A lithium manganese iron phosphate cathode material with a three-zone synergistic interface structure and a preparation method thereof

Abstract

The application discloses a kind of lithium manganese iron phosphate positive electrode materials with three-zone synergistic interface structure and preparation method thereof, belong to lithium battery technical field.The material of the application includes: internal manganese concentration gradient core, sodium alginate-nanocellulose derived interpenetrating network carbon-based intermediate interface region, and TiO2 external functional shell with single-atom transition metal anchored.The application guides MnF2 Nanocrystalline in-situ accurate generation by gradient core and sodium alginate Mn 2+ Coordination effect, carbon matrix is toughened using nanocellulose carbon fiber, and interface impedance is reduced and side reaction is inhibited by means of single-atom modified TiO2 layer.Three-zone synergy realizes multiple inhibition of manganese dissolution and efficient transmission of electrons / ions, significantly improves the rate performance and cycle stability of the material.

Description

Technical Field

[0001] This invention relates to the field of lithium battery technology, specifically to a lithium manganese iron phosphate cathode material with a three-zone synergistic interface structure and its preparation method. Background Technology

[0002] Lithium manganese iron phosphate (LMFP) combines the high safety and long cycle life of lithium iron phosphate with the energy density advantage of lithium manganese phosphate's 4.1V high voltage platform, and has been recognized as one of the key technologies for next-generation power battery cathode materials. However, this material faces three major bottlenecks in practical applications: firstly, the manganese leaching problem, where Mn dissolution occurs during cycling. 3+ Mn generated by the disproportionation reaction in the electrolyte 2+ This not only damages the crystal structure on the positive electrode surface but also migrates to the negative electrode for deposition, accelerating SEI film thickening and irreversible consumption of active lithium; secondly, there is a contradiction between conductivity and lithium-ion transport, as the intrinsic electronic conductivity of LMFP is only about 10. -10 While existing carbon coating technology can improve composite conductivity, the graphitized carbon layer formed by high-temperature carbonization often has excessively high density, which hinders lithium-ion penetration. If the carbon layer is too thick, it will seriously sacrifice volumetric energy density. It is difficult to achieve a balance between conductivity, ion permeability and energy density. Thirdly, the mechanical stability of the carbon coating layer is insufficient. During the charging and discharging process, the particles undergo anisotropic volumetric strain. Traditional homogeneous carbon layers, due to their poor toughness, gradually develop microcracks and break down during long cycles. The exposed fresh surface accelerates side reactions, resulting in a "plunge" in performance degradation.

[0003] To address the aforementioned problems, existing technologies have explored various approaches, but all have significant shortcomings. Fluorination interface modification typically involves secondary blending and sintering of ammonium fluoride with pre-synthesized LMFP to generate metal fluorides and inhibit manganese dissolution. However, this route lacks a localization mechanism for the fluorination reaction. - Not only does it react with surface Mn, but it can also penetrate into the bulk phase, causing lattice disruption. Furthermore, the density of the fluorinated layer is constrained by the random distribution of surface Mn, resulting in unstable quality. Sodium alginate (SA), due to its rich carboxyl groups in its molecular chain, possesses Mn... 2+ It has coordination ability, but it is currently only used as an electrode binder or a single carbon source. Its multifunctional potential for inducing in-situ chemical reactions and constructing multi-level composite interfaces has not been fully explored.

[0004] Therefore, how to achieve multiple inhibitions of manganese leaching, construct an efficient electron / ion transport network, and improve the mechanical stability of the interface layer without sacrificing energy density and cycle life has become an urgent technical challenge in the current research of LMFP cathode materials. Summary of the Invention

[0005] The technical problem to be solved by this invention is to overcome the shortcomings of the prior art and provide a lithium manganese iron phosphate cathode material with a three-zone synergistic interface structure and its preparation method, through a gradient core and sodium alginate Mn 2+ Coordination guides the precise in-situ generation of MnF2 nanocrystals; nanocellulose carbon fibers are used to toughen the carbon matrix; and a single-atom modified TiO2 layer is used to reduce interfacial impedance and suppress side reactions. The three-zone synergy achieves multiple inhibition of manganese dissolution and efficient electron / ion transport, significantly improving the rate performance and cycle stability of the material.

