A lead-free piezoelectric gradient composite lithium manganese iron phosphate cathode material and a preparation method thereof

Abstract

The application relates to the technical field of lithium ion battery cathodes, and discloses a lead-free piezoelectric gradient composite lithium manganese iron phosphate cathode material and a preparation method thereof. The material comprises LMFP@NBBT composite core layer particles and a multiple gradient functional network layer coated on the surface of the particles. The network layer raw materials comprise polyvinylidene fluoride-trifluoroethylene copolymer, multi-walled carbon nanotubes, polyethylene glycol and titanate coupling agent. In the preparation, the LMFP core layer is prepared through a hydrothermal method, the lead-free piezoelectric NBBT layer is coated through a sol-gel and sintering process, and the functional network is constructed on the surface through an electrospinning and annealing process. The application utilizes the alternating built-in electric field generated by the double piezoelectric phase polarization to reduce the ion transmission activation energy, combines the carbon nanotubes to construct a continuous electron transmission channel, and relies on the coupling agent to realize interface chemical bonding, thereby synergistically improving the charge transmission kinetics of the material and inhibiting the peeling of the coating layer in the cycle process.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery cathode technology, specifically to a lead-free piezoelectric graded composite lithium manganese iron phosphate cathode material and its preparation method. Background Technology

[0002] Lithium manganese iron phosphate (LMP), as a cathode material for lithium-ion batteries, combines the high safety and low cost of lithium iron phosphate with the high voltage platform of lithium manganese phosphate, possessing significant potential for energy density and broad application prospects in the new energy field. However, in practical applications, LMP is limited by its crystal structure characteristics, resulting in inherent technical defects. The material has low electronic conductivity, hindering charge transport under high-rate charge-discharge conditions. During battery cycling, trivalent manganese ions are prone to Gaines-Taylor distortion, leading to manganese ion dissolution and local structural phase transitions, reducing the material's long-term cycling stability. Simultaneously, the increased migration barrier of lithium ions at low temperatures causes a decrease in the discharge capacity of the cathode material.

[0003] To address the aforementioned issues of ion diffusion kinetics and structural stability, existing technologies typically combine piezoelectric materials with lithium manganese iron phosphate, attempting to utilize the built-in electric field generated by the piezoelectric effect to accelerate lithium-ion interfacial migration. However, in practical applications, existing piezoelectric composite schemes have multiple limitations. Current piezoelectric modification schemes mostly employ lead-containing ceramic systems such as lead zirconate titanate. These materials have a high lead content, posing environmental pollution and heavy metal leaching risks during material production, use, and waste recycling, and thus failing to meet current industry environmental standards.

[0004] Furthermore, existing piezoelectric composite structures are mostly simple single inorganic layer coatings. On the one hand, a single piezoelectric coating layer cannot construct continuous electron transport channels on the material surface and between particles, making it difficult to achieve a synergistic improvement in electron conductivity and ion diffusion rate, resulting in limited improvement in charge-discharge kinetics. On the other hand, there is a difference in the coefficient of thermal expansion between the inorganic piezoelectric coating material and the lithium manganese iron phosphate core layer, and there is a lack of effective chemical bonding between the heterogeneous material interfaces. During long-cycle charge-discharge, the cathode material undergoes volume shrinkage and expansion due to lithium insertion / extraction, and alternating stress can lead to microcracks or even interface delamination between the coating layer and the core layer. Failure of the coating structure will expose the cathode surface directly to the electrolyte, accelerating the occurrence of interfacial side reactions and shortening the battery cycle life. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a lead-free piezoelectric graded composite lithium manganese iron phosphate cathode material and its preparation method, aiming to solve the problems of slow charge transport dynamics in existing lithium manganese iron phosphate cathode materials, as well as the risks of heavy metal lead pollution and easy peeling failure of the coating interface in existing piezoelectric composite modification schemes.

[0006] To achieve the above objectives, the present invention provides the following technical solution: Firstly, the present invention provides a lead-free piezoelectric graded composite lithium manganese iron phosphate cathode material, employing the following technical solution: A lead-free piezoelectric graded composite lithium manganese iron phosphate cathode material includes LMFP@NBBT composite core particles and a multi-gradient functional network layer coated on the surface of the composite core particles. The multi-gradient functional network layer is made from raw materials comprising the following parts by weight: 45 to 55 parts of polyvinylidene fluoride-trifluoroethylene copolymer; 25 to 35 parts of multi-walled carbon nanotubes; 15 to 25 parts of polyethylene glycol; and 1 to 2 parts of titanate coupling agent. The NBBT inorganic coating layer outside the LMFP@NBBT composite core particles and the polyvinylidene fluoride-trifluoroethylene copolymer in the multi-gradient functional network layer constitute a dual piezoelectric response structure; the polyethylene glycol and multi-walled carbon nanotubes constitute an electron transport network; and the titanate coupling agent connects the inorganic coating layer and the organic network layer.

[0007] By adopting the above technical solution, due to the structural design of combining LMFP@NBBT composite core particles with a multi-gradient functional network layer, and utilizing the composite configuration of inorganic and organic dual piezoelectric phases and conductive networks, the following technical effects are achieved: (1) Piezoelectric polarization promotes ion transport: During the charging and discharging process of the battery, the lithium manganese iron phosphate lattice undergoes volume strain caused by lithium insertion and extraction. This stress is transmitted outward to the inorganic coating layer of NBBT and the external polyvinylidene fluoride-trifluoroethylene copolymer network. The ceramic phase of NBBT and the organic phase of polyvinylidene fluoride-trifluoroethylene copolymer are polarized under stress, forming an alternating built-in electric field. The direction of this built-in electric field is coupled with the direction of lithium ion migration, which reduces the activation energy of lithium ions crossing the interface between the two phases and increases the transport rate of lithium ions at the solid phase interface.

[0008] (2) Construction of three-dimensional conductive network: Polyethylene glycol undergoes micro-region melting during sintering or annealing, which fixes multi-walled carbon nanotubes on the particle surface, forming a continuous electron transport channel and reducing the contact resistance between particles.

[0009] (3) Interfacial chemical bonding: The titanate coupling agent forms chemical bonds by reacting with the hydroxyl groups on the surface of NBBT through the alkoxy groups. At the same time, its organic functional groups physically entangle or chemically crosslink with polyvinylidene fluoride-trifluoroethylene copolymer and carbon nanotubes, thereby improving the bonding force between the inorganic ceramic layer and the organic polymer layer and preventing the coating layer from peeling off during the cycling process.

[0010] Preferably, the LMFP@NBBT composite core layer particles consist of an inner lithium manganese iron phosphate core layer and an outer NBBT inorganic coating layer. In the chemical formula of the lithium manganese iron phosphate core layer, the molar ratio of manganese to iron is x:(1-x), where x ranges from 0.4 to 0.6. The molar ratio of the lithium manganese iron phosphate core layer to the NBBT inorganic coating layer is 14:1 to 15:1. By adopting the above technical solution, the manganese-iron ratio is controlled between 0.4 and 0.6. The addition of iron improves the conductivity of the manganese-based material, and the high voltage platform of manganese enhances the energy density. Controlling the coating molar ratio between 14:1 and 15:1 ensures that the NBBT layer thickness is within the range capable of transmitting stress-induced piezoelectric effects, while preventing the inorganic layer from being too thick and hindering lithium-ion transport.

[0011] Preferably, the inorganic coating layer of the NBBT is Na. 0.5 Bi 0.5 The TiO3-BaTiO3 system of lead-free piezoelectric ceramics uses sodium nitrate, bismuth nitrate pentahydrate, barium nitrate and tetrabutyl titanate as precursor materials.

[0012] By adopting the above technical solution, Na is selected. 0.5 Bi 0.5 The TiO3-BaTiO3 system serves as a coating layer. This material exhibits piezoelectric response characteristics near the quasi-isomorphic phase boundary, enabling it to generate a potential response to lattice strain, and its composition does not contain lead.

[0013] Preferably, the number average molecular weight of the polyethylene glycol is 4,000 to 6,000; and the titanate coupling agent is isopropyltris(dioctylpyrophosphate) titanate.

[0014] By employing the above technical solution, polyethylene glycol with a molecular weight of 4000 to 6000 is selected and maintained at a predetermined viscosity at the annealing temperature, allowing carbon nanotubes to spread and bond on the particle surface. The pyrophosphate oxy group of isopropyl tris(dioctylpyrophosphate)titanate has coordination ability to the oxide surface, improving the interfacial wettability between the inorganic and organic layers.

[0015] Preferably, the thickness of the multi-gradient functional network layer on the surface of the composite core layer particles is 1 micrometer to 3 micrometers.

[0016] By adopting the above technical solution, the thickness of the functional layer is controlled between 1 micrometer and 3 micrometers. While retaining the conductive network and piezoelectric active material, it prevents the proportion of inactive material from being too high, which would lead to a decrease in the volumetric energy density of the battery.

