Garnet-type solid-state mixed ionic and electronic conductor

Low ion and electron conductivities of the cathode and poor cathode interface performance severely limit the application of garnet solid-state batteries. While various approaches have been developed to improve the cathode interface contact or enhance ion and electron transport, none of these methods can simultaneously achieve high interfacial electrochemical activity and chemical stability. In this work, we propose to develop a single-phase garnet-type mixed ionic and electronic conductor (MIEC) as the cathode framework, which is expected to achieve the unification of high activity and high stability at the cathode interface. Combining first-principles calculations and ultrafast sintering techniques, we have synthesized and screened a series of garnet-type MIECs by introducing transition metals into garnet SSEs. Among the garnet-type MIECs, Li43Fe3La24Zr12Ta4O96 (3Fe) not only exhibits an excellent electronic conductivity of up to 2.87 × 10−4 S cm−1 but also maintains an ionic conductivity of 4.57 × 10−5 S cm−1. The high electronic conductivity is believed to originate from the high-temperature reduction phase formed during the rapid sintering process under inert conditions. A small polaron hopping mechanism is proposed to explain the electronic conductivity based on electronic structure calculations and activation energy analysis. Since garnet-type MIECs have the same crystal structure as garnet solid-state electrolytes (SSEs) and transition metal elements similar to those of oxide cathode materials, they potentially have good cosintering stability with both electrolytes and cathodes. This work provides a new strategy to solve the cathode interface problem in garnet solid-state batteries.