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

With the aid of first principle calculations, structural characteristics, mechanical stability, and electronic and electrochemical properties of two polymorphs of manganese-based pyrophosphate alpha- and beta-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 alpha- and beta-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 alpha- and beta-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 alpha- and beta-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 (v) for alpha- and beta-phases of Li2MnP2O7 and their delithiated configurations are also presented to explore the hardness and bond characteristics.