An organic phosphorus additive, an electrolyte and a preparation method thereof, and a lithium ion battery

Abstract

The application provides an organic phosphorus additive, an electrolyte and a preparation method thereof and a lithium ion battery, relates to the technical field of lithium ion batteries, and the organic phosphorus additive is used for the electrolyte of a lithium ion battery, and the organic phosphorus additive comprises at least one organic phosphorus compound containing a P=S double bond.The additive of the lithium ion battery electrolyte of the application is an organic phosphorus compound containing a P=S double bond, which can effectively improve the flame retardant performance of the electrolyte, and on the other hand, can form a protective film on the surface of an electrode, the protective film can inhibit the contact reaction of the electrode and the electrolyte, reduce the decomposition of the electrolyte and the corrosion of the electrode surface, improve the high-voltage resistance of the battery, make the cycle performance more stable, can improve the safety of the battery while ensuring that the battery has good electrochemical performance.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically to an organophosphorus additive, an electrolyte and its preparation method, and a lithium-ion battery. Background Technology

[0002] With the widespread application of lithium-ion batteries in electric vehicles, energy storage systems, and other fields, their safety has become an increasingly important concern. Traditional lithium-ion battery electrolytes often use organic carbonates as solvents, which are flammable. If the battery experiences thermal runaway, it can easily cause a fire or even an explosion, seriously threatening life and property. Therefore, developing flame-retardant electrolytes has become one of the key approaches to solving battery safety issues.

[0003] Among numerous flame retardants, phosphate esters have been widely studied due to their excellent flame-retardant properties. For example, trimethyl phosphate (TMP) and triethyl phosphate (TEP) can reduce self-extinguishing time and improve flame-retardant effects in electrolytes. However, phosphate esters suffer from severe incompatibility with graphite anodes. On one hand, graphite has catalytic activity, which can accelerate the decomposition of phosphate esters, leading to changes in electrolyte composition and affecting battery performance. On the other hand, phosphate esters have strong coordination with lithium ions. During lithium ion intercalation into graphite, phosphate esters are intercalated along with the lithium ions, causing the graphite electrode to peel off, damaging the electrode structure, and resulting in a significant decline in the battery's electrochemical performance, such as low initial coulombic efficiency, rapid capacity decay, and shortened cycle life. This often limits the addition of phosphate esters to electrolytes in practical applications, preventing them from fully realizing their flame-retardant advantages. A large amount of flammable carbonate solvents still dominate the electrolyte, and safety hazards remain. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the present invention provides an organophosphorus additive, an electrolyte, a preparation method thereof, and a lithium-ion battery. This not only effectively improves the flame retardancy of the electrolyte but also addresses the poor compatibility between traditional phosphate esters and graphite anodes, thereby enhancing the safety of lithium-ion batteries while ensuring their excellent electrochemical performance.

[0005] To achieve the above objectives, in a first aspect, the present invention provides an organophosphorus additive for use in lithium-ion battery electrolytes, wherein the organophosphorus additive comprises at least one organophosphorus compound containing a P=S double bond.

[0006] Preferably, the organophosphorus compound further contains C2-C4 branches.

[0007] Preferably, the organophosphorus additive is selected from one or more of organophosphorus compound 1, organophosphorus compound 2, organophosphorus compound 3 and organophosphorus compound 4; wherein the chemical formula of organophosphorus compound 1 is shown as formula (I), the chemical formula of organophosphorus compound 2 is shown as formula (II), the chemical formula of organophosphorus compound 3 is shown as formula (III), and the chemical formula of organophosphorus compound 4 is shown as formula (IV).

[0008] (I) (II)

[0009] (III) (IV).

[0010] Preferably, the second organophosphorus compound is prepared by an oxidation reaction between the first organophosphorus compound and benzoyl peroxide.

[0011] Preferably, in the oxidation reaction, the molar ratio of organophosphorus compound I to benzoyl peroxide is 1:(1-1.5).

[0012] Preferably, the specific steps of the oxidation reaction include: first, slowly adding benzoyl peroxide dropwise to organophosphorus compound one at 0-10°C; after the addition is complete, raising the temperature to room temperature and continuing the reaction for 2-4 hours.

[0013] In a second aspect, the present invention also provides an electrolyte comprising the organophosphorus additives as described in the first aspect.

[0014] Preferably, by mass percentage, it comprises: 70-85% organic solvent, 10-25% lithium salt, and 0.5-5% of the organophosphorus additive.

[0015] Preferably, the organic solvent is selected from one or more of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC).

[0016] Preferably, the organic solvent is composed of ethylene carbonate EC, dimethyl carbonate DMC, and ethyl methyl carbonate EMC.

[0017] Preferably, the mass ratio of ethylene carbonate EC, dimethyl carbonate DMC and ethyl methyl carbonate EMC is (1-3):(2-4):(3-5).

[0018] Preferably, the lithium salt is lithium hexafluorophosphate (LiPF6) and / or lithium bisfluorosulfonyl imide (LiFSI).

[0019] Preferably, the organophosphorus additive is selected from one or more of organophosphorus compounds I, II, III, and IV; Among them, the chemical formula of organophosphorus compound one is shown in formula (I), the chemical formula of organophosphorus compound two is shown in formula (II), the chemical formula of organophosphorus compound three is shown in formula (III), and the chemical formula of organophosphorus compound four is shown in formula (IV).

[0020] Thirdly, the present invention also provides a method for preparing the electrolyte as described in the second aspect, the method comprising adding an organophosphorus additive to an organic solvent in which lithium salt is dissolved, to obtain the electrolyte.

[0021] Fourthly, the present invention also provides a lithium-ion battery, comprising the electrolyte as described in the second aspect, or an electrolyte prepared by the preparation method described in the third aspect.

[0022] Compared with the prior art, the advantages of the present invention are as follows: (1) The additive in the lithium-ion battery electrolyte of the present invention is an organophosphorus compound containing P=S double bonds. On the one hand, it can effectively improve the flame retardant performance of the electrolyte; on the other hand, it can form a protective film on the electrode surface. This protective film can inhibit the contact reaction between the electrode and the electrolyte, reduce electrolyte decomposition and electrode surface corrosion, improve the battery's high-voltage resistance, and make the cycle performance more stable. Therefore, the organophosphorus additive of this application can improve battery safety while ensuring that the battery has good electrochemical performance.

