A battery positive electrode material and its preparation method and application

Abstract

The present application provides a battery positive electrode material, comprising a core and a first shell layer arranged on the core, wherein the core comprises LiMn x Fe 1‑x PO4, first shell includes LiMn y Fe 1‑y PO4, wherein 0<x≤0.4, 0.6≤y≤0.9. The positive electrode material of the battery has a multi-layer distribution structure, which can shorten the transmission distance of electrons and ions, thereby improving the electrochemical performance of the positive electrode material. Applying it in a battery can effectively improve the rate performance of the battery. The present application also provides a preparation method of the above-mentioned positive electrode material for a battery and its application.

Description

Technical Field

[0001] The present application relates to the field of lithium-ion batteries, and in particular to a battery positive electrode material and a preparation method and application thereof. Background Art

[0002] Polyanionic lithium iron phosphate LiFePO4 is recognized as the safest cathode material for lithium-ion batteries, but its theoretical energy density is low (586Wh / kg), which greatly limits its widespread application. Lithium manganese phosphate LiMnPO4 has a similar structure to lithium iron phosphate and a higher theoretical energy density (701Wh / kg), which has great application prospects. However, the electronic conductivity and lithium ion diffusion rate of lithium manganese phosphate (<10 -10 S / cm and 10 -15 cm 2 / s) than lithium iron phosphate (10 -9 S / cm and 10 -14 cm 2 / s) is lower, and there is a problem of divalent Mn dissolution, which makes it difficult for lithium manganese phosphate to obtain excellent electrochemical performance. At present, a more feasible solution is to partially replace Mn with Fe to form LiMn x Fe 1-x PO4 (0<x<1), a compromise is made between energy density and kinetic performance, and on this basis, necessary modifications are made to the material to obtain a material with excellent performance. Summary of the invention

[0003] In view of this, the present application provides a battery positive electrode material, which is a LiMn with high manganese content and poor electronic and ionic conductivity. x Fe 1-x PO4 (0.6≤x≤1) is placed in the outer layer, while LiMn with low manganese content and good electronic and ionic conductivity is placed in the outer layer. x Fe 1-x The PO4 (0<x≤0.4) part is placed in the inner layer, which effectively shortens the distance in the part where electron and ion transmission is difficult, and improves the electrochemical performance of the positive electrode material.

[0004] The present application provides a battery positive electrode material in a first aspect, comprising a core and a first shell layer arranged on the core, wherein the core comprises LiMn x Fe 1-x PO4, the first shell layer includes LiMn y Fe 1-y PO4, where 0<x≤0.4, 0.6≤y≤0.9.

[0005] In the positive electrode material of the battery, the diffusion path of ions and electrons from the inner core of the material to the outside is long, which is not conducive to the transmission of ions and electrons. In the present application, Fe element is doped into LiMnPO4 to improve the conductivity of the positive electrode material. The higher proportion of iron element in the inner core of the material can increase the diffusion rate of ions and electrons, thereby improving the electrochemical performance of the positive electrode material. The first shell layer has a higher proportion of manganese element, thereby ensuring that the positive electrode material of the battery has a higher energy density. Through this structural setting, the positive electrode material of the battery can have a higher energy density and better conductivity. When it is applied to the battery, the battery can have a higher charge and discharge capacity.

[0006] Optionally, the LiMn x Fe 1-x In PO4, 0<x≤0.4.

[0007] Optionally, the mass ratio of the inner core to the first shell is 1:(1.5-9).

[0008] Optionally, the inner core has a diameter of 0.5 μm-5 μm.

[0009] Optionally, the thickness of the first shell layer is 0.09 μm-2.7 μm.

[0010] Optionally, in the battery positive electrode material, the molar ratio of Fe element to Mn element is 1:(1-9).

[0011] Optionally, a second shell layer is further disposed on the surface of the first shell layer, and the second shell layer comprises LiFePO4 and carbon.

[0012] Optionally, the second shell layer has a thickness of 20nm-600nm.

[0013] Optionally, in the battery positive electrode material, the mass percentage of the inner core is 10%-40%; the mass percentage of the first shell layer is 45%-80%; and the mass percentage of the second shell layer is 10%-20%.

[0014] Optionally, the carbon accounts for 1%-3% of the mass percentage of the battery positive electrode material.

[0015] In a second aspect, the present application provides a method for preparing a positive electrode material for a battery, comprising:

[0016] A lithium source, an iron source, a manganese source, a phosphorus source and a solvent are placed in a reaction kettle for a first reaction to obtain a core, wherein the core includes LiMn x Fe 1-x PO4, wherein 0<x≤0.4; the temperature of the first reaction is 375℃-500℃, and the pressure of the first reaction is greater than or equal to 23Mpa;

[0017] A lithium source, an iron source, a manganese source, a phosphorus source, a solvent and the core are placed in a reactor for a second reaction, and a first shell layer is coated on the surface of the core to obtain a core provided with a first shell layer; the first shell layer includes LiMn y Fe 1-y PO4, wherein 0.6≤y≤0.9; the temperature of the second reaction is 375°C-500°C, and the pressure of the second reaction is greater than or equal to 23Mpa;

[0018] The inner core provided with the first shell layer is calcined at 500° C.-800° C. to obtain a positive electrode material for a battery.

[0019] Optionally, before calcination, the inner core provided with the first shell layer further includes: placing a lithium source, an iron source, a phosphorus source, a carbon source, a solvent and the inner core provided with the first shell layer in a reactor for a third reaction, coating the surface of the first shell layer with a second shell layer, wherein the second shell layer includes LiFePO4 and carbon.

[0020] Optionally, the lithium source includes an inorganic lithium salt and an organic lithium salt; the inorganic lithium salt includes one or more of lithium carbonate, lithium bicarbonate, lithium dihydrogen phosphate, lithium monohydrogen phosphate, lithium phosphate, lithium nitrate, lithium sulfate, lithium chromate and lithium hydroxide; the organic lithium salt includes one or more of lithium oxalate, lithium acetate, lithium benzoate, lithium citrate and lithium benzoate.

[0021] Optionally, the iron source includes one or more of ferrous oxide, ferrous carbonate, ferrous oxalate, ferrous sulfate, ferrous chloride and ferrous acetate.

[0022] Optionally, the manganese source includes one or more of manganese monoxide, manganese dioxide, manganese hydroxide, manganese carbonate, manganese oxalate, manganese sulfate, manganese nitrate, manganese sulfate, manganese chloride and manganese acetate.

[0023] Optionally, the phosphorus source includes one or more of phosphoric acid, ammonium dihydrogen phosphate, ammonium monohydrogen phosphate, and lithium dihydrogen phosphate.

[0024] Optionally, the carbon source includes one or more of glucose, sucrose, polyvinyl alcohol, starch, and citric acid.

[0025] Optionally, the solvent comprises water.

[0026] Optionally, the reaction time of the first reaction, the second reaction and the third reaction is 0.5h-10h.

