Construction of polysulfides defense system for greatly improving the long cycle life of metal sulfide anodes for sodium-ion batteries

Metal sulfides are promising anode materials for sodium ion batteries (SIBs) due to their high theoretical specific capacity and abundant source. Nevertheless, significant challenges, including large volume change, sluggish Na+ transport kinetics and polysulfides intermediates, have greatly affect their long cycle stability. Unfortunately, the majority of current studies only focus on the first two aspects, but lack of sufficient attention and insights into the effect of polysulfides intermediates. Here, a porous of CoSx (P-CoSx) electrode material is fabricated as an example to investigate the influence of polysulfides on its cycling performance. The results show that polysulfides cause a slight loss of reversible capacity during the battery cycling, while the failure of the battery is due to its significant fluctuations in reversible capacity after extensive cycles. Detailed analyses demonstrate that the intense fluctuation in capacity originates from the faster growth of dendrites caused by the reaction of sodium polysulfides with sodium foil and/or the reaction of elemental sulfur with sodium foil to penetrate the separator, resulting in a local short circuit. To suppress these undesirable side reaction, N, S co-doped porous carbon tubes (N, S-PC) rich in C–S and C–N bonds have been added to adsorb polysulfides and alleviate their reaction with sodium foil. As a result, the capacity of the P-CoSx electrode with N, S-PC (P-CoSx/N, S-PC) remains stable without significant fluctuations for 1000 cycles, which is much better than that of the pure P-CoSx electrode (intense fluctuation in capacity after 320 cycles). Our work offers insights into the crucial influence of polysulfides on the cycle performance of the P-CoSx anode and provides a feasible strategy to prolong the cycle life of metal sulfide anode for SIBs.