Activation of the Multi-Step Redox Reactions with the Radical-Mediated Pathway by N-Doped Carbon-Embedded Ti_0.95Co_0.05N Nanowires as a Multifunctional Separator with a High Donor-Number Solvent for Lithium-Sulfur Batteries

To commercialize high-energy density Lithium-sulfur batteries, achieving high cell performance in a lean electrolyte condition with high sulfur loading is imperative. However, continuously produced lithium polysulfides during cycles cannot be readily solvated in the limited amount of electrolyte. The accumulation of soluble lithium polysulfides increases the viscosity of the electrolyte with poor wettability and reduced ionic conductivity, transforming it from a liquid to a semi-solid phase.<br/>Based on the dissolution-deposition mechanism of lithium-sulfur batteries, electrolyte solvent modification with a high donor-number solvent can enhance the solvation ability of lithium polysulfides and sulfur utilization by modifying a Li⁺ solvated structure according to Pearson’s Hard and Soft Acids and Bases theory. The Li⁺ solvated structure with high donor-number solvents (soft acid) preferentially stabilizes soluble long-chain polysulfides and radicals (S₃^–, S₆^2– and S₈^2–, soft base), leading to good solvation and high sulfur utilization under the lean electrolyte lithium-sulfur batteries. However, due to the poor compatibility of Li metal with the solvent, optimizing the high donor-number solvent is crucial for achieving high sulfur utilization at the cathode and ensuring Li metal compatibility at the anode, especially under lean electrolyte conditions. Consequently, we aimed to identify a Li metal-compatible high donicity electrolyte by incorporating a 1 vol% of <i>N, N</i>-dimethylacetamide (DMA) into a 1,2-dimethoxyethane (DME)/1,3-dioxolane (DOL) co-solvent (v/v, 1/1) (1 v% DMA, DMA:DME:DOL=1:49.5:49.5 vol%) for the advanced Li-S batteries.<br/>Some research has reported that the high donor-number solvent can create ‘radical-mediated pathways with tri-sulfur radicals’. However, <u>no relationship or elucidation between the radical-mediated pathway and effective electrocatalyst has yet been studied in the lithium-sulfur battery field.</u> Recent research in the oxygen evolution reaction field has declared ferromagnetic catalysts can facilitate an O=O dissociation reaction with the parallel spin alignment of oxygen radicals. Changes in reaction kinetics can also be expected in the case of sulfur, which has similar chemical properties to oxygen. Moreover, the ferromagnetic catalysts have been shown to strengthen the adsorption of lithium polysulfides and weaken the binding energy of the S-S bond in Li₂S₂ boosting the conversion of Li₂S due to the spontaneous spin polarization in the lithium-sulfur battery field.<br/>In this work, we employed N-doped carbon-embedded cobalt-doped titanium nitride nanowires as a multifunctional separator in 1 v% DMA electrolyte (1 v% DMA@NC-Ti_0.95Co_0.05N NWs@PP) for lithium-sulfur batteries. The 1 v% DMA@NC-Ti_0.95Co_0.05N NWs@PP achieved lower polarization of Li₂S and S₈ formation during cycles, higher Q_lower/Q_upper ratio showing Li₂S conversion efficiency, and a discharge capacity of 464.4 mA h g⁻¹ even after 200 cycles with a decay rate of 0.093% per cycle with a sulfur loading of 3.6 mg cm⁻² and a low electrolyte-to-sulfur (E/S) ratio of 10 μL mg⁻¹. Ferromagnetic active domain with d-band shift through cobalt doping into titanium nitride lattice can help adsorb lithium polysulfides including tri-sulfur radicals. Moreover, the cobalt-doped sample activates the radical-mediated disproportionation and chain-breaking reaction (S₈^2– → 2S₆^2– + (1/4)S₈ and S₆^2– ↔ 2S₃^–) and facilitates Li₂S₂ dissociation with spin-polarized electrons in the ferromagnetic domain.