Modified lithium manganese iron phosphate positive electrode material, preparation method thereof and lithium ion battery

Abstract

The application provides a modified lithium manganese iron phosphate positive material, a preparation method thereof and a lithium ion battery. The modified lithium manganese iron phosphate positive material comprises, from inside to outside, a core, at least one modified lithium manganese iron phosphate coating layer and a polysiloxane organic coating layer; the core comprises a lithium iron phosphate core and a first carbon coating layer arranged on the surface of the lithium iron phosphate core; each modified lithium manganese iron phosphate coating layer comprises modified lithium manganese iron phosphate particles, and the modified lithium manganese iron phosphate particles comprise metal element doped lithium manganese iron phosphate particles and a second carbon coating layer arranged on the surface of the metal element doped lithium manganese iron phosphate particles; the metal element is selected from one or more of elements in group ⅡA, group ⅢB, group ⅣB and group ⅤB; and the material of the polysiloxane organic coating layer is a polysiloxane organic compound. The modified lithium manganese iron phosphate positive material has high conductivity, energy density and cycle stability, and especially has good high-temperature (45-65 DEG C) cycle stability.

Description

Technical Field

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

[0002] Compared to lithium iron phosphate (LFP), lithium manganese iron phosphate (LFP) offers higher voltage, higher energy density, and better low-temperature performance. Currently, the energy density of mainstream LFP cathode materials on the market has almost reached its limit, while the high voltage characteristics of manganese in LFP give it a higher voltage platform than LFP, thus potentially breaking through the current upper limit of battery energy density.

[0003] However, LiFe x Mn 1-x Although the conductivity of PO4 is improved compared to LiMnPO4, the improvement is limited, making it difficult to fully realize the electrochemical performance of the material. Lithium manganese iron phosphate suffers from low electronic conductivity and lithium-ion diffusivity, and problems such as low compaction density, poor cycling performance, and manganese ion dissolution during high-temperature cycling urgently need to be addressed.

[0004] Existing literature (publication number CN114843507A) discloses a single-core multi-shell lithium manganese iron phosphate cathode material and its preparation method, as well as a secondary battery. The composite material comprises a carbon-coated lithium iron phosphate core and multiple lithium manganese iron phosphate coating layers covering the outer surface of the core. Each lithium manganese iron phosphate coating layer includes lithium manganese iron phosphate particles and carbon material coating the particles. The particle size of the lithium manganese iron phosphate particles in the multiple coating layers increases radially from the inside to the outside. However, this lithium manganese iron phosphate cathode material suffers from the problem of easy dissolution of manganese ions during high-temperature cycling, resulting in poor cycle stability, and its electrochemical performance needs further improvement.

[0005] In summary, researching and developing a modified lithium manganese iron phosphate cathode material with excellent conductivity, low manganese ion dissolution, and a simple and efficient preparation method is of great significance for improving the cycle stability and electrochemical performance of lithium-ion batteries. Summary of the Invention

[0006] The main objective of this invention is to provide a modified lithium manganese iron phosphate cathode material, its preparation method, and a lithium-ion battery, in order to solve the problems of low conductivity, poor cycle performance, and easy dissolution of manganese ions under high temperature conditions (45-65°C) in the existing lithium manganese iron phosphate cathode material.

[0007] To achieve the above objectives, the present invention provides a modified lithium manganese iron phosphate cathode material, which, from the inside out, sequentially comprises a core, at least one modified lithium manganese iron phosphate coating layer, and a polysiloxane-based organic coating layer; the core comprises a lithium iron phosphate core and a first carbon coating layer disposed on the surface of the lithium iron phosphate core; each modified lithium manganese iron phosphate coating layer comprises modified lithium manganese iron phosphate particles, the modified lithium manganese iron phosphate particles comprising metal-doped lithium manganese iron phosphate particles and a second carbon coating layer disposed on their surface; the metal element is selected from one or more elements in Group IIA, Group IIIB, Group IVB, and Group VB; the material of the polysiloxane-based organic coating layer is a polysiloxane-based organic compound.

[0008] This application uses lithium iron phosphate (LFP) core as the core material, which has better conductivity and a more stable structure compared to LFP core material. By depositing at least one modified LFP coating layer on the surface of the core, the high-voltage characteristics of manganese in LFP can be utilized, thereby improving the energy density of the modified LFP cathode material. The silicon in the polysiloxane-based organic coating layer has a strong affinity for fluoride ions in the electrolyte. When the modified LFP cathode material of this application is used in a lithium-ion battery, silicon preferentially binds to fluoride ions, thereby inhibiting their reaction with manganese ions and suppressing the dissolution of manganese ions during high-temperature cycling. The modified LFP particles in this application include LFP particles doped with the aforementioned specific metal elements. Compared to other types, using these metal elements can improve the conductivity of LFP particles, thereby improving the conductivity of the modified LFP cathode material and thus improving the electrochemical performance (such as initial coulombic efficiency and energy density) of the resulting lithium-ion battery.

[0009] In summary, the modified lithium manganese iron phosphate cathode material provided in this application has high conductivity, energy density and cycle stability, especially good high temperature (45-65℃) cycle stability.

[0010] Furthermore, the doping amount of the metal element is 5 to 15 wt%, based on the weight percentage of the metal-doped lithium manganese iron phosphate particles; preferably, the metal element is selected from one or more of Y, V, Sr, Ti and Mg.

[0011] Compared to other types of metal elements, using the aforementioned types of metal elements for doping, and limiting the doping amount of the metal elements within the above range, is beneficial to improving the lithium-ion conductivity of modified lithium manganese iron phosphate particles, thereby improving the conductivity of the modified lithium manganese iron phosphate cathode material, and further improving the electrochemical performance of lithium-ion batteries made from the above cathode material, such as the first coulombic efficiency and energy density.

[0012] Furthermore, the thickness of the second carbon coating layer is 2–5 nm.

[0013] Compared to other ranges, limiting the thickness of the second carbon coating layer to the above range is beneficial to increasing the specific surface area and conductivity of the modified lithium manganese iron phosphate particles, thereby improving the electrochemical performance (such as energy density and conductivity) and cycle stability of the modified lithium manganese iron phosphate cathode material.

[0014] Furthermore, the average particle size of the kernel is 500–1000 nm, preferably 600–700 nm.

[0015] Compared to other ranges, limiting the average particle size of the core to the above range is beneficial to maximizing the electrochemical performance of the core, thereby improving the conductivity of the modified lithium manganese iron phosphate cathode material and enhancing its electrochemical performance when applied to lithium-ion batteries.

[0016] Furthermore, the thickness of the modified lithium manganese iron phosphate coating layer is 300–4600 nm, preferably 400–3400 nm.

[0017] Compared to other ranges, limiting the thickness of the lithium manganese iron phosphate coating layer to the above range is beneficial to improving the energy density and structural stability of the modified lithium manganese iron phosphate cathode material, thereby improving the electrochemical performance and cycle stability of lithium-ion batteries.

[0018] Furthermore, the thickness of the polysiloxane-based organic coating layer is 2–15 nm, preferably 5–10 nm.

[0019] Compared to other ranges, limiting the thickness of the polysiloxane-based organic coating layer to the above range is beneficial for suppressing the reaction between fluoride ions and manganese ions in the electrolyte, and for suppressing the dissolution of manganese ions during high-temperature cycling of lithium-ion batteries, thereby improving the cycle stability of lithium-ion batteries.

[0020] Furthermore, the thickness of the first carbon coating layer is 2–5 nm.

[0021] Compared to other ranges, limiting the thickness of the first carbon coating layer to the above range is beneficial to improving the specific surface area, conductivity and stability of the core, thereby improving the conductivity and cycle stability of the modified lithium manganese iron phosphate cathode material.

[0022] Furthermore, the modified lithium manganese iron phosphate cathode material includes three layers of modified lithium manganese iron phosphate coating, which are, from the inside out, a first modified coating layer, a second modified coating layer, and a third modified coating layer.

[0023] Setting up a three-layer modified lithium manganese iron phosphate coating layer is beneficial to leveraging the high voltage characteristics of manganese in lithium manganese iron phosphate, thereby improving the energy density and structural stability of the modified lithium manganese iron phosphate cathode material, and further improving the electrochemical performance and cycle stability of the prepared lithium-ion battery, such as the initial coulombic efficiency and energy density.

[0024] Furthermore, the thicknesses of the first modified coating layer, the second modified coating layer, and the third modified coating layer increase sequentially, and the average particle size of the metal element-doped lithium manganese iron phosphate particles in the first modified coating layer, the second modified coating layer, and the third modified coating layer increases sequentially.

[0025] The thicknesses of the first, second, and third modified coating layers, as well as the average particle size of the metal-doped lithium manganese iron phosphate particles therein, increase sequentially. On the one hand, this helps to reduce the diffusion difficulty of lithium ions in the inner layer of the modified lithium manganese iron phosphate cathode material, improve lithium ion transport efficiency, and thus improve the lithium ion conductivity of the modified lithium manganese iron phosphate cathode material. On the other hand, it also helps to increase the compaction density of the modified lithium manganese iron phosphate cathode material, thereby improving the discharge specific capacity and cycle stability of the prepared lithium-ion battery.

[0026] Furthermore, the thickness of the first modified coating layer is 300–600 nm, and the average particle size of the metal-doped lithium manganese iron phosphate particles therein is 50–200 nm; the thickness of the second modified coating layer is 800–1500 nm, and the average particle size of the metal-doped lithium manganese iron phosphate particles therein is 100–250 nm; the thickness of the third modified coating layer is 1400–2500 nm, and the average particle size of the metal-doped lithium manganese iron phosphate particles therein is 150–300 nm. More preferably, the thickness of the first modified coating layer is 300-400 nm, and the average particle size of the metal-doped lithium manganese iron phosphate particles therein is 50-100 nm; the thickness of the second modified coating layer is 800-1000 nm, and the average particle size of the metal-doped lithium manganese iron phosphate particles therein is 100-150 nm; the thickness of the third modified coating layer is 1400-2000 nm, and the average particle size of the metal-doped lithium manganese iron phosphate particles therein is 150-200 nm.

[0027] Compared to other ranges, limiting the thickness of the first modified coating layer, the second modified coating layer, and the third modified coating layer, as well as the average particle size of the metal element-doped lithium manganese iron phosphate particles therein, to the above-mentioned ranges is beneficial in two ways: firstly, it improves the lithium-ion transport efficiency in the inner layer of the modified lithium manganese iron phosphate cathode material, thereby improving the discharge specific capacity of the modified lithium manganese iron phosphate cathode material; secondly, it also helps to improve the compaction density of the modified lithium manganese iron phosphate cathode material, thereby improving the discharge specific capacity and cycle stability of the prepared lithium-ion battery.

[0028] Furthermore, the polysiloxane organic compound is selected from one or more of the group consisting of polydimethylsiloxane, polydiethylsiloxane, polymethylvinylsiloxane, and poly(3-carboxypropylmethylsiloxane).

[0029] Compared to other types, the use of the above-mentioned polysiloxane organic materials is beneficial to improving the binding affinity of silicon to fluoride ions in the electrolyte, thereby helping to suppress the reaction between silicon and manganese ions in the modified lithium manganese iron phosphate cathode material, and helping to suppress the dissolution of manganese ions during high-temperature cycling of lithium-ion batteries, thus improving the cycle stability and service life of the prepared lithium-ion batteries.

[0030] Furthermore, the weight ratio of the core, the first modified coating layer, the second modified coating layer, the third modified coating layer and the polysiloxane organic coating layer is (0.5~1):(5~8):(20~40):(80~120):(5~10).

