Composite modified fast-charging negative electrode material, preparation method thereof and secondary battery

Abstract

The invention provides a composite modified fast-charging negative electrode material, a preparation method thereof and a secondary battery. The composite modified fast-charging negative electrode material has a core-shell structure, the core-shell structure comprises an inner core material and a shell layer material, the inner core material comprises a silicon-containing material, and the shell layer material comprises a copolymer; the copolymer comprises a halogenated ester structural unit and an aromatic sulfonic acid structural unit. The composite modified fast-charging negative electrode material provided by the invention has good thermal stability and structural strength, and has relatively low interface impedance with an electrolyte, so that the electrochemical performance of the fast-charging negative electrode material at a high temperature is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of negative electrode materials, and specifically relates to a composite modified fast-charging negative electrode material, a preparation method thereof, and a secondary battery. Background Art

[0002] With the continuous development of science and technology, application scenarios such as electric vehicles, portable electronic devices and energy storage systems have put forward higher requirements for the energy density and high-temperature performance of secondary batteries. Among them, silicon-based negative electrode materials have become one of the research hotspots due to their high theoretical specific capacity, but the following problems still need to be solved in the application process: Silicon materials have a huge volume expansion (over 300%) during the charging and discharging process, which will cause the electrode structure to be destroyed and the capacity of the secondary battery to decay rapidly. At the same time, the cycle stability and ion transmission efficiency of secondary batteries in high temperature environments are also problems that need to be solved urgently.

[0003] At present, in order to further improve the electrochemical performance of silicon-based negative electrode materials, most researchers use traditional coating materials for modification. However, these coating materials have limited technical effects in improving the thermal stability and ion conductivity of silicon materials. For example, although some coating materials can alleviate the volume expansion phenomenon of silicon materials to a certain extent, they cannot effectively prevent the failure of secondary batteries in high-temperature environments, and their effect on reducing interfacial resistance is not obvious, making it difficult to meet the application requirements of high-power and long-life batteries. In addition, the preparation processes and technical means disclosed in the prior art have deficiencies in controlling the structure and performance of the coating layer, so it is impossible to accurately control the various performance indicators of the SEI film.

[0004] Therefore, how to ensure that the silicon-based negative electrode material has a high energy density and that the secondary battery assembled therefrom has both good safety and high-temperature performance is a technical problem that needs to be urgently solved by technicians in this field. Summary of the Invention

[0005] In response to the shortcomings of the prior art, the present invention aims to provide a composite-modified fast-charging negative electrode material, a preparation method thereof, and a secondary battery. The composite-modified fast-charging negative electrode material provided by the present invention has good thermal stability and structural strength, and at the same time has a low interfacial impedance with the electrolyte, thereby improving the electrochemical performance of the fast-charging negative electrode material at high temperatures.

[0006] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0007] In a first aspect, the present invention provides a composite modified fast-charging negative electrode material, wherein the composite modified fast-charging negative electrode material has a core-shell structure, wherein the core-shell structure comprises a core material and a shell material, wherein the core material comprises a silicon-containing material, and the shell material comprises a copolymer;

[0008] The copolymer includes a halogenated ester structural unit and an aromatic sulfonic acid structural unit.

[0009] The present invention regulates the structure and composition of the fast-charging negative electrode material, so that the composite modified fast-charging negative electrode material has good thermal stability, ion transport performance and structural strength, thereby playing a role similar to that of the SEI film, which is specifically embodied as follows:

[0010] (1) The copolymer shell material provided by the present invention effectively improves the thermal stability of the SEI film, preventing the electrochemical performance of the secondary battery from degrading or failing in a high-temperature environment, thereby extending its service life and improving its safety. Specifically, the halogen groups in the copolymer structure can significantly improve the thermal stability of the negative electrode material. At the same time, in a high-temperature environment, the halogen groups can also effectively inhibit the decomposition of the electrolyte and its destructive effects on the SEI film, thereby reducing the risk of thermal runaway of the secondary battery.

[0011] (2) The copolymer shell material provided by the present invention enhances the ion conductivity of the SEI membrane, improves the transmission rate of active ions, and optimizes the interface structure between the electrolyte and the fast-charge negative electrode material, thereby reducing the interface resistance between the two, thereby meeting the use requirements of the secondary battery to be able to charge and discharge quickly. Specifically, the introduced sulfonic acid polar groups with high dielectric constants can provide more ion transmission channels, ultimately improving the transmission rate of active ions. Furthermore, the sulfonic acid groups can strongly interact with the active ions to form efficient ion transmission channels, thereby enhancing the transmission capacity of active ions. In addition, when the negative electrode film is formed, the sulfonic acid groups are enriched at the interface between the SEI membrane and the electrolyte, promoting the rapid migration of active ions, thereby significantly reducing the interface resistance between the two.

[0012] (3) The ester groups in the copolymer structure can be tightly combined with the fast-charging negative electrode material through hydrogen bonding, thereby enhancing the structural strength of the fast-charging negative electrode material itself, thereby effectively solving the technical problems of structural destruction and capacity attenuation caused by volume expansion of inorganic active materials during the charging and discharging process.

[0013] Preferably, the monomer forming the haloester structural unit comprises a haloalkenoate.

[0014] Preferably, the haloenoate includes ethyl (E)-4-bromobut-2-enoate (CAS No.: 37746-78-4) and / or ethyl (E)-4-bromo-4,4-difluorobut-2-enoate (CAS No.: 1824708-65-7).

[0015] Preferably, the monomer forming the aromatic sulfonic acid structural unit includes an aromatic sulfonic acid compound containing an unsaturated double bond.

