WO2023236509A1

WO2023236509A1 - Electrolyte solution and preparation method therefor, and lithium-ion battery

Info

Publication number: WO2023236509A1
Application number: PCT/CN2022/141936
Authority: WO
Inventors: 刘世琦; 刘建文; 何思聪; 王石泉
Original assignee: 湖北万润新能源科技股份有限公司
Priority date: 2022-06-07
Filing date: 2022-12-26
Publication date: 2023-12-14
Also published as: CN114784381A

Abstract

An electrolyte solution and a preparation method therefor, and a lithium-ion battery. The electrolyte solution comprises a lithium salt, an organic solvent and a film-forming additive, wherein the lithium salt comprises a first lithium salt and a second lithium salt; the first lithium salt comprises lithium bis(trifluoromethanesulfonyl)imide and/or lithium bis(fluorosulfonyl)imide; and the second lithium salt comprises at least one of lithium difluoro(oxalate)borate, lithium tetrafluoroborate, lithium perchlorate and lithium bis(oxalate)borate.

Description

Electrolyte and preparation method thereof, lithium-ion battery

This application claims the priority of the Chinese invention patent application submitted on June 7, 2022, with the application number 202210637294.3 and the invention title "An electrolyte and its preparation method, lithium ion battery". The disclosure content of this application is cited incorporated into this article.

Technical field

The present invention relates to the technical field of lithium electronic batteries, and specifically to an electrolyte solution and a preparation method thereof, and a lithium ion battery.

Background technique

Lithium-ion batteries have the advantages of high voltage, high energy density, multiple cycles and green environmental protection. At present, lithium-ion batteries have been widely used in electronic equipment such as mobile phones, laptops, and smart robots, as well as in aerospace, ships, military facilities and other fields. In addition, lithium-ion batteries are also widely used as power batteries in electric vehicles. As the commercial scale of power batteries continues to expand, there is a need to develop batteries with higher efficiency, higher energy density and better safety performance.

Carbonate solvent is a commonly used electrolyte solvent for lithium-ion batteries. It has the advantages of high conductivity, low cost, and good compatibility with electrodes. However, in high-voltage battery systems, carbonate solvents are extremely easy to decompose. The by-products of the oxidative decomposition of the solvent will be deposited on the electrode surface, greatly increasing the battery impedance and reducing battery performance.

In addition, power battery performance is greatly affected by temperature, and electric vehicles are sometimes used in extreme weather. At ultra-low temperatures, the conductivity of the electrolyte drops sharply, which will lead to rapid fading of battery capacity, which hinders the large-scale use of power batteries.

Therefore, it is of great significance to provide a power lithium-ion battery electrolyte that has both high voltage performance and low temperature performance.

In view of this, the present invention is proposed.

Contents of the invention

The first object of the present invention is to provide an electrolyte that can form an excellent SEI film (solid electrolyte interface) by using a specific type of first lithium salt, inhibit excessive decomposition of the electrolyte solvent, and is beneficial to Intercalation and extraction of lithium ions; by introducing the second lithium salt and using it in conjunction with the first lithium salt, it can participate in SEI film formation while passivating the aluminum foil. Therefore, the electrolyte has high conductivity at low temperatures, and the battery produced has good cycle performance at high voltages. Moreover, by introducing film-forming additives, the present invention can further improve the cycle performance of the lithium-ion battery at high voltage and the conductivity of the electrolyte at low temperatures.

The second object of the present invention is to provide a method for preparing an electrolyte, which has the advantages of simple operation, mild conditions, short process flow, and is suitable for mass production.

The third object of the present invention is to provide a lithium-ion battery that has both high electrical performance and low-temperature performance.

In order to achieve the above objects of the present invention, the following technical solutions are adopted:

The invention provides an electrolyte, including lithium salt, organic solvent and film-forming additives;

Wherein, the lithium salt includes a first lithium salt and a second lithium salt;

The first lithium salt includes lithium bistrifluoromethanesulfonyl imide (LiTFSI) and/or lithium bisfluorosulfonyl imide (LiFSI).

