Battery electrolyte additives, lithium-ion battery electrolyte, lithium-ion battery

Abstract

The application discloses a battery electrolyte additive, which comprises at least formula I or formula II, wherein R1 and R2 are independently selected from a silicon-containing group, and X is an organic group with 2-20 carbon atoms; and in formula II, R1 and R2 are independently selected from a silicon-containing group, and X1 and X2 are independently selected from an organic group with 2-20 carbon atoms.

Description

Technical Field

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

[0002] Lithium-ion secondary batteries (LIBs) are the most competitive new generation of batteries, known as "green and environmentally friendly energy," and are the technology of choice for addressing contemporary environmental pollution and energy challenges. While LIBs have achieved tremendous success in the high-energy battery field in recent years, consumers still expect batteries with even higher overall performance, which relies on the research and development of new electrode materials and electrolyte systems. Currently, electronic digital products such as smartphones and tablets require increasingly higher energy density, making it difficult for commercial LIBs to meet these demands. Improving battery energy density can be achieved through two approaches: selecting high-capacity and high-density cathode and anode materials; and increasing the battery's operating voltage.

[0003] Pure silicon anodes have a theoretical capacity of up to 4200 mAh / g. However, when used as negative electrodes in lithium-ion secondary batteries, they suffer from severe battery expansion and electrode pulverization due to the volume effect, resulting in poor cycle performance. Furthermore, silicon-based materials have poor electrical conductivity, resulting in poor low-temperature performance. Research is considering combining silicon and carbon materials to form silicon-carbon composites, which could significantly increase the material's specific capacity and conductivity while also reducing the volume effect of silicon-based materials. When paired with a high-capacity, high-nickel cathode, silicon-carbon composites can achieve energy densities exceeding 300 Wh / Kg. Consequently, compatible electrolytes have emerged, becoming a hot topic in lithium-ion secondary battery electrolyte research.

[0004] Fluoroethylene carbonate can form a uniform and stable SEI film on the surface of the silicon-carbon negative electrode. Due to the particularity of the silicon-carbon negative electrode material (battery expansion and serious electrode powdering), its electrolyte system often requires more film-forming additives than the graphite negative electrode system, and usually requires the use of a large amount of fluoroethylene carbonate. However, fluoroethylene carbonate is easily decomposed in a high-temperature environment or a high-nickel positive electrode battery system, and cannot meet the high-temperature use requirements of the battery, etc. The use of fluoroethylene carbonate alone has many disadvantages. In order to solve the flatulence problem of lithium-ion secondary batteries containing fluoroethylene carbonate during high-temperature storage, CN201110157665 adds an organic dinitrile substance (NC-(CH2) n -CN, where n=2-4) to inhibit flatulence. US 2008 / 0311481 Al discloses ether / aryl compounds containing two nitrile groups, which can improve battery flatulence under high voltage and high temperature conditions and improve high-temperature storage performance. However, the use of nitrile compounds in ternary high-nickel cathode material systems can increase battery polarization, severely degrading cycle performance and low-temperature characteristics.

[0005] Technical issues

[0006] The embodiments of the present invention provide a battery electrolyte additive, a lithium-ion battery electrolyte, and a lithium-ion battery, aiming to solve the problem in existing lithium-ion batteries that, under high temperature conditions, the electrolyte oxidizes and decomposes, resulting in a problem in which the high-temperature storage performance and low-temperature discharge performance of the battery cannot be taken into account.

[0007] Technical Solutions

[0008] In order to achieve the above-mentioned object of the invention, the technical solution adopted by the present invention is as follows:

[0009] A battery electrolyte additive, the additive comprising at least Formula I shown below or Formula II shown below,

[0010]

[0011] In formula I, R1 and R2 are independently selected from silicon-containing groups, and X is selected from organic groups having 2 to 20 carbon atoms;

[0012] In formula II, R1 and R2 are independently selected from silicon-containing groups, and X1 and X2 are independently selected from organic groups having 2 to 20 carbon atoms.

[0013] Preferably, R1 and R2 are independently selected from trimethylsilyl and dimethyl-tert-butylsilyl.

[0014] Preferably, X is selected from a methylene-containing organic group having 2 to 6 carbon atoms, a carbonyl-containing organic group having 2 to 20 carbon atoms, a phenyl-containing organic group having 6 to 20 carbon atoms, and a heteroatom-containing organic group having 2 to 20 carbon atoms.

[0015] Preferably, the X is selected from -(CH2) n -, -C(CH3)2CO-, -C(Ph)2-, -CH2CH=CHCH2-, -(CH2) n CO-, -PhCO-, -Ph(CH2) n CO-, -C(C2H5)2CO-, -COC(C3H7)(C3H5)CO-, -C(Ph)[PhOSi(CH3)2]CO-, -COC(C3H7)(C2H5)CO-, -COC(C4H9)(C3H5)CO-, -COC(C5H 11 )(C2H5)CO-, -COC(Ph)(C2H5)CO-, wherein the value of n is a positive integer ranging from 1 to 6.

[0016] Preferably, X1 and X2 are independently selected from an alkyl group having 1 to 5 carbon atoms, a fluorinated alkyl group having 1 to 5 carbon atoms, a phenyl group, a tolyl group, a trifluoromethylphenyl group, a naphthyl group, and a benzoyl group.

[0017] Preferably, the formula I is selected from at least one of the compounds represented by the following structures:

[0018]

[0019] Preferably, the formula II is selected from at least one of the compounds shown in the following structures:

[0020]

[0021] And, a lithium-ion battery electrolyte, the electrolyte comprising a non-aqueous organic solvent, a lithium salt and an additive, wherein the additive is the battery electrolyte additive described in the present invention.