[0016] Preferably, the aromatic sulfonic acid compound containing an unsaturated double bond includes vinylbenzenesulfonic acid (CAS No.: 26914-43-2) and / or 3-ethyl-4-vinylbenzenesulfonic acid (CAS No.: 756758-86-8).

[0017] Preferably, the decomposition temperature of the copolymer is not lower than 200°C, preferably not lower than 220°C, for example, it can be 200°C, 205°C, 210°C, 215°C, 220°C, 225°C, 230°C or 250°C, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0018] The present invention regulates the decomposition temperature of the copolymer to ensure good stability at high temperatures. If the decomposition temperature of the copolymer is too low, the copolymer will easily decompose during the charge and discharge process of the secondary battery, especially in a high-temperature environment, resulting in failure of the SEI film.

[0019] Preferably, the glass transition temperature of the copolymer is -20°C to 10°C, preferably -7°C to -3°C, for example, it can be -20°C, -18°C, -15°C, -12°C, -10°C, -8°C, -7°C, -6°C, -5°C, -3°C, -2°C, 0°C, 2°C, 5°C, 8°C or 10°C, and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0020] The present invention regulates the glass transition temperature (Tg) of the copolymer to achieve good flexibility and mechanical strength. A low Tg of the copolymer results in poor mechanical properties, which in turn affects the stability of the SEI film. A high Tg of the copolymer results in a glassy state within the operating temperature range of the secondary battery, poor flexibility, and susceptibility to breakage.

[0021] Preferably, the number average molecular weight of the copolymer is 12000Da to 50000Da, preferably 20000Da to 45000Da, for example, it can be 12000Da, 15000Da, 18000Da, 20000Da, 22000Da, 25000Da, 28000Da, 30000Da, 32000Da, 35000Da, 38000Da, 40000Da, 42000Da, 45000Da, 48000Da or 50000Da, etc., but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0022] The present invention regulates the number-average molecular weight of the copolymer to achieve good processing, mechanical, and ion-conducting properties. Using a copolymer with a lower number-average molecular weight results in poor film-forming and mechanical properties, making it ineffective in protecting fast-charge anode materials. Using a copolymer with a higher number-average molecular weight results in poor solubility and processing, hindering the coating process and potentially leading to a decrease in ion-conducting properties.

[0023] Preferably, based on the total mass of the composite modified fast-charging negative electrode material as 100%, the mass percentage of the shell material is 1% to 5%, for example, it can be 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.2%, 4.5%, 4.8% or 5%, etc., not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0024] The present invention regulates the mass percentage of the shell material to form a shell protective layer with suitable thickness and uniform composition, thereby further optimizing the interface compatibility and ion conductivity, and reducing the occurrence of electrolyte decomposition and side reactions.

[0025] Preferably, the thickness of the shell layer in the composite modified fast-charging negative electrode material is 32nm to 70nm, for example, it can be 32nm, 35nm, 38nm, 40nm, 42nm, 45nm, 48nm, 50nm, 52nm, 55nm, 58nm, 60nm, 62nm, 65nm, 68nm or 70nm, etc., but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0026] The present invention regulates the shell thickness to ensure both good structural stability and high ionic conductivity, thereby comprehensively improving the cycle performance and rate performance of the assembled secondary battery. A thinner shell layer can easily break, leading to loss of active ions; a thicker shell layer can result in poor ion transport performance, and the cycle performance and rate performance of the assembled secondary battery can be reduced.

[0027] Preferably, the silicon-containing material includes a nitrogen-doped silicon-carbon material.

[0028] Preferably, the mass percentage of nitrogen element in the nitrogen-doped silicon-carbon material is 1% to 5%, for example, it can be 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.2%, 4.5%, 4.8% or 5%, etc., and is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0029] The present invention further improves the conductivity and electrochemical activity of the silicon-carbon material by regulating the mass percentage of the nitrogen element in the nitrogen-doped silicon-carbon material, thereby improving the first coulombic efficiency of the secondary battery.

[0030] Preferably, the mass percentage of silicon material in the nitrogen-doped silicon-carbon material is 40% to 52%, for example, it can be 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51% or 52%, etc., and is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0031] The present invention regulates the mass percentage of silicon in the nitrogen-doped silicon-carbon material, resulting in a nitrogen-doped silicon-carbon material with a higher gram capacity and higher first coulombic efficiency while exhibiting less volume expansion. Using a nitrogen-doped silicon-carbon material with a lower silicon content results in lower gram capacity and first coulombic efficiency; using a nitrogen-doped silicon-carbon material with a higher silicon content results in greater volume expansion.

[0032] In a second aspect, the present invention provides a method for preparing the composite modified fast-charging negative electrode material according to the first aspect, the method comprising the following steps:

[0033] The silicon-containing material is mixed with a copolymer solution including a halogenated ester structural unit and an aromatic sulfonic acid structural unit, and the mixture is spray-dried to obtain the composite modified fast-charging negative electrode material.

[0034] Preferably, the mass concentration of the copolymer solution is 5% to 19%, for example, it can be 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18% or 19%, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0035] Preferably, the method for preparing the copolymer in the copolymer solution comprises the following steps:

[0036] The copolymer is obtained by copolymerizing a halogenated acrylate monomer, an aromatic sulfonic acid compound monomer containing an unsaturated double bond, an initiator and an organic solvent.

