Modified graphite negative active material, preparation method, application and lithium secondary battery

Abstract

This invention belongs to the field of graphite materials, specifically disclosing a modified graphite anode active material, its preparation method, application, and lithium secondary battery. The invention pre-modifies graphite by ball milling with a structure-directing agent to pre-construct micro-nano defects and dislocations. Subsequently, it undergoes chemical modification using a special catalytic-oxidation-perchloric acid-sulfuric acid-phosphoric acid mixed system, followed by a three-stage substitution of water-alcohol-aprotic solvent. This optimizes surface sites and interlayer structure. Further, a two-stage reaction involving modification of solution A followed by solution B and hydrogen reduction allows for the confined induction synthesis of a Li-Si-P-S-O composite phase. Finally, grafting with a bifunctional coupling agent and functionalized crosslinking treatment optimizes the interlayer structure, thereby enhancing bulk ion transport and interfacial desolvation, significantly improving its low-temperature performance.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery anode material technology, specifically to the field of graphite anode active materials. Background Technology

[0002] Lithium-ion batteries experience a sharp performance degradation at low temperatures (such as below 0°C), limiting their application in electric vehicles, energy storage systems, and cold-weather equipment. The core bottleneck lies on the negative electrode side: on the one hand, the electrolyte viscosity increases at low temperatures, slowing down the solid-phase diffusion kinetics of lithium ions within the electrolyte and electrode materials; on the other hand, before solvated lithium ions can embed themselves on the graphite surface, they must overcome a high desolvation energy barrier, a process that becomes particularly difficult at low temperatures.

[0003] Currently, research on low-temperature modification of graphite anodes mainly focuses on a single dimension: one is to dope the graphite bulk phase with heteroatoms (such as phosphorus, sulfur, and nitrogen) or create pores to expand the interlayer spacing and introduce defects as active sites, aiming to improve the bulk diffusion rate of lithium ions; the other is to coat the graphite surface with carbon or modify it with metal oxides to improve interface stability. For example, patent document CN119349567A discloses a transition metal element-doped graphite anode material and its preparation method, as well as an anode and a lithium-ion battery. The preparation method includes: mixing graphite and nitrogen-containing materials to obtain a graphite mixture, and then heat-treating the graphite mixture and a transition metal element source in an oxygen-free environment to obtain the anode material. The transition metal element source includes any one of iron, nickel, cobalt, and manganese sources. Patent document CN116454272A discloses a phosphorus-doped graphite anode material, its preparation method, anode sheet, and lithium-ion battery. The method involves mixing solvent a, a phosphorus-containing compound, and a carbon-coated precursor to obtain mixture A; mixing mixture A with graphite powder to obtain mixture B; placing mixture B in a carbonization furnace and carbonizing it under an inert atmosphere; subsequently cooling it to room temperature to obtain material C; and processing the calcined material C to obtain the phosphorus-doped graphite anode material. Similarly, patent document CN104934579A discloses a method for preparing porous graphite-doped and carbon-coated graphite anode materials, specifically disclosing the use of porous carbon doping and carbon coating to modify graphite and solve the problem of poor high-rate performance of graphite.

[0004] In summary, existing technologies disclose numerous doping and coating modification schemes. However, these methods have certain limitations. For example, bulk modification often neglects interface issues, and simple layer expansion or doping cannot effectively reduce the desolvation energy barrier of Li⁺ at the electrode / electrolyte interface. At low temperatures, the interface remains a rate-determining step. Surface modification is difficult to balance conduction and catalysis. Although conventional carbon coatings can improve electronic conductivity, they do not promote the desolvation process of Li⁺ and may even increase ion transport resistance due to the formation of a dense inert layer. Some inorganic coatings may not bond well with the substrate and peel off during cycling.

[0005] In summary, existing technologies mostly treat bulk phase modification and surface modification as independent steps, failing to achieve a gradient connection and synergy between the bulk phase and the interface in terms of structure and function through integrated design, resulting in limited modification effects.

[0006] Therefore, developing a graphite anode modification technology that can simultaneously address the dual challenges of bulk ion transport and interfacial desolvation, and whose process can be scaled up, has significant practical implications and industrial value. Summary of the Invention

[0007] To address the problems existing in the prior art, the primary objective of this invention is to provide a method for preparing modified graphite anode active materials, aiming to obtain graphite anode materials that combine excellent bulk ion transport and interfacial desolvation, thereby improving low-temperature cycling stability.

[0008] The second objective of this invention is to provide the modified graphite anode active material prepared by the aforementioned method and its application in lithium secondary batteries.

[0009] A third objective of this invention is to provide a lithium secondary battery comprising the modified graphite anode active material.

[0010] A method for preparing a modified graphite anode active material, comprising the following steps:

[0011] Step 1:

[0012] The graphite raw material and the structure directing agent are ball-milled for the first stage of treatment to obtain the first modified graphite; the structure directing agent is a component that enables the graphite raw material to form micro-nano defects and dislocations;

[0013] Step 2:

[0014] The first modified graphite was subjected to a second modification in a modification solution, and then the second modified product was washed with water, C1-C4 alcohol and anhydrous aprotic solvent to obtain the second modified graphite.