[0017] Secondly, this invention provides a method for preparing a lead-free piezoelectric graded composite lithium manganese iron phosphate cathode material, employing the following technical solution: A method for preparing a lead-free piezoelectric graded composite lithium manganese iron phosphate cathode material includes the following steps: S1, prepare a multi-component precursor solution containing lithium source, manganese source, iron source and phosphorus source and carry out hydrothermal reaction, and obtain lithium manganese iron phosphate core layer particles after drying. S2, prepare NBBT precursor sol, add the lithium manganese iron phosphate core layer particles to the NBBT precursor sol and ultrasonically disperse to form a gel, and after pre-calcination, sintering and pulverization, obtain LMFP@NBBT composite core layer particles; S3, a spinning solution with a target mass concentration is prepared by dispersing polyvinylidene fluoride-trifluoroethylene copolymer, multi-walled carbon nanotubes, polyethylene glycol and titanate coupling agent in a solvent and then ultrasonically treating it. S4. Using an electrospinning process, under the conditions of applying DC high voltage and substrate heating, the spinning solution is uniformly deposited on the surface of the LMFP@NBBT composite core layer particles to form a composite material coated with a functional layer precursor network. S5, the composite material is placed under a protective atmosphere for heat preservation and annealing treatment, and after cooling, lead-free pressure gradient composite lithium manganese iron phosphate cathode material is obtained.

[0018] By adopting the above technical solution, and by using a hydrothermal method combined with sol-gel and electrospinning processes, the coating and network construction of inorganic ceramic phases and organic polymer phases on the particle surface are achieved. The specific reaction mechanism and action process correspond to the aforementioned steps as follows: In step S1: Lithium ions, manganese ions, iron ions and phosphate ions in the solution undergo co-precipitation and crystallization reactions in a high-temperature and high-pressure water environment to generate a solid solution of lithium manganese iron phosphate with an olivine structure.

[0019] In step S2: Tetrabutyl titanate undergoes hydrolysis and condensation reactions to form a three-dimensional network gel, encapsulating sodium, bismuth, and barium metal cations within it. The pre-calcination process volatilizes and decomposes the organic solvents and ligands in the gel; high-temperature sintering promotes a solid-phase reaction in the amorphous precursor, resulting in in-situ nucleation and crystallization of Na on the surface of lithium manganese iron phosphate particles. 0.5 Bi 0.5 TiO3-BaTiO3 phase.

[0020] In steps S3 and S4: the high-voltage electrostatic field applied by electrospinning causes the molecular chains of polyvinylidene fluoride-trifluoroethylene copolymer to undergo polarization orientation, inducing the generation of a β-phase crystal structure with piezoelectric activity; the substrate heating causes the solvent to evaporate before the jet reaches the particle surface, constructing a three-dimensional porous polymer network.

[0021] In step S5: Thermal annealing causes micro-region melting of polyethylene glycol, bonding multi-walled carbon nanotubes to the node positions of the particle network. Under heating conditions, the isopropyl group at one end of the titanate coupling agent undergoes a condensation reaction with the hydroxyl groups on the NBBT surface to form a titanium-oxygen covalent bond, while the other end physically entangles with the polymer molecular chain, achieving structural anchoring of the interface.

[0022] Preferably, in step S2, the specific process of pre-firing and sintering is as follows: in a protective atmosphere, the temperature is first raised to 300 to 400°C at a heating rate of 1 to 5°C / min for 1 to 3 hours for pre-firing, and then the temperature is raised to 600 to 700°C at a heating rate of 1 to 5°C / min for sintering for 6 to 8 hours.

[0023] By adopting the above technical solution, controlling the heating rate of 1 to 5 °C / min prevents the violent decomposition of organic matter in the gel from causing the coating layer to crack. Pre-firing at 300 to 400 °C provides sufficient heat to volatilize low-boiling-point organic matter and initially decompose nitrates; sintering at 600 to 700 °C provides thermodynamic energy for the formation of the ceramic phase, while preventing excessively high temperatures from causing the agglomeration of lithium manganese iron phosphate core particles or the destruction of the internal crystal structure.

[0024] Preferably, in step S3, the solvent is N,N-dimethylformamide, and the mass concentration of the solute in the spinning solution is controlled at 10% to 12%; the conditions for ultrasonic treatment are: rotation speed 80 to 120 r / min, frequency 10 to 20 Hz, and ultrasonic dispersion time 40 to 60 min.

[0025] By employing the above technical solution, N,N-dimethylformamide is selected as the solvent to dissolve and disperse carbon nanotubes in the polyvinylidene fluoride-trifluoroethylene copolymer. The solute concentration is controlled at 10% to 12% to maintain the viscosity of the spinning solution and form a continuous fiber network, preventing electrospraying due to excessively low concentration or clogging of the spinning needles due to excessively high concentration. Ultrasonic treatment is performed at a rotation speed of 80 to 120 r / min and a frequency of 10 to 20 Hz to provide suitable shear force and cavitation effect, promoting uniform dispersion of carbon nanotubes in the polymer matrix while preventing excessive ultrasonication from damaging the polymer chain structure.

[0026] Preferably, in step S4, the parameters of the electrospinning process are controlled as follows: the applied DC high voltage is 15 to 25 kV, the injection pump speed is 0.8 to 1.5 mL / h, and the distance between the spinning needle and the receiving substrate is 10 to 15 cm; the substrate heating is turned on simultaneously, and the temperature of the receiving substrate is controlled to be constant at 80 to 100°C.

[0027] By employing the above technical solution, a 15-25kV DC high voltage provides an electric field driving force to stretch the polymer jet and induce dipole orientation. Matched propulsion speed and receiving distance provide space for jet flight and solvent evaporation. Setting the receiving substrate temperature to 80-100℃ accelerates residual solvent removal and improves the adhesion of the deposited network to the composite particle surface.

[0028] Preferably, in step S5, the parameters for the heat preservation annealing treatment are set as follows: the heating rate is set to 1 to 3℃ / min, the temperature is raised to 120 to 140℃, and the heat preservation annealing treatment is carried out for 4 to 6 hours.

[0029] By employing the above technical solution, the annealing temperature is controlled between 120 and 140°C. This temperature falls between the Curie temperature and melting point of the polyvinylidene fluoride-trifluoroethylene copolymer, eliminating internal stress generated during electrospinning, improving the crystallinity of the organic phase, and completing the chemical cross-linking reaction at the interface with the participation of a coupling agent. Controlling the heating rate prevents microcracks caused by thermal stress, ensuring the integrity of the gradient network structure.

[0030] This invention provides a lead-free piezoelectric graded composite lithium manganese iron phosphate cathode material and its preparation method. It has the following beneficial effects: 1. This invention uses Na 0.5 Bi 0.5 The TiO3-BaTiO3 (NBBT) lead-free piezoelectric ceramic system serves as a coating material, replacing traditional lead-containing materials such as lead zirconate titanate (PZT). This technical solution eliminates the environmental pollution risks posed by heavy metal lead during material preparation, use, and recycling, while ensuring piezoelectric response characteristics and improving the product's environmental compatibility.

[0031] 2. This invention utilizes a sol-gel process to achieve in-situ growth of NBBT on the surface of lithium manganese iron phosphate particles, and introduces a titanate coupling agent for interlayer anchoring. The in-situ growth process ensures a tight bond between the inorganic coating layer and the core layer. Combined with the chemical bonding effect formed by the coupling agent, this solves the problem of weak interfacial bonding between heterogeneous materials, effectively suppressing coating peeling and microcrack formation caused by volume expansion during charging and discharging, and improving the cycle stability of the material.

