# Intercalating Sn/Fe Nanoparticles in Compact Carbon Monolith for Enhanced Lithium Ion Storage
## 3 Results and Discussion
The comparatively low ICE is mainly ascribed to the formation of the solid electrolyte interface (SEI) film and the irreversible part of the conversion reactions of SnO\({}_{2}\) and Li because the increasing surface areas of electrodes supply the sufficient contact areas between electrode and electrolyte and consume the increasing quantity of Li ions [60]. This much-improved ICE of our system expresses that the binary metal-in-carbon compact monolith bears stronger capability to confine and stabilize the Sn/Li\({}_{2}\)O nanocrystals against coarsening and hence shun the adverse side reactions (e.g., the irreversible conversion events between Sn/SnO\({}_{2}\) and Li\({}^{+}\) ions and the electrolyte dissociation reaction) to promote the conversion reaction reversibility, in good line with the CV data. Interestingly, ICE of SFO@C-350, SFO@C-400, and SFO@C-450 are all higher than that of the SFO electrode, which could be attributed to the fact that the very low surface area of SFO@C-350, SFO@C-400, and SFO@C-450 reduces the irreversible capacity loss caused by formation of a thick SEI film. Moreover, coulombic efficiency of SFO@C-400 rapidly increases to 92.8% and maintains over 98% in subsequent cycles, suggesting a high reversible capacity. To probe cyclability and Li\({}^{+}\) insertion/extraction processes, Figure 4b describes CV characteristics for the initial three consecutive cycles of the SFO@C-400 electrode at a sweep rate of 0.1 mV s\({}^{-1}\) from 0.005 to 3.0 V, to scrutinize the electrochemical reaction pathways. All CV curves basically reproduce the analogous electrochemical behaviors. In the first sweep, several irreversible peaks are detected. According to the prior reports, during the first discharging stage, the predominant cathodic peak observed at 1.41 V can pertain to the reduction of SnO\({}_{2}\) and Fe\({}_{2}\)O\({}_{3}\) to metallic Sn and/or Fe (conversion reaction), accompanied by formation of Li\({}_{2}\)O phases that act as buffer matrix to mitigate the volume fluctuation for the subsequent alloying reaction [32; 41; 42; 43; 44]. Upon further discharging, the subsequent peak at 0.77 V is ascribed to production of a solid electrolyte interface (SEI) film (related to electrolyte decomposition). With a continuous cathodic sweeping, the following broad cathodic peak at \(<\)0.7 V originates from the ensuing alloying process of Li\({}^{+}\) inserting into Sn to form the Li-Sn compound due to the multistep lithiation mechanism, in which case the Sn comes from the previously formed Sn via the reduction of SnO\({}_{2}\). Some rather weak redox peaks at high voltages such as \(\sim\)2.4 V plausibly derive from the Li insertion/extraction from oxygen-containing functional groups on carbon [61]. After the initial cycle, it can be clearly found that almost no apparent changes take place in the reduction peaks, indicating that the irreversible reaction primarily exists during the first cycle, and creation of the solid electrolyte interphase (SEI) film on the electrode surfaces merely arises in the first cycle. It needs to be stressed that, because of the electrochemical inertness between Fe and lithium, Fe works as inactive buffer toward regulating the thickness of the SEI layer and securing the nanoparticles' structural alteration [43]. Notwithstanding, from a second scan onward, all (de-)lithiation events progressively turn reversible. During the following anodic scan process (the charging step), these first cathodic peaks are replaced by two strong anodic/oxidation peaks at 0.6 and 1.3 V, which are sequentially caused due to the phase transition process of dealloying of various Li\({}_{x}\)Sn alloys to Sn phases and the partially reversible conversion reaction of Sn and Li\({}_{2}\)O to yield the SnO\({}_{2}\) nanocrystals [62]. The weak peak at 1.8 V is typical of insertion of Li\({}^{+}\) into Fe\({}_{2}\)O\({}_{3}\). The reduction peaks are less sharp than oxidation peaks, showing the slower reaction kinetics of alloying than dealloying.