Electrolyte and lithium-ion battery

Abstract

The present disclosure provides an electrolyte and a lithium-ion battery. The electrolyte comprises a lithium salt, an organic solvent, and an additive, wherein the additive comprises a tetranitrile siloxane compound additive A having a structure of Formula I and a boron-containing lithium salt additive B having a structure of Formula II. When the tetranitrile siloxane compound additive A and the boron-containing lithium salt additive B are used together in the electrolyte, they can inhibit the decomposition of the organic solvent and the interfacial film, reduce the cost of the lithium salt, improve the interfacial film performance, and significantly improve the low-temperature cycle performance, high-temperature cycle performance, thermal shock resistance, and high-temperature storage performance of the lithium-ion battery, thereby achieving a good synergistic effect.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and in particular to an electrolyte and a lithium-ion battery. Background Art

[0002] Lithium-ion batteries are widely used in consumer electronics and power batteries due to their advantages such as high specific energy, fast charging and discharging capabilities, and low self-discharge.

[0003] However, during the high-temperature cycling process of lithium-ion batteries, transition metals (cobalt, nickel, iron, manganese, etc.) in the positive electrode material are easily dissolved, causing damage to the positive electrode material structure and making it impossible to normally deintercalate and deintercalate lithium ions. In addition, the lithium salt (lithium hexafluorophosphate) in the electrolyte is easily decomposed at high temperatures to produce HF, which will corrode the electrode interface film and cause battery cycle capacity loss, which is not conducive to the low-temperature cycling performance of lithium-ion batteries and the improvement of cycling performance.

[0004] In order to solve the above technical problems, some scholars have proposed that the amount of lithium salt or organic solvent in the electrolyte can be appropriately increased to increase the lithium ions in the electrolyte that bear the transmission effect, which will be beneficial to the improvement of the low-temperature cycle performance and cycle performance of lithium-ion batteries. However, increasing the amount of lithium salt or organic solvent will increase the battery manufacturing cost to a certain extent, and when its amount is high, the lithium salt or organic solvent is more likely to decompose and produce gas under high temperature conditions to erode the electrode interface film, thereby causing the high-temperature storage performance and thermal shock performance of the lithium-ion battery to deteriorate.

[0005] Therefore, there is an urgent need to develop a new type of lithium-ion electrolyte. Summary of the Invention

[0006] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a new electrolyte and lithium ion battery that improves the low-temperature cycle performance, high-temperature cycle performance, thermal shock performance and high-temperature storage performance of lithium ion batteries, while also inhibiting the decomposition of organic solvents, removing by-products, protecting the positive electrode structure, and increasing the amount of organic solvent used, inhibiting the decomposition of lithium salts, reducing the amount of lithium salt used, and having low interfacial impedance.

[0007] The object of the present invention is achieved through the following technical solutions:

[0008] An electrolyte comprises a lithium salt, an organic solvent and an additive, wherein the additive comprises a tetranitrile siloxane compound additive A having a structure of formula I and a boron-containing lithium salt additive B having a structure of formula II.

[0009] The tetranitrile siloxane compound additive A of formula I has the following general formula:

[0010]

[0011] wherein R1 to R6 are selected from any one of methane, ethyl, propane, butane, phenyl, trifluoromethyl, alkenyl, fluoroalkenyl and alkynyl;

[0012] The boron-containing lithium salt compound additive B of formula II has the following general formula:

[0013]

[0014] Wherein, X1, X2, X3, and X4 are selected from any one of an alkane group, a fluorine atom, an oxygen atom, a hydrogen atom, a halogenated alkane group, an alkene group, an alkyne group, an aromatic hydrocarbon group, a halogenated aromatic hydrocarbon group, and a siloxane group.

[0015] In one embodiment, the structure of Formula I has the following general formula:

[0016]

[0017] In one embodiment, the mass percentage of the tetranitrile siloxane compound additive A in the electrolyte is defined as a%, and the range of a% is 0.1%≤a%≤5%.

[0018] In one embodiment, the mass proportion of the boron-containing lithium salt additive B in the electrolyte is defined as b%, and the range of b% is 0.1%≤b%≤5%.

[0019] In one embodiment, the ratio of b% to a% is in the range of 0.1≤b% / a%≤10.

[0020] In one embodiment, the organic solvent includes at least two of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl formate, ethyl formate, ethyl propionate, propyl propionate, methyl butyrate, and tetrahydrofuran; wherein the organic solvent contains at least one ethylene carbonate.

[0021] In one embodiment, the mass proportion of the ethylene carbonate in the electrolyte is defined as W1%, and the range of W1% is 10%≤W1%≤30%.

[0022] In one embodiment, the lithium salt is selected from LiPF6, LiSbF6, LiAsF6, LiTaF6, LiAlC l4 、Li2B 10 Cl 10 、Li2B 10 F 10 , LiClO4, LiCF3SO3, chelated lithium orthoborate salt and chelated lithium orthophosphate salt, wherein the lithium salt includes at least one LiPF6.

