Carbon coating step in silicon (Si) anode research has been considered as a necessity in order to enhance cycling performance by increasing electronic conductivity and bolstering stable Solid-Electrolyte Interphase (SEI) layer between Si host and liquid electrolytes^1-5. Many strategies have been employed to deposit carbon layer on the surface of Si, including chemical vapor deposition (CVD), sol-gel/carbonization method, etc. Although those processes can offer uniform coating on diverse types of Si nanostructures (nanowires, nanotubes, and nanoparticles) with improved battery performance, they require costly procedures; sophisticated equipment and, oftentimes, toxic precursors, that make the overall fabrication process unsuitable to scale up and advance towards commercialization.

In this work, we demonstrate a novel strategy that can abridge the additional carbon coating step and yet simultaneously obtain carbon coating and porous structure on Si host via simple/one-pot thermal treatment where CO₂ is reduced down to carbon by inorganic magnesium silicide (Mg₂Si). Based on the Ellingham diagram which describes oxidative nature of pure metallic elements, Mg has a strong tendency of reducing other oxygen-containing molecules, such as CO₂ or silica (SiO₂). The Gibbs free energy of the CO₂ reduction reaction with Mg₂Si is estimated to be -519 kJ/mol at 700 ^oC, indicating a thermodynamically feasible process. During the thermal treatment, CO₂ gas and Mg₂Si powder undergo spontaneous redox reaction and result in composite of MgO, Si, and C, confirmed by in-situ high temperature X-ray diffraction analysis. In this redox reaction, element Mg act as a reductant that takes O atoms away from CO₂ molecules, leaving elemental carbon layer between MgO and Si. After the MgO removal (sacrificing template for porous structure) via acid leaching, microstructural characterization reveals porous structures in Si host and uniform carbon layers (thickness: 20 ~ 40 nm) on the Si surface. Most importantly, obtaining micron-sized Si with macro-porous structures, the tap density of the material is 3~4 times higher than commercially available nano Si powder, which is greatly beneficial to develop Li-ion batteries with high volumetric densities. Cycling performance tests with CR2032-type coin cell (vs. Li/Li⁺) also show high initial coulombic efficiency (85~88%), good rate capability (>1000 mAh/g at 1.0 C-rate), and excellent long-term cycling performance (~1300 mAh/g after 300 cycles at 0.5 C-rate & ~1000 mAh/g after 500 cycles at 1.0 C-rate).

1 Lu, Z. et al. Nonfilling Carbon Coating of Porous Silicon Micrometer-Sized Particles for High-Performance Lithium Battery Anodes. ACS Nano 9, 2540-2547, doi:10.1021/nn505410q (2015).

2 de Guzman, R. C., Yang, J., Cheng, M. M.-C., Salley, S. O. & Simon Ng, K. Y. Effects of graphene and carbon coating modifications on electrochemical performance of silicon nanoparticle/graphene composite anode. Journal of Power Sources 246, 335-345, doi:http://dx.doi.org/10.1016/j.jpowsour.2013.07.100 (2014).

3 Shen, L., Wang, Z. & Chen, L. Carbon-coated hierarchically porous silicon as anode material for lithium ion batteries. RSC Advances 4, 15314-15318, doi:10.1039/C4RA01255K (2014).

4 Yi, R., Dai, F., Gordin, M. L., Sohn, H. & Wang, D. Influence of Silicon Nanoscale Building Blocks Size and Carbon Coating on the Performance of Micro-Sized Si–C Composite Li-Ion Anodes. Advanced Energy Materials 3, 1507-1515, doi:10.1002/aenm.201300496 (2013).

5 Gao, P., Tang, H., Xing, A. & Bao, Z. Porous silicon from the magnesiothermic reaction as a high-performance anode material for lithium ion battery applications. Electrochimica Acta 228, 545-552, doi:http://dx.doi.org/10.1016/j.electacta.2017.01.119 (2017).