Acetonitrile-based electrolyte, secondary battery and preparation method of secondary battery

Abstract

The invention relates to the technical field of secondary batteries, in particular to an acetonitrile-based electrolyte, a secondary battery and a preparation method thereof.The acetonitrile-based electrolyte comprises a first electrolyte and a second electrolyte, the first electrolyte is used for liquid injection before the secondary battery is subjected to pre-charging formation treatment, and the second electrolyte is used for liquid injection before the secondary battery is subjected to pre-charging formation treatment; the second electrolyte is used for liquid injection after the secondary battery is subjected to pre-charging formation treatment, and the volume fraction of acetonitrile in the first electrolyte is smaller than the volume fraction of acetonitrile in the second electrolyte. The volume fraction of acetonitrile in the first electrolyte is relatively small, so that the degree of side reaction of acetonitrile on the negative pole piece in the pre-charging formation stage is reduced, the gas production rate in the pre-charging formation stage can be reduced, and the structural stability of the battery cell in the pre-charging formation stage is improved; the volume fraction of acetonitrile in the second electrolyte is large, and the total content of acetonitrile in the electrolyte in the battery cell can be increased, so that the conductivity and the active ion migration rate of the electrolyte in the battery cell can be improved, and the rate capability of the battery cell can be improved.

Description

Technical Field

[0001] This invention relates to the field of secondary battery technology, specifically to an acetonitrile-based electrolyte, a secondary battery, and a method for preparing the same. Background Technology

[0002] With the increasing severity of global ecological and environmental problems, the secondary battery and even the electric vehicle industry are receiving more and more attention. A typical secondary battery cell includes a positive electrode, a negative electrode, a separator, and an electrolyte. Acetonitrile-based electrolytes are ion-conducting media with acetonitrile as the core solvent. Acetonitrile has a dielectric constant of approximately 37.5 at 25°C, far exceeding that of traditional solvents like carbonates, enabling efficient dissolution of electrolyte salts and promoting the dissociation of active ions. Acetonitrile's viscosity at 25°C is approximately 0.35 mPa·s. This low viscosity provides less resistance to the migration of active ions, resulting in a higher migration rate. Simultaneously, its low viscosity increases the electrolyte's wetting ability of the active materials in the electrode, facilitating the extraction and insertion of active ions, improving the rate performance of the secondary battery, and enabling fast charging. Acetonitrile has a melting point of -45.7°C and a boiling point of 81.6°C, exhibiting low melting point, high boiling point, and a wide liquid range, giving acetonitrile-based electrolytes good conductivity at low temperatures, making them suitable for energy storage devices in cold regions.

[0003] However, acetonitrile-based electrolytes present serious interfacial stability problems in battery cells, thus affecting the cycle life of the cells. Summary of the Invention

[0004] In view of this, the present invention provides an acetonitrile-based electrolyte, a secondary battery and a method for preparing the same, in order to improve the rate performance and cycle life of the battery cell.

[0005] In a first aspect, the present invention provides an acetonitrile-based electrolyte, comprising a first electrolyte and a second electrolyte, wherein the first electrolyte is used for injection before the secondary battery undergoes pre-charge formation treatment, and the second electrolyte is used for injection after the secondary battery undergoes pre-charge formation treatment, wherein the volume fraction of acetonitrile in the first electrolyte is less than the volume fraction of acetonitrile in the second electrolyte.

[0006] The first electrolyte has a relatively low volume fraction of acetonitrile. Injecting it into the cell before pre-charge formation helps reduce the degree of side reactions of acetonitrile on the negative electrode during the pre-charge formation stage. This not only reduces the amount of gaseous byproducts generated during pre-charge formation, thus reducing the degree of cell casing bulging and deformation, but also lowers the risk of cell casing rupture, improving the structural stability of the cell during pre-charge formation. Furthermore, it improves the density and stability of the SEI film, enhancing its formation quality and reducing interfacial impedance, thereby improving the cell's rate performance and cycle life. The second electrolyte has a relatively high volume fraction of acetonitrile. Injecting it into the cell after pre-charge formation increases the total acetonitrile content in the internal electrolyte, improving the conductivity and active ion migration rate of the internal electrolyte. This further enhances the cell's rate performance, enabling fast charging of the secondary battery and making it suitable for cold regions. In addition, after the pre-charge formation process is completed, an venting operation is usually performed. After venting, the injection of a second electrolyte can replenish the components that are consumed, volatilized or carried out by the gas due to the pre-charge formation, which is beneficial to improve the wetting degree of the active material, thereby reducing the interfacial resistance in the cell and thus improving the rate performance of the cell.

[0007] In some alternative embodiments, the volume fraction A of acetonitrile in the first electrolyte and the volume fraction B of acetonitrile in the second electrolyte satisfy: 1 ​​≤ B / A ≤ 5.

[0008] In some optional embodiments, the volume fraction A of acetonitrile in the first electrolyte satisfies: A≤50%, preferably 10%-50%.

[0009] In some optional embodiments, the volume fraction B of acetonitrile in the second electrolyte satisfies: B≤100%, preferably 20%-90%.