[0006] The technical solution of this invention is as follows: On one hand, the present invention provides a method for preparing a lithium manganese iron phosphate cathode material with a three-region synergistic interface structure, comprising the following steps: Synthesis of S1 manganese concentration gradient kernel Prepare manganese source solutions and iron source solutions separately, with the Mn:Fe molar ratio of the two solutions according to LiMn x Fe 1-x PO4 was calculated, where 0.5 ≤ x ≤ 1; under inert atmosphere protection and stirring conditions, the two solutions were pumped into a co-precipitation reactor, and a precipitant was added dropwise simultaneously to maintain the pH of the reaction system at 9.5 ± 0.2; during the feeding process, the pumping rate of the manganese source solution was controlled to decrease linearly, while the pumping rate of the iron source solution was correspondingly increased linearly, until the pumping rate of the manganese source solution dropped to 60-80% of the initial pumping rate, while maintaining the total metal ion feeding rate constant; the obtained precipitate was washed and vacuum dried to obtain the precursor; The precursor was mixed with lithium and phosphorus sources at a molar ratio of Li:(Mn+Fe):P=1.03:1:1. After ball milling and drying, the mixture was pre-calcined at 350-400℃ for 4-5 hours under an Ar atmosphere, and then sintered at 650-700℃ for 8-10 hours. After natural cooling, the mixture was pulverized to obtain gradient LMFP powder. Preparation of S2 Sodium Alginate-Nanocellulose Composite Modified Dispersion Prepare a sodium alginate solution and a nanocellulose dispersion, mix and stir them together; then, add ammonium fluoride and urea in sequence, and continue stirring until the solid is completely dissolved to obtain a composite modified dispersion; S3 one-step firing structure, three-zone structure The gradient LMFP powder obtained in step S1 was added to the composite modified dispersion; after ball milling and mixing, it was spray-dried and granulated to collect spherical precursor powder; tetrabutyl titanate vapor and cobalt acetylacetone vapor were introduced into the spherical precursor powder with Ar gas as the carrier gas for sintering. The temperature was first raised to 350-400℃ and held for 2-4 hours to allow sodium alginate to undergo low-temperature carbonization. At the same time, the HF decomposed by ammonium fluoride reacted with the Mn sites chelated by SA to generate MnF2 nanocrystals; then the temperature was raised to 650-680℃ and held for 3-4 hours to complete the deep carbonization and TiO2 / single-atom Co anchoring reaction; after natural cooling to room temperature in the furnace, it was taken out and pulverized to obtain lithium manganese iron phosphate cathode material.

[0007] The lithium iron phosphate cathode material of the present invention has a three-region structure: (a) Internal bulk region (core): consisting of primary particles or secondary aggregates, with the general chemical formula LiMn x Fe 1-x PO4, where 0.5≤x≤1, and the manganese content (x value) decreases in a gradient from the core to the surface; (b) Intermediate interface region: tightly wrapped around the surface of the internal bulk region, it is an interpenetrating network carbon matrix composed of sodium alginate-derived carbon and nanocellulose-derived carbon fibers, in which nitrogen doping sites and manganese fluoride (MnF2) nanocrystals are uniformly dispersed. (c) External functional shell: covering the intermediate interface region, composed of nano-titanium dioxide particles anchored with single-atom transition metals.

[0008] This patent achieves comprehensive suppression of manganese leaching and efficient electron / ion transport through the synergistic cooperation of the three regions—the internal bulk region, the intermediate interface region, and the external functional shell—in terms of spatial location and function.

[0009] Preferably, in step S1, the precipitant is a NaOH solution; the vacuum drying temperature is 120-130℃, and the time is 10-12h.

[0010] Preferably, in step S1, the lithium source is Li2CO3 and the phosphorus source is NH4H2PO4.

[0011] Preferably, in step S2, the concentration of sodium alginate solution is 3-5 wt.%, and the concentration of nanocellulose dispersion is 1-2 wt.%; when mixing, the mass ratio of sodium alginate to nanocellulose is (5-10):1.

[0012] Preferably, in step S2, the mass ratio of sodium alginate, ammonium fluoride and urea is 12:(2-2.5):3.

[0013] Preferably, in step S3, the mass ratio of gradient LMFP powder to sodium alginate is 100:(2-4).

[0014] Preferably, in step S3, during spray drying granulation, the inlet air temperature is 200-220℃, the outlet air temperature is 100-120℃, and the feeding rate is 15-20mL / min.

[0015] On the other hand, the present invention provides a lithium manganese iron phosphate cathode material with a three-zone synergistic interface structure, which is prepared by the above-mentioned preparation method of lithium manganese iron phosphate cathode material with a three-zone synergistic interface structure.

[0016] Compared with the prior art, the present invention has the following advantages: 1. Responsive interaction between gradient kernel and intermediate interface: forming a denser fluorinated interface with less manganese. The gradient kernel reduces the manganese content on the particle surface compared to conventional homogeneous LMFP by precisely controlling the Mn / Fe feed rate during the co-precipitation stage.