[0032] 3. This invention constructs a dual-functional synergistic network of piezoelectric field and electron transport. Multi-walled carbon nanotubes, aided by polyethylene glycol, form a continuous electronic conduction network, enhancing the material's electronic conductivity. Simultaneously, the PVDF-TrFE organic network and the NBBT inorganic layer generate a dual piezoelectric response under charge-discharge stress, and the excited built-in electric field lowers the activation energy for lithium ions to cross the interface. The synergistic effect of these two elements simultaneously improves the electron transport and lithium-ion diffusion kinetics of the cathode material. Attached Figure Description

[0033] Figure 1 The piezoelectric response constant d of the material powder in the embodiments and comparative examples of the present invention is given. 33 A bar chart comparing the two bars; Figure 2 This is a graph showing the trend of electronic conductivity values ​​in the embodiments and comparative examples of the present invention. Figure 3 The following is a comparison of charge transfer resistance and lithium-ion migration activation energy of each embodiment and comparative example in the test examples of the present invention. Sub-figure (a) shows the difference in the distribution of charge transfer resistance values ​​of each group of materials at three test temperatures of 25℃, 0℃ and -20℃ in a logarithmic coordinate system; Sub-figure (b) shows the comparison of lithium-ion migration activation energy at the macroscopic level of each group of materials. Figure 4 This is a comparison chart of the mechanical peel strength of the electrode and the amount of manganese ion dissolution in the electrolyte of each embodiment and comparative example in the test examples of the present invention. Among them, sub-figure (a) is a distribution chart of the average steady-state peel force of the coated electrode in the 180-degree peel test; sub-figure (b) is a chart of the absolute concentration of manganese in the electrolyte of each group of batteries measured in 200 cycles at a high temperature of 45°C. Figure 5 The figures show a comparison of the wide-temperature range and high-rate charge-discharge performance of each embodiment and comparative example in the test examples of this invention. Sub-figure (a) shows the difference in specific capacity decay between each group of batteries under 0.5C current during charge-discharge at 25°C and -20°C extreme cold conditions. Sub-figure (b) shows the rate response trajectory of the material at room temperature as the current density continuously increases from 0.1C to 10C. Figure 6 This is a comparison chart of the full-cell long-cycle capacity retention rate and median voltage decay of each embodiment and comparative example in the test examples of the present invention. Among them, sub-chart (a) is a chart of the absolute capacity retention rate of each group of batteries after 500 deep charge-discharge cycles at 1C rate; sub-chart (b) is a chart of the median voltage drop of each group of batteries. Detailed Implementation

[0034] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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.

[0035] To accommodate the processing capacity and batch preparation scale of laboratory-level electrospinning equipment, a standardized conversion of "0.1g is equivalent to 1 part by weight" is set as the benchmark.

[0036] Preparation Examples 1-6: Preparation Example 1: This preparation example provides a method for preparing LMFP@NBBT composite particles, using Mn:Fe = 4:6 and a coating molar ratio of 15:1. The method includes the following steps: (1) 66.7g lithium hydroxide monohydrate, 147.1g manganese acetate tetrahydrate, 221.3g ferrous acetate tetrahydrate and 173.0g phosphoric acid with a mass fraction of 85% were dissolved in 15.5L of deionized water, and 288.2g of anhydrous citric acid was added. The mixture was stirred for 30min using a planetary stirrer with a revolution speed of 10r / min and a rotation speed of 2000r / min. The mixture was transferred to a hydrothermal reactor and reacted at 180℃ for 12h. After cooling, it was washed twice with deionized water and dried under vacuum at 80℃ to obtain LMFP (lithium manganese iron phosphate) core layer particles.

[0037] (2) Dissolve 4.04g sodium nitrate, 23.04g bismuth nitrate pentahydrate, 1.31g barium nitrate and 34.0g tetrabutyl titanate in 500mL ethylene glycol methyl ether, add 5.0g acetylacetone, and ultrasonically stir for 30min at 100r / min and 10Hz to prepare 0.1mol NBBT precursor sol.

[0038] (3) Weigh 236.0g of the LMFP core layer particles obtained in step (1) above and add them to the NBBT precursor sol. Disperse the sol with ultrasound at 100r / min and 10Hz for 30min to form a gel.

[0039] (4) The gel was transferred to a sintering sagger and placed in a sintering furnace with high-purity nitrogen (protective atmosphere). The temperature was first raised to 350℃ at 3℃ / min for 2 hours, and then raised to 650℃ at 3℃ / min for 6 hours. After natural cooling, the LMFP@NBBT composite particles were obtained by mechanical crushing and air jet pulverization.

[0040] Preparation Example 2: This preparation example provides a method for preparing LMFP@NBBT composite particles, using Mn:Fe = 5:5 and a coating molar ratio of 15:1. The method includes the following steps: (1) 66.7g lithium hydroxide monohydrate, 183.8g manganese acetate tetrahydrate, 184.4g ferrous acetate tetrahydrate and 173.0g phosphoric acid with a mass fraction of 85% were dissolved in 15.5L of deionized water, and 288.2g of anhydrous citric acid was added. The mixture was stirred for 45min using a planetary stirrer with a revolution speed of 15r / min and a rotation speed of 1800r / min. The mixture was transferred to a hydrothermal reactor and reacted at 190℃ for 14h. After cooling, the mixture was washed twice with deionized water and dried under vacuum at 82℃ to obtain LMFP core layer particles.

[0041] (2) Dissolve 4.04g sodium nitrate, 23.04g bismuth nitrate pentahydrate, 1.31g barium nitrate and 34.0g tetrabutyl titanate in 500mL ethylene glycol methyl ether, add 5.0g acetylacetone, and ultrasonically stir for 30min at 100r / min speed and 10Hz to prepare NBBT sol.

[0042] (3) Weigh 236.0g of the LMFP core layer particles obtained in step (1) above and add them to the NBBT sol. Disperse the sol with ultrasound at 100r / min and 10Hz for 30min to form a gel.

[0043] (4) The gel was transferred to a sintering sagger and placed in a sintering furnace with high-purity nitrogen (protective atmosphere). The temperature was first raised to 300℃ at 1℃ / min for 1h, and then raised to 600℃ at 1℃ / min for 8h. After natural cooling, the LMFP@NBBT composite particles were obtained by mechanical crushing and air jet pulverization.

[0044] Preparation Example 3: This preparation example provides a method for preparing LMFP@NBBT composite particles, using Mn:Fe = 6:4 and a coating molar ratio of 15:1. The method includes the following steps: (1) 66.7g lithium hydroxide monohydrate, 220.6g manganese acetate tetrahydrate, 147.6g ferrous acetate tetrahydrate and 173.0g phosphoric acid with a mass fraction of 85% were dissolved in 15.5L of deionized water, and 288.2g of anhydrous citric acid was added. The mixture was stirred for 60min using a planetary stirrer with a revolution speed of 20r / min and a rotation speed of 1500r / min. The mixture was transferred to a hydrothermal reactor and reacted at 200℃ for 16h. After cooling, the mixture was washed twice with deionized water and dried under vacuum at 85℃ to obtain LMFP core layer particles.

[0045] (2) Dissolve 4.04g sodium nitrate, 23.04g bismuth nitrate pentahydrate, 1.31g barium nitrate and 34.0g tetrabutyl titanate in 500mL ethylene glycol methyl ether, add 5.0g acetylacetone, and ultrasonically stir for 30min at 100r / min speed and 10Hz to prepare NBBT sol.

[0046] (3) Weigh 236.0g of the LMFP core layer particles obtained in step (1) above and add them to the NBBT sol. Disperse the sol with ultrasound at 100r / min and 10Hz for 30min to form a gel.

[0047] (4) The gel was transferred to a sintering sagger and placed in a sintering furnace with high-purity nitrogen (protective atmosphere). The temperature was first raised to 400℃ at 5℃ / min for 3 hours, and then raised to 700℃ at 5℃ / min for 6 hours. After natural cooling, the gel was mechanically crushed and air-jet pulverized to obtain LMFP@NBBT composite particles.

[0048] Preparation Example 4: This preparation example provides a method for preparing LMFP@NBBT composite particles, using Mn:Fe = 4:6 and a coating molar ratio of 15:1. The method includes the following steps: (1) 66.7 g lithium hydroxide monohydrate, 147.1 g manganese acetate tetrahydrate, 221.3 g ferrous acetate tetrahydrate and 173.0 g phosphoric acid with a mass fraction of 85% were dissolved in 15.5 L of deionized water, and 288.2 g anhydrous citric acid was added. The reaction conditions and post-treatment were the same as in Preparation Example 1 to obtain LMFP core layer particles.

[0049] (2) Dissolve 4.16g sodium nitrate, 23.77g bismuth nitrate pentahydrate, 0.52g barium nitrate and 34.0g tetrabutyl titanate in 500mL ethylene glycol methyl ether, add 5.0g acetylacetone, and ultrasonically stir for 30min at 100r / min speed and 10Hz to prepare NBBT sol.

[0050] (3) Weigh 236.0g of the LMFP core layer particles prepared above and add them to the NBBT sol. Disperse the sol with ultrasound at 100r / min and 10Hz for 30min to form a gel.

[0051] (4) The gel was transferred to a sintering sagger and placed in a sintering furnace with high-purity nitrogen (protective atmosphere). The temperature was first raised to 320°C at 2°C / min for 1.5 h, and then raised to 650°C at 2°C / min for 6 h. After natural cooling, the LMFP@NBBT composite particles were obtained by mechanical crushing and air jet pulverization.

[0052] Preparation Example 5: This preparation example provides a method for preparing LMFP@NBBT composite particles, using Mn:Fe = 4:6 and a coating molar ratio of 15:1. The method includes the following steps: (1) 66.7 g lithium hydroxide monohydrate, 147.1 g manganese acetate tetrahydrate, 221.3 g ferrous acetate tetrahydrate and 173.0 g phosphoric acid with a mass fraction of 85% were dissolved in 15.5 L of deionized water, and 288.2 g anhydrous citric acid was added. The reaction conditions and post-treatment were the same as in Preparation Example 1 to obtain LMFP core layer particles.

