Electrolyte, secondary battery, battery module, battery pack, and device

Abstract

To provide a secondary battery that combines good output performance, high-temperature cycle performance, and high-temperature storage performance.SOLUTION: An electrolyte includes an electrolyte salt and an organic solvent. The electrolyte salt includes lithium bis(fluorosulfonyl)imide (LiFSI), and the mass % content of the lithium bis(fluorosulfonyl)imide (LiFSI) in the electrolyte is 4.5% to 11%. The organic solvent includes ethylene carbonate, and the mass % contents of the ethylene carbonate and the lithium bis(fluorosulfonyl)imide satisfy 0.9≤WLiFSI / (16.77%-WEC)≤2.9. The electrolyte according to the present invention allows a battery to have both low cell internal resistance, excellent high-temperature storage performance, and high-temperature cycle performance.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to the technical field of batteries, and in particular to an electrolyte, a secondary battery, a battery module, a battery pack, and an apparatus. [Background technology]

[0002] In recent years, with the daily depletion of fossil energy and the increasing pressure of environmental pollution, secondary batteries have gained unprecedented attention and development. The application scope of secondary batteries is becoming wider and wider, and they are applied in multiple fields such as energy storage power supply systems such as hydroelectric, thermal, wind and solar power stations, as well as electric tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace and so on.

[0003] With the continuous development of secondary batteries, users are putting forward stricter requirements for use (e.g., batteries must have output performance, cycle performance, and storage performance, etc.), so there is an urgent need to develop secondary batteries with good comprehensive performance. Summary of the Invention

[0004] The present invention has been made in view of the above problems, and an object of the present invention is to provide an electrolyte, a secondary battery, a battery module, a battery pack, and an apparatus, and a secondary battery containing the electrolyte has good output performance (low cell internal resistance), high-temperature cycle performance, and high-temperature storage performance.

[0005] In order to achieve the above object, a first aspect of the present invention provides an electrolytic solution comprising an electrolyte salt and an organic solvent, the electrolyte salt comprising lithium bis(fluorosulfonyl)imide (LiFSI), a mass % content of the lithium bis(fluorosulfonyl)imide (LiFSI) in the electrolytic solution being 4.5% to 11%, the organic solvent comprising ethylene carbonate (EC), and a pH of 0.9≦W LiFSI / (16.77%-W EC )≦2.9(W LiFSIis the mass% content of lithium bis(fluorosulfonyl)imide (LiFSI) in the electrolyte, and W EC is the mass % content of the ethylene carbonate (EC) in the electrolyte solution).

[0006] When the electrolyte satisfies all of the above conditions, the secondary battery can have good output performance (low cell internal resistance), high-temperature cycle performance, and high-temperature storage performance.

[0007] In any of the embodiments, optionally the electrolyte has a pH of 1.05≦W LiFSI / (16.77%-W EC When the electrolyte satisfies this condition, the output performance, high-temperature cycle performance, and high-temperature storage performance of the battery can be further improved.

[0008] In any of the embodiments, optionally, the electrolyte further comprises an additive comprising lithium difluorophosphate (LiPO2F2).

[0009] In any of the embodiments, optionally, the mass ratio of the lithium bis(fluorosulfonyl)imide (LiFSI) to the lithium difluorophosphate (LiPO2F2) is 7 to 26, and optionally, 11 to 22. When the mass relationship between the lithium bis(fluorosulfonyl)imide (LiFSI) and the lithium difluorophosphate (LiPO2F2) is controlled within the above range, the high temperature cycle performance of the battery can be further improved.

[0010] In any of the embodiments, optionally, the ratio of the total mass of the ethylene carbonate and the lithium bis(fluorosulfonyl)imide to the mass of the lithium difluorophosphate is 20 to 68, and optionally, 28 to 51. When the mass relationship of ethylene carbonate, lithium bis(fluorosulfonyl)imide, and lithium difluorophosphate is controlled within the above range, the high temperature cycle performance of the battery can be further improved.

[0011] In any of the embodiments, optionally, the mass % content of the lithium difluorophosphate (LiPO2F2) in the electrolyte is 1% or less, and optionally 0.3% to 0.7%. When the additive in the electrolyte contains lithium difluorophosphate, and in particular, the mass ratio is controlled within the above range, the high-temperature cycle performance of the battery can be further improved.

[0012] In any of the embodiments, optionally, the total mass% content of the ethylene carbonate (EC) and the lithium bis(fluorosulfonyl)imide (LiFSI) in the electrolyte is 16% to 24%, and optionally 17% to 24%. When the mass relationship between the ethylene carbonate and the lithium bis(fluorosulfonyl)imide is controlled within the above range, the output performance, high-temperature cycle performance, and high-temperature storage performance of the battery can be further improved.