[0037] Preferably, the molar ratio of the halogenated acrylate monomer to the aromatic sulfonic acid compound monomer containing an unsaturated double bond is (1-4):1, preferably (1.8-2.2):1, for example, it can be 1:1, 1.2:1, 1.5:1, 1.8:1, 2:1, 2.2:1, 2.5:1, 2.8:1, 3:1, 3.2:1, 3.5:1, 3.8:1 or 4:1, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0038] The present invention regulates the molar ratio of the halogenated acrylate monomer to the aromatic sulfonic acid compound monomer containing an unsaturated double bond, so that it can not only improve the thermal stability of the fast-charging negative electrode material and the stability of the interface film, but also have good ionic conductivity, thereby improving the rate performance of the secondary battery. If a halogenated acrylate monomer with a lower molar content is used, the thermal stability of the secondary battery will be reduced, and the safety performance will be reduced. In addition, due to the high proportion of aromatic groups, the rigidity of the copolymer is too large, causing the interface film to rupture easily; if a halogenated acrylate monomer with a higher molar content is used, the ionic conductivity of the interface film will be reduced, resulting in poor rate performance of the secondary battery.

[0039] Preferably, the copolymerization reaction is carried out under an inert atmosphere, which exemplarily includes argon and / or nitrogen.

[0040] Preferably, the copolymerization reaction temperature is 60°C to 80°C, for example, 60°C, 65°C, 70°C, 75°C or 80°C, etc., and is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0041] Preferably, the copolymerization reaction time is 5 h to 9 h, for example, 5 h, 6 h, 7 h, 8 h or 9 h, etc., and is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0042] In the present invention, the initiator illustratively includes at least one of azobisisobutyronitrile, azobisisoheptanenitrile, or dibenzoyl peroxide (BPO). Furthermore, based on the total mass of the comonomers as 100%, the mass percentage of the initiator is 0.5% to 1%, for example, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, or 1%, etc., and is not limited to the values ​​listed above. Other values ​​not listed within this numerical range are also applicable.

[0043] In the present invention, the organic solvent illustratively includes at least one of benzene, tetrahydrofuran (THF), N-methylpyrrolidone (NMP) or N,N-dimethylformamide (DMF).

[0044] In the present invention, after the copolymerization reaction, the method further comprises the following steps: adding the reaction solution after the copolymerization reaction into a precipitation solvent to obtain a copolymer precipitate, and then washing and drying the copolymer precipitate to obtain the copolymer.

[0045] In the present invention, the precipitation solvent illustratively includes at least one of propanol, isopropanol or acetone.

[0046] Preferably, the mixing temperature is 60°C to 100°C, for example, it can be 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, 90°C, 95°C or 100°C, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0047] Preferably, the mixing time is 5 h to 10 h, for example, 5 h, 6 h, 7 h, 8 h, 9 h or 10 h, etc., and is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0048] Preferably, the inlet temperature of the spray drying is 120°C to 200°C, for example, 120°C, 150°C, 180°C or 200°C, etc., and is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0049] Preferably, the outlet temperature of the spray drying is 60°C to 98°C, for example, it can be 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, 90°C or 98°C, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0050] Preferably, the silicon-containing material includes a nitrogen-doped silicon-carbon material.

[0051] Preferably, the method for preparing the nitrogen-doped silicon-carbon material comprises the following steps:

[0052] The silicon-carbon material is placed in an ammonia atmosphere for heat treatment to obtain the nitrogen-doped silicon-carbon material.

[0053] Preferably, the temperature of the heat treatment is 800° C. to 1000° C., and the time of the heat treatment is 2 h to 4 h.

[0054] Specifically, the temperature of the heat treatment can be, for example, 800°C, 850°C, 900°C, 950°C or 1000°C, etc., and is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable; the time of the heat treatment can be, for example, 2h, 2.5h, 3h, 3.5h or 4h, etc., and is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0055] In a third aspect, the present invention provides a secondary battery comprising a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator, wherein the negative electrode sheet comprises a negative electrode active material, and the negative electrode active material comprises the composite modified fast-charging negative electrode material according to the first aspect.

[0056] In the present invention, the electrolyte further includes a lithium salt additive, and the lithium salt additive is lithium bis(difluorosulfonylimide) (LiFSI).

[0057] In the present invention, the LiFSI lithium salt additive can produce a synergistic effect with the copolymer shell material, thereby further reducing the interfacial impedance.

[0058] Furthermore, the mass percentage of the lithium salt additive in the electrolyte is 1% to 3%, for example, it can be 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8% or 3%, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0059] The numerical range described in the present invention includes not only the point values ​​listed above, but also any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0060] Compared with the prior art, the present invention has the following beneficial effects:

[0061] The present invention provides a composite modified fast-charging negative electrode material. By regulating the structure and composition of the fast-charging negative electrode material, the composite modified fast-charging negative electrode material has good thermal stability, ion transport performance and structural strength, thereby playing a role similar to that of the SEI film. Specifically, it is:

[0062] (1) The copolymer shell material provided by the present invention effectively improves the thermal stability of the SEI film, preventing the electrochemical performance of the secondary battery from degrading or failing in a high-temperature environment, thereby extending its service life and improving its safety. Specifically, the halogen groups in the copolymer structure can significantly improve the thermal stability of the negative electrode material. At the same time, in a high-temperature environment, the halogen groups can also effectively inhibit the decomposition of the electrolyte and its destructive effects on the SEI film, thereby reducing the risk of thermal runaway of the secondary battery.

[0063] (2) The copolymer shell material provided by the present invention enhances the ion conductivity of the SEI membrane, improves the transmission rate of active ions, and optimizes the interface structure between the electrolyte and the fast-charge negative electrode material, thereby reducing the interface resistance between the two, thereby meeting the use requirements of the secondary battery to be able to charge and discharge quickly. Specifically, the introduced sulfonic acid polar groups with high dielectric constants can provide more ion transmission channels, ultimately improving the transmission rate of active ions. Furthermore, the sulfonic acid groups can strongly interact with the active ions to form efficient ion transmission channels, thereby enhancing the transmission capacity of active ions. In addition, when the negative electrode film is formed, the sulfonic acid groups are enriched at the interface between the SEI membrane and the electrolyte, promoting the rapid migration of active ions, thereby significantly reducing the interface resistance between the two.

