Novel electrolyte composition for sodium-ion batteries

Abstract

The present invention relates to an electrolyte composition comprising at least a sodium salt dissolved in at least one solvent and a combination of additives, wherein: the solvent is selected from the group consisting of ethylene glycol carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, propylene carbonate, ethyl acetate, ethyl propionate, methyl propionate, 4-fluorotoluene, 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, difluoroethylene glycol carbonate, ethyl difluoroacetate and mixtures thereof; and the combination of additives comprises at least sodium difluoro(oxalato)borate (NaODFB) and tris(trimethylsilyl)phosphite (TMSPi).

Description

[0001] The present invention relates to a novel electrolyte composition for sodium-ion batteries and its use as a non-aqueous liquid electrolyte in sodium-ion batteries.

[0002] Due to the high natural abundance and low cost of sodium compared to lithium, sodium ion (Na-ion) technology appears to be a promising alternative candidate for the next generation of batteries, especially in the field of stationary energy storage. This has led to a booming research on various components of Na-ion batteries in the past decade, and the development of new and excellent electrode materials for optimized Na-ion battery systems. 0.67 Mn 0.5 Fe 0.5 Of the two O2 technologies, the former has the highest performance in terms of power, cycle life and energy density.

[0003] However, for practical applications, other performance indicators, such as high temperature performance (55°C), self-discharge and durability, depend heavily on the solid electrolyte interface (SEI) that must be met. This requires in-depth research on the properties of the electrolyte used. In the early stages, researchers naively thought that we could simply extrapolate what is used for Li-ion technology, but this did not take into account that the solubility of sodium-based organic products is different from that of their lithium counterparts due to the Na + The acidity ratio of Li + Gentler.

[0004] In view of such discussion, the inventors experienced that the Na3V2(PO4)2F3 / C battery based on the classic EC-PC-DMC-1 mol / L NaPF6 electrolyte performed excellently at room temperature, but exhibited poor cycling and self-discharge performance at 55°C.

[0005] To date, the inventors have implemented specific electrolytes based on a mixture of EC-PC and NaPF6, to which some additives were added; sodium difluoro(oxalato)borate (NaODFB), sulfonitrile N≡C-(CH2)2-C≡N, 1,3-propane sultone and vinylene carbonate.

[0006] However, this electrolyte has some disadvantages, such as being unable to be used with commercial polyolefin separators due to poor wettability, high viscosity, and reduced battery power capability (highly resistive SEI).

[0007] There remains a need for improved electrolyte compositions that alleviate the above-mentioned limitations while retaining the advantages.

[0008] It was therefore an object of the present invention to provide novel electrolyte compositions for Na-ion batteries which have improved properties compared to known electrolyte compositions.

[0009] The present invention also aims to provide a non-aqueous liquid electrolyte that can minimize parasitic reactions to achieve high capacity retention and reduced self-discharge of Na-ion batteries at cycling temperatures ranging from ambient temperature (i.e., 20°C-25°C) to higher temperatures, in particular up to 55°C.

[0010] Another object of the present invention is to provide an electrolyte composition for Na-ion batteries having improved wettability properties.

[0011] Another object of the present invention is to provide an electrolyte composition for Na-ion batteries that can be used in combination with commercial polyolefin separators.

[0012] Another object of the present invention is to provide an electrolyte composition for Na-ion batteries that allows tuning of the SEI thickness.

[0013] The present invention therefore relates to an electrolyte composition comprising at least a sodium salt dissolved in at least one solvent, and a combination of additives, wherein:

[0014] - the solvent is selected from the group consisting of ethylene glycol carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, propylene carbonate, ethyl acetate, ethyl propionate, methyl propionate, 4-fluorotoluene, 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, difluoroethylene glycol carbonate, ethyl difluoroacetate and mixtures thereof;

[0015] - A combination of additives comprising at least sodium difluoro(oxalato)borate (NaODFB) and tris(trimethylsilyl)phosphite (TMSPi).