[0031] Compared to other ranges, limiting the weight ratio of the core, the first modified coating layer, the second modified coating layer, the third modified coating layer, and the polysiloxane-based organic coating layer to the above range is beneficial to improving the conductivity, energy density, and structural stability of the modified lithium manganese iron phosphate cathode material, thereby improving the electrochemical performance and cycle stability of the prepared lithium-ion battery, such as the initial coulombic efficiency and energy density.

[0032] To achieve the above objectives, a second aspect of the present invention also provides a method for preparing the modified lithium manganese iron phosphate cathode material, the method comprising: step S1, performing plasma ball milling on a first carbon source and lithium iron phosphate to obtain a core; step S2, performing plasma ball milling on a metal-organic compound and lithium manganese iron phosphate particles to obtain metal-doped lithium manganese iron phosphate particles; wherein the metal-organic compound includes metal elements; step S3, performing plasma ball milling on the metal-doped lithium manganese iron phosphate particles, the second carbon source, and the core, repeating step S3 to obtain a core with at least one layer of modified lithium manganese iron phosphate coating on its surface; step S4, performing wet ball milling on the core with at least one layer of modified lithium manganese iron phosphate coating, a polysiloxane-based organic compound, and a solvent, and drying to obtain the modified lithium manganese iron phosphate cathode material.

[0033] The preparation method of the modified lithium manganese iron phosphate cathode material provided in this application utilizes plasma ball milling in steps S1, S2, and S3, which improves the efficiency of ball milling and sintering, making the preparation process simpler and more efficient. The core obtained in step S1 has better conductivity and a more stable structure compared to the lithium manganese iron phosphate core. In step S2, plasma ball milling is performed on the organometallic compound and the lithium manganese iron phosphate particles. Using organometallic compounds allows for simultaneous metal element doping and carbon coating, improving the conductivity of the modified lithium manganese iron phosphate cathode material and thus enhancing the electrochemical performance of the resulting lithium-ion battery. (e.g., initial coulombic efficiency, energy density); In step S3, at least one layer of modified lithium manganese iron phosphate coating is applied to the core surface, which can take advantage of the high voltage characteristics of manganese in lithium manganese iron phosphate, thereby improving the energy density of the modified lithium manganese iron phosphate cathode material; In step S4, the core with at least one layer of modified lithium manganese iron phosphate coating, polysiloxane organic matter, and solvent are subjected to wet ball milling to obtain the modified lithium manganese iron phosphate cathode material. The introduction of polysiloxane organic matter can suppress the dissolution of manganese ions during high-temperature cycling of lithium-ion batteries. At the same time, wet ball milling can improve ball milling efficiency and save costs.

[0034] In summary, the modified lithium manganese iron phosphate cathode material prepared by the above preparation method provided in this application has high conductivity, energy density and cycle stability, especially good high temperature (45-65℃) cycle stability.

[0035] Further, the weight ratio of the organometallic compound to the lithium manganese iron phosphate particles is (0.1-2):1; preferably, the organometallic compound is selected from one or more of the group consisting of yttrium isooctanoate, vanadium isooctanoate, tristrontium citrate, tetra(octadecyl) titanate and magnesium pyridinecarboxylate.

[0036] Compared to other types, using the aforementioned organometallic compounds and limiting the weight ratio of the organometallic compounds to lithium manganese iron phosphate particles within the above-mentioned range is beneficial for improving the conductivity of lithium manganese iron phosphate particles, controlling the size of lithium manganese iron phosphate particles, and inhibiting their excessive growth. This, in turn, is beneficial for improving the conductivity of the modified lithium manganese iron phosphate cathode material, and consequently, for improving the electrochemical performance of the prepared lithium-ion battery, such as the initial coulombic efficiency and energy density.

[0037] Furthermore, the above preparation method further includes: repeating step S2, and using three types of lithium manganese iron phosphate particles with different average particle sizes to obtain corresponding lithium manganese iron phosphate particles doped with three metal elements through plasma ball milling; preferably, the first lithium manganese iron phosphate particle, the second lithium manganese iron phosphate particle, and the third lithium manganese iron phosphate particle are sequentially used to perform plasma ball milling to obtain the corresponding lithium manganese iron phosphate particles doped with three metal elements, and the average particle size of the first lithium manganese iron phosphate particle, the second lithium manganese iron phosphate particle, and the third lithium manganese iron phosphate particle increases sequentially; more preferably, the average particle size of the first lithium manganese iron phosphate particle is 50-150 nm, the average particle size of the second lithium manganese iron phosphate particle is 51-190 nm, and the average particle size of the third lithium manganese iron phosphate particle is 100-200 nm.

[0038] Repeat step S2 and use three types of lithium manganese iron phosphate particles with different average particle sizes to prepare corresponding lithium manganese iron phosphate particles doped with three metal elements. On the one hand, this helps to reduce the diffusion difficulty of lithium ions in the inner layer of the modified lithium manganese iron phosphate cathode material and improve the lithium ion transport efficiency, thereby improving the lithium ion conductivity of the modified lithium manganese iron phosphate cathode material. On the other hand, it also helps to increase the compaction density of the modified lithium manganese iron phosphate cathode material, thereby improving the discharge specific capacity and cycle stability of the prepared lithium-ion battery.

[0039] Further, in step S2, the temperature of the plasma ball milling treatment of the first, second, and third lithium manganese iron phosphate particles is increased sequentially to 600–650°C, 670–720°C, and 740–790°C, respectively; preferably, in step S2, the plasma ball milling treatment time for each of the first, second, and third lithium manganese iron phosphate particles is independently 5–10 hours; preferably, in step S2, the plasma ball milling treatment time for the first, second, and third lithium manganese iron phosphate particles is... The input power of each particle is independently 1 to 5 kW, the rotation speed is independently 900 to 1450 rpm, the filling rate is independently 30 to 70%, and the ball-to-material ratio is independently (5 to 10):1. Preferably, in step S2, the plasma ball milling of the first, second, and third lithium manganese iron phosphate particles is carried out independently under an inert atmosphere. Preferably, the pressure of the inert atmosphere is independently 0.1 to 0.5 MPa, and preferably, the inert atmosphere is independently selected from one or more of the group consisting of nitrogen, helium, and argon.

[0040] Compared to other ranges, this application limits the plasma ball milling process in step S2 to be carried out under an inert atmosphere, and limits the process parameters such as temperature, time, input power, rotation speed, filling rate and ball-to-material ratio of the plasma ball milling process to the above range. This is beneficial to improve the effect of plasma ball milling, and to make the metal element doping more uniform, thereby improving the conductivity of the modified lithium manganese iron phosphate cathode material.

[0041] Furthermore, in step S1, the plasma ball milling treatment is carried out at a temperature of 780–800°C for 5–10 hours and at a rotation speed of 950–1200 rpm, preferably under an inert atmosphere.

[0042] Compared to other ranges, this application limits the plasma ball milling process in step S1 to be carried out under an inert atmosphere, and limits its temperature, time and rotation speed to the above range, which is beneficial to improving the effect of plasma ball milling and also to improving the uniformity of coating, thereby improving the energy density, conductivity and cycle stability of the modified lithium manganese iron phosphate cathode material.

[0043] Furthermore, the above preparation method further includes: repeating step S3 three times to obtain a core with a three-layer modified lithium manganese iron phosphate coating; preferably, the temperature of each plasma ball milling treatment in step S3 is 500-650℃ and the time is 5-8h; preferably, the input power of each plasma ball milling treatment in step S3 is 1-3kW, the rotation speed is 1200-1300rpm, the filling rate is 40-50%, and the ball-to-material ratio is (5-7):1; preferably, each plasma ball milling treatment in step S3 is carried out under an inert atmosphere, preferably the pressure of the inert atmosphere is 0.1-0.3MPa, and preferably the inert atmosphere is selected from one or more of the group consisting of nitrogen, helium and argon.

[0044] Compared to other ranges, this application limits the plasma ball milling process in step S3 to be carried out under an inert atmosphere, and limits the process parameters such as temperature, time, input power, rotation speed, filling rate and ball-to-material ratio of the plasma ball milling process to the above range. This is beneficial to improving the effect of plasma ball milling, improving the uniformity of coating, and thus improving the energy density, conductivity and cycle stability of the modified lithium manganese iron phosphate cathode material.

[0045] Further, in step S4, the wet ball milling temperature is 20-25°C, the rotation speed is 10000-15000 rpm, and the time is 2-5 h; preferably, in step S4, the drying temperature is 80-120°C, and the time is 12-24 h; preferably, the solvent is selected from one or more of the group consisting of ethanol, water, and methanol.

[0046] Compared to other types, step S4 uses the above-mentioned solvents for the above-mentioned wet ball milling process, and compared to other ranges, it limits the temperature, rotation speed and time to the above-mentioned ranges, which is beneficial to improve the processing effect of wet ball milling. This is beneficial to improve the density of the polysiloxane organic coating layer and its bonding force with the inner modified lithium manganese iron phosphate coating layer, which in turn is beneficial to suppress the dissolution of manganese ions during high-temperature cycling of lithium-ion batteries and improve the cycle stability of modified lithium manganese iron phosphate cathode materials.

[0047] Furthermore, the first carbon source and the second carbon source are each independently selected from one or more of the group consisting of glucose, sucrose, starch, polyethylene glycol, polyvinyl alcohol, and citric acid.

[0048] Compared to other types, using the first and second carbon sources mentioned above is beneficial for improving the energy density, conductivity, and cycle stability of modified lithium manganese iron phosphate cathode materials.

[0049] Furthermore, a plasma ball mill was used for plasma ball milling.

[0050] Compared to other types of ball mills, using a plasma ball mill for plasma ball milling is beneficial to improving the efficiency of ball milling and sintering, making the preparation process simpler and more efficient.

[0051] To achieve the above objectives, a third aspect of the present invention also provides a lithium-ion battery, comprising a positive electrode, a negative electrode, an electrolyte, and a separator disposed between the positive electrode and the negative electrode, wherein the positive electrode comprises the above-mentioned modified lithium manganese iron phosphate positive electrode material.

[0052] Applying the modified lithium manganese iron phosphate cathode material provided in this application to lithium-ion batteries can enable lithium-ion batteries to have high first-time coulombic efficiency and energy density, as well as excellent cycle stability, especially good high-temperature (45-65℃) cycle stability. Attached Figure Description

[0053] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0054] Figure 1 The XRD pattern of the yttrium-doped lithium manganese iron phosphate particles prepared in step (2) of Example 1 of this application is shown;

[0055] Figure 2 The image shown is a scanning electron microscope (SEM) image of the modified lithium manganese iron phosphate cathode material prepared in Example 1 of this application;

[0056] Figure 3The image shown is a transmission electron microscope (TEM) image of the modified lithium manganese iron phosphate cathode material prepared in Example 1 of this application. Detailed Implementation

[0057] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the embodiments.

[0058] As described in the background section, existing lithium manganese iron phosphate cathode materials suffer from low conductivity, poor cycle performance, and easy dissolution of manganese ions under high-temperature conditions (45–65°C). To address these technical problems, this application provides a modified lithium manganese iron phosphate cathode material. From the inside out, the modified lithium manganese iron phosphate cathode material sequentially comprises a core, at least one modified lithium manganese iron phosphate coating layer, and a polysiloxane-based organic coating layer. The core includes a lithium iron phosphate core and a first carbon coating layer disposed on the surface of the lithium iron phosphate core. Each modified lithium manganese iron phosphate coating layer includes modified lithium manganese iron phosphate particles, which include metal-doped lithium manganese iron phosphate particles and a second carbon coating layer disposed on their surface. The metal elements include, but are not limited to, one or more elements from Group IIA, Group IIIB, Group IVB, and Group VB. The polysiloxane-based organic coating layer is made of a polysiloxane-based organic compound.

