Polymer–Inorganic Composite Protective Layer for Stable Na Metal Anodes

Na metal anodes have attracted great interest because of their low electrochemical potential, high theoretical specific capacity, and natural abundance. However, nonuniform Na ion flux induced by an inhomogeneous solid electrolyte interphase (SEI) usually leads to the formation of mossy or dendritic Na, resulting in a low Coulombic efficiency (CE) and potential safety hazards. Herein, a well-designed artificial protective layer consisting of poly(vinylidene fluoride) (PVDF) and Sn nanoparticles was successfully constructed on a Cu current collector (PSN@Cu current collector) by a scalable and facile doctor blade coating technology. The flexible PVDF matrix in the protective layer can accommodate interface fluctuations, whereas the sodiophilic Sn nanoparticles can provide sufficient ionic conductivity for uniform deposition/stripping and a high mechanical modulus against potential dendrite growth. As a result, a high average CE of 99.73% can be achieved for 2800 h at 2 mA cm–2 with the PSN@Cu current collector. Furthermore, the PSN@Cu current collector has a stable cycling lifetime of 2300 h at 1 mAh cm–2, which is more than 10 times higher than that of bare Cu current collector (∼220 h). In addition, the unique structure and composition of the PSN-derived SEI are carefully investigated and can explain the much improved stability and electrochemical performance of Na metal anodes. This approach highlights the significance of a hybrid protective layer with synergistic properties and presents a new opportunity for stabilizing Na metal anodes.