Electrolyte and sodium-ion battery

Abstract

The present application provides an electrolyte and a sodium-ion battery. The electrolyte comprises a first additive, a second additive, and a third additive, wherein the first additive is selected from diglycolic anhydride, the second additive is selected from at least one of sodium difluorophosphate and sodium difluoro(oxalato)borate, and the third additive is selected from hexamethylene diisocyanate. The synergistic effect of the three additives reduces impedance in sodium-ion batteries, and improves sodium-ion battery cycle performance and high-temperature storage performance.

Description

Electrolyte and sodium ion battery

[0001] The present application claims priority to the Chinese patent application No. 202411297884.1, filed on September 18, 2024, and entitled "Electrolyte and sodium ion battery", the content of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0002] The present application relates to the technical field of electrochemistry, in particular to an electrolyte and a sodium ion battery. BACKGROUND

[0003] Lithium ion batteries are widely used in portable electronic products, electric vehicles and energy storage fields due to their high energy density, no memory effect and high working voltage. However, as the market demand increases, the price of lithium resources rises, and therefore, sodium ion batteries with low cost and abundant resources become the most potential alternative to lithium ion batteries.

[0004] The basic principle of sodium ion batteries is similar to that of lithium ion batteries, but due to the fact that the radius of sodium ion is much larger than that of lithium ion, and the phase transformation of positive and negative active materials in the working process of sodium ion batteries is more complex, sodium ion batteries exhibit more severe volume change than lithium ion batteries. The large volume change leads to unstable interface, lattice fission along the sodium ion deintercalation channel, and accelerated structural rearrangement under high pressure, which results in rapid voltage decay, lattice oxygen escape destroys the crystal structure, and also intensifies the side reaction between the electrode and the electrolyte, leading to increased impedance of sodium ion batteries, and deteriorated cycle performance and high-temperature storage performance of sodium ion batteries. Therefore, how to reduce the impedance of sodium ion batteries and improve the cycle performance and high-temperature storage performance of sodium ion batteries becomes a problem to be solved. SUMMARY

[0005] The present application aims to provide an electrolyte and a sodium ion battery to reduce the impedance of sodium ion batteries and improve the cycle performance and high-temperature storage performance of sodium ion batteries. The specific technical solutions are as follows:

[0006] The first aspect of the present application provides an electrolyte, which comprises a solvent, an electrolyte and an additive, the additive comprising a first additive, a second additive and a third additive, the first additive being selected from diethanol anhydride, the second additive being selected from at least one of sodium difluorophosphate and sodium difluoro oxalate borate, and the third additive being selected from hexamethylene diisocyanate; the mass percentage content of the first additive is W1, the mass percentage content of the second additive is W2, and the mass percentage content of the third additive is W3, based on the mass of the electrolyte, and satisfy: 0.1%≤W1≤4%, 0.1%≤W2≤1%, and 0.05%≤W3≤1%.

[0007] In some embodiments of the present application, 1%≤W1≤3%.

[0008] In some embodiments of the present application, the electrolyte includes a fourth additive selected from at least one of vinylene carbonate, fluoroethylene carbonate, ethylene sulfate and 1,3-propane sultone; the mass percentage content of the fourth additive is W4 based on the mass of the electrolyte, 0.5%≤W4≤5%.

[0009] In some embodiments of the present application, 1%≤W4≤3%.

[0010] In some embodiments of the present application, the electrolyte is selected from at least one of sodium hexafluorophosphate and sodium bisfluorosulfonylimide; the mass percentage content of the electrolyte is W5 based on the mass of the electrolyte, 12%≤W5≤18%.

[0011] In some embodiments of the present application, the solvent is selected from at least one of C3-C6 carbonate compounds, C3-C8 carboxylic acid ester compounds, sulfone compounds and ether compounds; the C3-C6 carbonate compounds are selected from at least one of vinyl carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate and ethyl propyl carbonate; the C3-C8 carboxylic acid ester compounds are selected from at least one of γ-butyrolactone, methyl acetate, methyl propionate, methyl butyrate, ethyl acetate, ethyl propionate, ethyl butyrate, propyl acetate and propyl propionate; the sulfone compounds are selected from at least one of sulfolane; the ether compounds are selected from at least one of dimethyl glycol ether, triethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether; the mass percentage content of the solvent is W6 based on the mass of the electrolyte, 71%≤W6≤87%.

[0012] The second aspect of the present application provides a sodium ion battery, which includes a negative electrode sheet, a positive electrode sheet, a separator and the electrolyte provided by the first aspect of the present application.

[0013] In some embodiments of the present application, the positive electrode sheet includes a positive electrode material layer, the positive electrode material layer includes a positive electrode active material, and the positive electrode active material is selected from at least one of polyanion materials, layered oxide materials and Prussian blue materials; the polyanion material has a general formula of Na c P d (W a O b )Z e , wherein P is selected from at least one of Fe and V, W is selected from at least one of phosphorus, sulfur, silicon and tungsten, Z is selected from F, 0≤c≤10, 0≤d≤5, 0≤a≤10, 0≤b≤20, 0≤e≤5; the layered oxide material has a general formula of NaNi x Fey Mn z N 1-x-y-z O2, N is selected from at least one of Mg, Cu, Zn, Al, Sn, B, Ga, Cr, Sr, V and Ti, 0≤y≤1, 0≤x≤1, 0≤z≤1, x+y+z≤1; the general formula of the Prussian blue material is Na p M[M'(CN)6] m ·nH2O, wherein M and M' are each independently selected from at least one of Fe, Mn, Cu, Co and Ni, 0

[0014] In some embodiments of the present application, the positive active material is selected from a polyanion material selected from at least one of NaFePO4, Na2FeP2O7, Na4Fe3(PO4)2P2O7, Na3V2(PO4)3, Na2Fe(SO4)2.

