Lithium nano cellulose carbon coated lithium iron phosphate composite material, preparation method thereof, positive electrode material, positive electrode plate and battery

Abstract

The invention focuses on the technical field of lithium ion battery positive electrode materials, and particularly relates to a lithiated nanocellulose carbon coated lithium iron phosphate composite material, a preparation method of the material, and a positive electrode material, a positive plate and a battery based on the material. The lithium iron phosphate composite material comprises a lithium iron phosphate matrix and a carbon layer coated on the surface of the matrix, the carbon layer is formed by carbonizing lithiated nano cellulose gel and is a conductive layer with a three-dimensional continuous porous network structure; in the carbonization process, the lithium element in the lithiated nanocellulose gel can be migrated to a lithium iron phosphate matrix, so that an in-situ lithium compensation effect is exerted on the lithium iron phosphate matrix. According to the invention, a continuous three-dimensional conductive network is successfully constructed under the condition of extremely low carbon content, and the carbon layer is used as a structural framework, so that the particle aggregation phenomenon is effectively inhibited. Accurate and gradient compensation of lithium loss from the table to the interior is realized, and crystal lattices are effectively repaired. The obtained material has high specific capacity, excellent rate capability and ultra-long cycle life.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery cathode material technology, specifically to a lithium-ion nanocellulose carbon-coated lithium iron phosphate composite material, its preparation method, cathode material, cathode sheet, and battery. Background Technology

[0002] Lithium iron phosphate (LiFePO4) Cathode materials have high safety and long cycle life, and are widely used in lithium-ion batteries. However, their inherent low electronic conductivity and lithium-ion diffusion coefficient limit their performance in fast charge and discharge.

[0003] Carbon coating technology is an improvement One of the effective means of improving the electrochemical performance of materials is to optimize the structure and properties of the carbon coating layer using different materials, which can significantly improve the conductivity and lithium-ion diffusion rate of composite electrode materials. In the existing technology, natural small molecule organic compounds such as glucose, sucrose, citric acid, etc., or artificial polymers such as polyacrylonitrile, phenolic resin, etc. are often used as carbon sources. During the high-temperature carbonization process, these carbon sources often cause the following problems in the coating layer due to violent pyrolysis and shrinkage: (1) poor continuity, incomplete coating forming island-like or dot-like coatings, limiting the electronic conduction path; (2) dense structure, although improving electronic conductivity, may hinder the diffusion of lithium ions; (3) uneven bonding with the matrix, which easily causes secondary agglomeration of active particles. In addition, during the high-temperature solid-state sintering process, lithium migration of lithium iron phosphate is inevitable, which will cause the final product to deviate from the stoichiometry and generate such Impurities such as impurities disrupt the integrity of the crystal structure, leading to continuous irreversible consumption of active lithium and structural degradation during charging and discharging. This is a key intrinsic factor restricting the stability of its long cycle life.

[0004] Therefore, developing a novel composite material that integrates the two major functions of "uniformly constructing a conductive network" and "controllable lithium compensation" and achieves efficient coating with low carbon content, along with its simple preparation method, is crucial for breakthroughs in [the field of composite materials]. Overcoming the electrochemical performance bottleneck of cathode materials is of great significance for promoting the development of next-generation high-performance batteries.

[0005] In recent years, natural polymer materials have gained attention due to their wide availability and green, renewable nature. Cellulose, as a natural polymer material, is rich in hydroxyl functional groups. When decomposed to a microscale, it exhibits excellent hydrophilicity and three-dimensional network gelation ability, making it an ideal carbon precursor. The irradiation oxidation degradation method for preparing nanocellulose provides an efficient and clean way to obtain nano-carbon sources with high aspect ratios and high reactivity. This process does not require large amounts of strong acids or bases, making it environmentally friendly. After carbonization, this nanocellulose readily forms a porous network structure. Furthermore, after lithium modification and coating with lithium iron phosphate for carbonization, it forms a carbon network with synergistic enhancement of physical crosslinking and chemical bonding, resulting in more efficient electron transport and greater structural stability during cycling. Summary of the Invention