[0016] The electrolyte of the present invention is based on the use of two additives, NaODFB and TMSPi. The uniqueness of this new electrolyte is the use of TMSPi.

[0017] This new electrolyte is implemented by finding effective additives to appropriately and judiciously tune the SEI thickness and composition.

[0018] The electrolyte compositions of the present invention combine the positive attributes of earlier Na-based electrolytes (wettability) with the benefits provided by additives (tuning of SEI thickness) achieved through specific chemical adjustments.

[0019] According to an embodiment, the amount of sodium difluoro(oxalato)borate in the electrolyte composition of the present invention ranges from 0.05 wt.% to 10 wt.% relative to the total weight of the electrolyte composition.

[0020] According to an embodiment, the amount of tris(trimethylsilyl)phosphite in the electrolyte composition of the present invention ranges from 0.05 wt. % to 10 wt. % relative to the total weight of the electrolyte composition.

[0021] Preferably, the solvent as defined above is selected from the group consisting of ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, propylene carbonate, ethyl propionate, 4-fluorotoluene, 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether and mixtures thereof.

[0022] According to a particular embodiment of the present invention, the solvent is a mixture of at least two solvents comprising ethylene carbonate (EC) as a first solvent and a second solvent selected from the group consisting of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), propylene carbonate (PC), ethyl propionate (EP) and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE).

[0023] Preferably, the second solvent is PC or DMC, more preferably PC.

[0024] The volume ratio of the first solvent to the second solvent may be in the range of 1:20 to 20:1 and preferably 1:9 to 5:1.

[0025] According to the invention, a mixture of ethylene carbonate and propylene carbonate in a volume ratio of 1:1 is particularly preferred.

[0026] According to an embodiment, the electrolyte composition of the present invention includes sodium difluoro(oxalato)borate, tris(trimethylsilyl)phosphite, ethylene glycol carbonate, and propylene carbonate.

[0027] According to an embodiment, the electrolyte composition of the present invention further includes dimethyl carbonate.

[0028] According to an embodiment, the electrolyte composition of the present invention includes sodium difluoro(oxalato)borate, tris(trimethylsilyl)phosphite, ethylene glycol carbonate, and dimethyl carbonate.

[0029] According to an embodiment, the electrolyte composition of the present invention further comprises an additional carbonate selected from vinylene carbonate and ethylene glycol carbonate, preferably vinylene carbonate (VC).

[0030] Preferably, the amount of the additional carbonate in the electrolyte composition of the present invention ranges from 0.1 wt.% to 10.0 wt.% relative to the total weight of the electrolyte composition.

[0031] In particular, the addition of VC to the electrolyte composition of the present invention further improves electrolyte performance due to the synergistic effect between VC and TMSPi previously tested with lithium-ion technology. In addition, VC helps minimize the degradation of DMC by forming an elastomer, thereby inhibiting the continuous reduction of DMC and limiting the amount of soluble material that migrates to the positive electrode.

[0032] According to the present invention, the nature of the sodium salt is not so critical. The sodium salt can be selected from the commonly used salts in non-aqueous electrolytes suitable for Na-ion batteries. As examples of sodium salts, one of sodium hexafluorophosphate (NaPF6), sodium perchlorate (NaClO4), sodium bis(fluorosulfonyl)imide (NaFSI), sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), sodium bis(pentafluoroethylsulfonyl)imide (NaBETI), sodium tetrafluoroborate (NaBF4) and mixtures thereof can be mentioned.

[0033] Preferably, the sodium salt is NaPF6.

[0034] The amount of the sodium salt in the electrolyte composition may range from about 0.1 mol / L to 3.0 mol / L and preferably from about 0.5 mol / L to 2.0 mol / L.

