# Intercalating Sn/Fe Nanoparticles in Compact Carbon Monolith for Enhanced Lithium Ion Storage
## 3 Results and Discussion
Instead of being completely coated, these nanoparticles closely connect with the carbon matrix or get partially embedded inside the carbon layers, inducing a unique metal particles-in-carbon sandwich-like geometry. This distinct metal-intercalated configuration can be further evidenced by HRTEM image of Figure 2f, where the metal nanoparticles are evenly intercalated between carbon layers. The average particle size is about 5-10 nm, in excellent consistency with XRD results. Two sets of well-defined crystalline facet fringes with the marked lattice inter-spacings of 0.33 and 0.27 nm are clearly observed, which belong to the (110) facet of SnO2 and (104) plane of Fe2O3, respectively. More importantly, SnO2 and Fe2O3 are in intimate contact. These closely distributed/embedded metal oxide nanoparticles and the typical carbon layers must result in ample robust heterointerfaces or heterojunctions between two metal oxide nanoparticles (as highlighted by yellow dash lines across these joint particle boundary in Figure 2f), which can bridge these two nanocrystals and unleash huge possibility to create the synergic effect (particularly the internal potentials) during (de-)lithiation for the much enhanced electrochemical performance [41; 42; 43; 44; 45; 46; 47; 48]. Furthermore, the worm-like entities intimately surrounding the nanoparticles with both the high transparency and indistinct crystallographic diffraction stripes represent non-crystallized carbon tiers, accordant to the phase structures verified by XRD and Raman tools. Such a finding, indeed, signifies that the nanocrystals constructing the SnO2/Fe2O3@C micro-particulates are together conjoined via the strong chemical bonds in place of the physical stacking so that this metal-in-carbon geometry can be reinforced to much great extent. In this case, after lithiation, every SnO2 particle would change to a single/separate district constituted by Sn and Li2O. Even if the dimension of the initially generated Sn phases within such 5-10 nm SnO2 crystals is considerably smaller than 5-10 nm, they will endlessly grow and become bigger than the initial size of SnO2 in that the Sn is unable to diffuse from other adjacent Sn-Li2O zones. As a result, the lithiation-triggered coarsening of the Sn crystals can be avoidable within this SFO@C electrode. Moreover, no pronounced microporous characteristics can be found in these amorphous carbon regions, further corroborating the extremely small BET specific surface area. Such morphological uniqueness and structural uniformity may tolerate the large mechanic strains of the Sn nanoparticles, to alleviate the volume variation of SnO2, inhibit their pulverization and agglomeration during (de-)lithiation, and facilitate the electron transfer between the active materials, thereby giving rise to the improved rate capability and prolonged cycle stabilities. Figure 2: **(a)** SEM images of (a) SFO@C-350, (**b**) SFO@C-400, (**c**) SFO@C-450, and (**d**) SFO samples; (**e**) TEM images of SFO@C-400 sample and (**f**) corresponding HRTEM image. The surface electronic structure and chemical element valance state of the typical SFO@C-400 material was explored by utilizing the X-ray photoelectron spectroscopy (XPS), as delineated in Figure 3. In the survey-scan spectrum (Figure 3a), the noticeable peaks related with C, Sn, Fe, and O elements are detected, explicitly validating the co-presence of the C, Sn, Fe, and O elements in the composite. The fine spectrum of C1s core emission (Figure 3b) can be compartmentalized into three evident components, sourced from the sp\({}^{2}\)-hybridized graphitic C atoms of C-C/C=C (284.6 eV), sp\({}^{3}\)-hybridized C atoms of epoxy groups (C-O, 286 eV), and functional groups on the carbon surfaces such as carbonyl groups (C=O, 288.6 eV) [55]. The high-resolution Sn 3d spectrum (Figure 3c) clarifies the chemical bonding state of Sn, with a pair of strong envelops positioned at 487.6 and 495.8 eV being indexed to Sn 3d\({}_{5/2}\) and Sn 3d\({}_{3/2}\) for Sn\({}^{4+}\) of SnO\({}_{2}\), respectively [56].