Dr Arunabh Ghosh
LE STUDIUM RESEARCH FELLOW / ARD 2020 - LAVOISIER Programme
In residence at
Prof. Fouad Ghamouss
Design, formulation and characterisation of new safer electrolytes for electrochemical storage of energy
Developing a novel electrolyte for supercapacitor application is not a straightforward process. Beyond having a large potential window and high ionic conductivity, there are many other requirements, such as electrochemical stability, high ionic conductivity, suitable viscosity, that an electrolyte needs to meet in order to be promising for the performance of the device. Besides these, the electrolyte must have a large liquid range temperature, which is the main deciding factor of the device’s operating temperature range. Furthermore, the volatility and flammability of the electrolyte are the keys to deciding the safety grades of supercapacitor devices. Controlling all these parameters at the same time is what makes the development of electrolytes a very fascinating but a tricky process. Therefore, we can see that commercially available organic solvents still fail to provide an ideal solution, and they suffer from one or more following issues, like flammability, low potential window, and low operating temperature range.
In this context, we are formulating and designing a new set of electrolytes, one is based on a new organic solvent, and the other one is based on a mixture of two ionic liquids; with a strong focus on the optimization of all the required characteristics, which can potentially fulfill well-round application needs. The target is to fulfill application needs in sub-ambient temperatures, presenting good mobility, low flammability, and wide working potential window.
Publications in relation with the research project
The increasing need in the development of storage devices is calling for the formulation of alternative electrolytes, electrochemically stable and safe over a wide range of conditions. To achieve this goal, electrolyte chemistry must be explored to propose alternative solvents and salts to the current acetonitrile (ACN) and tetraethylammonium tetrafluoroborate (Et4NBF4) benchmarks, respectively. Herein, phenylacetonitrile (Ph-ACN) has been proposed as a novel alternative solvent to ACN in supercapacitors. To establish the main advantages and drawbacks of such a substitution, Ph-ACN + Et4NBF4 blends were formulated and characterized prior to being compared with the benchmark electrolyte and another alternative electrolyte based on adiponitrile (ADN). While promising results were obtained, the low Et4NBF4 solubility in Ph-ACN seems to be the main limiting factor. To solve such an issue, an ionic liquid (IL), namely 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl] imide (EmimTFSI), was proposed to replace Et4NBF4. Unsurprisingly, the Ph-ACN + EmimTFSI blend was found to be fully miscible over the whole range of composition giving thus the flexibility to optimize the electrolyte formulation over a large range of IL concentrations up to 4.0 M. The electrolyte containing 2.7 M of EmimTFSI in Ph-ACN was identified as the optimized blend thanks to its interesting transport properties. Furthermore, this blend possesses also the prerequisites of a safe electrolyte, with an operating liquid range from at least −60 °C to +130 °C, and operating window of 3.0 V and more importantly, a flash point of 125 °C. Finally, excellent electrochemical performances were observed by using this electrolyte in a symmetric supercapacitor configuration, showing another advantage of mixing an ionic liquid with Ph-ACN. We also supported key structural descriptors by density functional theory (DFT) and COnductor-like Screening Model for Real Solvents (COSMO-RS) calculations, which can be associated to physical and electrochemical properties of the resultant electrolytes.