Electrolyte, preparation method thereof and lithium ion battery

Abstract

The application provides an electrolyte, a preparation method thereof and a lithium ion battery. The electrolyte comprises an organic solvent, a lithium salt and an additive A. The additive A contains fluorine atoms, boron atoms, a carbonyl group, a siloxane group and a benzene ring in the structure, can form a solid electrolyte interface film with high mechanical strength and toughness on the surface of a negative electrode, inhibits volume expansion of a silicon-carbon negative electrode during charging and discharging, reduces damage of stress generated by volume expansion of the silicon-carbon negative electrode during charging and discharging to the solid electrolyte interface film, and thus improves cycle performance and storage performance of the battery.

Description

TECHNICAL FIELD

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

[0002] Lithium ion batteries are one of the main energy storage technologies in the new energy market due to their high energy density, portability, strong designability and high safety. Silicon-carbon negative electrodes have a high theoretical specific capacity and are expected to improve the mass and volume energy densities of lithium ion batteries.

[0003] However, silicon in the silicon-carbon negative electrode will undergo significant volume changes during lithium intercalation and deintercalation, which can easily lead to volume expansion of the silicon-carbon negative electrode material during charging and discharging. The solid electrolyte interface film on the surface of the silicon-carbon negative electrode material is difficult to withstand the damage caused by the expansion of the silicon negative electrode during battery cycling, affecting the cycle performance and storage performance of the lithium ion battery. SUMMARY

[0004] Embodiments of the present application provide an electrolyte, a preparation method thereof and a lithium ion battery, which can improve the technical problem that the solid electrolyte interface film on the surface of the silicon-carbon negative electrode material is difficult to withstand the damage caused by the expansion of the silicon negative electrode during battery cycling, affecting the cycle performance and storage performance of the lithium ion battery.

[0005] In a first aspect, embodiments of the present application provide an electrolyte, the electrolyte comprising an organic solvent, a lithium salt and an additive A;

[0006] The structure of the additive A comprises a fluorine atom, a boron atom, a carbonyl group, a siloxane group and a benzene ring.

[0007] In an embodiment, the structure of the additive A is shown in Formula I:

[0008]

[0009] In an embodiment, the mass percentage of the additive A in the electrolyte is 5%-10%.

[0010] In an embodiment, the electrolyte further comprises a film former, and the film former comprises at least one of vinyl sulfite, propylene sulfite, fluoroethylene carbonate and vinylene carbonate.

[0011] In an embodiment, the mass percentage of the film former in the electrolyte is 3%-5%.

[0012] In an embodiment, the lithium salt comprises at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium difluorophosphate, lithium trifluoromethylsulfonate and lithium bisfluorosulfonimide; and / or

[0013] The mass percentage of the lithium salt in the electrolyte is 12%-15%.

[0014] In an embodiment, the organic solvent comprises ethylene carbonate, dimethyl carbonate, and methyl ethyl carbonate, and the mass ratio of the ethylene carbonate, the dimethyl carbonate, and the methyl ethyl carbonate is (1-3):(1-3):(4-8); and / or

[0015] The mass percentage of the organic solvent in the electrolyte is 70%-80%.

[0016] In a second aspect, embodiments of the present application provide a preparation method of the electrolyte as described above, the method comprising the following steps:

[0017] Under a protective atmosphere, a formula amount of an organic solvent, a lithium salt, an additive A, and other raw materials are mixed uniformly to obtain the electrolyte.

[0018] In an embodiment, the other raw materials comprise a film-forming agent.

[0019] In an embodiment, the mixing is stirring mixing, the temperature of the mixing is 25-30℃, and / or the time of the mixing is 50-60min; and / or

[0020] The protective atmosphere comprises a nitrogen atmosphere and / or an argon atmosphere.

[0021] In a third aspect, embodiments of the present application provide a lithium ion battery, the lithium ion battery comprising the electrolyte as described above or the electrolyte prepared by the method as described above.

[0022] In an embodiment, the lithium ion battery further comprises a positive electrode, a negative electrode, and a separator;

[0023] The material of the positive electrode comprises lithium manganese iron phosphate and / or nickel-cobalt-manganese ternary material, and / or the material of the negative electrode comprises silicon-carbon material.

