Positive electrode material and application thereof

Abstract

The invention belongs to the technical field of battery material preparation, and particularly relates to a positive electrode material and application. The positive electrode material comprises an inner core and a coating layer coating the inner core, the inner core comprises first particles and second particles, the first particles and the second particles comprise sodium ion transition metal oxides containing Cu, Fe and Mn, and the median particle size of the first particles is larger than that of the second particles; the coating layer comprises a compound with a structural formula of NaAO2, and A comprises at least one of Al and Sb. The positive electrode material disclosed by the invention can enable the battery to have relatively good cycle performance and rate capability.

Description

Technical Field

[0001] This application belongs to the field of battery material preparation technology, specifically relating to a cathode material and its application. Background Technology

[0002] Sodium-ion batteries have abundant and low-cost raw materials for both positive and negative electrodes, giving them a significant advantage over lithium-ion batteries and making them highly competitive in low-speed transportation and energy storage markets. However, the relatively low energy density of sodium-ion batteries limits their commercial development. Sodium-ion batteries have an energy density of 120-160 Wh / kg, still lower than the 160-180 Wh / kg of lithium iron phosphate batteries. This is partly due to the lower crystal structure stability of the positive electrode material, making the material structure easily damaged; and partly because the radius of sodium ions is larger than that of lithium ions, resulting in greater changes in the cell volume of the positive electrode material during battery cycling, making the active material more prone to breakage and deteriorating cycle performance. Furthermore, side reactions between the positive electrode material surface and the electrolyte lead to increased interfacial impedance and decreased rate capacity. Current positive electrode materials cannot simultaneously achieve high discharge capacity, cycle performance, and rate performance. Summary of the Invention

[0003] Therefore, the technical problem to be solved by this application is to overcome the shortcomings of existing sodium-ion batteries that cannot simultaneously achieve high discharge capacity, rate performance and cycle performance, thereby providing a cathode material and its application.

[0004] Therefore, this application provides the following technical solution.

[0005] The first aspect of this application provides a cathode material comprising a core and a coating layer covering the core. The core comprises a first particle and a second particle, the first particle and the second particle comprising sodium ion transition metal oxides containing Cu, Fe and Mn, wherein the median particle size D1 of the first particle is greater than the median particle size D2 of the second particle; the coating layer comprises a compound with the structural formula NaAO2, wherein A comprises at least one of Al and Sb.

[0006] In some embodiments, the cross-sectional porosity of the positive electrode material is 1-10%.

[0007] In some embodiments, the median particle size D1 of the first particle is 2-10 μm.

[0008] In some embodiments, the median particle size D2 of the second particle is 0.5-2.5 μm.

[0009] In some embodiments, the median particle size D1 of the first particle and the median particle size D2 of the second particle satisfy the following relationship: D1≥3D2.

[0010] In some embodiments, the mass ratio of the first particle to the second particle is (1-3):1.

[0011] In some embodiments, the first particle comprises a structure having the structural formula Na. x1 Cu y1 Fe z1 Mn a1 G b1 Compounds containing O2, where G includes at least one of Ti, Sc, Co, Zn, and Sr, 0.6 <x1≤1,0<y1<0.33,0<z1<0.33,0<a1<0.33,0<b1<0.3,y1+z1+a1+b1=1。

[0012] In some embodiments, the second particle comprises a structure with the formula Na. x2 Cu y2 Fe z2 Mn a2 M b2 Q c A compound containing O2, wherein Q includes at least one of Nb, Ta, W, Mo, La, and Eu, M includes at least one of Ti, Sc, Co, Zn, and Sr, and 0.6 <x2≤1,0<y2≤0.3,0<z2≤0.3,0<a2<0.3,0<b2<0.3,0≤c<0.05,y2+z2+a2+b2+c=1。

[0013] In some embodiments, the thickness of the coating layer is 2-20 nm.

[0014] In some embodiments, the mass content of the coating layer in the cathode material is 0.2-1.5 wt%.

[0015] A second aspect of this application provides a secondary battery, including a positive electrode sheet, the positive electrode sheet including a current collector and an active material layer disposed on at least one surface of the current collector, the active material layer including the aforementioned positive electrode material.