[0022] Preferably, based on the total mass of the electrolyte being 100%, the sum of the mass percentages of Formula I and Formula II is 0.05% to 2%.

[0023] Preferably, the additive further comprises at least one of fluoroethylene carbonate, vinylene carbonate, 1,3-propane sultone, 1,4-butane sultone, 1,3-propylene sultone, vinyl sulfate, and propylene sulfate.

[0024] Preferably, based on the total mass of the electrolyte being 100%, the sum of the mass percentages of the additives is less than or equal to 15%.

[0025] Preferably, the non-aqueous organic solvent includes at least one of ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, γ-butyrolactone, γ-valerolactone, δ-valerolactone, and ε-caprolactone.

[0026] Preferably, based on the total mass of the electrolyte being 100%, the sum of the mass percentages of the non-aqueous organic solvents is 55% to 75%.

[0027] And, a lithium ion battery, the lithium ion battery comprising a positive electrode, a negative electrode, a separator and an electrolyte, and the electrolyte is the lithium ion battery electrolyte described in the present invention.

[0028] Preferably, the active material of the positive electrode is a transition metal oxide; and the active material of the negative electrode is graphite, a Si-containing composite material or lithium titanate.

[0029] Preferably, the transition metal oxide is LiNi x Co y Mn z L (1-x-y-z) O2, wherein L is one of Al, Sr, Mg, Ti, Ca, Zr, Zn, Si and Fe, and the values ​​of x, y and z satisfy: 0≤x<1, 0<y≤1, 0≤z<1, and 0<x+y+z≤1.

[0030] Beneficial effects

[0031] The lithium-ion battery electrolyte provided by the present invention contains at least an additive of the structure shown in Formula I or Formula II. The structures shown in Formula I and Formula II contain an imidazolone structure (one N atom is connected to each side of the carbonyl carbon). On the one hand, the carbonyl group in the imidazolone structure can react with PF5 (a decomposition product of LiPF6) and can be preferentially reduced at the negative electrode of the battery to form a low-impedance SEI film, thereby improving the low-temperature characteristics and power characteristics of the lithium-ion battery. On the other hand, the N atom in the imidazolone structure contains a lone pair of electrons and is easily oxidized to form a protective film by losing electrons at the positive electrode of the battery, thereby inhibiting the oxidative decomposition of the electrolyte at the positive electrode and improving the high-temperature storage performance of the lithium-ion battery. In addition, R1 and R2 contain silicon groups, which can undergo hydrolysis or polymerization reactions with HF and H2O containing active proton hydrogen in the electrolyte, thereby achieving the purpose of removing H2O and inhibiting HF, thereby improving the thermal stability of LiPF6 and improving the high-temperature performance of the battery. In summary, the battery electrolyte additive provided by the present invention, which contains at least an additive having a structure represented by Formula I or Formula II, can form a low-impedance protective film on the surface of the electrode (positive electrode and negative electrode), inhibit the side reactions between the electrode and the electrolyte, reduce the interfacial impedance, and take into account high and low temperature performance (having good low-temperature discharge performance, as well as good cycle performance and high-temperature storage performance), thereby improving the overall output performance of the lithium-ion battery.

[0032] The lithium-ion battery electrolyte provided by the present invention contains the battery electrolyte additive of the present invention, which can form a low-impedance protective film on the surface of the electrodes (positive and negative electrodes), inhibit side reactions between the electrodes and the electrolyte, reduce interfacial impedance, take into account high and low temperature performance (having good low-temperature discharge performance, as well as good cycle performance and high-temperature storage performance), and improve the overall output performance of the lithium-ion battery.

[0033] The lithium-ion battery provided by the present invention, because it contains the lithium-ion battery electrolyte of the present invention, can significantly improve the cycle performance and low-temperature discharge performance of the lithium-ion battery, and improve the overall output performance of the lithium-ion battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 1 is a linear voltammetric sweep (LSV) curve diagram provided by Example 1 and Comparative Example 1 of the present invention;

[0035] Figure 2 is a dQ / dV-V analysis chart of the capacity voltage differential curve provided by Example 1 and Comparative Example 1 of the present application.

[0036] Embodiment of the present application

[0037] In order to make the technical problems to be solved by the present application, technical solutions and beneficial effects more clear, the present application will be further described in detail below in combination with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application, and are not used to limit the present application.

[0038] In the description of the present application, it should be understood that the terms "first", "second" are only for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined with "first", "second" can explicitly or implicitly include one or more of the features. In the description of the present application, the meaning of "a plurality of" is two or more, unless otherwise specifically limited.

[0039] The embodiment of the present application provides an additive for battery electrolyte, the additive at least includes structure as shown in formula I or structure as shown in formula II,

[0040]

[0041] In formula I, R1, R2 are independently selected from silicon-containing groups, and X is selected from organic groups with 2-20 carbon atoms;

[0042] In formula II, R1, R2 are independently selected from silicon-containing groups, and X1, X2 are independently selected from organic groups with 2-20 carbon atoms.

