Effect of Sulfate Electrolyte Additives on LiNi1/3Mn1/3Co1/3O2/Graphite Pouch Cell Lifetime : Correlation between XPS Surface Studies and Electrochemical Test Results

The role of two homologous cyclic sulfate electrolyte additives, trimethylene sulfate (or 1,3,2-dioxathiane-2,2-dioxide, TMS) and ethylene sulfate (or 1,3,2-dioxathiolane-2,2-dioxide, DTD), used either alone or in combination with vinylene carbonate (VC) on the lifetime of LiNi1/3Mn1/3­Co1/3O2(NMC)/graphite pouch cells was studied by correlating data from gas chromatography/mass spectroscopy (GC–MS), dQ/dV analysis, ultrahigh precision coulometry, storage experiments, and X-ray photoelectron spectroscopy. For VC alone, more stable and protective SEI films were observed at the surface of both electrodes due to the formation of a polymer of VC, which results in higher capacity retention. For TMS, similar chemical SEI compositions were found compared to the TMS-free electrolytes. When VC was added to TMS, longer cell lifetime is attributed to VC. For DTD, a cell lifetime that competes with VC was explained by a preferential reduction potential and a much higher fraction of organic compounds in the SEI films. When VC was added to DTD, the contribution of both additives to the SEI films is consistent with the initial reactivity observed from dQ/dV and GC–MS analysis.