A lithium iron phosphate battery cathode material with high rate performance and a preparation method thereof

Abstract

The application discloses a lithium iron phosphate positive electrode material with high rate performance and a preparation method thereof, and relates to the technical field of lithium iron phosphate battery positive electrode materials. The application adopts pre-lithiation spray drying combined with an organic acid complexing sol method, realizes specific enrichment of Mg on the surface under neutral conditions through a strong complexing agent, generates a semi-coherent MgO heterogeneous phase layer in situ through low-temperature sintering, and forms a firm inorganic-organic composite interface regulation layer through ester exchange covalent grafting of a sulfonimide group and secondary heat treatment. The obtained material has a primary particle D50 of 60-120 nm, a 0.1 C specific capacity of 160.5-164.8 mAh / g, a 20 C retention rate of greater than or equal to 90.9%, a 10 C cycle retention rate at 45 DEG C of greater than or equal to 91.8% after 5000 cycles, a gas production of less than or equal to 0.08 mL / Ah, and a Fe elution amount of less than or equal to 0.13 ug / g, which are significantly superior to those of the prior art, and the material is suitable for high-power long-life lithium ion batteries.

Description

Technical Field

[0001] This invention relates to the technical field of lithium iron phosphate battery cathode materials, and in particular to a lithium iron phosphate battery cathode material with high rate performance and its preparation method. Background Technology

[0002] Lithium iron phosphate (LiFePO4) has been widely used in power batteries and energy storage due to its excellent structural stability, low cost, and environmental friendliness. To meet the demands of high-power applications such as power tools, two-wheeled vehicles, 48V mild hybrid systems, and 12V start-stop systems, high-rate lithium iron phosphate cathode materials have become a research and industrialization hotspot in recent years. Existing technologies mainly improve the electron / ion transport performance of materials through carbon coating, metal ion doping, and primary particle nano-sizing. Controlling the primary particle size to 100–300 nm and combining it with uniform carbon coating has enabled mainstream mass-produced materials to achieve 5C–10C discharge capabilities at room temperature, while also achieving a good balance between cycle life and energy density. Representative technologies include CN114725318A.

[0003] However, existing high-rate lithium iron phosphate batteries generally employ an extreme nano-sizing strategy (primary particle size < 70 nm) when pursuing further extreme rate performance. Although this can significantly shorten the Li-Phase I / O cycle time, the overall efficiency of the lithium iron phosphate production process is significantly reduced. + While achieving excellent rate and low-temperature performance through solid-phase diffusion in coin half-cells, this approach introduces serious issues related to interfacial stability and long cycle life. The drastic reduction in primary particle size leads to a significant increase in the material's specific surface area, resulting in a substantial increase in the interfacial reactivity between the cathode and electrolyte. Under high-rate charge-discharge conditions, especially above 10 C, the charging voltage easily exceeds the oxidation stability window of conventional carbonate electrolytes, inducing continuous oxidative decomposition of the electrolyte and the formation of a high-resistivity CEI film. This causes a rapid increase in battery polarization and a rapid decline in power characteristics. Simultaneously, the severe unsaturation of Fe atoms on the surface of nanoscale primary particles reduces the Fe-O bond energy, leading to Fe... 2+ The tendency for Fe dissolution is significantly enhanced. The dissolved iron ions migrate to the negative electrode and catalyze the continued growth of the SEI, further exacerbating the increase in internal resistance and capacity decay of the full cell. While existing technologies rely on carbon coating to suppress Fe dissolution to some extent, they fail to effectively passivate the highly active surface at the intrinsic structural design level. Therefore, the excellent cycle performance in laboratory stages (capacity retention >96% after 2500 cycles for coin cells) often shrinks significantly in actual full cells, failing to meet stringent requirements. Therefore, there is an urgent need for a novel high-rate lithium iron phosphate cathode material that, while maintaining excellent rate and low-temperature performance, systematically suppresses the high interfacial side reactions and Fe dissolution behavior caused by nano-sizing at the material structure level, achieving synergistic optimization of high power and long lifespan under full-cell conditions. Summary of the Invention

[0004] This application provides a method for preparing lithium iron phosphate cathode material, including the following steps:

[0005] S1: Prepare an organic acid complexing agent into a solution, add iron phosphate powder, lithium carbonate or lithium hydroxide into the solution according to the stoichiometric ratio and dissolve to obtain Li-Fe-P precursor slurry sol, spray dry the sol prepared above, and heat treat the obtained spray-dried powder in an air atmosphere at 330 to 380°C to obtain pre-lithiated lithium iron phosphate precursor.

[0006] S2: Prepare the pre-lithiated lithium iron phosphate precursor from step S1 into a slurry, add magnesium nitrate as a magnesium source to the precursor slurry, add an organic acid complexing agent, and age it at 85 to 95°C to obtain a coating precursor with magnesium enrichment on the surface.

[0007] S3: Mix the coating precursor obtained in step S2 with a carbon source and perform solid-state sintering at 730 to 780°C to form an inorganic heterogeneous MgO phase layer on the surface.

[0008] S4: Disperse the material obtained in step S3 in an organic precursor solution containing sulfonylimide groups, and carry out an ester exchange reaction at 45 to 65°C to bond the sulfonylimide groups to the surface of the MgO layer, thereby obtaining a prefunctionalized material.

[0009] S5: The prefunctionalized material obtained in step S4 is subjected to a second heat treatment at 330 to 380°C to form a composite interface control layer between the inorganic layer and the organic layer, thereby obtaining the lithium iron phosphate cathode material.

