Synthesis Of Co-based Oxide Nanomaterials And Their Applications In Electrochemical Energy Storages

With the rapid development of portable devices and electric vehicles,there is a urgent need for electrochemical energy storage with the advantages of environmentally friendly and efficiently.Among lots of the electrochemical energy storages,lithium-ion batteries?LIBs?and supercapacitors?SCs?have attracted extensive attention due to their high energy density?high power density and excellent cycling stability.Seeking for better electrode materials is the key to improve the electrochemical performance of lithium-ion batteries and supercapacitors.The commercial LIBs?graphite serves as the anode material?are far from meeting the applications,because of its intrinsic low theoretical specific capacity?372 mAh g-1?.Therefore,lots of studies have been carried out in seeking alternative anode materials for LIBs with high theoretical specific capacity,low cost and non toxicity.Transition metal oxides precisely meet these requirements which have become the promising electrode materials.Recently,some studies proved that preparing the unique nanostructures,coating carbon on the surface of the electrode and adulteration can effectively improve the conductivity of electrode and prevent the volume change of the electrode material during the charge/discharge process,which results in the better cycling stability.SCs can be divided into two types defined by their storage mechanisms,i.e.,electrical double layer capacitors?EDLCs?and pseudocapacitors?PCs?.Current commercial capacitors mainly belong to EDLCs,which based on carbonaceous materials including carbon aerogel,carbon fiber,carbon nanotube and grapheme.However,EDLCs have suffered from the low energy density which restricts its extensive applications.However,PCs can provide much higher energy density due to their fast and reversible faradic redox reactions.In order to enhance the electrochemical performance,we have focused much of our attention on building unique nanostructure electrode,improving the conductivity of and increasing the specific surface area of electrode.The specific expriments and results are as follows:In the third chapter of the thesis,we successfully synthesized the composite CUCo2O4/C nanofibers using a facile single-nozzle electrospinning method followed by the subsequent heating treatment.Besides,the electrochemical performance of the composite served as the anodic electrode for the Lithium ion batteries was investigated.The initial discharge capacity of CuCo2O4/C hybrid electrode was about 1100 mAh g'1,resulting in the first coulombic efficiency of 75%.After 100 cycles,the discharge capacity of the hybrid electrode still retains 592 mAh g-1 at the current density of 100mA g₁.Even when the current density increase to 4000mA g-1,the discharge capacity still retains 294 mAh g-1.In general,the hybrid electrode exhibited a better cycling stability and rate capability than the CuCo2O4 electrode.In the fourth chapter of the thesis,we successfully synthesized the 3D CuCo2O4@NiCo2O4 core/shell nanostructures by a two-step method and the corresponding heat-treatment process on Ni foam.Besides,the core/shell nanostructures were used directly as the electrode for SCs without using a polymer binder.The electrochemical tests were carried out in a three electrode electrochemical cell.The specific capacitance of the hybrid electrode is calculated to be 2029 F g-1,when the current density is 10 mA cm-2.After 4500 cycles,the specific capacitance of the hybrid electrode still retains 1548 F g-1.When the current density increases from 2 to 30 mA cm-2,the specific capacitance still retains 1551 F g-1?61.6%of the initial value?.The hybrid electrode exhibits the higher specific capacitance and much better rate performance than that of CuCo2O4 electrode.