601525 lipo battery.Research progress on synthesis methods of lithium manganate

　　Lithium-ion batteries are one of the most important power sources in today's portable electronic devices [1]. In recent years, lithium-ion batteries have provided uninterrupted power energy in mini portable electronic devices, special space and special technologies, power tools, etc. applications have attracted widespread attention from researchers. Currently, the commercialized lithium-ion cathode material is LiCoO2, but it has many shortcomings, such as being toxic, explosive, scarce raw materials, and high cost. The electrode materials that can now be chosen to replace it include LiMn2O4, Li(Ni,Mn,Co)O2[2], LiFePO4 and Li2FeSiO4, etc. 1 Commonly used methods for synthesizing lithium manganate materials in industry. From the early stages of research, the lithium manganate cathode material has adopted the traditional high-temperature solid phase method. This method has a simple process and is easy to realize industrialization. It is commonly used by international manufacturers of lithium manganate. . However, there are also obvious shortcomings, such as higher heat treatment temperature and longer heat treatment time, large differences in the composition, structure and particle size distribution of the products, and the difficulty in controlling the electrochemical properties of the materials. In order to overcome the shortcomings of high-temperature solid-state reaction methods, a variety of new soft chemical synthesis methods have been studied in recent years, including Pechini [3], sol-gel method [4], co-precipitation method, melt impregnation method and hydrothermal synthesis method. etc., so that the electrochemical properties of the materials have been improved to varying degrees. 1.1 High-temperature solid-phase reaction method High-temperature solid-phase reaction method [5] refers to a method in which solid-phase reactants are calcined at high temperature to synthesize target products. Initially, Siapkas et al. [6] used Li2CO3 and MnO2 as raw materials to prepare lithium-deficient and lithium-rich LiMn2O4 by high-temperature calcination. The results show that the micron-sized Li0.5Mn2O4 powder synthesized at 730°C has a higher initial capacity. Although the process is relatively simple and the production conditions are relatively easy to control, due to the poor uniformity of the mixing of the reactants, the performance of the products varies. The high-temperature solid-phase synthesis method [7] is simple to operate and the raw materials are easily available, but the phase mixing is uneven, the crystal grains are irregular in shape, the particle size distribution is wide, and the calcination time is long and the temperature is high. Although the production cycle of this method is long, the process is very simple and the preparation conditions are relatively easy to control. 1.2 Pechini method The principle of the Pechini method is to use certain acids to react with cations to form chelates, and the chelates can polymerize with polyhydric alcohols to form solid polymer resins. Liu et al. dissolved a certain amount of citric acid and adipic acid in water at 90°C, added appropriate amounts of LiNO3 and Mn(NO3)2, heated at 140°C, and dried under vacuum to generate a precursor. After calcination, LiMn2O4 powder was obtained. Shen Jun et al. used ethylene glycol as a chelating agent to synthesize black powder LiMn2O4. Experimental results show that the Pechini method is easy to prepare nanocrystalline spinel lithium manganate and cobalt-doped spinel lithium manganate. The suitable calcination temperature is 800°C. Extending the calcination time in air can increase the valence state of manganese and obtain oxygen-rich spinel. 1.3 Sol-gel method The sol-gel method is to hydrolyze metal alkoxide or inorganic salt to form a uniform sol of metal oxide or metal hydroxide, and then condense the solute through evaporation to polymerize into a transparent gel, and dry the gel. , roasting to remove organic components to obtain the required inorganic powder materials. Park et al. used the sol-gel method to coat LiCoO2 on LiMn2O4 powder, and finally found that the LiMn2O4 powder was coated with LiCoO2 with a mole fraction of 2.2%. Its first discharge specific capacity reached 122mAh/g, and after 100 cycles of charge and discharge at 1C rate The charge-discharge cycle specific capacity is 120mAh/g. Under the condition of high-rate charge and discharge at 20C, the first discharge specific capacity of the coated lithium manganate is also 120mAh/g. After 100 cycles, the specific capacity remains above 80%, which is sufficient. It shows that the coated LiCoO2 plays a role in improving the electrochemical properties of lithium manganate, especially the cycle performance of the material. 