Preparation And Energy-Storage Properties Of Ni-Co Sulfides(Phosphides) Micro-Nano Hierarchical Structures

Supercapacitor is a kind of energy storage between conventional capacitor and chemical battery.It has many advantages such as fast charging and discharging,high power density and long cycle life.However,the traditional pseudo-capacitance electrode materials of typical transition metal oxides have poor conductivity,unstable structure and weak mechanical properties.It is the key to design and develop high-performance pseudo-capacitive materials.Due to low electronegativity,high conductivity and excellent electrochemical activity,transition metal sulfides and phosphides are considered to be an ideal alternative,but complex micro-nano hierarchical structures cannot be obtained by the existing methods.1D nanowires(NWs)and nanotubes(NTs)with high anisotropy have unique superiority in energy storage than other irregular ones,but the strong Van der Waals force between them will lead to agglomeration,volume swelling and structural collapse during rapid oxidation reduction reactions.To this end,this paper focuses on the synthesis of 2D ternary transition metal sulfides(phosphides)hierarchical structures constructed by 1D NWs/NTs via a newly developed two-step ion-exchange method,and the involved formation mechanisms are discussed in detail.Besides,the ion-exchange method has expanded to other bimetallic system successfully.Effects of the composition,size and structure on electrochemical properties are analyzed in detail.Asymmetric all-solid-state devices were designed and assembled.The main contents are summarized as follows:(1)Based on "top-down" approach,namely micro-sized structures from the anisotropic growth are converted to desirable products via composition transformation and ion-etching at the nanometer scale.Hierarchically porous hexagonal microsheets consisting of NiCo2S4 interwoven NTs were successfully synthesized through two-step hydrothermal method.In the first step,through fine control over reaction time and the concentration,smooth NWs-intertwined porous hexagonal microsheets precursor were obtained.In the second step,precursor NWs was converted into NTs via the reaction rate-and time-dependent Kirkendall effect.The formation mechanism of such unique micro-nano structure was discussed in detail.Also,the method can be extended to other bimetallic sulfide MCo2S4(M = Ni,Co,Fe).This unique structure provides multiple channels for ion transport,greatly accelerates the ion diffusion,and the nanotube wall thickness of only 18 nm facilities ions to penetrate inside.Owing to the inter-welding of 1D NTs,not only the structure is stable,but also their resistance is much lower than that of surface contacting NTs.The optimized NiCo2S4 electrode material showed a specific capacitance reaching 1780 F g-1 at a current density of 1 A g-1;especially a high current density of 10 A g-1,the capacitance retention was up to 92.4%after 10000 cycles.An assembled all-solid-state asymmetric supercapacitor has a voltage window of 1.8 V,delivers an energy density 67.2 Wh kg-1(at a power density 900 W kg-1),and the capacitance retention is 92.1%after 5000 cycles.(2)Through low temperature phosphorization,porous hexagonal microsheets constructed by porous NiCoP-CoP NWs were successfully prepared in a tubular furnace via a reaction of the Ni-Co precursors prepared by the above-mentioned first-step hydrothermal with sodium hypophosphite.The thickness of microsheets is about 100 nm,their length is about 10 ?m,and the pore size in a single nanowire is3?5 nm.Due to the high specific surface area,high conductivity and synergistic effect,the NiCoP-CoP electrode material has a high specific capacitance of 1968.5 F g-1(at 1 A g-1).When the current density increases to 10 A g-1,the capacitance remains 83.1%.Especially the electrode shows high cycle stability with capacitance retention of 95%after 10,000 charge-discharge cycles at the current density of 20 A g-1.All-solid-state asymmetric supercapacitors with NiCoP-CoP as the positive electrode and the nitrogen-doped graphene hydrogel film as the negative electrode deliver an energy density of 38.7 Wh kg-1 at a power density of 1784 W kg-1.Additionally,a new type of multilayer capacitor in series connection was designed,and a single device with four layers has a wide voltage window of reaching 7.2 V.The proposed device will achieve higher performance and hold great promise in wearable applications for miniaturized electronic devices.