[0006] To address the problems existing in the prior art, this invention provides a lithium-ionized nanocellulose carbon-coated lithium iron phosphate composite material and its preparation method, as well as a cathode material, cathode sheet, and battery based on this material. This invention aims to solve the problems existing in the prior art. The present invention addresses the problems of incomplete conductive network and uncontrollable lithium cycling loss in low-carbon coated cathode materials. Specifically, these problems include: 1) how to achieve uniform and robust three-dimensional conductive coating under low carbon content conditions; 2) how to accurately and efficiently compensate for lithium loss during high-temperature preparation and repair lattice defects; and 3) how to strengthen the interface between the carbon layer and the substrate to improve the long cycle life of the material.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A lithium-ion nanocellulose carbon-coated lithium iron phosphate composite material comprises a lithium iron phosphate matrix and a carbon layer coated on its surface; the carbon layer is formed by carbonization of lithium-ion nanocellulose gel, and the carbon layer is a conductive layer with a three-dimensional continuous porous network structure; the lithium element in the lithium-ion nanocellulose gel can migrate to the lithium iron phosphate matrix during the carbonization process, playing an in-situ lithium compensation role.

[0009] The lithium-modified nanocellulose is obtained by dual modification of nanocellulose prepared by irradiation degradation and partial esterification. Its surface simultaneously contains lithium ions loaded through ion exchange and groups bonded to cellulose hydroxyl groups via phosphate ester bonds. In this invention, lithium ions are introduced into the lithium-modified nanocellulose through an ion exchange reaction. The selected lithium compounds include at least one of lithium hydroxide, lithium nitrate, and lithium acetate. These lithium compounds serve as lithium sources and can release lithium ions during the subsequent carbonization process, achieving in-situ lithium compensation of the lithium iron phosphate matrix. Lithium hydroxide and lithium acetate are explicitly mentioned in the preparation method as lithium salts used in step S2.

[0010] The mass ratio of the lithium-ionized nanocellulose gel to the lithium iron phosphate matrix is ​​1:1 to 10:1.

[0011] The thickness of the carbon layer is 5-50 nm, and the particle size range of the composite material is 100-500 nm.

[0012] The method for the three-dimensional network carbon-coated lithium iron phosphate composite material with gradient lithium compensation function is characterized by comprising the following steps:

[0013] S1: Preparation of lithium iron phosphate crystals;

[0014] S2: Preparation of lithium-modified nanocellulose gel: Using nanocellulose prepared by irradiation degradation as raw material, it is first subjected to ion exchange reaction with lithium salt solution, and then partially esterified with lithium dihydrogen phosphate solution to obtain modified nanocellulose gel.

[0015] S3: Core-shell precursor preparation: The lithium iron phosphate matrix powder obtained in step S1 is uniformly dispersed in the lithium-modified nanocellulose sol obtained in step S2. After drying, a dried precursor with a "lithium iron phosphate core-modified cellulose shell" structure is obtained.

[0016] S4. Segmented controlled-release carbonization: The dried precursor is placed in an inert atmosphere and subjected to a two-stage heat treatment:

[0017] First stage: Heat treatment at 300℃ to 500℃ to promote cross-linking of lithium-modified nanocellulose, release the first part of lithium, and then implement lithium compensation on the surface of lithium iron phosphate particles.

[0018] The second stage involves heat treatment at 500℃ to 750℃ to carbonize the cross-linked cellulose skeleton and form a three-dimensional network carbon layer. At the same time, a second portion of lithium is released, which diffuses into the interior of the lithium iron phosphate particles, forming a concentration gradient.

[0019] The lithium iron phosphate crystals are synthesized by a liquid-phase method, which includes mixing a lithium source, an iron source, and a phosphorus source in a molar ratio of 1:1:1-1.2 and reacting them at 120-200℃ for 2-8 hours.

[0020] In step S2, the lithium salt is lithium hydroxide or lithium acetate; the radiation used in the irradiation degradation method is gamma rays or electron beams.

[0021] The heat treatment is a staged calcination process, including:

[0022] Start heating from room temperature, until

[0023] The first stage of calcination temperature is 300-500℃, and the holding time is 2-4 hours;

[0024] The second stage of calcination is at a temperature of 500-750℃, and the holding time is 4-8 hours.

[0025] The heating rate is 5-15℃ / min.

[0026] A lithium-ion battery cathode material comprising the aforementioned nanocellulose carbon-coated lithium iron phosphate composite material.

[0027] A lithium-ion battery positive electrode sheet comprising the aforementioned positive electrode material.

[0028] A lithium-ion battery comprising the aforementioned positive electrode.

