A Strategy to Suppress the Electrochemo-Mechanical Degradation in Solvent-Free Electrodes for All-Solid-State Batteries

Lee, Yong Min

In response to the safety concerns on highly flammable liquid electrolytes in lithium-ion batteries (LIBs), the all-solid-state batteries (ASSBs) have emerged as promising alternatives for the next generation. The use of solid electrolytes with low flammability can provide resistance to fire/explosion incidents under abnormal conditions. In addition, it is expected that high power and high energy density can be achieved using high-voltage cathode materials due to the wide electrochemical window of solid electrolytes. Therefore, various solid electrolytes (polymer, inorganic, and composite solid electrolytes) were intensively studied.

Among the solid electrolytes, sulfide-based inorganic solid electrolytes have attracted much attention due to their advantages. They have the highest ionic conductivity (10^-2 ~ 10^-3 S cm^-1) at room temperature, comparable to a liquid electrolyte. Also, owing to its ductile characteristics, interfacial contact with electrode material is also good, and large-scale production is advantageous. However, there are many problems to be solved to realize ASSB with sulfide-based electrolytes. one of the main obstacles is to produce sheet-type electrodes with high active mass loading and good stability. The slurry process commonly used to manufacture sheet-type electrodes of LIBs cannot be directly applied to ASSBs because sulfide-based electrolytes exhibit high reactivity with most organic solvents. Even if the reactivity is not considered, in the slurry process, the bulk binder surrounds the electrode components and blocks the ion transfer pathway, thereby degrading the cell performance.

In this regard, solvent-free processing has emerged as a novel manufacturing process to overcome these issues. It is well known that polytetrafluoroethylene (PTFE) can fibrillate under shear stress conditions. When PTFE is added to the electrode and stress is applied, the PTFE is thinly fibrillated within the electrode. These well-dispersed fibers bind electrode materials and form sheet-type electrode film without delamination. This novel process has little concern about reactivity as no solvent is added, and PTFE does not react with electrode components. Moreover, since the thinly fibrous binder is evenly spread throughout the electrode, a sheet-type electrode can be formed with a smaller amount than in the slurry process, and the ion transfer pathway in the electrode can be less blocked. However, solvent-free processing also has problems to be solved. The thinly fibrillated binder cannot provide sufficient adhesion strength between the electrode components. The insufficient adhesion strength of the binder cannot prevent the reduction in the contact area between the electrode and the electrolyte material due to volume expansion during the charge/discharge process, resulting in rapid performance degradation.

In this work, we pre-treated PTFE to increase adhesion strength between electrode components and the cycling stability of the solvent-free electrode. When comparing the mechanical strength of solvent-free electrodes using the Surface and Interfacial Cut Analysis System (SAICAS) tool, electrodes with pre-treated PTFE had higher mechanical strength than electrodes with bare PTFE. Due to the higher mechanical strength, the pre-treated PTFE electrode had better cycling stability than the electrode with bare PTFE. This pre-treatment technology is expected to be a promising technology that can contribute to the realization of large-scale production and commercialization of ASSBs.