[0035] As an example of the electrolyte combination of the present invention, the following electrolyte composition may be mentioned, which comprises:

[0036] - NaODFB in an amount of 0.5 wt.%,

[0037] - TMSPi in an amount of 1.0 wt.%,

[0038] - sodium salts, especially 1 mol / L NaPF6, and

[0039] -EC / PC at a volume ratio of 1:1.

[0040] As another specific example of the electrolyte composition of the present invention, the following electrolyte composition may be mentioned, which includes:

[0041] - NaODFB in an amount of 0.5 wt.%,

[0042] - TMSPi in an amount of 1.0 wt.%,

[0043] - vinylene carbonate in an amount of 3.0 wt.%,

[0044] - sodium salts, especially 1 mol / L NaPF6, and

[0045] -EC / PC at a volume ratio of 1:1.

[0046] As another example of the electrolyte composition of the present invention, the following electrolyte composition may be mentioned, which comprises:

[0047] - NaODFB in an amount of 0.5 wt.%,

[0048] - TMSPi in an amount of 1.0 wt.%,

[0049] - sodium salts, especially 1 mol / L NaPF6, and

[0050] -EC / DMC at a volume ratio of 1:1.

[0051] As another specific example of the electrolyte composition of the present invention, the following electrolyte composition may be mentioned, which includes:

[0052] - NaODFB in an amount of 0.5 wt.%,

[0053] - TMSPi in an amount of 1.0 wt.%,

[0054] - vinylene carbonate in an amount of 3.0 wt.%,

[0055] - sodium salts, especially 1 mol / L NaPF6, and

[0056] -EC / DMC at a volume ratio of 1:1.

[0057] The electrolyte composition of the present invention can be prepared by any method known in the art, in particular by mixing the first solvent and the second solvent in a desired volume ratio, preferably under stirring, and then adding the sodium salt and the combination of additives to the solution. The order of addition can be changed, such as adding the combination of additives to the mixture of solvents and then adding the sodium salt. The entire process is preferably carried out under an inert atmosphere, for example, under argon or nitrogen.

[0058] As already mentioned, the electrolyte composition of the invention is particularly suitable for use in Na-ion batteries.

[0059] Therefore, the present invention also relates to the use of the electrolyte composition as defined above as a non-aqueous liquid electrolyte in a Na-ion battery, in particular in a Na-ion battery comprising a hard carbon negative electrode comprising a binder, in particular carboxymethyl cellulose.

[0060] The present invention also relates to the use of the electrolyte composition as defined above as a non-aqueous liquid electrolyte to reduce self-discharge and increase retention capacity in Na-ion batteries, in particular in Na-ion batteries comprising a hard carbon negative electrode comprising a binder, in particular carboxymethyl cellulose.

[0061] In accordance with this objective, a reduction in self-discharge is observed when the battery is cycled at temperatures ranging from about room temperature (20-25°C) to 55°C.

[0062] The present invention also relates to a Na-ion battery comprising:

[0063] - at least one positive electrode comprising at least one positive electrode active material and a current collector,

[0064] - at least one negative electrode comprising a negative electrode active material, and

[0065] - at least one separator impregnated with a non-aqueous liquid electrolyte, said separator being placed between said positive electrode and said negative electrode,

[0066] The non-aqueous liquid electrolyte is the electrolyte composition defined above.

[0067] The positive electrode active material is a material that can reversibly insert sodium ions and can be selected from: oxides such as Na x MO2, wherein M represents at least one metal element selected from the group consisting of Ni, Co, Mn, Fe, Cr, Ti, Cu, V, Al and Mg; and phosphates such as NaTi2(PO4)3, Na3V2(PO4)3, Na3V2(PO4)2F3, Na2MnP2O7, Na2MnPO4F, Na 1.5 VPO 4.8 F 0.7 and NaV 1-x Cr x Among these positive electrode active materials, Na3V2(PO4)2F3 (also known as NVPF), Na 2 / 3 Ni 1 / 3 Mn 2 / 3 O2 and mixtures thereof are particularly preferred.

