A C-LFP nanomaterial and its preparation method

Abstract

This invention belongs to the field of electrochemical materials, and particularly relates to a C-LFP nanomaterial and its preparation method. The method includes: 1) preparing a mixed solution containing an iron source and a carbon source, and performing a carbon-iron composite hydrothermal treatment to obtain a CF precursor; 2) carbonizing and calcining the CF precursor to obtain an Fe@C intermediate; 3) placing the Fe@C intermediate in a reaction reagent containing a phosphorus source and a lithium source for a secondary hydrothermal treatment to obtain an LFP@C intermediate; 4) placing the LFP@C intermediate in a nitrogen-containing modifier, stirring and modifying it, and then calcining it under a protective atmosphere to obtain the C-LFP nanomaterial. This invention constructs a C-LFP material with a core-double-shell hierarchical porous structure. The core is LFP nanocrystals, the inner shell is MOF-derived mesoporous carbon, and the outer shell is a nitrogen-doped carbon layer. The pore gradient and conductivity gradient of the double-layer carbon synergistically achieve electron-ion transport kinetic matching, exhibiting extremely low polarization and excellent cycling stability.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical materials, and particularly relates to a C-LFP nanomaterial and its preparation method. Background Technology

[0002] Lithium iron phosphate (LiFePO4, or LFP) is a cathode material for lithium-ion batteries. It has an olivine structure and, compared with traditional layered oxide cathode materials such as lithium cobalt oxide and ternary materials, LFP has outstanding advantages such as low raw material cost, environmental friendliness, excellent thermal stability and long cycle life. Therefore, it has been widely used in the fields of power batteries and energy storage.

[0003] However, lithium iron phosphate (LFP) has extremely low intrinsic electronic conductivity and a small lithium-ion diffusion coefficient, which severely limits its high-rate performance. To overcome these shortcomings, researchers have developed carbon-coated lithium iron phosphate (C-LFP) technology. This technology improves electronic conductivity by coating the LFP surface with a conductive carbon layer and shortens the lithium-ion diffusion path by constructing a porous structure.

[0004] The existing C-LFP preparation technologies mainly fall into the following three categories.

[0005] Solid-state method combined with mechanically mixed carbon source: Iron, lithium, phosphorus and carbon sources are mechanically mixed and then sintered at high temperature. This method is simple, but the contact between the carbon source and the LFP precursor depends on the randomness of mechanical mixing, making it difficult to achieve uniform coating. In addition, high-temperature sintering can easily lead to grain growth, making it difficult to achieve both high specific surface area and high crystallinity.

[0006] In-situ carbon coating using liquid-phase method: LFP / C composite materials are synthesized in one step via hydrothermal or solvothermal methods. Although the liquid-phase method is advantageous for controlling particle size, during the hydrothermal process, the organic carbon source often competes with LFP grain growth. Premature carbonization of the carbon source inhibits LFP crystallization, while excessive pursuit of LFP crystallinity leads to insufficient graphitization of the carbon layer and decreased conductivity. Furthermore, the one-step method makes it difficult to precisely control the pore structure of the carbon layer, often resulting in a dense, non-porous carbon layer that blocks lithium-ion transport channels.

[0007] Metal-organic framework derivatization: Using Fe-based MOFs as precursors, Fe@C composites are obtained through high-temperature carbonization, followed by sintering with lithium and phosphorus sources. This method can inherit the high specific surface area of ​​MOFs, but existing technologies typically employ high-temperature solid-state lithiation under an inert atmosphere or oxygen-free hydrothermal conversion. The former is prone to sintering of Fe nanoparticles and severe graphitization and shrinkage of the carbon framework due to prolonged high-temperature holding, resulting in a significant decrease in specific surface area. The latter, in the absence of an oxidant, also presents challenges in converting Fe to Fe2+. 2+ The problem of embedding olivine lattice leads to residual metallic iron impurities in the product, resulting in deterioration of electrochemical performance.

[0008] Furthermore, existing carbon coating technologies mostly employ a single carbon layer structure, which cannot simultaneously meet the dual requirements of "rapid internal electron transport" and "surface electrolyte wetting." How to construct C-LFP nanomaterials with a hierarchical porous structure and complete carbon layer coating, while resolving the contradiction between precise control of Fe valence state and grain size regulation, is a pressing technical problem in this field. Summary of the Invention

[0009] To address the problems that existing C-LFP preparation processes often result in residual impurities or lattice defects, making it difficult to simultaneously achieve high specific surface area and high crystallinity of the olivine phase, as well as the need to meet the requirements of rapid bulk electron transport and efficient surface ion embedding, this invention provides a C-LFP nanomaterial and its preparation method.

