Nowadays, it is a global trend to develop EVs e starting
from hybrid EVs to plug-in EVs and ultimately to pure EVs e
which use state-of-the-art lithium-ion batteries for propulsion,
as a measure to reduce the gasoline consumption and mitigate
CO2 emission [7]. However, due to the high cost and insufficient
energy density of current EV batteries, EVs account for a
small fraction of the car market. Currently, EVs powered with
lithium-ion batteries have a driving range limited to 160 km
upon a single charge [8]. Tremendous research efforts have
been dedicated to increase the energy density of lithium-ion
batteries. However, the energy density of lithium-ion batteries
is around 100e200 Wh kg1, which cannot achieve the
long term goal in term of EVs [9]. Therefore, novel energy
storage systems with higher energy density are extremely
desired.