CN116885281A

CN116885281A - Lithium ion battery electrolyte and lithium ion battery thereof

Info

Publication number: CN116885281A
Application number: CN202311066057.7A
Authority: CN
Inventors: 付敏; 时兰兰; 苏敏
Original assignee: Wanxiang A123 Systems Asia Co Ltd
Current assignee: Wanxiang A123 Systems Asia Co Ltd
Priority date: 2023-08-23
Filing date: 2023-08-23
Publication date: 2023-10-13

Abstract

The invention discloses a lithium ion battery electrolyte and a lithium ion battery thereof, wherein the lithium ion battery electrolyte comprises lithium salt, a film forming additive, fluorobenzene, a composition of the film forming additive for infiltration improvement and the balance of solvent. The solvent for reducing the surface tension is fluorobenzene, and the electrolyte additive for improving the interfacial film forming performance and reducing the surface tension is two novel electrolyte additives. One of which is a fluoronaphtho-silicone heterocyclic siloxane compound and the other of which is a fluorobenzo-silicone heterocyclic siloxane compound. The invention researches that fluorobenzene, fluoronaphtho-silicon heterocyclic siloxane compound and fluorobenzosilicon heterocyclic siloxane compound are added into the electrolyte, so that the surface tension of the electrolyte and the time for the electrolyte to infiltrate the battery cell are obviously reduced. Compared with the high-temperature circulation and the multiplying power performance of the battery core, the battery core has higher circulation life after being added with the infiltration improving solvent and the novel additive, and has higher capacity retention rate after multiplying power discharge.

Description

Lithium ion battery electrolyte and lithium ion battery thereof

Technical Field

The invention belongs to the field of lithium ion batteries, and relates to an electrolyte, in particular to a lithium ion battery electrolyte and a lithium ion battery thereof.

Background

Lithium ion batteries are widely used in the field of new energy automobiles, but the energy density of the lithium ion batteries cannot fully meet the mileage requirement of the automobiles. To solve the problem of relatively low lithium ion energy density in gasoline, a common approach is to increase the total amount of active materials. In order to increase the amount of active material added, the compacted density of the positive and negative electrode sheets must be increased. When the compaction density of the positive and negative plates is too high, the problem of poor wettability of electrolyte to the surfaces of the plates can occur, meanwhile, in the long-term circulation process, the electrolyte in the lithium ion battery can be obviously consumed, and the electrolyte wettability problem in the later circulation period is more outstanding.

Therefore, in order to increase the energy density and cycle life of the lithium ion battery, it is necessary to increase the wettability of the lithium ion battery with the electrolyte.

For the problem of poor electrolyte wettability caused by too high compaction density or lower filling coefficient of the pole piece, the patent number CN111640946A proposes that an infiltration improving additive is added into the preparation of the pole piece, and the infiltration improving additive in the pole piece can be dissolved into the electrolyte after filling, so that the position of the original additive in the pole piece generates gaps due to dissolution, thereby being beneficial to storing the electrolyte. The infiltration improving additive dissolved into the electrolyte improves the infiltration of the electrode plate interface in the subsequent charge and discharge process.

The patent number CN112366356A solves the problem of infiltration of the battery cell from the technical point of view, and the core invention is that the battery cell is rolled after being pre-charged. By comparing the battery cells which are not rolled, the inventor finds that the wettability of the battery cells which are rolled is obviously improved, the risk of lithium precipitation is reduced, the K value of the battery cells is reduced, and the capacity is improved.

Patent CN103151560BC proposes a lithium ion battery electrolyte and an additive, wherein the components of the additive include N-methyl-2-pyrrolidone-derived quaternary ammonium salt and SEI film forming additive. The authors compare the existing additive for improving the infiltration, and find that after the N-methyl-2-pyrrolidone derivative quaternary ammonium salt is added into the electrolyte, the infiltration of the electrolyte to the pole piece is obviously improved, and the problems of low-temperature discharge capacity and poor cycle life of the lithium ion battery are improved.

The electrolyte wettability problem has remarkable influence on the capacity exertion and the cycle life of the battery cell, and the problem can be overcome from three aspects, namely: cell design, process design and raw material development. There are problems with the approach of wettability improvement from the cell design point of view. The idea of improving wettability from the cell design point of view is: the compaction density of the positive and negative electrode plates is reduced, the surface density of the electrode plates is reduced, the size of the battery core is reduced, the liquid injection amount is improved, and the like, but the capacity deviation of the battery core is caused, and the cost is increased. For the current new energy vehicle field pursuing high energy density and high cost performance, the design cannot meet the market demand.

