An all-solid-state lithium battery using inorganic solid electrolytes requires safety assurance and improved energy density, both of which are issues in large-scale applications of lithium-ion batteries. Utilization of high-capacity lithium-excess electrode materials is effective for the further increase in energy density. However, they have never been applied to all-solid-state batteries. Operational difficulty of all-solid-state batteries using them generally lies in the construction of the electrode-electrolyte interface. By the amorphization of Li2RuO3 as a lithium-excess model material with Li2SO4, here, we have first demonstrated a reversible oxygen redox reaction in all-solid-state batteries. Amorphous nature of the Li2RuO3-Li2SO4 matrix enables inclusion of active material with high conductivity and ductility for achieving favorable interfaces with charge transfer capabilities, leading to the stable operation of all-solid-state batteries.

Amorphous LiCoO2-based positive electrode materials are synthesized by a mechanical milling technique. As a lithium oxy-acid, Li2SO4, Li3PO4, Li3BO3, Li2CO3, and LiNO3 are selected and milled with LiCoO2. XRD patterns indicate that reaction between LiCoO2 and these lithium oxy-acids proceeds. Amorphization mainly occurs, and several broad peaks attributable to cubic LiCoO2 are observed in all the samples. These amorphous active materials show mixed conductivities of electron and lithium ion. All-solid-state cells using the prepared amorphous active materials and the Li2.9B0.9S0.1O3.1 glass-ceramic electrolyte are fabricated and their charge-discharge properties are examined. The cells with only the 80LiCoO2·20Li2SO4 (mol%) and the 80LiCoO2·20Li3PO4 active materials function as secondary batteries. This is because higher lithium ionic conductivities are obtained in the 80LiCoO2·20Li2SO4 and 80LiCoO2·20Li3PO4 active materials than in the others. The largest capacity is obtained in the cell with the 80LiCoO2·20Li2SO4 active material because of its good formability and high lithium ionic conductivity. In addition, the cell with the 80LiCoO2·20Li2SO4 positive electrode active material shows the better cycle and rate performance than that with the crystalline LiCoO2. It is noted that the amorphization with lithium oxy-acids is a promising technique for achieving a novel active material with better electrochemical performance.

All-solid-state sodium batteries using NaZrSiPO (NASICON) solid electrolytes are promising candidates for safe and low-cost advanced rechargeable battery systems. Although NASICON electrolytes have intrinsically high sodium-ion conductivities, their high sintering temperatures interfere with the immediate development of high-performance batteries. In this work, sintering-free NASICON-based composites with NaPS (NPS) glass ceramics were prepared to combine the high grain-bulk conductivity of NASICON and the interfacial formation ability of NPS. Before the composite preparation, the NASICON/NPS interfacial resistance was investigated by modeling the interface between the NASICON sintered ceramic and the NPS glass thin film. The interfacial ion-transfer resistance was very small above room temperature; the area-specific resistances at 25 and 100 °C were 15.8 and 0.40 Ω cm, respectively. On the basis of this smooth ion transfer, NASICON-rich (70-90 wt %) NASICON-NPS composite powders were prepared by ball-milling fine powders of each component. The composite powders were well-densified by pressing at room temperature. Scanning electron microscopy observation showed highly dispersed sub-micrometer NASICON grains in a dense NPS matrix to form closed interfaces between the oxide and sulfide solid electrolytes. The composite green (unfired) compacts with 70 and 80 wt % NASICON exhibited high total conductivities at 100 °C of 1.1 × 10 and 6.8 × 10 S cm, respectively. An all-solid-state NaSn/TiS cell was constructed using the 70 wt % NASICON composite electrolyte by the uniaxial pressing of the powder materials, and its discharge properties were evaluated at 100 °C. The cell showed the reversible capacities of about 120 mAh g under the current density of 640 μA cm. The prepared oxide-based composite electrolytes were thus successfully applied in all-solid-state sodium rechargeable batteries without sintering.

Amorphous Na0.7CoO2–Na x MO y (M = N, S, P, B, or C) positive electrode active materials were synthesized by a mechanochemical technique to achieve high capacities and improved cyclabilities owing to their open and random structures. As none of the X-ray diffraction peaks are attributable to the starting materials, it was clear that the reaction between Na0.7CoO2 and Na x MO y had been successful. The prepared Na0.76Co0.8N0.2O2.2 (80Na0.7CoO2·20NaNO3 (mol %)) was easily densified by pressing at room temperature, and then applied as a positive electrode in bulk-type all-solid-state sodium cells (Na15Sn4/Na3PS4 glass-ceramic/Na0.7CoO2–Na x MO y ). The cell based on the Na0.76Co0.8N0.2O2.2 active material without any conductive additives in an ultrathick positive electrode layer (∼50 μm thickness) operated as a secondary battery at 25 °C. The average discharge voltage was 2.9 V, and the initial discharge capacity was 70 mAh g−1 of the positive electrode. This cell exhibited a higher discharge voltage and a larger capacity than cells employing crystalline Na0.7CoO2 or milled Na0.7CoO2 as the positive electrode. The electrochemical properties of Na0.7CoO2 were therefore improved by amorphization with NaNO3. Furthermore, the cell with the composite electrode containing a conducting additive gave a discharge capacity of 170 mAh g−1 of Na0.76Co0.8N0.2O2.2, which is the highest reported to date for all-solid-state sodium cells based on oxide positive electrodes. Therefore, the amorphization of layered transition-metal oxides with sodium oxy-acids is an effective way to achieve novel active materials with high capacities.