The second lithium salt includes lithium difluoroborate (LiODFB), lithium tetrafluoroborate (LiBF ₄ ), lithium perchlorate (LiClO ₄ ) and lithium bisoxaloborate (LiBOB, also known as lithium dioxalate borate, At least one of lithium dioxalate borate).

The electrolyte provided by the present invention can improve the cycle performance of lithium-ion batteries at high voltage and the conductivity of the electrolyte at low temperatures by using specific types of lithium salts and introducing film-forming additives.

Specifically, the first lithium salt provided by the present invention has a high solubility and can be formulated into a high-concentration electrolyte. Compared with traditional electrolytes, high-concentration electrolytes can form an excellent SEI film, mainly due to the decomposition of lithium salts, which can inhibit excessive decomposition of the electrolyte solvent. Moreover, the SEI film formed by high-concentration electrolyte is mainly lithium-rich compounds and has high conductivity, which is conducive to the insertion and extraction of lithium ions.

At the same time, by introducing the second lithium salt and used in conjunction with the first lithium salt, the present invention can effectively passivate the aluminum foil and prevent the first lithium salt from corroding the aluminum foil under high voltage; and the second lithium salt partially participates in SEI film formation, which can Further improve battery performance.

In addition, the present invention adds film-forming additives to the electrolyte, which can form an excellent interface protective film on the electrode surface to avoid side reactions between the solvent and the electrode material surface, thereby improving the high-voltage cycle performance of the battery.

In some specific embodiments of the present invention, the second lithium salt includes at least two of lithium difluoroborate, lithium tetrafluoroborate and lithium bisoxaloborate.

Preferably, the molar concentration of the first lithium salt in the electrolyte is 0.8-4mol/L, including but not limited to 0.9mol/L, 1mol/L, 1.2mol/L, 1.4mol/L, 1.5mol /L, 1.8mol/L, 2mol/L, 2.2mol/L, 2.4mol/L, 2.6mol/L, 2.8mol/L, 3mol/L, 3.2mol/L, 3.4mol/L, 3.6mol/L , 3.8 mol/L, or any range value between the two.

Using a molar concentration within this range is beneficial to further improving the cycle performance of lithium-ion batteries at high voltages and the conductivity of the electrolyte at low temperatures.

More preferably, the molar concentration of the first lithium salt in the electrolyte is 1˜3 mol/L.

Preferably, the molar concentration of the second lithium salt in the electrolyte is 0.05-0.7mol/L, including but not limited to 0.07mol/L, 0.09mol/L, 0.1mol/L, 0.2mol/L, The point value of any one of 0.3mol/L, 0.4mol/L, 0.5mol/L, and 0.6mol/L or the range value between any two.

More preferably, the molar concentration of the second lithium salt in the electrolyte is 0.1˜0.5 mol/L.

Preferably, the film-forming additives include lithium difluorophosphate (LiPO ₂ F ₂ ), vinylene carbonate (VC, also known as 1,3-dioxol-2-one, ethylene carbonate), sulfuric acid At least one of vinyl ester (DTD), dimethyl sulfate (DMS), butylene sulfite (BS) and fluoroethylene carbonate (FEC).

Film-forming additives can form an excellent interface protective film on the electrode surface to avoid side reactions between the solvent and the electrode material surface.

The use of the above types of film-forming additives will help further improve the high-voltage cycle performance of lithium-ion batteries.

In some specific embodiments of the present invention, the film-forming additives include lithium difluorophosphate (LiPO ₂ F ₂ ), vinylene carbonate (VC, also known as 1,3-dioxol-2-one , ethylene carbonate), vinyl sulfate (DTD), dimethyl sulfate (DMS), butylene sulfite (BS) and fluoroethylene carbonate (FEC).

