Single-crystal lithium manganese iron phosphate and its preparation method, lithium-ion batteries and electrical devices

Abstract

This application belongs to the field of battery technology, specifically relating to monocrystalline lithium manganese iron phosphate and its preparation method, lithium-ion batteries, and power devices. The average particle size of the monocrystalline lithium manganese iron phosphate is 0.1μm~0.4μm, and the maximum particle size is 0.4μm~0.7μm. In terms of particle quantity, particles with a size in the range of (0μm, 0.1μm) account for 20%~70%, and particles with a size in the range of (0.1μm, 0.2μm) account for 30%~80%. This monocrystalline lithium manganese iron phosphate combines high compaction density and long cycle performance.

Description

Technical Field

[0001] This application belongs to the field of battery technology, specifically relating to monocrystalline lithium manganese iron phosphate and its preparation method, lithium-ion batteries and power devices. Background Technology

[0002] With the continued acceleration of the development of the new energy storage industry, the market's requirements for core performance aspects of batteries, such as safety, energy density, and cycle life, have significantly increased. Among them, lithium manganese iron phosphate (LMFP) energy storage batteries have gradually gained market favor due to their outstanding advantages of low cost, high safety, long cycle life, and wide temperature range adaptability. They are considered the direction for the iterative upgrade of lithium iron phosphate (LFP) materials and are expected to become one of the mainstream technologies for the next generation of low-cost, high-energy-density energy storage batteries. However, LFP materials suffer from problems such as low intrinsic conductivity and poor structural stability leading to severe manganese ion dissolution. Currently, methods such as achieving wide particle size distribution through morphology control, using flexible conductive carbon-based coating, and optimizing sintering processes cannot simultaneously achieve long cycle life and high compaction performance. Therefore, related technologies for LFP materials still need improvement. Summary of the Invention

[0003] This invention aims to at least partially solve one of the technical problems in related technologies. To this end, this invention proposes a single-crystal lithium manganese iron phosphate compound that balances high solid density and long cycle performance, its preparation method, a lithium-ion battery, and an electrical device thereof.

[0004] In a first aspect, this application provides a monocrystalline lithium manganese iron phosphate. According to an embodiment of this application, the average particle size of the monocrystalline lithium manganese iron phosphate is 0.1 μm to 0.4 μm, and the maximum particle size is 0.4 μm to 0.7 μm; in terms of particle quantity, particles with a size in the range of (0 μm, 0.1 μm) account for 20% to 70%, and particles with a size in the range of (0.1 μm, 0.2 μm) account for 30% to 80%.

[0005] This single-crystal lithium manganese iron phosphate exhibits a wide particle size distribution, high compaction density, and low stress deformation, resulting in excellent processing adaptability. After electrode rolling, the particle size stability is strong, and it is not prone to microcracks and breakage. This effectively improves the problems of easy breakage, low compaction, and poor processability of traditional lithium manganese iron phosphate rolling. At the same time, the cycle performance is significantly optimized, effectively overcoming the bottleneck of difficulty in balancing compaction density and cycle stability in existing technologies.

[0006] According to embodiments of this application, monocrystalline lithium manganese iron phosphate satisfies at least one of the following conditions: Particles with a size between 0 μm and 0.1 μm account for 25% to 60% of the total. Particles with a size in the range of (0.1μm, 0.2μm) account for 32% to 70%.

[0007] According to an embodiment of this application, the particle size distribution curve of the single-crystal lithium manganese iron phosphate has a first characteristic peak, a second characteristic peak, and a third characteristic peak with successively increasing peak positions. The peak position of the first characteristic peak is less than or equal to 1 μm, the peak position of the second characteristic peak is greater than 1 μm, and the peak position of the third characteristic peak is greater than or equal to 10 μm.

[0008] According to embodiments of this application, monocrystalline lithium manganese iron phosphate satisfies at least one of the following conditions: The difference between the peak position of the second characteristic peak and the peak position of the first characteristic peak is 0.5 μm to 3 μm; The difference between the peak position of the third characteristic peak and the peak position of the first characteristic peak is 10μm~15μm; The ratio of the peak value corresponding to the first characteristic peak to the peak value corresponding to the second characteristic peak is 1 to 2.5; The ratio of the peak value corresponding to the first characteristic peak to the peak value corresponding to the third characteristic peak is 2 to 4.

[0009] According to embodiments of this application, the single-crystal lithium manganese iron phosphate D 50 The particle size is 0.3μm to 2.0μm, preferably 0.5μm to 1.5μm.

[0010] According to an embodiment of this application, the lattice constant a / c value of the single-crystal lithium manganese iron phosphate is 2.2010~2.2030; preferably 2.2015~2.2025.

[0011] According to embodiments of this application, monocrystalline lithium manganese iron phosphate satisfies at least one of the following conditions: The cell volume of the single-crystal lithium manganese iron phosphate is 290 Å. 3 ~298Å 3 Preferably 294Å 3 ~297 Å 3 ; The compaction density of the monocrystalline lithium manganese iron phosphate is 2.4 g / cc to 2.6 g / cc, preferably 2.45 g / cc to 2.58 g / cc.

[0012] According to embodiments of this application, a single-crystal lithium manganese iron phosphate comprises a lithium manganese iron phosphate core and a carbon coating layer covering the surface of the lithium manganese iron phosphate core, and satisfies at least one of the following conditions: (1) Based on the total weight of the monocrystalline lithium manganese iron phosphate, the content of the carbon coating layer is 1wt% to 2.5wt%, preferably 1.5wt% to 2.0wt%; (2) The lithium manganese iron phosphate core has the chemical composition shown in Formula 1: Lia Mn b Fe (1-b) M c (PO4) d General Formula 1 In general formula 1, 0.95≤a≤1.10, 0.3≤b≤0.7, 0≤c≤0.5, 0.95≤d≤1.10, and M includes at least one of Ti, Mg, Zn, Cu, Sr, Al, Zr, Y, Co, W, Ca, Nb, Sn, Sb, Mo, V, B, Si, and Na.

[0013] According to embodiments of this application, the discharge curve of a lithium metal coin cell using the monocrystalline lithium manganese iron phosphate as the positive electrode active material has a single discharge plateau.

