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 ( R CT ) 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 La 2 Li 0.5 Co 0.5 O 4, and markedly improves electrochemical performance. The R CT decreases by more than 1 order of magnitude compared to conventional sintering. Diffusion coefficient analysis revealed that the La 3+ diffusion was significantly suppressed under MMW irradiation compared to conventional sintering. This behavior arises from the lower ponderomotive force acting on La 3+ due to its smaller z 2 / 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.