(Invited) Solid Halide Electrolytes for All-Solid-State Lithium Ion Batteries

Abstract

In order to realize all-solid-state batteries (ASSBs) with high energy density and high-rate charging/discharging capability that surpass conventional LIBs, development of solid electrolytes (SEs) is one of the utmost remaining issues. The difficulty is that SEs need to satisfy stringent requirements particularly for large scale applications; not only high ionic conductivity, but also high electrochemical stability, chemical stability, and deformability. However, neither sulfide nor oxide SEs satisfy all these requirements by themselves so far. Here, we propose that, considering the electronic and chemical nature of halide anions, halide solid electrolytes (HSEs) have potential to satisfy all the above requirements, and demonstrated that HSEs are indeed suited as an SE material for ASSB application [Ref]. The lithium ionic conductivity of cold-pressed powders of the HSEs we developed surpassed 1 mS/cm at room temperature without any additional grain-boundary resistance. Such high ionic conductivity was realized even with close-packed anion sublattices whose ion pathways are unlike those of sulfide or oxide SEs, indicating that the design consideration for ionic conductors varies depeding on the material systems. The bulk-type ASSBs using these new HSE materials exhibited excellent charge/discharge performance with 4 V class cathode active materials without any extra coating; the initial coulomb efficiency over 94% and low interfacial resistance to be less than 10 Ωcm2, indicating high oxidation stability of HSE materials. Moreover, by appropriate selection of cation/anion pairs, electrochemical stability of HSE can be improved so that HSE can work with the anode active materials with Li-metal potential such as graphite. All these characteristics of halide materials clearly indicate that HSEs are promising candidate for ASSB application, in addition to sulfides and oxides, for large scale deployment. [Ref] T. Asano et al., Adv. Mater. 2018, 30, 1803075.

Electrochemical Impedance Spectroscopy for All‐Solid‐State Batteries: Theory, Methods and Future Outlook

“…Other classes of mechanically-soft inorganic SEs have been discovered, such as phosphides [29,104,105] and halides. [106][107][108][109] Approaches to EIS measurement and ECM modelling are very similar to the sulfides, e. g. cold-pressing of dense compacts and strongly overlapping bulk and grain boundary impedances at room temperature. Interestingly, a recently reported halide SE, Li 2 Sc 2/3 Cl 4 [110] does not require oxide-coating of cathode materials due to the greater electrochemical stability of this material class.…”