Simultaneous acceleration of sulfur reduction and oxidation on bifunctional electrocatalytic electrodes for quasi-solid-state Zn-S batteries

This study is led by Prof. Ke Lu (Institutes of Physical Science and Information Technology, Anhui University), Prof. Pengfei Cao (State Key Lab of Organic-Inorganic Composites, Beijing University of Chemical Technology), and Prof. Bin Song (Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University). This work mainly proposes the construction of a bifunctional catalytic carrier to facilitate the sulfur redox process for achieving a high-performance quasi-solid-state Zn-S battery.

Owing to their non-toxic aqueous electrolytes and the abundant and cost-effective Zn metal anodes, aqueous zinc ion batteries hold tremendous interest in sustainable energy-storage devices. However, the strong electrostatic interactions between the zinc ions and the insertion-type cathode lattice make the capacity and cycling stability undesirable. Therefore, efforts have been devoted to the research of cathodes that rely on conversion chemistry, in which the sulfur (S) cathode with a high theoretical capacity (1,675 mAh g⁻¹) renders the Zn-S electrochemistry with great potential. However, the research on aqueous Zn-S batteries is still in its infancy, and challenges still lie in the cathode side. One problem is the insulating nature of both sulfur and its discharge product of ZnS. Another issue is the huge volume expansion during the discharging process (50.3%). More importantly, in the recharging process, the high decomposition energy barrier of ZnS along with the sluggish redox kinetics renders the severe irreversible side reactions.

Thus, the construction of a bifunctional electrocatalytic cathode that anchors multicatalytic centers to simultaneously direct complete sulfur reduction and minimize the ZnS reoxidation energy barrier is highly anticipated to manipulate the bidirectional conversion kinetics of S/ZnS redox couples and realizing high-efficiency Zn-S batteries. Herein, researchers focus on the construction of a bidirectional electrocatalytic sulfur matrix for complete sulfur reduction and easy ZnS activation. In this context, they design the CoO nanocrystals anchored on N-doped porous carbon composite (NC-CoO) as an advanced sulfur host to efficiently lower the ZnS decomposition barrier and suppress the disproportionation side reactions, realizing a high-efficiency Zn-S electrochemistry. Furthermore, post-mortem analyses indicate that the degradation of Zn-S cells is mainly attributed to the limited reversibility of the zinc anode rather than to the decomposition and/or accumulation of ZnS. The bi-directional catalytic manipulation of sulfur redox provides a new perspective on realizing the theoretical potential of Zn-S cells.