The morphological properties of the

The morphological properties of the samples were visualized by the field-emission scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Fig. 1D and E, the as-fabricated NPHC was made up of arched tile-like carbon nanosheets with several hundred nanometers in size. As shown in Fig. S2, the thickness of the carbon sheets measured by AFM was about 1.5–5nm. Whereas the NPHC-1 (Fig. S3A) nanosheets were much condensed resembling the template of magadiite (Fig. S3B). It thus can be concluded that the completely space-confined synthesis of NPHC-1 lowered the stripping degree of carbon nanosheets during thermal treatment process. The sample NHC also showed tightly packed features with relatively small restacking nanosheets (Fig. S4). Transmission electron microscopy (TEM) image (Fig. 1F) disclosed that the sheet-like carbon with different sizes of pores on the surfaces. HRTEM image (Fig. 1G) further indicated that NPHC had an amorphous lamellar structure and the no complete continuous lattice spacing (dotted circle) leading to the formation of more defects. The lattice spacing was 0.377nm in some crystalline region. Elemental mapping confirmed homogeneous dispersions of O, N, and P species (Fig. 1H) and the energy dispersive spectroscopy (EDS) quantified the weight contents of C, N, O, and P were 74%, 6.4%, 16.2%, and 3.4%, respectively (Table S2). The high content of heteroatom led to the high disorder of carbon structures and can afforded considerable active sites for ions storage [30].