The Importance of Adhesion in Solid-State Batteries

Solid-state batteries are one of the key technologies to expedite the electrification of the economy. Compared with traditional liquid electrolytes, solid-state electrolytes are intrinsically safer and can enable high-energy designs by pairing with high-capacity cathodes and lithium metal anodes. However, maintaining the solid/solid contact between electrode and electrolyte during electrochemical cycling is non-trivial and the main challenge for all-solid-state batteries. The common strategy for combatting the contact issue is applying stack pressure, which is a sort of scientific shortcut that masks the real challenges of maintaining good contact between the different electrodes and electrolytes. Improving the adhesion between different battery components is a more effective and commercially viable strategy. We hypothesized that increasing the adhesion between electrode and electrolyte will improve electrode integrity, mitigate contact loss, and increase Coulombic efficiency and cycle life. There are very few studies on the adhesion of battery materials. This is primarily due to limited techniques to measure adhesion at the nanometer and micrometer scales relevant to the battery operation. In this work, we proposed to use atomic force microscopy (AFM) with customized probes to investigate adhesion between a model solid polymer electrolyte (SPE) consisting of crosslinked poly(ethylene oxide) (xPEO) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and a model cathode active material, LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC622). We showed that a two-fold increase in polymer stiffness (storage shear modulus, G’) from increasing the crosslinking density, leads to a 40% reduction in the bulk ionic conductivity in the SPE. However, we show evidence that this trade-off relationship between stiffness and ionic conductivity does not significantly impact the battery performance. In drastic contrast, it leads to a decrease in the work of adhesion at the electrolyte|cathode interface, which has a profound impact on the resulting discharge capacity of the battery. How the crosslinking density of the polymer affects the NMC-xPEO adhesion at the microscale and how this affects battery performance remain open questions, which the authors wish to gain insight into from the discussions.