[0053] (2) Dissolve 3.91g sodium nitrate, 22.31g bismuth nitrate pentahydrate, 2.09g barium nitrate and 34.0g tetrabutyl titanate in 500mL ethylene glycol methyl ether, add 5.0g acetylacetone, and ultrasonically stir for 30min at 100r / min speed and 10Hz to prepare NBBT sol.

[0054] (3) Weigh 236.0g of the LMFP core layer particles prepared above and add them to the NBBT sol. Disperse the sol with ultrasound at 100r / min and 10Hz for 30min to form a gel.

[0055] (4) The gel was transferred to a sintering sagger and placed in a sintering furnace with high-purity nitrogen (protective atmosphere). The temperature was first raised to 380℃ at 4℃ / min for 2.5h, and then raised to 650℃ at 4℃ / min for 6h. After natural cooling, the LMFP@NBBT composite particles were obtained by mechanical crushing and air jet pulverization.

[0056] Preparation Example 6: This preparation example provides a method for preparing LMFP@NBBT composite particles, using Mn:Fe = 4:6 and a lower limit of the coating molar ratio of 14:1. The method includes the following steps: (1) 66.7 g lithium hydroxide monohydrate, 147.1 g manganese acetate tetrahydrate, 221.3 g ferrous acetate tetrahydrate and 173.0 g phosphoric acid with a mass fraction of 85% were dissolved in 15.5 L of deionized water, and 288.2 g anhydrous citric acid was added. The reaction conditions and post-treatment were the same as in Preparation Example 1 to obtain LMFP core layer particles.

[0057] (2) Dissolve 4.04g sodium nitrate, 23.04g bismuth nitrate pentahydrate, 1.31g barium nitrate and 34.0g tetrabutyl titanate in 500mL ethylene glycol methyl ether, add 5.0g acetylacetone, and ultrasonically stir for 30min at 100r / min speed and 10Hz to prepare NBBT sol.

[0058] (3) Weigh 220.4g of the LMFP core layer particles prepared above and add them to the NBBT sol. Disperse the sol with ultrasound at 100r / min and 10Hz for 30min to form a gel.

[0059] (4) The gel was transferred to a sintering sagger and placed in a sintering furnace with high-purity nitrogen (protective atmosphere). The temperature was first raised to 350℃ at 3℃ / min for 2 hours, and then raised to 650℃ at 3℃ / min for 6 hours. After natural cooling, the LMFP@NBBT composite particles were obtained by mechanical crushing and air jet pulverization.

[0060] Examples 1-5: Example 1: This embodiment provides a method for preparing a lead-free, piezoelectric graded composite lithium manganese iron phosphate cathode material, including the following steps: (1) Weigh 5.0g of polyvinylidene fluoride-trifluoroethylene copolymer, 3.0g of multi-walled carbon nanotubes and 2.0g of polyethylene glycol (number average molecular weight of 4000), mix them and add them to 90.0g of N,N-dimethylformamide solvent to make the solute mass concentration 10%; add 0.1g of isopropyltris(dioctylpyrophosphoryloxy)titanate (accounting for 1% of the total mass of solute); and ultrasonically disperse at 100r / min and 15Hz for 40min to prepare spinning solution.

[0061] (2) An electrospinning device was used, with the LMFP@NBBT composite particles obtained in Preparation Example 1 supported by aluminum foil as the receiving substrate; the injection pump speed was set to 1.0 mL / h, the distance between the spinning needle and the receiving substrate was 12 cm, and the DC high voltage for spinning was 20 kV; the heating module at the bottom of the receiving substrate was turned on simultaneously to keep the substrate temperature constant at 90 °C; the electrospinning continued until a functional layer precursor network with a thickness of 2 μm was formed on the particle surface.

[0062] (3) The composite material coated with the spinning functional layer was scraped from the substrate and placed in a tube furnace with argon gas introduced; the heating rate was set to 2℃ / min, the temperature was raised to 130℃, and the heat treatment was carried out for 5h; after the annealing was completed, it was naturally cooled to room temperature to obtain lead-free pressure gradient composite lithium manganese iron phosphate cathode material.

[0063] Example 2: This embodiment provides a method for preparing a lead-free, piezoelectric graded composite lithium manganese iron phosphate cathode material, including the following steps: (1) Weigh 5.0g of polyvinylidene fluoride-trifluoroethylene copolymer, 3.0g of multi-walled carbon nanotubes and 2.0g of polyethylene glycol (number average molecular weight of 4000), mix them and add them to 73.3g of N,N-dimethylformamide solvent to make the solute mass concentration 12%; add an additional 0.1g of isopropyltris(dioctylpyrophosphoryloxy)titanate (accounting for 1% of the total mass of solute); and ultrasonically disperse at 80r / min and 10Hz for 60min to prepare spinning solution.

[0064] (2) An electrospinning device was used, with the LMFP@NBBT composite particles obtained in Preparation Example 2 supported by aluminum foil as the receiving substrate; the injection pump speed was set to 0.8 mL / h, the distance between the spinning needle and the receiving substrate was 10 cm, and the DC high voltage for spinning was 20 kV; the heating module at the bottom of the receiving substrate was turned on simultaneously to keep the substrate temperature constant at 90 °C; the electrospinning continued until a functional layer precursor network with a thickness of 1 μm was formed on the particle surface.

[0065] (3) The composite material coated with the spinning functional layer was scraped from the substrate and placed in a tube furnace with argon gas introduced; the heating rate was set to 1℃ / min, the temperature was raised to 120℃, and the heat treatment was carried out for 6h; after the annealing was completed, it was naturally cooled to room temperature to obtain lead-free pressure gradient composite lithium manganese iron phosphate cathode material.

[0066] Example 3: This embodiment provides a method for preparing a lead-free, piezoelectric graded composite lithium manganese iron phosphate cathode material, including the following steps: (1) Weigh 5.0g of polyvinylidene fluoride-trifluoroethylene copolymer, 3.0g of multi-walled carbon nanotubes and 2.0g of polyethylene glycol (number average molecular weight of 4000), mix them and add them to 80.9g of N,N-dimethylformamide solvent to make the solute mass concentration 11%; add an additional 0.15g of isopropyltris(dioctylpyrophosphoryloxy)titanate (accounting for 1.5% of the total mass of solute); ultrasonically disperse at 120r / min and 20Hz for 50min to prepare spinning solution.

[0067] (2) An electrospinning device was used, with the LMFP@NBBT composite particles obtained in Preparation Example 3 supported by aluminum foil as the receiving substrate; the injection pump speed was set to 1.5 mL / h, the distance between the spinning needle and the receiving substrate was 15 cm, and the DC high voltage for spinning was 20 kV; the heating module at the bottom of the receiving substrate was turned on simultaneously to keep the substrate temperature constant at 90 °C; the electrospinning continued until a functional layer precursor network with a thickness of 3 μm was formed on the particle surface.

[0068] (3) The composite material coated with the spinning functional layer was scraped from the substrate and placed in a tube furnace with argon gas introduced; the heating rate was set to 3℃ / min, the temperature was raised to 140℃, and the heat treatment was carried out for 4h; after the annealing was completed, it was naturally cooled to room temperature to obtain lead-free pressure gradient composite lithium manganese iron phosphate cathode material.

[0069] Example 4: This embodiment provides a method for preparing a lead-free, piezoelectric graded composite lithium manganese iron phosphate cathode material, including the following steps: (1) Weigh 5.0g of polyvinylidene fluoride-trifluoroethylene copolymer, 3.0g of multi-walled carbon nanotubes and 2.0g of polyethylene glycol (number average molecular weight of 4000), mix them and add them to 90.0g of N,N-dimethylformamide solvent to make the solute mass concentration 10%; add an additional 0.1g of isopropyltris(dioctylpyrophosphate)titanate (accounting for 1% of the total mass of solute); and ultrasonically disperse at 90r / min and 12Hz for 40min to prepare spinning solution.

[0070] (2) An electrospinning device was used, with the LMFP@NBBT composite particles obtained in Preparation Example 4 supported by aluminum foil as the receiving substrate; the injection pump speed was set to 1.0 mL / h, the distance between the spinning needle and the receiving substrate was 12 cm, and the DC high voltage for spinning was 15 kV; the heating module at the bottom of the receiving substrate was turned on simultaneously to keep the substrate temperature constant at 80 °C; the electrospinning continued until a functional layer precursor network with a thickness of 2 μm was formed on the particle surface.

[0071] (3) The composite material coated with the spinning functional layer was scraped from the substrate and placed in a tube furnace with argon gas introduced; the heating rate was set to 2℃ / min, the temperature was raised to 130℃, and the heat treatment was carried out for 5h; after the annealing was completed, it was naturally cooled to room temperature to obtain lead-free pressure gradient composite lithium manganese iron phosphate cathode material.