[0016] A third aspect of this application provides an electrical device including the aforementioned secondary battery, wherein the secondary battery serves as the power supply for the electrical device.

[0017] The technical solution of this application has the following advantages: 1. This application provides a cathode material comprising a core and a coating layer covering the core. The core comprises a first particle and a second particle, wherein the first particle and the second particle comprise sodium-ion transition metal oxides containing Cu, Fe, and Mn, and the median particle size D1 of the first particle is larger than the median particle size D2 of the second particle. The coating layer comprises a compound with the structural formula NaAO2, wherein A comprises at least one of Al and Sb. The core of this application comprises two particles of different sizes. The larger particle size increases the grain size of the material, mitigates stress and strain, reduces particle breakage, and is beneficial to improving the cycle stability of the material structure. The smaller particle size of the second particle accelerates the insertion and extraction rate of sodium ions, thereby improving not only charge and discharge capacity but also rate performance. This application employs a combination of two particle sizes, which can utilize the larger particles to improve structural stability and thus improve cycle performance, while also taking advantage of the smaller particles to improve capacity and rate performance. This application features a coating layer surrounding the core, comprising a compound with the structural formula NaAO2, where A includes at least one of Al and Sb. This coating layer is a sodium-conducting coating layer, uniformly coating the core surface, reducing side reactions between the electrolyte and the core surface, and further improving the long-cycle stability of the cathode material. The cathode material of this application features a sodium-conducting coating layer surrounding the core, and the core comprises particles of two different sizes, enabling the battery to achieve both good cycle performance, rate performance, and high energy density. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the specific embodiments of this application or the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0019] Figure 1 This is a schematic diagram of the positive electrode material structure in Embodiment 1 of this application; Figure label: 1-First particle; 2-Second particle; 3-Coating layer. Detailed Implementation

[0020] The following embodiments are provided to better understand this application and are not limited to the preferred embodiments described herein. They do not constitute a limitation on the content and scope of protection of this application. Any product that is the same as or similar to this application, derived by anyone under the guidance of this application or by combining features of this application with other prior art, falls within the scope of protection of this application.

[0021] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0022] To enable batteries to achieve a balance between energy density, cycle performance, and rate performance, this application provides a cathode material comprising a core and a coating layer covering the core. The core comprises a first particle and a second particle, wherein the first particle and the second particle comprise sodium-ion transition metal oxides containing Cu, Fe, and Mn, and the median particle size D1 of the first particle is greater than the median particle size D2 of the second particle. The coating layer comprises a compound with the structural formula NaAO2, wherein A comprises at least one of Al and Sb.

[0023] This application's core comprises two types of particles with different sizes. Larger particles increase the material's grain size, mitigate stress and strain, reduce particle breakage, and improve the material's structural cycle stability. Smaller particles accelerate the insertion / extraction of sodium ions, increasing both charge / discharge capacity and rate performance. This application utilizes a combination of two particle sizes, leveraging the larger particles to enhance structural stability and improve cycle performance while also taking advantage of the smaller particles' increased capacity and rate performance. The core is surrounded by a coating layer comprising a compound with the structural formula NaAO2, where A includes at least one of Al and Sb. This coating layer is a sodium-conducting coating, uniformly covering the core surface, reducing side reactions between the electrolyte and the core surface, and further improving the long-term cycle stability of the cathode material. The cathode material of this application features a sodium-conducting coating layer outside the core, and the core comprises two different particle sizes, enabling the battery to achieve a balance between good cycle performance, rate performance, and high energy density.

[0024] In some embodiments, the cross-sectional porosity of the cathode material is 1-10%. The cross-sectional porosity of the cathode material in this application meets the above range, which is beneficial for increasing ion transport paths and further improving rate performance while ensuring energy density. As examples, the cross-sectional porosity of the cathode material is 2%, 4%, 6%, 8%, etc.

[0025] In some embodiments, the median particle size D1 of the first particle is 2-10 μm; as an example, the median particle size D1 of the first particle is 2 μm, 4 μm, 6 μm, 8 μm, 10 μm, 3-5 μm, 7-9 μm, etc.