[0013] In any embodiment, optionally, the mass% content of the lithium bis(fluorosulfonyl)imide (LiFSI) in the electrolyte is 6% to 11%. When the content of lithium bis(fluorosulfonyl)imide is controlled within the above range, the output performance, high-temperature cycle performance, and high-temperature storage performance of the battery can be further improved.

[0014] In any of the embodiments, optionally, the mass percent content of the ethylene carbonate (EC) in the electrolyte is 8.3% to 15.2%, and optionally 10% to 15.2%. When the mass percentage of ethylene carbonate is controlled within the above range, the output performance, high-temperature cycle performance, and high-temperature storage performance of the battery can be further improved.

[0015] In any of the embodiments, optionally, the electrolyte salt further includes lithium hexafluorophosphate (LiPF6), and optionally, the total mass% content of the lithium hexafluorophosphate and the lithium bis(fluorosulfonyl)imide in the electrolyte is 15.7% to 17.2%. When the mass relationship between lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide is controlled within the above range, the Li + The concentration is maintained within an appropriate range and the electrolyte has good electrical conductivity.

[0016] In a second aspect of the present invention, there is provided a secondary battery comprising the electrolyte solution according to the first aspect of the present invention.

[0017] In a third aspect of the present invention, there is provided a battery module including the secondary battery according to the second aspect of the present invention.

[0018] In a fourth aspect of the present invention, there is provided a battery pack including the battery module according to the third aspect of the present invention.

[0019] In a fifth aspect of the present invention, there is provided an apparatus including at least one selected from the secondary battery according to the second aspect of the present invention, the battery module according to the third aspect of the present invention, or the battery pack according to the fourth aspect of the present invention. [Brief description of the drawings]

[0020] [Figure 1] FIG. 1 is a schematic diagram of a secondary battery according to an embodiment of the present invention. [Diagram 2] FIG. 2 is an exploded view of the secondary battery according to the embodiment of the present invention shown in FIG. [Diagram 3] FIG. 1 is a schematic diagram of a battery module according to an embodiment of the present invention. [Figure 4] 1 is a schematic diagram of a battery pack according to an embodiment of the present invention. [Diagram 5] FIG. 5 is an exploded view of the battery pack according to the embodiment of the present invention shown in FIG. [Figure 6]FIG. 1 is a schematic diagram of an apparatus using a secondary battery as a power source according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Hereinafter, the specifically disclosed embodiments of the electrolyte, secondary battery, battery module, battery pack, and device of the present invention will be described in detail with reference to the appropriate accompanying drawings. However, unnecessary detailed description may be omitted. For example, detailed description of well-known matters and redundant description of substantially the same structure may be omitted. This is to prevent the following description from becoming unnecessarily long and to facilitate understanding by those skilled in the art. Note that the drawings and the following description are provided to enable those skilled in the art to fully understand the present invention, and are not intended to limit the spirit of the claims.

[0022] In the present invention, in order to simplify the description, some numerical ranges are specifically disclosed. However, any lower limit and any upper limit may be combined to form an unspecified range, and any lower limit and another lower limit may be combined to form an unspecified range, and similarly any upper limit and any other upper limit may be combined to form an unspecified range. In addition, each point or single numerical value disclosed individually may be combined with any other point or single numerical value, or another lower limit or upper limit, as a lower limit or upper limit, to form an unspecified range.

[0023] In one embodiment of the present invention, the present invention provides an electrolyte solution including an electrolyte salt and an organic solvent, the electrolyte salt including lithium bis(fluorosulfonyl)imide (LiFSI), the mass% content of the lithium bis(fluorosulfonyl)imide (LiFSI) in the electrolyte solution being 4.5% to 11%, the organic solvent including ethylene carbonate (EC), the mass% content of the ethylene carbonate (EC) and the lithium bis(fluorosulfonyl)imide (LiFSI) being 0.9≦W LiFSI / (16.77%-W EC )≦2.9(W LiFSIis the mass% content of lithium bis(fluorosulfonyl)imide (LiFSI) in the electrolyte, and W EC is the mass % content of ethylene carbonate (EC) in the electrolyte solution).

[0024] The electrolyte of the present invention contains a specific type of electrolyte salt and an organic solvent, and the contents of the electrolyte salt and the organic solvent are specially designed so that a battery using the electrolyte has good output performance (low cell internal resistance), high temperature cycle performance, and high temperature storage performance.

