Adiponitrile-based electrolyte additive composition, electrolyte and lithium ion battery

Abstract

The invention discloses an adiponitrile-based electrolyte additive composition, an electrolyte and a lithium ion battery, and belongs to the technical field of lithium ion batteries, the adiponitrile-based electrolyte additive composition comprises adiponitrile and a functional additive; a single nitrile additive is incompatible with a graphite negative electrode, the negative electrode interface is destroyed in the later stage of high-temperature circulation to cause capacity diving, the negative electrode side reaction is remarkable in the high-temperature storage process, DCR is increased too fast, after the functional additive is added, when adiponitrile is decomposed, the functional additive is also decomposed together, and the capacity diving is caused. A high-quality composite coating film obtained by compounding decomposition products of adiponitrile and a functional additive is formed on the surface of the graphite negative electrode, decomposition of adiponitrile and the functional additive can be inhibited, subsequent electrolyte decomposition reaction is prevented, side reaction is inhibited, structural stability of a positive electrode material is protected, and dissolution of transition metal is reduced. By improving the chemical properties of the electrolyte and the physicochemical characteristics of the surface of the electrode, the problem of serious capacity fading in the high-temperature storage and circulation process of the battery is effectively solved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion battery electrolyte preparation, and particularly relates to a dicyanohexane-based electrolyte additive composition, an electrolyte and a lithium ion battery. BACKGROUND

[0002] Lithium ion batteries are widely used in electronic products, automotive industry and energy storage fields due to their high energy density, long cycle life and no memory effect. However, with the continuous expansion of application fields such as new energy vehicles and wearable devices, people have higher requirements for the high-temperature performance of lithium ion batteries. At present, the commercial lithium ion battery electrolyte is mainly composed of carbonate organic solvents and lithium hexafluorophosphate. Although the ethylene carbonate in the electrolyte can form an effective CEI / SEI film on the positive and negative electrode surfaces, the film has poor stability at high temperature. In addition, ethylene carbonate is easily oxidized and decomposed at high voltage to produce gas; fluorinated ethylene carbonate decomposes at high temperature to produce HF, which corrodes the positive electrode interface, leading to an increase in interface impedance and the dissolution of transition metal ions, causing problems such as battery gas production, internal resistance growth and cycle life decay.

[0003] In order to improve the high-temperature performance of lithium ion batteries, the prior art usually adopts methods such as optimizing lithium salts, optimizing electrolyte solvents and adding key electrolyte additives. However, these methods have certain limitations, such as the introduction of new lithium salts increasing the cost, the low conductivity and poor interface repairability of electrolyte without ethylene carbonate, the incompatibility of single nitrile additive with graphite negative electrode, the destruction of the negative electrode interface at the later stage of high-temperature cycling leading to capacity collapse, and the significant negative electrode side reaction during high-temperature storage and the rapid growth of internal resistance.

[0004] Chinese patent application No. CN110911743A discloses a lithium ion battery electrolyte additive, a lithium ion battery electrolyte and a lithium ion battery. The electrolyte additive of the patent comprises ethylene phosphate and ethylene sulfonyl fluoride, and the electrolyte has good flame retardance. The lithium ion battery using the electrolyte not only has good cycle stability when working at high voltage, but also has good safety performance at high temperature. However, the application still has problems of further improving the flame retardance and high-temperature stability of the electrolyte. Chinese patent application No. CN114865081A discloses an electrolyte additive, an electrolyte and a lithium ion battery. The electrolyte additive of the patent comprises additive A and additive B, wherein additive A is a fluorine-containing lithium salt and additive B is a specific compound. The electrolyte additive can significantly improve the high-temperature cycle performance and high-temperature storage performance of the lithium ion battery while effectively reducing the risk of power performance degradation of the lithium ion battery under the synergistic action of additive A and additive B. However, the application still has problems of improving the cycle performance and high-temperature storage performance of the electrolyte.

[0005] Therefore, the single nitrile additive is incompatible with the graphite negative electrode, the negative electrode interface is damaged in the later stage of high-temperature cycle, the capacity plummets, and the negative electrode side reaction is significant during high-temperature storage, and the internal resistance grows too fast. The existing electrolyte has poor stability in a high-temperature environment, and the positive and negative electrode interface films are prone to decomposition in a high-temperature environment, causing problems such as battery gas production, internal resistance growth, and cycle life attenuation. Although the non-ethylene carbonate electrolyte can inhibit the release of lattice oxygen and reduce gas production, the conductivity is low, the internal resistance is increased, and the interface repairability is poor, and the normal-temperature cycle performance is poor. Although the introduction of new lithium salts such as LiFSI can improve the high-temperature storage performance and cycle performance, it will increase the cost. Although the existing electrolyte additives such as SN, ADN and TMSP can remove HF in the electrolyte and protect the positive electrode from HF corrosion, there are still problems of further improving the flame retardance and high-temperature stability of the electrolyte. It is urgent to develop a new electrolyte additive combination to improve the comprehensive performance of the electrolyte in a high-temperature environment, so as to effectively improve the problems of low capacity retention rate and serious cycle attenuation of lithium ion batteries in the high-temperature storage and cycle process. SUMMARY

[0006] In order to overcome the above-mentioned defects of the prior art, the purpose of the present application is to provide a dicyanohexane-based electrolyte additive composition, electrolyte, method and application, so as to solve the technical problem of poor cycle performance and storage performance of the battery in a high-temperature environment.

