Electrolyte combination, secondary liquid injection method and lithium ion battery

Abstract

The application relates to an electrolyte combination, a secondary injection method and a lithium ion battery. The electrolyte combination is used for secondary injection of a lithium ion battery and comprises: a first electrolyte, the first electrolyte is used for first injection in the secondary injection, the first electrolyte comprises a first lithium salt, a first solvent and a first additive, the first lithium salt is lithium bisfluorosulfonylimide, the first solvent comprises fluorinated ethylene carbonate, and the first additive comprises a fluorine-containing lithium salt additive and a positive electrode film-forming additive; and a second electrolyte, the second electrolyte is used for second injection in the secondary injection, the second electrolyte comprises a second lithium salt, a second solvent and a second additive, the second lithium salt is lithium hexafluorophosphate, and the second additive comprises a gas generation inhibiting additive. The electrolyte combination provided by the application inhibits battery gas generation, inhibits dissolution of positive electrode metal ions, forms a flexible SEI film rich in LiF, improves interface stability, and improves capacity performance and cycle performance of the battery.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to an electrolyte, a secondary injection method and a lithium ion battery. BACKGROUND

[0002] With the continuous improvement of the energy density of lithium ion batteries, the application of high-capacity silicon-based anodes has become increasingly common. However, during charging and discharging, the silicon anode undergoes a huge volume change, causing damage to the electrode material and decomposition of the electrolyte, thereby reducing its electrochemical performance; in addition, silicon is a semiconductor, and its electrical conductivity is low. The irreversible capacity loss of the silicon anode is mainly due to the consumption of the SEI film formation and the "lithium consumption" of silicon (formation of Li5Si4 crystals).

[0003] Currently, fluoroethylene carbonate is an important negative electrode film-forming additive for the silicon system, which forms a SEI film containing LiF on the surface of the silicon particles, thereby inhibiting the "lithium consumption" phenomenon.

[0004] However, as a negative electrode film-forming additive, fluoroethylene carbonate has the following problems: (1) based on the demand for high capacity, it is necessary to increase the silicon content, thereby increasing the content of FEC or DFEC, however, high content of fluoroethylene carbonate can exacerbate the problem of battery gas production, and also cause the cycle life of the battery to decline; (2) at high temperatures, fluoroethylene carbonate is prone to decomposition to produce HF, which corrodes the positive electrode, causing the dissolution of positive electrode metal ions, resulting in a decrease in battery capacity and cycle performance; (3) the SEI film formed by fluoroethylene carbonate alone is rich in a certain amount of Li2CO3 and rigid polyolefins, and cannot maintain good interface stability during the deintercalation of lithium from the silicon anode. SUMMARY

[0005] Therefore, it is necessary to provide an electrolyte combination, a secondary injection method and a lithium ion battery to inhibit battery gas production, reduce the side reaction of fluoroethylene carbonate with the positive electrode, inhibit the dissolution of positive electrode metal ions, inhibit the "lithium consumption" phenomenon, form an elastic SEI film, improve the interface stability, and improve the capacity performance and cycle performance of the battery.

[0006] A first aspect of the present application provides an electrolyte combination for secondary injection of a lithium ion battery, the electrolyte combination comprising: a first electrolyte, the first electrolyte being used for a first injection in the secondary injection, the first electrolyte comprising a first lithium salt, a first solvent and a first additive, the first lithium salt being lithium bisfluorosulfonylimide, the first solvent comprising fluoroethylene carbonate, and the first additive comprising a fluorine-containing lithium salt additive and a positive electrode film-forming additive; a second electrolyte, the second electrolyte being used for a second injection in the secondary injection, the second electrolyte comprising a second lithium salt, a second solvent and a second additive, the second lithium salt being lithium hexafluorophosphate, and the second additive comprising a gas production inhibition additive.

[0007] In some embodiments, the mass percentage of fluoroethylene carbonate in the first electrolyte is 18% to 45%; and / or, the second additive further comprises fluoroethylene carbonate, and the mass percentage of fluoroethylene carbonate in the second electrolyte is 0.01% to 5%.

[0008] In some embodiments, the mass percentage of lithium difluorosulfonylimide in the first electrolyte is 3% to 7%.

[0009] In some embodiments, the mass percentage of the fluorine-containing lithium salt additive in the first electrolyte is 1% to 3%.

[0010] In some embodiments, the mass percentage of the gas generation inhibiting additive in the second electrolyte is 0.1% to 1%.

[0011] In some embodiments, the lithium ion battery comprises a negative electrode, the negative electrode comprises a current collector and a negative electrode coating, the negative electrode coating comprises a silicon-containing material, and the mass percentage a% of fluoroethylene carbonate in the electrolyte combination and the mass percentage x% of the silicon-containing material in the negative electrode satisfy: 0.5≤a / x≤0.7.