[0010] In some alternative embodiments, the second electrolyte further includes an additive used to reduce gas production during the charging process.

[0011] In some alternative embodiments, the first electrolyte further includes the additive, wherein the mass fraction of the additive in the first electrolyte is less than the mass fraction of the additive in the second electrolyte.

[0012] In some optional embodiments, the mass fraction C of the additive in the first electrolyte and the mass fraction D of the additive in the second electrolyte satisfy: 2≤D / C≤8.

[0013] In some optional embodiments, the mass fraction C of the additive in the first electrolyte satisfies: C ≤ 1 wt%, preferably 0.2 wt% - 1 wt%.

[0014] In some optional embodiments, the mass fraction D of the additive in the second electrolyte satisfies: D≤6wt%, preferably 0.4wt%-4wt%.

[0015] In some alternative embodiments, the additive includes 4,4-bis-1,3,2-dioxazothiophene-2,2-dioxide, 1,3-propanesulfonate lactone, methylene disulfonate, and vinyl sulfate.

[0016] In some optional embodiments, the first electrolyte further includes one or more of carboxylic acid ester solvents, carbonate solvents, and ether solvents.

[0017] In some optional embodiments, the second electrolyte further includes one or more of carboxylic acid ester solvents, carbonate solvents, and ether solvents.

[0018] In some alternative embodiments, the first electrolyte further contains an electrolyte salt, the concentration of which is 0.8 mol / L to 1.5 mol / L.

[0019] In some alternative embodiments, the second electrolyte further contains an electrolyte salt, the concentration of which is 0.8 mol / L to 1.5 mol / L.

[0020] In a second aspect, the present invention provides a method for preparing a secondary battery, comprising: injecting the first electrolyte described in the first aspect into a battery cell, performing a pre-charge formation treatment on the battery cell to obtain a pre-formed battery cell; and injecting the second electrolyte described in the first aspect into the pre-formed battery cell.

[0021] In some alternative implementations, the volume fraction F of the first electrolyte satisfies the following condition, based on the total volume of electrolyte injected into the cell: 80% ≤ F ≤ 90%.

[0022] In some alternative implementations, the volume fraction G of the second electrolyte satisfies the following condition, based on the total volume of the electrolyte injected into the cell: 10% ≤ G ≤ 20%.

[0023] In some alternative embodiments, the volume fraction S1 of acetonitrile, based on the total volume of electrolyte injected into the cell, satisfies: S1≤59%, preferably 8.2%-50%.

[0024] In some optional embodiments, the first electrolyte and the second electrolyte further include an additive used to reduce the amount of gas generated during the charging process. Based on the total mass of the electrolyte injected into the cell, the mass fraction S2 of the additive satisfies: S2≤1.9wt%, preferably 0.2wt%-1.6wt%.

[0025] In some alternative embodiments, the battery cell contains a positive electrode sheet containing a positive electrode active material, which includes lithium iron phosphate, lithium cobalt oxide, lithium nickel oxide, lithium nickel oxide, or lithium nickel cobalt aluminum oxide, preferably lithium iron phosphate.

[0026] Thirdly, the present invention provides a secondary battery prepared using the preparation method described in the second aspect. Detailed Implementation

[0027] The present invention will be further described in detail below with reference to the embodiments. It should be understood that the embodiments described herein are merely illustrative of the invention and not intended to limit it. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in the art or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0028] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit this application.

[0029] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0030] As described in the background section, acetonitrile-based electrolytes present serious interfacial stability problems in battery cells. Acetonitrile molecules have relatively low lowest unoccupied molecular orbitals (LUMOs), exhibiting strong reducing properties on the negative electrode surface during the pre-charge formation stage of secondary batteries. This causes acetonitrile molecules to readily gain electrons and decompose, generating a large amount of gaseous byproducts such as methane and ethane. These byproducts not only cause the cell casing to bulge and deform but also affect the structure of the surface interfacial film (SEI film) formed on the surface of the negative electrode active material, thus reducing the density and stability of the SEI film and significantly increasing the interfacial impedance of the cell. Especially at high temperatures, the kinetics of these side reactions accelerate significantly, and the large amount of gaseous byproducts can easily lead to cell casing rupture, reducing the structural stability of the cell and thus drastically shortening its cycle life.

[0031] Based on this, in a first aspect, the present invention provides an acetonitrile-based electrolyte, comprising a first electrolyte and a second electrolyte, wherein the first electrolyte is used for injection before the secondary battery undergoes pre-charge formation treatment, and the second electrolyte is used for injection after the secondary battery undergoes pre-charge formation treatment, wherein the volume fraction of acetonitrile in the first electrolyte is less than the volume fraction of acetonitrile in the second electrolyte.

[0032] The acetonitrile volume fraction in the first electrolyte is relatively small. Injecting it into the cell before pre-charge formation helps reduce the degree of side reactions occurring at the negative electrode during the pre-charge formation stage. This not only reduces the amount of gaseous byproducts generated during pre-charge formation, thus reducing the degree of cell casing bulging and deformation, but also lowers the risk of cell casing rupture during this stage, improving the structural stability of the cell during pre-charge formation. Furthermore, it improves the density and stability of the SEI film, enhancing its formation quality and reducing interfacial impedance within the cell, thereby improving the cell's rate performance and cycle life.