[0007] In order to achieve the above-mentioned purpose, the present application adopts the following technical solutions: The present application discloses a dicyanohexane-based electrolyte additive composition, comprising: dicyanohexane and a functional additive, and the general structure formula of the functional additive is: ; Among them, R is selected from substituted or unsubstituted phenyl, alkyl, amino and furan.

[0008] Preferably, in the structural formula of the functional additive: R is independently selected from substituted or unsubstituted alkyl with carbon atom number of 1-3, phenyl substituted or unsubstituted by halogen, amino with carbon atom number of 1-3, and furan with carbon atom number of 1-3.

[0009] Preferably, the functional additive is selected from at least one of the following compounds:

[0010] Preferably, the mass ratio of dicyanohexane to the functional additive is (0.3%-0.5%):(0.5%-1%).

[0011] Preferably, the dicyanohexane-based electrolyte additive composition further comprises fluoroethylene carbonate, vinylene carbonate, lithium bisoxalate borate and lithium difluorophosphate.

[0012] The application further discloses an electrolyte, which comprises an organic solvent, a lithium salt and a adiponitrile-based electrolyte additive; the adiponitrile-based electrolyte additive is the adiponitrile-based electrolyte additive described above.

[0013] Preferably, the organic solvent comprises at least one of methyl ethyl carbonate, ethylene carbonate and dimethyl carbonate; The organic solvent comprises, in terms of volume percentage, 20%-30% of methyl ethyl carbonate, 15%-20% of ethylene carbonate and 30%-50% of dimethyl carbonate.

[0014] Preferably, the lithium salt comprises at least one of lithium hexafluorophosphate and lithium bisfluorosulfonylimide.

[0015] Preferably, the molar concentration of the lithium salt in the electrolyte is 1.0-1.3 mol / L; and in the electrolyte, the mass percentage of the adiponitrile-based electrolyte additive composition, the organic solvent and the lithium salt is (5%-8%):(70%-83%):(12%-16%).

[0016] The application further discloses a lithium ion battery, which is assembled by using the electrolyte described above and a positive electrode sheet, a negative electrode sheet and a separator; the capacity retention rate of the lithium ion battery is 92.5%-94.3% when stored at 60 ℃ for 45 days; and the capacity retention rate is 86.4%-90.6% when cycled at 45 ℃ for 500 cycles.

[0017] Compared with the prior art, the application has the following beneficial effects: The application discloses a kind of adiponitrile-based electrolyte additive compositions, adiponitrile is preferentially oxidized and decomposed on the surface of positive electrode to form CEI film containing cyano, inhibit electrolyte oxidation and transition metal dissolution.Functional additives are coordinated with lithium ions on the surface of negative electrode through isothiocyanate group, and SEI film containing silicon oxygen structure is formed, and its three-dimensional network structure effectively closes active site between graphite layers.Crosslinking reaction occurs between the decomposition products of the two additives at the electrode interface, forming a composite protective layer with gradient distribution, the outer dense structure blocks electrolyte penetration, and the inner porous structure maintains ion transport channel.Synergistic regulation of each group makes the functional additive maintain moderate solubility in electrolyte, which avoids premature decomposition and loss, and ensures the directional adsorption and deposition on the electrode surface.Single adiponitrile (ADN) is incompatible with graphite negative electrode, which can damage the negative electrode interface in the later stage of high-temperature cycle, causing capacity drop, and the negative electrode side reaction is significant during high-temperature storage, and DCR increases too fast.After introducing the functional additive, the two work together to solve the problem of negative electrode side reaction caused by single nitrile additive.During the decomposition of adiponitrile, the functional additive is also decomposed to form a high-quality composite coating on the surface of graphite negative electrode, which is composed of the decomposition products of the two.The composite coating can inhibit the further decomposition of adiponitrile and functional additive, and prevent subsequent electrolyte decomposition reaction, thereby inhibiting side reactions.The composite coating can form stable CEI film and SEI film on the positive electrode and negative electrode of lithium ion battery respectively, reducing the side reactions of positive and negative electrode interfaces under high-temperature working conditions.It can inhibit side reactions and protect the structure stability of positive electrode materials, and reduce the dissolution of transition metals.The problems of low capacity retention rate during high-temperature storage and capacity decay during high-temperature cycle are solved.

[0018] Further, when R is selected as alkyl with 1-3 carbon atoms, the increase in electrolyte viscosity caused by long-chain alkyl can be avoided.When phenyl substituted by halogen is selected, halogen atoms inhibit the penetration of electrolyte decomposition products through steric hindrance effect, and their electronegativity can promote the directional arrangement of functional additives on the electrode surface.When amino with 2-3 carbon atoms is selected, it can form coordination complexes with dissolved transition metal ions (such as Co 2+ 、Mn 2+ ), preventing their reduction and deposition on the negative electrode surface.The synergistic effect of the three types of substituents enables the functional additive to form a dense and ionically conductive interface layer on the surface of the positive electrode, effectively blocking direct contact between the electrolyte and the high-activity positive electrode material.The precise regulation of the molecular structure of the functional additive enables it to form a composite interface film with three-dimensional network structure under high-temperature working conditions, ultimately improving the capacity retention rate and cycle stability of lithium ion batteries in high-temperature environments.