[0025] The inventors of the present invention found that lithium bis(fluorosulfonyl)imide (LiFSI) has good ionic conductivity and dissociation degree, and can be effectively dissociated in the electrolyte, but is easily oxidized on the surface of the positive electrode, which affects the high-temperature cycle performance of the secondary battery. When using lithium bis(fluorosulfonyl)imide (LiFSI) in the electrolyte to improve the output performance, ethylene carbonate (EC) is added to the organic solvent, and the content of these is adjusted so that the electrolyte is 0.9≦W LiFSI / (16.77%-W EC Through extensive research, it has been found that by adjusting the cell resistance so that the cell current density is 2.9 or less, the battery can have good output performance (low cell internal resistance), high-temperature cycle performance, and high-temperature storage performance.

[0026] In one embodiment, W LiFSI / (16.77%-W EC ) may be within a range of values ​​with any two of the following endpoints: 0.9, 0.92, 1.05, 1.11, 1.14, 1.4, 1.44, 1.47, 1.75, 1.77, 1.78, 1.79, 1.80, 1.82, 2.16, 2.2, 2.45, 2.49, 2.5, 2.84, 2.9. For example, 1.05≦W LiFSI / (16.77%-W EC )≦2.9,1.4≦W LiFSI / (16.77%-W EC )≦2.5,1.75≦W LiFSI / (16.77%-W EC)≦2.2, and although the inventors have shown the above values ​​in parallel, W LiFSI / (16.77%-W EC ) is within the range between any two of the above values, it does not mean that the same or similar performance can be obtained. LiFSI / (16.77%-W EC The preferred range of can be selected based on the specific considerations and specific experimental data below.

[0027] In certain embodiments, the electrolyte further comprises an additive comprising lithium difluorophosphate (LiPO2F2).

[0028] In one embodiment, the mass ratio of lithium bis(fluorosulfonyl)imide (LiFSI) to lithium difluorophosphate (LiPO2F2), W LiFSI / W LiPO2F2 is 7 to 26, optionally 11 to 22, except that W LiPO2F2 is the mass percent content of lithium difluorophosphate (LiPO2F2) in the electrolyte. Optionally, the mass ratio of lithium bis(fluorosulfonyl)imide (LiFSI) to lithium difluorophosphate (LiPO2F2) may be within a range of values ​​having any two of the following endpoints: 7, 7.70, 11.00, 12.32, 12.83, 15.40, 19.25, 21.56, 22, 25.67, and 26. For example, 7≦W LiFSI / W LiPO2F2 ≦26,11≦W LiFSI / W LiPO2F2 ≦26,11≦W LiFSI / W LiPO2F2 ≦22, and although the inventor has shown the above values ​​in parallel, W LiFSI / W LiPO2F2 It does not mean that the same or similar performance can be obtained if the W is within the range between any two of the above values. LiFSI / W LiPO2F2 The preferred range can be selected based on the following specific considerations and specific experimental data.

[0029] The inventors have found that when the additive of the electrolyte contains lithium difluorophosphate (LiPO2F2), and in particular, the mass relationship between lithium bis(fluorosulfonyl)imide (LiFSI) and lithium difluorophosphate (LiPO2F2) is controlled within the above range, lithium difluorophosphate (LiPO2F2) forms an interface film on the surface of the positive electrode in preference to lithium bis(fluorosulfonyl)imide (LiFSI), effectively reducing the probability of catalytic oxidation of lithium bis(fluorosulfonyl)imide (LiFSI) on the surface of the positive electrode, and further improving the high-temperature cycle performance of the battery.

[0030] In one embodiment, the ratio of the sum of the masses of ethylene carbonate and lithium bis(fluorosulfonyl)imide to the mass of lithium difluorophosphate (W LiFSI +W EC ) / W LiPO2F2 is 20 to 68, and optionally 28 to 51. Optionally, the ratio of the sum of the mass of ethylene carbonate and lithium bis(fluorosulfonyl)imide to the mass of the lithium difluorophosphate (W LiFSI +W EC ) / W LiPO2F2 can be in a range of values ​​with any two of the following endpoints: 20, 20.08, 28, 28.75, 33.57, 37.32, 40.32, 46.30, 50.43, 67.29, 68. For example, 20≦(W LiFSI +W EC ) / W LiPO2F2 ≦68,28≦(W LiFSI +W EC ) / W LiPO2F2 ≦68,28≦(W LiFSI +W EC ) / W LiPO2F2 ≦51,28≦(W LiFSI +W EC ) / W LiPO2F2 ≦46.3, and the inventors have shown the above values ​​in parallel, (W LiFSI +W EC ) / W LiPO2F2It does not mean that the same or similar performance can be obtained if the range between any two of the above values ​​is within the range. LiFSI +W EC ) / W LiPO2F2 The preferred range of can be selected based on the following specific considerations and specific experimental data. When the mass relationship between ethylene carbonate and lithium bis(fluorosulfonyl)imide is controlled within the above range, the high-temperature cycle performance of the battery can be further improved.