[0023] In one embodiment, the mass proportion of LiPF6 in the electrolyte is defined as W2%, and the range of W2% is 8%≤W2%≤15%.

[0024] A lithium-ion battery comprises a positive electrode sheet, a negative electrode sheet, a separator, and the electrolyte described in any one of the above embodiments, wherein the total amount of element B in the electrolyte accounts for no less than 100 ppm by mass of the electrolyte.

[0025] Compared with the prior art, the present invention has at least the following advantages:

[0026] In the above-mentioned electrolyte, the silicon oxygen functional group in the added tetranitrile siloxane compound additive A can remove HF in the electrolyte, and the cyano functional group can well protect the structural stability of the positive electrode material, effectively avoiding the transition metal (cobalt, nickel, iron, manganese, etc.) in the positive electrode material from being easily dissolved, causing the structural destruction of the positive electrode material and the inability to normally deintercalate lithium ions, and due to the high oxidation resistance of the tetranitrile siloxane compound additive A, the further decomposition of the organic solvent can be inhibited; at the same time, the by-products of the decomposition of the organic solvent, such as HF, can also be removed, and the decomposition of the positive electrode material can be inhibited to achieve a protective effect, thereby improving the thermal shock performance, high temperature cycle performance and storage safety performance of the lithium ion battery; the BO functional group in the boron-containing lithium salt additive B can simultaneously participate in the film formation on the positive and negative electrode surfaces to form ions. The interface film with faster electron transmission, thinner and good thermal stability can effectively improve the negative effect of high impedance brought by the tetranitrile siloxane compound additive A, and can also significantly improve the low-temperature cycle performance, thermal shock performance and high-temperature storage performance of the lithium-ion battery; it can also inhibit the decomposition of lithium salt in the electrolyte and reduce the amount of lithium salt used; therefore, when the tetranitrile siloxane compound additive A and the boron-containing lithium salt additive B are used in combination, they can inhibit the decomposition of organic solvents, increase the amount of organic solvents, inhibit the decomposition of the interface film, reduce the cost of lithium salts, improve the performance of the interface film, and significantly improve the low-temperature cycle performance, high-temperature cycle performance, thermal shock performance and high-temperature storage performance of the lithium-ion battery, thereby achieving a good synergistic effect and providing the market with a new type of lithium-ion electrolyte. Specific embodiments

[0027] To facilitate understanding of the present invention, the present invention will be described more fully below. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to enable a more thorough and comprehensive understanding of the disclosure of the present invention.

[0028] It should be noted that when an element is referred to as being "fixed to" another element, it may be directly attached to the other element or there may be an intermediate element. When an element is referred to as being "connected to" another element, it may be directly connected to the other element or there may be an intermediate element. The terms "vertical," "horizontal," "left," "right," and similar expressions used herein are for illustrative purposes only and do not represent the only embodiments.

[0029] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one skilled in the art to which this invention pertains. The terms used in this specification of the present invention are for the purpose of describing specific embodiments only and are not intended to limit the present invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0030] The present disclosure provides an electrolyte comprising a lithium salt, an organic solvent and an additive, wherein the additive comprises a tetranitrile siloxane compound additive A having a structure of formula I and a boron-containing lithium salt additive B having a structure of formula II. In the electrolyte, the silicon oxygen functional group in the tetranitrile siloxane compound additive A can remove HF in the electrolyte, while the cyano functional group can well protect the structural stability of the positive electrode material, effectively avoiding the phenomenon that the transition metal (cobalt, nickel, iron, manganese, etc.) in the positive electrode material is easily dissolved and causes the structural damage of the positive electrode material so that lithium ions cannot be normally deintercalated and deintercalated. In addition, due to the high oxidation resistance of the tetranitrile siloxane compound additive A, the further decomposition of the organic solvent can be inhibited; at the same time, the byproducts of the decomposition of the organic solvent, such as HF, can be removed, and the decomposition of the positive electrode material can be inhibited to achieve a protective effect, thereby improving the thermal shock performance, high temperature cycle performance and Storage and other safety performance; the BO functional group in the boron-containing lithium salt additive B can participate in the film formation on the positive and negative electrode surfaces at the same time to form an interface film with faster ion transfer, thinner and good thermal stability, which can effectively improve the high impedance negative effect brought by the tetranitrile siloxane compound additive A, and at the same time significantly improve the low-temperature cycle performance, thermal shock performance and high-temperature storage performance of the lithium-ion battery; it can also inhibit the decomposition of lithium salt in the electrolyte and reduce the amount of lithium salt used; therefore, when the tetranitrile siloxane compound additive A and the boron-containing lithium salt additive B are used in combination, they can inhibit the decomposition of organic solvents, the decomposition of the interface film, reduce the cost of lithium salts, improve the performance of the interface film, and significantly improve the low-temperature cycle performance, high-temperature cycle performance, thermal shock performance and high-temperature storage performance of the lithium-ion battery, thereby achieving a good synergistic effect, and providing the market with a new type of lithium-ion electrolyte.