[0024] The beneficial effects of embodiments of the present application are as follows:

[0025] In the embodiment of the present application, the additive A is added to the electrolyte. On the one hand, the additive A contains fluorine atoms in its structure, which can form a solid electrolyte interface film containing inorganic substances such as lithium fluoride on the surface of the negative electrode during the charging and discharging process of the battery. Lithium fluoride makes the formed solid electrolyte interface film have higher mechanical strength and stability, which can inhibit the volume expansion of the silicon-carbon negative electrode during the charging and discharging process, and reduce the damage of the stress generated by the volume expansion of the silicon-carbon negative electrode to the solid electrolyte interface film during the charging and discharging process. In addition, the additive A contains a carbonyl group in its structure, which can undergo a polymerization reaction on the surface of the negative electrode to form a polycarbonate polymer film. The polycarbonate polymer film can reduce the direct contact between the electrolyte and the negative electrode material, reduce the decomposition of the negative electrode material, and at the same time can alleviate the damage of the volume expansion of the silicon-carbon material to the electrode structure. On the other hand, the additive A contains boron atoms in its structure, which can be oxidized and decomposed during the charging and discharging process, and then form a stable borate film on the surface of the silicon-carbon negative electrode. The borate film has good flexibility and mechanical strength, which can adapt to the volume change of the silicon-carbon negative electrode during the charging and discharging process, thereby reducing the rupture of the solid electrolyte interface film on the surface of the silicon-carbon negative electrode and reducing the occurrence of side reactions between the electrolyte and the silicon-carbon negative electrode. In addition, the additive A contains a siloxane group in its structure, which can effectively form a Si-O-Si cross-linking structure on the surface of the silicon-carbon negative electrode, which can enhance the toughness of the solid electrolyte interface film on the surface of the silicon-carbon negative electrode, effectively inhibit the expansion of the silicon-carbon negative electrode, and thus improve the cycle performance and storage performance of the battery. DETAILED DESCRIPTION

[0026] The technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application. In addition, it should be understood that the specific embodiments described herein are only used to illustrate and explain the present application, and are not used to limit the present application. In the present application, the orientation words such as "upper" and "lower" generally refer to the upper and lower of the device in the actual use or working state, and "inner" and "outer" refer to the outline of the device.

[0027] In the related art, the silicon-carbon negative electrode material is prone to volume expansion during the charging and discharging process. The solid electrolyte interface film on the surface of the silicon-carbon negative electrode material is difficult to withstand the damage caused by the expansion of the silicon negative electrode during the cycle process of the battery, which affects the cycle performance and storage performance of the lithium ion battery, and needs to be further improved.

[0028] To solve the above problems, the embodiment of the present application provides an electrolyte, the electrolyte comprises an organic solvent, a lithium salt and an additive A, wherein the structure of the additive A comprises fluorine atoms, boron atoms, a carbonyl group, a siloxane group and a benzene ring.

[0029] In the embodiment, the additive A is added in the electrolyte. On the one hand, the structure of the additive A contains fluorine atoms, which can form a solid electrolyte interface film containing inorganic substances such as lithium fluoride on the surface of the negative electrode in the charging and discharging process of the battery. The lithium fluoride makes the formed solid electrolyte interface film have high mechanical strength and stability, can inhibit the volume expansion of the silicon-carbon negative electrode in the charging and discharging process, and reduce the damage of the stress generated by the volume expansion of the silicon-carbon negative electrode in the charging and discharging process to the solid electrolyte interface film. In addition, the structure of the additive A contains a carbonyl group, which can occur polymerization reaction on the surface of the negative electrode to form a polycarbonate polymer film. The polycarbonate polymer film can reduce the direct contact of the electrolyte with the negative electrode material, reduce the decomposition of the negative electrode material, and at the same time can alleviate the damage of the volume expansion of the silicon-carbon material to the electrode structure. On the other hand, the structure of the additive A contains boron atoms, which can be oxidized and decomposed in the charging and discharging process, and then form a stable borate film on the surface of the silicon-carbon negative electrode. The borate film has good flexibility and mechanical strength, can adapt to the volume change of the silicon-carbon negative electrode in the charging and discharging process, thereby reducing the rupture of the solid electrolyte interface film on the surface of the silicon-carbon negative electrode and reducing the occurrence of side reactions between the electrolyte and the silicon-carbon negative electrode. In addition, the structure of the additive A contains a siloxane group, which can effectively form a Si-O-Si cross-linking structure on the surface of the silicon-carbon negative electrode, can enhance the toughness of the solid electrolyte interface film on the surface of the silicon-carbon negative electrode, effectively inhibit the expansion of the silicon-carbon negative electrode, thereby improving the cycle performance and storage performance of the battery. In addition, the structure of the additive A contains a benzene ring, the additive A can form a protective film on the surface of the positive electrode, which can reduce the side reactions between the electrolyte and the positive electrode in the high-voltage system. In an embodiment, the structure of the additive A is shown as formula I:

[0030]

[0031] In the embodiment, the CAS number of the additive A is 1622303-27-8, the name of the additive A is: Benzamide, N-[2-[[[(1,1-dimethylethyl)dimethylsilyl]oxy]methyl]-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]-4-[1-(trifluoromethyl)cyclopropyl], the additive A can be directly purchased commercially, the raw material is easy to obtain, the synthesis steps of the additive A can be saved, and the production process of the electrolyte is simplified.

[0032] In an embodiment, the mass percentage of the additive A in the electrolyte is 5%-10%. In this embodiment, if the content of the additive A is too low, the additive A cannot effectively improve the mechanical strength and toughness of the solid electrolyte interface film; if the content of the additive A is too high, the additive A can easily cause an increase in gas production and an increase in impedance. Alternatively, the mass percentage of the additive A in the electrolyte can be any one of 5%, 6%, 7%, 8%, 9%, 10%, or a range between any two of them.

[0033] In an embodiment, the electrolyte further comprises a film-forming agent, and the film-forming agent comprises at least one of vinyl sulfite, propylene sulfite, fluoroethylene carbonate, and vinylene carbonate. In this embodiment, fluoroethylene carbonate and vinylene carbonate can form a dense solid electrolyte interface film on the surface of the silicon-carbon negative electrode; additives such as vinyl sulfite and propylene sulfite can assist in film formation on the positive and negative electrodes, and have high thermal stability. By adding the film-forming agent and the additive A to the electrolyte, the synergistic effect between the film-forming agent and the additive A can further improve the high-temperature storage performance and cycle performance of the battery.

[0034] In an embodiment, the mass percentage of the film-forming agent in the electrolyte is 3%-5%. In this embodiment, if the content of the film-forming agent is too low, the film-forming agent cannot effectively form a dense solid electrolyte interface film on the surface of the silicon-carbon negative electrode; if the content of the film-forming agent is too high, the film-forming agent can easily cause an increase in impedance and exacerbate high-temperature gas production. Alternatively, the mass percentage of the film-forming agent in the electrolyte can be any one of 3%, 3.5%, 4%, 4.5%, 5%, or a range between any two of them.

[0035] In an embodiment, the lithium salt comprises at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium difluorophosphate, lithium trifluoromethylsulfonate, and lithium bisfluorosulfonimide.

[0036] In an embodiment, the mass percentage of the lithium salt in the electrolyte is 12%-15%. Alternatively, the mass percentage of the lithium salt in the electrolyte can be any one of 12%, 12.5%, 13%, 13.5%, 14%, 14.5%, 15%, or a range between any two of them.

[0037] In an embodiment, the organic solvent comprises ethylene carbonate, dimethyl carbonate, and methyl ethyl carbonate, and the mass ratio of the ethylene carbonate, dimethyl carbonate, and methyl ethyl carbonate is (1-3):(1-3):(4-8). Alternatively, the mass ratio of the ethylene carbonate, dimethyl carbonate, and methyl ethyl carbonate can be any one of 1:1:4, 1:1:6, 1:1:8, 1:2:4, 1:3:4, 3:1:8, 3:3:8, or a range between any two of them.

[0038] In an embodiment, the mass percentage of the organic solvent in the electrolyte is 70%-80%. Alternatively, the mass percentage of the organic solvent in the electrolyte can be any one of 70%, 72%, 74%, 76%, 78%, 80%, or a range between any two of them.

[0039] The application also provides a preparation method of the electrolyte as described above, comprising the following steps:

[0040] S1, under a protective atmosphere, uniformly mixing a formula amount of an organic solvent, a lithium salt, an additive A, and other raw materials to obtain the electrolyte.

[0041] The preparation method provided by the application is simple in operation and short in process, and can meet the needs of large-scale industrial production.

[0042] In an embodiment, the other raw materials further include a film-forming agent.

[0043] In an embodiment, the protective atmosphere includes a nitrogen atmosphere and / or an argon atmosphere.