[0010] The main objective of this invention is: I. It is possible to construct C-LFP materials with multiple core-shell structures and hierarchical pore structures through layered and step-by-step template transformation; Second, it takes into account the synergistic effect of high specific surface area, high conductivity and fast lithium-ion transport of the material. Third, it possesses good structural stability, which gives C-LFP excellent rate tolerance.

[0011] To achieve the above objectives, the present invention adopts the following technical solution.

[0012] A method for preparing C-LFP nanomaterials, The method includes: 1) Prepare a mixed solution containing an iron source and a carbon source, perform carbon-iron composite hydrothermal treatment, filter and dry to obtain the CF precursor; 2) The CF precursor was subjected to carbonization and calcination in a protective atmosphere to obtain the Fe@C intermediate; 3) The Fe@C intermediate was placed in a reaction reagent containing phosphorus and lithium sources, soaked, and then subjected to a second hydrothermal treatment. After filtration and drying, the LFP@C intermediate was obtained. 4) Prepare a nitrogen-containing modifier. Place the LFP@C intermediate in the nitrogen-containing modifier and stir to modify it. Then, calcine it under a protective atmosphere to obtain C-LFP nanomaterials.

[0013] As a preferred option In step 1), the iron source in the mixed solution is ferric chloride and / or ferric nitrate, with a concentration of 0.08–0.12 mol / L. Step 1) The carbon source in the mixed solution is terephthalic acid, and its dosage is 0.95–1.05 mol / mol Fe. 3+ .

[0014] As a preferred option Step 1) The hydrothermal treatment of the carbon-iron composite is controlled at a hydrothermal temperature of 105-120 ℃ and a reaction time of 16-24 h.

[0015] As a preferred option Step 2) The protective atmosphere is an inert gas protective atmosphere; Step 2) The carbonization and calcination treatment is carried out at a constant temperature of 680-720 °C for 1.5-2.5 h.

[0016] As a preferred option In step 3), both phosphorus and its source in the reaction reagents are provided by lithium phosphate compounds; The lithium phosphate compound includes lithium dihydrogen phosphate; In step 3), the concentrations of Li and P in the reaction reagents are calculated based on the Fe content in the Fe@C intermediate, and the concentrations of Li and P are both 1.02–1.05 mol / mol Fe.

[0017] As a preferred option Step 3) involves maintaining ultrasonic treatment during the immersion process; Step 3) The secondary hydrothermal treatment process is carried out in an oxygen-containing atmosphere, with the hydrothermal temperature controlled at 165–190 °C and the holding time at 10–14 h.

[0018] As a preferred option Step 4) The modifier is prepared by dissolving dopamine hydrochloride in a buffer solvent with a pH of 8.2 to 8.8, wherein the concentration of dopamine hydrochloride is 1.5 to 3.0 mg / mL.

[0019] As a preferred option Step 4) The stirring modification involves completely immersing the LFP@C intermediate in a nitrogen-containing modifier and stirring continuously for 6–12 hours. Step 4) The protective atmosphere calcination involves placing the sample in an inert gas atmosphere or a nitrogen-inert gas protective atmosphere with a nitrogen content ≤5%VOL, and calcining it at a constant temperature of 680-720 ℃ for 2.0-3.0 h.

[0020] A C-LFP nanomaterial.

[0021] This invention is essentially a layered and distributed transformation preparation method based on MOF templates to achieve confined crystallization and conformal coating. Specifically, through hydrothermal coordination self-assembly of an iron source and a carbon source (terephthalic acid), a CF precursor, namely 101(Fe) crystal, with a quasi-octahedral cage-like structure is initially formed. This 101(Fe) crystal possesses three-dimensionally interconnected mesoporous channels and an extremely high specific surface area, enabling it to serve as a precisely confined template "reaction vessel."

[0022] Subsequently, a carbothermal reduction treatment is performed on this basis, which causes the organic ligand carbon source terephthalic acid to pyrolyze to form amorphous carbon. Then, the iron source is reduced to form elemental iron and / or Fe3C cementite phase. The Fe@C intermediate formed in this way has a unique pinning structure. Metallic iron / iron carbide nanoparticles are anchored in situ at the node positions of the mesoporous carbon framework. Unlike simple physical mixing, this binding form ensures the uniformity and stability of the distribution of metallic iron / iron carbide nanoparticles, avoids the migration and agglomeration of iron source in subsequent transformation, and provides spatially fixed active sites for the uniform nucleation of LFP.