[0072] Example 5: This embodiment provides a method for preparing a lead-free, piezoelectric graded composite lithium manganese iron phosphate cathode material, including the following steps: (1) Weigh 5.0g of polyvinylidene fluoride-trifluoroethylene copolymer, 3.0g of multi-walled carbon nanotubes and 2.0g of polyethylene glycol (number average molecular weight of 6000), mix them and add them to 90.0g of N,N-dimethylformamide solvent to make the solute mass concentration 10%; add an additional 0.2g of isopropyltris(dioctylpyrophosphate)titanate (accounting for 2% of the total mass of solute); and ultrasonically disperse at 110r / min and 18Hz for 40min to prepare spinning solution.

[0073] (2) An electrospinning device was used, with the LMFP@NBBT composite particles obtained in Preparation Example 5 supported by aluminum foil as the receiving substrate; the injection pump speed was set to 1.0 mL / h, the distance between the spinning needle and the receiving substrate was 12 cm, and the DC high voltage for spinning was 25 kV; the heating module at the bottom of the receiving substrate was turned on simultaneously to keep the substrate temperature constant at 100 °C; the electrospinning continued until a functional layer precursor network with a thickness of 2 μm was formed on the particle surface.

[0074] (3) The composite material coated with the spinning functional layer was scraped from the substrate and placed in a tube furnace with argon gas introduced; the heating rate was set to 2℃ / min, the temperature was raised to 130℃, and the heat treatment was carried out for 5h; after the annealing was completed, it was naturally cooled to room temperature to obtain lead-free pressure gradient composite lithium manganese iron phosphate cathode material.

[0075] Comparative Examples 1-6: Comparative Example 1: Compared with Example 1, the difference is that the receiving substrate used is the LMFP core layer particle without NBBT coating obtained in step (1) of Example 1, that is, the inorganic piezoelectric transition layer is missing, and the rest are the same.

[0076] Comparative Example 2: Compared with Example 1, the difference is that in the spinning solution preparation process of step (1), the polyvinylidene fluoride-trifluoroethylene copolymer is replaced with an equal mass of conventional nonpolar polyvinylidene fluoride (PVDF), that is, the organic piezoelectric polarization characteristics are missing, and the rest are the same.

[0077] Comparative Example 3: Compared with Example 1, the difference is that isopropyltris(dioctylpyrophosphate)titanate coupling agent is not added in the spinning solution preparation process of step (1), while the rest are the same.

[0078] Comparative Example 4: Compared with Example 1, the difference is that polyethylene glycol is not added in the preparation of the spinning solution in step (1), but the rest are the same.

[0079] Comparative Example 5: Compared with Example 1, the difference is that in step (2), the electrospinning equipment is not used for in-situ high-voltage polarization. Instead, the composite particles are directly immersed in the spinning solution and physically coated by conventional stirring and drying. The substrate is not heated. All other steps are the same.

[0080] Comparative Example 6: Compared with Example 1, the difference lies in the process sequence. First, the uncoated LMFP core layer particles are spun and coated and annealed, then immersed in NBBT sol for drying, and finally uniformly subjected to high-temperature sintering treatment at 650°C. All other aspects are the same.

[0081] Test Examples 1-5: Test Example 1: This test case provides quantitative tests on the macroscopic piezoelectric response constant and apparent electronic conductivity of the powder materials prepared in each embodiment and comparative example, to verify the internal piezoelectric polarization state and the construction of the three-dimensional conductive network of the material.

[0082] 1. Weigh 2.0g of the composite powder materials finally obtained in Examples 1 to 5 and Comparative Examples 1 to 6 respectively, place them in an insulating steel mold with an inner diameter of 15mm, apply a unidirectional axial pressure of 200MPa at room temperature using a powder tablet press, hold the pressure for 60s, and after demolding, obtain a dense powder tablet with a thickness of about 2.5mm. Prepare 3 samples in parallel for each test group.

[0083] 2. Place the powder tablets prepared above into ZJ-3 quasi-static pressure cells. 33 Between the test probes of the measuring instrument, adjust the preload of the test clamp to the equipment reference position, and read and record its longitudinal piezoelectric strain constant d under low-frequency alternating stress conditions of 100Hz. 33Take the arithmetic mean of the three samples.

[0084] 3. Take 5.0g of each group of powder materials and put them into the powder test chamber of the ST2722 powder resistivity tester. Start the hydraulic system to apply a constant test pressure of 10MPa to the powder by the test electrode. After the system pressure stabilizes, read the resistance value of the powder bed by the four-probe method and calculate the apparent electronic conductivity of the material based on the cross-sectional area of ​​the test chamber and the thickness of the powder bed.

[0085] Table 1. Piezoelectric response and conductivity test data of each embodiment and comparative example

[0086] Combined with Table 1 Figure 1 and Figure 2 The intuitive columnar distribution characteristics reveal that the gradient composite cathode materials prepared in Examples 1 to 5 consistently maintain a high piezoelectric response constant above 24 pC / N, exhibiting significant synergistic enhancement. In routine polarization behavior studies, the piezoelectric output of a single component is often limited by the shielding effect of the internal depletion layer. However, in Comparative Example 1, the inorganic piezoelectric transition layer NBBT was missing, and this structural deficiency directly led to a precipitous drop in the piezoelectric response constant within the bulk phase of the material. This phenomenon indirectly confirms that a single polymer layer cannot provide sufficient bulk polarization charge to penetrate the core under pressure. Correspondingly, the test results of Comparative Example 2 show that when conventional nonpolar PVDF is used to replace the highly polar PVDF-TrFE copolymer, the system loses the all-trans conformation crystallization region capable of directional response, and the macroscopic piezoelectric effect is almost completely lost. This thermodynamic and crystallographic limitation is also verified in the process, as observed in the data of Comparative Example 5 and... Figure 1 The extremely low column height distribution reveals the necessity of electrostatic field intervention. In systems constructed entirely through conventional mechanical stirring and physical coating, the perovskite ceramic grains and polymer segments do not undergo in-situ dipole orientation. This thermodynamic polarization deactivation ultimately results in piezoelectric properties remaining at an ineffective level of only 1.8 pC / N.

[0087] Having clarified the source mechanism of the piezoelectric response, the integrity of the conductive network directly determines whether charge can effectively penetrate the entire polarized system. (See Table 1 and...) Figure 2 The logarithmic coordinate data, specifically Comparative Example 4, vividly demonstrates the decisive role of the physical construction mechanism of the three-dimensional conductive network in the overall macroscopic electrical properties. In this control group, the absence of polyethylene glycol leads to severe structural discontinuities in the rigid tubular carbon nanotubes during conventional blending. Due to the inability to effectively adhere to the spherical core, the powder conductivity of the system exhibits a decrease spanning several orders of magnitude (dropping to 10). -5(On the order of S / cm). In practical interface engineering control, the introduction of polyethylene glycol of a specific molecular weight in the examples served as a key thermal fluid medium during the annealing stage. The low-viscosity melt physically anchored the highly conductive carbon mesh to the particle surface through capillary action. This micron-level welding mechanism fundamentally eliminates the electronic insulation barrier of the phosphate material itself, resulting in a general increase and stabilization of the conductivity of the example groups at 10. -2 The magnitude is on the order of S / cm. When exploring the coupling relationship between processes, the test feedback of Comparative Example 6 presents an extreme failure state. Figure 2 The near-bottom-out performance of this set of data indicates that reversing the original preparation process caused the polymer and carbon network to undergo complete carbonization and decomposition in a high-temperature sintering environment of 650℃. The collapse of the physical structure not only destroyed the conductive network but also deprived it of its chemical basis for becoming a piezoelectric medium. The huge contrast between the experimental records and the parameters presented in the charts illustrates that the multi-gradient structure constructed in this scheme and the specific in-situ process arrangement constitute a highly interlocked system; the absence of any single link will lead to a complete collapse of the material's physicochemical properties.

[0088] Test Example 2: This test example provides quantitative measurements of temperature-dependent electrochemical impedance (EIS) and lithium-ion migration activation energy (Ea) of the materials prepared in each embodiment and comparative example to verify the effect of the interfacial alternating piezoelectric field on improving low-temperature kinetic sluggishness and accelerating ion transport across the potential barrier.

[0089] 1. The powder materials obtained in Examples 1 to 5 and Comparative Examples 1 to 6 were mixed with conductive carbon black and polyvinylidene fluoride at a mass ratio of 8:1:1, respectively. An appropriate amount of N-methylpyrrolidone was added and the mixture was ground into a uniform slurry. The slurry was coated onto an aluminum foil current collector and dried under vacuum at 120°C for 12 hours. Then, it was rolled and punched into a positive electrode sheet with a diameter of 14 mm. Using a lithium metal sheet as the negative electrode and a 1 mol / L lithium hexafluorophosphate solution of ethylene carbonate / dimethyl carbonate / ethyl methyl carbonate (volume ratio 1:1:1) as the electrolyte, CR2032 coin cells were assembled in an argon-filled glove box. Five samples of each material were assembled in parallel.

