Ni-catalyzed Degradation of Polyimide Separators in Quasi-Solid-State Li-ion Batteries

Abstract

Polyimide (PI) is a promising separator material for polymer-based quasi-solid-state lithium-ion batteries (LIBs), due to its superior thermal stability, mechanical strength, and high porosity. Here we first report a critical issue that warrants keen attention in the quasi-solid state Li||LiNi0.8Co0.1Mn0.1O2 (NCM811) LIBs adopting PI separator. The PI undergoes chemical degradation after >2000 hours of storage or 1000 hours of cycling, accompanied by battery performance fade. Post-cycled analysis reveals the electrochemical degradation route, where the PI backbone is inclined to ring-opening depolymerization, and the imide groups are oxidized to aromatic quinone groups. This manifests as dissolution and visible color changes of the PI separator. The catalytic effect of Nickel (Ni), arising from either direct contact with NCM811 cathodes and/or with shuttled Ni species in the electrolyte, is key to triggering PI degradation. Interestingly, Cobalt (Co) and Manganese (Mn) do not show such a catalytic effect. Physically isolating the PI from both Li and NCM, via sandwiching the PI between two polyethylene separators, prevents the degradation and achieves stable operation of the solid LIBs. This further highlights the integral role of both oxidative potential and catalytic Nickel species. This work documents the intrinsic instability of PI with Ni-containing cathode, shedding light on the redesign of the separator in high-energy lithium-ion batteries.