# Intercalating Sn/Fe Nanoparticles in Compact Carbon Monolith for Enhanced Lithium Ion Storage
## 3 Results and Discussion
Furthermore, the correlative plateau zones of the electrode can be also determined by charge/discharge patterns (Figure 4a). After the initial cycle, the cathodic peaks at \(<\) 0.7 V sees a conspicuous displacement, whereas other peaks encounter the slight shift, documenting the influence of SEI layers. Contrarily, the oxidation peaks do not witness the notable variation, revealing the superior reversibility of Fe\({}_{2}\)O\({}_{3}\) and SnO\({}_{2}\). After the first cycle, the following second and third cathodic and anodic cycles practically overlap each other (namely the reduction/oxidation peaks remain unaltered in terms of the current densities and voltage positions), shedding light upon the outstanding cyclability and reversibility of these electrochemical reactions (namely the (de-)lithiation processes have no changes). This superior reversibility surely manifests that our sample can well manage structural stress of SnO\({}_{2}\) to arrest the volume variations and clustering of the Sn nanoparticles. Through making a comparison between the second/third cycle and the first cycle, one can clearly discover that the reduction peak at 0.77 V disappears and tends to be stable subsequent to the first cycle, stating the irreversible generation of SEI film because of the electrolyte decomposition. The diminished intensity regarding wide cathodic peaks within the scope between 0.005 and 1.0 V verifies the successful oppression of the gain in the SEI layer thickness. In addition, the recognizable negatively shifted anodic peaks (concomitant with discernable positively moved cathodic peaks) in the SFO@C-400 electrode expounds its improved dynamic performance, viz., easier mobility of Li ion across SEI layers and quicker charge transport. Relevant electrochemical events on lithiation/de-lithiation can be expressed as follows:  
\[\text{Conversion reactions: SnO}_{2}+4\text{Li}^{+}+4\text{e}^{-} \leftrightarrow\text{Sn}+2\text{Li}_{2}\text{O}\]  
\[\text{Fe}_{2}\text{O}_{3}+6\text{Li}^{+}+6\text{e}^{-} \leftrightarrow 2\text{Fe}+3\text{Li}_{2}\text{O}\]  
\[\text{Alloying/de-alloying process: Sn}+x\text{Li}^{+}+x\text{e}^{-} \leftrightarrow\text{Li}_{x}\text{Sn}\ (0\leq x\leq 4.4)\]  
Figure 4c plots the electrochemical kinetics and rate capability of these hybrids at a series of progressively increased current densities ranging from 0.2 to 3.2 A g\({}^{-1}\). The electrochemical performance is unstable, and the electrode capacity gradually decreases with the current density increment. Fortunately, when the current rate rises successively from 0.2, 0.4, 0.8, and 1.6 to 3.2 A g\({}^{-1}\), an average capacity of SFO@C-400 is calculated to be about 829, 732, 597, 451, and 331 mAh g\({}^{-1}\), respectively, showing the best rate capability. The steady capacity at 3.2 A g\({}^{-1}\) is as high as 40 % of the one at 0.2 A g\({}^{-1}\), significantly outperforming those of other three electrodes. Even at a high current density of 3.2 A g\({}^{-1}\), the average capacity of 331 mAh g\({}^{-1}\) is very close to the theoretical capacity of the graphite anode (374 mAh g\({}^{-1}\)). Markedly, when abruptly switching the current density from 3.2 A g\({}^{-1}\) back to 0.2 A g\({}^{-1}\) after 10 cycles under various current densities, the average capacity can instantly revert to 681 mAh g\({}^{-1}\) after 10 cycles, reinstating 83% of the original level, indicating a good structural stability. This reveals that the binary metal-in-carbon structure conduces to electron/ion migration, augmenting the rate capability. These values are undeniably superior to the counterparts of SFO@C-350 and SFO@C-450, which is due to the less sluggish Li\({}^{+}\) diffusion resistance and charge transfer impedance of SFO@C-400, as verified by EIS test (Figure 6a).