[0023] (2) The branched chains in the organophosphorus compounds of the present invention will participate in the formation process of the SEI film. The generated passivation film contains organic sulfur lithium compounds such as ROSO2Li, RSO3Li, compounds containing PS or P=S bonds, and inorganic S-type compounds such as Li2SO3 and Li2S. These products are highly flexible, have strong mechanical properties, and have strong lithium ion conduction capabilities. They can prevent the passivation film from cracking and remodeling due to volume expansion during repeated lithium insertion and delithiation at the positive electrode of high-voltage batteries. They have the advantages of low impedance and strong cycle resistance. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] This invention provides an organophosphorus additive, an electrolyte, a preparation method thereof, and a lithium-ion battery. The organophosphorus additive can not only effectively improve the flame retardancy of the electrolyte, but also form a protective film on the electrode surface, improving the poor compatibility between traditional phosphate esters and graphite anodes. Thus, while improving battery safety, it ensures that the battery has good electrochemical performance, solving the technical defects of conventional electrolytes in the prior art, which have low safety performance and reduce the electrochemical performance of lithium-ion batteries.

[0026] To achieve the above-mentioned technical effects, the overall concept of this invention is as follows: In a first aspect, embodiments of the present invention provide an organophosphorus additive for use in the electrolyte of a lithium-ion battery, wherein the organophosphorus additive comprises at least one organophosphorus compound containing a P=S double bond.

[0027] The additive in the lithium-ion battery electrolyte of this invention includes at least one organophosphorus compound containing a P=S double bond. Due to the presence of the P=S double bond, when the organophosphorus additive of this invention is added to the lithium-ion battery electrolyte, it effectively improves the flame-retardant properties of the electrolyte. Furthermore, because the P=S double bond energy is moderate, it is easily reduced / oxidized and broken on the electrode surface, preferentially undergoing electrochemical reactions before the electrolyte solvent. The reaction products mainly include phosphorus-containing sulfides (such as Li3PS4, Li2S), phosphorus oxides (Li3PO4), and a small amount of organosulfides. The reaction products deposit on the electrode surface to form a dense protective film. This protective film is a lithium-ion conductor and an electronic insulator, isolating the electrode from direct contact with the electrolyte, inhibiting solvent co-intercalation and decomposition, reducing electrolyte decomposition, and maintaining a stable film structure that is not easily broken under high voltage. This reduces electrode corrosion, improves the battery's high-voltage tolerance, and makes cycle performance more stable. Therefore, the organophosphorus additive of this application can improve battery safety while ensuring good electrochemical performance.

[0028] Preferably, the organophosphorus compound further contains C2-C4 branches.

[0029] The branched chains in the organophosphorus compounds of this invention participate in the formation process of the solid electrolyte interphase (SEI) film. The generated SEI passivation film contains organic sulfur lithium compounds such as ROSO2Li, RSO3Li, compounds containing PS or P=S bonds, and inorganic sulfur compounds such as Li2SO3 and Li2S. These products have high flexibility, strong mechanical properties, and strong lithium-ion conduction ability, which can avoid cracking and remodeling of the passivation film due to volume expansion during repeated lithium insertion and delithiation at the positive electrode of high-voltage batteries. It has the advantages of low impedance and strong cycle resistance.

[0030] Preferably, the organophosphorus additive is selected from one or more of organophosphorus compound 1, organophosphorus compound 2, organophosphorus compound 3 and organophosphorus compound 4; wherein the chemical formula of organophosphorus compound 1 is shown as formula (I), the chemical formula of organophosphorus compound 2 is shown as formula (II), the chemical formula of organophosphorus compound 3 is shown as formula (III), and the chemical formula of organophosphorus compound 4 is shown as formula (IV).

[0031]

[0032] (I) (II)

[0033] (III) (IV) Preferably, when the above four compounds are used in combination, the mass ratio of organophosphorus compound one, organophosphorus compound two, organophosphorus compound three, and organophosphorus compound four is (1-3):(0.5-1):(1-2):(2-3).

[0034] Preferably, the second organophosphorus compound is prepared by an oxidation reaction between the first organophosphorus compound and benzoyl peroxide. The chemical reactions involved in this oxidation reaction are as follows:

[0035] Preferably, in the oxidation reaction, the molar ratio of organophosphorus compound 1 to benzoyl peroxide is 1:(1-1.5). This ratio can improve the conversion efficiency of the reaction.

[0036] Preferably, the specific steps of the oxidation reaction include: first, slowly adding benzoyl peroxide dropwise to organophosphorus compound one at 0-10℃; after the addition is complete, raising the temperature to room temperature (unless otherwise specified, room temperature in this invention refers to 20-30℃) and continuing the reaction for 2-4 hours. This invention adds the compound at a low temperature first to ensure that the temperature does not rise drastically during mixing, allowing for a room-temperature reaction and accelerating the reaction rate.

[0037] Preferably, the organic solvent used in the oxidation reaction is selected from one or more of tetrahydrofuran, dichloromethane, or ethyl acetate.

[0038] In this embodiment of the invention, the method for preparing organophosphorus compound two from organophosphorus compound one includes the following steps: S11: Dissolve organophosphorus compound 1 in an organic solvent to obtain a solution of organophosphorus compound 1, wherein the organic solvent is selected from one or more of tetrahydrofuran, dichloromethane or ethyl acetate; S12: Dissolve peroxybenzoyl in an organic solvent to obtain a peroxybenzoyl solution. Under low temperature (0-10℃) conditions, slowly add the peroxybenzoyl solution to the organophosphorus compound I solution, wherein the molar ratio of organophosphorus compound I to peroxybenzoyl is 1:(1-1.5). After the addition is complete, raise the temperature to room temperature and continue the reaction for 2-4 hours. S13: After the reaction is completed, a post-processing step is also included. The post-processing step specifically includes: removing byproducts and solvents by sequentially washing, extracting, and distilling the reacted substances to obtain crude products; and further purifying them by column chromatography or recrystallization to obtain high-purity organophosphorus compound II.