[0017] Sodium alginate molecules contain a large number of carboxyl groups (-COO-), which are beneficial for Mn. 2+ It possesses specific coordination chelation capabilities. When the sodium alginate-nanocellulose composite modified dispersion is mixed with the gradient core, due to the limited and orderly distribution of the total surface manganese, the sodium alginate carboxyl groups can achieve precise chelation with almost "isostoichiometric ratio". Each chelation site defines a nucleation site for MnF2 nanocrystals.

[0018] In the subsequent sintering process, F - Precisely guided to these predefined chelation sites, ultrafine MnF2 nanocrystals are generated in situ at low temperatures. These nanocrystals are uniformly embedded in the interpenetrating carbon matrix formed by the carbonization of sodium alginate and nanocellulose. Due to the highly ordered and uniformly distributed nucleation sites, the generated MnF2 interface layer is extremely dense and ultrathin, and forms chemical bonds with the matrix through sodium alginate-derived carbon residues, with a binding force far exceeding that of physically deposited layers.

[0019] This counterintuitive effect of "the less manganese on the surface, the higher the quality of the fluorinated layer" constitutes a "responsive" collaborative mechanism between the gradient kernel and the intermediate interface, which is one of the core innovations that distinguishes this invention from the prior art.

[0020] 2. SA-CNF interpenetrating network carbon matrix achieves a three-in-one approach of "chemical chelation + physical interception + mechanical toughening". Sodium alginate and nanocellulose undergo self-assembly during solution and drying processes through strong hydrogen bonding between hydroxyl groups (-OH) and carboxyl groups (-COO-) on the molecular chain, forming a precursor complex with uniform molecular distribution.

[0021] During sintering, sodium alginate carbonizes into a conductive carbon matrix rich in residual oxygen-containing functional groups, while nanocellulose forms a high aspect ratio fibrous precursor under shear dispersion and hydrogen bonding guidance, which is then converted into a fibrous conductive network after carbonization. The two are not simply blended carbonization products, but rather form a nanoscale interpenetrating network structure.

[0022] The unique advantage of this structure is that: Chemical chelation: The oxygen-containing functional groups remaining in the sodium alginate-derived carbon matrix inherit the Mn group from the sodium alginate molecular chain. 2+ Coordination ability can provide chemical chelation and capture functions at the interface for a long time; Physical interception: The high specific surface area and oxygen-containing active sites of the nanocellulose carbonized fiber network effectively intercept trace amounts of Mn escaping from the MnF2 layer. 2+ It has physical adsorption and interception capabilities, and forms a dual manganese capture mechanism of "chemical locking + physical interception" through chemical chelation with sodium alginate-derived carbon. Mechanical toughening: The nanocellulose carbon fiber network acts like "nanosteel bars" in concrete. Through toughening mechanisms such as fiber bridging, crack deflection and pull-out, it significantly improves the tensile strength and fracture toughness of the interface layer, effectively resists the volume stress in charge-discharge cycles, and prevents the initiation and propagation of microcracks. Ion transport channels: Interpenetrating network structures generate a large number of micropores and mesopores in the carbon matrix, providing rapid transport channels for lithium ions and avoiding the common problem of poor lithium ion permeability in dense and homogeneous carbon layers.

[0023] 3. Single-atom modified TiO2 outer functional layer constructs highly conductive channels and high-voltage stability barriers on the surface. The outermost TiO2 nanoparticles cover the surface of the intermediate interface region in a discontinuous island-like morphology, forming a chemical and electrochemical stability barrier under high voltage, effectively suppressing side reactions at the electrolyte / LMFP interface above 4.1V.

[0024] The single-atom transition metal anchored on the TiO2 surface has a highly unsaturated coordination environment and a unique d-orbital electronic structure, which can act as an efficient "electronic bridge" between the carbon matrix and TiO2, significantly reducing the interfacial charge transfer impedance (Rct) and accelerating the transport of electrons from the current collector to the particle reaction site through the external conductive network.

[0025] The layer exists in the form of a discontinuous island-like cover, which is a key feature of the intentional design: the discontinuous cover provides sufficient interface protection while retaining ample channels for lithium ions to enter and exit, achieving a delicate balance between "accelerated electron transport" and "unimpeded ion transport". Detailed Implementation

[0026] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] Example 1 The preparation method of the lithium manganese iron phosphate cathode material with a three-region synergistic interface structure in this embodiment includes the following steps: Synthesis of S1 manganese concentration gradient kernel MnSO4·H2O and FeSO4·7H2O solutions were prepared separately, with a total metal ion concentration of 1 mol / L and an initial Mn:Fe molar ratio of 8.5:1.5. Under nitrogen protection and stirring, both solutions were pumped into a co-precipitation reactor, with 2 mol / L NaOH solution added dropwise to maintain the pH of the reaction system at 9.5 ± 0.2. During the feeding process, the pumping rate of the MnSO4·H2O solution was linearly decreased at a rate of 0.4% / min, while the pumping rate of the FeSO4·7H2O solution was increased accordingly, while maintaining a constant total metal ion feeding rate until the pumping rate of the MnSO4·H2O solution decreased to 76.5% of the initial pumping rate. The resulting precipitate was repeatedly washed with deionized water until the conductivity of the supernatant was <50 μS / cm, and then vacuum dried at 120℃ for 12 h to obtain the precursor.

