A lithium-ion battery

Abstract

The application discloses a lithium ion battery, which comprises a positive electrode sheet, a composite diaphragm, a negative electrode sheet and an electrolyte, the composite diaphragm comprises a base film and a swelling layer, the swelling layer is arranged on at least one surface of the base film, and the swelling rate of the swelling layer is Y; the electrolyte comprises a first electrolyte for primary injection and a second electrolyte for secondary injection, the injection amount of the primary injection is X1, the injection amount of the secondary injection is X2, and the total injection amount of the electrolyte is X; wherein X1, X2 and Y satisfy the following relationship: X2<=X1; 0.7Y<=10(X1+X2)-10.5<=1.5Y. When the lithium ion battery is charged and discharged, the electrolyte enters and exits the electrode sheet pores, the electrolyte outside the battery changes, and the thickness of the swelling layer on the diaphragm also changes. The swelling rate of the diaphragm, the injection amount of the primary injection and the injection amount of the secondary injection satisfy the relationship to inhibit the deformation of the electrode sheet, and the problem of broken sheet of the lithium ion battery during the charging and discharging cycle is solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of secondary batteries, in particular to a lithium ion battery. BACKGROUND

[0002] Lithium ion batteries are widely used in electronic digital products, electric vehicles, energy storage systems, aerospace and other fields due to their excellent high operating voltage, long cycle life, high energy density and environmental protection characteristics. With the progress of technology and the improvement of application requirements, further optimization of battery performance is particularly important, and rationalization of battery design is one of the key means to achieve this goal.

[0003] In the conventional design of lithium ion batteries, the battery structure usually includes connecting the positive and negative electrode sheets to metal conductors through ultrasonic welding, isolating the two electrode sheets by a separator, and using a winding technology to form the electrode sheets, and finally using a finishing tape to fix the battery cell. However, this structure has a significant problem in actual use: the electrode sheets will experience a thickness change of 0-50% during charging and discharging. Since copper foil is commonly used as a conductive material, its elongation rate is usually about 20%, and this thickness change often causes the copper foil to stretch beyond its threshold, causing the electrode sheet to break, seriously affecting the service life and safety of the battery.

[0004] Therefore, it is necessary to provide a new technical solution to solve the above technical problems. SUMMARY

[0005] The present application aims to: in view of the shortcomings of the prior art, provide a lithium ion battery which can effectively solve the problem of electrode sheet expansion and breakage during charging and discharging of the battery.

[0006] In order to achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0007] A lithium ion battery, comprising a positive electrode sheet, a composite separator, a negative electrode sheet and an electrolyte, the composite separator comprising a base film and a swelling layer, the swelling layer being provided on at least one surface of the base film, the swelling rate of the swelling layer being Y;

[0008] The electrolyte comprises a first electrolyte for primary injection and a second electrolyte for secondary injection, the injection amount of the primary injection being X1, the injection amount of the secondary injection being X2, and the total injection amount of the electrolyte being X;

[0009] Wherein, X1, X2 and Y satisfy the relationship: X2≤X1; 0.7Y≤10(X1+X2)-10.5≤1.5Y.

[0010] Preferably, the swelling layer has a swelling ratio Y of 150-250%, and the total electrolyte injection amount X of the electrolyte is 120-140%.

[0011] Preferably, the first injection amount X1 of the first injection is 60-70%, and the second injection amount X2 of the second injection is 50-60%.

[0012] Preferably, the swelling layer comprises a first component and a second component.

[0013] The first component is an ethylene-ester copolymer, accounting for 83-92% of the total mass of the swelling layer; and the second component is a crosslinking agent, accounting for 8-17% of the total mass of the swelling layer.

[0014] Preferably, the ethylene-ester copolymer comprises at least one of ethylene-vinyl acetate copolymer, ethylene-acrylic acid copolymer, ethylene-ethyl acrylate copolymer, ethylene-methyl methacrylate copolymer, and ethylene-acrylate copolymer.