[0064] (3) The ester groups in the copolymer structure can be tightly combined with the fast-charging negative electrode material through hydrogen bonding, thereby enhancing the structural strength of the fast-charging negative electrode material itself, thereby effectively solving the technical problems of structural destruction and capacity attenuation caused by volume expansion of inorganic active materials during the charging and discharging process. DETAILED DESCRIPTION

[0065] The technical solution of the present invention is further described below by way of specific embodiments. It should be understood by those skilled in the art that the embodiments are merely to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0066] Example 1

[0067] This embodiment provides a composite modified fast-charging negative electrode material, which has a core-shell structure. The core-shell structure includes a core material and a shell material, wherein the core material includes a nitrogen-doped silicon-carbon material, and the shell material includes a copolymer of (E)-4-bromobut-2-enoic acid ethyl ester and vinylbenzenesulfonic acid.

[0068] Among them, the decomposition temperature of the copolymer is 220°C, the glass transition temperature is -5°C, and the number average molecular weight is 31,000Da. Taking the total mass of the composite modified fast-charge negative electrode material as 100%, the mass percentage of the shell material is 3%, and the thickness of the shell is 51nm. The mass percentage of nitrogen in the nitrogen-doped silicon-carbon material is 3%, and the mass percentage of silicon material is 46%. The average particle size of the nitrogen-doped silicon-carbon material is 6μm, and the specific surface area is 4.5m 2 / g.

[0069] This embodiment also provides a method for preparing the composite modified fast-charging negative electrode material, which comprises the following steps:

[0070] The silicon-carbon material is placed in an ammonia atmosphere and annealed at 900° C. for 3 hours to obtain a nitrogen-doped silicon-carbon material;

[0071] Ethyl (E)-4-bromobut-2-enoate and vinylbenzenesulfonic acid in a molar ratio of 2:1 were added to a tetrahydrofuran solvent, and then azobisisobutyronitrile was added as an initiator. The mixture was heated to 70° C. under an argon atmosphere for a copolymerization reaction for 7 hours to obtain a reaction solution.

[0072] The reaction solution is added to a propanol solvent to precipitate a copolymer, and the copolymer precipitate is washed and dried to obtain a copolymer;

[0073] The copolymer was dissolved in tetrahydrofuran solvent to obtain a copolymer solution with a mass concentration of 12%. The nitrogen-doped silicon-carbon material was added to the copolymer solution and stirred at 80°C for 7 hours to obtain a mixed solution. The mixed solution was spray-dried at an inlet temperature of 160°C and an outlet temperature of 80°C to obtain a composite-modified fast-charging negative electrode material.

[0074] Example 2

[0075] This embodiment provides a composite modified fast-charging negative electrode material, which has a core-shell structure. The core-shell structure includes a core material and a shell material, wherein the core material includes a nitrogen-doped silicon-carbon material, and the shell material includes a copolymer of (E)-4-bromobut-2-enoic acid ethyl ester and vinylbenzenesulfonic acid.

[0076] Among them, the decomposition temperature of the copolymer is 220°C, the glass transition temperature is -7°C, and the number average molecular weight is 45,000Da. Taking the total mass of the composite modified fast-charge negative electrode material as 100%, the mass percentage of the shell material is 4%, and the thickness of the shell is 60nm. The mass percentage of nitrogen in the nitrogen-doped silicon-carbon material is 2%, and the mass percentage of silicon material is 50%. The average particle size of the nitrogen-doped silicon-carbon material is 6μm, and the specific surface area is 4.5m 2 / g.

[0077] This embodiment also provides a method for preparing the composite modified fast-charging negative electrode material, which comprises the following steps:

[0078] The silicon-carbon material is placed in an ammonia atmosphere and annealed at 900° C. for 3 hours to obtain a nitrogen-doped silicon-carbon material;

[0079] Ethyl (E)-4-bromobut-2-enoate and vinylbenzenesulfonic acid at a molar ratio of 1.8:1 were added to a tetrahydrofuran solvent, and then azobisisobutyronitrile was added as an initiator. The mixture was heated to 75° C. under an argon atmosphere for a copolymerization reaction for 6 hours to obtain a reaction solution.

[0080] The reaction solution is added to a propanol solvent to precipitate a copolymer, and the copolymer precipitate is washed and dried to obtain a copolymer;

[0081] The copolymer was dissolved in tetrahydrofuran solvent to obtain a copolymer solution with a mass concentration of 15%. The nitrogen-doped silicon-carbon material was added to the copolymer solution and stirred at 80°C for 7 hours to obtain a mixed solution. The mixed solution was spray-dried at an inlet temperature of 160°C and an outlet temperature of 80°C to obtain a composite-modified fast-charging negative electrode material.

[0082] Example 3

[0083] This embodiment provides a composite modified fast-charging negative electrode material, which has a core-shell structure. The core-shell structure includes a core material and a shell material, wherein the core material includes a nitrogen-doped silicon-carbon material, and the shell material includes a copolymer of (E)-4-bromobut-2-enoic acid ethyl ester and vinylbenzenesulfonic acid.

[0084] Among them, the decomposition temperature of the copolymer is 220°C, the glass transition temperature is -3°C, and the number average molecular weight is 20,000Da. Taking the total mass of the composite modified fast-charge negative electrode material as 100%, the mass percentage of the shell material is 2%, and the thickness of the shell is 40nm. The mass percentage of nitrogen in the nitrogen-doped silicon-carbon material is 4%, and the mass percentage of silicon material is 45%. The average particle size of the nitrogen-doped silicon-carbon material is 6μm, and the specific surface area is 4.5m 2 / g.

