Structural underpinnings of cathode protection by in situ generated lithium oxyfluorophosphates

The in situ strategy of cathode protection opportunistically uses PF6− anions in electrolyte as chemical reagents to convert inactive precursor molecules to active electrolyte additives. With our approach, a bifunctional x(SiMe3)2 precursor replaces fluoride in the hexafluorophosphate, forming either a bidentate monoanion P(x)F4− or a linear x(PF5−)2 dianion, depending on the bridge x (e.g., oxalato, malonato, succinato, and catecholato). While the efficiency of these species as cathode protective agents has been demonstrated, the mechanism for their beneficial action remains unknown. In this study, several molecular precursors have been synthesized, and the topology and energetics of secondary anions were correlated with their cycling performance. Regardless of their structural motif, all such additives demonstrated improved electrochemical performance by reducing initial cathode impedance and lowering the rate of capacity fade compared to the baseline electrolyte. However, bidentate monoanions with a malonato bridge showed significant advantage over other molecular designs for mitigating the impedance rise, suggesting that structural strain in the anion is important for easing surface modification of the cathode. •In situ synthesized lithium oxyfluorophosphates protect cathodes.•Lithium oxyfluorophosphates additives improve electrochemical performance.•Additives with a malonato bridge show significant advantage.•Structural strain in oxyfluorophosphates anion eases surface modification.