[0031] In order to better understand the technical solutions and beneficial effects of the present disclosure, the present disclosure is further described in detail below with reference to specific embodiments.

[0032] The electrolyte of one embodiment includes a lithium salt, an organic solvent, and an additive. The additive includes a tetranitrile siloxane compound additive A having a structure of Formula I and a boron-containing lithium salt additive B having a structure of Formula II. The tetranitrile siloxane compound additive A having a structure of Formula I has the following general formula:

[0033]

[0034] wherein R1 to R6 are selected from any one of methane, ethyl, propane, butane, phenyl, trifluoromethyl, alkenyl, fluoroalkenyl and alkynyl;

[0035] The boron-containing lithium salt compound additive B of formula II has the following general formula:

[0036]

[0037] Wherein, X1, X2, X3, and X4 are selected from any one of an alkane group, a fluorine atom, an oxygen atom, a hydrogen atom, a halogenated alkane group, an alkene group, an alkyne group, an aromatic hydrocarbon group, a halogenated aromatic hydrocarbon group, and a siloxane group.

[0038] It can be understood that the silicon oxygen functional group in the added tetranitrile siloxane compound additive A can remove HF in the electrolyte, and the cyano functional group can well protect the structural stability of the positive electrode material, effectively avoiding the transition metal (cobalt, nickel, iron, manganese, etc.) in the positive electrode material from being easily dissolved, causing the structural destruction of the positive electrode material and the inability to normally deintercalate lithium ions. Moreover, due to the high oxidation resistance of the tetranitrile siloxane compound additive A, the further decomposition of the organic solvent can be inhibited; at the same time, the by-products of the decomposition of the organic solvent, such as HF, can also be removed, and the decomposition of the positive electrode material can be inhibited to achieve a protective effect, thereby improving the thermal shock performance, high temperature cycle performance and storage safety performance of the lithium-ion battery; the BO functional group in the boron-containing lithium salt additive B can participate in the reaction on the positive and negative electrode surfaces at the same time. The film-forming interfacial film has faster ion transmission, is thinner, and has good thermal stability. It can effectively improve the negative effect of high impedance brought by the tetranitrile siloxane compound additive A, and can also significantly improve the low-temperature cycle performance, thermal shock performance and high-temperature storage performance of the lithium-ion battery; it can also inhibit the decomposition of lithium salt in the electrolyte and reduce the amount of lithium salt used; therefore, when the tetranitrile siloxane compound additive A and the boron-containing lithium salt additive B are used in combination, they can inhibit the decomposition of organic solvents and interfacial film, reduce the cost of lithium salts, improve the performance of the interfacial film, and significantly improve the low-temperature cycle performance, high-temperature cycle performance, thermal shock performance and high-temperature storage performance of the lithium-ion battery, thereby achieving a good synergistic effect and providing the market with a new type of lithium-ion electrolyte.

[0039] It should be noted that the presence of the tetranitrile siloxane compound additive A can increase the amount of organic solvent to a certain extent, and the boron-containing lithium salt additive B can inhibit the decomposition of the lithium salt to a certain extent and reduce the amount of lithium salt. In this way, while significantly improving the low-temperature cycle performance, high-temperature cycle performance, thermal shock performance and high-temperature storage performance of the lithium-ion battery, the electrolyte formula can be further optimized, that is, the amount of organic solvent is increased while the amount of lithium salt is reduced. This not only reduces the production cost of the lithium salt, but also effectively avoids the shortage of lithium resources caused by the growing development of new energy. The invention effectively avoids the phenomenon that the high dosage of traditional lithium salts or organic solvents is more likely to decompose and produce gas under high temperature conditions, thereby corroding the electrode interface film and deteriorating the high-temperature storage performance and thermal shock performance of the lithium-ion battery. Moreover, since the dosage of the organic solvent is increased and the dosage of the lithium salt is reduced, the fluidity of the electrolyte can be improved. Moreover, since the tetranitrile siloxane compound additive A has a linear symmetrical structure and good fluidity, the fluidity of the electrolyte is further improved, thereby increasing the transmission speed of lithium ions in the electrolyte, thereby better improving the low-temperature cycle performance, high-temperature cycle performance, thermal shock performance and high-temperature storage performance of the lithium-ion battery.

[0040] In one embodiment, the structure of Formula I has the following general formula:

[0041]

[0042] In a preferred embodiment, since the structure of I-II has four symmetrical nitrile siloxanes and the structure of II-I has four symmetrical BOs, the two can achieve a better synergistic effect when used in combination.

[0043] In one embodiment, the mass percentage of the tetranitrile siloxane compound additive A in the electrolyte is defined as a%, and the range of a% is 0.1%≤a%≤5%.

[0044] It can be understood that if the amount of the tetranitrile siloxane compound additive A is less than 0.1% or greater than 5%, the low-temperature cycle performance, high-temperature cycle performance and thermal shock performance of the lithium-ion battery cannot be effectively improved. Only when a% satisfies 0.1%≤a%≤5% can the further decomposition of the organic solvent be effectively inhibited. At the same time, the amount of the organic solvent can be increased to a certain extent, and the by-products of the decomposition of the organic solvent, such as HF, can be removed. It can also inhibit the decomposition of the positive electrode material to achieve a protective effect, and can also effectively improve the thermal shock performance, high-temperature cycle performance and storage safety performance of the lithium-ion battery.