[0012] In some embodiments, the mass percentage of the first electrolyte in the electrolyte combination is 20% to 40%; and / or, the mass percentage of the second electrolyte in the electrolyte combination is 60% to 80%.

[0013] The second aspect of the present application provides a secondary injection method for using the electrolyte combination provided in the first aspect above to perform secondary injection on a lithium ion battery, the secondary injection method comprising the following steps: using the first electrolyte to perform a first injection in the secondary injection on the lithium ion battery; sequentially performing standing and formation on the lithium ion battery after the first injection; using the second electrolyte to perform a second injection in the secondary injection on the lithium ion battery after the formation; and sequentially performing aging and capacity grading on the lithium ion battery after the second injection.

[0014] In some embodiments, the formation comprises a plurality of formation stages, and the current density of a later formation stage is less than that of an earlier formation stage.

[0015] The third aspect of the present application provides a lithium ion battery comprising the electrolyte combination provided in the first aspect above; or, the lithium ion battery is prepared by the secondary injection method provided in the second aspect above.

[0016] Compared with the prior art, the present application has at least the following beneficial effects:

[0017] The electrolyte combination provided by the application, the first electrolyte is used for the first time to inject liquid, through the synergistic effect of lithium bisfluorosulfonylimide, fluorinated ethylene carbonate and fluorine-containing lithium salt additives, the formation of SEI film with elasticity, rich in high LiF and low impedance is promoted, the battery charge and discharge performance and cycle performance are improved. The lithium bisfluorosulfonylimide salt in the first electrolyte not only can improve the stability of the electrolyte itself, and the reducibility of the large anion FSI - in lithium bisfluorosulfonylimide is also conducive to promoting the formation of LiF in the SEI film; the fluorine-containing lithium salt additive itself is easy to generate an interface film rich in LiF component at the positive and negative electrodes, inhibits the "eating lithium" phenomenon, and improves the cycle performance of the battery; the fluorine-containing lithium salt additive induces the direct defluorination of fluorinated ethylene carbonate, improves the content of LiF in the formed SEI film, inhibits the "eating lithium" phenomenon, and directly defluorinates the cyclic structure of fluorinated ethylene carbonate, promotes the defluorinated fluorinated ethylene carbonate to easily polymerize to form polyvinylidene carbonate, thereby improving the elasticity of the formed SEI film, thereby improving the interface stability, and improving the cycle stability of the battery.

[0018] In addition, the second electrolyte is used for the second time to inject liquid, the gas inhibition additive in the second electrolyte reacts with the positive and negative electrode surfaces at high temperature, repairs the interface film, ensures the interface stability of the positive and negative electrodes, inhibits the battery gas production, and improves the capacity performance and cycle performance of the battery.

[0019] In summary, the electrolyte combination of the application is used for the secondary injection of lithium ion batteries, which improves the interface stability of the positive and negative electrodes of the battery, and takes into account the capacity performance and cycle performance of the lithium ion battery. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 The flow chart of the secondary injection method in an embodiment of the application. DETAILED DESCRIPTION

[0021] The reference of the embodiments of the application will be provided in detail, and one or more examples thereof are described below. Each example is provided as an explanation rather than limiting the application. In fact, it is obvious to those skilled in the art that various modifications and changes can be made to the application without departing from the scope or spirit of the application. For example, the features described or illustrated as part of one embodiment can be used in another embodiment to produce further embodiments.

[0022] Therefore, it is intended that the application cover such modifications and changes as fall within the scope of the appended claims and their equivalents. Other objects, features, and aspects of the application are disclosed in or are apparent from the following detailed description. It is to be understood by the skilled person that the present discussion is merely a description of exemplary embodiments, and is not intended to limit the broader aspects of the application.

[0023] In the present application, the technical features described in an open way include both the closed technical solution consisting of the listed features and the open technical solution comprising the listed features.

[0024] In the present application, if no special description, the numerical range is considered to be continuous, and includes the minimum value and the maximum value of the range, and every value between the minimum value and the maximum value. Further, when the range refers to an integer, every integer between the minimum value and the maximum value of the range is included. In addition, when multiple ranges are provided to describe a feature or a characteristic, the ranges can be combined. In other words, unless otherwise specified, all ranges disclosed herein should be interpreted as including any and all sub-ranges subsumed therein.

[0025] If no special description, all embodiments and optional embodiments of the present application can be combined to form new technical solutions. If no special description, all technical features and optional technical features of the present application can be combined to form new technical solutions.