[0085] This embodiment also provides a method for preparing the composite modified fast-charging negative electrode material, which comprises the following steps:

[0086] The silicon-carbon material is placed in an ammonia atmosphere and annealed at 900° C. for 3 hours to obtain a nitrogen-doped silicon-carbon material;

[0087] Ethyl (E)-4-bromobut-2-enoate and vinylbenzenesulfonic acid at a molar ratio of 2.2:1 were added to a tetrahydrofuran solvent, and then azobisisobutyronitrile was added as an initiator. The mixture was heated to 65° C. under an argon atmosphere for a copolymerization reaction for 8 hours to obtain a reaction solution.

[0088] The reaction solution is added to a propanol solvent to precipitate a copolymer, and the copolymer precipitate is washed and dried to obtain a copolymer;

[0089] The copolymer was dissolved in tetrahydrofuran solvent to obtain a copolymer solution with a mass concentration of 8%. The nitrogen-doped silicon-carbon material was added to the copolymer solution and stirred at 80°C for 7 hours to obtain a mixed solution. The mixed solution was spray-dried at an inlet temperature of 160°C and an outlet temperature of 80°C to obtain a composite-modified fast-charging negative electrode material.

[0090] Example 4

[0091] This embodiment provides a composite modified fast-charging negative electrode material, which has a core-shell structure. The core-shell structure includes a core material and a shell material, wherein the core material includes a nitrogen-doped silicon-carbon material, and the shell material includes a copolymer of (E)-4-bromobut-2-enoic acid ethyl ester and vinylbenzenesulfonic acid.

[0092] Among them, the decomposition temperature of the copolymer is 200°C, the glass transition temperature is -20°C, and the number average molecular weight is 12000Da. Taking the total mass of the composite modified fast-charge negative electrode material as 100%, the mass percentage of the shell material is 1%, and the thickness of the shell is 32nm. The mass percentage of nitrogen in the nitrogen-doped silicon-carbon material is 1%, and the mass percentage of silicon material is 40%. The average particle size of the nitrogen-doped silicon-carbon material is 4μm, and the specific surface area is 8m 2 / g.

[0093] This embodiment also provides a method for preparing the composite modified fast-charging negative electrode material, which comprises the following steps:

[0094] The silicon-carbon material is placed in an ammonia atmosphere and annealed at 800° C. for 2 h to obtain a nitrogen-doped silicon-carbon material;

[0095] Ethyl (E)-4-bromobut-2-enoate and vinylbenzenesulfonic acid in a molar ratio of 1:1 were added to a tetrahydrofuran solvent, and then azobisisobutyronitrile was added as an initiator. The mixture was heated to 60° C. under an argon atmosphere for a copolymerization reaction for 9 hours to obtain a reaction solution.

[0096] The reaction solution is added to a propanol solvent to precipitate a copolymer, and the copolymer precipitate is washed and dried to obtain a copolymer;

[0097] The copolymer was dissolved in tetrahydrofuran solvent to obtain a copolymer solution with a mass concentration of 5%. The nitrogen-doped silicon-carbon material was added to the copolymer solution and stirred at 80°C for 7 hours to obtain a mixed solution. The mixed solution was spray-dried at an inlet temperature of 160°C and an outlet temperature of 80°C to obtain a composite modified fast-charging negative electrode material.

[0098] Example 5

[0099] This embodiment provides a composite modified fast-charging negative electrode material, which has a core-shell structure. The core-shell structure includes a core material and a shell material, wherein the core material includes a nitrogen-doped silicon-carbon material, and the shell material includes a copolymer of (E)-4-bromobut-2-enoic acid ethyl ester and vinylbenzenesulfonic acid.

[0100] Among them, the decomposition temperature of the copolymer is 230°C, the glass transition temperature is 10°C, and the number average molecular weight is 50,000Da. Taking the total mass of the composite modified fast-charge negative electrode material as 100%, the mass percentage of the shell material is 5%, and the thickness of the shell is 70nm. The mass percentage of nitrogen in the nitrogen-doped silicon-carbon material is 5%, and the mass percentage of silicon material is 52%. The average particle size of the nitrogen-doped silicon-carbon material is 8μm, and the specific surface area is 1m 2 / g.

[0101] This embodiment also provides a method for preparing the composite modified fast-charging negative electrode material, which comprises the following steps:

[0102] The silicon-carbon material is placed in an ammonia atmosphere and annealed at a temperature of 1000° C. for 4 hours to obtain a nitrogen-doped silicon-carbon material;

[0103] Ethyl (E)-4-bromobut-2-enoate and vinylbenzenesulfonic acid in a molar ratio of 4:1 were added to a tetrahydrofuran solvent, and then azobisisobutyronitrile was added as an initiator. The mixture was heated to 80° C. under an argon atmosphere for a copolymerization reaction for 5 hours to obtain a reaction solution.

[0104] The reaction solution is added to a propanol solvent to precipitate a copolymer, and the copolymer precipitate is washed and dried to obtain a copolymer;

[0105] The copolymer was dissolved in tetrahydrofuran solvent to obtain a copolymer solution with a mass concentration of 19%. The nitrogen-doped silicon-carbon material was added to the copolymer solution and stirred at 80°C for 7 hours to obtain a mixed solution. The mixed solution was spray-dried at an inlet temperature of 160°C and an outlet temperature of 80°C to obtain a composite-modified fast-charging negative electrode material.