[0014] In a second aspect, this application provides a method for preparing the aforementioned single-crystal lithium manganese iron phosphate. According to an embodiment of this application, the method includes: mixing a lithium manganese iron phosphate precursor and a lithium iron phosphate precursor to obtain a raw material mixture; heating the raw material mixture to T2 at a heating rate not exceeding 2°C / min and holding it at that temperature for 2h to 10h, where T2 is 600°C to 850°C; then cooling it to T2-80°C at a cooling rate not exceeding 2°C / min; and continuing to cool it to room temperature to obtain the single-crystal lithium manganese iron phosphate; wherein the average particle size of the lithium manganese iron phosphate precursor is 0.05μm to 0.2μm; and the average particle size of the lithium iron phosphate precursor is 0.2μm to 0.4μm.

[0015] This method involves mixing small-particle lithium manganese iron phosphate precursors with large-particle lithium iron phosphate precursors as raw materials. Combined with precise control of the high-temperature solid-state sintering curve, atomic-level homogeneous mixing of the two precursors is achieved, forming a single olivine solid solution. This results in a unified lattice environment, consistent iron plateaus, and uniform electron conduction and ion diffusion efficiency within the material. This avoids plateau shifts caused by conduction differences in the mixed state, ultimately producing single-crystal lithium manganese iron phosphate with a wide particle size distribution, high compaction density, high compressive strength, and low stress deformation. Furthermore, this material exhibits excellent processing adaptability, strong particle size stability after electrode rolling, and significantly optimized cycle performance, effectively solving the bottleneck of balancing compaction density and cycle stability in existing technologies.

[0016] According to embodiments of this application, the method satisfies at least one of the following conditions: (a) The lithium manganese iron phosphate precursor has the chemical composition shown in general formula 2: Li a2 Mn b2 Fe (1-b2) M c2 (PO4) d2 General Formula 2 (b) The lithium iron phosphate precursor has the chemical composition shown in general formula 3: Li a3 FeM c3 (PO4) d3 Formula 3 Wherein, 0.95≤a2≤1.10, 0.75≤b2<0.95, 0≤c2≤0.5, 0.95≤d2≤1.10, 0.95≤a3≤1.10, 0≤c3≤0.5, 0.95≤d3≤1.10, and M includes at least one of Ti, Mg, Zn, Cu, Sr, Al, Zr, Y, Co, W, Ca, Nb, Sn, Sb, Mo, V, B, Si, and Na.

[0017] According to an embodiment of this application, the mass of the lithium manganese iron phosphate precursor accounts for 30% to 90% of the total mass of the raw material mixture.

[0018] According to embodiments of this application, the raw material mixture further includes an auxiliary agent, which includes at least one of ammonium metavanadate and magnesium carbonate.

[0019] A third aspect of this application discloses a lithium-ion battery comprising monocrystalline lithium manganese iron phosphate as described in the first aspect of this application or monocrystalline lithium manganese iron phosphate prepared by the method described in the second aspect of this application. Therefore, this lithium-ion battery exhibits good cycle performance and high energy density.

[0020] The fourth aspect of this application discloses an electrical device comprising the monocrystalline lithium manganese iron phosphate described in the first aspect of this application, the monocrystalline lithium manganese iron phosphate prepared by the method described in the second aspect of this application, or the lithium-ion battery described in the third aspect of this application. Therefore, this electrical device has a long service life and good battery life. Attached Figure Description

[0021] Figure 1 This is a SEM image of the single-crystal lithium manganese iron phosphate of Example 1 of this application.

[0022] Figure 2 This is a distribution diagram of the particle size distribution of single-crystal lithium manganese iron phosphate in Embodiment 1 of this application.

[0023] Figure 3 This is the volume particle size distribution curve of the single-crystal lithium manganese iron phosphate of Example 1 of this application.

[0024] Figure 4 These are charge-discharge curves of batteries assembled from single-crystal lithium manganese iron phosphate materials in Examples 1 and 1 of this application.

[0025] Figure 5 These are the XRD patterns of single-crystal lithium manganese iron phosphate in Example 1 and Comparative Example 1 of this application.

[0026] Figure 6 This is a distribution map of Fe and Mn elements in single-crystal lithium manganese iron phosphate of Example 1 and Comparative Example 1 of this application. Detailed Implementation

[0027] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0028] In a first aspect, this application provides a single-crystal lithium manganese iron phosphate. According to embodiments of this application, the average particle size of the single-crystal lithium manganese iron phosphate is 0.1 μm to 0.4 μm (specifically, 0.1 μm, 0.15 μm, 0.2 μm, 0.25 μm, 0.3 μm, 0.35 μm, 0.4 μm or any two thereof), and the maximum particle size is 0.4 μm to 0.7 μm (specifically, 0.4 μm, 0.45 μm, 0.5 μm, 0.55 μm, 0.6 μm, 0.65 μm, 0.7 μm). The particle size is 7μm or any two of these sizes; in terms of particle count, particles with a size between 0μm and 0.1μm account for 20% to 70% (specifically, 20%, 30%, 40%, 50%, 60%, 70% or any two of these sizes), and particles with a size between 0.1μm and 0.2μm account for 30% to 80% (specifically, 30%, 40%, 50%, 60%, 70%, 80% or any two of these sizes).

[0029] This single-crystal lithium manganese iron phosphate exhibits a wide particle size distribution, high compaction density, and low stress deformation, resulting in excellent processing adaptability. After electrode rolling, the particle size stability is strong, and it is not prone to microcracks and breakage. This effectively improves the problems of easy breakage, low compaction, and poor processability of traditional lithium manganese iron phosphate rolling. At the same time, the cycle performance is significantly optimized, effectively overcoming the bottleneck of difficulty in balancing compaction density and cycle stability in existing technologies.

[0030] In this paper, particle size can be detected by statistical analysis of scanning electron microscope (SEM) images of single-crystal lithium manganese iron phosphate using Metis image analysis software. The SEM image shooting conditions are set as follows: accelerating voltage 10 kV and magnification 30 K. The specific detection steps are as follows: (1) After importing the SEM image, call the single-crystal particle-specific analysis module and select the large particle intermittent fitting model for particle contour recognition; (2) Do not set a limit threshold for particle size detection range, particle aspect ratio, or particle area ratio during the test process; (3) Particle size is uniformly calculated using caliper diameter, i.e., the arithmetic mean of the major and minor diameters of a single particle; (4) Select (0μm, 0.1μm], (0.1μm, 0.2μm] and so on for distribution editing; (5) Export the statistical results of the current SEM image to Excel to obtain the distribution of the current SEM image. The average particle size, maximum particle size, and particle ratio of the above particle size range can be obtained.

