Li [Ni 0.4 Mn 0.4 Co 0.2 ]O 2 (NMC442)/graphite pouch cells containing various electrolyte additives, either singly or in combination, were studied using cycling experiments up to 4.4 and 4.5 V coupled with simultaneous electrochemical impedance spectroscopy (EIS) measurements. The impedance of most cells increased dramatically at 4.4 and 4.5 V, but was nearly reversible over one cycle. However, during continued cycling, the impedance of all cells slowly increased at all potentials. Electrolyte additives were found to dramatically affect this behavior. The impacts of adding prop-1-ene-1,3-sultone (PES), vinylene carbonate (VC), triallyl phosphate (TAP), methylene methane disulfonate (MMDS), ethylene sulfate (DTD) and/or tris(-trimethyl-silyl)-phosphite (TTSPi) to 1M LiPF 6 ethylene carbonate:ethyl methyl carbonate (EC:EMC) electrolyte were studied. PES-containing cells had dramatically lower impedance and better capacity retention than VC and TAP-containing cells during both 4.4 and 4.5 V experiments. When MMDS, DTD and/or TTSPi were added in combination with PES, the performance was improved further. Finally, continuous charge-discharge cycling was compared to cycling with a 24-hour hold applied at the top of charge at 4.4 V. The high voltage hold led to severe impedance growth which could be partially overcome through the use of optimal additive combinations. Lithium-ion (Li-ion) batteries are currently used in phones, laptop computers and, more recently, electric vehicles. It is well known that electrolyte additives can have a dramatic effect on the performance and lifetime of Li-ion batteries.1,2 Vinylene carbonate (VC) is perhaps the most famous and widely used additive and has been shown to improve cycle and calendar life of Li-ion cells.3 VC is less effective, however, when used in cells cycling to potentials above 4.2 V 4 or at elevated temperatures.5 Sulfur-containing additives have recently been investigated by several research groups in the hopes of overcoming the temperature sensitivity of VC and extending the usable voltage range of Li-ion cells. [6][7][8] Prop-1-ene-1,3-sultone (PES) has been shown to function as a stable solid electrolyte interphase (SEI)-forming additive that improved coulombic efficiency (CE), reduced charge end point capacity slippage and self-discharge rates. 8,9 PES nearly eliminated all gas production during storage at 4.2 V and 60• C, whereas VC did not. 9,10The work by Xia et al. 9 and Nelson et al. 10 demonstrated the superiority of PES over VC as an electrolyte additive in NMC/graphite cells. Methylene methane disulfonate (MMDS) has been shown to reduce electrolyte oxidation at the positive electrode and reduce the volume of gas produced, as well as decrease the impedance and rate of parasitic reactions when compared to cells without MMDS.6,11 The additive ethylene sulfate or 1,3,2-dioxathiolane-2,2-dioxide (DTD) has been shown to function as a film-forming additive for the SEI on the negative electrode.12,13 The additive tris-(trimethyl-silyl) phosphite (TTSPi) has bee...