Flame retardancy in lithium-ion battery electrolytes has been significantly improved by the use of several new aromatic phosphorus-containing esters, which markedly suppress thermal runaway. The identification, preparation, and properties of these thermal runaway inhibitors (TRIs) are described. When used as an additive (2–5%) to the standard battery electrolyte system, these materials improve safety by preventing potential fire and explosion. The effects of these esters on electrochemical and thermal stabilities, ionic conductivity, and electrochemical cycling are presented.