A medium-entropy Li-garnet solid electrolyte with ultrahigh ionic conductivity and outstanding electrochemical stability for solid-state batteries

Solid-state batteries (SSBs) have gained attention as a next-generation energy storage technology, with solid-state electrolytes (SSEs) serving as a key component in their development. Garnet-type SSEs, known for their high stability, face limitations due to low ionic conductivity. High-entropy ceramics, with intrinsic structural disorder and tunable compositions, offer a pathway to overcome these challenges. In this study, we innovatively design and develop a medium-entropy garnet-type SSE of Li₆La₃Zr_0.5Ti_0.5Ta_0.5Nb_0.5O₁₂ (LLZTTNO) by partially substituting Zr with Ti, Ta, and Nb. The incorporation of these multiple elements introduces lattice disorder and facilitates fast ion transport through low-energy diffusion pathways, therefore, greatly enhances the electrochemical performance of the SSE. Consequently, LLZTTNO demonstrates excellent ionic conductivity (1.52 × 10^–3 S cm^-1), a low Li-ion activation energy for Li⁺ transport (0.18 eV), and minimal electronic conductivity (6.88 × 10^–10 S cm^-1). A Li-graphite (Li-C) composite was employed as an anode to enhance interfacial compatibility between the solid electrolyte and electrode. The Li-C/LLZTTNO/Li-C symmetric cell demonstrates long-term lithium plating/stripping for over 3600 h at 0.1 mA cm^-2, and achieves a critical current density of 2.5 mA cm^-2. The Li-C/LLZTTNO/LFP full cell also delivers a specific capacity of 143.87 mAh g^-1 and retains 83.4 % of its capacity over 150 cycles at a 0.1 C rate. This work supports the potential application of medium-entropy garnet SSEs for next-generation SSBs, providing valuable insights into their electrochemical performance and stability.