter of 10 to 15 pm were obtained (FIG. 5C). The main reason for this flat deposition is the formation of a uniform inorganic SEI. In sharp contrast, needle-like sodium dendrites can be observed on the sodium surface using the THF-based electrolyte (FIG. 5D). This rough deposition is primarily derived from the non-uniform organic SEI formed on the sodium surface. Sodium dendrites with a large specific surface area can also accelerate the side reactions between sodium and the solvent, ultimately leading to the rapid failure of the cell.Example 6

[0063] Effects of tris(trimcthylsilyl )borale (TMSB ) on the electrolyte

[0064] Na||NaNCM full cells were tested to demonstrate the practical application of the electrolyte. To suppress dioxolane (DOL) polymerization, an additional tris(trimethylsilyl)borate (TMSB) was added for improved cycling performance. In the rate capability test at - 40 °C, the DOL-based electrolyte exhibited the highest specific discharge capacity across all rates ranging from 0.2 C to 5.0 C (1 C = 170 mAh g-1), surpassing both the baseline and the THF-based electrolytes. Even at 5.0 C, a high specific discharge capacity of 20 mAh g1was achieved (FIG. 6A).

[0065] FIG. 6B shows the long-term cycling performance of Na||NaNCM full cells at charge / discharge rate of 0.2 C at -40 °C, with a high mass cathode loading of 15 mg cm2. When paired with the high mass loading cathode, the full cell using DOL-based electrolyte achieved an impressive initial specific capacity of 89.4 mAh g-1and maintained 92.4% capacity retention after 600 cycles, along with a high average Coulombic efficiency (CE) of 99.7% at the 0.2 C charge / discharge rate. In comparison, after activation, the THF-based electrolyte attained a specific discharge capacity of 68.7 mAh g4, with a lower capacity retention of 89.6% and an average CE of 99.1% after 600 cycles, which was lower than the DOL-based electrolyte. The baseline electrolyte exhibited a lower specific discharge capacity compared to the other electrolytes and quickly short-circuited at 350 cycles.

[0066] Figure 6C illustrates the charge-discharge curves after 200 cycles. The baseline electrolyte exhibited a high overpotential, resulting in a low charge / discharge capacity. In contrast, the DOL-based electrolyte demonstrated a significantly smaller overpotential, leading to a higher charge / discharge capacity. These results axe consistent with the XPS analysis.

[0067] To further demonstrate the superiority of the electrolyte of the present disclosure, full cell galvanostatic cycling at an even lower temperature of -78 °C, with a high mass loading cathode of 15 mg cm-2, was tested. FIG. 6D shows the cycling performance of the Na||NaNCM cells at -78 °C and a rate of 0.05 C, with a cathode loading of 15 mg cm'2. At a charge / discharge rate of 0.05 C, the sodium-mctal cell with the baseline electrolyte quickly short-circuited, and the sodium-metal cell with the THF-based electrolyte showed almost no capacity. Conversely, the sodium-mctal cell with the DOL-bascd electrolyte exhibited a decent initial discharge capacity of 75.3 mAh g’1, and even after 50 cycles, a capacity retention of 81 .7% was achieved.

[0068] FIG. 6E is a chart showing a comparison of the cycling performance of sodium-metal cells at -40 °C with previously reported results. The chart shows that the present work is the first to pair an ultralow-temperature electrolyte with a high mass loading cathode (15 mg cm'2), achieving high capacity retention after long-term cycling among all reported full-cell performances at -40 °C. The high-performance full cells, employing a high mass loading cathode of 15 mg cm'2, and cycling at -40 °C and even -78 °C, demonstrate the commercial viability of the low-temperature or ultralow-temperature electrolyte of the present disclosure. This achievement represents a significant advancement over previously reported low- temperature sodium-metal cells and / or batteries, which typically suffer from poor capacity retention, unstable cycling, or require lower cathode loadings.

[0069] In summary, the results show that the introduction of dioxolane (DOL) weak solvent to the baseline electrolyte with both high ESPmax and ESPmm results in the formation of an anion-rich solvation sheath. This anion-rich solvation sheath facilitates the formation of an inorganic-rich SEI, consequently lowering the desolvation barrier and improving ultralow- temperature cycling performance. The Na||Na symmetric cell with the DOL-based electrolyte exhibited extended cycling times exceeding 10,000 hours, with a low overpotential of 8 mV at -40 °C. Moreover, the Na||NaNCM full-cell using the DOL-based electrolyte with a high cathode loading of 15 mg cm'2demonstrated stable cycling for over 600 cycles at -40 °C and 50 cycles at -78 °C, respectively. The present disclosure presents an effective and straightforward electrolyte modification approach, paving the way for enhanced high-voltage sodium batteries in extremely cold environments. It also offers insights into the development of largescale sodium battery storage systems.

[0070] Although embodiments of the invention have been shown and described, the invention is not limited to the described embodiments. Instead, it would be appreciated by those skilled in the art that various modifications and variations can be made to the embodiments of the invention without departing from the scope of the invention, the scoop of which is set forth in the following claims.

Claims

CLAIMS1. A sodium-metal cell comprising:(a) an anode;(b) an electrolyte comprising: a base electrolyte containing diglyme and sodium hexafluorophosphate; dioxolane; and tris(trimcthylsilyl)boratc; and(c) a cathode comprising sodium or Na2 / 3Cu1 / 12Ni1 / 4Mn2 / 3sO2. having a high mass loading of at least 15 mg cm'2, wherein the sodium-metal cell enables stable cycling at a temperature of -40 °C or lower.

2. The sodium-metal cell according to claim 1, wherein the electrolyte is an ultralow- temperature electrolyte, operable at a temperature ranging from ambient temperature down to -86 °C.

3. The sodium-metal cell according to claim 1 or 2, wherein the tris(trimethylsilyl)borate is present in an amount of 3 wt.%, based on the total weight of the electrolyte.

4. The sodium-metal cell according to claim 1, wherein the electrolyte comprises diglyme and dioxolanc in a volume ratio ranging from 1:2 to 1:4.

5. The sodium-metal cell according to claim 1 or 3, wherein the sodium-metal cell is operable at a voltage of greater than 3.8V.

6. The sodium-metal cell according to claim 1, wherein the sodium-metal cell is characterized by a capacity retention of 92.4% after 600 cycles, cycled between 2.0 and 4.0 V, at an ultralow temperature of -40 °C, with a charge and discharge rate of 0.2 C, and an initial specific capacity of 89.4 mAh g4.

7. The sodium-metal cell according to claim 1 , wherein the sodium-metal cell is characterized by a capacity retention of 81.7% after 50 cycles, cycled between 2.0 and 4.0 V,at an ultralow temperature of -78 °C, with a charge and discharge rate of 0.05C and an initial specific capacity of 75.3 mAh g’1.

8. The sodium-metal cell according to claim 1, wherein the anode comprises sodium.

9. The sodium-metal cell according to claim 1, wherein the sodium hexafluorophosphate is present in the electrolyte at a concentration of 0.5 M.

10. A high-performance sodium-metal battery comprising one or more sodium-metal cells as defined in any one of the preceding claims.

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