[0015] Preferably, the crosslinking agent is at least one of 2,5-dimethyl-2,5-bis(tert-butyl peroxy) hexane, tert-butyl peroxy-2-ethylhexyl carbonate, 1,1-di-tert-butyl peroxy-3,3,5-trimethylcyclohexane, dicumyl peroxide, benzoyl peroxide, diisopropylbenzene hydroperoxide, and diethylenetriamine.

[0016] Preferably, the swelling layer is in a dot, line, or surface shape covering the surface of the base film, and the area of the swelling layer in orthographic projection accounts for 50-100% of the area of the base film in orthographic projection.

[0017] Preferably, the composite separator further comprises a ceramic layer; wherein the ceramic layer is arranged between the base film and the swelling layer.

[0018] And / or, the ceramic layer is arranged side by side with the swelling layer on the surface of the base film.

[0019] And / or, the ceramic layer is arranged on the outer surface of the swelling layer.

[0020] Preferably, the electrolyte comprises a first electrolyte for the first injection and a second electrolyte for the second injection; wherein the volume content of the first injection is 60-70% of the total electrolyte injection volume content, and the volume content of the second injection is 50-60% of the total electrolyte injection volume content.

[0021] Preferably, the first electrolyte comprises a first additive, and the first additive comprises fluoroethylene carbonate, vinylene carbonate, 1,3-propane sultone, and a nitrile compound.

[0022] Preferably, the second electrolyte comprises a second additive, and the second additive comprises fluoroethylene carbonate, lithium bisfluorosulfonylimide, 1,3-propane sultone and a nitrile compound.

[0023] Preferably, the nitrile compound is at least one of succinonitrile, adiponitrile, pivalonitrile, 1,3,6-hexanetristitnitrile, p-fluorobenzonitrile and p-methylbenzonitrile.

[0024] Preferably, the total content of fluoroethylene carbonate and vinylene carbonate in the first additive is 1-3% of the total mass of the electrolyte, the content of 1,3-propane sultone is 1-5% of the total mass of the electrolyte, and the content of the nitrile compound is 1-5% of the total mass of the electrolyte.

[0025] Preferably, the content of fluoroethylene carbonate and lithium bisfluorosulfonylimide in the second additive is 1-3% of the total mass of the electrolyte, the total content of 1,3-propane sultone is 0.01-0.2% of the total mass of the electrolyte, and the content of the nitrile compound is 0.1-1% of the total mass of the electrolyte.

[0026] Preferably, the first electrolyte and the second electrolyte further comprise a lithium salt and an organic solvent.

[0027] Preferably, the lithium salt is selected from one or more of lithium hexafluorophosphate, lithium difluorophosphate, lithium difluorobisoxalate phosphate, lithium tetrafluorooxalate phosphate, lithium oxalate phosphate, lithium bisoxalate borate, lithium difluorooxalate borate, lithium tetrafluoroborate, lithium bisfluorosulfonylimide salt and lithium bisfluorosulfonylimide, and the content of the lithium salt is 0.1-15.0wt% of the total mass of the electrolyte.

[0028] Preferably, the organic solvent is selected from one or more of ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl propionate, propyl propionate, ethyl acetate, ethyl n-butyrate and gamma-butyrolactone, and the content of the organic solvent is 20.0-70.0wt% of the total mass of the electrolyte.

[0029] Compared with the prior art, the present application has the following beneficial effects:

[0030] (1) The secondary battery provided by the present application sets a swelling layer on the base film. When the lithium ion battery is in a charge-discharge cycle, the electrolyte enters and exits the pores of the pole piece, and the electrolyte outside the battery also changes. The thickness of the swelling layer on the diaphragm also changes. The swelling rate of the diaphragm, the first liquid injection amount and the second liquid injection amount satisfy the relationship: X2≤X1; 0.7Y≤10(X1+X2)-10.5≤1.5Y, so as to inhibit the deformation of the pole piece and solve the problem of pole piece breakage of the lithium ion battery in the charge-discharge cycle.

