A core-shell structure chromium oxide positive electrode and its preparation method

Abstract

A core-shell structure chromium oxide positive electrode and its preparation method, belonging to the technical field of lithium ion batteries. The material is: the chemical formula of the core-shell structure chromium oxide positive electrode is M x Cr 8‑x O 21 ‑@LLZAO, including doped M x Cr 8‑x O 21 The core is a garnet electrolyte and the LLZAO coating is a shell, wherein M is one or more metal elements such as Zn, Mg, Ca, etc., and 0≤x≤1. 3+ Metal elements with similar ionic radius replace Cr8O 21 Cr with the lowest valence state in cathode materials 3+ , does not destroy Cr8O 21 structure and improve its discharge capacity, while introducing LLZAO garnet electrolyte coating layer, on the one hand, it improves the Li + The core-shell structure modification strategy enables the chromium oxide cathode material to possess high discharge capacity, excellent rate capability, and excellent cycling stability.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium ion batteries and relates to a core-shell structure chromium oxide positive electrode and a preparation method thereof. Background Art

[0002] In recent years, with the continuous expansion of applications in new energy vehicles and military power sources, the requirements for lithium batteries' energy density, power characteristics and other performance have become increasingly higher. The key to lithium battery performance lies in the selection of positive electrode materials. Currently, commercially available positive electrode materials such as LiCoO2, LiFePO4, LiMn2O4, and ternary positive electrodes (NCM, NCA) have an energy density that is difficult to exceed 300Wh / kg and their development has reached a bottleneck. Chromium oxide (Cr8O 21 ) as a positive electrode material for lithium batteries, it has a high theoretical specific capacity (~642mAh / g), a high voltage platform (3.0V, vs Li + / Li), and its lithium storage capacity is higher than other transition metal oxide materials, so it has attracted widespread attention from researchers. 21 It is mainly obtained by high-temperature solid-phase heat treatment of raw material CrO3. Affected by heat treatment temperature, time, heating rate and other conditions, it is difficult to obtain a single chromium oxide product, resulting in its discharge specific capacity being far less than the theoretical capacity. At the same time, the irreversible capacity loss of the chromium oxide positive electrode in the first cycle is very large, which greatly limits its practical application.

[0003] At present, the modification research around related issues mainly includes the following parts: (1) regulating the structural composition of chromium oxide by changing the process conditions (calcination temperature, time, atmosphere, calcination times, etc.); (2) stabilizing the structure of chromium oxide by adding metal oxides (such as Al2O3, TiO2, etc.) to coat it and improve the rate performance; (3) reducing the dissolution of chromium oxide in the electrolyte by doping it with exogenous elements to replace chromium ions, thereby improving its electrochemical performance. However, the above methods have not fundamentally improved the problems of low discharge capacity, poor rate performance and cycle stability of chromium oxide. In actual measurements, chromium oxide positive electrodes generally only cycle at a small rate below 0.1C, with a first-cycle discharge capacity of less than 400mAh / g and poor cycle stability, which further limits the commercial and military applications of this material.

[0004] Therefore, it is particularly important and challenging to develop a simple and easy method to improve the discharge capacity, rate performance and cycle stability of chromium oxide positive electrodes. Summary of the Invention

[0005] The purpose of the present invention is to solve the problems of low discharge specific capacity, poor rate performance and cycle stability of chromium oxide positive electrode materials, and to provide a core-shell structure chromium oxide positive electrode and a preparation method thereof.

[0006] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0007] A core-shell structure chromium oxide positive electrode, the chemical formula of the positive electrode is M x Cr 8-x O 21 -@LLZAO, including doped M x Cr 8-x O 21 The core and the garnet electrolyte LLZAO coating shell layer, wherein M is one or more of Zn, Mg, Ca, Ta, Ge, Nb, Sb, Sn, Ti and Ga, and 0≤x≤1.

[0008] Furthermore, the chemical formula of the LLZAO is Li 7-3x La3Zr2Al x O 12 , where 0≤x≤0.3, the thickness of the coating shell depends on LLZAO and M x Cr 8-x O 21 The stoichiometric ratio of the mass ratio is 1 to 10:100.

[0009] A method for preparing the core-shell structured chromium oxide positive electrode is as follows:

[0010] S1. CrO3 powder and a doping metal salt raw material are weighed and mixed according to a stoichiometric ratio, and then ground to uniformly disperse the raw materials. The mixture is placed in a tube furnace and calcined at a high temperature under an oxidizing atmosphere. After cooling, the powder is ground and sieved, washed with water to remove unreacted CrO3, and vacuum dried to obtain a doped chromium oxide active material.

