A lithium manganese iron phosphate cathode material, its preparation method, electrode sheet, and battery.

Abstract

This invention provides a lithium manganese iron phosphate cathode material, its preparation method, electrode sheet, and battery. The preparation method includes the following steps: instantaneously sintering n lithium manganese iron phosphate precursor powder layers to obtain the lithium manganese iron phosphate cathode material; wherein, among the n lithium manganese iron phosphate precursor powder layers, m lithium manganese iron phosphate precursor powder layers have different thicknesses, n≥2, m≥2, n≥m; the instantaneous sintering conditions for each lithium manganese iron phosphate precursor powder layer are the same; the lithium manganese iron phosphate precursor includes a phosphorus source, a manganese source, an iron source, and a lithium source. This method can simplify the process flow while taking into account the compaction density and cycle performance of the material.

Description

Technical Field

[0001] This invention relates to the field of energy storage technology, and in particular to a lithium manganese iron phosphate cathode material, its preparation method, electrode sheet, and battery. Background Technology

[0002] With the rapid development of new energy vehicles and the energy storage industry, the market has placed higher demands on the energy density of lithium-ion batteries. While lithium iron phosphate (LFP) materials have advantages such as low cost, stable structure, and good safety, their relatively low voltage platform (3.4V) makes them difficult to meet the needs of high-energy-density batteries. Meanwhile, lithium manganese iron phosphate (LFP) materials, with their Mn... 2+ / Mn 3+ With a reaction potential as high as 4.1V, it can increase the overall energy density of the battery by 21% while maintaining the high safety of the olivine structure, thus becoming the next generation of cathode material.

[0003] However, lithium manganese phosphate (LMP) has a wide intrinsic electronic bandgap (approximately 2 eV), classifying it as an insulator, and its electronic and ionic conductivity are significantly lower than that of lithium iron phosphate (LFP). To overcome this kinetic limitation and fully utilize its high-voltage platform capacity, the primary particle size of LMP materials is typically controlled at the nanometer level in actual production to shorten the lithium-ion diffusion path. However, a large number of nanoparticles inevitably leads to an increased specific surface area, higher interparticle porosity, and lower compaction density. This severely restricts the improvement of battery volumetric energy density, preventing the high-voltage advantage of LMP from being fully realized in practical applications. Currently, to improve the compaction density of LMP, existing technologies often employ physical gradation methods, such as preparing particles of different sizes and then mechanically mixing them, or performing multiple granulation and secondary sintering. While these methods can improve compaction density to some extent, they often degrade the cycle performance of LMP materials.

[0004] Therefore, how to balance the compaction density and cycle performance of lithium manganese iron phosphate cathode materials is an important problem that urgently needs to be solved in this field. Summary of the Invention

[0005] This invention provides a method for preparing lithium manganese iron phosphate cathode material, which simplifies the process while taking into account the material's compaction density and cycle performance.

[0006] This invention provides a lithium manganese iron phosphate cathode material that improves compaction density while maintaining excellent cycle life.

[0007] This invention provides an electrode that improves compaction density while maintaining excellent cycle life.

[0008] This invention provides a battery that improves compaction density while maintaining excellent cycle life.

[0009] In a first aspect, the present invention provides a method for preparing lithium manganese iron phosphate cathode material, comprising the following steps:

[0010] The lithium manganese iron phosphate precursor powder layers were subjected to instantaneous sintering treatment to obtain the lithium manganese iron phosphate cathode material.

[0011] Among the n lithium manganese iron phosphate precursor powder layers, the thickness of m lithium manganese iron phosphate precursor powder layers is different, n≥2, m≥2, n≥m; the sintering conditions of the instantaneous sintering treatment of each lithium manganese iron phosphate precursor powder layer are the same; the lithium manganese iron phosphate precursor includes phosphorus source, manganese source, iron source and lithium source.

[0012] In one possible implementation, as described above, prior to the instantaneous sintering process, the method includes: depositing a lithium manganese iron phosphate precursor onto the surface of a heated substrate to form n independent lithium manganese iron phosphate precursor powder layers.

[0013] In one possible implementation, as described above, in the order of increasing thickness, the ratio of the thickness of the previous lithium manganese iron phosphate precursor powder layer to the thickness of the next lithium manganese iron phosphate precursor powder layer in the m layers is 1:(1.3-3.0).

[0014] In one possible implementation, as described above, the thickness of the preceding lithium manganese iron phosphate precursor powder layer is 0.5-1 cm, and the thickness of the subsequent lithium manganese iron phosphate precursor powder layer is 1-2 cm.

[0015] In one possible implementation, as described above, n=m=2, and m of the lithium manganese iron phosphate precursor powder layers include a first lithium manganese iron phosphate precursor powder layer and a second lithium manganese iron phosphate precursor powder layer; wherein, the thickness of the first lithium manganese iron phosphate precursor powder layer is less than the thickness of the second lithium manganese iron phosphate precursor powder layer; and the mass ratio of the first lithium manganese iron phosphate precursor powder layer to the second lithium manganese iron phosphate precursor powder layer is (3:7)-(7:3).