[0033] The second electrolyte has a larger volume fraction of acetonitrile. After pre-charging and formation treatment, it is injected into the cell, which can increase the total acetonitrile content in the electrolyte inside the cell. This is beneficial to improving the conductivity and active ion migration rate of the electrolyte inside the cell, and thus improving the rate performance of the cell. This allows the secondary battery composed of the cell to achieve fast charging and is suitable for cold regions.

[0034] In addition, after the pre-charge formation process is completed, an venting operation is usually performed. After venting, the injection of a second electrolyte can replenish the components that are consumed, volatilized or carried out by the gas due to the pre-charge formation, which is beneficial to improve the wetting degree of the active material, thereby reducing the interfacial resistance in the cell and thus improving the rate performance of the cell.

[0035] In some optional embodiments, the volume fraction A of acetonitrile in the first electrolyte and the volume fraction B of acetonitrile in the second electrolyte can satisfy: 1 ​​≤ B / A ≤ 5, where B / A can be 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, or any range of the above values. If the value of B / A is too high, it indicates that the volume fraction of acetonitrile in the first electrolyte is too low, resulting in poor wetting ability of the first electrolyte on the negative electrode active material. This leads to the inability to form an effective SEI film in the localized liquid-deficient areas of the negative electrode active material layer, which is detrimental to improving the rate performance of the battery cell. A low B / A value may indicate that the volume fraction of acetonitrile in the second electrolyte is too low, making it difficult to effectively increase the total acetonitrile content in the electrolyte inside the cell, thus failing to effectively improve the conductivity and active ion migration rate of the electrolyte inside the cell. Alternatively, it may indicate that the volume fraction of acetonitrile in the first electrolyte is too high, making it difficult to effectively reduce the amount of gaseous byproducts generated during the pre-charge formation stage and failing to effectively improve the film quality of the SEI film, which is detrimental to improving the structural stability, rate performance, and cycle life of the cell. By limiting the B / A value to the above range, not only can the gas generation during the pre-charge formation stage be effectively reduced, but the density and stability of the SEI film can also be effectively improved, thereby effectively improving the film quality of the SEI film. At the same time, it allows the first electrolyte to have better wetting ability for the negative electrode active material, thereby effectively improving the coverage of the SEI film in the negative electrode active material layer. In addition, it also allows for a higher acetonitrile content in the electrolyte inside the cell, thereby effectively improving the conductivity and active ion migration rate of the electrolyte inside the cell. In summary, by limiting the B / A value to the above range, the structural stability, rate performance, and cycle life of the battery cell during the pre-charge formation stage can be effectively improved.

[0036] In some optional embodiments, the volume fraction A of acetonitrile in the first electrolyte can satisfy: A≤50%, preferably 10%-50%, such as 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, etc., or any range of the above values. If the volume fraction of acetonitrile in the first electrolyte is too low, it will limit the wettability of the first electrolyte on the active material in the negative electrode, resulting in the inability to form an effective SEI film in the localized liquid-deficient areas of the negative electrode active material layer, which is detrimental to improving the rate performance of the battery cell. If the volume fraction of acetonitrile in the first electrolyte is too high, it will lead to severe gas generation during the pre-charge formation stage and reduce the film quality of the SEI film, which is detrimental to improving the structural stability, rate performance, and cycle life of the battery cell. By limiting the volume fraction of acetonitrile in the first electrolyte within the aforementioned range, not only can the gas production during the pre-charge formation stage be effectively reduced, but the density and stability of the SEI film can also be effectively improved, thereby effectively improving the film formation quality of the SEI film. At the same time, the first electrolyte has a better wetting ability on the negative electrode active material, thereby effectively improving the coverage of the SEI film in the negative electrode active material layer. In summary, by limiting the volume fraction of acetonitrile in the first electrolyte within the aforementioned range, the structural stability, rate performance, and cycle life of the battery cell during the pre-charge formation stage can be effectively improved.

[0037] In some optional embodiments, the volume fraction B of acetonitrile in the second electrolyte can satisfy: B ≤ 100%, preferably 20%-90%, such as 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, etc., or any range of the above values. By limiting the volume fraction of acetonitrile in the second electrolyte to the above range, a higher content of acetonitrile in the electrolyte inside the battery cell can be achieved, thereby effectively improving the conductivity and active ion migration rate of the electrolyte inside the battery cell, and thus effectively improving the rate performance of the battery cell.

[0038] The first electrolyte further includes one or more of carboxylic acid ester solvents, carbonate solvents, and ether solvents; the second electrolyte further includes one or more of carboxylic acid ester solvents, carbonate solvents, and ether solvents.

[0039] Among them, carboxylic acid ester solvents include cyclic carboxylic acid ester solvents and linear carboxylic acid ester solvents. Cyclic carboxylic acid ester solvents include, but are not limited to, γ-butyrolactone (GBL), and linear carboxylic acid ester solvents include, but are not limited to, methyl formate (MF), methyl acetate (MA), methyl butyrate (MB), ethyl acetate (EA), and ethyl propionate (EP).