[0039] Experiments show that the purity of organophosphorus compound II prepared by the above method is 98-99%, and the yield is 74-80%. Therefore, the present invention can prepare organophosphorus compound II with high purity and high yield using the above method, which is suitable for large-scale production.

[0040] In a second aspect, the present invention also provides an electrolyte comprising an organophosphorus additive, wherein the organophosphorus additive comprises at least one organophosphorus compound containing a P=S double bond.

[0041] The additives in the lithium-ion battery electrolyte of the present invention include at least one organophosphorus compound containing a P=S double bond. Because of the P=S double bond, when the organophosphorus additives of the present invention are added to the lithium-ion battery electrolyte, on the one hand, they can effectively improve the flame retardant performance of the electrolyte; on the other hand, because the P=S double bond energy is moderate, it is easily reduced / oxidized and broken on the electrode surface, preferentially undergoing electrochemical reactions before the electrolyte solvent. The reaction products mainly include phosphorus-containing sulfides (such as Li3PS4, Li2S), phosphorus oxides (Li3PO4), and a small amount of organosulfides. The reaction products are deposited on the electrode surface to form a dense protective film. This protective film is a lithium-ion conductor and an electronic insulator, which can isolate the direct contact reaction between the electrode and the electrolyte, inhibit solvent co-intercalation and decomposition, reduce electrolyte decomposition, and at the same time, the film structure is stable and not easily broken under high voltage, which can reduce electrode corrosion, improve the battery's high-voltage resistance, and make the cycle performance more stable.

[0042] Therefore, by adding at least one organophosphorus compound containing a P=S double bond to the electrolyte, this application can improve battery safety while ensuring good electrochemical performance of the battery.

[0043] Preferably, the organophosphorus compound further contains C2-C4 branches.

[0044] Adding C2-C4 branched organophosphorus compounds to the electrolyte of lithium-ion batteries allows these branches to participate in the formation of the solid electrolyte interphase (SEI) film. The resulting SEI passivation film contains organosulfur lithium compounds such as ROSO2Li, RSO3Li, compounds containing PS or P=S bonds, and inorganic sulfur compounds such as Li2SO3 and Li2S. These products exhibit high flexibility, strong mechanical properties, and excellent lithium-ion conductivity, preventing cracking and remodeling of the passivation film due to volume expansion during repeated lithium insertion and delithiation at the positive electrode of high-voltage batteries. They also possess advantages such as low impedance and strong cycle resistance.

[0045] Preferably, the organophosphorus additive is selected from one or more of organophosphorus compound 1, organophosphorus compound 2, organophosphorus compound 3 and organophosphorus compound 4; wherein the chemical formula of organophosphorus compound 1 is shown as formula (I), the chemical formula of organophosphorus compound 2 is shown as formula (II), the chemical formula of organophosphorus compound 3 is shown as formula (III), and the chemical formula of organophosphorus compound 4 is shown as formula (IV).

[0046] (I) (II)

[0047] (III) (IV).

[0048] Preferably, when the above four compounds are used in combination, the mass ratio of organophosphorus compound one, organophosphorus compound two, organophosphorus compound three, and organophosphorus compound four is (1-3):(0.5-1):(1-2):(2-3).

[0049] Preferably, the second organophosphorus compound is prepared by an oxidation reaction between the first organophosphorus compound and benzoyl peroxide. The chemical reactions involved in this oxidation reaction are as follows:

[0050] Preferably, in the oxidation reaction, the molar ratio of organophosphorus compound 1 to benzoyl peroxide is 1:(1-1.5). This ratio can improve the conversion efficiency of the reaction.

[0051] Preferably, the specific steps of the oxidation reaction include: first, slowly adding benzoyl peroxide dropwise to organophosphorus compound one at 0-10℃; after the addition is complete, raising the temperature to room temperature (unless otherwise specified, room temperature in this invention refers to 20-30℃) and continuing the reaction for 2-4 hours. This invention adds the compound at a low temperature first to ensure that the temperature does not rise drastically during mixing, allowing for a room-temperature reaction and accelerating the reaction rate.

[0052] Preferably, the organic solvent used in the oxidation reaction is selected from one or more of tetrahydrofuran, dichloromethane, or ethyl acetate.

[0053] Preferably, the lithium-ion battery electrolyte comprises, by mass percentage: 70-85% organic solvent, 10-25% lithium salt, and 0.5-5% organic phosphorus additive.

[0054] Preferably, the organic solvent is selected from one or more of ethylene carbonate EC, dimethyl carbonate DMC, and ethyl methyl carbonate EMC. In a more preferred embodiment, the organic solvent is composed of ethylene carbonate EC, dimethyl carbonate DMC, and ethyl methyl carbonate EMC. When the organic solvent is composed of ethylene carbonate EC, dimethyl carbonate DMC, and ethyl methyl carbonate EMC, the preferred mass ratio of ethylene carbonate EC, dimethyl carbonate DMC, and ethyl methyl carbonate EMC is (1-3):(2-4):(3-5).

[0055] This invention demonstrates that EC provides a high dielectric constant, which facilitates the dissociation of lithium salts; DMC and EMC have low viscosity, which improves the ionic conductivity of the electrolyte. This invention creatively combines EC, DMC, and EMC in a rational ratio, enabling these three substances to work synergistically, resulting in an electrolyte with excellent overall performance.

[0056] Preferably, the lithium salt is lithium hexafluorophosphate (LiPF6) and / or lithium bis(fluorosulfonyl)imide (LiFSI). Of the lithium salts, LiPF6 has high conductivity and good overall performance, while LiFSI has high conductivity and a wide electrochemical window. In a more preferred embodiment, the lithium salt of the present invention is a mixture of LiPF6 and LiFSI in a mass ratio of (0.5–0.8):(0.2–0.5). The combination of the two lithium salts has a synergistic effect, giving the electrolyte good overall performance.