[0031] (2) The swelling layer of the diaphragm of the present application consumes a small amount of electrolyte. In order to keep the battery cell from having insufficient electrolyte and causing lithium precipitation, a secondary electrolyte injection is required, and the first electrolyte and the second electrolyte used for the primary electrolyte injection and the secondary electrolyte injection are configured to have different properties, thereby ensuring excellent cycle performance and high-temperature performance of the battery. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 A graph of the relationship between the swelling rate and the injection amount of the present application. DETAILED DESCRIPTION

[0033] In order to make the technical solutions and advantages of the present application clearer, the technical solutions of the present application will be described clearly and completely below in conjunction with specific embodiments. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0034] According to a first aspect of the present application, the present application aims to provide a lithium ion battery, comprising a positive electrode sheet, a composite diaphragm, a negative electrode sheet, and an electrolyte, the composite diaphragm comprising a base film and a swelling layer, the swelling layer being arranged on at least one surface of the base film, the swelling rate of the swelling layer being Y;

[0035] The electrolyte comprises a first electrolyte for primary electrolyte injection and a second electrolyte for secondary electrolyte injection, the injection amount of the primary electrolyte injection being X1, the injection amount of the secondary electrolyte injection being X2, and the total injection amount of the electrolyte being X;

[0036] wherein X1, X2, and Y satisfy the relationship: X2≤X1; 0.7Y≤10(X1+X2)-10.5≤1.5Y.

[0037] When the lithium ion battery is in a charge-discharge cycle, the electrolyte enters and exits the pores of the electrode sheet, and the electrolyte outside the battery cell also changes, and the thickness of the swelling layer on the diaphragm also changes, so that the thickness change of the electrode sheet is adapted to the elongation rate of the copper foil. The swelling rate of the diaphragm, the injection amount of the primary electrolyte injection, and the injection amount of the secondary electrolyte injection satisfy the relationship: X2≤X1; 0.7Y≤10(X1+X2)-10.5≤1.5Y, so as to inhibit the deformation of the electrode sheet and solve the problem of broken pieces of the lithium ion battery in the charge-discharge cycle. The injection amount X1 of the primary electrolyte injection is to ensure that the amount of electrolyte is sufficient during formation, and the injection amount X2 of the secondary electrolyte injection is to adjust the formula of the overall battery cell electrolyte solvent and additives.

[0038] When the lithium ion battery is charged, the thickness of the pole piece increases, the porosity increases, the electrolyte enters the pores of the pole piece, the free electrolyte outside the battery decreases, the thickness of the swelling layer of the diaphragm decreases, and the overall thickness of the battery does not change; when the lithium ion battery is discharged, the thickness of the pole piece decreases, the porosity decreases, the electrolyte flows out of the interstice of the pole piece, the free electrolyte outside the battery increases, the thickness of the swelling layer of the diaphragm increases, and the overall thickness of the battery does not change.

[0039] In some embodiments, the swelling rate Y of the swelling layer is 150-250%, for example, it can be 150%, 160%, 180%, 200%, 220% or 250%; the total amount of electrolyte X is 120-140%, for example, it can be 120%, 125%, 130%, 135% or 140%.

[0040] Wherein, when the swelling rate is too small, the swelling diaphragm cannot provide the reserved space for the expansion of the pole piece, and the pole piece is broken after cycling, and when the swelling rate is too large, the thickness of the swelling diaphragm will squeeze the pole piece.

[0041] In some embodiments, the amount of electrolyte X1 of the first injection is 60-70%, for example, it can be 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69% or 70%; the amount of electrolyte X2 of the second injection is 50-60%, for example, it can be 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59% or 60%.

[0042] Wherein, the first injection improves the thermal stability of SEI and the density of SEI film through specific additives, and the second injection ensures the overall rate and low-temperature performance of the battery through the use of a solvent system with a high EP component of more than 40%.