[0015] Advantages of the present application:

[0016] The present application provides an electrolyte and a sodium ion battery, the electrolyte comprising a solvent, an electrolyte and an additive, the additive comprising a first additive, a second additive and a third additive, the first additive being selected from diethanol anhydride, the second additive being selected from at least one of sodium difluorophosphate and sodium difluoro oxalate borate, and the third additive being selected from hexamethylene diisocyanate, the first additive, the second additive and the third additive synergistically acting together, which is conducive to the formation of a stable solid electrolyte interface film (SEI film) at the negative electrode interface, and also conducive to the formation of a stable positive electrolyte interface film (CEI film) at the positive electrode interface, improving the stability of the positive and negative electrode interfaces, improving the structure stability of the positive electrode material, reducing the side reactions between the electrolyte and the electrode active material, effectively inhibiting the further decomposition of the electrolyte, reducing gas production, promoting the transmission of sodium ions at the electrode interface, thereby reducing the impedance of the sodium ion battery and improving the cycle performance and high-temperature storage performance of the sodium ion battery.

[0017] Of course, implementing any product or method of the present application does not necessarily require achieving all the advantages described above at the same time. DETAILED DESCRIPTION

[0018] The technical solutions in the embodiments of the present application will be described below in a clear and complete manner. Obviously, the described embodiments are only some of the embodiments of the present application, not all. All other embodiments obtained by those skilled in the art based on the present application are within the scope of protection of the present application.

[0019] The first aspect of the present application provides an electrolyte, comprising a solvent, an electrolyte and an additive, the additive comprising a first additive, a second additive and a third additive, the first additive being selected from di-glycolic anhydride, the second additive being selected from at least one of sodium difluorophosphate and sodium difluoro oxalate borate, and the third additive being selected from hexamethylene diisocyanate; the mass percentage of the first additive is W1, the mass percentage of the second additive is W2, and the mass percentage of the third additive is W3, based on the mass of the electrolyte, and satisfying: 0.1%≤W1≤4%, 0.1%≤W2≤1%, and 0.05%≤W3≤1%. For example, the value of W1 can be 0.1%, 0.3%, 0.5%, 0.8%, 1%, 1.3%, 1.5%, 1.8%, 2%, 2.3%, 2.5%, 2.8%, 3%, 3.3%, 3.5%, 3.8%, 4%, or a range between any two of them; the value of W2 can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, or a range between any two of them; and the value of W3 can be 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, or a range between any two of them.

[0020] The inventors found that the binding energy between the first additive diethanol acid anhydride (DGA) and sodium ions is lower than the binding energy between the solvent and sodium ions, and DGA can be preferentially adsorbed on the electrode surface, and during the first charging process of the sodium ion battery, DGA can preferentially undergo an oxidative polymerization reaction on the positive electrode surface to form a dense positive electrolyte interface film (CEI film) on the positive electrode surface, and can also undergo a reduction polymerization reaction on the negative electrode surface to form a dense solid electrolyte interface film (SEI film) on the negative electrode surface, reducing the side reaction between the electrolyte and the electrode active material, reducing gas production, and stabilizing the structure of the positive electrode material, reducing lattice oxygen loss, and improving the cycle performance of the sodium ion battery. However, after the polymerization reaction of DGA on the electrode surface, there are many alkoxycarbonate salts on the electrode interface, which increases the interface impedance. Therefore, by introducing a second additive into the electrolyte, the second additive is selected from at least one of sodium difluorophosphate (NaDFP) and sodium difluoro oxalate borate (NaODFB), on the one hand, the second additive can form a loose and porous interface film rich in NaF on the electrode surface, which helps to improve the diffusion ability of sodium ions on the electrode interface and improve the cycle performance of the sodium ion battery; on the other hand, a thin film rich in F, P and / or B compounds is formed on the electrode surface, which can effectively reduce the interface impedance and improve the high-temperature storage performance of the sodium ion battery. In addition, by further introducing a third additive hexamethylene diisocyanate (HDI) into the electrolyte, on the one hand, hexamethylene diisocyanate can undergo a polymerization reaction on the electrode surface to generate a substance containing a polyamide group, which can further improve the ability of the SEI film and the CEI film to conduct sodium ions; on the other hand, the sodium element in the positive active material of the sodium ion battery has the defect of easy moisture absorption, and HDI can react with H2O and HF in the electrolyte, which can effectively inhibit the decomposition of sodium salt and the collapse of the electrode structure caused by water, thereby further improving the structural stability of the electrode material, reducing gas production and reducing the dissolution of transition metals, thereby further reducing the impedance, and improving the cycle performance and high-temperature storage performance of the sodium ion battery.

[0021] When the value of W1 is too small, for example less than 0.1%, it is not conducive to the formation of a dense interface film on the positive and negative electrode surfaces, and it is not conducive to reducing the side reaction of electrolyte and electrode active material, thereby not conducive to improving the cycle performance of the sodium ion battery; when the value of W1 is too large, for example greater than 4%, it will cause the SEI film formed to be too thick, the sodium ion battery impedance is large, and the capacity loss is obvious. When the value of W2 is too small, for example less than 0.1%, the impedance reduction effect is poor; when the value of W2 is too large, for example greater than 1%, the content of the second additive is too high, and its solubility in the solvent is low, which causes the electrolyte solid content to rise, adhere to the separator and electrode, and is not conducive to the transmission of sodium ions. When the value of W3 is too small, for example less than 0.05%, the generation of substances containing polyamide groups on the electrode surface is reduced, the interface film forming effect is poor, and the HDI content is too low, the water and acid suppression effect is poor; when the value of W3 is too large, for example greater than 1%, the interface film formed is thick, which is not conducive to reducing the impedance. The electrolyte of the present application simultaneously includes the first additive, the second additive and the third additive, and the mass percentage contents of the three are controlled within the range of the present application, and the three synergistically act, which is conducive to the formation of stable CEI film on the positive electrode and stable SEI film on the negative electrode, reduces the side reaction of electrolyte and electrode active material, reduces gas production, and stabilizes the structure of the positive electrode material, reduces the loss of lattice oxygen, further improves the diffusion ability of sodium ions at the electrode interface, further reduces the impedance, and improves the cycle performance and high-temperature storage performance of the sodium ion battery.