[0057] Thirdly, the present invention also provides a method for preparing an electrolyte, the method comprising: adding an organophosphorus additive to an organic solvent in which lithium salt is dissolved to obtain the electrolyte, wherein the organophosphorus additive comprises at least one organophosphorus compound containing a P=S double bond.

[0058] The additive in the lithium-ion battery electrolyte of the present invention is an organophosphorus compound containing P=S double bonds. On the one hand, it can effectively improve the flame retardant performance of the electrolyte. On the other hand, it can also form a protective film on the electrode surface. This protective film can inhibit the contact reaction between the electrode and the electrolyte, reduce electrolyte decomposition and electrode surface corrosion, improve the battery's high voltage resistance, and make the cycle performance more stable.

[0059] Therefore, by adding the aforementioned organophosphorus compounds containing P=S double bonds to the electrolyte, this application can improve battery safety while ensuring that the battery has good electrochemical performance.

[0060] Preferably, the organophosphorus additive also contains C2-C4 branches.

[0061] Adding C2-C4 branched organophosphorus compounds to the electrolyte allows these branches to participate in the formation of the solid electrolyte interphase (SEI) film. The resulting SEI passivation film contains organosulfur lithium compounds such as ROSO2Li, RSO3Li, compounds containing PS or P=S bonds, and inorganic sulfur compounds such as Li2SO3 and Li2S. These products exhibit high flexibility, strong mechanical properties, and excellent lithium-ion conductivity, preventing cracking and remodeling of the passivation film due to volume expansion during repeated lithium insertion and extraction at the positive electrode of high-voltage batteries. They also offer advantages such as low impedance and strong cycle resistance.

[0062] Preferably, the organophosphorus additive is selected from one or more of organophosphorus compound 1, organophosphorus compound 2, organophosphorus compound 3 and organophosphorus compound 4; wherein the chemical formula of organophosphorus compound 1 is shown as formula (I), the chemical formula of organophosphorus compound 2 is shown as formula (II), the chemical formula of organophosphorus compound 3 is shown as formula (III), and the chemical formula of organophosphorus compound 4 is shown as formula (IV).

[0063] (I) (II)

[0064] (III) (IV).

[0065] Preferably, when the above four compounds are used in combination, the mass ratio of organophosphorus compound one, organophosphorus compound two, organophosphorus compound three, and organophosphorus compound four is (1-3):(0.5-1):(1-2):(2-3).

[0066] Preferably, the second organophosphorus compound is prepared by an oxidation reaction between the first organophosphorus compound and benzoyl peroxide. The chemical reactions involved in this oxidation reaction are as follows:

[0067] Preferably, in the oxidation reaction, the molar ratio of organophosphorus compound 1 to benzoyl peroxide is 1:(1-1.5). This ratio can improve the conversion efficiency of the reaction.

[0068] Preferably, the specific steps of the oxidation reaction include: first, slowly adding benzoyl peroxide dropwise to organophosphorus compound one at 0-10℃; after the addition is complete, raising the temperature to room temperature (unless otherwise specified, room temperature in this invention refers to 20-30℃) and continuing the reaction for 2-4 hours. This invention adds the compound at a low temperature first to ensure that the temperature does not rise drastically during mixing, allowing for a room-temperature reaction and accelerating the reaction rate.

[0069] Preferably, the organic solvent used in the oxidation reaction is selected from one or more of tetrahydrofuran, dichloromethane, or ethyl acetate.

[0070] Preferably, the lithium-ion battery electrolyte comprises, by mass percentage: 70-85% organic solvent, 10-25% lithium salt, and 0.5-5% organic phosphorus additive.

[0071] Preferably, the organic solvent is selected from one or more of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC).

[0072] Preferably, the organic solvent is composed of ethylene carbonate EC, dimethyl carbonate DMC, and ethyl methyl carbonate EMC.

[0073] Preferably, the mass ratio of ethylene carbonate EC, dimethyl carbonate DMC and ethyl methyl carbonate EMC is (1-3):(2-4):(3-5).

[0074] Studies have shown that EC provides a high dielectric constant, which facilitates the dissociation of lithium salts; DMC and EMC have low viscosity, which can improve the ionic conductivity of the electrolyte. This invention creatively combines EC, DMC, and EMC in a reasonable ratio, enabling the three substances to work synergistically, thus giving the electrolyte excellent overall performance.

[0075] Preferably, the lithium salt is lithium hexafluorophosphate (LiPF6) and / or lithium bisfluorosulfonyl imide (LiFSI). Of the lithium salts, LiPF6 has high conductivity and good overall performance; LiFSI has high conductivity and a wide electrochemical window. This invention allows for the selection and compounding of one or more of these salts according to actual needs.

[0076] In this embodiment of the invention, the method for preparing the electrolyte specifically includes the following steps: S21: Weigh the organic solvent, lithium salt, and organophosphorus additive according to the ratio of 70-85% organic solvent, 10-25% lithium salt, and 0.5-5% organophosphorus additive, wherein the organic solvent is selected from one or more of ethylene carbonate EC, dimethyl carbonate DMC, and ethyl methyl carbonate EMC.

[0077] S22: A lithium salt solution formed by slowly adding lithium salt to an organic solvent at room temperature (20-30℃) until it is completely dissolved; S23: Add additives to the lithium salt solution and continue stirring for 1-2 hours to fully disperse the additives and obtain the electrolyte for lithium-ion batteries.

[0078] Fourthly, the present invention also provides a lithium-ion battery, comprising an electrolyte, wherein the electrolyte comprises an organophosphorus additive, the organophosphorus additive comprising at least one organophosphorus compound containing a P=S double bond.

