(303c) Dry Granulation of Cathode Active Material and Solid Electrolyte Powders for All-Solid-State Batteries

Transition from gasoline-powered automobiles to electric vehicles (EVs) has progressed to realize a carbon neutralization. The prerequisite for EVs is the advanced secondary batteries which are safe, high-capacity, and capable of faster charging/discharging. All-solid-state batteries are attracting attention as the next-generation batteries that can fulfill these requirements. As a manufacturing process for all-solid-state batteries, a low-cost and environmentally-friendly dry process, which is solvent-free and does not require drying, is desirable. However, as original materials for all-solid-state batteries are fine cohesive powders, it is necessary to improve their flowability by means of a dry granulation process. For dry granulation, selection and handling of binder particles is of utmost importance. Poly(1,1,2,2-tetrafluoroethylene) (PTFE) is a promising candidate of solid binder particles, because it has been utilized for dry sheeting of fine particles for battery electrodes [1]. However, the fundamental phenomenon of particle agglomeration by PTFE remains unclear.

In this study, we investigated the dry granulation of battery particles using PTFE solid binder in three types of dry granulation processes, which were horizontal high-shear mixer (HSM), planetary ball mill (PBM), and twin-screw hot kneader (TSHK). Generally, in these processes, the dominant external forces exerting the powder are as follows: shear force in the HSM; shear and compressive forces in the PBM; shear and compressive forces with heat in the TSHK. By comparing these different granulation processes, we clarified the requirements for dry granulation with PTFE.

We measured mass fraction of ungranulated fine particles within the granules prepared from three types of granulation processes. It was found that in the case of the TSHK mass fraction of ungranulated fine particles remarkably decreased and the PTFE solid particles were more likely to disperse by transforming to fibrous morphology, leading to agglomerating the fine original particles. This result revealed that by exerting the compressive and shear forces under high temperatures, the PTFE solid binder promotes granulation of the original fine battery particles. Moreover, with an increase in the granulation temperature in the TSHK, the size of granules significantly increased. This was caused by an increasing the number of PTFE fibers and decreasing the diameter of the PTFE fibers under higher temperatures. We then fabricated the electrode sheets by a roller compactor from granules prepared using the TSHK. The tensile strength of the sheets exhibited sufficient strength as compared to that in reference [1]. Furthermore, the sheets fabricated from granules prepared at higher temperatures showed higher tensile strength. We demonstrate that the TSHK process can be employed for the continuous production of electrode sheets for all-solid-state batteries.

[1] F. Hippauf et al, Energy Storage Materials, 21, 390–398 (2019)