[0015] The modified solution is a mixed solution containing a catalyst, an oxidant, and a composite acid; the composite acid includes perchloric acid, concentrated sulfuric acid, and concentrated phosphoric acid.

[0016] Step 3:

[0017] The second modified graphite was first mixed with solution A containing silicate ester and sulfur-phosphorus source, and subjected to the first stage of heat preservation treatment. Then it was mixed with solution B containing lithium source and boron source, and subjected to the second stage of heat preservation treatment. Finally, it was calcined in a reducing atmosphere to obtain the third modified graphite.

[0018] Step 4:

[0019] The modified graphite and bifunctional silane coupling agent were grafted and modified, and then crosslinked with organic functional molecules containing Lewis basic groups and crosslinking agents. After curing, the modified graphite anode active material was obtained.

[0020] Bifunctional silane coupling agents are coupling agents containing at least one functional group selected from amino, hydroxyl, mercapto, and -NCO groups.

[0021] This invention pre-modifies graphite by ball milling with a structure-directing agent to pre-construct micro-nano defects and dislocations. Subsequently, it undergoes chemical modification using a special catalytic-oxidation-perchloric acid-sulfuric acid-phosphoric acid mixed system, followed by a three-stage substitution of water-alcohol-aprotic solvent. This optimizes surface sites and interlayer structure. Further, combined with a two-stage reaction of first modifying solution A and then solution B, and hydrogen reduction, it can confine and induce the synthesis of Li-Si-PSO composite phase. Finally, grafting with a bifunctional coupling agent and functionalized crosslinking treatment can optimize the interlayer structure, thereby enhancing bulk ion transport and interfacial desolvation, and significantly improving its low-temperature performance.

[0022] In this invention, the graphite raw material is natural graphite with a D50 particle size of 5~20 μm, and more specifically 8~12 μm.

[0023] In this invention, the structure-directing agent includes nano-silica, nano-alumina, and zinc stearate (Zn(C)). 18 H 35 At least one of O2)2) or polyvinylpyrrolidone;

[0024] Preferably, the structure-directing agent is 1% to 5% of the weight of the graphite raw material, and more preferably 1% to 3%;

[0025] Preferably, the ball milling is a dry ball milling or a wet ball milling;

[0026] Preferably, in ball milling, the ball-to-material ratio is 10~30:1;

[0027] Preferably, the ball mill rotation speed is 200~400 r / min, more preferably 300~350 r / min;

[0028] Preferably, the ball milling time is 2 to 10 hours, and more preferably 3 to 7 hours.

[0029] In this invention, the catalyst in the modified liquid includes at least one of V2O5, MnO2, Fe2O3, or I2; the amount of catalyst used is 0.5% to 3% of the weight of the graphite raw material, preferably 1% to 2%;

[0030] Preferably, in the mixed acid, the volume ratio of perchloric acid, concentrated sulfuric acid, and concentrated phosphoric acid is (2~4):(5~7):(1~3);

[0031] Preferably, the liquid-to-solid ratio of the mixed acid and graphite is 50-200 mL / g, and more preferably 80-120 mL / g;

[0032] Preferably, the oxidant includes at least one of permanganate and persulfate;

[0033] Preferably, the oxidant is 1 to 5 times the weight of the graphite raw material, and more preferably 2 to 3 times;

[0034] Preferably, the temperature for the second stage of modification is 0~25℃, and more preferably 5~15℃;

[0035] Preferably, the second stage of modification takes 4 to 12 hours, and more preferably 6 to 10 hours;

[0036] Preferably, the anhydrous aprotic solvent is selected from at least one of anhydrous acetonitrile (CH3CN), anhydrous tetrahydrofuran (THF), or anhydrous 1,2-dimethoxyethane (DME).

[0037] In this invention, in solution A, the silicate ester is a tetraalkyl silicate, and the alkyl group in the tetraalkyl silicate is preferably a C1-C4 alkyl group;

[0038] Preferably, the sulfur and phosphorus source is phosphorus sulfide;

[0039] Preferably, in solution A, the molar ratio of silicate ester to sulfur-phosphorus source is 1:2.5~3.5;

[0040] Preferably, the solvent in solution A includes at least one of acetonitrile, tetrahydrofuran, and 1,2-dimethoxyethane;

[0041] Preferably, the weight ratio of the second modified graphite to the silicate ester in solution A is 1:0.01~0.15; more preferably, it can be 1:0.03~0.06.

[0042] Preferably, the temperature of the first heat preservation treatment is 30~60℃, and more preferably 45~55℃; the time of the first heat preservation treatment is 2~15h, and more preferably 6~10h.

[0043] In solution B, the lithium source includes at least one of LiTFSI, LiBF4, and LiOTf;

[0044] Preferably, the boron source includes at least one of borate esters, boric acid (H3BO3), boron oxide / boric anhydride (B2O3), and BF3·Et2O;

[0045] Preferably, in solution B, the molar ratio of lithium source to boron source is 4~6:1;

[0046] Preferably, the solvent in solution B includes at least one of acetonitrile, tetrahydrofuran, and 1,2-dimethoxyethane;

[0047] Preferably, the weight ratio of the second modified graphite to the lithium source in solution B is 1:0.03~0.2, more preferably 0.05~0.15;

[0048] Preferably, the temperature of the second stage of heat preservation treatment is 30~60℃, and more preferably 45~55℃;

[0049] The second stage of heat preservation treatment takes 5-25 hours, and can be further extended to 10-20 hours.

