Preparation And Energy Storage Performance Of Multidimensional Micro-nano Structural Transition Metal Compounds Materials

Supercapacitor is a new kind of electrical energy storage device which has potential applications in solving the intermittent storage problem of sustainable green energy,such as solar energy and wind energy.Because of theirs high power density,fast charging/discharging rate and extremely long cycle life,supercapacitors possess great advantages in the areas of consumer electronics,urban transportation,large power communications,military aerospace,mechanical engineering,etc.To date,the major unsolved issues during supercapacitors’development is that the low energy density could not meet the demond of high energy density for electronic devices.According to the equation of calculating the energy density（E）of supercapacitors,E=₂¹××(1²,where C is the specific capacitance and V is the operational voltage,there are two approaches to boost the energy density of supercapacitors.First,it is important to increase the specific capacitance by replacingconventional carbon based electrodes with pseudo-capacitive electrodes.Second,it is crucial to expand the operational voltage of the supercapacitor device.Based on the research of preparation of transition metal compounds in our group,the research contents of this thesis is focus on the preparation of transition metal compounds multidimensional materials by controlling their structures and compositions,and their enhanced energy storage performance for supercapacitors.The detailed imformation is as follows:1.Synthesis of acanthosphere-like NiCo₂O₄@α-MnO₂ microspheres and its high specific capacitance in supercapacitorIn order to achieve high specific capacitance of supercapacitors,we design and fabricated a heterostructures architecture with an ultrathinα-MnO₂ nanosheets on the surface of acanthosphere-like NiCo₂O₄ nanosheres using an“in situ growth”technique of three-steps hydrothermal method.The as-prepared acanthosphere-like NiCo₂O₄@α-MnO₂ microspheres posess a ultrahigh specific surface area by controlling the morphology ofα-MnO₂ at different reaction time during the hydrothermal process.The as-designed heterostructures architecture improved the electrons transit rate and shorten the ion diffusion pathway,provding high electronic double layer capacitance and simultaneously pseudocapacitance derived from the intercalation/deintercalation of Li⁺.Therefore,the branched hybrid electrodes exhibits exceptional specific capacitances of 695 F g^-1(2.17 F cm^-2)and good rate capability.However,the electrodes only maintains 92%of its initial value after 6000 cycles due to the structure pulverization and dissolution ofα-MnO₂ causing the active material peeling off the electrode surface.2.Synthesis of hollow tubular V₂O₃@MoS₂ nanomaterial and its high energy density in supercapacitorIn order to broaden the potential voltage of supercapacitors,We synthesized a hollow tubular V₂O₃@MoS₂ composite for the first time by using a component optimization strategy.To address the restacking and aggregation issues of MoS₂-based sheet-like pseudocapacitive materials,a second component of V₂O₃ was added into the system of MoS₂ nanosheets by a facile hydrothermal reaction.The introduced V₂O₃increased the electronic conductivity of MoS₂ and also prevented the restacking propensity of nanosheets,achieving an ultra-broad operational potential window between-1.0 V and+1.0 V and thus high energy denisty.The V₂O₃@MoS₂ composite single electrode exhibited a high reversible capacitance of 655 F g^-1.Moreover,an asymmetric pseudocapacitor with our composite as a positive electrode in combination with active carbon negative electrode achieved a large potential window（0¹.6 V）,high specific capacitance(108.7 F g^-1 at 0.5 A g^-1),and high energy density(31.8 W h kg^-1)at a power density of 0.37 kW kg^-1.Besides,the“electro-activation”process was proposed to analyze the excellent cycling stability by using cyclic voltammetry（CV）and galvanostatic charge-discharge（GCD）measurements.During the continuing charging and discharging process,electrolyte ions are easily intercalated into the van der Waals layers of MoS₂ and V₂O₃ followed by expansion of the interlayer distance.Thus,the specific capacitance of electrode increased with the increasing the active sites during the Faradic reaction process.3.Synthesis of 2D amorphous NiCo LDH 3D electrode nanostructure and its high energy density in supercapacitorTo achieve the goal of high specific capacitance and high energy density of supercapacitors,we synthesized an amorphous binary NiCo layered double hydroxide（NiCo LDH）for the first time by a green,economical and practicable ultrahigh voltage electrophoretic deposition method（EPD）.Three kinds of different morphologies,including nanospheres,nanosheets and nanofilms,was explored by adjusting the mixed electrolytes of electrophoretic deposition system.The low crystallinity of amorphous NiCo LDH provided high density of the grain boundaries leading to more efficient diffusion channels for electrolyte and thus contributed to pseudocapacitance.The as-obtained amorphous NiCo LDH/CFP self-supported positive electrode exhibited a high specific capacitance of 347 mF cm^-3.Moreover,an asymmetric supercapacitor based on the amorphous NiCo LDH/CFP and commercial activated carbon（AC）offers an energy density of 41.67 W h cm^-3 at the power density of 0.5 kW cm^-33 and excellent cycle life（only 0.3%deterioration of its initial specific capacitance after 10000 cycles）.The good performance of amorphous NiCo LDH nanosheets in capacitance is ascribed to the high inner charge storage and efficient ion transportation.This research provides a new method to control the morphologies of amorphous nanomaterials by EPD technique in the future.4.Mechanism analysis of electrochemical energy storage characteristics in supercapacitorsThe electrochemical energy storage mechanism of NiCo₂O₄@α-MnO₂,V₂O₃@MoS₂ and amorphous NiCo LDH multi-dimensional micro-nano structures was studied by using electrochemical characterization methods,such as cyclic voltammetry（CV）and galvanostatic charge-discharge（GCD）.The results showed that the three kinds of nanomaterials belong to typical pseudo-capacitor electrode materials.The effect of porous structure on energy storage performance was studied by nitrogen adsorption-desorption measurements.The results showed that the multi-scale pore structure（micropores-mesopores-macropores）is beneficial to the electrolyte infiltration and it increases the specific capacitance contribution of the active materials.The Trasatti method was used to analyze the contribution of the electric double layer capacitance and the Pseudo-capacitance of the active materials,and the cause of the difference in these contributions.