Octahedral tin dioxidenanocrystals as high capacity anode materials for Na-ion batteries

Abstract

Single crystalline SnO₂ nanocrystals (∼60 nm in size) with a uniform octahedral shape were synthesised using a hydrothermal method. Their phase and morphology were characterized by XRD and FESEM observation. TEM and HRTEM analyses identified that SnO₂ octahedral nanocrystals grow along the [001] direction, consisting of dominantly exposed {221} high energy facets. When applied as anode materials for Na-ion batteries, SnO₂ nanocrystals exhibited high reversible sodium storage capacity and excellent cyclability (432 mA h g⁻¹ after 100 cycles). In particular, SnO₂ nanocrystals also demonstrated a good high rate performance. Ex situ TEM analysis revealed the reaction mechanism of SnO₂ nanocrystals for reversible Na ion storage. It was found that Na ions first insert into SnO₂ crystals at the high voltage plateau (from 3 V to ∼0.8 V), and that the exposed (1 × 1) tunnel-structure could facilitate the initial insertion of Na ions. Subsequently, Na ions react with SnO₂ to form Na_xSn alloys and Na₂O in the low voltage range (from ∼0.8 V to 0.01 V). The superior cyclability of SnO₂ nanocrystals could be mainly ascribed to the reversible Na–Sn alloying and de-alloying reactions. Furthermore, the reduced Na₂O “matrix” may help retard the aggregation of tin nanocrystals, leading to an enhanced electrochemical performance.