Layered tin pyrophosphate composite negative electrode material and preparation method thereof

Abstract

The invention belongs to the technical field of lithium ion battery electrode materials, and particularly relates to a layered tin pyrophosphate composite negative electrode material and a preparation method thereof. The preparation method comprises the following steps: dispersing a tin source, an organic phosphorus source and oxidized worm carbon in water, and heating in a water bath to obtain a precursor; and calcining the precursor in an inert atmosphere to obtain the catalyst. The obtained composite negative electrode material is composed of tin pyrophosphate, phosphorus-doped carbon and oxidized wormlike carbon, the tin pyrophosphate is uniformly dispersed on a conductive network constructed by the phosphorus-doped carbon and the oxidized wormlike carbon in a layered structure, and the composite negative electrode material has high specific discharge capacity and excellent cycling stability and rate capability, and can be widely applied to lithium ion batteries.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery electrode material technology, specifically relating to a layered tin pyrophosphate composite anode material and its preparation method. Background Technology

[0002] In recent years, the issue of energy sustainability has driven the development of various new clean energy sources, namely renewable energy such as solar, wind, hydro, and geothermal energy. However, current mainstream renewable energy sources are characterized by intermittency. Therefore, developing alkaline-ion batteries with high energy density and long lifespan to meet the growing demand for renewable energy is urgent. Lithium-ion batteries have many advantages, such as high energy density, long cycle life, and environmental friendliness, playing a crucial role in electric vehicles, electronic devices, and energy storage. However, commercial graphite anodes, due to their low specific capacity, poor rate performance, and safety hazards, are difficult to meet the future development needs of lithium-ion batteries. Therefore, developing high-performance anode materials to meet the requirements of next-generation lithium-ion batteries is crucial.

[0003] Tin pyrophosphate (SnP2O7), as a novel anode material, exhibits outstanding application advantages: First, it possesses a high theoretical specific capacity and a suitable lithium insertion / extraction potential, which can effectively suppress lithium dendrite growth caused by overcharging under high current density, significantly improving battery safety performance; Second, the polyanionic groups in its crystal structure are large in volume and highly stable, serving as a structurally stable alloy matrix, which not only provides a large buffer space for lithium storage but also suppresses the occurrence of side reactions.

[0004] In terms of preparation methods, tin pyrophosphate commonly uses hydrothermal, solvothermal, and solid-phase phosphating methods, which generally suffer from insufficient safety, complex processes, and high production costs. Traditional tin pyrophosphate preparation processes often employ inorganic phosphorus sources, which presents several limitations: Na₂H₂PO₂, NH₄H₂PO₄, and (NH₄)₂HPO₄ release toxic gases such as PH₃ ​​and NH₃ upon high-temperature heating; red phosphorus, black phosphorus, and yellow phosphorus are flammable and easily oxidized, making storage difficult. These problems significantly hinder the large-scale production of tin pyrophosphate. Furthermore, tin pyrophosphate itself exhibits slow electron transfer kinetics and low electronic conductivity. During lithium insertion / extraction / dealloying reactions, significant volume shrinkage and expansion occur, resulting in substantial volume changes and insufficient cycle stability, further impacting its application performance. Therefore, developing a green, environmentally friendly, and simple method for synthesizing SnP₂O₇ to improve its electrochemical performance has become an important research direction in this field.

[0005] Chinese invention patent application CN117438559A discloses a tin pyrophosphate composite anode material and its preparation method. The preparation method includes the following steps: First, a precipitation method is used to mix a precipitant and a tin source, precipitate, centrifuge, wash, dry, and grind the resulting precipitate, then mix it uniformly with phytic acid and a nitrogen-containing carbon source. Then, most of the solvent is evaporated by water bath heating until a slurry is formed, followed by drying to obtain a precursor. Finally, the precursor is transferred to a tube furnace and calcined under an inert atmosphere. The resulting heat-treated product is cooled to room temperature and ground to obtain the final product. The tin pyrophosphate composite anode material obtained by this invention has the molecular formula SnP2O7 / CP@CN, exhibiting high discharge specific capacity, good cycle and rate performance, and a simple, rapid, high-yield, low-energy-consumption, low-cost, and environmentally friendly preparation method, showing broad application prospects in the lithium-ion battery field. However, the phosphorus source in this patent is limited to phytic acid, restricting its application.