[0016] In one possible implementation, as described above, the area ratio of the first lithium manganese iron phosphate precursor powder layer to the second lithium manganese iron phosphate precursor powder layer is 1:(0.3-0.5).

[0017] In one possible implementation, as described above, the instantaneous sintering process includes: heating the n lithium manganese iron phosphate precursor powder layers to 700-1000°C within 0.1-5s and maintaining the temperature for 1-60s.

[0018] In a second aspect, the present invention provides a lithium manganese iron phosphate cathode material, which is prepared by the method described in any one of the first aspects of the present invention.

[0019] Thirdly, the present invention provides an electrode sheet comprising the lithium manganese iron phosphate cathode material described in the second aspect of the present invention.

[0020] Fourthly, the present invention provides a battery comprising the electrode sheet described in the third aspect of the present invention.

[0021] This invention provides a method for preparing lithium manganese iron phosphate (LFP) cathode materials. By instantaneously sintering n LFP precursor powder layers, with m of these layers having different thicknesses and all layers subjected to the same sintering conditions, a microscopic temperature gradient is instantaneously constructed within a single reaction system by utilizing the thermal conductivity attenuation effect caused by the thickness difference under high-energy thermal shock. The thinner precursor powder layers undergo high-temperature in-situ ripening, directionally growing into micron-sized large particles; while the thicker precursor powder layers undergo explosive nucleation induced by relatively low temperatures, generating nano-sized small particles. This strategy requires only one ultra-fast sintering to achieve in-situ, absolutely uniform particle topology and self-assembly interstitial filling. This invention's preparation method is simple and easy to implement, improving the compaction density of LFP from a new dimension of thermodynamic spatial distribution, and this method does not reduce the cycle life of the LFP cathode material. Attached Figure Description

[0022] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0023] Figure 1 XRD pattern of lithium manganese iron phosphate cathode material provided in Example 1 of this invention;

[0024] Figure 2 SEM image of lithium manganese iron phosphate cathode material provided in Example 1 of this invention;

[0025] Figure 3 SEM image of lithium iron phosphate cathode material (Comparative Example 1) provided by this invention;

[0026] Figure 4 SEM image of the comparative example 2 lithium manganese iron phosphate cathode material provided by the present invention.

[0027] The accompanying drawings have illustrated specific embodiments of the invention, which will be described in more detail below. These drawings and descriptions are not intended to limit the scope of the invention in any way, but rather to illustrate the concept of the invention to those skilled in the art through reference to particular embodiments. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0029] The field of lithium-ion battery cathode material preparation has long revolved around the particle size distribution, surface modification, and sintering process of lithium manganese iron phosphate (LFP). Related applications are mainly concentrated in the production of cathode powders and electrode fabrication for power batteries, energy storage batteries, and other high-energy-density batteries. Since the cathode material directly determines the battery's volumetric energy density, rate performance, and cycle stability, balancing particle packing density and material cycle life during the preparation stage has become a key issue to be addressed in the industrialization of LFP. In existing technologies, to improve the compaction density of LFP, optimization methods such as subsequent mixing of nanoparticles and microparticles, surface coating, or multi-step sintering are commonly used. In actual production, these approaches often rely on multiple heat treatments or subsequent mechanical mixing to combine particles of different sizes, resulting in a long process chain and high requirements for sintering conditions, mixing uniformity, and powder morphology control.

[0030] In the prior art, since nanoparticles and microparticles usually come from different sources or are prepared in different processes, uneven interface distribution, local agglomeration and component segregation are likely to occur between particles, resulting in fluctuations in the Mn / Fe ratio in local areas, which in turn leads to increased electrochemical polarization and poor voltage plateau consistency, resulting in a decrease in cycle life.

[0031] Furthermore, the gradation structure formed solely through post-mixing is significantly limited by the mixing precision in terms of particle contact and gap-filling effect, making it difficult to consistently achieve high compaction density. Moreover, traditional multi-step sintering processes not only consume more energy and have longer production cycles, but repeated heating can also exacerbate particle growth or structural inhomogeneity, further weakening rate performance and cycle stability.

[0032] Therefore, existing technologies still present a significant contradiction between simplifying the process, improving compaction density, and ensuring elemental uniformity. In view of this, how to balance the compaction density and cycle performance of lithium manganese iron phosphate cathode materials while simplifying the preparation process is a crucial problem that urgently needs to be solved in this field.

[0033] In a first aspect, the present invention provides a method for preparing lithium manganese iron phosphate cathode material, comprising the following steps: performing instantaneous sintering treatment on n lithium manganese iron phosphate precursor powder layers to obtain lithium manganese iron phosphate cathode material; wherein, among the n lithium manganese iron phosphate precursor powder layers, the thickness of m lithium manganese iron phosphate precursor powder layers is different, n≥2, m≥2, n≥m; the sintering conditions of the instantaneous sintering treatment of each lithium manganese iron phosphate precursor powder layer are the same; the lithium manganese iron phosphate precursor includes a phosphorus source, a manganese source, an iron source, and a lithium source.