[0106] Example 6

[0107] The difference between this embodiment and Example 1 is that the (E)-4-bromobut-2-enoic acid ethyl ester monomer is replaced by an equimolar content of (E)-4-bromo-4,4-difluorobut-2-enoic acid ethyl ester, and the vinylbenzenesulfonic acid monomer is replaced by an equimolar content of 3-ethyl-4-vinylbenzenesulfonic acid. The rest is the same as Example 1.

[0108] Example 7

[0109] The difference between this embodiment and embodiment 1 is that the molar ratio of (E)-4-bromobut-2-enoic acid ethyl ester to vinylbenzenesulfonic acid is 0.5:1, and the other conditions are the same as those in embodiment 1.

[0110] Example 8

[0111] The difference between this embodiment and embodiment 1 is that the molar ratio of (E)-4-bromobut-2-enoic acid ethyl ester to vinylbenzenesulfonic acid is 8:1, and the other conditions are the same as those in embodiment 1.

[0112] Example 9

[0113] The difference between this embodiment and embodiment 1 is that the decomposition temperature of the copolymer is 180° C., and the other aspects are the same as those of embodiment 1.

[0114] Example 10

[0115] The difference between this embodiment and embodiment 1 is that the glass transition temperature of the copolymer is -30°C, and the other aspects are the same as those of embodiment 1.

[0116] Example 11

[0117] The difference between this embodiment and embodiment 1 is that the glass transition temperature of the copolymer is 15° C., and the other aspects are the same as those of embodiment 1.

[0118] Comparative Example 1

[0119] The difference between this comparative example and Example 1 is that no modification treatment is performed on the nitrogen-doped silicon-carbon material, and only one nitrogen-doped silicon-carbon material is provided. The average particle size, specific surface area, mass percentage of the silicon material, and preparation method of the provided nitrogen-doped silicon-carbon material are the same as those in Example 1.

[0120] Comparative Example 2

[0121] The difference between this comparative example and Example 1 is that the (E)-4-bromobut-2-enoic acid ethyl ester monomer is replaced by an equimolar amount of vinylbenzenesulfonic acid monomer, and the other conditions are the same as those in Example 1.

[0122] Comparative Example 3

[0123] The difference between this comparative example and Example 1 is that the vinylbenzenesulfonic acid monomer is replaced by an equimolar amount of (E)-4-bromobut-2-enoic acid ethyl ester monomer, and the other conditions are the same as those in Example 1.

[0124] Application Examples 1-11 and Comparative Application Examples 1-3

[0125] The fast-charging negative electrode materials provided in Examples 1 to 11 and Comparative Examples 1 to 3 were used to prepare negative electrode sheets, and then assembled to obtain lithium-ion batteries. The specific preparation method is as follows:

[0126] Preparation of negative electrode sheet:

[0127] The fast-charging negative electrode material, conductive carbon black Super-P, single-walled carbon nanotubes (SWCNTs) and polyacrylic acid binder (PAA) provided in the above embodiments and comparative examples were mixed and stirred evenly with water in a mass ratio of 80:9:1:10 to obtain a negative electrode slurry, and the solid content was controlled to be 30%. The negative electrode slurry was then coated on a copper foil current collector through a coating process, and then vacuum dried and cold pressed to obtain a negative electrode sheet.

[0128] Preparation of positive electrode:

[0129] The ternary cathode material NCM811 (LiNi 0.8 Co 0.1 Mn 0.1 The active material (O2), polyvinylidene fluoride binder (PVDF) and conductive carbon black Super-P are mixed and stirred evenly with N-methylpyrrolidone solvent in a mass ratio of 96:2:2 to obtain a positive electrode slurry. The positive electrode slurry is then coated on an aluminum foil through a coating process, and then dried and cold pressed to obtain a positive electrode sheet.

[0130] Electrolyte:

[0131] Ethylene carbonate, dimethyl carbonate, diethyl carbonate, and fluoroethylene carbonate are mixed in a volume ratio of 20:40:30:10 to obtain an organic solvent. A fully dried lithium salt, LiPF6, is then dissolved in the organic solvent to prepare an electrolyte with a LiPF6 concentration of 1 mol / L. The electrolyte also includes a LiFSI lithium salt additive, and the mass percentage of LiFSI is 2% based on the total mass of the electrolyte being 100%.

[0132] Preparation of lithium-ion batteries:

[0133] The positive electrode sheet, separator (including a polyethylene-based film and a ceramic coating provided on one side of the polyethylene-based film) and negative electrode sheet are stacked in order, with the separator placed between the positive electrode sheet and the negative electrode sheet to play an isolating role, and then wound to obtain a bare battery cell; the bare battery cell is placed in an outer packaging shell, dried, and then injected with electrolyte, and after vacuum packaging, standing, forming and shaping processes, a lithium-ion battery is obtained.

[0134] Comparative Application Example 4

[0135] The difference between this comparative application example and application example 1 is that all LiFSI lithium salt additives are replaced with LiPF6 lithium salt, and the rest are the same as application example 1.

[0136] Test conditions

[0137] The lithium-ion batteries provided in Application Examples 1 to 11 and Comparative Application Examples 1 to 4 were subjected to performance tests. The rate performance of the batteries was tested under lithium-ion battery test conditions. The tests were conducted on the LAND battery test system of Wuhan Jinnuo Electronics Co., Ltd. at room temperature (25°C). The charge and discharge voltages were limited to 2.5V to 4.2V. The test conditions are as follows:

[0138] (1) First Coulombic efficiency

[0139] At 25°C, the lithium-ion battery was charged to 4.2V at a constant current and constant voltage rate of 0.33C and allowed to stand for 10 minutes. Then, the lithium-ion battery was discharged to 2.5V at a constant current rate of 0.33C and allowed to stand for 10 minutes. The first coulombic efficiency of the lithium-ion battery was calculated.

