Pressure-Driven and Creep-Enabled Interface Evolution in Sodium Metal Batteries

All-solid-state batteries (ASSBs) using an alkali metal anode and a solid-state electrolyte (SE) face several problems due to poor physical and electrical contact. Recent experiments have shown that applying a stack pressure can improve the interface contact and suppress void formation. The mechanical properties of Na metal are different from those of Li metal, leading to differences in the mechanisms of the pressure-dependent interface evolution. Herein, we report a three-dimensional time-dependent model for tracking the evolution of interfaces formed between Na metal and Na-β″-alumina SE. Our results show that Na metal contacts more conformally with the SE, providing a lower interfacial resistance, compared with Li metal, assuming equal resistance due to contamination. The differences due to contact elastoplasticity are larger than the differences in metal creep effects. In fact, we show that increased stack pressure can lead to lower creep because the contact is more conformal at high pressures. Our excellent agreement with recent experiments determines an effective hardness of Na in the Na-SE batteries to be 15 MPa. The results further reveal that the pressure dependence of void suppression is dominated by contact elastoplasticity.