[0034] Among them, lithium manganese iron phosphate precursor powder layer is a layered structure with continuous geometric boundaries formed by spreading the precursor powder to a certain thickness. Its key characteristics include: a defined thickness (usually on the order of millimeters to centimeters) and an interior of porous, non-densely packed particles. This structure makes thickness a core parameter determining temperature distribution, thus affecting the subsequent reaction pathway.

[0035] n represents the total number of lithium manganese iron phosphate precursor powder layers; m represents the number of powder layers with deliberately different thicknesses, which is a subset of n, thus satisfying n≥m≥2. n≥2 means that there are at least two powder layers; m≥2 means that at least two powder layers have different thicknesses to introduce an effective temperature field difference; n≥m means that not all powder layers must have different thicknesses, and there can be combinations of some powder layers with the same thickness and some with different thicknesses.

[0036] Instantaneous sintering refers to the process of heating (e.g., heating to 700-1000℃), reaction, and crystallization within an extremely short time (e.g., 0.1-5s). The heating rate is extremely high, the total processing time is on the order of seconds, and the system is in a strongly non-equilibrium state.

[0037] Sintering conditions can include heating rate, target temperature, holding time, cooling method and rate, heating method, atmosphere, and pressure. Furthermore, identical sintering conditions emphasize that all powder layers must have completely identical parameters, meaning they are placed simultaneously in the same equipment and subjected to the same heating program (identical temperature profile, time, and atmosphere), without any individual adjustment of the temperature or time for any particular powder layer.

[0038] The core of this invention lies in introducing a differentiation between the spatial temperature field and reaction kinetics through the introduction of thickness difference and instantaneous sintering—a highly non-equilibrium process. From the perspective of heat transfer mechanism, the thickness of the powder layer directly determines the heat diffusion path and thermal resistance. Thinner layers have a shorter heat diffusion distance and a significantly reduced thermal conductivity time constant, thus allowing for a faster attainment of a quasi-isothermal state close to the sintering condition temperature. In contrast, thicker layers experience limited heat transfer in the vertical direction, resulting in significant temperature lag and gradient decay in the internal region—that is, a higher surface temperature and a significantly lower internal temperature. This difference is gradually smoothed out during conventional long-duration sintering, but forms a stable spatial temperature distribution difference on a timescale of seconds.

[0039] This temperature difference directly alters the competitive relationship between nucleation and growth. According to classical nucleation theory, the nucleation rate is closely related to supersaturation and temperature, while grain growth depends on atomic diffusion capacity. In thin-layer regions, due to the higher actual temperature, the diffusion coefficient increases significantly, and the system tends towards a low nucleation density and high growth rate path—that is, a small number of nuclei grow rapidly to form micron-sized particles. Conversely, in thick-layer regions, due to the lower effective temperature but still within the reaction activation range, the system exhibits characteristics of high supersaturation and restricted diffusion, resulting in a nucleation rate much higher than the growth rate, forming a large number of nanoscale nuclei while maintaining a small size.

[0040] Based on this, n independent powder layers are formed, allowing the thickness, area, and mass of each powder layer to be set individually. Each powder layer has a gradient along the thickness direction but does not experience significant thermal coupling, thus ensuring that the temperature field of the thick and thin layers is always controllable, thereby improving the gradation effect.

[0041] Furthermore, since all powder layers originate from the same precursor system and complete the reaction within the same time window, the element diffusion paths are consistent, and there is no compositional segregation problem caused by macroscopic batch or multi-step mixing. Therefore, what is obtained is a particle group with completely consistent composition and different size distribution, that is, a true homologous gradation.

[0042] In terms of compaction density, this in-situ formed bimodal or multimodal particle size distribution conforms to classical particle packing theory. Large particles form the skeletal structure, while small particles fill the pores between them, which can significantly reduce porosity and increase packing density and compaction density. Since the particles are generated and contacted in situ during sintering, the interface bonding is tighter, reducing the loose contact and secondary porosity commonly found in mechanical mixing.

[0043] The improvement in cycle life is the result of multiple mechanisms. Micron-sized particles reduce the specific surface area, thereby reducing electrolyte side reactions and interfacial film growth, and improving interfacial stability; nano-sized particles shorten the lithium-ion diffusion path, improving rate performance and reaction uniformity. Since the two types of particles are completely identical in composition and crystal structure, there is no electrochemical mismatch problem caused by heterogeneous interfaces, thus avoiding interfacial failure and manganese dissolution during cycling.

[0044] In one possible embodiment, the lithium manganese iron phosphate precursor includes a phosphorus source, a manganese source, an iron source, a lithium source, and a carbon source. For example, the lithium source may include lithium carbonate or lithium hydroxide; the phosphorus source may include ammonium dihydrogen phosphate or iron phosphate; the iron source may include ferrous oxalate; and the manganese source may include manganese trioxide, manganese oxide, or manganese carbonate. Further, a carbon source may be included to improve conductivity, such as glucose, sucrose, or starch. For example, in the lithium manganese iron phosphate precursor, the molar ratio of lithium, manganese, iron, phosphorus, and oxygen is 1:x:y:1:4, 0.3≤x≤0.7, and y=1-x; the lithium manganese iron phosphate precursor also includes 10-15 wt.% carbon.

