At present, the ternary cathode materials NCM111, NCM523 and NCM622 for lithium-ion batteries have been put into mass production. From the perspective of cathode materials, the increase of nickel content will lead to the intensification of Li/Ni mixing in the ternary material and shorten the cycle life.
More seriously, the increase of nickel content will lead to a significant increase in the residual alkaline impurities between particles, which will lead to serious gas production during the charging and discharging process, resulting in bulging deformation of the battery, shortening of cycle and shelf life, and potential safety hazards. The residual alkaline impurities have become the key to restrict the application of high-nickel ternary materials in high-energy-density power batteries for electric vehicles.
In addition, in recent years, methods such as various anion and cation doping or coating have been adopted to stabilize the bulk phase structure of ternary materials and achieve the effect of improving cycling and storage performance. These methods are difficult to solve the problem of high residual alkali impurities in high nickel materials. To this end, the author of this paper studied the residual alkali impurities under different sintering temperatures and lithium/metal ratios when NCM811 materials were prepared by high-temperature solid-phase method, and verified the alkali reduction effect of various post-treatment systems.
The high-nickel ternary cathode material NCM811 was prepared by a high-temperature solid-phase synthesis process, and the product was α-NaFeO2 structure without impurity phase. Using the sintering conditions of low lithium ratio and high sintering temperature, the residual basic impurity content of the sample is still higher than that of NCM523, NCM111 and other low nickel content ternary materials, indicating that high basic impurities are the common characteristics of high nickel ternary materials.
In a liquid phase environment, NCM811 was treated with different amounts of ammonium dihydrogen phosphate and secondary sintered. The results show that the residual alkali content on the surface of the material is reduced, and the crystal structure does not change, but electrochemically inert substances remain on the surface of the particles, resulting in a significant decrease in capacity and cycle retention. This shows that the idea of reducing residual alkaline impurities by conversion is feasible, but it is necessary to further optimize the amount of phosphate added and the temperature of backburning.
Using water washing can greatly reduce the content of alkaline impurities, and it is lower than the level of imported products. Compared with the sample before treatment, the specific capacity increased by about 1.5mAh/g, and the capacity retention rate after 100 cycles also increased from 90.8% before treatment to 94.1%.
The above results show that water washing is a convenient and effective method to control the content of alkaline impurities in high-nickel ternary materials and improve the properties of materials. As the basic process of controlling residual alkali in high nickel ternary materials, water washing can exert the performance of the material itself. In the follow-up, it is necessary to focus on how to combine water washing and alkali reduction with coating, so as to directly convert the residual basic impurities of alkaline impurities into the coating layer that can realize Li+ conduction on the surface of the primary particles of the positive electrode material, so as to further improve the utilization rate of lithium resources and at the same time. Improve the electrochemical performance of cathode material products such as cycle and thermal stability.