After activation, the nano-Si anode

After activation, the nano-Si anode can deliver a reversible
capacity of 1120 mA h g21 at 1C (4.2 A g21) and 644 mA h g21 at 2C
(8.4 A g21) rates, respectively (Supplementary Fig. S12). Moreover,
the high internal porosity enables the volume expansion of Si without
rupturing the solid-electrolyte interphase (SEI) at the outer surface.
Instead of repetitively breaking and reforming SEI during extended
cycling, a thin and stable SEI conformally covers the surface of the
nano-Si electrode (Supplementary Fig. S11a), which results in high
reversibility and therefore good cycling stability22,37. Coulombic efficiency
(CE), an indicator of SEI formation and stability in anodes, is
as high as 99.3% when averaged from the 100th to the 300th cycle
(Fig. 4a). The first cycle CE is,70% because the initial SEI formation
and the lithiation of the native oxide consumes some lithium; this
value could be increased by performing prelithiation38.