# Intercalating Sn/Fe Nanoparticles in Compact Carbon Monolith for Enhanced Lithium Ion Storage
## 3 Results and Discussion
Two depressed semicircles on a series of SFO@C electrodes tend to merge into one at high-medium frequency particularly for SFO@C-400, showing the fast assembly of the thin and stable SEI layers, further indicating that the metal-in-carbon monolith structure cannot only mitigate the  
Figure 2:0 (**a**) Electrochemical impedance spectra of the SFO@C electrodes and SFO electrode after the first cycle in a fully charged state; (**b**) the equivalent circuit and (**c**) the corresponding fitted results of the resistance of SEI layer (R\({}_{\text{SEI}}\)) and the charge transfer impedance (R\({}_{\text{ct}}\)) of SFO@C electrodes and SFO electrode; (**d**) Warburg impedance spectra. volume effect but also improve the electron conductivity and Li\({}^{+}\) diffusion. The EIS plots are well fitted, using the equivalent circuit as shown in Figure 6b, and the corresponding fitted results are provided in Figure 6c, which includes resistance of SEI layer (R\({}_{\text{SEI}}\)) and charge transfer impedance (R\({}_{\text{ct}}\)). For the equivalent electrical circuit, R\({}_{\text{e}}\) means the electrolyte impedance derived from the intercept of high-frequency half-loop on the Z' axis, CPE1 is the constant phase element (CPE) in the high-frequency regions, and CPE2 represents the related double layer capacitors corresponding to the semicircle in the medium-frequency regions [55; 72]. In the case of R\({}_{\text{e}}\), all four electrodes do not present appreciable discrepancy, showing that the corresponding cells get suitably constructed and measured under the identical circumstances. Apparently, the R\({}_{\text{SEI}}\) and R\({}_{\text{ct}}\) of the SFO@C-400 electrode are 4.37 and 11.27 \(\Omega\), respectively, which are slightly lower than those of the SFO@C-350 and SFO@C-450 electrodes, manifesting the superiority of SFO@C-400 in augmenting the activation and dynamics of electrochemical events. This could be due to a stable formation of the thin SEI layers, likewise in good line with the irreversible capacity decay in the initial several cycles. The SEI film as a Li\({}^{+}\) reservoir is able to achieve a supererogatory Li\({}^{+}\) storage over surfaces of SFO@C, which telescopes the Li\({}^{+}\) migration pathways. In reality, such \"stored or trapped\" Li\({}^{+}\) in SEI layer is highly itinerant relative to their counterparts in the electrolyte and electrode interiors, bringing about the expedited surface charge transfer [73]. The unvaried SEI film can likewise be envisaged to be a Li\({}^{+}\) swapping and electron transport film, guaranteeing the boosted coulombic efficiency and outstanding rate capability [65]. However, the R\({}_{\text{SEI}}\) and R\({}_{\text{ct}}\) of the SFO electrode are 71.63 and 229.7 \(\Omega\), respectively, which are almost 20 times larger than those of SFO@C-400 electrode. The significant differences in values of R\({}_{\text{SEI}}\) and R\({}_{\text{ct}}\) between SFO@C electrodes and SFO electrode can be attributed to the following two reasons: (i) The carbon matrix in the SFO@C electrodes effectively enhances the conductivity of electrodes; and (ii) the Li\({}_{2}\)O generated during the discharge process, which has been proven to be not only electrochemically inactive, but also a bad electric conductor [74], has been completely converted back to Li by the reversible conversion reactions in the SFO@C electrodes. Furthermore, the Warburg impedance spectra of the electrodes (Figure 6d) demonstrate that the slope of linear plots for SFO@C electrodes is very close. However, the slope of linear plot for the SFO electrode is 232.8, considerably larger than that (122.7) of the SFO@C-400 electrode, clearly disclosing a faster Li\({}^{+}\) ion diffusion in the latter electrode than the former electrodes.