[0011] S2, weighing the doped chromium oxide, surfactant and LLZAO garnet powder according to the metering ratio, mixing and grinding, placing in a tube furnace, calcining at high temperature under an oxidizing atmosphere, grinding after cooling and sieving the powder to obtain M x Cr 8-x O 21 -@LLZAO core-shell structure positive electrode material.

[0012] Furthermore, in step S1, the doping metal salt is one of metal acetate, oxalate, sulfate or nitrate.

[0013] Furthermore, in step S1, the high-temperature calcination temperature is 270° C. to 300° C., the time is 12 to 24 hours, and the heating rate is 1 to 10° C. / min.

[0014] Furthermore, in steps S1 and S2, the grinding is planetary ball milling, the ball milling speed is 300-500 rpm, the time is 6-24 hours, and the ball-to-material ratio is 1 / 2-1 / 3.

[0015] Furthermore, in steps S1 and S2, the oxidizing atmosphere is oxygen or air, and the mesh size of the sieving is 400 to 1000 meshes.

[0016] Furthermore, in step S2, the surfactant is one of PVP, PEG, PVA, and SDBS, and the mass ratio of the surfactant to the doped chromium oxide is 1 to 5:100.

[0017] Furthermore, in step S2, the high-temperature calcination temperature is 320°C to 400°C, the heating rate is 1 to 10°C / min, and the time is 6 to 24 hours.

[0018] Compared with the prior art, the beneficial results of the present invention are:

[0019] (1) The metal elements doped into the doped chromium oxide core designed in the core-shell structure chromium oxide positive electrode material provided by the present invention include elements from the third period, the fourth period, and the fifth period, and also include transition metal elements. The doped elements are of a wide variety and have universal applicability. They can be extended to other lithium-ion battery oxide positive electrode systems for electrode material modification.

[0020] (2) The garnet electrolyte coating layer of the present invention has high mechanical strength, which can inhibit the volume expansion of the chromium oxide positive electrode particles during the cycle, protect the structure of the positive electrode active material particles during the cycle, thereby ensuring the stability of the cycle process. At the same time, it acts as an ion conductor to promote the Li + transmission efficiency.

[0021] (3) The doping element of the present invention replaces Cr 3+ The sites improve the discharge capacity and rate performance of chromium oxide, and the designed coating layer stabilizes the electrochemical cycle of the material. This method uses simple grinding and calcination treatment, which is simple to operate, has obvious effects, and can be easily extended to other battery material modification systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Zn in Example 1 0.75 Cr 7.5 O 21 -First cycle discharge capacity diagram of batteries assembled with LLZAO cathode material at 0.047C (30mA / g) and 1C;

[0023] Figure 2 Zn in Example 1 0.75 Cr 7.5 O21 -100-cycle discharge capacity diagram of the battery assembled with LLZAO cathode material at 0.047C (30mA / g);

[0024] Figure 3 Cr8O in Comparative Example 1 21 The first cycle discharge specific capacity diagram of the battery assembled with the positive electrode material at 0.047C (30mA / g) and 1C;

[0025] Figure 4 Cr8O in Comparative Example 1 21 100-cycle discharge capacity diagram of the battery assembled with the positive electrode material at 0.047C (30mA / g). DETAILED DESCRIPTION

[0026] The technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are only a part of the embodiments of the invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work fall within the scope of protection of the present invention. If specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or the conditions recommended by the manufacturer. If the manufacturer is not specified for the reagents or instruments used, they are all conventional products that can be purchased commercially.

[0027] The present invention introduces Cr 3+ Metal elements with similar ionic radius replace Cr8O 21 Cr with the lowest valence state in cathode materials 3+ , does not destroy Cr8O 21 structure and improve its discharge capacity, while introducing LLZAO garnet electrolyte coating layer, on the one hand, it improves the Li + The core-shell structure modification strategy enables the chromium oxide cathode material to possess high discharge capacity, excellent rate capability, and excellent cycling stability.

[0028] Example 1

[0029] Weigh 0.219g of zinc acetate dihydrate and 1g of chromium trioxide, grind for 10 minutes to mix the block chromium trioxide and zinc acetate dihydrate evenly, dry grind with a planetary ball mill for 12 hours, the ball mill speed is 400rpm, and the ball-to-material ratio is 1 / 3. The mixed powder after ball milling is placed in a tubular furnace, and high-purity oxygen is introduced into the tubular furnace. The temperature is raised to 270°C at a heating rate of 5°C / min, kept warm for 12 hours, and taken out after cooling to room temperature. The heat-treated sample is ground into uniform fine powder, dispersed in 10ml of deionized water, and magnetically stirred for 10 minutes to fully dissolve the unreacted chromium trioxide and zinc acetate dihydrate in the deionized water. The aqueous solution is removed by suction, and the washing process is repeated three times. The washed material is moved into a vacuum oven and dried at 60°C for 24 hours to remove residual water in the sample, and Zn 0.75 Cr 7.5 O 21 Positive electrode material: 0.02g PVP, 0.05g LLZAO and 1g Zn 0.75 Cr 7.5 O 21 The mixture was mixed and ground for 10 minutes, then planetary ball milled for 6 hours, placed in a tube furnace with air atmosphere, heated to 350°C at a heating rate of 5°C / min, kept at this temperature for 18 hours, cooled to room temperature and taken out to obtain Zn 0.75 Cr 7.5 O 21 -LLZAO.