[0031] According to embodiments of this application, the proportion of monocrystalline lithium manganese iron phosphate particles with a size of (0 μm, 0.1 μm) is 25% to 60%. Within this range, the ion diffusion path can be guaranteed to meet the electrical performance requirements, while the small particles can effectively fill the gaps between the large particles, significantly improving the particle packing efficiency and thus significantly increasing the compaction density of monocrystalline lithium manganese iron phosphate.

[0032] According to embodiments of this application, the proportion of monocrystalline lithium manganese iron phosphate particles with a size of (0.1 μm, 0.2 μm) is 32% to 70%. Within this range, small particles can effectively fill the gaps between large particles, significantly improving particle packing efficiency and resulting in a significant increase in the compaction density of monocrystalline lithium manganese iron phosphate.

[0033] According to an embodiment of this application, the particle size distribution curve of the monocrystalline lithium manganese iron phosphate has a first characteristic peak, a second characteristic peak, and a third characteristic peak with successively increasing peak positions. The peak position of the first characteristic peak is less than or equal to 1 μm, the peak position of the second characteristic peak is greater than 1 μm, and the peak position of the third characteristic peak is greater than or equal to 10 μm. By satisfying the above requirements, the monocrystalline lithium manganese iron phosphate accurately constructs a wide particle size distribution system, with large particles forming a structural support framework and small particles effectively filling the gaps, significantly improving particle packing efficiency and resulting in a significant increase in material compaction density. Simultaneously, the material is less prone to microcracks and breakage during electrode rolling, effectively improving the problems of brittleness, low compaction, and poor processability associated with traditional lithium manganese iron phosphate rolling.

[0034] In this article, the peak position of the characteristic peak in the particle size distribution curve of single-crystal lithium manganese iron phosphate refers to the particle size value corresponding to the peak value of the characteristic peak. The D of single-crystal lithium manganese iron phosphate... 50 Particle size refers to the particle size value corresponding to a cumulative volume distribution of 50% of the total particle size. The particle size distribution curve can be measured using a Malvern particle size analyzer.

[0035] According to an embodiment of this application, the difference between the peak position of the second characteristic peak and the peak position of the first characteristic peak is 0.5 μm to 3 μm (specifically, 0.5 μm, 0.8 μm, 1.0 μm, 1.2 μm, 1.5 μm, 1.8 μm, 2.0 μm, 2.2 μm, 2.5 μm, 2.8 μm, 3 μm, or any two of these ranges). Therefore, a more suitable wide particle size distribution system can be constructed, further improving particle packing efficiency and compaction density.

[0036] According to an embodiment of this application, the difference between the peak position of the third characteristic peak and the peak position of the first characteristic peak is 10 μm to 15 μm (specifically, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, or any range between two of them). Therefore, a more suitable wide particle size distribution system can be constructed, further improving particle packing efficiency and compaction density.

[0037] According to an embodiment of this application, the ratio of the peak value corresponding to the first characteristic peak to the peak value corresponding to the second characteristic peak is 1 to 2.5 (specifically, such as 1, 1.2, 1.4, 1.6, 1.8, 2.0, 2.2, 2.4, 2.5 or any two of them). Thus, a more suitable wide particle size distribution system can be constructed, further improving particle packing efficiency and compaction density.

[0038] According to an embodiment of this application, the ratio of the peak value corresponding to the first characteristic peak to the peak value corresponding to the third characteristic peak is 2 to 4 (specifically, such as 2, 2.2, 2.4, 2.6, 2.8, 3.0, 3.2, 3.4, 3.6, 3.8, 4.0, or any two of them). Therefore, a more suitable wide particle size distribution system can be constructed, further improving particle packing efficiency and compaction density.

[0039] In this article, the peak value corresponding to the characteristic peak in the particle size and volume distribution curve of single-crystal lithium manganese iron phosphate refers to the volume fraction corresponding to the characteristic peak, that is, the value corresponding to the peak height.

[0040] According to an embodiment of this application, the lattice constant a / c value of the single-crystal lithium manganese iron phosphate is 2.2010~2.2030 (specifically, 2.2010, 2.2015, 2.2020, 2.2025, 2.2030, or any two of them). Within the above range, the lattice constant a / c value shows a moderately increasing trend compared to conventional single-crystal lithium manganese iron phosphate, resulting in a reasonably stretched lattice, a widened one-dimensional lithium-ion diffusion channel pore size, and an effective reduction in the lithium-ion migration energy barrier. Simultaneously, its lattice distortion remains within a reasonable range, without excessive structural distortion. While maintaining good structural stability, the ion diffusion kinetics are significantly improved, and the material's rate discharge performance is significantly enhanced.

[0041] In some embodiments, the lattice constant a / c of ​​the single-crystal lithium manganese iron phosphate is 2.2015~2.2025. Within this range, the pore size of the lithium-ion diffusion channel can be further widened, the lithium-ion migration barrier can be reduced, and good structural stability can be maintained, thereby further improving the ion diffusion kinetics and rate discharge performance.

[0042] According to an embodiment of this application, the unit cell volume of the single-crystal lithium manganese iron phosphate is 290 Å. 3 ~298Å 3 Specifically, 294Å 3 ~297 Å 3 As an example, the cell volume of single-crystal lithium manganese iron phosphate can specifically be 290 Å. 3 291Å 3 292Å 3 293Å 3 294Å 3 295Å 3 296Å 3 297Å 3 298Å 3 Or a range between either or both. Meeting the above requirements, single-crystal lithium manganese iron phosphate has a wider lithium-ion diffusion channel pore size, a lower lithium-ion migration energy barrier, and retains better structural stability, thus facilitating further improvement in ion diffusion kinetics and rate discharge performance.

[0043] In this paper, the lattice constant a / c and the unit cell volume can be detected by XRD method.

[0044] According to embodiments of this application, the compaction density of the monocrystalline lithium manganese iron phosphate is 2.4 g / cc to 2.6 g / cc, specifically 2.45 g / cc to 2.58 g / cc. As an example, the compaction density of the monocrystalline lithium manganese iron phosphate is 2.4 g / cc, 2.42 g / cc, 2.44 g / cc, 2.46 g / cc, 2.48 g / cc, 2.5 g / cc, 2.52 g / cc, 2.54 g / cc, 2.56 g / cc, 2.58 g / cc, 2.6 g / cc, or any range thereof. This improves energy density and thus enhances battery life.

