Electrolyte, secondary battery, and electric device

Abstract

Embodiments of the present application provide electrolyte, secondary battery and electric device, the electrolyte includes the additive with following structural formula (I): wherein, R1, R2 Hydrogen or alkyl group of carbon atom number 1-4, and wherein, the mass fraction of additive is 0.1% to 1% based on the total mass of the electrolyte.In the present application, the additive with the above structural formula (I) has amide bond, the amide bond has basicity, can neutralize the acidic substance (HF) generated in secondary battery, thereby inhibiting the reaction of HF and cylindrical steel shell, inhibiting the corrosion problem of secondary battery.

Description

TECHNICAL FIELD

[0001] Embodiments of the present application relate to the field of batteries, in particular, to electrolyte, secondary battery and power device. BACKGROUND

[0002] In the field of new energy, electric vehicles have rapidly occupied the automobile market with the advantages of low energy consumption, no carbon emission, etc., and are favored by the majority of users. With the increase of popularity, the problem of short driving range of electric vehicles has also been gradually exposed, which requires lithium ion batteries to further improve the energy density to meet the increasingly high demand for driving range. In order to improve the energy density of the battery, the positive electrode material develops towards high-nickel ternary, the negative electrode material develops towards high-silicon, and the structure of the battery gradually transitions from soft package, square shell to cylindrical form. In this field, the appearance of known Tesla 4680 (battery diameter of 46 mm, height of 80 mm) large cylindrical battery has attracted the attention of the industry, which has higher energy density and safety, and is increasingly favored by other electric vehicle enterprises, so that various battery manufacturers are competing to develop cylindrical batteries. The design and structure of cylindrical batteries are different from soft package and square shell batteries. Due to the large tension of the cylindrical battery pole roll and the small internal space, the shell is commonly made of stainless steel to meet the sufficient pressure resistance. Compared with other materials (such as the aluminum shell commonly used in square shell batteries), the corrosion resistance of stainless steel in cylindrical batteries is weak, which is prone to both electrochemical corrosion (electrochemical oxidation) and chemical corrosion (such as acid corrosion). Since the steel shell is immersed in the electrolyte, the composition of the electrolyte will cause corrosion risk to the steel shell, which poses a higher challenge to the electrolyte.

[0003] However, at present, the commonly used electrolyte design strategy in the industry for cylindrical batteries mainly focuses on improving the wettability of the electrolyte, and there is no special solution to the corrosion problem of the steel shell, or the design strategy of high-voltage electrolyte is followed, and a protective film is formed on the surface of the steel shell through the film-forming passivation of specific additives, which can inhibit the corrosion phenomenon to a certain extent. However, this method is a temporary solution and does not solve the root problem. In the long-term working process, the additives in the electrolyte are consumed, the protective effect on the steel shell is lost, and the corrosion and short circuit problems still occur. SUMMARY

[0004] In view of the current corrosion problem of the steel shell of the cylindrical battery, the present application proposes a kind of additive and the design of corresponding electrolyte formula. The additive proposed in the present application can effectively neutralize the acidic components in the electrolyte of the cylindrical battery, thereby inhibiting the corrosion reaction of HF (acidic substance) and the steel shell, and inhibiting the corrosion of the steel shell. In addition, through reasonable formula design, the electrolyte proposed in the present application can improve the high-temperature storage performance of the battery without significantly deteriorating the impedance.

[0005] The application provides an electrolyte, which comprises an additive with the following structural formula (I):

[0006]

[0007] wherein R1 and R2 are hydrogen or alkyl with 1-4 carbon atoms, and wherein the mass fraction of the additive is 0.1%-1%, optionally 0.3%, based on the total mass of the electrolyte. In the application, the additive with the above structural formula (I) has an amide bond, which is alkaline and can neutralize the acidic substances (HF) generated in the secondary battery, thereby inhibiting the reaction between HF and the cylindrical steel shell and inhibiting the corrosion problem of the secondary battery. Further, when 0.3% of the additive with the above structural formula (I) is added, the corrosion problem of the steel shell of the battery no longer occurs, and the storage capacity of the battery is improved. In addition, when the amount of the additive with the above structural formula (I) is too low (for example, the mass fraction is less than 0.1%), the battery steel shell is obviously corroded, and when the amount of the additive with the above structural formula (I) is too large (for example, the mass fraction is higher than 1%), the internal resistance of the battery is obviously deteriorated.

