Postgraduate research opportunities Mitigating effects of polysulphide shuttle via catholyte additives in lithium polysulphide flow batteries

Energy storage is seen as a crucial component of a net zero energy landscape, and redox flow batteries have particular advantages over other battery types to enable flexible, long-lasting and long-duration storage. This project will work with flow battery manufacturer StorTera Ltd to develop a lithium polysulphide single-liquid flow battery, which promises to be a very low cost, sustainable solution thanks to the high abundance of sulphur and ability to use recycled lithium.

Lithium-sulphur batteries have been under development for many years, with the promise of 10x higher energy density (2600 W h kg-1)[1] than Li-ion batteries (up to 260 W h kg-1)[2]. There are major challenges to commercialise the Li-S cell such as a low coulombic efficiency due to the transport and reduction of polysulphides, known as polysulphide shuttle. Polymers of lithium and sulphur (Li2Sn, n=3-8), which get shorter as the battery discharges, should stay on the cathode side but can diffuse to the anode and cause self-discharge. A variety of technologies have been applied to inhibit the shuttle such as making new electrode constructions and formulations as well as the addition of lithium passivating agents (LiNO3) to inhibit the shuttle by forming a solid electrolyte interphase (SEI) on the anode.

An alternative option is to configure a battery where the cathode consists of a solution of the soluble polysulphide compounds and is kept separate from the electrodes. This configuration has a lithium metal anode and graphite cathode with the polysulphide solution (catholyte) being pumped through the cell. This Single Liquid Flow Battery (SLIQ) concept is being developed by StorTera Ltd and the Brightman group at the University of Strathclyde. The batteries can easily be scaled to suit grid energy requirements due to the scalability of the catholyte volume and hence the capacity.

This project seeks to understand the polysulphide shuttle and anode corrosion behaviour in the SLIQ battery. You will investigate the use of additives (LiTFSI, LiNO3, LiPAA, LiBF4) at various concentrations to mitigate the effects of the polysulfide shuttle and improve the cell life and capacity of the cell.

The aim of this project would be to observe the impact to the polysulphide shuttle due to the catholyte additives by measuring the capacity, coulombic efficiency, as well as the viscosity and conductivity of the catholyte formulation.

Different catholyte formations will be prepared by a solution of DME/DOL[3] with varying concentrations of lithium-passivating additives (LiNO3, LiTFSI, etc.) in an inert atmosphere to avoid the introduction of moisture to the lithium electrode. Cells will be tested to determine the coulombic efficiency of the cells, capacity retention and redox kinetics. The catholyte formulations will be characterised by the conductivity and viscosity which are known to directly impact battery performance.

The current of the polysulphide shuttle can be measured directly in the cell[4] and as such determine the effect the additive has on the shuttle.

The analysis of this data would involve comparison to an established benchmark to investigate the impact on cycle life. The additives will be tested at different concentrations, allowing formulations to be compared to determine the optimal concentration for the additive. From the conductivity and viscosity measurements will allow the comparison of the effects of additives to each other as these parameters have a direct impact on battery performance. The long-term effects of the additives will be analysed to discover whether the salts can be used for larger scale systems.

Screening of additives for lithium polysulphide batteries may enable the technology to be competitive with that of Li-ion. From the variety of the additives used it may be able to see that a specific structure or composition of additive may work best and could be developed further, potentially developing a bespoke solution for this shuttle effect. The findings of this project will be of interest to both the Brightman research group and StorTera who could apply this information to assist their research.

In addition to undertaking cutting edge research, students are also registered for the Postgraduate Certificate in Researcher Development (PGCert), which is a supplementary qualification that develops a student’s skills, networks and career prospects.

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• Chemical & Process Engineering
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