The Importance of Achieving Intimate Contact In All-Solid-State Batteries

All-solid-state batteries (ASSBs) have gained significant interest as an attractive alternative to traditional lithium-ion batteries (LIBs) due to their improved safety and potential for higher energy density. Among the various types of solid electrolytes (SEs) including oxide-based and polymer SEs, sulfide-based SEs have attracted the most attention, largely due to their high ionic conductivity and electrochemical stability. However, the commercialization of ASSBs is being hindered by several unresolved challenges, namely various side reactions at electrode-electrolyte interfaces, dendritic behaviors at the anode, and poor interfacial contact. To note, the contact issue is unique to ASSBs compared to LIBs, and therefore has not been investigated deeply. While the high ductility of the sulfide-based SEs enables plastic deformation of SE particles to fill the voids between SE particles, when SEs contact materials of higher strength, the lack of deformation in SEs leads to the formation of voids at material interfaces. Herein, the impact of intimate contact in sulfide-based ASSBs is highlighted. By investigating pellet, wet-processed, and dry-processed electrodes, we quantified a crucial factor, ‘coverage’ – the areal ratio between SE-contacted CAM surfaces to the CAM surfaces. Coverage is a key factor since lithium-ion can only be conducted through SEs to be intercalated into the CAM. Through a simplified single-particle two-dimensional electrochemical model, we were able to visualize the effects of coverage on the behaviors of lithium-ions inside the CAM during operation. In cases of low contact coverage, especially at high C-rates, the lithium-ion kinetics are limited by solid-state diffusion within the CAM particles, rather than by ionic conduction at SEs. Electrochemical tests were conducted to demonstrate the effects of coverage on internal resistances and rate capabilities in lab-scale pressure-cells, hence emphasizing the importance of intimate contact at the CAM-SE interface.<br/>Intimate contact at the lithium metal-SE interface is less challenging to achieve compared to the cathode side owing to the decent ductility of lithium metal. However, incorporating bare lithium metal as the anode material is very challenging due to the dendritic growth of lithium metal. To suppress this, many studies have been conducted to introduce a protective layer between the anode-SE interface, including metallic, organic, and inorganic layers. Here, we were able to fabricate a compact nano-silicon (nSi) layer through in-situ lithiation sintering. One of the key features of the nSi layer is the of intimate contact at the nSi-SE interface induced by the compact layer. The nSi-Li layer enables a very reversible stripping/plating of lithium metal with exceptional critical current density (CCD) and areal capacity, proven through electrochemical analyses at a pouch-cell scale. In contrast, the composite layer fabricated using micro-silion (μSi) did not achieve the same level of enhancement as the nSi layer. Through a two-dimensional electrochemical model, we were able to attribute one of the reasons behind the disparity in performance to the degree of intimate contact. The limited contact induces high localized current densities, resulting in a shorter cyclability and lower CCD. In conclusion, these studies highlight the importance of achieving ideal morphological traits in ASSBs by modifying fabrication processes and material selection, with the specific focus on achieving intimate contact at both the CAM-SE interfaces and the protective layer-SE interface.