[0040] Carbonate solvents include cyclic carbonate solvents and linear carbonate solvents. Cyclic carbonate solvents include, but are not limited to, propylene carbonate (PC) and ethylene carbonate (EC). Linear carbonate solvents include, but are not limited to, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and methyl propyl carbonate (MPC).

[0041] Ether solvents include cyclic ether solvents and chain ether solvents. Cyclic ether solvents include, but are not limited to, tetrahydrofuran (THF), 2-methyltetrahydrofuran (2Me-THF), and 1,3-dioxocyclopentane (DOL). Chain ether solvents include, but are not limited to, dimethoxymethane (DMM), 1,2-dimethoxyethylene (DME), and diethylene glycol dimethyl ether (DG).

[0042] The materials of the solvents other than acetonitrile (ACN) in the first electrolyte and the second electrolyte can be exactly the same, partially the same, or completely different. For example, the solvent of the first electrolyte is a mixed solvent of acetonitrile (ACN), ethyl acetate (EA), ethyl methyl carbonate (EMC), and ethylene carbonate (EC), and the solvent of the second electrolyte is a mixed solvent of acetonitrile (ACN), ethyl acetate (EA), and ethylene carbonate (EC).

[0043] The acetonitrile-based electrolyte further includes an electrolyte salt, which can be a lithium salt or a sodium salt, depending on the type of secondary battery. Lithium salts include, but are not limited to, lithium hexafluorophosphate (LiPF6), lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), lithium bis(oxalate-borate)borate (LiBOB), lithium difluorooxalate-borate (LiDFOB), lithium bis(difluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), with lithium hexafluorophosphate (LiPF6) being preferred. Sodium salts include, but are not limited to, sodium perchlorate, sodium hexafluorophosphate, sodium bis(fluorosulfonyl)imide, sodium tetrafluoroborate, and sodium trifluoromethanesulfonylimide. The electrolyte salt in the first electrolyte and the electrolyte salt in the second electrolyte can be made of the same or different materials.

[0044] Specifically, the concentration of the electrolyte salt in the first electrolyte solution is 0.8 mol / L-1.5 mol / L, such as 0.8 mol / L, 0.9 mol / L, 1.0 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, 1.4 mol / L, 1.5 mol / L, or any range of the above values. The concentration of the electrolyte salt in the second electrolyte solution is 0.8 mol / L-1.5 mol / L, such as 0.8 mol / L, 0.9 mol / L, 1.0 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, 1.4 mol / L, 1.5 mol / L, or any range of the above values.

[0045] The electrolyte salts in the first electrolyte and the second electrolyte can be made of the same or different materials; preferably, the electrolyte salts in the first electrolyte and the second electrolyte are made of the same materials. The total concentration of the electrolyte salts in the first electrolyte and the second electrolyte can be the same or different; preferably, the total concentration of the electrolyte salts in the first electrolyte and the second electrolyte are the same.

[0046] The electrolyte salt in the first electrolyte may include a first electrolyte salt and a second electrolyte salt. The first electrolyte salt includes, but is not limited to, LiPF6, and its concentration may be 0.5 mol / L-1.5 mol / L, such as 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1.0 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, 1.4 mol / L, 1.5 mol / L, or any range of the above values. The second electrolyte salt includes, but is not limited to, LiFSI, and its concentration may be 0-1 mol / L, such as 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1 mol / L, or any range of the above values. That is, when the concentration of the first electrolyte salt in the first electrolyte is high, the second electrolyte salt may not be added, and the first electrolyte contains a single electrolyte salt.

[0047] The electrolyte salt in the second electrolyte may include a third electrolyte salt and a fourth electrolyte salt. The third electrolyte salt includes, but is not limited to, LiPF6, and its concentration may be 0.5 mol / L-1.5 mol / L, such as 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1.0 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, 1.4 mol / L, 1.5 mol / L, or any range of the above values. The fourth electrolyte salt includes, but is not limited to, LiFSI, and its concentration may be 0-1 mol / L, such as 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1 mol / L, or any range of the above values. That is, when the concentration of the third electrolyte salt in the second electrolyte is high, the fourth electrolyte salt may not be added, and the second electrolyte contains a single electrolyte salt.

[0048] In some optional embodiments, the second electrolyte may further include additives to reduce gas generation during the charging process. The charging operation during cell cycling can also cause acetonitrile to decompose on the surface of the negative electrode, generating gaseous byproducts that can lead to bulging, deformation, or even rupture of the cell casing, affecting the cell's cycle life. Adding additives to the second electrolyte can reduce the amount of gaseous byproducts generated during cell cycling, thus helping to extend the cell's cycle life.

[0049] In some optional embodiments, the first electrolyte may further include the additive, wherein the mass fraction of the additive in the first electrolyte is less than the mass fraction of the additive in the second electrolyte. The additive preferentially decomposes on the surface of the negative electrode active material during the pre-charge formation stage to promote the formation of the SEI film, facilitating the acquisition of a dense and stable SEI film. The SEI film can prevent the electrolyte from reacting with the negative electrode active material, thereby further reducing the amount of gaseous byproducts generated during the pre-charge formation stage, further reducing the risk of cell casing rupture during the pre-charge formation stage, and further improving the structural stability of the cell during the pre-charge formation stage. The film-forming (SEI film) potential of the additive is >2V.

