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Increasing power demands and environmental concerns leave lithium-sulfur batteries a promising prospect for the future of energy. OXIS Energy is one of a handful of companies that stand poised to usher in this energy revolution.
Lithium-ion batteries are the current ruler of the rechargeable battery market, with applications including personal electronic devices, mobile phones, laptops, electric vehicles, and wind farms.
Advancement in technology is not always about raw power alone, and, as devices become more sophisticated, the need for advanced batteries increases.
Many potential battery models are snapping at the heels of lithium-ion batteries, but one of the most promising is the lithium-sulfur battery. With the rechargeable battery market set to reach an approximated value of $90 billion by 2024, the struggle to become a market leader is increasing.
One company striving for this position, and hoping that lithium-sulfur batteries will be the technology to help them do it, is Oxfordshire-based OXIS Energy.
The first battery was created in 1780 by Italian scientist Alessandro Volta to prove a point to a fellow scientist, Luigi Galvani.
Volta's battery was nothing more than layers of silver and zinc interspersed with layers of cloth or paper soaked in saltwater. As Volta applied a wire to both ends of this crude pile of material, he found a current flowed through it.
Volta and the scientists that followed found that the key to increasing these early batteries was experimenting with the materials used in their construction.
Today's batteries take a pretty standard form as a whole, comprising of an anode, cathode, and electrolyte suspension. Electricity is generated when electrons flow through a battery, from an anode to a cathode. The electrons are created thanks to a reaction between the anode and an electrolyte solution in which it sits.
How is the Potential Difference Between the Cathode and the Anode Maintained to Keep Electrons Flowing?
Electrons keep flowing as the cathode creates positive ions that balance the charge of the electrons and leak into the electrolyte. Once here, these ions flow to the anode, but not via wires which are reserved for electrons, in the electrolyte, and past a semi-permeable membrane.
A rechargeable battery uses electrical energy to reverse this process and the chemical reaction that drives it, pushing the ions back to the cathode and the electrons back to the anode.
Within a lithium-sulfur battery, the anode comprises lithium, while the cathode is sulfur-based. But what is it about this set-up that makes it more favorable than lithium-ion batteries?
Lithium-Sulfur Batteries: Cleaner, Greener and Cheaper
OXIS Energy's primary focus is applying lithium-sulfur batteries to the electric vehicle market, with them offering products for use in aviation, defense and marine vehicles. Perhaps the area in which lithium-sulfur batteries stand to make the most significant impact is the car market.
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Cars are one of the most significant contributors to greenhouse gases in the atmosphere, so turning drivers on to electric vehicles that cut these gas emissions while also conserving fossil fuels is incredibly desirable. As more consumers turn to electric vehicles, the need to squeeze more mileage out of the batteries that power them is of vital importance. Replacing the currently favored lithium-ion battery with a lithium-sulfur alternative could be the key to enabling this change.
In terms of availability, as a material, sulfur is hard to beat. The Earth's crust contains abundant amounts of sulfur, making it an extremely cheap resource. The US alone produces 8.8 million tons of sulfur per year, the majority of which is a by-product of the oil industry.
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OXIS Energy says that its battery eliminates the need to use dangerous and environmentally damaging heavy metals such as cobalt, replacing them with a recycled material that is also a by-product of the oil industry.
Comparing Energy Densities: Lithium-Sulfur VS Lithium-Ion
Concerns for cleaner energy are paramount, but when it comes to industry, better performance is the crucial consideration. Fortunately, lithium-sulfur batteries are a greener and more eco-friendly choice.
The best-performing commercially available lithium-ion batteries have an energy density of around 300 Wh/kg. Theoretically, lithium-sodium could reach an energy density of approximately 2710 Wh/kg.
A much smaller lithium-sulfur battery could deliver the same energy as its considerably larger and heavier lithium-ion counterpart. This energy density improvement means that significantly less material is used than in the manufacture of a similar power lithium-ion unit. As you may imagine, this would be a considerable benefit for many industries such as the aviation sector.
In addition to being lightweight, the pouches that OXIS Energy is offering can be molded and shaped into a variety of sizes. In terms of safety, the lithium-sulfur cell that OXIS Energy produces has been tested and found to resist piercing with nails and bullet impacts.
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Why are Lithium-Sulfur Batteries Not Currently the Battery of Choice?
Unfortunately, when it comes to lithium-sulfur batteries, there is a vast gulf between academia and industry. The results achieved in the lab have not been scaled up for industrial purposes. This disparity includes energy density and the number of recharge cycles that a lithium-sulfur battery can undergo.
Whereas theory suggests an energy density of 2510 Wh/kg for lithium-sulfur batteries, OXIS currently reports that its batteries have reached 471 Wh/kg . This is still a considerable improvement when compared with lithium-ion, with hopes to hit a 600 Wh/kg milestone in the next few years.
Current lab models that suggest a 1000 recharge lifecycle have not yet been met on a mass-manufactured unit. OXIS is currently working towards a 500-cycle cell.
In an attempt to close this gap, OXIS Energy regularly collaborates with academia, providing a template for how these very different fields could begin to work in harmony.
During the discharge section of the batteries cycle, soluble lithium polysulfides (LiPS) form at the sulfur-based cathode. These are the positive ions mentioned above, and as with those positive ions, they leak into the electrolyte substrate and travel to the lithium anode. But, unlike other batteries, once there, LiPS begin to dissolve the anode.
This anode dissolution issue severely reduces the lifespan of such batteries, and cause the pouches to distort. In extreme cases, they even pose a safety concern. Researchers across the globe are working on solutions to the problem to hopefully get past the final hurdle standing between lithium-sulfur and market domination.
For OXIS Energy, there is no question that lithium-sulfur batteries are the way forward. OXIS Energy has teamed with Minas Gerais Development Company CODEMGE to invest $50 million into a 20,000 square plant in Brazil to create lithium-sulfur batteries.
By 2023, OXIS hopes to have the plant, which is leased for 15 years from Mercedes Benz, operational, and estimates that when it is at full capacity, it will be able to produce 5 million lithium-sulfur cells per year.
OXIS Energy and its lithium-sulfur battery stand poised to carry humanity to exciting new horizons.
References and Further Reading
Zhu, K., Wang, C., Chi Z., et al.  How Far Away Are Lithium-Sulfur Batteries From Commercialization? Frontiers in Energy Research. https://doi.org/10.3389/fenrg.2019.00123
Richardson, J.  Lithium-Sulfur Batteries Could Be Cheaper & More Energy Dense. [Online] Clean Technica. Available at: https://cleantechnica.com/2020/02/11/lithium-sulfur-batteries-could-be-cheaper-more-energy-dense/ (Accessed on 29 June 2020).
Hill, M.  Oxis Energy battery cells nearing aviation applications. [Online] PV Magazine. Available at: https://www.pv-magazine.com/2020/02/03/oxis-energy-battery-cells-nearing-aviation-applications/ (Accessed on 29 June 2020).
'Sulfur Production worldwide 2019,' Statista, https://www.statista.com/statistics/1031181/sulfur-production-globally-by-country/
OXIS Energy. Pioneering patented technology. [Online] Available at: https://www.oxisenergy.com (Accessed on 29 June 2020).