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^-2. When tested in full Na || Na₃V₂(PO₄)₃ coin cell, the coated separator enables the delivery of a stable capacity of about 100 mAh g^-1 for 500 cycles (90% capacity retention) at a specific current of 235 mA g^-1 and satisfactory rate capability performances (i.e., 75 mAh g^-1 at 3.5 A g^-1).