Electrolyte for sodium secondary battery, sodium secondary battery and electrical device

Abstract

The present application provides an electrolyte for a sodium secondary battery, a sodium secondary battery, and an electrical device. The electrolyte for a sodium secondary battery includes a solvent, and the ratio of the amount of free solvent material to the total amount of the solvent material in the electrolyte is 50% or less. This electrolyte can enhance the high-temperature cycle performance of the battery, reduce the high-temperature gas generation phenomenon of the battery, and improve the electrochemical performance and safety performance of the battery at high temperatures. In one embodiment, the electrolyte includes a sodium salt, and the molar concentration of the sodium salt in the electrolyte is 2 to 7 mol / L, optionally 3 to 7 mol / L.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application references Chinese patent application No. 202310075575.9, filed on January 16, 2023, entitled "Electrolyte for Sodium Secondary Battery, Sodium Secondary Battery and Electrical Device," the entire contents of which are incorporated herein by reference.

[0002] The present application relates to the technical field of sodium secondary batteries, and in particular to electrolytes for sodium secondary batteries, sodium secondary batteries, and electrical devices. [Background technology]

[0003] As battery technology continues to develop, lithium secondary batteries have become dominant in the portable electronics market and have gradually expanded into large-scale electrical energy storage applications. However, traditional lithium resources are no longer able to meet the urgent need for large-scale, high-energy-density energy storage systems. Therefore, sodium secondary batteries have emerged, which have abundant sodium reserves, low cost, and electrochemical properties similar to those of lithium secondary batteries. However, sodium secondary batteries, which are currently widely studied, suffer from poor high-temperature performance and serious high-temperature gas generation phenomena, significantly limiting their further application. Summary of the Invention [Means for solving the problem]

[0004] In view of the problems in the background art, the present application provides an electrolyte for a sodium secondary battery that enhances the high-temperature cycle performance of the sodium secondary battery, reduces the high-temperature gas generation phenomenon of the sodium secondary battery, and improves the electrochemical performance and safety performance of the sodium secondary battery at high temperatures.

[0005] A first aspect of the present application provides an electrolyte for a sodium secondary battery, the electrolyte containing a solvent, wherein the ratio of the amount of free solvent material to the total amount of solvent material in the electrolyte is 50% or less.

[0006] By controlling the ratio of the amount of free solvent material to the total amount of solvent material in the electrolyte to be 50% or less, the proportion of free solvent molecules in the electrolyte is reduced, side reactions between the free solvent molecules and the positive and negative electrodes are reduced, the high-temperature cycle performance of the battery is improved, the gas generation phenomenon during high-temperature cycles of sodium secondary batteries is suppressed, and the electrochemical performance and safety performance of the battery at high temperatures are improved.

[0007] In any embodiment, the electrolyte comprises a sodium salt, and the molar concentration of the sodium salt in the electrolyte is 2 to 7 mol / L, optionally 3 to 7 mol / L.

[0008] By controlling the molar concentration of the sodium salt within an appropriate range, the proportion of solvent molecules that coordinate with sodium ions in the electrolyte can be increased and the proportion of free solvent molecules in the electrolyte can be reduced, resulting in a battery with excellent room temperature cycle performance and high temperature cycle performance and low high temperature gas generation.

[0009] In any embodiment, the sodium salt comprises a first sodium salt, and the solubility of the first sodium salt in the solvent at room temperature is greater than 4 mol / L.

[0010] The first sodium salt having excellent solubility can ensure that the sodium salt has an appropriate molar concentration in the electrolyte, and the battery has excellent room temperature cycling performance and high temperature cycling performance, and produces little high-temperature gas.

[0011] In any embodiment, the first sodium salt comprises one or more of sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethanesulfonyl)imide, sodium (fluorosulfonyl)(trifluoromethanesulfonyl)imide, and optionally one or two of sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethanesulfonyl)imide.

[0012] In any embodiment, the molar concentration of the first sodium salt in the electrolyte is 0.01 mol / L or greater.

[0013] The first sodium salt has an appropriate molar concentration in the electrolyte, and the battery has excellent room temperature cycling performance and high temperature cycling performance, and exhibits little high temperature gas generation phenomenon.

[0014] In any embodiment, the sodium salt comprises a second sodium salt, and the solubility of the second sodium salt in the solvent at room temperature is less than 3 mol / L.

[0015] In any embodiment, the second sodium salt comprises one or more of sodium hexafluorophosphate, sodium tetrafluoroborate, sodium hexafluoroarsenate, sodium trifluoroacetate, sodium difluoro(oxalato)borate, sodium bis(oxalato)borate, sodium trifluoromethanesulfonate.

[0016] In any embodiment, the molar concentration of the second sodium salt in the electrolyte is less than 2.7 mol / L, optionally between 0.2 and 2.7 mol / L.

[0017] The second sodium salt has an appropriate molar concentration in the electrolyte, and the battery has excellent room temperature cycling performance and high temperature cycling performance, and exhibits little high temperature gas generation phenomenon.

[0018] In an optional embodiment, the electrolyte comprises a lithium salt.

[0019] The inclusion of a lithium salt in the electrolyte can improve the room temperature cycle performance and high temperature cycle performance of the battery, reduce the high temperature gas generation phenomenon of the battery, and improve the electrochemical performance and safety performance of the battery.

[0020] In any embodiment, the lithium salt comprises one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium trifluoroacetate, lithium tetraphenylborate, lithium trifluoromethanesulfonate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide.

[0021] In any embodiment, the solvent comprises one or more of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol ethyl methyl ether, ethylene glycol dibutyl ether.

[0022] A suitable solvent can improve the normal temperature cycling performance and high temperature cycling performance of the battery, reduce the high temperature gas generation phenomenon of the battery, and improve the electrochemical performance and safety performance of the battery.

[0023] A second aspect of the present application provides a sodium secondary battery including the electrolyte according to the first aspect.

[0024] In any embodiment, a sodium secondary battery includes a negative electrode plate, the negative electrode plate including a negative electrode current collector and an undercoat layer formed on at least one side of a surface of the negative electrode current collector, and the electrolyte includes a lithium salt; Sodium secondary batteries satisfy the relationship 0.84≦Y-0.01X≦5.84, X g / m 2 is the areal density of the undercoat layer, and Y% is the mass content of the lithium salt based on the total mass of the electrolyte.

[0025] The surface density of the undercoat layer on at least one side of the negative electrode current collector is X g / m 2 and the mass content Y% of the lithium salt based on the total mass of the electrolyte is controlled to satisfy the relational expression 0.84≦Y−0.01X≦5.84, thereby improving the room temperature cycle performance and high temperature cycle performance of the battery and improving the electrochemical performance of the battery.

[0026] In any embodiment, the areal density of the primer layer is 5 to 50 g / m 2 is.

[0027] When the undercoat layer has an appropriate areal density, it is possible to enhance the room temperature cycle performance and high temperature cycle performance of the battery and improve the electrochemical performance of the battery.

[0028] In any embodiment, the primer layer comprises one or more of a carbon coating, an alloy coating, a metal oxide coating, and optionally a carbon coating.

[0029] In an optional embodiment, the primer layer comprises one or more of superconducting carbon black, ketjen black, acetylene black, carbon nanotubes, and graphene.