[0023] The most crucial steps are then confined oxidation and in-situ crystallization growth. During the secondary hydrothermal process, in a limited oxygen atmosphere, the Fe@C intermediate is first etched by dissolved oxygen in the aqueous solution and oxygen in the air to form an iron oxide transition layer, which gradually transforms into an iron oxide layer. The formed iron oxide undergoes ion exchange and structural rearrangement in the acidic hydrothermal environment, thereby forming olivine-type LFP in situ. During this process, the pore walls of the carbon framework act as a physical barrier, preventing the long-range diffusion of Fe ions. This strictly confines the oxidation-crystallization process within the mesopores, effectively limiting the excessive growth of LFP and restricting its particle size to a scale of a few nanometers to tens of nanometers. Furthermore, because the LFP nuclei of this invention are heterogeneous at the solid-liquid interface, unlike conventional homogeneous nucleation in the liquid phase, the LFP crystals and the carbon framework form a coherent interface, generating strong interaction forces. This ensures that the LFP particles do not detach from the conductive network during subsequent electrochemical cycles, which greatly guarantees the cycling stability of the C-LFP nanomaterials of this invention. Furthermore, based on this, the present invention utilizes the conformal adhesion properties of polydopamine. Dopamine hydrochloride undergoes oxidative self-polymerization in a weakly alkaline Tris buffer to form polydopamine. Through the strong coordination between the catechol groups on its surface and the Fe-O bonds and carbon surface defect sites on the LFP@C intermediate surface, a nanoscale coating layer is formed. This coating layer can accurately replicate the rough surface morphology of the LFP@C intermediate and transforms into a nitrogen-doped carbon layer after heat treatment. The formation of this nitrided carbon layer can further provide additional electronic state density and improve intrinsic electronic conductivity. Most importantly, it can be integrated into the carbon framework through SP2 hybridization, significantly enhancing structural stability. The overall synergy between the aforementioned carbon framework confinement and the restricted growth of LFP results in the overall C-LFP nanomaterial exhibiting extremely excellent cycling stability and rate tolerance.

[0024] The resulting C-LFP material has a unique multi-level core-double-shell structure. The core is a highly crystalline LFP nanocrystal, while the inner shell is a MOF-derived mesoporous carbon framework that inherits a regular pore structure to provide a fast ion transport channel. The outer shell is a polydopamine-derived nitrogen-doped carbon layer that provides a high electronic conductivity network and stabilizes the electrode / electrolyte interface.

[0025] The mesoporous structure of the inner shell allows for full electrolyte wetting, ensuring that Li + Rapid exchange at the interface, the microporous structure of the outer shell inhibits the dissolution of byproducts such as lithium polyphosphide, and reduces interfacial side reactions. The synergistic effect of the pore gradient and conductivity gradient of the bilayer carbon achieves kinetic matching of electron-ion transport, which is the main reason why the material exhibits extremely low polarization and excellent rate performance.

[0026] The beneficial effects of this invention are as follows: 1. A core-double-shell hierarchical porous structure was constructed: Through a hierarchical step-by-step transformation strategy, a three-level structure of LFP nanocrystals@mesoporous carbon@nitrogen-doped carbon was formed. The inner mesoporous carbon provides ion transport channels, and the outer nitrogen-doped carbon provides an electronic conductivity network, realizing the kinetic synergy of electron-ion transport. 2. It achieves high specific capacity and excellent rate performance. The material has a first discharge specific capacity of up to 164 mAh / g at 0.2 C rate and still maintains 128 mAh / g at a high rate of 10 C. 3. It exhibits extremely low electrochemical polarization. CV tests show that the redox peak potential difference is only about 98 mV, which is significantly lower than that of conventional C-LFP materials, indicating that the charge transfer resistance is greatly reduced and the reaction kinetics are excellent. 4. It exhibits excellent cycle stability and structural tolerance, and its capacity can be quickly recovered after high-rate cycling, proving that the core-double-shell structure effectively buffers the volumetric strain during the charging and discharging process. Attached Figure Description

[0027] Figure 1 This is a CV curve characterization diagram of the sample in Example 1 of the present invention. Detailed Implementation

[0028] The present invention will be further described clearly and in detail below with reference to specific embodiments and the accompanying drawings. Those skilled in the art will be able to implement the present invention based on these descriptions. Furthermore, the embodiments of the present invention described below are generally only some, not all, of the embodiments of the present invention. Therefore, all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention.

[0029] Unless otherwise specified, all raw materials used in the embodiments of the present invention are commercially available or obtainable by those skilled in the art; unless otherwise specified, all methods used in the embodiments of the present invention are methods mastered by those skilled in the art.

