Effect Of The Core-shell Structured Metal Oxides/Sulfides On The Performance Of All-Solid-State Supercapacitors

As a promising energy storage device,supercapacitors have attracted tremendous interest due to high power density,fast charge/discharge rate and long cycle life.As the critical component of supercapacitors,electrode materials play a decisive role in improving the performance of supercapacitors.Metal oxides utilizing fast redox reactions to store ions in the electrode/solution interface have larger capacitance than electric double-layer capacitors(EDLCs)which using physical adsorption to store ions,attracting considerable attention in the research community.However,its lower conductivity and structural stability limit its further application.Therefore,it is very important to design and synthesize metal oxides/sulfides electrode materials with high conductivity and high stability in this field.In general,two strategies have been used to improve the electrochemical performance of metal oxygen/sulfide electrode materials:(1)Structural design:As for nanomaterials,morphology and structure both have a significant effect on their electrochemical performances.Therefore,the ingeniously structural design of the electrode material can greatly improve its performance.(2)Chemical composition:Compared with single-component materials,multicomponent composite materials can not only make full use of the merits of individual component and a potential synergistic effect but also make up for the shortage of single material.Based on the above design concepts,we have successful prepared a series of core-shell electrode materials,such as hollow core-shell CuCo₂O₄,hollow core-shell CuCo₂S₄,Cu-doped Ni₃S₂ core-shell nanoarray,multi-dimensional micro-nano CuCo₂O₄/NiMoO₄and 2D/2D CuCo₂O₄/NiMoO₄core-shell array.In addition,asymmetric supercapacitor device assembled by these electrode materials as energy storage devices show excellent electrochemical performance and potential practical application.The main research contents are as follows:(1)Owing to the advantages of 1D nanostructure for effective charge transport,2D nanoflakes for largely exposed active sites,we have designed and synthesized 3D dandelion-like CuCo₂O₄microspheres/branched NiMoO₄ nanosheets heterostructures via a facile and stepwise hydrothermal approach.The urchin-like CuCo₂O₄ microspheres serves as the framework and branched NiMoO₄nanosheets as secondary building units,which simultaneously integrates the composition and spatial structure advantages based on highly conductive 1D structure,highly active 2D structure and robust3D structure.Due to the multi-dimensional architecture and double compositions,the unique structure exhibited high specific capacitance of 2215 F g^-1 at 1 A g^-1,good rate capability and excellent cycling stability of 98.3%over 8000 cycles.More importantly,at a high current density of 10 A g^-1,the specific capacitance remained at 1071 F g^-1,which was 48.2%of the value obtained at 1 A g^-1,revealing excellent rate capability.The results indicated that the dandelion-like CuCo₂O₄/NiMoO₄3D electrode was a promising candidate for supercapacitors.(2)Nanosheets have showed great potential due to abundant active sites and large specific surface area.However,self-aggregation and restacking of nanosheets during preparation and electrochemical process limit its further application.Micro/nanostructure would be helpful to address the issue because micrometer scale structure provide favorable mechanics and stability to preserve 2D nanostructure,thus realizing the full merits of 2D structure.Based on above analysis,we proposed a strategy by combining 2D spatial confinement effect with micro/nano structure to design and synthesize multilayered hamburger-like structure.The vertically aligned CuCo₂O₄ multilayered microsheets acted as sliced bread to provide robust mechanical strength and efficient charge transport channels.Definite interspace of each-layer microsheets not only ensured aligned growth for subsequent NiMoO₄ nanosheets but also provided a spatial confinement effect to buffer the large volume changes during cycling.The constructed 3D CuCo₂O₄/NiMoO₄heterostructures exhibit superior pseudocapacitive performance with an ultrahigh capacitances of 2350 F/g at 1 A/g as well as an excellent cycling stability of 91.5%over 5000 cycles.An impressive capacitance as high as1235 F/g can be achieved even at a current density of 10 A/g.52.5%of the initial capacitance is retained when the current density increases from 1.0 to 10 A/g,indicating excellent high-rate capability.(3)Designing multi-shell hollow structure materials is also one of the important strategies to improve electrochemical performance because multi-shell hollow structures possess higher density and larger specific surface area.Compared with the current multi-step template method for the hollow materials,we report a simple strategy to synthesize a uniform T-CuCo₂O₄ via a solvothermal/calcination procedure.Shell numbers can be controlled by adjusting the reaction time.The hierarchical hollow structure with the specific surface area(42.5 m²/g)and pore diameter(20.0 nm)can be beneficial to the charge storage for Faradic redox reactions and shorten channels for electrolyte diffusion.The T-CuCo₂O₄ electrode acting as a three-electrode cell demonstrated the specific capacitance of 691 F/g(current density:1 A/g),rate capability of 470 F/g(current density:20 A/g)and the capacitance retention of 93%after 6000 cycles.Similarly,hollow precursors are vulcanized with thioacetamide to prepare hollow CuCo₂S₄ hollow structures.The capacity of hollow CuCo₂S₄ is significantly higher than that of CuCo₂O₄owing to lower electronegativity of sulfur than oxygen.However,its cycle stability is weaker than that of CuCo₂O₄,which may be due to large volume changes causing the sphere to break.(4)Owing to low electronegativity of sulfur,metal sulfides tend to have better conductivity.Therefore,metal sulfides show excellent power performance when they are used as electrode materials in electrochemical energy storage.However,its inferior capacity retention during charging-discharging process hinder large-scale practical application of supercapacitors.Both experimental and theoretical calculation study were performed to study effect of Cu doped Ni₃S₂ on structural and electrochemical properties.From the results of electrochemical tests,we find that introducing Cu into Ni₃S₂can effectively improve electrochemical performance of Ni₃S₂.According to the first principle calculations,doping Cu can induce a large number of free carriers around the Fermi level,contributing to an elevated charge transfer efficiency and rate capability.The optimized Cu-doped Ni₃S₂ electrode exhibited higher specific capacitance compared with the pristine Ni₃S₂ electrode at10 A/g,and it showed capacitance retention of about 94.0%after 5000 cycles at 5 A/g.