[0090] 2. The completed coin cells were placed in high and low temperature alternating test chambers at set temperatures of 25℃, 0℃, and -20℃, respectively, and kept at a constant temperature for 4 hours at each temperature node to allow the cells to reach thermal equilibrium. AC impedance testing was performed using an electrochemical workstation under AC amplitude of 5mV and a frequency range of 100kHz to 0.01Hz. The charge transfer resistance data after equivalent circuit fitting was extracted and the arithmetic mean was calculated.

[0091] 3. Based on the charge transfer resistance extracted at the three temperature nodes, the reciprocal of the absolute temperature is used as the abscissa and the natural logarithm of the reciprocal of the charge transfer resistance is used as the ordinate for Arrhenius linear fitting. The lithium-ion migration activation energy at the material interface is extracted by calculating the slope of the fitted line.

[0092] Table 2. Temperature-dependent charge transfer resistance and lithium-ion migration activation energy data for each embodiment and comparative example.

[0093] Combining the data in Table 2 with Figure 3 The columnar distribution of the neutron diagram (b) reveals that the lithium-ion migration activation energies exhibited in Examples 1 to 5 remain in the low range of 39.8 to 45.2 kJ / mol. In the investigation of the low-temperature electrochemical behavior of conventional lithium manganese iron phosphate cathode materials, the charge transfer process at the interface is dominated by thermodynamic factors and is highly susceptible to kinetic freezing under extremely cold conditions. This typical temperature sensitivity... Figure 3 The impedance data at -20°C on the logarithmic coordinate axis of the neutron diagram (a) is intuitively reflected in this. The charge transfer resistance increase in the example group under low-temperature conditions is much lower than conventionally expected. Combining the crystallographic variation law and the mechanism deduction of this system, the volume contraction and expansion of the core layer lattice caused by battery charge-discharge cycles will generate periodic mechanical stress. This mechanical energy can be directly converted into an alternating built-in electric field under the in-situ intervention of the double piezoelectric layer. The resulting local Coulomb force acts as an additional driving force to offset part of the thermodynamic energy barrier limitation, which essentially accelerates the desolvation and cross-grain boundary migration of lithium ions at the solid-liquid interface.

[0094] When investigating the structural dependence of this alternating built-in electric field, the data from the comparative examples provide a physical basis for reverse verification. Comparative Example 1, lacking an inorganic piezoelectric transition layer, exhibits... Figure 3 The activation energy in neutron diagram (b) spikes dramatically to 62.4 kJ / mol, exhibiting a significant barrier transition compared to the previous example. This change demonstrates that a single polymer coating structure cannot establish an effective polarization electric field in a micro-strain environment, leading to a deterioration in the interfacial impedance of the material to over 3000 Ω at -20 °C. Similar thermodynamic degradation occurs in designs employing nonpolar PVDF as an alternative; in Comparative Example 2, the response path of the polymer phase dipole is blocked, and its impedance behavior at all temperature gradients degrades to the conventional level of traditional physical coating materials. Further observation... Figure 3 The column height jump in Comparative Example 5 in neutron diagram (a) excludes the spontaneous supporting effect of the intrinsic piezoelectric constant of the material from the perspective of process intervention. The composite powder that has not undergone electrospinning high-voltage polarization treatment failed to achieve the ordered arrangement of internal dipole moments, and the disordered state of the microstructure made the construction of the alternating built-in electric field completely fail.

[0095] Besides the piezoelectric effect-driven barrier modulation, the integrity of the interfacial physical conductivity pathways also profoundly influences apparent electrochemical kinetics. When discussing performance degradation caused by non-piezoelectric factors, Figure 3 The anomalous impedance surge observed in Comparative Example 4 prompted an investigation into the carrier transport coupling mechanism. The absence of polyethylene glycol, a specific component, disrupted the capillary fusion process between the carbon nanotube network and the spherical core particles. The extremely poor electronic conductivity led to severe interfacial charge accumulation, directly causing the apparent activation energy during the measurement fitting process to soar to 72.8 kJ / mol. This phenomenon reveals the strong coupling between ion transport rate and electron transport capability at the solid-phase interface; that is, when interfacial electrons cannot be dissipated in time, local electric field distortion will inversely inhibit the lithium-ion insertion / extraction process. The timing of the preparation process also dominates the final fate of the material. The high-temperature carbonization destruction caused by the incorrect process sequence in Comparative Example 6 destroyed the electrochemical activity of the material from both the impedance base and temperature sensitivity levels. Its extreme impedance value of over 45,000 Ω at -20℃ indicates that a dense insulating ion-resisting layer has formed on the material surface, completely losing the feasibility of practical applications under extreme conditions. These interconnected data gradients and graph features collectively demonstrate the irreplaceable role of each physicochemical factor in maintaining a low interfacial barrier environment within the gradient network configuration of this invention.

[0096] Test Example 3: This test example provides quantitative tests on the electrode peel strength and manganese ion dissolution after long cycling of the materials prepared in each embodiment and comparative example, to verify the effect of titanate coupling agent in constructing a chemical bonding network between the rigid ceramic layer and the flexible polymer layer, and its actual role in inhibiting interfacial peeling and structural collapse.

[0097] 1. Prepare positive electrode sheets for each group according to the slurry ratio and coating process in Test Example 2, controlling the single-sided coating density to be 15.0 mg / cm². After rolling, cut into rectangular electrode strips of 15 mm × 100 mm. Attach the coated side of the electrode sheet to 3M standard test tape and roll it back and forth three times using a 2 kg standard pressure roller, then let it stand for 30 min. Fix the test strips in the upper and lower clamps of a universal testing machine and perform a 180-degree continuous peel test at a tensile rate of 50 mm / min. Record the steady-state peel force range in the force-displacement curve, calculate the average peel strength, and take the arithmetic mean of 5 parallel samples for each group of materials.

[0098] 2. Using the coin cell assembly method in Test Example 2, battery samples for each embodiment and comparative example were prepared. The batteries were placed in a constant temperature chamber to maintain an ambient temperature of 45°C, and subjected to 200 charge-discharge cycles at a current rate of 1C within a voltage range of 3.0V to 4.3V.

[0099] 3. After the cycle test, the battery in the discharged state was disassembled in an argon-filled glove box. The separator and negative electrode were collected and placed together with the residual electrolyte in deionized water containing 5% nitric acid for ultrasonic extraction for 60 min. The extract was filtered and diluted to a 50 mL volumetric flask. The absolute concentration of manganese in the solution was measured using inductively coupled plasma optical emission spectrometry (ICP-OES) to characterize the dissolution of manganese from the positive electrode active material during the cycle.

[0100] Table 3. Test data on electrode peel strength and manganese ion leaching amount in each embodiment and comparative example.

[0101] Combining the data in Table 3 with Figure 4 The scattered and needle-like distribution characteristics of the materials in the example groups indicate a consistent stability feedback between the macroscopic interface adhesion performance and the microscopic lattice anti-dissolution capability. In routine high-temperature and long-cycle coin cell failure analysis, the lattice stress release caused by the distortion of trivalent manganese ions inside lithium manganese iron phosphate materials is an unavoidable problem. Conventional materials are prone to pulverization and physical peeling of the surface coating structure under the continuous effect of such periodic lattice volume changes. Experimentally measured peel strengths of Examples 1 to 5 were steadily maintained in the range of 39.8 to 45.1 N / m, corresponding to a manganese ion dissolution in the electrolyte that was strictly controlled below 22 ppm. Exploring the source of this dual macroscopic and microscopic steady-state characteristic can be compared with... Figure 4 The abnormal data trend in Comparative Example 3 without the titanate coupling agent. In the control system without the specific coupling agent, the peel strength of the electrode plummeted to 12.3 N / m, exhibiting a powdery characteristic that was extremely easy to detach during actual disassembly. Simultaneously, the manganese concentration measured by ICP-OES spectroscopy surged to 94.8 ppm. This simultaneous deterioration of physical failure and chemical dissolution explains the fundamental difference between conventional physical contact and interfacial chemical bonding. In the examples, the specifically introduced coupling agent molecules, after alkoxy hydrolysis, can undergo a dehydration condensation reaction with the hydroxyl groups on the surface of the NBBT piezoelectric ceramic. The resulting Ti-O-Ti covalent network constructs a molecular-level shock-absorbing buffer zone between the rigid ceramic grains and the flexible polymer skeleton. Based on this chemical anchoring effect, the mechanical stress caused by repeated charging and discharging of the internal crystals is effectively dispersed, blocking the exposure of the newly formed active interface and the physical channel for subsequent dissolution of manganese ions into the electrolyte.