[0044] In an embodiment, the mixing is stirring mixing, the mixing temperature is 25-30℃, and the mixing time is 50-60 min. In this embodiment, the mixing temperature is 25-30℃, which can reduce the decomposition of the lithium salt, the additive A, and the film-forming agent at high temperature. Alternatively, the mixing temperature can be any one of 25℃, 26℃, 27℃, 28℃, 29℃, 30℃, or a range between any two of them; and the mixing time can be any one of 50 min, 52 min, 54 min, 56 min, 58 min, 60 min, or a range between any two of them.

[0045] The application also provides a lithium ion battery, which includes the electrolyte as described above or the electrolyte prepared by the method as described above.

[0046] In the embodiment, the electrolyte of the lithium ion battery contains the additive A. On the one hand, the additive A contains fluorine atoms in the structure, which can form a solid electrolyte interface film containing inorganic substances such as lithium fluoride on the surface of the negative electrode in the charging and discharging process of the battery. The lithium fluoride makes the formed solid electrolyte interface film have higher mechanical strength and stability, which can inhibit the volume expansion of the silicon-carbon negative electrode in the charging and discharging process and reduce the damage of the stress generated by the volume expansion of the silicon-carbon negative electrode to the solid electrolyte interface film in the charging and discharging process. In addition, the additive A contains a carbonyl group in the structure, which can occur polymerization on the surface of the negative electrode to form a polycarbonate polymer film. The polycarbonate polymer film can reduce the direct contact between the electrolyte and the negative electrode material, reduce the decomposition of the negative electrode material, and at the same time can alleviate the damage of the volume expansion of the silicon-carbon negative electrode material to the electrode structure. On the other hand, the additive A contains boron atoms in the structure. The additive A can be oxidized and decomposed in the charging and discharging process, and then form a stable borate film on the surface of the silicon-carbon negative electrode. The borate film has good flexibility and mechanical strength, which can adapt to the volume change of the silicon-carbon negative electrode in the charging and discharging process, improve the flexibility of the solid electrolyte interface film, thereby reducing the rupture of the solid electrolyte interface film on the surface of the silicon-carbon negative electrode and reducing the occurrence of side reactions between the electrolyte and the silicon-carbon negative electrode. In addition, the additive A contains a siloxane group in the structure, which can effectively form a Si-O-Si cross-linking structure on the surface of the silicon-carbon negative electrode, which can enhance the toughness of the solid electrolyte interface film on the surface of the silicon-carbon negative electrode and effectively inhibit the expansion of the silicon-carbon negative electrode. In addition, the additive A contains a benzene ring in the structure, which can form a protective film on the surface of the positive electrode, which can reduce the side reactions between the electrolyte and the positive electrode under high voltage system. The additive A can improve the mechanical strength, toughness and stability of the solid electrolyte interface film on the surface of the silicon-carbon negative electrode, and then improve the cycle performance and storage performance of the battery.

[0047] In an embodiment, the lithium ion battery further comprises a positive electrode, a negative electrode and a separator; the material of the positive electrode comprises lithium manganese iron phosphate and / or nickel cobalt manganese ternary material; the material of the negative electrode comprises silicon-carbon material.

[0048] The above scheme is further described in combination with specific examples. The preferred embodiments of the present application are described in detail as follows:

[0049] Embodiment 1

[0050] The present embodiment provides an electrolyte, which is composed of an organic solvent, a lithium salt, a film-forming agent and an additive A;

[0051] The organic solvent is composed of ethylene carbonate, dimethyl carbonate and methyl ethyl carbonate, and the mass ratio of ethylene carbonate, dimethyl carbonate and methyl ethyl carbonate is 15:15:70. The mass fraction of the organic solvent in the electrolyte is 79.5%.

[0052] The lithium salt is lithium hexafluorophosphate, and the mass fraction of the lithium salt in the electrolyte is 12.5%;

[0053] The film-forming agent is vinylene carbonate, and the mass fraction of the film-forming agent in the electrolyte is 3%;

[0054] The mass fraction of the additive A in the electrolyte is 5%.

[0055] The embodiment also provides a preparation method of the electrolyte.

[0056] (1) under an argon atmosphere, adding a formula amount of lithium salt into an organic solvent, then adding a film-forming agent, finally adding an additive A, and stirring and mixing at 25 DEG C to obtain an electrolyte.

[0057] Embodiment 2

[0058] The embodiment provides an electrolyte, which is composed of an organic solvent, a lithium salt, a film-forming agent and an additive A.

[0059] The organic solvent is composed of vinylene carbonate, dimethyl carbonate and methyl ethyl carbonate, the mass ratio of the vinylene carbonate, the dimethyl carbonate and the methyl ethyl carbonate is 15:15:70, and the mass fraction of the organic solvent in the electrolyte is 77.5%.