[0010] It should be noted that in the S1 stage of this invention, after dissolving iron phosphate and lithium source with an organic acid complexing agent to form a uniform Li-Fe-P sol, the sol is spray-dried and subjected to pre-lithiation heat treatment at 330–380 °C in an air atmosphere, so that the Li in the sol... + Fe 3+ PO4 3- At low temperatures, a preliminary reaction is conducted to generate crystalline or semi-crystalline primary lithium iron phosphate particles (pre-lithiated lithium iron phosphate precursors). This step retains the advantages of atomic-level mixing in the sol-gel method and provides a clear and stable crystalline particle surface for the subsequent S2 stage. In the S2 stage, the pre-lithiated precursor is re-slurryed and then magnesium nitrate and a strong organic acid complexing agent are added. During aging at 85–95 °C, the bulk lattice has initially formed and the Mg... 2+Strongly bound by the complexing agent, it is almost impossible for it to enter the existing LiFePO4 lattice, and it is specifically adsorbed and enriched on the outermost surface of the particles. During the S3 stage, when the surface magnesium-rich layer is carbothermally sintered at 730-780 °C, it is transformed in situ into a rock salt structure MgO heterogeneous phase layer under the assistance of a reducing atmosphere, and forms a semi-coherent interface with the matrix. In the S4-S5 stages, sulfonylimide groups are covalently grafted through ester exchange and secondary heat treatment to form an inorganic-organic composite interface control layer. Finally, a continuous and dense protective layer is constructed on the surface of the primary particles at 60-120 nm, thereby effectively suppressing Fe dissolution and interfacial side reactions, and improving the high-rate performance and long cycle life of the material.

[0011] In a preferred embodiment of a method for preparing lithium iron phosphate cathode material, in steps S1 and S2, the organic acid complexing agent is selected from at least one of citric acid, tartaric acid, ethylenediaminetetraacetic acid, and oxalic acid.

[0012] It should be noted that in steps S1 and S2, the organic acid complexing agent is selected from at least one of citric acid, tartaric acid, ethylenediaminetetraacetic acid, and oxalic acid. In stage S1, these polydentate organic acids react with Fe through multiple carboxyl groups. 3+ A chelate with extremely high stability constant is formed, achieving complete macroscopic dissolution of FePO4 at 60–90 °C and forming a uniform Li-Fe-P sol with the lithium source. After spray drying and pre-lithiation heat treatment at 330–380 °C, pre-lithiated lithium iron phosphate primary particles are generated. In the S2 stage, the same or similar complexing agent continues to react with the newly added Mg. 2+ It forms a chelate with a high stability constant, which significantly reduces the free Mg in the solution under aging conditions of 85–95 °C. 2+ Concentration, making Mg 2+ Due to the limited diffusion kinetics and the pre-formed bulk lattice, it is almost impossible to enter the interior of the LiFePO4 lattice. It specifically adsorbs and enriches on the outermost surface of the particles, thereby ensuring that the magnesium-rich layer on the surface can be completely and uniformly transformed in situ into the MgO heterogeneous phase layer during subsequent sintering.

[0013] In a preferred technical solution for the preparation of lithium iron phosphate cathode material, the heat treatment time in step S1 is 0.5 to 3 h.

[0014] It should be noted that, by subjecting the spray-dried sol to a moderate pre-lithiation heat treatment at 330–380 °C for 0.5–3 h in S1, this invention can form crystalline or semi-crystalline lithium iron phosphate primary particles while maintaining atomic-level mixing uniformity, thereby significantly enhancing the surface structural stability of the particles and making the Mg in the S2 stage... 2+ The surface enrichment under the action of strong complexing agents is more thorough and controllable, ultimately ensuring that the MgO heterogeneous phase layer is continuous and dense, the semi-coherent interface is firmly bonded, significantly reducing Fe dissolution and improving high-rate cycling and high-temperature storage performance.

[0015] In a preferred embodiment of a method for preparing lithium iron phosphate cathode material, in step S3, the solid-state sintering time is 6–10 h, and the carbon source is selected from at least one of glucose, sucrose, and polyvinylpyrrolidone.

[0016] It should be noted that step S3 of this invention is limited to using glucose, sucrose, or polyvinylpyrrolidone as a carbon source. These three types of carbon sources can achieve mild and controllable pyrolysis at 730–780 °C, providing a sufficient reducing atmosphere to prevent Fe from being degraded. 2+ Oxidation results in extremely low residual carbon and the absence of a thick carbon layer. More importantly, the active carbon species produced by its pyrolysis and the transient reduction microenvironment can precisely induce the in-situ crystallization of the magnesium-rich surface layer into pure-phase MgO, forming a semi-coherent interface with the LiFePO4 matrix, ensuring rapid Li... + Transport channels and surface hydroxyl groups are exposed.

[0017] In a preferred technical solution for the preparation of lithium iron phosphate cathode material, the secondary heat treatment time in step S5 is 0.5 to 3 hours.

[0018] It should be noted that the secondary heat treatment time in step S5 must be controlled within 0.5 to 3 hours to allow for adequate and sufficient Si-OC bond breaking and rearrangement between the organic layer and the MgO layer, forming a strong Mg-O-Si-O-sulfonamide covalent cross-linked network. If the time is less than 0.5 hours, the cross-linking will be incomplete and the organic layer will easily fall off. If the time exceeds 3 hours, the sulfonamide groups will thermally decompose and fail, both of which will lead to rapid instability of the composite interface in the electrolyte.

[0019] Furthermore, the lithium iron phosphate cathode material prepared by the above-described method has an inorganic-organic composite interface control layer on the surface of its primary particles. This composite interface control layer comprises, from the inside out:

[0020] An inorganic heterogeneous phase layer tightly coated on the surface of LiFePO4, wherein the inorganic heterogeneous phase layer is MgO;

[0021] An organically functionalized layer chemically bonded to the surface of MgO, wherein the organically functionalized layer contains sulfonylimide groups;

[0022] The D50 of primary particles is 60–120 nm.

[0023] It should be noted that the lithium iron phosphate cathode material utilizes a unique process described in this application to construct an in-situ, inner-to-outer, double-layer inorganic-organic composite interface control layer on the surface of primary particles in the 60–120 nm range. The inner MgO inorganic heterophase is semi-coherent with the LiFePO4 matrix, completely covering the surface-coordinated unsaturated Fe sites and intrinsically blocking Fe. 2+It dissolves into the electrolyte, and due to the high lattice matching, forms a low-energy-barrier Li. + Fast transport channel; the outer sulfonamide organic functionalized layer is firmly anchored to the MgO surface through stable Mg-O-Si covalent bonds, and its strong electron-withdrawing properties significantly reduce Li + It breaks down solubilization barriers and accelerates interfacial insertion / extraction, while efficiently capturing acidic species such as HF and PF5, inhibiting excessive CEI growth. The inner and outer layers form a continuous and dense ion-electron insulating protective network through covalent cross-linking, which completely passivates the side reactions of high specific surface area brought about by extreme nano-sizing, ultimately achieving a synergistic breakthrough in high-rate discharge, low-temperature output, and ultra-long cycle life.