[0027] Optionally, the calcination time is 2h-10h.

[0028] Optionally, the calcination is performed under an inert atmosphere, and the inert atmosphere includes one or more of nitrogen, argon, helium, neon, krypton, xenon, and radon.

[0029] In a second aspect, the present application provides a positive electrode plate, comprising a current collector and a positive electrode material layer disposed on the current collector, wherein the positive electrode material layer comprises the battery positive electrode material as described in the first aspect.

[0030] In a third aspect, the present application provides a secondary battery, comprising a positive electrode, a negative electrode, a separator and an electrolyte, wherein the positive electrode comprises the positive electrode plate as described in the third aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 A positive electrode material for a battery provided in one embodiment of the present application;

[0032] Figure 2 Schematic diagram of the structure of a battery positive electrode material of structure A;

[0033] Figure 3 Schematic diagram of the structure of the positive electrode material of structure B;

[0034] Figure 4 A method for preparing a positive electrode material for a battery provided in one embodiment of the present application;

[0035] Figure 5 A method for preparing a positive electrode material for a battery is provided in another embodiment of the present application. DETAILED DESCRIPTION

[0036] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0037] Although lithium manganese phosphate LiMnPO4 has a higher theoretical energy density than lithium iron phosphate, its electronic conductivity and lithium ion diffusion rate are lower than those of lithium iron phosphate, which is not conducive to its capacity. In addition, in the positive electrode material of the battery, the distance for the ions and electrons inside the material to be transmitted to the outside is long, and the ion and electron transmission is more difficult, which further limits the capacity of the lithium manganese phosphate positive electrode material. In order to improve the electrochemical performance of the lithium manganese phosphate positive electrode material, the present application provides a battery positive electrode material. Please refer to Figure 1 , Figure 1A battery positive electrode material provided in one embodiment of the present application. The battery positive electrode material of the present application includes a core 10 and a first shell 20 disposed on the surface of the core 10, and the first shell 20 completely covers the core 10. The core 10 includes a lithium iron manganese phosphate compound as shown in Formula I, and the first shell 20 includes a lithium iron manganese phosphate compound as shown in Formula II.

[0038] LiMn x Fe 1-x PO4, formula I, wherein 0<x≤0.4.

[0039] LiMn y Fe 1-y PO4, formula II, wherein 0.6≤y≤0.9.

[0040] In the embodiment of the present application, LiMn x Fe 1-x The value of x in PO4 can be, but is not limited to, 0.1, 0.2, 0.3 or 0.4, LiMn y Fe 1-y In PO4, the value of y can be, but is not limited to, 0.6, 0.7, 0.8 or 0.9. Controlling x to be less than or equal to 0.4 can ensure that the inner layer has a higher electrical conductivity, and controlling y to be greater than or equal to 0.6 can ensure that the material as a whole has a higher energy density. In some embodiments of the present application, the value of x is 0.2-0.3, and the value of y is 0.8-0.9. In the battery positive electrode material of the present application, the mass percentage of iron in the inner core is higher than the mass percentage of iron in the first shell layer. The higher proportion of iron in the inner core can make the inner core have better conductivity, increase the transmission speed of ions and electrons at the inner core position, and promote the transmission of ions and electrons inside the battery positive electrode material; the first shell layer has a higher proportion of manganese content, thereby ensuring that the battery positive electrode material as a whole has a higher energy density.

[0041] The following is a comparison of the positive electrode material with structure B to illustrate that the positive electrode material of the present application can improve the electrochemical performance of the material. The positive electrode material of the present application takes structure A as an example. The composition of the positive electrode material of the battery with structure A from the inside to the outside is LiMn 0.2 F e0.8 PO4(40%) / LiMn 0.8 Fe 0.2 PO4 (60%), of which LiMn 0.2 Fe 0.8 PO4 accounts for 40% of the mass percentage of the battery positive electrode material, LiMn 0.8 Fe 0.2 PO4 accounts for 60% of the mass percentage of the positive electrode material of the battery. The positive electrode material of structure B is composed of LiMn from the inside to the outside. 0.8 Fe0.2 PO4(60%) / LiMn 0.2 F e0.8 PO4 (40%), see Figure 2 and Figure 3 , Figure 2 Schematic diagram of the structure of the battery positive electrode material of structure A, Figure 3 Figure 1 is a schematic diagram of the structure of the positive electrode material of structure B. The arrangement positions of the two layers of materials in structure A and structure B are opposite, that is, LiMn as the shell layer in structure A 0.8 Fe 0.2 PO4 is the core in structure B, and LiMn 0.8 Fe 0.2 PO4 and LiMn 0.2 F e0.8 The mass ratio of PO4 is 3:2, that is, the energy density of the positive electrode material of the battery structure A and the positive electrode material of the structure B is the same. 0.8 Fe 0.2 PO4 and LiMn 0.2 F e0.8 The material density of PO4 is almost the same, and although the structural arrangements of structure A and structure B are different, the overall size of their particles is similar.

[0042] For the positive electrode material of structure B, its core LiMn 0.8 Fe 0.2 The manganese content in PO4 is high and the iron content is low, so it is difficult for electrons and ions to be transported in the inner core, that is, the inner core is a difficult-to-transmit layer. For the positive electrode material of structure B, the maximum distance for ions and electrons to be transported in the difficult-to-transmit layer is 0.843R. The calculation of this transport distance is as follows: Assuming that the radius of the positive electrode material is R and the radius of the inner core is R1, since the inner core LiMn 0.8 Fe 0.2 PO4 accounts for 60% of the mass percentage of the positive electrode material, so R and R1 have the following relationship:

[0043]

[0044]

[0045] From the above formula, we can get R1=0.843R, that is, for the positive electrode material of structure B, the maximum transmission distance of ions and electrons in its transport-difficult layer is 0.843R.

[0046] For the battery positive electrode material of structure A, its shell layer is the difficult transmission layer. Assuming that the radius of the battery positive electrode material is R and the radius of the inner core is R2, the maximum transmission distance of ions and electrons in the difficult transmission layer of the battery positive electrode material of structure A is R-R2, and the calculation is as follows:

[0047]

[0048]

[0049] From the above formula, we can get R2 = 0.737R. For the positive electrode material of structure A, the maximum transmission distance of ions and electrons in the transmission difficulty layer is 0.263R. 0.8 Fe 0.2 In the positive electrode material with PO4 placed in the inner core, the transmission distance of the electrons of the ions in the transmission-difficult layer in the battery positive electrode material of structure A is shorter, and the distance shortening ratio is 69%, wherein the distance shortening ratio is calculated as: (0.843R-0.263R) / 0.843R.

