Although cobalt sulfide is a promising electrode material for supercapacitors, its wide application is limited by relative poor electrochemical performance, low electrical conductivity, and inefficient nanostructure. Here, we demonstrated that the electrochemical activity of cobalt sulfide could be significantly improved by Al doping. We designed and fabricated hierarchical core-branch Al-doped cobalt sulfide nanosheets anchored on Ni nanotube arrays combined with carbon cloth (denoted as CC/H-Ni@Al-Co-S) as an excellent self-standing cathode for asymmetric supercapacitors (ASCs). The combination of structural and compositional advantages endows the CC/H-Ni@Al-Co-S electrode with superior electrochemical performance with high specific capacitance (1830 F g/2434 F g at 5 mV s/1 A g) and excellent rate capability (57.2%/72.3% retention at 1000 mV s/100 A g). The corresponding all-solid-state ASCs with CC/H-Ni@Al-Co-S and multilayer graphene/CNT film as cathode and anode, respectively, achieve a high energy density up to 65.7 W h kg as well as superb cycling stability (90.6% retention after 10 000 cycles). Moreover, the ASCs also exhibit good flexibility and stability under different bending conditions. This work provides a general, effective route to prepare high-performance electrode materials for flexible all-solid-state energy storage devices.

Transition-metal phosphates/phosphides possess promising theoretical electrochemical characteristics and exhibit great potential in advanced supercapacitors. Unfortunately, limited by the processing techniques and overall structure, their specific capacity and rate performance are still unsatisfactory. Herein, we report the fabrication of transition-metal phosphate electrodes with an ultrathin sheetlike array structure by one-step electrodeposition at room temperature. As a proof-of-concept, a transition-metal phosphate member of NiCo­(HPO4)2·3H2O with an ultrathin nanosheet structure (thickness ∼2.3 nm) was synthesized and investigated. The as-prepared NiCo­(HPO4)2·3H2O electrode showcases an ultrahigh specific capacity of 1768.5 C g–1 at 2 A g–1 (the highest value for transition-metal phosphates/phosphides reported to date), superb rate performance of 1144.8 C g–1 at 100 A g–1, and excellent electrochemical stability. Moreover, the transition-metal phosphate nanosheet array can be uniformly deposited on various conductive substrates, demonstrating the generality of our strategy. Therefore, this simple electrodeposition strategy provides an opportunity to fabricate ultrathin transition-metal phosphate nanosheet materials that can be used for energy storage/conversion, electrocatalysis, and other electrochemical energy-related devices.

Asymmetric supercapacitors (ASCs) can substantially broaden their working voltage range, benefiting from the advantages of both cathode and anode while breaking through the energy storage limitations of corresponding symmetric cells. Wide voltage aqueous ASCs hold great promise for future electronic systems that require satisfied energy density, power density, and cycle life, due to the advantages of aqueous electrolyte in terms of low cost, operational safety, facile manufacture, environment-friendly, and high ionic conductivity. This review will first briefly present an overview of the historical developments, charge storage mechanisms, and matching principles of wide voltage aqueous ASCs. Then, the cathode and anode materials with wide potential windows for building wide voltage aqueous ASCs over the last few decades are summarized. The next section details the optimization methods of aqueous electrolyte related to wide voltage aqueous ASCs. In addition, the basic device configurations of wide voltage aqueous ASCs are classified and discussed. Furthermore, several strategies are proposed for achieving highperformance wide voltage aqueous ASCs in terms of voltage window, specific capacitance, rate performance, and electrochemical stability. Finally, to motivate further research and development, several key scientific challenges and the perspectives are discussed.

Under the big background of sustainable development, energy utilization is undergoing a fast and large shift toward electricity