Pseudo Jahn-Teller Origin of Buckling Deformation of Two-dimensional Group-IV-Based Triphosphides as an Anode of Sodium-Ion Batteries

Two-dimensional group-IV-based triphosphides MP₃ (M = C, Ge, and Sn) with blue-type structures are regarded as potential and promising anode materials for sodium-ion batteries (SIBs). It is very important to understand the relationship between the structure and Na storage capacity of anode materials. In the present study, we revealed the origin of buckled configurations of both blue-type and unreported black-type MP₃ (M = C, Ge, and Sn) by employing pseudo Jahn-Teller effect (PJTE) theory and ab initio calculations. The difference in buckling height of MP₃ (M = C, Ge, and Sn) is strongly related to the PJTE intensity and the energy gap of the coupled excited states. Upon Na adsorption, the buckling height of blue GeP₃ and SnP₃ decreases, and the resulting extensively exposed surface could improve the storage capacity of Na and contribute to their performance as anode materials of SIBs. This process can be explained by the suppressed role of PJTE because of the increased energy gaps of the coupled electronic states and consequent weakening of the PJTE coupling intensity. In comparison, others are not preferable as anode materials for SIBs because of their poor Na adsorption ability.