[0050] Similarly, when LiMn 0.8 Fe 0.2 When the mass percentage of PO4 in the positive electrode material is 70%, the transmission distance of the difficult layer is 0.331R for the positive electrode material of structure A, and 0.888R for the positive electrode material of structure B. That is, compared with structure B, LiMn 0.8 Fe 0.2 The distribution structure of PO4 placed in the core, the transmission distance of the electrons in the transmission difficulty layer of the battery positive electrode material of structure A is shorter, and the distance reduction ratio is 63%. This application also lists the transmission distance comparison of positive electrode materials with other content compositions, please refer to Table 1, Table 1 shows different contents of LiMn 0.8 Fe 0.2 Transmission distance comparison table of PO4 positive electrode materials.

[0051] Table 1 LiMn with different contents 0.8 Fe 0.2 Transmission distance comparison table of PO4 positive electrode materials

[0052] serial number <![CDATA[LiMn 0.8 Fe 0.2 PO4 content]]> Structure A Structure B Distance reduction ratio 1 60％ 0.263R 0.843R 69％ 2 70％ 0.331R 0.888R 63％ 3 80％ 0.415R 0.928R 55％ 4 90％ 0.536R 0.965R 44％

[0053] It can be clearly seen from the above calculations that, under the premise of having the same energy density, the structural distribution of the present application can effectively shorten the transmission distance of ions and electrons in the difficult-to-transmit layer in the battery positive electrode material, thereby improving the electrochemical properties of the battery positive electrode material, so that the battery positive electrode material can effectively improve the battery's rate performance.

[0054] In the embodiment of the present application, the mass proportion of the first shell layer in the positive electrode material of the battery is greater than the mass proportion of the core, and further, the mass ratio of the core to the first shell layer is 1:(1.5-9). The mass ratio of the core to the first shell layer can be, but is not limited to, 1:1.5, 1:2, 1:4, 1:5, 1:6, 1:7, 1:8 or 1:9. Controlling the mass ratio of the first shell layer within the above range can ensure that the positive electrode material of the battery has a higher energy density, and the material inside can also have a higher conductivity. If the mass proportion of the first shell layer is too large, the core-shell distribution structure has a weak regulating effect on the positive electrode material, and the electrochemical performance of the positive electrode material cannot be effectively improved.

[0055] In an embodiment of the present application, the diameter of the inner core is 0.5 μm-5 μm, and the diameter of the inner core can be, but is not limited to, 0.5 μm, 0.8 μm, 1 μm, 2 μm, 4 μm or 5 μm. In an embodiment of the present application, the thickness of the first shell layer is 0.09 μm-2.7 μm. The thickness of the first shell layer can be, but is not limited to, 0.09 μm, 0.1 μm, 0.5 μm, 1 μm, 2 μm or 2.7 μm.

[0056] In the embodiment of the present application, the molar ratio of Fe element to Mn element in the positive electrode material of the battery is 1:(1-9). The molar ratio of Fe element to Mn element in the positive electrode material of the battery can be, but is not limited to, 1:1, 1:3, 1:5 or 1:9. For example, when the composition of the positive electrode material of the battery from the inside to the outside is LiMn 0.2 F e0.8 PO4(20%) / LiMn 0.8 Fe 0.2 When PO4 (80%) is used, the molar ratio of Fe to Mn is 1:2.125. Controlling the molar ratio of Fe to Mn in the positive electrode material of the battery within the above range can ensure that the battery has a higher energy density.

[0057] In some embodiments of the present application, the positive electrode material of the battery also includes doping elements, and the doping elements include one or more of Ti, V, Co, Ni, Cu, Zn, Mg, Al, and Ca. The doping elements can improve the ionic and electronic conductivity of the positive electrode material of the battery. In the present application, both the core and the first shell of the positive electrode material of the battery can be doped with elements to improve conductivity. In some embodiments of the present application, the doping elements are located in the first shell of the positive electrode material of the battery. In some embodiments of the present application, the mass percentage of the doping elements in the positive electrode material of the battery is 0.1%-0.5%. The mass percentage of the doping elements in the positive electrode material of the battery can be specifically but not limited to 0.1%, 0.3%, 0.4% or 0.5%.

[0058] In some embodiments of the present application, a second shell layer is also provided on the surface of the first shell layer of the positive electrode material of the battery, and the second shell layer includes LiFePO4 and carbon. Since Mn will dissolve in the electrolyte and deposit at the interface between the electrode and the electrolyte to form a high resistance layer, increasing the internal resistance of the battery and reducing the rate performance and cycle performance of the battery, coating LiFePO4 on the surface of the first shell layer can isolate the first shell layer from the electrolyte, thereby inhibiting the dissolution of Mn. In some embodiments of the present application, the mass percentage of LiFePO4 in the positive electrode material of the battery is 10%-20%. The mass percentage of LiFePO4 in the positive electrode material of the battery can be, but not limited to, 10%, 13%, 15% or 20%. In the present application, the carbon in the second shell layer can improve the conductivity of the positive electrode material of the battery and realize charging and discharging at a larger rate. In some embodiments of the present application, the mass percentage of carbon in the positive electrode material of the battery is 1%-5%. The mass percentage of carbon in the positive electrode material of the battery can be, but not limited to, 1%, 3%, 4% or 5%. In some embodiments of the present application, the thickness of the second shell layer is 20nm-600nm. The thickness of the second shell layer can be, but is not limited to, 20nm, 50nm, 100nm, 200nm, 300nm, 400nm, 500nm or 600nm. In some embodiments of the present application, the thickness of the second shell layer is 25nm-400nm.

[0059] The battery positive electrode material provided in the present application not only has a high energy density, but also has good conductivity and can achieve high-rate charging and discharging. Applying it in a battery can effectively improve the cycle performance and rate performance of the battery.

[0060] This application also provides a method for preparing a positive electrode material for a battery. Figure 4 , Figure 4 A method for preparing a positive electrode material for a battery provided in one embodiment of the present application comprises:

[0061] Step 100: placing a lithium source, an iron source, a manganese source, a phosphorus source and a solvent in a reaction kettle for a first reaction to obtain a core, the core comprising LiMn x Fe 1-x PO4, wherein 0<x≤0.4; the temperature of the first reaction is 375℃-500℃, and the pressure of the first reaction is greater than or equal to 23Mpa;

[0062] Step 200: placing a lithium source, an iron source, a manganese source, a phosphorus source, a solvent and a core in a reactor to carry out a second reaction, coating the surface of the core with a first shell layer to obtain a core provided with a first shell layer; the first shell layer includes LiMn y Fe 1-yPO4, wherein 0.6≤y≤0.9; the temperature of the second reaction is 375°C-500°C, and the pressure of the second reaction is greater than or equal to 23Mpa;

[0063] Step 300: calcining the inner core provided with the first shell layer at 500° C.-800° C. to obtain a positive electrode material for a battery.

