image: (a) Reaction pathways of conventional all-solid-state Li-S batteries. In ASSLSBs, the solid-solid conversion between S8 and Li2S at constrained three-phase interfaces involves high energy barriers, typically resulting in incomplete sulfur conversion. (b) Reaction pathways of all-solid-state Li-S batteries with tandem catalysis. Tandem catalysis significantly reduces these energy barriers and promotes smoother and more complete reduction of S8 to Li2S via Li2S2 intermediates.
Credit: ©Science China Press
All-solid-state lithium–sulfur (Li-S) batteries (ASSLSBs) employing nonflammable inorganic solid electrolytes are regarded as a promising next-generation energy-storage system. ASSLSBs not only intrinsically eliminate the notorious shuttle effect of lithium polysulfides in liquid Li-S batteries, but also offer the advantage of intrinsic safety. However, ASSLSBs still face critical challenges, particularly the high energy barriers and unclear redox mechanism of the solid-state S8/Li2S conversion, which result in low sulfur utilization and poor battery capacity.
To overcome these difficulties, the joint research team at Tianjin University, Zhengzhou University and Soochow University, led by Prof. Quan-Hong Yang, Prof. Chunpeng Yang, Prof. Xu Zhang and Prof. Liang Zhang, has proposed to exploit deep conversion of S8 to Li2S via intermediate Li2S2 by tandem catalysis for high-capacity ASSLSBs, employing a Co@MX catalyst. Earlier studies generally assume that ASSLSBs take a one-step reduction reaction from S8 to Li2S; however, recent studies hold that partial S8 is finally reduced only to Li2S2. By contrast, this work shows that tandem catalysis achieves stepwise S8 reduction to Li2S via Li2S2, during which atomically dispersed Co sites break S-S bonds and the polar MXene surface facilitates Li+ diffusion, significantly reducing the conversion energy barriers and exploiting deep sulfur conversion capacity. This research was published online in National Science Review.
Structure of Co@MX and interaction of Li2S/Co@MX
Co SAs anchored on Ti3C2Cl2 MXene were synthesized by Lewis acidic molten salts etching method. The surface Cl-groups on MXene show distinct electron-withdrawing effect on Li in Li2S, and the Co-S coordination between Co SAs and S in Li2S has been proved. These results illustrate the strong chemical interaction between Li2S and Co@MX, which plays a key role in accelerating sulfur redox process.
Reaction pathway of ASSLSBs with tandem catalysis
Density functional theory calculations reveal that Co@MX can facilitate S-S bond cleavage and the polar Cl-rich surface is beneficial to fast Li+ diffusion. In addition, Co@MX synergistically decreases the energy barrier of each reaction step and renders the rate-limiting Li2S2→Li2S reduction reaction more thermodynamically favorable. A series of characterization techniques collectively confirm the greatly enhanced sulfur conversion process and especially that the Li2S2→Li2S reduction step is effectively promoted.
Reaction kinetics and electrochemical performance of ASSLSBs
Due to the unique tandem catalytic effect, the Co@MX-based sulfur cathode exhibits improved reaction kinetics with a lower polarization and apparent activation energy, rapid ion transport and charge transfer. Consequently, Co@MX tandem catalyst enables ASSLSBs with high-rate, high-loading and long-time cycling. The Co@MX-based ASSLSB delivers a reversible capacity of 1329 mAh gS−1 after 2000 cycles at 2.8 mA cm−2 at room temperature.
This work provides valuable insight into the electrocatalyst design for tailoring the solid-state sulfur redox process, and deepens understanding of solid-state catalytic mechanisms, laying a solid foundation for the practical use of ASSLSBs.