Structure, Defect Chemistry, and Lithium Transport Pathway of Lithium Transition Metal Pyrophosphates (Li₂MP₂O₇, M: Mn, Fe, and Co): Atomistic Simulation Study

Abstract

Lithium transition metal pyrophosphate materials (Li₂MP₂O₇, M: Mn, Fe, and Co) have been proposed as promising novel cathode materials for lithium ion batteries. Using atomistic simulation with empirical potential parameters, which has been validated on various cathode materials by Islam et al. [<i>Phil. Trans. R. Soc. A</i> <b>2010</b>, <i>368</i>, 3255–3267], these new pyrophosphates are investigated to elucidate structure, defect chemistry, and Li⁺ ion transport pathway. The core–shell model with empirical force fields reproduces the experimental unit-cell parameters, and formation energies of intrinsic defects (Frenkel and antisite) are calculated. From migration energy calculation, it is found that the pyrophosphates without partial occupation have a 2D Li⁺ ion pathway. Meanwhile, under the condition of partial occupancies of Li and transition metal atoms, the diffusion pathway of Li⁺ ions is a 3D network