[0064] In some embodiments of the present application, the solvent includes water. In some embodiments of the present application, the lithium source includes an inorganic lithium salt and an organic lithium salt, wherein the inorganic lithium salt includes one or more of lithium carbonate, lithium bicarbonate, lithium dihydrogen phosphate, lithium monohydrogen phosphate, lithium phosphate, lithium nitrate, lithium sulfate, lithium chromate and lithium hydroxide, and the organic lithium salt includes one or more of lithium oxalate, lithium acetate, lithium benzoate, lithium citrate and lithium benzoate. In some embodiments of the present application, the iron source includes one or more of iron oxide, ferrous carbonate, ferrous oxalate, ferrous sulfate, ferrous chloride and ferrous acetate. In some embodiments of the present application, the manganese source includes one or more of manganese monoxide, manganese dioxide, manganese hydroxide, manganese carbonate, manganese oxalate, manganese sulfate, manganese nitrate, manganese sulfate, manganese chloride and manganese acetate. In some embodiments of the present application, the phosphorus source includes one or more of phosphoric acid, ammonium dihydrogen phosphate, ammonium monohydrogen phosphate and lithium dihydrogen phosphate.

[0065] In an embodiment of the present application, the temperature of the first reaction is 375°C-500°C, the pressure of the first reaction is 23MPa-60Mpa, and the reaction time of the first reaction is 0.5h-10h. In some embodiments of the present application, the reaction time of the first reaction is 0.5h-3h, and a structurally stable inner core can be formed at a reaction time of 0.5h-3h. The reaction time of the first reaction can be specifically but not limited to 0.5h, 1h, 2h, 3h, 5h or 10h. In an embodiment of the present application, the temperature of the second reaction is 375°C-500°C, the pressure of the second reaction is 23MPa-60Mpa, and the reaction time of the second reaction is 0.5h-10h. In some embodiments of the present application, the reaction time of the second reaction is 0.5h-3h, and a structurally stable first shell can be formed at a reaction time of 0.5h-3h. In an embodiment of the present application, the first reaction and the second reaction are reacted in an inert atmosphere, and the inert atmosphere includes one or more of nitrogen, argon, helium, neon, krypton, xenon, and radon.

[0066] In the embodiment of the present application, the calcination temperature is 500°C-800°C, and the calcination time is 2h-10h. The calcination temperature can be, but not limited to, 500°C, 600°C, 700°C or 800°C, and the calcination time can be, but not limited to, 2h, 4h, 6h or 10h.

[0067] The present application adopts a supercritical fluid method to prepare the inner core and the first shell layer. When the reaction temperature is greater than or equal to 375°C and the pressure is greater than or equal to 23Mpa, the solvent in the reactor is in a supercritical fluid state, which can greatly improve the activity of the reaction and is conducive to the formation of a dense crystal structure. In addition, the method is easy to operate and the battery positive electrode material can be obtained by one sintering, which greatly shortens the reaction time.

[0068] In some embodiments of the present application, the method for preparing the positive electrode material of the battery further includes preparing a second shell layer. Figure 5 , Figure 5 A method for preparing a positive electrode material for a battery provided in another embodiment of the present application comprises:

[0069] Step 100: placing a lithium source, an iron source, a manganese source, a phosphorus source and a solvent in a reaction kettle for a first reaction to obtain a core, the core comprising LiMn x Fe 1-x PO4, wherein 0<x≤0.4; the temperature of the first reaction is 375℃-500℃, and the pressure of the first reaction is greater than or equal to 23Mpa;

[0070] Step 200: placing a lithium source, an iron source, a manganese source, a phosphorus source, a solvent and a core in a reactor to carry out a second reaction, coating the surface of the core with a first shell layer to obtain a core provided with a first shell layer; the first shell layer includes LiMn y Fe 1-y PO4, wherein 0.6≤y≤0.9; the temperature of the second reaction is 375°C-500°C, and the pressure of the second reaction is greater than or equal to 23Mpa;

[0071] Step 300: placing a lithium source, an iron source, a phosphorus source, a carbon source, a solvent and a core having a first shell layer in a reactor to carry out a third reaction, coating the surface of the first shell layer with a second shell layer, wherein the second shell layer comprises LiFePO4 and carbon; the temperature of the third reaction is 375° C.-500° C., and the pressure of the third reaction is greater than or equal to 23 MPa;

[0072] Step 400: calcining the inner core provided with the first and second shell layers at 500° C.-800° C. to obtain a positive electrode material for a battery.

[0073] In some embodiments of the present application, the carbon source includes one or more of glucose, sucrose, polyvinyl alcohol, starch, and citric acid. The temperature of the third reaction is 375°C-500°C, the pressure of the third reaction is 23MPa-60Mpa, and the reaction time of the third reaction is 0.5h-10h. The use of a supercritical fluid method is conducive to the formation of a dense second shell layer, thereby more effectively inhibiting the dissolution of Mn. In an embodiment of the present application, the third reaction is carried out in an inert atmosphere.

[0074] The preparation method of the battery positive electrode material provided in the present application is simple and highly operable, and can produce a battery positive electrode material with excellent rate performance and cycle performance.

[0075] The present application also provides a positive electrode sheet, which includes a current collector and a positive electrode material layer disposed on the current collector, wherein the positive electrode material layer includes the battery positive electrode material of the present application. In the embodiment of the present application, the preparation of the positive electrode material layer can be to mix the battery positive electrode material, the conductive agent, the binder and the solvent to form a positive electrode slurry, and the positive electrode slurry is coated and dried to obtain the positive electrode material layer. When preparing the positive electrode slurry, the binder and the solvent can be mixed first, and then the conductive agent is added after sufficient stirring, and then the battery positive electrode material is added after stirring, and the positive electrode slurry is obtained by sieving after stirring. Among them, the conductive agent, the binder and the solvent are conventional choices in the field of batteries. For example, the binder can be selected from one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl alcohol (PVA), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), polyimide (PI), polyacrylic acid (PAA), polyacrylate, polyolefin, sodium carboxymethyl cellulose (CMC) and sodium alginate. The conductive agent can be selected from one or more of carbon nanotubes, carbon black and graphene.

[0076] The present application also provides a secondary battery, which includes a positive electrode, a negative electrode, an electrolyte, and a separator located between the positive electrode and the negative electrode, wherein the positive electrode includes the positive electrode plate provided in the present application.

[0077] In the present application, the negative electrode of the secondary battery may be any negative electrode known in the art. In the embodiment of the present application, the negative electrode may include one or more of a carbon-based negative electrode, a silicon-based negative electrode, a tin-based negative electrode and a lithium negative electrode. Among them, the carbon-based negative electrode may include graphite, hard carbon, soft carbon, graphene, etc.; the silicon-based negative electrode may include silicon, silicon carbon, silicon oxygen, silicon metal compounds, etc.; the tin-based negative electrode may include tin, tin carbon, tin oxygen, tin metal compounds; the lithium negative electrode may include metallic lithium or a lithium alloy. The lithium alloy may specifically be at least one of a lithium silicon alloy, a lithium sodium alloy, a lithium potassium alloy, a lithium aluminum alloy, a lithium tin alloy and a lithium indium alloy. In some embodiments of the present application, the current collector of the negative electrode is copper foil, and the negative electrode active material includes one or more of natural graphite, artificial graphite, hard carbon, soft carbon, lithium titanate, iron oxide, lithium titanium phosphate, titanium dioxide, silicon, silicon oxide, tin and antimony; the binder includes one or more of polyacrylic acid (PAA), polyvinylidene fluoride (PVDF), carboxymethyl cellulose (CMC) and styrene butadiene latex (SBR); the conductive agent includes one or more of acetylene black, ketjen black, Super-P, carbon nanotubes, carbon nanofibers, activated carbon and graphene. In the present application, the preparation method of the negative electrode can adopt any method known in the art.