[0045] In one embodiment, the mass proportion of the boron-containing lithium salt additive B in the electrolyte is defined as b%, and the range of b% is 0.1%≤b%≤5%.

[0046] It can be understood that if the dosage of the boron-containing lithium salt additive B is less than 0.1% or greater than 5%, it will not be able to effectively improve the low-temperature cycle performance, high-temperature cycle performance and thermal shock performance of the lithium-ion battery. Only when b% satisfies 0.1%≤b%≤5% can it effectively inhibit the decomposition of the lithium salt in the electrolyte and reduce the dosage of the lithium salt. At the same time, it can also participate in the film formation to form an interface film with faster ion transport, thinner and better thermal stability. It can effectively improve the negative effect of high impedance brought by the tetranitrile siloxane compound additive A, and can also significantly improve the low-temperature cycle performance, thermal shock performance and high-temperature storage performance of the lithium-ion battery.

[0047] In one embodiment, the ratio of b% to a% is in the range of 0.1≤b% / a%≤10.

[0048] It is understandable that adding too much tetranitrile siloxane compound additive A will increase the interfacial impedance, thereby deteriorating the low-temperature cycling performance of the lithium-ion battery, while adding the boron-containing lithium salt additive B can effectively improve the interfacial impedance, thereby improving the low-temperature cycling performance of the lithium-ion battery. However, if too much or too little boron-containing lithium salt additive B is added, the ratio of tetranitrile siloxane compound additive A to boron-containing lithium salt additive B will be unbalanced, making it impossible to effectively ensure that the tetranitrile siloxane compound additive A and the boron-containing lithium salt additive B can play a better synergistic role. Therefore, only when the tetranitrile siloxane compound additive A and the boron-containing lithium salt additive B simultaneously meet 0.1% ≤ a% ≤ 5%, 0.1 ≤ b% / a% ≤ 10, and 0.1 ≤ b% / a% ≤ 10, can the two play a good synergistic role, that is, increase the amount of organic solvent and reduce the amount of lithium salt to ensure that the electrolyte has better fluidity, thereby significantly improving the low-temperature cycling performance, thermal shock resistance and high-temperature storage performance of the lithium-ion battery.

[0049] In one embodiment, the organic solvent includes at least two of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl formate, ethyl formate, ethyl propionate, propyl propionate, methyl butyrate, and tetrahydrofuran; wherein the organic solvent contains at least one ethylene carbonate.

[0050] It can be understood that by combining ethylene carbonate with any one of propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl formate, ethyl formate, ethyl propionate, propyl propionate, methyl butyrate, and tetrahydrofuran, it is ensured that the lithium salt can be well dissolved in the organic solvent to obtain an electrolyte with a stable system and good fluidity, and to ensure a good synergistic effect with the subsequently added tetranitrile siloxane compound additive A, that is, to ensure that the tetranitrile siloxane compound additive A can well inhibit the decomposition of ethylene carbonate, and at the same time, the amount of ethylene carbonate can be increased to a certain extent to further improve the fluidity of the electrolyte.

[0051] In one embodiment, the mass proportion of the ethylene carbonate in the electrolyte is defined as W1%, and the range of W1% is 10%≤W1%≤30%.

[0052] It is understood that if the weight percentage of ethylene carbonate in the electrolyte is less than 10% or greater than 30%, the addition of the tetranitrile siloxane compound additive A will not effectively improve the low-temperature cycling performance, high-temperature cycling performance, thermal shock resistance, and high-temperature storage performance of the lithium-ion battery. Therefore, when the weight percentage of ethylene carbonate in the electrolyte satisfies 10% ≤ W1% ≤ 30%, while ensuring improvements in the low-temperature cycling performance, high-temperature cycling performance, thermal shock resistance, and high-temperature storage performance of the lithium-ion battery, the amount of ethylene carbonate can be increased to a certain extent to further improve the fluidity of the electrolyte.

[0053] In a preferred embodiment, the range of W1% is 20%≤W1%≤25%.

[0054] In one embodiment, when the ratio of a% to W1% is in the range of 0.033≤a / W1≤0.1, the ratio of the tetranitrile siloxane compound additive A to ethylene carbonate is ensured to be more appropriate, thereby ensuring that the added tetranitrile siloxane compound additive A and the subsequently added boron-containing lithium salt additive B can exert a better synergistic effect.

[0055] In one embodiment, the lithium salt is selected from LiPF6, LiSbF6, LiAsF6, LiTaF6, LiAlC l4 、Li2B 10 Cl 10 、Li2B 10 F 10 , LiClO4, LiCF3SO3, chelated lithium orthoborate salt and chelated lithium orthophosphate salt, wherein the lithium salt includes at least one LiPF6.