[0019] Further, the appropriate mass ratio is the key to ensure that adiponitrile and functional additives work synergistically, ensuring proper reaction during decomposition and forming a composite coating with good performance. If the proportion is not appropriate, it may lead to excess or insufficient of one side, affecting the formation quality and effect of the composite coating, unable to effectively inhibit the side reaction, reduce the protection effect of the electrode interface, and thus affect the performance of the battery during high-temperature storage and cycling, such as capacity retention rate and capacity decay. The mass ratio relationship of the two additives achieves dynamic balance of the interface film formation process by regulating the decomposition rate. When the content of adiponitrile is less than 0.3%, the complexation of transition metal ions in the positive electrode interface film is insufficient; when it exceeds 0.5%, the unreacted adiponitrile continues to decompose on the negative electrode surface, leading to an increase in interface impedance. When the content of functional additives is less than 0.5%, a complete negative electrode protection layer cannot be formed; when it exceeds 1%, it may cause abnormal increase in the viscosity of the electrolyte. This proportion range allows the positive CEI film and negative SEI film formation processes to proceed synchronously, for example, at the beginning of high-temperature cycling, the functional additives preferentially form a dense passivation layer on the negative electrode, while adiponitrile forms a transition metal complex layer on the positive electrode surface, and the synergistic effect of the two inhibits the subsequent decomposition of the electrolyte.

[0020] Further, fluoroethylene carbonate is preferentially reduced on the negative electrode surface to form a fluorine-rich solid electrolyte interface film, inhibiting the side reaction between the electrolyte and the graphite material at high temperature. Vinylene carbonate undergoes electrochemical polymerization on the positive electrode surface to form a three-dimensional network structure interface layer, effectively blocking the dissolution of transition metal ions and the release of lattice oxygen. Lithium bisoxalate borate decomposes during charging and discharging to produce borate substances, which react with metal oxides on the electrode surface to form a flexible composite interface layer. Lithium difluorophosphate releases PO2F2 - Anions neutralize the free hydrofluoric acid in the electrolyte, reducing the corrosion of acidic substances on the electrode material. The four components form a synergistic effect in three dimensions of positive electrode interface construction, negative electrode interface protection, and electrolyte purification, and together with the adiponitrile-based system, they form a multi-level protection mechanism. The side reactions at the electrode / electrolyte interface under high-temperature conditions are effectively inhibited, and the electrolyte oxidation decomposition caused by transition metal catalysis is reduced. The composite interface film significantly improves the structural stability of the electrode material, reduces the active lithium loss during high-temperature cycling, and thus improves the capacity retention characteristics of the battery under high-temperature storage and cycling conditions.

[0021] The application also discloses an electrolyte, which comprises an organic solvent, a lithium salt and the above-mentioned adiponitrile-based electrolyte additive. The organic solvent, the lithium salt and the additive cooperate with each other to build an electrolyte system with better performance. The additive improves the chemical properties of the electrolyte and the physical and chemical properties of the electrode surface through the formation of a composite coating and other effects, effectively improves the problems of serious capacity attenuation of the battery in the high-temperature storage and cycle process from the aspects of protecting the battery interface, reducing side reactions, reducing internal resistance and improving the stability of the electrolyte. The adiponitrile and the trimethylsilyl isothiocyanate in the electrolyte system form a complementary action mechanism. The adiponitrile preferentially adsorbs and complexes transition metal ions at the positive electrode interface, inhibits the electrolyte oxidative decomposition caused by the release of lattice oxygen, but when it exists alone, it will cause side reactions at the negative electrode interface to cause the rupture of the SEI film. The isothiocyanate group of the trimethylsilyl isothiocyanate has strong adsorption, decomposes to produce sulfides and siloxane substances on the negative electrode surface, forms a dense and elastic composite SEI film, and effectively blocks the direct contact of adiponitrile with the graphite negative electrode. Fluoroethylene carbonate and vinylene carbonate act as auxiliary film formers, oxidize to generate CEI films containing fluorine and olefin structures on the positive electrode surface, and together with the cyanide decomposition products of adiponitrile, form a multilayer protection structure. Lithium bis(oxalato)borate and lithium difluorophosphate cover the active sites on the electrode surface through coordination, reducing the reactivity between the electrolyte and the high-activity electrode material.

