# Intercalating Sn/Fe Nanoparticles in Compact Carbon Monolith for Enhanced Lithium Ion Storage
## 1 Introduction  
Lithium ion batteries (LIBs) presently prevail among the power source markets for the portable electronics and constitute the most fascinating clean-energy technology for the flourishing large-scale stationary energy storage, smart grids, and vehicular electrics. However, traditional LIBs (depending upon the insertion composite substances, including the graphite-positive electrode and lithium transition metal oxide or phosphate negative electrodes) have reached their inherent ceiling regardingspecific energy (roughly 210 Wh kg\({}^{-1}\)), which falls short of satiating the ever-increasing requirement of ever-larger energy and power. To pursue the novel higher-capacity electrode substances owning the stable lifetime to supplant the traditional graphite anodes (low capacity of 372 mA h g\({}^{-1}\)) is in high demand for LIBs [1, 2, 3, 4, 5]. Both the alloy-type and conversion-type substances feature the rather great theoretical capacities as anodes because of the multiple electron reactions and transportations [6, 7, 8, 9]. Among them, the tin dioxides (SnO\({}_{2}\)) emerge as one most potential anode alternative, given its very low discharge potential versus Li/Li\({}^{+}\) and great capacity contributed by two electrochemical events of (de-)alloying and (de-)conversion, as expressed [10, 11, 12]:  
\[\text{SnO}_{2}+4\text{Li}^{+}+4\text{e}^{-}\rightarrow\text{Sn}+2\text{Li}_{2}\text {O} \tag{1}\]  
\[\text{Sn}+4.4\text{Li}^{+}+4.4\text{e}^{-}\leftrightarrow\text{Sn}\text{Li}_{ 4.4} \tag{2}\]  
Production of an electrochemically passive and resistive Li\({}_{2}\)O component during the first conversion stage described by Equation (1) can deprive an irreversible capacity up to 731 mA h g\({}^{-1}\) during initial lithiation events, inducing a limited initial Coulombic efficiency (\(<50\%\)) [13, 14]. Though, the second alloying process (Equation (2)) can deliver a high theoretical reversible capacity of 783 mA h g\({}^{-1}\), an extremely serious volume swelling beyond 300% ensues, hence thwarting its practical usage [15, 16, 17]. Electrochemical lithiation triggers generation of boundary areas between Li-rich Li\({}_{x}\)Sn and Sn crystals and resultantly yields the stress reinforcement. The gain in the strain energy engenders an anisotropic expansion and exotic volume fluctuation, giving rise to some parasitic aftereffects, including the grim surface fracture, even electrode pulverization, and electrical disconnection with the current collector. The phenomena supra re-expose the fresh Sn surfaces to electrolyte, calling forth a continuous rise in the solid electrolyte interface (SEI) tier thickness over the prolonged battery cycling and eventually an acute battery performance deterioration [18, 19, 20, 21]. Pursuant to the rationale that structural strains can come under assuagement via the particulate downscaling toward the nanometer, a threshold dimension for the fracture reproduction and spread has turned out to be 150 nm [18, 19, 20, 21]. Thus, multitudinous elaborate structural modification strategies (via delicate nanotechnology) involving generating the sophisticated nanostructures and hybridizing with a powerful surface coating (carbon, SiO\({}_{x}\), TiO\({}_{2}\), SiOC, and so forth) have been universally adopted to fulfil the buffering of structural strains of SnO\({}_{2}\) and improve the ICE in the course of (de-)lithiation [22, 23, 24, 25, 26, 27, 28]. Typically, signally cushioning the volume expansion and enhancing the cyclic stabilities have come true by constructing the well-designed fine structures ranging from 1D to 3D, for example, SnO\({}_{2}\) nanotubes, SnO\({}_{2}\) hollow nanospheres, Sn/SnO\({}_{2}\) yolk-shell configurations, hierarchically porous organizations, and so on.