[0029] Preparation with This is a carbon-coated composite cathode material with a core-shell structure, based on lithium iron phosphate particles. The surface of these particles is coated with an ultrathin, three-dimensional, continuous porous carbon network. The carbon layer is formed by carbonizing lithium-modified nanocellulose through a dual process of "ion exchange" and "partial esterification," with a carbon content controlled between 0.5-2.0 wt%. Its core innovation lies in the lithium concentration gradient within the lithium iron phosphate particles, from the shell to the core. The shell surface is rich in lithium and carbon, while the lithium content inside the core is relatively flat, thus repairing bulk defects.

[0030] The preparation method is as follows, which includes four key steps:

[0031] Matrix preparation steps: Synthesize lithium iron phosphate matrix powder.

[0032] Precursor modification steps: Nanocellulose was prepared by irradiation degradation, followed by dual chemical modification of ion exchange and partial esterification to introduce two lithium species, one weakly bound and one strongly bound.

[0033] Core-shell structure construction: The matrix powder is uniformly dispersed in modified cellulose sol and then spray-dried or freeze-dried to form a "lithium iron phosphate core-modified cellulose shell" precursor.

[0034] Segmented controlled-release carbonization: Segmented heat treatment is carried out under an inert atmosphere. The first stage (heating from room temperature to 300-500℃) causes cellulose to crosslink and releases weakly bound lithium to compensate for the particle surface; the second stage (500-750℃) completes carbonization to form a conductive network, while strongly bound lithium is slowly released and diffuses into the particle interior, forming a lithium concentration gradient.

[0035] Compared to existing technologies, the beneficial effect of this invention lies in achieving the organic unity of three major technical effects:

[0036] Conductivity and structure are unified: a continuous three-dimensional conductive network was successfully constructed under extremely low carbon content conditions. At the same time, the carbon layer, as a structural framework, effectively suppressed particle aggregation.

[0037] Unified Lithium Supplementation and Repair: The organic combination of "dual lithium source" and "segmented carbonization" processes enables precise and gradient compensation for lithium loss from the surface to the interior, thereby effectively repairing the crystal lattice.

[0038] Performance and process are unified: the resulting materials exhibit high specific capacity, excellent rate performance, and ultra-long cycle life. For example, the composite cathode material patent applied for by Suzhou Emmet Materials Technology Co., Ltd., and the easily prepared fibrous phosphorus as a lithium-ion battery anode material, both demonstrate excellent electrochemical performance. At the same time, the process is based on biomass materials, making it green and easy to scale up. Attached Figure Description

[0039] Figure 1 This is a scanning electron microscope (SEM) image of the three-dimensional network carbon-coated lithium iron phosphate composite material (LFP@GLCNF) with gradient lithium compensation function prepared in Example 1 of the present invention.

[0040] Figure 2 In this invention Figure 1 SEM image of the sample at high magnification.

[0041] Figure 3 This is a SEM image of Comparative Example 1 (uncoated raw lithium iron phosphate) in this invention.

[0042] Figure 4 This is a SEM image of the sample prepared in Comparative Example 2 of this invention (coated with unlithiated pure nanocellulose).

[0043] Figure 5 This is a SEM image of the sample prepared in Comparative Example 3 of the present invention (using a physical mixture of glucose and lithium carbonate as a carbon source and a lithium source).

[0044] in: Figure 1 This demonstrates a nanonetwork structure that is uniformly covered on the particle surface.

[0045] Figure 2 The morphology of the lithium iron phosphate particle distribution is shown more clearly.

[0046] Figure 3 It showed obvious particle aggregation.

[0047] Figure 4 The carbon layer structure shows poor continuity.

[0048] Figure 5 This indicates that the carbon coating is uneven and there is a large amount of flocculent material. Detailed Implementation

[0049] The present invention will be further described in detail below with the aid of embodiments. It should be noted that the embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention.

[0050] A lithium-ion nanocellulose carbon-coated lithium iron phosphate composite material comprises a lithium iron phosphate matrix and a carbon layer coated on its surface; the carbon layer is formed by carbonization of lithium-ion nanocellulose gel, and the carbon layer is a conductive layer with a three-dimensional continuous porous network structure; the lithium element in the lithium-ion nanocellulose gel can migrate to the lithium iron phosphate matrix during the carbonization process, playing an in-situ lithium compensation role.

