Interfacial Decomposition Behaviour of Triethyl Phosphate-Based Electrolytes for Lithium-Ion Batteries

Triethyl phosphate (TEP) is a cheap, environmentally benign, and non-flammable electrolyte solvent, whose implementation in lithium-ion batteries is held back by its co-intercalation into graphite anodes, resulting in exfoliation of the graphite structure. In this work, the electrode-electrolyte interface behaviour of electrolytes containing up to 100 % TEP is investigated and correlated to electrochemical performance. High capacity and stable cycling are maintained with up to 30 % TEP in carbonate ester-based electrolytes, but above this threshold the reversibility of Li⁺ intercalation into graphite drops sharply to almost zero. This represents a potential route to improved battery safety, while TEP can also improve safety indirectly by enabling the use of lithium bis(oxalato)borate, a fluorine-free salt with limited solubility in traditional electrolytes. To understand the poor performance at TEP concentrations of >30 %, its solvation behaviour and interfacial reaction chemistry were studied. Nuclear magnetic resonance spectroscopy data confirms changes in the Li⁺ solvation shell above 30 % TEP, while operando gas analysis indicates extensive gas evolution from TEP decomposition at the electrode above the threshold concentration, which is almost entirely absent below it. X-ray photoelectron spectroscopy depth profiling of electrodes demonstrates poor passivation by the solid electrolyte interphase above 30 % TEP and significant graphite exfoliation.