[0043] In some embodiments, the swelling layer comprises a first component and a second component;

[0044] The first component is an ethylene-ester copolymer, which accounts for 83%-92% of the total mass of the swelling layer, for example, it can be 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91% or 92%.

[0045] The second component is a crosslinking agent, which accounts for 8%-17% of the total mass of the swelling layer, for example, it can be 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16% or 17%. When the content of the crosslinking agent is too low, the adhesion between the base film and the swelling layer will be not firm and easy to fall off.

[0046] In some embodiments, the ethylene-ester copolymer includes at least one of ethylene-vinyl acetate copolymer, ethylene-acrylic acid copolymer, ethylene-ethyl acrylate copolymer, ethylene-methyl methacrylate copolymer, and ethylene-acrylate copolymer.

[0047] In some embodiments, the cross-linking agent is at least one of 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexane, tert-butylperoxy-2-ethylhexyl carbonate, 1,1-di-tert-butylperoxy-3,3,5-trimethylcyclohexane, dicumyl peroxide, benzoyl peroxide, diisopropylbenzene hydroperoxide, and diethylenetriamine.

[0048] In some embodiments, the swelling layer covers the surface of the base film in a dot, line or plane shape, and the area of the forward projection of the swelling layer accounts for 50-100% of the area of the forward projection of the base film, for example, it can be 50%, 60%, 70%, 80%, 90% or 100%.

[0049] In some embodiments, the composite separator further includes a ceramic layer; wherein the ceramic layer is arranged between the base film and the swelling layer;

[0050] and / or, the ceramic layer is arranged side by side with the swelling layer on the surface of the base film;

[0051] and / or, the ceramic layer is arranged on the outer surface of the swelling layer.

[0052] In some embodiments, after the residual electrolyte and the gas generated by the formation are extracted, the second electrolyte is added to 80% of the total amount of the electrolyte.

[0053] In some embodiments, the first electrolyte includes a first additive, and the first additive includes fluoroethylene carbonate, vinylene carbonate, 1,3-propane sultone, and nitrile compounds.

[0054] Wherein the main products of the decomposition of fluoroethylene carbonate in the battery are LiF and -CHF-OCO2-type compounds, which can help to form a more stable SEI film, thereby preventing further decomposition of the electrolyte on the electrode surface and improving the cycle performance of the battery. The decomposition of fluoroethylene carbonate can react with Si negative electrode during the delithiation process, generating adhesion between Si particles and SEI, improving the stability of the SEI film and the cycle performance of the Si negative electrode battery;

[0055] Vinylene carbonate as a film-forming additive, will occur on the surface of the lithium-ion battery negative polymerization reaction, forming a dense SEI film, to prevent further reduction of electrolyte decomposition on the negative electrode surface. The decomposition products of vinylene carbonate include Li2CO3, Li2C2O4 and HCO2Li, etc., these inorganic and organic compounds together constitute SEI film, improve the cycle stability and safety of the battery;

[0056] 1,3-propane sulfone lactone is usually used to improve the high temperature performance of electrolyte, it can reduce the decomposition of electrolyte at high temperature, improve the thermal stability of the battery. 1,3-propane sulfone lactone decomposition products may help to form a stable SEI film, improve the safety and cycle stability of the battery.

[0057] Fluorinated ethylene carbonate, vinylene carbonate and 1,3-propane sulfone lactone as a combination of synergistic effect, in which fluorinated ethylene carbonate and vinylene carbonate help to form a stable SEI film on the negative electrode, and 1,3-propane sulfone lactone may help to maintain the stability of the electrolyte at high temperature.

[0058] In some embodiments, the second electrolyte includes a second additive, the second additive includes fluorinated ethylene carbonate, lithium bisfluorosulfonylimide, 1,3-propane sulfone lactone and nitrile compounds.

