Site-Occupation-Tuned Superionic LixScCl3+xHalide Solid Electrolytes for All-Solid-State Batteries

The enabling of high energy density of all-solid-state lithium batteries (ASSLBs) requires the development of highly Li⁺-conductive solid-state electrolytes (SSEs) with good chemical and electrochemical stability. Recently, halide SSEs based on different material design principles have opened new opportunities for ASSLBs. Here, we discovered a series of Li_xScCl_3+x SSEs (x = 2.5, 3, 3.5, and 4) based on the cubic close-packed anion sublattice with room-temperature ionic conductivities up to 3 × 10^-3 S cm^-1. Owing to the low eutectic temperature between LiCl and ScCl₃, Li_xScCl_3+x SSEs can be synthesized by a simple co-melting strategy. Preferred orientation is observed for all the samples. The influence of the value of x in Li_xScCl_3+x on the structure and Li⁺ diffusivity were systematically explored. With increasing x value, higher Li⁺, lower vacancy concentration, and less blocking effects from Sc ions are achieved, enabling the ability to tune the Li⁺ migration. The electrochemical performance shows that Li₃ScCl₆ possesses a wide electrochemical window of 0.9-4.3 V vs Li⁺/Li, stable electrochemical plating/stripping of Li for over 2500 h, as well as good compatibility with LiCoO₂. LiCoO₂/Li₃ScCl₆/In ASSLB exhibits a reversible capacity of 104.5 mAh g^-1 with good cycle life retention for 160 cycles. The observed changes in the ionic conductivity and tuning of the site occupations provide an additional approach toward the design of better SSEs.

Coordination numbers and physical properties in molten salts and their mixtures

Mixtures of trivalent metal halides with alkali halides are involved in many technologies but, from a more fundamental and general perspective, are worthy of study as interesting systems in which to examine the relationship between atomic-scale structure and physical properties. Here we examine the relationship between the viscosity and local and longer range structural measures in such mixtures where the trivalent metal cations span a significant size range and exhibit different behaviours in the dependence of their viscosity on the mixture composition. We characterise the structure and dynamics of the first coordination shell and the relationship between its structural relaxation time and the shear relaxation time of the mixture (the Maxwell relaxation time). We are then led to an examination of the structure of the networks which progressively form between the trivalent metal cations as their concentration increases in the mixtures. Here we find significant differences between small and larger cations, sufficient to explain the different behaviour of their viscosities. We draw attention to the similarities and differences of these networks with those which form in highly viscous, glass-forming materials like BeF₂:LiF.