The wettability problem of the electrolyte to the pole piece generally occurs in a low-injection-coefficient cell, a high-compaction-density cell and a cell in the later cycle, and is summarized as an initial cell wettability problem and a cell wettability problem in the later life. The method for improving wettability by changing the production process belongs to the range of solving the problem of initial cell wettability. The process steps for improving the wettability of the electrolyte to the polar plate are mainly in the working section during and after the liquid injection. Although the wettability of the battery core in the initial stage can be solved by improving the production process, the problem that the wettability of the battery core to the pole piece by the electrolyte in the later period of circulation is poor cannot be changed. The reason for poor wettability in the latter stage of the cycle is mainly due to the consumption of the solvent, and the problem of solvent consumption cannot be solved by changing the production process. The electrolyte is used for solving the problem of poor wettability of the electrolyte to the pole piece in the later cycle, the electrolyte is required to be started, and the problem of poor wettability of the electrolyte to the battery cell in the later cycle is improved by improving the formula of the electrolyte, so that the electrolyte is simpler and more convenient, and the effect is obvious.

Some patents modify the electrolyte in combination with two ways of thinking to improve wettability, which are: 1. reducing the consumption rate of the solvent; 2. the interfacial tension and viscosity of the solvent are reduced. The film forming additive is added into the electrolyte to reduce the consumption rate of the solvent and improve the content of the solvent with small surface tension and viscosity. In the addition of electrolyte additives, the effect of the additives on other properties of the cell, in particular calendar life, must also be considered.

The current commonly used additives for improving the wettability of the electrolyte can affect the DCR and calendar life of the battery cell, and to solve the defects of the wettability improving additives, new additives and new additive combination modes must be designed.

Determining the infiltration performance of the electrolyte to the electrode plates of the battery is a main consideration point of the electrolyte and the design of the battery, because the infiltration quality of the electrolyte to the electrode plates directly affects the first effect and the capacity in the formation process and also affects the cycle life of the battery. When the cell is formed, the electrolyte or process is selected to be unsuitable for the requirements of the cell design, and the problem of poor wettability of the electrolyte to the pole piece may occur. The electrolyte is poor in infiltration of the electrode plates, the electrolyte is further unevenly distributed in the battery core, the problems of poor compactness, poor stability and the like of a local SEI film formed during formation are caused, the SEI film cannot completely prevent continuous decomposition of the electrolyte in the subsequent capacity division process, the initial efficiency is reduced, the capacity of the capacity is low, and the lithium precipitation abnormality even possibly occurs due to the increase of the impedance of the local SEI film.

Some electrolyte has better wettability to the pole piece in the formation stage, can meet the requirement of the battery cell in the formation stage, and cannot cause a series of problems in the formation stage, but in the later cycle stage of the battery cell, the electrolyte consumes too fast solvent, so that the wettability of the electrolyte to the pole piece in the later cycle stage is reduced.

In order to improve the chemical composition and the infiltration performance of electrolyte to the pole piece in the circulating process, relevant schools and companies all put forward own improvement schemes and apply for more patents. The general thinking is to improve from the directions of cell design, production process, raw material selection and the like, but most patents have more or less problems due to the limitation of thinking, and some patents only solve the wettability problem in the process of forming the components and the components, but do not even deteriorate the wettability problem in the circulating process, and some patents solve the wettability problem in the process of forming the components and the circulating process, but also bring other electrical performance problems.

Disclosure of Invention

Based on the problem of poor wettability of the battery core in the formation and circulation processes, the invention improves the commonly used electrolyte, and provides the lithium ion battery electrolyte and the lithium ion battery thereof. The solvent for reducing the surface tension is fluorobenzene, and the electrolyte additive for improving the interfacial film forming performance and reducing the surface tension is two novel electrolyte additives. One of the electrolyte additives is a fluoronaphtho-silicone heterocyclic siloxane compound and the other electrolyte additive is a fluorobenzosilicone heterocyclic siloxane compound. The invention researches that fluorobenzene, fluoronaphtho-silicon heterocyclic siloxane compound and fluorobenzosilicon heterocyclic siloxane compound are added into electrolyte, and discovers that the surface tension of the electrolyte and the time for the electrolyte to infiltrate into a battery cell can be obviously reduced. Compared with the high-temperature circulation and the multiplying power performance of the battery core, the battery core has higher circulation life after being added with the infiltration improving solvent and the novel additive, and has higher capacity retention rate after multiplying power discharge.