[0068] In addition to the positive electrode active material, the positive electrode may further include a polymer binder and optionally an electronically conductive agent.

[0069] As examples of polymer binders, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), cellulose fibers, cellulose derivatives such as starch, carboxymethyl cellulose (CMC), diacetyl cellulose, hydroxyethyl cellulose or hydroxypropyl cellulose, styrene butadiene rubber (SBR) and mixtures thereof may be mentioned. Among these binders, PVdF is preferred.

[0070] The conductive agent may be carbon black, Super P carbon black, acetylene black, Ketjen black, channel black, natural graphite or synthetic graphite, carbon fiber, carbon nanotube, vapor-grown carbon fiber, or a mixture thereof.

[0071] The weight ratio relative to the total weight of the positive electrode is preferably:

[0072] - Positive electrode active material: 80% to 98%

[0073] -Electronic conductive agent: 1% to 10%

[0074] -Polymer binder: 1% to 10%.

[0075] The negative electrode active material for the negative electrode may be selected from carbon materials, in particular hard carbon, soft carbon, carbon nanofibers or carbon felt, antimony, tin and phosphorus.

[0076] According to a preferred embodiment of the present invention, the negative electrode active material is a carbon material and the negative electrode further comprises a polymer binder, which may be selected from the same polymer binders as those mentioned above for the positive electrode, and is preferably selected from cellulose derivative binders.

[0077] As mentioned for the positive electrode, the negative electrode may further comprise a conductive agent, which may be selected from the same conductive agents as those mentioned above for the positive electrode.

[0078] The negative electrode may also include a current collector.

[0079] The current collectors of the positive and negative electrodes may be composed of an electrically conductive material, more particularly a metallic material that may be chosen from aluminum, copper, nickel, titanium and steel.

[0080] The partition may be a The separators include conventional polymer based separators of polypropylene and / or polyethylene or such Glass fibers of borosilicate glass fiber separators or such Cellulose-based separators of nonwoven nanofiber separators.

[0081] The Na-ion battery according to the present invention can consist of a single electrochemical cell comprising two electrodes (i.e., one positive electrode and one negative electrode) separated by an electrolyte; or of a plurality of chemical cells assembled in series; or of a plurality of chemical cells assembled in parallel; or of a combination of both types of assembly. BRIEF DESCRIPTION OF THE DRAWINGS

[0082] Figure 1 The capacity (in mAh g based on the mass of the positive electrode active material) of a Na-ion battery comprising comparative composition EC1 as the non-aqueous liquid electrolyte is given. -1 Evolution of voltage (V) as a function of the meter.

[0083] Figure 2The capacity (in mAh g based on the mass of the positive electrode active material) as a function of the cycle number is given for a Na-ion battery comprising comparative composition EC1 as non-aqueous liquid electrolyte. -1 The evolution of planning.

[0084] Figure 3 The capacity (based on the mass of the positive electrode active material, in mAh g) of a Na-ion battery comprising composition EC2 as the non-aqueous liquid electrolyte is given. -1 Evolution of voltage (V) as a function of the meter).

[0085] Figure 4 The capacity (in mAh g based on the mass of the positive electrode active material) as a function of the cycle number is given for Na-ion batteries comprising composition EC2 as the non-aqueous liquid electrolyte. -1 The evolution of planning.

[0086] Figure 5 The capacity (in mAh g based on the mass of the positive electrode active material) is given for a Na-ion battery comprising the composition EC3 according to the invention as non-aqueous liquid electrolyte. -1 Evolution of voltage (V) as a function of the meter).

[0087] Figure 6 The capacity (in mAh g based on the mass of the positive electrode active material) as a function of the number of cycles is given for Na-ion batteries comprising the composition EC3 according to the invention as non-aqueous liquid electrolyte. -1 The evolution of planning.