[0027] If no special description, all steps of the present application can be performed in sequence or randomly, preferably in sequence. For example, the method comprises steps (a) and (b), which means that the method can comprise steps (a) and (b) in sequence, or steps (b) and (a) in sequence. For example, it is mentioned that the method can further comprise step (c), which means that step (c) can be added to the method in any order, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0028] If no special description, the "includes" and "contains" mentioned in the present application means open, and can also be closed. For example, the "includes" and "contains" can mean that other components not listed can also be included or contained, or only the listed components can be included or contained.

[0029] If no special description, in the present application, the term "or" 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).

[0030] The first aspect of the present application provides an electrolyte combination for secondary injection of a lithium ion battery, the electrolyte combination comprising a first electrolyte and a second electrolyte. The first electrolyte is used for the first injection, and the first electrolyte comprises a first lithium salt, a first solvent and a first additive. The first lithium salt is lithium bisfluorosulfonylimide. The first solvent comprises fluoroethylene carbonate. The first additive comprises a fluorine-containing lithium salt additive and a positive electrode film-forming additive. The second electrolyte is used for the second injection, and the second electrolyte comprises a second lithium salt, a second solvent and a second additive. The second lithium salt is lithium hexafluorophosphate. The second additive comprises a gas generation inhibiting additive.

[0031] The electrolyte combination provided by the present application is used for the first injection. Through the synergistic effect of lithium bisfluorosulfonylimide, fluoroethylene carbonate and the fluorine-containing lithium salt additive, the lithium bisfluorosulfonylimide salt not only improves the stability of the electrolyte itself in the first electrolyte, but also has good reducibility of the large anion FSI - in the lithium bisfluorosulfonylimide, which is conducive to the generation of LiF in the SEI film. The fluorine-containing lithium salt additive itself is easy to generate an interface film rich in LiF component at the positive and negative electrodes, inhibits the "lithium eating" phenomenon, and improves the cycle performance of the battery. The fluorine-containing lithium salt additive induces the direct defluorination of fluoroethylene carbonate, improves the content of LiF in the formed SEI film, inhibits the "lithium eating" phenomenon, and directly defluorinates the cyclic structure of fluoroethylene carbonate, which promotes the polymerization of defluorinated fluoroethylene carbonate to form polyvinylidene carbonate, thereby improving the elasticity of the formed SEI film, improving the interface stability, and improving the cycle stability of the battery.

[0032] In addition, the second electrolyte is used for the second injection. The gas generation inhibiting additive in the second electrolyte reacts with the positive and negative electrode surfaces at high temperature, repairs the interface film, ensures the interface stability of the positive and negative electrodes, inhibits the generation of gas in the battery, and improves the capacity performance and cycle performance of the battery.

[0033] In summary, the use of the electrolyte combination of the present application for the secondary injection of a lithium ion battery improves the interface stability of the positive and negative electrodes of the battery, and balances the capacity performance and cycle performance of the lithium ion battery.

[0034] It can be understood that the selection of the first lithium salt and the second lithium salt is based on the following reasons:

[0035] (1) The first lithium salt of the first electrolyte is not LiPF6, but LiFSI.

[0036] PF6 -Although LiF can be generated by hydrolysis, the formed SEI film is in small amount and uneven growth. The fluorinated ethylene carbonate-based electrolyte interacts with PF5, a strong Lewis acid generated by the decomposition of LiPF6 under high temperature conditions, and then undergoes defluorination to generate VC and HF and other acidic products, resulting in an increase in the acidity of the electrolyte and corrosion of the positive electrode.

[0037] LiFSI is easily soluble in the bipolar aprotic solvent (i.e., fluorinated ethylene carbonate) due to the delocalized charge of FSI - , can be synergistically coordinated with fluorinated ethylene carbonate, and is more conducive to the generation of a high-content LiF and uniformly grown SEI film during electrochemical reduction.

[0038] (2) The second lithium salt of the second electrolyte is not LiFSI, so that LiFSI fully participates in film formation while minimizing residual LiFSI, thereby avoiding the phenomenon of positive electrode high-temperature gas generation and current collector corrosion due to excessive residual LiFSI in the electrolyte during battery cycling and storage.

[0039] In some embodiments, the fluorinated ethylene carbonate includes one or more of monofluorinated ethylene carbonate (FEC) and difluorinated ethylene carbonate (DFEC).

[0040] In some embodiments, the mass percentage of fluorinated ethylene carbonate in the first electrolyte is 18% to 45%, including but not limited to 18%, 20%, 25%, 30%, 35%, 40%, and 45%. Within the above mass percentage range, the use of high content of fluorinated ethylene carbonate during the first injection of electrolyte can form a SEI film rich in LiF on the surface of silicon particles, inhibit the formation of Li5Si4, reduce silicon expansion and consumption of lithium ions, and the introduction of high content of fluorinated ethylene carbonate during the first injection can reduce the addition of fluorinated ethylene carbonate during the second injection, reducing the high temperature deterioration caused by fluorinated ethylene carbonate.

