Battery technology What is the truth about all-solid-state lithium-sulfur batteries?

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All-solid-state lithium-sulfur batteries (ASSLSBs) are considered promising candidates for the next generation of energy storage due to their high specific energy, safety, and cost-efficiency. But what exactly is behind the technology, and are all problems truly solved?

All-solid-state lithium-sulfur batteries could usher in the next generation of energy storage. However, there has been a major obstacle to their practical application. The slow solid-solid sulfur redox reaction (SSSRR) at the three-phase boundaries has led to low performance and short lifespan.

But what does this mean? Sulfur and its reduction products (such as Li₂S) are in a solid state. There is no liquid phase as a mediator. The advantage of a liquid phase as a mediator mainly lies in improved reaction kinetics, efficient ion and electron transfer, and more homogeneous reaction distribution.

Several problems arise due to the solid state:

• Limited ion and electron transport: In traditional lithium-sulfur batteries with liquid electrolyte, sulfur and polysulfides partially dissolve in the liquid, facilitating reactions. In all-solid-state systems, these dissolved species are absent, so the reaction progress heavily depends on the direct contact area between solid materials.
• Reaction kinetics are extremely slow: The conversion of elemental sulfur (S₈) to Li₂S requires a series of electron and ion transfers. Since solid phases often mix poorly and diffusion in solids is slower than in liquids, this reaction in ASSLSBs is very sluggish.
• Passivation and contact loss: During discharge, Li₂S forms as a reaction product. Since Li₂S is electrically insulating, it can deposit as a solid layer on the electrodes and hinder further reaction. This leads to capacity losses and rapid degradation of the battery.
• Insufficient interfacial stability: The interfaces between the sulfur cathode, solid electrolyte, and lithium anode are often unstable. Mechanical stresses from charge-discharge cycles can cause microcracks, worsening the contact between phases and further impeding ion flow.

A study has been published promising a remedy. Researchers present a solution to this problem through the use of lithium thioborophosphate iodide (LBPSI) as a glassy solid electrolyte. This solid electrolyte not only serves as a superionic conductor but also as a surface redox mediator (a chemical compound that facilitates the transfer of electrons between two reaction partners by acting as an electronic or ionic mediator). Through the reversible redox reaction between iodide (I⁻) and iodine/iodide complexes (I₂/I₃⁻), it facilitates otherwise sluggish reactions at the solid-solid interfaces, thereby significantly increasing the density of active reaction centers.

Due to this mechanism, the developed ASSLSBs demonstrate impressive performance. They allow for fast charging and achieve a specific capacity of nearly 1,500 mAh g⁻¹ (based on sulfur) at a charging rate of 2C (30 °C or 86°F). Even at an extremely high charging rate of 20C, a capacity of 784 mAh g⁻¹ is still achieved. Additionally, the battery is expected to endure 25,000 cycles with a residual capacity of 80.2 percent. (mr)