[0140] First coulombic efficiency (%)=(first discharge total capacity of the lithium-ion battery at a rate of 0.33C / first charge total capacity of the lithium-ion battery at a rate of 0.33C)×100%.

[0141] (2) Capacity retention after 1000 cycles at 1C / 2C at room temperature

[0142] At 25°C, the lithium-ion battery is charged to 4.2V at a constant current and constant voltage rate of 1C, with a cut-off current of 0.05C, and allowed to stand for 10 minutes. Then, the lithium-ion battery is discharged to 2.5V at a constant current rate of 2C and allowed to stand for 10 minutes. This is considered one charge and discharge cycle. The lithium-ion battery is charged and discharged 1200 times according to the above method. The capacity retention rate of the lithium-ion battery after 1000 charge and discharge cycles at 1C / 2C is calculated.

[0143] Capacity retention rate (%) of a lithium-ion battery after N cycles = (discharge capacity at the Nth cycle / initial discharge capacity) × 100%, where N is the number of cycles of the lithium-ion battery.

[0144] (3) Room temperature 6C rate performance - constant current charging ratio

[0145] At 25°C, the lithium-ion battery was discharged at a constant current rate of 1C to 2.5V and allowed to stand for 10 minutes. The lithium-ion battery was then charged at a constant current and constant voltage rate of 6C to 4.2V with a cut-off current of 0.05C and allowed to stand for 10 minutes. The constant current charging capacity Q1 and the total constant current and constant voltage charging capacity Q2 of the lithium-ion battery were recorded. The constant current charging ratio of the 6C rate charging was calculated according to the following formula: 6C rate charging constant current charging ratio = (constant current charging capacity Q1 / total constant current and constant voltage charging capacity Q2) × 100%.

[0146] (4) 1C / 8C discharge capacity retention rate at room temperature

[0147] At 25°C, the divided lithium-ion battery is charged to 4.2V at a constant current and constant voltage rate of 1C, with a cut-off current of 0.05C; it is left to stand for 10 minutes; then the lithium-ion battery is discharged to 2.5V at a constant current rate of 1C, and its discharge capacity Q is recorded. 1C As the initial discharge capacity; then at 25 ° C, the lithium ion battery is charged to 4.2V at a constant current and constant voltage rate of 1C, with a cut-off current of 0.05C; let it stand for 10 minutes; then the fully charged lithium ion battery is discharged to 2.5V at a constant current rate of 8C, and its discharge capacity Q is recorded. 8C ; Calculate the discharge capacity retention rate (%) of lithium-ion batteries at 1C / 8C rate = (discharge capacity Q at 8C rate) 8C / Discharge capacity Q at 1C rate 1C )×100%.

[0148] (5) Initial expansion rate of negative electrode

[0149] ① Before assembling the lithium-ion battery, use a micrometer to measure the initial thickness of the negative electrode sheet and record it as h1. The thickness of the negative electrode current collector is recorded as h0.

[0150] ② Fully charged: At 25°C, after assembling the battery cells, charge the lithium-ion battery at a constant current and constant voltage rate of 0.33C to 4.2V, with a cut-off current of 0.05C, and let it stand for 120 minutes;

[0151] ③ Disassemble the fully charged lithium-ion battery to obtain the negative electrode sheet, clean the negative electrode sheet with dimethyl carbonate, and measure the thickness of the cleaned negative electrode sheet, which is recorded as h2;

[0152] ④The initial expansion rate of the negative electrode sheet is: (h2-h1) / (h1-h0)×100%.

[0153] (6) Battery cell thermal runaway ARC test

[0154] Start the ARC adiabatic thermal runaway test (the test sample is heated from room temperature to 45±2°C in the chamber, and after 90 minutes, the temperature rise rate of the lithium-ion battery is detected). If the temperature rise exceeds 0.2°C within 10 minutes (i.e., the self-heating temperature rise rate, referred to as SHR, SHR>0.02°C / min), it is considered that a self-exothermic reaction has occurred inside the lithium-ion battery, and the adiabatic environment is maintained until the lithium-ion battery thermal runaway occurs; if the temperature rise does not exceed 0.2°C within 10 minutes (i.e., SHR≤0.02°C / min), continue to the next step temperature rise test; each temperature step is 5°C, and the steps are repeated at each temperature step. The ARC test temperature range is 45°C-300°C, the starting temperature of self-heating is T1 (temperature rise rate SHR>0.02°C / min), and the starting temperature of thermal runaway is T2 (temperature rise rate SHR>1°C / min).

[0155] The test results are shown in Table 1:

[0156] Table 1

[0157]

[0158]

[0159] It can be seen from Table 1 that compared with Comparative Application Example 1, Application Examples 1 to Application Examples 5 of the present invention regulate the structure and composition of the fast-charging negative electrode material, so that the composite modified fast-charging negative electrode material has good thermal stability, ion transport performance and structural strength, thereby improving the electrochemical performance of lithium-ion batteries at high temperatures.

[0160] By comparing Application Example 1 with Application Example 6, it can be seen that the copolymer shell material prepared by replacing the (E)-4-bromobut-2-enoic acid ethyl ester monomer with (E)-4-bromo-4,4-difluorobut-2-enoic acid ethyl ester and replacing the vinylbenzenesulfonic acid monomer with 3-ethyl-4-vinylbenzenesulfonic acid in the present invention also effectively improves the thermal stability and ion conductivity of the SEI film, thereby avoiding the degradation or failure of the electrochemical performance of the lithium-ion battery in a high temperature environment.