[0050] Preferably, after the second stage of heat preservation treatment, an evaporation and desolvation treatment is performed, followed by calcination in a reducing atmosphere.

[0051] In this invention, the reducing atmosphere is a hydrogen-containing atmosphere; preferably, the hydrogen content in the hydrogen-containing atmosphere is 3~10% vol%.

[0052] Preferably, the roasting temperature is 200~500℃; more preferably 250~450℃; and even more preferably 300~400℃.

[0053] Preferably, the roasting time is 0.5 to 2 hours.

[0054] In this invention, the bifunctional silane coupling agent includes at least one of 3-propyltriethoxysilane, 3-aminopropyltriethoxysilane, or 3-mercaptopropyltrimethoxysilane;

[0055] The bifunctional silane coupling agent is 0.5-8% of the weight of the third modified graphite, preferably 1-5%; more preferably 1-2.5%.

[0056] Preferably, the solvent used in the grafting modification process includes a hydrophobic organic solvent, such as EA, DCM, alkanes, etc.

[0057] Preferably, the temperature of the grafting modification process is 40~100℃, and more preferably 60~80℃;

[0058] Preferably, the grafting modification time is 4 to 12 hours, and more preferably 6 to 10 hours;

[0059] Preferably, the organic functional molecule containing a Lewis basic group is selected from one or more of terminal amino polyethers, 4-cyanobenzonic acid, or crown ether derivatives;

[0060] Preferably, the organic functional molecule containing Lewis basic groups accounts for 1% to 5% of the weight of the third modified graphite;

[0061] Preferably, the crosslinking agent is selected from at least one of pyromellitic acid, glutaraldehyde, or hexamethylene diisocyanate;

[0062] Preferably, the molar ratio of the crosslinking agent to the organic functional molecule is 1:10 to 1:2, and more preferably 1:3 to 6;

[0063] Preferably, the temperature of the crosslinking modification process is 50~90℃, and the reaction time is 6~24 hours, or more preferably 8~15 hours.

[0064] Preferably, the curing process atmosphere includes at least one of nitrogen and rare gases;

[0065] Preferably, the curing temperature is 100~200℃, and more preferably 130~170℃;

[0066] Preferably, the curing time is 1 to 4 hours.

[0067] The present invention also provides a modified graphite anode active material prepared by the preparation method described above.

[0068] The preparation method described in this invention can endow the prepared material with special physicochemical properties, and the material prepared by the method can unexpectedly enhance the material's low-temperature properties.

[0069] The present invention also provides an application of the modified graphite anode active material prepared by the above preparation method, which is used as an anode active material in the preparation of lithium secondary batteries.

[0070] The present invention also provides a lithium secondary battery comprising the modified graphite anode active material prepared by the aforementioned preparation method.

[0071] The lithium secondary battery of the present invention, except for the active material described in the present invention, may have other known components and structural relationships.

[0072] Beneficial effects

[0073] 1. A unique "core-shell-interface" gradient functional structure was created: It retains the high electronic conductivity and high capacity of graphite itself; within and between the graphite layers, an in-situ confined amorphous sulfide network constitutes an ion-conductivity-enhancing phase, whose room-temperature ion conductivity is several orders of magnitude higher than that of conventional solid electrolyte interphase (SEI) films, greatly accelerating bulk Li⁺ migration; through organic functional molecular layers chemically bridged by the liquid phase, a clearly catalytic Li⁺-enhancing phase is formed. + An artificial interface layer that desolvates the solvent. This layer is present before the first cycle of the battery and can induce the formation of a thinner, more stable SEI film with higher ionic conductivity.

[0074] 2. Achieved synergistic optimization of bulk transport and interfacial reaction kinetics:

[0075] This invention employs pre-activation with a structure-directing agent, modification with a special modifying liquid, and a three-stage solvent replacement treatment, which helps optimize the interlayer structure and structural surface. Furthermore, the synthesis strategy of using solution A first, followed by solution B, enables highly selective optimization of the controllable growth of the Li-Si-PSO composite phase. Finally, subsequent functionalization grafting and crosslinking treatments optimize the interface, thereby improving ion conduction and desolvation. This gradient structure decomposes and optimizes the Li⁺ intercalation process, specifically manifested in rapid desolvation at the interface, rapid migration through the surface molecular layer, and high-speed diffusion to storage sites within the bulk fast ion conductor network. This synergistic effect is particularly significant under low-temperature conditions.

[0076] The material described in this invention exhibits excellent low-temperature performance: lithium-ion batteries assembled using this material are expected to retain over 80% of their room-temperature capacity at -20°C, while also possessing considerable discharge rate capability. Excellent fast-charging performance: Due to the significantly improved bulk ionic conductivity, the material supports fast charging at 5C or higher at room temperature with high capacity retention. The artificially constructed interfacial bifunctional layer effectively reduces interfacial charge transfer impedance and minimizes polarization. Simultaneously, the robust gradient structure buffers volume changes, suppressing structural pulverization and interfacial degradation during cycling, thereby extending battery life.