In order to achieve the above purpose, the present invention adopts the following technical scheme:

the lithium ion battery electrolyte comprises, by weight, 10% -17% of lithium salt, 1.8% -3.7% of film forming additive, 3% -20% of fluorobenzene, 0.5% -10% of infiltration improvement film forming additive composition and the balance of solvent; wherein the infiltration improvement film-forming additive composition comprises a compound represented by formula 1 and/or formula 2:

wherein, R1, R2, R3, R4, R5 and R6 are all selected from hydrogen, halogen, substituted or unsubstituted alkane, alkene, alkyne, benzene ring or heterocyclic compound.

The invention designs a solution scheme of lithium ion battery electrolyte for improving infiltration, which comprises lithium salt, a film forming additive, a solvent, an infiltration improving solvent and the infiltration improving film forming additive. Wherein the infiltration improving solvent is mainly fluorobenzene, the infiltration improving film-forming additive is two additives, one of which is fluoro-naphtho-silicon heterocyclic siloxane compound and the other of which is fluoro-benzo-silicon heterocyclic siloxane compound. The adding amount of fluorobenzene serving as a wetting improvement solvent in the electrolyte is 3% -20%, the mass fraction of the wetting improvement film-forming additive composition in the electrolyte is 0.5% -10%, wherein the proportion of fluoronaphtho-silicon heterocyclic siloxane compound in the electrolyte is 0.5% -5%, and the adding amount of the fluorobenzo-silicon heterocyclic siloxane compound accounts for 0.5% -5% of the total mass.

Preferably, the mass fraction of fluorobenzene in the electrolyte is 3-5%. The mass fraction of the fluoro-naphtho-silicone heterocyclic siloxane compound is 0.5-3%, and the mass fraction of the fluoro-benzo-silicone heterocyclic siloxane compound is 0.5-3%. The total mass fraction of the infiltration improving additive combination in the electrolyte is 1-6%.

As a preferred embodiment of the present invention, the compounds of formula 1 and formula 2 are selected from at least one of the following compounds represented by T1 to T6:

as a preferred embodiment of the present invention, the infiltration improvement film-forming additive composition is a combination of a compound represented by T1-T3 and a compound represented by T4-T6.

As a preferred embodiment of the present invention, the infiltration improvement film-forming additive composition is a combination of compounds represented by T3 and T6.

As a preferable scheme of the invention, the addition amount of the compound shown in T3 accounts for 0.5-1% of the total mass of the electrolyte, and the addition amount of the compound shown in T6 accounts for 1% -1.5% of the total mass of the electrolyte.

In the present invention, two novel electrolyte additives of formula 1 and formula 2, which may be used together with fluorobenzene alone or in combination, have been studied. The combination of formula 1 with formula 2 and the use of fluorobenzene together provides superior results compared to the use of one of the additives alone in combination with fluorobenzene. Preferably, one electrolyte additive is selected from T1-T3 and T4-T6 respectively, and is used together with fluorobenzene, so that wettability can be better improved, and cycle life can be prolonged.

The contents of the T1-T3 and T4-T6 compounds in the electrolyte directly affect the wettability of the electrolyte, but since the T1-T3 and T4-T6 additives participate in the formation of SEI films, the addition amounts thereof affect the impedance and stability of SEI formed in the formation process, and the addition amounts of the additives need to be further determined. Preferably, the addition amount of the T1-T3 additive accounts for 0.5-1.5% of the total mass of the electrolyte, and the addition amount of the T4-T6 additive accounts for 1% -2% of the total mass of the electrolyte.

Further preferably, since the compound having a fluoro group is dehydrated under the catalysis of the decomposition product of lithium hexafluorophosphate at a high temperature, the hydrogen fluoride may cause an increase in the elution amount of the transition metal of the positive electrode, and the eluted transition metal is precipitated at the negative electrode, which may increase the self-discharge, increase the DCR of the battery cell and deteriorate the stability of the SEI film, thereby resulting in a deviation in the high temperature storage performance. The silane groups can absorb hydrofluoric acid in the electrolyte, so that the dissolution of transition metal is reduced, the high-temperature stability of the SEI film is improved, and the improvement of the content of T4-T6 is beneficial to the improvement of storage. Meanwhile, fluoroether groups and aromatic groups connected to the compound are beneficial to improving the wettability of the electrolyte to the polar plate. T6 is selected from T4-T6. In the T1-T3 additive, the T3 additive is also connected with a fluoroether group, and the fluoroether group is beneficial to improving the wettability of the additive, and T3 is selected from T1-T3.