[0045] In one possible embodiment, the precursor can be a wet precursor obtained by ball milling or sand milling, or it can be a composite particle obtained by spray granulation, or a powder-spreadable material formed by dry mixing followed by granulation. For example, a raw material system including phosphorus source, manganese source, iron source, and lithium source can be ball-milled at a speed of 100-3000 r / min for 0.5-24 h to obtain a lithium manganese iron phosphate precursor.

[0046] In one possible embodiment, the lithium manganese iron phosphate precursor powder layer is preferably a loose powder layer, a compacted powder layer, or a semi-compacted powder bed. The loose powder layer is suitable for rapid laying and forming larger pore spaces, the compacted powder layer is suitable for improving initial packing stability, and the semi-compacted powder bed balances laying efficiency and morphology retention. During operation, multiple independent powder layers can be formed using methods such as scraper leveling, die positioning, zoned powder laying, or programmed powder distribution, with each layer maintaining a different thickness during laying. It should be understood that the above examples are merely illustrative and not limiting.

[0047] In the preparation of lithium manganese iron phosphate cathode materials, one or more mixed inert gases, such as argon, helium, or nitrogen, can be used for protection to improve stability.

[0048] In one possible embodiment, as described above, prior to the instantaneous sintering process, the method includes: depositing a lithium manganese iron phosphate precursor onto the surface of a heated substrate to form n independent lithium manganese iron phosphate precursor powder layers.

[0049] In this embodiment, while maintaining the same sintering conditions for instantaneous sintering, to further control the temperature field and particle size differentiation of the lithium manganese iron phosphate precursor powder layer, the precursor is directly laid on the surface of the heated substrate, thereby constructing a unilateral forced heat input system. Heat is primarily conducted unidirectionally from the substrate to the powder layer, rather than relying on atmospheric convection or radiation for uniform heating as in traditional furnace sintering. This bottom heat source structure has two key characteristics: first, the heat flux is maximum and the temperature response is fastest at the interface; second, a stable temperature decay gradient is formed along the thickness direction. It is this clear heat transfer directionality that allows the mechanism by which subsequent thickness differences cause temperature differences, and consequently affect particle size differences, to be amplified and stably reproduced.

[0050] For example, the heating substrate can be made of flexible carbon cloth, graphite paper, stainless steel foil, nickel foil, molybdenum foil, titanium foil, conductive ceramics, or other flexible conductive materials with high conductivity and high electrothermal conversion efficiency.

[0051] In one possible embodiment, as described above, in the order of increasing thickness, the ratio of the thickness of the previous lithium manganese iron phosphate precursor powder layer to the thickness of the next lithium manganese iron phosphate precursor powder layer is 1:(1.3-3.0).

[0052] In this embodiment, since the characteristic heating time of the powder layer is approximately proportional to the square of its thickness, the thickness ratio is controlled at 1:(1.3-3.0), corresponding to a separation of approximately 3-6 times the thermal response time. This means that within the same heating window, the thinner layer can essentially complete rapid heating and high-temperature residence, while the thicker layer remains in the low-temperature range of delayed heating. This temperature ensures that the thinner layer is in the high-temperature growth region, and the thicker layer is in the medium-temperature high-nucleation region, with clearly separated kinetic paths for both, but both within the effective reaction range.

[0053] In one possible embodiment, as described above, the thickness of the first lithium manganese iron phosphate precursor powder layer is 0.5-1 cm, and the thickness of the second lithium manganese iron phosphate precursor powder layer is 1-2 cm.

[0054] In this embodiment, by further limiting the thickness of the lithium manganese iron phosphate precursor powder layer, the generation of zoned particles can be matched more efficiently. A powder layer thickness of 0.5-1 cm means that heat can penetrate most of the powder layer in a short time, bringing the overall temperature close to the substrate temperature. Most of the powder layer can enter the high-temperature reaction zone, tending towards a low nucleation density and rapid grain growth path, forming micron-sized particles. In contrast, a powder layer thickness of 1-2 cm results in heat being transferred rapidly only to the surface layer. The internal region remains in a state of temperature lag throughout the entire sintering cycle, but is still in the reaction activation zone. At this time, it is easier to form a high supersaturation environment, promoting the generation of a large number of crystal nuclei. However, due to limited diffusion, the crystal nuclei are difficult to grow and ultimately remain at the nanometer or submicron scale.

[0055] In one possible embodiment, as described above, n=m=2, and the m lithium manganese iron phosphate precursor powder layers include a first lithium manganese iron phosphate precursor powder layer and a second lithium manganese iron phosphate precursor powder layer; wherein, the thickness of the first lithium manganese iron phosphate precursor powder layer is less than the thickness of the second lithium manganese iron phosphate precursor powder layer; the mass ratio of the first lithium manganese iron phosphate precursor powder layer to the second lithium manganese iron phosphate precursor powder layer is (3:7)-(7:3).