[0041] In some embodiments, the first solvent further includes a linear carbonate, and the mass ratio of fluorinated ethylene carbonate to linear carbonate in the first solvent is 1:1 to 1:5, including but not limited to 1:1, 2:3, 3:7, 1:3, 1:4, and 1:5. Within the above mass ratio range, the combination of fluorinated ethylene carbonate and linear carbonate is beneficial to optimizing the formation of SEI film, improving the stability and elasticity of SEI film. A higher proportion of fluorinated ethylene carbonate ensures the formation of a SEI film rich in LiF, thereby inhibiting the "lithium consumption" phenomenon of the silicon negative electrode and prolonging the cycle life of the battery. The addition of linear carbonate improves the stability of the electrolyte and reduces the viscosity, thereby improving the conductivity of the electrolyte.

[0042] In some embodiments, the linear carbonate includes one or more of ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), propyl propionate (PP), and ethyl propionate (EP).

[0043] Further, the mass percentage of lithium bisfluorosulfonylimide in the first electrolyte is 3% to 7%, including but not limited to 3%, 4%, 5%, 6%, and 7%. Within the above mass percentage range, the content of LiFSI in the first electrolyte is relatively low. On the one hand, after the first injection, a small amount of LiFSI directly participates in the formation of the film, which improves the low-temperature and cycle performance of the battery. At the same time, an appropriate amount of LiFSI can reduce the high-temperature gas production and current collector corrosion caused by excessive LiFSI residue in the electrolyte. On the other hand, the low content of LiFSI can effectively improve the wettability, and the low concentration of the first lithium salt LiFSI can induce a high overpotential during the low-current formation process, which is conducive to the formation of inorganic components such as LiF in the SEI layer formed by the high-fluorinated electrolyte.

[0044] In some embodiments, the fluorine-containing lithium salt additive includes one or more of lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), lithium triflate (LiFSA), lithium difluoro oxalate borate (LiODFB), lithium difluoro dioxalate phosphate (LiODFP), and lithium tetrafluoro oxalate borate (LiOTFP).

[0045] In some embodiments, the mass percentage of the fluorine-containing lithium salt additive in the first electrolyte is 1% to 3%, including but not limited to 1%, 1.5%, 2%, 2.5%, and 3%. Within the above mass percentage range, the fluorine-containing lithium salt additive is conducive to inducing the direct defluorination of fluoroethylene carbonate to form an inorganic elastic SEI film. If the content of the fluorine-containing lithium salt additive is too high, there will be residue in the battery after the filling, and the fluorine-containing lithium salt additive is unstable and prone to react with the negative electrode to produce gas, which affects the electrochemical performance.

[0046] In some embodiments, the second electrolyte does not include a fluorine-containing lithium salt additive. If the second electrolyte also includes a fluorine-containing lithium salt additive, it has high reactivity in the electrolyte and is prone to react to produce gas at high temperatures, which will cause the high-temperature performance to deteriorate.

[0047] In some embodiments, the second electrolyte also includes fluoroethylene carbonate, and the mass percentage of fluoroethylene carbonate in the second electrolyte is 0.01% to 5%, including but not limited to 0.01%, 0.5%, 1%, 2%, 3%, 4%, and 5%. Within the above mass percentage range, an appropriate amount of fluoroethylene carbonate is conducive to repairing the SEI film in the later stage of battery cycling, reducing gas production and side reactions, and improving the cycle performance and capacity performance of the battery.

[0048] In some embodiments, the gas evolution inhibiting additive includes one or more of vinyl sulfate (DTD), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), vinyl sulfite (ES), methylene malonate (MMDS), 1,3-propane sultone (PS), propenyl-1,3-sultone (PST), tetra-vinylsilane (TVSI), and tripropargyl phosphate (TPP).

[0049] In some embodiments, the mass percentage of the gas evolution inhibiting additive in the second electrolyte is 0.1% to 1%, including but not limited to 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%. Within the above range of mass percentage, the gas evolution is inhibited, and at the same time, the battery impedance is reduced. If too much gas evolution inhibiting additive is added, it will gradually participate in film formation during the use of the battery, which will increase the battery impedance and deteriorate the battery kinetics, and is not conducive to the charge-discharge performance and cycle performance of the battery.

[0050] It can be understood that the gas evolution inhibiting additive is added to the second electrolyte, but not to the first electrolyte. After the gas evolution inhibiting additive is formed into a film, the impedance is large. In order not to make the initial impedance of the battery too large, the performance of the battery such as the rate is affected.

