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NiCo2O4 has been proved to be a promising electrochemical material. In this work, a hollow NiCo2O4 material with an urchin-like structure is constructed by a self-template of Ni–Co complex compound based on the Ostwald ripening mechanism. During the thermal annealing in air, the residual carbon from the organic ligands of the Ni–Co complex compound leads to the generation of oxygen vacancies in urchin-like NiCo2O4. The as-obtained hollow NiCo2O4 (H-NiCo2O4-12) material manifests admirable specific capacitance of 886 F g−1 at 1 A g−1 and preserves 80% of capacitance retention after 5000 cycles. Moreover, the assembled asymmetric supercapacitor of H-NiCo2O4-12//AC delivers an energy density of 33.4 W h kg−1 at 747.7 W kg−1 and a high coulombic efficiency of about 95%. The improved supercapacitive performance of H-NiCo2O4-12 is attributed to its synergistic effect of hollow structure and oxygen vacancies. The hollow urchin-like structure of NiCo2O4 can buffer the volume expansion, expose abundant reaction active sites, and favor the transport of ions; while oxygen vacancies can adjust the electronic structure of NiCo2O4, which can favor the transfer of electrons. This work may throw some light on designing a virtue supercapacitor electrode material with hollow nanoporous morphology.

To meet the requirements of high-performance and long lifespan supercapacitors, the development of electrode materials with better performance and stable structure is in high demand. Herein, an urchin-like NiCoP nanoarray coated with a carbon layer hybrid electrode (NiCoP@C-ULAs) has been successfully synthesized on a carbon cloth by an effective and goal-directed approach. The three-dimensional (3D) urchin-like precursor prepared via the solvothermal method integrates fascinating architectures, in which 1D nanospines favor charge transport and 2D ultrathin nanoflakes provide large active sites and short paths for ion diffusion, thus leading to the highest performance among the precursors. Subsequently, carbon coating and phosphorization were employed to synthesize the NiCoP@C-ULAs composite. The resulting NiCoP@C-ULAs exhibited a high specific area (201.9 m2 g−1), small average pore size (4.4 nm), ultrahigh specific capacity (1046 C g−1 at 1 A g−1) and superior rate capability (76.5% retention even at 20 A g−1). Remarkably, the cycling stability was enhanced from 55.1% to 86.3% at 8 A g−1 after 5000 cycles as compared with NiCoP-ULAs. The main reason for the significant improvement in the electrochemical performances is due to the unique structure and the synergistic effect of NiCoP with high specific capacity and the carbon coating with good mechanical stability. Furthermore, the all-solid-state NiCoP@C-ULAs//AC asymmetric supercapacitor delivers a high energy density of 37.1 W h kg−1 at a power density of 792.8 W kg−1 and outstanding cycling stability (91.4% of specific capacity retention after 10 000 cycles). The LEDs could be lit up by the assembled capacitors for several minutes, which suggests that the NiCoP@C-ULA electrodes possess enormous potential as energy storage and conversion devices in future.

To meet the demand for high-power-density and long lifespan surpercapacitors (SCs), the Ni–P@Ni HL/CC-1h with a core–shell structure (Ni–P sphere as the core and nanoflake with the Ni and Ni–P heterogeneous layer as shell) was constructed via a facile strategy. The strategy included hydrothermal synthesis of Ni–P spheres with large Ni surface layer on carbon cloth (Ni–P@Ni HL/CC) and subsequent chemical dealloying using HCl as etching solution in order to remove the redundant Ni substances. The morphology, composition, and electrochemical performances of raw Ni–P@Ni HL/CC and the corresponding samples obtained by different dealloying times (0.5, 1, and 2 h) were characterized. Interestingly, the Ni–P@Ni HL/CC-1h presents a unique structure with a nanoflake shell and a porous core, which can provide a large number of exposed active sites, accelerate electrolyte ion diffusion and support ultra-long cycling. Furthermore, the Ni species existing in the outer flake can increase the conductivity and promote the capacitance during the charge–discharge processes. The Ni–P@Ni HL/CC-1h exhibited high specific capacity of 280.8 C g−1 at current density of 1 mA cm−2, high rate retention of 76.2% at 20 mA cm−2. The maximum specific capacity could reach 388.8 C g−1 at 8 mA cm−2, and maintained the 92.6% retention after 3000 cycles. Moreover, the Ni–P@Ni HL/CC-1h//AC all-solid-state asymmetric supercapacitor (ASC) exhibited high specific capacity, 86.0% retention after 10,000 cycles and high energy density of 27.6 Wh kg−1 at power density of 942.8 W kg−1.