[0051] The lithium-modified nanocellulose is obtained by irradiation degradation of nanocellulose, followed by dual modification through ion exchange and partial esterification. Its surface contains not only lithium ions loaded through ion exchange but also substances bonded to the cellulose hydroxyl groups via phosphate ester bonds. In this invention, lithium ions are introduced into the lithium-modified nanocellulose through an ion exchange reaction, and the selected lithium compounds include at least one of lithium hydroxide, lithium nitrate, and lithium acetate. These lithium compounds serve as lithium sources, releasing lithium ions during the subsequent carbonization process to achieve in-situ lithium compensation of the lithium iron phosphate matrix. Lithium hydroxide and lithium acetate are explicitly mentioned in the preparation method as lithium salts used in step S2.

[0052] The mass ratio of the lithium-ionized nanocellulose gel to the lithium iron phosphate matrix is ​​1:1 to 10:1.

[0053] The thickness of the carbon layer is 5-50 nm, and the particle size range of the composite material is 100-500 nm.

[0054] The method for the three-dimensional network carbon-coated lithium iron phosphate composite material with gradient lithium compensation function is characterized by comprising the following steps:

[0055] S1: Preparation of lithium iron phosphate matrix;

[0056] S2: Preparation of lithium-modified nanocellulose gel: Using nanocellulose prepared by irradiation degradation as raw material, it is first subjected to ion exchange reaction with lithium salt solution, and then partially esterified with lithium dihydrogen phosphate solution to obtain modified nanocellulose gel.

[0057] S3. Preparation of core-shell precursor: The lithium iron phosphate matrix powder obtained in step S1 is uniformly dispersed in the lithium-modified nanocellulose sol prepared in step S2, and after drying, a dried precursor with a "lithium iron phosphate core-modified cellulose shell" structure is obtained.

[0058] S4. Segmented controlled-release carbonization: The dried precursor is subjected to a two-stage heat treatment under an inert atmosphere:

[0059] The first stage involves heat treatment at 300℃ to 500℃ to crosslink the lithium-modified nanocellulose and release the first portion of lithium, thereby compensating for lithium on the surface of the lithium iron phosphate particles.

[0060] The second stage involves heat treatment at 500℃ to 750℃ to carbonize the cross-linked cellulose skeleton, forming a three-dimensional network carbon layer and releasing a second portion of lithium, which diffuses into the interior of the lithium iron phosphate particles to form a concentration gradient.

[0061] The lithium iron phosphate matrix is ​​synthesized by a liquid-phase method, which includes mixing a lithium source, an iron source, and a phosphorus source in a molar ratio of 1:1:1-1.2 and reacting them at 120-200℃ for 2-8 hours.

[0062] In step S2, the lithium salt is lithium hydroxide or lithium acetate; the radiation used in the irradiation degradation method is gamma rays or electron beams.

[0063] The heat treatment is a staged calcination process, including:

[0064] Start heating from room temperature, until

[0065] The first stage of calcination temperature is 300-500℃, and the holding time is 2-4 hours;

[0066] The second stage of calcination is at a temperature of 500-750℃, and the holding time is 4-8 hours.

[0067] The heating rate is 5-15℃ / min.

[0068] A lithium-ion battery cathode material comprising the aforementioned nanocellulose carbon-coated lithium iron phosphate composite material.

[0069] A lithium-ion battery positive electrode sheet comprising the aforementioned positive electrode material.

[0070] A lithium-ion battery comprising the aforementioned positive electrode.

[0071] Preparation with A core-shell structured carbon-coated composite cathode material: This material uses lithium iron phosphate particles as the matrix, with an ultrathin three-dimensional continuous porous carbon network coated on the surface. The carbon layer is formed by carbonizing lithium-modified nanocellulose through dual modification of "ion exchange" and "partial esterification," with the carbon content controlled between 0.5-2.0 wt%. Its core innovation lies in the lithium concentration gradient within the lithium iron phosphate particles, from the shell to the core. The shell surface is rich in lithium and carbon, while the lithium content inside the core is gradual, thus repairing bulk defects.

[0072] Preparation method: This method includes four key steps:

[0073] Matrix preparation: Synthesize lithium iron phosphate matrix powder.

[0074] Precursor modification: Nanocellulose was prepared by irradiation degradation and then subjected to dual chemical modification by ion exchange and partial esterification to introduce two lithium species, one weakly bound and one strongly bound.

[0075] Core-shell structure construction: The matrix powder is uniformly dispersed in modified cellulose sol and then spray-dried or freeze-dried to form a "lithium iron phosphate core-modified cellulose shell" precursor.