[0059] This application uses lithium iron phosphate (LFP) core as the core material, which has better conductivity and a more stable structure compared to LFP core material. By depositing at least one modified LFP coating layer on the surface of the core, the high-voltage characteristics of manganese in LFP can be utilized, thereby improving the energy density of the modified LFP cathode material. The silicon in the polysiloxane-based organic coating layer has a strong affinity for fluoride ions in the electrolyte. When the modified LFP cathode material of this application is used in a lithium-ion battery, silicon preferentially binds to fluoride ions, thereby inhibiting their reaction with manganese ions and suppressing the dissolution of manganese ions during high-temperature cycling. The modified LFP particles in this application include LFP particles doped with the aforementioned specific metal elements. Compared to other types, using these metal elements can improve the conductivity of LFP particles, thereby improving the conductivity of the modified LFP cathode material and thus improving the electrochemical performance (such as initial coulombic efficiency and energy density) of the resulting lithium-ion battery.

[0060] In summary, the modified lithium manganese iron phosphate cathode material provided in this application has high conductivity, energy density and cycle stability, especially good high temperature (45-65℃) cycle stability.

[0061] In a preferred embodiment, the doping amount of the metal element is 5 to 15 wt%, based on the weight percentage of the lithium manganese iron phosphate particles doped with the metal element. The doping amount of the metal element includes, but is not limited to, the above range. Limiting it to this range is beneficial for improving the lithium-ion conductivity of the modified lithium manganese iron phosphate particles, thereby improving the conductivity of the modified lithium manganese iron phosphate cathode material, and further beneficial for improving the electrochemical performance of the lithium-ion battery made from the above cathode material, such as the initial coulombic efficiency and energy density.

[0062] To further improve the lithium-ion conductivity of modified lithium manganese iron phosphate particles, and thus further improve the conductivity of the modified lithium manganese iron phosphate cathode material, preferably, the metal element includes, but is not limited to, one or more of Y, V, Sr, Ti and Mg.

[0063] In a preferred embodiment, the thickness of the second carbon coating layer is 2–5 nm. The thickness of the second carbon coating layer includes, but is not limited to, the above range. Limiting it to the above range is beneficial to increasing the specific surface area and conductivity of the modified lithium manganese iron phosphate particles, thereby improving the electrochemical performance of the modified lithium manganese iron phosphate cathode material, such as energy density and conductivity. It is also beneficial to improve the cycle stability of the modified lithium manganese iron phosphate cathode material.

[0064] In a preferred embodiment, the average particle size of the core is 500–1000 nm. The average particle size of the core includes, but is not limited to, the above range. Limiting it to this range is beneficial for maximizing the electrochemical performance of the core, thereby improving the conductivity of the modified lithium manganese iron phosphate cathode material and enhancing its electrochemical performance when applied in lithium-ion batteries.

[0065] To further improve the conductivity of the modified lithium manganese iron phosphate cathode material, preferably, the average particle size of the core is 600-700 nm.

[0066] In a preferred embodiment, the thickness of the first carbon coating layer is 2–5 nm. The thickness of the first carbon coating layer includes, but is not limited to, the above range. Limiting it to the above range is beneficial to improving the specific surface area, conductivity, and stability of the core, thereby improving the conductivity and cycle stability of the modified lithium manganese iron phosphate cathode material.

[0067] In a preferred embodiment, the thickness of the modified lithium manganese iron phosphate coating layer is 300–4600 nm. The thickness of the modified lithium manganese iron phosphate coating layer includes, but is not limited to, the above range. Limiting it to this range is beneficial for improving the energy density and structural stability of the modified lithium manganese iron phosphate cathode material, thereby improving the electrochemical performance and cycle stability of the prepared lithium-ion battery, such as its initial coulombic efficiency and energy density.

[0068] To further improve the energy density and structural stability of the modified lithium manganese iron phosphate cathode material, and to further improve the electrochemical performance and cycle stability of lithium-ion batteries, such as the first coulombic efficiency and energy density, the thickness of the modified lithium manganese iron phosphate coating layer is preferably 400–3400 nm.

[0069] In a preferred embodiment, the thickness of the polysiloxane-based organic coating layer is 2–15 nm. The thickness of the polysiloxane-based organic coating layer includes, but is not limited to, the above range. Limiting it to this range helps to suppress the reaction between fluoride ions and manganese ions in the electrolyte, and helps to suppress the dissolution of manganese ions during high-temperature cycling of the lithium-ion battery, thereby improving the cycle stability of the lithium-ion battery.

[0070] To further suppress the dissolution of manganese ions during high-temperature cycling of lithium-ion batteries and thus further improve the cycle stability of lithium-ion batteries, the thickness of the polysiloxane organic coating layer is preferably 5-10 nm.

[0071] In a preferred embodiment, the modified lithium manganese iron phosphate cathode material comprises three modified lithium manganese iron phosphate coating layers, which are, from the inside out, a first modified coating layer, a second modified coating layer, and a third modified coating layer. The three-layer modified lithium manganese iron phosphate coating layer is beneficial for leveraging the high-voltage characteristics of manganese in lithium manganese iron phosphate, thereby improving the energy density and structural stability of the modified lithium manganese iron phosphate cathode material. This, in turn, helps improve the electrochemical performance and cycle stability of the resulting lithium-ion battery, including its initial coulombic efficiency, energy density, and other properties.

[0072] In a preferred embodiment, the thicknesses of the first, second, and third modified coating layers increase sequentially, as does the average particle size of the metal-doped lithium manganese iron phosphate particles within them. This sequential increase in thickness and average particle size of the metal-doped lithium manganese iron phosphate particles helps to reduce the diffusion difficulty of lithium ions within the inner layers of the modified lithium manganese iron phosphate cathode material, improving lithium ion transport efficiency and thus increasing the lithium-ion conductivity of the modified lithium manganese iron phosphate cathode material. Furthermore, it also helps to increase the compaction density of the modified lithium manganese iron phosphate cathode material, thereby improving the discharge specific capacity and cycle stability of the resulting lithium-ion battery.

[0073] In a preferred embodiment, the thickness of the first modified coating layer is 300–600 nm, and the average particle size of the metal-doped lithium manganese iron phosphate particles therein is 50–200 nm; the thickness of the second modified coating layer is 800–1500 nm, and the average particle size of the metal-doped lithium manganese iron phosphate particles therein is 100–250 nm; the thickness of the third modified coating layer is 1400–2500 nm, and the average particle size of the metal-doped lithium manganese iron phosphate particles therein is 150–300 nm. The thicknesses of the first modified coating layer, the second modified coating layer, and the third modified coating layer, and the average particle size of the lithium manganese iron phosphate particles doped with metal elements contained in each of them, are not limited to the ranges mentioned above. Limiting them to the ranges above is beneficial in two ways: firstly, it improves the lithium-ion transport efficiency in the inner layer of the modified lithium manganese iron phosphate cathode material, thereby improving the discharge specific capacity of the modified lithium manganese iron phosphate cathode material; secondly, it is also beneficial in improving the compaction density of the modified lithium manganese iron phosphate cathode material, thereby improving the discharge specific capacity and cycle stability of the prepared lithium-ion battery.

[0074] To further improve the lithium-ion transport efficiency in the inner layer of the modified lithium manganese iron phosphate cathode material, further improve the compaction density of the modified lithium manganese iron phosphate cathode material, and thus further improve the discharge specific capacity and cycle stability of the prepared lithium-ion battery, preferably, the thickness of the first modified coating layer is 300-400 nm, and the average particle size of the metal-doped lithium manganese iron phosphate particles therein is 50-100 nm; the thickness of the second modified coating layer is 800-1000 nm, and the average particle size of the metal-doped lithium manganese iron phosphate particles therein is 100-150 nm; the thickness of the third modified coating layer is 1400-2000 nm, and the average particle size of the metal-doped lithium manganese iron phosphate particles therein is 150-200 nm.

[0075] In a preferred embodiment, the polysiloxane organic compounds include, but are not limited to, one or more of the group consisting of polydimethylsiloxane, polydiethylsiloxane, polymethylvinylsiloxane, and poly(3-carboxypropylmethylsiloxane). Compared to other types, using the above-mentioned polysiloxane organic compounds is beneficial for increasing the binding affinity of silicon to fluoride ions in the electrolyte, thereby helping to suppress its reaction with manganese ions in the modified lithium manganese iron phosphate cathode material, and helping to suppress the dissolution of manganese ions during high-temperature cycling of the lithium-ion battery, thus improving the cycle stability and service life of the resulting lithium-ion battery.

[0076] In a preferred embodiment, the weight ratio of the core, the first modified coating layer, the second modified coating layer, the third modified coating layer, and the polysiloxane-based organic coating layer is (0.5–1):(5–8):(20–40):(80–120):(5–10). The weight ratio of the core and the aforementioned layers includes, but is not limited to, the range described above. Limiting it to this range is beneficial for improving the conductivity, energy density, and structural stability of the modified lithium manganese iron phosphate cathode material, thereby improving the electrochemical performance and cycle stability of the resulting lithium-ion battery, such as its initial coulombic efficiency and energy density.

[0077] The second aspect of this application also provides a method for preparing the above-mentioned modified lithium manganese iron phosphate cathode material, the method comprising: step S1, performing plasma ball milling on a first carbon source and lithium iron phosphate to obtain a core; step S2, performing plasma ball milling on a metal-organic compound and lithium manganese iron phosphate particles to obtain metal-doped lithium manganese iron phosphate particles; wherein the metal-organic compound includes metal elements; step S3, performing plasma ball milling on the metal-doped lithium manganese iron phosphate particles, a second carbon source and a core, repeating step S3 to obtain a core with at least one layer of modified lithium manganese iron phosphate coating on its surface; step S4, performing wet ball milling on the core with at least one layer of modified lithium manganese iron phosphate coating, a polysiloxane organic compound and a solvent, and drying to obtain the modified lithium manganese iron phosphate cathode material.

[0078] In step S1, the first carbon source and lithium iron phosphate are subjected to plasma ball milling to obtain a core (i.e., a carbon-coated lithium iron phosphate core), which has better electrical conductivity and a more stable structure compared to the lithium manganese iron phosphate core. Compared to traditional ball milling processes, plasma ball milling technology utilizes plasma-assisted ball milling. By introducing a plasma field and combining it with mechanical ball milling during the process, the synergistic effect of mechanical energy and plasma energy is achieved. Plasma ball milling technology combines heating effects, high-energy electron bombardment effects, and grinding mechanical impact effects, enabling the ball milling and sintering processes to be completed in the same device. Using plasma ball milling can improve the efficiency of ball milling and sintering, making the preparation process simpler and more efficient. In step S2, the organometallic compound and lithium manganese iron phosphate particles are subjected to plasma ball milling to obtain metal-doped lithium manganese iron phosphate particles. Organometallic compounds (MMCs) include metal elements and carbon-containing organic components. Using MMCs allows for simultaneous metal doping and carbon coating. On one hand, the metal elements in the MMC improve the conductivity of lithium manganese iron phosphate (LFP) particles, thereby increasing the conductivity of the modified LFP cathode material. On the other hand, the carbon chains (containing carbon-containing organic components) of the MMC ensure uniform dispersion of the metal elements, acting as a reducer and partially replacing the conductive agent. Simultaneously, the carbon chains of the MMC coat the surface of the LFP particles, controlling their size and inhibiting excessive growth. Based on these two advantages, combined with subsequent processing steps, the conductivity of the modified LFP cathode material can be improved, thus enhancing the electrochemical performance of the resulting lithium-ion battery (e.g., initial coulombic efficiency and energy density). In step S3, at least one layer of modified LFP coating is applied to the core surface, leveraging the high-voltage characteristics of manganese in LFP to increase the energy density of the modified LFP cathode material. In step S4, the core, which has at least one layer of modified lithium manganese iron phosphate coating, polysiloxane organic material, and solvent are wet-milled to obtain modified lithium manganese iron phosphate cathode material. The silicon in the polysiloxane organic material has a strong binding affinity for fluoride ions in the electrolyte. When the modified lithium manganese iron phosphate cathode material of this application is applied to a lithium-ion battery, silicon preferentially binds to fluoride ions, thereby inhibiting its reaction with manganese ions, and thus inhibiting the dissolution of manganese ions during high-temperature cycling of the lithium-ion battery. Wet ball milling has high ball milling efficiency, low energy consumption, and simple operation. Compared with other types of ball milling processes, wet ball milling can improve ball milling efficiency and save costs.