Metal-organic frameworks featuring controllable morphologies and tunable compositions and pore structures are widely used as precursors for supercapacitors electrode materials. A zinc/cobalt-based zeolitic imidazolate framework was utilized as a precursor to synthesize N, S-codoped porous carbon supported CoS2 nanocomposites, using a two-step (carbonization and sulfurization) procedure. The as-prepared materials maintained the rhombic dodecahedral structure of the precursors, and carbon nanotubes were rooted onto the nanocomposite surface. The N, S-codoped porous carbon supported CoS2 nanocomposite hybrid electrode exhibited excellent electrochemical performance: high specific capacitance (635.8 F/g at 1 A/g), good rate capability (544.2 F/g at 10 A/g), and long-term cycling stability (88.0% retained after 2000 cycles). Moreover, the energy density of the N, S-codoped porous carbon-supported CoS2 nanocomposites//activated carbon asymmetric supercapacitor reached 14.1 Wh/kg at the power density of 0.71 kW/kg, and its capacitance remained 92.2% after 2000 cycles in the 0–1.4 V potential range.

NiCo2O4 has been proved to have promising electrochemical performance. In this work, a new type of hollow urchin-like NiCo2O4 material is constructed by a self-template of Ni-Co complex compound based on the Ostwald ripening mechanism. During the thermal annealing in air, the residual carbon from the organic ligands of Ni-Co complex compound leads to the generation of oxygen vacancies in urchin-like NiCo2O4. The as-obtained hollow NiCo2O4 (H-NiCo2O4-12) material manifests admirable specific capacitance of 886 Fg-1 at 1Ag-1, and preserves 80% of capacitance retention after 5000 cycles. Meanwhile, the assembled asymmetric supercapacitor of H-NiCo2O4-12//AC delivers an energy density of 33.4 Wh g-1 at 747.7 W kg-1. The improved supercapacitive performance of H-NiCo2O4-12 should attribute to its synergistic effect of hollow structure and oxygen vacancies. The hollow urchin-like structure of NiCo2O4 can buffer the volume expansion, expose abundant reaction active sites and favor the transport of ions; while oxygen vacancies can adjust the electronic structure of NiCo2O4 which can favor the transfer of electrons. This work may throw some light on designing a virtue supercapacitor electrode material with hollow nanoporous morphology.

Transition metal phosphides (TMPs) have attracted great interest owing to the metallic properties and high specific capacities. Here, we designed hollow NiCoP nanocubes with increased specific surface area using a Ni-Co Prussian blue analogue as a self-template and NH3·H2O as an etching agent. During the synthesis, both carbonization and phosphorization are completed in one step. The obtained hollow structure alleviates the volume variation of electrode material during reversible electrochemical reaction. Meanwhile, the residual carbon distributed uniformly in NiCoP at the molecular level, resulting in a high conductivity. DFT calculations further reveal that the electrical conductivity of NiCoP is superior to those of monometallic phosphide and metal oxide. Therefore, the optimized NiCoP-4–500 displays a high specific capacity (1590 F g−1 at 1 A g−1) and outstanding cycling stability (78.2% retention after 12,000 cycles). Moreover, a prepared hybrid supercapacitor device delivers an energy density of 38.4 W h kg−1 with a power density of 799.9 W kg−1 at 1 A g−1. The results indicate that the obtained high-performance TMPs with hollow structures have an application potential for energy storage devices.