[0059] In which, lithium bisfluorosulfonylimide has high conductivity and high thermal stability.

[0060] In some embodiments, the nitrile compound is at least one of succinonitrile, adiponitrile, suberondinitrile, 1,3,6-hexanetriazine, p-fluorobenzonitrile and p-methylbenzonitrile.

[0061] In which, the nitrile compound can promote the formation of CEI protective layer on the surface of the positive active material particles, and can block the reaction of electrolyte with the active site on the surface of the positive active material particles.

[0062] In some embodiments, the total content of fluorinated ethylene carbonate (FEC) and vinylene carbonate (VC) in the first additive accounts for 1-3% of the total mass of the electrolyte, for example, it can be 1%, 2% or 3%; the content of 1,3-propane sulfone lactone (PS) accounts for 1-5% of the total mass of the electrolyte, for example, it can be 1%, 2%, 3%, 4% or 5%; the content of nitrile compound accounts for 1-5% of the total mass of the electrolyte, for example, it can be 1%, 2%, 3%, 4% or 5%.

[0063] The content of each substance in the first additive is controlled in the range in order to ensure the high temperature stability of the negative SEI film through the synergistic effect of the additive.

[0064] In some embodiments, the content of fluoroethylene carbonate (FEC) and lithium bisfluorosulfonylimide (LiFSI) in the second additive accounts for 1-3% of the total mass of the electrolyte, for example, it can be 1%, 2% or 3%; the total content of 1,3-propane sultone (PS) accounts for 0.01-0.2% of the total mass of the electrolyte, for example, it can be 0.01%, 0.05%, 0.1%, 0.15% or 0.2%; the content of nitrile compounds accounts for 0.1-1% of the total mass of the electrolyte, for example, it can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.8%, 0.9% or 1%.

[0065] The content of each substance in the second additive is controlled within the range in order to control the content of the additive to ensure the kinetic performance of the electrolyte.

[0066] In some embodiments, the electrolyte further comprises a lithium salt and an organic solvent.

[0067] In some embodiments, the lithium salt is selected from one or more of lithium hexafluorophosphate, lithium difluorophosphate, lithium difluorobis(oxalato)phosphate, lithium tetrafluoro(oxalato)phosphate, lithium oxalate phosphate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium tetrafluoroborate, lithium bisfluorosulfonylimide salt and lithium bisfluorosulfonylimide, and the content of the lithium salt accounts for 0.1-15.0 wt% of the total mass of the electrolyte.

[0068] In some embodiments, the concentration of the lithium salt is 0.8-1.2 mol / L.

[0069] In some embodiments, the organic solvent is selected from one or more of ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl propionate, propyl propionate, ethyl acetate, ethyl n-butyrate and γ-butyrolactone, and the content of the organic solvent accounts for 20.0-70.0 wt% of the total mass of the electrolyte.

[0070] In some embodiments, the negative electrode tab comprises a negative electrode current collector and a negative electrode active material layer coated on at least one surface of the negative electrode current collector, and the negative electrode active material layer can be one or more of, but not limited to, graphite, soft carbon, hard carbon, carbon fiber, mesocarbon microbeads, silicon-based material, tin-based material, lithium titanate or other metals capable of forming alloys with lithium, etc. Among them, the graphite can be selected from one or more of artificial graphite, natural graphite and modified graphite; the silicon-based material can be selected from one or more of elemental silicon, silicon oxide compounds, silicon-carbon composites and silicon alloys; and the tin-based material can be selected from one or more of elemental tin, tin oxide compounds and tin alloys. The negative electrode current collector is usually a structure or part that collects current, and the negative electrode current collector can be various materials suitable for use as a negative electrode current collector of a lithium ion battery in the art, for example, the negative electrode current collector can be, but not limited to, a metal foil, and more specifically, can be, but not limited to, a copper foil, etc.

