Lithium-ion batteries have become an ideal energy source for active development in the 21st century due to their high operating voltage, high specific energy, no memory effect, no pollution, small self-discharge, and long cycle life. The key to the development of lithium-ion batteries is the development of electrode materials. At present, compared with the negative electrode materials of lithium-ion batteries, the development of positive electrode materials is slow, which has become the main factor inhibiting the improvement of battery specific energy. The cathode materials mainly focus on: cobalt, manganese, nickel oxide lithium salts and olivine phosphates and their dopant series materials. Lithium manganate is considered to be one of the most promising cathode materials for lithium ion batteries due to its advantages of abundant resources, no pollution, low price, resistance to overcharge and overdischarge, and good thermal stability. Lithium manganate has two structures: spinel and layered. Due to the poor cycling performance of spinel-type lithium manganate, especially the serious capacity attenuation at high temperatures, it has not been widely used yet. Layered LiMnO2 has the advantages of non-toxic, safe, and high theoretical capacity (about 285mAh/g). It has become a hot spot in current research and has very broad application prospects in fields such as electric vehicles and space technology. my country's manganese reserves are very abundant, and the application of manganese oxide lithium cathode materials can greatly reduce the cost of current commercial lithium-ion batteries. Accelerating the research on LiMnO2 cathode materials is of great significance to my country's lithium-ion battery industry. 1. Research progress of layered lithium manganate 1.1 Structure of layered lithium manganate The ideal layered LiMnO2 structure belongs to the trigonal crystal system. O atoms are arranged in a slightly twisted cubic tightly packed arrangement. Mn atoms are located in the octahedral layer, while Li atoms in adjacent octahedral layers. But LiMnO2 is actually a homogeneous polycrystalline compound with two structures: orthorhombic and monoclinic. Orthorhombic LiMnO2 has a layered rock salt structure, and monoclinic LiMnO2 has an α-NaFeO2 type layered structure. This structure is not thermodynamically stable. phase, which is difficult to synthesize under equilibrium conditions. The electrochemical performance of layered LiMnO2 is closely related to its synthesis method. The material particle size, morphology, specific surface area, crystallinity and lattice defects obtained by different synthesis methods are significantly different. These factors affect the "embedding and extraction" of lithium ions. Reaction has a decisive influence. 1.2 Synthesis of layered lithium manganate Layered LiMnO2 can be synthesized using a variety of techniques, such as high-temperature solid-phase synthesis, sol-gel synthesis, template method, hydrothermal synthesis, co-precipitation synthesis, etc. (1) High-temperature solid-phase synthesis method High-temperature solid-phase synthesis of LiMnO2, that is, lithium salts (such as Li2CO3, LiNO3, LiOH·H2O, etc.) and manganese salts [such as Mn(NO3)2, MnCO3, etc.] or manganese oxides (such as Mn3O4 , MnO2, etc.) are ground and mixed in a certain way, and then sintered at high temperature for a period of time in a suitable furnace atmosphere (Ar or N2). The raw materials undergo solid phase reaction directly at high temperature to obtain the required sample. Li Yibing et al. roasted MnO2 to obtain Mn2O3, and then mixed it with LiOH·H2O and roasted it to obtain synthetic o-LiMnO2 powder. The solid-phase reaction method has a simple process and is suitable for industrial mass production. However, the contact between the reactants is uneven and the reaction is insufficient. Therefore, soft chemical methods are mainly used in current experiments to prepare materials with excellent electrochemical properties. The disadvantages are: serious loss of lithium, difficulty in controlling the stoichiometric ratio, difficulty