[0031] In one embodiment, the mass % content of W in the electrolyte solution of lithium difluorophosphate (LiPO2F2) is LiPO2F2 is 1% or less, and optionally 0.3% to 0.7%. Optionally, W is the mass % content in the lithium difluorophosphate (LiPO2F2) electrolyte. LiPO2F2 can be within a range of values ​​with any two of the following endpoints: 0%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 1.0%. For example, 0%≦W LiPO2F2 ≦1%, 0.3%≦W LiPO2F2 ≦1%, 0.3%≦W LiPO2F2 ≦0.7%, 0.4%≦W LiPO2F2 ≦0.7%, 0.5%≦W LiPO2F2 ≦0.7%, and although the inventors have shown the above values ​​in parallel, W LiPO2F2 It does not mean that the same or similar performance can be obtained if the W is within the range between any two of the above values. LiPO2F2 The preferred range can be selected based on the following specific considerations and specific experimental data.

[0032] In one embodiment, the total mass % content of ethylene carbonate (EC) and lithium bis(fluorosulfonyl)imide (LiFSI) in the electrolyte is 16% to 24%, and optionally 17% to 24%. Optionally, the total mass % content of ethylene carbonate (EC) and lithium bis(fluorosulfonyl)imide (LiFSI) (W LiFSI +W EC) may be within a numerical range with any two of the following as endpoints: 16%, 16.06%, 17.24%, 17.73%, 18.66%, 18.74%, 19.76%, 20.08%, 20.13%, 20.14%, 20.16%, 20.17%, 20.19%, 20.23%, 21.73%, 23.15%, 23.23%, and 24%. When the mass relationship between ethylene carbonate (EC) and lithium bis(fluorosulfonyl)imide (LiFSI) is controlled within the above range, the output performance, high-temperature cycle performance, and high-temperature storage performance of the battery can be further improved.

[0033] In one embodiment, the mass% content of lithium bis(fluorosulfonyl)imide (LiFSI) in the electrolyte is 6% to 11%. Optionally, the mass% content of lithium bis(fluorosulfonyl)imide (LiFSI) in the electrolyte may be within a numerical range having any two of the following endpoints: 4.5%, 4.62%, 6%, 6.16%, 7.70%, 9.24%, 10.78%, and 11%. When the content of lithium bis(fluorosulfonyl)imide is controlled within the above range, the output performance, high-temperature cycle performance, and high-temperature storage performance of the battery can be further improved.

[0034] In an embodiment, the mass% content of ethylene carbonate (EC) in the electrolyte is 8.3% to 15.2%. Optionally, it is 10% to 15.2%. Optionally, the mass% content of ethylene carbonate (EC) in the electrolyte may be 8.36%, 10%, 10.03%, 12.37%, 12.38%, 12.43%, 12.44%, 12.45%, 12.46%, 12.47%, 12.49%, 12.50%, 12.53%, 12.58%, 12.62%, 15.14%, 15.2%, or any two of the above values ​​combined. When the content of ethylene carbonate is controlled within the above range, the output performance, high temperature cycle performance, and high temperature storage performance of the battery can be further improved.

[0035] In one embodiment, the electrolyte salt further includes lithium hexafluorophosphate (LiPF6), and optionally, W, which is the total mass% content of the lithium hexafluorophosphate and the lithium bis(fluorosulfonyl)imide in the electrolyte solution. LiFSI +W LiPF6 is 15.7% to 17.2%, where W LiPF6 is the mass % content of lithium hexafluorophosphate in the electrolyte. Optionally, the sum of the mass % contents of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide in the electrolyte may be 15.7%, 15.87%, 16.16%, 16.45%, 16.74%, 17.03%, 17.2%, or the value is within a range obtained by combining any two of the above values. When the mass % contents of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide in the electrolyte are controlled within the above range, the Li + The concentration is maintained within an appropriate range and the electrolyte has good conductivity.

[0036] In one embodiment, the organic solvent further comprises at least one of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, methyl formate, ethyl formate, methyl acetate, ethyl acetate, propyl propionate, ethyl butanoate, ethyl propionate, propyl butyrate, ethylene carbonate, propylene carbonate, butylene carbonate, γ-butyrolactone, and tetrahydrofuran.

[0037] In certain embodiments, the electrolyte salt may optionally include one or more of lithium tetrafluoroborate (LiBF), lithium perchlorate (LiClO), lithium hexafluoroarsenate (LiAsF), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluoro(oxalato)borate (LiDFOB), lithium bis(oxalato)borate (LiBOB), lithium difluorobis(oxalato)phosphate (LiDFOP), and lithium tetrafluoro(oxalato)phosphate (LiTFOP), in addition to lithium hexafluorophosphate (LiPF) and lithium bis(fluorosulfonyl)imide (LiFSI).