[0045] In this paper, the compaction density can be tested by the following method: 1. Weigh 1g of single-crystal lithium manganese iron phosphate and add it into a compaction mold (13mm in diameter). Vibrate the mold to make the powder flat on the surface; 2. Place the treated mold in the Sansi compaction test equipment and slowly pressurize it to the specified pressure of 225MPa. After holding it for 30s, remove it to obtain the compaction density result.

[0046] According to an embodiment of this application, a single-crystal lithium manganese iron phosphate includes a lithium manganese iron phosphate core and a carbon coating layer covering the surface of the lithium manganese iron phosphate core, wherein the lithium manganese iron phosphate core has the chemical composition shown in general formula 1: Li a Mn b Fe (1-b) M c (PO4) d General Formula 1 In general formula 1, 0.95≤a≤1.10, 0.3≤b≤0.7, 0≤c≤0.5, 0.95≤d≤1.10, and M includes at least one of Ti, Mg, Zn, Cu, Sr, Al, Zr, Y, Co, W, Ca, Nb, Sn, Sb, Mo, V, B, Si, and Na. Having the above chemical formula, this single-crystal lithium manganese iron phosphate exhibits an orthorhombic olivine structure, and XRD analysis shows a pure phase without impurities (no LiMnPO4 or FePO4 impurities), belonging to space group Pnma, which is beneficial for obtaining better overall performance.

[0047] According to embodiments of this application, based on the total weight of the monocrystalline lithium manganese iron phosphate, the content of the carbon coating layer is 1wt% to 2.5wt%, specifically 1.5wt% to 2.0wt%. As an example, based on the total weight of the monocrystalline lithium manganese iron phosphate, the content of the carbon coating layer can specifically be 1wt%, 1.2wt%, 1.4wt%, 1.6wt%, 1.8wt%, 2.0wt%, 2.2wt%, 2.4wt%, 2.5wt%, or any range between two of these. This improves the conductivity of the monocrystalline lithium manganese iron phosphate while having virtually no adverse effect on the material's compaction density and processing characteristics.

[0048] In this paper, the content of carbon coating can be detected using a high-frequency infrared carbon-sulfur analyzer. The specific steps are as follows: (1) Place the single-crystal lithium manganese iron phosphate powder in a vacuum oven at 100 ℃ for 2 h, cool it to room temperature, remove the adsorbed moisture, and avoid water vapor interfering with the infrared signal. (2) Accurately weigh 0.2 g of dried single-crystal lithium manganese iron phosphate powder and place it in a pretreated ceramic crucible; add iron flux and tungsten flux in sequence to cover the sample. (3) Send the crucible into a high-frequency induction furnace, introduce high-purity oxygen, and ignite it at high frequency. The carbon in the sample is fully oxidized to CO2; the CO2 concentration is detected by the infrared absorption cell, and the instrument automatically collects the signal to obtain the content of carbon coating.

[0049] According to embodiments of this application, the discharge curve of a lithium metal coin cell using monocrystalline lithium manganese iron phosphate as the positive electrode active material exhibits a single discharge plateau. Specifically, the presence of a single discharge plateau indicates that the Mn and Fe atoms in the monocrystalline lithium manganese iron phosphate undergo sufficient interdiffusion, forming a uniformly structured olivine solid solution without the problem of independent coexistence of two phases, thereby allowing the Fe atoms in the monocrystalline lithium manganese iron phosphate to fully interdiffuse, forming a uniform olivine solid solution. 2+ / Fe 3+ With a consistent lattice environment and thermodynamic potential, the iron plateaus in the charge-discharge curves highly overlap, with no obvious steps or potential splits, resulting in more stable cell voltage output, higher consistency, and significantly improved cycle stability.

[0050] A second aspect of this application provides a method for preparing the aforementioned single-crystal lithium manganese iron phosphate. According to an embodiment of this application, the method includes: mixing a lithium manganese iron phosphate precursor and a lithium iron phosphate precursor to obtain a raw material mixture; heating the raw material mixture to T2 at a heating rate not exceeding 2°C / min and holding it at that temperature for 2h~10h (specifically, 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h or any two thereof), where T2 is 600°C~850°C (specifically, 600°C, 620°C, 650°C, 680°C, 700°C, 720°C, 750°C, 780°C, 800°C, 820°C, 850°C or any two thereof); and then cooling it to T2-80°C at a cooling rate not exceeding 2°C / min. The temperature is further reduced to room temperature to obtain the single-crystal lithium manganese iron phosphate; the average particle size of the lithium manganese iron phosphate precursor is 0.05μm~0.2μm (specifically, 0.05μm, 0.08μm, 0.1μm, 0.12μm, 0.15μm, 0.18μm, 0.2μm or any two of them); the average particle size of the lithium iron phosphate precursor is 0.2μm~0.4μm (specifically, 0.2μm, 0.22μm, 0.24μm, 0.26μm, 0.28μm, 0.3μm, 0.32μm, 0.34μm, 0.36μm, 0.38μm, 0.4μm or any two of them).

[0051] This method involves mixing small-particle lithium manganese iron phosphate precursors with large-particle lithium iron phosphate precursors as raw materials. Combined with precise control of the high-temperature solid-state sintering curve, atomic-level homogeneous mixing of the two precursors is achieved, forming a single olivine solid solution. This results in a unified crystal lattice environment, consistent iron plateaus, and uniform electron conduction and ion diffusion efficiency within the material. This avoids plateau shifts caused by conduction differences in the mixed state, ultimately producing single-crystal lithium manganese iron phosphate with a wide particle size distribution, high compaction density, high compressive strength, and low stress deformation. Furthermore, this material exhibits excellent processing adaptability, strong particle size stability after electrode rolling, and significantly optimized cycle performance, effectively solving the bottleneck of balancing compaction density and cycle stability in existing technologies. Additionally, scanning electron microscopy (SEM) testing of the single-crystal lithium manganese iron phosphate reveals excellent particle size distribution, roundness, and no obvious abnormal growth. Locally agglomerated particles show complete melting and growth, dense grain boundaries, and uniform particle size. EDS testing shows no significant component segregation and no local manganese-poor / manganese-rich agglomerations.

[0052] According to an embodiment of this application, the lithium manganese iron phosphate precursor has the chemical composition shown in general formula 2: Li a2 Mn b2 Fe (1-b2) M c2 (PO4) d2 General Formula 2 The lithium iron phosphate precursor has the chemical composition shown in general formula 3: Li a3 FeM c3 (PO4) d3 Formula 3 Wherein, 0.95≤a2≤1.10, 0.75≤b2<0.95, 0≤c2≤0.5, 0.95≤d2≤1.10, 0.95≤a3≤1.10, 0≤c3≤0.5, 0.95≤d3≤1.10, and M includes at least one of Ti, Mg, Zn, Cu, Sr, Al, Zr, Y, Co, W, Ca, Nb, Sn, Sb, Mo, V, B, Si, and Na.