Preferably, the mass fraction of the film-forming additive in the electrolyte is 0.1% to 6%; including but not limited to 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9 A point value of any one of %, 1%, 1.5%, 2%, 3%, 3.5%, 4%, 5%, 5.5% or a range value between any two.

Using film-forming additives in the above dosage range will help further improve the high-voltage cycle performance of lithium-ion batteries.

More preferably, the mass fraction of the film-forming additive in the electrolyte is 0.5% to 5%.

Preferably, the organic solvent includes ether solvents and/or nitrile solvents.

Ether solvents have good compatibility with lithium anodes and carbon anodes, and are not prone to side reactions with the anode.

Nitrile solvents have high electrical conductivity and can significantly improve the shuttle ability of lithium ions in the electrolyte solvent. At the same time, nitrile solvents have high oxidation resistance. The cyano functional group of nitrile solvents can form a stable complex with the metal ions of the positive electrode, thereby improving the high-pressure stability of the positive electrode.

More preferably, the organic solvent includes a mixture of the ether solvent and the nitrile solvent, and the mass ratio of the ether solvent to the nitrile solvent is 1 to 3:1.

In some specific embodiments of the present invention, the mass ratio of the ether solvent and the nitrile solvent may be 1:1, or 2:1, or 3:1.

Using the above mass ratio of ether solvents and nitrile solvents is beneficial to further improving the conductivity of the electrolyte.

Preferably, the ether solvent includes at least one of ethylene glycol dimethyl ether, tetrahydrofuran and dioxolane (also known as 1,3-dioxolane).

Among them, ethylene glycol dimethyl ether solvent has good compatibility with lithium negative electrode and carbon negative electrode, and is not prone to side reactions with the negative electrode.

Preferably, the nitrile solvent includes at least one, more preferably at least two, of acetonitrile, butyronitrile, succinonitrile, glutaronitrile and adiponitrile.

The present invention also provides a method for preparing the electrolyte as described above, which includes the following steps:

Mix the lithium salt, organic solvent and film-forming additive evenly.

The preparation method has the advantages of simple operation, convenience, mild conditions, short process flow, and is suitable for mass production.

In some specific embodiments of the invention, the mixing is performed in an argon-filled glove box.

In some specific embodiments of the present invention, the organic solvent undergoes dehydration and purification. So that the prepared electrolyte does not contain water.

Preferably, the preparation method includes the following steps:

After the ether solvent and the nitrile solvent are mixed evenly, the first lithium salt, the second lithium salt and the film-forming additive are added thereto in sequence, and the electrolyte solution is obtained after mixing.

In some specific embodiments of the present invention, the preparation method of the electrolyte specifically includes the following steps: in a glove box filled with argon gas, the ether solvent and the nitrile solvent after dehydration and purification are mixed in proportion (formulation ratio) and mix evenly to obtain a mixed solvent; then, add the first lithium salt therein, and after the first lithium salt is completely dissolved, add the second lithium salt; after the first lithium salt and the second lithium salt are both completely dissolved, Then add a film-forming additive thereto, and after mixing evenly, obtain the electrolyte solution.

The present invention also provides a lithium ion battery, including the electrolyte solution as described above, or the electrolyte solution prepared by the electrolyte solution preparation method as described above.

The lithium-ion battery has good cycle performance at high voltage and higher energy density; and the electrolyte in the lithium-ion battery has good conductivity at low temperatures and has a high capacity retention rate.

Compared with the prior art, the beneficial effects of the present invention are:

(1) The electrolyte provided by the present invention can be formulated into a high-concentration electrolyte by introducing a first lithium salt with high solubility. The high-concentration electrolyte can form an excellent SEI film and can inhibit excessive decomposition of the electrolyte solvent. Moreover, the SEI film formed by high-concentration electrolyte is mainly lithium-rich compounds and has high conductivity, which is conducive to the insertion and extraction of lithium ions.