[0038] In some embodiments, the electrolyte may further include other types of additives, if desired. For example, the additives may include anode film-forming additives, cathode film-forming additives, and additives for improving certain battery performance, such as additives for improving the overcharge performance of the battery, additives for improving the high-temperature performance of the battery, and additives for improving the low-temperature performance of the battery.

[0039] The secondary battery, battery module, battery pack, and device of the present invention will be described below with reference to the appropriate drawings.

[0040] In one embodiment of the present invention, a secondary battery is provided.

[0041] Generally, a secondary battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator. During the charging and discharging process of the battery, active ions shuttle between the positive electrode sheet and the negative electrode sheet to be inserted and removed. The electrolyte plays a role in conducting ions between the positive electrode sheet and the negative electrode sheet. The separator is provided between the positive electrode sheet and the negative electrode sheet, and mainly plays a role in preventing short-circuiting between the positive and negative electrodes, while allowing ions to pass through.

[0042] [Positive electrode sheet] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer provided on at least one surface of the positive electrode current collector, the positive electrode film layer includes a positive electrode active material, and the positive electrode active material includes, but is not limited to, lithium cobalt oxide, lithium nickel manganese cobalt oxide, lithium nickel manganese aluminate, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium iron silicate, lithium vanadium silicate, lithium cobalt silicate, lithium manganese silicate, lithium manganese spinel oxide, lithium nickel manganese spinel oxide, lithium titanate, etc. The positive electrode active material can be one or more of these.

[0043] For example, the positive electrode current collector has two surfaces opposing each other in the thickness direction of the positive electrode current collector, and the positive electrode film layer is provided on either one or both of the two opposing surfaces of the positive electrode current collector.

[0044] In the secondary battery of the present invention, a metal foil sheet or a composite current collector is used for the positive electrode current collector. For example, an aluminum foil is used as the metal foil sheet. The composite current collector can include a lower layer of a polymeric material and a metal layer formed on at least one surface of the lower layer of a polymeric material. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymeric material substrate (e.g., a substrate such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0045] The positive electrode membrane layer further optionally includes a conductive agent. However, there is no specific limit to the type of conductive agent, and those skilled in the art can select it according to actual needs. For example, the conductive agent used in the positive electrode membrane layer may be one or more selected from superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0046] In the present invention, the positive electrode sheet can be prepared by a method known in the art. For example, the positive electrode active material, conductive agent, and binder in the present invention can be dispersed in a solvent (e.g., N-methylpyrrolidone (NMP)) to form a uniform positive electrode slurry, and the positive electrode slurry can be applied to a positive electrode current collector, and the positive electrode sheet can be obtained through processes such as oven drying and cold pressing.

[0047] [Negative electrode sheet] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer provided on at least one surface of the negative electrode current collector, the negative electrode film layer including a negative electrode active material.

[0048] In the secondary battery of the present invention, the negative electrode active material may be a negative electrode active material generally used in the field for producing a negative electrode of a secondary battery, and examples of the negative electrode active material include artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based material, tin-based material, and lithium titanate. The silicon-based material may be one or more selected from silicon alone, silicon oxide (e.g., silicon monoxide), silicon-carbon complex, silicon-nitrogen complex, and silicon alloy. The tin-based material may be one or more selected from tin alone, tin oxide, and tin alloy.

[0049] For example, the negative electrode current collector has two surfaces opposing each other in the thickness direction of the negative electrode current collector, and the negative electrode film layer is provided on either one or both of the two opposing surfaces of the negative electrode current collector.

[0050] In the secondary battery of the present invention, a metal foil sheet or a composite current collector is used for the negative electrode current collector. For example, copper foil is used as the metal foil sheet. The composite current collector can include a lower layer of a polymeric material and a metal layer formed on at least one surface of a substrate of a polymeric material. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymeric substrate (e.g., substrate such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE) etc.).

[0051] In the secondary battery of the present invention, the negative electrode film layer usually contains, in addition to the negative electrode active material, an optional binder, an optional conductive agent, and an optional other auxiliary agent, and is usually formed by applying and drying a negative electrode slurry. The negative electrode slurry is usually formed by dispersing the negative electrode active material and the optional conductive agent and binder in a solvent and stirring the mixture uniformly. The solvent may be N-methylpyrrolidone (NMP) or deionized water.

[0052] The conductive agent may be, by way of example, one or more selected from superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0053] The binder may be, by way of example, one or more selected from styrene butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA) and carboxymethyl chitosan (CMCS).

[0054] Other optional auxiliary agents include, for example, thickeners (eg, sodium carboxymethylcellulose (CMC-Na)).

