Enhanced Li+ conductivity of Li7La3Zr2O12 by increasing lattice entropy and atomic redistribution via Spark Plasma Sintering

Abstract

Cubic Li7La3Zr2O12 (c-LLZO) is an ideal solid-state electrolyte for Li-ion batteries. The Li diffusivity at 24d site is a rate-determining step of the ionic conductivity. Aluminium is commonly used to synthesise c-LLZO. However, Al3+ prefers to occupy the 24d over 96h. To improve the ionic conductivity of Al-doped LLZO, minimizing the occupation of Al at 24d is shown to be crucial. In this work, SPS (Spark Plasma Sintering) is employed primarily due to its capability of metastable phase stabilization via a non-equilibrium process and ion redistribution related electromigration process. X-ray powder diffraction refinement reveals that occupancy of Al at 24d site decreased and lattice distortion index increase of SPS processed Al-doped c-LLZO. The lattice distortion index is closely tied to lattice entropy, i.e., configuration and vibrational entropy. Increasing lattice entropy can reduce the energy barriers between Al and Li’s 24d and 96h sites, stabilising Al3+ occupancy at 96h site. After SPS parameter optimisation, the high ionic conductivity of 2.6 × 10–4 S·cm-1 and low electronic conductivity of 10–7 S·cm-1. This study offers a new perspective on using SPS to modify the lattice structure, with potential benefits beyond simply densifying the samples. Highlighting the importance of high lattice entropy is favourable in enhancing ionic conductivity.