[0076] Segmented controlled-release carbonization: Segmented heat treatment is carried out under an inert atmosphere. The first stage (heating from room temperature to 300-500℃) causes cellulose to crosslink and releases weakly bound lithium to compensate for the particle surface; the second stage (500-750℃) completes carbonization to form a conductive network, while strongly bound lithium is slowly released and diffuses into the particle interior, forming a lithium concentration gradient.

[0077] Example 1:

[0078] Preparation of lithium iron phosphate matrix: Iron, lithium, and phosphorus sources were dissolved in deionized water at a Li:Fe:P molar ratio of 1.05:1:1, and the pH was adjusted to approximately 8.5 with ammonia. The mixed solution was transferred to a high-pressure reactor lined with polytetrafluoroethylene and reacted at 180°C for 10 hours. After the reaction, the mixture was allowed to cool naturally. The product was centrifuged, washed repeatedly with deionized water and ethanol, and then vacuum dried at 80°C for 12 hours to obtain the final product. Precursor powder. The powder was calcined at 650°C for 8 hours under a nitrogen atmosphere at a rate of 5°C / min to obtain a well-crystallized lithium iron phosphate matrix powder (denoted as LFP-01).

[0079] Preparation of lithium-modified nanocellulose gel:

[0080] a. Take 100g of nanocellulose hydrogel (solid content 2.0 wt%, fiber diameter about 5-20 nm, length several micrometers) prepared by electron beam irradiation degradation method.

[0081] b. Ion exchange procedure: 200 mL of 0.2 mol / L LiOH solution was added to the above gel, and the mixture was magnetically stirred at 40 °C for 6 hours. After the reaction was completed, centrifugation was performed, followed by washing with deionized water until neutral, finally yielding primary lithium-ionized cellulose nanoparticles.

[0082] c. Esterification: The primary lithium-ionized cellulose nanoparticles were redispersed in 200 mL of deionized water, and 50 mL of 0.1 mol / L [agent / solvent] was added. The solution was stirred and reacted in a 60°C water bath for 4 hours. After the reaction, centrifugation was performed, followed by washing with deionized water until near neutral. A homogeneous lithium-modified nanocellulose gel with a solid content of approximately 1.8 wt% was obtained. A portion of this gel was taken and diluted with deionized water to a 0.5 wt% sol for later use.

[0083] Core-shell precursor preparation: Weigh 10.0 g of LFP-01 powder prepared in step 1 and slowly add it to 200 g (equivalent to 1.0 g dry weight) of the above 0.5 wt% lithium-modified nanocellulose sol (i.e., dry basis mass ratio LFP : modified cellulose = 10 : 1). Place the mixture in a planetary ball mill and ball mill at 350 rpm for 3 hours to form a highly homogeneous slurry. Subsequently, dry the slurry using a spray dryer with an inlet temperature set at 180℃ and an outlet temperature of approximately 90℃ to obtain a dry precursor powder with good flowability.

[0084] Segmented controlled-release carbonization: The precursor powder is placed in an alumina boat within a tube furnace, and then high-purity nitrogen gas at a flow rate of 200 mL / min is introduced as a protective gas. Heat treatment is then performed according to the following procedure:

[0085] First stage: Increase the temperature from room temperature to 400℃ at a rate of 5℃ / min, and maintain the temperature at 400℃ for 2 hours.

[0086] Second stage: Continue to increase the temperature to 600℃ at a rate of 5℃ / min, and maintain the temperature at 600℃ for 6 hours.

[0087] After the process was completed, the product was allowed to cool naturally to room temperature. It was then removed, lightly ground, and passed through a 400-mesh sieve to obtain the final three-dimensional network carbon-coated lithium iron phosphate composite material, denoted as LFP@GLCNF-1. The carbon content was determined to be approximately 1.05 wt% using an elemental analyzer.

[0088] Example 2:

[0089] The main difference between this embodiment and Embodiment 1 lies in the process parameters of segmented carbonization, in order to explore the influence of different temperatures on gradient formation.

[0090] In step 4, the carbonization process is adjusted as follows:

[0091] First stage: Increase the temperature to 400℃ at a rate of 5℃ / min and hold for 1.5 hours.

[0092] Second stage: Increase the temperature to 650℃ at a rate of 2℃ / min and hold for 5 hours.

[0093] The remaining steps are exactly the same as in Example 1. The resulting product is designated LFP@GLCNF-2. Its carbon content is approximately 1.15 wt%.