[0088] Figure 7 The capacity (based on the mass of the positive electrode active material, in mAh g) of a Na-ion battery comprising comparative composition EC4 as the non-aqueous liquid electrolyte is given. -1 Evolution of voltage (V) as a function of the meter).

[0089] Figure 8 The capacity (in mAh g based on the mass of the positive electrode active material) as a function of the cycle number is given for Na-ion batteries comprising comparative composition EC4 as non-aqueous liquid electrolyte. -1 The evolution of planning.

[0090] Figure 9 The capacity (based on the mass of the positive electrode active material, in mAh g) of a Na-ion battery comprising comparative composition EC5 as the non-aqueous liquid electrolyte is given. -1 Evolution of voltage (V) as a function of the meter).

[0091] Figure 10The capacity (in mAh g based on the mass of the positive electrode active material) as a function of the cycle number is given for Na-ion batteries comprising comparative composition EC5 as non-aqueous liquid electrolyte. -1 The evolution of planning.

[0092] Figure 11 The capacity (based on the mass of the positive electrode active material, in mAh g) of a Na-ion battery comprising comparative composition EC6 as the non-aqueous liquid electrolyte is given. -1 Evolution of voltage (V) as a function of the meter).

[0093] Figure 12 The capacity (in mAh g based on the mass of the positive electrode active material) as a function of the cycle number is given for Na-ion batteries comprising comparative composition EC6 as non-aqueous liquid electrolyte. -1 The evolution of planning.

[0094] Figure 13 The capacities (in mAh g based on the mass of the positive electrode active material) are given for Na-ion batteries comprising the composition EC7 according to the invention as non-aqueous liquid electrolyte. -1 Evolution of voltage (V) as a function of the meter).

[0095] Figure 14 The capacity (in mAh g based on the mass of the positive electrode active material) as a function of the number of cycles is given for Na-ion batteries comprising the composition EC7 according to the invention as non-aqueous liquid electrolyte. -1 The evolution of planning.

[0096] Figure 15 The capacity (in mAh g based on the mass of the positive electrode active material) is given for a Na-ion battery comprising the composition EC8 according to the invention as non-aqueous liquid electrolyte. -1 Evolution of voltage (V) as a function of the meter).

[0097] Figure 16 The capacity (in mAh g based on the mass of the positive electrode active material) as a function of the number of cycles is given for Na-ion batteries comprising the composition EC8 according to the invention as non-aqueous liquid electrolyte. -1 The evolution of planning.

[0098] Figure 17 Images are given of comparative electrolyte composition EC1 on a PE separator.

[0099] Figure 18 Images of the electrolyte composition EC4 according to the invention on a PE separator are given. Example

[0100] Example 1: Preparation of electrolyte composition

[0101] The following electrolyte compositions, EC1 (comparative composition) and EC2 and EC3 according to the present invention, comprising the additives detailed in the table below (in wt.%), have been prepared in a mixture of solvent EC / PC in a volume ratio of 1:1 containing 1 mol / L NaPF6:

[0102] Electrolyte composition NaODFB TMSPi vinylene carbonate EC1 (comparison) - - - EC2 (present invention) 0.5 1.0 - EC3 (present invention) 0.5 1.0 3.0

[0103] Electrolyte compositions EC1 and EC2 have been prepared by adding additives to EC / PC solvent containing 1 mol / L of NaPF6.

[0104] Some other compositions (comparative compositions and compositions according to the invention) were also prepared. They contained the additives detailed in the table below (in wt.%) and were prepared in a mixture of solvent EC / DMC in a volume ratio of 1:1 containing 1 mol / L NaPF6:

[0105] Electrolyte composition NaODFB TMSPi vinylene carbonate EC4 (comparison) - - - EC5 (comparison) - 1.0 - EC6 (comparison) - - 3.0 EC7 (present invention) 0.5 1.0 - EC8 (present invention) 0.5 1.0 3.0

[0106] Example 2: Electrochemical performance of sodium-ion batteries containing the electrolyte composition of the present invention

[0107] The effects of all the above mentioned electrolyte compositions have been tested in button cells with the following configuration.