[0071] In some embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, the positive electrode active material layer including a positive electrode active material, which can be one or more of a combination of compounds represented by the chemical formulae Li a Ni x Co y M z O 2-b N b (0.95≤a≤1.2, x>0, y≥0, z≥0, and x+y+z=1, 0≤b≤1, M is selected from a combination of one or more of Mn, Al, and N is selected from a combination of one or more of F, P, and S), and can be one or more of a combination of LiCoO2, LiNiO2, LiVO2, LiCrO2, LiMn2O4, LiCoMnO4, Li2NiMn3O8, LiNi 0.5 Mn 1.5 O4, LiCoPO4, LiMnPO4, LiFePO4, LiNiPO4, LiCoFSO4, CuS2, FeS2, MoS2, NiS, TiS2, and the like. The positive electrode active material can also be modified, and methods for modifying the positive electrode active material will be known to those skilled in the art, for example, the positive electrode active material can be modified by coating, doping, and the like, and the material used for the modification can be one or more of a combination of Al, B, P, Zr, Si, Ti, Ge, Sn, Mg, Ce, W, and the like. The positive electrode current collector is generally a structure or component that collects current, and can be any of a variety of materials suitable for use as a positive electrode current collector for a lithium ion battery, for example, the positive electrode current collector can be one or more of a combination of metal foils and the like, and more specifically can be one or more of a combination of aluminum foils and the like.

[0072] In order to make the technical solutions and advantages of the present application clearer, the present application and its beneficial effects will be described in further detail below with reference to specific embodiments, but the embodiments of the present application are not limited thereto.

[0073] Example 1

[0074] (1) Preparation of the Separator

[0075] The slurry is sprayed on the base film by rotary spraying, and the slurry droplets are sprayed on the surface of the diaphragm to form a point coating layer by the centrifugal force of the rotating disc. The raw materials for preparing the swelling layer are melt blended at 100°C by a twin-screw extruder to obtain a melt, and the melt is extruded to obtain material particles of the swelling layer, and the temperature of the melt is controlled at 100°C. The material particles of the swelling layer are prepared into a film under heating, the heating temperature is 100°C, and the film is subjected to high-temperature vulcanization treatment at 130°C to obtain the swelling layer, and the thickness of the swelling layer is 5μm. The swelling layer is attached to the point coating layer, and the swelling layer and the base film are compounded at 2Mpa and 80°C to obtain the diaphragm.

[0076] The swelling layer comprises 90% ethylene-vinyl acetate copolymer and 10% 2,5-dimethyl-2,5-bis(tert-butyl peroxy)hexane, the crosslinking agent is diethylene triamine, the organic peroxide is 2,5-dimethyl-2,5-bis(tert-butyl peroxy)hexane, the base film is a polyethylene film with a porosity of 40-50%, the thickness of the swelling layer is 4μm, and the porosity is 40%.

[0077] (2) Preparation of electrolyte

[0078] The total content of fluoroethylene carbonate and vinylene carbonate in the first electrolyte accounts for 3% of the total mass of the electrolyte, the content of 1,3-propane sultone accounts for 5% of the total mass of the electrolyte, and the content of the nitrile compound accounts for 5% of the total mass of the electrolyte.

[0079] The content of fluoroethylene carbonate and lithium bisfluorosulfonylimide in the second additive accounts for 3% of the total mass of the electrolyte, the total content of 1,3-propane sultone accounts for 0.2% of the total mass of the electrolyte, and the content of succinonitrile accounts for 1% of the total mass of the electrolyte.

[0080] The total injection volume content is 100%, the volume content of the first injection is 60%-70% of the total injection volume content, and the volume content of the second injection is 50%-60% of the total injection volume content.

[0081] The volume content of the first injection is 60% of the total injection volume content, and the volume content of the second injection is 60% of the total injection volume content.