[0022] Further, the organic solvent comprises at least one of methyl ethyl carbonate, ethylene carbonate and dimethyl carbonate. These organic solvents have good ability to dissolve lithium salts, can form an electrolyte with certain ion conductivity, and meet the demand of ion conduction for normal work of lithium ion batteries. In cooperation with the adiponitrile-based electrolyte additive, the additive helps to form a stable interface film on the electrode surface, further improves the stability of the electrolyte at high temperature, and reduces the occurrence of side reactions. The synergistic effect of ethylene carbonate and dimethyl carbonate can regulate the film formation characteristics of the electrode interface. The high dielectric constant of ethylene carbonate ensures the complete dissociation of lithium salt, and the low viscosity of dimethyl carbonate promotes the rapid migration of lithium ions. The ratio of the three types of solvents forms a thermodynamically stable solvent network through complementary physicochemical properties, wherein the ratio of ethylene carbonate and dimethyl carbonate can adjust the density of the interface film, and the ratio of methyl ethyl carbonate and dimethyl carbonate can control the balance of the viscosity and polarity of the electrolyte. The multi-component synergistic effect effectively reduces the amount of solvent decomposition gas in the high-temperature cycle process.

[0023] Further, the organic solvents include, in volume percentage, methyl ethyl carbonate 20%-30%, ethylene carbonate 15%-20%, and dimethyl carbonate 30%-50%. Different organic solvents have different properties, and the electrolyte has suitable viscosity, dielectric constant, and boiling point, etc. by being collocated in this proportion. For example, ethylene carbonate helps to form a stable SEI film, and low-viscosity solvents such as dimethyl carbonate can improve the wettability and ion conduction rate of the electrolyte. This proportion range can maintain the stability and good ion conduction ability of the electrolyte in a high-temperature environment, and reduce the performance degradation of the battery caused by solvent problems. Reasonable solvent composition is conducive to the uniform adsorption and decomposition of additives on the electrode surface, forming a better composite coating and enhancing the protection effect on the electrode interface. The balance between the thermodynamic stability and kinetic performance of the electrolyte solvent system under high-temperature conditions is achieved. The optimization of the content of ethylene carbonate reduces the release of lattice oxygen at the positive electrode interface, and the high proportion of dimethyl carbonate reduces the activation energy of the bulk decomposition reaction of the electrolyte. The synergistic effect of each solvent maintains a stable ion transport channel of the electrolyte during high-temperature cycling, while inhibiting the dissolution of transition metals and the rupture of the electrode interface film, thereby prolonging the high-temperature working life of the battery.

[0024] Further, the lithium salt includes at least one of lithium hexafluorophosphate and lithium bisfluorosulfonylimide. These lithium salts can dissociate lithium ions in the organic solvent, provide stable lithium ion conduction for the charging and discharging process of the battery, and ensure the normal operation of the battery. They have good compatibility with the selected organic solvent and adiponitrile-based electrolyte additives, and are not prone to severe side reactions in a high-temperature environment, which together maintain the stability of the electrolyte and help to improve the capacity retention rate and cycle performance of the battery at high temperature. Lithium hexafluorophosphate as the basic lithium salt of the electrolyte maintains ion conductivity, but its defect of easy decomposition at high temperature is compensated by the introduction of lithium bisfluorosulfonylimide. The bisfluorosulfonyl group in lithium bisfluorosulfonylimide preferentially decomposes to form a stable intermediate at high temperature, blocking the chain decomposition reaction of lithium hexafluorophosphate. The synergistic effect of the two lithium salts respectively builds a physical barrier and a chemical stable layer at the positive and negative electrode interfaces, forming a multi-level high-temperature protection mechanism. The electrolyte acidification phenomenon caused by lithium salt decomposition in a high-temperature environment is effectively inhibited, the damage to the electrode structure caused by the dissolution of transition metals is reduced, and the ion conduction stability of the electrolyte system during high-temperature cycling is maintained, solving the problem of battery high-temperature attenuation caused by the performance limitations of single lithium salt.

[0025] Further, the lithium salt concentration of 1.0-1.3 mol / L can ensure sufficient lithium ion concentration in the electrolyte, meet the demand of high capacity and high power output of the battery, and will not cause the viscosity to increase due to too high concentration, affect the ion conduction rate, maintain stable ion conduction at high temperature, and reduce the capacity attenuation caused by the increase of internal resistance. The additive mass percentage of 5%-8% ensures that there is enough adiponitrile and functional additive to react on the electrode surface to form an effective composite coating film to inhibit the side reaction. If the concentration is too low, it may not form a complete coating film, and the protection effect is insufficient; if the concentration is too high, it may cause too many impurities in the electrolyte, affecting the performance of the electrolyte, and this range can achieve the best protection effect and performance improvement.