[0028] The precursor was mixed with Li₂CO₃ and NH₄H₂PO₄ at a molar ratio of Li:(Mn+Fe):P = 1.03:1:1, and dispersed in anhydrous ethanol. The mixture was then ball-milled in a planetary ball mill at 300 rpm for 4 h. After drying, the mixture was placed in a tube furnace and pre-calcined at 350 °C at a rate of 3 °C / min under an Ar atmosphere for 5 h. The temperature was then increased to 650 °C at a rate of 5 °C / min and sintered for 10 h. After natural cooling, the mixture was subjected to air jet milling to obtain gradient LMFP powder.

[0029] Preparation of S2 Sodium Alginate-Nanocellulose Composite Modified Dispersion 3g of sodium alginate was dissolved in 97g of deionized water and mechanically stirred for 4 hours until completely dissolved, yielding a 3wt.% sodium alginate solution. Separately, 0.6g of nanocellulose was dispersed in 49.4g of deionized water and ultrasonically treated with a probe for 30 minutes (300W power, pulse mode) to obtain a uniform 1.2wt.% semi-transparent nanocellulose dispersion.

[0030] Take 80g of the above sodium alginate solution and mix it with 20g of nanocellulose dispersion, and stir magnetically for 1 hour. Then, add 0.4g of ammonium fluoride and 0.6g of urea in sequence, and continue stirring until the solid is completely dissolved to obtain the composite modified dispersion.

[0031] S3 one-step sintering Add 100g of the above-mentioned gradient LMFP powder to the prepared composite modified dispersion, and add an appropriate amount of deionized water to adjust the slurry solid content to 40%. Use zirconia balls (2mm in diameter) as the grinding medium and ball mill at 300 rpm for 1 hour in a planetary ball mill. Immediately spray dry the resulting slurry for granulation: inlet air temperature 200℃, outlet air temperature 100℃, feed rate 15mL / min. Collect the spherical precursor powder.

[0032] Spherical precursor powder was placed in a tube furnace, and a high-purity Ar atmosphere (purity ≥99.999%, flow rate 100 sccm) was introduced. Ar gas (flow rate 20 sccm) was used as the carrier gas, sequentially passing through tetrabutyl titanate vapor and cobalt acetylacetonate vapor before converging into the main gas path, carrying the tetrabutyl titanate vapor and cobalt acetylacetonate vapor into the furnace tube. The sintering heating program was as follows: the temperature was increased to 350℃ at a rate of 5℃ / min and held for 4 hours to allow sodium alginate to undergo low-temperature carbonization. Simultaneously, the HF from the decomposition of ammonium fluoride reacted with the Mn sites chelated by SA to generate MnF2 nanocrystals. Then, the temperature was further increased to 650℃ at a rate of 5℃ / min and held for 4 hours to complete deep carbonization and the TiO2 / single-atom Co anchoring reaction. After natural cooling to room temperature in the furnace, the material was removed and subjected to air-jet pulverization to obtain lithium manganese iron phosphate cathode material.

[0033] Example 2 The preparation method of the lithium manganese iron phosphate cathode material with a three-region synergistic interface structure in this embodiment includes the following steps: Synthesis of S1 manganese concentration gradient kernel MnSO4·H2O and FeSO4·7H2O solutions were prepared separately, with a total metal ion concentration of 1 mol / L and an initial Mn:Fe molar ratio of 8.5:1.5. Under nitrogen protection and stirring, both solutions were pumped into a co-precipitation reactor, with 2 mol / L NaOH solution added dropwise simultaneously to maintain the pH of the reaction system at 9.5 ± 0.2. During the feeding process, the pumping rate of the MnSO4·H2O solution was controlled to decrease linearly at a rate of 0.4% / min, while the pumping rate of the FeSO4·7H2O solution increased accordingly, while maintaining a constant total metal ion feeding rate until the pumping rate of the MnSO4·H2O solution decreased to 76.5% of the initial pumping rate. The resulting precipitate was repeatedly washed with deionized water until the conductivity of the supernatant was <50 μS / cm, and then vacuum dried at 125℃ for 11 h to obtain the precursor.

