Diffusion Control at the Interface in Oxide-Based Solid-State Batteries via Nonequilibrium Millimeter-Wave Irradiation

Oxide-based all-solid-state batteries (ASSBs) offer high safety and chemical stability but suffer from large interfacial charge transfer resistance (RCT) between electrodes and solid electrolytes (SEs). This study introduces a 24 GHz millimeter-wave (MMW) irradiation technique for interface engineering that densifies SEs while suppressing interdiffusion. MMW irradiation promotes uniform microstructure formation, inhibits the growth of resistive interphases such as La2Li0.5Co0.5O4, and markedly improves electrochemical performance. The RCT decreases by more than 1 order of magnitude compared to conventional sintering. Diffusion coefficient analysis revealed that the La3+ diffusion was significantly suppressed under MMW irradiation compared to conventional sintering. This behavior arises from the lower ponderomotive force acting on La3+ due to its smaller z2/m value. The MMW-assisted sintering strategy provides a thermally nonequilibrium pathway to achieve simultaneous SE densification and sustained interfacial Li-ion transport activity in oxide-based ASSBs.