Insights into the microstructural regulation and electrochemical performance of Ge/V dual-doped garnet-type solid electrolytes

Ge doping stabilizes the cubic phase, enhances ionic conductivity and improves the sintering activity of garnet LLZO. • Further V co-doping introduces high-valence V ions to tune Li vacancy concentration and create ample Li + migration channels. • Co-doping optimizes interfacial Li + distribution and inhibits insulating Li 2 CO 3 formation. • Smaller Ge 4+ and V 5+ substitute Zr sites to widen Li + migration channels. Garnet-type solid electrolyte lithium lanthanum zirconium oxide (Li 7 La 3 Zr 2 O 12 , LLZO) has attracted extensive research attention owing to the high ionic conductivity of its cubic phase. However, cubic-phase LLZO readily transforms into tetragonal-phase LLZO at room temperature, which degrades its ionic conductivity. In addition, conventional cation doping strategies frequently yield poor densification as a result of inferior sintering activity. In this work, Ge/V bimetallic co-doped LLZO solid electrolytes are synthesized via the conventional solid-state sintering method, and the effects of Ge/V co-doping on the cubic phase stability and ion transport properties of LLZO are systematically investigated. Material characterization results reveal that Ge/V co-doping distorts the lattice and generates lithium vacancies, which collectively stabilize the cubic crystal phase and optimize the Li + migration bottlenecks. Meanwhile, Ge/V co-doping improves the sintering activity and enhances the surface densification of LLZO. The LLZO-GV electrolyte delivers an ionic conductivity of 3.61 × 10 −4  S·cm −1 at room-temperature with a lithium-ion transference number of 0.54, with an electrochemical stability window up to 5.33 V. Electrochemical characterizations of the assembled LFP/LLZO-GV/Li full cell reveal that it delivers a specific capacity of 161.39 mAh g −1 at 25 °C and 0.2C, and retains a capacity retention of 96.6% after 100 cycles at 25 °C and 0.1C. Moreover, the Li//Li symmetric cell can stably cycle for 600 h at a current density of 0.3 mA cm −2 and exhibits superior rate capability under various current densities. This bimetallic co-doping strategy provides an effective route to develop high-performance all-solid-state lithium batteries.