[0034] The precursor was mixed with Li₂CO₃ and NH₄H₂PO₄ at a molar ratio of Li:(Mn+Fe):P = 1.03:1:1, and dispersed in anhydrous ethanol. The mixture was ball-milled in a planetary ball mill at 300 rpm for 4 h. After drying, the mixture was placed in a tube furnace and pre-calcined at 370 °C for 4.5 h under an Ar atmosphere, followed by sintering at 670 °C for 9 h at a rate of 5 °C / min. After natural cooling, the mixture was subjected to air jet milling to obtain gradient LMFP powder.

[0035] Preparation of S2 Sodium Alginate-Nanocellulose Composite Modified Dispersion 4g of sodium alginate was dissolved in 96g of deionized water and mechanically stirred for 4 hours until completely dissolved, yielding a 4wt.% sodium alginate solution. Separately, 1g of nanocellulose was dispersed in 49g of deionized water and ultrasonically treated with a probe for 30 minutes (300W power, pulse mode) to obtain a uniform 2wt.% semi-transparent nanocellulose dispersion.

[0036] Take 50g of the above sodium alginate solution and mix it with 20g of nanocellulose dispersion, and stir magnetically for 1 hour. Then, add 0.4g of ammonium fluoride and 0.5g of urea in sequence, and continue stirring until the solid is completely dissolved to obtain the composite modified dispersion.

[0037] S3 one-step sintering Add 100g of the above-mentioned gradient LMFP powder to the prepared composite modified dispersion, and add an appropriate amount of deionized water to adjust the slurry solid content to 40%. Use zirconia balls (2mm in diameter) as the grinding medium and ball mill at 300 rpm for 1 hour in a planetary ball mill. Immediately spray dry the resulting slurry for granulation: inlet air temperature 210℃, outlet air temperature 110℃, feed rate 16mL / min. Collect the spherical precursor powder.

[0038] Spherical precursor powder was placed in a tube furnace, and a high-purity Ar atmosphere (purity ≥99.999%, flow rate 100 sccm) was introduced. Ar gas (flow rate 20 sccm) was used as the carrier gas, sequentially passing through tetrabutyl titanate vapor and cobalt acetylacetonate vapor before converging into the main gas path, carrying the tetrabutyl titanate vapor and cobalt acetylacetonate vapor into the furnace tube. The sintering heating program was as follows: the temperature was increased to 380℃ at a rate of 5℃ / min and held for 3 hours to allow sodium alginate to undergo low-temperature carbonization. Simultaneously, the HF from the decomposition of ammonium fluoride reacted with the Mn sites chelated by SA to generate MnF2 nanocrystals. Then, the temperature was further increased to 660℃ at a rate of 5℃ / min and held for 3.5 hours to complete deep carbonization and the TiO2 / single-atom Co anchoring reaction. After natural cooling to room temperature in the furnace, the material was removed and subjected to air-jet pulverization to obtain lithium manganese iron phosphate cathode material.

[0039] Example 3 The preparation method of the lithium manganese iron phosphate cathode material with a three-region synergistic interface structure in this embodiment includes the following steps: Synthesis of S1 manganese concentration gradient kernel MnSO4·H2O and FeSO4·7H2O solutions were prepared separately, with a total metal ion concentration of 1 mol / L and an initial Mn:Fe molar ratio of 8.5:1.5. Under nitrogen protection and stirring, both solutions were pumped into a co-precipitation reactor, with 2 mol / L NaOH solution added dropwise to maintain the pH of the reaction system at 9.5 ± 0.2. During the feeding process, the pumping rate of the MnSO4·H2O solution was linearly decreased at a rate of 0.4% / min, while the pumping rate of the FeSO4·7H2O solution was increased accordingly, while maintaining a constant total metal ion feeding rate until the pumping rate of the MnSO4·H2O solution decreased to 76.5% of the initial pumping rate. The resulting precipitate was repeatedly washed with deionized water until the conductivity of the supernatant was <50 μS / cm, and then vacuum dried at 130℃ for 10 h to obtain the precursor.

[0040] The precursor was mixed with Li₂CO₃ and NH₄H₂PO₄ at a molar ratio of Li:(Mn+Fe):P = 1.03:1:1, and dispersed in anhydrous ethanol. The mixture was ball-milled in a planetary ball mill at 300 rpm for 4 h. After drying, the mixture was placed in a tube furnace and pre-calcined at 400 °C at a rate of 3 °C / min under an Ar atmosphere for 4 h, followed by sintering at 700 °C at a rate of 5 °C / min for 8 h. After natural cooling, the mixture was subjected to air jet milling to obtain gradient LMFP powder.

