Fast ion-conducting high-entropy garnet solid-state electrolytes with excellent air stability

The garnet-type electrolyte is one of the most promising solid-state electrolytes (SSEs) due to its high ionic conductivity (σ) and wide electrochemical window. However, such electrolyte generates lithium carbonate (Li₂CO₃) in air, leading to an increase in impedance, which greatly limits their practical applications. In turn, high-entropy ceramics (HECs) can improve phase stability due to high-entropy effect. Herein, high-entropy garnet (HEG) Li_6.2La₃(Zr_0.2Hf_0.2Ti_0.2Nb_0.2Ta_0.2)₂O₁₂ (LL(ZrHfTiNbTa)O) SSEs were synthesized by the solid-state reaction method. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM) characterizations indicated that the LL(ZrHfTiNbTa)O electrolyte has excellent air stability. Room-temperature conductivity of LL(ZrHfTiNbTa)O can be maintained at ~1.42×10⁻⁴ S/cm after exposure to air for 2 months. Single-element-doped garnets were synthesized to explain the role of different elements and the mechanism of air stabilization. In addition, a lithium (Li)/LL(ZrHfTiNbTa)O/Li symmetric cell cycle is stable over 600 h, and the critical current density (CCD) is 1.24 mA/cm², indicating remarkable stability of the Li/LL(ZrHfTiNbTa)O interface. Moreover, the LiFePO₄/LL(ZrHfTiNbTa)O/Li cell shows excellent rate performance at 30 ℃. These results suggest that HECs can be one of the strategies for improving the performance of SSEs in the future due to their unique effects.