Li+ Transport Mechanism at the Heterogeneous Cathode/Solid Electrolyte Interface in an All-Solid-State Battery via the First-Principles Structure Prediction Scheme

High interfacial resistance between a cathode and solid electrolyte (SE) has been a long-standing problem for all-solid-state batteries (ASSBs). Though thermodynamic approaches suggested possible phase transformations at the interfaces, direct analyses of the ionic and electronic states at the solid/solid interfaces are still crucial. Here, we used our newly constructed scheme for predicting heterogeneous interface structures via the swarm-intelligence-based crystal structure analysis by particle swarm optimization method, combined with density functional theory calculations, and systematically investigated the mechanism of Li-ion (Li+) transport at the interface in LiCoO2 cathode/β-Li3PS4 SE, a representative ASSB system. The sampled favorable interface structures indicate that the interfacial reaction layer is formed with both mixing of Co and P cations and mixing of O and S anions. The calculated site-dependent Li chemical potentials μLi(r) and potential energy surfaces for Li+ migration across the interfaces reveal that interfacial Li+ sites with higher μLi(r) values cause dynamic Li+ depletion with the interfacial electron transfer in the initial stage of charging. The Li+-depleted space can allow oxidative decomposition of SE materials. These pieces of evidence theoretically confirm the primary origin of the observed interfacial resistance in ASSBs and the mechanism of the resistance decrease observed with oxide buffer layers (e.g., LiNbO3) and oxide SE. The present study also provides a perspective for the structure sampling of disordered heterogeneous solid/solid interfaces on the atomic scale.