[0026] The application also discloses a lithium ion battery, which is assembled by adopting the electrolyte, positive and negative electrode sheets and a separator, and has a capacity retention rate of 92.5%-94.3% after being stored at 60 DEG C for 45 days. Compared with the low high-temperature storage capacity retention rate in the prior art, the electrolyte effectively inhibits the side reaction in the high-temperature storage process by forming a stable composite coating film, reduces the capacity loss, and has excellent high-temperature storage stability. The synergistic effect of adiponitrile and functional additives is reflected in the high-temperature cycle process: adiponitrile is preferentially oxidized and decomposed on the positive electrode surface to form a CEI film rich in cyano groups, thereby inhibiting the continuous oxidation of the electrolyte; and the functional additives are reduced and decomposed on the negative electrode surface to form a SEI film containing sulfur, thereby improving the interface stability. The specific ratio combination of ethylene carbonate and methyl ethyl carbonate in the organic solvent forms a solvent system with low viscosity and high stability at high temperature, thereby reducing the solvent decomposition gas. The synergistic control of the lithium salt concentration and the additive content not only guarantees the electrochemical window of the electrolyte, but also reduces the risk of high-temperature side reactions through the thermal stability of lithium bisfluorosulfonylimide. The matching design of the high-nickel ternary material positive electrode and the graphite negative electrode, combined with the high-temperature resistance of the separator, forms a complete battery system architecture. The interface stability of the lithium ion battery under high-temperature working conditions is improved, the composite structure of the positive electrode CEI film and the negative electrode SEI film effectively inhibits the electrolyte decomposition and transition metal dissolution, and the optimized balance of the solvent system and the lithium salt concentration balances the electrical conductivity and the thermal stability, so that the battery can maintain a high capacity retention rate under the conditions of long-term storage at 60 DEG C and 45 DEG C cycle. The problem of fast high-temperature cycle attenuation in the prior art is solved, the application performance of the lithium ion battery at high temperature is significantly improved, and the use temperature range and service life of the battery are widened. DETAILED DESCRIPTION

[0027] The technical solutions of the application will be described below clearly and completely. Obviously, the described embodiments are part of the embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor belong to the protection scope of the application.

[0028] In the present application, all the embodiments and preferred embodiments mentioned in the present application can be combined to form new technical solutions, if not otherwise specified.

[0029] In the present application, all the technical features and preferred features mentioned in the present application can be combined to form new technical solutions, if not otherwise specified.

[0030] In the present application, percentage (%) or part refers to the percentage by weight or weight parts of the composition, if not otherwise specified.

[0031] In the present application, each component or its preferred component involved can be combined to form new technical solutions, if not otherwise specified.

[0032] In the present application, unless otherwise specified, the numerical range "a~b" represents a shorthand notation for any real number combination between a and b, wherein a and b are both real numbers. For example, the numerical range "6~22" represents that all the real numbers between "6~22" have been listed herein, and "6~22" is only a shorthand notation for these numerical combinations.

[0033] The lower limit and upper limit of the range disclosed in the present application can be one or more lower limits and one or more upper limits, respectively.

[0034] In the present application, the term "and / or" used herein means any combination of one or more of the associated listed items and all possible combinations, and includes these combinations.

[0035] In the present application, unless otherwise specified, each reaction or operation step can be carried out sequentially or according to the sequence. Preferably, the reaction method herein is carried out sequentially.

[0036] Unless otherwise specified, the professional and scientific terms used herein have the same meaning as those familiar to those skilled in the art. In addition, any method or material similar or equivalent to the described content can also be applied in the present application.

[0037] In the first aspect of the present application, a adiponitrile-based electrolyte additive composition is provided, comprising adiponitrile (ADN) and a functional additive, and the general structure of the functional additive is: ; wherein R is independently selected from substituted or unsubstituted alkyl with 1-3 carbon atoms, phenyl substituted or unsubstituted by halogen, amino with 1-3 carbon atoms, furan with 1-3 carbon atoms.

[0038] In a second aspect, the present application provides an electrolyte, the electrolyte comprising an organic solvent, a lithium salt and the dinitrile-based electrolyte additive composition of the first aspect.

[0039] The organic solvent comprises at least one of methyl ethyl carbonate, ethylene carbonate and dimethyl carbonate. The organic solvent comprises, in terms of volume percentage, 20-30% of methyl ethyl carbonate, 15-20% of ethylene carbonate and 30-50% of dimethyl carbonate.

[0040] The lithium salt comprises at least one of lithium hexafluorophosphate and lithium bisfluorosulfonylimide.

[0041] The concentration of the lithium salt in the electrolyte is 1.0-1.3 mol / L, the mass percentage of the electrolyte additive in the electrolyte is 5-8%, and the rest is the organic solvent.

[0042] In a third aspect, the present application provides a preparation method of the electrolyte, comprising: under an inert gas atmosphere, adding a functional additive, fluoroethylene carbonate, vinylene carbonate, lithium bisfluorosulfoxylate and lithium difluorophosphate into an organic solvent, stirring and mixing at low temperature to obtain the electrolyte. Under an inert gas atmosphere, adding a lithium salt and an additive composition into an organic solvent, stirring and mixing uniformly to obtain the electrolyte. The inert gas (such as argon) atmosphere can eliminate oxygen, moisture and the like in the air, avoid side reactions such as oxidation and hydrolysis of the lithium salt, the additive and the like with components in the air, and ensure the purity and performance stability of the electrolyte, especially in high-temperature applications, reduce side reactions and capacity decay caused by impurities. Stirring and mixing uniformly enables the lithium salt, the additive and the organic solvent to fully contact, forming a uniform and stable electrolyte system, which is conducive to the additive uniformly playing a role on the electrode surface in the subsequent battery assembly and use process, forming a stable interface film, and improving the consistency and stability of the battery performance.