[0030] In any embodiment, the sodium secondary battery is a negative electrode-free sodium metal battery.

[0031] The anode-free sodium metal battery can enhance the room temperature cycling performance and high temperature cycling performance of the battery and improve the electrochemical performance of the battery.

[0032] A third aspect of the present application provides a battery module including the sodium secondary battery according to the second aspect.

[0033] A fourth aspect of the present application provides a battery pack including the sodium secondary battery according to the second aspect or the battery module according to the third aspect.

[0034] A fifth aspect of the present application provides an electric device including at least one of the sodium secondary battery according to the second aspect, the battery module according to the third aspect, or the battery pack according to the fourth aspect. [Brief explanation of the drawings]

[0035] [Figure 1] FIG. 1 is a schematic diagram of a sodium secondary battery according to an embodiment of the present application. [Figure 2] FIG. 2 is an exploded view of the sodium secondary battery according to the embodiment of the present application shown in FIG. 1. [Figure 3] 1 is a schematic diagram of a battery module according to an embodiment of the present application; [Figure 4] 1 is a schematic diagram of a battery pack according to an embodiment of the present application. [Figure 5] FIG. 5 is an exploded view of the battery pack shown in FIG. 4 according to an embodiment of the present application. [Figure 6] 1 is a schematic diagram of an electrical device that uses a sodium secondary battery according to an embodiment of the present application as a power source. DETAILED DESCRIPTION OF THE INVENTION

[0036] Hereinafter, with appropriate reference to the drawings, embodiments specifically disclosing the secondary battery, battery module, battery pack, and electric device of the present application will be described in detail. However, unnecessary details may be omitted. For example, detailed descriptions of well-known matters and redundant descriptions of substantially the same structures may be omitted. This is to avoid unnecessary redundancy in the following description and to facilitate easy understanding by those skilled in the art. Furthermore, the drawings and the following description are provided to enable those skilled in the art to fully understand the present application and are not intended to limit the scope of the claims.

[0037] The "ranges" disclosed herein are defined in the form of lower and upper limits. A given range is defined by selecting one lower limit and one upper limit, and the selected lower and upper limits define the boundaries of the given range. Such defined ranges may be inclusive or exclusive of the endpoints and may be arbitrarily combined; i.e., any lower limit may be combined with any upper limit to form a single range. For example, if ranges of 60 to 120 and 80 to 110 are recited for a given parameter, it is understood that ranges of 60 to 110 and 80 to 120 are also contemplated. Furthermore, if 1 and 2 are recited as minimum range values ​​and 3, 4, and 5 are recited as maximum range values, then the ranges of 1 to 3, 1 to 4, 1 to 5, 2 to 3, 2 to 4, and 2 to 5 are all contemplated. Unless otherwise specified herein, a numerical range "a to b" represents a shorthand notation for any combination of real numbers between a and b, where a and b are both real numbers. For example, the numerical range "0 to 5" means that all real numbers between "0 and 5" are listed herein, and "0 to 5" is simply shorthand for combinations of these numbers. Note that describing a parameter as an integer ≧2 is equivalent to disclosing that the parameter is, for example, the integers 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0038] Unless otherwise stated, all embodiments and alternative embodiments in the present application can be combined with each other to form new technical solutions.

[0039] Unless otherwise stated, all technical features and optional technical features of the present application can be combined with each other to form new technical solutions.

[0040] Unless otherwise specified, the terms "comprise" and "comprises" used herein may be open-ended or closed-ended. For example, the terms "comprise" and "comprises" may indicate that the compound may further include or include other components not listed, or may include or include only the listed components.

[0041] Unless otherwise stated, the term "or" in this application is inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, any one of the following conditions satisfies the condition "A or B": A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or A and B are both true (or exist).

[0042] When a sodium secondary battery is used or stored at high temperatures, the reaction activity between the positive and negative electrodes and the electrolyte becomes stronger, which significantly increases side reactions during the battery cycle process and generates a large amount of gas, which makes the battery prone to volume expansion and, in serious cases, may even cause a short circuit inside the battery, seriously affecting the electrochemical performance and safety performance of the battery.

[0043] [Electrolyte for sodium secondary batteries] The present application provides an electrolyte for a sodium secondary battery, the electrolyte including a solvent, wherein the ratio of the amount of free solvent material to the total amount of solvent material in the electrolyte is 50% or less.

[0044] In this specification, the term "sodium secondary battery" refers to a secondary battery that uses sodium ions as a charge carrier, and the negative electrode active material of a sodium secondary battery includes a carbon-based material, a titanium-based material, an alloy material, a layered transition metal oxide, a layered transition metal selenide, an organic material, or metallic sodium, and a sodium secondary battery that uses metallic sodium as the negative electrode active material is called a sodium metal battery.

[0045] In some embodiments, the negative electrode active material of the sodium secondary battery includes one or more of a graphitic carbon material, an amorphous carbon material, and a nanocarbon material.

[0046] In some embodiments, the negative electrode active material of the sodium secondary battery is hard carbon.

[0047] In some embodiments, the negative electrode active material of a sodium secondary battery is metallic sodium.

[0048] As used herein, the term "anode-free sodium metal battery" refers to a sodium metal battery that employs a current collector as the anode during packaging, and in which sodium ions are deposited on the current collector during charging to form a sodium metal anode.

[0049] As used herein, the term "electrolyte" refers to a carrier that performs ion transfer in a sodium secondary battery, and includes a liquid electrolyte, a solid electrolyte, or a quasi-solid electrolyte.

[0050] As used herein, the term "free solvent" refers to solvent molecules that are not coordinated or associated with sodium ions, and that are free to move without any hydrogen bonding or hydrogen bond-like interactions between the solvent and the solvent molecules. For example, free solvent does not participate in the solvation structure of sodium ions.

[0051] Regarding solvated structures, since the binding force of solvent molecules to cations in electrolytes is much stronger than that to anions, after a sodium salt is dissolved in a solvent, the solvent aggregates around the sodium ions, and the aggregates formed by the sodium ions and the solvent are called solvated structures.

[0052] In some embodiments, the ratio of the amount of free solvent material to the total amount of solvent material in the electrolyte is no more than any one of 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, and 50%.

[0053] By controlling the ratio of the amount of free solvent material to the total amount of solvent material in the electrolyte to be 50% or less, the proportion of free solvent molecules in the electrolyte is reduced, side reactions between the free solvent molecules and the positive and negative electrodes are reduced, the high-temperature cycle performance of the battery is improved, the gas generation phenomenon during high-temperature cycles of sodium secondary batteries is suppressed, and the electrochemical performance and safety performance of the battery at high temperatures are improved.