[0082] (3) Preparation of battery

[0083] The positive electrode sheet, the negative electrode sheet and the separator are wound to form an electric core, the positive and negative electrode tabs are led out from the opposite ends of the electric core, and the adhesive tape is wound on the outside to fix the electric core. After the negative electrode end cover, the gasket, the sealing ring, the positive electrode end cover and the pole are mechanically punched and formed in a concentric stack, the electrolyte prepared above is injected into the dried battery, and after packaging, standing, primary formation, secondary injection, secondary formation, shaping and distribution, a lithium ion battery is finally obtained.

[0084] Comparative Example 1

[0085] Different from example 1, the separator in the comparative example does not contain a swelling layer.

[0086] The rest is the same as example 1, which will not be repeated here.

[0087] Among them, the preparation method of examples 2-7 and comparative examples 2-8 is the same as that of example 1, except that the thickness of the swelling layer, the porosity, the composition of the first additive and the second additive, the injection amount and the parameters, see the following table 1-2.

[0088] Table 1

[0089]

[0090]

[0091] Table 2

[0092]

[0093]

[0094] The lithium ion batteries prepared in examples and comparative examples were respectively tested for the following performance tests:

[0095] (1) Capacity retention rate test:

[0096] The initial capacity detection was full charged at 0.5C and discharged at 0.2C. The battery / cell was charged at 5C, rested for 5min, then discharged at 1C, rested for 5min. This was one cycle. After every 50 cycles, the capacity was detected by full charging at 0.5C and discharging at 0.2C.

[0097] (2) Battery roll core thickness change test:

[0098] The battery outer package was disassembled, and then the roll core thickness or diameter was tested by a constant pressure caliper (300g)

[0099] The results of the above performance tests are shown in tables 3 and 4.

[0100] Table 3

[0101]

[0102]

[0103] Table 4

[0104] SOC Example 1 cell diameter (mm) Comparative Example 1 cell diameter (mm) 0％ 10.35 10.14 25％ 10.3 10.21 50％ 10.37 10.28 75％ 10.36 10.3 100％ 10.32 10.35

[0105] As can be seen from the comparison of the experimental data of examples 1-7 and comparative examples 1-9 in table 3-4, when the separator is provided with a swelling layer and the thickness and porosity of the swelling layer are within the set range, the deformation of the electrode sheet can be inhibited, and the problem of the battery breaking during the charge-discharge cycle can be effectively solved. When the value of the relationship exceeds the preset range, the electrolyte will overflow, and when the value of the relationship is less than the preset range, the electrolyte will be insufficient, the wetting effect will be poor, and lithium precipitation will occur in the electrode sheet.

[0106] As can be seen from the comparison of the experimental data of examples 1 and comparative example 1 in table 4, compared with the conventional separator without a swelling layer, the separator provided in the application can allow the electrolyte to enter and exit the pores of the electrode sheet during the charge-discharge cycle, the electrolyte outside the battery also changes, and the thickness of the swelling layer on the separator also changes, so that the diameter of the battery in example 1 changes less than that of the battery in comparative example 1 after charge-discharge, and the problem of the battery breaking during the charge-discharge cycle can be effectively solved.

[0107] As can be seen from the comparison of the experimental results of examples 2-3 and comparative examples 2-4, when the amount of the first injection and the amount of the second injection exceed the limited range, the electrode sheet will be slightly purple; when no second injection is performed, lithium precipitation will occur in the electrode sheet due to insufficient electrolyte.

[0108] As can be seen from the comparison of the experimental results of example 1 and comparative examples 4, 7-8, when the first additive is added with FEC, VC, PS and nitrile compounds at the same time, the battery has excellent high-temperature performance.

[0109] Based on the disclosure and teachings of the above specification, those skilled in the art can make changes and modifications to the above embodiments. Therefore, the present application is not limited to the above specific embodiments, and any obvious improvements, replacements or modifications made by those skilled in the art based on the present application are within the scope of the present application. In addition, although some specific terms are used in the specification, these terms are only for convenience of explanation and do not constitute any limitation on the present application.

