A facile two‐step method is developed for large‐scale growth of ultrathin mesoporous nickel cobaltite (NiCo2O4) nanosheets on conductive nickel foam with robust adhesion as a high‐performance electrode for electrochemical capacitors. The synthesis involves the co‐electrodeposition of a bimetallic (Ni, Co) hydroxide precursor on a Ni foam support and subsequent thermal transformation to spinel mesoporous NiCo2O4. The as‐prepared ultrathin NiCo2O4 nanosheets with the thickness of a few nanometers possess many interparticle mesopores with a size range from 2 to 5 nm. The nickel foam supported ultrathin mesoporous NiCo2O4 nanosheets promise fast electron and ion transport, large electroactive surface area, and excellent structural stability. As a result, superior pseudocapacitive performance is achieved with an ultrahigh specific capacitance of 1450 F g−1, even at a very high current density of 20 A g−1, and excellent cycling performance at high rates, suggesting its promising application as an efficient electrode for electrochemical capacitors.

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Binary metal oxides has been regarded as a promising class of electrode materials for high‐performance energy storage devices since it offers higher electrochemical activity and higher capacity than mono‐metal oxide. Besides, rational design of electrode architectures is an effective solution to further enhance electrochemical performance of energy storage devices. Here, the advanced electrode architectures consisting of carbon textiles uniformally covered by mesoporous NiCo2O4 nanowire arrays (NWAs) are successfully fabricated by a simple surfactant‐assisted hydrothermal method combined with a short post annealing treatment, which can be directly applied as self‐supported electrodes for energy storage devices, such as Li‐ion batteries, supercapacitors. The as‐prepared mesoporous NiCo2O4 nanowires consist of numerous highly crystalline nanoparticles, leaving a large number of mesopores to alleviate the volume change during the charge/discharge process. Electrode architectures presented here promise fast electron transport by direct connection to the growth substrate and facile ion diffusion path provided by both the abundant mesoporous structure in nanowires and large open spaces between neighboring nanowires, which ensures every nanowire participates in the ultrafast electrochemical reaction. Benefiting from the intrinsic materials and architectures features, the unique binder‐free NiCo2O4/carbon textiles exhibit high specific capacity/capacitance, excellent rate capability, and cycling stability.

mers usually suffer from poor cyclic stability in long term charge-discharge processes. [ 15 ] The low electron conductivity of transition metal oxides leads to inferior rate capability. [ 16 ] In the search for high performance electrode materials, transition-metal sulfi des have been extensively studied as new class of pseudocapacitive materials for supercapacitors. [17][18][19][20] In particular, NiCo 2 S 4 possess higher electrochemical activity and higher capacity than mono-metal sulphides based on richer redox reactions. [ 21,22 ] More signifi cantly, NiCo 2 S 4 exhibited an excellent electrical conductivity, at least two orders of magnitude higher than that of NiCo 2 O 4 . [ 23 ] The development of novel nanostructured materials will effectively improve the utilization of active materials because of their high surface area, and short electron-and ion-transport pathways. Several different types of NiCo 2 S 4 nanostructures, including nanoplates, [ 24 ] nanoprisms, [ 25 ] nanotubes, [ 26,27 ] microspheres, [ 28 ] have been recently synthesized and their electrochemical performance was investigated. For example, Lou et al. [ 25 ] reported the fabrication of Ni x Co 3-x S 4 hollow nanoprisms through a simple sacrifi cial template method and a high reversible capacity (895 F g −1 under 1 A g −1 ) can be achieved. However, it should be noted that the introduction of a conductive agent and a polymer binder during the thin fi lm electrode preparation not only increase extra contact resistance but also inevitably compromises the overall energy storage capacity that still seriously limit their performance. [ 29,30 ] Three dimensional (3D) electrode architectures have emerged as a new direction because it can provide 3D interconnected network of both electron and ion pathways, allowing for effi cient charge and mass exchange during faradic redox reactions. Greatly enhanced performance has kindled the interest of researchers in the fi eld of 3D electrode architectures designs, such as copper pillar array, [ 31 ] nickel network, [ 32,33 ] stainless steel mesh, [ 34 ] carbonaceous interpenetrating structures. [ 35 ] The rigid electrode with metals as current collectors lead to the devices less fl exible, and they also have a low energy density. Carbon nanotubes sponge and graphene foams have been recently fabricated using chemical vapor deposition and used as electrode architectures for supercapacitors. [36][37][38] However, it is diffi cult to produce carbonaceous foams in large-scale because of their relatively high-cost and complex preparation processes. Additionally, due to the potential incompatibility issue between To push the energy density limit of supercapacitors, a new class of electrode materials with favorable architectures is strongly needed. Binary metal sulfi des hold great promise as an electrode material for high-performance energy storage devices because they offer higher electrochemical activity and higher capacity than mono-metal sulfi des. Here, the rational design and fabrication of NiCo 2 S 4 nan...

While the synthesis of hollow structures of transition metal oxides is well established, it is extremely challenging to fabricate complex hollow structures for mixed transition metal sulfides. Here we report an anion exchange method to synthesize a complex ternary metal sulfides hollow structure, namely nickel cobalt sulfide ball-in-ball hollow spheres. Uniform nickel cobalt glycerate solid spheres are first synthesized as the precursor and subsequently chemically transformed into nickel cobalt sulfide ball-in-ball hollow spheres. When used as electrode materials for electrochemical capacitors, these nickel cobalt sulfide hollow spheres deliver a specific capacitance of 1,036 F g À 1 at a current density of 1.0 A g À 1 . An asymmetric supercapacitor based on these ball-in-ball structures shows long-term cycling performance with a high energy density of 42.3 Wh kg À 1 at a power density of 476 W kg À 1 , suggesting their potential application in high-performance electrochemical capacitors.

Sodium-ion batteries (SIBs) have attracted increasing attention in the past decades, because of high overall abundance of precursors, their even geographical distribution, and low cost. Apart from inherent thermodynamic disadvantages, SIBs have to overcome multiple kinetic problems, such as fast capacity decay, low rate capacities and low Coulombic efficiencies. A special case is sodium super ion conductor (NASICON)-based electrode materials as they exhibit - besides pronounced structural stability - exceptionally high ion conductivity, rendering them most promising for sodium storage. Owing to the limiting, comparatively low electronic conductivity, nano-structuring is a prerequisite for achieving satisfactory rate-capability. In this review, we analyze advantages and disadvantages of NASICON-type electrode materials and highlight electrode structure design principles for obtaining the desired electrochemical performance. Moreover, we give an overview of recent approaches to enhance electrical conductivity and structural stability of cathode and anode materials based on NASICON structure. We believe that this review provides a pertinent insight into relevant design principles and inspires further research in this respect.

Despite the significant advancement in preparing metal oxide hollow structures, most approaches rely on template-based multistep procedures for tailoring the interior structure. In this work, we develop a new generally applicable strategy toward the synthesis of mixed-metal-oxide complex hollow spheres. Starting with metal glycerate solid spheres, we show that subsequent thermal annealing in air leads to the formation of complex hollow spheres of the resulting metal oxide. We demonstrate the concept by synthesizing highly uniform NiCo2O4 hollow spheres with a complex interior structure. With the small primary building nanoparticles, high structural integrity, complex interior architectures, and enlarged surface area, these unique NiCo2O4 hollow spheres exhibit superior electrochemical performances as advanced electrode materials for both lithium-ion batteries and supercapacitors. This approach can be an efficient self-templated strategy for the preparation of mixed-metal-oxide hollow spheres with complex interior structures and functionalities.