On the effect of sulfur strand length on the electrochemical performance of dual-redox-active sulfur-naphthalene diimide cathode materials

Sulfur-rich copolymer networks prepared via inverse vulcanization are attractive materials for lithium–sulfur (Li–S) batteries. Maximizing the S content is beneficial for capacity but leads to long S-strands that are prone to the formation of lithium polysulfides, dissolution in the electrolyte, and deposition elsewhere, thus affecting the long-term stability of the battery. The effect of S-strand length on battery stability is therefore crucial and often underestimated when preparing inversely vulcanized cathode materials. Here, we study the impact of the S-strand length of dual-redox-active S-naphthalene diimide (NDI) networks SNDI on the capacity and stability of Li-SNDI batteries. Network composition is broadly varied with average S-strand lengths n of 10.0 ≥ n ≥ 0.6. n is directly determined from simulated and experimental near-edge X-ray absorption fine-structure (NEXAFS) spectra. For larger n, capacity of Li-SNDI batteries increases but cycling stability decreases (n = 10: initial specific capacity ≥ 350 mA·h/g, capacity retention 20% after 200 cycles). For very small n, specific capacity stabilizes at ∼100 mA·h/g featuring an excellent cycling stability of over 1000 cycles. Here, n ∼ 1 corresponds to electrochemically inactive thioether linkages that are possible as homopolymerization of the NDI monomer is hampered. As the redox potentials of NDI and sulfur are similar, the loss of capacity at small n is partially compensated for by the increasing presence of NDI cross-links. The performance of the NDI networks is comparable to the best nonconjugated NDI-based organic batteries in terms of capacity but superior regarding long-term stability. The herein presented approach addresses the stability of cathode materials and provides a simple means to generate dense organic networks for polymer batteries.