[0056] In one embodiment, the mass proportion of LiPF6 in the electrolyte is defined as W2%, and the range of W2% is 8%≤W2%≤15%, so as to ensure that the ratio of added LiPF6 to the boron-containing lithium salt additive B is more appropriate, thereby ensuring that the two can play a better synergistic role.

[0057] In a preferred embodiment, when the ratio of b% to W2% is in the range of 0.067≤b / W2≤0.125, especially when combined with 0.1%≤a%≤5%, 0.1≤b% / a%≤10, 0.1≤b% / a%≤10, and 0.033≤a / W1≤0.1, the added boron-containing lithium salt additive B can not only meet the requirements of more comprehensive improvement of the high interfacial impedance caused by the tetranitrile siloxane compound additive A; it can also effectively inhibit the decomposition of the lithium salt in the electrolyte while reducing the amount of lithium salt used, thereby reducing the amount of lithium salt used, not only reducing the production cost but also improving the fluidity of the electrolyte; at the same time, it can also ensure that the addition of the tetranitrile The base siloxane compound additive A can effectively inhibit the decomposition of the organic solvent and remove its by-products. It can also well protect the structural stability of the positive electrode material, effectively avoiding the easy dissolution of transition metals (cobalt, nickel, iron, manganese, etc.) in the positive electrode material, causing the structural damage of the positive electrode material and the inability to normally deintercalate and deintercalate lithium ions. It can also increase the amount of organic solvent to a certain extent to further improve the fluidity of the electrolyte, not only better ensure the rapid transmission of lithium ions in the electrolyte, thereby better improving the low-temperature cycle performance, high-temperature cycle performance, high-temperature storage performance and thermal shock performance of the lithium-ion battery, but also reduce the production cost of lithium salts, so as to effectively solve the shortage of lithium resources.

[0058] The applicant has found that in a more preferred embodiment, when the tetranitrile siloxane compound additive A and the boron-containing lithium salt additive B satisfy (a:b)=(0.1-5):(0.1-5), and 0.1≤b / a≤10, the amount of the tetranitrile siloxane compound additive A relative to ethylene carbonate satisfies 0.033≤a / W1≤0.1, and the amount of the boron-containing lithium salt additive B relative to lithium hexafluorophosphate satisfies 0.067≤b / W2≤0.125, when the amount of vinyl carbonate and lithium hexafluorophosphate is reduced at the same time, the tetranitrile siloxane compound additive A and the boron-containing lithium salt additive B can play a better synergistic role, that is, not only improving the low-temperature cycle performance, high-temperature cycle performance, high-temperature storage performance and thermal shock performance of the lithium-ion battery, but also reducing the amount of lithium salt and ethylene carbonate, effectively reducing the production cost of the lithium-ion battery.

[0059] The present disclosure also provides a lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet, a separator, and the electrolyte described in any of the above embodiments, wherein the total amount of element B in the electrolyte accounts for no less than 100 ppm by mass in the electrolyte, to ensure that the BO functional groups in the added tetranitrile siloxane compound additive A and the boron lithium salt additive B can play a good synergistic role. In this way, while reducing the amount of lithium salt, the amount of organic solvent used is increased, thereby better improving the fluidity of the electrolyte, being more conducive to the transmission of lithium ions, and thereby improving the low-temperature cycle performance, high-temperature cycle performance, thermal shock performance, and high-temperature storage performance of the lithium-ion battery.

[0060] Compared with the prior art, the present invention has at least the following advantages:

[0061] In the above-mentioned electrolyte, the silicon oxygen functional group in the added tetranitrile siloxane compound additive A can remove HF in the electrolyte, and the cyano functional group can well protect the structural stability of the positive electrode material, effectively avoiding the transition metal (cobalt, nickel, iron, manganese, etc.) in the positive electrode material from being easily dissolved, causing the structural destruction of the positive electrode material and the inability to normally deintercalate lithium ions. Moreover, due to the high oxidation resistance of the tetranitrile siloxane compound additive A, the further decomposition of the organic solvent can be inhibited; at the same time, the byproducts of the decomposition of the organic solvent, such as HF, can also be removed, and the decomposition of the positive electrode material can be inhibited to achieve a protective effect, thereby improving the thermal shock performance, high temperature cycle performance and storage safety performance of the lithium ion battery; the BO functional group in the boron-containing lithium salt additive B can simultaneously participate in the oxidation of the positive and negative electrodes. The interfacial film with faster ion transmission, thinner and better thermal stability formed by the film formation can effectively improve the negative effect of high impedance brought by the tetranitrile siloxane compound additive A, and can also significantly improve the low-temperature cycle performance, thermal shock performance and high-temperature storage performance of the lithium-ion battery; it can also inhibit the decomposition of lithium salt in the electrolyte and reduce the amount of lithium salt used; therefore, when the tetranitrile siloxane compound additive A and the boron-containing lithium salt additive B are used in combination, they can inhibit the decomposition of organic solvents and interfacial film, reduce the cost of lithium salts, improve the performance of the interfacial film, and significantly improve the low-temperature cycle performance, high-temperature cycle performance, thermal shock performance and high-temperature storage performance of the lithium-ion battery, thereby achieving a good synergistic effect and providing the market with a new type of lithium-ion electrolyte.

