Cationic-Dependent Cooperative Catalysis of Ni0.261Co0.739S2/N-Doped Carbon Nanotubes for Achieving Superior Electrocatalytic Activity and Cycling Stability at High Rate in Li-S Batteries
Authors: Ji Junhyuk , Park Minseon , Kim Minho , Kang Song Kyu , Park Gwan Hyeon , Seo Min Ho , Kim Won Bae 
Published: December 2024

Lithium-sulfur (Li-S) batteries are increasingly recognized for their exceptional energy storage capabilities, offering an energy density of approximately 2600 Wh kg-1. This is largely attributed to the use of sulfur in the cathode—a material both abundantly available and cost-effective. However, the widespread adoption of Li-S batteries faces significant challenges, including the low electrical conductivity of sulfur and its discharge byproducts (e.g., Li2S2, Li2S), the migration of polysulfides within the electrolyte, and electrode volume expansion, which collectively impede their practical application.

In the realm of recent advancements, transition metal (TM) sulfides have attracted considerable interest for their superior electrical conductivity and the presence of effective binding sites for lithium polysulfides (LiPSs), arising from a high density of valence electrons. This is a consequence of the soft basic nature of S2-/S2
2- anions, as opposed to the hard basic O2- ions. Particularly, TM sulfides with pyrite-type cubic crystalline structures, including NiS2 and CoS2, have proven to be potent electrocatalysts. They facilitate enhanced polysulfide redox reactions owing to their conductivity surpassing that of TM oxides. Furthermore, the integration of multi-cationic species into a single composite electrocatalyst has emerged as a strategic approach to refine the electronic configuration and augment the catalytic synergy of TM sulfides. Despite these developments, the discrete roles and synergistic contributions of cations within bimetallic TM sulfides remain largely unexplored, especially in comparison to the unveiled catalytic mechanisms of TM oxides.

This study introduces a novel, highly active, and durable sulfur catalyst system composed of NixCo1-xS2 nanocrystals dispersed on N-doped porous carbon nanotubes (NixCo1-xS2@NPCTs). This system serves as an efficient cathode electrocatalyst for rechargeable Li-S batteries. The carbonized, porous architecture offers extensive surface area and buffering capacity for cyclic redox processes, alongside numerous chemical binding sites on the CNT framework. The ingeniously designed dual-active TM sulfides, characterized by NiOh
2+−S−CoOh
2+ bonds within the octahedral TMS6 structure, effectively catalyze sulfur cathode reactions. They fulfill multifaceted roles; specifically, CoOh
2+ sites with vacancies robustly interact with LiPSs to mitigate the shuttle effect. Concurrently, the NiOh
2+ species, with an optimal doping concentration, precisely modulate the chemical interaction with LiPSs to facilitate a sequential redox reaction, thereby promoting sustained cooperative catalysis and reducing Li2S passivation on the catalyst surface. The Ni0.261Co0.739S2 catalyst, supported by NPCT carbon, excels by achieving excellent discharge capacity and exhibiting remarkable long-term electrochemical stability. Even under harsh conditions with a low electrolyte-to-sulfur ratio (E/S = 10.0 μL mg-1), this catalyst system demonstrates superior cycling performance and durability. Notably, the Ni0.261Co0.739S2@NPCTs catalyst system showcases exceptional cyclic endurance, maintaining a capacity of 511 mAh g-1 with a mere 0.055% decay per cycle at a 5.0 C rate over 1000 cycles, alongside a significant areal capacity of 2.20 mAh cm-2 under a sulfur loading of 4.61 mg cm-2 after 200 cycles at 0.2 C. Complementary theoretical DFT calculations corroborate the experimental results, enhancing our understanding of the efficacy of the catalyst system, particularly in terms of weakend sulfur bonding strength and optimzed binding energy which are pivotal for the catalytic performance of NixCo1-xS2@NPCTs in interactions with TM sulfides and lithium polysulfides. Our findings could highlight the importance of leveraging the synergistic potential of advanced carbon hosts and optimized bimetallic sulfide electrocatalysts, while also illuminating the intricate atomic configurations crucial for augmenting the redox activity of Li-S batteries in practical applications.

Figure 1