# Intercalating Sn/Fe Nanoparticles in Compact Carbon Monolith for Enhanced Lithium Ion Storage
## 3 Results and Discussion
The peak separation or distance between these two bands is about 8.2 eV, which is very consistent with the splitting binding energy of Sn(IV) ion (indexed Standard ESCA Spectra of the Elements and Line Energy Information, \(\Phi\) Co., USA). It is noteworthy that these binding energy values are somewhat smaller than the counterparts reported before, and this most possibly results from fewer oxygen neighbors around Sn on average because of a certain reduction of SnO\({}_{2}\) over surfaces by the closely contacted amorphous carbon [57]. This shrunken binding energy for Sn\({}^{4+}\) also mirrors the strong interaction between SnO\({}_{2}\) and carbon, further supporting that the Sn metal nanoparticles are strongly bonded to carbon layers instead of the simple physical stacking, as analyzed from TEM images. The narrow-scan Fe2p core-level spectrum (Figure 3d) consists of two prominent spin-orbit doublets emerging at around 712.1 and 716.8 eV, which are relevant to the characteristic Fe\({}^{3+}\) 2p\({}_{3/2}\) and Fe\({}^{3+}\) 2p\({}_{1/2}\), respectively [43]. Overall, the data imply the coexistence of Sn\({}^{4+}\) and Fe\({}^{3+}\) in SFO@C-400, and further manifest that the Sn and Fe atoms are in the form of SnO\({}_{2}\) and Fe\({}_{2}\)O\({}_{3}\), respectively. The galvanostatic charge/discharge measurement in the range of 0.005-3.0 V (vs. Li/Li\({}^{+}\)) was carried out in coin-type cells with Li counter electrodes to evaluate the electrochemical performance of all samples. Figure 4a presents the initial discharge/charge curves of the SFO@C-350, SFO@C-400, SFO@C-450, and SFO electrodes, which state representative characteristics of the Sn-carbon composite  
Figure 3: XPS spectra of SFO@C-400 sample: (**a**) survey spectrum, (**b**) C 1s, (**c**) Sn 3d, and (**d**) Fe 2p. anodes. In the discharge profile, the voltage declines rapidly below 1.2 V and expresses a discharge plateaus at 0.8-1.2 V, linked with reduction of SnO\({}_{2}\) to Sn. The discharge plateaus spanning from 0.3 to 0.5 V deal with the Li-Sn alloying toward creation of Li\({}_{x}\)Sn alloy, which accords with the CV plots analyzed later [33; 34]. Correspondingly, the charge profile also shows two sloping charge plateaus located at about 0.5 and 1.25 V, which are attributed to dealloying of Li\({}_{x}\)Sn and the reversible reaction of Sn and Li\({}_{2}\)O to SnO\({}_{2}\), which can be more clearly confirmed by differential capacity (dQ/dV) curves, as seen in Figure 5. Note that, as a result of relatively sluggish Li\({}^{+}\) diffusion kinetics within SnO\({}_{2}\), the potential plateaus of conversion reaction shifts to below 1.0 V, triggering off a lower capacity produced between open current voltage and 1.0 V during the discharging. Under such a circumstance, the portion of the conversion process and alloying event superposes at a potential smaller than 1.0 V, inducing a considerably greater capacity than the value beyond 1.0 V. Nevertheless, the overpotential will enable the re-conversion event occurring at potential above 1.0 V in the course of charging, bringing about a bigger capacity at 1.0-3.0 V in the course of discharging. Thus, the SFO@C-350, SFO@C-400, SFO@C-450, and SFO electrodes deliver an initial discharge/charge capacity of 1100/723, 1301/876, 1468/955, and 1353/842 mAh g\({}^{-1}\), yielding an initial coulombic efficiency (ICE) of roughly 65.7%, 67.3%, 65%, and 62.2% for SFO@C-350, SFO@C-400, SFO@C-450, and SFO, respectively. Albeit still smaller than the pre-lithiation transition metal oxides like Li\({}_{3}\)V(MoO\({}_{4}\))\({}_{3}\) or pre-lithiation SnO\({}_{2}\) foil [58; 59], the ICE values here surpass these of multitudes of tailor-made SnO\({}_{2}\)-based hybrid materials (\(<50\%\)) reported so far.