Below is a paragraph from an academic paper. Polish the writing to meet the academic style, improve the spelling, grammar, clarity, concision and overall readability. When necessary, rewrite the whole sentence. Output the full text after rewriting. Original paragraph: FeSe2@NC can be synthesized through the following steps. Firstly, the FeOOH precursor was synthesized by a simple hydrothermal method. Then, the FeOOH precursor was coated with dopamine to obtain FeOOH@polydopamine (FeOOH@PDA). Finally, the FeOOH@PDA was evenly mixed with selenium powder and annealed at 450 °C for 10 h in N2 to obtain FeSe2@NC. As shown in Fig. 1a, a series of obvious diffraction  peaks located at 31.1°, 34.9°, 36.2°, 48.2°, 50.9°, 51.0°, and 54.0° in XRD patterns of two samples correspond to (101), (111), (120), (211), (002), (130), and (031) lattice planes of orthogonal FeSe2 (PDF 82-0296). No impurities are detected in both samples, indicating the high purity of the materials. The effect of the mixing ratio between FeOOH and selenium powder on the synthesis of pure FeSe2 is also explored. After gradually increasing the proportion of selenium powder, pure FeSe2 is finally obtained at a ratio of 1:8 and the excess selenium powder evaporates as selenium vapor (Figs. 1a and S1). The surface elements of Fe, Se, N, and C for FeSe2@NC are explicitly recorded by X-ray photoelectron spectroscopy (XPS) in Fig. 1b. To be specific, Fig. 1c shows the Se 3d spectrum with two significant peaks at 54.2 eV for Se 3d5/2 and 55.1 eV for Se 3d3/2. The weak peak at 59.1 eV belongs to SeOx, which comes from the oxidation on the surface of the sample in the air [26, 47]. Fig. 1d gives the high-resolution Fe 2p spectrum, which contains two spin-orbit doublets accompanied by two satellites. Two sharp peaks observed at 707.0 and 719.9 eV can be indexed to Fe 2p3/2 and Fe 2p1/2 for FeSe2@NC [35]. The peaks at 286.4 and 285.2 eV in the C 1s spectrum (Fig. 1e) are attributed to C-N and C=C bonds, respectively, implying the existence of N-doped carbon [47]. In addition, in the N 1s spectrum (Fig. 1f), pyridine-N, pyrrole-N, and oxidized-N are represented by three minor peaks at 398.8, 400.5, and 405.8 eV, respectively. N-doping can improve the conductivity of electrodes and enhance the Na-storage capability [48, 49]. The carbon content of the FeSe2@NC was confirmed by thermogravimetry (TG) in air shown in Fig. 1g. The curve mainly consists of four stages: in the initial heating stage, the slight mass loss is due to the release of absorbed water from the material. The mass then increases at around 250 ℃, as the sample reacts with O2 to form solid FeOx, SeO2, and CO2 gases. The mass drops quickly as the temperature rises over 400 °C owing to the evaporation of SeO2. After calculation, the carbon content in FeSe2@NC is about 12.3%. As the specific surface area and pore structure of electrode material greatly influence Na+ storage capacity, the specific surface areas and pore size distribution of the samples are investigated by the nitrogen adsorption/desorption isotherms and Barrett-Joyner-Halenda (BJH) pore size distribution curves in Fig. 1h and 1i, showing that the specific surface area is 56.5 m2 g-1 for FeSe2@NC, bigger than the value for FeSe2 (54.4 m2 g-1). BJH curves evidence that FeSe2@NC has mesopores with a size of 4 nm, while FeSe2 has no obvious pores.