[0050] It should be noted that if the additive is added only to the first electrolyte and not to the second electrolyte, the additive in the first electrolyte may be largely consumed during the pre-charge formation stage, and the remaining additive cannot effectively reduce gas generation during cell cycling. An excessively high mass fraction of the additive in the first electrolyte can negatively impact the SEI film formation quality, potentially leading to higher interfacial impedance and hindering the improvement of the cell's rate performance. To improve the SEI film formation quality and effectively reduce gas generation during cell cycling, the additive is divided into two portions and added to the first and second electrolytes respectively, with the mass fraction of the additive in the first electrolyte being less than that in the second electrolyte.

[0051] In some optional embodiments, the mass fraction C of the additive in the first electrolyte and the mass fraction D of the additive in the second electrolyte can satisfy: 2 ≤ D / C ≤ 8, where D / C can be 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, or any range of the above values. A high D / C value may indicate that the mass fraction of the additive in the second electrolyte is too large. While this can reduce the amount of gaseous byproducts generated during cell cycling and extend the cell's cycle life, it can also lead to excess additives during cell cycling, causing waste and hindering the control of cell manufacturing costs. Alternatively, it may indicate that the mass fraction of the additive in the first electrolyte is too small, thus failing to effectively reduce the gas production during the pre-charge formation stage. A low D / C value may indicate that the mass fraction of the additive in the first electrolyte is too high, which could negatively impact the SEI film formation quality, leading to higher interfacial impedance and hindering the improvement of the cell's rate performance. Alternatively, it may indicate that the mass fraction of the additive in the second electrolyte is too low, failing to effectively reduce the amount of gaseous byproducts generated during cell cycling and thus failing to effectively extend the cell's cycle life. By limiting the D / C value to the aforementioned range, not only can the gas generation during the pre-charge formation stage and cycling be reduced, thus extending the cell's cycle life, but also excess additives during cell cycling can be avoided, thereby reducing the cell's manufacturing cost.

[0052] In some optional embodiments, the mass fraction C of the additive in the first electrolyte can satisfy: C ≤ 1 wt%, preferably 0.2 wt% - 1 wt%, such as 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1 wt%, etc., or a range of any of the above values. By limiting the mass fraction of the additive in the first electrolyte to the above range, not only can the degree of side reaction of acetonitrile on the negative electrode sheet during the pre-charge formation stage be reduced, thereby effectively reducing the gas production during the pre-charge formation stage and thus effectively improving the structural stability of the cell during the pre-charge formation stage, but it can also ensure that the SEI film has good film formation quality, which is beneficial to improving the rate performance of the cell.

[0053] In some optional embodiments, the mass fraction D of the additive in the second electrolyte can satisfy: D ≤ 6 wt%, preferably 0.4 wt%-4 wt%, such as 0.4 wt%, 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, 3 wt%, 3.5 wt%, 4 wt%, etc., or a range of any of the above values. By limiting the mass fraction of the additive in the second electrolyte to the above range, not only can the amount of gaseous byproducts generated during cell cycling be reduced, which is beneficial to extending the cycle life of the cell, but also the excess of additives during cell cycling can be avoided, which is beneficial to reducing the manufacturing cost of the cell.

[0054] In some alternative embodiments, the additives include, but are not limited to, 4,4-bis-1,3,2-dioxazothiophene-2,2-dioxide (BDTD), 1,3-propanesulfonate lactone (PS), methylene disulfonate (MMDS), and vinyl sulfate (DTD). The materials of the additives in the first electrolyte and the second electrolyte may be the same or different.

[0055] The first electrolyte and the second electrolyte may also contain other types of additives such as conductive additives and flame retardant additives.

[0056] In this application, "too high" means exceeding the upper limit of the corresponding data range, and "too low" means being less than the lower limit of the corresponding data range.

[0057] Secondly, the present invention provides a method for preparing a secondary battery, comprising:

[0058] The first electrolyte described in the first aspect is injected into the battery cell, and the battery cell is pre-charged and formed to obtain a pre-formed battery cell;

[0059] The second electrolyte described in the first aspect is injected into the preformed battery cell.

[0060] In some optional embodiments, based on the total volume of electrolyte injected into the cell, the volume fraction F of the first electrolyte satisfies: 80% ≤ F ≤ 90%, such as 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, etc., or any range of the above values; or, the volume fraction G of the second electrolyte satisfies: 10% ≤ G ≤ 20%, such as 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, etc., or any range of the above values. F reflects the amount of the first electrolyte injected, and G reflects the amount of the second electrolyte injected. By limiting F and G to the above ranges, it is not only beneficial to form a good interface film during the pre-charge formation stage, but also beneficial to improve the wetting performance and fast charging capability of the electrolyte in the cell. Preferably, the volume fraction F of the first electrolyte is 82%, and the volume fraction G of the second electrolyte is 18%.