[0022] In some embodiments of the present application, the electrolyte includes a fourth additive, the fourth additive is selected from at least one of vinylene carbonate (VC), fluoroethylene carbonate (FEC), diethyl sulfate (DTD) and 1,3-propane sultone (PS); the mass percentage content of the fourth additive is W4, 0.5%≤W4≤5%, based on the mass of the electrolyte; preferably, 1%≤W4≤3%. For example, the value of W4 can be 0.5%, 0.8%, 1%, 1.3%, 1.5%, 1.8%, 2%, 2.3%, 2.5%, 2.8%, 3%, 3.3%, 3.5%, 3.8%, 4%, 4.3%, 4.5%, 4.8%, 5%, or a range formed by any two of them. The electrolyte includes the fourth additive within the range of the present application and controls the mass percentage content of the fourth additive within the range of the present application, and the first additive, the second additive, the third additive synergistically act, which is conducive to the formation of an interface film on the positive and negative electrodes, improves the stability of the interface film, improves the wettability of the electrolyte, and further improves the cycle performance and high-temperature storage performance of the electrochemical device.

[0023] In some embodiments of the present application, the electrolyte is selected from at least one of sodium hexafluorophosphate and sodium bisfluorosulfonimide; the mass percentage of the electrolyte is W5, 12%≤W5≤18% based on the mass of the electrolyte. For example, the value of W5 can be 12%, 12.5%, 13%, 13.5%, 14%, 14.5%, 15%, 15.5%, 16%, 16.5%, 17%, 17.5%, 18%, or a range between any two of them. The electrolyte includes the electrolyte in the above range and regulates the mass percentage of the electrolyte within the scope of the present application, so that the electrolyte has high ionic conductivity and good electrochemical stability, and the cycle performance and high-temperature storage performance of the sodium ion battery can be further improved.

[0024] In some embodiments of the present application, the solvent is selected from at least one of C3-C6 carbonate compounds, C3-C8 carboxylic acid ester compounds, sulfone compounds, and ether compounds; the C3-C6 carbonate compounds are selected from at least one of ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, and ethyl propyl carbonate; the C3-C8 carboxylic acid ester compounds are selected from at least one of γ-butyrolactone, methyl acetate, methyl propionate, methyl butyrate, ethyl acetate, ethyl propionate, ethyl butyrate, propyl acetate, and propyl propionate; the sulfone compounds are selected from at least one of sulfolane; the ether compounds are selected from at least one of dimethyl glycol ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether; the mass percentage of the solvent is W6, 71%≤W6≤87% based on the mass of the electrolyte. For example, the value of W6 can be 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, or a range between any two of them. The electrolyte includes the solvent in the above range and regulates the mass percentage of the solvent within the scope of the present application, so that the electrolyte has suitable viscosity, high ionic conductivity, and good electrochemical stability, and the cycle performance and high-temperature storage performance of the sodium ion battery can be further improved.

[0025] The second aspect of the present application provides a sodium ion battery, which comprises a negative electrode sheet, a positive electrode sheet, a separator and the electrolyte provided by the first aspect of the present application. Generally, during the charging and discharging process of the sodium ion battery, metal active ions (sodium ions) are embedded and extracted between the positive electrode sheet and the negative electrode sheet. The electrolyte plays a role in conducting ions between the positive electrode sheet and the negative electrode sheet. The separator is arranged between the positive electrode sheet and the negative electrode sheet, mainly playing a role in preventing short circuit of the positive and negative electrodes, while allowing ions to pass through. The sodium ion battery of the present application comprises the electrolyte provided by the first aspect of the present application, which is beneficial to forming a stable CEI film on the positive electrode and a stable SEI film on the negative electrode, reducing side reactions between the electrolyte and the electrode active material, reducing gas production, and stabilizing the structure of the positive electrode material, reducing lattice oxygen loss, further improving the diffusion capacity of sodium ions at the electrode interface, further reducing the impedance, and improving the cycle performance and high-temperature storage performance of the sodium ion battery.

[0026] In some embodiments of the present application, the positive electrode sheet comprises a positive electrode material layer, the positive electrode material layer comprises a positive electrode active material, and the positive electrode active material is selected from at least one of a polyanion material, a layered oxide material and a Prussian blue type material; the polyanion material has a general formula of Na c P d (W a O b )Z e , wherein P is selected from at least one of Fe and V, W is selected from at least one of phosphorus, sulfur, silicon and tungsten, Z is selected from F, 0≤c≤10, 0≤d≤5, 0≤a≤10, 0≤b≤20, 0≤e≤5; the layered oxide material has a general formula of NaNi x Fe y Mn z N 1-x-y-z O2, N is selected from at least one of Mg, Cu, Zn, Al, Sn, B, Ga, Cr, Sr, V and Ti, 0≤y≤1, 0≤x≤1, 0≤z≤1, x+y+z≤1; the Prussian blue type material has a general formula of Na p M[M′(CN)6] m ·nH2O, wherein M and M′ are each independently selected from at least one of Fe, Mn, Cu, Co and Ni, 0 1 / 3 Mn 2 / 3 O2, NaFe 0.5 Ni 0.5 O2, NaNi1 / 3 Fe 1 / 3 Mn 1 / 3 O2; Prussian blue type materials can include, but are not limited to, Na2Fe[Fe(CN)6], Na2Mn[Mn(CN)6]. The positive electrode tab includes a positive electrode active material within the scope of the present application, which is conducive to improving the cycle performance and high-temperature storage performance of the sodium-ion battery.

[0027] In some embodiments of the present application, the positive electrode active material is selected from a polyanion material selected from at least one of NaFePO4, Na2FeP2O7, Na4Fe3(PO4)2P2O7, Na3V2(PO4)3, and Na2Fe(SO4)2. The polyanion material within the scope of the present application has good structural stability, which can further improve the cycle performance and high-temperature storage performance of the sodium-ion battery.

[0028] The positive electrode tab includes a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector. The above-mentioned "positive electrode material layer disposed on at least one surface of the positive electrode current collector" means that the positive electrode material layer can be disposed on one surface of the positive electrode current collector along the thickness direction of the positive electrode current collector, or can be disposed on two surfaces of the positive electrode current collector along the thickness direction of the positive electrode current collector. It should be noted that the "surface" here can be the entire area of the surface of the positive electrode current collector, or can be part of the area of the surface of the positive electrode current collector, which is not particularly limited in the present application as long as the purpose of the present application can be achieved. The present application does not particularly limit the positive electrode current collector as long as the purpose of the present application can be achieved, for example, the positive electrode current collector can be an aluminum foil, an aluminum alloy foil, or a composite positive electrode current collector. The above-mentioned composite positive electrode current collector can be a polymer material base layer and a metal layer formed on at least one surface of the polymer material base material, the material of the above-mentioned polymer material base layer can include, but is not limited to, at least one of polypropylene (PP), polyethylene terephthalate (PET), or polybutylene terephthalate (PBT), and the material of the above-mentioned metal layer can include, but is not limited to, at least one of aluminum, aluminum alloy, nickel, or nickel alloy. The thickness of the positive electrode material layer and the positive electrode current collector is not particularly limited in the present application as long as the purpose of the present application can be achieved, for example, the thickness of the single-sided positive electrode material layer is 50 μm to 250 μm, and the thickness of the positive electrode current collector is 7 μm to 20 μm.

