Toward New Solvents for EDLCs: From Computational Screening to Electrochemical Validation

The development of innovative electrolytes is a key aspect of improving electrochemical double layer capacitors (EDLCs). New solvents, new conducting salts as well as new ionic liquids need to be considered. To avoid time-consuming “trial and error” experiments, it is desirable to “rationalize” this search for new materials. An important step in this direction is the systematic application of computational screening approaches. Via the fast prediction of the properties of a large number of compounds, for instance all reasonable candidates within a given compound class, such approaches should allow to identify of the most promising candidates for subsequent experiments. In this work we consider the toy system of all reasonable nitrile solvents up to 12 heavy atoms. To investigate if our recently proposed computational screening strategy is a feasible tool for the purpose of rationalizing the search for new EDLC electrolyte materials, we correlatein the case of EDLCs for the first timecomputational screening results with experimental findings. For this, experiments are performed on those compounds for which experimental data is not available from the literature. We find that our screening approach is well suited to pick good candidates out of the set of all reasonable nitriles, comprising almost 5000 compounds

Insights into Bulk Electrolyte Effects on the Operative Voltage of Electrochemical Double-Layer Capacitors

Electrochemical double-layer capacitors (EDLCs) are robust, high-power, and fast-charging energy storage devices. Rational design of novel electrolyte materials could further improve the performance of EDLCs. Computational methods offer immense scope in aiding the development of such materials. Trends in experimentally observed operative voltages nevertheless remain difficult to predict and understand. We discuss here the intriguing case of adiponitrile (ADN) versus 2-methyl-glutaronitrile (2MGN) based electrolytes, which result in very different operative voltages in EDLCs despite structural similarity. As a preliminary step, bulk electrolyte effects on electrochemical stability are investigated by <i>ab initio</i> molecular dynamics (AIMD) and static, cluster-based quantum chemistry calculations

Impact of Selected LiPF₆ Hydrolysis Products on the High Voltage Stability of Lithium-Ion Battery Cells

Diverse LiPF₆ hydrolysis products evolve during lithium-ion battery cell operation at elevated operation temperatures and high operation voltages. However, their impact on the formation and stability of the electrode/electrolyte interfaces is not yet investigated and understood. In this work, literature-known hydrolysis products of LiPF₆ dimethyl fluorophosphate (DMFP) and diethyl fluorophosphate (DEFP) were synthesized and characterized. The use of DMFP and DEFP as electrolyte additive in 1 M LiPF₆ in EC:EMC (1:1, by wt) was investigated in LiNi_1/3Mn_1/3Co_1/3O₂/Li half cells. When charged to a cutoff potential of 4.6 V vs Li/Li⁺, the additive containing cells showed improved cycling stability, increased Coulombic efficiencies, and prolonged shelf life. Furthermore, low amounts (1 wt % in this study) of the aforementioned additives did not show any negative effect on the cycling stability of graphite/Li half cells. DMFP and DEFP are susceptible to oxidation and contribute to the formation of an effective cathode/electrolyte interphase as confirmed by means of electrochemical stability window determination, and X-ray photoelectron spectroscopy characterization of pristine and cycled electrodes, and they are supported by computational calculations