[0054] In some embodiments, the electrolyte includes a sodium salt, and the molar concentration of the sodium salt in the electrolyte is 2 to 7 mol / L. In some embodiments, the molar concentration of the sodium salt in the electrolyte is optionally 2 to 3 mol / L, 2 to 3.5 mol / L, 2 to 4 mol / L, 2 to 4.5 mol / L, 2 to 5 mol / L, 2 to 5.5 mol / L, 2 to 6 mol / L, 2 to 6.5 mol / L, 2 to 7 mol / L, 3 to 3.5 mol / L, 3 to 4 mol / L, 3 to 4.5 mol / L, 3 to 5 mol / L, 3 to 5.5 mol / L, 3 to 6 mol / L, 3 to 6.5 mol / L, 3 to 7 mol / L, 3.5 to 4 mol / L, 3.5 to 4.5 mol / L, 3.5 to 5 mol / L, 3.5 to 5.5 mol / L, or 3.5 to 6 mol / L. L, 3.5~6.5mol / L, 3.5~7mol / L, 4~4.5mol / L, 4~5mol / L, 4~5.5mol / L, 4~6mol / L, 4~6.5mol / L, 4~7mol / L, 4.5~5mol / L, 4.5~5.5mol / L, 4.5~6mol / L, 4.5~6.5mol / L, 4.5~7mol / L, 5~5.5mol / L, 5~6mol / L, 5~6.5mol / L, 5~7mol / L, 5.5~6mol / L, 5.5~6.5mol / L, 5.5~7mol / L, 6~6.5mol / L, 6~7mol / L, and 6.5~7mol / L.

[0055] By controlling the molar concentration of the sodium salt within an appropriate range, the proportion of solvent molecules that coordinate with sodium ions in the electrolyte can be increased, the proportion of free solvent molecules in the electrolyte can be reduced, and side reactions between the free solvent molecules and the positive and negative electrodes can be reduced. In addition, with an appropriate sodium salt concentration, the electrolyte has appropriate viscosity and excellent ionic conductivity, and the battery has excellent room temperature cycle performance and high temperature cycle performance, and has low high-temperature gas generation.

[0056] In some embodiments, the electrolyte includes a sodium salt, and the molar concentration of the sodium salt in the electrolyte is 3 to 7 mol / L. In some embodiments, the molar concentration of the sodium salt in the electrolyte is optionally 3 to 3.5 mol / L, 3 to 4 mol / L, 3 to 4.5 mol / L, 3 to 5 mol / L, 3 to 5.5 mol / L, 3 to 6 mol / L, 3 to 6.5 mol / L, 3 to 7 mol / L, 3.5 to 4 mol / L, 3.5 to 4.5 mol / L, 3.5 to 5 mol / L, 3.5 to 5.5 mol / L, 3.5 to 6 mol / L, 3.5 to 6.5 mol / L, 3.5 to 7 mol / L, 4 to 4.5 mol / L, 4 to 5 mol / L, L, 4~5.5mol / L, 4~6mol / L, 4~6.5mol / L, 4~7mol / L, 4.5~5mol / L, 4.5~5.5mol / L, 4.5~6mol / L, 4.5~6.5mol / L, 4.5~7mol / L, 5~5.5mol / L, 5~6mol / L, 5~6.5mol / L, 5~7mol / L, 5.5~6mol / L, 5.5~6.5mol / L, 5.5~7mol / L, 6~6.5mol / L, 6~7mol / L, and any one of 6.5~7mol / L.

[0057] By controlling the molar concentration of the sodium salt within an appropriate range, the proportion of solvent molecules that coordinate with sodium ions in the electrolyte can be increased, the proportion of free solvent molecules in the electrolyte can be reduced, and side reactions between the free solvent molecules and the positive and negative electrodes can be reduced, further reducing the high-temperature gas generation phenomenon in the battery. In addition, with an appropriate sodium salt concentration, the electrolyte has appropriate viscosity and excellent ionic conductivity, which contributes to the electrochemical performance of the battery.

[0058] In some embodiments, the sodium salt comprises a first sodium salt, and the solubility of the first sodium salt in the solvent at room temperature is greater than 4 mol / L. In some embodiments, the sodium salt comprises a first sodium salt, and the solubility of the first sodium salt in the solvent at room temperature is greater than any one of 4 mol / L, 4.5 mol / L, 5 mol / L, 5.5 mol / L, 6 mol / L, 6.5 mol / L, or 7 mol / L.

[0059] As used herein, the term "room temperature" refers to a temperature of 25°C.

[0060] The first sodium salt having excellent solubility can ensure that the sodium salt has an appropriate molar concentration in the electrolyte, and the battery has excellent room temperature cycling performance and high temperature cycling performance, and produces little high-temperature gas.

[0061] In some embodiments, the first sodium salt comprises one or more of sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethanesulfonyl)imide, sodium (fluorosulfonyl)(trifluoromethanesulfonyl)imide.

[0062] In some embodiments, the compound is one or two of sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethanesulfonyl)imide.

[0063] The first sodium salt comprises a sulfur-containing organic sodium salt, which is likely to be involved in the formation of a structurally stable and uniformly distributed SEI, and is used to improve the high-temperature cycling performance of the battery and reduce the high-temperature gas generation phenomenon of the battery. Furthermore, with the appropriate first sodium salt, the electrolyte has appropriate viscosity and excellent ionic conductivity, and the battery has excellent room-temperature cycling performance and high-temperature cycling performance, and the amount of high-temperature gas generation is low.

[0064] In some embodiments, the molarity of the first sodium salt in the electrolyte is 0.01 mol / L or greater, or any one or more of 0.01 mol / L, 0.5 mol / L, 1 mol / L, 1.5 mol / L, 2 mol / L, 2.5 mol / L, 3 mol / L, 3.5 mol / L, 4 mol / L, 4.5 mol / L, 5 mol / L, 5.5 mol / L, 6 mol / L, 6.5 mol / L, and 7 mol / L.

[0065] The first sodium salt having an appropriate molar concentration in the electrolyte ensures that a sufficient amount of sodium salt is involved in the formation of a solvate structure, reducing the proportion of free solvent in the electrolyte. Furthermore, the first sodium salt having an appropriate molar concentration allows the electrolyte to have appropriate viscosity and excellent ionic conductivity, and the battery to have excellent room temperature cycling performance and high temperature cycling performance, with little high-temperature gas generation.

[0066] In some embodiments, the sodium salt comprises a second sodium salt, and the solubility of the second sodium salt in the solvent at room temperature is less than 3 mol / L. In some embodiments, the sodium salt comprises a second sodium salt, and the solubility of the second sodium salt in the solvent at room temperature is less than any one of 1 mol / L, 1.5 mol / L, 2 mol / L, 2.5 mol / L, or 3 mol / L.

[0067] In some embodiments, the second sodium salt comprises one or more of sodium hexafluorophosphate, sodium tetrafluoroborate, sodium hexafluoroarsenate, sodium trifluoroacetate, sodium difluoro(oxalato)borate, sodium bis(oxalato)borate, sodium trifluoromethanesulfonate.

[0068] The anion in the second sodium salt has a weak bond with the sodium ion, enabling rapid deposition and release of sodium metal, contributing to improved dynamic performance of the battery. Particularly in anode-free sodium metal batteries, the second sodium salt can effectively ensure the cycle performance of the battery. Furthermore, with the appropriate second sodium salt, the electrolyte has appropriate viscosity and excellent ionic conductivity, resulting in excellent room-temperature cycle performance and high-temperature cycle performance, with low high-temperature gas generation.

[0069] In some embodiments, the molar concentration of the second sodium salt in the electrolyte is less than 2.7 mol / L, or less than any one of 2.7 mol / L, 2.6 mol / L, 2.5 mol / L, 2.4 mol / L, 2.3 mol / L, 2.2 mol / L, 2.1 mol / L, 2 mol / L, 1.9 mol / L, 1.8 mol / L, 1.7 mol / L, 1.6 mol / L, 1.5 mol / L, 1.4 mol / L, 1.3 mol / L, 1.2 mol / L, 1.1 mol / L, 1 mol / L, 0.9 mol / L, 0.8 mol / L, 0.7 mol / L, 0.6 mol / L, 0.5 mol / L, 0.4 mol / L, 0.3 mol / L, 0.2 mol / L, and 0.1 mol / L.

