A Consideration of Electrolyte Additives for LiNi0.5Mn1.5O4/Li4Ti5O12Li-Ion Cells

LNMO/LTO cells represent an excellent vehicle with which to test the impact of electrolyte additives on cell performance due to the high potential of LNMO which leads to electrolyte oxidation. Additives that prevent this oxidation are desired. Electrolyte additive researchers normally compare the capacity retention versus cycle number of cells with and without additives. If the capacity retention is improved, the additive is "good". In this paper, this logic is shown to be flawed. One additive, LiO-t-C4F9, which does improve capacity retention versus cycle number, is shown to do so by increasing electrolyte oxidation at the positive electrode. This causes increased charge end point capacity slippage and increased self discharge rates. As such, these additives, like many promoted in the literature, are not "good", but are "bad". On the other hand, one additive, Al(HFiP)3, is shown to reduce parasitic reactions at the LNMO electrode but does not lead to improved capacity retention. A full understanding of the impact of the additive on the parasitic reaction rates at both positive and negative electrodes is required before a sound judgment about the impact of an additive can be made. Therefore, additive researchers must pay attention to coulombic efficiency, capacity end point slippage, reversible and irreversible capacity loss during storage as well as voltage drop during storage in addition to the long-time cycling performance for cells to get an overall evaluation of electrolyte additives.