Solid-phase reactions in inorganic chemistry, especially high-temperature solid-phase reactions, have always been one of the main methods for people to synthesize new solid materials. In order to obtain a metastable solid-phase reaction product and expand the choice of materials, it is necessary to lower the temperature of the solid-phase reaction. The solid-phase chemical reaction under room temperature or low heat temperature is a new research hotspot developed recently. Low-temperature solid-phase reactions have played an important role in the synthesis of cluster compounds, new polyacid compounds, metal complexes, etc., and have been successfully applied to the synthesis of oxides and single-component nanopowders. This method not only greatly simplifies the synthesis process and reduces the cost, but also reduces the defects such as impure product, particle agglomeration, and difficulty in recovery caused by intermediate steps and high-temperature solid-phase reactions. It provides a cheap and simple new method for the synthesis of solid materials, and also finds extremely valuable applications in material chemistry for low-heat solid-phase reactions. The low-heat solid-phase reaction does not use solvents, is environmentally friendly, and is energy-saving, high-efficiency, pollution-free, and simple in the process. It has become one of the important means of green synthetic chemistry. It is of great significance in the synthesis of high-performance battery active materials. Therefore, the low-heat solid-phase reaction method is one of the important methods for synthesizing battery active materials.
The basic principle of the low-temperature solid-phase chemical reaction method is:
First, the solid-phase metal complex prepared at room temperature or low temperature is decomposed, that is, the solid-phase complex is thermally decomposed at a relatively high temperature to obtain oxide or composite oxide ultrafine powder. For example, LiNO3, Mn(CH3COO)·4H2O and citric acid are used as raw materials and mixed uniformly in a certain amount ratio, and then fully ground for 2h at room temperature to obtain a solid-phase coordination precursor, and then the precursor is calcined at a certain temperature for a period of time to obtain LiMn2O4 ultrafine powder. Another example is the use of Li2CO3 and Mn(OAc)2 as the reactants, by adding a small amount of citric acid or oxalic acid to them, grinding them thoroughly, and calcining at 550°C for 4 hours to obtain spinel-type LiMn2O4.
Compared with the traditional high-temperature solid-phase reaction method, the solid-phase coordination reaction has the advantages of low calcination temperature and short time. The prepared LiMn2O4 material has uniform particles and relatively regular morphology.
The traditional LiCoO2 synthesis method is mainly solid-phase reaction method, which is divided into high-temperature solid-phase synthesis and low-temperature solid-phase synthesis. Among them, the high-temperature synthesis method is based on Li2CO3 and CoCO3 (or CoO, Co3O4) as raw materials, prepared according to the amount ratio of Li:Co=1:1, and calcined in the air atmosphere at 700~900℃ to obtain LiCoO2; the low-temperature solid-phase synthesis method is to heat the mixed Li2CO3 and CoCO3 in the air at a constant speed to 400°C and keep it for several days to generate LiCoO3 powder. Someone synthesized LiCoO2 powder material by solid-phase reaction at room temperature and then calcination method. The synthesis process is to ball mill Co(Ac)2·4H2O and LiOH·H2O in a ball mill to obtain a purple powder; then heat it in an oven at 40~50℃ to obtain an intermediate product; then it was dried in a vacuum at 0.08MPa at 100°C for 6 hours, and then placed at 600°C for heat treatment for 16 hours to obtain LiCoO2 material. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) indicate that the particle size is about 45nm, and the specific surface area measured by the BET method is about 50m2/g. Research on the electrochemical performance of LiCoO2 prepared by the low-temperature solid-phase reaction method shows that its cycle stability is better and the discharge voltage platform is higher (3.9V); the initial capacity of the mixed sample obtained by adding nano-LiCoO2 to ordinary LiCoO2 at the optimal ratio (7.5%) has been significantly increased, the discharge voltage platform is higher, and the cycle stability is better.
Lithium hydroxide and oxalic acid were mixed in the same amount, and after grinding for 30 minutes, an equimolar amount of cobalt acetate was added, and a pink paste intermediate was obtained after grinding for 1 hour. The intermediate was vacuum-dried at 150°C for 24h to obtain the precursor. The precursor is calcined in an air atmosphere at a temperature of 500~800°C for 6 hours to obtain LiCoO2 powder with a grain size of less than 100nm. With the increase of the calcination temperature, the degree of crystallization and grain size of the sample increase, and the unit cell parameters show a trend of elongation of the a-axis and shortening of the c-axis. The test results of charge and discharge performance show that the sample calcined at 700℃ has good electrochemical performance. The initial charge/discharge specific capacity is 169.4/115.3mA·h/g, and the discharge specific capacity after 30 cycles is still greater than 101mA·h/g .
The low-heat solid-phase reaction method can also be used to prepare LiCo0.8Ni0.2O2 powder. Studies have shown that the sample particles are agglomerated by many small spherical crystal grains, and are irregular, loose and porous. This material is conducive to the penetration of electrolyte and the diffusion of lithium ions; the charge-discharge performance test shows that the initial specific capacity of the sample obtained at 700~800℃ is 145mA·h/g, and the capacity is reduced by about 11% after 50 cycles.
Discussion on the mechanism of low-heat solid-phase reaction
Most solid phase reactions are difficult to carry out at lower temperatures, and some molecular solids with lower melting points or inorganic substances containing crystal water and most organic substances can form solid complexes.
It can undergo a solid phase reaction at room temperature or even at 0°C. Studies have shown that the presence of water of crystallization in the compound does not change the direction and limit of the reaction, it plays a role in reducing the solid-phase reaction temperature and accelerating the reaction rate. The size of the particles has little to do with the length of the grinding time, which is mainly affected by the reaction and preparation methods, but the shape of the particles is closely related to the length of the grinding time. Grinding the reactants sufficiently to be fine and uniform, so that the surface area of the particles increases sharply with the decrease of their particle size, which is also an important means to shorten the reaction time and promote the reaction. Someone explored the mechanism of low-heat solid-phase reaction, proposed and verified the four stages of solid-phase reaction with experiments, namely diffusion-reaction-nucleation growth. Each step may be the determining step of the reaction rate. In the solid phase, the reactant uses a small amount of crystal water to provide a place to accelerate the reaction, and the particles collide with each other to rapidly nucleate. However, due to the slow diffusion of ions through each phase, especially the product phase, the crystal nucleus cannot grow up quickly. According to the principle of crystallography, when the nucleation speed is fast and the nucleus growth speed is slow, it is easy to produce products with small crystal grains; otherwise, the produced crystal grains are larger. This may be one of the reasons why the low-heat solid-phase reaction can obtain small-grained particles.
In the process of using low-heat solid-phase reaction to synthesize battery cathode materials, the raw materials of the reactants often contain crystal water, and a small amount of crystal water can reduce the solid-phase reaction temperature and accelerate the reaction speed, which is conducive to obtaining products with fine grains; adequate grinding of the solid phase under low temperature conditions is essential for the formation of fine particles. Some people think that grinding can uniformly mix the reactants required for the synthesis of LiCoO2 from the microscopic level, thereby reducing the diffusion distance of lithium ions during the calcination of the intermediate, thereby forming a powder particle material with uniform particle size.