Sodiated NaxSnSb nanoparticles embedded in N-doped graphene sponges direct uniform Na nucleation and smooth plating for high efficiency Na metal batteries†
The low-cost sodium (Na) metal anode has high specific capacity and is one of the most promising electrode materials for next generation high-energy density batteries. Unfortunately, it is also extremely reactive and suffers from severe side reactions with almost all electrolytes, which results in detrimental failures including uncontrollable growth of mossy structures, fast electrolyte depletion and short cycle life. We report here the in situ synthesis of composite sponges with sodiated NaxSnSb particles embedded in N-doped graphene (NG) frameworks for modulating Na anodes with significantly improved stability and fast charging capability. The sodiated NaxSnSb composites function as seeding particles that guide low-overpotential and smooth growth of Na metal whereas the NG wrapping layers protect the freshly plated Na metal from detrimental side reactions with the battery electrolyte. As a result, as high as 95% of the freshly plated Na metal can be reversibly stripped during battery operation and this efficiency can last hundreds of cycles. We further show that the Na3Sb phase in the NaxSnSb composites played critical roles as sponges with only the Na15Sn4 phase have much lower performance. Overall, the synergy of guided growth from NaxSnSb and shielding from NG allows multifunctional hosts that support Na anodes to cycle with nearly unchanged overpotential and charge transfer resistance for >500 cycles in both symmetric cells and full cells. The Na metal remained dense without measurable formation of unsafe dendrite or inactive mossy structures in these novel sponges even after aggressive cycling.