[0062] The following examples illustrate some specific embodiments, where percentages are expressed by weight. It should be noted that the following examples do not exhaust all possible situations, and that the materials used in the following examples, unless otherwise specified, can be obtained from commercial sources.

[0063] Table 1

[0064]

[0065]

[0066]

[0067]

[0068]

[0069] The specific structure of the additive B used in Comparative Examples 1 to 11 and Examples 1 to 36 and the specific structure of the additive A are shown in Table 2 below.

[0070] Table 2

[0071]

[0072]

[0073] The preparation method of the electrolyte of Examples 1 to 36 and Comparative Examples 1 to 11 is as follows: ethylene carbonate (EC), diethyl carbonate (DEC), and propylene carbonate (PC) are mixed in a mass ratio of 1:1:1 to obtain an organic solvent, and then additives A and additive B are added to the organic solvent in the mass percentage amounts shown in Table 1 for Examples 1 to 36 and Comparative Examples 1 to 11, respectively, and mixed evenly, and finally lithium salt (LiPF6) is added to obtain an electrolyte.

[0074] Preparation of positive electrode sheet: The positive electrode active material lithium cobalt oxide (LiCoO2), the conductive agent CNT (Carbon Nanotube, carbon nanotube), and the binder PVDF (polyvinylidene fluoride) are mixed in an organic solvent of N-methylpyrrolidone at a mass ratio of 97:1.5:1.5 to form a uniform positive electrode slurry. This positive electrode slurry is then coated on the positive electrode current collector aluminum foil, followed by drying and cold pressing to obtain the positive electrode sheet.

[0075] Preparation of negative electrode sheet: The negative electrode active material graphite, the conductive agent acetylene black, the binder styrene-butadiene rubber, and the thickener sodium carboxymethyl cellulose are thoroughly stirred and mixed in an appropriate amount of deionized water and organic solvent in a mass ratio of 96:1.2:1.5:1.3 to form a uniform negative electrode slurry; then this negative electrode slurry is coated on the negative electrode current collector copper foil, followed by drying and cold pressing to obtain the negative electrode sheet.

[0076] Production of lithium-ion batteries: PE porous polymer film is used as the separator.

[0077] The positive electrode sheet, separator and negative electrode sheet are stacked in order, with the separator placed between the positive and negative electrodes to act as an isolation. The stacked electrode sheets and separator are then wound to obtain a roll core, which is placed in an aluminum-plastic film bag that has been punched and formed. The electrolyte prepared above is injected into the baked and dried battery cells. After vacuum packaging, standing, formation and other processes, the preparation of the lithium-ion battery is completed.

[0078] Lithium-ion battery testing:

[0079] 45℃ cycle test:

[0080] The test method is: in a constant temperature box at 45±2℃, charge the lithium-ion battery at a constant current and constant voltage of 1C to 4.5V, cut off the current at 0.05C, and then discharge it at 1C to 3V. Perform multiple charge and discharge cycles under the above conditions, and calculate the capacity retention rate of the battery after 600 cycles. There are 5 batteries in each group.

[0081] Capacity retention rate (%) = discharge capacity corresponding to the number of cycles (mAh) / discharge capacity of the third cycle (mAh) * 100%

[0082] The average value of the capacity retention rate of each group of 5 batteries after different cycles is recorded in Table 1.

[0083] 60℃ high temperature storage test

[0084] After the lithium-ion battery is left at 25±2°C for 2 hours, it is charged and discharged at 1C / 0.5C, with a charge and discharge voltage of 3.0-4.5V. The discharge capacity is the first discharge capacity, and then the battery is fully charged. It is then placed in storage at 60°C, and the residual capacity retention rate of the battery after storage is calculated using the following formula: Residual capacity retention rate on day n (%) = (residual discharge capacity on day n) / (first cycle discharge capacity) * 100%; the thickness expansion rate of the battery after storage is calculated using the following formula: Thickness expansion rate on day n (%) = (battery thickness after storage on day n) / (initial battery thickness) * 100%;

[0085] -30℃ low temperature cycle performance test:

[0086] The test method is as follows: charge the lithium-ion battery to 4.5V at 1C constant current and constant voltage in a 25±2℃ constant temperature box, with a cut-off current of 0.05C, and then discharge it to 3V at 0.2C, and record the initial discharge capacity C0. Then place the lithium-ion battery in a 25±2℃ constant temperature box and charge it to 4.5V at 1C constant current and constant voltage, with a cut-off current of 0.05C. After charging, place the lithium-ion battery in a -30±2℃ constant temperature box for 2H, and then discharge it to 3.0V at 0.2C, and record the discharge capacity as C1.