[0024] As a preferred technical solution for lithium iron phosphate cathode material, the inorganic heterophase layer is MgO with a rock salt structure. In grazing incidence X-ray diffraction (incident angle 0.3°~0.5°), there is an obvious MgO(200) crystal plane diffraction peak (2θ≈42.8°~43.1°), and the peak position of this peak in φ scan is completely consistent with the φ scan peak position of LiFePO4(100) crystal plane.

[0025] It should be noted that MgO forms a highly oriented epitaxial semi-coherent interface with the LiFePO4 matrix. This interface bonding is extremely strong and almost stress-free, allowing the coating layer to maintain structural integrity even under extremely high rates and ultra-long cycling conditions. This thoroughly and permanently passivates unsaturated Fe sites on the surface, fundamentally preventing Fe dissolution. Simultaneously, this orientation-matched interface is Li + It provides a fast transport channel that seamlessly connects with the bulk crystallographic direction, significantly reducing the interface insertion / extraction energy barrier and achieving extremely high rate performance. In addition, the highly ordered crystalline MgO surface provides an ideal anchoring platform for the uniform covalent grafting of subsequent sulfonylimide groups, making the inorganic-organic composite interface more compact and stable, and synergistically inhibiting electrolyte decomposition, HF erosion and CEI runaway growth, thereby achieving comprehensive synergistic optimization between high power output, ultra-long cycle life, high temperature storage stability and safety performance.

[0026] This invention employs a complete technical route combining pre-lithiation spray drying with organic acid complexation sol-gel method, surface Mg specific enrichment aging, low-temperature in-situ generation of semi-coherent MgO heterophase, sulfonylimide group transesterification covalent grafting, and secondary heat treatment to form a robust inorganic-organic covalent cross-linked interface. This successfully constructs a highly oriented and extremely robust bilayer MgO-sulfonylimide composite interface control layer on the surface of ultra-nanosized primary particles (60–120 nm). This completely passivates the surface unsaturated Fe sites caused by the high specific surface area at the intrinsic structural level of the material and constructs a low-energy-barrier Li. +The fast transmission channel achieves a comprehensive balance between high power and ultra-long lifespan, with specific benefits including: a 0.1C initial discharge specific capacity of 160.5–164.8 mAh / g, an initial efficiency of 99.0%–99.3%, a 20C discharge capacity retention of 90.9%–93.5%, a 5000-cycle capacity retention at 10C under 45℃ of 91.8%–94.2%, a 3C discharge retention of 77.5%–80.1% under -20℃, a gas production of only 0.05–0.08 mL / Ah after 30 days of storage at 45℃, a low post-cycle interfacial impedance (Rcei) of 18.2–22.6 Ω, and a low Fe dissolution rate of 0.08–0.13 μg / g. All key indicators significantly surpass the existing high-rate lithium iron phosphate technology, providing a high-performance solution for power tools, two-wheeled vehicles, 48V mild hybrids, and 12V electric vehicles. It provides a truly industrializable lithium iron phosphate cathode material solution with ultra-long life and high safety for the most demanding high-power application scenarios such as V-start-stop. Attached Figure Description

[0027] Figure 1 Here is a scanning electron microscope image of the lithium iron phosphate cathode material prepared in Example 3;

[0028] Figure 2 The particle size distribution curves are for the cathode materials prepared in Examples 1 to 4.

[0029] Figure 3 X-ray diffraction patterns of the MgO heterophase layer on the surface of the lithium iron phosphate cathode material prepared in Example 3 are shown, where: a is the XRD pattern under conventional θ-2θ scanning mode; b is the grazing incidence X-ray diffraction (GIXRD, incident angle 0.4°) pattern; and c is a φ scan comparison image.

[0030] Figure 4 The full spectrum of the material in Example 1 (XPS Survey);

[0031] Figure 5 The N 1s high-resolution spectrum of the material in Example 1;

[0032] Figure 6 The F 1s high-resolution spectrum of the material in Example 1;

[0033] Figure 7 The high-resolution Mg 2p spectrum of the material in Example 1;

[0034] Figure 8 The high-resolution C 1s spectrum of the material in Example 1;

[0035] Figure 9 The high-resolution spectrum of Fe 2p of the material in Example 1 is shown. Detailed Implementation

[0036] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the examples in the specification.

[0037] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0038] Secondly, the term "an embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places throughout this specification does not necessarily refer to the same embodiment, nor is it a single embodiment or an embodiment selectively excluded from other embodiments.

[0039] Example

[0040] Example 1

[0041] This embodiment provides a method for preparing lithium iron phosphate cathode material, including the following steps:

[0042] S1: Dissolve 60 g of citric acid in 800 mL of deionized water, heat to 80 °C, and while stirring, add 36.2 g (0.2 mol) of iron phosphate dihydrate (FePO4·2H2O) and 15.1 g (0.204 mol, Li / Fe=1.02) sequentially. Continue stirring at 80 °C for 4 h until the solution is completely transparent, forming a light yellow Li-Fe-P sol. Spray dry the sol using a centrifugal spray dryer (inlet air temperature 220 °C, outlet air temperature 95 °C, peristaltic pump speed 25 r / min) to obtain a light yellow spray-dried powder. Place the spray-dried powder in a muffle furnace and heat-treat at 360 °C for 1.5 h in air atmosphere (heating rate 5 °C / min, cooled with the furnace) to obtain a pre-lithiated lithium iron phosphate precursor.

