The formulation of solvent systems can have a severe impact on the lifetime, rate performance, and temperature performance of lithium ion cells. Methyl acetate (MA) has been found to increase rate and temperature performance of carbonate solvents, but decreased cell lifetime. This work used ultra-high precision coulometry, in-situ gas analysis, and isothermal microcalorimetry to investigate a recently reported promising high rate additive blend of fluoroethylene carbonate (FEC) and 1, 3, 2-dioxathiolane-2, 2-dioxide (DTD) in LiNi 0.5 Mn 0.3 Co 0.2 O 2/graphite pouch cells. Solvent systems composed of blends of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were investigated, with additions of 0%, 20% and 40% wt MA. MA was found to decrease the coulombic efficiency and increase slippage, parasitic heat flow, and gas volume. The addition of just 1% wt of DTD to 2% FEC improved the performance of cells containing 20% and 40% MA to that of cells containing no MA and 20% MA, respectively, under 4.3 V. Results suggest the negative impact of MA originates from increased electrolyte oxidation at the positive electrode. Additionally, blends of MA in EC: EMC and EC: DMC (3: 7 wt) were tested head-to-head, yielding a small improvement to parasitic heat flow and UHPC performance when cells used EC: EMC.