Tin oxide (SnO₂) is considered as a promising material for both Li- and Na-ion batteries due to its high theoretical capacities (1494 mAh/g with Li and 1378 mAh/g with Na; 2~3 times higher than common carbon-based anode)^1-3. However, the initial irreversible capacity loss induced by inactive Li₂O/Na₂O formation and volume change (260% with Li and 420% with Na) during charge/discharge process need to be addressed in order to improve cycling performance⁴. In addition, the poor electronic conductivity of above mentioned alkali metal oxides and gradual aggregation of Sn particles in the electrode structure during operation lead to poor rate capability and rapid capacity fading. Many studies have strived to address these issues and result in the development of diverse types of SnO₂-based nanocomposites with carbonaceous materials including reduced graphene oxides^5-7, carbon nanofibers^8,9, carbon nanotubes^10,11, and disordered carbons^12,13 to enhance initial coulombic efficiency and electronic conductivity in the electrode structure as well as to prohibit Sn particle aggregation by introducing physical barriers between active materials. However, the cycling performance and initial irreversible capacity loss of the currently reported composites still remain insufficient to be adopted in practical cells.

In this work, carbon-coated porous Sn/SnO₂ composite (Sn/SnO₂@C) is synthesized via inorganic CO₂ reduction route with magnesium stannide (Mg₂Sn) for Li- and Na-ion batteries. High purity Mg₂Sn powder is prepared by solid-state reaction and then thermally treated under CO₂ flow environment. During the second heat treatment, gaseous CO₂ molecules becomes reduced down to elemental C via interaction with Mg which is known to be highly reductive in nature (Mg₂Sn + CO₂ à 2MgO + Sn + C, ΔG = -690 kJ/mol). The resultant Sn is partially oxidized to form SnO₂ which eventually results in Sn/SnO₂@C composite. Electrodes with this composition and structure exhibit enhanced initial coulombic efficiency and stable cycling performance. It appears that a nanocomposite matrix in which active materials are distributed without aggregation in intimate contact with C is essential for realizing SnO₂-based anodes for Li- and Na-ion batteries with long cycle life.

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2 Kim, Y., Yoon, Y. & Shin, D. Fabrication of Sn/SnO2 composite powder for anode of lithium ion battery by aerosol flame deposition. Journal of Analytical and Applied Pyrolysis 85, 557-560, doi:https://doi.org/10.1016/j.jaap.2008.06.005 (2009).

3 Lee, Y. et al. Hollow Sn–SnO2 Nanocrystal/Graphite Composites and Their Lithium Storage Properties. ACS Applied Materials & Interfaces 4, 3459-3464, doi:10.1021/am3005237 (2012).

4 Sivashanmugam, A. et al. Electrochemical behavior of Sn/SnO2 mixtures for use as anode in lithium rechargeable batteries. Journal of Power Sources 144, 197-203, doi:https://doi.org/10.1016/j.jpowsour.2004.12.047 (2005).

5 Hu, X., Zeng, G., Chen, J., Lu, C. & Wen, Z. 3D graphene network encapsulating SnO2 hollow spheres as a high-performance anode material for lithium-ion batteries. Journal of Materials Chemistry A 5, 4535-4542, doi:10.1039/C6TA10301D (2017).

6 Fan, L. et al. Controlled SnO2 Crystallinity Effectively Dominating Sodium Storage Performance. Advanced Energy Materials 6, 1502057-n/a, doi:10.1002/aenm.201502057 (2016).

7 Tian, R. et al. The effect of annealing on a 3D SnO2/graphene foam as an advanced lithium-ion battery anode. Scientific Reports 6, 19195, doi:10.1038/srep19195

https://www.nature.com/articles/srep19195#supplementary-information (2016).

8 Wang, M., Li, S., Zhang, Y. & Huang, J. Hierarchical SnO2/Carbon Nanofibrous Composite Derived from Cellulose Substance as Anode Material for Lithium-Ion Batteries. Chemistry – A European Journal 21, 16195-16202, doi:10.1002/chem.201502833 (2015).

9 Liu, Y. et al. Enhanced electrochemical performance of hybrid SnO2@MOx (M = Ni, Co, Mn) core-shell nanostructures grown on flexible carbon fibers as the supercapacitor electrode materials. Journal of Materials Chemistry A 3, 3676-3682, doi:10.1039/C4TA06339B (2015).

10 Cui, J. et al. Enhanced conversion reaction kinetics in low crystallinity SnO2/CNT anodes for Na-ion batteries. Journal of Materials Chemistry A 4, 10964-10973, doi:10.1039/C6TA03541H (2016).

11 Chen, S. et al. Branched CNT@SnO2 nanorods@carbon hierarchical heterostructures for lithium ion batteries with high reversibility and rate capability. Journal of Materials Chemistry A 2, 15582-15589, doi:10.1039/C4TA03218G (2014).

12 Fan, J. et al. Ordered, Nanostructured Tin-Based Oxides/Carbon Composite as the Negative-Electrode Material for Lithium-Ion Batteries. Advanced Materials 16, 1432-1436, doi:10.1002/adma.200400106 (2004).

13 Pol, V. G., Wen, J., Miller, D. J. & Thackeray, M. M. Sonochemical Deposition of Sn, SnO2 and Sb on Spherical Hard Carbon Electrodes for Li-Ion Batteries. Journal of The Electrochemical Society 161, A777-A782, doi:10.1149/2.064405jes (2014).