[0030] Example 1 A C-LFP nanomaterial is prepared by the following method: 1) Prepare a mixed aqueous solution containing 0.10 mol / L ferric chloride and 0.1 mol / L terephthalic acid, perform a carbon-iron composite hydrothermal treatment at 110℃ for 24 h, filter and vacuum dry at 80℃ to obtain the CF precursor; 2) The CF precursor was subjected to carbonization and calcination at 700 °C for 2 h in an argon atmosphere to obtain the Fe@C intermediate; 3) Place the Fe@C intermediate in a 0.105 mol / L lithium dihydrogen phosphate aqueous solution. The volume of lithium dihydrogen phosphate used is equal to that used in step 1) the mixed aqueous solution of ferric chloride and terephthalic acid. After being fully immersed in the solution for 30 min on an ultrasonic test bench, it is subjected to a secondary hydrothermal treatment at 175 °C for 12 h in an air atmosphere. After filtration, it is vacuum dried at 80 °C to obtain the LFP@C intermediate. 4) Using Tris buffer solution with a pH of 8.5 as the solvent, dopamine hydrochloride was added to prepare a modifier with a dopamine hydrochloride concentration of 2.5 mg / mL. The LFP@C intermediate was completely immersed in the modifier and continuously stirred for 9 h. Then, it was transferred to a nitrogen-argon mixture with a nitrogen volume content of 5% VOL for constant temperature calcination heat treatment at 700 °C for 2.5 h to obtain C-LFP nanomaterials.

[0031] The product was characterized, and the nitrogen adsorption-desorption isotherm exhibited a typical Type IV curve with an H4-type hysteresis loop, indicating that the material has a rich mesoporous structure and an overall specific surface area as high as 139 m². 2 / g, the surface has a high specific surface area and abundant pores, which is conducive to sufficient contact of electrolyte and rapid transport of lithium ions, while the XRD calculation shows that the grain size is about 30 to 40 nm.

[0032] In addition, cyclic voltammetry (CV) tests were performed, and the test results were as follows: Figure 1 As shown, at a scan rate of 0.1 mV / s, the redox peak potential difference is very small, only about 98 mV, which is much smaller than that of conventional C-LFP materials. The sharp and symmetrical peak shape indicates that the electrode process is mainly controlled by the surface. The carbon coating layer effectively improves the electronic conductivity and reduces the polarization loss. Moreover, the absolute values ​​of the oxidation peak and reduction peak current are almost equal, indicating that the electrochemical reaction has good reversibility, small polarization, and fast kinetic rate. In addition, the peak shape has no broadening or tailing, the CV curve baseline is stable, and there is no obvious pseudocapacitive contribution, indicating that the mesoporous structure provides sufficient ion transport channels, which is conducive to the high-rate performance.

[0033] In addition, specific capacity testing, rate performance testing, and cycle stability testing are conducted.

[0034] Test results show that at a 0.2 C rate, the initial discharge specific capacity reaches as high as 164 mAh / g, with an initial coulombic efficiency of approximately 97.1%. Under test conditions at 0.5 C, 1.0 C, 2.0 C, and 5.0 C rates, the capacities also reach high levels of 164 mAh / g, 161 mAh / g, 156 mAh / g, and 146 mAh / g, respectively, demonstrating excellent performance from low to high rates, indicating extremely superior electrochemical and microstructural stability. Even at an ultra-high rate of 10.0 C, it maintains an excellent rate performance of 128 mAh / g. Furthermore, after 10 charge-discharge cycles at an ultra-high rate of 10.0 C, when the capacity was tested again at a 0.2 C rate, it rapidly recovered to a high capacity of approximately 163 mAh / g within two cycles, with almost no capacity decay and near-complete capacity recovery. This demonstrates that the product exhibits extremely high structural stability and exceptional rate tolerance.

[0035] Furthermore, in the cycle stability test, after 500 cycles at 1.0 C rate, the capacity retention reached 96.3%, and after 1000 cycles, the capacity retention was still as high as about 92.2%. After 1000 cycles at 5.0 C high rate, its capacity was still able to retain about 83.9%, demonstrating excellent cycle stability.

[0036] Example 2 A C-LFP nanomaterial is prepared by the following method: 1) Prepare a mixed aqueous solution containing 0.08 mol / L ferric chloride and 0.076 mol / L terephthalic acid, and subject it to a carbon-iron composite hydrothermal treatment at 105℃ for 24 h; 2) The CF precursor was subjected to carbonization and calcination at 680 °C for 2.5 h in an argon atmosphere to obtain the Fe@C intermediate; 3) The Fe@C intermediate was placed in a 0.082 mol / L lithium dihydrogen phosphate aqueous solution. The volume of lithium dihydrogen phosphate was equal to that of the mixed aqueous solution of ferric chloride and terephthalic acid in step 1). After being fully immersed in the solution for 30 min on an ultrasonic test bench, it was subjected to a secondary hydrothermal treatment at 165 °C for 14 h in an air atmosphere. The solution was then filtered and vacuum dried at 80 °C to obtain the LFP@C intermediate. 4) Prepare a dopamine hydrochloride concentration of 2.0 mg / mL using Tris buffer solution with pH 8.2. Immerse and stir the LFP@C intermediate for 12 h, and then calcine it at 680 °C for 3.0 h in a nitrogen-argon mixture with a nitrogen volume content of 5% VOL to obtain C-LFP nanomaterials.