[0030] Zn 0.75 Cr 7.5 O 21 -LLZAO cathode active material, Super P conductive carbon black, and PVDF were dissolved in N-methylpyrrolidone (NMP) at a mass ratio of 8:1:1 and stirred for 24 hours. The slurry was applied to an Al current collector using a doctor blade and dried in a vacuum oven at 120°C for 24 hours to obtain a core-shell chromium oxide cathode. CR2025 button cells were assembled using a 1M LiPF6+EC / DEC / DMC (1:1:1) electrolyte, the prepared chromium oxide cathode, a PP separator, and metallic lithium as the negative electrode. The battery's charge and discharge performance was tested at 30 mA / g (0.047 C) and 1 C, with a voltage test range of 1.5 to 4.5 V.

[0031] Example 2

[0032] Weigh 0.030g of magnesium sulfate and 1g of chromium trioxide, grind for 10 minutes to mix the block chromium trioxide and magnesium sulfate evenly, dry grind for 12 hours with a planetary ball mill, the ball mill speed is 350rpm, the ball-to-material ratio is 1 / 3, and the mixed powder after ball milling is placed in a tubular furnace. High-purity oxygen is introduced into the tubular furnace, and the temperature is increased to 270°C at a heating rate of 5°C / min, kept warm for 12 hours, and taken out after cooling to room temperature. The heat-treated sample is ground into uniform fine powder, dispersed in 10ml of deionized water, and magnetically stirred for 10 minutes to fully dissolve the unreacted chromium trioxide and magnesium sulfate in the deionized water. The aqueous solution is removed by filtration, and the washing process is repeated three times. The washed material is moved into a vacuum oven and dried at 60°C for 24 hours to remove residual water in the sample, and Mg is obtained. 0.2 Cr 7.9 O 21 Positive electrode material: 0.02g PVA, 0.1g LLZAO and 1g Mg 0.2 Cr 7.9 O 21 The mixture was mixed and ground for 10 min, then planetary ball milled for 6 h, placed in a tube furnace with air atmosphere, heated to 320 ° C at a heating rate of 5 ° C / min, kept at this temperature for 12 h, cooled to room temperature and taken out to obtain Mg 0.2 Cr 7.9 O 21 -LLZAO.

[0033] The positive electrode preparation and battery assembly test were the same as in Example 1.

[0034] Example 3

[0035] Weigh 0.081g of calcium acetate and 1g of chromium trioxide, grind for 10 minutes to mix the block chromium trioxide and calcium acetate evenly, dry grind for 12 hours using a planetary ball mill, with a ball mill speed of 350rpm and a ball-to-material ratio of 1 / 3. Place the milled mixed powder in a tubular furnace, introduce high-purity oxygen into the tubular furnace, heat it to 270°C at a heating rate of 5°C / min, keep it warm for 12 hours, cool it to room temperature and take it out. Grind the heat-treated sample into uniform fine powder, disperse it in 10ml of deionized water, and stir it magnetically for 10 minutes to fully dissolve the unreacted chromium trioxide and calcium acetate in the deionized water. Filter and remove the aqueous solution. Repeat the washing process three times. Move the washed material into a vacuum oven and dry it at 60°C for 24 hours to remove residual water in the sample, and obtain Ca 0.4 Cr 7.8 O 21 Positive electrode material: 0.05gSDBS, 0.03gLLZAO and 1gCa 0.4 Cr 7.8 O 21The mixture was mixed and ground for 10 minutes, then planetary ball milled for 12 hours, placed in a tube furnace, high-purity oxygen was introduced into the tube furnace, and the temperature was increased to 380°C at a heating rate of 5°C / min, kept at this temperature for 12 hours, cooled to room temperature and taken out to obtain Ca 0.4 Cr 7.8 O 21 -LLZAO.

[0036] The positive electrode preparation and battery assembly test were the same as in Example 1.