[0079] In summary, the modified lithium manganese iron phosphate cathode material prepared by the above preparation method provided in this application has high conductivity, energy density and cycle stability, especially good high temperature (45-65℃) cycle stability.

[0080] In a preferred embodiment, the weight ratio of the organometallic compound to the lithium iron phosphate particles is (0.1–2):1. This weight ratio includes, but is not limited to, the range described above. Limiting it to this range is beneficial for improving the conductivity of the lithium iron phosphate particles, for controlling their size, and for suppressing excessive growth. This, in turn, improves the conductivity of the modified lithium iron phosphate cathode material, and consequently, for enhancing the electrochemical performance of the resulting lithium-ion battery, such as its initial coulombic efficiency and energy density.

[0081] To further improve the electrical conductivity of lithium manganese iron phosphate particles, further regulate the size of lithium manganese iron phosphate particles, and inhibit their excessive growth, thereby further improving the electrical conductivity of modified lithium manganese iron phosphate cathode materials, preferably, the organometallic compounds include, but are not limited to, one or more of the group consisting of yttrium isooctanoate, vanadium isooctanoate, tristrontium citrate, tetra(octadecyl) titanate and magnesium pyridinecarboxylate.

[0082] In a preferred embodiment, the above preparation method further includes: repeating step S2, using three types of lithium manganese iron phosphate particles with different average particle sizes to obtain lithium manganese iron phosphate particles doped with the corresponding three metal elements through plasma ball milling. Repeating step S2 to obtain lithium manganese iron phosphate particles doped with the corresponding three metal elements using three types of lithium manganese iron phosphate particles with different average particle sizes has two advantages: firstly, it reduces the diffusion difficulty of lithium ions in the inner layer of the modified lithium manganese iron phosphate cathode material, improves lithium ion transport efficiency, and thus improves the lithium-ion conductivity of the modified lithium manganese iron phosphate cathode material; secondly, it also helps to increase the compaction density of the modified lithium manganese iron phosphate cathode material, thereby improving the discharge specific capacity and cycle stability of the resulting lithium-ion battery.

[0083] To further reduce the diffusion difficulty of lithium ions in the inner layer of the modified lithium manganese iron phosphate cathode material and further improve the compaction density of the modified lithium manganese iron phosphate cathode material, preferably, the three types of metal element-doped lithium manganese iron phosphate particles are obtained by plasma ball milling sequentially using first lithium manganese iron phosphate particles, second lithium manganese iron phosphate particles, and third lithium manganese iron phosphate particles, and the average particle size of the first lithium manganese iron phosphate particles, second lithium manganese iron phosphate particles, and third lithium manganese iron phosphate particles increases sequentially; more preferably, the average particle size of the first lithium manganese iron phosphate particles is 50-150 nm, the average particle size of the second lithium manganese iron phosphate particles is 51-190 nm, and the average particle size of the third lithium manganese iron phosphate particles is 100-200 nm.

[0084] In a preferred embodiment, in step S2, the temperatures of the first, second, and third lithium manganese iron phosphate particles undergoing plasma ball milling are sequentially increased to 600–650°C, 670–720°C, and 740–790°C, respectively. The temperatures of the three plasma ball milling processes include, but are not limited to, the above ranges. Limiting these temperatures to these ranges is beneficial for improving the effectiveness of the plasma ball milling process and for achieving more uniform doping of the metal elements, thereby improving the energy density, conductivity, and cycle stability of the modified lithium manganese iron phosphate cathode material.

[0085] In a preferred embodiment, in step S2, the plasma ball milling treatment time for the first, second, and third lithium manganese iron phosphate particles is independently 5–10 hours. The plasma ball milling treatment time includes, but is not limited to, the above range. Limiting it to this range is beneficial for improving the effectiveness of the plasma ball milling treatment and for achieving more uniform metal element doping, thereby improving the energy density, conductivity, and cycle stability of the modified lithium manganese iron phosphate cathode material.

[0086] It should be noted that the filling rate in this application refers to the percentage of the volume of the milled material in the plasma ball mill to the effective volume of the plasma ball mill; the ball milling media are added during the plasma ball milling process in this application, wherein the ball-to-material ratio refers to the weight ratio of the ball milling media to the milled material. The ball milling media used in each plasma ball milling process in this application is zirconium oxide. In a preferred embodiment, in step S2, the input power of the plasma ball milling process for the first, second, and third lithium manganese iron phosphate particles is independently 1-5 kW, the rotation speed is independently 900-1450 rpm, the filling rate is independently 30-70%, and the ball-to-material ratio is independently (5-10):1. The ranges of the input power, rotation speed, filling rate, and ball-to-material ratio for each of the above plasma ball milling processes include, but are not limited to, the above ranges. Limiting them to the above ranges is beneficial to improving the effect of plasma ball milling, and is beneficial to making the metal element doping more uniform, thereby improving the conductivity of the modified lithium manganese iron phosphate cathode material.

[0087] In a preferred embodiment, in step S2, the plasma ball milling of the first, second, and third lithium manganese iron phosphate particles is carried out independently under an inert atmosphere. Compared to performing the plasma ball milling treatment under other atmospheres, performing it under an inert atmosphere helps to suppress the generation of harmful substances during the ball milling process, protects the ball milling material and equipment from damage, improves the effectiveness of the plasma ball milling treatment, and allows for more uniform doping of metal elements, thereby improving the conductivity of the modified lithium manganese iron phosphate cathode material.

[0088] To further improve the effect of plasma ball milling and make the metal element doping more uniform, thereby further improving the conductivity of the modified lithium manganese iron phosphate cathode material, preferably, the pressure of the inert atmosphere is independently 0.1 to 0.5 MPa, and the inert atmosphere is independently selected from one or more of the group consisting of nitrogen, helium and argon.

[0089] In a preferred embodiment, the plasma ball milling treatment in step S1 is carried out at a temperature of 780–800°C for 5–10 hours and at a rotation speed of 950–1200 rpm. Compared to other ranges, limiting the temperature, time, and rotation speed of the plasma ball milling treatment in step S1 to the above ranges is beneficial to improving the effect of the plasma ball milling treatment and also to improving the uniformity of the coating, thereby improving the energy density, conductivity, and cycle stability of the modified lithium manganese iron phosphate cathode material.

[0090] To further improve the effect of plasma ball milling, further improve the uniformity of coating, and thus further improve the energy density, conductivity and cycle stability of modified lithium manganese iron phosphate cathode material, plasma ball milling is preferably carried out under an inert atmosphere.

[0091] In a preferred embodiment, the above preparation method further includes repeating step S3 three times to obtain a core with a three-layer modified lithium manganese iron phosphate coating. This process is beneficial for improving the energy density and conductivity of the modified lithium manganese iron phosphate cathode material, and the multi-layer coating is beneficial for improving the compaction density of the modified lithium manganese iron phosphate cathode material, thereby improving the cycle stability of the prepared lithium-ion battery.

[0092] In a preferred embodiment, the temperature of each plasma ball milling treatment in step S3 is 500-650°C and the time is 5-8 hours. Compared with other ranges, limiting the temperature and time of the three plasma ball milling treatments in step S3 to the above range is beneficial to improving the effect of plasma ball milling treatment, improving the uniformity of coating, and thus improving the energy density, conductivity and cycle stability of the modified lithium manganese iron phosphate cathode material.

[0093] In a preferred embodiment, the input power for each plasma ball milling process in step S3 is 1–3 kW, the rotation speed is 1200–1300 rpm, the filling rate is 40–50%, and the ball-to-material ratio is (5–7):1. The ranges of input power, rotation speed, filling rate, and ball-to-material ratio for the three plasma ball milling processes in step S3 include, but are not limited to, the ranges described above. Limiting these ranges is beneficial for improving the effectiveness of the plasma ball milling process, improving the uniformity of the coating, and thus improving the energy density, conductivity, and cycle stability of the modified lithium manganese iron phosphate cathode material.

[0094] In a preferred embodiment, each plasma ball milling process in step S3 is performed under an inert atmosphere. Compared to performing the plasma ball milling process under other atmospheres, performing it under an inert atmosphere is beneficial for improving product purity, protecting the milled material and equipment from damage, enhancing the effectiveness of the plasma ball milling process, and improving the uniformity of coating. This, in turn, helps to improve the energy density, conductivity, and cycle stability of the modified lithium manganese iron phosphate cathode material.

[0095] To further improve the effect of plasma ball milling, further improve the uniformity of coating, and thus further improve the energy density, conductivity and cycle stability of modified lithium manganese iron phosphate cathode material, preferably, the pressure of the inert atmosphere in each plasma ball milling process in step S3 is 0.1 to 0.3 MPa, and the inert atmosphere includes, but is not limited to, one or more of the group consisting of nitrogen, helium and argon.

[0096] In a preferred embodiment, the wet ball milling temperature in step S4 is 20–25°C, the rotation speed is 10,000–15,000 rpm, and the time is 2–5 hours. The temperature, rotation speed, and time of the wet ball milling are not limited to the above ranges. Limiting them within these ranges is beneficial for improving the processing effect of the wet ball milling, thereby improving the density of the polysiloxane-based organic coating layer and its bonding force with the inner modified lithium manganese iron phosphate coating layer. This helps to suppress the reaction between fluoride ions and manganese ions in the electrolyte, thus inhibiting the dissolution of manganese ions during high-temperature cycling of the lithium-ion battery; it also helps to improve the overall efficiency of the preparation process.

[0097] To further improve the ball milling efficiency of wet ball milling, preferably, the solvent includes, but is not limited to, one or more of the group consisting of ethanol, water, and methanol.

[0098] In a preferred embodiment, the drying temperature in step S4 is 80–120°C, and the drying time is 12–24 hours. Compared to other ranges, limiting the drying temperature and time to the above range is beneficial to improving the solvent removal rate, thereby improving the purity of the modified lithium manganese iron phosphate cathode material.

[0099] In a preferred embodiment, the first carbon source and the second carbon source are each independently selected from, but not limited to, one or more of, the group consisting of glucose, sucrose, starch, polyethylene glycol, polyvinyl alcohol, and citric acid. Compared to other types, using the above-mentioned carbon sources is beneficial for improving the energy density, conductivity, and cycle stability of the modified lithium manganese iron phosphate cathode material.

[0100] Plasma ball milling technology utilizes plasma-assisted ball milling. By organically combining a plasma field with mechanical ball milling during the milling process, it achieves a synergistic effect between mechanical energy and plasma energy. Plasma ball milling technology integrates heating effects, high-energy electron bombardment effects, and grinding mechanical impact effects, enabling the ball milling and sintering processes to be completed within the same device. In a preferred embodiment, a plasma ball mill is used for plasma ball milling. Using a plasma ball mill improves the efficiency of both milling and sintering, making the preparation process simpler and more efficient.

[0101] A third aspect of this application also provides a lithium-ion battery, including a positive electrode, a negative electrode, an electrolyte, and a separator disposed between the positive and negative electrodes, wherein the positive electrode of the lithium-ion battery includes the modified lithium manganese iron phosphate positive electrode material provided in this application.

