Failure mechanism of sulfurized polyacrylonitrile (SPAN) cathode induced by boron-contained lithium salt

Sulfurized polyacrylonitrile (SPAN) is deemed as the most promising lithium-sulfur (Li-S) batteries cathode owing to high sulfur utilization degree and stable cycling performance. However, abnormal high initial capacity of 2683.2 mA h g^-1 and severe degradation (100.5 mA h g^-1, 100 cycles) induced by LiDFOB salt are observed in our work. To conduct in-depth research on related mechanism, LiPF₆ and LiTFSI based batteries are tested as fair comparisons and relatively cycling performances are exhibited. The electrochemical performance of electrolyte and the interfacial properties of cycled Li anode are compared, then the impact of Li ion transfer and parasitic interface reactions are excluded. Synchrotron-based pair distribution function (PDF) and Raman spectroscopy tests indicate that new B-S bonds are generated on SPAN during the first discharge process in LiDFOB based battery, while the insertion of Li ions on S sites are greatly suppressed. Density functional theory method suggests that active S sites after S-S bond cleavage will be attacked and bonded by B from DFOB^-, which is hard to break and continuously inhibit effective reactions between Li ions and S, leading to serious irreversible battery degradation. The failure mechanism of SPAN cathode induced by boron-contained lithium salt are further verified by LiBOB.