[0043] The lithium ion battery electrolyte provided by the embodiment of the present application contains at least an additive with the structure shown in Formula I or Formula II. The structure shown in Formula I or Formula II contains an imidazolone structure (two N atoms are connected to the carbon of the carbonyl group on both sides), on the one hand, the carbonyl group in the imidazolone structure can react with PF5 (the decomposition product of LiPF6) to preferentially form a low-impedance SEI film on the negative electrode of the battery, thereby improving the low-temperature characteristics and power characteristics of the lithium ion battery; on the other hand, the N atom in the imidazolone structure contains a lone pair of electrons, which is easy to lose electrons to be oxidized into a protective film on the positive electrode of the battery, thereby inhibiting the oxidative decomposition of the electrolyte on the positive electrode and improving the high-temperature storage performance of the lithium ion battery. In addition, the silicon-containing group in R1 and R2 can hydrolyze or polymerize with active proton-containing hydrogen in the electrolyte, such as HF and H2O, to achieve the purpose of removing H2O and inhibiting HF, thereby further improving the thermal stability of LiPF6 and the high-temperature performance of the battery. In summary, the battery electrolyte additive provided by the embodiment of the present application contains at least an additive with the structure shown in Formula I or Formula II, which can form a low-impedance protective film on the surface of the electrode (positive electrode and negative electrode), inhibit the side reaction of the electrode and the electrolyte, reduce the interface impedance, and balance the high-temperature and low-temperature performance (good low-temperature discharge performance, good cycle performance and good high-temperature storage performance), thereby improving the overall output performance of the lithium ion battery.

[0044] In the embodiment of the present application, the additive is mainly used to improve the film forming performance during the first charge and discharge.

[0045] Formula I is a cyclic structure, and Formula II is a linear structure, but both of them contain an imidazolone structure. In addition, R1 and R2 in the structure shown in Formula I or Formula II are independently selected from a silicon-containing group, thereby ensuring the realization of the above technical effects.

[0046] Preferably, the silicon-containing group is a silane group. Specifically preferably, R1 and R2 are independently selected from a trimethylsilyl group and a dimethyl-tert-butylsilyl group. The additive with R1 and R2 selected from a trimethylsilyl group and a dimethyl-tert-butylsilyl group has more excellent effects in improving the cycle performance, high-temperature storage performance and low-temperature discharge performance of the lithium ion battery.

[0047] In Formula I of the embodiment of the present application, X is selected from an organic group with 2-20 carbon atoms. On the one hand, X is an organic group that does not produce active protons, thereby avoiding the influence of reducing the protons on the performance of the battery; on the other hand, the organic group with 2-20 carbon atoms has a suitable molecular structure and good compatibility with non-aqueous organic solvents. If the number of carbon atoms of X is too large, not only the solubility is reduced, but also due to the too large steric hindrance, some reaction sites are shielded, the reaction difficulty is increased, the reaction activity is reduced, and finally the formation of the protective film (SEI) on the electrode surface is affected.

[0048] Preferably, the X is selected from a methylene-containing organic group with 2 to 6 carbon atoms, a carbonyl-containing organic group with 2 to 20 carbon atoms, a phenyl-containing organic group with 6 to 20 carbon atoms, and a heteroatom-containing organic group with 2 to 20 carbon atoms. The preferred X group has a suitable spatial structure and good reactivity with the positive and negative electrodes, which is conducive to forming a low-impedance SEI film at the positive and negative electrodes, improving the low-temperature characteristics and power characteristics of the lithium-ion battery, inhibiting the oxidative decomposition of the electrolyte at the positive electrode, and improving the high-temperature storage performance of the lithium-ion battery. The methylene-containing organic group with 2 to 6 carbon atoms includes but is not limited to -(CH2) n -, wherein n is a positive integer ranging from 1 to 6; the carbonyl-containing organic group having 2 to 20 carbon atoms includes but is not limited to -C(CH3)2CO-, -(CH2) n CO-, -PhCO-, -Ph(CH2) n CO-, -C(C2H5)2CO-, -COC(C3H7)(C3H5)CO-, -C(Ph)[PhOSi(CH3)2]CO-, -COC(C3H7)(C2H5)CO-, -COC(C4H9)(C3H5)CO-, -COC(C5H 11 )(C2H5)CO-, -COC(Ph)(C2H5)CO-, wherein the value of n is a positive integer ranging from 1 to 6; the organic group containing phenyl with 6 to 20 carbon atoms includes but is not limited to -PhCO-, -Ph(CH2) n CO-, -C(Ph)[PhOSi(CH3)2]CO-, -COC(Ph)(C2H5)CO-, wherein the value of n is a positive integer ranging from 1 to 6; the organic group containing heteroatoms with 2 to 20 carbon atoms includes but is not limited to -C(Ph)[PhOSi(CH3)2]CO-, or an organic group containing atoms such as N, S, and O. The introduction of silicon atoms can enhance the effect of "hydrolysis or polymerization reaction with HF and H2O containing active proton hydrogen in the electrolyte to achieve the purpose of removing H2O and inhibiting HF, thereby improving the thermal stability of LiPF6". Preferably, the X is selected from -(CH2) n -, -C(CH3)2CO-, -C(Ph)2-, -CH2CH=CHCH2-, -(CH2) n CO-, -PhCO-, -Ph(CH2) n CO-, -C(C2H5)2CO-, -COC(C3H7)(C3H5)CO-, -C(Ph)[PhOSi(CH3)2]CO-, -COC(C3H7)(C2H5)CO-, -COC(C4H9)(C3H5)CO-, -COC(C5H 11)(C2H5)CO-, -COC(Ph)(C2H5)CO-, wherein the value of n is a positive integer ranging from 1 to 6.

[0049] Specifically preferably, the formula I is selected from at least one of the compounds represented by the following structures:

[0050]

[0051]

[0052] The preferred compound represented by formula I is used as an additive for lithium-ion battery electrolyte, and has a more excellent effect in improving the cycle performance, high-temperature storage performance and low-temperature discharge performance of lithium-ion batteries.

