The requirements of large scale lithium ion batteries such as electric vehicles (EV) and energy storage systems (ESS) have been attention. However, lithium ion batteries using liquid electrolytes have been badly reported of safety issues by its flammability. To resolve this issue, all-solid-state lithium ion batteries have been considered for candidate of next generation lithium ion batteries. The key material to realize all-solid-state lithium ion batteries is solid electrolytes with high conductivity and safety.

The ionic conductivity of solid electrolytes are currently attracting great attention. Several types of sulfide based electrolytes have been shown favorable lithium ion conductivities of 10^-3-10^-2 S/cm, which are comparable to those of liquid electrolytes, have been reported. In addition, those have several advantages of single Li ion conduction, good electrochemical stability over wide potential range.

However, all-solid-state cell has obstacle that is energy density of composite electrode lower than battery system using liquid electrolytes. It is difficult to make composite cathode with solid electrolytes for developing all-solid-state cells, that was required the continuous lithium ion and electron conducting paths. Inorganic solid electrolytes had lower contact area with active materials than liquid electrolytes, it was caused restrict ionic paths with active materials, and low electrochemical performance. In a large number of articles on all-solid-state lithium ion batteries using sulfide based electrolytes, composite electrodes were prepared by dry mixing. However, it is very difficult to control the particle mixture by dry mixing, it was caused low electrochemical performance. To conclude, an effective method for homogeneously mixed composite electrodes can improve the electrochemical performance of the all-solid-state lithium ion batteries.

In the present study, the LiCoO₂/Li₂S-P₂S₅ electrolyte composite cathodes were successfully fabricated by using the ESSD method for archive homogeneous thick film composite electrodes. The ESSD (Electrostatic Slurry Spray Deposition) process is unique method for thick film fabrication. The ESSD technique uses electrical energy to atomize and spray solutions. The ESSD technique is a method of spraying the slurry containing prepared powders. Dense film without any cracks and agglomeration can be easily fabricated by ESSD.

The composite cathodes were produced with differential cathode material / solid electrolyte ratio. Solid electrolytes were synthesized by mechanical milling method. In the deposited composite cathode thick films, all components homogeneously mixed, which were investigated by SEM and EDX. In addition, we made all-solid-state lithium ion batteries with composite cathode / solid electrolyte / In structure by uniaxial cold pressing method, which were successfully tested.