[0094] Example 3:

[0095] The difference between this embodiment and Embodiment 1 lies in the dry-basis mass ratio of lithium iron phosphate to modified cellulose in the precursor.

[0096] In step 3, 5.0 g of LFP-01 powder was weighed and added to a sol containing the same dry weight (1.0 g) of modified cellulose (i.e., the mass ratio of LFP : modified cellulose = 5 : 1, and the total mass of the sol was adjusted accordingly). The spray drying and subsequent carbonization processes were exactly the same as in Example 1. The resulting product was designated LFP@GLCNF-3. Its carbon content was approximately 1.8 wt%.

[0097] Comparative Example 1 (Uncoated Sample):

[0098] Take 5.0 g of LFP-01 powder prepared in step 1 of Example 1, and calcine it directly at 700°C for 2 hours under a nitrogen atmosphere at a rate of 5°C / min to simulate the thermal history during the carbonization process. The uncoated control sample is obtained and is denoted as N-LFP.

[0099] Comparative Example 2 (Unlithiated cellulose nanoparticles coated):

[0100] Pure irradiated degraded nanocellulose gel, prepared using the same method as step a in Example 1 but without any lithium modification, was prepared as a 0.5 wt% sol. A precursor was prepared by mixing LFP-01 and pure cellulose dry basis in the same ratio as step 3 in Example 1 (LFP-01: pure cellulose dry basis = 10:1) and treated using the exact same carbonization procedure (380℃ + 620℃) as step 4 in Example 1. The resulting sample was designated LFP@CNF.

[0101] Comparative Example 3 (Traditional Hybrid Lithium Source Coating):

[0102] Weigh 10.0 g of LFP-01 powder, and mix it with 0.5 g of glucose (as a carbon source) and 0.12 g of... (As a lithium source, a molar amount of lithium equivalent to that in the modified cellulose of Example 1 is provided.) The mixture is thoroughly physically mixed in a mortar for 1 hour. The mixture is then placed in a tube furnace and heated directly to 700°C at a rate of 5°C / min under nitrogen protection and held for 6 hours. The resulting sample is designated LFP@Glu+Li.

[0103] Table 1

[0104] Sample number 1C discharge specific capacity (mAh / g) 10C / 0.1C capacity retention 1C rate 100-cycle capacity retention LFP@GLCNF-1 155.8 88.5% 99.2% LFP@GLCNF-2 156.3 89.1% 99.0% LFP@GLCNF-3 152.1 85.0% 98.5% N-LFP 120.5 45.2% 92.1% LFP@CNF 142.3 70.3% 95.8% LFP@Glu+Li 135.6 60.8% 93.5%

[0105] As shown in Table 1, the sample products (LFP@GLCNF series) exhibit excellent performance across all indicators. In particular, LFP@GLCNF-1 and LFP@GLCNF-2 maintain high specific capacity while also demonstrating excellent high-rate performance and cycling stability (capacity retention >96% after 100 cycles). This is directly attributable to their unique structure: a low-content yet continuous three-dimensional conductive carbon network ensures rapid electron transport; a gradient lithium compensation mechanism maximally repairs lattice defects and optimizes ion diffusion kinetics; and a reinforced interface ensures structural stability during long-term cycling.

[0106] Comparative Example 2 (LFP@CNF) showed limited performance improvement due to the lack of effective lithium compensation, with more significant degradation during cycling. Comparative Example 3 (LFP@Glu+Li) exhibited the worst performance due to uneven carbon layer and uncontrollable lithium compensation.

[0107] The above embodiments, combined with the figures, fully demonstrate that this invention, through the material design of lithium-ionized cellulose nanoparticles and the innovative core-shell carbon coating process, successfully combines low carbon content, high uniformity coating, high conductivity, and gradient lithium compensation, resulting in a significant improvement in overall performance. This synergistically solves the two core problems of poor conductivity and intrinsic cycle degradation in LFP materials. The provided three-dimensional network carbon-coated LFP composite material with gradient lithium compensation and its preparation method exhibit significant effectiveness and superiority.

[0108] 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. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and such modifications or substitutions should be covered within the scope of the claims of the present invention.

