5a shows
the galvanostatic discharge-charge voltage curves of the nonsupported
SnO2 nanowire arrays for the first, the second and the
sixtieth cycles at a constant current density of 160mAg1 within a
cut-off voltage window of 0.01–1.2V. Two obvious slope regions
can be observed in the first discharge process with a high initial
capacity of 1983mA h g1, which is mainly attributed to the
reduction of SnO2 to Sn. In addition, the unique structure of the
SnO2 nanowire arrays with mesoporous interspaces provides large
amount of reactive sites and interface between active materials
and the electrolyte, making the Li+ diffusion and charge transport
much easier by shortening the diffusion length, which is also
favorable to the storage of lithium ion. The subsequent charge
curve shows a capacity of 652mA h g1 up to 1.2V. Such an initial
irreversible loss of 67.1% is mainly caused by the irreversible
reduction of SnO2 to Sn and the formation of a solid electrolyte
interface (SEI) on the surface of the active materials. For the second
cycle, the non-supported SnO2 nanowire arrays exhibit a high
discharge capacity of 792mA h g1 and a charge capacity of
640mA h g1, showing a higher Coulombic efficiency of 80.8%.
However, a relatively low discharge capacity of 403mA h g1 is
retained after 60 cycles due to the volume variation, structural
degradation and aggregation of non-supported SnO2 nanowires
upon insertion/extraction of lithium ions.