[0006] Chinese invention patent application CN119612465A discloses a method for preparing an amorphous lithium-ion battery anode material, comprising the following steps: (1) mixing a tin source, inositol hexaphosphate, and a carbon source in an inert atmosphere and stirring thoroughly to obtain a mixed solution; (2) freeze-drying the obtained mixed solution to obtain a precursor material; (3) calcining the precursor material in an inert atmosphere to obtain an amorphous lithium-ion battery anode material. This invention method can prepare amorphous tin pyrophosphate. The amorphous structure, when used as a lithium-ion battery anode material, can provide abundant ion diffusion pathways, enabling rapid diffusion of internal lithium ions. It can also adapt to volume changes in the anode material during charging and discharging, enhancing cycle stability and effectively improving the electrochemical performance of the electrode material. However, the stirring in the first step of this patent needs to be carried out in an inert atmosphere, which is cumbersome and increases costs. Furthermore, the phosphorus source used is limited to inositol hexaphosphate.

[0007] Chinese invention patent application CN114597406A discloses a method for preparing carbon-coated tin pyrophosphate and its application. The method includes: (1) reacting a mixture containing a tin source, a phosphorus source, and compound A through a complexation reaction to obtain an intermediate product; (2) calcining the intermediate product under an inactive atmosphere to obtain the carbon-coated tin pyrophosphate. Compound A is selected from at least one of citric acid, acetylacetone, tartaric acid, and oxalic acid. The preparation process of carbon-coated tin pyrophosphate provided by this patent is simple, the raw material composition is concise, and it is suitable for mass production. At the same time, when this carbon-coated tin pyrophosphate material is applied to the negative electrode of sodium-ion batteries, it can exhibit excellent long cycle performance and rate performance. However, this patent uses ammonium dihydrogen phosphate and diammonium hydrogen phosphate as inorganic phosphorus sources, which cannot form a phosphorus-doped carbon skeleton in situ. It will release toxic NH3 gas when heated at high temperature, and requires the tin source, phosphorus source, and compound A to be dissolved in water, thus limiting the selection of raw materials. Summary of the Invention

[0008] To overcome the shortcomings of tin pyrophosphate (SnP2O7) such as slow electron transfer kinetics, significant volume change, and insufficient cycle stability, the present invention aims to provide a layered tin pyrophosphate composite anode material and its preparation method. The resulting composite anode material has high discharge specific capacity, excellent cycle stability, and rate performance, and can be widely used in lithium-ion batteries.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing a layered tin pyrophosphate composite anode material includes the following steps: A tin source, an organophosphorus source, and oxidized worm carbon are dispersed in water and heated in a water bath to obtain a precursor; the precursor is then calcined under an inert atmosphere to obtain the final product.

[0010] Preferably, the method for preparing the oxidized worm carbon is as follows: a strong acid solution is added to a reaction flask, expanded graphite is added, and then the mixture is stirred under reflux conditions to carry out an oxidation reaction, thereby obtaining the carbon.

[0011] Preferably, the strong acid solution is a mixture of concentrated sulfuric acid and concentrated nitric acid, and the ratio of expanded graphite, concentrated sulfuric acid, and concentrated nitric acid is 1-3g: 10-20mL: 30-60mL.

[0012] Preferably, the oxidation reaction temperature is 70-90℃ and the time is 1-3h.

[0013] Preferably, the tin source is one or more of SnCl4·5H2O, SnCl2·2H2O, SnCl4, SnCl2, tin acetate, tin hydroxide, tin dioxide, stannous oxide, stannous oxalate, and stannous citrate.

[0014] Preferably, the organophosphorus source is one or more of phytic acid, hydroxyethylidene diphosphonic acid, and ethylenediaminetetramethylenephosphonic acid.

[0015] Preferably, the molar ratio of tin in the tin source to phosphorus in the organic phosphorus source is 1:2.4-10, and the mass of the oxidized worm carbon is 10-20% of the theoretical mass of the generated tin pyrophosphate.

[0016] Preferably, the water bath heating temperature is 60-90℃, and the time is 1-3 hours.

[0017] Preferably, the calcination treatment is as follows: pre-calcination at 300-400℃ for 1-3 hours, followed by heating to 500-700℃ at a rate of 5-10℃ / min, and continuing calcination for 3-5 hours.

[0018] A layered tin pyrophosphate composite anode material prepared by the above preparation method.

