New lithium halide solid-electrolyte materials, Li YCl and Li YBr , are found to exhibit high lithium-ion conductivity, high deformability, and high chemical and electrochemical stability, which are required properties for all-solid-state battery (ASSB) applications, particularly for large-scale deployment. The lithium-ion conductivities of cold-pressed powders surpass 1 mS cm at room temperature without additional intergrain or grain boundary resistances. Bulk-type ASSB cells employing these new halide solid electrolyte materials exhibit coulombic efficiencies as high as 94% with an active cathode material of LiCoO without any extra coating. These superior electrochemical characteristics, as well as their material stability, indicate that lithium halide salts are another promising candidate for ASSB solid electrolytes in addition to sulfides or oxides.

Roles of antisite transition metals interchanging with Li atoms in electrode materials of Li transition-metal complex oxides were clarified using a newly developed direct labeling method, termed powder diffraction anomalous fine structure (P-DAFS) near the Ni K-edge. We site-selectively investigated the valence states and local structures of Ni in Li0.89Ni1.11O2, where Ni atoms occupy mainly the NiO2 host-layer sites and partially the interlayer Li sites in-between the host layers, during electrochemical Li insertion/extraction in a lithium-ion battery (LIB). The site-selective X-ray near edge structure evaluated via the P-DAFS method revealed that the interlayer Ni atoms exhibited much lower electrochemical activity as compared to those at the host-layer site. Furthermore, the present analyses of site-selective extended X-ray absorption fine structure performed using the P-DAFS method indicates local structural changes around the residual Ni atoms at the interlayer space during the initial charge; it tends to gather to form rock-salt NiO-like domains around the interlayer Ni. The presence of the NiO-like domains in the interlayer space locally diminishes the interlayer distance and would yield strain energy because of the lattice mismatch, which retards the subsequent Li insertion both thermodynamically and kinetically. Such restrictions on the Li insertion inevitably make the NiO-like domains electrochemically inactive, resulting in an appreciable irreversible capacity after the initial charge but an achievement of robust linkage of neighboring NiO2 layers that tend to be dissociated without the Li occupation. The P-DAFS characterization of antisite transition metals interchanging with Li atoms complements the understanding of the detailed charge-compensation and degradation mechanisms in the electrode materials.

Li-rich layered oxide (LLO) is a promising cathode material for lithium-ion batteries because of its large capacity in comparison with conventional layered rock-salt structure materials. In contrast to the conventional materials, it is known that LLO of 3d transition metal has a nanodomain microstructure; however, roles of each domain and effects of strain, induced by the microstructure, on electrode properties are still unclear. In this study, the influence of the strain on an electronic structure is studied to elucidate the stabilization mechanism of LLO material Li­[Li0.2Ni0.2Mn0.6]­O2 in the charged state by using resonant X-ray diffraction spectroscopy (RXDS), X-ray diffraction, and X-ray absorption spectroscopy (XAS) in combination with ab initio calculation. RXDS of a superlattice peak and XAS at Mn and Ni K-edges unveil that this material has a microstructure consisting of Mn-rich and Ni-rich domains, whose structures are similar to Li2MnO3 and LiNiO2, respectively. In the Ni-rich domain, trigonal distortion in the NiO6 octahedral cluster is induced by an elastic constraint due to the microstructure. Hybridization between oxygen p- and nickel d-orbitals is enhanced by the distortion as revealed both by XAS and by ab initio calculation, accounting for stabilization of the charged state by alleviating the direct hole formation on oxygen p-orbital that usually destabilizes the charged material.

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.