[0077] This invention constructs a graphite anode material with a gradient functional structure, which fundamentally and synergistically improves the transport and reaction kinetics of lithium ions under low temperature and fast charging conditions, providing a key anode material solution for the development of high-performance low-temperature lithium-ion batteries. Attached Figure Description

[0078] Figure 1 SEM image of the modified graphite obtained in Example 1;

[0079] Figure 2The cycling performance of the modified graphite prepared in Example 1 at -20 °C and 0.1C (1C=372 mAh / g) current is shown.

[0080] Figure 3 The rate performance of the modified graphite prepared in Example 1 at -20 °C and 0.1~1C (1C=372 mAh / g) current was obtained. Detailed Implementation

[0081] The optional preparation method of the present invention may include the following steps:

[0082] Step 1

[0083] Natural graphite powder (particle size D50 of 5~20 μm) was mixed with a structure-directing agent and subjected to low-speed ball milling under an inert atmosphere. Mechanical shearing force was used to induce uniform micro- and nano-defects and dislocations on the surface and edges of the graphite particles, while simultaneously allowing the structure-directing agent to embed into the graphite surface, reserving channels for subsequent deep penetration of the oxidant.

[0084] The structure-directing agent is selected from nano-silica (Nano-SiO2), nano-alumina (Nano-Al2O3), and zinc stearate (Zn(C)2O3). 18 H 35 The amount of one of O2)2) or polyvinylpyrrolidone (PVP) added is 1% to 5% of the mass of natural graphite, preferably 1% to 3%.

[0085] The low-speed ball milling process parameters are as follows: the ball-to-material mass ratio is controlled at (10~30):1, with a preferred ratio of 15:1; the ball milling speed is controlled at 200~400 r / min, with a preferred speed of 250~300 r / min; the ball milling time is 2~6 hours, with a preferred time of 3~4 hours. Zirconia balls or agate balls are used as the milling media, and the ball milling process is carried out in a closed system free of water and oxygen.

[0086] Step 2

[0087] The mechanically activated graphite precursor prepared in step 1 was uniformly mixed with the catalyst, and then the mixture was added to a mixed acid system in an ice bath (0~5℃) and stirred to form a suspension. Then, under continuous stirring and temperature control in the ice bath, the oxidant was slowly added, and the reaction was carried out at 5~15℃ for 4~12 hours. After the reaction was completed, the product was subjected to gradient solvent replacement, washing successively with deionized water, anhydrous ethanol, and anhydrous aprotic solvent until the water content of the system was <50 ppm, yielding anhydrous graphite oxide precursor.

[0088] The catalyst is selected from one of V2O5, MnO2, Fe2O3 or I2, and its addition amount is 0.5% to 3% of the mass of natural graphite. The preferred range is 1% to 2%.

[0089] The mixed acid system is composed of perchloric acid (HClO4, concentration 60-70%), concentrated sulfuric acid (H2SO4, concentration 98%), and concentrated phosphoric acid (H3PO4, concentration 85%) in a volume ratio of (2-4):(5-7):(1-3), with a preferred ratio of 3:6:1. The ratio of total acid volume (mL) to graphite mass (g) is 50:1 to 200:1.

[0090] The oxidant is potassium permanganate (KMnO4), ammonium persulfate ((NH4)2S2O8), or a compound thereof. When a composite oxidant is used, the mass ratio of KMnO4 to (NH4)2S2O8 is 1:1 to 1:3. The ratio of the total mass of the oxidant to the mass of graphite is 2:1 to 5:1.

[0091] The reaction temperature is 5~15℃.

[0092] The anhydrous aprotic solvent is selected from one of anhydrous acetonitrile (CH3CN), anhydrous tetrahydrofuran (THF), or anhydrous 1,2-dimethoxyethane (DME).

[0093] Step 3

[0094] Two solutions were prepared: Solution A (TEOS and P2S5 dissolved in acetonitrile at a molar ratio of 1:(2.5~3.5)); Solution B (LiTFSI and B(OEt)3 dissolved in acetonitrile at a molar ratio of (4:1) to (6:1). Under argon protection, Solution A was first slowly added dropwise to the graphite slurry obtained in step 2 over 2 hours, and stirred at 50±2℃ for 6~12 hours; then Solution B was slowly added dropwise to the above system over 3~5 hours, and the reaction continued for 12~24 hours. After the reaction was completed, the solvent was removed by rotary evaporation under reduced pressure at 60±10℃. The resulting dry powder was then heat-treated in a tube furnace for 0.5~2 hours, followed by natural cooling to obtain the composite material.

[0095] Furthermore, in solution A, the molar ratio of TEOS to P2S5 is preferably 1:3. The total amount used should ensure that the theoretical content of (Si+P) elements in the final product accounts for 1% to 10% of the total mass of the composite material.

