Insights into the Pseudocapacitive Behavior of Sulfurized Polymer Electrodes for Li-S Batteries

Practical applications of sulfurized polymer (SP) materials in Li-S batteries (LSBs) are often written off due to their low S content (≈35 wt%). Unlike conventional S₈ /C composite cathodes, SP materials are shown to function as pseudocapacitors with an active carbon backbone using a comprehensive array of tools including in situ Raman and electrochemical impedance spectroscopy. Critical metric analysis of LSBs containing SP materials with an active carbon skeleton shows that SP cathodes with 35 wt% S are suitable for 350 Wh kg^-1 target at the cell level if S loading >5 mg cm^-2 , electrolyte-to-sulfur ratio <2 µL mg^-1 , and negative-to-positive ratio <5 can be achieved. Although 3D current collectors can enable such high loadings, they often add excess mass decreasing the total capacity. An "active" carbon nanotube bucky sandwich current collector developed here offsets its excess weight by contributing to the electric double layer capacity. SP cathodes (35 wt% S) with ≈5.5 mg cm^-2 of S loading (≈15.8 mg cm^-2 of SP loading) yield a sulfur-level gravimetric capacity ≈1360 mAh g_s ^-1 (≈690 mAh g_s ^-1 ), electrode level capacity 200 mAh g_electrode ^-1 (100 mAh g_electrode ^-1 ), and areal capacity ≈7.8 mAh cm^-2 (≈4.0 mAh cm^-2 ) at 0.1C (1C) rate for ≈100 cycles at E/S ratio = 7 µL mg^-1 .