[0102] The modified lithium manganese iron phosphate cathode material provided in this application has high conductivity, energy density and structural stability. Its application in lithium-ion batteries can enable lithium-ion batteries to have high electrochemical performance such as high initial coulombic efficiency and energy density, as well as good cycle stability, especially good high temperature (45-65℃) cycle stability.

[0103] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0104] Example 1

[0105] A method for preparing a modified lithium manganese iron phosphate cathode material includes the following steps:

[0106] (1) Weigh 10g of lithium iron phosphate with an average particle size of 400nm and 1g of glucose and place them in a plasma ball mill (Guangzhou Huagong Opto-Mechatronics Technology Co., Ltd., Plasma-BM-L). The plasma ball milling is carried out at 780℃ and under a nitrogen atmosphere for 5h to obtain a core with an average particle size of 600nm. The rotation speed of the plasma ball mill is 950rpm.

[0107] (2) Weigh 80g of lithium manganese iron phosphate particles with an average particle size of 50nm and 12g of yttrium isooctanoate and place them in a plasma ball mill of the same model as in step (1). Perform plasma ball milling at 600℃ under a nitrogen atmosphere for 5h to obtain yttrium-doped lithium manganese iron phosphate particles with an average particle size of 60nm. The plasma ball mill has an input power of 2kW, a rotation speed of 950rpm, a filling rate of 30%, a ball-to-material ratio of 5:1, and a nitrogen pressure of 0.1MPa. The yttrium doping content is 9wt% based on the weight percentage of the yttrium-doped lithium manganese iron phosphate particles. The XRD pattern of the obtained yttrium-doped lithium manganese iron phosphate particles is shown below. Figure 1 As shown, by Figure 1 It can be seen that the diffraction peaks of yttrium-doped lithium manganese iron phosphate particles are consistent with the characteristic peaks of LiMnPO4, indicating that it has a good crystal phase and no impurity peaks.

[0108] (3) Weigh 400g of lithium manganese iron phosphate particles with an average particle size of 60nm and 60g of yttrium isooctanoate and place them in a plasma ball mill. The plasma ball milling treatment was carried out at 670℃ and under a nitrogen atmosphere for 5h to obtain yttrium-doped lithium manganese iron phosphate particles with an average particle size of 100nm. The input power of the plasma ball mill was 2.7kW, the rotation speed was 950rpm, the filling rate was 30%, the ball-to-material ratio was 5:1, the nitrogen pressure was 0.1MPa, and the doping amount of yttrium was 9wt% based on the weight percentage of the yttrium-doped lithium manganese iron phosphate particles.

[0109] (4) Weigh 1200g of lithium manganese iron phosphate particles with an average particle size of 100nm and 180g of yttrium isooctanoate and place them in a plasma ball mill. The plasma ball milling is carried out at 740℃ and under a nitrogen atmosphere for 5h to obtain yttrium-doped lithium manganese iron phosphate particles with an average particle size of 150nm. The input power of the plasma ball mill is 3.4kW, the rotation speed is 950rpm, the filling rate is 30%, the ball-to-material ratio is 5:1, the nitrogen pressure is 0.1MPa, and the doping amount of yttrium is 9wt% based on the weight percentage of the yttrium-doped lithium manganese iron phosphate particles.

[0110] (5) The core obtained in step (1) and the yttrium-doped lithium manganese iron phosphate particles obtained in step (2) are mixed with 9g of sucrose and placed in a plasma ball mill. The mixture is then subjected to plasma ball milling at 500℃ and under a nitrogen atmosphere for 5h to obtain a core with a coating thickness of 300nm and a modified lithium manganese iron phosphate coating layer on the surface. The plasma ball mill has an input power of 1kW, a rotation speed of 1200rpm, a filling rate of 40%, a ball-to-material ratio of 5:1, and a nitrogen pressure of 0.1MPa.

[0111] (6) The core with a modified lithium manganese iron phosphate coating layer obtained in step (5) and the yttrium-doped lithium manganese iron phosphate particles obtained in step (3) are mixed with 45g of sucrose and placed in a plasma ball mill. The mixture is then subjected to plasma ball milling at 520℃ and under a nitrogen atmosphere for 5h to obtain a core with two modified lithium manganese iron phosphate coating layers. The thickness of the second modified lithium manganese iron phosphate coating layer from the inside to the outside is 800nm. The input power of the plasma ball mill is 1.2kW, the rotation speed is 1200rpm, the filling rate is 40%, the ball-to-material ratio is 5:1, and the nitrogen pressure is 0.1MPa.

[0112] (7) The core with two layers of modified lithium manganese iron phosphate coating obtained in step (6) and the yttrium-doped lithium manganese iron phosphate particles obtained in step (4) are mixed with 150g of sucrose and placed in a plasma ball mill. The mixture is then subjected to plasma ball milling at 550℃ and under a nitrogen atmosphere for 5h to obtain a core with three layers of modified lithium manganese iron phosphate coating. The thickness of the third layer of modified lithium manganese iron phosphate coating from the inside to the outside is 1400nm. The input power of the plasma ball mill is 1.5kW, the rotation speed is 1200rpm, the filling rate is 40%, the ball-to-material ratio is 5:1, and the nitrogen pressure is 0.1MPa.

[0113] (8) The core with a three-layer modified lithium manganese iron phosphate coating obtained in step (7), 100g of polydimethylsiloxane and 500g of ethanol were mixed and placed in a ball mill (manufacturer: Changsha Tianchuang Powder Technology Co., Ltd., model: XQM-4) for wet ball milling. After ball milling, it was placed in an oven at 80℃ and dried for 12h to obtain a modified lithium manganese iron phosphate cathode material with a thickness of 2.4μm. The wet ball milling temperature was 25℃, the time was 2h, and the speed of the ball mill was 10000rpm.

[0114] The SEM image of the modified lithium manganese iron phosphate cathode material prepared in Example 1 is shown below. Figure 2 As shown, the TEM image of the modified lithium manganese iron phosphate cathode material prepared in Example 1 is as follows. Figure 3 As shown. By Figure 2 It can be seen that the modified lithium manganese iron phosphate cathode material prepared in Example 1 has an irregular blocky particle morphology and a relatively rough surface; Figure 3 It can be seen that the thickness of the outermost polysiloxane organic coating layer in the modified lithium manganese iron phosphate cathode material prepared in Example 1 is 6 nm.

[0115] Example 2

[0116] (1) Weigh 10g of lithium iron phosphate with an average particle size of 500nm and 1g of sucrose and place them in a plasma ball mill. Then, treat the plasma ball mill at 790℃ and under a nitrogen atmosphere for 6h to obtain a core with an average particle size of 700nm. The rotation speed of the plasma ball mill is 1000rpm.

[0117] (2) Weigh 80g of lithium manganese iron phosphate particles with an average particle size of 80nm and 40g of strontium citrate and place them in a plasma ball mill. The plasma ball milling treatment was carried out at 620℃ and under a nitrogen atmosphere for 6h to obtain strontium-doped lithium manganese iron phosphate particles with an average particle size of 100nm. The input power of the plasma ball mill was 2.2kW, the rotation speed was 1000rpm, the filling rate was 40%, the ball-to-material ratio was 6:1, the nitrogen pressure was 0.2MPa, and the strontium doping amount was 14wt% based on the weight percentage of the strontium-doped lithium manganese iron phosphate particles.

[0118] (3) Weigh 400g of lithium manganese iron phosphate particles with an average particle size of 100nm and 200g of strontium citrate and place them in a plasma ball mill. The plasma ball milling treatment was carried out at 690℃ and under a nitrogen atmosphere for 6h to obtain strontium-doped lithium manganese iron phosphate particles with an average particle size of 140nm. The input power of the plasma ball mill was 2.9kW, the rotation speed was 1000rpm, the filling rate was 40%, the ball-to-material ratio was 6:1, the nitrogen pressure was 0.2MPa, and the strontium doping amount was 14wt% based on the weight percentage of the strontium-doped lithium manganese iron phosphate particles.

[0119] (4) Weigh 1000g of lithium manganese iron phosphate particles with an average particle size of 120nm and 500g of strontium citrate and place them in a plasma ball mill. Perform plasma ball milling treatment at 760℃ and under nitrogen atmosphere for 6h to obtain strontium-doped lithium manganese iron phosphate particles with an average particle size of 190nm. The input power of the plasma ball mill is 3.6kW, the rotation speed is 1000rpm, the filling rate is 40%, the ball-to-material ratio is 6:1, the nitrogen pressure is 0.2MPa, and the strontium doping amount is 14wt% based on the weight percentage of the strontium-doped lithium manganese iron phosphate particles.

[0120] (5) The core obtained in step (1) and the strontium-doped lithium manganese iron phosphate particles obtained in step (2) are mixed with 8g of starch and placed in a plasma ball mill. The mixture is then subjected to plasma ball milling at 520℃ and under a nitrogen atmosphere for 6h to obtain a core with a coating thickness of 320nm and a modified lithium manganese iron phosphate coating layer on the surface. The plasma ball mill has an input power of 1.2kW, a rotation speed of 1220rpm, a filling rate of 42%, a ball-to-material ratio of 6:1, and a nitrogen pressure of 0.15MPa.

[0121] (6) The core with a modified lithium manganese iron phosphate coating layer obtained in step (5), the strontium-doped lithium manganese iron phosphate particles obtained in step (3), and 40g of starch were mixed and placed in a plasma ball mill. The mixture was then subjected to plasma ball milling at 540℃ and under a nitrogen atmosphere for 6h to obtain a core with two modified lithium manganese iron phosphate coating layers. The thickness of the second modified lithium manganese iron phosphate coating layer from the inside to the outside was 900nm. The input power of the plasma ball mill was 1.4kW, the rotation speed was 1220rpm, the filling rate was 42%, the ball-to-material ratio was 6:1, and the nitrogen pressure was 0.15MPa.

[0122] (7) The core with two layers of modified lithium manganese iron phosphate coating obtained in step (6), the strontium-doped lithium manganese iron phosphate particles obtained in step (4), and 140g of starch were mixed and placed in a plasma ball mill. The mixture was plasma ball milled at 560℃ under a nitrogen atmosphere for 6h to obtain a core with three layers of modified lithium manganese iron phosphate coating. The thickness of the third layer of modified lithium manganese iron phosphate coating from the inside to the outside was 1600nm. The input power of the plasma ball mill was 1.6kW, the rotation speed was 1220rpm, the filling rate was 42%, the ball-to-material ratio was 6:1, and the nitrogen pressure was 0.15MPa.

[0123] (8) The core with a three-layer modified lithium manganese iron phosphate coating obtained in step (7), 90g of polydiethylsiloxane and 600g of ethanol are mixed and placed in a ball mill for wet ball milling. After ball milling, it is placed in an oven at 90°C and dried for 14h to obtain a modified lithium manganese iron phosphate cathode material with a thickness of 2.7μm. The wet ball milling temperature is 25°C, the time is 3h, and the speed of the ball mill is 10000rpm.

[0124] Example 3

[0125] (1) Weigh 10g of lithium iron phosphate with an average particle size of 400nm and 0.8g of starch and place them in a plasma ball mill. Then, treat the plasma ball mill at 790℃ and under a nitrogen atmosphere for 8h to obtain a core with an average particle size of 800nm. The rotation speed of the plasma ball mill is 1100rpm.