in controlling the particle size of the synthesized product, uneven distribution, and irregular morphology. (2) Sol-gel method Sathiya et al. mixed metal alkoxides LiNO3, Mn(NO3)2, Co(NO3)2, and NiNO3 in a solution according to a certain stoichiometric ratio, used citric acid as a chelating agent, and added ethylenediamine The pH value of the solution is adjusted, and a sol is formed through the reaction. The mixture is then stirred and undergoes dehydration and polycondensation reaction to form a gel. The gel is vacuum dried and solidified, and then sintered to form the sample to be prepared. This method has a lower reaction temperature and shorter reaction time, but the raw materials are more expensive. (3) Co-precipitation method Co-precipitation method is to add a precipitant to a mixed solution composed of soluble salts with different chemical components in a solution state to form insoluble ultrafine particles, that is, precursor precipitates, and then precipitate these A method of drying or calcining materials to obtain corresponding ultrafine particles. Fan et al. added NH4HCO3 as a precipitant to the MnCl2 solution to prepare S-MnCO3. MnCO3 is calcined in air to obtain Mn2O3, which is mixed with LiOH·H2O and calcined under argon gas protection to obtain So-LiMnO2. (4) Melting method Wang Chengwei and others evenly mixed LiCl and LiNiO3 (the mass ratio of the two is 1:3) into a porcelain crucible, raised the temperature to 300°C in a muffle furnace, and added NaMnO2 powder to the crucible after the mixture was melted. Stir evenly, treat at constant temperature for 4 hours, then cool and wash with deionized water. The sample is vacuum dried at 120°C to obtain LiMnO2. (5) Hydrothermal synthesis is a synthesis in which substances in the solvent undergo chemical reactions under certain conditions of temperature (100~1000℃) and pressure (1~100MPa). Zhou et al. used homemade γ-MnOOH to disperse and dissolve in LiOH·H2O solution in a high-pressure reactor to obtain o-LiMnO2 with a diameter of 80 to 100 nm. Komaba et al. added O2 to the Mn(CH3COO)2·4H2O and KOH solutions, stirred them, filtered out the precipitate to obtain Mn3O4, mixed it with LiOH·H2O aqueous solution and put it into an autoclave, and reacted to obtain o-LiMnO2. Hydrothermal synthesis is conducive to the generation of low valence state, intermediate valence state and special valence state compounds, and can be doped uniformly. Powders obtained by hydrothermal synthesis generally have high crystallinity and small crystal defects, and the size, uniformity, shape, composition, etc. of the powder can be strictly controlled. (6) Ion exchange method Ion exchange method is a method that uses the mutual exchange reaction between anions or cations in solid ion exchangers and ions in liquid to separate, purify or prepare new substances. The use of this method to prepare LiMnO2 is based on the fact that NaMnO2 has a greater affinity for lithium ions than sodium ions, thereby producing LiMnO2 through an exchange reaction with lithium ions in the lithium salt solution. Xu Tianjun et al. used ball milling solid phase reaction to obtain NaMnO2, and then immersed it in a mixed molten salt of LiNO3 and LiCl to prepare m-LiMnO2. SEM and TEM observation results show that the particle size of LiMnO2 is 300~500nm, and the synthesized LiMnO2 shows good electrochemical cycle performance. (7) Template method Zhou Zhenhua et al. dissolved lithium acetate and manganese acetate in absolute ethanol, weighed a certain amount of silica gel and soaked it in the solution. It was first calcined in a muffle furnace at a constant temperature of 300°C for 4 hours, and then in a nitrogen atmosphere furnace at 700 Calcined at ℃ for 10h. Then dissolve the silica gel template in 4 mol/L NaOH solution to obtain nano-sized LiMnO2 powder. After characterization, the MnO2 prepared by the template method has good crystallinity, high purity, and a particle size of about 30nm. 2. Doping modification of layered lithium manganate Doping modification is a common method to stabilize the structure and performance of materials. Doping in layered lithium manganate introduces a small amount of other metal ions, such as Co3+, Al3+, Co3+, Ni3+ Equal doping can increase the valence state of Mn, effectively suppress the Jahn Teller effect, significantly improve the cycling stability of layered lithium manganese oxide at room temperature, and also improve the cycling stability at high temperatures. 