Design of a Novel Apparatus for the in-situ Formulation and Characterization of Safer Electrolytes

LIB and SC are both alternative energy storage devices, which store energy via orbital electron exchange (chemically) and by electrostatically (physically) separating positive and negative charges, respectively. Most liquid electrolytes used in these commercial electrochemical devices are obtained by dissolving a salt like lithium salt, or tetraethylammonium tetrafluoroborate in a specific solvent like alkylcarbonate mixture or the acetonitrile (ACN), respectively. Nevertheless, many issues often linked to the stability of selected electrolytes limit their lifetime. For example, it is reported in the literature that the oxidation reaction of the LIB electrolyte at high potentials leads to the formation of gases like methane, ethane, carbon dioxide and carbon monoxide when high potentials are applied to the electrode, which increases the pressure inside the sealed cell and modify then the physical properties of the electrolytes. While in the case of SC, the main issue links to the use of the tetraethylammonium tetrafluoroborate and ACN-based electrolyte is its safety since under certain special conditions ACN can produce hydrogen cyanide (HCN) when is burned. In fact, amount of HCN produced depends on volume of ACN, temperature of fire and environment. Nevertheless, these issues cannot be evaluated without the simultaneous knowledge of their thermodynamic properties, fluid phase equilibria and electrochemical properties in presence and in absence of external stimulus (presence of gas, temperature, etc.).
This project will address this perspective by developing first a new and original apparatus based on an isochoric saturation technique, supplied by an analysis of the gas phase composition using a Gas Chromatography, combined with different electrochemical devices (voltamperometer, potentiostat or gavalnostat) that allows the simultaneous determination in real conditions of the fluid phase equilibria through pVTxy measurements and the electrochemical properties of a wide range of electrolytes for LIB and SC applications in order to understand factors, which limit electrolytes lifetime and safety.
Additionally, during this project, an approach which consists to cover these properties with the objectives to study ionic liquids, ILs, as an additive component, and not as a media, will be also investigated in useful technological solvents like alkylcarbonates (for their uses in LIB), acetonitrile and other dinitrile structures (for their uses in SC). This choice was made with the objective to keep the advantages of each solvent and improve their disadvantages (corrosive, volatile) by adding small quantity of IL. To clarify the differences between ILs and molten salts, similar experiences will be also investigated using the tetraethylammonium tetrafluoroborate for its use in SC.
For that, we plan to focus then our effort on the effect of the electrolyte formulation on their unique properties like the solvation, volumetric, thermal, electrochemical, thermodynamic and transport properties by using the original apparatus proposed in this project and different experimental and theoretical techniques already available and developed at QUB. Additionally, the electrochemical stability of each electrolyte will be determined by applying different charge-discharge cycles and by analysing then the gas phase composition changes over the time and temperature. Furthermore, the effect of presence of degradation products, like ethane and carbon dioxide, on the properties of the electrolytes will be also investigated in situ. The determination of the ethane and carbon dioxide solubility in the selected electrolytes will be then determined to dress some conclusions on the main advantages and disadvantages to use ILs rather than classical molten salts, as well as, to propose key parameters to improve the stability of electrolytes for LIB and SC applications.

The development of new electrolytes for high-performance LIB and SC is part of the general problem of optimizing the management of on-board energy. Classic organic electrolytes are commonly composed of an organic solvent (or rather a mixture of solvents) and a dissolved salt. These electrolytes present many advantages except safety owing to their high vapour pressure and flammability. Their use in large-scale batteries or SC in applications such as electrical vehicles (EV) or hybrid electrical vehicles (HEV) can thus be dangerous. For this reason the search of new safer electrolytes is required for the development of electrical energy storage systems. However, this requires also a clearer understanding of the "safety issues" related to current state-of-art electrolytes for both LIB and SC to obtain the technological improvements for a safer, more secure society and substantially benefit the environment and economy.

This project will advance knowledge, understanding and readiness of technologies necessary for the further development of safer electrolytes. This will be driven by the development and the utilisation of an original apparatus to provide information of the decomposition of the electrolytes and the quantification of gaseous products during cycling of lithium ion batteries and supercapacitors and how these are affected by electrolyte composition. Furthermore, the impact of this project will be also enhanced by the successful development of a novel electrolyte for LIB and SC, and associated understanding gained within this project will be a significant enabler for future industrial research, product development and ultimately enhance the viability of such systems for larger scale use. This will benefit the project team; its associates and stakeholders in the wider battery supply chain growing within the UK. Additionally, during this project improvements to electrolytes stability and safety will also be generated, leading to associated scale up work within the UK in the future, for materials, battery packs and ultimately systems.

Information on the designed apparatus, novel materials, cell testing and system feasibility generated will be disseminated via conferences, publications as appropriate. Research is increasing worldwide on advanced battery and supercapacitor materials topics, especially in Asia and the USA and this project will accelerate the progress of research on these topics in the UK.

Results from the project also overlap into other technology areas for example, the use of high stability electrolytes in Li-Air batteries and system aspects with other metal - battery technologies such as Lithium-Sulfur or Sodium-ion batteries.