In-situ coating method for chromium oxide positive electrode material

Abstract

A method for in-situ coating of a chromium oxide positive electrode material belongs to the technical field of preparation of lithium ion battery positive electrode materials, and the specific scheme is as follows: the chromium oxide material is composed of an inner-layer high-specific-capacity chromium oxide and an outer-layer coated high-chromium-content oxide material. Aiming at the prominent problem of low structural stability of a high-capacity chromium oxide material, a layer of chromium oxide material with good structural stability is coated in situ through high-temperature quenching, and the storage performance and the cycling stability of the material are improved through the stabilizing effect of an outer coating layer.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium battery positive electrode materials, and in particular relates to a method for in-situ coating of chromium oxide positive electrode materials. Background Art

[0002] Compared with other rechargeable batteries, lithium-ion batteries (LIBs) have higher energy density and cycle life, making them widely used in various fields. The specific energy of commercial cathode materials such as LiCoO2, LiFePO4, LiMn2O4, and ternary cathodes (NCM or NCA) has increased from 120Wh / kg to 300Wh / kg, but it is close to the theoretical limit, so researchers have turned their attention to the development of new materials. In recent years, chromium oxides have attracted much attention due to their high energy density and high capacity at low discharge rates. Multi-electron reactive chromium oxides, such as Cr8O 21 (Cr(III)2[Cr(VI)O4]2Cr(VI)4O 13 ), which has attracted much attention due to its high theoretical specific capacity (642mAh / g, 1210Wh / kg) and high operating voltage (3.0V, vs.Li+ / Li). Its theoretical specific capacity is the highest among conventional ternary materials LiNi 0.8 Co 0.1 Mn 0.1 O2 (280mAh / g). Its cost as a positive electrode material for lithium-ion batteries is also low, so it has good competitiveness and prospects.

[0003] In recent years, chromium oxide, as an important cathode material, has received extensive attention in the field of batteries. Traditionally, the research on chromium oxide has focused on the optimization of sintering process, the combination with other active materials to improve battery performance, and the improvement of its cycle performance by material coating. x / Cr8O 21 Composite positive electrodes can improve battery capacity to a certain extent, but they also have problems that affect rate performance. In addition, although the use of atomic layer deposition (ALD) technology to coat the Al2O3 layer on the surface of chromium oxide can effectively improve the cycle stability of the battery, it significantly increases the production cost of the battery. In order to overcome the above problems, related research is exploring simpler and more economical coating technologies to maintain the excellent cycle performance of chromium oxide and avoid cost increases. Summary of the invention

[0004] The purpose of the present invention is to solve the problems of high cost and poor rate performance of existing chromium oxides, and to provide a method for in-situ coating of chromium oxide positive electrode materials, specifically, the surface of the active chromium oxide material is in-situ coated with a layer of CrO xChromium oxide protective layer material. Compared with traditional chromium oxide materials, the modified material has superior surface structure stability, which significantly improves the material's air stability. x The chromium oxide protective layer reduces the side reactions between the positive electrode and the electrolyte, improves the cycle life and storage life of the chromium oxide positive electrode material, and the assembled lithium-chromium oxide battery significantly improves the cycle stability.

[0005] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0006] A method for in-situ coating of a chromium oxide positive electrode material, the method comprising:

[0007] Step 1: calcining chromium trioxide at high temperature and grinding it to obtain active chromium oxide material;

[0008] Step 2: In-situ coating of protective chromium oxide on the surface of the active chromium oxide material in step 1.

[0009] Furthermore, in step 1, the high temperature calcination time is 6-48h, the temperature is 250-375°C, and the heating rate is 1-10°C / min. According to the different sintering temperature and sintering time, the active chromium oxide material obtained is Cr8O 21 、Cr2O5、Cr5O 12 Or a mixture thereof.

[0010] Furthermore, in step one, the grinding includes manual grinding, ball milling or crushing with a wall breaking machine.

[0011] Furthermore, in step 2, the in-situ coating is specifically to quench the active chromium oxide material in step 1 at a high temperature to generate CrO x Chromium oxide protective layer.

[0012] Furthermore, in step 2, the specific conditions of the in-situ coating are: quenching temperature is 375-1000°C, and the time is 10s-60min. x The specific structure of the chromium oxide protective layer material is Cr2O5, Cr5O 12 With Cr2O3 or a mixed phase thereof. Due to the different quenching temperature and time, the CrO x The coating thickness of the chromium oxide protective layer material is 1nm-50μm.

[0013] Compared with the prior art, the present invention has the following beneficial effects:

[0014] With Cr8O 21 In comparison, Cr2O5 and Cr5O 12The chromium oxides have a higher chemical stability and are not easy to react with the electrolyte, thus effectively avoiding the degradation of the battery performance due to material degradation or reaction with the electrolyte during use. The present invention proposes a method of coating Cr2O5, Cr5O3 on the surface of the chromium oxide material by a simple in-situ coating technology. 12 Or Cr2O3 and other high-stability chromium oxides. This coating method can not only significantly improve the air stability of chromium oxide, but also effectively improve its cycle stability and long-term performance without significantly increasing the production cost of the material. The application of this type of coating technology has significant advantages, can provide stronger guarantees for the long-term stable operation of chromium oxide batteries, and has broad application prospects.

