Processing and Characterization of Secondary Solid-State Li-Ion Batteries

Conventional lithium ion batteries use organic liquid electrolyte which is flammable and brings safety issues. Solid-state lithium ion batteries employ inorganic lithium ion conductive solid electrolytes which offer improved safety, reliability and leakage-free properties. Developing rechargeable solid-state batteries was the main goal of this research. The obstacles to the development of solid-state batteries are the lack of highly conductive solid electrolyte materials and electrode/electrolyte interface issues (e.g. large interface resistance and interface integrity problem).

At the beginning of this work, thin film solid-state batteries were developed, because a thin film configuration can very effectively reduce the cell resistance from a solid electrolyte. However, the thin film batteries encountered short circuit issues caused by film defects (e.g. pin holes, particles) from the deposition process.

Subsequently, bulk solid-state batteries were developed to overcome the short circuit problem. To build a bulk solid-state battery, highly conductive garnet-type Li7La3Zr2O12 (LLZO) solid electrolyte was synthesized first. A polymerized complex sol-gel method (Pechini method) was developed for synthesis, and Al-doping influence was investigated. Finally, the synthesis process was greatly simplified, and the optimal level of Al-doping was also determined. Bulk solid-state batteries with Al-LLZO solid electrolyte, Cu0.1V2O5 cathode, and Li anode were prepared and tested. The batteries exhibited high initial discharge capacities, but they encountered dramatic capacity decay problems when tested at RT. It was found that the RT capacity decay problem was caused by water vapor in air. By preventing exposure to humid air, the cell life time at RT increased.

In the final chapter, a 3-dimensional (3D) solid electrolyte with a porous cathode side and a dense anode side was created. The intent was for the porous structure to greatly increase the contact area between solid electrolyte and electrode. Thus, the battery energy and powder density will increase. However, results suggest that the synthesis process need to be improved in order to get a more uniform porous structure.