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A thin coating layer between the cathode active materials (CAMs) and solid electrolytes (SEs) is indispensable for alleviating the reaction at the CAM/SE interface and thereby enhancing the reversible capacity of all-solid-state Li-ion batteries (ASSLIBs). It is well established that the interfacial resistance in the CAM/coating layer/SE is sensitive to post-annealing processes. However, the underlying mechanism remains unclear; the influence of the electrochemical/physicochemical phenomena on capacity degradation, particularly the impact of the annealing temperature, has not been adequately elucidated. Herein, we applied various annealing temperatures to LiNbO3-coated LiCoO2 (LNO-coated LCO) particles, and initially adopted a double-coating method to investigate the behaviour of the LCO/LNO/Li10GeP2S12 interfaces. We observed that the electronic conductivity of LNO-coated LCO increased with increasing annealing temperature, presumably because of the LNO segregation at the LCO surface and Co diffusion to the LNO layer, both of which occurred during the dynamic LNO phase change from amorphous (350 °C annealed state) to crystalline (700 °C annealed state). The increased electronic conductivity triggers the interfacial reaction between the LNO layer and Li10GeP2S12, which is the primary cause of reversible capacity loss, whereas the ionic conductivity in LNO has minimal effect on the reversible capacity at low C-rates. This study highlights the importance of having a suitable electronic conductivity in the coating layer to improve the performance of bulk-type ASSLIBs.

In the field of all-solid-state Li-ion batteries, understanding and controlling contact loss and interfacial reactions in composite electrodes over long-term cycling remains challenging. This study investigated how various physicochemical phenomena individually affect the discharge profile upon cycling for a solid-state cell with the following composition: LiNbO3-coated LiCoO2 + Li10GeP2S12|Li10GeP2S12|In–Li alloy. Two specially designed cells were prepared to separately characterise contact loss or interfacial chemical reactions, by means of electrochemical measurements and chemical analysis. It was found that in the discharge process, contact loss induced a certain overpotential compared to the initial value, while the chemical reaction accounted for the capacity loss caused by an additionally increased overpotential in the final stage of the discharge process. The overpotential from the undesired chemical reaction was proposed to be due to the high Li-ion transfer/migration resistance induced by sulphate formation and the decrease in the lithium content at the LiNbO3/Li10GeP2S12 interface.

A practical material search using a recommender system is demonstrated to obtain novel lithium ion conducting oxides. The synthesis of unknown chemically relevant compositions (CRCs) proposed by the recommender system and their related materials effectively reveals two kinds of novel lithium ion conductors using different approaches. In the Li2O–GeO2–P2O5 system, Li6Ge2P4O17 is found, which has the same composition as the recommended unknown CRC. Less-than-10-time synthesis following the ranking order in the diagram provides evidence of the discovered new phase. In the other quasi-ternary diagram of the Li2O–ZnO–GeO2 system, Li3Zn0.65Ge4.35O10.85 is discovered by a combination of a recommender system and synthetic chemistry, because the composition of the novel phase is different from that of the recommended unknown CRC. Here too, the required time for material discovery is reduced to one-third of that required for random search without the recommender system. Phase identification and elemental analysis suggest that these discovered materials could have unique compositions and crystal structures. The room-temperature (∼300 K) ionic conductivity (10−9–10−6 S cm−1) of the novel phases can be improved by compositional and structural optimisations. The recommender system could emerge as a practical material search tool for enhancing the discovery rate of solid lithium ion conductors.

Hydride-ion conductors have attracted great interest as solid electrolytes due to the highly negative redox potential of the hydride ion. This study provides direction for effective design of materials that enhance H− conductivity by exploring the relationship between cation size and hydride-ion conductivity using LnSrLiH2O2 (Ln = La, Pr, Nd, Sm, Gd), which are K2NiF4-type oxyhydride materials, synthesised under ambient and high pressures. The size of the A-site cation and the activation energy for H− conduction were found to be strongly correlated. GdSrLiH2O2 exhibits the lowest activation energy of 67 kJ mol−1. Crystal structure analysis and first principles calculations revealed that H− is more displaced along the conduction pathway for smaller A-site cations in LnSrLiH2O2 due to lower repulsion between A-site cations and H−. These findings contribute to the further development of K2NiF4-type oxyhydrides with high H− conductivities.

A solid solution of the lithium superionic conductor Li10+δGe1+δP2−δS12 (0 ≤ δ ≤ 0.35) was synthesized and its structure and ionic conductivity were examined. The highest ionic conductivity value of 1.42 × 10−2 S cm−1 was obtained at 300 K with a sintered pellet of the sample having the highest solid solution lithium content of δ = 0.35. The Arrhenius conductivity curves obtained for this material exhibited a gradual change in slope over the temperature range of 193–373 K and the activation energy for ionic conduction decreased from 26 kJ mol−1 below 373 K to 7 kJ mol−1 above 573 K, which is typical of highly ionic conducting solids. The crystal structures of the solid solutions were determined using neutron diffraction, and conduction pathways were visualized through analysis by applying the maximum entropy method. The lithium distribution was found to disperse significantly throughout a one-dimensional conduction pathway as the temperature was increased from 4.8 K to 750 K. In addition, two-dimensional distribution of lithium along the ab plane became apparent at high temperatures, suggesting that the conduction mechanism changes from one-dimensional to three-dimensional with increasing temperature.