Site-Occupation-Tuned Superionic Li_xScCl_3+xHalide Solid Electrolytes for All-Solid-State Batteries

Abstract

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 x ScCl3+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 x ScCl3+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 x ScCl3+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.

Tailoring the Cation Lattice for Chloride Lithium‐Ion Conductors

“…Our calculated Li-ion conductivity of Li 3 InCl 6 (C2/m) is 1.2 mS cm −1 , in good agreement with the experimental reported value of 1.49 mS cm −1 . [18,[21][22][23] A recent experiment study reports Li-ion conductivity of Li 3 YCl 6 to be 0.51 mS cm −1 , which is slightly lower than our calculated Li-ion conductivity of 14 mS cm −1 , with error bounds in the range of 5 to 47 mS cm −1 and the difference may be attributed to the existence of grain boundary [18] and blocking defects. [19] In addition, a wide range of Zr-doped Li (Figure 2e).…”

“…[ 24 ] Another recent study on Li x ScCl 3+ x showed increased cation concentration leads to lower Li‐ion conductivity, which is also consistent with our design principle regarding cation concentration. [ 23 ] As observed in experiments, [ 23,24 ] cation redistribution or reordering may happen in these Li 3 MCl 6 systems after substitution, affecting the ion conduction. Given our computation method for materials substitution did not consider these cation redistributions during cation substitution, our computation‐predicted Li‐ion conductivities may differ from experimental values for some substituted materials, and the effect of Li concentration and cation distribution in substituted Li 3 MCl 6 materials merits further study.…”

“…Our results further confirm recent computation and experimental studies that Li 3 MCl 6 are promising Li SICs for a wide range of cation and substitutions. [18][19][20][21][22][23][24][25] Most Li x MCl 4 and Li x M 1/2 Cl 4 materials even with substantial levels of substitution have Li-ion conductivities much lower than 200 mS cm −1 at 600 K, and 24 of 51 materials exhibit a negligible amount of Li-ion hopping to obtain a statistically reliable ionic diffusivity. There are a few Li-ion conductors as the exceptions in the Li x MCl 4 category, as a result of their distinct local cation coordination, which are further discussed in Section 3.…”

“…In addition, doping and substitutions on these chloride SICs to tune Li concentration and cation concentration were demonstrated to further improve ionic conductivities. [22][23][24][25] It is not clear why certain doping strategies lead to improved Li-ion conductivity. To rationally guide future materials design of new halide Li-ion conductors, a scientific understanding about the effects of cation concentration, cation configuration, and Li concentration on Li-ion conduction is needed.…”

“…[ 19 ] Recent experimental studies reported a series of Li‐containing chlorides Li 3 MCl 6 (M = In, Er, Sc) and their doped variations that achieved Li‐ion conductivities on the order of 1 mS cm −1 at RT. [ 19,21–25 ] While the discovery of new SIC systems in oxides and sulfides is greatly limited by the unique crystal structures required for achieving fast Li‐ion conduction, [ 26–28 ] Li‐containing chlorides with common fcc and hcp anion sublattices exhibit adequately low Li‐ion migration barrier, and are a promising chemical space for new Li‐ion conductors.…”