# Intercalating Sn/Fe Nanoparticles in Compact Carbon Monolith for Enhanced Lithium Ion Storage
## 3 Results and Discussion
The carbon crystal structure can be further substantiated through Raman spectroscopy. No Raman scattering characteristic bands indexed to SnO\({}_{2}\) and Fe\({}_{2}\)O\({}_{3}\) are detected. The two sharp bumps located at around 1344 and 1588 cm\({}^{-1}\) well matching with the D and G peaks of the carbon materials are discerned [49]. It has been reported that the D band typifies a defect-induced breathing mode of sp\({}^{2}\) rings, which stems from the strongly dispersed LO phonons around the K point activated in double resonance processes, namely, k-point phonons of A\({}_{\rm 1g}\) symmetry. The G band is responsible for the characteristic doubly degenerate E\({}_{\rm 2g}\) mode (iTO and LO) of the sp\({}^{2}\) domains or atoms, i.e., the first-order phonon scattering vibrations in the in-plane of the ideal graphite. To be more precise, a Kohn anomaly at K has been indicated as the real source of the D band due to the greatest optical branch starting from the K-A\({}_{1}\) mode, which features the largest electron-phonon coupling among K phonons. Thus, the D band can elucidate the degree of disorder and structural defects in the graphite layer of carbon materials, whose quantitative level can be appraised by the intensity ratio of D to G band (I\({}_{\rm D}\)/I\({}_{\rm G}\)) [50, 51, 52, 53]. Commonly, an improved I\({}_{\rm D}\)/I\({}_{\rm G}\) value implies the smaller sp\({}^{2}\) domains within the carbon materials and restoration of sp\({}^{2}\) carbon. This value on the typical SFO@C-400 sample is as high as 1.14, disclosing that the carbon in SFO@C-400 is the amorphous nanocarbon with the disorder structure or small sp  
Figure 1: (**a**) XRD patterns; (**b**) N\({}_{2}\) adsorption/desorption isotherm curves, and TGA curves (**c**) of all samples; (**d**) Raman spectra of SFO@C-400 sample. microdomain [54]. This partial graphitization of carbon matrix can simultaneously support the decent electrical conductivity and mechanic elasticity or flexibility, which can be beneficial for accelerating the charge transfer rate, stabilizing the metal oxide/metal heterointerfaces, and boosting the electrode mechanic stability. The microstructures and morphology of the samples were further observed via SEM and TEM. As shown in Figure 2a-c, all SFO@C-350, SFO@C-400, and SFO@C-450 samples exhibit a similar bulk-like structure with a size of 10-15 \(\upmu\)m and no bulk metal aggregations. This monolithic morphology evolves with the pyrolysis temperature. Under a lower temperature of 350 and 400 \({}^{\circ}\)C (Figure 2a,b), no observable macropores exist on surfaces of monoliths, whereas at a higher temperature of 450 \({}^{\circ}\)C many irregular macropores/cavities of 1-2 \(\upmu\)m in diameter abound (Figure 2c). This could be caused by the gas generated during decomposition of carbon precursors and accounts for the huge difference in the BET specific surface area between SFO@C-350, SFO@C-400, and SFO@C-450. The magnified SEM image of SFO@C-400 in the inset of Figure 3b demonstrates that the surface of SFO@C-400 is made up of the closely packed ultrafine particles and no discernable interparticle small openings/voids are monitored, symptomatic of a dense configuration of SFO@C-400. However, the micron-sized bulk structure can no longer be well maintained in the case of SFO, which manifests a loosely stacked organization comprising lots of agglomerates with an average size of 200 nm. This clearly evidences that the carbon matrix is indispensable to result in unique structural motifs of micro-dimensionality and compactness. TEM measurements were carried out to acquire more detailed information of the microstructure of the typical SFO@C-400 sample. Commonly speaking, Figure 2e presents a typical panorama possessing no detectable porosity (involving the mesopores and macropores), confirming the non-porous and dense nature from the BET data. It is clearly found that numerous metal oxide nanoparticles are uniformly distributed in a continuous carbon matrix.