[0079] The electrolyte additive of the lithium-ion battery of the present invention includes at least one organophosphorus compound containing a P=S double bond. Because it contains a P=S double bond, when the organophosphorus additive of the present invention is added to the electrolyte of the lithium-ion battery, it can effectively improve the flame retardant performance of the electrolyte. Furthermore, because the P=S double bond energy is moderate, it is easily reduced / oxidized and broken on the electrode surface, preferentially undergoing electrochemical reactions before the electrolyte solvent. The reaction products mainly include phosphorus-containing sulfides (such as Li3PS4, Li2S), phosphorus oxides (Li3PO4), and a small amount of organosulfides. The reaction products are deposited on the electrode surface to form a dense protective film. This protective film is a lithium-ion conductor and an electronic insulator, which can isolate the direct contact reaction between the electrode and the electrolyte, inhibit solvent co-intercalation and decomposition, reduce electrolyte decomposition, and simultaneously ensure a stable film structure that is not easily broken under high voltage. This reduces electrode corrosion, improves the battery's high-voltage tolerance, and makes the cycle performance more stable.

[0080] Therefore, by adding at least one organophosphorus compound containing a P=S double bond to the electrolyte, this application can improve the safety of lithium-ion batteries while ensuring that they have good electrochemical performance.

[0081] Preferably, the organophosphorus compound further contains C2-C4 branches.

[0082] Adding C2-C4 branched organophosphorus compounds to the electrolyte of lithium-ion batteries allows these branches to participate in the formation of the solid electrolyte interphase (SEI) film. The resulting SEI passivation film contains organosulfur lithium compounds such as ROSO2Li, RSO3Li, compounds containing PS or P=S bonds, and inorganic sulfur compounds such as Li2SO3 and Li2S. These products exhibit high flexibility, strong mechanical properties, and excellent lithium-ion conductivity, preventing cracking and remodeling of the passivation film due to volume expansion during repeated lithium insertion and delithiation at the positive electrode of high-voltage batteries. They also possess advantages such as low impedance and strong cycle resistance.

[0083] Preferably, the organophosphorus additive is selected from one or more of organophosphorus compound 1, organophosphorus compound 2, organophosphorus compound 3 and organophosphorus compound 4; wherein the chemical formula of organophosphorus compound 1 is shown as formula (I), the chemical formula of organophosphorus compound 2 is shown as formula (II), the chemical formula of organophosphorus compound 3 is shown as formula (III), and the chemical formula of organophosphorus compound 4 is shown as formula (IV).

[0084] (I) (II)

[0085] (III) (IV).

[0086] Preferably, when the above four compounds are used in combination, the mass ratio of organophosphorus compound one, organophosphorus compound two, organophosphorus compound three, and organophosphorus compound four is (1-3):(0.5-1):(1-2):(2-3).

[0087] Preferably, the second organophosphorus compound is prepared by an oxidation reaction between the first organophosphorus compound and benzoyl peroxide. The chemical reactions involved in this oxidation reaction are as follows:

[0088] Preferably, in the oxidation reaction, the molar ratio of organophosphorus compound 1 to benzoyl peroxide is 1:(1-1.5). This ratio can improve the conversion efficiency of the reaction.

[0089] Preferably, the specific steps of the oxidation reaction include: first, slowly adding benzoyl peroxide dropwise to organophosphorus compound one at 0-10℃; after the addition is complete, raising the temperature to room temperature (unless otherwise specified, room temperature in this invention refers to 20-30℃) and continuing the reaction for 2-4 hours. This invention adds the compound at a low temperature first to ensure that the temperature does not rise drastically during mixing, allowing for a room-temperature reaction and accelerating the reaction rate.

[0090] Preferably, the organic solvent used in the oxidation reaction is selected from one or more of tetrahydrofuran, dichloromethane, or ethyl acetate.

[0091] Preferably, the lithium-ion battery electrolyte comprises, by mass percentage: 70-85% organic solvent, 10-25% lithium salt, and 0.5-5% organic phosphorus additive.

[0092] Preferably, the organic solvent is selected from one or more of ethylene carbonate EC, dimethyl carbonate DMC, and ethyl methyl carbonate EMC. In a more preferred embodiment, the organic solvent is composed of ethylene carbonate EC, dimethyl carbonate DMC, and ethyl methyl carbonate EMC. When the organic solvent is composed of ethylene carbonate EC, dimethyl carbonate DMC, and ethyl methyl carbonate EMC, the preferred mass ratio of ethylene carbonate EC, dimethyl carbonate DMC, and ethyl methyl carbonate EMC is (1-3):(2-4):(3-5).

[0093] Studies have shown that EC provides a high dielectric constant, which facilitates the dissociation of lithium salts; DMC and EMC have low viscosity, which can improve the ionic conductivity of the electrolyte. This invention creatively combines EC, DMC, and EMC in a reasonable ratio, enabling the three substances to work synergistically, resulting in an electrolyte with excellent overall performance.

[0094] Preferably, the lithium salt is lithium hexafluorophosphate (LiPF6) and / or lithium bis(fluorosulfonyl)imide (LiFSI). Of the lithium salts, LiPF6 has high conductivity and good overall performance, while LiFSI has high conductivity and a wide electrochemical window. In a more preferred embodiment, the lithium salt of the present invention is a mixture of LiPF6 and LiFSI in a mass ratio of (0.5–0.8):(0.2–0.5). The combination of the two lithium salts has a synergistic effect, giving the electrolyte good overall performance.

[0095] Preferably, the lithium-ion battery of the present invention further includes a negative electrode and a positive electrode, wherein the negative electrode is selected from graphite, silicon-carbon, and silicon-oxygen / C (SiO2). x One or more of the following are selected from lithium iron phosphate and nickel-cobalt-manganese ternary materials (LiNi). x Co y Mn z O2), nickel-cobalt-aluminum ternary materials (LiNi) x Co y Al z One or more of O2 and lithium cobalt oxide.

[0096] The following specific examples illustrate the organophosphorus additives, electrolytes, their preparation methods, and lithium-ion batteries of the present invention. All reagents and raw materials used in the embodiments of the present invention are commercially available analytical grade (AR). Unless otherwise specified, they can be purchased through conventional commercial channels. Organophosphorus compound one: Chinese name: methyl sulfone, CAS number 2588-04-7, purchased from Aladdin Reagent (Shanghai) Co., Ltd.; Organophosphorus compound three: Chinese name: ethyl malathion, CAS number 3700-86-5, purchased from Hubei Qingbei Yunyan Pharmaceutical Technology Co., Ltd.; Organophosphorus compound four: Chinese name: sparganium, CAS number 64249-01-0, purchased from Aladdin Reagent (Shanghai) Co., Ltd.; Organophosphorus compound two is prepared by reacting organophosphorus compound one with benzoyl peroxide.