[0051] In some embodiments, the second solvent includes a cyclic carbonate and a linear carbonate. The cyclic carbonate includes one or more of vinyl carbonate (EC), propylene carbonate (PC); the linear carbonate includes one or more of methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), propyl propionate (PP), and ethyl propionate (EP). The use of cyclic carbonate and linear carbonate together is conducive to promoting the sufficient dissolution of the second lithium salt and the second additive, promoting the role of each component in the electrolyte, and improving the improvement effect of the electrolyte on the performance of the battery.

[0052] In some embodiments, the second lithium salt includes one or more of lithium hexafluorophosphate, lithium bisfluorosulfonylimide, lithium tetrafluoroborate, lithium difluoro(oxalato)borate, lithium bis(trifluoromethanesulfonyl)imide.

[0053] In some embodiments, the mass percentage of the second lithium salt in the second electrolyte is 13% to 18%.

[0054] In some embodiments, the mass percentage of the first electrolyte in the electrolyte combination is 20% to 40%, including but not limited to 20%, 25%, 30%, 35%, 40%. Controlling the proportion of the first electrolyte in the electrolyte combination within the above range is conducive to forming a good SEI film at the initial stage and stabilizing the positive and negative electrode interface.

[0055] In some embodiments, the second electrolyte has a mass percentage of 60-80% in the electrolyte combination, including but not limited to 60%, 65%, 70%, 75%, and 80%. Controlling the proportion of the second electrolyte in the electrolyte combination within the above range is conducive to continuing to optimize the SEI film in the later stage of battery cycling and improving the long cycle performance and capacity performance of the battery.

[0056] In some embodiments, the lithium ion battery comprises a negative electrode, the negative electrode comprises a current collector and a negative electrode coating, the negative electrode coating comprises a silicon-containing material, and the mass percentage a% of fluoroethylene carbonate in the electrolyte combination and the mass percentage x% of the silicon-containing material in the negative electrode satisfy: 0.5≤a / x≤0.7. Within the above proportion range, the amount of fluoroethylene carbonate matches the content of silicon in the negative electrode, so that the formed SEI film can better inhibit the "lithium consumption" phenomenon.

[0057] In some embodiments, the silicon-containing material comprises one or more of silicon-carbon material, silicon-oxygen material, and pre-lithiated silicon material.

[0058] The second aspect of the present application provides a secondary injection method, as shown in the figure, the secondary injection method uses the electrolyte combination provided by the first aspect of the present application to perform secondary injection on the lithium ion battery, and the secondary injection method comprises the following steps: Figure 1

[0059] S1, using the first electrolyte to perform the first injection on the lithium ion battery.

[0060] S2, sequentially performing standing and formation on the lithium ion battery after the first injection.

[0061] S3, using the second electrolyte to perform the second injection on the lithium ion battery after the formation.

[0062] S4, sequentially performing aging and capacity distribution on the lithium ion battery after the second injection.

[0063] In some embodiments, the formation comprises a plurality of formation stages, and the current density of the latter formation stage is less than that of the former formation stage.

[0064] The injection and formation method of the present application first adopts a high overpotential formation method, cooperates the potential difference generated by uneven infiltration and the high polarization generated by large current to induce high overpotential at the interface, promotes the growth of a large amount of inorganic LiF in the induced SEI film, and then further forms a uniform and stable SEI film with inorganic and organic alternation through small current formation. The SEI film formed by this method contains a large number of large particles of LiF inside, and the inner layer is loose and porous, which is conducive to the transmission of lithium ions.

[0065] ​The third aspect of the present application provides a lithium ion battery comprising the electrolyte combination provided by the first aspect; or the lithium ion battery is prepared by the secondary injection method provided by the second aspect.

[0066] In some embodiments, the lithium ion battery further comprises a positive electrode sheet and a negative electrode sheet.

[0067] The present application will be further described below in combination with specific examples and comparative examples.

[0068] Example 1

[0069] The present embodiment provides a secondary injection method for a lithium ion battery, which comprises the following steps:

[0070] S1, using a first electrolyte to perform first injection on the lithium ion battery.

[0071] The first electrolyte comprises a first lithium salt, a first solvent and a first additive; the first lithium salt is 5wt% of LiFSI; the first solvent is 23.0wt% of FEC and 69.5wt% of EMC, the mass ratio of FEC to EMC is 1:3; the first additive comprises 1wt% of LiPO2F2, 1.0wt% of LiODFB and 0.5wt% of TMSP. The mass percentage of the first electrolyte in the electrolyte combination for secondary injection of the lithium ion battery is 30%.

[0072] S2, after the first injection, the lithium ion battery is placed at 45℃ for 24h, and then subjected to 0.5C formation to 3.0V, 0.2C formation to 3.4V, and 0.05C formation to 3.75V.

[0073] S3, using a second electrolyte to perform second injection on the formed lithium ion battery.