[0029] The positive electrode material layer can further include a positive electrode conductive agent and a positive electrode binder. The present application does not have a particular limitation on the types of the positive electrode conductive agent and the positive electrode binder, as long as the purpose of the present application can be achieved. For example, the positive electrode conductive agent can include, but is not limited to, at least one of conductive carbon black (Super P), acetylene black, Ketjen black, carbon nanotube, graphene, carbon dot, or carbon fiber. The carbon nanotube can include, but is not limited to, single-walled carbon nanotube and / or multi-walled carbon nanotube. The carbon fiber can include, but is not limited to, vapor grown carbon fiber (VGCF) and / or nanocarbon fiber. For example, the positive electrode binder can include, but is not limited to, at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, or fluorine-containing acrylic ester resin. The present application does not have a particular limitation on the mass ratio of the positive electrode active material, the conductive agent, and the binder in the positive electrode material layer, and a person skilled in the art can select according to actual needs, as long as the purpose of the present application can be achieved.

[0030] The present application does not have a particular limitation on the preparation method of the positive electrode tab, as long as the purpose of the present application can be achieved. For example, the preparation method of the positive electrode tab can include, but is not limited to, the following steps: dispersing the components for preparing the positive electrode tab, such as the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder, in a solvent (for example, N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry on the positive electrode current collector, and after processes such as drying and cold pressing, the positive electrode tab can be obtained.

[0031] In the present application, the negative electrode tab includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. The above-mentioned "the negative electrode material layer is disposed on at least one surface of the negative electrode current collector" means that the negative electrode material layer can be disposed on one surface of the negative electrode current collector along the thickness direction of the negative electrode current collector, or can be disposed on both surfaces of the negative electrode current collector along the thickness direction of the negative electrode current collector. It should be noted that the "surface" herein can be the entire area of the surface of the negative electrode current collector, or can be part of the area of the surface of the negative electrode current collector, which is not particularly limited in the present application as long as the purpose of the present application can be achieved. The negative electrode current collector is not particularly limited in the present application as long as the purpose of the present application can be achieved, for example, the negative electrode current collector can be a copper foil, a copper alloy foil, an aluminum foil, a nickel foil, a stainless steel foil, a titanium foil, a foamed nickel or a foamed copper, or a composite negative electrode current collector. The above-mentioned composite negative electrode current collector can be a high polymer material base layer and a metal layer formed on at least one surface of the high polymer material base layer, and the material of the above-mentioned high polymer material base layer can include but is not limited to at least one of polypropylene (PP), polyethylene terephthalate (PET) or polybutylene terephthalate (PBT), and the material of the above-mentioned metal layer can include but is not limited to at least one of copper, copper alloy, nickel or nickel alloy. The thickness of the negative electrode material layer and the negative electrode current collector is not particularly limited in the present application as long as the purpose of the present application can be achieved, for example, the thickness of the single-sided negative electrode material layer is 50 μm to 180 μm, and the thickness of the negative electrode current collector is 3 μm to 15 μm.

[0032] In the present application, the negative electrode material layer includes a negative electrode active material, and the type of the negative electrode active material is not particularly limited in the present application as long as the purpose of the present application can be achieved, for example, the negative electrode active material can include but is not limited to soft carbon, hard carbon, sodium titanate, sodium metal or sodium alloy, etc. In some embodiments of the present application, the negative electrode material layer can further include a negative electrode conductive agent and a negative electrode binder, and the type of the negative electrode conductive agent and the negative electrode binder is not particularly limited in the present application as long as the purpose of the present application can be achieved. For example, the negative electrode conductive agent can include but is not limited to at least one of conductive carbon black (Super P), acetylene black, ketjen black, carbon nanotube, graphene, carbon dot or carbon fiber. The above-mentioned carbon nanotube can include but is not limited to single-walled carbon nanotube and / or multi-walled carbon nanotube. The above-mentioned carbon fiber can include but is not limited to vapor grown carbon fiber (VGCF) and / or nano carbon fiber. For example, the negative electrode binder can include but is not limited to at least one of styrene butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA) or carboxymethyl chitosan (CMCS). In some embodiments of the present application, the negative electrode material layer can optionally further include other auxiliary agents, for example, a thickening agent, which can include but is not limited to carboxymethyl cellulose sodium (CMC-Na) and the like.

[0033] The preparation method of the negative electrode sheet is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the preparation method of the negative electrode sheet can include, but is not limited to, the following steps: dispersing the components for preparing the negative electrode sheet, such as the negative electrode active material, the negative electrode conductive agent, the negative electrode binder and other additives, in a solvent (for example, deionized water) to form a negative electrode slurry; coating the negative electrode slurry on the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0034] The separator is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the material of the separator can include, but is not limited to, at least one of polyethylene (PE), polypropylene (PP), glass fiber, polyester (for example, polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA). The type of the separator can include at least one of woven film, non-woven fabric, microporous film, composite film, calendered film or spunlaced film. In the present application, the thickness of the separator is not particularly limited, as long as the purpose of the present application can be achieved, for example, the thickness of the separator can be 4 μm to 20 μm.

[0035] In the present application, the sodium ion battery further includes a shell for accommodating the positive electrode sheet, the separator, the negative electrode sheet and the electrolyte, and other components known in the field of sodium ion batteries, and the present application does not limit the above-mentioned other components. The shell is not particularly limited in the present application, and can be a shell known in the art, as long as the purpose of the present application can be achieved. For example, the shell can be a hard shell or a flexible shell. The material of the hard shell can be metal, and the type of metal is not limited in the present application, and a metal hard shell known in the art can be used, as long as the purpose of the present application can be achieved. The flexible shell can be a metal plastic film, such as an aluminum plastic film, a steel plastic film, etc.

