Atomic-Scale Insights into Comparative Mechanisms and Kinetics of Na–S and Li–S Batteries

The Na–S and Li–S batteries are in the forefront to supplant ubiquitously used lithium-ion batteries. The understanding of mechanistic differences between Na–S and Li–S is critical to enable the inter-transfer of developed technologies toward designing high-performance cathode materials. The anchoring materials (AMs) are required to overcome the performance-limiting factors such as sluggish kinetics of metal polysulfides' (M2Sn, M = Na and Li, n = 1–8) conversion reactions and their dissolution into electrolytes. This study undertakes the challenges to critically understand the role of AMs on the polysulfide chemistry in both the batteries. We employ first-principles density functional theory simulations to comprehensively examine the adsorption mechanisms of M2Sn and the kinetics of sulfur reduction reactions (SRRs) and the catalytic decomposition of short-chain polysulfides across Na–S and Li–S batteries on pristine and vanadium (V) single-atom catalyst embedded WSe2 (V@WSe2) substrates. We found that pristine WSe2 cannot immobilize the higher-order M2Sn; however, V@WSe2 endows adequate binding energies to trap the higher-order M2Sn. The degree of M2Sn adsorption strengths and the effectiveness of the V@WSe2 varies between Na–S and Li–S systems. We elucidate the underlying mechanistic details with the aid of charge transfer, bond strength, and density of state analysis. Importantly, our simulations reveal that, in V@WSe2, the rate-limiting step of the SRR is kinetically faster in Li–S, whereas the oxidative decomposition of the discharge end product M2S exhibits accelerated kinetics in Na–S batteries. These findings are pivotal to understand the role of AMs in the design of cathode materials for addressing the performance-limiting factors in Na–S and Li–S batteries, in particular, and metal–sulfur batteries, in general.