It is further preferable that the addition amount of fluorobenzene is controlled to be between 3 and 10%. When the amount of fluorobenzene added exceeds a certain range, the high-temperature cycle and storage performance are lowered due to the generation of hydrogen fluoride, so that the amount of fluorobenzene added is controlled to be in a minimum range. Data for comparative examples and comparative examples found: the best results can be obtained when the fluorobenzene is added in an amount of 3-4%. The T6 additive can not only improve the wettability of the electrolyte, but also absorb hydrogen fluoride generated in the electrolyte, and can also participate in film formation, and the SEI film with stable performance, and the addition amount of the T6 is further optimized to be 1-1.5%. The T3 additive has similar structure and functional group as T6, so that the wettability of the electrolyte is obviously improved, and a stable SEI film can be formed at an electrode interface, so that the cycle life is improved. Further optimizing the addition amount of T3 to be 0.5-1%.

As a preferable scheme of the invention, the film forming additive is one or more of vinylene carbonate, fluoroethylene carbonate, propane sultone, ethylene sulfate, methane sulfonyl dimethyl ester, tri (trimethyl) silicon-based phosphate, lithium difluoro oxalate borate, lithium tetrafluorooxalate phosphate or lithium difluoro phosphate.

As a preferred embodiment of the present invention, the film-forming additive is a combination of vinylene carbonate, vinyl sulfate and tri (trimethyl) silicon-based phosphate.

The invention mainly introduces two electrolyte additives for improving infiltration and the advantages of the two additives and fluorobenzene in combination. These two new additives cannot replace other film forming additives, and other additives need to be added to further improve the stability of the SEI film. The common film forming additive is added into the electrolyte containing fluorobenzene and one or two novel additives, so that the performance of the lithium ion battery can be further optimized, and particularly the cycle life and the multiplying power performance of the lithium ion battery are improved. Mainly because the addition of the common film forming additive can form CEI film and SEI film on the anode and the cathode before the decomposition of the novel additive, the stability of the interface film is further enhanced, and the cycle performance is further improved.

Specifically, the film-forming additive used in the invention is one or more of a carbon-containing additive, a sulfur-containing additive, a phosphorus-containing additive, a boron-containing additive, a nitrogen-containing additive and a silicon-containing additive.

The addition of the film-forming electrolyte additive can affect the cycle performance, the storage performance and the rate capability of the battery cell. The content of the film forming additive is increased, so that the cycle performance of the battery cell can be improved, but the direct current internal resistance of the battery cell can be obviously increased, and the rate capability is reduced. In order to balance the effect of the film forming additives on the electrical properties, it is desirable to optimize the amount of film forming electrolyte additives added. The patent selects VC, DTD and TMSP as standard film forming additives, and the VC, DTD and TMSP are required to be optimized.

Preferably, the addition amount of VC is between 1 and 2 percent, the addition amount of DTD is between 0.5 and 1.2 percent, and the addition amount of TMSP is between 0.3 and 0.5 percent.

As a preferred embodiment of the present invention, the solvent includes at least two of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, dipropyl carbonate, ethyl acetate, methyl acetate, ethyl propionate, propyl acetate, or propyl propionate.

The invention adopts one or more solvents commonly used in the lithium ion battery field to prepare electrolyte, for example: ethylene Carbonate (EC), propylene Carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC), dipropyl carbonate (DPC), ethyl Acetate (EA), methyl Acetate (MA), ethyl Propionate (EP), propyl Propionate (PP), and the like. The solvent used in the present invention includes two or more of the above solvents, and the ratio between the solvents is not limited. The invention also uses another solvent fluorobenzene, and the addition of the solvent mainly improves the wettability of the electrolyte to the polar plate, which is the solvent used in the invention.

As a preferred embodiment of the present invention, the lithium salt includes one or more of lithium hexafluorophosphate, lithium bis (fluorosulfonyl) imide, or lithium bis (trifluoromethylsulfonyl) imide.

The mass fraction of the lithium salt and the solvent in the electrolyte is not particularly limited, and can be limited by referring to the mass fraction of the lithium salt and the solvent in the electrolyte commonly used in the lithium ion battery, or can be determined according to factors such as the positive electrode material, the negative electrode material, the diaphragm, the cell design, the development requirement and the like of the lithium ion battery. In a specific embodiment of the invention, the content of lithium salt is controlled between 10% and 17%.

