Stable freestanding Li-ion battery cathodes by in situ conformal coating of conducting polypyrrole on NiS-carbon nanofiber films

Nickel sulfide (NiS) is an attractive and promising cathode material for lithium ion batteries (LIBs) with a high theoretical capacity of 590 mAh g−1, which is much higher than that of LiMO2 (M = Co, Fe, Mn, etc.). Here, we demonstrate a facile synthesis of hybrid materials by in situ polymerization of a conducting polymer of polypyrrole (PPy) on NiS-carbon nanofiber (CNF) films to form high-performance freestanding LIB cathodes. PPy coating maintains the structural integrity and accommodates volumetric change of NiS upon cycling, and also helps enhance electronic conduction. CNF film provides a good mechanical and effective electrical interconnection in the entire electrode. Using the freestanding NiS-PPy-CNF hybrid film with a high NiS loading of 5.5 mg cm−2 as a cathode, we demonstrate a high discharge capacity of 635 mAh g−1 at 0.1 A g−1 and exceptional areal capacity of 3.03 mAh cm−2 at 0.7 mA cm−2 with long cycling life over 700 cycles, representing the best performance of NiS-based electrodes so far.