[0008] In some embodiments, the additive is the following structure: In the application, when the additive is the structure, the characteristics of the amide bond can be effectively played, so that the acidic substances (HF) generated in the secondary battery can be better neutralized.

[0009] In the application, the electrolyte further comprises a cyclic carbonate, which comprises but is not limited to one or more of EC (ethylene carbonate), PC (propylene carbonate), FEC (fluoroethylene carbonate) and the like, and the mass fraction of the cyclic carbonate is not less than 8% and not higher than 24%, based on the total mass of the electrolyte.

[0010] In some embodiments, the cyclic carbonate comprises fluoroethylene carbonate (FEC), and the mass fraction of the fluoroethylene carbonate is not higher than 8% and not less than 2%, based on the total mass of the electrolyte.

[0011] In some embodiments, the electrolyte further comprises a chain carbonate, and the chain carbonate comprises DMC (dimethyl carbonate), and the mass fraction of the DMC (dimethyl carbonate) is not less than 45%, based on the total mass of the electrolyte.

[0012] In some embodiments, the electrolyte further comprises an unsaturated inorganic additive, which is an additive containing an unsaturated bond and centered on an inorganic atom, and wherein the mass fraction of the unsaturated inorganic additive is not less than 0.3% and not higher than 0.8% based on the total mass of the electrolyte. In some embodiments, the unsaturated inorganic additive comprises one or more of triphenyl phosphate (TPP), 1-propene-1,3-sultone (PST), and tetra-vinyl silane (TVS), etc. In the present application, when the unsaturated inorganic additive is too little (e.g., the mass fraction is less than 0.3%), the positive electrode film formation is insufficient, and the high-temperature storage performance of the secondary battery is deteriorated, and when the unsaturated inorganic additive is excessive (e.g., the mass fraction is higher than 0.8%), the initial DCR of the secondary battery will increase dramatically, therefore, it is necessary to limit the mass fraction of the unsaturated inorganic additive to not less than 0.3% and not higher than 0.8%.

[0013] Some other embodiments of the present application provide a secondary battery, comprising: a positive electrode comprising a positive electrode active material; a negative electrode comprising a negative electrode active material; and an electrolyte, the electrolyte being the above-mentioned electrolyte, wherein the positive electrode active material comprises lithium nickel cobalt manganese oxide, and the mass fraction of the nickel is not less than 80% based on the total mass of the positive electrode active material, the negative electrode active material comprises a silicon-oxygen material or a silicon-carbon material, and the mass fraction of the silicon compound is not higher than 5% based on the total mass of the negative electrode active material, and the upper limit of the operating voltage of the secondary battery is not less than 4.15V.

[0014] In some embodiments, the secondary battery is a cylindrical lithium ion battery, and the diameter size of the cylindrical lithium ion battery is not less than 46mm.

[0015] Still some other embodiments of the present application provide an electric device comprising the above-mentioned secondary battery.

[0016] In summary, the additive (I) proposed in the present application has an amide bond in its structure, which has basicity and can neutralize the acidic substances (HF) generated in the secondary battery, thereby inhibiting the reaction of HF and the cylindrical steel shell and inhibiting the corrosion problem. Through reasonable formulation design, the electrolyte proposed in the present application can be well matched with the high-nickel positive electrode, effectively solving the poor storage performance problem of the high-nickel cylindrical system, and simultaneously solving the steel shell corrosion problem. DETAILED DESCRIPTION

[0017] In the development of secondary batteries such as cylindrical batteries in the field of new energy, there is a problem of steel shell corrosion. Based on this, the present application can neutralize the acidic substances (HF) generated in the secondary battery by adding an additive in the electrolyte, thereby inhibiting the reaction of HF and the cylindrical steel shell and inhibiting the corrosion problem.

