Potential-Controlled Switchable-Resistance Polymer Layer for Enhanced Safety of Lithium-Ion Batteries with NMC-Type Cathodes

Preventing thermal runaway in lithium-ion batteries is crucial to ensure their safe operation. In this study, we report application of a poly[Ni(CH 3 Osalen)] polymer layer at the current collector to protect active materials in high-performance energy storage devices based on NMC532 cathodes. Poly[Ni(CH 3 Osalen)] has a conductivity window matching well the operational voltage of NMC materials, and it transitions to a nonconducting state when the potential exceeds safe limits upon overcharge, overdischarge, or short circuit. According to the stress tests performed in coin prototypes and ex situ XPS, EDX, and XRD studies, the polymer layer effectively limits current flow under extreme conditions and prevents degradation of internal components of the cell. While securing operational safety of the cell, the polymer layer allows for retention of up to ca. 90% of the capacity value of unprotected samples at an extended operational voltage range of 2.8–5.0 V. According to electrochemical impedance spectroscopy, the protective action of poly[Ni(CH 3 Osalen)] stems from the 10-fold increase in the charge transfer resistance at the polymer layer/cathode material interface, which compensates the sharp voltage change.