Interfaces in all-solid-state Li-ion batteries

Lithium-ion batteries (LIB) have been widely applied to portable electronic devices in the past decades. However, there has been a growing safety concern on Li-ion batteries, stemming from the flammability of the conventional liquid electrolytes. Replacing the organic liquid electrolytes with solid electrolytes (SEs) is generally viewed as the best potential solution to this challenge. Even though ceramic solid electrolytes are nonflammable and can tolerate extreme temperatures, there is still a long way before commercialization of solid state batteries (SSBs) is possible. The high interfacial resistance between SEs and electrodes is considered as the biggest roadblock standing in the way of the practical realization of SSBs. Such interfacial resistance could cause great capacity loss, as well as poor cycling performance. Therefore, in this thesis, research efforts have been made to understand the origins of the observed large interfacial resistance, and explore possible approaches to alleviate its impact on battery performance.

In this thesis, we studied interfaces include cathode-electrolyte and anode-electrolyte interfaces. In the study of the cathode-electrolyte interface, we applied Spark Plasma Sintering (SPS) to prepare Li_0.33La_0.57TiO₃(LLTO)/LiMn₂O₄, Li_1.3Al_0.3Ti_1.7(PO₄)₃/ LiMn₂O₄, Li_0.33La_0.57TiO₃/LiCoO₂ half-cells. Along with Scanning Electron Microscopy/ Transmission Electron Microscopy to characterize interfacial microstructure. The results showed that the interdiffusion between cathode and electrolyte materials leads to the formation of a micron-thick interdiffusion layer, and result in interfacial resistances on the level of 10⁵ Ω, which is about 40 times higher than the resistance of individual SEs. Thus, the formation of interdiffusion layers is the dominant origin of high electrical resistance in cathode-electrolyte interfaces. Cold sintering has been applied to prepare LLTO/LMO half-cells. The interdiffusion proved to be somewhat alleviated by cold sintering. However, more work is needed to improve particle compaction and surface contact.

Towards the understanding of anode-electrolyte interface properties, it is the dendrite formation in LLTO that was studied herein. We found that the reaction between Li and LLTO could lead to an enhanced durability of LLTO against Li dendrites. The reaction product has better conductivity than LLTO. More evidence on LLTO battery performance is still needed, however, this discovery has the potential of solving the dendrite problem for Li-metal SSBs.