[0041] Preparation of S2 Sodium Alginate-Nanocellulose Composite Modified Dispersion Weigh 5g of sodium alginate and dissolve it in 95g of deionized water. Stir mechanically for 4 hours until completely dissolved to obtain a 5wt.% sodium alginate solution. Separately weigh 0.5g of nanocellulose and disperse it in 49.5g of deionized water. Sonicate the solution using a probe for 30 minutes (300W power, pulse mode) to obtain a uniform 1wt.% semi-transparent nanocellulose dispersion.

[0042] Take 80g of the above sodium alginate solution and mix it with 20g of nanocellulose dispersion, and stir magnetically for 1 hour. Then, add 0.7g of ammonium fluoride and 1g of urea in sequence, and continue stirring until the solid is completely dissolved to obtain the composite modified dispersion.

[0043] S3 one-step sintering Add 100g of the above-mentioned gradient LMFP powder to the prepared composite modified dispersion, and add an appropriate amount of deionized water to adjust the slurry solid content to 40%. Use zirconia balls (2mm in diameter) as the grinding medium and ball mill at 300 rpm for 1 hour in a planetary ball mill. Immediately spray dry the resulting slurry for granulation: inlet air temperature 220℃, outlet air temperature 120℃, feed rate 20mL / min. Collect the spherical precursor powder.

[0044] Spherical precursor powder was placed in a tube furnace, and a high-purity Ar atmosphere (purity ≥99.999%, flow rate 100 sccm) was introduced. Ar gas (flow rate 20 sccm) was used as the carrier gas, sequentially passing through tetrabutyl titanate vapor and cobalt acetylacetonate vapor before converging into the main gas path, carrying the tetrabutyl titanate vapor and cobalt acetylacetonate vapor into the furnace tube. The sintering heating program was as follows: the temperature was increased to 400℃ at a rate of 5℃ / min and held for 2 hours to allow sodium alginate to undergo low-temperature carbonization. Simultaneously, the HF from the decomposition of ammonium fluoride reacted with the Mn sites chelated by SA to generate MnF2 nanocrystals. Then, the temperature was further increased to 680℃ at a rate of 5℃ / min and held for 3 hours to complete deep carbonization and the TiO2 / single-atom Co anchoring reaction. After natural cooling to room temperature in the furnace, the material was removed and subjected to air-jet pulverization to obtain lithium manganese iron phosphate cathode material.

[0045] Comparative Example 1 The difference from Example 1 is that in step S1, the pumping rate of MnSO4·H2O solution and FeSO4·7H2O solution is kept constant, maintaining the Mn:Fe molar ratio of 7.5:2.5.

[0046] Comparative Example 2 The difference from Example 1 is that in step S2, the composite modified dispersion is replaced with a pure sodium alginate solution with the same sodium alginate content.

[0047] Comparative Example 3 The difference from Example 1 is that in step S3, tetrabutyl titanate vapor and cobalt acetylacetone vapor are not introduced during the one-step sintering process.

[0048] Comparative Example 4 Using commercially available LiMn 0.75 Fe 0.25 PO4 product.

[0049] Comparative Example 5 The preparation method of the lithium manganese iron phosphate cathode material of Comparative Example 5 includes the following steps: Synthesis of S1 manganese concentration gradient kernel Same as step S1 in Example 1.

[0050] S2 carbon-coated gradient LMFP synthesis 120g of gradient LMFP powder obtained in step S1 was mixed with 9.6g of glucose and 2.4g of PVDF by wet ball milling and sintered at 750℃ for 4h under Ar atmosphere to obtain carbon-coated gradient LMFP.

[0051] S3 Post-fluorination Treatment 100g of the above-mentioned carbon-coated gradient LMFP was mixed with 1g of ammonium fluoride by dry ball milling, and then heat-treated at 400℃ for 2h under Ar atmosphere for post-fluorination treatment. After natural cooling to room temperature in the furnace, it was taken out and subjected to air jet milling to obtain lithium manganese iron phosphate cathode material.

[0052] The positive electrode materials, conductive agent Super P, and binder PVDF of each embodiment and comparative example were mixed at a mass ratio of 90:5:5, and slurry was prepared using N-methylpyrrolidone (NMP) as solvent. The slurry was coated onto an aluminum foil current collector, vacuum dried at 120°C for 12 hours, and then rolled and cut into positive electrode sheets. Using lithium metal sheets as the negative electrode, 1 mol / L LiPF6 dissolved in EC / DEC / EMC (volume ratio 1:1:1, containing 2% FEC additive) was used as the electrolyte, and Celgard 2500 polypropylene membrane was used as the separator. CR2032 coin cells were assembled in an argon-filled glove box. Electrochemical tests were conducted at room temperature (25±1°C) using a blue electric shock testing system. The test voltage window was 2.5-4.5V (vs. Li + / Li).