[0074] The second electrolyte comprises a second lithium salt, a second solvent and a second additive; the second lithium salt is 15wt% of LiPF6; the second additive is 3.0wt% of FEC and 0.5wt% of VEC; the balance is the second solvent, the second solvent is a mixed solvent of EC and EMC, the volume ratio of EC to EMC is 3:7. The mass percentage of the second electrolyte in the electrolyte combination for secondary injection of the lithium ion battery is 70%.

[0075] S4, sequentially performing aging and capacity grading on the lithium ion battery after the second injection.

[0076] The lithium ion battery:

[0077] Prepared by the above secondary injection method, including a positive electrode sheet, a negative electrode sheet, and an electrolyte combination injected by the above secondary injection method, wherein the mass percentage a% of FEC in the electrolyte combination and the mass percentage x% of the silicon-containing material in the negative electrode satisfy: a / x = 0.6 (x% = 15%).

[0078] The positive electrode sheet is prepared by the following preparation method:

[0079] The positive electrode material comprises 97.5wt.% of LiNi 90 Co 5.5 Mn 3.5 Al1O2 positive electrode active material, 1.5wt.% of Super P conductive agent, and 1.0wt.% of polyvinylidene fluoride binder; the above components are mixed, and then N-methylpyrrolidone solvent is added and uniformly mixed to obtain a positive electrode slurry, the positive electrode slurry is coated on the surface of a carbon-coated aluminum foil, dried, and then sliced to obtain a positive electrode sheet;

[0080] The negative electrode sheet is prepared by the following preparation method:

[0081] Graphite, silicon-carbon composite material (purchased from Lanxi Zhi Xin New Energy Material Co., Ltd., model S0310), single-walled carbon nanotubes, Super P conductive agent, polyacrylic acid, and butadiene-styrene latex are mixed in a mass ratio of 80.1:15:0.1:0.6:3.3:0.9, and then deionized water is added and stirred uniformly to obtain a negative electrode slurry, the negative electrode slurry is coated on the surface of a copper foil, dried, and then sliced to obtain a negative electrode sheet.

[0082] Example 2

[0083] The secondary injection method of the lithium ion battery and the lithium ion battery in this embodiment are basically the same as those in Example 1, and the difference lies in that:

[0084] In step S1, the mass percentage of the first electrolyte in the electrolyte combination for secondary injection of the lithium ion battery is 40%, and the mass percentage of FEC in the first electrolyte is 18wt.%.

[0085] In step S3, the mass percentage of the second electrolyte in the electrolyte combination for secondary injection of the lithium ion battery is 60%, and the mass percentage of FEC in the second electrolyte is 3wt.%.

[0086] The mass percentage a% of FEC in the electrolyte combination and the mass percentage x% of the silicon-containing material in the negative electrode satisfy: a / x = 0.6 (x% = 15%).

[0087] Example 3

[0088] The secondary injection method of the lithium ion battery and the lithium ion battery in this embodiment are basically the same as those in embodiment 1, except that:

[0089] In step S1, the mass percentage of FEC in the first electrolyte is 18wt%.

[0090] In step S3, the mass percentage of FEC in the second electrolyte is 3wt%.

[0091] The mass percentage a% of FEC in the electrolyte combination and the mass percentage x% of the silicon-containing material in the negative electrode satisfy: a / x = 0.5 (x% = 15%).

[0092] Embodiment 4

[0093] The secondary injection method of the lithium ion battery and the lithium ion battery in this embodiment are basically the same as those in embodiment 1, except that:

[0094] In step S1, the mass percentage of FEC in the first electrolyte is 40.5wt%, and the mass percentage of the first electrolyte in the electrolyte combination for the secondary injection of the lithium ion battery is 20%.

[0095] In step S3, the mass percentage of FEC in the second electrolyte is 3wt%, and the mass percentage of the second electrolyte in the electrolyte combination for the secondary injection of the lithium ion battery is 80%.

[0096] The mass percentage a% of FEC in the electrolyte combination and the mass percentage x% of the silicon-containing material in the negative electrode satisfy: a / x = 0.7 (x% = 15%).

[0097] Embodiment 5

[0098] The secondary injection method of the lithium ion battery and the lithium ion battery in this embodiment are basically the same as those in embodiment 1, except that:

[0099] In step S1, the mass percentage of LiFSI in the first electrolyte is 3wt%, and the mass percentage of EMC in the first electrolyte is 71.5wt%.

[0100] Embodiment 6

[0101] The secondary injection method of the lithium ion battery and the lithium ion battery in this embodiment are basically the same as those in embodiment 1, except that:

[0102] In step S1, the mass percentage of LiFSI in the first electrolyte is 7wt%, and the mass percentage of EMC in the first electrolyte is 67.5wt%.