[0026] And / or, the median particle size D2 of the second particle is 0.5-2.5 μm; as an example, the median particle size D2 of the second particle is 0.5 μm, 0.8 μm, 1.1 μm, 1.5 μm, 1.8 μm, 2.1 μm, 2.5 μm, 0.6-2.3 μm, 1.2-2.2 μm, 1.7-2 μm, etc.

[0027] The median particle size D1 of the first particle in this application meets the above-mentioned range, which can further suppress structural damage caused by the drastic contraction and expansion of the unit cell and improve structural stability. The median particle size D2 of the second particle in this application meets the above-mentioned range, which can accelerate the insertion and extraction rate of sodium ions and further improve the rate performance and energy density of the battery. In this application, the median particle size refers to the particle size when the cumulative particle volume reaches 50%. The median particle size can be determined by laser particle size analyzer according to GB / T 19077-2016 Particle Size Distribution Laser Diffraction Method. The testing instrument can be the Mastersizer 3000 laser particle size analyzer from Malvern Instruments Ltd., UK.

[0028] In some embodiments, the median particle size D1 of the first particle and the median particle size D2 of the second particle satisfy the following relationship: D1≥3D2.

[0029] The median particle size of the first and second particles in this application satisfies the above-mentioned relationship. By utilizing the difference in particle size distribution between the two types of particles, the cycle performance is guaranteed while the charge / discharge capacity and rate performance are improved. As an example, D1 and D2 satisfy the following relationships: D1=3D2, D1=3.5D2, D1=5D2, D1=10D2, D1=12D2, D1=15D2, D1=18D2, etc.

[0030] In some embodiments, the mass ratio of the first particle to the second particle is (1-3):1. A suitable mass ratio between the first and second particles in the cathode material is beneficial for improving both the rate performance and cycle performance of the battery. Examples of mass ratios between the first and second particles include 1:1, 2:1, and 3:1.

[0031] In some embodiments, the first particle comprises a structure having the structural formula Na. x1 Cu y1 Fe z1 Mn a1 G b1Compounds of O2, G includes at least one of Ti, Sc, Co, Zn, and Sr, 0.6 < x1 ≤ 1, 0 < y1 < 0.33, 0 < z1 < 0.33, 0 < a1 < 0.33, 0 < b1 < 0.3, y1 + z1 + a1 + b1 = 1; Doping elements such as Ti, Sc, Co, Zn, and Sr are introduced into the first particles, which have the effects of inhibiting phase transformation and reducing stress and strain, and are beneficial to reducing the structural damage of the cathode material caused by the shrinkage or expansion of the unit cell volume of the cathode material during charge and discharge, improving the structural stability of the cathode material, and improving the cycling performance.

[0032] And / or, the second particle includes a compound with the structural formula Na x2 Cu y2 Fe z2 Mn a2 M b2 Q c Compounds of O2, where Q includes at least one of Nb, Ta, W, Mo, La, and Eu, M includes at least one of Ti, Sc, Co, Zn, and Sr, 0.6 < x2 ≤ 1, 0 < y2 ≤ 0.3, 0 < z2 ≤ 0.3, 0 < a2 < 0.3, 0 < b2 < 0.3, 0 ≤ c < 0.05, y2 + z2 + a2 + b2 + c = 1. Doping elements such as Nb, Ta, W, Mo, La, and Eu are introduced into the second particles, which is beneficial to inhibiting surface phase transformation, reducing the migration of metal ions and oxygen loss, and improving the structural stability of the surface layer or interface layer.

[0033] In some embodiments, the thickness of the coating layer is 2 - 20 nm; And / or, the mass content of the coating layer in the cathode material is 0.2 - 1.5 wt%. In this application, regulating the thickness and mass content of the coating layer is beneficial to improving sodium ion conduction and reducing the occurrence of side reactions in the electrolyte. As examples, the thickness of the coating layer is 2 nm, 4 nm, 8 nm, 12 nm, 16 nm, 20 nm, 5 - 18 nm, 10 - 15 nm, etc.; the mass content of the coating layer in the cathode material is 0.2 wt%, 0.5 wt%, 0.8 wt%, 1 wt%, 1.2 wt%, 1.5 wt%, etc.

