Use of phosphoranimines to reduce organic carbonate content in Li-ion battery electrolytes

The use of phosphoranimines (PAs), a class of linear, monomeric phosphazenes, as electrolytes for Li-ion battery applications has been investigated as a route to improve safety and stability for Li-ion batteries. Of the potential PAs for use in battery applications, this work focuses on the initial synthetic preparation and analysis of N-trimethylsilyl-P,P-bis((2-methoxyethoxy)ethoxy)-P-ethylphosphoranimine (PA-5). PA-5 has high LiPF₆ solubility in excess of 2 M, high thermal stability with a melting point below -80° C and high thermal stability as a neat compound to at least 250° C. As part of electrolyte blends, the inclusion of PA-5 shifts the onset of thermal degradation by close to 40° C at 35% loading and by 20° C at a 10% loading, improves the low temperature performance of the electrolyte, and when used as a primary solvent leads to increases in the flash point (by 20° C) when compared to more traditional EC:EMC blends. Cycling capabilities of full-coin cells with graphite negative electrodes and Li_1+w[Ni_0.5Mn_0.3Co_0.2]_1-wO₂ positive electrodes using PA-5:EC:EMC electrolyte blends are comparable with the performance seen for traditional EC:EMC blends. Analysis of the impact of the use of additives such as vinylene carbonate in PA-5:EC:EMC blended electrolyte results in enhanced capacity retention and improved coulombic efficiency.