SnOx/Carbon Core Shell Structure: An Efficient Anode for Rechargeable Sodium Ion Batteries

Lithium-ion batteries have been widely used as energy storage media in portable electronic devices, electrical vehicles, hybrid electrical vehicles, etc. In particular, they have become indispensable energy storage devices for intermittent energy conversion applications such as solar cells and wind power. However, the low abundance and uneven distribution of lithium resources show the potential difficulties of the long-term and large-scale applications of lithium-ion batteries in aspects of their availability and cost. Accordingly, the development of new types of batteries, such as sodium-ion and magnesium-ion batteries, is necessary. Among them, sodium (Na)-ion batteries (NIBs) possess electrochemical working principles that are similar to LIBs. In addition, sodium is inexpensive and abundant. Therefore, NIBs could substitute LIBs in applications such as smart grids and large-scale energy storage for renewable solar power and wind power.

The major obstacle in realizing NIBs has been the absence of suitable anodes. Most of all, graphite which has been a commercially well-known anode material for LIBs cannot be used as an insertion host for Na ions due to their large ionic size. Moreover, only few noticeable studies on negative electrode materials, e.g., lead and hard carbon have been reported thus far. Metal oxides are considered to be important electrode materials, as they follow a conversion reaction mechanism. However, metal oxides suffer from several problems when used as anode materials which limit their use, such as swelling and shrinking of active material particles upon the insertion and extraction of Li⁺ or Na⁺ ions, which can induce poor contact between the active materials and the electron conducting agents, leading to low electric and ionic conductivity.

In this study, we report the electrochemical activity of carbon coated SnO_x nanostructures for NIBs. The nanostructured SnO_x were synthesized by a hydrothermal method and their surfaces were coated by carbon to improve the electric conductivity. The effect of solvent used for hydrothermal process and annealing temperature on the structure, morphology and Na storage capacity was investigated. The single phase SnO₂ was obtained using ethanol solvent, while the blend of SnO and SnO₂ was obtained using DI water and ethylene glycol solvents. The transmission electron microscopic images conform the presence of carbon coating on the SnO_x nanoparticles. Further, the anodes made of carbon coated SnO_x prepared in ethanol solvent exhibited stable cycling performance and attained the capacity of about 443 mAh/g in the first charge. With the help of the conductive carbon, the carbon coated SnO₂ delivered more capacity at high rates; 257 mAh/g at the 1 C-rate, 156 mAh/g at the 5 C-rate and 70 mAh/g at the 10 C-rate. The excellent cyclability and high-rate capability are attributed to the formation of a mixed conducting network and the uniform carbon coating formed on the SnO₂ nanoparticles.