Claims

1. A lithium-ion battery, characterized by, The battery comprises a positive electrode sheet, a composite diaphragm, a negative electrode sheet and an electrolyte, and is subjected to once liquid injection and formation and then secondary liquid injection, the composite diaphragm comprises a base film and a swelling layer, the swelling layer is arranged on at least one surface of the base film, and the swelling rate of the swelling layer is Y, wherein the swelling rate Y of the swelling layer is 150-250%. The electrolyte comprises a first electrolyte for once liquid injection and a second electrolyte for secondary liquid injection, the liquid injection amount of the once liquid injection is X1, the liquid injection amount of the secondary liquid injection is X2, and the total liquid injection amount of the electrolyte is X, wherein the total liquid injection amount X of the electrolyte is 120-140%. Wherein, X1, X2 and Y satisfy the relationship: X2≤X1, 0.7Y≤10 (X1+X2)-10.5≤1.5Y. The first electrolyte comprises a first additive, and the first additive comprises fluoroethylene carbonate, vinylene carbonate, 1,3-propane sulfone lactone and nitrile compounds, the total content of fluoroethylene carbonate and vinylene carbonate in the first additive accounts for 1-3% of the total mass of the electrolyte, the content of 1,3-propane sulfone lactone accounts for 1-5% of the total mass of the electrolyte, and the content of nitrile compounds accounts for 1-5% of the total mass of the electrolyte. The second electrolyte comprises a second additive, and the second additive comprises fluoroethylene carbonate, lithium bisfluorosulfonylimide, 1,3-propane sulfone lactone and nitrile compounds, the content of fluoroethylene carbonate and lithium bisfluorosulfonylimide in the second additive accounts for 1-3% of the total mass of the electrolyte, the total content of 1,3-propane sulfone lactone accounts for 0.01-0.2% of the total mass of the electrolyte, and the content of nitrile compounds accounts for 0.1-1% of the total mass of the electrolyte.

2. The lithium-ion battery of claim 1, wherein, The liquid injection amount X1 of the once liquid injection is 60-70%, and the liquid injection amount X2 of the secondary liquid injection is 50-60%.

3. The lithium-ion battery of claim 1, wherein, The swelling layer comprises a first component and a second component; The first component is an ethylene-ester copolymer, and accounts for 83%-92% of the total mass of the swelling layer; The second component is a crosslinking agent, and accounts for 8%-17% of the total mass of the swelling layer.

4. The lithium-ion battery of claim 3, wherein, The ethylene-ester copolymer comprises at least one of ethylene-ethyl acrylate copolymer, ethylene-methyl methacrylate copolymer and ethylene-acrylate copolymer.

5. The lithium-ion battery of claim 3, wherein, The crosslinking agent is at least one of 2,5-dimethyl-2,5-bis(tert-butyl peroxy) hexane, tert-butyl peroxy carbonate-2-ethyl hexyl ester, 1,1-di-tert-butyl peroxy-3,3,5-trimethyl cyclohexane, dicumyl peroxide, benzoyl peroxide, diisopropylbenzene hydroperoxide and diethylene triamine.

6. The lithium-ion battery of claim 1, wherein, The swelling layer is in the form of dots, lines or surfaces covering the surface of the base film, and the area of the forward projection of the swelling layer accounts for 50-100% of the area of the forward projection of the base film.

7. The lithium-ion battery of claim 1, wherein, The composite diaphragm further comprises a ceramic layer, wherein the ceramic layer is arranged between the base film and the swelling layer; And / or, the ceramic layer and the swelling layer are arranged side by side on the surface of the base film; And / or, the ceramic layer is arranged on the outer surface of the swelling layer.

8. The lithium-ion battery of claim 7, wherein, The nitrile compound is at least one of succinonitrile, adiponitrile, suberonitrile, 1,3,6-hexanetricarbonitrile, p-fluorobenzonitrile, and p-methylbenzonitrile.

Images