[0070] The second sodium salt has an appropriate molar concentration in the electrolyte, and the battery has excellent room temperature cycling performance and high temperature cycling performance, and exhibits little high temperature gas generation phenomenon.

[0071] In some embodiments, the molar concentration of the second sodium salt in the electrolyte is 0.2 to 2.7 mol / L. In some embodiments, the molar concentration of the second sodium salt in the electrolyte is optionally any one of 0.2 to 0.5 mol / L, 0.2 to 1 mol / L, 0.2 to 1.5 mol / L, 0.2 to 2 mol / L, 0.2 to 2.5 mol / L, 0.2 to 2.7 mol / L, 0.5 to 1 mol / L, 0.5 to 1.5 mol / L, 0.5 to 2 mol / L, 0.5 to 2.5 mol / L, 0.5 to 2.7 mol / L, 1 to 1.5 mol / L, 1 to 2 mol / L, 1 to 2.5 mol / L, 1 to 2.7 mol / L, 1.5 to 2 mol / L, 1.5 to 2.5 mol / L, 1.5 to 2.7 mol / L, 2 to 2.5 mol / L, 2 to 2.7 mol / L, and 2.5 to 2.7 mol / L.

[0072] When the second sodium salt has an appropriate molar concentration in the electrolyte, it is possible to improve the room temperature cycle performance and high temperature cycle performance of the battery, reduce the high temperature gas generation phenomenon of the battery, and improve the electrochemical performance and safety performance of the battery.

[0073] In some embodiments, the electrolyte comprises a lithium salt.

[0074] In some embodiments, the lithium salt comprises one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium trifluoroacetate, lithium tetraphenylborate, lithium trifluoromethanesulfonate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide.

[0075] Taking a sodium metal battery as an example, the introduction of lithium ions can further effectively inhibit the growth of sodium dendrites during the deposition process of sodium metal. During the charging process of a sodium metal battery, an electrochemical reaction occurs at the negative electrode in which sodium ions gain electrons and are reduced to sodium metal. In the initial stage of the reaction, sodium metal deposits unevenly on the surface of the current collector, forming dendrites, the tips of which have a high negative charge density due to the tip effect. Both lithium ions and sodium ions are positive monovalent cations and carry a single unit positive charge. However, due to their smaller radius, lithium ions have a higher positive charge density and are distributed to the tips of dendrites before sodium ions, effectively reducing the continued deposition of sodium ions at the tips of sodium dendrites, inhibiting the growth of sodium dendrites, and greatly improving the cycle performance of the battery.

[0076] The inclusion of a lithium salt in the electrolyte can improve the room temperature cycle performance and high temperature cycle performance of the battery, reduce the high temperature gas generation phenomenon of the battery, and improve the electrochemical performance and safety performance of the battery.

[0077] In some embodiments, the solvent comprises one or more of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol ethyl methyl ether, ethylene glycol dibutyl ether.

[0078] The molecules of the ether solvent can establish a stable electrode / electrolyte interface on the surface of the negative electrode, form a stable SEI, reduce electrochemical polarization, improve the room temperature cycling performance and high temperature cycling performance of the battery, reduce the high temperature gas generation phenomenon of the battery, and improve the electrochemical performance and safety of the battery.

[0079] [Sodium secondary battery] The present application provides a sodium secondary battery, which includes an electrolyte in any embodiment.

[0080] In some embodiments, a sodium secondary battery includes a negative electrode plate, the negative electrode plate including a negative electrode current collector and an undercoat layer formed on at least one side of a surface of the negative electrode current collector, and the electrolyte includes a lithium salt; Sodium secondary batteries satisfy the relationship 0.84≦Y-0.01X≦5.84, X g / m 2 is the areal density of the undercoat layer, and Y% is the mass content of the lithium salt based on the total mass of the electrolyte.

[0081] In some embodiments, a sodium secondary battery includes a negative electrode plate, the negative electrode plate including a negative electrode current collector and an undercoat layer formed on at least one side of the negative electrode current collector, the electrolyte including a lithium salt, and the sodium secondary battery has the following properties: 0.84≦Y−0.01X≦5.84, 0.84≦Y−0.01X≦1.84, 0.84≦Y−0.01X≦2.84, 0.84≦Y−0.01X≦3.84, 0.84≦Y−0.01X≦4.84, 0.84≦Y−0.01X≦5.84, 1.84≦Y−0.01X≦2.84, 1.84≦Y−0.01X≦3.84, 1.84≦Y−0.01X≦4.84 X ≤ 4.84, 1.84 ≤ Y-0.01X ≤ 5.84, 2.84 ≤ Y-0.01X ≤ 3.84, 2.84 ≤ Y-0.01X ≤ 4.84, 2.84 ≤ Y-0.01X ≤ 5.84, 3.84 ≤ Y-0.01X ≤ 4.84, 3.84 ≤ Y-0.01X ≤ 5.84, 4.84 ≤ Y-0. 0.01X≦5.84, 0.84≦Y-0.01X≦1.5, 0.84≦Y-0.01X≦2, 0.84≦Y-0.01X≦2.5, 0.84≦Y-0.01X≦3, 0.84≦Y-0.01X≦3.5, 0.84≦Y-0.01X≦4, 0.84≦Y-0.01X≦4.5, 0.8 4≦Y-0.01X≦5, 1.84≦Y-0.01X≦5.5, 1.5≦Y-0.01X≦2, 1.5≦Y-0.01X≦2.5, 1.5≦Y-0.01X≦3, 1.5≦Y-0.01X≦3.5, 1.5≦Y-0.01X≦4, 1.5≦Y-0.01X≦4.5, 1.5≦Y -0.01X≦5, 1.5≦Y-0.01X≦5.5, 2≦Y-0.01X≦2.5, 2≦Y-0.01X≦3, 2≦Y-0.01X≦3.5, 2≦Y-0.01X≦4, 2≦Y-0.01X≦4.5, 2≦Y-0.01X≦5, 2≦Y-0.01X≦5.5, 2.5≦Y-0. 0.01X≦3, 2.5≦Y-0.01X≦3.5, 2.5≦Y-0.01X≦4, 2.5≦Y-0.01X≦4.5, 2.5≦Y-0.01X≦5, 2.5≦Y-0.01X≦5.5, 3≦Y-0.01X≦3.5, 3≦Y-0.01X≦4, 3≦Y-0.01X≦4.5, 3≦Y -0.01X≦5, 3≦Y-0.01X≦5.5, 3.5≦Y-0.01X≦4, 3.5≦Y-0.01X≦4.5, 3.5≦Y-0.01X≦5, 3.5≦Y-0.01X≦5.5, 4≦Y-0.01X≦4.5, 4≦Y-0.01X≦5, 4≦Y-0.01X≦5.5, 4.Satisfies one of the following relations: 5≦Y-0.01X≦5, 4.5≦Y-0.01X≦5.5, 5≦Y-0.01X≦5.5. X g / m 2 is the areal density of the undercoat layer, and Y% is the mass content of the lithium salt based on the total mass of the electrolyte.