The invention provides a lithium ion battery adopting the lithium ion electrolyte, which comprises a positive plate containing a positive electrode active material, a negative plate containing a negative electrode active material and a diaphragm, wherein the positive electrode active material comprises one or more of lithium cobaltate, lithium manganate, lithium nickelate, lithium iron phosphate, nickel cobalt manganese ternary material, nickel cobalt aluminum ternary material or lithium-rich manganese material, and the negative electrode active material comprises one or more of artificial graphite, natural graphite, soft carbon, hard carbon, silicon carbon or silicon oxygen.

Compared with the prior art, the invention has the following beneficial effects:

1) The two novel additives provided by the invention mainly comprise silicon groups capable of absorbing HF and reducing surface tension and fluorobenzene and fluoronaphthalene groups capable of reducing surface tension. By using the fluorine-containing compound together with fluorobenzene, the problem of performance deviation caused by HF generation at high temperature of fluorobenzene can be overcome, and wettability of electrolyte can be provided. The advantages of the present invention are verified against the high temperature cycle and rate performance data.

2) From various patent and literature reports: if the wettability of the electrolyte to the pole piece in the lithium ion battery needs to be improved, researches are needed from the angles of cell design, process design and electrolyte design. The electrolyte is improved in the infiltration capacity of the electrode plate through the cell design, and the principle is that the porosity of the electrode plate is improved, enough space is provided for storing the electrolyte, or the liquid injection amount of the electrolyte is improved, so that the lithium ion battery stores enough electrolyte in the formation and volume and circulation stages. The method is only applicable to power type lithium ion batteries, but not applicable to energy type lithium ion batteries. Because the excessive holes of the pole pieces or the excessive liquid injection amount can reduce the volume density and the mass density of the battery cell. The improvement of the liquid injection process during liquid injection can only improve the wettability of the electrolyte to the polar plate during chemical composition, and can not solve the wettability problem caused by insufficient electrolyte in the long-term circulation process. The design from the electrolyte point of view is an economical and simple method without sacrificing energy density. The invention is designed from the angles of solvent and additive, mainly uses the solvent and additive for improving the infiltration performance, obviously improves the cycle life and the multiplying power performance of the lithium ion battery through the design, and obviously improves the infiltration performance of electrolyte to the pole piece.

Detailed Description

The technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention, and it is apparent that the described embodiments are only some embodiments of the present invention, not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.

The invention selects a lithium ion battery for verifying the influence of the novel additive on the electrolyte and the performance of the battery core. According to the invention, electrolyte is prepared according to the amount of the trial-produced battery core, and then the prepared electrolyte is injected into the lithium ion battery. The lithium ion battery used by the invention not only comprises electrolyte to be verified, but also comprises a positive electrode, a negative electrode and a diaphragm.

The positive active material of the lithium ion battery used in the present invention includes one or more of all commonly used materials, such as lithium cobaltate, lithium manganate, lithium nickelate, lithium iron phosphate, lithium manganese iron phosphate, nickel cobalt manganese ternary material, nickel cobalt aluminum ternary material, lithium-rich manganese, etc.

The main preparation method of the positive plate comprises the steps of selecting one or more positive active materials, a conductive agent and a binder polyvinylidene fluoride (PVDF) according to the mass ratio of 95.5:2:2.5, dispersing the materials into a proper amount of N-methylpyrrolidone, and fully and uniformly stirring the materials according to a homogenization process step. Uniformly coating the uniformly dispersed positive electrode slurry on an aluminum foil, and baking, rolling, slitting and punching to obtain the positive electrode plate.

The invention does not strictly limit the types of the anode active materials, and all the anode active materials commonly used at present can meet the requirements of the invention, and one or more anode materials are selected. The negative electrode active material comprises natural graphite, artificial graphite, hard carbon, soft carbon, silicon carbon, silica, lithium titanate, etc

The specific manufacturing process of the negative plate comprises the following steps: one or more negative electrode active materials, a conductive agent, styrene Butadiene Rubber (SBR) and sodium carboxymethylcellulose (CMC) are selected, all raw materials are put into a star stirring tank according to the mass ratio of 96:1:2:1, and uniformly dispersed negative electrode slurry is prepared according to a pulping process. And then uniformly coating the negative electrode slurry on a copper foil, and baking, rolling, slitting and punching to obtain the negative electrode plate.

The invention is not restricted to the selection of membranes, and commonly used membranes meet the requirements of the invention, for example: one of polypropylene membrane (PP), polyethylene membrane (PE), polyethylene/polypropylene double-layer composite membrane, polyimide electrostatic spinning membrane (PI), polypropylene/polyethylene/polypropylene three-layer composite membrane (PP/PE/PP), ceramic membrane, PVDF glue coating membrane, etc.

