The preparation of methylene ethylene carbonate (MEC) and the incorporation of MEC into lithium ion batteries as an electrolyte additive were investigated. MEC is prepared in good yield by mercury catalyzed cyclization. Addition of low concentrations of MEC (1–2%) to 1M LiPF6 in 3:7 ethylene carbonate/ethyl methyl carbonate improves the capacity retention of lithium ion batteries cycled at elevated temperature (60°C). Ex situ surface analysis (XPS and FTIR) of the electrodes supports the presence of poly(methylene ethylene carbonate) on the anode surface. Modification of the anode solid electrolyte interphase (SEI) correlates with significant improvements in the cycling performance at 60°C.