Ionic liquid-based electrolytes for Na-ion batteries

Lead Research Organisation: University of Oxford
Department Name: Materials

Abstract

Today, renewable energy is both competitively economic and efficient, however, without a solution to its inherent unpredictability and intermittency, the capability to displace significant proportions of conventional fossil fuel-based generation is limited. The grid requires generation which can manageably, and reliably, match energy supply with demand, and therefore, the solution is energy storage. Currently, battery technology is the only viable form of energy storage independent of geography. The predominant battery technology is lithium-ion batteries (LIBs), however, LIBs are ill-suited to large-scale energy storage because lithium is expensive and has limited finite resources, as do other critical LIB components such as cobalt. Lithium is now often considered to become the next 'oil'. Sodium-ion batteries (NIBs) present a highly promising solution; sodium is inexpensive and of high abundance, having practically infinite resources in the sea, thus providing a potentially sustainable solution. In addition, the manufacturing process for NIBs is similar to LIBs, which enables the industrial LIB manufacturing to be easily adapted for NIB manufacturing.

The most critical factors for high-energy battery storage applications are safety, long cycle life and low cost. Currently, organic electrolytes (e.g. ethylene carbonate), are used in both NIBs and LIBs, however, they pose significant risk due to their high volatility, flammability and poor thermal stability. Room temperature ionic liquids (RTILs) are an alternative electrolyte, which present a potential solution; RTILs are inherently non-flammable, have high thermal stability, wide electrochemical stability, and low vapour pressure. However, RTILs as electrolytes present their own technical challenges; they have a relatively high viscosity, and hence a resultant low conductivity.

To improve the performance of RTILs as electrolytes it is important therefore, to be able to accurately measure and control the factors of the electrolyte which drive resistance to dendrites and improve ion transport efficiency, such as the transference number. Accurate measurement of the transference number will enable effective RTIL modifications to be identified. The solid electrolyte interphase (SEI) is a deposit of insoluble inorganic salts and organic products on the electrode formed from decomposition of the electrolyte. The SEI is critical to cell performance as it must be both electrically insulating and ionically conductive, yet also protect the electrode from solvent molecules whilst being mechanically and electrochemically stable during cycling. Therefore, it is critical to understand the formation characteristics of the SEI as this will influence RTIL design.

Bis(fluoromethanesulfonyl)imide (FSI)-based RTILs have shown low viscosity and outstanding chemical stability, making them a promising electrolyte option. The electrochemical properties of FSI-based RTILs in NIBs will therefore, be characterised. Techniques that will be used for characterisation include: X-Ray Photoelectron Spectroscopy (XPS), Fourier Transform Infrared Spectroscopy (FTIR), Electrochemical Impedance Spectroscopy (EIS), Scanning Electron Microscopy (SEM) and X-Ray Reflectivity. To measure electrochemical properties, cyclic voltammetry (CV) and galvanostatic cycling techniques will be used. Additionally, to measure transference numbers, the Hittorf method and Bruce-Vincent method will be used. The evolution of the SEI will be characterised by EIS and correlated to its chemical composition by surface x-ray techniques in collaboration with Beamline i07 at the Diamond Light Source. Techno-economic analysis will be carried out in collaboration with Royal Dutch Shell.

This project falls within the EPSRC energy research area.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/R513295/1 01/10/2018 30/09/2023
2267211 Studentship EP/R513295/1 01/10/2019 31/03/2023 Shobhan Dhir