A polyaniline/molybdenum trioxide/activated carbon cloth (PANI/MoO3/ACC) composite was successfully synthesized via facile hydrothermal and in-situ polymerization reactions. The morphology and microstructure of the PANI/MoO3/ACC composite were investigated by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The electrochemical performance of the as-prepared electrode was evaluated by cyclic voltammetry, galvanostatic charge-discharge measurements, and electrochemical impedance spectroscopy. The coating PANI layer in PANI/MoO3/ACC increases the electrochemical activity of MoO3 and accelerates the ion diffusion and electron transfer. Due to synergistic effects between the components, PANI/MoO3/ACC exhibits a specific capacitance of 1050 F g−1, significantly higher than that of MoO3/ACC (296.7 F g−1) and PANI/CC (500.0 F g−1) at a current density of 0.5 A g−1 in 1 M H2SO4 aqueous electrolyte, as well as good cycling stability for 2000 cycles with 71% retention. It also displays lower ion diffusion and charge transfer resistance, as evidenced by the electrochemical impedance data. PANI/MoO3/ACC exhibits excellent electrochemical performance and is thus a promising electrode material for supercapacitors.

Transition metal sulfides (TMS) are of great interest as promising battery-type electrode materials, however, the poor conductivity and sluggish reaction kinetics seriously limit their application. Here, we designed a hollow structured precursor of Ni-based metal-organic frameworks (Ni-MOFs) via Ostwald ripening mechanism. Based on this unique precursor, a hollow carbon-coated nickel sulfide nanocrystal (H-NiS1-X/C) with sulfur vacancies was further synthesized through an ion exchange strategy and thermal annealing. By optimizing the content of sulfur source, the sample with appropriate S-vacancies (H-NiS1-X/C-50) was developed. Benefiting from its hollow structure and S-vacancies, this H-NiS1-X/C-50 displayed a high reversible specific capacity (1728 F g−1, 1 A g−1), stable cycling (72% capacity retention over 8000 cycles) and superior rate capability. After assembling the asymmetric supercapacitor, a high energy density of 36.88 Wh kg−1 was achieved. Experimental results and DFT calculations demonstrate that introducing S-vacancies builds an embedded electric field and produces lattice distortions in H-NiS1-X/C, thus enhancing the conductivity of the material. Our strategy also provides a facile way to construct high-performance TMS with unique hollow structure and S-vacancies for developing advanced energy storage devices.

A series of Zr4+ and/or Al3+ doped LiNi0.83Co0.11Mn0.06O2 are synthesized via a solid phase method. TEM shows that the co-doped specimen consists of a layered core and a coherent interface cation-mixed shell. XRD and XPS results clarify that Zr4+ doping increases the Li+/Ni2+ cation site exchange degree and the concentration of Ni2+. The morphology and structural integrity of the co-doped electrode maintains well, and it provides a higher cycle retention (89.7%) than blank (60.1%) after 150 cycles and a superior capacity of 152.8 mAh g−1 at 7C (only 57.7 mAh g−1 for blank) and an enhanced elevated temperature cycle retention (83.0%) than blank (33.7%) after 100 cycles at 55 °C. Generally, Zr4+ raises the concentration of Ni2+ to keep electric neutrality and then reconstruct a stable interfacial structure to inhibit side reactions, the structural stability could be also enhanced by the strong Zr–O and Al–O bonds. Meanwhile, Al3+ served as centers of positive charge can suppress the phase transformation. Benefiting from the synergistic effect of Zr4+/Al3+, as well as the induced coherent interface cation-mixed shell, co-doped specimen shows enhanced structural stability and the superior electrochemical performance. This study provides a route to prepare advanced layered cathodes for lithium-ion batteries.