[0053] In Formula II of the embodiment of the present invention, X1 and X2 are independently selected from organic groups with 2 to 20 carbon atoms. On the one hand, X1 and X2 are organic groups that do not generate active protons, thereby avoiding the impact of protons on battery performance. On the other hand, organic groups with 2 to 20 carbon atoms have a suitable molecular structure and good compatibility with non-aqueous organic solvents. If the number of carbon atoms in X1 and X2 is too large, not only will their solubility be reduced, but due to excessive steric hindrance, some reaction sites will be obscured, increasing the difficulty of the reaction, reducing the reaction activity, and ultimately affecting the formation of the electrode surface protective film (SEI).

[0054] Preferably, X1 and X2 are independently selected from an alkyl group with 1 to 5 carbon atoms, a fluorinated alkyl group with 1 to 5 carbon atoms, a phenyl group, a tolyl group, a trifluoromethylphenyl group, a naphthyl group, and a benzoyl group. Preferably, the X1 and X2 groups have a suitable spatial structure and good reactivity with the positive and negative electrodes, which is conducive to the formation of a low-impedance SEI film at the positive and negative electrodes, improving the low-temperature characteristics and power characteristics of the lithium-ion battery, inhibiting the oxidative decomposition of the electrolyte at the positive electrode, and improving the high-temperature storage performance of the lithium-ion battery. On this basis, heteroatoms can also be introduced into X1 and X2, including but not limited to organic groups of atoms such as Si, N, S, and O. Among them, the introduction of silicon atoms can enhance the effect of "hydrolysis or polymerization reaction with HF and H2O containing active proton hydrogen in the electrolyte, achieving the purpose of removing H2O and inhibiting HF, thereby improving the thermal stability of LiPF6".

[0055] The alkyl group having 1 to 5 carbon atoms includes, but is not limited to, methyl, ethyl, propyl, butyl, and pentyl. To avoid excessive steric hindrance, when both X1 and X2 are alkyl groups having 3 or more carbon atoms, linear alkyl groups with a low degree of branching are preferred.

[0056] The fluorinated alkyl group having 1 to 5 carbon atoms includes, but is not limited to, a fluorinated methyl group, a fluorinated ethyl group, a fluorinated propyl group, a fluorinated butyl group, and a fluorinated pentyl group, and specifically may be a trifluoromethyl group or a trifluoroethyl group. To avoid excessive steric hindrance, when both X1 and X2 are fluorinated alkyl groups having 3 or more carbon atoms, a linear fluorinated alkyl group with a low degree of branching is preferably selected.

[0057] Specifically preferably, the formula II is selected from at least one of the compounds represented by the following structures:

[0058]

[0059] The preferred compound represented by formula II is used as an additive for lithium-ion battery electrolyte, and has a more excellent effect in improving the cycle performance, high-temperature storage performance and low-temperature discharge performance of lithium-ion batteries.

[0060] Accordingly, an embodiment of the present invention provides a lithium-ion battery electrolyte, the electrolyte comprising a non-aqueous organic solvent, a lithium salt, and an additive, wherein the additive is the aforementioned battery electrolyte additive. Specifically, the battery electrolyte additive comprises at least Formula I shown below or Formula II shown below.

[0061]

[0062] In formula I, R1 and R2 are independently selected from silicon-containing groups, and X is selected from organic groups having 2 to 20 carbon atoms;

[0063] In formula II, R1 and R2 are independently selected from silicon-containing groups, and X1 and X2 are independently selected from organic groups having 2 to 20 carbon atoms.

[0064] The lithium ion battery electrolyte provided by the embodiment of the present application contains at least an additive with the structure shown in Formula I or Formula II. The structure shown in Formula I or Formula II contains an imidazole ketone structure (two N atoms are connected to the carbonyl carbon on both sides), on the one hand, the carbonyl in the imidazole ketone structure can react with PF5 (LiPF6 decomposition product), can be preferentially reduced at the negative electrode to form a low impedance SEI film, and can improve the low temperature characteristics and power characteristics of the lithium ion battery; on the other hand, the N atom in the imidazole ketone structure contains a lone pair of electrons, is easy to lose electrons and oxidize into a protective film at the positive electrode, inhibits the oxidative decomposition of the electrolyte at the positive electrode, and improves the high temperature storage performance of the lithium ion battery. In addition, the silicon-containing group in R1 and R2 can hydrolyze or polymerize with active proton-containing hydrogen in the electrolyte, such as HF and H2O, to achieve the purpose of removing H2O and inhibiting HF, and further improve the thermal stability of LiPF6 and the high temperature performance of the battery. In summary, the lithium ion battery electrolyte provided by the present application contains at least an additive with the structure shown in Formula I or Formula II, which can form a low impedance protective film on the surface of the electrode (positive electrode and negative electrode), inhibit the side reaction of the electrode and the electrolyte, reduce the interface impedance, and take into account the high and low temperature performance (good low temperature discharge performance, good cycle performance and good high temperature storage performance), and improve the overall output performance of the lithium ion battery.

[0065] The components of the lithium ion battery electrolyte will be described in detail below.

[0066] Non-aqueous organic solvent

[0067] Water has a certain influence on the formation of SEI of the lithium ion battery and the performance of the battery, which is specifically manifested in that the battery capacity becomes smaller, the discharge time becomes shorter, the internal resistance increases, the cycle capacity attenuates, the battery expands, etc. The embodiment of the present application uses a non-aqueous organic solvent as the solvent component of the electrolyte.

[0068] Preferably, the non-aqueous organic solvent includes at least one of ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, γ-butyrolactone, γ-valerolactone, δ-valerolactone, and ε-caprolactone. The addition of the preferred non-aqueous organic solvent can improve the comprehensive performance of the lithium ion secondary battery.

