Understanding the Thermal and Mechanical Stabilities of Olivine-Type LiMPO4 (M = Fe, Mn) as Cathode Materials for Rechargeable Lithium Batteries from First Principles

摘要

To elucidate the microscopic origin of the difference behaviors, first-principles calculations were performed to investigate the thermal and mechanical stabilities of Li_xFePO4 and Li_xMlnPO4. The calculated free energies suggested that LiFePO4 and LiMnPO4 are thermal stable with respect to relevant oxides both in their pristine and fully delithiated states. According to the calculations, it can be identified that the shear deformations are more easier to occur with respect to the volume compressions in Li_xPO4 and Li_xMnPO^ and this phenomenon is related to M—O(I) and M-O(II) bonds. Typically for MnPO4, Li~+ extraction from the host structures further weakens the Mn—O(I) bonds by about 33%, and it thus becomes very brittle. The shear anisotropy (A_G) of MnPO4 is abnormally large and has already reached 19.05 %, which is about 6 times as large as that of FePO4. Therefore, shear deformations and dislocations occur easily in MnPO4. Moreover, as the Mn-O(I) bonds in MnPO4 are mainly spread within the {101} and {101} crystal planes, the relevant slip systems thus allow the recombination of bonds at the interfaces, leading to the experimentally observed phase transformation. It can be concluded that mechanical reason will play an important role for the poor cycling performance of MnPO4.