[0018] The present application provides an electrolyte, which comprises an additive having the following structural formula (I):

[0019]

[0020] wherein R1 and R2 are hydrogen or alkyl with 1-4 carbon atoms, and wherein the mass fraction of the additive is 0.1%-1%, optionally 0.3%, based on the total mass of the electrolyte. In the present application, the additive having the above structural formula (I) has an amide bond, which is basic and can neutralize the acidic substances (HF) generated in the secondary battery, thereby inhibiting the reaction between HF and the cylindrical steel shell and inhibiting the corrosion problem. Further, when 0.3% of the additive having the above structural formula (I) is added, the corrosion problem of the steel shell of the battery no longer occurs, and the storage capacity of the battery is improved. In addition, when the amount of the additive having the above structural formula (I) is too low (e.g., the mass fraction is less than 0.1%), the battery steel shell is significantly corroded, and when the amount of the additive having the above structural formula (I) is too large (e.g., the mass fraction is greater than 1%), the internal resistance of the battery is significantly deteriorated.

[0021] In some embodiments, the additive has the following structure: In the present application, when the additive has this structure, the properties of the amide bond can be effectively utilized, thereby better neutralizing the acidic substances (HF) generated in the secondary battery.

[0022] In the present application, the electrolyte further comprises a cyclic carbonate, which includes but is not limited to one or more of EC (ethylene carbonate), PC (propylene carbonate), FEC (fluoroethylene carbonate), etc., and the mass fraction of the cyclic carbonate is not less than 8% and not more than 24%, based on the total mass of the electrolyte. In the present application, the content of the cyclic carbonate has an important influence on the initial DCR (impedance) of the secondary battery (has an important influence on wettability), and a too low mass fraction of the cyclic carbonate (e.g., a mass fraction less than 8%) will result in low conductivity and large DCR, and a too high mass fraction of the cyclic carbonate (e.g., a mass fraction greater than 24%) will result in too large viscosity of the electrolyte, also increasing the DCR.

[0023] In some embodiments, the cyclic carbonate comprises fluoroethylene carbonate (FEC), and the mass fraction of the fluoroethylene carbonate is not more than 8% and not less than 2%, based on the total mass of the electrolyte. In the present application, if the mass fraction of FEC is too low (e.g., less than 2%), an effective SEI film (solid electrolyte interface film) cannot be formed on the surface of the silicon compound of the negative electrode, which will result in a sharp drop in the storage capacity of the secondary battery; and a too high mass fraction of FEC (e.g., greater than 8%) will deteriorate the initial DCR of the secondary battery and also deteriorate the storage performance of the secondary battery.

[0024] In some embodiments, the electrolyte further includes a chain carbonate, the chain carbonate including one or more of EMC (ethyl methyl carbonate), DMC (dimethyl carbonate), DEC (diethyl carbonate), and the like. In the present application, the chain carbonate includes DMC (dimethyl carbonate), and the mass fraction of DMC (dimethyl carbonate) is not less than 45% based on the total mass of the electrolyte, because DMC (dimethyl carbonate) plays an important role in reducing the DCR of the secondary battery, and too low a mass fraction of DMC (dimethyl carbonate) (i.e., less than 45%) will result in an increase in the DCR of the battery, and therefore, the mass fraction of DMC (dimethyl carbonate) needs to be limited to not less than 45%.

[0025] In some embodiments, the electrolyte further includes an unsaturated inorganic additive, the unsaturated inorganic additive being an additive containing an unsaturated bond and having an inorganic atom as a central atom, and wherein the mass fraction of the unsaturated inorganic additive is not less than 0.3% and not more than 0.8% based on the total mass of the electrolyte. In some embodiments, the unsaturated inorganic additive includes one or more of triphenyl phosphate (TPP), 1-propene-1,3-sultone (PST), and tetra-vinyl silane (TVS), and the like. In the present application, when the unsaturated inorganic additive is too little (e.g., the mass fraction is less than 0.3%), the positive electrode film formation is insufficient, and the high-temperature storage performance of the secondary battery deteriorates, and when the unsaturated inorganic additive is too much (e.g., the mass fraction is more than 0.8%), the initial DCR of the secondary battery will increase dramatically, and therefore, the mass fraction of the unsaturated inorganic additive needs to be limited to not less than 0.3% and not more than 0.8%.

[0026] Some other embodiments of the present application provide a secondary battery, the secondary battery including: a positive electrode including a positive electrode active material; a negative electrode including a negative electrode active material; and an electrolyte, the electrolyte being the above-mentioned electrolyte, wherein the positive electrode active material includes lithium nickel cobalt manganese oxide (e.g., LiNi 0.9 Co 0.05 Mn 0.05 O2), and the mass fraction of nickel is not less than 80% based on the total mass of the positive electrode active material, the negative electrode active material includes a silicon-oxygen material or a silicon-carbon material, and the mass fraction of silicon compound is not more than 5% based on the total mass of the negative electrode active material, and the upper limit of the operating voltage of the secondary battery is not less than 4.15V. In some embodiments, the secondary battery is a cylindrical lithium ion battery, and the diameter size of the cylindrical lithium ion battery is not less than 46mm.

