Diethyl(thiophen-2-ylmethyl)phosphonate: A novel multifunctional electrolyte additive for high voltage batteries

Carbonate-based electrolytes used in Li-ion batteries encounter various challenges in extreme electrochemical environments, and hence their application requires various additives, especially when used with high voltage cathode materials. These additives are designed to form protective interphases that prevent parasitic carbonate oxidation, while in certain cases they stabilize electrolytes from reduction at anode surfaces or even serve as flame-retardants that postpone the thermal runaway during overcharge. However, most of these additives casts negative effects, lowering ionic conductivity of electrolyte or impairing the compatibility between cathode and electrolyte. An ideal solution of minimizing the presence of these inert molecules is to identify an additive that structurally integrates these multiple functions into a single compound. In this work, we report a novel additive, diethyl(thiophen-2-ylmethyl)phosphonate (DTYP). Its 0.5% presence in a base electrolyte dramatically improves the capacity retention of a high voltage Li-ion cell using LiNi_0.5Mn_1.5O₄ from 18% to 85% after 280 cycles at 1C at 60 °C, increases the endothermic reaction onset temperature from 193 °C to 223 °C, and reduces the self-extinguishing time of the electrolyte from 88 s to 77 s. Thus, such a multifunctional additive presents a cost-efficient solution to the issues often faced in high voltage lithium-ion batteries.