[0053] By using a precursor with the above chemical composition, the atomic-level uniform mixing of the two precursors can be promoted by controlling the different manganese content, lithium-phosphorus ratio, and doping element content of the precursor, forming a single olivine solid solution with a unified lattice environment, consistent iron plateau, and uniform electron conduction and ion diffusion efficiency inside the material. This is beneficial for producing single-crystal lithium manganese iron phosphate with high compressive strength, low stress deformation, high solid density, and long cycle performance.

[0054] According to the embodiments of this application, there are no particular limitations on the preparation methods of the two precursors, and they can be flexibly selected according to actual needs.

[0055] In some embodiments, the lithium manganese iron phosphate precursor can be prepared by the following steps: iron phosphate, manganese tetroxide, lithium source, phosphorus source, M source and carbon source are weighed and mixed according to the molar ratio of Li, Mn, Fe, M and P in general formula 2. The amount of carbon source added is such that the residual carbon content of the final lithium manganese iron phosphate precursor is 1.5wt% to 2.5wt%. Then, it is dispersed in a solvent (including but not limited to deionized water) with a certain solid content (e.g. ≥35%), and then ground once. After grinding to the target particle size, it is spray-dried once. Then, the spray-dried material is sintered at low temperature to obtain the lithium manganese iron phosphate precursor.

[0056] In some embodiments, the temperature for the aforementioned low-temperature sintering can be 450℃~650℃ (specifically, 450℃, 480℃, 500℃, 520℃, 550℃, 580℃, 600℃, 620℃, 650℃, or any two of these ranges), and the sintering time can be 3h~12h (specifically, 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, 11h, 12h, or any two of these ranges). Through precise temperature control, lithium manganese iron phosphate precursors with suitable particle size and superior performance can be obtained.

[0057] In some embodiments, the lithium iron phosphate precursor can be prepared by the following steps: weighing and mixing iron phosphate, lithium source, phosphorus source, M source and carbon source according to the molar ratio of Li, Fe, M and P in general formula 3, the amount of carbon source added is sufficient to meet the final target of 1.0wt%~2.0wt% residual carbon content of the prepared lithium iron phosphate precursor, then dispersing it in a solvent (including but not limited to deionized water) with a certain solid content (e.g. ≥35%), then grinding it once, grinding it to the target particle size, then spray drying it once, and then sintering the obtained spray-dried material at high temperature to obtain the lithium iron phosphate precursor.

[0058] In some embodiments, the high-temperature sintering temperature can be 750℃~850℃ (specifically, 750℃, 760℃, 770℃, 780℃, 790℃, 800℃, 810℃, 820℃, 830℃, 840℃, 850℃ or any two of these ranges), and the sintering time can be 3h~12h (specifically, 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, 11h, 12h or any two of these ranges). Through precise temperature control, lithium iron phosphate precursors with suitable particle size and superior performance can be obtained.

[0059] In some embodiments, the iron phosphate, lithium source, phosphorus source, M source, and carbon source used to prepare the above two precursors are not specifically limited in this invention. As an example, the lithium source may include at least one of lithium carbonate, lithium hydroxide, lithium phosphate, lithium nitrate, anhydrous lithium hydroxide, and lithium oxide; the phosphorus source may include at least one of phosphoric acid, lithium dihydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, manganese phosphate, lithium phosphate, and pyrophosphate; the M source may include at least one of M oxide, M hydroxide, M carbonate, M fluoride, M sulfuric acid, M phosphoric acid, and M chloride; the carbon source may be at least one of glucose, sucrose, phenolic resin, and organic polymer; the iron phosphate may be iron phosphate with an iron-to-phosphorus molar ratio Fe / P of 0.940-0.990; and the manganese content of manganese tetroxide may be 0.68-0.75%.

[0060] According to embodiments of this application, the mass ratio of the lithium manganese iron phosphate precursor to the total mass of the raw material mixture is 30% to 90%, specifically 30%, 40%, 50%, 60%, 70%, 80%, 90%, or any two of these ratios. Alternatively, the mass ratio of the lithium manganese iron phosphate precursor to the lithium iron phosphate precursor is 30-90:70-10. Within the above ratio range, the two precursors can achieve atomically homogeneous mixing to form a single olivine solid solution, thereby obtaining monocrystalline lithium manganese iron phosphate with high compressive strength, low stress deformation, high solid density, and long cycle performance.

[0061] According to embodiments of this application, the raw material mixture further includes an additive, which includes at least one selected from ammonium metavanadate and magnesium carbonate. This improves the ionic conductivity of monocrystalline lithium manganese iron phosphate, thereby enhancing its kinetic performance, facilitating capacity utilization, and improving rate performance.

[0062] A third aspect of this application discloses a lithium-ion battery comprising monocrystalline lithium manganese iron phosphate as described in the first aspect of this application or monocrystalline lithium manganese iron phosphate prepared by the method described in the second aspect of this application. Therefore, this lithium-ion battery exhibits good cycle performance and high energy density.

[0063] It is understandable that there are no particular restrictions on the specific type of lithium-ion battery; it can be a primary battery or a secondary battery. The shape of the battery can be cylindrical, square, or any other shape. According to the outer packaging, the battery can be a hard-shell battery, a soft-pack battery, etc.

[0064] Typically, a lithium-ion battery includes a positive electrode, a negative electrode, an electrolyte, and a separator. The positive electrode, negative electrode, and separator are fabricated into an electrode assembly using winding or stacking processes. The electrode assembly and electrolyte are housed in an outer packaging. During charging and discharging, lithium ions repeatedly insert and extract between the positive and negative electrodes. The electrolyte acts as a conductor between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing lithium ions to pass through.

[0065] In some embodiments, the positive electrode sheet may include a positive current collector and a positive active material layer disposed on at least one side surface of the positive current collector. The positive active material layer includes the aforementioned monocrystalline manganese iron phosphate, conductive agent, and binder. The positive current collector may include a metal foil, for example, aluminum foil. The conductive agent may include acetylene black, single-walled carbon nanotubes, and materials conventional in the art. The binder may be polyvinylidene fluoride (PVDF) and materials conventional in the art.