[0034] In some embodiments, the preparation method of the above cathode material includes: (1) Mix a sodium source, a copper source, an iron source, a manganese source, and a compound containing element G, and obtain the first particles through ball milling and first calcination; Mix a sodium source, a copper source, an iron source, a manganese source, and a compound containing element M, obtain an intermediate product through ball milling and second calcination, add a compound containing element Q and mix, and obtain the second particles through grinding and third calcination; (2) The first particle, the second particle and the solvent are mixed evenly, and the particles of different sizes are chemically adsorbed through the solution. Then, the positive electrode material particles of different sizes are obtained by spray drying. The positive electrode material particles, sodium source, and A-containing compound are mixed and then calcined.

[0035] Step (1), in preparing the two types of particles, the mixing step also includes adding a solvent, which includes, but is not limited to, at least one of methanol, ethanol, and isopropanol; the sodium source includes, but is not limited to, at least one of NaOH, Na2CO3, and NaHCO3; the copper source includes, but is not limited to, copper oxides; the iron source includes, but is not limited to, iron oxides; the manganese source includes, but is not limited to, manganese oxides; the compounds containing G, M, Q, and A are all selected from, but not limited to, their oxides.

[0036] The first calcination temperature is 850-950℃, and the time is 6-12h; the second calcination temperature is 800-900℃, and the time is 6-12h; the third calcination temperature is 600-800℃, and the time is 2-6h.

[0037] Step (2) uses a solvent including, but not limited to, at least one of deionized water and ethanol; the calcination temperature is 600-800℃, and the time is 6-12h. During the chemical adsorption process of particles of different sizes through the solution, the first particle with a larger median particle size has a stronger adsorption capacity for the second particle with a smaller median particle size, usually forming a structure where multiple second particles are adsorbed around a single first particle, as shown in [reference needed]. Figure 1 As shown.

[0038] In another embodiment, this application provides a secondary battery including a positive electrode sheet, the positive electrode sheet including a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, the positive active material layer including the aforementioned positive electrode material.

[0039] In some embodiments, the positive electrode active material layer further includes conductive agents, binders, etc.; the conductive agents include, but are not limited to, at least one of acetylene black, needle coke, carbon nanotubes, and graphene; the binders include, but are not limited to, at least one of polyethylene terephthalate, styrene-butadiene rubber, nitrile rubber, fluororubber, styrene-butadiene-styrene block copolymer or its hydride, ethylene-propylene-diene terpolymer, styrene-ethylene-butadiene-ethylene copolymer, styrene-isoprene-styrene block copolymer or its hydride, polyvinyl acetate, ethylene-vinyl acetate copolymer, propylene-α-olefin copolymer, polyvinylidene fluoride, and polytetrafluoroethylene.

[0040] In some embodiments, the secondary battery of this application includes a negative electrode sheet, which includes a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector. The negative active material layer includes at least one of carbon material, silicon-carbon material, and silicon material. The negative active material layer also includes a conductive agent, a thickener, a binder, etc.; the conductive agent, thickener, and binder are all made from conventional raw materials in the art. For example, the conductive agent includes acetylene black, the thickener includes CMC, and the binder includes SBR, etc.

[0041] In some embodiments, the secondary battery of this application further includes an electrolyte, which is a conventional electrolyte in the art; for example, the electrolyte includes an electrolyte and a solvent, the electrolyte including but not limited to at least one of sodium hexafluorophosphate (NaPF6) and sodium perchlorate (NaClO4); the solvent including but not limited to at least one of ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and propylene carbonate (PC).

[0042] In some embodiments, the secondary battery of this application further includes a separator; the separator is a conventional separator in the art; for example, the separator includes, but is not limited to, at least one of polypropylene membrane, polyethylene membrane, polyvinylidene fluoride membrane, spandex membrane, and aramid membrane.

[0043] The secondary battery of this application has high energy density, good structural stability and excellent long cycle performance, which can provide longer battery life for electrical devices.

[0044] In another embodiment, this application provides an electrical device including the aforementioned secondary battery, wherein the secondary battery serves as the power supply for the electrical device.

