High-temperature reaction method for preparing positive materials


The compound containing cobalt, nickel and manganese sources and the lithium salt are mixed uniformly according to a certain proportion, at a given temperature, air is roasted for a certain period of time, cooled to room temperature, crushed, and sieved to obtain the product. In the sintering process, it often includes a variety of physical, chemical and physical chemical changes. Generally, it is accompanied by processes such as dehydration, thermal decomposition, phase change, eutectic, melting, dissolution, crystallization and crystal growth. The high temperature solid phase method is easy to realize industrialization due to its simple process flow.

In the sintering process of the cathode material precursor, the reduction of the free energy of the sintering system is the driving force of the reaction, including the following aspects:

① Due to the increase of the particle bonding surface and the flattening of the particle surface, the total specific surface area and total surface free energy of the powder decrease;

②The total volume and total surface area of pores in the sintered body are reduced;

③ Elimination of lattice distortion in powder particles.

The excess free energy existing in the powder or powder compact before sintering, including surface energy and lattice distortion energy. The former refers to the surface free energy of particles and pores in contact with the atmosphere, and the latter refers to the increase in energy caused by excess vacancies, dislocations and stress in the particles. The surface energy is smaller than the lattice distortion energy. For example, the surface energy of extremely fine powder is several hundred cal/mol, while the lattice distortion energy is as high as several kilocalories/mol. However, for the sintering process, especially in the initial stage, the surface energy plays a major role. Theoretically, the low-energy state after sintering is at most the equilibrium defect concentration corresponding to the single crystal. In fact, the sintered body always has more polycrystals with more thermal equilibrium defects. Therefore, the absolute value of the reduction in lattice distortion energy during the sintering process is still secondary to the reduction in surface energy. A certain number of thermal equilibrium vacancies, vacancies and dislocation networks are always retained in the sintered body.

The pore size changes during the sintering process, regardless of whether the total porosity is reduced or not, the total surface area of the pores always decreases. After the isolation pores are formed, the reduction of the surface area is mainly due to the spheroidization of the pores under the condition that the pore volume remains unchanged, and the continuous shrinkage and disappearance of the spherical pores can also further reduce the total surface area. Therefore, no matter what stage of the sintering process, the reduction of the surface free energy of the hole is always the driving force of the sintering process.

The mechanical energy consumed by the powder in the process of crushing and grinding is stored in the powder in the form of surface energy. In addition, due to the crystal lattice defects caused by pulverization, the powder has a higher activity due to the large surface area. Compared with the sintered body, the powder is in an energy unstable state. The surface energy of the powdery material is greater than the grain boundary energy of the polycrystalline sintered body, which is the driving force for sintering. After the powder is sintered, the grain boundary energy replaces the surface energy, which is the reason for the stable existence of polycrystalline materials. For example, in the high-temperature solid-phase synthesis of lithium manganese oxide, sufficient grinding of the raw materials can not only improve the electrochemical performance of the product, but also effectively reduce the temperature required for the reaction.


Synthesis of lithium manganese oxide by high temperature solid phase method

Synthesis of lithium manganese oxide by high temperature solid phase method


The following discussion takes lithium manganese oxide as an example.

Lithium manganese oxide is synthesized by the solid-phase method, that is, lithium salt (such as Li2CO3, LiNO3, LiOH·H2O, etc.) and manganese salt [such as MnCO3, Mn(NO3)2, etc.] or manganese oxide (such as electrolytic MnO2, Mn2O3, Mn3O4, etc.) are ground and mixed in a certain way, then fired at high temperature for a long time, and the solid phase reaction occurs directly. Its characteristic is that the solid raw material mixture is directly reacted in solid form. In order to ensure a sufficient reaction rate, the solid material must be heated to above 750°C. For the solid-phase synthesis reaction of lithium manganese oxide, there are at least three phases: lithium salt, manganese dioxide and lithium manganese oxide. In the process of solid-phase reaction between solid-phase mixtures, atoms or ions need to pass through the interface of each phase and pass through each phase area, which forms the mutual diffusion of atoms or ions in multiple solid phases. Therefore, kinetic factors play a decisive role in the reaction rate.

Due to the large number of microstructure rearrangements involved in the formation of the product LiMn2O4 during the reaction, which involves the breaking and recombination of chemical bonds, atoms or ions have to migrate over a considerable distance (on the atomic scale). Therefore, a sufficiently high temperature is required to allow these atoms or ions to diffuse to the new reaction interface, and at the same time, electric heating or microwave heating is required to achieve high-temperature solid-phase reactions.

The high temperature solid phase method was used to synthesize the spinel cathode material LiMn2O4 with Li2CO3 as the lithium source, chemical MnO2(CMD) and electrochemical MnO2 (EMD) as the manganese source, and ethanol-water mixture as the dispersion medium. The specific method is: weigh a certain proportion of Li2CO3 and MnO2, mix and grind mechanically, then add a certain proportion of ethanol/water mixed solution, soak for 24h under stirring to obtain a colloidal mixture, evaporate to dryness, and dry under vacuum (<113kPa) at 100°C for 2h, grind into fine powder, then pre-baked in air at 550°C for several hours, at about 650°C for several hours, and finally at 750°C for more than ten hours, and then natural cooling to obtain the sample. The reaction equation is:

The reaction equation


Characterized by XRD, BET, TEM and electrochemical test materials. It shows that the samples prepared at 750°C have a good spinel structure, the specific surface area is about 420m2/g and 220m2/g, the product has a uniform particle size distribution, and the average particle size is 200nm. Under constant current charge and discharge under the conditions of 410-4A/cm2 and 3.0~4.35V, the first discharge capacity is greater than 110mA·h/g, the efficiency is greater than 90%, and it has good cycle reversibility.