[0037] Comparative Example 1

[0038] Weigh 1g of chromium trioxide powder, grind it for 10 minutes, place it in a tube furnace, introduce high-purity oxygen, heat it to 270℃ at a heating rate of 10℃ / min, keep it warm for 24 hours, cool it down to room temperature and take it out. The heat-treated sample is ground into a uniform fine powder and dispersed in 10ml of deionized water. Stir it magnetically for 10 minutes to fully dissolve the unreacted chromium trioxide in the deionized water. Filter and remove the aqueous solution. Repeat this washing process three times. The washed material is transferred to a vacuum oven and dried at 60℃ for 24 hours to remove the residual water in the sample to obtain pure Cr8O 21 positive electrode material.

[0039] The positive electrode preparation and battery assembly test were the same as in Example 1.

[0040] Table 1. Comparison of first cycle discharge capacity and discharge rate of Examples 1 to 3 and Comparative Example 1

[0041]

[0042] Figures 1-2 The discharge capacity and cycle performance of the doped positive electrode material at different rates in Example 1 are shown. Figure 3-4 Conventional Cr8O 21 Discharge capacity and cycle performance at two rates. 21 The first cycle discharge capacity at 30mA / g (0.047C) is 355mAh / g, and the first cycle discharge capacity at 1C is 287mAh / g, which is much smaller than that of Cr8O 21 The theoretical discharge capacity is 642 mAh / g. After different doping and coating, the first cycle discharge capacity of the batteries in the three examples at 30 mA / g (0.047 C) exceeds 400 mAh / g. Under the condition of high rate discharge of 1 C, they also have an ultra-high discharge capacity of >350 mAh / g. At the same time, after 100 cycles, they have a higher capacity than Cr8O 21Improved cycling stability. This study demonstrates that the preparation method for core-shell chromium oxide cathode materials significantly improves the discharge capacity, rate capability, and cycling stability of chromium oxide, providing guidance and reference for the research and modification of chromium oxide and other oxide cathode materials.

[0043] The embodiments described above are only part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative work are within the scope of protection of the present invention.

[0044] Without departing from the principles of the present invention, various improvements and substitutions made within the technical scope disclosed by the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a core-shell structured chromium oxide positive electrode, characterized in that: The chemical formula of the positive electrode material is M x Cr 8-x O 21 -@LLZAO, including doped M x Cr 8-x O 21 The core and the garnet electrolyte LLZAO coating shell layer, wherein M is one or more of Zn, Mg, Ca, Ta, Ge, Nb, Sb, Sn, Ti and Ga, and 0≤x≤1; The chemical formula of the LLZAO is Li 7- 3x La3Zr2Al x O 12 , where 0≤x≤0.3, the thickness of the coating shell depends on LLZAO and M x Cr 8-x O 21 The stoichiometric ratio of the mass ratio is 1 to 10:100; the method is: S1. CrO3 powder and a doping metal salt raw material are weighed and mixed according to a stoichiometric ratio, and then ground to uniformly disperse the raw materials. The mixture is placed in a tube furnace and calcined at a high temperature under an oxidizing atmosphere. After cooling, the powder is ground and sieved, washed with water to remove unreacted CrO3, and vacuum dried to obtain a doped chromium oxide active material. S2, weighing the doped chromium oxide, surfactant and LLZAO garnet powder according to the metering ratio, mixing and grinding, placing in a tube furnace, calcining at high temperature under an oxidizing atmosphere, grinding after cooling and sieving the powder to obtain M x Cr 8- x O 21 -@LLZAO core-shell structure positive electrode material; the surfactant is one of PVP, PEG, PVA, and SDBS, and the mass ratio of the surfactant to the doped chromium oxide is 1 to 5:

100.

2. The method for preparing a core-shell structured chromium oxide positive electrode according to claim 1, wherein: In step S1, the doping metal salt is one of metal acetate, oxalate, sulfate or nitrate.

3. The method for preparing a core-shell structured chromium oxide positive electrode according to claim 1, wherein: In step S1, the high-temperature calcination temperature is 270° C. to 300° C., the time is 12 to 24 hours, and the heating rate is 1 to 10° C. / min.

4. The method for preparing a core-shell structured chromium oxide positive electrode according to claim 1, wherein: In steps S1 and S2, the grinding is planetary ball milling, the ball milling speed is 300-500 rpm, the time is 6-24 hours, and the ball-to-material ratio is 1 / 2-1 / 3.

5. The method for preparing a core-shell structured chromium oxide positive electrode according to claim 1, wherein: In steps S1 and S2, the oxidizing atmosphere is oxygen or air, and the mesh size of the sieving is 400 to 1000 meshes.

6. The method for preparing a core-shell structured chromium oxide positive electrode according to claim 1, wherein: In step S2, the high-temperature calcination temperature is 320°C to 400°C, the heating rate is 1 to 10°C / min, and the time is 6 to 24 hours.

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