[0055] [Separator] A secondary battery generally further includes a separator. The separator separates the positive electrode sheet and the negative electrode sheet, prevents short circuiting inside the battery, and allows active ions to pass through the separator and move between the positive and negative electrodes. In the present invention, there is no particular limitation on the type of separator, and any known separator with a porous structure having good chemical stability and mechanical stability may be used. In an embodiment, the material of the separator may be one or more selected from glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator may be a single-layer film or a multi-layer composite film, and there is no particular limitation. When the separator is a multi-layer composite film, the materials of each layer may be the same or different, and there is no particular limitation.

[0056] In some embodiments, the positive electrode sheet, the negative electrode sheet and the separator can be formed into an electrode unit by a winding process or a stacking process.

[0057] In an embodiment, the secondary battery may include an exterior packaging, which is used to package the electrode unit and the electrolyte.

[0058] In an embodiment, the exterior of the secondary battery may be a hard housing, such as a hard plastic housing, an aluminum housing, a steel housing, etc. The exterior of the secondary battery may be a soft package, such as a pouch-type soft package. The material of the soft package may be plastic, and examples of the plastic include polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0059] In the present invention, the shape of the secondary battery is not particularly limited, and it may be cylindrical, rectangular, or any other shape. For example, Fig. 1 shows a secondary battery 5 having a rectangular structure as an example.

[0060] Referring to FIG. 2, in an embodiment, the exterior may include a housing 51 and a cover 53. However, the housing 51 includes a bottom plate and a side plate connected to the bottom plate, and a receiving chamber is formed by being surrounded by the bottom plate and the side plate. The housing 51 has an opening connected to the receiving chamber, and the cover 53 is placed over the opening to close the receiving chamber. The positive electrode sheet, the negative electrode sheet, and the separator form an electrode unit 52 by a winding process or a stacking process. The electrode unit 52 is packaged in the receiving chamber. The electrolyte is impregnated into the electrode unit 52. The number of electrode units 52 included in the secondary battery 5 may be one or more, and can be selected by those skilled in the art according to actual needs.

[0061] In an embodiment, the secondary battery may be assembled into a battery module, and the number of secondary batteries included in the battery module may be one or more, and the specific number can be selected by a person skilled in the art according to the application and capacity of the battery module.

[0062] Fig. 3 shows an example of a battery module 4. Referring to Fig. 3, in the battery module 4, the secondary batteries 5 may be arranged in sequence along the longitudinal direction of the battery module 4. Of course, they may be arranged in any other manner. Furthermore, the secondary batteries 5 may be fixed by a fastener.

[0063] Optionally, the battery module 4 may further include a housing having an accommodating space, and the multiple secondary batteries 5 are accommodated in the accommodating space.

[0064] In an embodiment, the battery modules may be assembled into a battery pack, and the number of battery modules included in the battery pack can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0065] 4 and 5 show an example of a battery pack 1. Referring to Fig. 4 and Fig. 5, the battery pack 1 may include a battery case and a plurality of battery modules 4 arranged in the battery case. The battery case may include an upper case 2 and a lower case 3, and the upper case 2 may be covered by the lower case 3 to form a sealed space for accommodating the battery modules 4. The plurality of battery modules 4 may be arranged in the battery case in any manner.

[0066] The present invention also provides an apparatus including one or more of the secondary batteries, battery modules, or battery packs provided by the present invention. The secondary batteries, battery modules, or battery packs can be used as a power source for the apparatus or as an energy storage unit for the apparatus. The apparatus can be, but is not limited to, a mobile device (e.g., a mobile phone, a laptop, etc.), an electric vehicle (e.g., a pure electric vehicle, a hybrid vehicle, a plug-in hybrid vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), a train, a ship, a satellite, an energy storage system, etc.

[0067] The device can select a secondary battery, a battery module, or a battery pack according to the requirements of its use.

[0068] An example of an apparatus is shown in Figure 6. The apparatus may be a pure electric vehicle, a hybrid vehicle, or a plug-in hybrid vehicle. To meet the high power and high energy density requirements for the secondary battery of the apparatus, a battery pack or a battery module may be used.

[0069] Other exemplary devices include mobile phones, tablets, notebook computers, etc. These devices are generally required to be lightweight and thin, and can use secondary batteries as their power source. Working Example

[0070] The following describes examples of the present invention. The examples described below are illustrative and are used only to explain the present invention, and should not be construed as limiting the present invention. In the examples, unless a specific technique or condition is shown, the technique or condition is described in the literature in the field, or the product instructions are followed. For reagents or equipment used without a manufacturer, normal products can be purchased commercially. The content of each component in the examples of the present invention is based on mass unless otherwise specified.

[0071] Example 1 All of the lithium-ion batteries (also simply called batteries) were fabricated by the following method.