2.1 Cation doping The current suppression of Jahn Teller distortion mainly uses cation doping: ① Incorporating low-valent metal ions to increase the average valence state of manganese, thereby effectively suppressing the occurrence of the Jaln-Teller effect; ② Incorporating a larger radius elements, improve the unit cell parameters of the material, increase the unit cell volume, and promote the diffusion of lithium ions. 2.1.1 Single atom doping (1) The ionic radius and octahedral site selection energy of Co-doped Co3+ are similar to those of Mn3+. After layered LiMnO2 is doped with Co, Co3+ occupies the original Mn3+ octahedral position, causing the Jahn-Teller effect on Mn3+ It has a certain inhibitory effect, thereby stabilizing the layered structure and improving the electrochemical properties of the material. Oh et al. used ultrasonic spray drying to synthesize Li[Ni0.5Mn0.5]1-xCoxO2. When the content of Co3+ increases, the discharge capacity of the material increases. In LiNi0.5Mn0.5O2, the valence states of Ni and Mn are each +2. , +4. The addition of Ni2+ increases the average valence state of Mn and improves the stability of the material. (2) Cr doping Myung et al. used Cr doping LiMnO2 to obtain LiMn1-xCrxO2. Research points out that the doping of Cr helps to improve the electrochemical performance. The less the amount of Cr doping, the higher the capacitance of the material. The ionic radius of Cr3+ is smaller than that of Mn3+, which leads to the shortening of the Mn-O bond and the reduction of the grain size, which stabilizes the Mn3+ in the octahedral position to a certain extent, thus improving the stability. (3) Ni doping Since Ni is difficult to oxidize under normal conditions, after Ni is introduced into layered LiMnO2, the oxidation state of Ni is generally 2 valence, Ni2+ occupies the position of Mn3+, and the valence state of the transition metal is maintained at +3, resulting in Mn The average valence of Mn3+ increases, reducing the Jahn-Teller effect of Mn3+, thereby stabilizing the layered structure and improving the structural stability of the material. Yong et al. synthesized Li[NixLi(1/3-2/3)Mn(2/3-x/3)]O2 (x=0.17, 0.25, 0.33, 0.5). Through comparison, it was found that with the increase of Ni component Reduced, the voltage platform becomes longer and the phase structure becomes more complete. Research shows that lower Ni content can also increase charge-discharge capacity and initial irreversible capacity. He Pingping used the precipitation method to synthesize LiNixMn1-xO2 and analyzed its structure. He found that when n(Ni)/n(Mn)<1, as the Ni content increases, Ni's occupation of Li sites will become more serious. The more Ni occupies Li in the 3a position, the smaller the amount of Li+ desorption and insertion, which in turn affects the electrochemical properties of the cathode material. (4) Fe and Zn doping Suresh et al. used deionization method to prepare LiMn1-xMxO2 (M=Fe, Zn). By doping Fe and Zn, they found that doping Fe and Zn can improve the cycle performance of the material. Resulting in a decrease in capacitance, LiMn0.95Zn0.05O2 and LiMn0.95Fe0.05O2 reached maximum capacitances of 180mAh/g and 193mAh/g respectively, and still had very good capacities after several cycles. The study found that the decrease in capacity caused by the increase in Zn component was mainly due to the decrease in electrochemically active Mn3+, while the decrease in capacity caused by the increase in Fe was due to the increase in Fe occupying Li positions, causing spinel deformation. Single element doping is not ideal for improving the performance of LiMnO2. If two or more component dopings are combined, the overall performance of LiMnO2 can be comprehensively improved. 