[0043] S2: Take 50 g of the above pre-lithiated precursor and redisperse it in 400 mL of deionized water. Stir thoroughly for 30 min to form a homogeneous slurry. Add 0.64 g of magnesium nitrate hexahydrate (Mg(NO3)2·6H2O) (Mg / Fe molar ratio 0.5%), followed by 2.5 g of citric acid (citric acid and Mg... 2+ The mixture was prepared by adjusting the pH to 7.0 with dilute ammonia water (molar ratio 1:1), heating to 95 °C, and aging with vigorous stirring at 450 rpm for 8 h to obtain a magnesium-enriched coating precursor.

[0044] S3: The coating precursor obtained in step S2 was centrifuged (6000 rpm, 10 min), washed three times with deionized water, and dried at 80 ℃ for 12 h. The dried powder was then mixed with glucose at a mass ratio of 100:10 by planetary ball milling (300 rpm, 4 h). The mixture was heated to 750 ℃ ​​at 5 ℃ / min under nitrogen protection in a tube furnace, held for 10 h, and then naturally cooled to obtain a LiFePO4 / C intermediate with a MgO heterogeneous phase layer on the surface.

[0045] S4: Take 10 g of the above intermediate and disperse it in 50 mL of anhydrous isopropanol solution containing 1.0 wt% lithium bis(trifluoromethanesulfonyl)imide. Stir the mixture at 45 ℃ for 6 h. After the reaction is completed, centrifuge at 8000 rpm for 10 min, discard the supernatant, wash 3 times with anhydrous ethanol, and vacuum dry at 80 ℃ for 12 h to obtain the prefunctionalized material.

[0046] S5: The prefunctionalized material is placed in a muffle furnace and subjected to a secondary heat treatment at 330 °C for 3 h in an air atmosphere (heating rate 3 °C / min, cooled with the furnace) to obtain the target lithium iron phosphate cathode material with a primary particle D50 of 60 nm.

[0047] Example 2

[0048] This embodiment provides a method for preparing lithium iron phosphate cathode material, including the following steps:

[0049] S1: Dissolve 32 g of citric acid and 33 g of tartaric acid (total mass 65 g, molar ratio approximately 1:1) in 850 mL of deionized water, heat to 75 °C, and add 36.2 g (0.2 mol) of iron phosphate dihydrate (FePO4·2H2O) while stirring. Continue stirring for 30 min, then add 17.6 g (0.205 mol) of lithium hydroxide monohydrate (LiOH·H2O, Li / Fe = 1.025). Maintain the temperature at 75 °C and stir vigorously for 5 h until the solution is completely transparent and light yellow, forming a Li-Fe-P sol. Spray dry using a centrifugal spray dryer (inlet air temperature 220 °C, outlet air temperature 93 °C, peristaltic pump speed 28 r / min) to obtain a light yellow spray-dried powder. Place the spray-dried powder in a muffle furnace under an air atmosphere and heat to 330 °C at 5 °C / min, hold for 3 h, and cool with the furnace to obtain a pre-lithiated lithium iron phosphate precursor.

[0050] S2: Take 50 g of the above pre-lithiated precursor and redisperse it in 420 mL of deionized water. Stir at high speed for 30 min to prepare a homogeneous slurry. Add 1.02 g of magnesium nitrate hexahydrate (Mg(NO3)2·6H2O) (Mg / Fe molar ratio 0.8%), followed by 3.9 g of citric acid (citric acid and Mg...). 2+The precursor of the coating with magnesium enrichment on the surface was obtained by adjusting the pH to 7.0 with dilute ammonia water (molar ratio 1.05:1), heating to 90 °C, and aging with vigorous stirring at 450 rpm for 7 h.

[0051] S3: The coating precursor obtained in step S2 was centrifuged at 6000 rpm for 10 min, washed three times with deionized water, and dried at 80 ℃ for 12 h. The dried powder was then mixed with sucrose at a mass ratio of 100:12 in a planetary ball mill jar and ball milled at 300 rpm for 4 h. The mixture was heated to 760 ℃ at 5 ℃ / min under nitrogen protection in a tube furnace, held at that temperature for 6 h, and then cooled naturally to obtain a LiFePO4 / C intermediate with a MgO heterogeneous phase layer on the surface.

[0052] S4: Take 10 g of the above intermediate and disperse it in 50 mL of anhydrous ethanol solution containing 1.5 wt% lithium bis(trifluoromethanesulfonyl)imide. Stir the mixture at 50 °C for 5 h. After the reaction is complete, centrifuge at 8000 rpm for 10 min, discard the supernatant, wash 3 times with anhydrous ethanol, and dry under vacuum at 80 °C for 12 h to obtain the prefunctionalized material.

[0053] S5: The prefunctionalized material is placed in an air atmosphere in a muffle furnace and heated to 360°C at 3°C / min. After a second heat treatment for 2 hours, it is cooled with the furnace to obtain the target lithium iron phosphate cathode material with a primary particle size of D50 = 80 nm.

[0054] Example 3

[0055] This embodiment provides a method for preparing lithium iron phosphate cathode material, including the following steps:

[0056] S1: Dissolve 75 g of ethylenediaminetetraacetic acid (EDTA) in 900 mL of deionized water, heat to 85 °C, slowly add 30.2 g (0.2 mol) of anhydrous iron phosphate (FePO4), stir for 1 h, then add 15.2 g (0.205 mol, Li / Fe = 1.03) of lithium carbonate, and continue stirring vigorously at 85 °C for 6 h until the solution is completely clear and transparent, forming a Li-Fe-P sol; spray dry using a centrifugal spray dryer (inlet air temperature 225 °C, outlet air temperature 98 °C, peristaltic pump speed 26 r / min) to obtain a white to light yellow spray-dried powder; heat the spray-dried powder to 380 °C in an air atmosphere in a muffle furnace at 5 °C / min, hold for 0.5 h, and cool with the furnace to obtain a pre-lithiated lithium iron phosphate precursor;

[0057] S2: Disperse 50 g of the above pre-lithiated precursor in 450 mL of deionized water, and shear at high speed for 30 min to form a homogeneous slurry. Add 1.28 g of magnesium nitrate hexahydrate (Mg / Fe molar ratio 1.0%), followed by 5.2 g of disodium EDTA (EDTA and Mg...). 2+ The precursor of the coating with magnesium enrichment on the surface was obtained by adjusting the pH to 7.0 with dilute ammonia water (molar ratio 1.1:1), heating to 85 °C, and stirring at 450 rpm for 6 h.

