Solid state battery (SSB) has attracted much attention as a promising successor to Li-ion battery (LIB). For full commercialization, however, there still remain crucial issues; finding super-ionic conductors, and control of the bottlenecks, which often appear around various types of interfaces, for ion transport as well as degradation. These interfaces, in fact, are affected by strain/stress owing to the intrinsic lattice mismatch around the interfaces and the volume change of electrode materials during charging/discharging. Though strain/stress effects have been examined on the meso- or macro-scopic scale so far, the atomistic studies were still limited. In this work, we explored the microscopic effects of strain/stress on the Li-ion transport by investigating (1) cathode materials (Li_xCoO₂, x<1), and (2) cathode / coating layer interfaces (LiCoO₂/LiNbO₃, LiTaO₃) by density functional theory (DFT) calculations.

First, we calculated Li-ion self-diffusion coefficients (D^*_Li)of Li_xCoO₂,(x=0.81 and 0.69) under some strain/stress conditions via extensive DFT molecular dynamics (MD) simulations. We found that "Li layer distance" does not alter much upon the strain/stress conditions, and D^*_Li is more correlated with the “Co layer distance", which is in contrast to the chemical intuition. The detailed examinations demonstrated that the Co-Li electrostatic repulsion dominantly affects the Li-ion diffusion, and the Li-Li correlation also has effects. Besides, activation volume (V_a) for cathode material was evaluated with the extension into the tensor form, which is theoretically for the first time. This extension can be utilized under different mechanical cases and anisotropic structures like a layered rock-salt structure.

For Li-ion migration across interface, we also investigated LiCoO₂ interfaces with the coating materials LiNbO₃ and LiTaO₃, which have identical structures with slightly different lattice constants. Thus, the strain effects may be manifested in their comparison. Migration barriers across the interfaces by DFT calculations with Nudged Elastic Band method indicated that LiTaO₃ has smaller barriers than LiNbO₃. This tendency was consistent with the calculated Li chemical potential (minus of Li vacancy formation energy), in which the difference between LiCoO₂ and the coating material is smaller in LiTaO₃. Such difference can be attributed to the interfacial strain effect related to the different lattice mismatches in both coating materials. With more detailed analysis of the atomic structures during the migration, we extracted the characters correlated with the migration barriers.

In the talk, I will show more recent results and discuss the strain/stress effects from more general viewpoint, which will contribute to understanding and control of Li-ion transport in SSBs.