[0096] In solution B, the preferred molar ratio of LiTFSI to B(OEt)3 is 5:1. The amount of LiTFSI used should be such that the molar ratio of Li:(Si+P) is between 2:1 and 4:1 to ensure a sufficient lithium source.

[0097] The heat treatment atmosphere is a hydrogen-containing inert gas, with hydrogen accounting for 3% to 10% of the volume, used to reduce some oxygen-containing functional groups and promote the formation of sulfide networks. The heat treatment temperature is 200 to 500°C; preferably 250 to 450°C; more preferably 300 to 400°C.

[0098] Step 4

[0099] The composite material obtained in step 3 was dispersed in an anhydrous organic solvent containing a bifunctional silane coupling agent and subjected to a pre-reaction under isothermal reflux or stirring under an inert atmosphere. Subsequently, organic functional molecules containing Lewis basic groups were introduced into the system to facilitate rapid chemical bridging with the suspended functional groups on the material surface. A trace amount of crosslinking agent was added to initiate an in-situ crosslinking reaction, utilizing the condensation or addition effects between multifunctional groups to weave a desolvated catalytic film with a three-dimensional network topology on the outermost layer of the material. After the reaction, the mixture was centrifuged, and the solid product was washed sequentially with anhydrous ethanol and anhydrous acetone. The product was then vacuum-dried at 60–80 °C. Finally, the dried powder was kept at 100–150 °C for 1–4 h under an inert atmosphere, and after cooling, the final gradient functionalized anode material was obtained.

[0100] The bifunctional silane coupling agent is selected from one or more of 3-propyltriethoxysilane (IPTS, containing -NCO group), 3-aminopropyltriethoxysilane (APTES, containing -NH2 group) or 3-mercaptopropyltrimethoxysilane (MPTMS, containing -SH group); its mass concentration in the solvent is 0.5% to 5%, preferably 1% to 2%.

[0101] The anhydrous organic solvent is selected from anhydrous toluene, anhydrous xylene, or anhydrous n-hexane; the reaction temperature is controlled at 40~100℃, preferably 60~80℃; and the reaction time is 4~12 hours.

[0102] The organic functional molecule containing Lewis basic groups is selected from one or more of terminal amino polyethers, 4-cyanobenzonic acid, or crown ether derivatives; its addition amount is 1% to 5% of the mass of the composite material.

[0103] The crosslinking agent is selected from one of pyromellitic acid, glutaraldehyde, or hexamethylene diisocyanate, and the molar ratio of the crosslinking agent to the organic functional molecule is 1:10 to 1:2. The reaction temperature is controlled at 50 to 90°C, and the reaction time is 6 to 24 hours.

[0104] Example 1

[0105] Step 1: Mechanochemical pre-activation

[0106] Weigh 10 g of natural graphite powder (D50 is 10.1 μm), add 2 wt.% of structure-directing agent nano-silica (Nano-SiO2), and ball mill at 300 r / min for 4 hours under argon protection. The ball-to-material mass ratio is 15:1.

[0107] Step 2: Catalytic oxidation layer expansion and gradient anhydrous solvent replacement

[0108] The mechanically activated graphite precursor was mixed with a MnO2 catalyst at a concentration of 1.5% of the graphite mass. The mixture was then added to a mixed acid system in an ice bath at 5°C. The mixed acid composition was: perchloric acid: concentrated sulfuric acid: concentrated phosphoric acid = 3:6:1 (volume ratio), with a total acid volume to graphite mass ratio of 100:1 (mL:g). Under continuous stirring and an ice bath, the oxidant KMnO4 was slowly added at a concentration three times the graphite mass. The reaction temperature was controlled at 5°C, and the reaction time was 8 h. After the reaction, a gradient solvent exchange was performed sequentially using deionized water, anhydrous ethanol, and anhydrous acetonitrile (a non-protic solvent) until the water content of the system was below 50 ppm, yielding anhydrous graphite oxide precursor.

[0109] Step 3: In-situ confined growth of amorphous sulfide networks

[0110] 5 mmol TEOS and 15 mmol P2S5 were dissolved in 50 mL acetonitrile to prepare a homogeneous solution A. 25 mmol LiTFSI and 5 mmol B(OEt)3 were dissolved in 50 mL acetonitrile to form solution B. Under argon protection, solution A was slowly added dropwise over 2 h to the graphite slurry obtained in step 2 (where the weight ratio of graphite to TEOS in solution A was controlled at 1:0.05), and the reaction was stirred at 50 °C for 8 h (first stage treatment). Subsequently, solution B was added dropwise to the system over 4 h, and the reaction continued for 18 h (second stage treatment). After the reaction was completed, the solvent was removed by rotary evaporation under reduced pressure at 60 °C to obtain a dry powder. The obtained dry powder was placed in a tube furnace and heated to 350 °C (heat treatment temperature) at a rate of 2 °C / min under a 5% H2 / 95% Ar atmosphere, held at that temperature for 1 h (heat treatment time), and then allowed to cool naturally to obtain the composite material.