[0058] S3: After centrifuging the coating precursor at 6000 rpm, washing it three times with water, and drying it at 80 ℃ for 12 h, the dried powder was ball-milled with polyvinylpyrrolidone (PVP K30) at a mass ratio of 100:13 for 4 h; the mixture was heated to 730 ℃ at 5 ℃ / min under nitrogen protection, held at that temperature for 8 h, and then naturally cooled to obtain the surface MgO heterophase LiFePO4 / C intermediate.

[0059] S4: Take 10 g of intermediate and disperse it in 50 mL of anhydrous isopropanol solution containing 2.0 wt% lithium bis(trifluoromethanesulfonyl)imide. Stir the reaction at 55 °C for 4 h, centrifuge at 8000 rpm, wash 3 times with anhydrous ethanol, and vacuum dry at 80 °C for 12 h to obtain the prefunctionalized material.

[0060] S5: The prefunctionalized material is heated to 380 ℃ in air in a muffle furnace at 3 ℃ / min, subjected to a second heat treatment for 1.5 h, and cooled with the furnace to obtain the target lithium iron phosphate cathode material with a primary particle D50 of 95 nm.

[0061] Example 4

[0062] This embodiment provides a method for preparing lithium iron phosphate cathode material, including the following steps:

[0063] S1: Dissolve 58 g of citric acid in 750 mL of deionized water, heat to 80 °C, add 36.2 g (0.2 mol) of iron phosphate dihydrate, stir for 40 min, then add 17.8 g (0.21 mol, Li / Fe = 1.05) of lithium hydroxide monohydrate, and continue stirring at 80 °C for 4.5 h until completely transparent; spray dry using a centrifugal spray dryer (inlet air temperature 220 °C, outlet air temperature 95 °C, rotation speed 27 r / min) to obtain spray-dried powder; heat the spray-dried powder to 350 °C in an air atmosphere in a muffle furnace at 5 °C / min, hold for 1 h, and cool with the furnace to obtain the pre-lithiated lithium iron phosphate precursor;

[0064] S2: Disperse 50 g of the pre-lithiated precursor in 380 mL of deionized water, and shear it at high speed to form a homogeneous slurry. Add 1.92 g of magnesium nitrate hexahydrate (Mg / Fe molar ratio 1.5%), followed by 5.8 g of tartaric acid (tartaric acid and Mg...). 2+ The precursor for the magnesium enrichment coating on the surface was obtained by adjusting the pH to 7.0 with dilute ammonia water (molar ratio 1.2:1), heating to 95 °C, and aging at 450 rpm for 5 h.

[0065] S3: The coating precursor was centrifuged, washed with water 3 times, dried at 80 ℃ for 12 h, and then ball-milled with sucrose at a mass ratio of 100:15 for 4 h. The mixture was heated to 780 ℃ at 5 ℃ / min under nitrogen protection, held for 7 h, and then naturally cooled to obtain the surface MgO heterophase LiFePO4 / C intermediate.

[0066] S4: Take 10 g of the intermediate and disperse it in 50 mL of anhydrous isopropanol solution containing 2.5 wt% lithium bis(trifluoromethanesulfonyl)imide. Stir the reaction at 65 °C for 3 h, centrifuge at 8000 rpm, wash 3 times with ethanol, and vacuum dry at 80 °C for 12 h to obtain the prefunctionalized material.

[0067] S5: The prefunctionalized material is heated to 370 °C in air in a muffle furnace at a rate of 3 °C / min, subjected to a second heat treatment for 0.5 h, and then cooled with the furnace to obtain the target lithium iron phosphate cathode material with a primary particle size of D50 = 120 nm.

[0068] Comparison Example

[0069] Comparative Example 1

[0070] The difference between this comparative example and Example 1 is that magnesium nitrate is not added; that is, in step S2, only citric acid is added without a magnesium source.

[0071] Comparative Example 2

[0072] The difference between this comparative example and Example 1 is that citric acid is not added in step S2, only magnesium nitrate is added (the Mg / Fe molar ratio is still 0.5%).

[0073] Comparative Example 3

[0074] The difference between this comparative example and Example 1 is that the carbon source in step S3 is changed to pitch (the mass ratio is still 10:100), and the sintering temperature is still 750 ℃.

[0075] Comparative Example 4

[0076] The difference between this comparative example and Example 1 is that step S4 uses a 1.0 wt% 3-aminopropyltriethoxysilane (KH-550) isopropanol solution instead of a lithium bis(trifluoromethanesulfonyl)imide solution.

[0077] Comparative Example 5

[0078] The difference between this comparative example and Example 1 is that the secondary heat treatment at 330 °C in S5 is skipped, and the material is used directly after S4 as the final material.

[0079] Application examples

[0080] Application Example 1

[0081] First, the positive electrode sheet was prepared: The lithium iron phosphate positive electrode materials obtained in Examples 1 to 4 and Comparative Examples 1 to 6 were weighed and mixed with conductive carbon black (Super P) and PVDF binder at a mass ratio of 80:10:10. NMP solvent was added and stirred for 4 h to form a uniform slurry with a solid content of 40 wt%. The slurry was coated onto an aluminum foil current collector (wet film thickness 100 μm), dried in a vacuum oven at 80 ℃ for 8 h, and then cut into circular electrode sheets with a diameter of 14 mm. The sheets were then compacted to a loading of 2.5 mg / cm² using a roller mill. 2 The thickness is 50 μm. The negative electrode is a lithium metal foil (1 mm thick). The electrolyte is a mixed solvent of 1 mol / L LiPF6 dissolved in EC / DMC / EMC (volume ratio 1:1:1). The separator is a Celgard 2400 polypropylene microporous membrane. CR2032 coin cells were assembled in an argon glove box with a water and oxygen content of <0.1 ppm.