[0015] Cr8O 21 It has a higher electrochemical capacity, but its structure is not the most stable form, and the capacity layer can be dissolved by water, so its air stability is poor. 12 Compared with Cr2O3 materials, they have better structural stability and can be stored in the air for a long time. Therefore, the active chromium oxide materials are coated with stable Cr2O5 and Cr5O 12 With Cr2O3, its storage capacity can be improved; at the same time, Cr2O5, Cr5O 12 It has good chemical stability with Cr2O3, is not prone to side reactions with the electrolyte, and significantly improves the cycle stability of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 Cr2O3 coated Cr8O 21 SEM photos of the materials;

[0017] Figure 2 Li||Cr8O in Example 1 21 @Long cycle curve of Cr2O3 battery;

[0018] Figure 3 This is a long cycle curve diagram of the Li||Cr2O5@Cr2O3 battery in Example 2;

[0019] Figure 4 Li||Cr8O in Example 3 21 @Long cycle curve of Cr2O5 battery;

[0020] Figure 5 Li||Cr8O in Example 4 21 @Cr5O 12 Long cycle curve of the battery;

[0021] Figure 6 Li||Cr8O in Comparative Example 1 21 Long cycle curve of the battery;

[0022] Figure 7 Li||Cr8O in Example 1 21 Long cycle curve of the battery. DETAILED DESCRIPTION

[0023] The technical solution of the present invention will be clearly and completely described below in conjunction with the drawings and embodiments. Obviously, the described embodiments are only 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 ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0024] The present invention aims to provide a method for in-situ coating of chromium oxide positive electrode materials to improve their cycle life and storage life. The specific method is to coat a layer of CrO on the surface of the active chromium oxide by high temperature quenching. x Chromium oxide protective layer material. Compared with traditional chromium oxide materials, the modified material can isolate air due to its inert surface, which significantly improves the air stability of the material. x The protective layer reduces the side reactions between the positive electrode and the electrolyte, thereby alleviating the capacity decay of chromium oxide as a battery material when it is left alone.

[0025] Example 1

[0026] Weigh 5.0g of chromium trioxide crystal powder and place it in a ceramic ark, then place it in a tube furnace and continue pyrolysis at 270℃ for 24h in an air atmosphere with a heating rate of 5℃ / min. Grind and sieve the product to obtain the product Cr8O 21 The obtained Cr8O 21 The Cr2O3-coated Cr8O 21 The morphology of the obtained material is as follows Figure 1 The specific battery assembly process and electrochemical performance test are as follows:

[0027] The Cr2O3-coated Cr8O 21The material, Super P and PVDF were dissolved in N-methylpyrrolidone (NMP) at a mass ratio of 8:1:1 and stirred for 12 hours. The slurry was coated on the Al current collector with a coater and placed in a vacuum oven and dried at 120°C for 12 hours to obtain a coated chromium oxide positive electrode. Lithium metal discs were used as negative electrodes to assemble CR2025 button batteries. EC / DEC (1:1)-LiPF6 (1 mol / L) was used as the electrolyte, the electrolyte dosage was 50 μL, and the diaphragm was a commercial PP diaphragm. The charge and discharge performance of the battery was tested at 0.1C, and the voltage test range was 1.5 to 4.5V. The battery test results are as follows Figure 2 shown.

[0028] Example 2

[0029] Weigh 5.0g of chromium trioxide crystal powder and place it in a ceramic ark, then place it in a tube furnace and continue to pyrolyze it at 350℃ in an air atmosphere for 12h, with a heating rate of 5℃ / min. Grind and sieve the product to obtain the product Cr2O5. The obtained Cr2O5 is placed in an empty tube furnace at 800℃ for pyrolysis for 10s, and quenched to obtain Cr2O3-coated Cr2O5 material. The specific battery assembly process and electrochemical performance test are as follows:

[0030] The Cr2O3-coated Cr2O5 material obtained by the above process was dissolved in N-methylpyrrolidone (NMP) with Super P and PVDF in a mass ratio of 8:1:1 and stirred for 12 hours. The slurry was coated on the Al current collector with a coater and placed in a vacuum oven and dried at 120°C for 12 hours to obtain a coated chromium oxide positive electrode. Lithium metal discs were used as negative electrodes to assemble CR2025 button batteries. EC / DEC (1:1)-LiPF6 (1 mol / L) was used as the electrolyte, and the amount of electrolyte was 50 μL. A commercial PP diaphragm was used as the diaphragm. The charge and discharge performance of the battery was tested at 0.1C, and the voltage test range was 1.5 to 4.5V. The capacity retention rate of the battery after being placed at 55°C for 1 month was 98%. The test results of the battery at room temperature are as follows Figure 3 shown.