[0126] (2) Weigh 70g of lithium manganese iron phosphate particles with an average particle size of 100nm and 120g of tetra(octadecyl) titanate and place them in a plasma ball mill. The plasma ball milling treatment was carried out at 630℃ and under nitrogen atmosphere for 8h to obtain titanium-doped lithium manganese iron phosphate particles with an average particle size of 140nm. The input power of the plasma ball mill was 2.3kW, the rotation speed was 1100rpm, the filling rate was 50%, the ball-to-material ratio was 7:1, the nitrogen pressure was 0.3MPa, and the titanium doping content was 7wt% based on the weight percentage of the titanium-doped lithium manganese iron phosphate particles.

[0127] (3) Weigh 350g of lithium manganese iron phosphate particles with an average particle size of 120nm and 600g of tetra(octadecyl) titanate and place them in a plasma ball mill. Perform plasma ball milling treatment at 700℃ and under nitrogen atmosphere for 8h to obtain titanium-doped lithium manganese iron phosphate particles with an average particle size of 170nm. The input power of the plasma ball mill is 3kW, the rotation speed is 1100rpm, the filling rate is 50%, the ball-to-material ratio is 7:1, the nitrogen pressure is 0.3MPa, and the titanium doping content is 7wt% based on the weight percentage of the titanium-doped lithium manganese iron phosphate particles.

[0128] (4) Weigh 1000g of lithium manganese iron phosphate particles with an average particle size of 160nm and 1710g of tetra(octadecyl) titanate and place them in a plasma ball mill. Perform plasma ball milling treatment at 740℃ and under nitrogen atmosphere for 8h to obtain titanium-doped lithium manganese iron phosphate particles with an average particle size of 230nm. The input power of the plasma ball mill is 3.4kW, the rotation speed is 1100rpm, the filling rate is 50%, the ball-to-material ratio is 7:1, the nitrogen pressure is 0.3MPa, and the titanium doping content is 7wt% based on the weight percentage of the titanium-doped lithium manganese iron phosphate particles.

[0129] (5) The core obtained in step (1) and the titanium-doped lithium manganese iron phosphate particles obtained in step (2) are mixed with 8g of PEG and placed in a plasma ball mill. The mixture is then subjected to plasma ball milling at 540℃ and under a nitrogen atmosphere for 8h to obtain a core with a thickness of 440nm and a modified lithium manganese iron phosphate coating layer on the surface. The plasma ball mill has an input power of 1.4kW, a rotation speed of 1240rpm, a filling rate of 44%, a ball-to-material ratio of 6:1, and a nitrogen pressure of 0.2MPa.

[0130] (6) The core with a modified lithium manganese iron phosphate coating layer obtained in step (5), the titanium-doped lithium manganese iron phosphate particles obtained in step (3), and 40g of PEG were mixed and placed in a plasma ball mill. The mixture was plasma ball milled at 560℃ under a nitrogen atmosphere for 8h to obtain a core with two modified lithium manganese iron phosphate coating layers. The thickness of the second modified lithium manganese iron phosphate coating layer from the inside to the outside was 1200nm. The input power of the plasma ball mill was 1.6kW, the rotation speed was 1240rpm, the filling rate was 44%, the ball-to-material ratio was 6:1, and the nitrogen pressure was 0.2MPa.

[0131] (7) The core with two layers of modified lithium manganese iron phosphate coating obtained in step (6), the titanium-doped lithium manganese iron phosphate particles obtained in step (4), and 140g of PEG were mixed and placed in a plasma ball mill. The mixture was plasma ball milled at 580℃ under a nitrogen atmosphere for 8h to obtain a core with three layers of modified lithium manganese iron phosphate coating. The thickness of the third layer of modified lithium manganese iron phosphate coating from the inside to the outside was 1900nm. The input power of the plasma ball mill was 1.8kW, the rotation speed was 1240rpm, the filling rate was 44%, the ball-to-material ratio was 6:1, and the nitrogen pressure was 0.2MPa.

[0132] (8) The core with a three-layer modified lithium manganese iron phosphate coating obtained in step (7), 80g of polymethyl vinyl siloxane and 600g of ethanol are mixed and placed in a ball mill for wet ball milling. After ball milling, it is placed in an oven at 80°C and dried for 24h to obtain a modified lithium manganese iron phosphate cathode material with a thickness of 3.5μm. The wet ball milling temperature is 25°C, the time is 3h, and the speed of the ball mill is 12000rpm.

[0133] Example 4

[0134] (1) Weigh 10g of lithium iron phosphate with an average particle size of 500nm and 0.8g of PEG and place them in a plasma ball mill. Then, treat the plasma ball mill at 790℃ and under a nitrogen atmosphere for 8h to obtain a core with an average particle size of 900nm. The rotation speed of the plasma ball mill is 1150rpm.

[0135] (2) Weigh 60g of lithium manganese iron phosphate particles with an average particle size of 120nm and 70g of magnesium pyridine carboxylate and place them in a plasma ball mill. The plasma ball milling treatment was carried out at 640℃ and under a nitrogen atmosphere for 9h to obtain magnesium-doped lithium manganese iron phosphate particles with an average particle size of 180nm. The input power of the plasma ball mill was 2.4kW, the rotation speed was 1300rpm, the filling rate was 60%, the ball-to-material ratio was 8:1, the nitrogen pressure was 0.4MPa, and the magnesium doping amount was 10wt% based on the weight percentage of the magnesium-doped lithium manganese iron phosphate particles.

[0136] (3) Weigh 300g of lithium manganese iron phosphate particles with an average particle size of 160nm and 350g of magnesium pyridine carboxylate and place them in a plasma ball mill. The plasma ball milling treatment was carried out at 710℃ and under a nitrogen atmosphere for 9h to obtain magnesium-doped lithium manganese iron phosphate particles with an average particle size of 210nm. The input power of the plasma ball mill was 3.1kW, the rotation speed was 1300rpm, the filling rate was 60%, the ball-to-material ratio was 8:1, the nitrogen pressure was 0.4MPa, and the magnesium doping amount was 10wt% based on the weight percentage of the magnesium-doped lithium manganese iron phosphate particles.

[0137] (4) Weigh 800g of lithium manganese iron phosphate particles with an average particle size of 200nm and 935g of magnesium pyridine carboxylate and place them in a plasma ball mill. The plasma ball milling treatment was carried out at 780℃ and under a nitrogen atmosphere for 9h to obtain vanadium-doped lithium manganese iron phosphate particles with an average particle size of 270nm. The input power of the plasma ball mill was 3.8kW, the rotation speed was 1300rpm, the filling rate was 60%, the ball-to-material ratio was 8:1, the nitrogen pressure was 0.4MPa, and the vanadium doping content was 10wt% based on the weight percentage of the vanadium-doped lithium manganese iron phosphate particles.

[0138] (5) The core obtained in step (1) and the magnesium-doped lithium manganese iron phosphate particles obtained in step (2) are mixed with 7g of PVA and placed in a plasma ball mill. The mixture is then subjected to plasma ball milling at 580℃ and under a nitrogen atmosphere for 9h to obtain a core with a thickness of 510nm and a modified lithium manganese iron phosphate coating layer on the surface. The plasma ball mill has an input power of 1.8kW, a rotation speed of 1280rpm, a filling rate of 46%, a ball-to-material ratio of 7:1, and a nitrogen pressure of 0.25MPa.

[0139] (6) The core with a modified lithium manganese iron phosphate coating layer obtained in step (5), the magnesium-doped lithium manganese iron phosphate particles obtained in step (3), and 37g PVA were mixed and placed in a plasma ball mill. The mixture was plasma ball milled at 600℃ under a nitrogen atmosphere for 9h to obtain a core with two modified lithium manganese iron phosphate coating layers. The thickness of the second modified lithium manganese iron phosphate coating layer from the inside to the outside was 1400nm. The input power of the plasma ball mill was 2.0kW, the rotation speed was 1280rpm, the filling rate was 46%, the ball-to-material ratio was 7:1, and the nitrogen pressure was 0.25MPa.

[0140] (7) The core with two layers of modified lithium manganese iron phosphate coating obtained in step (6), the vanadium-doped lithium manganese iron phosphate particles obtained in step (4), and 120g PVA were mixed and placed in a plasma ball mill. The mixture was plasma ball milled at 620℃ under a nitrogen atmosphere for 9h to obtain a core with three layers of modified lithium manganese iron phosphate coating. The thickness of the third layer of modified lithium manganese iron phosphate coating from the inside to the outside was 2200nm. The input power of the plasma ball mill was 2.2kW, the rotation speed was 1280rpm, the filling rate was 46%, the ball-to-material ratio was 7:1, and the nitrogen pressure was 0.25MPa.

[0141] (8) The core with a three-layer modified lithium manganese iron phosphate coating obtained in step (7), 70g of poly(3-carboxypropylmethylsiloxane) and 800g of ethanol were mixed and placed in a ball mill for wet ball milling. After ball milling, it was placed in an oven at 100℃ for drying for 24h to obtain a modified lithium manganese iron phosphate cathode material with a thickness of 4.1μm. The wet ball milling temperature was 25℃, the time was 3h, and the speed of the ball mill was 13000rpm.

[0142] Example 5

[0143] (1) Weigh 10g of lithium iron phosphate with an average particle size of 600nm and 0.5g of glucose and place them in a plasma ball mill. Then, treat the plasma ball mill at 800℃ and under a nitrogen atmosphere for 7h to obtain a core with an average particle size of 1000nm. The rotation speed of the plasma ball mill is 1200rpm.

[0144] (2) Weigh 60g of lithium manganese iron phosphate particles with an average particle size of 140nm and 110g of vanadium isooctanoate and place them in a plasma ball mill. The plasma ball milling treatment was carried out at 650℃ and under a nitrogen atmosphere for 10h to obtain vanadium-doped lithium manganese iron phosphate particles with an average particle size of 200nm. The input power of the plasma ball mill was 2.5kW, the rotation speed was 1450rpm, the filling rate was 70%, the ball-to-material ratio was 10:1, the nitrogen pressure was 0.5MPa, and the vanadium doping content was 11wt% based on the weight percentage of the vanadium-doped lithium manganese iron phosphate particles.

[0145] (3) Weigh 300g of lithium manganese iron phosphate particles with an average particle size of 190nm and 550g of vanadium isooctanoate and place them in a plasma ball mill. The plasma ball milling treatment was carried out at 720℃ and under a nitrogen atmosphere for 10h to obtain vanadium-doped lithium manganese iron phosphate particles with an average particle size of 250nm. The input power of the plasma ball mill was 3.2kW, the rotation speed was 1450rpm, the filling rate was 70%, the ball-to-material ratio was 10:1, the nitrogen pressure was 0.5MPa, and the vanadium doping content was 11wt% based on the weight percentage of the vanadium-doped lithium manganese iron phosphate particles.

[0146] (4) Weigh 800g of lithium manganese iron phosphate particles with an average particle size of 200nm and 1467g of vanadium isooctanoate and place them in a plasma ball mill. The plasma ball milling is carried out at 790℃ and under a nitrogen atmosphere for 10h to obtain vanadium-doped lithium manganese iron phosphate particles with an average particle size of 300nm. The input power of the plasma ball mill is 3.9kW, the rotation speed is 1450rpm, the filling rate is 70%, the ball-to-material ratio is 10:1, the nitrogen pressure is 0.5MPa, and the vanadium doping content is 11wt% based on the weight percentage of the vanadium-doped lithium manganese iron phosphate particles.

[0147] (5) The core obtained in step (1) and the vanadium-doped lithium manganese iron phosphate particles obtained in step (2) are mixed with 5g of citric acid and placed in a plasma ball mill. The mixture is then subjected to plasma ball milling at 600℃ and under a nitrogen atmosphere for 10h to obtain a core with a thickness of 600nm coated with a modified lithium manganese iron phosphate coating layer. The plasma ball mill has an input power of 2kW, a rotation speed of 1300rpm, a filling rate of 50%, a ball-to-material ratio of 7:1, and a nitrogen pressure of 0.3MPa.

