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 S 8 /Li 2 S 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 S 8 to Li 2 S via intermediate Li 2 S 2 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 S 8 to Li 2 S; however, recent studies hold that partial S 8 is finally reduced only to Li 2 S 2 . By contrast, this work shows that tandem catalysis achieves stepwise S 8 reduction to Li 2 S via Li 2 S 2 , 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 Li 2 S/Co@MX
Co SAs anchored on Ti 3 C 2 Cl 2 MXene were synthesized by Lewis acidic molten salts etching method. The surface Cl-groups on MXene show distinct electron-withdrawing effect on Li in Li 2 S, and the Co-S coordination between Co SAs and S in Li 2 S has been proved. These results illustrate the strong chemical interaction between Li 2 S 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 Li 2 S 2 →Li 2 S reduction reaction more thermodynamically favorable. A series of characterization techniques collectively confirm the greatly enhanced sulfur conversion process and especially that the Li 2 S 2 →Li 2 S 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 g S −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.
National Science Review
Experimental study