Lithium-sulfur (Li-S) battery system is endowed with tremendous energy density, resulting from the complex sulfur electrochemistry involving multielectron redox reactions and phase transformations. Originated from the slow redox kinetics of polysulfide intermediates, the flood of polysulfides in the batteries during cycling induced low sulfur utilization, severe polarization, low energy efficiency, deteriorated polysulfide shuttle, and short cycling life. Herein, sulfiphilic cobalt disulfide (CoS2) was incorporated into carbon/sulfur cathodes, introducing strong interaction between lithium polysulfides and CoS2 under working conditions. The interfaces between CoS2 and electrolyte served as strong adsorption and activation sites for polar polysulfides and therefore accelerated redox reactions of polysulfides. The high polysulfide reactivity not only guaranteed effective polarization mitigation and promoted energy efficiency by 10% but also promised high discharge capacity and stable cycling performance during 2000 cycles. A slow capacity decay rate of 0.034%/cycle at 2.0 C and a high initial capacity of 1368 mAh g(-1) at 0.5 C were achieved. Since the propelling redox reaction is not limited to Li-S system, we foresee the reported strategy herein can be applied in other high-power devices through the systems with controllable redox reactions.

We developed two-step solution-phase reactions to form hybrid materials of Mn 3 O 4 nanoparticles on reduced graphene oxide (RGO) sheets for lithium ion battery applications. Mn 3 O 4 nanoparticles grown selectively on RGO sheets over free particle growth in solution allowed for the electrically insulating Mn 3 O 4 nanoparticles wired up to a current collector through the underlying conducting graphene network. The Mn 3 O 4 nanoparticles formed on RGO show a high specific capacity up to ~900mAh/g near its theoretical capacity with good rate capability and cycling stability, owing to the intimate interactions between the graphene substrates and the Mn 3 O 4 nanoparticles grown atop. The Mn 3 O 4 /RGO hybrid could be a promising candidate material for high-capacity, low-cost, and environmentally friendly anode for lithium ion batteries. Our growth-on-graphene approach should offer a new technique for design and synthesis of battery electrodes based on highly insulating materials.

In order to achieve high energy and power densities, we developed a high-voltage asymmetric electrochemical capacitor (EC) based on graphene as negative electrode and a MnO(2) nanowire/graphene composite (MGC) as positive electrode in a neutral aqueous Na(2)SO(4) solution as electrolyte. MGC was prepared by solution-phase assembly of graphene sheets and α-MnO(2) nanowires. Such aqueous electrolyte-based asymmetric ECs can be cycled reversibly in the high-voltage region of 0-2.0 V and exhibit a superior energy density of 30.4 Wh kg(-1), which is much higher than those of symmetric ECs based on graphene//graphene (2.8 Wh kg(-1)) and MGC//MGC (5.2 Wh kg(-1)). Moreover, they present a high power density (5000 W kg(-1) at 7.0 Wh kg(-1)) and acceptable cycling performance of ∼79% retention after 1000 cycles. These findings open up the possibility of graphene-based composites for applications in safe aqueous electrolyte-based high-voltage asymmetric ECs with high energy and power densities.

Flexible energy storage devices 1Ϫ3 have many potential applications in portable electronic devices, 4Ϫ6 including roll-up display, electronic paper, stretchable integrated circuits, and wearable systems for personal multimedia, computing, or medical devices. Flexible supercapacitors are available with large power density, moderate energy density, good operational safety, and long cycling life and hence are highly desirable as a modern energy storage system. 7 A freestanding binder-free electrode with favorable mechanical strength and large capacitance is a vital component of a flexible supercapacitor. Although transition metal oxides and conducting polymers have been widely studied as supercapacitor electrode materials, only carbon-based materials have shown favorable flexibility and hence been promising as freestanding soft electrodes. Papers, films, and/or clothes made from carbon nanotubes/fibers have been demonstrated to be suitable as freestanding electrodes.2,8Ϫ13 Nevertheless, the less active surface of carbon materials always prevents them from high capacitance performance. The incorporation of an electrochemically active second phase in a carbonbased freestanding electrode can dramatically enhance the electrode capacitance.14 Graphene is an intriguing twodimensional carbon material and has attracted much research attention due to several breakthroughs in fundamental research and promising practical applications. 15Ϫ30Chemical modified graphene exhibits enormous active edges and oxygen functional groups. It has extraordinary electrochemical and mechanical properties comparable to or even better than carbon nanotubes. 21,26,27 Flexible papers with graphene sheet or graphene oxide sheet as sole building block have already been fabricated by flow-directed assembly. 16,25,31,32 Graphene paper presents excellent tensile modulus up to 35 GPa and room temperature electrical conductivity of 7200 S m Ϫ1. 25 These intriguing characteristics enable graphene paper as a freestanding electrode. Various conducting polymers have been widely studied as electrode materials for supercapacitors because of their high capacitance, easy production, and low cost. However, poor conductivity and weak flexibility of conducting polymers limit them from usage in high-performance flexible supercapacitors. It has been confirmed that graphene can enhance not only the electric conductivity of silica 18 but especially the mechanical strength of polymer composites. 21 This work is aimed to prepare graphene-conducting polymer composite paper as a flexible electrode combining the advantages of graphene paper (high