Relaxation of the Interface Resistance between Solid Electrolyte and 5 V-Class Positive Electrode

Solid-state Li batteries using 5 V-class positive electrode materials display a higher energy density. However, the high resistance at the interface of the electrolyte and positive electrode (interface resistance, R_i) hinders their practical applications. Here, we report the relaxation of R_i between a solid electrolyte (Li₃PO₄) and a 5 V-class electrode (LiCo_0.5Mn_1.5O₄). Although R_i is small at the Mn^3+/4+ redox voltage of 4.0 V vs Li/Li⁺ (11 Ω cm²), it rapidly increases by more than 2 orders of magnitude as the voltage increases above the Co^3+/4+ redox voltage of 5.2 V vs Li/Li⁺. After the applied voltage is reduced to 4.0 V vs Li/Li⁺, R_i decays to the original value after 3 h. The relaxation of R_i after exposure to high voltages suggests that the increase in R_i above 5 V vs Li/Li⁺ is attributable to the formation of an interfacial layer at the LPO/LCMO interface.