[0066] In some embodiments, the negative electrode sheet may include a negative electrode current collector and a layer of negative electrode active material disposed on at least one side surface of the negative electrode current collector. The negative electrode active material layer may include a negative electrode active material, a thickener, a conductive agent, and a binder. The negative electrode current collector may be a metal foil, for example, copper foil. The negative electrode active material may include artificial graphite, natural graphite, silicon-carbon based composite materials, lithium metal composite materials, lithium metal materials, and other commonly used negative electrode active materials in the art. The thickener may be sodium carboxymethyl cellulose (CMC-Na) and other conventional materials in the art. The conductive agent may be acetylene black and other conventional materials in the art. The binder may be styrene-butadiene rubber and other conventional materials in the art.

[0067] In some embodiments, the separator may be a separator known in the art that can be used in lithium-ion batteries and is stable to the electrolyte used, such as a polyethylene separator, a polypropylene separator, a polyethylene / polypropylene composite separator, etc.

[0068] The fourth aspect of this application discloses an electrical device comprising the monocrystalline lithium manganese iron phosphate described in the first aspect of this application, the monocrystalline lithium manganese iron phosphate prepared by the method described in the second aspect of this application, or the lithium-ion battery described in the third aspect of this application. Therefore, this electrical device has a long service life and good battery life.

[0069] It is understood that there are no particular restrictions on the specific type of electrical device; it can be any device that uses a lithium-ion battery as a power source or energy storage unit. For example, electrical devices include, but are not limited to, electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), mobile terminals (such as mobile phones, laptops, game consoles, wearable devices, etc.), drones, aerospace equipment, satellites, ships, energy storage systems, and so on.

[0070] It is understandable that, in addition to the lithium-ion battery mentioned above, the electrical device also includes necessary structures and components, all of which can be made with reference to conventional technologies. For example, an electric vehicle may include a body, chassis, tires, navigation system, radar system, steering system, braking system, lubrication system, cooling system, driving system, etc., which will not be described in detail here.

[0071] The embodiments of this application are described in detail below.

[0072] Example 1 Step S1: Weigh and mix lithium carbonate, manganese tetroxide, iron phosphate, titanium dioxide, ammonium metavanadate, magnesium carbonate, and lithium dihydrogen phosphate according to the molar ratio of Li, Mn, Fe, Ti, V, Mg, and P of 1.045:0.75:0.25:0.02:0.03:0.04:1.020. Glucose, the carbon source, accounts for 7.4% of the theoretical yield of lithium manganese iron phosphate, and polyethylene glycol accounts for 3.6%. Add them to deionized water as the solvent, controlling the solid content at 35 wt%. Grind using a ball mill (2000 rpm). 50 The machine is stopped when the particle size reaches 0.2 μm, and the first grinding material I is obtained; Step S2: The primary ground material I obtained in Step S1 is fed into the feed inlet of an atomizing disc dryer (inlet temperature set at 245±5℃, outlet temperature controlled at 100±5℃) with a solid content of 35% for spray drying to obtain primary spray-dried material. This dried material is heated to 650℃ at a heating rate of 2.2℃ / min under N2 atmosphere, held at that temperature for 8 hours, then cooled to 330℃ at a rate of 2.8℃ / min, and then naturally cooled to room temperature. After cooling in the furnace, it is then broken down and dissociated by an air jet mill. 50 The process was stopped when the particle size reached 0.8 μm, and the lithium manganese iron phosphate precursor (i.e., precursor 1) was obtained. Step S3: Weigh and mix lithium carbonate, iron phosphate, titanium oxide, ammonium metavanadate, and lithium dihydrogen phosphate according to the molar ratio of Li, Fe, Ti, V, and P of 1.049:1:0.04:0.02:1.025. Sucrose, the carbon source, accounts for 5.4% of the theoretical yield of lithium manganese iron phosphate, and polyethylene glycol accounts for 2.8%. Add them to deionized water as the solvent, controlling the solid content at 40 wt%. Grind the mixture using a ball mill (2000 rpm). 50 The milling process is stopped when the particle size reaches 0.4 μm, yielding primary milled material II. Step S4: The primary ground material II obtained in Step S3 is fed into the feed inlet of an atomizing disc dryer (inlet temperature set to 245±5℃, outlet temperature controlled at 100±5℃) with a solid content of 40% for spray drying to obtain primary spray-dried material. This dried material is heated to 800℃ at a heating rate of 2.6℃ / min under N2 atmosphere, held at that temperature for 8 hours, then cooled to 330℃ at a rate of 3.6℃ / min, and then allowed to cool naturally to room temperature. After cooling in the furnace, the material is then broken down and dissociated by an air jet mill. 50 The process was stopped when the thickness reached 1.2 μm, and the lithium iron phosphate precursor (i.e., precursor 2) was obtained. Step S5 (reheating): The lithium manganese iron phosphate precursor obtained in step S2, the lithium iron phosphate precursor obtained in step S4, and ammonium metavanadate are weighed and batched in a mass ratio of 0.54:0.46:0.004. The mixture is then dry-mixed using a high-speed mixer. The resulting raw material mixture is then heated to 800°C at a heating rate of 1°C / min under a N2 atmosphere and held at that temperature for 10 hours. It is then cooled to room temperature at a rate of 1.8°C / min and cooled in the furnace to obtain single-crystal lithium manganese iron phosphate.

[0073] Example 2-16 Same as Example 1, with specific differences shown in Table 1.

[0074] Comparative Example 1 The sintering temperature in step S5 of Example 1 was adjusted to 500°C.

[0075] Comparative Example 2 In step S5 of Example 1, only dry mixing is performed, and the high-temperature reheating process is omitted.

[0076] Comparative Example 3 Only steps S1 and S2 of Comparative Example 1 were performed. In step S1, lithium carbonate, manganese tetroxide, iron phosphate, titanium oxide, ammonium metavanadate, magnesium carbonate, and lithium dihydrogen phosphate were weighed and mixed according to the molar ratio of Li, Mn, Fe, Ti, V, Mg, and P of 1.045:0.60:0.40:0.02:0.03:0.04:1.020. Glucose, the carbon source, accounted for 7.4% of the theoretical yield of lithium manganese iron phosphate, and polyethylene glycol accounted for 3.6% of the theoretical yield. These were added to deionized water as the solvent, with the solid content controlled at 35 wt%. The mixture was then ground using a ball mill (at 2000 rpm). 50 The machine stops when it reaches 0.3μm.

