Lithium sulfur is perhaps the most mature of the ‘beyond Li-ion’ battery chemistries with a potential energy density of >600Whkg-1. Also with the potential for substantially reduced costs and improved safety. However, a number of challenges mean that cycle life has been poor. Hence the potential attracts attention and much needed research at the fundamental chemistry stage.
There are a number of issues with the current Li-S cells:
- Poor cycle life ~100 cycles
- Low power capability compared to lithium-ion
- Maximum operating temperature ~40°C
- Low maximum charge rate ~1C
- Energy density not meeting projected values – the EU project ALISE [ref2.] targeted 550Wh/litre and 500Wh/kg at cell level, however, they achieved 340Wh/litre and 325Wh/kg
A major issue with the Lithium Sulfur cell is the complex conversion from elemental sulfur (S8) to lithium sulfide (Li2S). During the discharge cycle the sulfur is reduced into a soluble form of intermediate species: Li2S3, Li2S4, Li2S5, Li2S6, Li2S7 and Li2S8 these are the lithium polysulfides. These lithium polysulfides are soluble in the electrolyte and diffuse out from the cathode structure. As the discharge process continues, the length of the soluble polysulfides chain is getting reduced, which affects the viscosity of the electrolyte. The final discharge products are Li2S2 and Li2S, these are a solid and an insulating material which passivates the electronically conductive surface of the cathode. The result is an increase of the internal resistance that prematurely ends the discharge. This increased resistance equals an increase in heat generation.
In the charge process the solid products (Li2S2 and Li2S are oxidized back to the soluble form, which in turn are converted to elemental sulfur.
The charge / discharge process and the polysulfide intermediates is known as the Polysulfide Shuttle. This tortuous process can lead to the loss of lithium and hence to decrease in the cell capacity. This shuttle mechanism is fundamentally limiting the cycle life of the Lithium Sulfur cell. Hence a large part of the research is looking at research into additives that limit these intermediates.
References:
- James B Robinson et al, “2021 roadmap on lithium sulfur batteries”, 2021 J. Phys. Energy 3 031501
- ALISE – EU funded developing Li-S materials and technology at TRL 4 level.
- Abbas Fotouhi, Daniel J. Auger, Laura O’Neill, Tom Cleaver and Sylwia Walus, “Lithium-Sulfur Battery Technology Readiness and Applications—A Review”, Energies 2017
- Lithium-sulfur batteries: lightweight technology for multiple sectors, FARADAY INSIGHTS – ISSUE 8: JULY 2020
See also: Cathode, Battery Chemistry A-Z