Xie Ying, Yu Hai-Tao, Yi Ting-Feng, Zhu Yan-Rong
Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University), Ministry of Education, School of Chemistry and Materials Science, Heilongjiang University , Harbin 150080, PR China.
ACS Appl Mater Interfaces. 2014 Mar 26;6(6):4033-42. doi: 10.1021/am4054833. Epub 2014 Mar 11.
To elucidate the microscopic origin of the difference behaviors, first-principles calculations were performed to investigate the thermal and mechanical stabilities of LixFePO4 and LixMnPO4. 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 LixFePO4 and LixMnPO4, 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 (AG) 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 {1̅01} 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.
为了阐明不同行为的微观起源，进行了第一性原理计算以研究Li x FePO 4 和Li x MnPO 4 的热稳定性和机械稳定性。计算得到的自由能表明，LiFePO 4 和LiMnPO 4 在其原始状态和完全脱锂状态下相对于相关氧化物都是热稳定的。根据计算，可以确定在Li x FePO 4 和Li x MnPO 4 中，相对于体积压缩，剪切变形更容易发生，并且这种现象与M - O(I)和M - O(II)键有关。以MnPO 4 为例，从主体结构中提取Li(+)会使Mn - O(I)键进一步减弱约33%，因此它变得非常脆。MnPO 4 的剪切各向异性(AG)异常大，已经达到19.05%，约为FePO 4 的6倍。因此，MnPO 4 中容易发生剪切变形和位错。此外，由于MnPO 4 中的Mn - O(I)键主要分布在{101}和{1̅01}晶面内，相关的滑移系统因此允许界面处的键重新组合，导致实验观察到的相变。可以得出结论，机械原因对MnPO 4 较差的循环性能起重要作用。