[0027] Some other embodiments of the present application provide an electric device including the above-mentioned secondary battery. In the present application, the secondary battery includes a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and an electrolyte, and the related preparation method of the secondary battery is as follows.

[0028] (1) Preparation of the positive electrode

[0029] The positive electrode active material ternary NMC (LiNi 0.9 Co 0.05 Mn 0.05 O2), polyvinylidene fluoride as the binder, and Super P (conductive carbon black) as the conductive agent were mixed in a weight ratio of 97-99:0.5-1.5:0.5-1.5, N-methyl pyrrolidone (NMP) was added, and the system was stirred to be uniform and transparent under the action of a vacuum stirrer to obtain a positive electrode slurry. The positive electrode slurry was uniformly coated on the positive electrode current collector aluminum foil, and then the aluminum foil was dried at room temperature and transferred to an oven for drying, followed by cold pressing, slitting to obtain the positive electrode (polar piece).

[0030] (2) Preparation of the negative electrode

[0031] The artificial graphite with a mass fraction of 96%-98%, SiC (silicon carbide) with a mass fraction of 2%-4% as the negative electrode active material, Super P as the conductive agent, carboxymethyl cellulose sodium (CMC-Na) as the thickening agent, and styrene butadiene rubber (SBR) as the binder were mixed in a mass ratio of 94-98:0.5-1.5:0.5-1.5:1-3, deionized water was added, and a negative electrode slurry was obtained under the action of a vacuum stirrer. The negative electrode slurry was uniformly coated on the negative electrode current collector copper foil, and then the copper foil was dried at room temperature and transferred to an oven for drying, followed by cold pressing, slitting to obtain the negative electrode (polar piece).

[0032] (3) Preparation of the electrolyte:

[0033] In an argon atmosphere glove box with a water content of <10 ppm, battery-grade ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate (FEC), methyl ethyl carbonate (EMC), dimethyl carbonate (DMC) were mixed in a mass fraction of 1%-11%, 1%-11%, 2%-8%, 6%-10%, and more than 45% of the total mass of the total electrolyte to form an organic solvent. Then LiPF6, TPP, PST, TVS, and additives were added in a mass fraction of 14%, 0.1%-0.2%, 0.1%-0.4%, 0.1%-0.2%, and 0.1%-1% of the total mass of the electrolyte, and mixed uniformly to obtain the electrolyte. In this application, the content of all components is the mass percentage calculated based on the total mass of the electrolyte. In addition, in this application, the additive has the following structural formula (I):

[0034]

[0035] wherein R1 and R2 are hydrogen or an alkyl group with 1-4 carbon atoms.

[0036] (4) Preparation of the separator film:

[0037] A polypropylene film was used as the separator film.

[0038] (5) Preparation of the secondary battery:

[0039] A polypropylene film (PP) with a thickness of 12 μm was used as the separator film. The above-prepared positive electrode, the separator film, and the negative electrode were sequentially wound into a roll core, so that the separator film was positioned between the positive electrode and the negative electrode to play a role of separation. Then, the roll core was loaded into a large cylindrical battery shell with a size of 4695 (a battery diameter of 46 mm and a height of 95 mm), and 1.7 g / Ah of the above-prepared electrolyte was injected into the battery shell, followed by sealing. After that, electrolyte solution was formed, and finally, a cylindrical lithium ion battery (i.e., a lithium ion battery) was prepared.

[0040] Those skilled in the art will understand that the above-described method for preparing the secondary battery is only an example. Other methods commonly used in the art can be employed without departing from the content disclosed in the present application.

[0041] The reagents and raw materials used in the present application are commercially available.

[0042] The above-described secondary battery can be applied to any suitable electric device, including but not limited to electric vehicles, etc.

[0043] Some specific examples and comparative examples are listed below to better illustrate the present application.

