Stephan Röser

Lifetime limit of tris(trimethylsilyl) phosphite as electrolyte additive for high voltage lithium ion batteries

The tris(trimethylsilyl) phosphite (TMSPi) is considered as an ideal electrolyte additive for lithium ion batteries. In this work, its positive effect as well as its failure mechanism in a LiPF6 containing electrolyte was studied by means of selected electrochemical, structural and analytical techniques. The LiNi0.5Co0.2Mn0.3O2/graphite cells with TMSPi as electrolyte additive were cycled between 2.8 and 4.6 V. Thanks to the compact cathode electrolyte interphase formed by the oxidative decomposition of TMSPi in a freshly prepared TMSPi containing electrolyte, both the discharge capacity and the cycling stability of cells were enhanced. However, our results also show that TMSPi actually reacts with LiPF6 at room temperature. TMSPi is consumed by this spontaneous reaction after aging for certain time. In addition, a part of the fluorophosphates, generated from the hydrolysis of LiPF6, is bonded to one or two TMS groups, causing a decrease in the fluorophosphate content in the CEI film. Consequently, the cycling stability of the lithium ion cells with aged TMSPi containing electrolyte deteriorates. The obtained results offer important insights into the practical application of TMSPi, which means that TMSPi can only be used as an effective additive in a freshly prepared LiPF6 containing electrolyte.

Alternative Single-Solvent Electrolytes Based on Cyanoesters for Safer Lithium-Ion Batteries.

To identify alternative single-solvent-based electrolytes for application in lithium-ion batteries (LIBs), adequate computational methods were applied to screen specified physicochemical and electrochemical properties of new cyanoester-based compounds. Out of 2747 possible target compounds, two promising candidates and two structurally equivalent components were chosen. A constructive selection process including evaluation of basic physicochemical properties as well assessing the compatibility towards graphitic anodes was initiated to identify the most promising candidates. With addition of a film-forming additive in a low concentration, the most promising candidate showed an adequate long-term cycling stability with LiNi1/3 Mn1/3 Co1/3 O2 [NMC(111)] in a full-cell setup using graphite as anode material. The main advantages of the new electrolyte formulation are related to its good thermal behavior, especially with regard to safety in combination with satisfying electrochemical performance.