[0148] (6) The core with a modified lithium manganese iron phosphate coating layer obtained in step (5) and the vanadium-doped lithium manganese iron phosphate particles obtained in step (3) are mixed with 20g of citric acid and placed in a plasma ball mill. The mixture is then subjected to plasma ball milling at 620℃ and under a nitrogen atmosphere for 10h to obtain a core with two modified lithium manganese iron phosphate coating layers. The thickness of the second modified lithium manganese iron phosphate coating layer from the inside to the outside is 1500nm. The input power of the plasma ball mill is 2.2kW, the rotation speed is 1300rpm, the filling rate is 50%, the ball-to-material ratio is 7:1, and the nitrogen pressure is 0.3MPa.

[0149] (7) The core with two layers of modified lithium manganese iron phosphate coating obtained in step (6), the vanadium-doped lithium manganese iron phosphate particles obtained in step (4), and 90g of starch were mixed and placed in a plasma ball mill. The mixture was plasma ball milled at 640℃ under a nitrogen atmosphere for 10h to obtain a core with three layers of modified lithium manganese iron phosphate coating. The thickness of the third layer of modified lithium manganese iron phosphate coating from the inside to the outside was 2500nm. The input power of the plasma ball mill was 2.4kW, the rotation speed was 1300rpm, the filling rate was 50%, the ball-to-material ratio was 7:1, and the nitrogen pressure was 0.3MPa.

[0150] (8) The core with a three-layer modified lithium manganese iron phosphate coating obtained in step (7), 60g of polydimethylsiloxane and 800g of ethanol are mixed and placed in a ball mill for wet ball milling. After ball milling, it is placed in an oven at 120°C and dried for 24 hours to obtain a modified lithium manganese iron phosphate cathode material with a thickness of 4.6μm. The wet ball milling temperature is 25°C, the time is 2h, and the speed of the ball mill is 14000rpm.

[0151] Example 6

[0152] The difference from Example 1 is that the weight ratio of yttrium isooctanoate to lithium manganese iron phosphate particles in steps (2), (3) and (4) is 0.1:1, and the other steps are the same as in Example 1.

[0153] Based on the weight percentage of yttrium-doped lithium manganese iron phosphate particles, the yttrium doping content is 6 wt%.

[0154] Example 7

[0155] The difference from Example 1 is that the weight ratio of yttrium isooctanoate to lithium manganese iron phosphate particles in steps (2), (3) and (4) is 2:1, and the other steps are the same as in Example 1.

[0156] Based on the weight percentage of yttrium-doped lithium manganese iron phosphate particles, the yttrium doping content is 55 wt%.

[0157] Example 8

[0158] The difference from Example 1 is that the temperature of plasma ball milling in step (1) is 800°C and the time is 10h, while the rest of the steps are the same as in Example 1.

[0159] Example 9

[0160] The difference from Example 1 is that the plasma ball milling treatment in step (1) is at a temperature of 650°C for 3 hours, while the other steps are the same as in Example 1.

[0161] Example 10

[0162] The difference from Example 1 is that in steps (2), (3) and (4), the input power of the plasma ball mill is 0.5kW, the rotation speed is 800rpm, the filling rate is 80%, the ball-to-material ratio is 4:1, and the nitrogen pressure is 0.6MPa. The remaining steps are the same as in Example 1.

[0163] Example 11

[0164] The difference from Example 1 is that in steps (5), (6) and (7), the input power of the plasma ball mill is 5kW, the rotation speed is 1000rpm, the filling rate is 60%, the ball-to-material ratio is 4:1, and the nitrogen pressure is 0.4MPa. The remaining steps are the same as in Example 1.

[0165] Example 12

[0166] The difference from Example 1 is that step (6) includes: mixing the core with a modified lithium manganese iron phosphate coating layer obtained in step (5) and the yttrium-doped lithium manganese iron phosphate particles obtained in step (2) with 9g of sucrose, placing them in a plasma ball mill, and plasma ball milling them at 500°C and under a nitrogen atmosphere for 5h to obtain a core with two modified lithium manganese iron phosphate coating layers, wherein the thickness of the second modified lithium manganese iron phosphate coating layer from the inside to the outside is 300nm;

[0167] Step (7) includes: mixing the core with two layers of modified lithium manganese iron phosphate coating obtained in step (6), the yttrium-doped lithium manganese iron phosphate particles obtained in step (2), and 9g of sucrose, placing them in a plasma ball mill, and treating them with plasma ball milling at 500°C and under a nitrogen atmosphere for 5h to obtain a core with three layers of modified lithium manganese iron phosphate coating, wherein the thickness of the third layer of modified lithium manganese iron phosphate coating from the inside to the outside is 300nm; the remaining steps are the same as in Example 1.

[0168] Comparative Example 1

[0169] A method for preparing a modified lithium manganese iron phosphate cathode material includes the following steps:

[0170] (1) Weigh 10g of lithium iron phosphate with an average particle size of 400nm and 0.5g of glucose and place them in a plasma ball mill. Then, treat the plasma ball mill at 780℃ and under a nitrogen atmosphere for 5h to obtain a core with an average particle size of 600nm. The rotation speed of the plasma ball mill is 950rpm.

[0171] (2) Weigh 60g of lithium manganese iron phosphate particles with an average particle size of 50nm and 5g of glucose and place them in a plasma ball mill. The plasma ball milling treatment is carried out at 600℃ and under a nitrogen atmosphere for 5h to obtain first carbon-coated lithium manganese iron phosphate particles with an average particle size of 60nm. The input power of the plasma ball mill is 2kW, the rotation speed is 950rpm, the filling rate is 30%, the ball-to-material ratio is 5:1, and the nitrogen pressure is 0.1MPa.

[0172] (3) Weigh 300g of lithium manganese iron phosphate particles with an average particle size of 60nm and 30g of glucose and place them in a plasma ball mill. The plasma ball milling is carried out at 670℃ and under a nitrogen atmosphere for 5h to obtain second carbon-coated lithium manganese iron phosphate particles with an average particle size of 90nm. The input power of the plasma ball mill is 2.7kW, the rotation speed is 950rpm, the filling rate is 30%, the ball-to-material ratio is 5:1, and the nitrogen pressure is 0.1MPa.

[0173] (4) Weigh 800g of lithium manganese iron phosphate particles with an average particle size of 100nm and 80g of glucose and place them in a plasma ball mill. The plasma ball milling is carried out at 740℃ and under a nitrogen atmosphere for 5h to obtain third carbon-coated lithium manganese iron phosphate particles with an average particle size of 150nm. The input power of the plasma ball mill is 3.4kW, the rotation speed is 950rpm, the filling rate is 30%, the ball-to-material ratio is 5:1, and the nitrogen pressure is 0.1MPa.

[0174] (5) The core obtained in step (1) and the first carbon-coated lithium manganese iron phosphate particles obtained in step (2) are mixed with 5g of glucose and placed in a plasma ball mill. The mixture is then subjected to plasma ball milling at 500℃ and under a nitrogen atmosphere for 5h to obtain a core with a thickness of 300nm and a modified lithium manganese iron phosphate coating layer on the surface. The plasma ball mill has an input power of 1kW, a rotation speed of 1200rpm, a filling rate of 40%, a ball-to-material ratio of 5:1, and a nitrogen pressure of 0.1MPa.

[0175] (6) The core with a modified lithium manganese iron phosphate coating layer obtained in step (5) and the second carbon-coated lithium manganese iron phosphate particles obtained in step (3) are mixed with 20g of glucose and placed in a plasma ball mill. The mixture is then subjected to plasma ball milling at 520℃ and under a nitrogen atmosphere for 5h to obtain a core with two modified lithium manganese iron phosphate coating layers. The plasma ball mill has an input power of 1.2kW, a rotation speed of 1200rpm, a filling rate of 40%, a ball-to-material ratio of 5:1, a nitrogen pressure of 0.1MPa, and a thickness of 700nm for the second modified lithium manganese iron phosphate coating layer from the inside out.

[0176] (7) The core with two layers of modified lithium manganese iron phosphate coating obtained in step (6) and the third carbon-coated lithium manganese iron phosphate particles obtained in step (4) are mixed with 90g of glucose and placed in a plasma ball mill. The plasma ball mill is treated at 550℃ and under nitrogen atmosphere for 5h to obtain a core with three layers of modified lithium manganese iron phosphate coating. The input power of the plasma ball mill is 1.5kW, the rotation speed is 1200rpm, the filling rate is 40%, the ball-to-material ratio is 5:1, the nitrogen pressure is 0.1MPa, and the thickness of the third layer of modified lithium manganese iron phosphate coating from the inside to the outside is 1300nm.

[0177] (8) The core with a three-layer modified lithium manganese iron phosphate coating obtained in step (7), 60g of polydimethylsiloxane and 500g of ethanol are mixed and placed in a ball mill for wet ball milling. After ball milling, it is placed in an oven at 80°C and dried for 12h to obtain a lithium manganese iron phosphate cathode material with a thickness of 2.4μm. The wet ball milling temperature is 25°C, the time is 2h, and the speed of the ball mill is 10000rpm.

[0178] Comparative Example 2

[0179] The difference from Example 1 is that steps (2), (3), (4) and (8) are omitted, and the core obtained in step (1) is directly used to perform steps (5) to (7) to obtain a core with three layers of modified lithium manganese iron phosphate coating on the surface. Each layer of modified lithium manganese iron phosphate coating is not doped with yttrium. The remaining steps are the same as in Example 1.

[0180] Comparative Example 3

[0181] The difference from Example 1 is that step (1) is omitted, and lithium iron phosphate is used to replace the core in step (5). The remaining steps are the same as in Example 1.

[0182] Comparative Example 4

[0183] The difference from Example 1 is that steps (2) to (8) are omitted, and the core obtained in step (1) is used directly as the positive electrode material for the assembly of lithium-ion batteries.

[0184] Using the modified lithium manganese iron phosphate cathode material and cathode material prepared in Examples 1 to 12 and Comparative Examples 1 to 4 as the positive electrode active material, artificial graphite as the negative electrode material, 1 mol / L LiPF6 / EC+DEC (volume ratio 1:1) as the electrolyte, and Celgard 2400 membrane as the separator, a lithium-ion battery was assembled.

[0185] The following performance tests were performed on all lithium-ion batteries obtained in this application:

[0186] (1) The compaction density of the modified lithium manganese iron phosphate cathode material was measured according to the method of GB / T 30835-2014 "Carbon Composite Lithium Iron Phosphate Cathode Material for Lithium-ion Batteries".

[0187] (2) The conductivity of the modified lithium manganese iron phosphate cathode material was measured according to the method of GB / T 30835-2014 "Carbon Composite Lithium Iron Phosphate Cathode Material for Lithium-ion Batteries".

[0188] (3) The conditions for the first charge specific capacity and the first coulombic efficiency test are as follows: 0.2C constant current charging, voltage range is 2.0~4.5V;

[0189] (4) The test conditions for discharge specific capacity are as follows: constant current discharge at 1C rate, voltage range is 4.5~2V;

[0190] (5) The test conditions for capacity retention are as follows: 55℃, voltage range 2~4.5V, 500 cycles.

[0191] The test results are shown in Table 1.

