Studies of the Effect of High Voltage on the Impedance and Cycling Performance of Li[Ni0.4Mn0.4Co0.2]O2/Graphite Lithium-Ion Pouch Cells

Abstract

Li[Ni0.4Mn0.4Co0.2]O2 (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 LiPF6 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.