[0037] The C-LFP nanomaterials prepared in this example were characterized in the same manner as in Example 1. The main characterization results showed that the particle size of the product in this example was approximately 28–35 nm, and the specific surface area was 135 m². 2 / g, the initial discharge specific capacity at 0.2 C rate is 162 mAh / g, the initial coulombic efficiency is 96.5%, the capacity at 10 C rate is 125 mAh / g, and the capacity retention after 1000 cycles at 1 C is 91.5%.

[0038] Example 3 A C-LFP nanomaterial is prepared by the following method: 1) Prepare a mixed aqueous solution containing 0.12 mol / L ferric nitrate and 0.126 mol / L terephthalic acid, perform a carbon-iron composite hydrothermal treatment at 120℃ for 16 h, filter and vacuum dry at 80℃ to obtain the CF precursor; 2) The CF precursor was subjected to carbonization and calcination at 720 °C for 1.5 h in an argon atmosphere to obtain the Fe@C intermediate; 3) The Fe@C intermediate was placed in a 0.125 mol / L lithium dihydrogen phosphate aqueous solution. The volume of lithium dihydrogen phosphate was equal to that of the mixed aqueous solution of ferric nitrate and terephthalic acid in step 1). After being fully immersed in the solution for 30 min on an ultrasonic test bench, it was subjected to a secondary hydrothermal treatment at 190 °C for 10 h in an air atmosphere. The solution was then filtered and vacuum dried at 80 °C to obtain the LFP@C intermediate. 4) Prepare a modifier with a dopamine hydrochloride concentration of 3.0 mg / mL using Tris buffer solution with a pH of 8.8. Completely immerse the LFP@C intermediate in the modifier and continuously stir for 6 h. Then transfer it to a nitrogen-argon mixture with a nitrogen volume content of 5% VOL for constant temperature calcination heat treatment at 720 ℃ for 2.0 h to obtain C-LFP nanomaterials.

[0039] The C-LFP nanomaterials prepared in this example were characterized in the same manner as in Example 1. The main characterization results showed that the particle size of the product in this example was approximately 38–45 nm, and the specific surface area was 142 m². 2 / g, the initial discharge specific capacity at 0.2 C rate is 163 mAh / g, the initial coulombic efficiency is 96.8%, the capacity at 10 C rate is 126 mAh / g, and the capacity retention rate after 1000 cycles at 1 C is 91.8%.

[0040] Example 4 A C-LFP nanomaterial is prepared by the following method: 1) Prepare a mixed aqueous solution containing 0.09 mol / L ferric chloride and 0.09 mol / L terephthalic acid, perform a carbon-iron composite hydrothermal treatment at 115℃ for 20 h, filter and vacuum dry at 80℃ to obtain the CF precursor; 2) The CF precursor was subjected to carbonization and calcination at 700 °C for 2 h in an argon atmosphere to obtain the Fe@C intermediate; 3) The Fe@C intermediate was placed in a 0.093 mol / L lithium dihydrogen phosphate aqueous solution, with the volume of lithium dihydrogen phosphate being equal to that of the mixed aqueous solution of ferric chloride and terephthalic acid in step 1). After being fully immersed in the solution for 45 min on an ultrasonic test bench, it was subjected to a secondary hydrothermal treatment at 180 °C for 12 h in an oxygen-nitrogen mixed atmosphere with an oxygen volume content of 10%. The solution was then filtered and vacuum dried at 80 °C to obtain the LFP@C intermediate. 4) Prepare a modifier with a dopamine hydrochloride concentration of 2.0 mg / mL using Tris buffer solution with a pH of 8.5. Completely immerse the LFP@C intermediate in the modifier and continuously stir for 8 h. Then transfer it to a pure argon atmosphere for constant temperature calcination heat treatment at 700 ℃ for 2.5 h to obtain C-LFP nanomaterials.

[0041] The C-LFP nanomaterials prepared in this example were characterized in the same manner as in Example 1. The main characterization results showed that the particle size of the product in this example was approximately 28–35 nm, and the specific surface area was 140 m². 2 / g, the initial discharge specific capacity at 0.2 C rate is 163 mAh / g, the initial coulombic efficiency is 97.2%, the capacity at 10 C rate is 127 mAh / g, and the capacity retention after 1000 cycles at 1 C is 92.0%.