[0082] In this specification, the term "an undercoat layer on at least one side of a negative electrode current collector" refers to an undercoat layer being applied to one or both sides of the current collector, and the undercoat layer may be in direct contact with the current collector, i.e., no other structure may be present between the current collector and the undercoat layer, or the undercoat layer may not be in direct contact with the current collector, i.e., no other structure may be present between the current collector and the undercoat layer.

[0083] The undercoat layer has the characteristic of low metal nucleation potential, which can effectively improve the metal deposition / dissolution performance, while also improving the large volume change of the cell caused by the metal deposition / dissolution process, making the cell structure stable, and improving the room temperature cycle performance and high temperature cycle performance of the battery, thereby improving the electrochemical performance of the battery.

[0084] The free electron-containing undercoat layer electrostatically attracts lithium ions with a high positive charge density, which then pass through the SEI layer and are deposited on the undercoat layer. This effectively reduces the overpotential caused by sodium metal deposition, suppresses the formation of sodium dendrites, and contributes to improving the cycle performance of the battery.

[0085] The applicant has determined that the surface density of the undercoat layer on at least one side of the negative electrode current collector is X g / m 2 The inventors have unexpectedly discovered that by controlling the mass content Y% of the lithium salt based on the total mass of the electrolyte to satisfy the relational expression 0.84≦Y-0.01X≦5.84, it is possible to improve the room temperature cycle performance and high temperature cycle performance of the battery and to improve the electrochemical performance of the battery.

[0086] In some embodiments, the areal density of the primer layer is between 5 and 50 g / m 2In some embodiments, the areal density of the primer layer is preferably 5 to 10 g / m 2 , 5~20g / m 2 , 5 to 30 g / m 2 , 5~40g / m 2 , 5~50g / m 2 , 10-20g / m 2 , 10-30g / m 2 , 10~40g / m 2 , 10~50g / m 2 , 20-30g / m 2 , 20~40g / m 2 , 20~50g / m 2 , 30-40g / m 2 , 30~50g / m 2 , 40-50g / m 2 It is one of the following.

[0087] A suitable area density of the undercoat layer can improve the room temperature cycle performance and high temperature cycle performance of the battery, and at the same time, improve the energy density of the battery to meet the needs of battery use.

[0088] In some embodiments, the primer layer comprises one or more of a carbon coating, an alloy coating, and a metal oxide coating.

[0089] In some embodiments, the primer layer comprises a carbon coating.

[0090] In some embodiments, the primer layer comprises one or more of superconducting carbon black, ketjen black, acetylene black, carbon nanotubes, and graphene.

[0091] In some embodiments, the primer layer comprises a binder, and the binder comprises any one of polyvinylidene fluoride, sodium carboxymethyl cellulose, styrene butadiene rubber, sodium alginate, lithium / sodium polyacrylate, polytetrafluoroethylene, polyimide, and polyurethane.

[0092] In some embodiments, the undercoat layer is a metal coating, the metal has a body-centered cubic structure, and the metal comprises any one of α-Fe, V, Nb, Cr, Mo, Ta, and W.

[0093] In some embodiments, the undercoat layer is an alloy coating, and the alloy comprises any one or more of the following metals: Au, Ag, Sn, Sb.

[0094] In some embodiments, the primer layer is a metal oxide coating, and the oxide comprises at least one of copper oxide, aluminum oxide.

[0095] In some embodiments, the primer layer is a conductive polymer coating, and the conductive polymer comprises any one of polyaniline, polythiophene, polypyrrole, and polyphenylacetylene.

[0096] In some embodiments, the primer layer is a conductive ceramic coating, and the conductive ceramic material comprises at least one of TiB2, TiC, and B4C3.

[0097] In some embodiments, the undercoat layer is a conductive carbon coating, and the conductive carbon comprises at least one of conductive carbon black, graphite, carbon fiber, single-walled carbon nanotubes, multi-walled carbon nanotubes, graphene, and fullerene. In any embodiment, the sodium secondary battery is a negative-electrode-free sodium metal battery.

[0098] In some embodiments, the sodium secondary battery is a negative electrode-free sodium metal battery.

[0099] Anode-free sodium metal batteries do not use anode active material, but only use the anode current collector as the anode. During the initial charge process, sodium is plated onto the anode, which then returns to the cathode during discharge, achieving a charge-discharge cycle. By using only the anode current collector and no anode material, anode-free sodium metal batteries effectively overcome the shortcomings of sodium metal batteries and can achieve a higher energy density than metallic sodium anodes.

[0100] A sodium secondary battery is a negative electrode-free sodium metal battery, which can enhance the room temperature cycle performance and high temperature cycle performance of the battery and improve the electrochemical performance of the battery.

[0101] In some embodiments, the CB value of the negative electrode-free sodium metal battery is 0.1 or less.

[0102] The CB value is the capacity per unit area of ​​the negative electrode plate in a sodium secondary battery divided by the capacity per unit area of ​​the positive electrode plate. Because anode-free sodium metal batteries do not contain negative electrode active material, the capacity per unit area of ​​the negative electrode plate is small, and the CB value of the secondary battery is 0.1 or less.

[0103] In some embodiments, the negative electrode current collector used in the negative plate includes at least one of a metal foil current collector, a metal foam current collector, a metal mesh current collector, a carbon felt current collector, a carbon cloth current collector, and a carbon paper current collector. Because sodium ions do not form an alloy with aluminum and because cost and weight considerations are taken into account, an aluminum-based current collector is preferentially adopted for sodium secondary batteries. The aluminum-based current collector is any one of aluminum foil, aluminum alloy foil, and aluminum-based composite current collector. The aluminum-based composite current collector includes a polymer-based film and aluminum foil and / or aluminum alloy foil formed on both sides of the polymer-based film. Specifically, the aluminum-based composite current collector has a "sandwich" structure, with a polymer-based film located in the center and aluminum foils on both sides, or aluminum foils on both sides. Alternatively, an aluminum alloy foil may be provided on one side of a polymer base film and an aluminum alloy foil on the other side, and the polymer base film may be made of any one of polyamide, polyterephthalate, polyimide, polyethylene, polypropylene, polystyrene, polyvinyl chloride, acrylonitrile-butadiene-styrene copolymer, polybutylene terephthalate, poly-p-phenylene terephthalamide, polyphenylene ether, polyformaldehyde, epoxy resin, phenolic resin, polytetrafluoroethylene, polyvinylidene fluoride, silicone rubber, and polycarbonate.

[0104] [Positive electrode] In some embodiments, the positive electrode plate includes a positive electrode current collector and a positive electrode active material layer formed on a surface of at least a portion of the positive electrode current collector, the positive electrode active material layer including a positive electrode active material, and the positive electrode active material can include at least one of a layered transition metal oxide, a polyanion-type compound, and a Prussian blue compound.

[0105] The transition metal in the layered transition metal oxide may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. Optionally, the layered transition metal oxide may be, for example, NaxMO2, where M is one or more of Ti, V, Mn, Co, Ni, Fe, Cr, and Cu, and 0 <x≦1である。

[0106] Polyanionic compounds include metal ions, transition metal ions, and tetrahedral (YO4) n- The compound may have an anionic unit, wherein the metal ion is optionally one of sodium ion, lithium ion, potassium ion, and zinc ion, the transition metal is optionally at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce, Y is optionally at least one of P, S, and Si, and n is (YO4) n- represents the valence of the atom.