[0111] Step 4: Constructing an interfacial bifunctional layer using liquid-phase chemical bridging

[0112] The composite material obtained in step 3 was dispersed in a solution of propyltriethoxysilane (a bifunctional silane coupling agent), with the silane concentration in the solvent controlled at 1.5% (1% of the composite material weight). The reaction was carried out under argon protection at 70°C with stirring for 8 h. Subsequently, organic functional molecules (terminated amino polyether, accounting for 3% of the composite material mass) and crosslinking agent glutaraldehyde were added to the system, with a molar ratio of crosslinking agent to terminated amino polyether of 1:5. The reaction was carried out at 80°C for 12 h. After the reaction, the mixture was centrifuged, and the resulting solid was washed successively with anhydrous ethanol and anhydrous acetone. It was then vacuum dried at 80°C for 12 h. Finally, the dried product was kept at 150°C for 2 h under an argon atmosphere and allowed to cool naturally to obtain the final graded functionalized graphite anode material.

[0113] Example 2

[0114] Compared with Example 1, the only difference is that the type and amount of structure-directing agent and the ball milling time in step 1 are changed. The experimental groups are as follows:

[0115] Group A: The structure-directing agent is nano-alumina, the addition amount is 1% of the graphite mass, the ball milling speed is 350 r / min, the ball milling treatment is 6 hours; the ball-to-material ratio is 10:1.

[0116] Group B: The structure-directing agent is zinc stearate, with an addition amount of 4% of the graphite mass, and ball milling treatment for 3 hours;

[0117] Group C: The structure-directing agent is polyvinylpyrrolidone, added at 5% of the graphite mass, and ball-milled for 2 hours.

[0118] All other operations and parameters are the same as in Example 1.

[0119] Example 3

[0120] Compared to Example 1, the only difference is that the types and amounts of catalyst and oxidant added in step 2 are changed. The experimental groups are as follows:

[0121] Group A: The catalyst is V2O5, and the amount added is 0.5% of the graphite mass;

[0122] Group B: The catalyst is Fe2O3, and the amount added is 3% of the graphite mass;

[0123] Group C: The oxidant is ammonium persulfate, and the amount of oxidant added is twice the mass of graphite. The mixed acid composition is: perchloric acid: concentrated sulfuric acid: concentrated phosphoric acid = 3.5:5.5:1 (volume ratio), and the aprotic solvent is tetrahydrofuran;

[0124] All other operations and parameters are the same as in Example 1.

[0125] Example 4

[0126] Compared with Example 1, the only difference is in the molar ratio of TEOS to P2S5 and the molar ratio of LiTFSI to B(OEt)3 in step 3. The experimental groups are as follows:

[0127] Group A: The amount of TEOS remains unchanged, and the molar ratio of TEOS to P2S5 is controlled at 1:2.5;

[0128] Group B: The amount of TEOS remained unchanged, and the molar ratio of TEOS to P2S5 was controlled at 1:3.5; the reaction was stirred at 45℃ for 10 h (first stage treatment).

[0129] Group C: The amount of B(OEt)3 remained unchanged, and the molar ratio of LiTFSI to B(OEt)3 was controlled at 4:1. Solution B was added dropwise to the system within 5 hours, and the reaction continued for 10 hours (second stage treatment).

[0130] All other operations and parameters are the same as in Example 1.

[0131] Example 5

[0132] Compared to Example 1, the only difference lies in the temperature and time of the heat treatment in step 3. The experimental groups are as follows:

[0133] Group A: The heat treatment temperature is 250℃, and the holding time is 2 hours. The atmosphere is a 10% H2-Ar mixture.

[0134] Group B: The heat treatment temperature is 450℃, and the holding time is 0.5 hours.

[0135] All other operations and parameters are the same as in Example 1.

[0136] Example 6

[0137] Compared to Example 1, the only difference lies in the type of bifunctional silane coupling agent used in step 4. The experimental groups are as follows:

[0138] Group A: The type of bifunctional silane coupling agent is APTES; the dosage is 1.8% of the weight of the composite material.

[0139] Group B: The type of bifunctional silane coupling agent is MPTMS;

[0140] All other operations and parameters are the same as in Example 1.

[0141] Example 7

[0142] Compared to Example 1, the only difference lies in the types and amounts of organic functional molecules added in step 4. The experimental groups are as follows:

[0143] Group A: Organic functional molecule 4-cyanobenzonic acid; added to the composite material at a mass of 1%;

[0144] Group B: Organic functional molecule terminal amino polyether, added to the composite material at 5% mass.

[0145] All other operations and parameters are the same as in Example 1.

[0146] Comparative Example 1

[0147] Compared with Example 1, the only difference is that step 1 is not performed. Natural graphite is directly subjected to step 2 and subsequent processing. All other operations and parameters are the same as in Example 1.

[0148] Comparative Example 2

[0149] Compared with Example 1, the only difference is that no structure guiding agent was added in step 1, while the other operations and parameters are the same as in Example 1.

[0150] Comparative Example 3

[0151] Compared to Example 1, the only difference is that the conditions in step 2 are changed; the experimental group is as follows:

[0152] Group A: No concentrated phosphoric acid was added to the mixed acid. The proportions of other acidic components and the total amount of the mixed acid remained unchanged. All other operations and parameters were the same as in Example 1.

