Unlocking the hidden chemical space in cubic-phase garnet solid electrolyte for long-term stable solid-state batteries

Compared to the tetragonal phase, the cubic phase polymorphism of garnet-type solid electrolytes (Li7La3M2O12) has attracted considerable interest for all-solid-state batteries owing to their high ionic conductivity. Herein, we successfully synthesized a cubic-phase garnet without vacancy formation (Li = 7.0) by applying multicomponent (Hf, Sn, Sc, and Ta) substitutions in the Zr site, considering defect formation energy, site-exchange energy, and charge balance. The entropy-driven stabilization allowed access to the hidden chemical space in cubic-phase garnet (Li > 6.6), and signified the feasibility of low-temperature synthesis as the nucleation temperature of cubic phase decreased from 750 to 400 ℃ in the solid-state reaction. Specifically, Li = 7.0 cubic-phase garnet-type solid electrolyte exhibited superior reduction stability against lithium metal compared to that with low lithium contents (Li = 6.6) and identical atomic species, which further demonstrated the long-term cycle operational stability of such solid-state batteries. The present findings exemplified the superior reduction stability of the cubic-phase garnet with higher chemical potential of lithium to design solid electrolytes.