[0043] In a fourth aspect, the present application provides an electrochemical device, graphite is used as a negative electrode active material, graphite, conductive agent acetylene black, binder CMC and SBR are prepared into a negative electrode slurry at a mass percentage of 96.8:0.5:1.4:1.3, the negative electrode slurry is coated on a copper foil current collector, vacuum dried, and a negative electrode sheet is prepared; NCM811 is used as a positive electrode active material, the positive electrode active material, conductive agent acetylene black and binder PVDF are prepared into a positive electrode slurry at a mass ratio of 96.5:1.2:2.3, the positive electrode slurry is coated on an aluminum foil current collector, vacuum dried, and a positive electrode sheet is prepared; the electrolyte prepared in the examples and the comparative examples is assembled into a 18650 battery together with the above-mentioned positive electrode sheet, negative electrode sheet and separator.

[0044] The prior art battery has the problems of low capacity retention rate and fast high-temperature cycle decay during high-temperature storage. The application provides an electrolyte additive composition, which comprises a nitrile additive (ADN) and a functional additive. The single nitrile additive is incompatible with a graphite negative electrode, destroys the negative electrode interface in the later stage of high-temperature cycle, and causes capacity to drop, and the negative electrode side reaction is significant during high-temperature storage, and the DCR increases too fast. After the functional additive is added, the functional additive will also decompose when the ADN decomposes, and as a result, a high-quality composite coating formed by the decomposition products of the ADN and the functional additive compound is formed on the surface of the graphite negative electrode, and the further decomposition of the ADN and the functional additive is inhibited when the composite coating is formed. The composite coating can prevent subsequent electrolyte decomposition reactions, inhibit side reactions, protect the structure stability of the positive electrode material, and reduce the dissolution of transition metals.

[0045] To make the objects, technical solutions, and advantages of the embodiments of the present application clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are some but not all of the embodiments of the present application. Generally, the components of the embodiments of the present application can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present application is not intended to limit the scope of the claimed present application, but only represents selected embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the scope of protection of the present application.

[0046] Embodiment 1 The embodiment provides a dicyanohexane-based electrolyte additive composition, which comprises a nitrile additive ADN and a functional additive, and further comprises fluoroethylene carbonate, vinylene carbonate, lithium difluoro(oxalato)borate, and lithium difluorophosphate.

[0047] The functional additive is 4-fluorobenzoyl isothiocyanate, and the structural formula is: .

[0048] The embodiment also provides an electrolyte, which is composed of an organic solvent, lithium hexafluorophosphate and the above-mentioned adiponitrile-based electrolyte additive composition. The concentration of lithium hexafluorophosphate in the electrolyte is 1.15 mol / L. The organic solvent includes, in terms of volume percentage, 20% of methyl ethyl carbonate, 20% of ethylene carbonate and 60% of dimethyl carbonate. In terms of the total mass of the electrolyte being 100%, the mass percentage of adiponitrile ADN is 0.5%, the mass percentage of the functional additive is 0.5%, the mass percentage of fluoroethylene carbonate is 5%, the mass percentage of vinylene carbonate is 0.3%, the mass percentage of lithium difluoro(oxalato)borate is 0.5%, the mass percentage of lithium difluorophosphate is 0.8%, the mass percentage of the organic solvent is 77.9%, and the mass percentage of lithium salt is 14.5%.

[0049] The electrolyte is prepared by the following method: under an argon atmosphere, functional additives, fluoroethylene carbonate, vinylene carbonate, lithium difluoro(oxalato)borate and lithium difluorophosphate are added to an organic solvent formed by mixing methyl ethyl carbonate, ethylene carbonate and dimethyl carbonate, and stirred and mixed at 10 DEG C for 2 hours to obtain the electrolyte.

[0050] Example 2 The embodiment provides an electrolyte additive and electrolyte, which are different from those of example 1 only in that the mass percentage of the functional additive is adjusted to 0.7%, and the other conditions are the same as those of example 1.

[0051] Example 3 The embodiment provides an electrolyte additive and electrolyte, which are different from those of example 1 only in that the mass percentage of the functional additive is adjusted to 1%, and the other conditions are the same as those of example 1.

[0052] Example 4 The embodiment provides an adiponitrile-based electrolyte additive composition, which includes: a nitrile additive ADN and a functional additive; and further includes fluoroethylene carbonate, vinylene carbonate, lithium difluoro(oxalato)borate and lithium difluorophosphate. The functional additive is 3-methylbenzoylisothiocyanate, and the structural formula is: .

[0053] The embodiment also provides an electrolyte composed of an organic solvent, lithium bisfluorosulfonylimide and the above-mentioned adiponitrile-based electrolyte additive composition. The concentration of lithium bisfluorosulfonylimide in the electrolyte is 1 mol / L. The organic solvent includes, in terms of volume percentage, 27% of methyl ethyl carbonate, 18% of vinyl carbonate and 55% of dimethyl carbonate. In terms of total mass of the electrolyte being 100%, the mass percentage of adiponitrile ADN is 0.3%, the mass percentage of the functional additive is 0.6%, the mass percentage of fluoroethylene carbonate is 5%, the mass percentage of vinylene carbonate is 0.3%, the mass percentage of lithium bisfluorotetrafluoroborate is 0.5%, the mass percentage of lithium difluorophosphate is 0.8%, the mass percentage of the organic solvent is 80% and the mass percentage of lithium salt is 12.5%.

