Sulfide-based solid electrolytes can be pressed at room temperature to form an interface with the disappearance of grain boundaries. This enables fabrication of bulk-type batteries. It is considered to be the closest to practical application. On the other hand, oxide-based solid electrolytes have excellent atmospheric stability and heat resistance, but their hard mechanical properties make it difficult to form an interface between the oxide-based solid electrolyte and anode active materials by pressing at room temperature. Therefore, high-temperature heat treatment over 700 °C is required for the interface bonding between particles, but elemental diffusion occurs at the active material/solid electrolyte interface, resulting in the formation of a hetero-phase, which is a high-resistance layer.

In this study, we applied a sintering free solidification process to TiO₂ anode active material and oxide-based solid electrolyte to form a good interface between oxide-based solid electrolyte and TiO₂ anode at room temperature. The surface activation treatment produces Ti unbonded hands (dangling bonds) by grinding with a ball mill, and the unbonded hands are joined to each other to form an interface.

Amorphous TiO₂ with large specific surface area was synthesized by the method described in Ref.
2). TiO₂ and surface of oxide solid electrolyte Li_1.4Al_0.4Ti_1.6(PO₄)₃ (LATP) were activated by planetary ball mill and the formation of dangling bond was examined by ESR measurement. In addition, a bulk type of half-cell was fabricated by layering the anode composites (TiO₂:LATP:AB = 4:6:1 (weight ratio)), solid electrolyte layer (Li₃PS₄ (LPS)), and counter electrode (Li-In), and its electrical characteristics were evaluated.

The ESR of LATP, TiO₂ alone, and LATP/TiO₂ composites were measured. The untreated sample showed no peaks for LATP, TiO₂ or LATP/TiO₂, while the surface-activated TiO₂ sample showed a peak at g=2.00268, which was assigned to radicals, that is dangling bonds. For the surface-activated LATP sample, two peaks derived from dangling bond and Ti³⁺ were observed around g=2.00721 and g=1.9202. The LATP/TiO2 mixture also shows peaks for the dangling bond and Ti³⁺, but the peak ratio (g=2.00/g=1.92) decreased compared to that of activated LATP alone. This is thought to be due to formation of bonding between the dangling bonds of TiO₂ and LATP, resulting in a decrease in the number of radicals.

Cross-sectional SEM images showed that the surface-activated LATP/TiO₂ anode composites were densified by pressure molding, with a small amount of porosity and the disappearance of grain boundaries. In contrast, the untreated anode composites were unformed when the pressure was removed. In other words, the surface activation treatment makes the dangling bonds to bond with each other, resulting in interface formation in the molecular level. LATP/TiO₂ anode half-cell were assembled from surface activated TiO₂ and LATP and cycle characteristics were measured. As a result,
the initial discharge capacity was 27 mAh/g and increased to 81 mAh/g at 50th cycle. This discharge capacity was almost the same to that of half cells assembled with LPS and TiO₂ without surface activation. Coulomb efficiency also improved with the number of charge/discharge cycles as well. It can be said that even with hard oxides, a good interface was formed by surface activation with mechanical milling and charge-discharge cycles.