[0153] Group B: No third-stage treatment with aprotic solvents was performed;

[0154] Group C: The three solvents are mixed together to form a mixture, and then the graphite is subjected to a one-stage washing treatment. All other operations and parameters are the same as in Example 1.

[0155] Comparative Example 4

[0156] Compared to Example 1, the only difference is that the conditions in step 3 are changed, and the experimental groups are as follows:

[0157] Group A: After mixing solutions A and B, they were kept at the same temperature for the same duration and other operations and parameters as in Example 1.

[0158] Group B: First, incubate with solution B for the first stage, then incubate with solution A for the second stage. That is, the preparation order of solution B and solution A is reversed. All other operations and parameters are the same as in Example 1.

[0159] Comparative Example 5

[0160] Compared to Example 1, the only difference is that the conditions in step 4 are changed, and the experimental groups are as follows:

[0161] Group A: The bifunctional coupling agent was replaced by a single coupling agent, 3-chloropropyltriethoxysilane.

[0162] Group B: Replace the aforementioned organic functional molecules with polyvinyl alcohol;

[0163] Group C: The bifunctional coupling agent, organic functional molecule and crosslinking agent are mixed together and then subjected to a modification treatment.

[0164] All other operations and parameters are the same as in Example 1.

[0165] Graphite-lithium half-cell assembly and performance testing

[0166] The modified graphite active material, conductive carbon black, and polyvinylidene fluoride obtained from each case were mixed uniformly at a mass ratio of 8:1:1, and an appropriate amount of NMP was added as a dispersant to form a homogeneous slurry. The slurry was then coated onto the surface of a copper foil current collector and vacuum dried at 60°C for 12 hours to obtain the negative electrode sheet. This negative electrode sheet was then cut into circular electrodes with a diameter of approximately 10 mm using a cutting machine, resulting in an areal load of approximately 1 mg / cm². 2 The CR2025 coin cell was assembled in the following order: positive electrode casing, positive lithium electrode sheet, Celgard 2400 single-layer PP separator, negative electrode sheet, nickel foam, and negative electrode casing. LB-002 electrolyte (1M LiPF6 in DMC:EC:EMC=1:1:1 Vol%) was added, and the cells were sealed using a battery packaging machine. The assembled coin cell lithium-ion battery was charged and discharged at -20℃ using a 0.1C current (1C=372 mAh / g) to investigate the electrochemical performance of the materials. The voltage range was 0.01V~2.0V, and the instrument used was the Blue Electric electrochemical measurement system. The results are shown in Tables 1 and 2.

[0167]

[0168]

[0169] As demonstrated in Examples 1 and Comparative Examples 1-5, pre-modifying graphite by ball milling with a structure-directing agent pre-constructs micro-nano defects and dislocations. Subsequently, chemical modification is carried out using the special catalytic-oxidation-perchloric acid-sulfuric acid-phosphoric acid mixed system, followed by a three-stage replacement of water-alcohol-aprotic solvent. This optimizes the surface sites and interlayer structure. Further, combined with the subsequent two-stage reaction of first modifying solution A and then solution B, as well as hydrogen reduction, the Li-Si-PSO composite phase can be synthesized in a confined and induced manner. Finally, grafting with a bifunctional coupling agent and functionalized crosslinking treatment can optimize the interlayer structure, thereby enhancing bulk ion transport and interfacial desolvation, and significantly improving its low-temperature performance.

Claims

1. A method for preparing a modified graphite negative electrode active material, characterized by the steps of include: Step 1: The graphite raw material and the structure directing agent are ball-milled for the first stage of treatment to obtain the first modified graphite; the structure directing agent is a component that enables the graphite raw material to form micro-nano defects and dislocations; Step 2: The first modified graphite was subjected to a second modification in a modification solution, and then the second modified product was washed with water, C1-C4 alcohol and anhydrous aprotic solvent to obtain the second modified graphite. The modified solution is a mixed solution containing a catalyst, an oxidant, and a composite acid; the composite acid includes perchloric acid, concentrated sulfuric acid, and concentrated phosphoric acid. Step 3: The second modified graphite was first mixed with solution A containing silicate ester and sulfur-phosphorus source, and subjected to the first stage of heat preservation treatment. Then it was mixed with solution B containing lithium source and boron source, and subjected to the second stage of heat preservation treatment. Finally, it was calcined in a reducing atmosphere to obtain the third modified graphite. Step 4: The modified graphite and bifunctional silane coupling agent were grafted and modified, and then crosslinked with organic functional molecules containing Lewis basic groups and crosslinking agents. After curing, the modified graphite anode active material was obtained. Bifunctional silane coupling agents are coupling agents containing at least one functional group selected from amino, hydroxyl, mercapto, and -NCO groups.

2. The method for producing a modified graphite negative electrode active material according to claim 1, characterized by, The graphite raw material is natural graphite with a D50 particle size of 5~20 μm.

3. The method for producing a modified graphite negative electrode active material according to claim 1, characterized by, The structure directing agent includes at least one of nanosilica, nanoalumina, zinc stearate (Zn(C 18 H 35 O2)2) or polyvinylpyrrolidone. Preferably, the structure-directing agent is 1% to 5% of the weight of the graphite raw material, and more preferably 1% to 3%; Preferably, the ball milling is a dry ball milling or a wet ball milling; Preferably, in ball milling, the ball-to-material ratio is 10~30:1; Preferably, the rotational speed of the ball mill is 200~400 r / min; Preferably, the ball milling time is 2 to 10 hours.