[0054] Embodiment 5 The embodiment provides an adiponitrile-based electrolyte additive composition, which includes a nitrile additive ADN and a functional additive and further includes fluoroethylene carbonate, vinylene carbonate, lithium bisfluorotetrafluoroborate and lithium difluorophosphate. The functional additive is 2-furfuryl formyl isothiocyanate, and the structural formula is: .

[0055] The embodiment also provides an electrolyte composed of an organic solvent, lithium tetrafluoroborate and the above-mentioned adiponitrile-based electrolyte additive composition. The concentration of lithium tetrafluoroborate in the electrolyte is 1.1 mol / L. The organic solvent includes, in terms of volume percentage, 24% of methyl ethyl carbonate, 21% of vinyl carbonate and 55% of dimethyl carbonate. In terms of total mass of the electrolyte being 100%, the mass percentage of adiponitrile ADN is 0.35%, the mass percentage of the functional additive is 0.7%, the mass percentage of fluoroethylene carbonate is 5%, the mass percentage of vinylene carbonate is 0.3%, the mass percentage of lithium bisfluorotetrafluoroborate is 0.5%, the mass percentage of lithium difluorophosphate is 0.8%, the mass percentage of the organic solvent is 78.35% and the mass percentage of lithium salt is 14%.

[0056] Embodiment 6 The embodiment provides an adiponitrile-based electrolyte additive composition, which includes a nitrile additive ADN and a functional additive and further includes fluoroethylene carbonate, vinylene carbonate, lithium bisfluorotetrafluoroborate and lithium difluorophosphate. The functional additive is acetyl isothiocyanate, and the structural formula is: .

[0057] The embodiment also provides an electrolyte, which is composed of an organic solvent, lithium difluorophosphate and the above-mentioned adiponitrile-based electrolyte additive composition. The concentration of lithium difluorophosphate in the electrolyte is 1.2 mol / L. The organic solvent includes, in terms of volume percentage, 25% of methyl ethyl carbonate, 20% of ethylene carbonate and 55% of dimethyl carbonate. In terms of the total mass of the electrolyte being 100%, the mass percentage of adiponitrile ADN is 0.4%, the mass percentage of the functional additive is 0.8%, the mass percentage of fluoroethylene carbonate is 5%, the mass percentage of vinylene carbonate is 0.3%, the mass percentage of lithium difluorophosphate is 0.8%, the mass percentage of the organic solvent is 77.2% and the mass percentage of lithium salt is 15%.

[0058] Embodiment 7 The embodiment provides an adiponitrile-based electrolyte additive composition, which includes a nitrile additive ADN and a functional additive and further includes fluoroethylene carbonate, vinylene carbonate, lithium difluorophosphate and lithium difluorophosphate. The functional additive is N,N-dimethylcarbamoyl isothiocyanate, and the structural formula is: .

[0059] The embodiment also provides an electrolyte, which is composed of an organic solvent, lithium difluorophosphate and the above-mentioned adiponitrile-based electrolyte additive composition. The concentration of lithium difluorophosphate in the electrolyte is 1.2 mol / L. The organic solvent includes, in terms of volume percentage, 25% of methyl ethyl carbonate, 20% of ethylene carbonate and 55% of dimethyl carbonate. In terms of the total mass of the electrolyte being 100%, the mass percentage of adiponitrile ADN is 0.4%, the mass percentage of the functional additive is 0.8%, the mass percentage of fluoroethylene carbonate is 5%, the mass percentage of vinylene carbonate is 0.3%, the mass percentage of lithium difluorophosphate is 0.8%, the mass percentage of the organic solvent is 77.2% and the mass percentage of lithium salt is 15%.

[0060] Table 1: names and structural formulas of the functional additives corresponding to embodiments 1-7

[0061] Table 1 is the name and structural formula of the functional additive corresponding to embodiments 1-7; from the table, it can be clearly seen that the structure and name of the functional additive selected in each experimental example.

[0062] Comparative Example 1 The comparative example provides an electrolyte, which is different from the embodiment 1 only in that the comparative example does not contain adiponitrile and a functional additive, and is the same as the embodiment 1 in other aspects.

[0063] Comparative Example 2 This comparative example provides an electrolyte which is identical to Example 1 except that the functional additive is not contained in the comparative example.

