# Intercalating Sn/Fe Nanoparticles in Compact Carbon Monolith for Enhanced Lithium Ion Storage
## 3 Results and Discussion  
The powder XRD measurements of final samples were utilized to investigate the component and crystal structure. Figure 1a describes XRD patterns of the as-formed composites pyrolyzed at various temperatures and the carbon-free SFO specimen. All products share the similar reflections peaks that correspond to the tetragonal rutile SnO\({}_{2}\) (JCPDS card No. 41-1445) and the rhombohedral Fe\({}_{2}\)O\({}_{3}\) (JCPDS card No. 33-0664), confirming the successful conversion of metal ions into coexistence of SnO\({}_{2}\) and Fe\({}_{2}\)O\({}_{3}\) phases and their nanocrystalline nature. Moreover, no visible reflection signals linked with graphite emerge, possibly revealing that the carbon in all samples may be amorphous in crystallinity or inappreciable in quantity. The sharp diffraction peaks with the small full width at half-maximum (FWHM) values unravel the excellent crystallinity of the Sn and Fe nanograins. No other impurities but both the Fe\({}_{2}\)O\({}_{3}\) and SnO\({}_{2}\) phases are found. On account of the simplicity, scalability, and most significantly the circumvention of harnessing expensive/eco-unfriendly raw materials, our current way considerably gains an advantage over the prior studies regarding preparing the carbon/Sn/Fe hybrids, and thus is more amenable to practical adhiitions [16; 22; 25; 26; 27; 28]. The average size of the SnO\({}_{2}\) nano-crystallites within hybrids is computed as roughly 9 nm according to the Scherrer formula (d = 0.9\(\upmu\)/gcos\(\upmu\), wherein \(\upmu\) represents the Bragg angle, \(\upmu\) means FWHM in radian, \(\uplambda\) refers to the X-ray wavelength, and d stands for average crystalline particle size), employing Sn (100) peak. The textural properties were clarified via N\({}_{2}\) porosimetry, and the corresponding results were depicted in Figure 1c. The type III isotherms without the distinguishable hysteresis loop are obtained for all products, uncovering the distinctive signatures of non-porous materials. BET specific surface areas of SFO@C-350, SFO@C-400, SFO@C-450, and SFO are estimated to be 9.1, 14.2, 56.4, and 141.2 m\({}^{2}\) g\({}^{-1}\), respectively. The large value of SFO can be attributed to internal void structures. It is worth noting that the surface areas rise dramatically with the increase of oxidation temperature, which may be related to the gas derived from the decomposition of carbon source in gel precursor. The BET surface area of B-SFO@C-400 approaches only 14.2 m\({}^{2}\) g\({}^{-1}\), one order of magnitude below a host of the nanometer SnO\({}_{2}\)-based structures (\(>\)100 m\({}^{2}\) g\({}^{-1}\)) reported previously [29; 30]. This textural characteristic unveils the compact and non-porous nature of B-SFO@C-400, finally resulting in a large tap density of about 0.70 g cm\({}^{-3}\), 2-3 times outnumbering a great deal of the hitherto-reported elaborated SnO\({}_{2}\)-based materials with the highly hierarchical porosity and morphology [29; 30]. The carbon contents in all samples are identified via the thermogravity analysis (TGA) at the wide temperature range of 25-800 \({}^{\circ}\)C in air, as illustrated in Figure 1b. In TGA curves, a slight weight loss below 1% observed up to 110 \({}^{\circ}\)C ought to originate from removal of physically absorbed and free water from atmosphere. The total weight loss at high temperature is assigned to the combustion of carbon into CO\({}_{2}\) in air upon heating. Thus, the weight percentage of carbon in these composites can be calculated to be roughly 54.7 wt.% (SFO@C-350), 39 wt.% (SFO@C-400), and 32.7 wt.% (SFO@C-450). That is to say, the total metal or/and metal oxide fraction sequentially equals 45.3%, 61%, and 67.3%, revealing that an appropriate carbonization temperature can result in a rather high metal loading concentration. Combined with absence of XRD diffraction peaks assignable to graphite, the relatively high content proportion also suggests that the carbons in composites are in a non-crystalline state.