[0053] The performance test results are shown in Table 1: Table 1 Performance test results of the batteries assembled in each embodiment and comparative example As shown in Table 1, Comparative Example 1 uses a uniform LMFP core and has a high surface manganese concentration. A large amount of Mn... 2+The sodium alginate carboxyl chelating sites become saturated, losing their selectivity. Subsequent fluorination occurs randomly on the dense and disordered Mn sites, resulting in adjacent MnF2 nanocrystals that fuse and grow, ultimately forming a coarse, agglomerated, and unevenly distributed fluorinated layer. During long cycling, severe stress concentration occurs within the uneven fluorinated layer, and microcracks preferentially initiate and propagate from weaker areas, with attenuation accelerating after 500 cycles. The gradient core in Example 1 reduces the surface manganese concentration. Despite insufficient total Mn, the sodium alginate carboxyl groups achieve near-stoichiometric chelation, with each chelating site orderly distributed, leading to the in-situ generation of highly uniform ultrafine MnF2 nanocrystals. This ultrathin, dense interfacial layer exhibits excellent crack resistance under volumetric stress, ensuring the integrity of the three-zone structure throughout 1000 cycles. This result reveals a counterintuitive mechanism: with the cooperation of a gradient kernel, the less manganese on the surface, the more refined (thin, dense, and uniform) the fluorinated layer generated via sodium alginate chelation and localization. This minimal effort yields greater results and multiplies efficiency—this is the most fundamental difference between this invention and any known fluorination route.

[0054] Comparative Example 2 showed little difference from Example 1 in initial capacity and rate performance, indicating that nanocellulose is not the dominant factor in initial performance. The key difference lies in the later stages of long-term cycling (after 500 cycles). While the homogeneous carbon layer of sodium alginate in Comparative Example 2 provides good fluorination protection in the early stages of film formation, it is inherently a brittle material. Under the cyclic volumetric stress caused by cycling, microcracks gradually initiate and accumulate in the fiber-unreinforced homogeneous carbon layer. Once the cracks propagate and expose a fresh surface, electrolyte penetration, new side reactions, and accelerated Mn dissolution re-accelerate, creating a vicious cycle. Comparative Example 2 showed a significantly increased decay rate after 500 cycles, and the amount of Mn dissolution was twice that of Example 1. In the sodium alginate-nanocellulose interpenetrating network structure of Example 1, the "nanosteel bars" formed by the carbonized nanocellulose fibers inhibited the initiation of microcracks through fiber bridging effects. Even if cracks formed, the fibers provided bridging forces to prevent further crack propagation. The integrity of the coating layer was maintained over 1000 cycles, with the dual mechanism of "chemical fluorination protection + physical interception" operating effectively and at a consistently low rate of degradation. Nanocellulose toughening is the structural key to upgrading the "limited-term shelf life" of chemical fluorination protection to "long-term protection." Without this layer of "physical insurance," even the finest fluorinated interface would lose its protective function due to the mechanical failure of the carbon layer.

[0055] Although Comparative Example 3 also possesses a gradient core and a sodium alginate-cellulose nanocomposite interface, its performance significantly collapses at high rates. Furthermore, lacking the high-voltage physical barrier of TiO2, Comparative Example 3 exhibits slightly more electrolyte side reactions during long cycles, resulting in a lower retention rate at 1000 cycles compared to Example 1. The core contribution of the single-atom Co-modified TiO2 outer functional layer lies in the fact that single-atom Co possesses a highly unsaturated coordination environment and a unique d-orbital electronic structure, which can act as an efficient "electron bridge" between the carbon matrix and TiO2, significantly reducing the charge transfer impedance at the particle-electrolyte interface. This allows electrons to be rapidly injected into reaction sites at high current densities, overcoming the bottleneck of excessively high Rct rates when the carbon matrix directly faces the electrolyte. The discontinuous island-like coverage of TiO2 provides sufficient electron transport channels while retaining lithium-ion entry and exit channels, achieving the design objective of "accelerating electrons without blocking ions."

