As the development of electric vehicles (EVs) is progressing recently, it is urgent to develop high energy density batteries. Currently, Li-ion batteries (LIBs) have been mainly used for EVs. In LIBs, the most representative commercial anode material for LIBs is graphite. Unfortunately, its low theoretical capacity (372 mAh g⁻¹) and rate capability is not sufficient to be used in emerging applications, such as EVs and large-scale ESS. Si is a promising anode material for LIBs due to its large theoretical capacity (3600 mAh g^-1 for Li₁₅Si₄), low working potential (< 0.4 V vs. Li/Li⁺), low cost, large amount resources, low toxicity, and environmental friendliness. However, Si based anodes in LIBs have several drawbacks. During alloying with Li, Si exhibits large volume expansion of >300%, leading to crush of the active materials and capacity drop. Si electrode also has low intrinsic electrical conductivity compared with graphite. In addition, solid electrolyte interface (SEI) layer is not stable when Si is exposed to organic electrolytes, leading to the consumption of electrolyte and low Coulombic efficiency. Recently, studies on using a buffer layer coating have been conducted to alleviate the stress induced by volume change of Si. Recently, we reported an ultra-simple Mg-thermal-reduction method for the production of mass-scalable coral-like Si powders with a high surface area, and 3-dimensionally (3D)-interconnected Si structures. It is noticeable that simple Mg-reduction process can be used at low cost in ambient air.

In the present work, we employed carbon coating to improve the conductivity of the previously obtained porous nano-sized Si electrode and the stability of SEI layer. We prepared a nano-porous Si/C composite through a facile sol-gel route with various amounts of carbon from citric acid precursor. The morphology and phase purity of the synthesized Si/C composites were analyzed by XRD, FE-SEM, TEM, Raman spectroscopy, and BET analysis. Furthermore, electrochemical characteristics of Si/C electrode was analyzed by electrochemical impedance spectroscopy (EIS), cyclic voltammetry, and galvanostatic charge-discharge test. The Si/C electrode exhibited a long cycling stability, good rate capability, and high reversible capacity. In particular, the Coulombic efficiency in the first cycle was improved significantly. This superior performance of the electrode is attributed to the improvement in electrical conductivity, Li-ion accessibility, stability of SEI layer, and the suppression of volume expansion owing to porous nano-sized Si and carbon coating.