1.4 Co-precipitation method Co-precipitation method is to mix lithium salt and manganese-containing solution, adjust the pH value to form a precipitate, obtain the precursor through filtration, washing and drying, and then roast it at a certain temperature to obtain lithium manganate powder [8] method. Qiu et al. used LiCl, MnC12 and KOH to react in an ethanol solution to co-precipitate LiOH and Mn(OH)2 precipitates, which were washed, dried and roasted. The final test found that the particle size distribution of lithium manganate powder was relatively uniform and the electrochemical performance was good. 1.5 Molten salt impregnation method The molten salt impregnation method is to heat the mixture before the reaction to melt the lithium salt (Li2CO3 or LiOH) and immerse it into the gaps of the manganese salt, and then perform the heating reaction. The melt impregnation method is the reaction of solid and molten salt, and its rate is faster than the solid reaction. Xia et al. used a melt impregnation method to synthesize LiMn2O4. In order to allow the lithium salt to fully penetrate into the MnO2 micropores, the lithium salt and manganese salt were mixed evenly and heated to the melting point of the lithium salt, and it was necessary to heat at 600 to 750°C for a period of time. The optimal synthesis conditions were obtained through a series of experiments. Through electrochemical tests, it was found that the initial capacity of the product can reach 120-130mAh/g, and the cycle performance is also relatively ideal. The molten salt impregnation method can shorten the preparation time and process. The reaction mixture is heated at the melting point of the lithium salt for several hours, thereby reducing the temperature of the final heat treatment material, greatly shortening the reaction time, and the performance of the final lithium manganate is relatively excellent and uniform. . Therefore, this method is currently an effective method for preparing high-performance LiMn2O4. However, since this method requires relatively few types of molten salt and the temperature of the molten salt is difficult to control, it is not conducive to industrial production. 1.6 Hydrothermal synthesis method Hydrothermal synthesis method is a method of preparing powder materials through chemical reactions in fluids such as aqueous solution or water vapor at a lower temperature (usually 100-350°C) and high pressure. Kanaskau et al. used a hydrothermal synthesis method to dissolve MnOOH powder in LiOH aqueous solutions of different concentrations, react at a constant temperature of 130 to 170°C for 48 hours, and filter to obtain the LiMn2O4 product. Research has found that the synthesis of lithium manganate is mainly affected by temperature and LiOH concentration. The hydrothermal synthesis of lithium manganate generally includes three steps: preparation, hydrothermal reaction, and filtration and washing. Compared with the solid-phase method and the sol-gel method, the process is relatively simple, which has great advantages in industrial applications and is a method with greater development potential. 2. Some new methods for synthesizing lithium manganate materials. With the development of cathode materials for lithium-ion batteries, the research methods for spinel lithium manganate materials are becoming more and more extensive. The electrochemical properties of electrode materials are affected by phase crystallinity, purity, and particle size. Aspects such as size and particle size distribution are greatly affected, and these properties are closely related to the synthesis method of the material. In order to meet the current social demand for power batteries, people have further researched and developed some new methods for synthesizing lithium manganate materials. 2.1 Cellulose-citric acid method The cellulose citric acid method [9] is a process in which lithium manganate is further synthesized in the second step of the step-by-step synthesis of cellulose. Among them, Shen et al. used the cellulose-citric acid method to synthesize lithium manganate, that is, the first step is to synthesize cellulose; the second step is to add a certain amount of lithium acetate and manganese acetate to the prepared cellulose and heat it to obtain a solid precursor, and then 800°C LiMn2O4 is obtained by calcination at low temperature. Its first discharge specific capacity was 134mAh/g. After 40 cycles of charge and discharge, the capacity only dropped by 6.7%. The lithium manganate powder synthesized by this method has good electrochemical properties. The most important thing is that this method combines the advantages of two different previous methods, Pechini and citric acid adsorption methods. It not only reduces consumption, but also has simple synthesis steps and Lithium manganate powder with uniform and good properties was obtained. 