Study Of Cobalt Compounds As The Electrode Materials Of Super Capacitor And Li-ion Battery

Surpercapacitor is a new energy storage device with high energy density,high charge-discharge efficiency,excellent reversibility,long cyclic performance and high power density.Lithium-ion batteries are considered as the most promising power sources because of their high energy density,high voltage and long cycle life.Transition metal oxides have been widely used as electrode materials for Supercapacitor and Li-ion Battery due to their high theoretical capacity and excellent electrochemical properties.In this paper,Co（OH）₂,Co₃O₄ and NiCo₂O₄ micro-nanowire electrodes were synthesized using a template-free synthetic route,and characterized by SEM,XRD and TEM.The electrochemical performances of electrodes were investigated by cyclic voltammetric measurements,constant current charge/discharge measurements and electrochemical impedance spectroscopic measurements.Hybrid capacitors,with Co（OH）₂,Co₃O₄ and NiCo₂O₄ micro-nanowire electrodes as anode and activated carbon electrode as cathode,were fabricated separately.Coin type cell were fabricated separately using Co₃O₄ and NiCo₂O₄ micro-nanowire electrodes as work electrode and lithium metal sheet as both auxiliary and reference electrode.The performances of capacitors and cells were investigated by cyclic voltammetric measurements,constant current charge/discharge measurements and electrochemical impedance spectroscopic measurements.XRD result shows that Co（OH）₂ is brucite structure,and Co₃O₄ and NiCo₂O₄ nanowire arrays are spinel structure with high structural integrality.SEM and TEM results show that the Co（OH）₂ and Co₃O₄ arrays were composed of micro-nanowires with diameters of about 500 nm and lengths up to 10～15 μm.The NiCo₂O₄ arrays were made up of micro-nano wires with diameters of around 540 nn and lengths up to 10～15 μm.The maximum specific capacitance of Co（OH）₂ micro-nanowire electrode reached 635.3 F·g-t measured by constant current charge/discharge experiments.After 500 cycles,the specific capacitance maintain at 466.7 F·g^-1.EIS result shows that charge transfer resistance is only 0.13 Ω and increased only by 0.03Ω after 500 cycles.With the increase of KOH electrolyte concentration,the specific capacitance increased first and then decreased.The maximum specific capacitance of Co（OH）₂//AC hybrid capacitor reached 47.5 F·g^-1.After 500 cycles the capacitance maintain at 36.3 F·g^-1.EIS result shows that charge transfer resistance is only 0.7Ω.The maximum specific capacitance of Co₃O₄ micro-nanowire electrode reached 746.5 F·g^-1 measured by constant current charge/discharge experiments.After 500 cycles,the specific capacitance maintain at 626.9 F·g^-1.EIS shows that charge transfer resistance is only 0.35 Ω and increased only by 0.04 Ω after 500 cycles.The specific capacitance increased with the increase of KOH electrolyte concentration.The maximum specific capacitance of Co₃O₄//AC hybrid capacitor reached 49.3 F·g^-1.The specific capacitance maintain at 33.0 F·g^-1 after 500 cycles.EIS result shows that charge transfer resistance is only 3.0 Ω.The specific capacity of the original Co₃O₄ micro-nanowire Li-ion coin type cell are 1400 mAh g^-1 and 854 mAh g^-1 in the first discharge and charge at 0.1 C,respectively.After 10 cycles,the discharge specific capacity maintain at 880 mAh·g^-1.The maximum specific capacitance of NiCo₂O₄ micro-nanowire electrode reached 739 F·g^-1 measured by constant current charge/discharge experiments.The specific capacitance retained 548.4 F·g^-1 after 500 cycles.EIS result shows that charge transfer resistance is only 0.26Ω,and increased only by 0.12Ω after 500 cycles.The maximum specific capacitance of the NiCo₂O₄//AC capacitor reached 75 F-g"1.The specific capacitance maintain at 43 F·g^-1 after 500 cycles.EIS result shows that charge transfer resistance is only 1.05 Ω.The specific capacity of the NiCo₂O₄ micro-nanowire Li-ion coin type cell are 994.4 mAh g^-1 and 686.7 mAh g^-1 in the first discharge and charge at 0.1C,respectively.After 10 cycles,the reversible specific capacity maintain at 667 mAh·g^-1.When the discharge/charge rate increased to 1 C,29%of the capacity can be retained.Considering the easy synthesis of the micro-nanowire arrays electrode without conducting material and binder,the free standing NiCo₂O₄ micro-nanowire material will be considered as anode for practical Li-ion Batteries.