[0069] Further preferably, the sum of the mass percentage contents of the non-aqueous organic solvents is 55% to 75% based on 100% of the total mass of the electrolyte.

[0070] Lithium salt

[0071] In an embodiment of the present invention, the lithium salt may be selected from lithium salts commonly used in lithium-ion batteries, including but not limited to one or more of lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium bisoxalatoborate, lithium bisfluorooxalatoborate, lithium bis(trifluoromethylsulfonyl)imide, and lithium bis(fluorosulfonyl)imide. Furthermore, the lithium salt may be used in an amount of 10% to 18% by weight of the lithium-ion secondary battery electrolyte.

[0072] additive

[0073] In the embodiment of the present invention, the additive is mainly used to improve the film forming performance during the first charge and discharge.

[0074] Formula I is a cyclic structure, while Formula II is a linear structure, but both contain an imidazolone structure. Furthermore, R1 and R2 in the structures shown in Formula I and Formula II are independently selected from silicon-containing groups, thereby ensuring the achievement of the above-mentioned technical effects.

[0075] Preferably, the silicon-containing group is a silyl group. Specifically, preferably, R1 and R2 are independently selected from trimethylsilyl and dimethyl-tert-butylsilyl. Additives in which R1 and R2 are selected from trimethylsilyl and dimethyl-tert-butylsilyl groups have a more excellent effect in improving the cycling performance, high-temperature storage performance, and low-temperature discharge performance of lithium-ion batteries.

[0076] In Formula I of the present invention, X is selected from an organic group with 2 to 20 carbon atoms. On the one hand, X is an organic group that does not generate active protons, thereby avoiding the impact of protons on battery performance. On the other hand, organic groups with 2 to 20 carbon atoms have a suitable molecular structure and good compatibility with non-aqueous organic solvents. If X has too many carbon atoms, not only will its solubility be reduced, but due to excessive steric hindrance, it will block some reaction sites, increase the reaction difficulty, reduce reaction activity, and ultimately affect the formation of the electrode surface protective film (SEI).

[0077] Preferably, the X is selected from a methylene-containing organic group with 2 to 6 carbon atoms, a carbonyl-containing organic group with 2 to 20 carbon atoms, a phenyl-containing organic group with 6 to 20 carbon atoms, and a heteroatom-containing organic group with 2 to 20 carbon atoms. The preferred X group has a suitable spatial structure and good reactivity with the positive and negative electrodes, which is conducive to forming a low-impedance SEI film at the positive and negative electrodes, improving the low-temperature characteristics and power characteristics of the lithium-ion battery, inhibiting the oxidative decomposition of the electrolyte at the positive electrode, and improving the high-temperature storage performance of the lithium-ion battery. The methylene-containing organic group with 2 to 6 carbon atoms includes but is not limited to -(CH2) n -, wherein n is a positive integer ranging from 1 to 6; the carbonyl-containing organic group having 2 to 20 carbon atoms includes but is not limited to -C(CH3)2CO-, -(CH2)n CO-, -PhCO-, -Ph(CH2) n CO-, -C(C2H5)2CO-, -COC(C3H7)(C3H5)CO-, -C(Ph)[PhOSi(CH3)2]CO-, -COC(C3H7)(C2H5)CO-, -COC(C4H9)(C3H5)CO-, -COC(C5H 11 )(C2H5)CO-, -COC(Ph)(C2H5)CO-, wherein the value of n is a positive integer ranging from 1 to 6; the organic group containing phenyl with 6 to 20 carbon atoms includes but is not limited to -PhCO-, -Ph(CH2) n CO-, -C(Ph)[PhOSi(CH3)2]CO-, -COC(Ph)(C2H5)CO-, wherein the value of n is a positive integer ranging from 1 to 6; the organic group containing heteroatoms with 2 to 20 carbon atoms includes but is not limited to -C(Ph)[PhOSi(CH3)2]CO-, or an organic group containing atoms such as N, S, and O. The introduction of silicon atoms can enhance the effect of "hydrolysis or polymerization reaction with HF and H2O containing active proton hydrogen in the electrolyte to achieve the purpose of removing H2O and inhibiting HF, thereby improving the thermal stability of LiPF6". Preferably, the X is selected from -(CH2) n -, -C(CH3)2CO-, -C(Ph)2-, -CH2CH=CHCH2-, -(CH2) n CO-, -PhCO-, -Ph(CH2) n CO-, -C(C2H5)2CO-, -COC(C3H7)(C3H5)CO-, -C(Ph)[PhOSi(CH3)2]CO-, -COC(C3H7)(C2H5)CO-, -COC(C4H9)(C3H5)CO-, -COC(C5H 11 )(C2H5)CO-, -COC(Ph)(C2H5)CO-, wherein the value of n is a positive integer ranging from 1 to 6.

[0078] Specifically preferably, the formula I is selected from at least one of the compounds represented by the following structures:

[0079]

[0080] The preferred compound represented by formula I is used as an additive for lithium-ion battery electrolyte, and has a more excellent effect in improving the cycle performance, high-temperature storage performance and low-temperature discharge performance of lithium-ion batteries.

[0081] In Formula II of the embodiment of the present invention, X1 and X2 are independently selected from organic groups with 2 to 20 carbon atoms. On the one hand, X1 and X2 are organic groups that do not generate active protons, thereby avoiding the impact of protons on battery performance. On the other hand, organic groups with 2 to 20 carbon atoms have a suitable molecular structure and good compatibility with non-aqueous organic solvents. If the number of carbon atoms in X1 and X2 is too large, not only will their solubility be reduced, but due to excessive steric hindrance, some reaction sites will be obscured, increasing the difficulty of the reaction, reducing the reaction activity, and ultimately affecting the formation of the electrode surface protective film (SEI).

