Amorphous oxyhalide solid electrolytes with improved ionic conductivity and reductive stability for all-solid-state batteries

Abstract

Halides have emerged as a promising class of solid electrolytes (SEs), exhibiting superior ionic conductivities comparable to those of state-of-the-art sulfide-based SEs, while also offering oxidative stability comparable to oxide-based SEs, making them suitable for high-energy-density all-solid-state batteries (ASSBs). Although extensive research has been conducted on cubic close-packed (ccp) and hexagonal close-packed (hcp) halide-based SEs, oxyhalide SEs have recently attracted significant attention owing to their disordered amorphous structures and high ionic conductivities. However, the intrinsic reductive instability of halides and limitations associated with conventional oxygen sources have impeded their practical application. Herein, we report a novel series of amorphous oxyhalide SEs, Li4xLaxTa1-xO2xCl5-2x (x = 0.25, 0.35, and 0.5), wherein lithium lanthanate (LiLaO2) is strategically employed as an oxygen source instead of the conventional Li2O. Using complementary X-ray diffraction, scanning electron microscope, Raman spectroscopy, X-ray photoelectron spectroscopy, and electrochemical impedance spectroscopy, insights into the structures and local environments that affect the ion transport are provided. These results indicate that the synergetic O/Cl ratio, La/Ta ratio, and Li+ carrier concentration play a pivotal role in structures and properties of the amorphous oxyhalide SEs. Notably, the Li1.4La0.35Ta0.65O0.7Cl4.3 not only delivers an impressive room-temperature of 4.9 mS cm-1, but also demonstrates an exceptionally high critical current density (CCD) of 40 mA cm-2 in symmetric Li cells using bare Li metal.