[0056] In this embodiment, the thickness relationship between the thin first layer and the thick second layer determines the generation path of the two types of particles: the first layer corresponds to micron-sized framework particles, and the second layer corresponds to nano / submicron-sized interstitial particles. Based on this, the introduction of a mass ratio constraint essentially controls the volume fraction or number fraction of the two types of particles, thereby directly controlling the shape and peak ratio of the final particle size distribution function (PSD). Since both layers use the same precursor and are sintered simultaneously, the mass distribution is almost equivalent to the proportional distribution of different particle size groups in the final product. This is a more direct and less loss-inducing control method than later mixing.

[0057] When the mass ratio is close to (3:7), the system is dominated by small particles, with large particles forming a sparse framework. This provides sufficient interstitial filling but weak framework support, making it suitable for applications requiring high reactivity or high rate performance. When the mass ratio is close to (7:3), large particles dominate, resulting in a more compact framework structure. Small particles only fill local gaps, which helps reduce specific surface area and side reactions. In the intermediate range, a better balance between packing density and structural stability can usually be achieved. Therefore, this range is not a single optimal point but rather provides an adjustable window, allowing the gradation structure to be engineered and optimized according to target performance (compaction density, rate, and cycle life).

[0058] In one possible embodiment, as described above, the area ratio of the first lithium manganese iron phosphate precursor powder layer to the second lithium manganese iron phosphate precursor powder layer is 1:(0.3-0.5).

[0059] In this embodiment, since adjusting the thickness alters the heat transfer and nucleation mechanisms, the mass ratio can be more precisely adjusted by regulating the area. Simultaneously, the larger particles generated in the first powder layer (with a larger area) are more spatially continuous, forming a uniform skeletal source; while the smaller particles generated in the second powder layer (with a smaller area) are more controlled in number, resembling interstitial components. During subsequent collection and mixing, this source distribution helps to form a uniform gradation, rather than excessive enrichment of small or large particles in localized areas.

[0060] In one possible embodiment, as described above, the instantaneous sintering process includes: heating n layers of lithium manganese iron phosphate precursor powder to 700-1000°C within 0.1-5s and maintaining the temperature for 1-60s.

[0061] In this embodiment, the rapid heating of 0.1-5s means that the heating rate of the system is much higher than that of conventional sintering. At this time, the powder has not yet had time to achieve temperature homogenization through internal conduction, and the thermal field is dominated by boundary conditions rather than bulk diffusion. As a result, the thin layer rapidly enters the high-temperature region as a whole, while the thick layer only heats up the surface and the interior lags significantly. Due to the extremely short time scale, this temperature difference will not be smoothed out, directly pushing different powder layers into different reaction paths. Setting the target temperature within 700-1000℃, combined with instantaneous heating, allows different powder layers to correspond to different mechanisms under the same external conditions: diffusion dominates in the thin layer, and nucleation dominates in the thick layer. The holding time of 1-60s is used to complete two key processes: first, the phase transformation and ordering of the precursor to the target crystalline phase; and second, the controlled subsequent evolution of the generated nuclei.

[0062] In a second aspect, the present invention provides a lithium manganese iron phosphate cathode material, which is prepared by any of the methods described in the first aspect of the present invention.

[0063] This invention yields a particle system with a homogeneous multi-peak particle size distribution: micron-sized large particles and nano / submicron-sized small particles are completely identical in composition, crystal structure, and formation history, differing only in size. This determines two key results. First, in terms of packing behavior, large particles form the supporting framework, while small particles fill the pores of the framework, forming a near-optimal interstitial particle gradation, significantly reducing initial porosity and increasing packing density and subsequent compaction potential. Second, in terms of interfacial chemistry, due to the absence of surface differences caused by heterogeneous particles or multiple batches, the interparticle contact and subsequent electrolyte interface (CEI) are consistent, avoiding localized electrochemical inhomogeneities.

[0064] From an electrochemical mechanism perspective, large particles reduce specific surface area, suppressing electrolyte side reactions and excessive interfacial film growth; small particles shorten lithium-ion diffusion paths, improving reaction kinetics and current distribution uniformity. The coupling of these two factors ensures the low specific surface area required for high-pressure tamping without sacrificing rate capability and reaction uniformity, providing a foundation for long-term high-pressure tamping cycling from the material's bulk. Specifically, the particle size of large particles can be 1-3 μm, and the particle size of small particles can be 200-400 nm.

[0065] Thirdly, the present invention provides an electrode, comprising the lithium manganese iron phosphate cathode material of the second aspect of the present invention.

[0066] The electrode sheet of this invention specifically includes a positive current collector and a positive active layer formed by the lithium manganese iron phosphate positive electrode material of the second aspect of this invention, disposed on the surface of the positive current collector. In this invention, small particles can fill the gaps between large particles, reducing the effective void volume fraction in the slurry, improving processability under solid content, and forming a more compact particle contact network after drying. During the compaction stage, large particles form a continuous force chain skeleton, bearing the main compressive stress; small particles rearrange and fill the gaps between the skeleton, reducing the residue of closed pores and large pores, causing the pore size distribution to converge towards a smaller and narrower range, thereby obtaining a higher compaction density under the same compaction pressure.