[0042] Comparative Example 1 A C-LFP nanomaterial is prepared by the following method: 1) Prepare a mixed aqueous solution containing 0.10 mol / L ferric chloride and 0.1 mol / L terephthalic acid, perform a carbon-iron composite hydrothermal treatment at 110℃ for 24 h, filter and vacuum dry at 80℃ to obtain the CF precursor; 2) The CF precursor was subjected to carbonization and calcination at 700 °C for 2 h in an argon atmosphere to obtain the Fe@C intermediate; 3) The Fe@C intermediate was placed in a 0.105 mol / L lithium dihydrogen phosphate aqueous solution, with the volume of lithium dihydrogen phosphate being equal to that of the mixed aqueous solution of ferric chloride and terephthalic acid in step 1). After being fully immersed in the solution for 30 min on an ultrasonic test bench, it was subjected to a secondary hydrothermal treatment at 175 °C for 12 h in an argon atmosphere. The solution was then filtered and vacuum dried at 80 °C to obtain the LFP@C intermediate. 4) Using Tris buffer solution with a pH of 8.5 as the solvent, dopamine hydrochloride was added to prepare a modifier with a dopamine hydrochloride concentration of 2.5 mg / mL. The LFP@C intermediate was completely immersed in the modifier and continuously stirred for 9 h. Then, it was transferred to a nitrogen-argon mixture with a nitrogen volume content of 5% VOL for constant temperature calcination heat treatment at 700 °C for 2.5 h to obtain C-LFP nanomaterials.

[0043] The C-LFP nanomaterials prepared in this example were characterized using the same methods as in Example 1. The XRD characterization results showed that the particle size of the sample in this example fluctuated greatly, with crystalline products existing in the 20–80 nm range. Furthermore, the XRD characterization results showed significant Fe content. 0 The elemental diffraction peaks (2θ = 44.7°) and trace amounts of Fe3O4 impurities indicate that Fe... 0 Failed to be effectively converted to Fe 2+ In addition, it exhibits a first discharge capacity of only 118 mAh / g at 0.2 C, an initial coulombic efficiency of 82%, a 10 C rate capacity of <80 mAh / g, and a capacity decay to 65% of the initial capacity after 100 cycles.

[0044] Characterization results show that, under an inert atmosphere, Fe in the Fe@C intermediate... 0 Fe3C cannot be effectively oxidized to Fe. 2+ Due to the lack of an oxidant in the hydrothermal system, Fe Only slight passivation occurs on the surface, forming a small amount of Fe3O4, while the interior remains in a metallic state. This leads to subsequent interaction with Li... 3 / PO4 3- During the reaction, only a portion of the Fe source participates in the formation of LFP, while the remaining Fe... 0 The residual elemental form not only reduces the utilization rate of active materials but also triggers a micro-battery effect during charge and discharge, leading to a sharp drop in initial coulombic efficiency and rapid capacity decay. Furthermore, the unoxidized Fe3C partially dissolves in an acidic hydrothermal environment, causing damage to the carbon framework structure and pore collapse, although the specific surface area remains at 125 m². 2 / g, but the number of effective electrochemical active sites is significantly reduced.

[0045] Comparative Example 2 A C-LFP nanomaterial is prepared by the following method: 1) Prepare a mixed aqueous solution containing 0.10 mol / L ferric chloride and 0.1 mol / L terephthalic acid, perform a carbon-iron composite hydrothermal treatment at 150℃ for 12 h, filter and vacuum dry at 80℃ to obtain the CF precursor; 2) The CF precursor was subjected to carbonization and calcination at 700 °C for 2 h in an argon atmosphere to obtain the Fe@C intermediate; 3) Place the Fe@C intermediate in a 0.105 mol / L lithium dihydrogen phosphate aqueous solution. The volume of lithium dihydrogen phosphate used is equal to that used in step 1) the mixed aqueous solution of ferric chloride and terephthalic acid. After being fully immersed in the solution for 30 min on an ultrasonic test bench, it is subjected to a secondary hydrothermal treatment at 175 °C for 12 h in an air atmosphere. After filtration, it is vacuum dried at 80 °C to obtain the LFP@C intermediate. 4) Using Tris buffer solution with a pH of 8.5 as the solvent, dopamine hydrochloride was added to prepare a modifier with a dopamine hydrochloride concentration of 2.5 mg / mL. The LFP@C intermediate was completely immersed in the modifier and continuously stirred for 9 h. Then, it was transferred to a nitrogen-argon mixture with a nitrogen volume content of 5% VOL for constant temperature calcination heat treatment at 700 °C for 2.5 h to obtain C-LFP nanomaterials.