[0036] The preparation process of the sodium ion battery of the present application is well known to those skilled in the art, and the present application is not particularly limited. For example, the preparation process of the sodium ion battery can include, but is not limited to, the following steps: stacking the positive electrode sheet, the separator and the negative electrode sheet in order, and according to the need, winding, folding and other operations to obtain a wound structure of the electrode assembly, placing the electrode assembly into the shell, injecting the electrolyte into the shell and sealing to obtain the sodium ion battery. Alternatively, the positive electrode sheet, the separator and the negative electrode sheet are stacked in order, and then the four corners of the entire stack structure are fixed with adhesive tape to obtain an electrode assembly of a stack structure, the electrode assembly is placed into the shell, the electrolyte is injected into the shell and sealed to obtain the sodium ion battery. In addition, the overcurrent prevention element, the guide plate and the like can also be placed in the shell according to the need, so as to prevent the pressure rise in the sodium ion battery and overcharge and discharge.

[0037] Examples

[0038] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0039] Test methods and equipment:

[0040] Cyclic performance test

[0041] The sodium-ion battery was placed in an environment of 25°C and left to stand for 4 hours. Then, it was charged at a constant current of 1C to a voltage of 3.6V, followed by constant voltage charging at 3.6V to a current of 0.05C. Finally, it was discharged at a constant current of 1C to a voltage of 1.5V. The initial discharge capacity was recorded as C1. This constituted one charge-discharge cycle. This process was repeated 400 times, and the discharge capacity after the 400th cycle was recorded as C1. 400 The charge / discharge cycle test instrument was BTS from Xinwei.

[0042] 25℃ capacity retention rate (%) = C 400 / C1×100%; The cycle performance of sodium-ion batteries is evaluated by the capacity retention rate at 25℃. The higher the capacity retention rate at 25℃, the better the cycle performance of sodium-ion batteries.

[0043] When performing cycle performance tests on the sodium-ion batteries of Examples 1-20 and Comparative Example 4, the upper limit voltage of 3.6V in the above steps was adjusted to 4.0V. The other examples and comparative examples were tested with an upper limit voltage of 3.6V.

[0044] High-temperature storage performance test

[0045] Storage thickness expansion rate test

[0046] The sodium-ion battery was charged at a constant current of 1C to a voltage of 3.6V, and then charged at a constant voltage of 3.6V to a current of 0.05C. Before storage, the thickness of the fully charged sodium-ion battery was measured with a flat plate thickness gauge and recorded as d0. Then, the sodium-ion battery was placed in a 60℃ explosion-proof oven and stored for 40 days. After that, the thermal thickness d1 of the battery was tested in a constant temperature chamber. The storage thickness expansion rate = (d1-d0) / d0×100%.

[0047] Storage capacity retention test

[0048] The sodium ion battery was placed in an environment at 25°C for 4h, then the sodium ion battery was charged at 1C constant current to a voltage of 3.6V, then charged at 3.6V constant voltage to a current of 0.05C, then discharged at 1C constant current to a voltage of 1.5V, and the initial discharge capacity was recorded as Q1, then the sodium ion battery was charged at 1C constant current to a voltage of 3.6V, then charged at 3.6V constant voltage to a current of 0.05C, then the sodium ion battery was placed in a 60°C explosion-proof oven, and after 40 days of storage, the sodium ion battery was taken out and cooled to room temperature, then the sodium ion battery was discharged at 1C constant current to a voltage of 1.5V at 25°C, and the discharge capacity at this time was recorded as Q2, and the storage capacity retention rate (%) = Q2 / Q1x100%.

[0049] The thickness expansion rate and the storage capacity retention rate at 60°C were used to evaluate the high-temperature storage performance of the sodium ion battery. The smaller the thickness expansion rate and the larger the storage capacity retention rate, the better the high-temperature storage performance of the sodium ion battery.

[0050] When the sodium ion batteries of Examples 1-20 and Comparative Example 4 were tested for high-temperature storage performance, the upper voltage 3.6V in the above steps was adjusted to 4.0V, and the rest of the examples and comparative examples were tested according to the upper voltage of 3.6V.

[0051] Test of direct current resistance (DCR)

[0052] Test of DCR1 of sodium ion battery before storage: The sodium ion battery was placed in an environment at 25°C, and charged at 1C constant current to a voltage of 3.6V, then charged at 3.6V constant voltage to a current of 0.05C, and left for 30min, then discharged at 1C constant current for 30min (adjusted to 50% SOC, SOC refers to the state of charge of the battery), and the end voltage was recorded as V1, left for 1h, then discharged at 2C constant current for 10s, and the end voltage was recorded as V2, and the DCR1 before storage was calculated as (V1-V2) / 2C;

[0053] Test of DCR2 of sodium ion battery after storage: The above sodium ion battery was placed in a 60°C explosion-proof oven, and after 7 days of storage, it was taken out, and the sodium ion battery was placed in an environment at 25°C, and charged at 1C constant current to a voltage of 3.6V, then charged at 3.6V constant voltage to a current of 0.05C, and left for 30min, then discharged at 1C constant current for 30min (adjusted to 50% SOC, SOC refers to the state of charge of the battery), and the end voltage was recorded as V3, left for 1h, then discharged at 2C constant current for 10s, and the end voltage was recorded as V4, and the DCR2 after storage was calculated as (V3-V4) / 2C

[0054] Impedance growth rate (%) = (DCR2-DCR1) / DCR1x100%.

[0055] The upper limit voltage of 3.6V in the above step is adjusted to 4.0V when performing direct current resistance test on the sodium ion battery of Examples 1-20 and Comparative Example 4, and the rest of the examples and comparative examples are tested according to the upper limit voltage of 3.6V.