[0067] Meanwhile, the dual-scale structure responds more gently to volume changes. The volume changes of large particles are relatively dispersed, while the small particles fill the gaps and act as a buffer and stress release agent, reducing the probability of local stress concentration and microcrack initiation. At the same time, since the particles are homologous, the element distribution is uniform and the interface properties are consistent, problems such as manganese dissolution and weak interface debonding are less likely to occur after compaction, resulting in a more stable electrode structure and better cycle life.

[0068] In one possible embodiment, the electrode sheet described above can be prepared by the following steps: dispersing lithium manganese iron phosphate cathode material and binder in an appropriate amount of N-methylpyrrolidone (NMP) solvent, and may also add a conductive agent, and then thoroughly stirring and mixing to form a uniform cathode slurry; uniformly coating the cathode slurry onto the cathode current collector, and then drying, rolling and slitting to obtain the cathode sheet.

[0069] In one specific embodiment, the positive electrode active layer comprises, by weight percentage, 70-99 wt% of positive electrode active material, 0.5-15 wt% of conductive agent, and 0.5-15 wt% of binder; more specifically, it comprises 80-98 wt% of positive electrode active material, 1-10 wt% of conductive agent, and 1-10 wt% of binder.

[0070] The positive current collector can be made of at least one of aluminum foil or nickel foil; the conductive agent can be selected from at least one of carbon black, acetylene black, graphene, Ketjen black, and carbon fiber; and the binder can be selected from at least one of polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, ethylene oxide-containing polymers, polyvinylpyrrolidone, and polyurethane.

[0071] Fourthly, the present invention provides a battery comprising the electrode sheet of the third aspect of the present invention.

[0072] High compaction density directly improves the volumetric energy density of a battery, a definite benefit resulting from increased active material content per unit volume. Traditional methods of increasing compaction often come at the cost of reduced rate capability or cycle life. This structure, however, combines large particles with low specific surface area with small particles with high kinetics. On one hand, the large particles reduce electrolyte consumption and side reactions, mitigating CEI thickening and lithium inventory loss. On the other hand, the small particles ensure uniform reaction within the electrode, reducing concentration polarization and preventing structural degradation caused by localized overcharging / overdischarging.

[0073] In addition, the uniform composition generated simultaneously reduces the electrochemical inhomogeneity inside the electrode, which helps to maintain a uniform current distribution under high load or high rate conditions, reduce hot spots and local aging, and further extend cycle life at the system level.

[0074] In addition to the aforementioned positive electrode, the battery of the present invention also includes a negative electrode, an electrolyte, and a separator. The present invention does not strictly limit the negative electrode active material in the negative electrode; it can be at least one of the negative electrode active materials commonly used in batteries, such as graphite, hard carbon, soft carbon, mesophase carbon microspheres, silicon-based negative electrode materials (mainly including silicon suboxide and silicon-carbon negative electrodes), and tin-based negative electrode materials (mainly including tin and tin alloys).

[0075] This invention does not strictly limit the choice of electrolyte, and may include one or more solvents commonly used in lithium-ion battery electrolytes, as well as lithium salts commonly used in lithium-ion electrolytes. For example, the solvent may be ethylene carbonate, propylene carbonate, butene carbonate, fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), diethyl carbonate (DEC), difluoroethylene carbonate (DFEC), dipropyl carbonate, methyl ethyl carbonate (EMC), ethyl acetate, ethyl propionate, propyl acetate, propyl propionate, sulfolane, γ-butyrolactone, etc.; the lithium salt may be one or more of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0076] This invention does not strictly limit the choice of separator material. It can be one of the separator materials commonly used in lithium-ion batteries, such as polypropylene separator (PP), polyethylene separator (PE), polypropylene / polyethylene double-layer composite membrane (PP / PE), polyimide electrospun separator (PI), polypropylene / polyethylene / polypropylene triple-layer composite membrane (PP / PE / PP), cellulose nonwoven separator, and separator with ceramic coating.

[0077] In one possible embodiment, the battery described above is prepared by the following steps: a bare cell is obtained by winding or stacking a positive electrode, a separator, and a negative electrode; the bare cell is then packaged into a pre-stamped aluminum-plastic film bag. After the packaged battery is dried at 85°C, electrolyte is injected into the dried battery. The battery undergoes resting, formation, and secondary sealing to complete the preparation of the lithium-ion battery.

[0078] The present invention will be described below through specific embodiments.

[0079] Example 1

[0080] The preparation method of the lithium iron phosphate cathode material in this embodiment includes the following steps:

[0081] 1) Weigh 0.23g of lithium source (lithium carbonate), 1.03g of manganese source (manganese carbonate), 0.86g of iron source (iron phosphate), 0.89g of phosphorus source (lithium dihydrogen phosphate), and 0.45g of carbon source (glucose) accounting for 15% of the total mass. Add the above mixture to 6ml of deionized water, place it in a ball mill, and ball mill at 1000 r / min for 2h. After ball milling, dry at 100℃ for 12h to obtain the lithium manganese iron phosphate precursor.

[0082] 2) The lithium manganese iron phosphate precursor is laid on the surface of a heated substrate (carbon cloth) to form two independent lithium manganese iron phosphate precursor powder layers.