[0072] (1) Preparation of positive electrode sheet Ternary materials (LiNi 0.8 Co 0.1 Mn 0.1 ), polyvinylidene fluoride as a binder, and acetylene black as a conductive agent were mixed in a weight ratio of 90:5:5, N-methylpyrrolidone (NMP) was added, and the system was stirred using a vacuum mixer until the system was uniform and transparent to obtain a positive electrode slurry. The positive electrode slurry was evenly applied to aluminum foil, and the aluminum foil was dried at room temperature and then transferred to a 120°C oven and dried for 1 hour. The aluminum foil was then cold pressed and cut to obtain a positive electrode sheet.

[0073] (2) Preparation of negative electrode sheet Artificial graphite as the negative electrode active material, carbon black as the conductive agent, and SBR as the binder were mixed in a mass ratio of 92:2:6, deionized water was added, and a negative electrode slurry was obtained by a vacuum mixer. The negative electrode slurry was uniformly applied to copper foil as a negative electrode current collector, and the copper foil was dried at room temperature and then transferred to an oven at 120°C to dry for 1 hour, and then cold pressed and cut to obtain a negative electrode sheet.

[0074] (3) Preparation of electrolyte: In a glove box with an argon atmosphere and a water content of <10 ppm, ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed as organic solvents in a mass ratio of EC:EMC:DEC = 15:55:30 to obtain an organic solvent mixture. Predetermined amounts of lithium bis(fluorosulfonyl)imide (LiFSI) and lithium hexafluorophosphate (LiPF6) were mixed in this order, with the mass% content of lithium bis(fluorosulfonyl)imide (LiFSI) in the electrolyte being 4.62%, the mass% content of lithium hexafluorophosphate (LiPF6) in the electrolyte being 11.25%, the mass% content of ethylene carbonate (EC) in the electrolyte being 12.62%, and the mass% content of W LiFSI / (16.77%-W EC ) was added to the mixture so that the concentration became 1.11.

[0075] (4) Preparation of secondary battery: The positive electrode sheet, separator, and negative electrode sheet were stacked in this order and rolled up so that the separator played a role in isolating the positive and negative electrode sheets. The electrode unit was then set in an exterior case, the electrolyte prepared above was injected, and a secondary battery was obtained through processes such as vacuum packaging, leaving it, chemical conversion, and shaping.

[0076] (Examples 2 to 17, Comparative Examples 1 to 5) Examples 2 to 17 and Comparative Examples 1 to 5 were prepared in the same manner as Example 1, but product parameters such as the mass ratio of ethylene carbonate (EC), the mass ratio of lithium bis(fluorosulfonyl)imide (LiFSI), the mass ratio of lithium hexafluorophosphate (LiPF6), and the mass ratio of lithium difluorophosphate (LiPO2F2) were adjusted as shown in Table 1. However, in Examples 10 to 17, predetermined amounts of lithium bis(fluorosulfonyl)imide (LiFSI) and lithium hexafluorophosphate (LiPF6) were added to the organic solvent mixture, and then lithium difluorophosphate (LiPO2F2) was added to the mixture.

[0077] [Table 1-1] [Table 1-2]

[0078] [Table 2]

[0079] Performance Testing (1) Lithium-ion battery expansion rate test at 60°C At 60°C, the lithium-ion battery was charged at a constant current of 0.5C until the voltage reached 4.35V, then charged at a constant voltage until the current dropped to 0.05C, and the thickness of the lithium-ion battery was measured and designated as h0. The lithium-ion battery was then placed in an incubator at 60°C and stored for 30 days, after which it was taken out and the thickness of the lithium-ion battery was measured and designated as h1. The thickness expansion rate of the lithium-ion battery after 30 days of storage = [(h1-h0) / h0] x 100%.

[0080] (2) Initial DCR performance test for lithium-ion batteries A newly prepared battery is charged at a constant current of 1C to 4.35V, then charged at a constant voltage until the current drops to 0.05C, then discharged at a constant current of 1C for 30 minutes, the voltage at this time is V1, then discharged at a constant current of 4C(I) for 30 seconds, the voltage at the end of discharge is V2, sampled at 0.1 seconds, the discharge DCR of the cell at 50% SOC is (V1-V2) / I.

[0081] (3) Battery cycle capacity test At 45°C, the secondary battery was charged at a constant current of 1C until it reached 4.25V, then charged at a constant voltage of 4.25V until the current reached 0.05C, left for 5 minutes, and then discharged at a constant current of 1C until it reached the lower limit of the cut-off voltage. This was the first charge / discharge cycle of the secondary battery, and the discharge capacity at this time was taken as the discharge capacity of the first cycle of the secondary battery. The secondary battery was charged / discharged 800 times using the above method, and the discharge capacity of the secondary battery after 800 cycles was obtained. The capacity retention rate (%) of the secondary battery after 800 cycles at 45°C = (discharge capacity of the secondary battery after 800 cycles / initial discharge capacity of the secondary battery) x 100%.