After punching the positive and negative electrodes, placing the positive electrode in a baking oven at 110-140 ℃ and placing the negative electrode in a baking oven at 90-100 ℃ for baking for 20-30 hours. When the moisture of the pole piece meets the requirement, the positive pole piece, the negative pole piece and the diaphragm are placed in a lamination machine to be laminated into a bare cell, and then the bare cell is packaged in a punched plastic-aluminum film bag. The encapsulated dry battery cell is dried for 8-15 hours at 80-95 ℃, the electrolyte is injected into the dry battery cell, and the battery cell is subjected to standing, formation, high Wen Gezhi, air extraction sealing and capacity division to obtain the lithium ion battery.

Example 1

The infiltration improving solvent used in this embodiment is fluorobenzene, and the infiltration improving film forming additive T1, wherein the addition of fluorobenzene accounts for 3% of the mass fraction of the electrolyte, and the addition of T1 accounts for 1% of the mass fraction of the electrolyte.

Example 2

The infiltration improving solvent used in this embodiment is fluorobenzene, the infiltration improving film forming additive T2, wherein the addition of fluorobenzene accounts for 3% of the mass fraction of the electrolyte, and the addition of T2 accounts for 1% of the mass fraction of the electrolyte.

Example 3

The infiltration improving solvent used in this embodiment is fluorobenzene, and the infiltration improving film forming additive T3, wherein the addition of fluorobenzene accounts for 3% of the mass fraction of the electrolyte, and the addition of T3 accounts for 1% of the mass fraction of the electrolyte.

Example 4

The infiltration improving solvent used in this embodiment is fluorobenzene, the infiltration improving film forming additive T4, wherein the addition of fluorobenzene accounts for 3% of the mass fraction of the electrolyte, and the addition of T4 accounts for 1.5% of the mass fraction of the electrolyte.

Example 5

The infiltration improving solvent used in this embodiment is fluorobenzene, the infiltration improving film forming additive T5, wherein the addition of fluorobenzene accounts for 3% of the mass fraction of the electrolyte, and the addition of T5 accounts for 1.5% of the mass fraction of the electrolyte.

Example 6

The infiltration improving solvent used in this example is fluorobenzene, the infiltration improving film forming additive T6, wherein the addition of fluorobenzene accounts for 3% of the mass fraction of the electrolyte, and the addition of T6 accounts for 1.5% of the mass fraction of the electrolyte.

Example 7

The infiltration improving solvent used in the embodiment is fluorobenzene, and infiltration improving film forming additives T1 and T5, wherein the addition of fluorobenzene accounts for 3% of the mass fraction of the electrolyte, the addition of T1 accounts for 1% of the mass fraction of the electrolyte, and the addition of T5 accounts for 1.5% of the mass fraction of the electrolyte.

Example 8

The infiltration improving solvent used in the embodiment is fluorobenzene, and infiltration improving film forming additives T2 and T5, wherein the addition of fluorobenzene accounts for 3% of the mass fraction of the electrolyte, the addition of T2 accounts for 1% of the mass fraction of the electrolyte, and the addition of T5 accounts for 1.5% of the mass fraction of the electrolyte.

Example 9

The infiltration improving solvent used in the embodiment is fluorobenzene, and infiltration improving film forming additives T3 and T5, wherein the addition of fluorobenzene accounts for 3% of the mass fraction of the electrolyte, the addition of T3 accounts for 1% of the mass fraction of the electrolyte, and the addition of T5 accounts for 1.5% of the mass fraction of the electrolyte.

Example 10

The infiltration improving solvent used in the embodiment is fluorobenzene, and infiltration improving film forming additives T1 and T6, wherein the addition of fluorobenzene accounts for 3% of the mass fraction of the electrolyte, the addition of T1 accounts for 1% of the mass fraction of the electrolyte, and the addition of T6 accounts for 1.5% of the mass fraction of the electrolyte.

Example 11

The infiltration improving solvent used in the embodiment is fluorobenzene, and infiltration improving film forming additives T2 and T6, wherein the addition of fluorobenzene accounts for 3% of the mass fraction of the electrolyte, the addition of T2 accounts for 1% of the mass fraction of the electrolyte, and the addition of T6 accounts for 1.5% of the mass fraction of the electrolyte.