[0019] The positive and beneficial effects of this invention are: 1. The organophosphorus source of this invention serves as both a phosphorus source and a carbon source, exhibiting strong chelating properties. It coordinates with tin ions to form stable complexes, thereby improving the dispersion uniformity of tin ions. Furthermore, during high-temperature carbonization, the organophosphorus source reacts with the tin source to form tin pyrophosphate while simultaneously forming a phosphorus-doped carbon (CP) framework. Phosphorus-doped carbon (CP) can introduce defects into the carbon-based network to further enhance conductivity. In addition, the organophosphorus source is non-toxic, harmless, and environmentally friendly. This invention uses oxidized worm carbon (Owc) as the second carbon source. The surface of oxidized worm carbon is rich in hydroxyl, carbonyl, and carboxyl groups. These oxygen-containing functional groups enable oxidized worm carbon (Owc) to effectively combine with tin and phosphorus sources, resulting in more uniform dispersion of raw materials. This facilitates the formation of high-purity tin pyrophosphate during calcination. In the final product, phosphorus-doped carbon (CP) and oxidized worm carbon together construct a continuous conductive network. Tin pyrophosphate (SnP2O7) particles are uniformly anchored on this conductive network in a layered structure. This not only improves the electronic conductivity of the material but also effectively buffers the volume shrinkage and expansion of tin pyrophosphate during lithium-ion insertion / extraction. At the same time, the layered structure of this composite material significantly reduces the interfacial charge transfer resistance, improves the electronic conduction efficiency and lithium-ion diffusion coefficient, and increases the pseudocapacitance contribution, thereby comprehensively improving the material's cycle stability, rate performance, and discharge specific capacity electrochemical performance.

[0020] 2. In this invention, a precursor is obtained by heating a tin source, an organophosphorus source, and oxidized worm-like carbon in a water bath. The precursor is then transferred to a tube furnace and calcined under an inert atmosphere. During the calcination process, the tin source reacts with the organophosphorus source to form cubic tin pyrophosphate (SnP2O7), while the organophosphorus source decomposes to form a phosphorus-doped carbon (CP) framework. The phosphorus-doped carbon (CP) and oxidized worm-like carbon together construct a continuous conductive network. The resulting composite anode material is composed of tin pyrophosphate, phosphorus-doped carbon, and oxidized worm-like carbon. Tin pyrophosphate particles are uniformly anchored on the continuous conductive network constructed by the oxidized worm-like carbon and phosphorus-doped carbon. Finally, a high-performance layered composite anode material SnP2O7 / CP / Owc is successfully synthesized. The preparation process of this invention is simple and fast, requires no high-pressure reactor, and is highly safe; water bath heating (60-90℃) replaces the traditional hydrothermal method (180-220℃), the reaction conditions are mild, and no inert atmosphere is required; the calcination temperature is low and the time is short, significantly reducing energy consumption and cost; the raw materials used are widely available, environmentally friendly, and do not produce toxic gases, making it suitable for large-scale production.

[0021] 3. This invention achieves a lithium-ion diffusion coefficient of up to 3.1×10⁻¹²cm² / s in the composite anode material through the synergistic effect of oxidized worm carbon and phosphorus-doped carbon, which is 11 times that of pure SnP₂O₇. The charge transfer resistance is reduced to 20.7Ω, significantly improving reaction kinetics. Furthermore, the pseudocapacitance contribution reaches 75.29% at a scan rate of 2mV / s, ensuring high rate performance.

[0022] 4. The layered composite anode material SnP2O7 / CP / Owc obtained in this invention exhibits high discharge specific capacity, excellent cycle stability, and rate performance. After 150 cycles at a current density of 0.5 A / g, the discharge specific capacity reaches as high as 368.5 mAh / g; furthermore, LiNi 0.5 Mn 1.5 The O4 / / SnP2O7 / CP / Owc full cell exhibits an initial discharge specific capacity of 351.0 mAh / g at 1.4-4.4V and 0.6C, and retains a discharge specific capacity of 242.6 mAh / g after 200 cycles. This full cell boasts an energy density of 471.4 Wh / kg and a power density of 654.7 W / kg, enabling it to stably drive LED bulbs and demonstrate strong practical application capabilities. It shows great promise for applications in the full cell field. Attached Figure Description