[0107] The Prussian blue compound may be a compound having a sodium ion, a transition metal ion, and a cyanide ion (CN-). The transition metal may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. The Prussian blue compound may be, for example, Na a Me b Me' c (CN)6, where Me and Me' are each independently at least one of Ni, Cu, Fe, Mn, Co, and Zn; <a≦2、0<b<1、0<c<1である。

[0108] The positive electrode active material layer may further include a conductive agent to improve the conductive performance of the positive electrode, which may be one or more of Super P, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphite, graphene, and carbon nanofibers.

[0109] The positive electrode active material layer may further include a binder to firmly bond the positive electrode active material and optional conductive agent to the positive electrode current collector, and the binder may optionally be at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), lithium polyacrylate (PAA), polyethylene alcohol (PVA), ethylene vinyl acetate copolymer (EVA), styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), sodium alginate (SA), polymethacrylic acid (PMA), and carboxymethyl chitosan (CMCS).

[0110] The positive electrode current collector may be a conductive carbon sheet, metal foil, carbon-coated metal foil, porous metal plate, or composite current collector. The conductive carbon material of the conductive carbon sheet may be one or more of Super P, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphite, graphene, and carbon nanofibers. The metal material of the metal foil, carbon-coated metal foil, and porous metal plate may each independently be at least one selected from copper, aluminum, nickel, and stainless steel. The composite current collector may be a composite current collector formed by combining a metal foil with a polymer-based film.

[0111] In some embodiments, a positive electrode plate can be manufactured by the following method. The components for manufacturing the positive electrode plate described above, such as the positive electrode active material, conductive agent, binder, and any other optional components, are dispersed in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry. The positive electrode slurry is applied to a positive electrode current collector, and after processes such as baking and cold pressing, a positive electrode plate can be obtained.

[0112] [Separator] In some embodiments, the sodium secondary battery further includes a separator. In the present application, the type of separator is not particularly limited, and any separator with a known porous structure having good chemical stability and mechanical stability can be selected.

[0113] In some embodiments, the separator may be made of at least one of polyethylene, polypropylene, polyvinylidene fluoride, aramid fiber, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, and natural fiber. The separator may be a single-layer film or a multi-layer composite film, and is not particularly limited. When the separator is a multi-layer composite film, the materials of the layers may be the same or different, and are not particularly limited.

[0114] In some embodiments, the positive electrode plates, negative electrode plates, and separator can be manufactured into an electrode assembly by a winding process or a stacking process.

[0115] In some embodiments, the sodium secondary battery may include an exterior case that can be used to package the electrode assembly and the electrolyte.

[0116] In some embodiments, the exterior of the sodium secondary battery may be a hard case, such as a hard plastic case, an aluminum case, or a steel case. The exterior of the sodium secondary battery may be a soft pack, such as a bag-type soft pack. The material of the soft pack may be plastic, and examples of plastic include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0117] In the present application, the shape of the sodium secondary battery is not particularly limited and may be cylindrical, rectangular, or any other shape. For example, Fig. 1 shows a sodium secondary battery 5 having a rectangular structure as an example.

[0118] In some embodiments, referring to FIG. 2 , the exterior body may include a case 51 and a cover plate 53. The case 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate are enclosed to form a storage chamber. The case 51 may have an opening communicating with the storage chamber, and the cover plate 53 may cover the opening to seal the storage chamber. The positive electrode plate, the negative electrode plate, and the separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is packaged in the storage chamber. An electrolyte is impregnated into the electrode assembly 52. ​​The sodium secondary battery 5 may include one or more electrode assemblies 52, and this can be selected by those skilled in the art according to specific actual needs.

[0119] In some embodiments, the sodium secondary batteries can be assembled into a battery module, and the number of sodium secondary batteries included in the battery module can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0120] Fig. 3 shows an example of a battery module 4. Referring to Fig. 3, in the battery module 4, a plurality of sodium secondary batteries 5 may be arranged in order along the longitudinal direction of the battery module 4. Of course, they may be arranged in any other form. Furthermore, the plurality of sodium secondary batteries 5 may be fixed by fastening members.

[0121] Optionally, the battery module 4 may further include a housing having an accommodating space, and the plurality of sodium secondary batteries 5 are accommodated in the accommodating space.

[0122] In some embodiments, the battery modules can be further assembled into a battery pack, and the number of battery modules included in the battery pack can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0123] 4 and 5 show an example of a battery pack 1. Referring to FIGS. 4 and 5, the battery pack 1 may include a battery box and a plurality of battery modules 4 provided in the battery box. The battery box may include an upper box 2 and a lower box 3, and the upper box 2 may cover the lower box 3 to form an enclosed space for accommodating the battery modules 4. The plurality of battery modules 4 may be arranged in the battery box in any manner.

[0124] The present application also provides an electric device including at least one of the sodium secondary battery, battery module, or battery pack provided herein. The sodium secondary battery, battery module, or battery pack may be used as a power source for the electric device or as an energy storage unit for the electric device. The electric device may include, but is not limited to, mobile devices (e.g., mobile phones, notebook computers, etc.), electric vehicles (e.g., rechargeable battery electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric cars, ships and satellites, energy storage systems, etc.

[0125] The electrical device can be selected as a sodium secondary battery, a battery module, or a battery pack depending on the needs of its use.

[0126] 6 shows an example of an electric device, such as a secondary battery electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the sodium secondary battery of the electric device, a battery pack or a battery module can be adopted.

[0127] Another example of the device may be a mobile phone, a tablet, a laptop, etc. Such devices usually require light weight and thinness, and may employ a sodium secondary battery as a power source.

[0128] Examples are as follows. Examples of the present application are described below. The examples described below are illustrative and are intended merely to interpret the present application and should not be understood as limiting the present application. If specific techniques or conditions are not specified in the examples, they are performed according to the techniques or conditions described in literature in the field or according to product specifications. If the manufacturer of the reagents or equipment used is not specified, they are all commercially available, ordinary products. In the following examples, only cases where the secondary battery is a sodium-ion battery are shown, but the present application is not limited thereto.

[0129] 1. Manufacturing method Example 1 is as follows. 1) Electrolyte production In an argon atmosphere glove box with a water content of <10 ppm, the second sodium salt, sodium hexafluorophosphate (NaPF6), was dissolved in ethylene glycol dimethyl ether (DME) and stirred to obtain a mixed solution. Subsequently, the lithium salt, lithium hexafluorophosphate (LiPF6), and the first sodium salt, sodium bis(fluorosulfonyl)imide (NaFSI), were dissolved in the mixed solution to obtain an electrolyte, in which the molar concentrations of sodium bis(fluorosulfonyl)imide were 4 mol / L and 1 mol / L, respectively, and the mass content of lithium hexafluorophosphate was 2% based on the total mass of the electrolyte.