(2) The electrolyte provided by the present invention can effectively passivate the aluminum foil by introducing a specific type of second lithium salt to be used in conjunction with the first lithium salt, and prevent the first lithium salt from corroding the aluminum foil at high voltage; and, the second Lithium salt participates in SEI film formation, which can further improve battery performance.

(3) The electrolyte provided by the present invention can form an excellent interface protective film on the electrode surface by introducing specific types of film-forming additives to avoid side reactions between the solvent and the electrode material surface, thereby improving the high voltage cycle performance of the battery. .

(4) The specific type of ether solvent in the electrolyte provided by the present invention has good compatibility with the lithium negative electrode and the carbon negative electrode, and is not prone to side reactions with the negative electrode.

(5) The specific type of nitrile solvent in the electrolyte provided by the present invention has high conductivity and can significantly improve the shuttle ability of lithium ions in the electrolyte solvent. In addition, nitrile solvents have high oxidation resistance. The cyano functional group of nitrile solvents can form a stable complex with the metal ions of the positive electrode, thus improving the high-voltage stability of lithium-ion batteries.

(6) The lithium-ion battery provided by the present invention has the advantages of high energy density and high capacity retention rate.

Description of drawings

In order to more clearly explain the specific embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings that need to be used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings in the following description The drawings illustrate some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without exerting any creative effort.

Figure 1 is a comparison chart of battery cycle performance provided by the present invention.

Detailed ways

The technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings and specific implementation modes. However, those skilled in the art will understand that the following described embodiments are some of the embodiments of the present invention, rather than all of them. They are only used to illustrate the invention and should not be construed as limiting the scope of the invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention. If the specific conditions are not specified in the examples, the conditions should be carried out according to the conventional conditions or the conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not indicated, they are all conventional products that can be purchased commercially.

Example 1

The electrolyte provided in this embodiment is composed of lithium salt, organic solvent and film-forming additives.

Wherein, the lithium salt consists of a first lithium salt and a second lithium salt. The first lithium salt is lithium bistrifluoromethanesulfonimide (LiTFSI), and the second lithium salt is lithium difluoroxalateborate (LiODFB). The organic solvent consists of an ether solvent and a nitrile solvent with a mass ratio of 3:1. The ether solvent is ethylene glycol dimethyl ether and the nitrile solvent is acetonitrile. The film-forming additive is lithium difluorophosphate (LiPO ₂ F ₂ ).

The molar concentration of the first lithium salt in the electrolyte solution is 1 mol/L. The molar concentration of the second lithium salt in the electrolyte solution is 0.1 mol/L. The mass fraction of the film-forming additive in the electrolyte is 0.5%.

The preparation method of the electrolyte provided in this embodiment includes the following steps: in a glove box filled with argon, mix the ether solvent and the nitrile solvent after dehydration and purification evenly according to the above ratio to obtain a mixed solvent; then, Add the first lithium salt to it, wait until the first lithium salt is completely dissolved, then add the second lithium salt; wait until both the first lithium salt and the second lithium salt are completely dissolved, then add a film-forming additive to it, and mix evenly , get the electrolyte.

Example 2

Wherein, the lithium salt consists of a first lithium salt and a second lithium salt. The first lithium salt is lithium bistrifluoromethanesulfonimide (LiTFSI), and the second lithium salt is lithium difluoroxalateborate (LiODFB). The organic solvent consists of an ether solvent and a nitrile solvent with a mass ratio of 3:1. The ether solvent is ethylene glycol dimethyl ether and the nitrile solvent is butyronitrile. The film-forming additive is lithium difluorophosphate (LiPO ₂ F ₂ ).

The preparation method of the electrolyte provided in this embodiment is the same as that in Embodiment 1.