[0082] Performance testing

[0083] 1. Initial discharge specific capacity and initial coulombic efficiency: After the CR2032 coin cell was left to stand for 12 hours in a constant temperature chamber at 25±1 ℃, it was charged to 4.2 V (cutoff current 0.01 C) at a constant current and constant voltage of 0.1 C (1 C = 170 mA / g) using a Newway or Blue Electric testing system. After standing for 5 minutes, it was discharged to 2.0 V at a constant current of 0.1 C. The discharge specific capacity was recorded as the initial discharge specific capacity, and the initial coulombic efficiency was calculated as (initial discharge capacity / initial charge capacity) × 100%.

[0084] 2. Rate Performance (20 C Discharge Capacity Retention): The coin cell was charged at 25±1 ℃ with a constant current and constant voltage of 0.2 C to 4.2 V (cutoff current 0.02 C), allowed to stand for 10 min, and then discharged at constant currents of 0.2 C, 1 C, 5 C, 10 C, 15 C, and 20 C to 2.0 V. The discharge specific capacity at each rate was recorded. 20 C discharge capacity retention = (20 C discharge specific capacity / 0.2 C discharge specific capacity) × 100%.

[0085] 3.10 C Long Cycle Life (Capacity Retention after 5000 Cycles): The coin cell half-cell was charged at 1 C constant current and constant voltage to 4.2 V (cutoff current 0.05 C) in a 45±1 ℃ constant temperature chamber, allowed to stand for 10 min, and then discharged at 10 C constant current to 2.0 V, constituting one cycle. This process was repeated 5000 times. Capacity retention rate = (5000th discharge capacity / 1st discharge capacity) × 100%.

[0086] 4. Low-temperature discharge performance (3C discharge capacity retention rate at -20 ℃): The coin cell was charged to 4.2 V at 0.2 C at 25 ℃, then placed in a high and low temperature chamber at -20±1 ℃ for 4 h, and then discharged at a constant current of 3 C to 2.0 V. Low-temperature discharge capacity retention rate = (3C discharge specific capacity at -20 ℃ / 0.2C discharge specific capacity at 25 ℃) × 100%.

[0087] 5. Gas production during high-temperature storage: Fully charged (4.2 V) pouch cells (capacity 2 Ah) were stored in a constant temperature chamber at 45±1 ℃ for 30 days. The volume change of the cells was measured every 5 days using the water displacement method. The volume difference before and after storage was the gas production. Gas production was expressed in mL / Ah.

[0088] 6. Cyclic impedance (EIS): The coin half-cell was tested at 25 °C and 50% SOC using a Shanghai Chenhua CHI660E electrochemical workstation with a frequency range of 100 kHz to 0.01 Hz and an amplitude of 5 mV. The Nyquist plot was recorded, and the interfacial impedance Rcei and charge transfer impedance Rct were obtained by fitting with ZView software.

[0089] 7. Fe dissolution detection after cycling: After 5000 cycles, the coin cell was disassembled in a glove box, the negative electrode lithium sheet was taken, and it was ultrasonically dissolved with 1 wt% dilute hydrochloric acid for 30 min. After the solution was diluted, the Fe element content was determined by inductively coupled plasma optical emission spectrometry (ICP-OES, Agilent 5110) and converted into the Fe mass (μg / g) dissolved per gram of positive electrode material.

[0090] Table 1 shows the experimental data for the materials prepared in Examples 1 to 4 and Comparative Examples 1 to 5.

[0091]

[0092] Table 1 lists the test results of the lithium iron phosphate cathode materials prepared in Examples 1-4 and Comparative Examples 1-5 in terms of specific capacity, rate performance, cycle life, low temperature performance, gas production, interfacial impedance, and Fe dissolution, which are used to compare and verify the comprehensive performance advantages of the materials of the present invention.

[0093] Combined with Example 3 and Figure 1As can be seen, the visible material is composed of primary particles with extremely uniform particle size and a D50 of about 95 nm. The particles are spherical or ellipsoidal in shape, with excellent dispersibility and almost no hard agglomeration. The particle surface is smooth and clean, and there is no rough amorphous carbon layer or particle adhesion phenomenon commonly seen in traditional carbon coating. This indicates that the low-temperature glucose carbon source used in this invention only forms a very thin conductive residual carbon, without causing any obstruction to the subsequent MgO surface exposure and sulfonylimide grafting.

[0094] Combined with Example 3 and Figure 3 It can be seen that in conventional θ-2θ scanning, the signal of the extremely thin surface MgO layer is completely submerged by the strong peaks of the LiFePO4 bulk phase and is not visible. After using GIXRD with a grazing incidence angle of 0.4°, the matrix signal is significantly attenuated, while the MgO(200) diffraction peaks are clearly highlighted, proving that the MgO layer is a highly crystalline rock salt structure and exists only on the outermost surface of the particles. More importantly, the φ scanning results show that the diffraction intensity of MgO(200) forms a sharp peak only at the same azimuth angle where LiFePO4(100) diffracts, which is in stark contrast to the flat baseline (black dashed line) that should be presented by randomly oriented MgO. This highly consistent orientation relationship directly proves that the MgO grains are templated by the LiFePO4(100) crystal plane and grow epitaxially along it, forming a semi-coherent interface with strict crystallographic orientation, thus providing a basis for ultrafast Li + Interfacial transport and permanent passivation of surface Fe sites provide the most robust structural basis.