[0056] Example 1-1

[0057] Preparation of electrolyte

[0058] Under the inert atmosphere environment with water content less than 0.1 ppm and oxygen content less than 1 ppm, propylene carbonate (PC) and methyl ethyl carbonate (EMC) are mixed according to the mass ratio of 3:7 to obtain a base solvent, and then the base solvent is added with electrolyte sodium hexafluorophosphate (NaPF6) and sodium bisfluorosulfonylimide (NaFSI), first additive diethylene glycol anhydride (DGA), second additive sodium difluorophosphate (NaDFP) and third additive hexamethylene diisocyanate (HDI) to mix uniformly to obtain an electrolyte. Among them, the mass percentage content of NaPF6 is 12%, the mass percentage content of NaFSI is 2%, the mass percentage content of DGA is 2%, the mass percentage content of NaDFP is 0.5%, the mass percentage content of HDI is 0.3%, and the rest of the base solvent is 83.2% based on the mass of the electrolyte.

[0059] Preparation of positive electrode sheet

[0060] The positive active material polyanion material Na4Fe3(PO4)2(P2O7), the conductive agent conductive carbon black (Super P), the carbon nanotube and the binder polyvinylidene fluoride (PVDF) are mixed according to the mass ratio of 94.5:2.5:0.5:2.5, and N-methyl pyrrolidone (NMP) is added as a solvent to be formulated into a slurry with a solid content of 55wt%, and the positive electrode slurry is obtained after uniform stirring in vacuum. The positive electrode slurry is uniformly coated on one surface of the positive electrode current collector aluminum foil with a thickness of 16μm to obtain a positive electrode sheet with a single-side coated positive electrode material layer. Then the above steps are repeated on the other surface of the aluminum foil, i.e. a positive electrode sheet with a double-side coated positive electrode material layer is obtained. After drying at 85℃, cold pressing, edge cutting, sheet cutting, striping, vacuum drying at 75℃ for 10 hours, and welding of the tabs, a positive electrode sheet with a double-side areal density of 30mg / cm 2

[0061] Preparation of negative electrode sheet

[0062] ​The negative active material hard carbon, conductive agent conductive carbon black (Super P), thickening agent carboxymethyl cellulose sodium (CMC) and binder styrene-butadiene rubber (SBR) are mixed in a mass ratio of 95:1.5:2.0:1.5, deionized water is added as a solvent, and a slurry with a solid content of 49wt% is prepared. After uniform stirring in a vacuum stirrer, a negative electrode slurry is obtained. The negative electrode slurry is uniformly coated on one surface of a negative electrode current collector aluminum foil with a thickness of 9μm to obtain a negative electrode sheet with a single-side coated negative electrode material layer. Then the above steps are repeated on the other surface of the aluminum foil to obtain a negative electrode sheet with a double-side coated negative electrode material layer. After drying at 85℃, cold pressing, edge cutting, sheet cutting, striping, vacuum drying at 85℃ for 10 hours, and welding of the tabs, a negative electrode sheet with a double-side areal density of 14.2mg / cm 2 is obtained. The thickness of the single-side negative electrode material layer is 140μm.

[0063] <Preparation of the separator>

[0064] Polyethylene (PE), aluminum oxide and polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 7:3:2, N-methyl pyrrolidone (NMP) is added as a solvent, and the above coating slurry is uniformly coated on both surfaces of a polyethylene porous polymer film substrate with a thickness of 9μm. After drying at 60℃, a double-side coated separator is obtained. The thickness of the single-side coating is 2μm.

[0065] <Preparation of the sodium-ion battery>

[0066] The above positive electrode sheet, separator and negative electrode sheet are sequentially stacked and then subjected to a winding process to produce an electrode assembly with a theoretical capacity of 1000mAh. The electrode assembly is placed in an aluminum-plastic film packaging bag, vacuum baked at 85℃ for 48 hours, and then the above electrolyte is injected. After vacuum packaging, standing, formation, aging, and capacity distribution processes, a sodium-ion battery is obtained.

[0067] Examples 1-2 to 1-14

[0068] Except for adjusting the relevant parameters in <Preparation of the electrolyte> according to Table 1, the rest is the same as Example 1-1. When the values of W1, W2 and W3 in the electrolyte change, the mass percentage content of the solvent W6 changes accordingly, and the mass percentage content of the electrolyte W5 remains unchanged.

[0069] Examples 1-15 to 1-18

[0070] The rest is the same as Example 1-1 except that the type and mass percentage of the second additive are adjusted according to Table 1 in the preparation of the electrolyte. Among them, when the value of W2 in the electrolyte changes, the mass percentage of the solvent W6 changes accordingly, and the values of W1, W3 and W5 in the electrolyte remain unchanged. The second additive sodium difluorophosphate is abbreviated as NaDFP, and the second additive sodium difluoro oxalate borate is abbreviated as NaODFB.

[0071] Examples 1-19 to 1-21

[0072] The rest is the same as Example 1-1 except that the type of the positive active material is adjusted according to Table 1 in the preparation of the positive electrode sheet.

[0073] Examples 2-1 to 2-7

[0074] The rest is the same as Example 1-1 except that the fourth additive fluoroethylene carbonate (FEC) is added to the electrolyte and the mass percentage of FEC W4 is adjusted according to Table 2 in the preparation of the electrolyte. Among them, when the value of W4 in the electrolyte changes, the mass percentage of the solvent W6 changes accordingly, and the values of W1, W2, W3 and W5 in the electrolyte remain unchanged.

[0075] Examples 2-8 to 2-13

[0076] The rest is the same as Example 1-1 except that the type of the fourth additive is adjusted and the mass percentage of the fourth additive W4 is adjusted according to Table 2 in the preparation of the electrolyte. Among them, when the value of W4 in the electrolyte changes, the mass percentage of the solvent W6 changes accordingly, and the values of W1, W2, W3 and W5 in the electrolyte remain unchanged. Fluoroethylene carbonate is abbreviated as FEC, vinylene carbonate is abbreviated as VC, ethylene sulfate is abbreviated as DTD, and 1,3-propane sultone is abbreviated as PS.

[0077] Example 2-14

[0078] The rest is the same as Example 1-1 except that no electrolyte sodium bisfluorosulfonylimide (NaFSI) is added to the electrolyte in the preparation of the electrolyte, and the mass percentage of sodium hexafluorophosphate (NaPF6) is adjusted to 14%.

[0079] Example 2-15

[0080] The rest is the same as Example 1-1 except that the mass percentage of the electrolyte sodium hexafluorophosphate (NaPF6) is adjusted to 10% and the mass percentage of the electrolyte sodium bisfluorosulfonylimide (NaFSI) is 2%, i.e. the value of W5 is 12% in the preparation of the electrolyte. Among them, when the value of W5 in the electrolyte changes, the mass percentage of the solvent W6 changes accordingly, and the values of W1, W2, W3 in the electrolyte remain unchanged.