[0077] Comparative Example 4 Step S5 is added to the process of Comparative Example 3, while the rest of the process remains the same as that of Comparative Example 3.

[0078] Table 1

[0079] Performance testing: 1. Particle size: Metis image analysis software was used to perform particle size statistical analysis on scanning electron microscope (SEM) images of single-crystal lithium manganese iron phosphate. The SEM image shooting conditions were set as follows: accelerating voltage 10 kV, magnification 30 K, and at least 500 single-crystal particles were counted. The specific detection steps are as follows: (1) After importing the SEM image, call the single-crystal particle dedicated analysis module and select the large particle intermittent fitting model for particle contour recognition; (2) No limit threshold is set for particle size detection interval, particle aspect ratio, or particle area ratio during the test process; (3) The particle size is uniformly calculated using caliper diameter, that is, the arithmetic mean of the major and minor diameters of a single particle; (4) The distribution is edited to select (0μm, 0.1μm], (0.1μm, 0.2μm], and so on; (5) The results of the current SEM image are statistically exported to Excel to obtain the distribution of the current SEM image. The average particle size, the maximum particle size, and the particle ratio of the above particle size intervals can be obtained.

[0080] 2. Malvern particle size test: The Malvern 3000 instrument was used to test the particle size; the upper and lower limits of the instrument's light shielding were 10%-20%; the stirring speed of the sample cell was 2000 rpm / min; the specific steps were as follows: (1) Weigh 0.03-0.04g of sample, add 1g of 10% NP40, (2) Add 9g of pure water (about 10ml in total), and sonicate externally for 3min (40kHz), (3) After dispersion, transfer all the liquid to the sample cell, click on the sample measurement, and perform particle size distribution test to obtain the particle size-volume curve and D. 50 Particle size.

[0081] 3. a / c ratio and cell volume: X-ray polycrystalline diffractometer was used: target source: Cu Kα, voltage 40 kV, current 40 mA; scanning mode: continuous step scan; scanning range 2θ: 10°~80° (covering all characteristic diffraction peaks of olivine); step size: 0.02°; dwell time per step: 0.3~0.5 s; anti-scattering slit and receiving slit were configured according to the instrument standard, and the baseline was stable and free of impurities.

[0082] The specific testing steps are as follows: (1) Powder drying: Single crystal lithium manganese iron phosphate powder is vacuum dried at 100 ℃ for 4 h to remove adsorbed moisture; (2) Grinding and refining: Grind thoroughly in an agate mortar for 5~10 min to eliminate preferred orientation of grains and reduce stress; (3) Mounting and flattening: Fill the glass sample cell with single crystal lithium manganese iron phosphate powder evenly, and press the glass slide lightly to ensure that the surface is flat and without protrusions, so as to avoid preferred orientation from interfering with diffraction intensity; (4) Import the original diffraction spectrum into the refinement software Jade 6.5, import the standard PDF cards: LiFePO4PDF#40-1499, LiMnPO4PDF#77-0178, export the refined lattice parameters a, b, c, calculate the a / c value, and calculate the unit cell volume of the orthorhombic crystal system according to V=a×b×c.

[0083] 4. Compacted Density: 1. Weigh 1g of single-crystal lithium manganese iron phosphate and add it to a compaction mold (13mm in diameter). Vibrate the mold to flatten the powder onto a smooth surface. 2. Place the treated mold in a Sansi compaction testing device and slowly apply pressure to the specified pressure of 225MPa. Hold for 30 seconds and then remove the mold to obtain the compacted density result. 5. Carbon content: The carbon-sulfur analyzer was used for detection. The specific steps are as follows: (1) The single-crystal lithium manganese iron phosphate powder was dried in a vacuum oven at 100 ℃ for 2 h, cooled to room temperature, and the adsorbed moisture was removed to avoid water vapor interfering with the infrared signal. (2) 0.2 g of dried single-crystal lithium manganese iron phosphate powder was accurately weighed and placed in a pretreated ceramic crucible; iron flux and tungsten flux were added in sequence to cover the sample. (3) The crucible was sent into a high-frequency induction furnace, high-purity oxygen was introduced, and high-frequency ignition was performed. The carbon in the sample was fully oxidized to CO2; the CO2 concentration was detected by the infrared absorption cell, and the instrument automatically collected the signal to obtain the carbon coating content.

[0084] 6. Specific surface area: Tested using a Micrometer 3030 instrument. The specific steps are as follows: (1) After weighing the empty tube, weigh 2g±0.01g of sample into a 1 / 4 ratio tube, place it in the degassing station and purge with nitrogen at 100℃ for two hours. After degassing, allow it to cool naturally to room temperature, weigh the total weight after degassing, and record it as the sample mass. (2) The degassed sample tubes were put on the instrument and tested using a Mc3030 surface area tester. The method was nitrogen adsorption and the relative pressure was from 0.05 to 0.2 at 10 points. After the test, the specific surface area of ​​the material was calculated using the multi-point BET equation.

[0085] 7. 0.1C discharge capacity and 45℃ cycling performance: Positive electrode sheet production: (1) The prepared single-crystal lithium manganese iron phosphate, conductive agent carbon black, and binder polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 90:5:5 and mixed with an appropriate amount of solvent N-methyl-2-pyrrolidone (NMP) to obtain a positive electrode slurry; (2) The obtained positive electrode slurry was coated onto an aluminum foil (20 μm thick) serving as the current collector and dried at 120 °C for 5 minutes. The coating weight of the positive electrode active material layer on one side was 80 mg / cm². 2 .

[0086] (3) Compress and shape the product using a roller press, controlling the compaction to 2.0 g / cm³. 3 The rolled electrode sheets are punched and weighed, and then punched into a disc shape with a diameter of 14mm to ensure that the edges of the electrode sheets are free of gaps and wrinkles, thus obtaining the positive electrode sheet.

[0087] Button battery making: In a glove box under an argon atmosphere, the obtained positive electrode, separator, negative electrode, and electrolyte were assembled into a coin cell. The negative electrode used a 15mm diameter lithium metal sheet; the separator used a 23μm thick polypropylene membrane; and the electrolyte used was a 1mol / L mixture of equal parts LiPF6, ethylene carbonate (EC), and diethyl carbonate (DEC).