[0108] By adding Na3V2(PO4)2F3(NVPF, mass loading: 12.6 mg / cm 2 A positive electrode was prepared by mixing PVdF (4.0 wt.%) with carbon black (4.0 wt.%). The resulting mixture was dispersed in N-methyl-2-pyrrolidone to form a homogenous slurry, which was then cast onto a current collector made of aluminum foil. The positive electrode was dried at 120°C and then rolled.

[0109] By hard carbon (mass loading: 6.1 mg / cm 2 A negative electrode was prepared by mixing 2.5 wt.% carboxymethyl cellulose (2.5 wt.%) and carbon black (3.0 wt.%). The resulting mixture was dispersed in water to form a homogenous slurry, which was then cast onto an aluminum foil current collector. The negative electrode was dried at 120°C and then rolled.

[0110] The positive electrode, separator (glass fiber separator The positive electrode (GF / D) and the negative electrode (GF / D) were dried at 85°C under vacuum (less than 100 mbar) for 24 hours. Then, a 2032-type button cell was assembled by stacking the positive electrode, separator, and negative electrode in an inert atmosphere, with water and oxygen contents less than 1 ppm. Finally, different electrolyte compositions EC1 to EC8 were injected into the cells, and the cells were packaged to assemble the corresponding Na-ion batteries Na-B1 to Na-B8, each including EC1 to EC8 as a non-aqueous liquid electrolyte.

[0111] Self-discharge and cycling test procedures:

[0112] All tests were performed at 55°C.

[0113] Voltage range: 2.0V-4.3V

[0114] 1. After assembly, Na3V2(PO4)2F3 / hard carbon full cells (injected with different electrolytes) were charged at C / 10 (1C = 128 mAg -1 ) Charge and discharge for 10 cycles.

[0115] 2. Charge the battery to 4.3 V (100% SoC), then let it sit for 1 week without applying current but monitoring the voltage, and then discharge the battery to 2.0 V. The entire cycle is called a self-discharge cycle, as shown in the charge and discharge curve in the figure.

[0116] 3. The battery is then charged and discharged at C / 10, which is also called a recovery cycle as shown in the figure.

[0117] 4. Finally, the battery was cycled at C / 10 for 50 cycles.

[0118] As disclosed in Comparative Examples (EC1, EC4, EC5 and EC6), the electrolytes without any additives or with only a single additive can neither achieve the purpose of reducing self-discharge nor improve the capacity retention ability at 55°C.

[0119] On the contrary, the combination of appropriately selected additives according to the present invention (such as EC2 and EC7) can suppress the side reactions occurring at both the positive and negative electrodes to varying degrees, thereby achieving good cycling performance. However, for better results, i.e., for full optimization, with higher capacity retention and less capacity loss during the self-discharge test, as well as high capacity recovery after the self-discharge test, it is preferred to add vinylene carbonate.

[0120] The synergistic effect arises from the combination of additives with functional groups (e.g., phosphide groups that bond to transition metal ions to cover catalytic sites), thereby changing the chemical properties and characteristic bonding structure of the SEI layer on the surfaces of the positive and negative electrodes.

[0121] Example 3: Wettability of the Electrolyte Composition of the Present Invention

[0122] To evaluate the wettability of the electrolytes, EC1 and EC4 electrolytes were wetted on polyethylene (PE) using a KRUSS DSA100 apparatus within 10 s. Contact angle measurements are performed on a PE separator membrane. First, a PE separator is placed flat on a glass substrate. Then, 10 μL of electrolyte is applied to the separator surface. Finally, a camera in the KRUSS DSA100 device captures an image of the liquid on the separator and analyzes the contact angle.

[0123] Obviously, EC1 electrolyte cannot wet the PE separator, while EC4 can fully wet the PE separator.