[0095] In conjunction with Example 1 and Figures 4 to 9 As can be seen, the clearly visible signals of Mg 1s (1303.5 eV), Mg 2p (50.4 eV), N 1s (399.8 eV), F 1s (688.5 eV), and S 2p (168.2 eV) in the full spectrum indicate that the surface of primary lithium iron phosphate particles simultaneously contains an inorganic heterophase of MgO and an organic functionalized layer of sulfonylimide. The high-resolution spectrum further shows that Mg 2p at 50.4 eV is attributed to a Mg-O bond, N 1s at 399.8 eV and F 1s at 688.5 eV are typical characteristics of sulfonylimide groups, while Fe 2p only exhibits pure Fe. 2+ State, Fe-free 3+ Satellite peaks demonstrate that the MgO layer completely covers the unsaturated Fe sites on the surface, effectively blocking the Fe dissolution pathway. These spectroscopic characteristics are consistent with the invention's process of "organic acid complexation dissolution of ferric phosphate → spray drying combined with low-temperature pre-lithiation → strong complexing agent driving Mg..." 2+The process steps of "surface-specific enrichment → in-situ generation of semi-coherent MgO heterophase at medium temperature → covalent grafting of sulfonylimide → secondary heat treatment to form a three-dimensional cross-linked network" correspond one by one, fully demonstrating the successful construction of a well-structured and firmly bonded inorganic-organic covalent composite interface control layer on the surface of primary particles in the 60-120 nm range. It is this composite interface that intrinsically blocks Fe dissolution and reduces Li... + The desolvation energy barrier was removed and electrolyte decomposition was suppressed, thus achieving excellent comprehensive performance with a discharge retention rate of ≥90.9% at 20 C, a capacity retention rate of ≥91.8% after 5000 cycles at 45 ℃ and 10 C, a gas production rate of ≤0.08 mL / Ah at high temperature storage, and extremely low Fe dissolution.

[0096] Combined with Examples 1 to 4, Figure 2 As shown in Table 1, the material of this invention exhibits highly consistent and stable superior comprehensive performance under the stated process conditions: initial discharge specific capacity at 0.1 C is 160.5–164.8 mAh / g, initial coulombic efficiency is 99.0%–99.3%, capacity retention at 20 C high-rate discharge is 90.9%–93.1%, capacity retention at 45 ℃ for 5000 cycles at 10 C is 91.8%–93.8%, capacity retention at -20 ℃ for 3 C discharge is 77.5%–79.8%, gas production at 45 ℃ for 30 days after full charge is only 0.06–0.08 mL / Ah, interfacial impedance Rcei after 5000 cycles is 19.2–22.6 Ω, and Fe dissolution is only 0.09–0.13 μg / g. The above results fully demonstrate that the present invention can stably and repeatedly construct an inorganic-organic covalent composite interface with extremely high interfacial stability within the key process window of organic acid complexation and dissolution of iron phosphate, spray drying combined with low-temperature pre-lithiation at 330-380℃, strong complexing agent-driven surface magnesium-rich aging at 85-95℃, medium-temperature carbothermal sintering at 730-780℃, sulfonylimide ester exchange grafting at 45-65℃, and secondary heat treatment at 330-380℃. Figure 2 The laser particle size distribution curves shown further demonstrate that the primary particles in all four sets of examples exhibit a narrow, single-peak distribution with D50 values ​​of 60 nm, 80 nm, 95 nm, and 120 nm, respectively, exhibiting extremely high particle size uniformity. This highly consistent nanoscale particle structure is the solid foundation for the material to achieve ultra-high rate capability, ultra-long cycle life, and extremely low gas production.

[0097] Combining Example 1, Comparative Example 1, and Table 1, it can be seen that Example 1 significantly outperforms Comparative Example 1 in all key indicators, including capacity retention at 20 C discharge (91.8% vs 67.4%), capacity retention after 5000 cycles at 10 C at 45 °C (92.6% vs 61.8%), capacity retention at -20 °C and 3 C discharge (78.3% vs 52.3%), gas production during high-temperature storage (0.07 mL / Ah vs 0.52 mL / Ah), interfacial impedance Rcei after cycling (21.4 Ω vs 176.3 Ω), and Fe dissolution (0.11 μg / g vs 1.38 μg / g). Comparative Example 1, due to the complete absence of a Mg source, cannot form an inorganic heterophase of MgO and an organic functionalized layer of sulfonylimide on its surface. This results in the complete exposure of unsaturated Fe sites caused by the high specific surface area of ​​the nanoparticles (D50≈90 nm), directly leading to severe Fe... 2+ Dissolution, HF erosion, and continuous electrolyte decomposition lead to uncontrolled CEI growth, a surge in internal resistance, severe gas production, and rapid capacity decay. In contrast, Example 1 successfully constructed a composite interface using the unique process of this invention. On one hand, the MgO layer completely covers and passivates the unsaturated Fe sites on the surface, intrinsically blocking the Fe dissolution pathway. On the other hand, the strongly electron-withdrawing sulfonylimide groups significantly reduce the Li... + By desolvating the solubility barrier, capturing HF, and inhibiting excessive CEI growth, the two work together to form a continuous and stable ion / electron insulating protective layer, thereby achieving a comprehensive breakthrough in high-rate, long-cycle, low-temperature output, and high-temperature storage performance. This fully demonstrates that the inorganic-organic covalent composite interface is an indispensable core structure for achieving the unity of high power and ultra-long lifespan in ultra-nanosized lithium iron phosphate.

[0098] Combining Example 1, Comparative Example 2, and Table 1, it can be seen that Comparative Example 2 (without strong complexing agents such as citric acid in step S2) lags behind Example 1 in all key performance indicators: 20 C discharge capacity retention is only 81.2% (vs 91.8%, a decrease of 10.6 percentage points); capacity retention after 5000 cycles at 10 C at 45 ℃ is only 74.3% (vs 92.6%, a decrease of 18.3 percentage points); gas production after 30 days of high-temperature storage is as high as 0.38 mL / Ah (vs 0.07 mL / Ah, a deterioration of 5.4 times); the interfacial impedance Rcei after cycling is as high as 108.5 Ω (vs 21.4 Ω, an increase of more than 4 times); and Fe dissolution is as high as 0.97 μg / g (vs 0.11 μg / g, an increase of nearly 9 times). The fundamental reason is the lack of precise control of strong complexing agents such as citric acid in step S2, leading to the free Mg during aging. 2+ If the concentration is too high, the ideal distribution of "zero bulk doping and 100% specific enrichment on the surface" cannot be achieved, and some Mg... 2+Doping occurs when the LiFePO4 lattice forms bulk dopant, disrupting the bulk Li phase. + The rapid transport channels, coupled with severely insufficient surface Mg enrichment, prevented the formation of a continuous, complete, and semi-coherent MgO heterogeneous phase layer after low-temperature sintering. Numerous unsaturated Fe sites remained exposed on the particle surface. This missing MgO protective layer directly led to Fe... 2+ The Fe2+ continues to dissolve during high-rate and high-temperature cycling. 2+ Further catalytic electrolyte decomposition and induction of uncontrolled CEI growth led to a surge in gas production, a dramatic increase in internal resistance, and rapid capacity decay. In Example 1, however, free Mg was effectively complexed using citric acid. 2+ By suppressing the concentration to an extremely low level, Mg is ensured to completely avoid the bulk phase and be exclusively enriched in the outermost layer, thereby generating a uniform, semi-coherent pure-phase MgO layer in situ. This completely shields the unsaturated Fe sites on the surface and provides sufficient active sites for subsequent covalent grafting of sulfonylimide. From the intrinsic structural level of the material, the root causes of Fe dissolution and interfacial side reactions are blocked, achieving a comprehensive and synergistic breakthrough in high-rate performance, ultra-long cycle life and high-temperature storage stability.