[0045] Example 1 This embodiment provides a positive electrode material, the structural schematic diagram of which is shown below. Figure 1 As shown, the device includes a core and a coating layer 3 covering the core. The core includes a first particle 1 and a second particle 2. The median particle size of the first particle 1 is larger than the median particle size of the second particle 2. The preparation method includes the following steps: (1) Preparation of the first particle: Na2CO3, CuO, Fe2O3, MnO2, and TiO2 were added to a ball mill jar in a molar ratio of 0.5:0.3:0.15:0.2:0.2, with 53g of Na2CO3. 1000ml of ethanol was then added, and the mixture was ball-milled for 4h at 400rpm / min. After drying, the mixture was kept at 900℃ for 10h as the first calcination, and then cooled to room temperature with the furnace. After cooling, the mixture was ground into powder, sieved, and the median particle size D1 was set to 5μm to obtain NaCu 0.3 Fe 0.3 Mn 0.2 Ti0.2 The first particle of O2; Second particle preparation: Take 10.6g of Na2CO3, and add Na2CO3, CuO, Fe2O3, MnO2, and TiO2 to a ball mill jar in a molar ratio of 0.5:0.3:0.15:0.2:0.19. Then add 1000ml of ethanol and ball mill at 400rpm / min for 4h. Dry, and then keep at 850℃ for 6h as the second calcination, and cool to room temperature with the furnace. Then mix with 464mg of WO3, grind evenly, and keep at 700℃ for 6h as the third calcination, and cool to room temperature with the furnace. After cooling, grind into powder, sieve, and make the median particle size D2 1.5μm to obtain the molecular formula NaCu. 0.3 Fe 0.3 Mn 0.2 Ti 0.19 W 0.01 The second particle of O2.

[0046] (2) Preparation of cathode material: The first particle and the second particle with a mass ratio of 2:1 were added to 200 ml of deionized water in a ball mill jar, mixed evenly, and then spray-dried. The dried mixture was then added to the ball mill jar with 132 mg of Na2CO3 and 127 mg of Al2O3, and dry-mixed evenly. The mixture was calcined at 700 °C for 8 h, cooled to room temperature with the furnace, ground, and sieved to obtain cathode material with a NaAlO2 coating layer. The amount of the first particle and the second particle added was controlled so that the mass content of the coating layer in the cathode material was 0.2 wt%. The mass of the coating layer was calculated based on the mass of NaAlO2 that could be generated from Na2CO3 and Al2O3 in this step.

[0047] This embodiment also provides a battery, including: Positive electrode sheet: The above-mentioned positive electrode material, conductive agent acetylene black, dispersant PVP, and binder polyvinylidene fluoride are dispersed in NMP at a mass ratio of 97.3:0.6:0.2:1.9 to prepare a slurry. This slurry is then coated on both sides of an aluminum foil, and after baking, rolling, and cutting, a positive electrode sheet (area density 0.22 g / 1540.25 mm²) is obtained. 2 ).

[0048] Negative electrode sheet: Hard carbon, conductive agent acetylene black, thickener CMC, and binder SBR are mixed and dispersed in deionized water at a mass ratio of 96.4:0.8:1.5:1.3 to prepare a slurry. This slurry is then coated on both sides of copper foil, and after baking, rolling, and cutting, the negative electrode sheet (area density 0.135g / 1540.25mm²) is obtained. 2 ).

[0049] Electrolyte: Mix EC and DMC at a volume ratio of 1:1, then add sodium hexafluorophosphate in a glove box to prepare the electrolyte. The concentration of sodium hexafluorophosphate in the electrolyte is 1 mol / L.

[0050] The prepared positive electrode sheet, polyethylene separator, and negative electrode sheet are stacked in sequence, with the separator in the middle of the positive and negative electrode sheets. After winding, hot pressing and shaping, and welding of the tabs, a bare battery is obtained. The bare battery is placed in an outer packaging aluminum-plastic film and baked in an oven at 85±10℃ for 24 hours. The electrolyte is injected into the dried battery. After standing, formation, and capacity testing, a sodium-ion soft-pack battery is obtained.

