Solvent-induced structural insight into improved electrochemical performance of olivine type high-voltage lithium cobalt phosphate cathode materials

The increasing demand for lithium-ion batteries with high energy density has driven research into olivine-type high-voltage cathode materials . However, the development of these materials is hindered by two significant challenges: poor electronic conductivity and a low lithium-ion diffusion coefficient . Herein, carbon-coated LiCoPO 4 materials were synthesized using precursors with different space groups via a solvothermal method, employing a binary mixture of diethylene glycol (DEG) and water as solvents. The influence of the solvent ratio on space group regulation, structural characteristics, and electrochemical performance was systematically investigated. When the solvent ratio (V DEG : V H2O ) is <2.3, the resulting cathode material exhibits a pure phase belonging to the Pnma space group. Conversely, when the solvent ratio exceeds this threshold, a metastable phase corresponding to the Pn 21 a space group is formed. Electrochemical testing revealed that the performance of LiCoPO 4 materials in the Pnma space group significantly surpasses that of materials in the Pn 21 a space group. Notably, the LiCoPO 4 /C material synthesized with a V DEG : V H2O ratio of 2.3, which crystallizes in the Pnma space group, demonstrates the most outstanding performance among the samples studied. This material achieves a specific discharge capacity of 157.1 mAh g −1 at 0.1C and 91.9 mAh g −1 at 2C. Additionally, the lithium-ion diffusion coefficient of this material reaches 1.01 × 10 −14  cm 2  s −1 . In summary, the electronic conductivity and lithium-ion diffusion coefficient of LiCoPO 4 were effectively enhanced through carbon coating and precise control of the crystal phase of the primary grains. This study provides valuable insights into strategies for further improving the performance of polyanionic-based cathode materials, offering a foundation for future advancements in high-energy-density lithium-ion batteries.