[0087] -30℃ low temperature cycle performance capacity retention rate (%) = C1 / C0*100%

[0088] Thermal shock test:

[0089] Ten batteries were discharged at 1C to 3.0V at room temperature, then charged at 1C constant current and constant voltage to 4.5V, with a cut-off of 0.05C.

[0090] Place the fully charged battery in an oven, raise the temperature to 150°C ± 2°C at a rate of (5°C ± 2°C) / min, and keep warm for 60 minutes;

[0091] The battery passes if it does not catch fire or explode.

[0092] The data from Comparative Example 2 and Comparative Example 1 show that although the addition of Additive A alone in Comparative Example 2 can improve the high-temperature cycle performance, thermal shock performance and high-temperature storage performance of the lithium-ion battery, it deteriorates the low-temperature cycle performance of the lithium-ion battery. This is mainly due to the large impedance of the interfacial film formed by the tetranitrile siloxane compound additive A.

[0093] The data from Comparative Example 3 and Comparative Example 1 show that the addition of boron-containing lithium salt additive B alone in Comparative Example 3 can improve the low-temperature cycle performance of lithium-ion batteries. This is due to the fact that the boron-containing lithium salt additive B can participate in forming a faster and thinner interface film on the surface of the positive and negative electrodes for ion transport.

[0094] Therefore, the data from Comparative Examples 1 to 3 show that when additive A or additive B is added alone, the high-temperature cycle performance, thermal shock performance and high-temperature storage performance of the lithium-ion battery can be improved. However, the degree of improvement is small and still cannot meet the growing market demand.

[0095] The data from Comparative Examples 4 to 5 and Examples 1 to 36 show that only when the tetranitrile siloxane compound additive A and the boron-containing lithium salt additive B are added simultaneously can a better synergistic effect be exerted, significantly improving the comprehensive performance of the lithium-ion battery.

[0096] Furthermore, the data from Comparative Examples 6 to 7 show that when the amount of tetranitrile siloxane compound additive A or the amount of boron-containing lithium salt additive B is too high, the viscosity of the electrolyte will increase, which not only reduces the fluidity of the electrolyte, but also increases the impedance of the interface film, thereby deteriorating the low-temperature cycle performance.

[0097] The data from Examples 1 to 13 show that when the tetranitrile siloxane compound additive A and the boron-containing lithium salt additive B satisfy (a:b) = (0.1 to 5): (0.1 to 5), and 0.1≤b / a≤10, the low-temperature cycle performance, thermal shock performance and high-temperature storage performance of the lithium-ion battery are significantly improved, among which the comprehensive performance of Example 3 is better.

[0098] The data from Examples 16 to 19 and Examples 1 to 4 show that the added tetranitrile siloxane compound additive A can effectively inhibit the decomposition and gas production of ethylene carbonate. However, when the amount of the tetranitrile siloxane compound additive A relative to ethylene carbonate is too low (less than 0.033), the inhibitory effect is not good, resulting in a significant deterioration in the thermal shock and high-temperature storage performance of the lithium-ion battery. For example, the amount of the tetranitrile siloxane compound additive A relative to ethylene carbonate in Example 1 is too low (0.005) and the amount of the tetranitrile siloxane compound additive A relative to ethylene carbonate in Example 2 is too low (0.025); the thermal shock pass rate and 60°C storage-30D thickness expansion rate of Examples 1 to 2 are worse than those of Examples 16 to 19 and Examples 3 to 4.

[0099] The data from Examples 16 to 19 and Examples 3 to 7 show that when the amount of tetranitrile siloxane compound additive A relative to ethylene carbonate is too high (greater than 0.1), the impedance of the interface film formed by it is large, thereby affecting the low-temperature cycling performance of the lithium-ion battery. For example, the -30°C low-temperature cycling performance capacity retention rate of Examples 5 to 7 is worse than the -30°C low-temperature cycling performance capacity retention rate of Examples 17 to 19. Therefore, only when the amount of tetranitrile siloxane compound additive A relative to ethylene carbonate satisfies 0.033≤a / W1≤0.1, the comprehensive performance of the lithium-ion batteries of Examples 3 to 7 and Examples 16 to 19 is better.

[0100] The data from Examples 20 to 26 and Examples 8 to 9 show that when the amount of boron-containing lithium salt additive B relative to lithium hexafluorophosphate is too low (less than 0.067), the comprehensive performance of Examples 8 to 9 is worse than that of Examples 20 to 26.

[0101] The data from Examples 20 to 26 and Example 10 show that when the amount of the boron-containing lithium salt additive B relative to lithium hexafluorophosphate is too high (greater than 0.125), as in Example 10, the comprehensive performance of each embodiment is inferior to that of Examples 20 to 26. Therefore, when the amount of the boron-containing lithium salt additive B relative to lithium hexafluorophosphate satisfies 0.067≤b / W2≤0.125, the boron-containing lithium salt additive B and lithium hexafluorophosphate can exert a good synergistic effect, resulting in the comprehensive performance of the lithium-ion batteries of Examples 20 to 26 being superior to that of Examples 8 to 10.