[0109] Electrode fabrication and battery assembly:

[0110] Weigh the above materials according to the mass ratio of composite electrode material, acetylene black, and polyvinylidene fluoride (PVDF) of 7:1:2. First, place the electrode material and acetylene black in a mortar and stir thoroughly to make them uniform before grinding. Then, add PVDF and grind thoroughly again. After all the substances are fully mixed, add 1 mL of deionized water and continue grinding for 5 minutes. Finally, use a spatula to evenly spread the viscous liquid onto the copper foil and place it in an 80°C oven to dry the moisture. Then, place it in a vacuum drying oven overnight. Use a slicer to cut the prepared sample into electrode sheets with a diameter of 10 mm and place them in a glove box to assemble button batteries.

Claims

1. A lithium-ionized nanocellulose carbon-coated lithium iron phosphate composite material, characterized in that: It includes a lithium iron phosphate matrix and a carbon layer coated on its surface; the carbon layer is formed by carbonization of lithium-ionized nanocellulose gel and is a conductive layer with a three-dimensional continuous porous network structure. During the carbonization process, the lithium element in the lithium-ionized nanocellulose gel can migrate to the lithium iron phosphate matrix, thereby providing in-situ lithium compensation.

2. The lithium-ionized nanocellulose carbon-coated lithium iron phosphate composite material according to claim 1, characterized in that, The lithium-modified nanocellulose is obtained by dual modification of nanocellulose prepared by irradiation degradation method through ion exchange and partial esterification. Its surface contains lithium ions loaded by ion exchange, as well as substances that are bound to cellulose hydroxyl groups through phosphate ester bonds.

3. The lithium-ionized nanocellulose carbon-coated lithium iron phosphate composite material according to claim 1, characterized in that, The mass ratio of the lithium-ionized nanocellulose gel to the lithium iron phosphate matrix is ​​1:1 to 10:

1.

4. The lithium-ionized nanocellulose carbon-coated lithium iron phosphate composite material according to claim 1, characterized in that, The thickness of the carbon layer is 5-50 nm, and the particle size range of the composite material is 100-500 nm.

5. A method for preparing the lithium-ionized nanocellulose carbon-coated lithium iron phosphate composite material according to claims 1-3, characterized in that, Includes the following steps: S1: Preparation of lithium iron phosphate matrix; S2: Preparation of lithium-modified nanocellulose gel: Using nanocellulose prepared by irradiation degradation as raw material, it is first subjected to ion exchange reaction with lithium salt solution, and then partially esterified with lithium dihydrogen phosphate solution to obtain modified nanocellulose gel. S3: Preparation of core-shell precursor: The lithium iron phosphate matrix powder obtained in step S1 is uniformly dispersed in the lithium-modified nanocellulose sol obtained in step S2. After drying, a dried precursor with a "lithium iron phosphate core-modified cellulose shell" structure is obtained. S4. Segmented controlled-release carbonization: The dried precursor undergoes a two-stage heat treatment in an inert atmosphere. First stage: Heat treatment at 300℃ to 500℃ to promote cross-linking of lithium-modified nanocellulose and release the first portion of lithium, and to implement lithium compensation on the surface of lithium iron phosphate particles. The second stage involves heat treatment at 500℃ to 750℃ to carbonize the cross-linked cellulose skeleton, construct a three-dimensional network carbon layer, and release a second portion of lithium, which diffuses into the interior of the lithium iron phosphate particles to form a concentration gradient. The lithium iron phosphate matrix is ​​synthesized by a liquid-phase method, which includes mixing a lithium source, an iron source, and a phosphorus source in a molar ratio of 1:1:1-1.2 and reacting them at 120-200℃ for 2-8 hours.

6. The method for lithium-ionized nanocellulose carbon-coated lithium iron phosphate composite material according to claim 5, characterized in that, In step S2, the lithium salt is lithium hydroxide or lithium acetate; the radiation used in the irradiation degradation method is gamma rays or electron beams.

7. The method for preparing the lithium-ionized nanocellulose carbon-coated lithium iron phosphate composite material according to claim 5, characterized in that, The heat treatment is a staged calcination process, including: Start heating from room temperature until... The first stage of calcination temperature is 300-500℃, and the holding time is 2-4 hours; The second stage of calcination is at a temperature of 500-750℃, and the holding time is 4-8 hours. The heating rate is 5-15℃ / min.

8. A lithium-ion battery cathode material comprising the lithium-ion nanocellulose carbon-coated lithium iron phosphate composite material as described in any one of claims 1-3.

9. A lithium-ion battery cathode sheet comprising the cathode material as described in claim 8.

10. A lithium-ion battery comprising the positive electrode as described in claim 9.

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