[0103] Embodiment 7

[0104] The secondary injection method of the lithium ion battery and the lithium ion battery in this embodiment are basically the same as those in Embodiment 1, except that:

[0105] In step S1, the fluorine-containing lithium salt additive LiPO2F2 and LiODFB are replaced by 2.0wt% LiFSA.

[0106] Embodiment 8

[0107] The secondary injection method of the lithium ion battery and the lithium ion battery in this embodiment are basically the same as those in Embodiment 1, except that:

[0108] In step S1, the mass percentage of the first electrolyte in the electrolyte combination for the secondary injection of the lithium ion battery is 60%.

[0109] In step S3, the mass percentage of the second electrolyte in the electrolyte combination for the secondary injection of the lithium ion battery is 40%.

[0110] Embodiment 9

[0111] The secondary injection method of the lithium ion battery and the lithium ion battery in this embodiment are basically the same as those in Embodiment 1, except that:

[0112] In step S1, the mass percentage of the first electrolyte in the electrolyte combination for the secondary injection of the lithium ion battery is 10%.

[0113] In step S3, the mass percentage of the second electrolyte in the electrolyte combination for the secondary injection of the lithium ion battery is 90%.

[0114] Embodiment 10

[0115] The secondary injection method of the lithium ion battery and the lithium ion battery in this embodiment are basically the same as those in Embodiment 1, except that:

[0116] In step S1, the mass percentage of LiFSI in the first electrolyte is 12%, and the mass percentage of EMC in the first electrolyte is 62.5wt%.

[0117] Embodiment 11

[0118] The secondary injection method of the lithium ion battery and the lithium ion battery in this embodiment are basically the same as those in Embodiment 1, except that:

[0119] In step S2, 0.05C formation to 3.0V, 0.2C formation to 3.4V, and then 0.5C formation to 3.75V are adopted.

[0120] Embodiment 12

[0121] The secondary injection method of the lithium ion battery and the lithium ion battery in this embodiment are basically the same as those in embodiment 1, except that:

[0122] In step S3, the second electrolyte further includes 1wt% of LiPO2F2 and 0.5wt% of LiODFB.

[0123] Comparative example 1

[0124] The secondary injection method of the lithium ion battery and the lithium ion battery in this comparative example are basically the same as those in embodiment 1, except that:

[0125] In step S1, the first solvent does not include FEC, and the mass percentage of EMC in the first electrolyte is 92.7%.

[0126] Comparative example 2

[0127] The secondary injection method of the lithium ion battery and the lithium ion battery in this comparative example are basically the same as those in embodiment 1, except that:

[0128] In step S1, the first additive does not include LiPO2F2 and LiODFB, and the mass percentage of EMC in the first electrolyte is 71.5%.

[0129] Comparative example 3

[0130] The secondary injection method of the lithium ion battery and the lithium ion battery in this comparative example are basically the same as those in embodiment 1, except that:

[0131] In step S1, the first lithium salt is replaced by LiPF6.

[0132] Comparative example 4

[0133] The secondary injection method of the lithium ion battery and the lithium ion battery in this comparative example are basically the same as those in embodiment 1, except that:

[0134] In step S3, the second lithium salt is replaced by LiFSI.

[0135] Comparative example 5

[0136] The secondary injection method of the lithium ion battery and the lithium ion battery in this comparative example are basically the same as those in embodiment 1, except that:

[0137] In step S3, the second additive does not include VEC.

[0138] Performance test

[0139] (1) Discharge specific capacity test

[0140] The lithium ion batteries in the examples and comparative examples were subjected to capacity test, standing for 1 min, discharging at 0.1 C to the cut-off voltage 2.5 V, and the first discharge capacity was recorded. The results are shown in Table 1 below.

[0141] (2) High temperature cycle performance test

[0142] The lithium ion batteries in the examples and comparative examples were placed in a 45°C thermostat, charged at constant current 1 C to 4.2 V, charged at constant voltage 4.2 V to 0.05 C, and discharged at 1 C to 2.5 V, which was recorded as one charge-discharge cycle process, and the initial discharge capacity was recorded. The capacity retention rate = (residual discharge capacity / initial discharge capacity) x 100%, and the cycle number when the secondary battery capacity retention rate was 80% was recorded.

[0143] (3) High temperature storage performance:

[0144] The lithium ion batteries in the examples and comparative examples were subjected to capacity test, standing for 1 min, discharging at 0.1 C to the cut-off voltage 2.5 V, and the first discharge capacity was recorded. The results are shown in Table 1 below.