[0082] Preferably, X1 and X2 are independently selected from an alkyl group with 1 to 5 carbon atoms, a fluorinated alkyl group with 1 to 5 carbon atoms, a phenyl group, a tolyl group, a trifluoromethylphenyl group, a naphthyl group, and a benzoyl group. Preferably, the X1 and X2 groups have a suitable spatial structure and good reactivity with the positive and negative electrodes, which is conducive to the formation of a low-impedance SEI film at the positive and negative electrodes, improving the low-temperature characteristics and power characteristics of the lithium-ion battery, inhibiting the oxidative decomposition of the electrolyte at the positive electrode, and improving the high-temperature storage performance of the lithium-ion battery. On this basis, heteroatoms can also be introduced into X1 and X2, including but not limited to organic groups of atoms such as Si, N, S, and O. Among them, the introduction of silicon atoms can enhance the effect of "hydrolysis or polymerization reaction with HF and H2O containing active proton hydrogen in the electrolyte, achieving the purpose of removing H2O and inhibiting HF, thereby improving the thermal stability of LiPF6".

[0083] The alkyl group with 1 to 5 carbon atoms includes, but is not limited to, methyl, ethyl, propyl, butyl, and pentyl. To avoid excessive steric hindrance, when both X1 and X2 are alkyl groups with 3 or more carbon atoms, linear alkyl groups with low branching are preferably selected.

[0084] The fluorinated alkyl group having 1 to 5 carbon atoms includes, but is not limited to, a fluorinated methyl group, a fluorinated ethyl group, a fluorinated propyl group, a fluorinated butyl group, and a fluorinated pentyl group, and specifically may be a trifluoromethyl group or a trifluoroethyl group. To avoid excessive steric hindrance, when both X1 and X2 are fluorinated alkyl groups having 3 or more carbon atoms, a linear fluorinated alkyl group with a low degree of branching is preferably selected.

[0085] Specifically preferably, the formula II is selected from at least one of the compounds represented by the following structures:

[0086]

[0087] The preferred compound represented by formula II is used as an additive for lithium-ion battery electrolyte, and has a more excellent effect in improving the cycle performance, high-temperature storage performance and low-temperature discharge performance of lithium-ion batteries.

[0088] On the basis of the above-mentioned embodiments, further preferably, the total mass percentage of the compounds of Formula I and Formula II is 0.05% to 2% based on the total mass of the electrolyte. If the mass percentage of the compounds of Formula I and Formula II is less than 0.05%, a stable protective film cannot be formed on the electrode surface, the improvement effect of "inhibiting the side reaction of the electrode and the electrolyte, reducing the interface impedance, and comprehensively improving the output performance of the battery" cannot be achieved, and in addition, the low-temperature discharge performance of the high-nickel and silicon-carbon system cannot be improved. If the mass percentage of the compounds of Formula I and Formula II is greater than 2%, the protective film formed on the electrode surface is too thick, the battery polarization increases, and the battery performance deteriorates.

[0089] In the embodiments of the present application, on the basis of the compounds of Formula I and Formula II as additives, other additives can be further added to optimize the performance of the lithium ion battery. Preferably, the additives further include at least one of fluoroethylene carbonate, vinylene carbonate, 1,3-propane sultone, 1,4-butane sultone, 1,3-propylene sulfite, ethylene sulfate, and propylene sulfate.

[0090] Further preferably, the total mass percentage of the additives is less than or equal to 15% based on the total mass of the electrolyte. On this basis, the mass percentage of any preferred additive (fluoroethylene carbonate, vinylene carbonate, 1,3-propane sultone, 1,4-butane sultone, 1,3-propylene sulfite, ethylene sulfate, and propylene sulfate) alone accounts for 0.1% to 10% of the total mass of the electrolyte.

[0091] In addition, the embodiments of the present application provide a lithium ion battery, which includes a positive electrode, a negative electrode, a separator, and an electrolyte, and the electrolyte is the lithium ion battery electrolyte of the present application.

[0092] The lithium ion secondary battery provided by the embodiments of the present application can significantly improve the cycle performance and low-temperature discharge performance of the lithium ion battery and improve the overall output performance of the lithium ion battery due to the presence of the lithium ion secondary battery electrolyte of the present application.

[0093] In the embodiments of the present application, the composition of the electrolyte, the selection, content, and preferred type of each component, and the basis for the selection are as described above, and will not be described again here in order to save space.

[0094] The positive electrode includes a positive electrode active material, and the positive electrode active material commonly used in lithium ion batteries can be used in the embodiments of the present application. Preferably, the active material of the positive electrode is a transition metal oxide.

[0095] Specifically preferably, the transition metal oxide is LiNi x Co y Mnz L (1-x-y-z) O2, wherein L is one of Al, Sr, Mg, Ti, Ca, Zr, Zn, Si and Fe, and x, y, z satisfy 0≤x<1, 0<y≤1, 0≤z<1, and 0<x+y+z≤1.

[0096] The negative electrode comprises a negative electrode active material. The negative electrode active material commonly used in lithium ion batteries can be used in the embodiments of the present application. Preferably, the active material of the negative electrode is graphite, a Si-containing composite material or lithium titanate.

[0097] The separator can be selected from, but not limited to, a single-layer polyethylene (PE) separator, a single-layer polypropylene (PP) separator, a double-layer PP / PE separator, a three-layer PP / PE / PP separator or a ceramic separator.

[0098] The embodiments will be described below in detail.