[0060] The lithium salt is lithium hexafluorophosphate, and the mass fraction of the lithium salt in the electrolyte is 12.5%;

[0061] The film-forming agent is vinylene carbonate, and the mass fraction of the film-forming agent in the electrolyte is 3%;

[0062] The mass fraction of the additive A in the electrolyte is 7%.

[0063] The embodiment also provides a preparation method of the electrolyte.

[0064] (1) under an argon atmosphere, adding a formula amount of lithium salt into an organic solvent, then adding a film-forming agent, finally adding an additive A, and stirring and mixing at 25 DEG C to obtain an electrolyte.

[0065] Embodiment 3

[0066] The embodiment provides an electrolyte, which is composed of an organic solvent, a lithium salt, a film-forming agent and an additive A.

[0067] The organic solvent is composed of vinylene carbonate, dimethyl carbonate and methyl ethyl carbonate, the mass ratio of the vinylene carbonate, the dimethyl carbonate and the methyl ethyl carbonate is 15:15:70, and the mass fraction of the organic solvent in the electrolyte is 75.5%.

[0068] The lithium salt is lithium hexafluorophosphate, and the mass fraction of the lithium salt in the electrolyte is 12.5%;

[0069] The film-forming agent is vinylene carbonate, and the mass fraction of the film-forming agent in the electrolyte is 3%;

[0070] The mass fraction of the additive A in the electrolyte is 9%.

[0071] The embodiment also provides a preparation method of the electrolyte.

[0072] (1) under an argon atmosphere, adding a formula amount of lithium salt into an organic solvent, then adding a film-forming agent, finally adding an additive A, and stirring and mixing at 25 DEG C to obtain an electrolyte.

[0073] Embodiment 4

[0074] The embodiment provides an electrolyte, which is composed of an organic solvent, a lithium salt, a film-forming agent and an additive A.

[0075] The organic solvent is composed of vinylene carbonate, dimethyl carbonate and methyl ethyl carbonate, the mass ratio of the vinylene carbonate, the dimethyl carbonate and the methyl ethyl carbonate is 15:15:70, and the mass fraction of the organic solvent in the electrolyte is 76.5%;

[0076] The lithium salt is lithium hexafluorophosphate, and the mass fraction of the lithium salt in the electrolyte is 12.5%;

[0077] The film-forming agent is vinylene carbonate, and the mass fraction of the film-forming agent in the electrolyte is 4%;

[0078] The mass fraction of the additive A in the electrolyte is 7%.

[0079] The embodiment also provides a preparation method of the electrolyte.

[0080] (1) under an argon atmosphere, adding a formula amount of lithium salt into an organic solvent, then adding a film-forming agent, finally adding an additive A, and stirring and mixing at 25 DEG C to obtain an electrolyte.

[0081] Embodiment 5

[0082] The embodiment provides an electrolyte, which is composed of an organic solvent, a lithium salt, a film-forming agent and an additive A.

[0083] The organic solvent is composed of vinylene carbonate, dimethyl carbonate and methyl ethyl carbonate, the mass ratio of the vinylene carbonate, the dimethyl carbonate and the methyl ethyl carbonate is 15:15:70, and the mass fraction of the organic solvent in the electrolyte is 75.5%;

[0084] The lithium salt is lithium hexafluorophosphate, and the mass fraction of the lithium salt in the electrolyte is 12.5%;

[0085] The film-forming agent is vinylene carbonate, and the mass fraction of the film-forming agent in the electrolyte is 5%;

[0086] The mass fraction of the additive A in the electrolyte is 7%.

[0087] The embodiment also provides a preparation method of the electrolyte, and the specific method is as follows:

[0088] (1) under an argon atmosphere, a formula amount of lithium salt is added to an organic solvent, then a film-forming agent is added, finally, an additive A is added, and stirring and mixing are performed at 25 DEG C to obtain an electrolyte.

[0089] Embodiment 6

[0090] The embodiment provides an electrolyte, and the electrolyte is composed of an organic solvent, a lithium salt, a film-forming agent and an additive A;

[0091] The organic solvent is composed of vinylene carbonate, dimethyl carbonate and methyl ethyl carbonate, the mass ratio of the vinylene carbonate, the dimethyl carbonate and the methyl ethyl carbonate is 15:15:70, and the mass fraction of the organic solvent in the electrolyte is 81.5%;

[0092] The lithium salt is lithium hexafluorophosphate, and the mass fraction of the lithium salt in the electrolyte is 12.5%;

[0093] The film-forming agent is vinylene carbonate, and the mass fraction of the film-forming agent in the electrolyte is 3%;

[0094] The mass fraction of the additive A in the electrolyte is 3%.

