How do Li atoms pass through the Al₂O₃ coating layer during lithiation in Li-ion batteries?

We studied the lithiation of Al₂O₃ and found the energetically most favorable composition of Li_3.4Al₂O ₃ using ab initio molecular dynamics simulations. The calculated Li/Al ratio and corresponding volume expansion ratio agree well with reported experimental observations. The Al atoms accept electrons from the incoming Li atoms during lithiation, leading to the formation of various Al structures, that is, isolated atoms, dimers, trimers, and ring- and chain-type clusters. The Li atoms in the optimal concentration diffuse faster by four (five) orders of magnitude than the Al (O) atoms, and they also diffuse faster by four orders of magnitude than the Li atoms in a dilute Li concentration. We suggest that in Li-ion batteries the lithiation of the Al₂O₃ coating layer proceeds until a thermodynamically stable phase is reached; then, extra Li atoms overflow into the electrode by passing through the coating layer.