[0097] Example 1 An electrolyte containing an organophosphorus additive, comprising the following components by mass percentage: Organic solvent: 70%, composed of EC, DMC and EMC, wherein the mass ratio of EC, DMC and EMC is 2:3:5; Lithium salt: 25%, LiPF6; Organophosphorus additive: 5%, which is an organophosphorus compound.

[0098] The preparation method of the electrolyte includes: weighing each component according to the above ratio, slowly adding the lithium salt to the organic solvent, stirring at room temperature until completely dissolved, adding the organic phosphorus additive, and continuing to stir for 1.5 hours to obtain the electrolyte for lithium-ion batteries.

[0099] Example 2 An electrolyte containing an organophosphorus additive, comprising the following components by mass percentage: Organic solvent: 85%, composed of EC, DMC and EMC, wherein the mass ratio of EC, DMC and EMC is 1:2:4; Lithium salt: 14.5%, composed of LiPF6 and LiFSI, wherein the mass ratio of LiPF6 to LiFSI is 1:1; Organophosphorus additive: 0.5%, which is organophosphorus compound II. The preparation method of organophosphorus compound II specifically includes the following steps: dissolving 10g (0.04mol) of organophosphorus compound I in 100mL of tetrahydrofuran and stirring until homogeneous to obtain a solution of organophosphorus compound I; dissolving 12g (0.048mol) of benzoyl peroxide in 50mL of tetrahydrofuran to obtain a benzoyl peroxide solution; and slowly adding the benzoyl peroxide solution dropwise to the organophosphorus compound I solution at 0℃, controlling the dropwise addition... The reaction temperature was kept below 10°C. After the addition was complete, the temperature was raised to room temperature and the reaction continued for 3 hours. After the reaction was completed, 100 mL of water was added to the reaction solution, and the mixture was extracted with dichloromethane (3 × 50 mL). The organic phases were combined, dried with anhydrous sodium sulfate, filtered, and the solvent was removed by vacuum distillation to obtain the crude product. The crude product was purified by silica gel column chromatography (eluent: petroleum ether / ethyl acetate = 5:1) to obtain 8.5 g of white solid organophosphorus compound II, with a yield of 75% and a purity of 98% (HPLC).

[0100] The proton and carbon spectra of organophosphorus compound II are as follows: 1 H NMR (CDCl3, 400MHz): δ H 4.13 (m, 4H, 2×OCH2CH3), 3.75 (s, 2H, SCH2S), 2.65 (q, J=7.4 Hz, 2H, SCH2CH3), 1.33 (t, J=7.1 Hz, 6H, 2×OCH2CH3), 1.30 (t, J=7.4 Hz, 3H, SCH2CH3).

[0101] 13 C NMR (CDCl3, 100 MHz): δ c 66.2(d, J=6.8Hz, 2C, 2×OCH2CH3), 35.3 (s, 1C,S-CH2-S), 26.4 (s, 1C, -S-CH2CH3), 16.3 (q, J=3.2Hz, 2C, 2×OCH2CH3), 14.7 (s,1C, -S-CH2CH3).

[0102] The preparation method of the electrolyte includes: weighing each component according to the above ratio, slowly adding the lithium salt to the organic solvent, stirring at room temperature until completely dissolved, adding the organic phosphorus additive, and continuing to stir for 2 hours to obtain the electrolyte for lithium-ion batteries.

[0103] Example 3 An electrolyte containing an organophosphorus additive, comprising the following components by mass percentage: Organic solvent: 78%, composed of EC, DMC and EMC, wherein the mass ratio of EC, DMC and EMC is 2:3:5; Lithium salt: 18%, composed of LiPF6 and LiFSI, wherein the mass ratio of LiPF6 to LiFSI is 1:1; Organophosphorus additive: 2%, which is an organophosphorus compound.

[0104] The preparation method of the electrolyte includes: weighing each component according to the above ratio, slowly adding the lithium salt to the organic solvent, stirring at room temperature until completely dissolved, adding the organic phosphorus additive, and continuing to stir for 1.5 hours to obtain the electrolyte for lithium-ion batteries.

[0105] Example 4 An electrolyte containing an organophosphorus additive, comprising the following components by mass percentage: Organic solvent: 78%, composed of EC, DMC and EMC, wherein the mass ratio of EC, DMC and EMC is 2:3:5; Lithium salt: 19%, composed of LiPF6 and LiFSI, wherein the mass ratio of LiPF6 to LiFSI is 1:1; Organophosphorus additive: 3%, which is an organophosphorus compound.

[0106] The preparation method of the electrolyte includes: weighing each component according to the above ratio, slowly adding the lithium salt to the organic solvent, stirring at room temperature until completely dissolved, adding the organic phosphorus additive, and continuing to stir for 1.5 hours to obtain the electrolyte for lithium-ion batteries.

[0107] Example 5 An electrolyte containing an organophosphorus additive, comprising the following components by mass percentage: Organic solvent: 70%, composed of EC, DMC and EMC, wherein the mass ratio of EC, DMC and EMC is 2:3:5; Lithium salt: 25%, LiPF6; Organophosphorus additive: 5%, composed of organophosphorus compound one and organophosphorus compound two, wherein the mass ratio of organophosphorus compound one to organophosphorus compound two is 1:1, wherein organophosphorus compound two is prepared by the same preparation method as in Example 2.

[0108] The preparation method of the electrolyte includes: weighing each component according to the above ratio, slowly adding the lithium salt to the organic solvent, stirring at room temperature until completely dissolved, adding the organic phosphorus additive, and continuing to stir for 1.5 hours to obtain the electrolyte for lithium-ion batteries.