[0095] The embodiment also provides a preparation method of the electrolyte, and the specific method is as follows:

[0096] (1) under an argon atmosphere, a formula amount of lithium salt is added to an organic solvent, then a film-forming agent is added, finally, an additive A is added, and stirring and mixing are performed at 25 DEG C to obtain an electrolyte.

[0097] Embodiment 7

[0098] The embodiment provides an electrolyte, and the electrolyte is composed of an organic solvent, a lithium salt, a film-forming agent and an additive A;

[0099] The organic solvent is composed of vinylene carbonate, dimethyl carbonate and methyl ethyl carbonate, the mass ratio of the vinylene carbonate, the dimethyl carbonate and the methyl ethyl carbonate is 15:15:70, and the mass fraction of the organic solvent in the electrolyte is 72.5%;

[0100] The lithium salt is lithium hexafluorophosphate, and the mass fraction of the lithium salt in the electrolyte is 12.5%;

[0101] The film-forming agent is vinylene carbonate, and the mass fraction of the film-forming agent in the electrolyte is 3%;

[0102] The mass fraction of the additive A in the electrolyte is 12%.

[0103] The embodiment also provides a preparation method of the electrolyte.

[0104] (1) under an argon atmosphere, adding a formula amount of lithium salt into an organic solvent, then adding a film-forming agent, finally adding an additive A, and stirring and mixing at 25°C to obtain an electrolyte.

[0105] Embodiment 8

[0106] The embodiment provides an electrolyte, which is composed of an organic solvent, a lithium salt, a film-forming agent and an additive A.

[0107] The organic solvent is composed of vinylene carbonate, dimethyl carbonate and methyl ethyl carbonate, the mass ratio of the vinylene carbonate, the dimethyl carbonate and the methyl ethyl carbonate is 15:15:70, and the mass fraction of the organic solvent in the electrolyte is 78.5%.

[0108] The lithium salt is lithium hexafluorophosphate, and the mass fraction of the lithium salt in the electrolyte is 12.5%.

[0109] The film-forming agent is vinylene carbonate, and the mass fraction of the film-forming agent in the electrolyte is 2%.

[0110] The mass fraction of the additive A in the electrolyte is 7%.

[0111] The embodiment also provides a preparation method of the electrolyte.

[0112] (1) under an argon atmosphere, adding a formula amount of lithium salt into an organic solvent, then adding a film-forming agent, finally adding an additive A, and stirring and mixing at 25°C to obtain an electrolyte.

[0113] Embodiment 9

[0114] The embodiment provides an electrolyte, which is composed of an organic solvent, a lithium salt, a film-forming agent and an additive A.

[0115] The organic solvent is composed of vinylene carbonate, dimethyl carbonate and methyl ethyl carbonate, the mass ratio of the vinylene carbonate, the dimethyl carbonate and the methyl ethyl carbonate is 15:15:70, and the mass fraction of the organic solvent in the electrolyte is 73.5%.

[0116] The lithium salt is lithium hexafluorophosphate, and the mass fraction of the lithium salt in the electrolyte is 12.5%.

[0117] The film forming agent is vinylene carbonate, and the mass fraction of the film forming agent in the electrolyte is 7%.

[0118] The mass fraction of the additive A in the electrolyte is 7%.

[0119] The embodiment also provides a preparation method of the electrolyte.

[0120] (1) under an argon atmosphere, a formula amount of lithium salt is added to an organic solvent, then a film forming agent is added, finally, an additive A is added, and stirring and mixing are performed at 25°C to obtain an electrolyte.

[0121] Example 10

[0122] The embodiment provides an electrolyte, which is composed of an organic solvent, a lithium salt, a film forming agent and an additive A.

[0123] The organic solvent is composed of vinylene carbonate, dimethyl carbonate and methyl ethyl carbonate, the mass ratio of the vinylene carbonate, the dimethyl carbonate and the methyl ethyl carbonate is 15:15:70, and the mass fraction of the organic solvent in the electrolyte is 74.5%.

[0124] The lithium salt is lithium hexafluorophosphate, and the mass fraction of the lithium salt in the electrolyte is 12.5%.

[0125] The film forming agent is vinylene carbonate, and the mass fraction of the film forming agent in the electrolyte is 3%.

