Highly Adhesive, Fluorine-Free Binders Enabled by Chemical Anchoring for Durable High-Nickel Cathodes in Lithium-Ion Batteries

Abstract

High-nickel cathodes are pursued for high-energy-density lithium-ion batteries (LIBs) that require high-voltage operation and lean binder contents. Despite its conventional use, poly(vinylidene fluoride) (PVDF) exhibits limited mechanical adhesion and electrochemical stability under such demanding conditions. Moreover, its poor degradability and classification as a per- and polyfluoroalkyl substance (PFAS) increasingly challenge its long-term viability. To address these issues, we propose a fluorine-free polynorbornene-based terpolymer (PNB) binder that introduces a fundamentally new chemical anchoring mechanism. The PNB binder undergoes an in-situ ring-opening reaction with LiNixCoyMnzO2 (NCM), forming robust covalent ester linkages at the cathode-binder interface. This covalent bonding significantly enhances adhesion and structural integrity of cathodes, enabling mechanically stable cathodes even at a low binder content of 2 wt%. The unique chemical anchoring of PNB also enables stable cathode operation up to 4.5 V under aggressive oxidative conditions by forming a uniform protective layer that suppresses interfacial degradation. Comprehensive post-mortem analyses further reveal that the PNB network stabilizes the cathode surface via strong chemical anchoring, thereby mitigating the formation of rock-salt-like surface phases and capturing dissolved transition metals to inhibit cross-electrode degradation. This work highlights a distinct covalent chemical anchoring strategy, providing a design guideline for fluorine-free, high-performance binders in high-energy-density LIBs.