[0109] Example 6 An electrolyte containing an organophosphorus additive, comprising the following components by mass percentage: Organic solvent: 70%, composed of EC, DMC and EMC, wherein the mass ratio of EC, DMC and EMC is 2:3:5; Lithium salt: 25%, LiPF6; Organophosphorus additive: 5%, composed of organophosphorus compound II and organophosphorus compound III, wherein the mass ratio of organophosphorus compound II to organophosphorus compound III is 1:1, wherein organophosphorus compound II is prepared by the same preparation method as in Example 2.

[0110] The preparation method of the electrolyte includes: weighing each component according to the above ratio, slowly adding the lithium salt to the organic solvent, stirring at room temperature until completely dissolved, adding the organic phosphorus additive, and continuing to stir for 1.5 hours to obtain the electrolyte for lithium-ion batteries.

[0111] Example 7 An electrolyte containing an organophosphorus additive, comprising the following components by mass percentage: Organic solvent: 70%, composed of EC, DMC and EMC, wherein the mass ratio of EC, DMC and EMC is 2:3:5; Lithium salt: 25%, LiPF6; Organophosphorus additive: 5%, composed of organophosphorus compound I and organophosphorus compound IV, wherein the mass ratio of organophosphorus compound I to organophosphorus compound IV is 1:1.

[0112] The preparation method of the electrolyte includes: weighing each component according to the above ratio, slowly adding the lithium salt to the organic solvent, stirring at room temperature until completely dissolved, adding the organic phosphorus additive, and continuing to stir for 1.5 hours to obtain the electrolyte for lithium-ion batteries.

[0113] Example 8 An electrolyte containing an organophosphorus additive, comprising the following components by mass percentage: Organic solvent: 70%, composed of EC, DMC and EMC, wherein the mass ratio of EC, DMC and EMC is 2:3:5; Lithium salt: 25%, LiPF6; Organophosphorus additive: 5%, composed of organophosphorus compound I, organophosphorus compound II, and organophosphorus compound III, wherein the mass ratio of organophosphorus compound I, organophosphorus compound II, and organophosphorus compound III is 1:1:1.

[0114] The preparation method of the electrolyte includes: weighing each component according to the above ratio, slowly adding the lithium salt to the organic solvent, stirring at room temperature until completely dissolved, adding the organic phosphorus additive, and continuing to stir for 1.5 hours to obtain the electrolyte for lithium-ion batteries.

[0115] Example 9 An electrolyte containing an organophosphorus additive, comprising the following components by mass percentage: Organic solvent: 70%, composed of EC, DMC and EMC, wherein the mass ratio of EC, DMC and EMC is 2:3:5; Lithium salt: 25%, LiPF6; Organophosphorus additive: 5%, composed of organophosphorus compound I, organophosphorus compound II, and organophosphorus compound IV, wherein the mass ratio of organophosphorus compound I, organophosphorus compound II, and organophosphorus compound IV is 1:1:1.

[0116] The preparation method of the electrolyte includes: weighing each component according to the above ratio, slowly adding the lithium salt to the organic solvent, stirring at room temperature until completely dissolved, adding the organic phosphorus additive, and continuing to stir for 1.5 hours to obtain the electrolyte for lithium-ion batteries.

[0117] Example 10 An electrolyte containing an organophosphorus additive, comprising the following components by mass percentage: Organic solvent: 70%, composed of EC, DMC and EMC, wherein the mass ratio of EC, DMC and EMC is 2:3:5; Lithium salt: 25%, LiPF6; Organophosphorus additive: 5%, composed of organophosphorus compound I, organophosphorus compound II, organophosphorus compound III, and organophosphorus compound IV, wherein the mass ratio of organophosphorus compound I, organophosphorus compound II, organophosphorus compound III, and organophosphorus compound IV is 1:1:1:2.

[0118] The preparation method of the electrolyte includes: weighing each component according to the above ratio, slowly adding the lithium salt to the organic solvent, stirring at room temperature until completely dissolved, adding the organic phosphorus additive, and continuing to stir for 1.5 hours to obtain the electrolyte for lithium-ion batteries.

[0119] Comparative Example 1 A lithium-ion battery electrolyte without organophosphorus additives, comprising the following components by mass percentage: Organic solvent: 75%, composed of EC, DMC and EMC, wherein the mass ratio of EC, DMC and EMC is 2:3:5; Lithium salt: 25%, LiPF6; Additives: 0%.

[0120] The preparation method of the electrolyte includes: weighing each component according to the above ratio, slowly adding the lithium salt to the organic solvent, stirring at room temperature until completely dissolved, and obtaining the electrolyte.

[0121] Comparative Example 2 A lithium-ion battery electrolyte, comprising the following components by mass percentage: Organic solvent: 70%, composed of EC, DMC and EMC, wherein the mass ratio of EC, DMC and EMC is 2:3:5; Lithium salt: 25%, LiPF6; Additive: 5%, which is trimethyl phosphate (TMP).

[0122] The preparation method of the electrolyte includes: weighing each component according to the above ratio, slowly adding the lithium salt to the organic solvent, stirring at room temperature until completely dissolved, adding the additive, and continuing to stir for 1.5 hours to obtain the electrolyte.

[0123] Performance testing The electrolytes prepared in the above examples and comparative examples were respectively assembled into lithium-ion batteries with graphite as the negative electrode and lithium iron phosphate as the positive electrode, and the following performance tests were performed: Flame retardant performance test: The flame retardant performance of the electrolyte was determined by the self-extinguishing time test method. The specific method is as follows: a certain amount (1 mL) of electrolyte is placed in a standard combustion dish, ignited with an igniter, and the time from the removal of the igniter to the complete extinguishing of the flame is recorded. The test is repeated 3 times and the average value is taken.

[0124] Cyclic performance test: The lithium-ion battery with graphite as the negative electrode was subjected to charge-discharge cycle test at a rate of 0.5C.

[0125] First-cycle coulombic efficiency test: The first charge-discharge test of the lithium-ion battery with graphite as the negative electrode was carried out at a rate of 0.1C, and the first-cycle coulombic efficiency (discharge capacity / charge capacity × 100%) was calculated.