[0023] Figure 1 The images are scanning electron microscope images of the expanded graphite (worm carbon) used in Examples 1-4 at low magnification. Figure 2 The images are scanning electron microscope images of the oxidized worm carbon prepared in Examples 1-4 at low magnification. Figure 3 The images are scanning electron microscope images of the expanded graphite (worm carbon) used in Examples 1-4 at high magnification. Figure 4 These are scanning electron microscope images of the oxidized worm carbon prepared in Examples 1-4 at high magnification; Figure 5 FT-IR images of the oxidized worm carbon prepared in Examples 1-4; Figure 6 The XRD pattern of the product prepared in Example 1; Figure 7 This is a high-resolution XPS image of the C element in the product prepared in Example 1; Figure 8 SEM image of the product prepared in Example 1; Figure 9 TEM image of the product prepared in Example 1; Figure 10The rate performance of the half-cell assembled from the product prepared in Example 1 at current densities of 0.1, 0.2, 0.5, 1, 2, and 0.1 A / g is shown in the graph. Figure 11 The impedance diagram of the half-cell assembled from the product prepared in Example 1 before cycling; Figure 12 The graph shows the proportion of pseudocapacitive contribution of the half-cell assembled from the product prepared in Example 1 at a scan rate of 2mV / s. Figure 13 The graph shows the cycling performance of the half-cell assembled from the product prepared in Example 1 at a current density of 0.5 A / g. Figure 14 This is a comparison chart of the cycle performance of full cells assembled with the product prepared in Example 1 at different N / P ratios (capacity ratio of negative electrode and positive electrode); Figure 15 The image shows the LED light emission pattern of a full-cell battery with an N / P ratio (anode to cathode capacity ratio) of 1:1.4, prepared for Example 1. Figure 16 The image shows the XRD pattern of the product prepared in Example 2. Figure 17 SEM image of the product prepared in Example 2; Figure 18 TEM image of the product prepared in Example 2; Figure 19 The rate performance of the half-cell assembled from the product prepared in Example 2 at current densities of 0.1, 0.2, 0.5, 1, 2, and 0.1 A / g is shown in the graph. Figure 20 The graph shows the contribution percentage of pseudocapacitance to the half-cell assembled from the product prepared in Example 2 at a scan rate of 2mV / s. Figure 21 The image shows the XRD pattern of the product prepared in Example 3. Figure 22 SEM image of the product prepared in Example 3; Figure 23 The image shows the XRD pattern of the product prepared in Example 4; Figure 24 This is a SEM image of the product prepared in Example 4. Detailed Implementation

[0024] The present invention will be further described below with reference to some specific embodiments.

[0025] The preparation method of oxidized worm carbon in this embodiment of the invention is as follows: 15 mL of concentrated sulfuric acid (mass concentration of 98%) and 45 mL of concentrated nitric acid (mass concentration of 68%) are placed in a round-bottom three-necked flask, 2 g of expanded graphite (worm carbon) is added, and the mixture is stirred and refluxed in an 80°C constant temperature water bath for 2 h. The mixture is then poured into a beaker containing deionized water and allowed to stand for 24 h. The supernatant is discarded, the bottom layer is centrifuged until neutral, the solid phase product is collected, and the product is freeze-dried to obtain oxidized worm carbon (Owc).

[0026] The expanded graphite (worm carbon) is a loose and porous worm-like material with a fixed carbon content of ≥99% and a particle size of 2500 mesh.

[0027] Figure 1 The image shown is a scanning electron microscope image of the expanded graphite (worm carbon) used in this embodiment at low magnification. It can be seen that the expanded graphite (worm carbon) is composed of worm-like aggregates. Figure 2 The image shown is a scanning electron microscope image of the oxidized worm-like carbon prepared in this embodiment at low magnification. It can be seen that the originally dense worm-like aggregates decomposed after acid treatment.

[0028] Figure 3 The image shown is a scanning electron microscope image of the expanded graphite (worm carbon) used in this embodiment at high magnification. The irregular bending and protruding structures on its surface can be observed, resembling the segments or folds of a worm. Figure 4 The image shown is a scanning electron microscope image of the oxidized worm carbon prepared in this embodiment at high magnification. It can be seen that the oxidized worm carbon exhibits a sheet-like structure, and these sheets are stacked together to form a relatively loose morphology.

[0029] Figure 5 The FT-IR image of the oxidized worm carbon (Owc) prepared in this embodiment shows a large number of surface hydroxyl groups (-OH), carbonyl groups (C=O) and carboxyl groups (-COO⁻). These oxygen-containing functional groups enable the oxidized worm carbon (Owc) to effectively combine with the tin and phosphorus sources, which can make the raw materials more uniformly dispersed and is conducive to the formation of high-purity tin pyrophosphate during calcination.