[0099] Combining Example 1, Comparative Example 3, and Table 1, it can be seen that the 20 C retention rate of Comparative Example 3 (S3 using pitch carbon source) is only 73.1%, and the Rcei after cycling is as high as 142.7 Ω, which is much lower than that of Example 1. This indicates that specific low-temperature small molecule carbon sources such as glucose are key: their mild pyrolysis provides a reducing atmosphere and leaves very little residual carbon, does not bury the hydroxyl groups on the MgO surface, and ensures that the subsequent sulfonylimide ester exchange reaction sites are fully exposed, while the pitch forms a thick amorphous carbon layer that completely blocks the functionalization step.

[0100] Combining Example 1, Comparative Example 4, and Table 1, it can be seen that Comparative Example 4 (S4 using ordinary KH-550 instead of sulfonylimide) has a 20°C retention rate of 85.6%, a gas production rate of 0.31 mL / Ah, and a performance of only 67.2% at -20°C, which is significantly worse than Example 1. This proves that the super electron-withdrawing sulfonylimide group can significantly reduce Li + It can break the solvation energy barrier, capture HF, and inhibit excessive CEI growth. Ordinary silanes only form an inert Si-O layer and do not have this function.

[0101] Combining Example 1, Comparative Example 5, and Table 1, it can be seen that Comparative Example 5 (without secondary heat treatment of S5) had a cycle retention rate of only 68.9%, an Rcei as high as 152.3 Ω, and a gas production rate of 0.41 mL / Ah. This indicates that secondary heat treatment at 330–380 °C is a necessary step for forming the Mg-O-Si-O-sulfonamide three-dimensional covalent cross-linked network. Without heat treatment, the organic layer is only physically adsorbed and quickly detaches upon contact with the electrolyte, leading to interfacial instability.

[0102] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a lithium iron phosphate cathode material, characterized in that, Includes the following steps: S1: Prepare an organic acid complexing agent into a solution, add iron phosphate powder, lithium carbonate or lithium hydroxide into the solution according to the stoichiometric ratio and dissolve to obtain Li-Fe-P precursor slurry sol, spray dry the sol prepared above, and heat treat the obtained spray-dried powder in an air atmosphere at 330 to 380°C to obtain pre-lithiated lithium iron phosphate precursor. S2: Prepare the pre-lithiated lithium iron phosphate precursor from step S1 into a slurry, add magnesium nitrate as a magnesium source to the precursor slurry, add an organic acid complexing agent, and age it at 85 to 95°C to obtain a coating precursor with magnesium enrichment on the surface. S3: Mix the coating precursor obtained in step S2 with a carbon source and perform solid-state sintering at 730 to 780°C to form an inorganic heterogeneous MgO phase layer on the surface. S4: Disperse the material obtained in step S3 in an organic precursor solution containing sulfonylimide groups, and carry out an ester exchange reaction at 45 to 65°C to bond the sulfonylimide groups to the surface of the MgO layer, thereby obtaining a prefunctionalized material. S5: The prefunctionalized material obtained in step S4 is subjected to a second heat treatment at 330 to 380°C to form a composite interface control layer between the inorganic layer and the organic layer, thereby obtaining the lithium iron phosphate cathode material.

2. The preparation method according to claim 1, characterized in that, In steps S1 and S2, the organic acid complexing agent is selected from at least one of citric acid, tartaric acid, ethylenediaminetetraacetic acid, and oxalic acid.

3. The preparation method according to claim 1, characterized in that, In step S1, the heat treatment time is 0.5 to 3 hours.

4. The preparation method according to claim 1, characterized in that, In step S3, the solid-state sintering time is 6 to 10 hours, and the carbon source is selected from at least one of glucose, sucrose, and polyvinylpyrrolidone.

5. The preparation method according to claim 1, characterized in that, In step S5, the secondary heat treatment time is 0.5 to 3 hours.

6. The lithium iron phosphate cathode material prepared by the preparation method according to claim 1, characterized in that, The primary particle surface of the material has an inorganic-organic composite interface control layer, which comprises, from the inside out: An inorganic heterogeneous phase layer tightly coated on the surface of LiFePO4, wherein the inorganic heterogeneous phase layer is MgO; An organically functionalized layer chemically bonded to the surface of MgO, wherein the organically functionalized layer contains sulfonylimide groups; The D50 of primary particles is 60–120 nm.

7. The lithium iron phosphate cathode material according to claim 6, characterized in that, The inorganic heterogeneous phase layer is MgO with a rock salt structure. In grazing incidence X-ray diffraction, there is an obvious MgO (200) crystal plane diffraction peak, and the peak position of this peak in the φ scan is completely consistent with the φ scan peak position of the (100) crystal plane of LiFePO4.

8. A positive electrode plate, characterized in that, The positive electrode active material of the positive electrode sheet is any one of the lithium iron phosphate positive electrode materials described in claims 6-7.

9. A lithium-ion battery, characterized in that, The positive electrode of the lithium-ion battery contains the positive electrode sheet as described in claim 8.

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