Example 3

Wherein, the lithium salt consists of a first lithium salt and a second lithium salt. The first lithium salt is lithium bistrifluoromethanesulfonimide (LiTFSI), and the second lithium salt is lithium difluoroxalateborate (LiODFB). The organic solvent consists of an ether solvent and a nitrile solvent with a mass ratio of 3:1. The ether solvent is ethylene glycol dimethyl ether and the nitrile solvent is adiponitrile. The film-forming additive is lithium difluorophosphate (LiPO ₂ F ₂ ).

Example 4

Wherein, the lithium salt consists of a first lithium salt and a second lithium salt. The first lithium salt is lithium bistrifluoromethanesulfonimide (LiTFSI), and the second lithium salt is lithium difluoroxalateborate (LiODFB). The organic solvent consists of an ether solvent and a nitrile solvent with a mass ratio of 1:1. The ether solvent is ethylene glycol dimethyl ether and the nitrile solvent is butyronitrile. The film-forming additive is lithium difluorophosphate (LiPO ₂ F ₂ ).

Example 5

Wherein, the lithium salt consists of a first lithium salt and a second lithium salt. The first lithium salt is lithium bistrifluoromethanesulfonimide (LiTFSI), and the second lithium salt is lithium tetrafluoroborate (LiBF ₄ ). The organic solvent consists of an ether solvent and a nitrile solvent with a mass ratio of 3:1. The ether solvent is ethylene glycol dimethyl ether and the nitrile solvent is butyronitrile. The film-forming additive is lithium difluorophosphate (LiPO ₂ F ₂ ).

Example 6

The composition of the electrolyte provided in this embodiment is basically the same as that in Example 5. The only difference is that lithium tetrafluoroborate is replaced by lithium bisoxaloborate (LiBOB) (but its dosage remains unchanged, that is, the second lithium in the electrolyte The molar concentration of the salt is still 0.1mol/L).

Example 7

Wherein, the lithium salt consists of a first lithium salt and a second lithium salt. The first lithium salt is lithium bistrifluoromethanesulfonyl imide (LiTFSI), and the second lithium salt is lithium difluoroxalate borate (LiODFB). The organic solvent consists of an ether solvent and a nitrile solvent with a mass ratio of 3:1. The ether solvent is ethylene glycol dimethyl ether and the nitrile solvent is butyronitrile. The film-forming additive is vinylene carbonate (VC).

The molar concentration of the first lithium salt in the electrolyte solution is 1 mol/L. The molar concentration of the second lithium salt in the electrolyte solution is 0.1 mol/L. The mass fraction of the film-forming additive in the electrolyte is 5%.

Example 8

The composition of the electrolyte provided in this embodiment is basically the same as that in Example 7. The only difference is that vinylene carbonate is replaced by fluoroethylene carbonate (FEC) (but its dosage is kept unchanged, that is, the film formation in the electrolyte is The mass fraction of additives is still 5%).

Example 9

Wherein, the lithium salt consists of a first lithium salt and a second lithium salt. The first lithium salt is lithium bistrifluoromethanesulfonimide (LiTFSI), and the second lithium salt is lithium difluoroxalateborate (LiODFB). The organic solvent consists of an ether solvent and a nitrile solvent with a mass ratio of 3:1. The ether solvent is ethylene glycol dimethyl ether and the nitrile solvent is butyronitrile. The film-forming additive is fluoroethylene carbonate (FEC).

The molar concentration of the first lithium salt in the electrolyte solution is 2 mol/L. The molar concentration of the second lithium salt in the electrolyte solution is 0.5 mol/L. The mass fraction of the film-forming additive in the electrolyte is 5%.

Example 10

The composition of the electrolyte provided in this embodiment is basically the same as that in Embodiment 9. The only difference is that the molar concentration of the first lithium salt in the electrolyte is replaced with 3 mol/L.

Example 11

Wherein, the lithium salt is composed of a first lithium salt and a second lithium salt. The first lithium salt is lithium bisfluorosulfonimide (LiFSI), and the second lithium salt is lithium difluoroxalateborate (LiODFB). The organic solvent consists of an ether solvent and a nitrile solvent with a mass ratio of 3:1. The ether solvent is ethylene glycol dimethyl ether and the nitrile solvent is butyronitrile. The film-forming additive is fluoroethylene carbonate (FEC).

