Aged Tris(trimethylsilyl) Phosphite (TMSPi) As a L.I.B. Electrolyte Additive

The use of electrolyte additives has been and continues to be an economical and effective approach to enhance the performance and increase the lifetime of lithium ion battery systems. In the presence of additives, the baseline performance of electrolytes can be improved due to the formation of surface films on the electrodes, HF scavenging in the bulk electrolyte, etc. However, the interaction between electrolytes and small molecule additives are rarely studied due to the assumed inertness of the system. While this inert relationship may hold true for many additives, we have found that one commonly used additive, tris(trimethylsilyl) phosphite (TMSPi), demonstrates a dynamic (reactive) relationship when used with a LiPF 6 -based electrolyte (1.2 M LiPF 6 in EC/EMC, 3/7 wt/wt). Our findings suggest that immediately after the addition of TMSPi into the electrolyte, the nature of the electrolyte changes and the properties of the bulk electrolyte are drastically different between immediately making the solution and waiting less than a week. We present electrochemical data comparing freshly prepared (same day) and “aged” (one week) TMSPi-based electrolyte formulations. Our results indicate that the effect of electrolyte aging on cell electrochemistry (LiNi 0.5 Mn 0.3 Co 0.2 O 2 /graphite full cells operating between 3.0-4.4 V) can be dramatic with observable differences in impedance, redox behavior, surface film formation and transition metal dissolution from the positive electrode. The data presented sheds considerable light on the possible interactions between additives and electrolyte and are of utmost importance when considering electrolyte formulations for lithium ion batteries.