[0051] Example 2-3 This embodiment provides a cathode material that is basically the same as that in Embodiment 1. The main difference is that the rotation speed and time of the ball mill jar are adjusted during the preparation of the first and second particles so that the cross-sectional porosity of the cathode material is 2% and 8%, respectively.

[0052] Examples 2-3 also provide a battery, which uses the positive electrode material provided in Examples 2-3 instead of the positive electrode material provided in Example 1, and is otherwise basically the same as Example 1.

[0053] Example 4 This embodiment provides a cathode material that is basically the same as that in Embodiment 1, except that the median particle size D1 of the first particle is adjusted to 2 μm and the median particle size D2 of the second particle is adjusted to 0.6 μm. The adjustment of the median particle size D1 and median particle size D2 can be achieved by adjusting the sieve mesh size during the preparation of the first and second particles, and the same applies below.

[0054] Example 4 also provides a battery, which uses the positive electrode material provided in Example 4 instead of the positive electrode material provided in Example 1, and is otherwise basically the same as in Example 1.

[0055] Example 5 This embodiment provides a cathode material that is basically the same as that in Embodiment 1, except that the median particle size D1 of the first particle is adjusted to 10 μm.

[0056] Example 5 also provides a battery, which uses the positive electrode material provided in Example 5 instead of the positive electrode material provided in Example 1, and is otherwise basically the same as in Example 1.

[0057] Example 6 This embodiment provides a cathode material that is basically the same as that in Embodiment 1, except that the median particle size D2 of the second particle is adjusted to 0.5 μm.

[0058] Example 6 also provides a battery, which uses the positive electrode material provided in Example 6 instead of the positive electrode material provided in Example 1, and is otherwise basically the same as in Example 1.

[0059] Example 7 This embodiment provides a cathode material that is basically the same as that in Embodiment 1, except that the median particle size D1 of the first particle is adjusted to 8 μm and the median particle size D2 of the second particle is adjusted to 2.5 μm.

[0060] Example 7 also provides a battery, which uses the positive electrode material provided in Example 7 instead of the positive electrode material provided in Example 1, and is otherwise basically the same as in Example 1.

[0061] Examples 8-9 This embodiment provides a positive electrode material that is basically the same as that in Embodiment 1. The main difference is that the mass ratio of the first particle and the second particle is adjusted. In Embodiment 8, it is 1:1, and in Embodiment 9, it is 3:1.

[0062] Examples 8-9 also provide a battery, which uses the positive electrode material provided in Examples 8-9 instead of the positive electrode material provided in Example 1, and is otherwise basically the same as Example 1.

[0063] Examples 10-11 This embodiment provides a cathode material, which is basically the same as that in Embodiment 1. The main difference lies in adjusting the type of dopant source during the preparation of the first particle to obtain first particles with different molecular formulas. In Embodiment 10, the first particle is NaCu. 0.3 Fe 0.3 Mn 0.2 Sc 0.2 O2, the first particle in Example 11 is NaCu 0.3 Fe 0.3 Mn 0.2 Co 0.2 O2.

[0064] Examples 10-11 also provide a battery, which uses the positive electrode material provided in Examples 10-11 instead of the positive electrode material provided in Example 1, and is otherwise basically the same as Example 1.

[0065] Examples 12-13 This embodiment provides a cathode material, which is basically the same as that in Embodiment 1. The main difference lies in adjusting the type of dopant source during the preparation of the second particle to obtain second particles with different molecular formulas. In Embodiment 12, the second particle is NaCu. 0.3 Fe 0.3 Mn 0.2 Ti 0.18 Nb 0.02 O2, the second NaCu particle in Example 13 0.3 Fe 0.3 Mn 0.2 Ti0.18 Ta 0.02 O2.

[0066] Examples 12-13 also provide a battery, which uses the positive electrode material provided in Examples 12-13 instead of the positive electrode material provided in Example 1, and is otherwise basically the same as Example 1.

[0067] Examples 14-16 This embodiment provides a cathode material that is basically the same as that in Embodiment 1, except that at least one of the following is adjusted: the type of coating layer, the thickness of the coating layer, and the mass content of the coating layer.