Factors affecting product performance in solid phase method

Factors affecting product performance in solid phase method


The greater the lattice energy in the crystal, the stronger the bond of ions, the more difficult the diffusion of ions, and the higher the sintering temperature required. The bonding conditions of various crystals are different, so the sintering temperature is also very different, even the crystallinity of the same crystal is not a fixed value. Increasing the sintering temperature is beneficial to mass transfer such as solid diffusion or dissolution and precipitation. However, simply increasing the sintering temperature not only wastes fuel, but also promotes secondary recrystallization, which deteriorates the performance of the product. Especially in sintering with liquid phase, too high temperature will increase the amount of liquid phase and decrease the viscosity to deform the product. Therefore, the sintering temperature of different products must be combined with differential thermal and thermogravimetric analysis, and carefully analyzed to determine.

When synthesizing LiMn2O4 spinel by high temperature solid phase reaction, the suitable synthesis temperature is 650~850℃. LiMn2O4 changes from a cubic phase to a tetragonal phase in the air at 840℃. When the sintering temperature is higher than 750℃, it begins to lose oxygen, and as the quenching temperature increases and the cooling speed increases, the oxygen deficiency becomes more and more serious. And the electrochemical performance of anoxic spinel LiMn2O4 is poor. When the pressure at 750℃ is greater than 0.016MPa, LiMn2O4 will not lose oxygen. Therefore, the preparation of LiMn2O4 and its doped spinel compound is carried out at 750°C under the condition that po2 is equal to 0.02MPa in the air. The quenched sample is compared with the slowly cooled sample, quenching is more likely to cause ion mixing between the octahedral and tetrahedral positions. The higher the quenching temperature, the more severe the ion mixing, which is more conducive to manganese ions occupying the 8a position, which leads to difficulties in Li+ diffusion and poor electrochemical performance of electrode materials. Under normal circumstances, the LiMn2O4 spinel obtained by high-temperature solid-phase reaction has irregular morphology, uneven particles, often contains more impurities, and has poor conductivity and reversibility.

It can be known from the sintering mechanism that only volume diffusion leads to densification of the green body, and surface diffusion can only change the shape of the pores and cannot cause the difference in the center distance of the particles, so there is no densification process. Figure 1 shows the relationship between surface diffusion, volume diffusion and temperature. In the high temperature stage of sintering, volume diffusion is the main focus, while in the low temperature stage, the surface diffusion is the main focus. If the material is sintered at a low temperature for a long time, not only will it not cause densification, but will change the pore shape due to diffusion, which will cause loss of product performance. Therefore, if the true density and tap density of the material are to be improved, theoretical analysis should be as quickly as possible from low temperature to high temperature to create conditions for volume diffusion. However, as a positive electrode material, lithium ions are required to be released quickly. If the material is too dense and the agglomeration is serious, the electrochemical performance of the material will be reduced. Therefore, in the process of preparing various cathode materials at high temperature, a lot of experiments are needed to determine the best sintering time. During the experiment, the sintering process should be reasonably determined by considering various factors such as the heat transfer coefficient of the material, the secondary recrystallization temperature, and the diffusion coefficient.

Figure 1 - Diffusion coefficient and temperature relationship

Figure 1 - Diffusion coefficient and temperature relationship


Whether in the solid or liquid heating process, the fine particles increase the driving force of sintering, shorten the atomic diffusion distance and increase the solubility of the particles in the liquid phase, which leads to the acceleration of the sintering process. If the sintering rate is proportional to the 13th power of the initial particle size, theoretically, when the initial particle size is reduced from 24m to 0.5um, the sintering rate will increase by 64 times. This is equivalent to reducing the sintering temperature of powder with small particle size by 150~300℃.

Under normal conditions, the original ingredients are added in the form of salts, and sintered in the form of oxides after heating. Salts have a certain structure, such as a layered structure. When they are decomposed, this structure often cannot be completely destroyed. If the structural relationship between the raw material salt and the product is maintained, the salt type, decomposition temperature and time will affect the structural defects and internal strain of the sintered oxide, thereby affecting the sintering rate and performance.

The sintering atmosphere is generally divided into three types: oxidation, reduction, and neutral. In the synthesis of lithium manganese oxide, it is generally an oxidizing atmosphere. Generally speaking, in diffusion-controlled lithium manganese oxide sintering, the influence of atmosphere is related to diffusion control factors, and to the diffusion and dissolution ability of gas in the pores. In the sintering process, it is controlled by the cation diffusion rate. Therefore, when sintering in an oxidizing atmosphere, a large amount of oxygen accumulates on the surface, which increases the cation vacancy, which is beneficial to the acceleration of the cation diffusion and promotes the sintering.

The smaller the atomic size of the gas entering the closed pores, the easier it is to diffuse, and the easier it is to eliminate the pores. Macromolecular gases such as argon or nitrogen are not easy to diffuse freely in the lithium manganese oxygen crystal lattice, and eventually remain in the green body. Especially in the synthesis process of the positive electrode material, because the sample contains lithium element, in order to prevent its volatilization at high temperature from affecting the chemical composition of the material, a certain partial pressure of lithium atmosphere must be controlled.