[0126] The mass fraction of the additive A in the electrolyte is 10%.

[0127] The embodiment also provides a preparation method of the electrolyte.

[0128] (1) under an argon atmosphere, a formula amount of lithium salt is added to an organic solvent, then a film forming agent is added, finally, an additive A is added, and stirring and mixing are performed at 25°C to obtain an electrolyte.

[0129] Comparative Example 1

[0130] The difference between the comparative example 1 and the example 1 is that:

[0131] The mass fraction of the lithium salt in the electrolyte is 12.5%, the mass fraction of the organic solvent in the electrolyte is 84.5%, the mass fraction of the film forming agent in the electrolyte is 3%, the mass fraction of the additive A in the electrolyte is 0, and the rest is the same as the example 1.

[0132] Comparative Example 2

[0133] The difference between the comparative example 2 and the example 1 is that:

[0134] The mass fraction of lithium salt in the electrolyte is 12.5%, the mass fraction of organic solvent in the electrolyte is 80.5%, the mass fraction of film-forming agent in the electrolyte is 0, the mass fraction of additive A in the electrolyte is 7%, and the rest is the same as in Example 1.

[0135] Test method

[0136] The electrolyte prepared in Example 1-10 and Comparative Example 1-2 was injected into LiMn 0.5 Fe 0.5 PO4-silicon-carbon system lithium ion battery, and the performance was tested by using lithium ion battery.

[0137] (I) The preparation method of the lithium ion battery is as follows:

[0138] Preparation of positive electrode sheet:

[0139] LiMn 0.5 Fe 0.5 PO4, conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) were fully stirred and mixed in N-methyl pyrrolidone (NMP) solvent at a weight ratio of 96:2:2 to form a uniform positive electrode slurry; the slurry was coated on the positive electrode current collector aluminum foil, dried and cold-pressed to obtain the positive electrode sheet.

[0140] Preparation of negative electrode sheet:

[0141] The negative electrode active material silicon-carbon, conductive agent acetylene black, binder styrene-butadiene rubber (SBR), and thickening agent sodium carboxymethyl cellulose (CMC) were fully stirred and mixed in a suitable amount of deionized water solvent at a weight ratio of 95:2:2:1 to form a uniform anode slurry; the slurry was coated on the anode current collector copper foil, dried and cold-pressed to obtain the negative electrode sheet.

[0142] Preparation of lithium ion battery:

[0143] The positive electrode sheet, PE separator, and negative electrode sheet were stacked in order, and then wound to obtain a bare cell; the bare cell was placed in an outer packaging bag, and the electrolyte prepared in Example 1-10 and Comparative Example 1-2 was injected into the dried battery, and then vacuum packaging, standing, formation, shaping and other processes were carried out to obtain the lithium ion battery.

[0144] (II) 60℃ storage for 30d test:

[0145] The lithium ion battery was charged at 1.0C constant current constant voltage to 4.5V at 25℃, rested for 5min, then discharged at 0.1C to 2.5V, the discharged capacity was recorded as the initial capacity C0, then charged at 1.0C constant current constant voltage to 4.5V, the initial thickness was measured, and the initial thickness was recorded as T0; then, the battery was stored at 60℃±2℃, after 30d open circuit storage, the battery was taken out, the hot thickness T1 was tested, then the battery was charged and discharged at 1.0C, the remaining capacity C1 and the recovery capacity C2 were tested, and the hot thickness change rate, the capacity retention rate and the capacity recovery rate were calculated, and the calculation formula was as follows:

[0146] The capacity retention rate of the lithium ion battery after 30d storage at 60℃ was (C1 / C0) x 100%;

[0147] The capacity recovery rate of the lithium ion battery after 30d storage at 60℃ was (C2 / C0) x 100%;

[0148] The hot thickness change rate of the lithium ion battery after 30d storage at 60℃ was (T1-T0) / T0 x 100%.

[0149] (III) 25℃ 1C charge-discharge cycle test:

[0150] The lithium ion battery was charged at 1C (nominal capacity) constant current to 4.2V at 25℃, then charged at 4.2V constant voltage to current≤0.05C, rested for 10min, then discharged at 1C constant current to cut-off voltage 2.8V, which was one charge-discharge cycle. The lithium ion battery was subjected to 1000 times of charge-discharge cycle at 25℃ according to the above conditions.

[0151] The capacity retention rate of the lithium ion battery after N cycles was (discharge capacity of the Nth cycle / first discharge capacity) x 100%, and N was the cycle number of the lithium ion battery.

