We have successfully fabricated an asymmetric supercapacitor with high energy and power densities using graphene hydrogel (GH) with 3D interconnected pores as the negative electrode and vertically aligned MnO(2) nanoplates on nickel foam (MnO(2)-NF) as the positive electrode in a neutral aqueous Na(2)SO(4) electrolyte. Because of the desirable porous structure, high specific capacitance and rate capability of GH and MnO(2)-NF, complementary potential window of the two electrodes, and the elimination of polymer binders and conducting additives, the asymmetric supercapacitor can be cycled reversibly in a wide potential window of 0-2.0 V and exhibits an energy density of 23.2 Wh kg(-1) with a power density of 1.0 kW kg(-1). Energy density of the asymmetric supercapacitor is significantly improved in comparison with those of symmetric supercapacitors based on GH (5.5 Wh kg(-1)) and MnO(2)-NF (6.7 Wh kg(-1)). Even at a high power density of 10.0 kW kg(-1), the asymmetric supercapacitor can deliver a high energy density of 14.9 Wh kg(-1). The asymmetric supercapacitor also presents stable cycling performance with 83.4% capacitance retention after 5000 cycles.

We present a one-step approach to polydopamine-modified graphene hydrogel, with dopamine serving as both reductant and surface functionalization agents. The synthetic method is based on the spontaneous polymerization of dopamine and the self-assembly of graphene nanosheets into porous hydrogel structures. Benefiting from the abundant functional groups of polydopamine and the high specific surface areas of graphene hydrogel with three-dimensional interconnected pores, the prepared material exhibits high adsorption capacities toward a wide spectrum of contaminants, including heavy metals, synthetic dyes, and aromatic pollutants. Importantly, the free-standing graphene hydrogel can be easily removed from water after adsorption process, and can be regenerated by altering the pH values of the solution for adsorbed heavy metals or using low-cost alcohols for synthetic dyes and aromatic molecules.

Potassium has as rich an abundance as sodium in the earth, but the development of a K-ion battery is lagging behind because of the higher mass and larger ionic size of K than that of Li and Na, which makes it difficult to identify a high-voltage and high-capacity intercalation cathode host. Here we propose a cyanoperovskite KMnFe(CN) (0 ≤ x ≤ 2) as a potassium cathode: high-spin Mn/Mn and low-spin Fe/Fe couples have similar energies and exhibit two close plateaus centered at 3.6 V; two active K per formula unit enable a theoretical specific capacity of 156 mAh g; Mn and Fe are the two most-desired transition metals for electrodes because they are cheap and environmental friendly. As a powder prepared by an inexpensive precipitation method, the cathode delivers a specific capacity of 142 mAh g. The observed voltage, capacity, and its low cost make it competitive in large-scale electricity storage applications.

Garnet-structured solid electrolytes have been extensively studied for a solid-state lithium rechargeable battery. Previous works have been mostly focused on the materials' development and basic electrochemical properties but not the cathode/electrolyte interface. Understanding the cathode interface is critical to enhance chemical stability and electrochemical performance of a solid-state battery cell. In this work, we studied thoroughly the cathode/electrolyte interface between LiCoO 2 and Li 7 La 3 Zr 2 O 12 (LLZO). It was found that the high-temperature process to fuse LiCoO 2 and LLZO induced cross-diffusion of elements and formation of the tetragonal LLZO phase at the interface. These degradations affected electrochemical performance, especially the initial Coulombic efficiency and cycle life. In a clean cathode interface without the thermal process, an irreversible electrochemical decomposition at > ∼ 3.0 V vs Li + /Li was identified. The decomposition was able to be avoided by a surface modification of LLZO (e.g., Co-diffused surface layer and/or presence of an interlayer, Li 3 BO 3 ), and the surface modification was equally important to suppress a reaction during air storage. In a LiCoO 2 / LLZO interface, it is important to separate direct contacts between LiCoO 2 and pure LLZO.

NASICON (Na super ionic conductor) structures of NaMV(PO) (M = Mn, Fe, Ni) were prepared, characterized by aberration-corrected STEM and synchrotron radiation, and demonstrated to be durable cathode materials for rechargeable sodium-ion batteries. In NaMnV(PO), two redox couples of Mn/Mn and V/V are accessed with two voltage plateaus located at 3.6 and 3.3 V and a capacity of 101 mAh g at 1 C. Furthermore, the NaMnV(PO) cathode delivers a high initial efficiency of 97%, long durability over 1000 cycles, and good rate performance to 10 C. The robust framework structure and stable electrochemical performance makes it a reliable cathode materials for sodium-ion batteries.