[0044] Example 1

[0045] (1) Preparation of the positive electrode:

[0046] The positive electrode active material ternary NMC (LiNi 0.9 Co 0.05 Mn 0.05 O2), polyvinylidene fluoride as the binder, and Super P as the conductive agent were mixed in a weight ratio of 98:1:1, N-methyl pyrrolidone (NMP) was added, and the system was stirred to be uniform and transparent under the action of a vacuum stirrer, to obtain a positive electrode slurry. The positive electrode slurry was uniformly coated on a positive electrode current collector aluminum foil, and then the aluminum foil was dried in an oven after being air-dried at room temperature, and then cold-pressed and cut to obtain a positive electrode (polar piece).

[0047] (2) Preparation of the negative electrode:

[0048] The 97% mass fraction of artificial graphite, 3% mass fraction of SiC as negative active material, Super P as conductive agent, carboxymethyl cellulose sodium (CMC-Na) as thickening agent, butadiene rubber (SBR) as binder, mixed in a mass ratio of 96:1:1:2, add deionized water, under the action of vacuum stirrer to obtain negative electrode slurry, the negative electrode slurry is uniformly coated on the negative electrode current collector copper foil, then the copper foil is dried at room temperature, then transferred to the oven for drying, then cold-pressed, cut to obtain the negative electrode (polar piece).

[0049] (3) Preparation of electrolyte:

[0050] In an argon atmosphere glove box with water content <10 ppm, battery grade ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate (FEC), methyl ethyl carbonate (EMC), dimethyl carbonate (DMC) are mixed in a mass fraction of 8%, 8%, 6%, 8%, 55.3% of the total mass of the total electrolyte to form an organic solvent. Then add LiPF6, TPP, PST, TVS, additives in a mass fraction of 14%, 0.1%, 0.2%, 0.1%, 0.3% of the total mass of the electrolyte, mix uniformly to obtain the electrolyte (i.e. Example 1 in Table 1 below), in this example 1, the additive has the following structural formula:

[0051] (4) Preparation of separator film:

[0052] The polypropylene film is used as the separator film.

[0053] (5) Preparation of secondary battery:

[0054] The polypropylene film (PP) with a thickness of 12 μm is used as the separator film, and the above prepared positive electrode, separator film and negative electrode are sequentially wound into a core to make the separator film in the middle of the positive and negative electrodes to play a role of isolation. Then put it into a 4695 size large cylindrical battery shell, inject 1.7 g / Ah of the above prepared electrolyte, then seal, electrolyte is formed, and finally a cylindrical lithium ion battery (i.e. lithium ion battery) is prepared.

[0055] Example 2

[0056] The preparation method of Example 1 is consistent, except that the mass fraction of battery grade ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate (FEC), methyl ethyl carbonate (EMC), dimethyl carbonate (DMC) in Example 2 is 1%, 1%, 6%, 22%, 55.3% respectively.

[0057] Example 3

[0058] The preparation method of Example 1 is consistent, except that the mass fraction of battery-grade ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate (FEC), methyl ethyl carbonate (EMC), dimethyl carbonate (DMC) in Example 3 is 9%, 9%, 6%, 6%, and 55.3%, respectively.

[0059] Example 4

[0060] The preparation method of Example 1 is consistent, except that the mass fraction of battery-grade ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate (FEC), methyl ethyl carbonate (EMC), dimethyl carbonate (DMC) in Example 4 is 11%, 11%, 2%, 6%, and 55.3%, respectively.

[0061] Example 5

[0062] The preparation method of Example 1 is consistent, except that the mass fraction of battery-grade ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate (FEC), methyl ethyl carbonate (EMC), dimethyl carbonate (DMC) in Example 5 is 7%, 7%, 8%, 8%, and 55.3%, respectively.

[0063] Example 6

[0064] The preparation method of Example 1 is consistent, except that the mass fraction of battery-grade ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate (FEC), methyl ethyl carbonate (EMC), dimethyl carbonate (DMC) in Example 6 is 8%, 8%, 6%, 18.3%, and 45%, respectively.

[0065] Example 7

[0066] The preparation method of Example 1 is consistent, except that the mass fraction of LiPF6, TPP, PST, TVS, and additives in Example 7 is 14.1%, 0.1%, 0.1%, 0.1%, and 0.3%, respectively.