[0030] Example 1 A method for preparing a layered tin pyrophosphate composite anode material (SnP2O7 / CP / Owc) includes the following steps: Step A: Disperse 0.4g of stannous citrate and 0.07g of carbon monoxide in 40mL of deionized water to obtain a suspension of stannous citrate and carbon monoxide. Disperse 0.824g of 70wt.% phytic acid aqueous solution in 16mL of deionized water, and then add the phytic acid solution dropwise to the suspension of stannous citrate and carbon monoxide. Sonicate for 30min. Heat the mixture in a 90℃ water bath for 2h. Freeze-dry the mixture under vacuum at -50℃ for 12h, and then freeze-dry it under vacuum at 120℃ for 12h. Freeze-drying can maintain a better morphology and prevent liquid splashing during direct vacuum drying. Step B: Transfer the precursor obtained in step A to a tube furnace, pre-calcine at 350°C for 2 hours under a nitrogen atmosphere, then raise the temperature to 650°C at a rate of 10°C / min and continue calcining for 4 hours. Step C: Cool the heat-treated product obtained in Step B to room temperature under flowing nitrogen, grind for 30 minutes, and obtain the product.

[0031] Preparation method of stannous citrate: 1.2g SnCl2·2H2O was placed in a beaker, and 40mL of anhydrous ethanol and 20mL of polyethylene glycol were added in sequence. The mixture was stirred until a homogeneous solution was formed. Then, 0.74g of citric acid monohydrate was weighed and dissolved in the above mixed solution. Deionized water was added dropwise at a rate of 1d / s, and a milky white precipitate was produced. The addition was continued until no more precipitate was formed. The mixture was stirred for 10min and then allowed to stand for 24h. The supernatant was discarded, and the residue was centrifuged and dried to obtain white stannous citrate.

[0032] Figure 6 The XRD pattern of the product prepared in this embodiment shows that all diffraction peaks can be attributed to tin pyrophosphate, indicating high purity of the material. High purity is beneficial to the electrochemical performance of the composite anode material.

[0033] Figure 7 The high-resolution XPS image of the carbon element in the product prepared in this embodiment shows three distinct peaks: CC, CP, and CO. The CP peak indicates that phosphorus has been successfully doped into carbon, which not only improves the material's conductivity but also generates many external defects and LiP. + The diffusion of active sites will greatly improve the electrochemical performance of composite anode materials.

[0034] Figure 8 This is a SEM image of the product prepared in this embodiment. The image shows that the tin pyrophosphate nanoparticles are well dispersed and uniformly anchored on a conductive network constructed from P-doped carbon and oxidized worm-like carbon. The material exhibits an irregular sheet-like or layered stacked structure, with a surface that is not smooth but has obvious wrinkles and undulations, presenting a loose and porous appearance, which is beneficial for Li… +The migration of these molecules improves the electrochemical performance of the composite anode material.

[0035] Figure 9 The TEM image of the product prepared in this embodiment shows that the average particle size is 13.4 nm and the carbon layer thickness is 8-10 nm. The nanoscale particle size can effectively shorten the diffusion path of Li⁺ and reduce the ion transport resistance. The presence of carbon can not only improve the electronic conductivity of the material, but also suppress the volume expansion of the material during the lithium ion insertion and extraction process, thereby improving the electrochemical performance of the material.

[0036] Figure 10 The half-cell assembled from the product prepared in this embodiment (where the working electrode is composed of 80 wt.% active material (the composite material of this embodiment), 10 wt.% conductive agent conductive carbon black and 10 wt.% binder polyvinylidene fluoride, and the counter electrode is a commercial lithium sheet) was cycled 10 times each at current densities of 0.1, 0.2, 0.5, 1 and 2 A / g, and then restored to the rate performance diagram at a current density of 0.1 A / g. After 40 cycles (current density of 1 A / g), the discharge specific capacity can still reach 317.9 mAh / g, and the material exhibits good rate performance.