Example 12

The infiltration improving solvent used in the embodiment is fluorobenzene, and infiltration improving film forming additives T3 and T6, wherein the addition of fluorobenzene accounts for 3% of the mass fraction of the electrolyte, the addition of T3 accounts for 1% of the mass fraction of the electrolyte, and the addition of T6 accounts for 1.5% of the mass fraction of the electrolyte.

Comparative example 1

In this comparative example, no fluorobenzene or any infiltration improving film forming additive was added.

Comparative example 2

In the comparative example, only the infiltration improvement solvent fluorobenzene is used, and no infiltration improvement film-forming additive is added, wherein the addition amount of fluorobenzene accounts for 3% of the mass fraction of the electrolyte.

The electrolytes and lithium ion batteries of examples and comparative examples were tested and compared for differences in performance using the following methods:

(1) Electrolyte wettability test

The invention determines the wetting performance of the electrolyte of the examples and the comparative examples on the pole piece through two tests, namely the test of the surface tension of the electrolyte and the test of the climbing time of the electrolyte on the diaphragm. For the testing of the surface tension of the electrolyte, the invention uses a surface tension meter. For the test of the climbing time of the test electrolyte on the diaphragm, the method comprises the following steps: 30ml of the electrolyte was taken, poured into a 50ml small beaker, one end of the membrane was immersed in the electrolyte, the immersed membrane was about 1cm in height, and the other end of the membrane was suspended on a holder with a thin wire. When the diaphragm is immersed in the electrolyte, timing is started, and the time for the electrolyte to spread to a designated position on the diaphragm is measured and is defined as the climbing time. The surface tension and ramp time of the electrolytes of examples and comparative examples in the present invention are shown in table 1. The smaller the surface tension is, the better the electrolyte wettability is; the shorter the climbing time, the better the electrolyte wettability.

(2) 45 ℃ high-temperature cycle test for lithium ion battery

The lithium ion battery cell is subjected to 45 ℃ high-temperature cycle test according to the following steps: and (3) taking the lithium ion battery with the fixed volume, and placing the lithium ion battery into an incubator with the temperature of 45 ℃. To sufficiently lower the temperature of the cell, the rest time of the cell is controlled to ensure 1 hour or more. Then, the charge was carried out at a constant current and constant voltage of 1C, the off-voltage was 3.65, and the off-current was 0.05C. Then the constant current is discharged to 2V at 1C. And (3) carrying out charge and discharge cycles on the battery cells according to the steps, and recording the discharge capacity retention rate after 1000 times and 2000 times of cycles. The discharge capacity retention rate of the battery cell after 1000 and 2000 cycles is calculated by dividing the discharge capacity of the battery cell after 1000 and 2000 cycles by the discharge capacity of the battery cell after the first cycle. The specific test data are shown in Table 2.

(3) Lithium ion battery rate performance test

The steps of the multiplying power performance test of the battery cell in the embodiment and the comparative example are as follows: discharging the constant-current battery cell with the fixed volume to 2V at the constant current of 1C, standing for 5 minutes, and then charging to 3.65V at the constant current and constant voltage of 0.5C, wherein the cut-off current is 0.05C. Standing for 30 min, and discharging to 2V at 0.2C, 0.5C, 1C, 2C, and 3C respectively. The discharge capacities under the conditions of 0.2C, 0.5C, 1C, 2C, and 3C were recorded. And respectively calculating the ratio of the discharge capacity of different multiplying powers to the discharge capacity of 1C by taking the discharge capacity of 1C as a reference capacity.

The relevant test results are shown in Table 3.

Table 1 surface tension and ramp time of electrolytes of examples and comparative examples

From the surface tension of table 1 and the ramp-up time of the electrolyte, it can be seen that: compared with a comparative example in which the novel infiltration improvement film-forming additive is not added into the electrolyte, the surface tension and the climbing time of the electrolyte are obviously reduced after the novel additive is added. When two infiltration improving film-forming additives are used together with fluorobenzene, the infiltration effect improvement on the electrolyte is more obvious, and particularly, the T3 and T6 are combined, and the minimum surface tension and the climbing time are achieved. The data are compared, so that the wettability of the additive provided by the invention to electrolyte is obviously improved.