[0088] The batteries in the aforementioned embodiments and comparative examples were tested as follows: Capacity retention: High-temperature testing was conducted at 45°C in a high-temperature chamber, with the charge / discharge voltage range set to 2.5V-4.35V; the charging step was set to CC-CV with a cutoff voltage of 4.35V, and the discharging step was set to CV discharge with a cutoff current of 0.05C; the nominal capacity at 1C was 140mAh / g; the discharge capacity at the first cycle was recorded as the 0.1C discharge capacity, and 80 charge / discharge cycles were performed. The capacity retention rate of the battery after 80 cycles was calculated and used as an evaluation of cycle durability.

[0089] Table 2

[0090] Table 3

[0091] In the description of this invention, it should be understood that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0092] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0093] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A single-crystal lithium manganese iron phosphate, characterized in that, The average particle size of the monocrystalline lithium manganese iron phosphate is 0.1 μm to 0.4 μm, and the maximum particle size is 0.4 μm to 0.7 μm. In terms of particle count, particles with a size of (0μm, 0.1μm) account for 20% to 70%, and particles with a size of (0.1μm, 0.2μm) account for 30% to 80%.

2. The single-crystal lithium manganese iron phosphate according to claim 1, characterized in that, At least one of the following conditions must be met: Particles with a size between 0 μm and 0.1 μm account for 25% to 60% of the total. Particles with a size in the range of (0.1μm, 0.2μm) account for 32% to 70%.

3. The single-crystal lithium manganese iron phosphate according to claim 1, characterized in that, The particle size distribution curve of the single-crystal lithium manganese iron phosphate has a first characteristic peak, a second characteristic peak, and a third characteristic peak with successively increasing peak positions. The peak position of the first characteristic peak is less than or equal to 1 μm, the peak position of the second characteristic peak is greater than 1 μm, and the peak position of the third characteristic peak is greater than or equal to 10 μm.

4. The single-crystal lithium manganese iron phosphate according to claim 3, characterized in that, At least one of the following conditions must be met: The difference between the peak position of the second characteristic peak and the peak position of the first characteristic peak is 0.5 μm to 3 μm; The difference between the peak position of the third characteristic peak and the peak position of the first characteristic peak is 10μm~15μm; The ratio of the peak value corresponding to the first characteristic peak to the peak value corresponding to the second characteristic peak is 1 to 2.5; The ratio of the peak value corresponding to the first characteristic peak to the peak value corresponding to the third characteristic peak is 2 to 4.

5. The single-crystal lithium manganese iron phosphate according to claim 1, characterized in that, The D of the monocrystalline lithium manganese iron phosphate 50 The particle size is 0.3μm to 2.0μm, preferably 0.5μm to 1.5μm.

6. The single-crystal lithium manganese iron phosphate according to claim 1, characterized in that, The lattice constant a / c of ​​the single-crystal lithium manganese iron phosphate is 2.2010~2.2030; preferably 2.2015~2.2025.

7. The single-crystal lithium manganese iron phosphate according to claim 1, characterized in that, At least one of the following conditions must be met: The cell volume of the single-crystal lithium manganese iron phosphate is 290 Å. 3 ~298Å 3 Preferably 294Å 3 ~297 Å 3 ; The compaction density of the monocrystalline lithium manganese iron phosphate is 2.4 g / cc to 2.6 g / cc, preferably 2.45 g / cc to 2.58 g / cc.

8. The single-crystal lithium manganese iron phosphate according to claim 1, characterized in that, It includes a lithium manganese iron phosphate core and a carbon coating layer covering the surface of the lithium manganese iron phosphate core, and satisfies at least one of the following conditions: (1) Based on the total weight of the monocrystalline lithium manganese iron phosphate, the content of the carbon coating layer is 1wt% to 2.5wt%, preferably 1.5wt% to 2.0wt%; (2) The lithium manganese iron phosphate core has the chemical composition shown in Formula 1: Li a Mn b Fe (1-b) M c (PO4) d General Formula 1 In general formula 1, 0.95≤a≤1.10, 0.3≤b≤0.7, 0≤c≤0.5, 0.95≤d≤1.10, and M includes at least one of Ti, Mg, Zn, Cu, Sr, Al, Zr, Y, Co, W, Ca, Nb, Sn, Sb, Mo, V, B, Si, and Na.

9. The single-crystal lithium manganese iron phosphate according to claim 1, characterized in that, The discharge curve of the lithium metal coin cell using the monocrystalline lithium manganese iron phosphate as the positive electrode active material has a single discharge plateau.

10. A method for preparing single-crystal lithium manganese iron phosphate according to any one of claims 1 to 9, characterized in that, include: The lithium manganese iron phosphate precursor and the lithium iron phosphate precursor are mixed to obtain a raw material mixture; The raw material mixture is heated to T2 at a heating rate not exceeding 2℃ / min and held at that temperature for 2h~10h, where T2 is 600℃~850℃. Then, it is cooled to T2-80℃ at a cooling rate not exceeding 2℃ / min, and then further cooled to room temperature to obtain the single crystal lithium manganese iron phosphate. The average particle size of the lithium manganese iron phosphate precursor is 0.05 μm to 0.2 μm; The average particle size of the lithium iron phosphate precursor is 0.2 μm to 0.4 μm.

11. The method according to claim 10, characterized in that, At least one of the following conditions must be met: (a) The lithium manganese iron phosphate precursor has the chemical composition shown in general formula 2: Li a2 Mn b2 Fe (1-b2) M c2 (PO4) d2 General Formula 2 (b) The lithium iron phosphate precursor has the chemical composition shown in general formula 3: Li a3 FeM c3 (PO4) d3 Formula 3 Wherein, 0.95≤a2≤1.10, 0.75≤b2<0.95, 0≤c2≤0.5, 0.95≤d2≤1.10, 0.95≤a3≤1.10, 0≤c3≤0.5, 0.95≤d3≤1.10, and M includes at least one of Ti, Mg, Zn, Cu, Sr, Al, Zr, Y, Co, W, Ca, Nb, Sn, Sb, Mo, V, B, Si, and Na.

12. The method according to claim 10, characterized in that, The mass of the lithium manganese iron phosphate precursor accounts for 30% to 90% of the total mass of the raw material mixture.

13. The method according to claim 10, characterized in that, The raw material mixture also includes additives, which include at least one of ammonium metavanadate and magnesium carbonate.

14. A lithium-ion battery, characterized in that, Includes monocrystalline lithium manganese iron phosphate as described in any one of claims 1 to 9.

15. An electrical appliance, characterized in that, Includes the monocrystalline lithium manganese iron phosphate according to any one of claims 1 to 9 or the lithium-ion battery according to claim 14.

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