[0061] In some optional embodiments, based on the total volume of the electrolyte injected into the cell, the volume fraction S1 of acetonitrile can satisfy: S1≤59%, preferably 8.2%-50%, such as 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, etc., or any range of the above values. By limiting the volume fraction of acetonitrile within the above range, not only can the conductivity and active ion migration rate of the electrolyte inside the cell be improved, thereby effectively improving the rate performance of the cell and enabling the cell to meet 4C-6C fast charging requirements, but it also helps to control the degree of side reactions of acetonitrile on the surface of the negative electrode during cell cycling, thereby controlling the amount of gas generated during cell cycling and improving the structural stability of the cell.

[0062] In some optional embodiments, the first electrolyte and the second electrolyte further include additives. These additives reduce gas generation during the charging process. After injection, based on the total mass of the electrolyte injected into the cell, the mass fraction S2 of the additive can satisfy: S2 ≤ 1.9 wt%, preferably 0.2 wt% - 1.6 wt%, such as 0.2 wt%, 0.4 wt%, 0.6 wt%, 0.8 wt%, 1.0 wt%, 1.2 wt%, 1.4 wt%, 1.6 wt%, etc., or any range of the above values. By limiting the mass fraction of the additive in the electrolyte to the above range, not only can the gas generation during cell cycling be effectively reduced, thereby effectively extending the cell's cycle life, but the cell's interfacial impedance can also be controlled, avoiding a large interfacial impedance due to excessive additive content, which is beneficial for the cell to obtain higher charging capacity.

[0063] In some optional embodiments, the battery cell contains a positive electrode sheet containing a positive electrode active material, including lithium iron phosphate, lithium cobalt oxide, lithium nickel oxide, lithium nickel oxide, or lithium nickel cobalt aluminum oxide, preferably lithium iron phosphate. Lithium iron phosphate has a high energy density, and when combined with the aforementioned acetonitrile-based electrolyte, it can produce a secondary battery with high energy density, a wide temperature range, and high rate of return, which is beneficial for meeting consumers' demands for long driving range and fast charging of electric vehicles.

[0064] The following is an exemplary description of the complete steps of the preparation method of a secondary battery: A positive electrode, a separator, and a negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes to provide isolation. The electrodes are then wound to obtain an electrode assembly. The electrode assembly is placed in a housing. A first electrolyte is injected into the housing, followed by a pre-charge formation process. After formation, the air is vented, and then a second electrolyte is injected into the housing. The housing is then sealed to obtain the secondary battery.

[0065] The shell can be a rigid shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The shell can also be a soft package, such as a pouch. The material of the soft package can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0066] Thirdly, the present invention provides a secondary battery prepared using the secondary battery preparation method described in the second aspect.

[0067] The secondary battery in this application can be a single secondary battery cell or a battery module or battery pack assembled from secondary battery cells. The number of secondary battery cells contained in the battery module or battery pack can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0068] K1 = B / A, K2 = D / C, and Y represents the stability of the battery cell at 4C-6C. Y satisfies: Y ≤ 1.66. By adjusting the volume fraction of acetonitrile and the mass fraction of additives in the first and second electrolytes, the value of Y is controlled within the above range to ensure a longer cycle life of the battery cell at 4C-6C.

[0069] The embodiments of the technical solution of this application are described in detail below. The following embodiments are only used to illustrate the technical solution of this application more clearly, and are therefore only examples, and should not be used to limit the scope of protection of this application.

[0070] Example 1

[0071] This embodiment provides a secondary battery, the preparation method of which is as follows:

[0072] (1) Preparation of positive electrode sheet

[0073] The positive electrode active material LiFePO4, conductive carbon black (SP), and polyvinylidene fluoride (PVDF) were mixed in the solvent N-methylpyrrolidone (NMP) at a weight ratio of 97:1:2 and stirred evenly to obtain the positive electrode slurry.

[0074] Aluminum foil was used as the positive electrode current collector. Positive electrode slurry was coated onto both sides of the current collector and baked at 120℃ for 1 hour. Subsequently, it was cold-pressed, cut, and slit to prepare the positive electrode sheet. The loading of positive active material on one side of the positive electrode sheet was 21.3 mg / cm³. 2 .

[0075] (2) Preparation of negative electrode sheet

[0076] The negative electrode active material artificial graphite, conductive agent conductive carbon black (SP), thickener sodium carboxymethyl cellulose (CMC) and binder styrene-butadiene rubber (SBR) are mixed in deionized water at a weight ratio of 96:1:1.5:1.5 and stirred evenly to obtain the negative electrode slurry.

[0077] Copper foil was used as the negative electrode current collector. Negative electrode slurry was coated onto both sides of the current collector, baked at 120℃ for 1 hour, followed by cold pressing, cutting, and slitting to prepare the negative electrode sheet. The loading of negative electrode active material on one side of the current collector was 9.7 mg / cm³. 2 .

[0078] (3) Prepare electrolyte

[0079] In an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), ACN (acetonitrile):EA (ethyl acetate):EMC (ethyl methyl carbonate):EC (ethylene carbonate) were mixed uniformly in a volume ratio of 20:40:20:20 to obtain a mixed solvent. Lithium salts LiPF6 and LiFSI, as well as additive 1,3-propanesulfonate lactone (PS), were added to the mixed solvent and mixed uniformly to obtain the first electrolyte. The concentration of LiPF6 in the first electrolyte was 1 mol / L, the concentration of LiFSI was 0.2 mol / L, and the concentration of PS was 0.5 wt%.