[0082] [Table 3]

[0083] As can be seen from the above results, in the electrolyte solutions obtained in Examples 1 to 17, the mass% content of lithium bis(fluorosulfonyl)imide (LiFSI) in the electrolyte solution was 4.5% to 11%, and the mass% content of ethylene carbonate (EC) and lithium bis(fluorosulfonyl)imide (LiFSI) was 0.9≦W LiFSI / (16.77%-W EC )≦2.9. The batteries fabricated using the electrolyte had good output performance, high-temperature storage performance, and high-temperature cycle performance. On the other hand, in Comparative Examples 1 to 5, the contents of each component were outside the above ranges, and the output performance, high-temperature storage performance, and high-temperature cycle performance were inferior to those of Examples 1 to 17.

[0084] In addition, as can be seen from the above results, the electrolyte solutions of Examples 10 to 17 further contained lithium difluorophosphate (LiPO2F2), and the batteries fabricated using these electrolyte solutions had better high-temperature cycle performance than those of Examples 1 to 9.

[0085] It should be noted that the present invention is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiment that has substantially the same configuration as the technical idea and achieves the same effect within the scope of the technical solution of the present invention is included in the technical scope of the present invention. Furthermore, various modifications that a person skilled in the art can make to the embodiments without departing from the gist of the present invention, and other forms formed by combining some of the components in the embodiments are also included in the scope of the present invention. [Explanation of symbols]

[0086] 1 Battery pack 2 Upper case 3 Lower case 4 Battery Module 5 Secondary battery 51 Housing 52 Electrode unit 53 Cover

Claims

1. An electrolytic solution comprising an electrolyte salt and an organic solvent, the electrolyte salt contains lithium bis(fluorosulfonyl)imide, and the mass% content of the lithium bis(fluorosulfonyl)imide in the electrolyte solution is 4.5% to 11%, the organic solvent contains ethylene carbonate, and the mass% content of the ethylene carbonate in the electrolytic solution is 4.6% to 15.2%; The electrolyte further comprises an additive comprising lithium difluorophosphate; The mass ratio of the lithium bis(fluorosulfonyl)imide to the lithium difluorophosphate is 11 to 26.

2. The electrolyte according to claim 1, wherein the mass% content of the lithium bis(fluorosulfonyl)imide in the electrolyte is 6% to 11%.

3. The electrolyte according to claim 1, wherein the mass% content of the ethylene carbonate in the electrolyte is 8.3% to 15.2%.

4. The electrolyte according to claim 1, wherein the mass% content of the ethylene carbonate in the electrolyte is 10% to 15.2%.

5. 2. The electrolyte solution according to claim 1, wherein a mass ratio of the lithium bis(fluorosulfonyl)imide to the lithium difluorophosphate is 11 to 22.

6. 2. The electrolyte solution according to claim 1, wherein a ratio of the total mass of the ethylene carbonate and the lithium bis(fluorosulfonyl)imide to the mass of the lithium difluorophosphate is 20 to 68.

7. 2. The electrolyte solution according to claim 1, wherein a ratio of the total mass of the ethylene carbonate and the lithium bis(fluorosulfonyl)imide to the mass of the lithium difluorophosphate is 28 to 51.

8. The electrolyte solution according to claim 1 , wherein the mass % content of the lithium difluorophosphate in the electrolyte solution is 1% or less.

9. The electrolyte according to claim 1, wherein the mass% content of the lithium difluorophosphate in the electrolyte is 0.3% to 0.7%.

10. The electrolyte according to claim 1, wherein the total mass% content of the ethylene carbonate and the lithium bis(fluorosulfonyl)imide in the electrolyte is 16% to 24%.

11. The electrolyte solution according to claim 1, wherein the total mass% content of the ethylene carbonate and the lithium bis(fluorosulfonyl)imide in the electrolyte solution is 17% to 24%.

12. The electrolyte solution according to claim 1 , wherein the electrolyte salt further comprises lithium hexafluorophosphate.

13. The electrolyte according to claim 12, wherein the total mass% content of the lithium hexafluorophosphate and the lithium bis(fluorosulfonyl)imide in the electrolyte is 15.7% to 17.2%.

14. A secondary battery comprising the electrolyte solution according to claim 1.

15. A battery module comprising the secondary battery according to claim 14.

16. A battery pack comprising the battery module according to claim 15.

17. An apparatus comprising at least one selected from the group consisting of the secondary battery according to claim 14, the battery module according to claim 15, and the battery pack according to claim 16.

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