In-Situ Synthesis of Sn/SnO2@C Composites for Lithium and Sodium-Ion Batteries
Authors: Cha Younghwan , Dong Panpan , Zhang Xiahui , Song Min-Kyu 
Published: February 2020

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 oxides5-7, carbon nanofibers8,9, carbon nanotubes10,11, and disordered carbons12,13 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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