[0067] Example 8

[0068] The preparation method of Example 1 is consistent, except that the mass fraction of LiPF6, TPP, PST, TVS, and additives in Example 8 is 9.1%, 0.2%, 0.4%, 0.2%, and 0.3%, respectively.

[0069] Example 9

[0070] The preparation method of Example 1 is consistent, except that the mass fraction of LiPF6, TPP, PST, TVS, and additives in Example 9 is 14.2%, 0.1%, 0.2%, 0.1%, and 0.1%, respectively.

[0071] Example 10

[0072] The preparation method is consistent with that of Example 1, except that the mass fractions of LiPF6, TPP, PST, TVS, and the additive in Example 10 are 13.3%, 0.1%, 0.2%, 0.1%, and 1%, respectively.

[0073] Example 11

[0074] The preparation method is consistent with that of Example 1, except that the additive in Example 11 has the following structural formula:

[0075] Example 12

[0076] The preparation method is consistent with that of Example 1, except that the additive in Example 3 has the following structural formula:

[0077] Example 13

[0078] The preparation method is consistent with that of Example 1, except that the additive in Example 3 has the following structural formula:

[0079] Comparative Example 1

[0080] The preparation method is consistent with that of Example 1, except that the mass fractions of battery-grade ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in Comparative Example 1 are 2%, 2%, 2%, 24%, and 55.3%, respectively.

[0081] Comparative Example 2

[0082] The preparation method is consistent with that of Example 1, except that the mass fractions of battery-grade ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in Comparative Example 2 are 10%, 10%, 8%, 2%, and 55.3%, respectively.

[0083] Comparative Example 3

[0084] The preparation method is consistent with that of Example 1, except that the mass fractions of battery-grade ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in Comparative Example 3 are 10%, 10%, 0%, 10%, and 55.3%, respectively.

[0085] Comparative Example 4

[0086] The preparation method of Example 1 is consistent, except that the mass fraction of battery-grade ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate (FEC), methyl ethyl carbonate (EMC), dimethyl carbonate (DMC) in Comparative Example 4 is 5%, 5%, 10%, 10%, 55.3%, respectively.

[0087] Comparative Example 5

[0088] The preparation method of Example 1 is consistent, except that the mass fraction of battery-grade ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate (FEC), methyl ethyl carbonate (EMC), dimethyl carbonate (DMC) in Comparative Example 5 is 8%, 8%, 6%, 28.3%, 35%, respectively.

[0089] Comparative Example 6

[0090] The preparation method of Example 1 is consistent, except that the mass fraction of LiPF6, TPP, PST, TVS, additives in Comparative Example 6 is 14.2%, 0.1%, 0%, 0.1%, 0.3%, respectively.

[0091] Comparative Example 7

[0092] The preparation method of Example 1 is consistent, except that the mass fraction of LiPF6, TPP, PST, TVS, additives in Comparative Example 7 is 13.4%, 0.3%, 0.4%, 0.3%, 0.3%, respectively.

[0093] Comparative Example 8

[0094] The preparation method of Example 1 is consistent, except that the mass fraction of LiPF6, TPP, PST, TVS, additives in Comparative Example 8 is 14.3%, 0.1%, 0.2%, 0.1%, 0%, respectively.

[0095] Comparative Example 9

[0096] The preparation method of Example 1 is consistent, except that the mass fraction of LiPF6, TPP, PST, TVS, additives in Comparative Example 9 is 13.1%, 0.1%, 0.2%, 0.1%, 1.2%, respectively.

[0097] The secondary batteries in Examples 1-13 and Comparative Examples 1-9 above can be tested by the following method:

[0098] (1) Normal temperature direct current resistance DCR test

[0099] The secondary batteries in Examples 1-13 and Comparative Examples 1-9 above were left to stand at room temperature for 2 h, charged at 1 C constant current to 50% SOC, i.e. 4.25 V, and then charged at 4.25 V constant voltage to 0.05 C cut-off, left to stand for 30 min, and then discharged at 0.33 C constant current to 2.5 V; the cycle was repeated twice, and the discharge capacity of the last cycle was recorded as C0; after standing for 30 min, the battery was discharged at 0.33 C to 50% C0, the SOC of the battery was adjusted to 50%, and the battery was left to stand for 30 min; the terminal voltage V1 was recorded, the battery was discharged at C0 constant current for 30 s, and the terminal voltage V2 and the current I were recorded; and the DCR was calculated as (V1-V2) / I.