[0152] The test results are shown in Table 1 as follows:

[0153] Table 1

[0154]

[0155]

[0156] According to the test results in Table 1, the electrolyte provided by the embodiment contains an additive A, the additive A can participate in the film formation of the negative electrode, improve the mechanical strength and toughness of the solid electrolyte interface film on the surface of the negative electrode, inhibit the volume expansion of the silicon-carbon negative electrode during the charge-discharge process, reduce the damage of the stress generated by the volume expansion of the silicon-carbon negative electrode to the solid electrolyte interface film during the charge-discharge process, reduce the side reaction between the electrolyte and the silicon-carbon negative electrode, and improve the cycle performance and storage performance of the battery.

[0157] From the analysis of Example 1, Example 2, Example 3, Example 10, Example 6 and Example 7, it can be seen that by controlling the mass percentage of additive A in the electrolyte within an appropriate range, the gas generation and the battery impedance can be reduced, the mechanical strength and toughness of the solid electrolyte interface film on the negative electrode surface can be ensured, and the cycle performance and storage performance of the battery can be more improved.

[0158] From the analysis of Example 1, Example 4, Example 5, Example 8 and Example 9, it can be seen that by controlling the mass percentage of the film-forming agent in the electrolyte within an appropriate range, the gas generation and the battery impedance can be reduced, the mechanical strength and toughness of the solid electrolyte interface film on the negative electrode surface can be ensured, and the cycle performance and storage performance of the battery can be more improved.

[0159] From the analysis of Example 1 to Example 10 and Comparative Example 2, it can be seen that by adding the film-forming agent and additive A in the electrolyte, the storage performance and cycle performance of the battery can be further improved by the synergistic effect between the film-forming agent and additive A.

[0160] The above describes the embodiments of the present application in detail, and the principles and implementation modes of the present application are described by applying specific examples; the above description of the embodiments is only used to help understand the method of the present application and its core idea; meanwhile, for those skilled in the art, according to the idea of the present application, the specific implementation modes and application ranges can be changed, and the above description of the present application should not be understood as a limitation of the present application.

Claims

1. An electrolyte, characterized by, The organic solvent, the lithium salt, the additive A and the film forming agent are mixed in a protective atmosphere to obtain the electrolyte. The additive A has a structure including a fluorine atom, a boron atom, a carbonyl group, a siloxane group and a benzene ring, and the structure of the additive A is shown as formula I. The film forming agent includes at least one of ethylene sulfite, propylene sulfite, fluoroethylene carbonate and vinylene carbonate.

2. The electrolyte of claim 1, wherein a mass percentage of the film forming agent in the electrolyte is 3%-5%.

3. The electrolyte of claim 1 or 2, wherein the lithium salt includes at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium difluorophosphate, lithium trifluoromethylsulfonate and lithium bisfluorosulfonimide; and / or a mass percentage of the lithium salt in the electrolyte is 12%-15%.

4. The electrolyte of claim 1 or 2, wherein the organic solvent includes ethylene carbonate, dimethyl carbonate and methyl ethyl carbonate, and a mass ratio of the ethylene carbonate, the dimethyl carbonate and the methyl ethyl carbonate is (1-3):(1-3):(4-8); and / or a mass percentage of the organic solvent in the electrolyte is 70%-80%. The method includes the following steps: The formula amount of the organic solvent, the lithium salt, the additive A and other raw materials are mixed uniformly in a protective atmosphere to obtain the electrolyte.

6. The method of claim 5, wherein the other raw materials include the film forming agent.

7. The method of claim 5 or 6, wherein the mixing is stirring mixing, a temperature of the mixing is 25-30℃, and / or a time of the mixing is 50-60min; and / or the protective atmosphere includes a nitrogen atmosphere and / or an argon atmosphere. The lithium ion battery includes the electrolyte of any one of claims 1-4 or the electrolyte prepared by the method of any one of claims 5-7.

5. A process for the preparation of an electrolyte as claimed in any one of claims 1 to 4, characterized in that The lithium ion battery further includes a positive electrode, a negative electrode and a separator. The material of the positive electrode includes lithium manganese iron phosphate and / or nickel cobalt manganese ternary material, and / or the material of the negative electrode includes silicon carbon material. ​ ​ ​ ​ ​ 8. A lithium-ion battery, characterized by ​ 9. The lithium-ion battery of claim 8, wherein, ​ ​

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