Simultaneous Realization of Electrochemical Stability at Lithium Metal Potential and High Lithium Ionic Conductivity in Lithium Halide Solid Electrolytes
Authors: Sakaida Masashi , Asano Tetsuya , Sakai Akihiro , Miyazaki Akinobu , Hasegawa Shinya 
Published: February 2020

Summary
Here we created new composition of the lithium halide solid electrolyte (HSE) material with high lithium ionic conductivity applicable for negative electrode active materials reacting around lithium metal potential. From some of halide ions’ natures , e.g. monovalent and fairly large ionic radii, halide materials form weak bonding with cations, which is advantageous for lithium ionic conductivity but is disadvantageous to electrochemical stability of constituent cation against reduction. By strategically selecting containing elements and exploring various compositions, we find the materials, dubbed “stabilized HSE (s-HSE)” hereafter, overcoming the trade-off derived from halide ions’ natures. We demonstrated the excellent charge/discharge capability of bulk-type all-solid-state batteries (ASSBs), using only lithium halide materials as solid electrolytes (LiCoO2 / Li3YCl6 / s-HSE / graphite). The initial discharge capacity and coulomb efficiency reached as high as 118 mAh/g and 91 %. In this presentation, we will reveal the details of the material composition, estimation of the electrochemical stability, and the battery characteristics.
Introduction
The ASSBs are one of the most promising candidates for the post-lithium ion batteries. Towards realization of its commercialization, it is essential to improve solid state electrolytes. We have reported HSEs1 that exhibit high ionic conductivity and deformability required for low inter-grain resistance, etc. They are promising materials that have some excellent features distinct from sulfides2, oxides3, and several other material systems4.
However, due to poor electrochemical reduction stability of HSEs, they have not been applicable for low redox potential negative electrode materials, e.g. graphite, silicon, and lithium metal. Owing to some of halide ion’s natures, e.g. monovalent and fairly large ionic radii, chemical bonds formed between halide anion and cation becomes relatively weak. While weak lithium-anion bonding leads to high lithium ionic conductivity, weak cation-anion bonding lower electrochemical reduction stability. This trade-off is a general issue for HSEs, not limited to our reported HSEs.
In this study we create new composition of HSE material overcoming the aforementioned trade-off. One may consider to strengthen bonding between containing cations (M) and anion. Such a simple approach, however, easily fails because the ionic conductivity is severely sacrificed. Here to solve this conflicting issue, we strategically select constituting elements and tailor their compositions so that the advantageous characteristics of elements remain while the disadvantages are alleviated.
Experiments, results and discussions
The ionic conductivity of the s-HSE material was measured on pressed pellets using electrochemical impedance spectroscopy (EIS). The ionic conductivity at room temperature was as high as σ = 1.1 × 10-4 S/cm． Electrochemical stability against reduction was verified by cyclic voltammetry (CV) using the following cell configuration, stainless steel / s-HSE / Li3PS4 / Li. The reduction of s-HSE was not observed in the potential range from open circuit voltage to 0 V vs. Li+/Li. This clearly indicates that the s-HSE material is stable against lithium metal, and expected to be used in negative electrode reacting around lithium metal potential. We performed charge-discharge test for graphite / s-HSE / Li3PS4 / In-Li cell. The discharge capacity and coulomb efficiency were 262 mAh/g and 97 %, close to characteristics of cells using only Li3PS4, commonly studied electrolytes and known to work with graphite, as solid electrolyte. We also confirmed that the valence of M in the above cell at charged state was not reduced by X-ray absorption fine structure (XAFS). These results indicate that this halide material is applicable for negative electrode reacting around lithium metal potential. Furthermore, we consisted bulk-type ASSB using only HSE materials as solid electrolytes (LiCoO2 / Li3YCl6 / s-HSE / graphite). It showed the excellent characteristics; discharge capacity and coulomb efficiency were 118 mAh/g and 91 %.
From all of the above characteristics of the s-HSE, we reveal that HSEs can be used for low redox potential negative electrode required for high battery voltage. By overcoming above trade-off derived from halide ion’s natures, high energy density batteries with HSEs can be realized. This study reveals remarkably high potential of HSEs for ASSBs.

1
T. Asano et al, Adv. Mater. 30, 1803075 (2018).

2 Y. Kato et al., Nat. Energy, 1, 16030 (2016).

3 X. Han et al, Nat. Materials 16, 572–579 (2017).

4 S. Kim et al, Nat. Commun. 10, 1081 (2019).