The molar concentration of the first lithium salt in the electrolyte solution is 2.5 mol/L. The molar concentration of the second lithium salt in the electrolyte solution is 0.3 mol/L. The mass fraction of the film-forming additive in the electrolyte is 3%.

Example 12

Wherein, the lithium salt consists of a first lithium salt and a second lithium salt. The first lithium salt is lithium bistrifluoromethanesulfonimide (LiTFSI), and the second lithium salt is lithium difluoroxalateborate (LiODFB). The organic solvent consists of an ether solvent and a nitrile solvent with a mass ratio of 3:1. The ether solvent is ethylene glycol dimethyl ether and the nitrile solvent is succinonitrile. The film-forming additive is fluoroethylene carbonate (FEC).

The molar concentration of the first lithium salt in the electrolyte solution is 1.5 mol/L. The molar concentration of the second lithium salt in the electrolyte solution is 0.2 mol/L. The mass fraction of the film-forming additive in the electrolyte is 4%.

Comparative example 1

The electrolyte provided in this comparative example is composed of lithium salt and organic solvent.

Among them, the lithium salt is lithium hexafluorophosphate (LiPF ₆ ). The organic solvent consists of ethylene carbonate, dimethyl carbonate and diethyl carbonate with a mass ratio of 1:1:1. The molar concentration of lithium salt (lithium hexafluorophosphate) in this electrolyte solution is 1 mol/L.

The preparation method of the electrolyte provided in this comparative example includes the following steps: in a glove box filled with argon, mix the ethylene carbonate, dimethyl carbonate and diethyl carbonate that have been dehydrated and purified according to the above ratio. A mixed solvent is obtained; then, lithium hexafluorophosphate is added to it, and after mixing evenly, an electrolyte solution is obtained.

Comparative example 2

The electrolyte provided in this comparative example consists of lithium salt, organic solvent and film-forming additives.

Wherein, the lithium salt is the first lithium salt, and the first lithium salt is lithium bistrifluoromethanesulfonimide (LiTFSI). The organic solvent consists of an ether solvent and a nitrile solvent with a mass ratio of 3:1. The ether solvent is ethylene glycol dimethyl ether and the nitrile solvent is butyronitrile. The film-forming additive is fluoroethylene carbonate (FEC).

The molar concentration of the first lithium salt in the electrolyte solution is 2.5 mol/L. The mass fraction of the film-forming additive in the electrolyte is 5%.

The preparation method of the electrolyte provided in this comparative example is basically the same as that of Example 9, except that the second lithium salt is not added.

Comparative example 3

Wherein, the lithium salt is the second lithium salt, and the second lithium salt is lithium difluoroxalate borate (LiODFB). The organic solvent consists of an ether solvent and a nitrile solvent with a mass ratio of 3:1. The ether solvent is ethylene glycol dimethyl ether and the nitrile solvent is butyronitrile. The film-forming additive is fluoroethylene carbonate (FEC).

The molar concentration of the second lithium salt in the electrolyte solution is 2.5 mol/L. The mass fraction of the film-forming additive in the electrolyte is 5%.

The preparation method of the electrolyte provided in this comparative example is basically the same as that of Example 9, except that the first lithium salt is not added.

Comparative example 4

The composition of the electrolyte provided in this comparative example is basically the same as that in Example 9. The only difference is that the molar concentration of the first lithium salt in the electrolyte is replaced with 0.5 mol/L, and the molar concentration of the second lithium salt in the electrolyte is changed. The molar concentration is 2mol/L.

The preparation method of the electrolyte provided in this comparative example is the same as that in Example 9.