[0046] The C-LFP nanomaterials prepared in this example were subjected to the same performance characterization as in Example 1, except that the specific surface area remained at only 82 m². 2 / g, and electron microscopy characterization results show that its carbon skeleton is severely collapsed, the pore structure is irregular, and graphitized microcrystals appear locally. This is mainly because when the hydrothermal temperature exceeds 120 ℃, the growth rate of 101(Fe) crystals is too fast, forming dense large-particle MOFs. In addition, the ligand terephthalic acid is prone to decarboxylation side reaction under high pressure, which leads to an abnormally high degree of graphitization of the carbon skeleton after carbonization, and the mesoporous structure collapses due to excessive shrinkage.

[0047] Although high temperature is beneficial to crystallinity, pore blockage prevents the electrolyte from fully wetting the LFP active sites, prolonging the solid-state diffusion path of lithium ions. This results in a sharp drop in high-rate performance, and the formation of local graphitized microcrystals disrupts the continuity of the carbon skeleton, leading to stress concentration and decreased structural stability during cycling.

[0048] In terms of electrochemical performance, the product in this example has a 5C rate discharge specific capacity of only 112 mAh / g, which is much lower than that of Example 1, and the capacity retention rate is only 78% after 100 cycles.

[0049] Comparative Example 3 A C-LFP nanomaterial is prepared by the following method: 1) Prepare a mixed aqueous solution containing 0.10 mol / L ferric chloride and 0.1 mol / L terephthalic acid, perform a carbon-iron composite hydrothermal treatment at 110℃ for 24 h, filter and vacuum dry at 80℃ to obtain the CF precursor; 2) The CF precursor was subjected to carbonization and calcination at 700 °C for 2 h in an argon atmosphere to obtain the Fe@C intermediate; 3) The Fe@C intermediate was placed in a 0.105 mol / L lithium dihydrogen phosphate aqueous solution. The volume of lithium dihydrogen phosphate was equal to that of the mixed aqueous solution of ferric chloride and terephthalic acid in step 1). After being fully immersed in the solution for 30 min on an ultrasonic test bench, it was subjected to a secondary hydrothermal treatment at 175 °C for 12 h in an air atmosphere. The solution was then filtered and vacuum dried at 80 °C to obtain the target C-LFP nanomaterial.

[0050] The C-LFP nanomaterials prepared in this example were characterized in the same way as in Example 1. The most significant performance difference was that the CV cycle test results showed a large redox peak potential difference of about 158 ​​mV. The capacity retention at 0.2 C rate was relatively good, reaching about 159 mAh / g, while the capacity at 10 C rate was only 95 mAh / g. After 500 cycles at 1 C, the capacity retention rate was 81.2%. In addition, after 10 charge-discharge cycles at an ultra-high rate of 10.0 C, when the capacity was tested again at 0.2 C rate, it could only recover to about 146 mAh / g within 2 cycles, showing extremely poor rate tolerance.

[0051] This is mainly because without an outer nitrogen-doped carbon layer, the material relies solely on the MOF-derived mesoporous carbon framework for conductivity, resulting in an incomplete electron transport network and reduced ΔE in CV measurements. p The voltage drop of up to 158 mV indicates a significant increase in charge transfer resistance (Rct) and sluggish reaction kinetics. At high rates of 10 C, severe surface polarization leads to a substantial reduction in the actual usable capacity. Furthermore, the absence of a polydopamine protective layer allows LFP@C to directly contact the electrolyte, exacerbating interfacial side reactions, resulting in decreased cycle stability, and irreversible structural damage after high-rate cycling.

[0052] Comparative Example 4 A C-LFP nanomaterial is prepared by the following method: 1) Prepare a mixed aqueous solution containing 0.10 mol / L ferric chloride and 0.1 mol / L terephthalic acid, perform a carbon-iron composite hydrothermal treatment at 110℃ for 24 h, filter and vacuum dry at 80℃ to obtain the CF precursor; 2) The CF precursor was subjected to carbonization and calcination at 700 °C for 2 h in an argon atmosphere to obtain the Fe@C intermediate; 3) Place the Fe@C intermediate in a 0.105 mol / L lithium dihydrogen phosphate aqueous solution. The volume of lithium dihydrogen phosphate used is equal to that used in step 1) the mixed aqueous solution of ferric chloride and terephthalic acid. After being fully immersed in the solution for 30 min on an ultrasonic test bench, it is subjected to a secondary hydrothermal treatment at 175 °C for 12 h in an air atmosphere. After filtration, it is vacuum dried at 80 °C to obtain the LFP@C intermediate. 4) Using Tris buffer solution with a pH of 8.5 as the solvent, dopamine hydrochloride was added to prepare a modifier with a dopamine hydrochloride concentration of 2.5 mg / mL. The LFP@C intermediate was completely immersed in the modifier and continuously stirred for 9 h. Then, it was transferred to a nitrogen-argon mixture with a nitrogen volume content of 5% VOL for constant temperature calcination heat treatment at 800 °C for 2.5 h to obtain C-LFP nanomaterials.