[0102] In fact, the maintenance of solid-liquid interface integrity does not depend on a single component, but rather on a dynamic process involving the synergistic effects of a multi-level network. Extraction Figure 4The data characteristics of Comparative Example 1 reveal the limitations of a single coating layer in providing protection under harsh conditions. Due to the lack of a dense barrier from the inorganic NBBT piezoelectric transition layer, the outer polymer film alone cannot withstand the long-term chemical erosion of trace amounts of free acid in a 45°C electrolyte; test data shows a manganese leaching rate of 45.6 ppm. Besides the chemical resistance of the coating layer itself, the physical connectivity of the three-dimensional conductive network also affects the overall interface's mechanical strength. In Comparative Example 4, the absence of polyethylene glycol caused the electrode peel strength to drop to 28.7 N / m, confirming that if the capillary-like physical interlocking of a specific molecular weight of polyethylene glycol is lost between the carbon nanotube network and the polymer matrix, a localized imbalance in the internal structural stress distribution will occur. When tracing the decisive influence of the preparation thermal history on the final material interface... Figure 4 Neutron diagram (a) and Figure 4 The extreme data from Example 6 at the tail end of neutron plot (b) provides a clear answer. Disorder in the process sequence during material preparation triggered an irreversible structural disaster. The extremely low adhesion of 4.2 N / m and the high manganese dissolution peak of 135.6 ppm exhibited in this group confirm the complete destruction of the polymer skeleton and coupling agent chemical bonding sites by the initial high-temperature carbonization process. This destructive control result indicates that structural peeling of the failed coating under long-term cycling exposes the originally encapsulated active core, making it completely exposed to the corrosive electrolyte and thus accelerating phase transition decomposition. Through the above-mentioned staggered experimental extraction and spectral comparison, it is confirmed that the precise construction of specific process timings and interfacial chemical bonds is a prerequisite for breaking through the conventional physical coating limits and maintaining the long-term stable operation of the positive electrode electrochemical network.

[0103] Test Example 4: This test case provides quantitative tests on the wide temperature range (especially -20°C extreme cold environment) and multi-step high-rate charge and discharge performance of the materials prepared in each embodiment and comparative example, to verify the effect of the alternating built-in electric field on improving the low-temperature dynamic sluggishness, and the supporting effect of the three-dimensional conductive network on the high-speed transport of high-current electrons.

[0104] 1. Using the methods and process parameters described in Test Example 2, the positive electrode materials of each group were assembled into CR2032 coin cells. The formed cells were connected to a multi-channel battery testing system and charged and discharged three times at an ambient temperature of 25°C with a constant current of 0.5C within a voltage range of 3.0V to 4.3V. The discharge specific capacity of the third cycle was taken as the room temperature reference capacity.

[0105] 2. Transfer the batteries to a high and low temperature alternating environment chamber, set the ambient temperature to -20℃, and allow them to stand at this temperature for 8 hours to ensure internal thermal equilibrium. Discharge the batteries at a constant current of 0.5C at -20℃ until the cutoff voltage reaches 3.0V. Record the low-temperature discharge specific capacity and compare it with the room-temperature reference capacity to calculate the low-temperature capacity retention rate. Perform tests on 5 parallel samples in each group, and take the arithmetic mean to eliminate assembly errors.

[0106] 3. Take another batch of batteries from the same batch that have completed formation and operate at room temperature, and conduct multi-step high-rate charge-discharge tests at 25℃. During the charging process, use a constant current and constant voltage of 0.2C to charge to 4.3V (cutoff current 0.05C). During the discharging process, use gradient currents of 0.1C, 1C, 3C, 5C, and 10C to discharge to 3.0V. Each rate step cycle is repeated 5 times. The discharge specific capacity data of the last cycle at each rate is extracted, recorded, and compared for analysis.

[0107] Table 4. Wide-temperature range and high-rate discharge specific capacity test data for each embodiment and comparative example.

[0108] Combining the data in Table 4 with Figure 5 The column height difference characteristic of the neutron diagram (a) reveals that the example groups exhibit high electrochemical tolerance under extreme low temperature and high current impact conditions. When evaluating the electrochemical behavior of conventional lithium manganese iron phosphate materials under extreme cold conditions in the laboratory, a sharp capacity drop is often observed due to the inherent lattice electronic structure of the material and the limitation of interfacial liquid phase mass transfer. However, in... Figure 5 In neutron plot (a), the 0.5C discharge specific capacity of Examples 1 to 5 at -20°C remained stable in the range of 116.2 to 124.6 mAh / g, with the corresponding capacity retention rate exceeding the 74% bottleneck. This physical phenomenon, deviating from the conventional kinetic decay characteristics, confirms the effectiveness of the aforementioned alternating electric field intervention mechanism. During charge-discharge cycles, the lattice stress induced by the phase transition of the electrode material, under the dual response of the NBBT piezoelectric ceramic and the PVDF-TrFE polar polymer, in situ excites an alternating built-in electric field. This localized polarized electric field provides additional coulombic driving force for lithium ions trapped in a thermodynamically frozen state, thereby forcibly reducing the cross-interface migration activation energy. Looking at the data trends of the comparative examples in the chart, the necessity of this piezoelectric field assistance is confirmed in the opposite direction. Comparative Example 1, lacking an inorganic piezoelectric layer, and Comparative Example 2, lacking a polar polymer response phase, both saw their capacities drop below 70 mAh / g at -20°C. The data in Comparative Example 5 also showed a similar decay trajectory. The internal dipoles of the powder that did not undergo the directional induction of the high voltage electric field during spinning were randomly distributed. The loss of macroscopic bulk piezoelectric activity directly resulted in the material being unable to establish an effective internal electric field to counteract the sluggish effect caused by low temperature.

[0109] When the evaluation dimension is shifted from temperature variable to current density variable, high-rate step tests visually reveal the density of the physical conductive network at the solid-phase interface. Figure 5 The multi-step decay pattern in the neutron diagram (b) clearly reveals the response state of carrier transport under high-current impact. The example groups still output specific capacities of 91.2 to 102.5 mAh / g at extreme discharge rates of 10C, reflecting that the electron supply rate at the interface perfectly matches the rapid insertion / extraction rhythm of lithium ions. The structural dependence of this high-speed carrier transport is investigated. Figure 5 The steep drop in the broken line of Comparative Example 4 in neutron plot (b) provides a typical case of ohmic polarization collapse. In the formulation system lacking polyethylene glycol of a specific molecular weight, the capacity of this control group drastically decreased to 32.5 mAh / g at 5C and only 12.4 mAh / g at 10C. This precipitous drop in macroscopic capacity reveals the risk of chain breakage in the microscopic conductive network; the carbon nanotubes, without capillary bonding by the low-viscosity PEG fluid, are only loosely attached to the periphery of the particles. Faced with high-speed electron extraction at high current density, the huge contact resistance caused by weak physical contact instantly triggers local overpotential, forcing the discharge process to reach the cutoff voltage prematurely. In the data acquisition and recording of the entire group, the rate response trajectory of Comparative Example 6 almost completely overlaps the horizontal axis. This extreme failure phenomenon indicates that the incorrect process of coating before high-temperature sintering not only burns the organic phase that plays a connecting role but also forms a poor-quality carbonized hard shell on the surface of the entire material, hindering electron and electron exchange. These multi-dimensional capacity feedback data provide the final quantitative confirmation of the coupling supporting role of the two core mechanisms—alternating electric field-accelerated ion transport and three-dimensional micronetwork-mediated electron conduction—in electrochemical performance.

[0110] Test Example 5: This test case provides an evaluation of the long cycle life and median voltage decay of the full cells for the materials prepared in each embodiment and comparative example, in order to verify the comprehensive effectiveness of the multigradient network in suppressing lattice structure collapse and mitigating polarization growth during long-term dynamic phase transitions.

[0111] 1. The positive electrode powder materials obtained in Examples 1 to 5 and Comparative Examples 1 to 6 were used to prepare positive electrode sheets with a single-sided areal density of 18.0 mg / cm². Commercially available artificial graphite was selected as the counter electrode to prepare negative electrode sheets, with the areal density designed to achieve an N / P capacity ratio (negative electrode to positive electrode capacity ratio) of 1.1. In a dry room with a dew point below -40°C, the positive and negative electrode sheets and a polyethylene separator were assembled into CR2032 coin cells. After injecting the prepared standard electrolyte, the cells were sealed and aged at room temperature for 24 hours to ensure sufficient wetting of the porous interface. Six battery samples were assembled in parallel for each group.