Comparative example 5

The composition of the electrolyte provided in this comparative example is basically the same as that in Example 9. The only difference is that fluoroethylene carbonate (FEC) is replaced by ethylene sulfite (ES) (but its dosage is kept unchanged, that is, the electrolyte The mass fraction of film-forming additives in is still 5%).

Comparative example 6

The composition of the electrolyte provided in this comparative example is basically the same as that in Example 9, and the only difference is that the mass fraction of the film-forming additive in the electrolyte is replaced with 8%.

Comparative example 7

The composition of the electrolyte provided in this comparative example is basically the same as that in Example 9. The only difference is that the mass ratio of the ether solvent and the nitrile solvent is replaced by 1:3.

Experimental example 1

Lithium-ion batteries were prepared using the electrolytes prepared in the above embodiments and comparative examples respectively, and electrochemical performance tests were performed on the lithium-ion batteries prepared in each group (3 to 4.4 at -20°C, 0.2C rate Test the electrochemical performance of the battery under V conditions, cycle 150 cycles), and the test results are shown in Table 1 below.

Among them, the battery preparation method includes: placing the cut positive electrode piece in the center of the positive electrode shell of the button battery, then placing a separator (2320 separator from Celgard Company), and then injecting an appropriate amount of electrolyte (the above embodiments and each pair proportion of the electrolyte), so that the separator is completely wetted. Then place the lithium sheet (as the negative electrode) above the separator, place gaskets and shrapnel above the lithium sheet, and finally cover the negative electrode shell. Use a battery sealing machine to compact the positive and negative electrode shells. After leaving it for 24 hours, you finally get the test result. of lithium-ion batteries.

Wherein, the preparation method of the positive electrode sheet includes: mixing the positive active material LiNi _0.5 Mn _0.3 Co _0.2 O ₂ , the conductive agent acetylene black and the binder polyvinylidene fluoride with a mass ratio of 8:1:1. , add the dispersant N-methylpyrrolidone and stir to obtain a uniform positive electrode slurry. The positive electrode slurry is coated on the aluminum foil current collector with a loading of 2 mg/cm ² , and then dried and sliced to obtain a positive electrode piece.

Table 1 Electrochemical performance test results of each group of lithium-ion batteries

组别Group	-20℃循环150圈后的容量保持率Capacity retention rate after 150 cycles at -20℃
实施例1Example 1	80.21％80.21%
实施例2Example 2	81.85％81.85%
实施例3Example 3	81.06％81.06%
实施例4Example 4	80.73％80.73%
实施例5Example 5	81.25％81.25%
实施例6Example 6	81.12％81.12%
实施例7Example 7	81.92％81.92%
实施例8Example 8	81.90％81.90%
实施例9Example 9	87.24％87.24%
实施例10Example 10	88.81％88.81%
实施例11Example 11	76.68％76.68%
实施例12Example 12	78.15％78.15%
对比例1Comparative example 1	44.33％44.33%
对比例2Comparative example 2	56.76％56.76%
对比例3Comparative example 3	53.02％53.02%
对比例4Comparative example 4	60.28％60.28%
对比例5Comparative example 5	48.17％48.17%
对比例6Comparative example 6	65.38％65.38%

Comparative example 7

58.43%

At the same time, the battery cycle performance comparison chart of Example 1, Example 9, Example 10 and Comparative Example 1 is shown in Figure 1.

Combining the data in Table 1 and Figure 1, it can be seen that mixing ethylene glycol dimethyl ether and nitrile solvents can improve the oxidation resistance and conductivity of the electrolyte. The first lithium salt (LiTFSI) has high conductivity and solubility, and film-forming additives (LiPO ₂ F ₂ , VC, FEC) can form a good interface film on the electrode surface. The combination of specific types of solvents, lithium salts and additives can significantly improve the discharge capacity retention rate of lithium-ion batteries at low temperatures.

