An elemental sulfur/CoS2- ionic liquid based anode for high-performance aqueous sodium-ion batteries

Despite having advantages of safety and dominating consumer and electric vehicle market, the progress of aqueous rechargeable battery research is hindered by low capacity anode materials. A game-changing strategy is to use an environmentally benign aqueous electrolyte with earth abundance Na-ion and sulfur. Sulfur promises to be a next-generation electrode material due to its high theoretical capacity and energy density. But the practical application of elemental sulfur in aqueous electrolyte suffer from sharp capacity decay due to the dissolution of polysulfides. Herein, we report 70 % elemental sulfur along with CoS2 and 1‑butyl‑3-methylimidazolium o,o-bis(2-ethylhexyl) dithiophosphate (BMIm-DDTP) ionic liquid (IL) as anode ([email protected]2-IL) for aqueous rechargeable sodium-ion/sulfur batteries (ARSSB) in 2 M aq. Na2SO4 electrolyte.The [email protected]2-IL anode delivers an outstanding capacity of 977 mA h g−1 (> 91 % of the theoretical capacity of elemental sulfur i.e. 1070 mA h g−1), at 0.5 C with a stable cycling performance over 100 cycles with > 98 % of capacity retention at 2 C with 100 % of coulombic efficiency. A negligible loss of S was confirmed by UV–Vis spectroscopy and potentiometric titration and anchoring of polysulfide at [email protected]2-IL anode were evidenced by NMR, ESI-mass, Raman and X-ray photoelectron spectroscopy. The synergy between highly active CoS2 and BMIm-DDTP IL from the dynamic coordination of dithiophosphate with CoS2 provides easy access for polysulfides anchoring and fast reaction kinetics which effectively suppresses the dissolution of discharge polysulfides and HS−. The full cell studies using [email protected]2-IL anode with standard Na0.44MnO2 cathode demonstrate an outstanding capacity of 778 mA h g−1 w.r.t the weight of S and 104.98 mA h g−1 (of total electrodes weight), which is close to the theoretical capacity of 109 mA h g−1 with 100 % coulombic efficiency and 91.8 % capacity retention over 400 cycles at 2 C. The excellent performance of this aqueous battery chemistry shows that [email protected]2-IL anode has great potential towards practical application in ARSSB.