A supercapacitor with graphene-based electrodes was found to exhibit a specific energy density of 85.6 Wh/kg at room temperature and 136 Wh/kg at 80 °C (all based on the total electrode weight), measured at a current density of 1 A/g. These energy density values are comparable to that of the Ni metal hydride battery, but the supercapacitor can be charged or discharged in seconds or minutes. The key to success was the ability to make full utilization of the highest intrinsic surface capacitance and specific surface area of single-layer graphene by preparing curved graphene sheets that will not restack face-to-face. The curved morphology enables the formation of mesopores accessible to and wettable by environmentally benign ionic liquids capable of operating at a voltage >4 V.

Hollow tin dioxide (SnO2) microspheres were synthesized by the simple heat treatment of a mixture composed of tin(IV) tetrachloride pentahydrate (SnCl4·5H2O) and resorcinol–formaldehyde gel (RF gel). Because hollow structures were formed during the heat treatment, the pre‐formation of template and the adsorption of target precursor on template are unnecessary in the current method, leading to simplified synthetic procedures and facilitating mass production. Field‐emission scanning electron microscopy (FE‐SEM) images showed 1.7–2.5 μm sized hollow spherical particles. Transmission electron microscopy (TEM) images showed that the produced spherical particles are composed of a hollow inner cavity and thin outer shell. When the hollow SnO2 microspheres were used as a lithium‐battery anode, they exhibited extraordinarily high discharge capacities and coulombic efficiency. The reported synthetic procedure is straightforward and inexpensive, and consequently can be readily adopted to produce large quantities of hollow SnO2 microspheres. This straightforward approach can be extended for the synthesis of other hollow microspheres including those obtained from ZrO2 and ZrO2/CeO2 solid solutions.

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b S Supporting Information L ithium ion batteries and electrochemical capacitors (supercapacitors), separately or in combination, are being considered for electric vehicle (EV), renewable energy storage, and smart grid applications. 1À5 A major scientific challenge is to either significantly increase the energy density of conventional supercapacitors or dramatically improve the power density of lithium ion batteries. 2,3 Supercapacitors work on two main charge storage mechanisms: surface ion adsorption (electric double layer capacitance, EDL) and redox reactions (pseudocapacitance). 1,2 Compared with batteries, supercapacitors deliver a higher power density, offer a much higher cycle-life, need a very simple charging circuit, and are generally much safer. However, supercapacitors exhibit very low energy densities (e.g., 5 Wh/kg cell for commercial activated carbon-based supercapacitors versus 100À150 Wh/ kg cell for the commercial Li-ion battery, all based on the total cell weight). 2 Previous attempts to increase the gravimetric energy of supercapacitors have included the use of electrode materials with enhanced gravimetric capacitances 6 or the pseudocapacitance provided by nanostructured transition metal oxides. 7À9 However, the prohibitively high cost of ruthenium-based oxides and the cycling instability of manganese-based oxides 9À11 have impeded the commercial application of these supercapacitors.In 2006, our research group reported graphene-based electrodes for both EDL and redox supercapacitors, 12 which has since become a topic of intensive research. 13À19 Significant progress has been made using pseudocapacitance (e.g., redox pairs between graphene oxideÀMnO 2 16 or grapheneÀpolyaniline 17,18 ) and ionic liquid electrolyte with a high operating voltage 13,19 for improved energy density. However, these supercapacitors have yet to exhibit a sufficiently high energy density or power density for EV and renewable energy applications.Lithium-ion batteries operate on Faradaic reactions in the bulk of the active material. This bulk storage mechanism provides a much higher energy density (120À150 Wh/kg cell ) as compared to supercapacitors. However, storing lithium in the bulk of a material implies that lithium must leave the interior of a cathode active particle and eventually enter the bulk of an anode active particle during recharge, and vice versa during discharge. Because of the extremely low solid-state diffusion rates, these processes are kinetics-limited. As a result, lithium ion batteries deliver a very low power density (100À1000 W/kg cell ), requiring typically hours for recharge.Several efforts have been made to increase the power characteristics of lithium-ion batteries by reducing the dimensions of lithium storage materials down to the nanometer scale, which would reduce the lithium diffusion time. 20À23 However, nanostructured lithium storage electrodes (e.g., nanoparticles of lithium titanate anode or lithium iron phosphate cathode) are still not capable of delivering a power density comparable ...