[0100] (2) Secondary battery high-temperature storage performance test

[0101] The batteries were stored in an oven at a specified temperature (60°C). First, the initial capacity of the batteries was tested at room temperature (25°C) at a current of 0.33 C; then the batteries obtained in Examples 1-13 and Comparative Examples 1-9 were fully charged and stored in the oven at 60°C for 15 days; after that, the batteries were taken out and the recovery capacity of the batteries was tested at a current of 0.33 C; and the batteries were fully charged and stored again. The process was repeated until 120 days, and the ratio of the recovery capacity to the initial capacity at 120 days was collected and recorded as the storage capacity recovery rate.

[0102] (3) Steel can corrosion condition test

[0103] The batteries in Examples 1-13 and Comparative Examples 1-9 stored at high temperature (60°C) for 120 days were disassembled, and the corrosion condition of the inner wall of the steel can was recorded.

[0104] The test results are shown in Table 1 below.

[0105] Table 1. Performance test results of the secondary batteries in Examples 1-13 and Comparative Examples 1-9

[0106]

[0107]

[0108]

[0109] As can be seen from the above Examples 1-13, the electrolyte comprises an additive having the following structural formula (I):

[0110]

[0111] When R1, R2 are hydrogen or alkyl with carbon number 1-4, and the mass fraction of the additive is 0.1%-1% based on the total mass of the electrolyte, the additive with the above structural formula (I) contains amide bond, which has alkalinity and can neutralize the acidic substances (HF) generated in the secondary battery, thereby inhibiting the reaction between HF and the cylindrical steel shell and inhibiting the corrosion problem. Further, from the comparison of Examples 1, 9 and 10 and Comparative Examples 8-9, when 0.3% of the additive with the above structural formula (I) is added, the steel shell of the battery no longer has corrosion problem, and the storage capacity of the battery is improved. In addition, from Comparative Examples 8-9, when the amount of the additive with the above structural formula (I) is too low (for example, the mass fraction is less than 0.1%, such as 0% in Comparative Example 8), the battery steel shell is obviously corroded, and when the amount of the additive with the above structural formula (I) is too large (for example, the mass fraction is greater than 1%, such as 1.2% in Comparative Example 9), the internal resistance of the battery is obviously deteriorated.

[0112] In addition, from Examples 1, 11-13, it can be seen that the additive is the following structure:

[0113] The properties of the amide bond can be effectively played, thereby better neutralizing the acidic substances (HF) generated in the secondary battery.

[0114] From the comparison of Examples 1-3 and Comparative Examples 1-2, it can be seen that the content of the cyclic carbonate EC (ethylene carbonate), PC (propylene carbonate), FEC (fluoroethylene carbonate) has an important influence on the initial DCR (impedance) of the secondary battery (has an important influence on wettability), and too low mass fraction of the cyclic carbonate (for example, the mass fraction is less than 8%, such as 6% in Comparative Example 1) will result in low conductivity and large DCR, and too high mass fraction of the cyclic carbonate (for example, the mass fraction is greater than 24%, such as 28% in Comparative Example 2) will result in too large viscosity of the electrolyte, also increasing the DCR, therefore, the mass fraction of the cyclic carbonate needs to be limited to not less than 8% and not more than 24%.

[0115] From the comparison of Examples 1, 4-5 and Comparative Examples 3-4, it can be seen that the mass fraction of FEC has an important influence on the performance of the secondary battery. If the mass fraction of FEC is too low (for example, less than 2%, such as 0% in Comparative Example 3), an effective SEI film (solid electrolyte interface film) cannot be formed on the surface of the silicon compound of the negative electrode, which will result in a sharp drop in the storage capacity of the secondary battery; and too high mass fraction of FEC (for example, greater than 8%, such as 10% in Comparative Example 4) will deteriorate the initial DCR of the secondary battery, and at the same time deteriorate the storage performance of the secondary battery, therefore, the mass fraction of fluoroethylene carbonate needs to be not more than 8% and not less than 2%.