[0099] In the embodiments, the Chinese explanations of the English abbreviations are as follows:

[0100] EC: ethylene carbonate

[0101] EMC: ethyl methyl carbonate

[0102] DMC: dimethyl carbonate

[0103] LiPF6: lithium hexafluorophosphate

[0104] FEC: fluoroethylene carbonate

[0105] DTD: ethylene sulfate

[0106] PS: 1,3-propane sultone

[0107] PST: 1,3-propene sultone

[0108] In the embodiments, the structures of the additives used and the corresponding letter codes are shown in Table 1.

[0109] Table 1

[0110]

[0111]

[0112]

[0113] Embodiment 1

[0114] A lithium-ion secondary battery comprises a positive electrode, a negative electrode, a separator and an electrolyte, wherein the positive electrode active material is lithium nickel cobalt manganese oxide (NCM811) material; the negative electrode active material is a silicon-carbon composite material (Si / C), and a method for preparing the lithium-ion secondary battery comprises the following steps:

[0115] The positive electrode active material NCM811, conductive carbon black and binder polyvinylidene fluoride are mixed in a mass ratio of 96.8:2.0:1.2 and dispersed in N-methyl-2-pyrrolidone to obtain a positive electrode slurry. The positive electrode slurry is evenly coated on both sides of an aluminum foil, and then dried, rolled and vacuum-dried. An aluminum lead wire is welded with an ultrasonic welder to obtain a positive electrode plate (positive electrode sheet). The thickness of the plate is between 100 and 115 μm.

[0116] A silicon-carbon composite material, conductive carbon black, a binder of styrene-butadiene rubber, and carboxymethyl cellulose are mixed in a mass ratio of 96:1:1.2:1.8, dispersed in deionized water, to obtain a negative electrode slurry, which is then coated on both sides of a copper foil, dried, rolled, and vacuum-dried, and then welded with nickel lead wires using an ultrasonic welder to obtain a negative electrode plate (negative electrode sheet), with a thickness of the plate between 115 and 135 μm;

[0117] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a mass ratio of 2:1:7, and after mixing, 12.5% ​​of lithium hexafluorophosphate based on the total mass of the electrolyte and 1% of P1 based on the total mass of the electrolyte were added to prepare an electrolyte.

[0118] A ceramic diaphragm prepared by single-side coating of Al2O3.

[0119] The prepared positive electrode sheet, separator, and negative electrode sheet are placed on an automatic winding machine and wound to obtain a bare battery cell; the bare battery cell is placed in a cylindrical steel shell, the negative electrode tab and the cap tab are welded, and the prepared electrolyte is injected into the dried battery cell. The battery cell is sealed, allowed to stand, pre-charged, aged, and capacity divided to complete the preparation of the lithium-ion secondary battery (18650-3.0Ah).

[0120] Example 2 to Example 35

[0121] In Examples 2 to 26, except for the different components in the electrolyte, the preparation of the remaining positive electrodes, negative electrodes, separators, and lithium-ion secondary batteries is the same as in Example 1. The selection and content of the compound represented by structural formula I in each example are shown in Table 2.

[0122] In Examples 27 to 35, in order to further improve the comprehensive output performance of the battery, film-forming additives were added on the basis of Examples 1 to 26 above. The selection of each component and its content are shown in Table 3.

[0123] Comparative Example 1 to Comparative Example 5

[0124] In Comparative Examples 1 to 5, except for the different types and contents of the non-aqueous organic solvent and additives in the electrolyte (based on the total mass of the electrolyte), the preparation of the remaining positive electrodes, negative electrodes, separators, and lithium-ion secondary batteries was the same as in Example 1. The types and contents of the non-aqueous organic solvent and additives in Comparative Example 1 are shown in Table 2, and the types and contents of the non-aqueous organic solvent and additives in Comparative Examples 2-4 are shown in Table 3.

[0125] The lithium ion secondary batteries prepared in Examples 1 to 35 and Comparative Examples 1 to 5 were subjected to performance tests, and the test methods were as follows:

[0126] 1) Linear sweep voltammetry (LSV)

[0127] With Pt as the working electrode and Li as the counter electrode and reference electrode, a three-electrode device was assembled and linear scanning was performed on an electrochemical workstation.

[0128] 2) Cycling performance test: At 25±2℃ / 45℃±2℃, charge the divided battery to 4.2V at 0.5C constant current and constant voltage (cut-off current is 0.01C), and then discharge it to 2.75V at 1C constant current. After N charge / discharge cycles, calculate the Nth cycle capacity retention rate. The calculation formula is as follows:

[0129] Nth cycle capacity retention rate (%) = (Nth cycle discharge capacity / 1st cycle discharge capacity) × 100%;

[0130] 3) High-temperature storage performance: The divided battery was charged to 4.2V at room temperature using a constant current and constant voltage of 0.5C (cut-off current of 0.01C). The initial discharge capacity of the battery was measured. After storage at 60°C for 7 days, the battery was discharged to 2.75V at 0.5C to measure the battery's retention capacity and recovery capacity. The calculation formula is as follows:

[0131] Battery capacity retention rate (%) = retained capacity / initial capacity × 100%;

[0132] Battery capacity recovery rate (%) = recovery capacity / initial capacity × 100%.

[0133] 4) Low-temperature discharge: Charge at room temperature at 0.5C constant current and constant voltage to 4.2V, let it sit for 5 minutes, and then discharge at 0.2C to 2.75V. Measure the initial capacity of the battery. Let it sit for 5 minutes, then charge at 0.5C constant current and constant voltage to 4.2V (cut-off current is 0.01C). Place the battery in a -20°C low-temperature box for 6 hours, and under this condition, discharge at 0.2C to 2.75V. Measure the discharge capacity at low temperature.

