Hierarchical Flower-Like Porous Carbon Nanosheets with Richly Exposed (002) Planes for High-Performance Sodium–Ion Batteries

Hard carbon is the most promising anode material for sodium–ion batteries (SIBs). However, its electrochemical performance is still unsatisfactory, especially at high-current densities. Herein, we propose a self-assembly strategy to synthesize hierarchical flower-like hard carbon nanosheets with richly exposed (002) planes (HFHCN) and combined micro- and mesopores, which provide more storage sites and faster reaction kinetics than the commercial hard carbon microspheres without pores (CHCM), resulting in better electrochemical performance, especially at high-current densities. The HFHCN electrode achieved a higher charge capacity (390.8 mA h g–1 at 0.02 A g–1) and better rate capability (104.8 mA h g–1 at 2.0 A g–1). With increasing current densities, the HFHCN electrodes maintain an almost unchanged initial Coulomb efficiency and electrochemical polarization and a slowly decreasing charge capacity at the low-voltage plateau, whereas the CHCM electrode shows a drastically deteriorated performance in these aspects. A turning point for their significant difference in electrochemical performance occurs at 0.5 A g–1, where the charging capacity at the low-voltage plateau is 72 mA h g–1 for the HFHCN electrode, but 0 for the CHCM electrode. When matched with the NVP (Na3V2(PO4)3) cathode, HFHCN∥NVP full-cells show a higher initial discharge capacity (273.5 mA h g–1 at 0.1 A g–1) and better rate performance (63.2 mA h g–1 at 5 A g–1) than those of the CHCM∥NVP full-cell (180.2 mA h g–1 at 0.1 A g–1 and 2.8 mA h g–1 at 5 A g–1). In addition, HFHCN∥NVP full-cells also have a longer operating plateau and lower electrochemical polarization than the CHCM∥NVP full-cell, which are also the main factors contributing to the higher energy density.