2.1.2 Polyatomic doping (1) Ni and Co doping Xiao et al. used co-precipitation method to dope Ni and Co elements to prepare LiNi0.9-yMnyCo0.1O2. When y is equal to 0.5, some Mn ions in LiNi0.4Mn0.5Co0.1O2 become Mn3+, but it is not enough to prevent the J-T effect. When y is less than 50%, the initial charge and discharge capacities of LiNi0.4Mn0.5Co0.1O2 are 218mAh/g and 181mAh/g respectively, but the rate performance is slightly worse. When the Mn content exceeds 50%, the capacity will decrease and the polarization phenomenon will increase strongly. Zhong et al. [32] used Ni(CH3COO)2?4H2O, Co(CH3COO)2?4H2O, Mn(CH3COO)2?4H2O as raw materials, and Na2O3 as the precipitant to prepare LiNi0.6Co0.2Mn0.2O2. LiNi0.6Co0.2Mn0.2O2 is prepared at different temperatures and calcination times, and the electrochemical properties have certain differences. LiNi0.6Co0.2Mn0.2O2 has the most uniform particle size when calcined at 850°C. LiNi0.6Co0.2Mn0 The first discharge capacity of .2O2 is 148mAh/g at 850℃, and the power loss is minimum 8.2% after 30 cycles. It shows that Ni and Co atom doping can improve the electrochemical performance to a certain extent. After Co is incorporated, it occupies the octahedral position of Mn3+, which can stabilize the structure and improve conductivity and thermal stability. At the same time, the incorporation of Ni can increase the average valence state of Mn and stabilize the structure. Let each element play its own different role to jointly improve the electrochemical performance of the material. (2) Cr and Li doping Tang Aidong et al. used a liquid phase method to mix manganese, chromium, and lithium reactants at the molecular and atomic levels, and synthesized Li/Li (CrxLi1/3-x/3Mn2/3-x/ 3)O2. Constant current charge and discharge tests were performed at 25°C, 2.5~4.5V, and the current density was 14mA/g. It was found that the maximum discharge capacity of the material could reach 221mAh/g, and the capacity was 180mAh/g after 10 cycles. Research has found that the unit cell parameters a and c of the material increase with the incorporation of Cr. Larger unit cell parameters are conducive to the drag and insertion of lithium ions in the material, thereby improving the electrochemical performance of the material. (3) Cr and Ni doped Kim and others synthesized LiCrxNi0.5-xMn0.5O2 (x=0, 0.05, 0.1) through co-precipitation method. The experimental results show that the first discharge capacity of LiNi0.5Mn0.5O2 is 185mAh/g, 15 After 1 cycle, it is 150mAh/g. Due to the effect of Cr doping, the capacity of Cr0.05Ni0.45Mn0.5O2 is higher than that of LiCr0.1Ni0.4Mn0.5O2. The less the amount of Cr doping, the higher the initial capacity, but the degree of capacity attenuation increases with the increase of Cr. And become more slowly. (4) Ni, Ti-doped Li, etc. are prepared by spray drying method to prepare LiNi0.5Mn0.5-xTixO2. As the Ti component increases, the first charge and discharge capacity decreases. When x=0, the first discharge capacity is 190mAh/ When g, x=0.15, the first discharge capacity is 150mAh/g. When the Ti component is less than 0.3, it exhibits good cycle performance. After analysis, as the Ti component increases, the degree of cation mixing also deepens, and the crystal structure gradually changes from layered to a chaotic rock salt structure. This phenomenon may be related to the intensification of cation mixing. 2.2 Anion-doped layered LiMnO2. In addition to doping cations to improve its electrochemical performance, doping anions will also achieve the same effect. After layered LiMnO2 is doped with anions, Mn2+ or Mn4+ will be produced, which will compensate for the charge of Mn3+, interfere with the magnetic interaction between Mn3+, and thus stabilize the layered structure. Li0.7MnO2-ySy synthesized by Park et al. using an ion exchange method has a particle diameter of 100 to 200nm and a first discharge capacity of 170mAh/g at 2 to 4.6V. The material will gradually change to spinel type during cycling, but the cycle

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