[0037] Figure 11 The AC impedance diagram of the half-cell assembled from the product of this embodiment before cycling is shown. The working electrode is composed of 80 wt.% active material (the composite material of this embodiment), 10 wt.% conductive agent conductive carbon black and 10 wt.% binder polyvinylidene fluoride. The counter electrode is a commercial lithium sheet with a lithium-ion diffusion coefficient of 3.1 × 10⁻¹² cm² / s, which is 11 times that of SnP₂O₇. The charge transfer resistance is reduced to 20.7 Ω, which significantly improves the reaction kinetics.

[0038] Figure 12 The half-cell assembled from the product prepared in this embodiment (where the working electrode is composed of 80 wt.% active material (the composite material of this embodiment), 10 wt.% conductive agent conductive carbon black and 10 wt.% binder polyvinylidene fluoride, and the counter electrode is a commercial lithium sheet) showed a pseudocapacitive contribution of 75.29% at a scan rate of 2 mV / s, directly revealing that the dominant lithium storage mechanism of the material is pseudocapacitive behavior, which is the main reason why the material has excellent rate performance.

[0039] Figure 13The diagram shows the cycling performance of the half-cell assembled from the product prepared in this embodiment at a current density of 0.5 A / g. The working electrode consists of 80 wt.% active material (the composite material in this embodiment), 10 wt.% conductive agent conductive carbon black, and 10 wt.% binder polyvinylidene fluoride. The counter electrode is a commercial lithium sheet. After 150 cycles at 0.5 A / g, the discharge specific capacity of the material is 368.5 mAh / g, demonstrating a high discharge specific capacity.

[0040] Figure 14 The graph shows a comparison of the cycle performance of full cells assembled with different N / P ratios (capacity ratio of negative electrode and positive electrode) for this embodiment, where the N / P ratio is 1:1.4 for LiNi. 0.5 Mn 1.5 O4 / / SnP2O7 / CP / Owc full cell (positive electrode composition: active material LiNi) 0.5 Mn 1.5 The composition of the negative electrode is as follows: 85 wt.% O4, 10 wt.% conductive carbon black, and 5 wt.% polyvinylidene fluoride binder; The negative electrode composition is: 80 wt.% active material (composite material of this embodiment), 10 wt.% conductive carbon black, and 10 wt.% polyvinylidene fluoride binder. At 1.4-4.4V and 0.6C, the initial discharge specific capacity is 351.0 mAh / g, and after 200 cycles, the discharge specific capacity remains as high as 242.6 mAh / g. Based on the formulas for calculating energy density and power density: Energy density (Wh / kg) = (Discharge capacity (mAh) × Average discharge voltage (V)) / Mass (g) and Power density (W / kg) = (Discharge current (mA) × Average discharge voltage (V)) / Mass (g), the energy density of this full cell can reach 471.4 Wh / kg, and the power density can reach 654.7 W / kg.

[0041] Figure 15 In this embodiment, the product was assembled into LiNi with an N / P ratio (anode to cathode capacity ratio) of 1:1.4. 0.5 Mn 1.5 O4 / / SnP2O7 / CP / Owc full cell (positive electrode composition: active material LiNi) 0.5 Mn 1.5 The composition of the negative electrode is as follows: the active material is the composite material of this embodiment (80 wt.%), the conductive agent is carbon black (10 wt.%), and the binder is polyvinylidene fluoride (5 wt.%). The LED light emission diagram shows that the full cell assembled with the material of this invention can stably drive the LED bulb to emit light continuously.

[0042] Example 2 A method for preparing a tin pyrophosphate composite anode material (SnP2O7 / CP / Owc) includes the following steps: Step A: Disperse 0.4g of stannous citrate and 0.05g of carbon monoxide in 40mL of deionized water to obtain a suspension of stannous citrate and carbon monoxide; disperse 0.824g of 70wt.% phytic acid aqueous solution in 16mL of deionized water, then add the phytic acid solution dropwise to the suspension of stannous citrate and carbon monoxide, sonicate for 30min, heat the mixture in a 90℃ water bath for 2h, freeze-dry the mixture under vacuum at -50℃ for 12h, and then freeze-dry it under vacuum at 120℃ for 12h. Step B: Transfer the precursor obtained in step A to a tube furnace, pre-calcine at 350°C for 2 hours under a nitrogen atmosphere, then raise the temperature to 650°C at a rate of 10°C / min and continue calcining for 4 hours. Step C: Cool the heat-treated product obtained in Step B to room temperature under flowing nitrogen, grind for 30 minutes, and obtain the product.