[0081] Example 2-16

[0082] Except that in the <preparation of electrolyte>, the mass percentage content of electrolyte sodium hexafluorophosphate (NaPF6) is adjusted to 12%, the mass percentage content of electrolyte sodium bisfluorosulfonylimide (NaFSI) is 6%, that is, the value of W5 is 18%, and the rest is the same as Example 1-1. Among them, when the value of W5 in the electrolyte changes, the mass percentage content of the solvent W6 changes accordingly, and the values of W1, W2 and W3 in the electrolyte remain unchanged.

[0083] Example 2-17

[0084] Except that in the <preparation of electrolyte>, ethyl acetate (EA), propylene carbonate (PC) and methyl ethyl carbonate (EMC) are mixed according to the mass ratio of 2:3:5 to obtain the base solvent, and the rest is the same as Example 1-1.

[0085] Example 2-18

[0086] Except that in the <preparation of electrolyte>, ethyl propionate (EP), propylene carbonate (PC) and methyl ethyl carbonate (EMC) are mixed according to the mass ratio of 2:3:5 to obtain the base solvent, and the rest is the same as Example 1-1.

[0087] Example 2-19

[0088] Except that in the <preparation of electrolyte>, dimethyl glycol ether (DME), propylene carbonate (PC) and methyl ethyl carbonate (EMC) are mixed according to the mass ratio of 2:3:5 to obtain the base solvent, and the rest is the same as Example 1-1.

[0089] Comparative Examples 1 to 3, Comparative Examples 6 to 11

[0090] Except that in the <preparation of electrolyte>, the relevant parameters are adjusted according to Table 1, and the rest is the same as Example 1-1. Among them, when the values of W1, W2 and W3 in the electrolyte change, the mass percentage content of the solvent W6 changes accordingly, and the mass percentage content of the electrolyte W5 remains unchanged.

[0091] Comparative Example 4

[0092] Except that in the <preparation of electrolyte>, no first additive is added, the mass percentage content of the solvent W6 changes accordingly, and the values of W2, W3 and W5 remain unchanged, and the rest is the same as Example 1-20.

[0093] Comparative Example 5

[0094] Except that no first additive is added in the preparation of the electrolyte, the mass percentage content W6 of the solvent is changed accordingly, and the values of W2, W3 and W5 remain unchanged, and the rest is the same as in Examples 1-21.

[0095] The preparation parameters and performance parameters of each example and comparative example are shown in Tables 1-2.

[0096] Table 1 Note: " / " in Table 1 means no relevant preparation parameter.

[0097] As can be seen from Examples 1-1 to 1-21 and Comparative Examples 1-11, the electrolyte includes the first additive, the second additive and the third additive within the scope of the present application, and the values of W1, W2 and W3 are controlled within the scope of the present application, and the three synergistically act together to enable the sodium ion battery to have a higher 25°C capacity retention rate, a higher storage capacity retention rate, a lower storage thickness expansion rate and a lower impedance growth rate, indicating that the sodium ion battery has good cycle performance, high-temperature storage performance and low impedance. However, Comparative Examples 1-11 do not simultaneously satisfy the above characteristics, and the sodium ion battery has a lower 25°C capacity retention rate, a lower storage capacity retention rate, a higher storage thickness expansion rate and a higher impedance growth rate, indicating that the cycle performance and high-temperature storage performance of the sodium ion battery are poor, and the impedance is large.

[0098] As can be seen from Example 1-1 to Example 1-5, Comparative Example 3 to Comparative Example 7, when the first additive is not included in the electrolyte or the mass percentage content W1 of the first additive is too small, less than 0.1%, it is not conducive to forming an interface film on the positive and negative electrode surfaces, the sodium ion battery has a lower 25°C capacity retention rate, a lower storage capacity retention rate, a higher storage thickness expansion rate and a higher impedance growth rate, indicating that the value of W1 is too small, which is not conducive to improving the cycle performance and high-temperature storage performance of the sodium ion battery, and is not conducive to reducing the impedance. When the value of W1 is too large, greater than 4%, the formed SEI film is relatively thick, the sodium ion battery has a lower 25°C capacity retention rate, a lower storage capacity retention rate, a higher storage thickness expansion rate and a higher impedance growth rate, indicating that the value of W1 is too large, which is also not conducive to improving the cycle performance and high-temperature storage performance of the sodium ion battery, and is not conducive to reducing the impedance. As can be seen from Example 1-1, Example 1-6 to Example 1-9, Comparative Example 2, Comparative Example 8 and Comparative Example 9, when the second additive is not included in the electrolyte or the mass percentage content W2 of the second additive is too small, less than 0.1%, the sodium ion battery has a lower 25°C capacity retention rate, a lower storage capacity retention rate, a higher storage thickness expansion rate and a higher impedance growth rate, indicating that the value of W2 is too small, which is not conducive to improving the cycle performance and high-temperature storage performance of the sodium ion battery, and is not conducive to reducing the impedance. When the value of W2 is too large, greater than 1%, it is not conducive to the transmission of sodium ions, the sodium ion battery has a lower 25°C capacity retention rate, a lower storage capacity retention rate, a higher storage thickness expansion rate and a higher impedance growth rate, indicating that the value of W2 is too large, which is also not conducive to improving the cycle performance and high-temperature storage performance of the sodium ion battery, and is not conducive to reducing the impedance. As can be seen from Example 1-1, Example 1-10 to Example 1-14, Comparative Example 1, Comparative Example 10 and Comparative Example 11, when the third additive is not included in the electrolyte or the mass percentage content W3 of the third additive is too small, less than 0.05%, the interface film has poor film forming effect, the water removal and acid suppression effect is poor, the sodium ion battery has a lower 25°C capacity retention rate, a lower storage capacity retention rate, a higher storage thickness expansion rate and a higher impedance growth rate, indicating that the value of W3 is too small, which is not conducive to improving the cycle performance and high-temperature storage performance of the sodium ion battery, and is not conducive to reducing the impedance. When the value of W3 is too large, greater than 1%, the formed interface film is relatively thick, the sodium ion battery has a lower 25°C capacity retention rate, a lower storage capacity retention rate, a higher storage thickness expansion rate and a higher impedance growth rate, indicating that the value of W3 is too large, which is also not conducive to improving the cycle performance and high-temperature storage performance of the sodium ion battery, and is not conducive to reducing the impedance.