[0078] In the present application, the separator of the secondary battery can be any separator known to those skilled in the art, for example, the separator can be one or more of polyolefin microporous membrane, polyethylene terephthalate, polyethylene felt, glass fiber felt or ultrafine glass fiber paper.

[0079] In the present application, the electrolyte of the secondary battery includes a solution of an electrolyte lithium salt in a non-aqueous solvent. In an embodiment of the present application, the electrolyte lithium salt includes lithium hexafluorophosphate (LiPF6), lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), lithium hexafluorosilicate (LiSiF6), lithium tetraphenylborate (LiB(C6H5)4), lithium chloride (LiCl), lithium bromide (LiBr), lithium chloroaluminate (LiAlCl4), lithium fluoroalkyl sulfonate (LiC(SO2CF3)3), LiCH3SO3, LiN(SO2CF3)2 and LiN(SO2C2F5)2. In some embodiments of the present application, the non-aqueous solvent includes one or more of a chain acid ester and a cyclic acid ester. In some embodiments of the present application, the chain acid ester includes one or more of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC) and dipropyl carbonate (DPC). In some embodiments of the present application, the chain acid ester includes chain organic esters containing fluorine, sulfur or unsaturated bonds. In some embodiments of the present application, the cyclic acid ester includes one or more of ethylene carbonate (EC), propylene carbonate (PC), vinylene carbonate (VC), γ-butyrolactone (γ-BL) and sultone. In some embodiments of the present application, the cyclic acid ester includes cyclic organic esters containing fluorine, sulfur or unsaturated bonds. In some embodiments of the present application, the non-aqueous solvent includes one or more of chain ethers and cyclic ether solutions. In some embodiments of the present application, the cyclic ether includes one or more of tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF), 1,3-dioxolane (DOL) and 4-methyl-1,3-dioxolane (4-MeDOL). In some embodiments of the present application, the cyclic ether includes a cyclic organic ether containing fluorine, sulfur or unsaturated bonds. In some embodiments of the present application, the chain ether includes one or more of dimethoxymethane (DMM), 1,2-dimethoxyethane (DME), 1,2-dimethoxypropane (DMP) and diethylene glycol dimethyl ether (DG). In some embodiments of the present application, the chain ether includes a chain organic ether containing fluorine, sulfur or unsaturated bonds. In an embodiment of the present application, the concentration of the electrolyte lithium salt in the electrolyte is 0.1mol / L-15mol / L. In some embodiments of the present application, the concentration of the electrolyte lithium salt is 1mol / L-10mol / L.

[0080] In the embodiments of the present application, the battery can be prepared by using either a lamination process or a winding process. In some embodiments of the present application, the battery is prepared by using a lamination process.

[0081] The secondary battery provided in the present application has good cycle performance and safety performance due to the use of the battery positive electrode sheet of the present application.

[0082] The technical solution of the present application is further described below with reference to a number of embodiments.

[0083] Example 1

[0084] A method for preparing a positive electrode material for a battery, wherein the structure of the positive electrode material for the battery is LiMn from inside to outside. 0.2 Fe 0.8 PO4(30%) / LiMn 0.8 Fe 0.2 PO4(60%) / LiFePO4(10%) / C, the general formula of which is LiMn 0.54 Fe 0.46 PO4 / C. Among them, LiMn 0.2 Fe 0.8 PO4(30%) represents LiMn 0.2 Fe 0.8 The mass percentage of PO4 in the positive electrode active material of the battery is 30%, and LiFePO4 (10%) means that the mass percentage of LiFePO4 in the positive electrode active material of the battery is 10%.

[0085] 1) Preparation of LiMn 0.2 Fe 0.8 PO4 core

[0086] Lithium carbonate, ferric acetate, manganese acetate and phosphoric acid were weighed according to the stoichiometric ratio and dissolved in water to obtain a mixed solution, the mixed solution was transferred to a high-pressure reactor, argon gas was introduced into the high-pressure reactor to maintain an inert atmosphere, the temperature was raised to 380°C under stirring, the pressure of the reactor was 24 MPa, the precipitate was collected after reacting for 1 hour, the precipitate was washed with water and ethanol and dried to obtain LiMn 0.2 Fe 0.8 PO4 core.

[0087] 2) Preparation of LiMn 0.8 Fe 0.2 PO4 first shell

[0088] Lithium carbonate, ferric acetate, manganese acetate and phosphoric acid were weighed according to the stoichiometric ratio and dissolved in water to obtain a mixed solution. The mixed solution was transferred to a high-pressure reactor and LiMn 0.2 Fe 0.8PO4 core, argon was introduced into the high-pressure reactor to maintain an inert atmosphere, and the temperature was raised to 380°C under stirring. The pressure of the reactor was 24 MPa. After reacting for 1 hour, the precipitate was collected, and the precipitate was washed with water and ethanol and dried to obtain a core-shell structure of LiMn 0.2 Fe 0.8 PO4 / LiMn 0.8 Fe 0.2 PO4.

[0089] 3) Preparation of the second shell

[0090] Lithium carbonate, ferric acetate, glucose and phosphoric acid were weighed according to the stoichiometric ratio and dissolved in water to obtain a mixed solution. The mixed solution was transferred to a high-pressure reactor and a core-shell structured LiMn 0.2 Fe 0.8 PO4 / LiMn 0.8 Fe 0.2 PO4, argon was introduced into the high-pressure reactor to maintain an inert atmosphere, and the temperature was raised to 380°C under stirring. The pressure of the reactor was 24Mpa. After reacting for 1 hour, the precipitate was collected, washed with water and ethanol and dried, and the product was calcined at 700°C for 5 hours to obtain the battery positive electrode material LiMn 0.2 Fe 0.8 PO4(30%) / LiMn 0.8 Fe 0.2 PO4(60%) / LiFePO4(10%) / C.

[0091] 4) Preparation of secondary battery

[0092] The prepared positive electrode material of the battery, the conductive agent and the binder are dispersed in N-methylpyrrolidone at a mass ratio of 90:5:5, wherein the conductive agent is carbon black and carbon nanotubes, the binder is PVDF5130, and the solid content of the slurry is 50%. The slurry is coated on the surface of aluminum foil, dried and cut into 61mm*72mm positive electrode sheets, and the positive electrode sheets are assembled with the separator and the graphite negative electrode to obtain a battery.