4. The method for producing a modified graphite negative electrode active material according to claim 1, characterized by, In the modified liquid, the catalyst includes at least one of V2O5, MnO2, Fe2O3, or I2; the amount of catalyst used is 0.5% to 3% of the weight of the graphite raw material, preferably 1% to 2%; Preferably, in the mixed acid, the volume ratio of perchloric acid, concentrated sulfuric acid, and concentrated phosphoric acid is (2~4):(5~7):(1~3); Preferably, the liquid-to-solid ratio of the mixed acid and graphite is 50~200mL / g; Preferably, the oxidant includes at least one of permanganate and persulfate; Preferably, the oxidant is 1 to 5 times the weight of the graphite raw material; Preferably, the temperature for the second stage of modification is 0~25℃; Preferably, the second stage of modification takes 4 to 12 hours; Preferably, the anhydrous aprotic solvent is selected from at least one of anhydrous acetonitrile (CH3CN), anhydrous tetrahydrofuran (THF), or anhydrous 1,2-dimethoxyethane (DME).

5. The method for preparing the modified graphite anode active material as described in claim 1, characterized in that, In solution A, the silicate ester is a tetraalkyl silicate, and the alkyl group in the tetraalkyl silicate is preferably a C1-C4 alkyl group; Preferably, the sulfur and phosphorus source is phosphorus sulfide; Preferably, in solution A, the molar ratio of silicate ester to sulfur-phosphorus source is 1:2.5~3.5; Preferably, the solvent in solution A includes at least one of acetonitrile, tetrahydrofuran, and 1,2-dimethoxyethane; Preferably, the weight ratio of the second modified graphite to the silicate ester in solution A is 1:0.01~0.15; Preferably, the temperature of the first stage of heat preservation is 30~60℃; the time of the first stage of heat preservation is 2~15h; In solution B, the lithium source includes at least one of LiTFSI, LiBF4, and LiOTf; Preferably, the boron source includes at least one of borate esters, boric acid (H3BO3), boron oxide / boric anhydride (B2O3), and BF3·Et2O; Preferably, in solution B, the molar ratio of lithium source to boron source is 4~6:1; Preferably, the solvent in solution B includes at least one of acetonitrile, tetrahydrofuran, and 1,2-dimethoxyethane; Preferably, the weight ratio of the second modified graphite to the lithium source in solution B is 1:0.03~0.2, more preferably 0.05~0.15; Preferably, the temperature of the second stage of heat preservation is 30~60℃; the time of the second stage of heat preservation is 5~25h; Preferably, after the second stage of heat preservation treatment, an evaporation and desolvation treatment is performed, followed by calcination in a reducing atmosphere.

6. The method for preparing the modified graphite anode active material as described in claim 1, characterized in that, The reducing atmosphere is a hydrogen-containing atmosphere; preferably, the hydrogen content in the hydrogen-containing atmosphere is 3~10% vol%. Preferably, the roasting temperature is 200~500℃; more preferably 250~450℃; and even more preferably 300~400℃. Preferably, the roasting time is 0.5 to 2 hours.

7. The method for preparing the modified graphite anode active material as described in claim 1, characterized in that, Bifunctional silane coupling agents include at least one of 3-propylisocyanatetriethoxysilane, 3-aminopropyltriethoxysilane or 3-mercaptopropyltrimethoxysilane; The bifunctional silane coupling agent is 0.5-8% of the weight of the third modified graphite, preferably 1-5%; Preferably, the solvent used in the grafting modification process includes a hydrophobic organic solvent; Preferably, the temperature of the grafting modification process is 40~100℃; preferably, the grafting modification time is 4~12 hours. Preferably, the organic functional molecule containing a Lewis basic group is selected from one or more of terminal amino polyethers, 4-cyanobenzonic acid, or crown ether derivatives; Preferably, the organic functional molecule containing Lewis basic groups accounts for 1% to 5% of the weight of the third modified graphite; Preferably, the crosslinking agent is selected from at least one of pyromellitic acid, glutaraldehyde, or hexamethylene diisocyanate; Preferably, the molar ratio of the crosslinking agent to the organic functional molecule is 1:10 to 1:2; Preferably, the temperature of the crosslinking modification process is 50~90℃, and the reaction time is 6~24 hours; Preferably, the curing process atmosphere includes at least one of nitrogen and rare gases; Preferably, the curing temperature is 100~200℃; Preferably, the curing time is 1 to 4 hours.

8. A modified graphite anode active material prepared by the preparation method according to any one of claims 1 to 7.

9. The application of a modified graphite anode active material prepared by the preparation method according to any one of claims 1 to 7, characterized in that, It is used as a negative electrode active material in the preparation of lithium secondary batteries.

10. A lithium secondary battery, characterized in that, It includes the modified graphite anode active material prepared by the preparation method according to any one of claims 1 to 7.

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