[0068] Examples 14-16 also provide a battery, which uses the positive electrode material provided in Examples 14-16 instead of the positive electrode material provided in Example 1, and is otherwise basically the same as Example 1.

[0069] The parameters for each of the above embodiments are shown in Table 1.

[0070] Comparative Example 1 This comparative example provides a cathode material that is essentially the same as that in Example 1, except that the coating layers respectively coat the first particle and the second particle. The preparation method of this comparative example cathode material includes the following steps: (1) Na2CO3, CuO, Fe2O3, MnO2, and TiO2 were added to a ball mill jar in a molar ratio of 0.5:0.3:0.15:0.2:0.2, with 10.6 g of Na2CO3. 200 ml of ethanol was then added, and the mixture was stirred at 400 rpm for 4 h. After drying and grinding evenly, the mixture was calcined at 900 °C for 10 h and then cooled to room temperature in the furnace. After cooling, the mixture was ground into powder and sieved to obtain Ti-doped oxide material with the molecular formula NaCu. 0.3 Fe 0.3 Mn 0.2 Ti 0.2 O2. 66 mg of Na2CO3 and 63.5 mg of Al2O3 were added to the Ti-doped oxide and placed in a ball mill jar. The mixture was dry-mixed until homogeneous, calcined at 850 °C for 8 h, cooled to room temperature in the furnace, ground and sieved to obtain the coated Ti-doped oxide cathode material.

[0071] (2) Take 2.12 g of Na2CO3, add Na2CO3, CuO, Fe2O3, MnO2 and TiO2 in a ball mill jar in a molar ratio of 0.5:0.3:0.15:0.2:0.19, then add 200 ml of ethanol, stir at 400 rpm / min for 4 h, dry, grind evenly, keep warm at 800℃ for 6 h, and cool to room temperature with the furnace; then mix with 92.8 mg of WO3, grind evenly, keep warm at 850℃ for 6 h, and cool to room temperature with the furnace; after cooling, grind into powder, sieve, and obtain Ti / W co-doped oxide with the molecular formula NaCu 0.3 Fe 0.3 Mn 0.2 Ti 0.19 W 0.01 O2. 66 mg of Na2CO3 and 63.5 mg of Al2O3 were added to the Ti / W co-doped oxide in a ball mill jar, dry-mixed evenly, calcined at 850 °C for 8 h, cooled to room temperature with the furnace, ground and sieved to obtain the coated Ti / W co-doped oxide cathode material.

[0072] Ti-doped oxide cathode material and Ti / W co-doped oxide cathode material are mixed at a mass ratio of 2:1 to obtain cathode material.

[0073] This comparative example also provides a battery that uses the positive electrode material provided in this comparative example, and is otherwise basically the same as in Example 1.

[0074] Comparative Example 2 This comparative example provides a cathode material that is basically the same as that in Example 1. The main difference is that Na2CO3 and Al2O3 in the cathode material preparation process are replaced with zirconium oxide, so that the coating layer of the prepared cathode material is zirconium oxide.

[0075] This comparative example also provides a battery that uses the positive electrode material provided in this embodiment, and is otherwise basically the same as in Example 1.

[0076] Table 1 Parameters for each embodiment and comparative example

[0077] Test case This test case provides the performance of the batteries in various embodiments and comparative examples. The specific methods are as follows, and the obtained data are shown in Table 2: Test method for cross-sectional porosity of cathode materials: The cathode material particles were cut using an ion beam milling (CP) instrument to obtain samples with observable particle cross-sections. Scanning electron microscopy (SEM) was used to capture images of the cross-sections. Then, image analysis software was used: 1) ImageJ was used to extract the outline of individual particles from the captured images, and the area of ​​the selected individual particle was calculated as S0; 2) ImageJ was used to extract the pore region of the cross-section of the selected individual particle in step 1), and the area of ​​the pore region, S1, was calculated; 3) The porosity (A1) of the individual particle was calculated using A1 = (S1 / S0) × 100%; 4) Each experimental sample required multiple repeated cutting of the cross-section, and the number of particles effectively used in the porosity calculation should not be less than 50. Finally, the average porosity of the sample's cross-section was obtained by calculating the average porosity of each particle involved in the calculation.