It can be seen that in the electrolyte for lithium-ion batteries with high-pressure and low-temperature performance provided by the present invention, ethylene glycol dimethyl ether has good solubility and negative electrode deformability, and the nitrile solvent has high conductivity and It has good compatibility with the high-voltage cathode. When the two solvents are mixed in a certain ratio, the high-voltage cycle performance of the battery and the discharge capacity retention rate at low temperatures are improved.

Moreover, the present invention uses LiTFSI as the main lithium salt (first lithium salt) of the electrolyte, which has the characteristics of high solubility and high conductivity. High-concentration LiTFSI can be decomposed in the solvent to form an excellent SEI film. At the same time, in addition to passivating the aluminum current collector, the second lithium salt also partially participates in SEI film formation. Film-forming additives (LiPO ₂ F ₂ , VC, FEC) can form a good SEI film on the electrode surface and suppress side reactions between the electrolyte and the electrode.

Although the present invention has been illustrated and described with specific embodiments, it should be realized that the above embodiments are only used to illustrate the technical solutions of the present invention, but not to limit it; those of ordinary skill in the art should understand that: Without departing from the spirit and scope of the present invention, the technical solutions described in the foregoing embodiments may be modified, or some or all of the technical features thereof may be equivalently substituted; however, these modifications or substitutions shall not make the corresponding technical solutions essentially depart from the scope of the technical solutions of the various embodiments of the present invention; therefore, it is meant that all such substitutions and modifications falling within the scope of the present invention are included in the appended claims.

Claims

An electrolyte, characterized in that it includes a lithium salt, an organic solvent and a film-forming additive;

Wherein, the lithium salt includes a first lithium salt and a second lithium salt;

The first lithium salt includes lithium bistrifluoromethanesulfonimide and/or lithium bisfluorosulfonimide;

The second lithium salt includes at least one of lithium difluoroborate, lithium tetrafluoroborate, lithium perchlorate and lithium bisoxaloborate.
The electrolyte solution according to claim 1, wherein the molar concentration of the first lithium salt in the electrolyte solution is 0.8-4 mol/L;

Preferably, the molar concentration of the first lithium salt in the electrolyte is 1˜3 mol/L.
The electrolyte solution according to claim 1, wherein the molar concentration of the second lithium salt in the electrolyte solution is 0.05-0.7 mol/L;

Preferably, the molar concentration of the second lithium salt in the electrolyte is 0.1-0.5 mol/L.
The electrolyte according to claim 1, wherein the film-forming additives include lithium difluorophosphate, vinylene carbonate, vinyl sulfate, dimethyl sulfate, butylene sulfite and fluoroethylene carbonate. at least one of them.
The electrolyte according to claim 1, wherein the mass fraction of the film-forming additive in the electrolyte is 0.1% to 6%;

Preferably, the mass fraction of the film-forming additive in the electrolyte is 0.5% to 5%.
The electrolyte according to any one of claims 1 to 5, wherein the organic solvent includes ether solvents and/or nitrile solvents;

Preferably, the organic solvent includes a mixture of the ether solvent and the nitrile solvent, and the mass ratio of the ether solvent to the nitrile solvent is 1 to 3:1.
The electrolyte solution according to claim 6, wherein the ether solvent includes at least one of glycol dimethyl ether, tetrahydrofuran and dioxolane;

Preferably, the nitrile solvent includes at least one of acetonitrile, butyronitrile, succinonitrile, glutaronitrile and adiponitrile.
The preparation method of electrolyte solution according to any one of claims 1 to 7, characterized in that it includes the following steps:

Mix the lithium salt, organic solvent and film-forming additive evenly.
The preparation method of electrolyte according to claim 8, characterized in that the preparation method includes the following steps:

After the ether solvent and the nitrile solvent are mixed evenly, the first lithium salt, the second lithium salt and the film-forming additive are added thereto in sequence, and the electrolyte solution is obtained after mixing.
A lithium ion battery, including the electrolyte according to any one of claims 1 to 7, or the electrolyte prepared by the method for preparing the electrolyte according to claim 8 or 9.