[0056] The retention rate of Comparative Example 5 after 1000 weeks (86.3%) was not only much lower than that of Example 1 (96.5%), but also lower than that of Comparative Examples 2 and 3, which only had two core elements. The main reasons are as follows: (1) Lack of fluorination positioning: In the post-fluorination process, the HF gas generated by the direct thermal decomposition of ammonium fluoride randomly contacts and penetrates the carbon layer to react with the LMFP surface. Without the precise positioning guidance of SA carboxyl groups, F - The reaction between ions and surface Mn is completely random, and the generated MnF2 particles are coarse and extremely unevenly distributed. Local overfluorination causes loss of active material, and local underfluorination forms a weak protective area. (2) Carbon layer is eroded by fluorination: At high temperature, HF gas will also etch the previously formed glucose-derived carbon layer while forming MnF2, causing a large number of nanopores and defects in the carbon layer, and damaging the integrity of the conductive network. This directly explains why the performance of the 5x ratio of the comparative example is at the same low level as that of the comparative example 4 without any interface optimization. (3) No nanocellulose toughening and no outer layer protection: The carbon layer weakened by fluorination etching has no fiber reinforcement and no TiO2 protection, and it will quickly break and fail under cyclic stress.

Claims

1. A method for preparing a lithium manganese iron phosphate cathode material with a three-region synergistic interface structure, characterized in that, Includes the following steps: Synthesis of S1 manganese concentration gradient kernel Prepare manganese source solutions and iron source solutions separately, with the Mn:Fe molar ratio of the two solutions according to LiMn x Fe 1-x PO4 calculation, where 0.5≤x≤1; Under inert atmosphere protection and stirring conditions, the two solutions were pumped into the co-precipitation reactor, and the precipitant was added dropwise simultaneously to maintain the pH of the reaction system at 9.5±0.2; During the feeding process, the pumping rate of the manganese source solution was controlled to decrease linearly, and the pumping rate of the iron source solution was correspondingly increased linearly until the pumping rate of the manganese source solution dropped to 60-80% of the initial pumping rate, while the total metal ion feeding rate was kept constant during the process; The precipitate was washed and vacuum dried to obtain the precursor. The precursor was mixed with lithium and phosphorus sources at a molar ratio of Li:(Mn+Fe):P=1.03:1:

1. After ball milling and drying, the mixture was pre-calcined at 350-400℃ for 4-5 hours under an Ar atmosphere, and then sintered at 650-700℃ for 8-10 hours. After natural cooling, the mixture was pulverized to obtain gradient LMFP powder. Preparation of S2 Sodium Alginate-Nanocellulose Composite Modified Dispersion Prepare a sodium alginate solution and a nanocellulose dispersion, mix and stir them together; then, add ammonium fluoride and urea in sequence, and continue stirring until the solid is completely dissolved to obtain a composite modified dispersion; S3 one-step sintering The gradient LMFP powder obtained in step S1 was added to the composite modified dispersion; after ball milling and mixing, it was spray-dried and granulated to collect spherical precursor powder; tetrabutyl titanate vapor and cobalt acetylacetone vapor were introduced into the spherical precursor powder with Ar gas as the carrier gas for sintering. The temperature was first raised to 350-400℃ and held for 2-4 hours, then raised to 650-680℃ and held for 3-4 hours; after natural cooling to room temperature in the furnace, it was taken out and pulverized to obtain lithium manganese iron phosphate cathode material.

2. The method for preparing lithium manganese iron phosphate cathode material with a three-region synergistic interface structure as described in claim 1, characterized in that, In step S1, the precipitant is NaOH solution; the vacuum drying temperature is 120-130℃, and the time is 10-12h.

3. The method for preparing the lithium manganese iron phosphate cathode material with a three-region synergistic interface structure as described in claim 1, characterized in that, In step S1, the lithium source is Li2CO3 and the phosphorus source is NH4H2PO4.

4. The method for preparing the lithium manganese iron phosphate cathode material with a three-region synergistic interface structure as described in claim 1, characterized in that, In step S2, the concentration of sodium alginate solution is 3-5 wt.%, and the concentration of nanocellulose dispersion is 1-2 wt.%; when mixing, the mass ratio of sodium alginate to nanocellulose is (5-10):

1.

5. The method for preparing lithium manganese iron phosphate cathode material with a three-region synergistic interface structure as described in claim 1, characterized in that, In step S2, the mass ratio of sodium alginate, ammonium fluoride and urea is 12:(2-2.5):

3.

6. The method for preparing the lithium manganese iron phosphate cathode material with a three-region synergistic interface structure as described in claim 1, characterized in that, In step S3, the mass ratio of gradient LMFP powder to sodium alginate is 100:(2-4).

7. The method for preparing lithium manganese iron phosphate cathode material with a three-region synergistic interface structure as described in claim 1, characterized in that, In step S3, during spray drying granulation, the inlet air temperature is 200-220℃, the outlet air temperature is 100-120℃, and the feeding rate is 15-20mL / min.

8. A lithium manganese iron phosphate cathode material with a three-region synergistic interface structure, characterized in that, The lithium manganese iron phosphate cathode material with a three-zone synergistic interface structure as described in any one of claims 1-7 was prepared.

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