[0130] 2) Manufacturing of positive electrode plates The positive electrode active material Na3V2(PO4)3, the binder polyvinylidene fluoride (PVDF), and the conductive carbon black (Super-P) were uniformly mixed in a mass ratio of 96%:2%:2% in N-methylpyrrolidone (NMP) solvent to prepare a positive electrode slurry. The positive electrode slurry was then coated on the surface of aluminum foil using an extrusion coater according to the mass per unit area of ​​the positive electrode active material, baked, and further pressed into a cold press to form a coated electrode plate at a density of 2.5 g / cm. 3 The final positive electrode plate was manufactured by compressing and cold pressing the design.

[0131] 3) Manufacturing of negative electrode plates Carbon nanotubes and sodium alginate were added to deionized water and stirred to form a uniform slurry. The slurry was applied to a negative electrode current collector, baked, and cut to obtain a negative electrode plate with a negative electrode-free structure. The surface density of the undercoat layer was 20 g / m. 2 It was.

[0132] 4) Separator A polyethylene film (PE membrane) was used as the separator.

[0133] 5) Battery manufacturing The positive electrode plate, separator, and negative electrode plate were stacked in this order, a separator was interposed between the positive and negative electrode plates to serve as an insulator, and the electrolyte was added to assemble a button battery.

[0134] Examples 2 to 31 are as follows. Except for the different electrolyte formulations and / or manufacturing parameters of the negative electrode plates, the other steps of Examples 2 to 31 are the same as Example 1. The manufacturing methods of the negative electrode plates of Examples 29 and 30 are as follows, and the specific parameters are shown in Tables 1 and 2.

[0135] In Example 29, carbon nanotubes and sodium alginate were added to deionized water and stirred to form a uniform slurry. The slurry was then applied to an aluminum foil negative electrode current collector, baked, and cut to obtain a negative electrode current collector with an undercoat layer. The areal density of the undercoat layer was 20 g / m. 2 The negative electrode active material, hard carbon, the conductive agent, acetylene black, the binder, styrene butadiene rubber (SBCs), and the thickener, hydroxymethyl cellulose (CMC), were mixed in a weight ratio of 90:5:4:1 in an appropriate amount of deionized water with sufficient stirring to form a uniform negative electrode slurry. The negative electrode slurry was then applied to a negative electrode current collector with an undercoat layer, dried at 100°C, and pressed to obtain a negative electrode plate.

[0136] In Example 30, the negative electrode plate was aluminum foil.

[0137] Comparative Examples 1 to 7 are as follows. Except for the different electrolyte formulations and / or negative electrode plate manufacturing parameters, the other steps of Comparative Examples 1 to 7 are the same as those of Example 1, Comparative Example 6 is the same as the negative electrode plate manufacturing method of Example 29, and Comparative Example 7 is the same as the negative electrode plate manufacturing method of Example 30, and the specific parameters are shown in Tables 1 and 2.

[0138] 2. Battery performance test 1) Mass content of free solvent The infrared spectrum of the electrolyte was examined using a Nicolet iS5 Fourier transform infrared spectrophotometer. After normalization of the spectrogram, the intensity of the characteristic peak of the solvent was recorded as I1. The electrolyte solvent was used as a reference sample, and its infrared spectrum was measured. After normalization of the spectrogram, the intensity of the corresponding characteristic peak of the solvent was recorded as I2. I1 / I2 was used as the ratio of the amount of free solvent to the amount of solvent in the electrolyte. The characteristic peak of the solvent was selected depending on the type of solvent. For ether solvents, the characteristic peak of CO was between 800 and 900 cm. -1 For ester solvents, the characteristic peak of C=O is located between 850 and 950 cm -1 Between 1700 and 1800 cm -1 When comparing the peak intensities, the peaks are located between 850 and 950 cm. -1 The peak between these two peaks was used as a reference for comparison.

[0139] 2) Room temperature cycle performance At 25°C and atmospheric pressure (0.1 MPa), the battery was charged at a constant current of 0.5 C until the voltage reached 3.5 V, and then discharged at a constant current of 1 C until the voltage reached 3.2 V, which constituted one charge-discharge cycle. The capacity of the initial discharge was taken as 100%, and the charge-discharge cycle was repeated 500 times. After the test was stopped, the cycle capacity retention rate was recorded, and the room temperature capacity retention rate was used as an evaluation index for the room temperature cycle performance of the battery.

[0140] 3) High-temperature cycle performance At 60°C and atmospheric pressure (0.1 MPa), the battery was charged at a constant current of 0.5 C until the voltage reached 3.5 V, and then discharged at a constant current of 1 C until the voltage reached 3.2 V, which constituted one charge-discharge cycle. The capacity of the initial discharge was taken as 100%, and the charge-discharge cycle was repeated 500 times. After the test was stopped, the cycle capacity retention rate was recorded, and the high-temperature capacity retention rate was used as an evaluation index for the high-temperature cycle performance of the battery.

[0141] 4) High-temperature gas generation performance At room temperature (25°C), the battery was charged to 3.5V at a constant current of 0.5C. After the battery was fully charged, the initial volume of the battery was measured using the drainage method. The battery was placed in an oven at 60°C for 24 days and then removed. It was left at room temperature for 60 minutes. After cooling to room temperature, the battery volume was measured using the drainage method within 60 minutes. The volume expansion rate of the battery was calculated based on the battery volume measured before storage. The volume expansion rate (%) of the battery after 24 days of storage at 60°C = (volume of battery measured after storage / volume of battery measured before storage) - 1.

[0142] The test procedures for the comparative example and other examples are as described above.

[0143] 3. Test results The test results of the above examples and comparative examples are shown in Tables 1 and 2.

[0144] [Table 1-1] [Table 1-2] [Table 1-3] [Table 1-4]

[0145] [Table 2-1] [Table 2-2] [Table 2-3]

[0146] As can be seen from the above results, the ratio of the amount of free solvent material to the total amount of solvent material in the sodium secondary battery electrolytes of Examples 1 to 31 is 50% or less. As can be seen from a comparison of Examples 1 to 16, 18 to 31 with Comparative Examples 1 to 4, 6 to 7, and Example 17 with Comparative Example 5, a ratio of the amount of free solvent material to the total amount of solvent material in the electrolyte of 50% or less can increase the high-temperature capacity retention rate of the battery, improve the high-temperature cycle performance of the battery, reduce the high-temperature expansion rate of the battery, reduce the high-temperature gas generation phenomenon of the battery, and improve the electrochemical performance and safety performance of the battery at high temperatures.

[0147] As can be seen from Examples 1 to 31, the electrolyte contains sodium salt, and the molar concentration of the sodium salt in the electrolyte is 2 to 7 mol / L, and the battery has excellent room temperature cycle performance and high temperature cycle performance, and the high temperature gas generation phenomenon is small.

[0148] As can be seen from the comparison of Examples 1, 3-5, 7-14, and 31 with Examples 2 and 6, the molar concentration of sodium salt in the electrolyte is 3-7 mol / L, which can further reduce the gas generation phenomenon in the battery and improve the safety performance of the battery at high temperatures.

[0149] The electrolytes in Examples 1 to 31 all contain a first sodium salt, which contains sodium bis(fluorosulfonyl)imide (NaFSI) or sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), and the molar concentration of the first sodium salt is 0.01 mol / L or more. The batteries have excellent room temperature cycle performance and high temperature cycle performance, and exhibit little high-temperature gas generation.