[0124] Such results clearly demonstrate that the EC4 electrolyte has considerably better wetting ability.

Claims

1. An electrolyte composition comprising at least a sodium salt dissolved in at least one solvent, and a combination of additives, wherein: - the solvent is selected from the group consisting of ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, propylene carbonate, ethyl acetate, ethyl propionate, methyl propionate, 4-fluorotoluene, 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, difluoroethylene carbonate, ethyl difluoroacetate and mixtures thereof; - the combination of additives comprises at least sodium difluoro(oxalato)borate (NaODFB) and tris(trimethylsilyl)phosphite (TMSPi), The electrolyte composition further includes vinylene carbonate or ethylene glycol carbonate. 2 . The electrolyte composition according to claim 1 , wherein the amount of sodium difluoro(oxalato)borate ranges from 0.05 wt. % to 10 wt. % relative to the total weight of the electrolyte composition. 3 . The electrolyte composition according to claim 1 , wherein the amount of tris(trimethylsilyl)phosphite ranges from 0.05 wt. % to 10 wt. % relative to the total weight of the electrolyte composition.

4. The electrolyte composition according to claim 1, wherein the solvent is a mixture of at least two solvents including ethylene glycol carbonate as a first solvent and a second solvent selected from the group consisting of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, propylene carbonate, ethyl propionate, and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether. 5 . The electrolyte composition according to claim 1 , wherein the mixture of at least two solvents comprises a mixture of ethylene carbonate and propylene carbonate in a volume ratio of 1:

1.

6. The electrolyte composition according to claim 1, comprising sodium difluoro(oxalato)borate, tris(trimethylsilyl)phosphite, ethylene glycol carbonate, and propylene carbonate.

7. The electrolyte composition of claim 1, comprising dimethyl carbonate. 8 . The electrolyte composition according to claim 1 , wherein the amount of vinylene carbonate or ethylene glycol carbonate ranges from 0.1 wt. % to 10.0 wt. % relative to the total weight of the electrolyte composition. 9 . Use of the electrolyte composition according to claim 1 as a non-aqueous liquid electrolyte in a Na-ion battery.

10. The use according to claim 9, wherein the Na-ion battery comprises a hard carbon negative electrode comprising a binder.

11. Use of the electrolyte composition according to any one of claims 1 to 8 as a non-aqueous liquid electrolyte in a Na-ion battery to reduce self-discharge and improve retention capacity.

12. A Na-ion battery comprising: - at least one positive electrode comprising at least one positive electrode active material and a current collector, - at least one negative electrode comprising a negative electrode active material, and - at least one separator impregnated with a non-aqueous liquid electrolyte, said separator being placed between said positive electrode and said negative electrode, The non-aqueous liquid electrolyte is the electrolyte composition defined in any one of claims 1 to 8. 13 . The Na-ion battery according to claim 12 , wherein the negative electrode active material of the negative electrode is a carbon material and the negative electrode further comprises a polymer binder.

14. The Na-ion battery according to claim 13, wherein the polymer binder is carboxymethyl cellulose.

15. The electrolyte composition of claim 1, wherein the sodium salt is selected from the group consisting of sodium hexafluorophosphate (NaPF6), sodium perchlorate (NaClO4), sodium bis(fluorosulfonyl)imide (NaFSI), sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), sodium bis(pentafluoroethylsulfonyl)imide (NaBETI), sodium tetrafluoroborate (NaBF4), and mixtures thereof.

16. The electrolyte composition of claim 1, wherein the sodium salt is sodium hexafluorophosphate (NaPF6). 17 . The electrolyte composition according to claim 1 , wherein the amount of sodium salt in the electrolyte composition ranges from 0.1 mol / L to 3.0 mol / L. 18 . The electrolyte composition according to claim 1 , wherein the amount of sodium salt in the electrolyte composition ranges from 0.5 mol / L to 2.0 mol / L.

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