[0083] One lithium manganese iron phosphate precursor has a powder layer thickness of 0.5 cm, and the other lithium manganese iron phosphate precursor has a powder layer thickness of 1.5 cm, with an area ratio of 1:0.31 and a mass ratio of 6:4.

[0084] 3) Under an argon atmosphere, two lithium manganese iron phosphate precursor powder layers were instantaneously sintered to obtain lithium manganese iron phosphate cathode material.

[0085] The instantaneous sintering conditions for each lithium manganese iron phosphate precursor powder layer are the same. The instantaneous sintering conditions include heating the two lithium manganese iron phosphate precursor powder layers to 900°C within 1 second and maintaining the temperature for 10 seconds.

[0086] Example 2

[0087] The preparation method of the lithium iron phosphate cathode material in this embodiment includes the following steps:

[0088] 1) Weigh 0.34 g of lithium source (lithium hydroxide), 0.61 g of manganese source (manganese oxide), 0.41 g of iron source (iron oxide), 1.64 g of phosphorus source (ammonium dihydrogen phosphate), and 0.3 g of carbon source (glucose) accounting for 10% of the total mass. Add the above mixture to 6 mL of deionized water, place it in a ball mill, and ball mill at 800 r / min for 5 h. After ball milling, dry at 100℃ for 12 h to obtain the lithium manganese iron phosphate precursor.

[0089] 2) Two lithium manganese iron phosphate precursors are laid on the surface of a heated substrate (stainless steel foil) to form two independent lithium manganese iron phosphate precursor powder layers.

[0090] One lithium manganese iron phosphate precursor has a powder layer thickness of 1 cm, and the other lithium manganese iron phosphate precursor has a powder layer thickness of 1.5 cm, with an area ratio of 1:0.5 and a mass ratio of 3:7.

[0091] 3) Under an argon atmosphere, two lithium manganese iron phosphate precursor powder layers were instantaneously sintered to obtain lithium manganese iron phosphate cathode material.

[0092] The instantaneous sintering conditions for each lithium manganese iron phosphate precursor powder layer are the same: the two lithium manganese iron phosphate precursor powder layers are heated to 1000℃ within 3s and held at that temperature for 5s.

[0093] Example 3

[0094] The preparation method of the lithium iron phosphate cathode material in this embodiment includes the following steps:

[0095] 1) Weigh 0.13g of lithium source (lithium hydroxide), 1.25g of manganese source (manganese trioxide), 0.80g of iron source (iron phosphate), 0.82g of phosphorus source (lithium dihydrogen phosphate), and 0.45g of carbon source (glucose) accounting for 15% of the total mass. Add the above mixture to 6mL of deionized water, place it in a ball mill, and ball mill at 300 r / min for 24h. After ball milling, dry at 100℃ for 12h to obtain the lithium manganese iron phosphate precursor.

[0096] 2) Two lithium manganese iron phosphate precursors are laid on the surface of a heated substrate (stainless steel foil) to form two independent lithium manganese iron phosphate precursor powder layers.

[0097] One lithium manganese iron phosphate precursor has a powder layer thickness of 0.9 cm, and the other has a powder layer thickness of 1.2 cm, with an area ratio of 1:0.4 and a mass ratio of 7:3.

[0098] 3) Under an argon atmosphere, two lithium manganese iron phosphate precursor powder layers were instantaneously sintered to obtain lithium manganese iron phosphate cathode material.

[0099] The instantaneous sintering treatment of each lithium manganese iron phosphate precursor powder layer is carried out under the same sintering conditions: the two lithium manganese iron phosphate precursor powder layers are heated to 800°C within 1 second and held at that temperature for 20 seconds.

[0100] Example 4

[0101] This embodiment is basically the same as Embodiment 1, except that the lithium manganese iron phosphate precursor powder layer is heated to 700°C within 5 seconds and maintained at that temperature for 30 seconds.

[0102] Example 5

[0103] This embodiment is basically the same as Embodiment 1, except that the lithium manganese iron phosphate precursor powder layer is heated to 1000°C within 10 seconds and maintained at that temperature for 30 seconds.

[0104] Comparative Example 1

[0105] Comparative Example 1 is basically the same as Example 1, except that the lithium manganese iron phosphate precursor is formed into two powder layers with a thickness of 1.5 cm.

[0106] Comparative Example 2

[0107] Comparative Example 2 is basically the same as Example 2, except that the lithium manganese iron phosphate precursor is formed into two powder layers with a thickness of 1 cm.

[0108] Test Example 1

[0109] After the lithium manganese iron phosphate cathode material obtained in Example 1 was removed from the heating substrate, it was mixed in a ball mill for 30 min and then characterized by X-ray diffraction (XRD). The test conditions were as follows: Cu Kα rays (λ=1.5406 Å) were used as the radiation source, the working voltage was 40 kV, the working current was 40 mA, the scanning range was 10°-80°, and the scanning rate was 5° / min.

[0110] like Figure 1 The image shown is the XRD pattern of the lithium manganese iron phosphate cathode material provided in Example 1 of this invention. All peaks are characteristic peaks of lithium manganese iron phosphate, and no impurity phase peaks are present, indicating that the lithium manganese iron phosphate cathode material has been successfully synthesized.