[0150] As can be seen from the comparison between Example 1 and Example 31, the electrolyte further containing a second sodium salt can improve the room temperature cycling performance and high temperature cycling performance of the battery, reduce the high temperature gas generation phenomenon of the battery, and improve the electrochemical performance and safety performance of the battery.

[0151] In Examples 1 to 6 and 9 to 30, the electrolyte contains a second sodium salt, which includes sodium hexafluorophosphate (NaPF), sodium tetrafluoroborate (NaBF), or sodium trifluoroacetate (CFCOONa). The molar concentration of the second sodium salt in the electrolyte is less than 2.7 mol / L. The battery has excellent room temperature cycle performance and high temperature cycle performance, and exhibits little high-temperature gas generation.

[0152] As can be seen from the comparison of Examples 1, 12, and 14 with Example 13, the molar concentration of the second sodium salt is 0.2 to 2.7 mol / L, which can improve the room temperature cycle performance and high temperature cycle performance of the battery, reduce the high temperature gas generation phenomenon of the battery, and improve the electrochemical performance and safety performance of the battery.

[0153] As can be seen from the comparison between Example 7 and Example 8, and between Examples 1, 19-20 and Example 18, the electrolyte contains a lithium salt, and the lithium salt includes sodium hexafluorophosphate (LiPF6), lithium difluoro(oxalato)borate (LiODFB) or lithium bis(fluorosulfonyl)imide (LiFSI), which can improve the room temperature cycle performance and high temperature cycle performance of the battery, reduce the high temperature gas generation phenomenon of the battery, and improve the electrochemical performance and safety performance of the battery.

[0154] As can be seen from the comparison of Examples 1, 15-16 and Example 17, the solvent containing ethylene glycol dimethyl ether, diethylene glycol dimethyl ether or ethylene glycol ethyl methyl ether can improve the room temperature cycle performance and high temperature cycle performance of the battery, reduce the high temperature gas generation phenomenon of the battery, and improve the electrochemical performance and safety performance of the battery.

[0155] As can be seen from the comparison between Examples 1 and 29 and Example 30, providing an undercoat layer on at least one side of the negative electrode current collector, where the undercoat layer includes a carbon coating, can improve the room temperature cycle performance and high temperature cycle performance of the battery, and can also improve the electrochemical performance of the battery.

[0156] As can be seen from the comparison of Examples 1, 21 and 22 with Examples 23 and 24, the surface density of the undercoat layer on at least one side of the negative electrode current collector is X g / m 2 and the mass content Y% of the lithium salt based on the total mass of the electrolyte is controlled to satisfy the relational expression 0.84≦Y−0.01X≦5.84, thereby improving the room temperature cycle performance and high temperature cycle performance of the battery and improving the electrochemical performance of the battery.

[0157] As can be seen from the comparison of Examples 1, 25-26 and Examples 27-28, the surface density of the undercoat layer is 5-50 g / m 2 This can improve the room temperature cycle performance and high temperature cycle performance of the battery and improve the electrochemical performance of the battery.

[0158] As can be seen from the comparison between Example 1 and Example 29, the sodium secondary battery is a negative electrode-free sodium metal battery, which can improve the room temperature cycle performance and high temperature cycle performance of the battery and improve the electrochemical performance of the battery.

[0159] It should be noted that the present application is not limited to the above-described embodiments. The above-described embodiments are merely examples, and all embodiments that have substantially the same configuration as the technical idea and exhibit the same functions and effects within the scope of the technical solution of the present application are encompassed within the technical scope of the present application. Furthermore, various modifications that a person skilled in the art can make to the embodiments and other forms constructed by combining some of the components of the embodiments are also encompassed within the scope of the present application, as long as they do not deviate from the gist of the present application. [Explanation of symbols]

[0160] 1 battery pack 2 Upper Box 3 Lower Box 4 Battery Module 5. Sodium secondary battery 51 cases 52 Electrode assembly 53 Cover plate

Claims

1. 1. An electrolyte for a sodium secondary battery, comprising a solvent, wherein a ratio of an amount of free solvent material to a total amount of solvent material in the electrolyte is 50% or less.

2. 2. The electrolyte according to claim 1, wherein the electrolyte contains a sodium salt, and the molar concentration of the sodium salt in the electrolyte is 2 to 7 mol / L, and optionally 3 to 7 mol / L.

3. 3. The electrolyte of claim 2, wherein the sodium salt comprises a first sodium salt, and the solubility of the first sodium salt in the solvent at room temperature is greater than 4 mol / L.

4. 4. The electrolyte of claim 3, wherein the first sodium salt comprises one or more of sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethanesulfonyl)imide, sodium (fluorosulfonyl)(trifluoromethanesulfonyl)imide, and optionally one or two of sodium bis(fluorosulfonyl)imide and sodium bis(trifluoromethanesulfonyl)imide.

5. 5. The electrolyte according to claim 3, wherein the molar concentration of the first sodium salt in the electrolyte is 0.01 mol / L or more.

6. The electrolyte according to any one of claims 2 to 5, wherein the sodium salt comprises a second sodium salt, and the solubility of the second sodium salt in the solvent at room temperature is less than 3 mol / L.

7. 7. The electrolyte of claim 6, wherein the second sodium salt comprises one or more of sodium hexafluorophosphate, sodium tetrafluoroborate, sodium hexafluoroarsenate, sodium trifluoroacetate, sodium difluoro(oxalato)borate, sodium bis(oxalato)borate, and sodium trifluoromethanesulfonate.

8. 8. The electrolyte according to claim 6, wherein the molar concentration of the second sodium salt in the electrolyte is less than 2.7 mol / L, and preferably 0.2 to 2.7 mol / L.

9. 9. The electrolyte according to claim 1, wherein the electrolyte contains a lithium salt.

10. 10. The electrolyte of claim 9, wherein the lithium salt comprises one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium trifluoroacetate, lithium tetraphenylborate, lithium trifluoromethanesulfonate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide.

11. 11. The electrolyte of any one of claims 1 to 10, wherein the solvent comprises one or more of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol ethyl methyl ether, ethylene glycol dibutyl ether.

12. A sodium secondary battery comprising the electrolyte according to any one of claims 1 to 11.

13. The sodium secondary battery includes a negative electrode plate, the negative electrode plate including a negative electrode current collector and an undercoat layer formed on at least one side of the negative electrode current collector, and the electrolyte includes a lithium salt; The sodium secondary battery satisfies the relational expression 0.84≦Y−0.01X≦5.84, X g / m ２ 13. The sodium secondary battery according to claim 12, wherein Y is the areal density of the undercoat layer, and Y % is the mass content of the lithium salt based on the total mass of the electrolyte.

14. The surface density of the undercoat layer is 5 to 50 g / m ２ 14. The sodium secondary battery according to claim 13,

15. 15. The sodium secondary battery according to claim 13, wherein the undercoat layer comprises one or more of a carbon coating, an alloy coating, and a metal oxide coating, and is optionally a carbon coating.

16. The sodium secondary battery according to any one of claims 13 to 15, characterized in that the undercoat layer contains one or more of superconducting carbon black, ketjen black, acetylene black, carbon nanotubes, and graphene.

17. The secondary battery according to any one of claims 12 to 16, wherein the sodium secondary battery is a negative electrode-free sodium metal battery.

18. An electrical device comprising the sodium secondary battery according to any one of claims 12 to 17.

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