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The mysterious power of lithium-sulfur batteries: How do they achieve an astonishing 550 Wh/kg energy density?
Lithium-sulfur battery (Li-S battery), as a rechargeable battery, has attracted much attention due to its high specific energy. In terms of lightweighting, the low atomic weights of lithium and sulfur make the battery weigh similar to water. As early as 2008, lithium-sulfur batteries were used in the historic flight of the Zephyr 6 drone, which was the longest unmanned solar-powered flight at the time. With the continuous advancement of technology, lithium-sulfur batteries are expected to replace the current mainstream lithium-ion batteries, mainly because the former provides an astonishing energy density of 550 Wh/kg, far exceeding the 150-260 Wh/kg of lithium-ion batteries.
The astonishing energy density of lithium-sulfur batteries means that the energy storage capacity per unit weight can far exceed that of existing technologies.
The competitive advantages of lithium-sulfur batteries mainly come from two aspects. First, using sulfur instead of more costly and less energy-dense cobalt or iron compounds significantly reduces production costs. Secondly, lithium-sulfur batteries use metallic lithium instead of lithium ion intercalation in lithium-ion batteries, which can achieve greater improvements in energy density because the use of metallic lithium reduces the need for other substances. At the same time, due to the gradual leakage of active materials, the charge and discharge cycle of lithium-sulfur batteries will be affected and lead to a reduction in battery life.
Since the early 2000s, scientists have conducted extensive research on the stability of lithium-sulfur batteries. By 2020, scientists from Rice University demonstrated a battery based on a carbon sulfide cathode that could retain more than 70% of its charge after 1,000 cycles. In addition, Texas startup Zeta Energy announced in 2023 that its lithium-sulfur batteries based on carbon sulfide cathodes have been independently verified by multiple national laboratories and are no longer affected by the polysulfide "shuttle" effect. This technological breakthrough paves the way for the commercialization of lithium-sulfur batteries.
The polysulfide shuttle effect of lithium-sulfur batteries is the main reason for its decline. Breakthroughs in this technology are the hope for the future.
The chemical reaction of lithium-sulfur batteries is the core of their function. During the discharge process, lithium metal dissolves from the anode surface and forms polysulfides with lithium ions in the electrolyte, which is then returned to the anode as lithium during charging. Although this reaction is efficient, it is also accompanied by stability problems, especially the unstable growth of the solid electrolyte interface, which accelerates the formation of dendritic lithium and ultimately leads to internal short circuits. In addition to reactions during charging and discharging, volume changes between the anode and cathode also present a battery design challenge.
In order to solve these problems, scientific researchers have gradually explored a variety of improvement plans. For example, some research has fused carbon nanofibers with sulfur to enhance conductivity. Such materials not only improve the stability of the overall structure but also reduce the loss of polysulfides. In addition, studies have shown that adding sugar-based anode additives can effectively reduce the contamination of the anode by the release of polysulfide chains from the cathode.
Current experiments have shown that specific electrolytes and improved materials can increase the life of lithium-sulfur batteries to more than 1,000 times.
Even in terms of safety, due to their high energy density and non-linear charge and discharge response, lithium-sulfur batteries often need to be managed with microcontrollers and other safety circuits to avoid dangers caused by excessive discharge. However, it is worth noting that multiple studies conducted in 2021 and 2022 have shown that selecting appropriate electrolytes and interfacial stabilizers can significantly enhance the stability of the battery and further improve its commercialization prospects.
Resources from all parties are currently being invested in the commercialization process of lithium-sulfur batteries, and Example companies such as Sion Power and OXIS Energy have made initial progress in this field. In collaboration with the aerospace industry, Sion Power’s lithium-sulfur batteries have undergone flight testing under real-world conditions, demonstrating their potential for practical applications. However, obstacles that still need to be overcome include issues such as the dissolution of polysulfides and the chemical stability of electrolytes.
With the discovery of new materials and technological innovation, the market prospects of lithium-sulfur batteries are gradually becoming clearer, driving revolutionary changes in the fields of renewable energy and electric vehicles. In the next few years, lithium-sulfur batteries may become a new benchmark in battery technology, challenging the status of traditional lithium-ion batteries. As this technology develops, we can’t help but ask: Can lithium-sulfur batteries truly be widely used and become the mainstream choice for the next generation of batteries?
