Evaluation of Tailored Phenothiazines for Overcharge Protection and Cathode Passivation in High Voltage Lithium-Ion Batteries

Redox-active organic compounds are added to lithium-ion battery electrolytes (i) to serve as redox shuttles to mitigate excess current during overcharge and (ii) to passivate electrode surfaces to stabilize interfaces that would otherwise lead to electrolyte decomposition. Especially when expanding the voltage range in charging of lithium-ion cells, overcharge protection efficacy of reported redox shuttles has been limited, yet this may not be a challenge if redox shuttle failure results in cathode passivation, which becomes more critical to cell longevity in higher voltage cells. In this study, we explored a series of phenothiazine-based electrolyte additives with oxidation potentials above 4 V vs. Li in high-voltage (e.g. NMC cathode-containing) lithium-ion cells. Using electrochemical and spectroscopic techniques, we sought to determine if these additives act in shuttling (reversible) or passivating (irreversible) mechanisms.