[0134] Low-temperature discharge retention rate (%) = low-temperature discharge capacity / initial capacity × 100%.

[0135] The test results are shown in Tables 2 and 3 below.

[0136] Table 2

[0137]

[0138]

[0139]

[0140]

[0141]

[0142]

[0143] A comparative analysis was conducted between Examples 1-26, whose battery systems consisted of NCM811 and silicon-carbon composite materials, and Comparative Example 1. Table 2 shows that Examples 1-26, employing the technical solutions of the present invention, exhibited excellent cycling performance, high-temperature storage performance, and low-temperature discharge performance. In contrast, the lithium-ion battery employing the electrolyte of Comparative Example 1 exhibited poor output performance, failing to balance high- and low-temperature performance with cycling performance.

[0144] Specifically, when comparing each Example with Comparative Example 1, Examples 1 to 26 containing compounds of Structural Formula I / Formula II have significantly better low-temperature discharge performance, high-temperature cycling, room-temperature cycling, and high-temperature storage performance than Comparative Example 1. This indicates that the presence of P1, P2, P3, P4, P5, P6, P7, P8, P9, P10, P11, P12, P13, P14, P15, P1+P11, and P2+P14 can effectively improve the overall output performance of the battery.

[0145] The electrolytes of Comparative Example 1 and Example 1 were used to perform LSV evaluation and capacity differential curve (d Q / d V~V) analysis. The results are shown in Figure 1 、 Figure 2 As shown. Figure 1 It can be seen that the oxidation peak of the sample containing P1 is earlier, which indicates that the additive is oxidized first before other solvents, further indicating that P1 is easily oxidized at the positive electrode to form a protective film, inhibiting the side reaction between the electrolyte and the high nickel material. Figure 2 It can be seen that samples containing P1 preferentially undergo EC reduction at the negative electrode, and the EC reduction and decomposition of samples containing P1 are suppressed, as evidenced by a decrease in peak intensity. Based on the dQ / dV~V and LSV structures, it can be determined that P1 can form films at both the positive and negative electrodes.

[0146] Table 3

[0147]

[0148]

[0149]

[0150] In order to further improve the comprehensive output performance of the battery, the film-forming additives: fluorocarbonate, 1,3-propane sulfolane, 1,3-propylene sulfonate lactone, and ethylene sulfate are added to the batteries of Examples 27 to 35 in Table 3 on the basis of the batteries of Examples 1 to 26.

[0151] As shown in Table 3, the batteries of Examples 27 to 35 containing the compound of structural formula I / II in the technical solution of the application also have good cycle performance, high-temperature storage performance, and low-temperature discharge performance; and the batteries using the electrolytes of Comparative Examples 2 to 5 have poor partial output performance and cannot simultaneously consider high-temperature and low-temperature performance and cycle performance.

[0152] In summary, through comparison of the examples and the comparative examples, it is found that the lithium ion secondary battery containing the non-aqueous electrolyte has good battery output performance by adding the compound of structural formula I / II to form a protective film on the positive and negative electrodes. This technical solution has obvious improvement effect when applied to the high-nickel positive electrode combined with the silicon-carbon composite negative electrode system.

[0153] The above only describes the preferred embodiments of the application and is not intended to limit the application. Any modification, equivalent replacement, and improvement within the spirit and principle of the application should be included in the protection scope of the application.

Claims

1. A battery electrolyte additive, characterized in that: The additive comprises at least the following formula I: Formula I In Formula I, R1 and R2 are independently selected from silicon-containing groups, the silicon-containing group is a silyl group, and X is selected from an organic group having 2 to 20 carbon atoms; The formula I is selected from at least one of the compounds shown in the following structures: 。 2. A lithium ion battery electrolyte, characterized in that: The electrolyte comprises a non-aqueous organic solvent, a lithium salt and an additive, wherein the additive is the battery electrolyte additive according to claim 1.

3. The lithium-ion battery electrolyte according to claim 2, wherein Based on the total mass of the electrolyte being 100%, the mass percentage of the compound of formula I is 0.05% to 2%.

4. The lithium-ion battery electrolyte according to claim 2, wherein The additive further comprises at least one of fluoroethylene carbonate, vinylene carbonate, 1,3-propane sultone, 1,4-butane sultone, 1,3-propylene sultone, vinyl sulfate, and propylene sulfate.

5. The lithium-ion battery electrolyte according to claim 4, wherein Taking the total mass of the electrolyte as 100%, the sum of the mass percentages of the additives is less than or equal to 15%.

6. The lithium-ion battery electrolyte according to any one of claims 2 to 5, wherein The non-aqueous organic solvent includes at least one of ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, γ-butyrolactone, γ-valerolactone, δ-valerolactone, and ε-caprolactone.

7. The lithium-ion battery electrolyte according to claim 6, wherein Taking the total mass of the electrolyte as 100%, the total mass percentage of the non-aqueous organic solvent is 55% to 75%.

8. A lithium ion battery, characterized in that: The lithium-ion battery comprises a positive electrode, a negative electrode, a separator and an electrolyte, and the electrolyte is the lithium-ion battery electrolyte according to any one of claims 2 to 7.

9. The lithium-ion battery according to claim 8, wherein The active material of the positive electrode is a transition metal oxide; the active material of the negative electrode is graphite, a Si-containing composite material or lithium titanate.

10. The lithium ion battery according to claim 9, wherein The transition metal oxide is LiNi x Co y Mn z L (1-x-y-z) O2, wherein L is one of Al, Sr, Mg, Ti, Ca, Zr, Zn, Si and Fe, and the values ​​of x, y and z satisfy: 0≤x<1, 0<y≤1, 0≤z<1, and 0<x+y+z≤1.

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