# Intercalating Sn/Fe Nanoparticles in Compact Carbon Monolith for Enhanced Lithium Ion Storage
## 1 Introduction
However, aside from the synthetic expensiveness, cumbersomeness, low throughout, and eco-malignance, realistic implementation of preceding avenues still confronts grand challenges about both a small tap density in the range of \(<0.4\) g cm\({}^{-3}\) of foregoing architectures caused by abundant empty volumes and other morphobstructural caveats [29, 30]. Worse still, protecting the pre-engineered void spaces militates against their virtues during tight calendaring procedures in that an overwhelming majority of the pores and shells will get totally impaired [31]. Till now, assembling the compact secondary layouts of inactive/active buffers by nanocomposites can largely augment tap density. Out of such dense configurations, a conductive and elastic carbon-based matrix has aroused tremendous interests to effectively relax the mechanic strains to mitigate the volume evolutions and curb the structural cracking of Sn [32]. Even if integration of the nanometer SnO\({}_{2}\) with carbon coating can elongate cyclic longevity via inhibiting the capacity decay, the substantial initial irreversible capacity degeneration still takes place, appreciably whittling down the coulombic efficiency. Because Li\({}_{2}\)O hardly participates in the reverse process of Equation (1), it makes a great difference to foil the Sn coarsening during lithiation, restrain the interparticle migration between Sn and Li\({}_{2}\)O, and manipulate the reversibility of the conversion reaction [33, 34, 35, 36]. In this context, building the multicomponent hybrids or intermetallic alloys to ease structural strains of Sn has been postulated, where nanoscale alien metals can impose multiple effects on the Li ion chemistry of SnO\({}_{2}\)[14; 32; 37; 38; 39; 40]. Particularly, in addition to serving as spacers/barriers against the diffusion/agglomeration of Sn in the hybrids, Fe can function like an efficient catalyst to accelerate the Li\({}_{2}\)O decomposition, leading SnO\({}_{2}\) to reversibly form through the backward process of Equation (1). Studies further evidence that Fe in intimate contact with Sn can form heterostructures with the crystal-mode-controlled charge transport as a result of the superior interface effects and of generation of built-in potentials (internal electric field) around hetero-interfaces [32; 41; 42; 43; 44; 45; 46; 47; 48]. This multicomponent synergy can notably aid in mitigating the structural strains to stabilize the morphological integrity of active materials, expediting the reaction dynamics and ion/electron transfer, and thus consolidating the electrochemical behavior (particularly the rate capability). Based upon the viewpoints above, coupling the advantages of the compact structures and the multicomponent collaboration (for energy storage on Sn-based materials) to construct a compatible Sn-Fe/carbon-based matrix composite with more stable structure and faster conductivity is one of most promising strategies toward coping with the microstructural defect issues of SnO\({}_{2}\) anodes, especially given that the carbon substrate is able to extra height electronic connection/conduction of the overall electrode. Herein, we show the fabrication and improved (de-)lithiation behavior of a novel anode material based upon the ultrasmall (sub-10 nm) SnO\({}_{2}\)/Fe\({}_{2}\)O\({}_{3}\) binary nanoparticles uniformly/densely implanted inside a micro-sized compact carbon skeleton. When we used a scalable and straightforward gel pyrolysis approach, the xerogels comprising the binary metal ion-homogeneously coordinated adducts produced via polymerizing a clear gel of citrate and metal salts experienced a thermal crystallization in N\({}_{2}\), at a fixed temperature, to result in the dense metal nanoparticles-intercalated carbon monolith, which excels in delivering the electrons/ions and modulating the structural stress during Li ion storages.