[0112] 2. Transfer the batteries that have completed the standard 0.1C formation process to a constant temperature test chamber at an ambient temperature of 25℃. Set the test program to perform constant current and constant voltage charging (cutoff current 0.05C) and constant current discharging at a current rate of 1C within the operating voltage window of 3.0V to 4.3V. Run 500 cycles of continuous testing, and the system collects the capacity data for each cycle in real time. Extract the discharge specific capacity of the first cycle and the 500th cycle to calculate the long-cycle capacity retention rate of each sample.

[0113] 3. Extract the integral discharge energy (mWh) and discharge capacity (mAh) data for the corresponding number of cycles using the host computer software of the testing system. Divide the discharge energy per cycle by the discharge capacity to obtain the average discharge operating voltage (i.e., median voltage) for that cycle. Calculate the absolute decrease in median voltage of the 500th cycle compared to the median voltage of the first cycle, and record it as the voltage decay.

[0114] Table 5. Full-cell long-cycle capacity retention and median voltage decay data for each embodiment and comparative example.

[0115] Combining the measured data in Table 5 with Figure 6 The long-term service characteristics of the materials in the example group established a clear upper limit for resisting structural fatigue caused by deep lithium insertion / extraction. In real-world laboratory full-cell aging tests, lithium manganese iron phosphate materials, limited by their slow electronic transition capabilities and the evolution of microcracks accumulated from repeated phase transitions, often exhibit complications such as a sharp drop in capacity and a downward shift in voltage plateau after hundreds of cycles. However, in Figure 6 In neutron diagram (a), Examples 1 to 5, after 500 high-intensity charge-discharge cycles, maintained a stable capacity retention rate within a narrow band of 88.4% to 92.1%, with the median voltage decay strictly suppressed to within 50 mV. Exploring the source of this high electrochemical stability under deep cycling conditions reveals, at the macroscopic level, the effective channeling of microscopic lattice stress by the multi-gradient structure. During the long-cycle ion throughput of the battery, the chemically bonded piezoelectric ceramic and polar polymer network played a dual intervention role. This not only relied on the built-in electric field excited by micro-strain to continuously improve the cross-interface transport dynamics of ions, but also, at the physical level, the robust buffer zone it constructed effectively restrained the lattice volume expansion caused by manganese octahedral distortion, cutting off the mechanical damage path of microcracks extending to the periphery from the source.

[0116] Without the aforementioned multi-dimensional protection mechanisms, the failure process of a single-interface engineering process during long-term dynamic phase transitions becomes traceable. The degradation trajectory of the comparative example provides a complete inverse reference scale; for example, due to the lack of rigid support from the inorganic NBBT layer, the capacity retention of Comparative Example 1 drops significantly to 61.3%. Figure 6In neutron diagram (b), the bubble area representing voltage decay significantly expands to 183 mV, indicating that the flexible polymer on the outside cannot withstand internal mechanical expansion and contraction in the long term. Repeated stress impacts lead to interface rupture, which in turn triggers a surge in electrolyte side reactions and continuously consumes the limited active lithium. The deposited insulating passivation layer eventually evolves into a dead zone that blocks electron conduction. Mapping this polarization surge phenomenon to the connection state of the microscopic conductive network, Comparative Example 4, lacking capillary fusion of the conductive medium, exhibits a more severe phenomenon. The extreme voltage drop of up to 268 mV in this group reveals a typical physical contact failure model, namely, the loosely attached carbon nanotube network undergoes irreversible physical peeling during thousands of breathing movements of the crystal particles. This gradual disruption of the electronic pathway causes a large number of active manganese-iron sites deeply buried inside the particles to become electrochemically dead phases. Tracing back to the thermodynamic misalignment in the preparation stage, the test results of Comparative Example 6 show that it has almost lost its basic ability as an energy storage medium. The mere 12.5 mAh / g residual capacity and over 400 mV voltage decay in the first cycle of this group confirm that the incorrect sintering sequence, leading to surface carbonization and inner layer decomposition, created an insurmountable kinetic barrier in the early stages of material contact with the electrolyte. These dynamic extraction data spanning hundreds of cycles fully demonstrate that the core path to overcoming the long-term fate of capacity and voltage decay in such cathode materials lies in the synergistic construction of a composite system integrating piezoelectric response, chemical anchoring, and physical conduction at the atomic, molecular, and micron scales.

Claims

1. A lead-free piezoelectric graded composite lithium manganese iron phosphate cathode material, characterized in that, It includes LMFP@NBBT composite core layer particles and a multi-gradient functional network layer coating the surface of the composite core layer particles; The multigradient function network layer is made from raw materials comprising the following parts by weight: 45-55 parts of polyvinylidene fluoride-trifluoroethylene copolymer; 25-35 parts of multi-walled carbon nanotubes; 15-25 parts of polyethylene glycol; 1-2 parts of titanate coupling agent; The NBBT inorganic coating layer on the outer layer of the LMFP@NBBT composite core particles and the polyvinylidene fluoride-trifluoroethylene copolymer in the multi-gradient functional network layer synergistically provide a dual piezoelectric response.

2. The lead-free piezoelectric graded composite lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The LMFP@NBBT composite core layer particles consist of an inner lithium manganese iron phosphate core layer and an outer NBBT inorganic coating layer. In the chemical formula of the lithium manganese iron phosphate core layer, the molar ratio of manganese to iron is x:(1-x), where 0.4≤x≤0.6; The molar ratio of the lithium manganese iron phosphate core layer to the NBBT inorganic coating layer is (14-15):

1.

3. The lead-free piezoelectric graded composite lithium manganese iron phosphate cathode material according to claim 2, characterized in that, The inorganic coating layer of the NBBT is Na. 0.5 Bi 0.5 The TiO3-BaTiO3 system of lead-free piezoelectric ceramics, wherein the precursor raw materials of the inorganic coating layer of the NBBT include sodium nitrate, bismuth nitrate pentahydrate, barium nitrate and tetrabutyl titanate.

4. The lead-free piezoelectric graded composite lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The number average molecular weight of the polyethylene glycol is 4000-6000; the titanate coupling agent is isopropyl tris(dioctylpyrophosphate) titanate.

5. The lead-free piezoelectric graded composite lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The thickness of the multigradient functional network layer on the surface of the composite core particles is 1–3 μm.

6. A method for preparing a lead-free, piezoelectric graded composite lithium manganese iron phosphate cathode material, characterized in that, The preparation of a lead-free piezoelectric graded composite lithium manganese iron phosphate cathode material according to any one of claims 1-5 includes the following steps: A solution containing lithium, manganese, iron and phosphorus sources was prepared and subjected to hydrothermal reaction. After drying, LMFP core layer particles were obtained. NBBT precursor sol was prepared, and the LMFP core layer particles were added to the NBBT precursor sol and ultrasonically dispersed to form a gel. After pre-calcination, sintering and pulverization, LMFP@NBBT composite core layer particles were obtained. Polyvinylidene fluoride-trifluoroethylene copolymer, multi-walled carbon nanotubes, polyethylene glycol and titanate coupling agent are dispersed in a solvent and ultrasonically treated to prepare a spinning solution. Using an electrospinning process, under the conditions of applying DC high voltage and substrate heating, the spinning solution is uniformly deposited on the surface of the LMFP@NBBT composite core layer particles to form a composite material coated with a functional layer precursor network. The composite material is subjected to thermal annealing under a protective atmosphere, and after cooling, the final lead-free pressure gradient composite lithium manganese iron phosphate cathode material is obtained.

7. The method for preparing a lead-free piezoelectric graded composite lithium manganese iron phosphate cathode material according to claim 6, characterized in that, The specific processes for pre-firing and sintering are as follows: In a protective atmosphere, the temperature is first raised to 300-400℃ at a heating rate of 1-5℃ / min for pre-firing for 1-3 hours, and then raised to 600-700℃ at a heating rate of 1-5℃ / min for sintering for 6-8 hours.

8. The method for preparing a lead-free piezoelectric graded composite lithium manganese iron phosphate cathode material according to claim 6, characterized in that, The solvent is N,N-dimethylformamide, and the mass concentration of the solute in the spinning solution is controlled at 10% to 12%. The conditions for ultrasonic treatment are: rotation speed 80-120 r / min, frequency 10-20 Hz, and ultrasonic dispersion time 40-60 min.

9. The method for preparing a lead-free piezoelectric graded composite lithium manganese iron phosphate cathode material according to claim 6, characterized in that, The parameters for the electrospinning process are controlled as follows: The applied DC high voltage is 15-25kV, the injection pump speed is 0.8-1.5mL / h, and the distance between the spinning needle and the receiving substrate is 10-15cm. Simultaneously activate the substrate heating and control the receiving substrate temperature to remain constant at 80–100°C.

10. The method for preparing a lead-free piezoelectric graded composite lithium manganese iron phosphate cathode material according to claim 6, characterized in that, The parameters for the heat preservation annealing process are set as follows: Set the heating rate to 1-3℃ / min, heat to 120-140℃, and hold for annealing for 4-6 hours.

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