[0093] Example 2

[0094] The difference between Example 2 and Example 1 is that the mass ratio of the core to the first shell is different. The positive electrode material structure of the battery in Example 2 is LiMn 0.2 Fe 0.8 PO4(10%) / LiMn 0.8 Fe 0.2 PO4(80%) / LiFePO4(10%) / C, the general formula of which is LiMn 0.66 Fe 0.34 PO4 / C. Among them, LiMn 0.2 Fe0.8 PO4 accounts for 10% of the mass percentage of the positive electrode active material of the battery, LiMn 0.8 Fe 0.2 The mass percentage of PO4 in the positive electrode active material of the battery is 80%. The battery is prepared by the same method as in Example 1.

[0095] Example 3

[0096] The difference between Example 3 and Example 1 is that the mass ratio of the core to the first shell is different. The positive electrode material structure of the battery in Example 3 is LiMn 0.2 Fe 0.8 PO4(40%) / LiMn 0.8 Fe 0.2 PO4(45%) / LiFePO4(15%) / C, the general formula of which is LiMn 0.44 Fe 0.56 PO4 / C. Among them, LiMn 0.2 Fe 0.8 PO4 accounts for 40% of the mass percentage of the positive electrode active material of the battery, LiMn 0.8 Fe 0.2 The mass percentage of PO4 in the positive electrode active material of the battery is 45%. The battery is prepared by the same method as in Example 1.

[0097] Example 4

[0098] The difference between Example 4 and Example 1 is that the mass ratio of the core to the first shell is different. The positive electrode material structure of the battery in Example 4 is LiMn 0.2 Fe 0.8 PO4(10%) / LiMn 0.8 Fe 0.2 PO4(75%) / LiFePO4(15%) / C, the general formula of which is LiMn 0.62 Fe 0.38 PO4 / C. Among them, LiMn 0.2 Fe 0.8 PO4 accounts for 10% of the mass percentage of the positive electrode active material of the battery, LiMn 0.8 Fe 0.2 The mass percentage of PO4 in the positive electrode active material of the battery is 75%. The battery is prepared by the same method as in Example 1.

[0099] Example 5

[0100] The difference between Example 5 and Example 1 is that the mass ratio of the core to the first shell is different. The positive electrode material structure of the battery in Example 5 is LiMn 0.2 Fe 0.8 PO4(20%) / LiMn0.8 Fe 0.2 PO4(70%) / LiFePO4(10%) / C, the general formula of which is LiMn 0.6 Fe 0.4 PO4 / C. Among them, LiMn 0.2 Fe 0.8 PO4 accounts for 20% of the mass percentage of the positive electrode active material of the battery, LiMn 0.8 Fe 0.2 The mass percentage of PO4 in the positive electrode active material of the battery is 70%. The battery is prepared by the same method as in Example 1.

[0101] Example 6

[0102] The difference between Example 6 and Example 5 is that the components of the core are different. The positive electrode material structure of the battery in Example 6 is LiMn 0.4 Fe 0.6 PO4(20%) / LiMn 0.8 Fe 0.2 PO4(70%) / LiFePO4(10%) / C, the general formula of which is LiMn 0.64 Fe 0.36 PO4 / C. Among them, LiMn 0.4 Fe 0.6 PO4 accounts for 20% of the mass percentage of the positive electrode active material of the battery, LiMn 0.8 Fe 0.2 The mass percentage of PO4 in the positive electrode active material of the battery is 70%. The battery is prepared by the same method as in Example 1.

[0103] Example 7

[0104] The difference between Example 7 and Example 5 is that the components of the core and the first shell are different. The positive electrode material structure of the battery in Example 7 is LiFePO4 (20%) / LiMn 0.8 Fe 0.2 PO4(70%) / LiFePO4(10%) / C, the general formula of which is LiMn 0.56 Fe 0.44 PO4 / C. Among them, LiFePO4 accounts for 20% of the mass percentage of the positive electrode active material of the battery, LiMn 0.8 Fe 0.2 The mass percentage of PO4 in the positive electrode active material of the battery is 70%. The battery is prepared by the same method as in Example 1.

[0105] Example 8

[0106] The difference between Example 8 and Example 5 is that the components of the core and the first shell are different. The positive electrode material structure of the battery in Example 8 is LiMn 0.2 Fe 0.8 PO4(20%) / LiMn 0.9 Fe 0.1 PO4(70%) / LiFePO4(10%) / C, the general formula of which is LiMn 0.67 Fe 0.33 PO4 / C. Among them, LiMn 0.2 Fe 0.8 PO4 accounts for 20% of the mass percentage of the positive electrode active material of the battery, LiMn 0.9 Fe 0.1 The mass percentage of PO4 in the positive electrode active material of the battery is 70%. The battery is prepared by the same method as in Example 1.

[0107] Example 9

[0108] The difference between Example 9 and Example 5 is that the components of the core and the first shell are different. The positive electrode material structure of the battery in Example 9 is LiMn 0.2 Fe 0.8 PO4(20%) / LiMn 0.6 Fe 0.4 PO4(70%) / LiFePO4(10%) / C, the general formula of which is LiMn 0.56 Fe 0.44 PO4 / C. Among them, LiMn 0.2 Fe 0.8 PO4 accounts for 20% of the mass percentage of the positive electrode active material of the battery, LiMn 0.6 Fe 0.4 The mass percentage of PO4 in the positive electrode active material of the battery is 70%. The battery is prepared by the same method as in Example 1.

[0109] Example 10

[0110] The difference between Example 10 and Example 5 is that the components of the core and the first shell are different. The core and the first shell of the battery positive electrode material of Example 10 are doped with Co. The structure of the battery positive electrode material is LiMn from inside to outside. 0.19 Fe 0.8 Co 0.01 PO4(20%) / LiMn 0.79 Fe 0.2 Co 0.01 PO4(70%) / LiFePO4(10%) / C, the general formula of which is LiMn 0.591 Fe 0.40 Co 0.009 PO4 / C.

[0111] Embodiment 10 The preparation method of the battery positive electrode material comprises:

[0112] 1) Preparation of LiMn 0.19 Fe 0.8 Co 0.01 PO Kernel

[0113] Lithium carbonate, ferric acetate, manganese acetate, cobalt acetate and phosphoric acid were weighed according to the stoichiometric ratio and dissolved in water to obtain a mixed solution, the mixed solution was transferred to a high-pressure reactor, argon gas was introduced into the high-pressure reactor to maintain an inert atmosphere, the temperature was raised to 380° C. under stirring, the pressure of the reactor was 24 MPa, the precipitate was collected after reacting for 1 hour, the precipitate was washed with water and ethanol and dried to obtain LiMn 0.19 Fe 0.8 Co 0.01 PO4 core.

