Counterion Lewis Acidity Determines the Rate of Hexafluorophosphate Hydrolysis in Nonaqueous Battery Electrolytes

The decomposition of LiPF6 in nonaqueous battery electrolytes is a well-studied, deleterious process that leads to hydrofluoric acid (HF) driven transition metal dissolution at the positive electrode and gas production (H2) at the anode, often attributed to the inherent moisture sensitivity of the hexafluorophosphate anion. In this work, we use in situ nuclear magnetic resonance (NMR) spectroscopy to demonstrate that the rate of PF6- hydrolysis significantly decreases in Na and K systems, where the Lewis acidity of the cation dictates the rate of decomposition according to Li+ > Na+ > K+. Despite the remarkable stability of Na and K electrolytes, we show that they are still susceptible to hydrolysis in the presence of protons, which can catalyze the breakdown of PF6-, indicating that these chemistries are not immune from decomposition when paired with solvent/cathode combinations that generate H+ at high voltage. Quantitative in situ multinuclear and multidimensional NMR of decomposed electrolytes shows that after long-term degradation, these systems contain HF, HPO2F2, and H2PO3F as well as a variety of defluorinated byproducts, such as organophosphates and phosphonates, that are structurally similar to herbicides/insecticides and may pose health and environmental risks. Taken together, these results have important implications for Na- and K-ion batteries where hazardous and harmful byproducts like HF, soluble transition metals, organophosphates, and phosphonates can be greatly reduced through cell design. Our results also suggest that next-generation chemistries present a pathway to safer batteries that contain lower quantities of flammable gases, like H2, if properly engineered.