[0078] Method for testing specific capacity at room temperature: At 25±2℃, the secondary batteries obtained in each example and comparative example are subjected to constant current charge-discharge test at a rate of 0.33C in the working range of 2.5-4.0V to obtain the specific capacity of discharge.

[0079] Test method for rate performance: At 25±2℃, the secondary batteries obtained in each example and comparative example were charged at 0.33C within the range of 2.5-4.0V, and discharged at a rate of 5C respectively, and the discharge specific capacity was recorded.

[0080] Room temperature cycling performance: The pouch batteries obtained in the examples and comparative examples were subjected to a 25°C cycle test, with 500 charge-discharge cycles at a charge-discharge rate of 0.33C within the range of 2.5-4.0V, and the discharge capacity retention rate was recorded. The capacity retention rate was calculated as: (500 discharge cycles capacity / initial discharge capacity) × 100%.

[0081] Table 2 Performance Test Results

[0082] As can be seen from the above results, the cathode material of this application includes a core and a coating layer covering the core. The core includes a first particle and a second particle. The first particle and the second particle include sodium ion transition metal oxides containing Cu, Fe and Mn. The median particle size of the first particle is larger than that of the second particle. The coating layer contains a compound with the structural formula NaAO2, where A includes at least one of Al and Sb. This allows the battery to achieve both high discharge capacity and good cycle performance and rate performance.

[0083] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this application.

Claims

1. A positive electrode material, characterized by, The positive electrode material comprises an inner core and a coating layer covering the inner core, the inner core comprises first particles and second particles, the first particles and the second particles comprise sodium ion transition metal oxides containing Cu, Fe and Mn, the median particle size D1 of the first particles is greater than the median particle size D2 of the second particles; the coating layer comprises a compound with a structural formula of NaAO2, and the A comprises at least one of Al and Sb.

2. The positive electrode material of claim 1, wherein, The cross-sectional porosity of the positive electrode material is 1-10%.

3. The positive electrode material of claim 1, wherein, The median particle size D1 of the first particles is 2-10 μm. And / or, the median particle size D2 of the second particles is 0.5-2.5 μm.

4. The positive electrode material according to claim 3, characterized in that, The median particle size D1 of the first particles and the median particle size D2 of the second particles satisfy a relationship formula: D1≥3D2.

5. The cathode material of claim 1, wherein, The mass ratio of the first particles to the second particles is (1-3):

1.

6. The cathode material of claim 1, wherein, The first particles include a compound of the formula Na x1 Cu y1 Fe z1 Mn a1 G b1 O2, G includes at least one of Ti, Sc, Co, Zn, and Sr, 0.6 < x1 < 1, 0 < y1 < 0.33, 0 < z1 < 0.33, 0 < a1 < 0.33, 0 < b1 < 0.3, y1 + z1 + a1 + b1 = 1.

7. The cathode material of claim 1, wherein, The second particle includes a compound of the structural formula Na x2 Cu y2 Fe z2 Mn a2 M b2 Q c O2, wherein Q includes at least one of Nb, Ta, W, Mo, La, and Eu, M includes at least one of Ti, Sc, Co, Zn, and Sr, 0.6 < x2 ≤ 1, 0 < y2 ≤ 0.3, 0 < z2 ≤ 0.3, 0 < a2 < 0.3, 0 < b2 < 0.3, 0 ≤ c < 0.05, y2 + z2 + a2 + b2 + c = 1.

8. The cathode material of claim 1, wherein, The thickness of the coating layer is 2-20 nm. And / or, the mass content of the coating layer in the positive electrode material is 0.2-1.5 wt%.

9. A secondary battery characterized by comprising: The positive electrode material comprises an inner core and a coating layer covering the inner core, the inner core comprises first particles and second particles, the first particles and the second particles comprise sodium ion transition metal oxides containing Cu, Fe and Mn, the median particle size D1 of the first particles is greater than the median particle size D2 of the second particles; the coating layer comprises a compound with a structural formula of NaAO2, and the A comprises at least one of Al and Sb.

10. An electric device, characterized by The secondary battery of claim 9 is used as a power supply for the electric device.

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