Table 2 lithium ion battery 45 ℃ high temperature cycling test

Table 2 shows the discharge capacity retention rates of the different cells after 1000 and 2000 cycles at 45 ℃ under high temperature. When the novel infiltration improvement film forming additive is not added into the electrolyte, the discharge capacity retention rate of the lithium ion battery after 1000 times and 2000 times of circulation is relatively low. When fluorobenzene serving as an infiltration improving solvent and one of infiltration improving film-forming additives are added into the electrolyte, the discharge capacity retention rate of the lithium ion battery after 1000 times of circulation can be slightly improved, and the discharge capacity retention rate after 2000 times of circulation can be obviously improved, because the electrolyte is not obvious in infiltration insufficiency of the pole piece in the initial stage of circulation, but the electrolyte of the battery after 2000 times of circulation is obviously reduced, and the problem of poor infiltration is more obvious. When fluorobenzene is used in combination with two infiltration improvement film-forming additives, the improvement of the discharge capacity retention rate of 1000 cycles and 2000 cycles is obvious. Wherein the retention rate of 45 ℃ cycle discharge capacity of fluorobenzene is highest 1000 times and 2000 times when the fluorobenzene is combined with T3 and T6.

Table 3 lithium ion battery rate capability test

Table 3 shows the capacity retention of the cells filled with different electrolytes at different rates. Compared with a lithium ion battery without fluorobenzene, the battery cell has slightly better multiplying power performance after fluorobenzene is added into the electrolyte, but the difference is not obvious. After the novel infiltration improvement film forming additive is added into the lithium ion battery containing fluorobenzene, the multiplying power performance of the lithium ion battery is obviously improved, and particularly, after two infiltration improvement additives are added, the multiplying power performance is further optimized. Wherein the best collocation is to add T3 and T6 additives into the electrolyte.

While the invention has been described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that various modifications and additions may be made without departing from the scope of the invention. Equivalent embodiments of the present invention will be apparent to those skilled in the art having the benefit of the teachings disclosed herein, when considered in the light of the foregoing disclosure, and without departing from the spirit and scope of the invention; meanwhile, any equivalent changes, modifications and evolution of the above embodiments according to the essential technology of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. The lithium ion battery electrolyte is characterized by comprising, by weight, 10% -17% of lithium salt, 1.8% -3.7% of film forming additive, 3% -20% of fluorobenzene, 0.5% -10% of infiltration improvement film forming additive composition and the balance of solvent; wherein the infiltration improvement film-forming additive composition comprises a compound represented by formula 1 and/or formula 2:

2. The lithium ion battery electrolyte according to claim 1, wherein the compound of formula 1 and formula 2 is selected from at least one of the following T1 to T6:

3. the lithium ion battery electrolyte of claim 2, wherein the wettability improving film-forming additive composition is a combination of a compound represented by T1-T3 and a compound represented by T4-T6.

4. A lithium ion battery electrolyte according to claim 3 wherein the wettability enhancing film forming additive composition is a combination of compounds represented by T3 and T6.

5. The lithium ion battery electrolyte according to claim 3, wherein the addition amount of the compound shown by T3 is 0.5-1% of the total mass of the electrolyte, and the addition amount of the compound shown by T6 is 1% -1.5% of the total mass of the electrolyte.

6. The lithium ion battery electrolyte according to claim 1, wherein the film forming additive is one or more of vinylene carbonate, fluoroethylene carbonate, propane sultone, ethylene sulfate, methane sulfonyl dimethyl ester, tri (trimethyl) silicon-based phosphate, lithium difluorooxalato borate, lithium tetrafluorooxalato phosphate or lithium difluoroato phosphate.

7. The lithium ion battery electrolyte of claim 6, wherein the film forming additive is a combination of vinylene carbonate, vinyl sulfate and tri (trimethyl) silicon based phosphate.

8. The lithium ion battery electrolyte of claim 1, wherein the solvent comprises at least two of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methylethyl carbonate, dipropyl carbonate, ethyl acetate, methyl acetate, ethyl propionate, propyl acetate, or propyl propionate in combination.

9. The lithium ion battery electrolyte of claim 1, wherein the lithium salt comprises one or more of lithium hexafluorophosphate, lithium bis (fluorosulfonyl) imide, or lithium bis (trifluoromethylsulfonyl) imide.

10. A lithium ion battery, comprising the lithium ion battery electrolyte of any one of claims 1-9, a positive electrode sheet containing a positive electrode active material, a negative electrode sheet containing a negative electrode active material, and a separator, wherein the positive electrode active material comprises one or more combinations of lithium cobaltate, lithium manganate, lithium nickelate, lithium iron phosphate, nickel cobalt manganese ternary material, nickel cobalt aluminum ternary material or lithium-rich manganese material, and the negative electrode active material comprises one or more combinations of artificial graphite, natural graphite, soft carbon, hard carbon, silicon carbon or silicon oxygen. .