[0099] The type of positive active material generally affects the cycle performance, high-temperature storage performance and impedance of the sodium-ion battery. As can be seen from Example 1-1, Example 1-19 to Example 1-21, the positive electrode tab comprises the positive active material within the scope of the present application, and the obtained sodium-ion battery has a higher 25°C capacity retention rate, a higher storage capacity retention rate, a lower storage thickness expansion rate and a lower impedance growth rate, indicating that the sodium-ion battery has good cycle performance, high-temperature storage performance and lower impedance.

[0100] Table 2 Note: " / " in Table 2 means no relevant preparation parameters.

[0101] The type and mass percentage content W4 of the fourth additive generally affect the cycle performance, high-temperature storage performance and impedance of the sodium-ion battery. As can be seen from Example 1-1, Example 2-1 to Example 2-13, the electrolyte comprises the fourth additive within the scope of the present application and regulates the value of the mass percentage content W4 of the fourth additive within the scope of the present application, and the obtained sodium-ion battery has a higher 25°C capacity retention rate, a higher storage capacity retention rate, a lower storage thickness expansion rate and a lower impedance growth rate, indicating that the sodium-ion battery has better cycle performance, high-temperature storage performance and lower impedance.

[0102] The type and mass percentage content W5 of the electrolyte generally affect the cycle performance, high-temperature storage performance and impedance of the sodium-ion battery. As can be seen from Example 1-1, Example 2-14 to Example 2-16, the electrolyte comprises the electrolyte within the scope of the present application and regulates the value of the mass percentage content W5 of the electrolyte within the scope of the present application, and the obtained sodium-ion battery has a higher 25°C capacity retention rate, a higher storage capacity retention rate, a lower storage thickness expansion rate and a lower impedance growth rate, indicating that the sodium-ion battery has good cycle performance, high-temperature storage performance and lower impedance.

[0103] The type and mass percentage content W6 of the solvent generally affect the cycle performance, high-temperature storage performance and impedance of the sodium-ion battery. As can be seen from Example 1-1, Example 2-1 to Example 2-19, the electrolyte comprises the solvent within the scope of the present application and regulates the value of the mass percentage content W6 of the solvent within the scope of the present application, and the obtained sodium-ion battery has a higher 25°C capacity retention rate, a higher storage capacity retention rate, a lower storage thickness expansion rate and a lower impedance growth rate, indicating that the sodium-ion battery has good cycle performance, high-temperature storage performance and lower impedance.

[0104] The above description is only the preferred embodiment of the present application, and is not used to limit the present application, any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included in the scope of protection of the present application.

Claims

1. An electrolyte, comprising a solvent, an electrolyte and an additive, the additive comprising a first additive, a second additive and a third additive, the first additive being selected from di-glycolic anhydride, the second additive being selected from at least one of sodium difluorophosphate and sodium difluoro oxalate borate, the third additive being selected from hexamethylene diisocyanate.

2. The electrolyte of claim 1, wherein, The mass percentage content of the first additive is W1, the mass percentage content of the second additive is W2, and the mass percentage content of the third additive is W3, based on the mass of the electrolyte, satisfying: 0.1%≤W1≤4%, 0.1%≤W2≤1%, and 0.05%≤W3≤1%.

3. The electrolyte of claim 2, wherein, 1％≤W1≤3％。 4. The electrolyte of claim 1, wherein, The electrolyte comprises a fourth additive selected from at least one of vinylene carbonate, fluoroethylene carbonate, ethylene sulfate and 1,3-propane sultone; the mass percentage content of the fourth additive is W4, 0.5%≤W4≤5%, based on the mass of the electrolyte.

5. The electrolyte of claim 4, wherein, 1％≤W4≤3％。 6. The electrolyte according to any one of claims 1 to 5, wherein, The electrolyte is selected from at least one of sodium hexafluorophosphate and sodium bisfluorosulfonylimide; the mass percentage content of the electrolyte is W5, 12%≤W5≤18%, based on the mass of the electrolyte.

7. The electrolyte of any one of claims 1 to 5, wherein, The solvent is selected from at least one of C3-C6 carbonate compounds, C3-C8 carboxylic acid ester compounds, sulfone compounds and ether compounds; the C3-C6 carbonate compounds are selected from at least one of vinyl carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate and ethyl propyl carbonate; the C3-C8 carboxylic acid ester compounds are selected from at least one of γ-butyrolactone, methyl acetate, methyl propionate, methyl butyrate, ethyl acetate, ethyl propionate, ethyl butyrate, propyl acetate and propyl propionate; the sulfone compounds are selected from sulfolane; the ether compounds are selected from at least one of dimethyl glycol ether, triethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether; the mass percentage content of the solvent is W6, 71%≤W6≤87%, based on the mass of the electrolyte. 8.A sodium ion battery, comprising a negative electrode sheet, a positive electrode sheet, a separator and the electrolyte according to any one of claims 1 to 7.

9. The sodium-ion battery of claim 8, the positive electrode sheet comprising a positive electrode material layer, the positive electrode material layer comprising a positive electrode active material selected from at least one of a polyanion material, a layered oxide material, and a Prussian blue analog material; the polyanion material having a general formula of Na c P d (W a O b )Z e wherein, P is selected from at least one of Fe and V, W is selected from at least one of phosphorous, sulfur, silicon and tungsten, Z is selected from F, 0 x Fe y Mn z N 1-x-y-z O2, N is selected from at least one of Mg, Cu, Zn, Al, Sn, B, Ga, Cr, Sr, V and Ti, 0 p M[M'(CN)6] m nH2O, wherein M and M' are each independently selected from at least one of Fe, Mn, Cu, Co and Ni, 0 < p < 2, 0.8 < m < 1, 0 < n < 20.

10. The sodium-ion battery of claim 9, wherein, The positive electrode active material is selected from polyanion materials selected from at least one of NaFePO4, Na2FeP2O7, Na4Fe3(PO4)2P2O7, Na3V2(PO4)3 and Na2Fe(SO4)2.

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