Synthesis of ultrathin carbon layer-coated LiNiPO4 nanoparticles by solvothermal method

The emerging market of high-power lithium-ion batteries (LIBs) highlights the importance of developing high-voltage cathodes. Lithium nickel phosphate (LiNiPO4) with the highest theoretical discharge potential (∼5.1V) has attracted increasing attention due to its stable olivine structure. In this study, ultrathin carbon layer-coated LiNiPO4 was synthesized using oleylamine-assisted solvothermal method under different solvothermal times, and the effect of solvothermal time (1h-18h) on LiNiPO4 samples is fully investigated. X-ray diffraction (XRD) shows that high-purity LiNiPO4 materials can be prepared after calcination of the solvothermal product under N2, but adding part of H2 to the calcination atmosphere will lead to the generation of impurities in the target product. As the solvothermal time increases, the product will agglomerate and grow, the particle size will increase, and the morphology will change from nanoparticle to rodlike structure. After the high-temperature calcination step, the oleylamine is completely carbonized, and a conductive carbon layer is successfully coated on the LiNiPO4 with the thickness of 2 nm. As the solvothermal time increases, the graphitization degree of oleylamine increases after carbonization. The existence of the carbon layer and the increase in the degree of graphitization are beneficial to improving the electrical conductivity and Li-ion diffusion rate of LiNiPO4. The initial discharge capacity of LNP@C-12h product prepared with 12 h solvothermal time is 35 mA h g-1 at 0.1 C. Rietveld analysis shows that the anti-site defect concentration of the LNP@C material is 1.07%, and the proportion increases to 2.83% after several cycles. This work provides a positive contribution to the synthesis and modification of high-voltage LiNiPO4 materials and explains the electrochemical performance degradation of LiNiPO4 from the perspective of increasing the concentration of anti-site defects.