[0126] Oxidation voltage test: Linear scan voltammetry (LSV) test was performed on a Li|electrolyte|stainless steel battery at a scan rate of 1 mV / s (range: 0–6 V vs. Li). + / Li), by observing the elution position of LSV, the voltage resistance characteristics of the components in the electrolyte can be determined.

[0127] The performance test results of the above embodiments and comparative examples are shown in Table 1 below: Table 1 Performance test results of embodiments and comparative examples of the present invention

[0128] As can be seen from the data in Table 1: (1) The self-extinguishing time of the electrolyte with the organic phosphorus additive of the present invention is significantly shorter than that of Comparative Examples 1 and 2, indicating that the organic phosphorus additive of the present application can improve the flame retardant performance of the electrolyte.

[0129] (2) The battery capacity retention rates of Examples 1-10 of the present invention are all above 95%, which is better than the battery capacity retention rate (92%) of the electrolyte without flame retardant additive in Comparative Example 1, and significantly higher than the battery capacity retention rate (13.2%) of the electrolyte with traditional flame retardant additive TMP. This shows that the electrolyte containing organic phosphorus additive of the present invention can more effectively improve the cycle stability of graphite anode batteries.

[0130] (3) The first-cycle coulombic efficiency of the battery in the embodiments of the present invention is above 94.9%, which is higher than the first-cycle coulombic efficiency of the battery without flame retardant additives in Comparative Example 1 (93.2%), and significantly higher than the first-cycle coulombic efficiency of the battery with traditional flame retardant additive TMP (43%), indicating that the organophosphorus compound of the present invention enhances the cycle stability of the graphite anode.

[0131] (4) As can be seen from Examples 1 and 5, the phosphorus functional group of organophosphorus compound one is compatible with the graphite anode + carbonate electrolyte system: it can form a dense, stable, and low-resistance SEI film on the surface of the graphite anode, inhibit the continuous decomposition of the electrolyte, and improve the first-cycle coulombic efficiency and long-cycle retention rate; the molecular antioxidant structure is stable and has strong resistance to high-voltage oxidation decomposition, so the oxidation voltage is as high as 5.23V, showing the advantage of good oxidation resistance of organophosphorus compound one; at the same time, its phosphorus skeleton has high flame retardant efficiency, and adding 5% can achieve an extremely short self-extinguishing time, and the balance between flame retardancy and electrochemical compatibility is the best.

[0132] (5) The oxidation potential of Examples 4 and 7 containing organophosphorus compound 4 is low, indicating that organophosphorus compound 4 undergoes a polymerization reaction, which can prevent the battery from being charged further.

[0133] It should be noted that, in this invention, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes the element.

[0134] This invention is not limited to the embodiments described above. Those skilled in the art can make various improvements and modifications without departing from the principles of this invention, and these improvements and modifications are also considered within the scope of protection of this invention. Contents not described in detail in this specification are prior art known to those skilled in the art.

Claims

1. An organophosphorus additive for use in lithium-ion battery electrolytes, characterized in that, The organophosphorus additive includes at least one organophosphorus compound containing a P=S double bond.

2. The organophosphorus additive as described in claim 1, characterized in that, The organophosphorus compound also contains C2-C4 branches.

3. The organophosphorus additive as described in claim 1 or 2, characterized in that, The organophosphorus additive is selected from one or more of organophosphorus compounds I, II, III and IV; wherein the chemical formula of organophosphorus compound I is shown in formula (I), the chemical formula of organophosphorus compound II is shown in formula (II), the chemical formula of organophosphorus compound III is shown in formula (III), and the chemical formula of organophosphorus compound IV is shown in formula (IV). （Ⅰ） （Ⅱ） （Ⅲ） （Ⅳ）。 4. The organophosphorus additive as described in claim 3, characterized in that, The second organophosphorus compound is prepared by the oxidation reaction of the first organophosphorus compound with benzoyl peroxide.

5. The organophosphorus additive as described in claim 4, characterized in that, In the oxidation reaction, the molar ratio of organophosphorus compound 1 to benzoyl peroxide is 1:(1-1.5).

6. The organophosphorus additive as described in claim 4, characterized in that, The specific steps of the oxidation reaction include: first, slowly adding peroxybenzoyl to organophosphorus compound one at 0-10℃; after the addition is complete, raising the temperature to room temperature and continuing the reaction for 2-4 hours.

7. An electrolyte, characterized in that, Includes the organophosphorus additives as described in claim 1.

8. The electrolyte as described in claim 7, characterized in that, By mass percentage, it comprises: 70-85% organic solvent, 10-25% lithium salt, and 0.5-5% of the organophosphorus additive.

9. The electrolyte as described in claim 7, characterized in that, The organic solvent is selected from one or more of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC).

10. The electrolyte as described in claim 9, characterized in that, The organic solvent is composed of ethylene carbonate EC, dimethyl carbonate DMC, and ethyl methyl carbonate EMC.

11. The electrolyte as described in claim 10, characterized in that, The mass ratio of ethylene carbonate EC, dimethyl carbonate DMC and ethyl methyl carbonate EMC is (1-3):(2-4):(3-5).

12. The electrolyte as described in claim 7, characterized in that, The lithium salt is lithium hexafluorophosphate (LiPF6) and / or lithium bis(fluorosulfonyl)imide (LiFSI).

13. The electrolyte as described in claim 7 or 8, characterized in that, The organophosphorus additive is selected from one or more of organophosphorus compounds I, II, III, and IV; Among them, the chemical formula of organophosphorus compound one is shown in formula (I), the chemical formula of organophosphorus compound two is shown in formula (II), the chemical formula of organophosphorus compound three is shown in formula (III), and the chemical formula of organophosphorus compound four is shown in formula (IV).

14. A method for preparing an electrolyte as described in any one of claims 7-13, characterized in that, The preparation method includes: adding an organophosphorus additive to an organic solvent containing dissolved lithium salt to obtain the electrolyte.

15. A lithium-ion battery, characterized in that, Includes the electrolyte as described in any one of claims 7-13, or the electrolyte prepared by the preparation method described in claim 14.

Images