[0031] Example 3

[0032] Weigh 5.0g of chromium trioxide crystal powder and place it in a ceramic ark, then place it in a tube furnace and continue pyrolysis at 270℃ for 24h in an air atmosphere with a heating rate of 5℃ / min. Grind and sieve the product to obtain the product Cr8O 21 The obtained Cr8O 21 The Cr2O5-coated Cr8O 21 Materials. The specific battery assembly process and electrochemical performance test are as follows:

[0033] The Cr2O5-coated Cr8O 21 The material, Super P and PVDF were dissolved in N-methylpyrrolidone (NMP) at a mass ratio of 8:1:1 and stirred for 12 hours. The slurry was coated on the Al current collector with a coater and placed in a vacuum oven and dried at 120°C for 12 hours to obtain a coated chromium oxide positive electrode. Lithium metal discs were used as negative electrodes to assemble CR2025 button batteries. EC / DEC (1:1)-LiPF6 (1 mol / L) was used as the electrolyte, the electrolyte dosage was 50 μL, and the diaphragm was a commercial PP diaphragm. The charge and discharge performance of the battery was tested at 0.1C, and the voltage test range was 1.5 to 4.5V. The battery test results are as follows Figure 4 shown.

[0034] Example 4

[0035] Weigh 5.0g of chromium trioxide crystal powder and place it in a ceramic ark, then place it in a tube furnace and continue pyrolysis at 270℃ for 12h in air atmosphere with a heating rate of 5℃ / min. Grind and sieve the product to obtain the product Cr8O 21 The obtained Cr8O 21 It was placed in a tube furnace at 400 °C for pyrolysis for 30 seconds and quenched to obtain Cr5O 12 Coated Cr8O 21 Materials. The specific battery assembly process and electrochemical performance test are as follows:

[0036] The Cr5O obtained in the above process 12 Coated Cr8O 21 The material, Super P and PVDF were dissolved in N-methylpyrrolidone (NMP) at a mass ratio of 8:1:1 and stirred for 12 hours. The slurry was coated on the Al current collector with a coater and placed in a vacuum oven and dried at 120°C for 12 hours to obtain a coated chromium oxide positive electrode. Lithium metal discs were used as negative electrodes to assemble CR2025 button batteries. EC / DEC (1:1)-LiPF6 (1 mol / L) was used as the electrolyte, the electrolyte dosage was 50 μL, and the diaphragm was a commercial PP diaphragm. The charge and discharge performance of the battery was tested at 0.1C, and the voltage test range was 1.5 to 4.5V. The battery test results are as follows Figure 5 shown.

[0037] Comparative Example 1

[0038] The difference between this comparative example and Example 1 is that the secondary pyrolysis of the obtained chromium oxide is not performed. The test results of Comparative Example 1 are as follows: Figure 6 shown.

[0039] Comparative Example 2

[0040] The difference between this comparative example and Example 2 is that the secondary pyrolysis operation of the obtained chromium oxide is not performed. The test results of Comparative Example 2 are as follows: Figure 7 shown.

[0041] The electrochemical performances of the embodiments and comparative examples are shown in Table 1.

[0042] Table 1 Comparison of cycle retention rates of Examples 1 to 4 and Comparative Examples 1 and 2

[0043] sample magnification Capacity retention rate (starting from the second week) Example 1 0.1C 76.4％ Example 2 0.1C 71.0％ Example 3 0.1C 61.2％ Example 4 0.1C 59.4％ Comparative Example 1 0.1C 33.9％ Comparative Example 2 0.1C 32.7％

[0044] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. A method for in-situ coating of a chromium oxide positive electrode material, characterized in that: The method is: Step 1: calcining chromium trioxide at high temperature and grinding it to obtain active chromium oxide material; Step 2: In-situ coating of protective chromium oxide on the surface of the active chromium oxide material in step 1.

2. The method for in-situ coating of a chromium oxide positive electrode material according to claim 1, characterized in that: In step 1, the high temperature calcination time is 6-48h, the temperature is 250-375°C, and the heating rate is 1-10°C / min.

3. The method for in-situ coating of a chromium oxide positive electrode material according to claim 1, characterized in that: In step 1, the grinding includes manual grinding, ball milling or crushing by a wall breaking machine.

4. The method for in-situ coating of a chromium oxide positive electrode material according to claim 1, characterized in that: In step 2, the in-situ coating is specifically to quench the active chromium oxide material in step 1 at a high temperature to generate CrO on its surface. x Chromium oxide protective layer.

5. The method for in-situ coating of a chromium oxide positive electrode material according to claim 1 or 4, characterized in that: In step 2, the specific conditions of the in-situ coating are: quenching temperature is 375-1000°C, and time is 10s-60min.

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