2.2 Silica gel template method The silica gel template method [10] is a process that uses commercial silica gel as a hard template to synthesize nanoscale lithium manganate powder. Cabana et al. dissolved LiNO3 and Mn(NO3)2·6H2O in ethanol at a ratio of Mn/Li of 1:2, then added silica gel to the mixed solution and calcined it at 600°C in air atmosphere. After 4 hours, dissolve the calcined product in 2 mol/L NaOH solution to obtain the product. The silica gel template method uses relatively cheap silica gel as a hard template to synthesize lithium manganate powder, so that the particle size of the synthesized lithium manganate powder is basically uniformly distributed, which greatly improves the electrochemical performance of the lithium manganate powder. Therefore, this method It is very promising in synthesizing power materials for high-rate charge and discharge. 2.3α-MnO2 precursor method The α-MnO2 precursor method is to synthesize an α-MnO2 with a one-dimensional tunnel structure and a crystal structure that is easy to absorb other ions as a precursor, and then use this structure to allow lithium ions to enter. Its crystal structure makes the process of synthesizing lithium manganate powder easier. Fan et al. used α-MnO2 as the manganese source to synthesize high crystallinity LiMn2O4 powder through a low-temperature solid-state method: (NH4)2S2O8, MnSO4·H2O and (NH4)2SO4 were used to synthesize α-MnO2 through a hydrothermal method. This method overcomes the shortcomings of lithium manganate prepared by using traditional commercial manganese dioxide, such as uneven particle size, poor crystallinity and uneven electrochemical performance. Although this method uses α-MnO2 nanowires to be synthesized separately, the steps to synthesize α-MnO2 nanowires are relatively simple, the raw material resources are relatively abundant and the price is low. Therefore, this method is likely to be expanded to achieve industrial production. 3. Cycle performance of lithium manganate materials. Before LiMn2O4 can be used as a commercial lithium-ion battery cathode material, it needs to be further improved due to the storage, poor cycleability and severe capacity fading of the material under high temperature conditions. Although the rationale behind capacity loss over multiple cycles is not entirely clear, several inducing factors may include Jahn-Teller deformation, manganese dissolution, lattice instability, electrolyte decomposition, and particle fragmentation. In order to improve the high-temperature performance, the electrochemical performance of this material can be improved by improving the method of synthesizing spinel lithium manganate or doping some other transition elements (or ions) in LiMn2O4 so that the manganese or oxygen atoms in LiMn2O4 are replaced. Changes occur, which can greatly improve the cycle performance of lithium manganate materials. 4 Conclusion and Outlook Among the cellulose-citric acid method, silica gel template method, and α-MnO2 precursor method, the α-MnO2 precursor method reduces the calcination temperature and time, and the particle size distribution of the obtained product is relatively uniform, and the material has a high First discharge specific capacity and cycle performance. The flame spray pyrolysis method uses a low-cost mixed solution of lithium salts and manganese salts, which greatly reduces production costs, and the parameters of the equipment are relatively easy to control. Therefore, these two methods are more promising for industrial production. Spinel lithium manganate has the advantages of abundant raw materials, low price, safety, environmental friendliness and easy recycling, so spinel lithium manganate material has always been one of the hot spots of research. In recent years, new methods of synthesizing lithium manganate have emerged that take into account energy consumption, and the synthesized materials have excellent properties. Therefore, it is necessary to continue to study the capacity fading problem of lithium manganate in the future, and strive to find the best method for industrial synthesis of high electrochemical performance lithium manganate as soon as possible to end China's dependence on imported cobalt resources.

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