In a recent study published in the journal National Science Review, an international group of researchers from Tsinghua Shenzhen International Graduate School, Tianjin University, Joint School of the National University of Singapore, and Tianjin University developed an electrochemical molecular imprinting technique appropriate for Li-S batteries using the irreversible delithiation characteristics of metal sulfides.
(a) Discharge and charge curves for MSs. (b) Illustration of electrochemical MI strategy. Image Credit: Science China Press
Future battery research must focus on high energy density. Due to their high theoretical energy density, lithium-sulfur (Li-S) batteries have attracted much interest.
However, inadequate Li2S conversion and electrode passivation results from the sluggish solid-liquid-solid conversion of sulfur, particularly from the oxidation of lithium sulfide (Li2S) during charging, which necessitates overcoming significant reaction barriers. Consequently, the batteries' energy density and cycle life remain insufficient for commercial use.
Adding catalysis has shown to be a successful tactic in recent times for improving cathode kinetics and raising sulfur consumption. However, the efficient reversible conversion of sulfur is severely limited by the limited contact and weak interaction between solid-phase catalysts and solid Li2S, particularly under high sulfur loading and lean electrolytes. This significantly limits the energy density and cycle stability of Li-S batteries.
In particular, lithiation/delithiation was used to pre-embed Li2S, and then alcohol washing was used to remove Li2S and create Li2S imprinting flaws in metal sulfides (MS). The structural characterization demonstrated that the removal of Li2S was the cause of the sulfur vacancy that developed in the catalyst. Because of this unique flaw, the catalyst can attach to the target product Li2S only.
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The catalyst performance is favorably connected with the sulfur vacancy content, suggesting that the defect tailored for Li2S in MSs can greatly enhance the reaction. The researchers also tested several MSs to show the method's universality.
Following the material screening, a Li2S activation potential experiment revealed the strong catalytic conversion effect of Li2S oxidation, and QCM showed the targeted adsorption impact of MI-Ni3S2 on Li2S.
DFT further clarified the mechanism: These specially designed flaws allow the catalyst to attach just to Li atoms in the Li2S reactant, lengthening the Li-S bond and lowering the reaction energy barrier during charging, thus accelerating the conversion of Li2S to sulfur.
When it came to battery performance, the built Ah-scale Li-S pouch cell cycled steadily for more than 100 cycles under realistic circumstances, obtaining an energy density of more than 300 Wh/kg, depending on the total mass.
Furthermore, the researchers used this catalyst to effectively produce batteries with an energy density of 502 Wh/kg, surpassing the performance of most already reported works with extremely low electrolytes (E/S=1.8 μL/mgS).
In summary, the suggested synthetic method provides a perfect solution for the challenging Li2S dissociation issue, which is essential for the successful manufacturing of Li-S batteries. More encouragingly, this study offers a rationale and a practical method for creating catalysts with a well-managed solid-solid interface that applies to high-energy sulfur-based batteries.
Journal Reference:
Zhao, Y., et al. (2024) Engineering catalytic defects via molecular imprinting for high energy Li-S pouch cells. National Science Review. doi.org/10.1093/nsr/nwae190