Achieving stable Na metal cycling via polydopamine/multilayer graphene coating of a polypropylene separator.

Sodium metal batteries are considered one of the most promising low-cost high-energy-density electrochemical energy storage systems. However, the growth of unfavourable Na metal deposition and the limited cell cycle life hamper the application of this battery system at a large scale. Here, we propose the use of polypropylene separator coated with a composite material comprising polydopamine and multilayer graphene to tackle these issues. The oxygen- and nitrogen- containing moieties as well as the nano- and meso- porous network of the coating allow cycling of Na metal electrodes in symmetric cell configuration for over 2000 h with a stable 4 mV overpotential at 1 mA cm. When tested in full Na || NaV(PO) coin cell, the coated separator enables the delivery of a stable capacity of about 100 mAh g for 500 cycles (90% capacity retention) at a specific current of 235 mA g and satisfactory rate capability performances (i.e., 75 mAh g at 3.5 A g).