[0053] The C-LFP nanomaterials prepared in this example were characterized in the same way as in Example 1. The XRD characterization results of this sample showed that the grain size reached about 70-90 nm according to the Scherrer formula. The grain size was significantly increased, indicating that the carbon skeleton was severely damaged and the grains were overgrown. At the same time, the specific capacity at 0.2 C was only 148 mAh / g and the rate capacity at 10 C was 105 mAh / g, indicating that the high temperature led to the extension of the ion transport path and the deterioration of electrochemical kinetics.

[0054] This is mainly because when the calcination temperature is too high, although the polydopamine carbonizes more fully, the LFP grains grow abnormally, exceeding the confined size of the carbon framework channels. This causes the carbon framework to be mechanically broken, the hierarchical pore structure to fail, and the grain growth significantly prolongs the solid-state diffusion path of lithium ions within the LFP, resulting in a decrease in specific capacity and an increase in polarization. At the same time, at high temperatures, some ferrous ions are oxidized to ferric ions, triggering lattice oxygen loss, which further reduces electrochemical activity. Although the conductivity of the carbon layer is improved, the intrinsic kinetics of the active material deteriorate, leading to a decrease in high-rate performance.

Claims

1. A method for preparing C-LFP nanomaterials, characterized in that, The method includes: 1) Prepare a mixed solution containing an iron source and a carbon source, perform carbon-iron composite hydrothermal treatment, filter and dry to obtain the CF precursor; 2) The CF precursor was subjected to carbonization and calcination in a protective atmosphere to obtain the Fe@C intermediate; 3) The Fe@C intermediate was placed in a reaction reagent containing phosphorus and lithium sources, soaked, and then subjected to a second hydrothermal treatment. After filtration and drying, the LFP@C intermediate was obtained. 4) Prepare a nitrogen-containing modifier. Place the LFP@C intermediate in the nitrogen-containing modifier and stir to modify it. Then, calcine it under a protective atmosphere to obtain C-LFP nanomaterials.

2. The method for preparing C-LFP nanomaterials according to claim 1, characterized in that, In step 1), the iron source in the mixed solution is ferric chloride and / or ferric nitrate, with a concentration of 0.08–0.12 mol / L. Step 1) the carbon source in the mixed solution is terephthalic acid, and the amount is 0.95-1.05 mol / mol Fe 3+ .

3. A method for preparing C-LFP nanomaterials according to claim 1 or 2, characterized in that, Step 1) The hydrothermal treatment of the carbon-iron composite is controlled at a hydrothermal temperature of 105-120 °C and a reaction time of 16-24 h.

4. The method for preparing C-LFP nanomaterials according to claim 1, characterized in that, Step 2) The protective atmosphere is an inert gas protective atmosphere; Step 2) The carbonization and calcination treatment is carried out at a constant temperature of 680-720 °C for 1.5-2.5 h.

5. The method for preparing C-LFP nanomaterials according to claim 1, characterized in that, In step 3), both phosphorus and its source in the reaction reagents are provided by lithium phosphate compounds; The lithium phosphate compound includes lithium dihydrogen phosphate; In step 3), the concentrations of Li and P in the reaction reagents are calculated based on the Fe content in the Fe@C intermediate, and the concentrations of Li and P are both 1.02–1.05 mol / mol Fe.

6. A method for preparing C-LFP nanomaterials according to claim 1 or 5, characterized in that, Step 3) involves maintaining ultrasonic treatment during the immersion process; Step 3) The secondary hydrothermal treatment process is carried out in an oxygen-containing atmosphere, with the hydrothermal temperature controlled at 165–190 °C and the holding time at 10–14 h.

7. The method for preparing C-LFP nanomaterials according to claim 1, characterized in that, Step 4) The modifier is prepared by dissolving dopamine hydrochloride in a buffer solvent with a pH of 8.2 to 8.8, wherein the concentration of dopamine hydrochloride is 1.5 to 3.0 mg / mL.

8. A method for preparing C-LFP nanomaterials according to claim 1 or 7, characterized in that, Step 4) The stirring modification involves completely immersing the LFP@C intermediate in a nitrogen-containing modifier and stirring continuously for 6–12 h; Step 4) The protective atmosphere calcination involves placing the sample in an inert gas atmosphere or a nitrogen-inert gas protective atmosphere with a nitrogen content ≤5%VOL, and calcining it at a constant temperature of 680-720 ℃ for 2.0-3.0 h.

9. A C-LFP nanomaterial prepared by any one of the methods of claims 1 to 8.

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