[0043] Preparation method of stannous citrate: 1.2g SnCl2·2H2O was placed in a beaker, and 40mL of anhydrous ethanol and 20mL of polyethylene glycol were added in sequence. The mixture was stirred until a homogeneous solution was formed. Then, 0.74g of citric acid monohydrate was weighed and dissolved in the above mixed solution. Deionized water was added dropwise at a rate of 1d / s, and a milky white precipitate was produced. The addition was continued until no more precipitate was formed. The mixture was stirred for 10min and then allowed to stand for 24h. The supernatant was discarded, and the residue was centrifuged and dried to obtain white stannous citrate.

[0044] Figure 16 The XRD pattern of the product prepared in this embodiment shows that all diffraction peaks can be attributed to tin pyrophosphate, indicating high purity of the material. High purity is beneficial to the electrochemical performance of the material.

[0045] Figure 17 The image shows a SEM image of the product prepared in this embodiment. It can be seen from the image that the nanoscale particles of tin pyrophosphate are anchored on a conductive network constructed by P-doped carbon and oxidized worm carbon. The material is mainly in the form of blocky and thick sheet structures, with the sheets stacked to form a layered structure. There are few wrinkles and undulations, and the surface is relatively flat.

[0046] Figure 18 The TEM image of the product prepared in this embodiment shows an average particle size of 11.7 nm, from which the carbon layer thickness can be determined to be 5.5-7 nm. The nanoscale particle size can effectively shorten the diffusion path of Li⁺ and reduce ion transport resistance. The presence of carbon can not only improve the electronic conductivity of the material but also suppress the volume expansion of the material during lithium-ion insertion / extraction, thereby improving the electrochemical performance of the material.

[0047] Figure 19 The half-cell assembled from the product prepared in this embodiment (where the working electrode is composed of 80 wt.% active material (the composite material of this embodiment), 10 wt.% conductive agent conductive carbon black and 10 wt.% binder polyvinylidene fluoride, and the counter electrode is a commercial lithium sheet) was cycled 10 times each at current densities of 0.1, 0.2, 0.5, 1 and 2 A / g, and then restored to the rate performance diagram at a current density of 0.1 A / g. After 40 cycles (current density of 1 A / g), the discharge specific capacity can reach 260.2 mAh / g.

[0048] Figure 20 The half-cell assembled from the product prepared in this embodiment (where the working electrode consists of 80 wt.% active material, 10 wt.% conductive agent and 10 wt.% binder, and the counter electrode is a commercial lithium sheet) showed a pseudocapacitive contribution of 70.2% at a scan rate of 2 mV / s, directly revealing that the dominant lithium storage mechanism of the material is pseudocapacitive behavior, which is the main reason why the material has good rate performance.

[0049] Example 3 A method for preparing a tin pyrophosphate composite anode material (SnP2O7 / CP / Owc) includes the following steps: Step A: 1.36g SnO2 and 0.39g oxidized worm carbon were sequentially dispersed in 140mL of deionized water to obtain a suspension of tin dioxide and oxidized worm carbon; 4.11g ethylenediaminetetramethylenephosphonic acid was dispersed in 40mL of deionized water, and then this mixture was added dropwise to the suspension of tin dioxide and oxidized worm carbon, stirred for 30min, and the mixture was heated in an 80℃ water bath for 3h. The mixture was then freeze-dried in a vacuum at -50℃ for 12h, and then vacuum-dried in a vacuum at 60℃ for 12h. Step B: Transfer the precursor obtained in step A to a tube furnace, pre-calcine at 400°C for 1 hour under a nitrogen atmosphere, then raise the temperature to 500°C at a rate of 10°C / min and continue calcining for 5 hours. Step C: Cool the heat-treated product obtained in step B to room temperature under flowing nitrogen, grind for 20 minutes, and obtain the product.

[0050] Figure 21 The XRD pattern of the product prepared in this embodiment shows that all diffraction peaks can be attributed to tin pyrophosphate, indicating high purity of the material. High purity is beneficial to the electrochemical performance of the material.

[0051] Figure 22 This is a SEM image of the product prepared in this embodiment. The image shows that the material is sheet-like, with many nanoscale particles attached to the surface of the sheets. The sheets are stacked to form a loose, layered structure, which is beneficial for Li… +The migration of these molecules improves the electrochemical performance of the composite anode material.