[0111] Test Example 2

[0112] After the lithium manganese iron phosphate cathode materials obtained in Examples 1, 1, and 2 were removed from the heating substrate, they were mixed in a ball mill for 30 minutes and then the microstructure of the materials was observed using a scanning electron microscope (SEM). The samples were sputter-coated with gold before testing.

[0113] like Figure 2 The image shown is a SEM image of the lithium iron phosphate cathode material of Example 1 provided by the present invention; as shown Figure 3 The image shown is a SEM image of the lithium iron phosphate cathode material of Comparative Example 1 provided by this invention; as shown... Figure 4 The image shown is a SEM image of the lithium iron phosphate cathode material of Comparative Example 2 provided by this invention.

[0114] As shown in the figure, in Example 1, large and small particles coexist. The large particles have a size of 1 μm, while the small particles have a size between 200-400 nm, indicating that the particles synthesized simultaneously in situ were successfully graded. In Comparative Example 1, the sample layer was thicker, and all the particles obtained were small. In Comparative Example 2, the sample layer was thinner, and all the particles obtained were large.

[0115] Test Example 3

[0116] Compacted density: The compacted density of the lithium manganese iron phosphate composite materials obtained in the above examples and comparative examples was measured using a Sansi compaction density meter. 1 mg was weighed and placed in a 13 mm diameter disc mold, and the pressure was maintained at 29.4 KN for 30 seconds before the data on the device was read.

[0117] The cycle life of the materials in Examples 1-5 and Comparative Examples 1-2 was tested using coin cells. Polyvinylidene fluoride (PVDF) was dissolved in N-methylpyrrolidone to prepare a 95% PVDF solution. The PVDF was weighed according to a mass ratio of lithium manganese iron phosphate composite material, conductive carbon black, and PVDF of 90:5:5, and mixed into a slurry. This slurry was then uniformly coated onto aluminum foil, dried at 100°C for 12 hours, cut into circular positive electrode sheets, and dried in a vacuum oven at 100°C for 12 hours. Constant current charge-discharge cycle tests were conducted at 25°C within a voltage range of 2-4.3V, with a cycle rate of 1C, and the battery capacity retention rate during the cycles was recorded. The results are shown in Table 1.

[0118]

[0119] As can be seen from the data in Table 1, the lithium manganese iron phosphate cathode material provided by the present invention improves the compaction density while also taking into account the cycle life.

[0120] Finally, it should be noted that other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein, and is not limited to the precise structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

Claims

1. A method for preparing a lithium manganese iron phosphate cathode material, characterized in that, Includes the following steps: The lithium manganese iron phosphate precursor powder layers were subjected to instantaneous sintering treatment to obtain the lithium manganese iron phosphate cathode material. Among the n lithium manganese iron phosphate precursor powder layers, the thickness of m lithium manganese iron phosphate precursor powder layers is different, n≥2, m≥2, n≥m; the sintering conditions of the instantaneous sintering treatment of each lithium manganese iron phosphate precursor powder layer are the same; the lithium manganese iron phosphate precursor includes phosphorus source, manganese source, iron source and lithium source.

2. The method according to claim 1, characterized in that, Prior to the instantaneous sintering process, the process includes: laying the lithium manganese iron phosphate precursor on the surface of a heated substrate to form n independent lithium manganese iron phosphate precursor powder layers.

3. The method according to claim 2, characterized in that, In order of increasing thickness, among the m lithium manganese iron phosphate precursor powder layers, the ratio of the thickness of the previous lithium manganese iron phosphate precursor powder layer to the thickness of the next lithium manganese iron phosphate precursor powder layer is 1:(1.3-3.0).

4. The method according to claim 3, characterized in that, The thickness of the first lithium manganese iron phosphate precursor powder layer is 0.5-1cm, and the thickness of the second lithium manganese iron phosphate precursor powder layer is 1-2cm.

5. The method according to claim 1, characterized in that, n=m=2, and the m lithium manganese iron phosphate precursor powder layers include a first lithium manganese iron phosphate precursor powder layer and a second lithium manganese iron phosphate precursor powder layer; wherein, the thickness of the first lithium manganese iron phosphate precursor powder layer is less than the thickness of the second lithium manganese iron phosphate precursor powder layer. The mass ratio of the first lithium manganese iron phosphate precursor powder layer to the second lithium manganese iron phosphate precursor powder layer is (3:7)-(7:3).

6. The method according to claim 5, characterized in that, The area ratio of the first lithium manganese iron phosphate precursor powder layer to the second lithium manganese iron phosphate precursor powder layer is 1:(0.3-0.5).

7. The method according to any one of claims 1-6, characterized in that, The instantaneous sintering process includes: heating the n lithium manganese iron phosphate precursor powder layers to 700-1000°C within 0.1-5s and maintaining the temperature for 1-60s.

8. A lithium manganese iron phosphate cathode material, characterized in that, Prepared by the method described in any one of claims 1-7.

9. An electrode sheet, characterized in that, Including the lithium manganese iron phosphate cathode material as described in claim 8.

10. A battery, characterized in that, Includes the electrode as described in claim 9.

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