# Intercalating Sn/Fe Nanoparticles in Compact Carbon Monolith for Enhanced Lithium Ion Storage
## 3 Results and Discussion
In comparison with the 20\({}^{\text{th}}\) cycles (one randomly chosen steady cycle), a reversible capacity retention of up to 97% can still be estimated for the 100 consecutive cycles, clearly unveiling that practically no capacity degradation occurs upon cycling and our designed electrode structure can well tackle the stress-induced huge volume expansion upon (de-)lithiation and inhibit the aggregation over cycling Li\({}^{+}\) insertion/extraction processes. This kind of electrochemical behavior, namely the charge/discharge capacity identical with the theoretical value, has also been reported in the literature, the reason for which can result from the unvaried SEI layer thickness on the electrode surfaces over cycling. To emphasize the robust nature of our designed structure of binary metal-in-carbon compact monolith, the long-term cyclic performance of SFO@C-350, SFO@C-400, SFO@C-450, and SFO electrodes was further assessed at a large current density of 1 A g\({}^{-1}\) for 200 cycles, in which the hybrids will be subjected to the fiercer Li\({}^{+}\) insertion and extraction events. It is worth mentioning that the first cycle was carried out at 0.2 A g\({}^{-1}\) to complete the formation of SEI film. Again, a good cycling performance without the obvious capacity decay is monitored on the SFO@C-400, giving a highest discharge capacity of 466 mAh/g (about 73% retention since the second capacity) after 200 cycles, which is still much higher than theoretical specific capacity of commercial graphite anodes. This further confirms the advance of our designed architecture in terms of the applications to LIBs. Strikingly, for SFO@C-400, all coulombic efficiencies at different current densities during cycling are well sustained almost above 98%, which is the symbol of a stable SEI films and a good (de-)lithiation [65]. In contrast, the quick specific capacity decays during the overall cycling at the different current densities for SFO must be due to huge volume changes of SnO\({}_{2}\) in the charge/discharge processes, which causes it to not be in close contact with each other, leading to the much increased resistance, difficult electron transfer, and electrochemical reactions. All in all, the improved electrochemical performance can be ascribed to advantages of nanomaterials and intercalation structure. To further grasp the origins of difference in cyclic stability between the SFO@C-400 and SFO electrodes, their differential charge capacity (dQ/dV) plots (DCCs) for the different cycles at a current density of 0.2 A/g are evaluated to characterize the de-lithiations of the SnO\({}_{2}\) material implicating dealloying and de-conversion reactions in diverse potential ranges. Thus, its charge capacity is reasonably divided into two portions on the basis of two potential ranges, that is, 0.005-1.0 V and 1.0-3 V. In the first cycle, the foregoing plots of both electrodes (Figure 5a,b) have the distinguished sharp peaks at 0.5 V and several wide bands nearby 1.25, 2.0, and 2.8 V, with ones below 1V belonging to the Li\({}_{x}\)Sn dealloying into Sn and the rest above 1V referring to the de-conversion reactions of Sn to SnO\({}_{2}\), as analyzed in CV curves. The great integral intensity of the peaks in 1-3 V unveils the elevated electroactivity of the reversed conversion of Sn into SnO\({}_{2}\) in the initial cycles. It elaborates that most of Sn have reacted with Li to generate Li\({}_{x}\)Sn phases. Although the DCC peaks of SFO@C-400 (Figure 5a) in the range of 1.0-2.5 V, become weakened to some extent after 50 cycles, they are still clearly visible with substantial intensities, suggesting that the majority of Sn metals can reversibly react with Li\({}_{2}\)O to form the SnO\({}_{2}\) nanocrystals during the long-term cycling. Besides, the DCC peaks at about 0.5 keep strong and split into two ones and progressively move toward larger potentials, meaning the growing polarization for the Li\({}_{x}\)Sn dealloying.