Polyvalent interaction and confinement to suppress polysulfide dissolution and improve electrocatalysis

Abstract

Sulfur undergoes various changes from solid S₈ to soluble lithium polysulfides (Li₂S₈–Li₂S₄) and insoluble Li₂S₂ and Li₂S during charge–discharge cycling of lithium sulfur (Li–S) batteries. The dissolution of sulfur-containing compounds in battery electrolytes and their movement between electrodes, known as the polysulfide shuttle effect, decreases the battery performance. In addition, the kinetics of sulfur redox reactions are sluggish. Different host materials have been explored to address these issues. Herein, nanofibres of conjugated polymers have been synthesised that have multiple electron transport pathways. The cross-linker is nickel phthalocyanine tetrasulfonic acid tetrasodium salt (NPTS). Sulfur is situated in the voids of cross-linked nanofibres of the polymer and Ni²⁺ present in NPTS attracts the negative charge-bearing polysulfides. Due to the confinement and polyvalent electrostatic attraction, the solubility of sulfur and polysulfide is suppressed. Density functional theory calculations revealed that S²⁻ interacts with Ni²⁺ and Li⁺ interacts with the pyrrolic nitrogens of PPy-NPTS. The overlap of the p-orbitals of sulfur and nickel is determined from the density of states calculations. The bond length of Li₂S is ideal for this interaction, hence this molecule showed the highest adsorption energy with the cross-linked polymeric host. The adsorption energy decreased upon an increase in the number of sulfur atoms in the polysulfide chain due to the bond length mismatch. However, due to electrostatic polyvalent interaction, the adsorption energy is sufficient to suppress polysulfide dissolution. Thus, the structure of this host material with nickel cations and pyrrolic nitrogens is suitable to adsorb lithium polysulfides irrespective of their length, unlike neutral hosts. This efficient binding also improved the electrocatalysis of the sulfur redox reaction. Hence, the Li–S battery containing these nanofibres showed a specific capacity of 1326 mA h g⁻¹ at 0.2C. Batteries fabricated considering practical parameters, such as low electrolyte to sulfur ratio of 5.0 μL mg⁻¹ with sulfur loading of 4.0 mg cm⁻², showed impressive performance.