[0102] The data from Examples 20 to 21 show that the boron-containing lithium salt additive B can inhibit the decomposition of lithium hexafluorophosphate to a certain extent and reduce the amount of lithium hexafluorophosphate used. It can also improve the low-temperature cycle performance, high-temperature cycle performance, thermal shock performance and high-temperature storage performance of lithium-ion batteries.

[0103] The data from Examples 3, 14, and 15 show that when the tetranitrile siloxane compound additive A and the boron-containing lithium salt additive B satisfy (a:b)=(0.1-5):(0.1-5), and 0.1≤b / a≤10; and the amount of the tetranitrile siloxane compound additive A relative to ethylene carbonate satisfies 0.033≤a / W1≤0.1, and the amount of the boron-containing lithium salt additive B relative to lithium hexafluorophosphate satisfies 0.067≤b / W2≤0.125, the tetranitrile siloxane compound additive A and the boron-containing lithium salt additive B can produce a better synergistic effect, that is, Under the condition of significantly improving the low-temperature cycle performance, high-temperature cycle performance, thermal shock performance and high-temperature storage performance of the lithium-ion battery, the amount of ethylene carbonate is increased and the amount of lithium hexafluorophosphate is reduced. This not only better improves the fluidity of the electrolyte and is more conducive to the transmission speed of lithium ions in the electrolyte, but also reduces the production cost of lithium salts, effectively solving the shortage of lithium resources. For example, the amount of ethylene carbonate in Example 15 is significantly more than that in Example 3, and the amount of lithium hexafluorophosphate is less than that in Example 3, so that the comprehensive performance of Example 15 is significantly better than that of Example 3. Furthermore, the applicant has found that when the tetranitrile siloxane compound additive A and the boron-containing lithium salt additive B satisfy (a:b) = (0.1~5): (0.1~5), and 0.1≤b / a≤10, the amount of the tetranitrile siloxane compound additive A relative to ethylene carbonate satisfies 0.033≤a / W1≤0.1, and the amount of the boron-containing lithium salt additive B relative to lithium hexafluorophosphate satisfies 0.067≤b / W2≤0.125, when the amount of vinyl carbonate and lithium hexafluorophosphate is reduced at the same time, the tetranitrile siloxane compound additive A and the boron-containing lithium salt additive B can play a better synergistic role, so that the comprehensive performance of Example 14 is optimized.

[0104] The data from Examples 27 to 36 show that when the tetranitrile siloxane compound additive A and the boron-containing lithium salt additive B can also adopt other structural formulas disclosed in the present invention, they can still achieve a good synergistic effect.

[0105] The above-described embodiments merely represent several embodiments of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be determined by the appended claims.

Claims

1. An electrolyte comprising a lithium salt, an organic solvent and an additive, characterized in that: The additives include a tetranitrile siloxane compound additive A having a structure of formula I and a boron-containing lithium salt additive B having a structure of formula II. The tetranitrile siloxane compound additive A of formula I has the following general formula: Formula I wherein R1 to R6 are selected from any one of methane, ethyl, propane, butane, phenyl, trifluoromethyl, alkenyl, fluoroalkenyl and alkynyl; The boron-containing lithium salt compound additive B of formula II has the following general formula: Ⅱ-Ι Ⅱ-Ⅱ Ⅱ-Ⅲ Ⅱ-Ⅳ Ⅱ-Ⅴ Ⅱ-Ⅵ Wherein, X1, X2, X3, and X4 are selected from any one of an alkane group, a fluorine atom, an oxygen atom, a hydrogen atom, a halogenated alkane group, an alkene group, an alkyne group, an aromatic group, a halogenated aromatic group, and a siloxane group; The mass proportion of the tetranitrile siloxane compound additive A in the electrolyte is defined as a%, and the range of a% is 0.1%≤a%≤5%; The organic solvent comprises at least one ethylene carbonate, and the mass proportion of the ethylene carbonate in the electrolyte is defined as W1%; The mass proportion of the boron-containing lithium salt additive B in the electrolyte is defined as b%, and the range of b% is 0.1%≤b%≤5%. The lithium salt includes at least one LiPF6; The mass proportion of LiPF6 in the electrolyte is defined as W2%; The ratio of a% to W1% is in the range of 0.033≤a / W1≤0.1; The ratio of b% to W2% is in the range of 0.067≤b / W2≤0.

125.

2. The electrolyte according to claim 1, characterized in that The structure of Formula I has the following general formula: 。 3. The electrolyte according to claim 1, characterized in that The ratio of b% to a% is in the range of 0.1≤b% / a%≤10.

4. The electrolyte according to claim 1, characterized in that The range of W1% is 10%≤W1%≤30%.

5. The electrolyte according to claim 1, characterized in that The range of W2% is 8%≤W2%≤15%.

6. A lithium-ion battery, characterized in that: The invention comprises a positive electrode sheet, a negative electrode sheet, a separator and the electrolyte according to any one of claims 1 to 5, wherein the total amount of element B in the electrolyte accounts for not less than 100 ppm by mass in the electrolyte.

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