With slow progress in electric vehicle driving range and smartphone stamina, the limitations of current lithium-ion (Li-ion) batteries have become glaring. First commercialized by Sony in 1991, the now dominant Li-ion technology is struggling to improve the performance of the existing chemistry, its incremental gains outpaced by consumer demand. As the next breakthrough in battery design is hotly anticipated, one technology shines with promise due to superior performance and the practicality of its materials: Li-S.
The lithium-sulfur (Li-S) battery design provides a theoretical energy density (amount of energy stored per volume) which is five times that of Li-ion. Despite sharing an element, the chemistry is quite different: while Li-ion depends on the intercalation of current-carrying Li+ ions into the graphite layers of its carbon electrode during charge, the Li-S battery uses a sulfur-based electrode where Li+ ions react (reversibly) to produce lithium polysulfides, decreasing in state from Li2S8 to Li2S as the battery is discharged. The energy density of the Li-S system is higher because the reaction assimilates more Li+ ions per host atom than the carbon intercalation used in Li-ion.
While the Li-S concept has been kicked around since the 1970’s, poor endurance and safety prevented commercialization—until now. In the past, the biggest problem was that intermediate polysulfides (with more than two sulfur atoms) are soluble in the liquid electrolytes; in other words, the transport medium which allows ion flow between the electrodes also dissolves part of the sulfur electrode, irreversibly decreasing the useful sulfur material in the battery and causing fast degradation. Recent developments (including a new solid electrolyte and the use of nanocomposite material which protects the sulfur from electrolyte contact) have nearly realized the goal of an Li-S battery that survives as many charge cycles as Li-ion and meets flammability regulations.
The implications of Li-S inheriting the battery crown stretch beyond tech because of the materials used. Much publicized is the fact that this technology repurposes the elemental sulfur waste product generated by refining petroleum. While this will surely make the battery cheaper to produce, what effect could it have on the global sulfur market?
The refining of crude oil and gas requires the removal of naturally occurring (and extremely toxic) H2S; this removed stream (known as acid gas) must undergo Claus processing to convert the H2S to harmless elemental sulfur. Enormous amounts of sulfur are generated by this process: the Alberta oil sands operations (1.6 million bpd) produce 1.5 million tons per year of sulfur. While there is significant global demand for sulfur, the price fluctuates heavily, such that the sulfur buildup often presents the refinery with an unwanted storage and transportation headache.
If Li-S technology successfully replaces Li-ion, the battery industry will have heavy demand for sulfur. As a precedent, rechargeable batteries accounted for roughly 27% of global lithium consumption last year, up from 8% in 2002. Moreover, if electric vehicle production soars on the heels of Li-S batteries (due to huge increases in drive distance between charges), this sector will introduce sulfur demand faster than any other. To give a sense of the electrode material required for this use, a current Li-ion battery for an electric vehicle consumes up to 40kg of lithium carbonate equivalent, compared with 30-40g in a laptop battery. Ironically, a future of Li-S powered vehicles will significantly increase revenue from recovered sulfur while potentially reducing dependence on petroleum in the transportation sector.
Global oversupply of sulfur is looming due to increased fossil fuel refining while traditional sulfur markets are not growing fast enough to match. The agrochemical industry, which uses sulfuric acid to produce phosphate fertilizers and accounts for 46% of global sulfur consumption, has cyclical demand which contributes to unstable sulfur pricing (fluctuating between $800/t and zero in the past few years).
Given this volatility, it makes sense that refineries have been reluctant to put effort into selling and transporting recovered sulfur—it’s viewed as overhead. It also illuminates how new, steady demand from Li-S batteries could change the economics of sulfur recovery, adding a highly reliable financial incentive to offset the cost of an incredibly important, environment-preserving process.
Applied Analytics provides the process analyzers which are necessary to optimize the Claus process and maintain high conversion efficiency to elemental sulfur. Our flagship solutions include tail gas analysis for measuring the H2S/SO2 ratio in the catalytic step, feed forward analysis for measuring H2S level in the feed gas, and sulfur pit analysis for detecting fires and other hazards.
Since sulfur recovery units (SRU’s) are our biggest customers, we strive to think like SRU operators and anticipate their future concerns so that we can best serve their analysis needs.
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