Insights into Electrochemistry and Mechanical Stability of α- and β-Li2MnP2O7 for Lithium-Ion Cathode Materials: First-Principles Comparison

3 years ago

Insights into Electrochemistry and Mechanical Stability of α- and β-Li2MnP2O7 for Lithium-Ion Cathode Materials: First-Principles Comparison

Haiming Xie, Hao Sun, Xiaoying Hou, Jing Liang, Jingping Zhang, Shuwei Tang, Tong Zhang, Yuhan Li

With the aid of first principle calculations, structural characteristics, mechanical stability, and electronic and electrochemical properties of two polymorphs of manganese-based pyrophosphate α- and β-phases of Li₂MnP₂O₇ and their relevant delithiated structures are explored for comparison. Our results indicate that, although these two polymorphs of Li₂MnP₂O₇ belong to the monoclinic space group, considerable differences are discovered in Mn local environment of crystal structures. The cell voltage vs Li/Li⁺ are 4.68 and 4.16 V for α- and β-phases of the Li₂MnP₂O₇/LiMnP₂O₇ platform, respectively, comparable to the experimental values (4.45 and 4.00 V) for first voltage plateaus. All of the lithium atoms are practically fully ionized in the α- and β-Li₂MnP₂O₇ and their relative half delithiated states, charge transfer mainly concentrated upon Mn and O, which leads to the oxidization state of Mn from Mn²⁺ to Mn³⁺ and then from Mn³⁺ to Mn⁴⁺. The band gaps of delithiated configurations decrease gradually with removing lithium ions, and the conductivity changed from insulator nature to conductor characteristic. By the elastic properties calculations, the Pugh ratios (B/G) are 3.28 and 2.86 for the α- and β-Li₂MnP₂O₇, respectively, indicating their high mechanical stability. However, small B/G values are observed for the relevant delithiated phases. In addition, Young’s modulus (E) and Poisson’s ratio (ν) for α- and β-phases of Li₂MnP₂O₇ and their delithiated configurations are also presented to explore the hardness and bond characteristics.

Publisher URL: http://dx.doi.org/10.1021/acs.jpcc.7b06814

DOI: 10.1021/acs.jpcc.7b06814