[0080] In an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), ACN (acetonitrile):EA (ethyl acetate):EC (ethylene carbonate) were mixed uniformly at a volume ratio of 60:30:10 to obtain a mixed solvent. Lithium salts LiPF6 and LiFSI, as well as additive 1,3-propanesulfonate lactone (PS), were added to the mixed solvent and mixed uniformly to obtain a second electrolyte. The concentration of LiPF6 in the second electrolyte was 1 mol / L, the concentration of LiFSI was 0.2 mol / L, and the concentration of PS was 3 wt%.

[0081] (4) Assemble the battery

[0082] The positive electrode sheet, the polypropylene separator, and the negative electrode sheet are stacked in sequence, with the separator between the positive and negative electrode sheets, and the electrode assembly is obtained by winding. The electrode assembly is then placed in an aluminum-plastic film housing.

[0083] The first electrolyte is injected into the casing, and then pre-charged at 45°C with a constant current of 0.1C to 60% SOC;

[0084] The following operation was then performed twice to complete the formation: charging to full capacity at a constant current of 0.33C at 30°C, followed by discharging to 2V at 0.33C;

[0085] After the formation is completed, the air is vented, and then the second electrolyte is injected into the casing. After sealing, a secondary battery is obtained. Based on the total volume of the electrolyte injected into the cell, the volume fraction of the first electrolyte is 82% and the volume fraction of the second electrolyte is 18%.

[0086] Examples 2-22, Comparative Examples 1-3

[0087] The differences between Examples 2-22, Comparative Examples 1-3 and Example 1 are detailed in Table 1.

[0088] The additive content of the first electrolyte in Examples 1-13 is the same, and the additive content of the second electrolyte is the same, therefore the K2 value is the same;

[0089] Examples 1-4 have the same K1 value, the only difference being that the S1 and Y values ​​are different due to the different acetonitrile content in the electrolyte;

[0090] Examples 5-6 have the same K1 value, the only difference being that the S1 and Y values ​​are different due to the different acetonitrile content in the electrolyte;

[0091] Examples 8-9 have the same K1 value, the only difference being that the S1 and Y values ​​are different due to the different acetonitrile content in the electrolyte;

[0092] Examples 10-13 have the same K1 value, the only difference being that the S1 and Y values ​​are different due to the different acetonitrile content in the electrolyte;

[0093] The first electrolyte in Example 1 and the second electrolyte in Examples 14-22 have the same acetonitrile content, and therefore the K1 value is the same.

[0094] Examples 14-16 have the same K2 value, the only difference being that the different additive contents in the electrolyte lead to different S2 and Y values;

[0095] Examples 17-19 have the same K2 value, the only difference being that the different additive contents in the electrolyte lead to different S2 and Y values;

[0096] Example 1 and Example 20 have the same K2 value, the only difference being that the different additive content in the electrolyte leads to different S2 and Y values;

[0097] Examples 21-22 have the same K2 value, the only difference being that the different additive contents in the electrolyte lead to different S2 and Y values. The additive in Example 22 is methylene disulfonate (MMDS).

[0098] The vinyl electrolytes in Comparative Examples 1-3 do not contain a second electrolyte. The only difference between the vinyl electrolytes and the first electrolyte is the content of acetonitrile. After the formation is completed, the electrolyte is vented and then sealed to obtain a secondary battery.

[0099] Performance testing

[0100] (1) 4C cycle count: The ambient temperature is controlled at 25℃. The battery cell is charged to 3.8V in a stepped charging manner. The average charging rate during the charging process is 4C. After standing for 10 minutes, it is discharged at 1C constant current to 1C. The charging and discharging cycle is carried out according to the above process. The number of cycles when the battery cell reaches 80% SOH is recorded as the 4C cycle count.

[0101] (2) 6C cycle count: The ambient temperature is controlled at 25℃. The battery cell is charged to 3.8V in a stepped charging manner. The average charging rate during the charging process is 6C. After standing for 10 minutes, it is discharged at 1C constant current to 1C. The charging and discharging cycle is carried out according to the above process. The number of cycles when the battery cell reaches 80% SOH is recorded as the 4C cycle count.

[0102] The cells in Comparative Examples 2-3 generated a large amount of gas during the pre-charge formation stage, causing significant bulging and deformation of the casing, making them unsuitable for subsequent production and application. Therefore, cycle testing was not conducted. Cycle testing was performed on the secondary batteries of Examples 1-22 and Comparative Example 1, and the test results are shown in Table 1.

[0103] Table 1: Cyclic Test Structure for Secondary Batteries

[0104]

[0105] As can be seen from the comparison of Examples 1-22 and Comparative Examples 1-3 in Table 1, the acetonitrile-based electrolyte is divided into a first electrolyte and a second electrolyte. The first electrolyte is used for injection before the secondary battery undergoes pre-charge formation treatment, and the second electrolyte is used for injection after the secondary battery undergoes pre-charge formation treatment. The volume fraction of acetonitrile in the first electrolyte is less than that in the second electrolyte, which enables the battery cell to have excellent cycle performance at high rates (4C-6C), and has both excellent fast charging performance and cycle performance.