[0114] 2) Preparation of LiMn 0.79 Fe 0.2 Co 0.01 PO4 first shell

[0115] Lithium carbonate, iron acetate, manganese acetate, cobalt acetate and phosphoric acid were weighed according to the stoichiometric ratio and dissolved in water to obtain a mixed solution. The mixed solution was transferred to a high-pressure reactor and LiMn 0.19 Fe 0.8 Co 0.01 PO4 core, argon was introduced into the high-pressure reactor to maintain an inert atmosphere, and the temperature was raised to 380°C under stirring. The pressure of the reactor was 24 MPa. After reacting for 1 hour, the precipitate was collected, and the precipitate was washed with water and ethanol and dried to obtain a core-shell structure of LiMn 0.19 Fe 0.8 Co 0.01 PO4 / LiMn 0.79 Fe 0.2 Co 0.01 PO4.

[0116] The second shell layer and the battery were prepared in the same manner as in Example 5.

[0117] Embodiment 11

[0118] The difference between Example 11 and Example 5 is that the second shell does not contain LiFePO4, and the positive electrode material structure of the battery in Example 11 is LiMn 0.2 Fe 0.8 PO4(25%) / LiMn 0.8 Fe 0.2 PO4(75%) / C, the general formula of which is LiMn 0.65 Fe 0.35PO4 / C. A battery was prepared in the same manner as in Example 1.

[0119] Example 12

[0120] The difference between Example 12 and Example 5 is that the mass proportion of each layer is different. The positive electrode material structure of the battery in Example 12 is LiMn 0.2 Fe 0.8 PO4(20%) / LiMn 0.8 Fe 0.2 PO4(75%) / LiFePO4(5%) / C, the general formula of which is LiMn 0.64 Fe 0.36 PO4 / C. A battery was prepared in the same manner as in Example 1.

[0121] Comparative Example 1

[0122] The difference between Comparative Example 1 and Example 1 is that the mass ratio of the core to the first shell is different. The positive electrode material structure of Comparative Example 1 is LiMn 0.2 Fe 0.8 PO4(60%) / LiMn 0.8 Fe 0.2 PO4(30%) / LiFePO4(10%) / C, the general formula of which is LiMn 0.36 Fe 0.64 PO4 / C. Among them, LiMn 0.2 Fe 0.8 PO4 accounts for 60% of the mass percentage of the positive electrode active material of the battery, LiMn 0.8 Fe 0.2 The mass percentage of PO4 in the positive electrode active material of the battery is 30%. The battery is prepared by the same method as in Example 1.

[0123] Comparative Example 2

[0124] The difference between Comparative Example 2 and Example 1 is that the structure of the positive electrode material is different. The positive electrode material structure of Comparative Example 2 is LiMn 0.8 Fe 0.2 PO4(70%) / LiMn 0.2 Fe 0.8 PO4(20%) / LiFePO4(10%) / C, the general formula of which is LiMn 0.6 Fe 0.3 PO4 / C. A battery was prepared in the same manner as in Example 1.

[0125] Comparative Example 3

[0126] The material structure prepared in Comparative Example 3 is the same as that prepared in Example 1, except that Comparative Example 3 is prepared by a hydrothermal method. The positive electrode material preparation method of Comparative Example 3 is:

[0127] 1) Preparation of LiMn 0.2 Fe 0.8 PO4 core

[0128] Lithium carbonate, ferric acetate, manganese acetate and phosphoric acid were weighed according to the stoichiometric ratio and dissolved in water to obtain a mixed solution, the mixed solution was transferred to a hydrothermal reactor, argon gas was introduced into the hydrothermal reactor to maintain an inert atmosphere, the temperature was raised to 175°C under stirring, the precipitate was collected after reacting for 8 hours, the precipitate was washed with water and ethanol and dried to obtain LiMn 0.2 Fe 0.8 PO4 core.

[0129] 2) Preparation of LiMn 0.8 Fe 0.2 PO4 first shell

[0130] Lithium carbonate, ferric acetate, manganese acetate and phosphoric acid were weighed according to the stoichiometric ratio and dissolved in water to obtain a mixed solution. The mixed solution was transferred to a hydrothermal reactor and LiMn 0.2 Fe 0.8 PO4 core, argon was introduced into the hydrothermal reactor to maintain an inert atmosphere, and the temperature was raised to 175°C under stirring. After reacting for 8 hours, the precipitate was collected and washed with water and ethanol and dried to obtain a core-shell structured LiMn 0.2 Fe 0.8 PO4 / LiMn 0.8 Fe 0.2 PO4.

[0131] 3) Preparation of the second shell

[0132] Lithium carbonate, ferric acetate, glucose and phosphoric acid were weighed according to the stoichiometric ratio and dissolved in water to obtain a mixed solution. The mixed solution was transferred to a hydrothermal reactor and the core-shell structured LiMn 0.2 Fe 0.8 PO4 / LiMn 0.8 Fe 0.2 PO4, argon was introduced into the hydrothermal reactor to maintain an inert atmosphere, and the temperature was raised to 175°C under stirring. After reacting for 6.5 hours, the precipitate was collected, washed with water and ethanol and dried, and the product was calcined at 720°C for 7 hours to obtain the battery positive electrode material LiMn 0.2 Fe 0.8 PO4(30%) / LiMn 0.8 Fe 0.2PO4 (60%) / LiFePO4 (10%) / C. A battery was prepared in the same manner as in Example 1.

[0133] Comparative Example 4

[0134] The difference between Comparative Example 4 and Example 5 is that the components of the first shell layer are different. The positive electrode material structure of Comparative Example 4 is LiMn 0.2 Fe 0.8 PO4(20%) / LiMn 0.5 Fe 0.5 PO4(70%) / LiFePO4(10%) / C, the general formula of which is LiMn 0.39 Fe 0.61 PO4 / C. Among them, LiMn 0.2 Fe 0.8 PO4 accounts for 20% of the mass percentage of the positive electrode active material of the battery, LiMn 0.5 Fe 0.5 The mass percentage of PO4 in the positive electrode active material of the battery is 70%. The battery is prepared by the same method as in Example 5.

[0135] Effect Example

[0136] In order to verify the performance of the battery positive electrode material prepared in the present application, the present application also provides an effect embodiment.

[0137] 1) The electrochemical performance test was performed on the batteries of Examples 1-12 and Comparative Examples 1-4. The batteries of Examples 1-12 and Comparative Examples 1-4 were charged at 0.1C constant current and constant voltage in the voltage test range of 2.8-4.3V at 25°C, with a cut-off current of 0.02C and a constant current discharge of 0.1C. The initial charge capacity and discharge capacity were recorded, and the battery parameters were calculated according to the following formulas: discharge specific capacity = battery initial discharge capacity (mAh) / positive electrode material weight (g), energy density = discharge specific capacity (mAh / g)*average discharge voltage (V). Please refer to Table 2 for the test results.