[0161] Comparison of Application Example 1, Application Example 7, and Application Example 8 shows that the molar ratio of (E)-ethyl 4-bromobut-2-enoate to vinylbenzenesulfonic acid has an important influence on the thermal stability and electrochemical performance of lithium-ion batteries.

[0162] Comparing Application Example 1 with Application Example 9, it can be seen that the present invention optimizes the decomposition temperature of the copolymer so that it has good stability at high temperatures, thereby improving the high-temperature performance of the lithium-ion battery.

[0163] By comparing Application Example 1, Application Example 10 and Application Example 11, it can be seen that the present invention optimizes the glass transition temperature of the copolymer so that the copolymer has good flexibility and mechanical strength, thereby enhancing the stability of the SEI film and thus improving the overall performance of the lithium-ion battery.

[0164] From Comparative Application Example 1, Comparative Application Example 2-Comparative Application Example 3, it can be seen that if only a homopolymer prepared from a certain monomer is used to form the negative electrode material, all the technical effects of the technical solution of the present invention cannot be achieved, indicating that there is a synergistic effect between the halogenated ester group and the aromatic sulfonic acid group in the copolymer structure.

[0165] Comparing Application Example 1 with Comparative Application Example 4, it can be seen that the specific type of copolymer shell material provided by the present invention has a synergistic effect with the lithium bis(difluorosulfonylimide) additive in the electrolyte, thereby further reducing the interfacial impedance between the electrode and the electrolyte and optimizing the overall performance of the lithium-ion battery.

[0166] The applicant declares that the above is only a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the scope of protection and disclosure of the present invention.

Claims

1. A composite modified fast-charging negative electrode material, characterized in that: The composite modified fast-charging negative electrode material has a core-shell structure, the core-shell structure comprising a core material and a shell material, wherein the core material comprises a silicon-containing material, and the shell material comprises a copolymer; The copolymer includes a halogenated ester structural unit and an aromatic sulfonic acid structural unit.

2. The composite modified fast-charging negative electrode material according to claim 1, characterized in that: The monomers forming the halogenated ester structural unit include halogenated alkenoate; Preferably, the haloenoate comprises ethyl (E)-4-bromobut-2-enoate and / or ethyl (E)-4-bromo-4,4-difluorobut-2-enoate; Preferably, the monomer forming the aromatic sulfonic acid structural unit includes an aromatic sulfonic acid compound containing an unsaturated double bond; Preferably, the aromatic sulfonic acid compound containing an unsaturated double bond includes vinylbenzenesulfonic acid and / or 3-ethyl-4-vinylbenzenesulfonic acid.

3. The composite modified fast-charging negative electrode material according to claim 1 or 2, characterized in that: The decomposition temperature of the copolymer is not less than 200°C, preferably not less than 220°C; Preferably, the glass transition temperature of the copolymer is -20°C to 10°C, preferably -7°C to -3°C.

4. The composite modified fast-charging negative electrode material according to any one of claims 1 to 3, characterized in that: The number average molecular weight of the copolymer is 12,000 Da to 50,000 Da, preferably 20,000 Da to 45,000 Da.

5. The composite modified fast-charging negative electrode material according to any one of claims 1 to 4, characterized in that: Based on the total mass of the composite modified fast-charging negative electrode material being 100%, the mass percentage of the shell material is 1% to 5%; Preferably, the thickness of the shell layer in the composite modified fast-charging negative electrode material is 32 nm to 70 nm.

6. The composite modified fast-charging negative electrode material according to any one of claims 1 to 5, characterized in that: The silicon-containing material includes a nitrogen-doped silicon-carbon material; Preferably, the mass percentage of nitrogen in the nitrogen-doped silicon-carbon material is 1% to 5%; Preferably, the mass percentage of silicon material in the nitrogen-doped silicon-carbon material is 40% to 52%.

7. A method for preparing a composite modified fast-charging negative electrode material according to any one of claims 1 to 6, characterized in that: The method comprises the following steps: The silicon-containing material is mixed with a copolymer solution including a halogenated ester structural unit and an aromatic sulfonic acid structural unit, and the mixture is spray-dried to obtain the composite modified fast-charging negative electrode material.

8. The method according to claim 7, characterized in that The mass concentration of the copolymer solution is 5% to 19%; Preferably, the method for preparing the copolymer in the copolymer solution comprises the following steps: The copolymer is obtained by copolymerizing a halogenated acrylate monomer, an aromatic sulfonic acid compound monomer containing an unsaturated double bond, an initiator and an organic solvent; The molar ratio of the halogenated acrylate monomer to the aromatic sulfonic acid compound monomer containing an unsaturated double bond is (1-4):1, preferably (1.8-2.2):1; Preferably, the copolymerization reaction is carried out under an inert atmosphere; Preferably, the copolymerization reaction temperature is 60°C to 80°C; Preferably, the copolymerization reaction time is 5h to 9h; Preferably, the mixing temperature is 60°C to 100°C; Preferably, the mixing time is 5h to 10h; Preferably, the inlet temperature of the spray drying is 120°C to 200°C; Preferably, the outlet temperature of the spray drying is 60°C to 98°C.

9. The method according to claim 7 or 8, characterized in that The silicon-containing material includes a nitrogen-doped silicon-carbon material; Preferably, the method for preparing the nitrogen-doped silicon-carbon material comprises the following steps: placing the silicon-carbon material in an ammonia atmosphere for heat treatment to obtain the nitrogen-doped silicon-carbon material; Preferably, the temperature of the heat treatment is 800° C. to 1000° C., and the time of the heat treatment is 2 h to 4 h.

10. A secondary battery, characterized in that: The secondary battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator. The negative electrode sheet includes a negative electrode active material. The negative electrode active material includes the composite modified fast-charging negative electrode material according to any one of claims 1 to 6.

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