[0064] Table 2: Comparison of capacity retention of lithium ion batteries prepared from Examples 1-7 and Comparative Examples 1-2

[0065] Table 2: Comparison of capacity retention of lithium ion batteries prepared from Examples 1-7 and Comparative Examples 1-2; from the table, it can be seen that the initial DCR (direct current resistance): the difference is very small, and there is no obvious performance impact; the initial DCR of Examples 1-3 is 18.3-18.4 mΩ, and that of Comparative Example 1 and Comparative Example 2 is 18.5 and 18.3 mΩ, respectively, which are very close in value, and the resistance difference is negligible, and the subsequent performance difference (such as capacity retention) is not caused by the initial resistance. Capacity retention after 60°C storage (45 days): the examples are better than the comparative examples; the capacity retention of Examples 1-7 is 92.5%-94.3%, and that of the comparative examples is 82.3-90.3%. The examples are higher than the comparative examples. Under the condition of high-temperature long-term cycling (60°C, 45 days), the battery capacity of the examples decays more slowly, and the high-temperature stability is better. Capacity retention after 45°C cycling (500 cycles): the examples have more stable advantages; the capacity retention of Examples 1-7 is 86.4%-90.6%, that of Comparative Example 1 is 85.4%, and that of Comparative Example 2 is 77.9%. Under the condition of medium-temperature long-term cycling (45°C, 500 cycles), the capacity retention of the examples is still better than that of the comparative examples, and the optimization direction of the examples (such as the parameters of Example 3) has a positive effect on the cycling stability. Under the premise of close initial resistance, the capacity retention of the lithium ion batteries prepared from Examples 1-7 is higher than that of the comparative examples under the conditions of high-temperature long-term storage (60°C, 45 days) and medium-temperature long-term cycling (45°C, 500 cycles), which indicates that the cycling stability (especially the capacity decay control after long-term use) of the examples is better.

[0066] In summary, the adiponitrile-based electrolyte additive composition and electrolyte provided by the present application can form stable CEI film and SEI film on the positive and negative electrodes of lithium ion batteries respectively by compounding adiponitrile with a functional additive with a specific structure in a suitable ratio, in combination with specific organic solvents, lithium salts and a preparation method, effectively inhibiting the interface side reaction of the positive and negative electrodes at high temperature, protecting the structure of the positive electrode material and reducing the dissolution of transition metals; reasonable component composition and ratio optimize the dissolution, conduction and other properties of the electrolyte, balance various characteristics and ensure that the additive fully plays a role; inert gas protection and suitable stirring conditions are used in the preparation process to ensure the quality of the electrolyte. Ultimately, when the electrolyte is applied to lithium ion batteries, the performance of the battery in the high-temperature storage and cycling process is significantly improved, the capacity retention rate is 92.5% to 94.3% after 45 days of storage at 60 DEG C, and the capacity retention rate is 86.4% to 90.6% after 500 cycles at 45 DEG C, effectively solving the problems of low capacity retention rate and fast high-temperature cycling decay in the prior art. The functional additive improves the chemical properties of the electrolyte and the physical and chemical properties of the electrode surface, effectively improves the capacity decay problem in the high-temperature storage and cycling process of the battery from the aspects of protecting the battery interface, reducing side reactions, reducing internal resistance and improving electrolyte stability.

[0067] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. An adiponitrile-based electrolyte additive composition characterized in that, The application relates to an adiponitrile-based electrolyte additive composition. The functional additive has a general structural formula as follows: ； R is selected from substituted or unsubstituted phenyl, alkyl, amino and furan.

2. The adiponitrile-based electrolyte additive composition of claim 1, wherein, In the structural formula of the functional additive, R is independently selected from substituted or unsubstituted alkyl with 1-3 carbon atoms, phenyl substituted or unsubstituted by halogen, amino with 1-3 carbon atoms and furan with 1-3 carbon atoms. The functional additive is selected from at least one of the following compounds:

3. The adiponitrile-based electrolyte additive composition of claim 1, wherein, The mass ratio of the adiponitrile and the functional additive is (0.3%-0.5%):(0.5%-1%). 。 4. The adiponitrile-based electrolyte additive composition of claim 1, wherein, The adiponitrile-based electrolyte additive composition further comprises fluoroethylene carbonate, vinylene carbonate, lithium bis(oxalato)borate and lithium difluorophosphate.

5. The adiponitrile-based electrolyte additive composition of claim 1, wherein, The application relates to an electrolyte, which comprises an organic solvent, a lithium salt and an adiponitrile-based electrolyte additive.

6. An electrolyte, characterized by The organic solvent comprises at least one of methyl ethyl carbonate, ethylene carbonate and dimethyl carbonate.

7. The electrolyte of claim 6, wherein, The organic solvent comprises, in terms of volume percentage, 20%-30% of methyl ethyl carbonate, 15%-20% of ethylene carbonate and 30%-50% of dimethyl carbonate. The lithium salt comprises at least one of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide.

8. The electrolyte of claim 6, wherein, The molar concentration of the lithium salt in the electrolyte is 1.0-1.3 mol / L; and the mass percentage of the adiponitrile-based electrolyte additive composition, the organic solvent and the lithium salt in the electrolyte is (5%-8%):(70%-83%):(12%-16%).

9. The electrolyte of claim 6, wherein, The electrolyte is used to assemble a lithium ion battery together with a positive electrode sheet, a negative electrode sheet and a separator; the capacity retention rate of the lithium ion battery is 92.5%-94.3% when stored at 60 DEG C for 45 days, and the capacity retention rate is 86.4%-90.6% when cycled at 45 DEG C for 500 cycles.

10. A lithium-ion battery, characterized by, ​

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