Negative Redox Potential Shift in Fire-Retardant Electrolytes and Consequences for High-Energy Hybrid Batteries

Fire-retardant electrolyte chemistries have attracted great attention given their potential to solve the grand challenges of alkali-ion batteries: safety, use of metallic anodes, and anodic stability. Whereas extensive analysis and correlations are drawn to explain their unusual electrochemical behavior, one essential property, their effects on redox potentials of battery components (redox potential shift) pervasively lack a strict description and quantification. Here we show that the strong solvation of lithium cations by organic phosphates, the widely used flame-retardant constituents, induces a negative redox potential shift by as much as 500 mV. We demonstrate that the redox potential shift is characteristic of Li-cation (de)solvation processes whereas it is negligible for other processes. This has important consequences for high energy hybrid battery concepts such as high voltage dual-ion graphite or organic batteries. These findings also shine a different light on the enhanced anodic stability of these nonconventional battery electrolyte formulations.