[0116] From the comparison of Example 1, 6 and Comparative Example 5, it is known that the chain carbonate in the electrolyte includes DMC (dimethyl carbonate), and the mass fraction of DMC (dimethyl carbonate) is not less than 45% based on the total mass of the electrolyte, because DMC (dimethyl carbonate) plays an important role in reducing the DCR of the secondary battery, and too low mass fraction of DMC (dimethyl carbonate) (i.e. lower than 45%, such as 35% in Comparative Example 5) will result in an increase in the DCR of the battery, therefore, it is necessary to limit the mass fraction of DMC (dimethyl carbonate) to be not less than 45%.

[0117] From the comparison of Example 1, 7-8 and Comparative Example 6-7, it is known that the unsaturated inorganic additive in the electrolyte plays an important role, and the unsaturated inorganic additive includes triphenyl phosphate (TPP), 1-propene-1, 3-sultone (PST) and tetra-vinyl silane (TVS), the mass fraction of the unsaturated inorganic additive is not less than 0.3% and not higher than 0.8% based on the total mass of the electrolyte, when the unsaturated inorganic additive is too little (e.g. mass fraction lower than 0.3%, such as 0.2% in Comparative Example 6), the positive electrode film formation is insufficient, and the high temperature storage performance of the secondary battery is deteriorated, while when the unsaturated inorganic additive is excessive (e.g. mass fraction higher than 0.8%, such as 1% in Comparative Example 7), the initial DCR of the secondary battery will increase dramatically, therefore, it is necessary to limit the mass fraction of the unsaturated inorganic additive to be not less than 0.3% and not higher than 0.8%.

[0118] The features of several embodiments have been summarized above in order to provide a better understanding of aspects of the application. Persons skilled in the art will readily appreciate that they can readily use the application as a basis for designing or modifying other processes and structures for carrying out the same purposes and / or achieving the same advantages of the embodiments introduced herein. Those skilled in the art will also realize that such equivalent constructions do not depart from the spirit and scope of the application, and that they can make various changes, substitutions and alterations thereto without departing from the spirit and scope of the application in its broadest form.

Claims

1. An electrolyte, characterized by, The additive comprises a structure formula (I) as follows: （I）， wherein R1 and R2 are hydrogen or alkyl with 1-4 carbon atoms, and the mass fraction of the additive is 0.1%-1% based on the total mass of the electrolyte.

2. The electrolyte according to claim 1, characterized in that, The additive has the following structure: 。 3. The electrolyte of claim 1, wherein, The mass fraction of the additive is 0.3% based on the total mass of the electrolyte.

4. The electrolyte of claim 1, wherein The electrolyte further comprises a cyclic carbonate, and the mass fraction of the cyclic carbonate is not less than 8% and not more than 24% based on the total mass of the electrolyte.

5. The electrolyte according to claim 4, characterized in that, The cyclic carbonate comprises fluoroethylene carbonate, and the mass fraction of the fluoroethylene carbonate is not more than 8% and not less than 2% based on the total mass of the electrolyte.

6. The electrolyte according to claim 5, characterized in that The electrolyte further comprises a chain carbonate, and the chain carbonate comprises dimethyl carbonate, and the mass fraction of the dimethyl carbonate is not less than 45% based on the total mass of the electrolyte.

7. The electrolyte of claim 1, wherein The electrolyte further comprises an unsaturated inorganic additive, and the mass fraction of the unsaturated inorganic additive is not less than 0.3% and not more than 0.8% based on the total mass of the electrolyte, and the unsaturated inorganic additive comprises one or more of triphenyl phosphate, 1-propene-1,3-sultone and tetraethenylsilane.

8. A secondary battery characterized by comprising: The electrolyte comprises: a positive electrode comprising a positive electrode active material; a negative electrode comprising a negative electrode active material; and an electrolyte according to any one of claims 1-7, wherein the positive electrode active material comprises lithium nickel cobalt manganese oxide, and the mass fraction of nickel is not less than 80% based on the total mass of the positive electrode active material, the negative electrode active material comprises a silicon-oxygen material or a silicon-carbon material, and the mass fraction of silicon compounds in the negative electrode active material is not more than 5% based on the total mass of the negative electrode active material, and the upper limit of the operating voltage of the secondary battery is not less than 4.15 V. The secondary battery is a cylindrical lithium ion battery, and the diameter of the cylindrical lithium ion battery is not less than 46 mm.

9. The secondary battery according to claim 8, characterized by The secondary battery according to any one of claims 8-9 is provided.

10. An electrical device, characterized by ​

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