[0138] Table 2 Electrochemical performance parameters of batteries of Examples 1-12 and Comparative Examples 1-4

[0139]

[0140]

[0141] In Example 1 of the present application, the first shell is LiMn 0.8 Fe 0.2 PO4 and LiMn 0.8 Fe 0.2 PO4 accounts for 60% of the mass of the positive electrode material of the battery, LiMn 0.2Fe 0.8 The mass proportion of PO4 is 30%. This structural distribution makes the positive electrode material of the battery have a higher conductivity, so that the battery has good rate performance. 0.8 Fe 0.2 PO4 accounts for 80% of the mass of the battery positive electrode material, which makes the battery have a higher energy density, but also because of LiMn 0.8 Fe 0.2 The PO4 layer is thicker than that in Example 1, so its rate performance is slightly worse. 0.8 Fe 0.2 PO4 accounts for 45% of the mass of the battery positive electrode material, so that the battery has a higher energy density, but its rate performance is better than that of Example 1. In Example 4, the mass of the second shell layer in the battery positive electrode material is 15%, and LiMn 0.8 Fe 0.2 The mass proportion of PO4 is 75%, and the overall material has a higher energy density than that of Example 1. 0.8 Fe 0.2 The mass proportion of PO4 is 70%, and its rate performance is similar to that of Example 1. The composition of the core in Example 6 is LiMn 0.4 Fe 0.6 PO4, compared with Example 5, Example 6 has a higher energy density, but its rate performance is relatively poor. The core of the positive electrode material of the battery in Example 7 is LiFePO4, which has good conductivity. Compared with Example 5, the battery in Example 7 has better rate performance. The first shell of Example 8 is LiMn 0.9 Fe 0.1 PO4, a higher manganese content makes the battery have a higher energy density, but its rate performance is poor. The first shell of Example 9 is LiMn 0.6 Fe 0.4 PO4, its manganese content is lower than that of Example 5, so its rate performance is better and its energy density is lower. The positive electrode material of the battery in Example 10 is doped with Co element, and the battery in Example 10 has better rate performance than that in Example 5. The second shell layer of Example 11 does not contain LiFePO4, and the content of the first shell layer is higher, so it has a higher energy density, and the content of the second shell layer is lower, so its effect of inhibiting manganese dissolution is poor, so the battery cycle performance is poor. The second shell layer of Example 12 contains a certain amount of LiFePO4, and the battery has better cycle performance than that of Example 11.

[0142] LiMn in Comparative Example 1 0.8 Fe 0.2 PO4 accounts for 30% of the mass of the positive electrode material of the battery, and the energy density of the battery is relatively low.0.8 Fe 0.2 PO4 is the core of the material, and its general composition formula is similar to that of Example 5, but its rate performance is much lower than that of Example 5. Comparative Example 3 uses a hydrothermal method to prepare the positive electrode material. Since the material prepared by the hydrothermal method has poor crystallinity, the shell layer and the core layer interpenetrate each other to form a transition structure during the subsequent sintering process. The stratification in the material is not obvious, and the expected design structure cannot be achieved. The conduction of electrons and ions in the material is not significantly improved. In Comparative Example 4, the first shell layer is LiMn 0.5 Fe 0.5 PO4, which has a lower manganese content, has a lower energy density in the battery.

[0143] The above is a preferred embodiment of the present application, but it should not be understood as limiting the scope of the present application. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present application, and these improvements and modifications are also considered to be within the scope of protection of the present application.

Claims

1. A battery positive electrode material, It is characterized in that The invention comprises a core and a first shell layer arranged on the surface of the core, wherein a second shell layer is arranged on the surface of the first shell layer, and the core comprises LiMn x Fe 1-x PO 4 , the first shell layer includes LiMn y Fe 1- y PO 4 , wherein 0<x≤0.4, 0.6≤y≤0.9, the second shell layer comprises LiFePO 4 and carbon, the mass ratio of the inner core to the first shell being 1:(1.5-9); In the battery positive electrode material, the mass percentage of the inner core is 10%-40%; the mass percentage of the first shell layer is 45%-80%; and the mass percentage of the second shell layer is 10%-20%.

2. The battery positive electrode material according to claim 1, It is characterized in that The diameter of the core is 0.5 μm-5 μm.

3. The battery positive electrode material according to any one of claims 1 to 2, It is characterized in that The thickness of the first shell layer is 0.09 μm-2.7 μm.

4. The battery positive electrode material according to any one of claims 1 to 3, It is characterized in that In the battery positive electrode material, the molar ratio of Fe element to Mn element is 1:(1-9).

5. The battery positive electrode material according to claim 1, It is characterized in that The thickness of the second shell layer is 20nm-600nm.

6. The battery positive electrode material according to any one of claims 1 to 5, It is characterized in that The battery positive electrode material also includes doping elements, which include one or more of Ti, V, Co, Ni, Cu, Zn, Mg, Al, Ca, Mo, and W; the mass percentage of the doping elements in the battery positive electrode material is 0.1%-0.5%.

7. The battery positive electrode material according to claim 6, It is characterized in that The first shell layer includes the doping element.

8. A method for preparing a positive electrode material for a battery as claimed in any one of claims 1 to 7, It is characterized in that include: A lithium source, an iron source, a manganese source, a phosphorus source and a solvent are placed in a reaction kettle for a first reaction to obtain a core, wherein the core includes LiMn x Fe 1-x PO 4 , wherein 0<x≤0.4; the temperature of the first reaction is 375℃-500℃, and the pressure of the first reaction is greater than or equal to 23Mpa; A lithium source, an iron source, a manganese source, a phosphorus source, a solvent and the core are placed in a reactor for a second reaction, and a first shell layer is coated on the surface of the core to obtain a core provided with a first shell layer; the first shell layer includes LiMn y Fe 1-y PO 4 , wherein 0.6≤y≤0.9; the temperature of the second reaction is 375°C-500°C, and the pressure of the second reaction is greater than or equal to 23Mpa; calcining the inner core provided with the first shell layer at 500° C.-800° C. to obtain the battery positive electrode material; The core provided with the first shell layer also includes, before calcination: placing a lithium source, an iron source, a phosphorus source, a carbon source, a solvent and the core provided with the first shell layer in a reactor for a third reaction; coating the surface of the first shell layer with a second shell layer, wherein the second shell layer includes LiFePO 4 and carbon; the temperature of the third reaction is 375°C-500°C, and the pressure of the third reaction is greater than or equal to 23Mpa.

9. A positive electrode sheet, It is characterized in that It comprises a current collector and a positive electrode material layer arranged on the current collector, wherein the positive electrode material layer comprises the battery positive electrode material according to any one of claims 1 to 7.

10. A secondary battery, It is characterized in that It comprises a positive electrode, a negative electrode, a separator and an electrolyte, wherein the positive electrode comprises the positive electrode sheet as claimed in claim 9.

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