[0192] Table 1

[0193]

[0194] As can be seen from the above description, the embodiments of the present invention achieve the following technical effects:

[0195] Comparing Example 1 and Comparative Examples 1 to 4, it can be seen that the modified lithium manganese iron phosphate particles in this application include lithium manganese iron phosphate particles doped with the aforementioned specific types of metal elements. Compared with other types, the use of the aforementioned types of metal elements can improve the conductivity of lithium manganese iron phosphate particles, thereby improving the conductivity of the modified lithium manganese iron phosphate cathode material, and thus improving the electrochemical performance (such as initial coulombic efficiency and energy density) of the prepared lithium-ion battery. The silicon in the polysiloxane organic coating layer has a strong binding affinity for fluoride ions in the electrolyte. When the modified lithium manganese iron phosphate cathode material of this application is applied to a lithium-ion battery, silicon preferentially binds to fluoride ions, thereby inhibiting its reaction with manganese ions, and thus inhibiting the dissolution of manganese ions during high-temperature cycling of the lithium-ion battery, improving the cycle stability of the lithium-ion battery.

[0196] Comparative examples 1 to 5 show that placing at least one layer of modified lithium manganese iron phosphate coating on the surface of the core can leverage the high-voltage characteristics of manganese in lithium manganese iron phosphate, thereby improving the energy density of the modified lithium manganese iron phosphate cathode material. The average particle size of the metal-doped lithium manganese iron phosphate particles and the thickness of the modified lithium manganese iron phosphate coating layer increase sequentially from the inside to the outside. On the one hand, this helps to reduce the diffusion difficulty of lithium ions in the inner layer of the modified lithium manganese iron phosphate cathode material, improve the lithium ion transport efficiency, and thus improve the lithium ion conductivity of the modified lithium manganese iron phosphate cathode material. On the other hand, it also helps to increase the compaction density of the modified lithium manganese iron phosphate cathode material, thereby improving the discharge specific capacity and cycle stability of the prepared lithium-ion battery.

[0197] Comparing Examples 1, 6, and 7, it can be seen that the weight ratio of organometallic compounds to lithium manganese iron phosphate particles includes, but is not limited to, the above-mentioned range. Limiting it to the above-mentioned range is beneficial to improving the conductivity of lithium manganese iron phosphate particles, controlling the size of lithium manganese iron phosphate particles, and inhibiting their excessive growth. This is beneficial to improving the conductivity of the modified lithium manganese iron phosphate cathode material, and thus beneficial to improving the electrochemical performance of the prepared lithium-ion battery, such as the first coulombic efficiency and energy density.

[0198] Comparing Examples 1, 8, and 9, it can be seen that limiting the temperature and time of the plasma ball milling treatment in step S1 to the above range, compared to other ranges, is beneficial to improving the effect of the plasma ball milling treatment and also beneficial to improving the uniformity of the coating, thereby improving the energy density, conductivity, and cycle stability of the modified lithium manganese iron phosphate cathode material.

[0199] Comparing Examples 1, 10, and 11, it can be seen that the process parameters of plasma ball milling in step S2, such as temperature, time, input power, rotation speed, filling rate, and ball-to-material ratio, are not limited to the above-mentioned ranges. Limiting them to the above-mentioned ranges is beneficial to improving the effect of plasma ball milling in step S2, and to making the metal element doping more uniform, thereby improving the conductivity of the modified lithium manganese iron phosphate cathode material. The process parameters of plasma ball milling in step S3, such as temperature, time, input power, rotation speed, filling rate, and ball-to-material ratio, are not limited to the above-mentioned ranges. Limiting them to the above-mentioned ranges is beneficial to improving the effect of plasma ball milling in step S3, and to improving the uniformity of coating, thereby improving the energy density, conductivity, and cycle stability of the modified lithium manganese iron phosphate cathode material.

[0200] Comparing Examples 1 and 12, it can be seen that the thickness of the first modified coating layer, the second modified coating layer, and the third modified coating layer, as well as the average particle size of the lithium manganese iron phosphate particles doped with metal elements contained in each of them, include, but are not limited to, the above-mentioned ranges. Limiting them to the above-mentioned ranges is beneficial in reducing the difficulty of ion diffusion in the inner layer of the modified lithium manganese iron phosphate cathode material, thereby improving the discharge specific capacity of the modified lithium manganese iron phosphate cathode material. On the other hand, it is also beneficial in increasing the compaction density of the modified lithium manganese iron phosphate cathode material, thereby improving the discharge specific capacity and cycle stability of the prepared lithium-ion battery.

[0201] It should be noted that the terms "first," "second," etc., used in the specification and claims of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such terms can be used interchangeably where appropriate, so that the embodiments of this application described herein can be implemented, for example, in orders other than those described herein.

[0202] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a modified lithium manganese iron phosphate cathode material, characterized in that, Step S1: The first carbon source and lithium iron phosphate are subjected to plasma ball milling to obtain the core. Step S2 involves plasma ball milling of the organometallic compound and lithium iron phosphate particles to obtain metal-doped lithium iron phosphate particles; the weight ratio of the organometallic compound to the lithium iron phosphate particles is (0.1-2):1; Step S2 is repeated to obtain three different metal-doped lithium iron phosphate particles by plasma ball milling using three different average particle sizes; the input power of the plasma ball milling process is independently 1-5 kW, the rotation speed is independently 900-1450 rpm, the filling rate is independently 30-70%, and the ball-to-material ratio is independently (5-10):

1. Step S3 involves plasma ball milling the metal-doped lithium manganese iron phosphate particles, the second carbon source, and the core. This step S3 is repeated three times to obtain a core with a three-layer modified lithium manganese iron phosphate coating. In each plasma ball milling process in step S3, the input power is 1-3 kW, the rotation speed is 1200-1300 rpm, the filling rate is 40-50%, and the ball-to-material ratio is (5-7):

1. Step S4: The core, polysiloxane organic compound, and solvent coated with three layers of modified lithium manganese iron phosphate are wet ball-milled and dried to obtain the modified lithium manganese iron phosphate cathode material. The modified lithium manganese iron phosphate cathode material comprises, from the inside out, the core, a three-layer modified lithium manganese iron phosphate coating layer, and a polysiloxane-based organic coating layer; the core includes a lithium iron phosphate core and a first carbon coating layer disposed on the surface of the lithium iron phosphate core; Each modified lithium manganese iron phosphate coating layer includes modified lithium manganese iron phosphate particles, wherein the modified lithium manganese iron phosphate particles include the metal element-doped lithium manganese iron phosphate particles and a second carbon coating layer disposed on their surface; the thickness of the three modified lithium manganese iron phosphate coating layers increases sequentially from the inside to the outside, and the average particle size of the metal element-doped lithium manganese iron phosphate particles in the three modified lithium manganese iron phosphate coating layers increases sequentially from the inside to the outside; The thickness of the polysiloxane-based organic coating layer is 2–15 nm.

2. The method for preparing the modified lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The organometallic compound is selected from one or more of yttrium isooctanoate, vanadium isooctanoate, tristrontium citrate, tetra(octadecyl) titanate, and magnesium pyridinecarboxylate.

3. The method for preparing the modified lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The doping amount of the metal element is 5 to 15 wt%, based on the weight percentage of the lithium manganese iron phosphate particles doped with the metal element; the metal element is selected from one or more of Y, V, Sr, Ti and Mg.

4. The method for preparing the modified lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The thickness of the second carbon coating layer is 2–5 nm.

5. The method for preparing the modified lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The average particle size of the core is 500-1000 nm; the thickness of the polysiloxane organic coating layer is 5-10 nm; and the thickness of the first carbon coating layer is 2-5 nm.

6. The method for preparing the modified lithium manganese iron phosphate cathode material according to claim 5, characterized in that, The average particle size of the kernel is 600–700 nm.

7. The method for preparing the modified lithium manganese iron phosphate cathode material according to any one of claims 1 to 6, characterized in that, The three-layer modified lithium manganese iron phosphate coating consists of a first modified coating layer, a second modified coating layer, and a third modified coating layer from the inside out. The thickness of the first modified coating layer is 300–600 nm, and the average particle size of the metal-doped lithium manganese iron phosphate particles therein is 50–200 nm. The thickness of the second modified coating layer is 800–1500 nm, and the average particle size of the metal-doped lithium manganese iron phosphate particles therein is 100–250 nm. The thickness of the third modified coating layer is 1400–2500 nm, and the average particle size of the metal-doped lithium manganese iron phosphate particles therein is 150–300 nm.

8. The method for preparing the modified lithium manganese iron phosphate cathode material according to claim 7, characterized in that, The thickness of the first modified coating layer is 300-400 nm, and the average particle size of the metal-doped lithium manganese iron phosphate particles therein is 50-100 nm; the thickness of the second modified coating layer is 800-1000 nm, and the average particle size of the metal-doped lithium manganese iron phosphate particles therein is 100-150 nm; the thickness of the third modified coating layer is 1400-2000 nm, and the average particle size of the metal-doped lithium manganese iron phosphate particles therein is 150-200 nm.

9. The method for preparing the modified lithium manganese iron phosphate cathode material according to claim 7, characterized in that, The polysiloxane organic compound is selected from one or more of the group consisting of polydimethylsiloxane, polydiethylsiloxane, polymethylvinylsiloxane, and poly(3-carboxypropylmethylsiloxane); and / or, The weight ratio of the core, the first modified coating layer, the second modified coating layer, the third modified coating layer and the polysiloxane organic coating layer is (0.5-1):(5-8):(20-40):(80-120):(5-10).

10. The method for preparing the modified lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In step S2, the three types of lithium manganese iron phosphate particles with different average particle sizes are a first lithium manganese iron phosphate particle, a second lithium manganese iron phosphate particle, and a third lithium manganese iron phosphate particle; the average particle size of the first lithium manganese iron phosphate particle is 50-150 nm, the average particle size of the second lithium manganese iron phosphate particle is 51-190 nm, and the average particle size of the third lithium manganese iron phosphate particle is 100-200 nm.

11. The method for preparing the modified lithium manganese iron phosphate cathode material according to claim 10, characterized in that, In step S2, the temperature of the plasma ball milling process for the first lithium manganese iron phosphate particles, the second lithium manganese iron phosphate particles, and the third lithium manganese iron phosphate particles is increased sequentially to 600-650°C, 670-720°C, and 740-790°C, respectively. In step S2, the plasma ball milling treatment time for the first lithium manganese iron phosphate particles, the second lithium manganese iron phosphate particles, and the third lithium manganese iron phosphate particles is independently 5 to 10 hours. In step S2, the plasma ball milling of the first, second, and third lithium manganese iron phosphate particles is carried out independently under a nitrogen atmosphere with a pressure of 0.1 to 0.5 MPa.

12. The method for preparing the modified lithium manganese iron phosphate cathode material according to claim 10, characterized in that, In step S1, the plasma ball milling process is carried out at a temperature of 780–800°C, for a time of 5–10 hours, and at a rotation speed of 950–1200 rpm, under a nitrogen atmosphere.

13. The method for preparing the modified lithium manganese iron phosphate cathode material according to claim 10, characterized in that, In step S3, the temperature of each plasma ball milling treatment is 500-650°C, and the time is 5-8 hours. In step S3, each plasma ball milling process is performed under a nitrogen atmosphere with a pressure of 0.1 to 0.3 MPa.

14. The method for preparing the modified lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In step S4, the wet ball milling temperature is 20-25°C, the rotation speed is 10000-15000 rpm, and the time is 2-5 hours. The drying temperature in step S4 is 80-120°C, and the drying time is 12-24 hours. The solvent is selected from one or more of the group consisting of ethanol, water and methanol.

15. The method for preparing the modified lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The first carbon source and the second carbon source are each independently selected from one or more of the group consisting of glucose, sucrose, starch, polyethylene glycol, polyvinyl alcohol and citric acid; The plasma ball milling process was performed using a plasma ball mill.

16. A lithium-ion battery, comprising a positive electrode, a negative electrode, an electrolyte, and a separator disposed between the positive electrode and the negative electrode, characterized in that, The positive electrode comprises the modified lithium manganese iron phosphate positive electrode material prepared by the preparation method of the modified lithium manganese iron phosphate positive electrode material according to claim 1.

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