[0052] Example 4 A method for preparing a tin pyrophosphate composite anode material (SnP2O7 / CP / Owc) includes the following steps: Step A: 1.36g SnO2 and 0.28g oxidized worm carbon were sequentially dispersed in 140mL of deionized water to obtain a suspension of tin dioxide and oxidized worm carbon; 4.11g ethylenediaminetetramethylenephosphonic acid was dispersed in 40mL of deionized water, and then this mixture was added dropwise to the suspension of tin dioxide and oxidized worm carbon, stirred for 30min, and the mixture was heated in an 80℃ water bath for 3h. The mixture was then freeze-dried in a vacuum at -50℃ for 12h, and then vacuum-dried in a vacuum at 60℃ for 12h. Step B: Transfer the precursor obtained in step A to a tube furnace, pre-calcine at 400°C for 1 hour under a nitrogen atmosphere, then raise the temperature to 600°C at a rate of 10°C / min and continue calcining for 5 hours. Step C: Cool the heat-treated product obtained in step B to room temperature under flowing nitrogen, grind for 15 minutes, and obtain the product.

[0053] Figure 23 The XRD pattern of the product prepared in this embodiment shows that all diffraction peaks can be attributed to tin pyrophosphate, indicating high purity of the material. High purity is beneficial to the electrochemical performance of the material.

[0054] Figure 24 This is a SEM image of the product prepared in this embodiment. The image shows that the material is sheet-like, with stacked layers forming a loose, layered structure. Many nanoscale particles are attached to the surface of the sheets, and wrinkles appear on the surface, which is beneficial for Li… + The migration of these molecules improves the electrochemical performance of the composite anode material.

[0055] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Any other modifications or equivalent substitutions made by those skilled in the art to the technical solutions of the present invention, as long as they do not depart from the spirit and scope of the technical solutions of the present invention, should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing a layered tin pyrophosphate composite negative electrode material, characterized by, It comprises the following steps: The tin source, the organic phosphorus source and the oxidized vermicular carbon are dispersed in water, and a precursor is obtained by water bath heating; the precursor is calcined under an inert atmosphere to obtain the layered tin pyrophosphate composite negative electrode material.

2. The method of claim 1, wherein the layered tin pyrophosphate composite negative electrode material is prepared by the following steps of: The oxidized vermicular carbon is prepared by adding a strong acid solution into a reaction bottle, adding expanded graphite, and then stirring and performing an oxidation reaction under reflux conditions. ​ 3. The method of claim 2, wherein the layered tin pyrophosphate composite negative electrode material is prepared by the following steps of: The strong acid solution is a mixture of concentrated sulfuric acid and concentrated nitric acid, and the amount ratio of the expanded graphite, the concentrated sulfuric acid and the concentrated nitric acid is 1-3g:10-20mL:30-60mL. ​ 4. The method of claim 2, wherein the layered tin pyrophosphate composite negative electrode material is prepared by the following steps of: The oxidation reaction temperature is 70-90℃, and the time is 1-3h. ​ 5. The method of claim 1, wherein the layered tin pyrophosphate composite negative electrode material is prepared by the following steps of: The tin source is one or more of SnCl4·5H2O, SnCl2·2H2O, SnCl4, SnCl2, tin acetate, tin hydroxide, tin dioxide, stannous oxide, stannous oxalate and stannous citrate. ​ 6. The method of claim 1, wherein the layered tin pyrophosphate composite negative electrode material is prepared by the following steps of: The organic phosphorus source is one or more of phytic acid, hydroxyethylidene-1,1-diphosphonic acid and ethylenediaminetetramethylene phosphonic acid. ​ 7. The method for preparing the layered tin pyrophosphate composite anode material according to claim 1, characterized in that, The molar ratio of tin in the tin source to phosphorus in the organic phosphorus source is 1:2.4-10, and the mass of the oxidized vermicular carbon is 10-20% of the theoretical mass of the generated tin pyrophosphate.

8. The method for preparing the layered tin pyrophosphate composite anode material according to claim 1, characterized in that, The water bath heating temperature is 60-90℃, and the time is 1-3h.

9. The method for preparing the layered tin pyrophosphate composite anode material according to claim 1, characterized in that, The calcination treatment is: pre-burning at 300-400℃ for 1-3h, then increasing the temperature to 500-700℃ at a rate of 5-10℃ / min, and continuing to calcine for 3-5h. 10.A layered tin pyrophosphate composite negative electrode material prepared by the preparation method of any one of claims 1-9.

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