[0145] (4) Gas production after high temperature storage:

[0146] The volume test method of lithium ion battery: a beaker containing deionized water was placed on an electronic balance, and the reading m1 of the electronic balance at this time was recorded. The lithium ion battery was fixed in the air by the clamp of the iron stand, and slowly immersed in the beaker containing deionized water until the lithium ion battery was completely immersed in the deionized water. The reading m2 of the electronic balance at this time was recorded. According to the formula mg=ρ H2O VP, the battery volume V=(m2-m1) x g / (ρ H2O x P) can be calculated. Wherein ρ H2O is the density of deionized water, g is the gravitational coefficient, P is a standard atmospheric pressure, and V is the volume of the secondary battery;

[0147] The volume test method described above was used to calculate the battery volume V1 of the secondary battery before high temperature storage, and then the volume V2 of the secondary battery after high temperature storage was calculated. The gas production of the battery after high temperature storage = V2-V1;

[0148] The method of high temperature storage is the same as (3) above.

[0149] Table 1

[0150]

[0151]

[0152] In combination with Table 1, it can be seen from Comparative Examples 1-12 and Comparative Examples 1-5 that the electrolyte combination, the secondary injection method and the lithium ion battery of the present application ensure the capacity performance, improve the high-temperature cycle performance and high-temperature storage performance of the battery, and reduce the gas production of the battery.

[0153] Any combination of the technical features of the above-described embodiments can be made, and in order to make the description simple, all possible combinations of the technical features in the above-described embodiments are not described, however, as long as the combination of the technical features does not exist, it should be considered as the scope of the present application.

[0154] The above-described embodiments only express several implementation manners of the present application, the description is more specific and detailed, but it should not be understood as a limitation on the patent scope of the present application. It should be pointed out that for ordinary skilled in the art, without departing from the technical concept of the present application, a number of modifications and improvements can be made, which all belong to the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the appended claims.

Claims

1. An electrolyte combination, characterized in that, For secondary electrolyte filling of lithium-ion batteries, the electrolyte assembly includes: The first electrolyte is used in the first injection of the secondary injection solution. The first electrolyte includes a first lithium salt, a first solvent and a first additive. The first lithium salt is lithium difluorosulfonylimide, the first solvent includes fluoroethylene carbonate, and the first additive includes fluorinated lithium salt additive and positive electrode film-forming additive. The mass percentage of the lithium bis(fluorosulfonyl)imide in the first electrolyte is 3% to 7%; The mass percentage of the fluorinated lithium salt additive in the first electrolyte is 1% to 3%; The second electrolyte is used in the second injection of the secondary injection solution. The second electrolyte includes a second lithium salt, a second solvent, and a second additive. The second lithium salt is lithium hexafluorophosphate, and the second additive includes a gas-inhibiting additive. The second solvent includes cyclic carbonates and linear carbonates; The gas-inhibiting additives include one or more of vinyl sulfate, vinylene carbonate, ethylene ethylene carbonate, vinyl sulfite, methylene disulfonate, 1,3-propane sulfonyl lactone, propenyl-1,3-sulfonyl lactone, tetravinylsilane, and tripropynyl phosphate.

2. The electrolyte combination according to claim 1, characterized in that, The fluoroethylene carbonate has a mass percentage of 18% to 45% in the first electrolyte; and / or, The second additive also includes fluoroethylene carbonate, wherein the fluoroethylene carbonate has a mass percentage of 0.01% to 5% in the second electrolyte.

3. The electrolyte combination according to claim 1, characterized in that, The mass percentage of the gas-inhibiting additive in the second electrolyte is 0.1% to 1%.

4. The electrolyte combination according to any one of claims 1 to 3, characterized in that, The lithium-ion battery includes a negative electrode, the negative electrode includes a current collector and a negative electrode coating, the negative electrode coating includes a silicon-containing material, and the mass percentage a% of the fluoroethylene carbonate in the electrolyte combination and the mass percentage x% of the silicon-containing material in the negative electrode coating satisfy: 0.5≤a / x≤0.

7.

5. The electrolyte combination according to any one of claims 1 to 3, characterized in that, The first electrolyte constitutes 20% to 40% by mass in the electrolyte combination; and / or, The second electrolyte accounts for 60% to 80% of the total electrolyte composition by mass.

6. A method for secondary injection, characterized in that, The lithium-ion battery is refilled with electrolyte using the electrolyte combination according to any one of claims 1 to 5, the refilling method comprising the following steps: The first electrolyte is used to perform the first electrolyte injection into the lithium-ion battery. The lithium-ion battery after the first liquid injection was subjected to a settling and formation process in sequence. The lithium-ion battery after formation is injected with the second electrolyte for a second time. The lithium-ion batteries after the second electrolyte injection were subjected to aging and capacity testing in sequence.

7. The secondary injection method according to claim 6, characterized in that, The formation includes multiple formation stages, and the current density of a later formation stage is less than the current density of a previous formation stage.

8. A lithium-ion battery, characterized in that, Includes the electrolyte combination according to any one of claims 1 to 5; or, The lithium-ion battery is prepared by the secondary liquid injection method according to claim 6 or 7.

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