[0106] When the acetonitrile content in the first electrolyte is high, appropriately reducing the acetonitrile content in the second electrolyte can enable the battery cell to achieve excellent cycle performance; when the additive content in the first electrolyte is high, appropriately reducing the acetonitrile content in the second electrolyte can also enable the battery cell to achieve excellent cycle performance.

[0107] In this document, the term "implementation" means that a specific feature, structure, or characteristic described in connection with an implementation can be included in at least one implementation of this application. The appearance of this phrase in various places in the specification does not necessarily refer to the same implementation, nor is it a separate or alternative implementation mutually exclusive with other implementations. It will be explicitly and implicitly understood by those skilled in the art that the implementations described herein can be combined with other implementations.

[0108] The above description is merely a preferred embodiment of the present invention and the technical principles employed. Those skilled in the art will understand that the present invention is not limited to the specific embodiments described above, and various obvious changes, readjustments, combinations, and substitutions can be made without departing from the scope of protection of the present invention. Therefore, although the present invention has been described in detail through the above embodiments, the present invention is not limited to the above embodiments, and may include many other equivalent embodiments without departing from the concept of the present invention. The scope of protection of the present invention is determined by the scope of the appended claims.

Claims

1. An acetonitrile-based electrolyte, characterized in that, The application relates to an acetonitrile-based electrolyte, comprising: a first electrolyte for injecting before pre-charging and forming treatment of a secondary battery; a second electrolyte for injecting after pre-charging and forming treatment of the secondary battery; the volume fraction of acetonitrile in the first electrolyte is less than that in the second electrolyte.

2. The acetonitrile-based electrolyte according to claim 1, characterized in that, the volume fraction A of acetonitrile in the first electrolyte and the volume fraction B of acetonitrile in the second electrolyte satisfy one or more of the following conditions: 1<=B / A<=5; A<=50%, preferably 10%-50%; B<=100%, preferably 20%-90%.

3. The acetonitrile-based electrolyte according to claim 1 or 2, characterized in that, the second electrolyte further comprises an additive for reducing the gas production during charging.

4. The acetonitrile-based electrolyte according to claim 3, characterized in that, the first electrolyte further comprises the additive, and the mass fraction of the additive in the first electrolyte is less than that in the second electrolyte; and / or the additive comprises 4,4-bis-1,3,2-dioxazolylthiophene-2,2-dioxide, 1,3-propane sulfone lactone, methane disulfonic acid methylene ester, and ethylene sulfate.

5. The acetonitrile-based electrolyte according to claim 3, wherein the mass fraction C of the additive in the first electrolyte and the mass fraction D of the additive in the second electrolyte satisfy one or more of the following conditions: 2<=D / C<=8; C<=1wt%, preferably 0.2wt%-1wt%; D<=6wt%, preferably 0.4wt%-4wt%.

6. The acetonitrile-based electrolyte according to any one of claims 1-5, wherein: the first electrolyte further comprises one or more of a carboxylic acid ester solvent, a carbonate solvent, and an ether solvent; and / or the second electrolyte further comprises one or more of a carboxylic acid ester solvent, a carbonate solvent, and an ether solvent; and / or the first electrolyte further comprises an electrolyte salt, and the concentration of the electrolyte salt in the first electrolyte is 0.8mol / L-1.5mol / L; and / or the second electrolyte further comprises an electrolyte salt, and the concentration of the electrolyte salt in the second electrolyte is 0.8mol / L-1.5mol / L.

7. A method for producing a secondary battery, characterized by, The application further relates to a method for manufacturing a secondary battery, comprising: injecting the first electrolyte according to any one of claims 1-6 into an electrode core, and performing pre-charging and forming treatment on the electrode core to obtain a pre-formed electrode core; injecting the second electrolyte according to any one of claims 1-6 into the pre-formed electrode core.

8. The production method according to claim 7, characterized by, the volume fraction F of the first electrolyte based on the total volume of the electrolyte injected into the electrode core satisfies 80%<=F<=90%, and / or the volume fraction G of the second electrolyte satisfies 10%<=G<=20%, and / or the volume fraction S1 of acetonitrile satisfies S1<=59%, preferably 8.2%-50%; and / or the first electrolyte and the second electrolyte further comprise an additive for reducing the gas production during charging, and the mass fraction S2 of the additive based on the total mass of the electrolyte injected into the electrode core satisfies S2<=1.9wt%, preferably 0.2wt%-1.6wt%.

9. The production method according to claim 7 or 8, characterized by, The electric cell contains a positive electrode sheet, the positive electrode sheet contains a positive electrode active material, and the positive electrode active material includes lithium iron phosphate, lithium cobaltate, lithium nickelate, lithium nickelate, or lithium nickel cobalt aluminumate, preferably lithium iron phosphate.

10. A secondary battery characterized by comprising: Prepared by the preparation method of any one of claims 7 to 9.

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