Design And Fabrication Of Co-based Nanocomposites And Study Of Their Electrochemical Performances

Supercapacitor(SCs)and lithium ion batteries(LIBs)now are effective and widely used energy storage devices.With the rapid development of the markets,such as portable electronic devices and electric cars,high powder and energy density are required desperately.Carbon materials are still the main electrode materials of commercial SCs and LIBs.But,because of the limitation of itself(low specific capacitance/capacity),carbon materials cannot meet the requirements of high powder and energy density.Transition metal oxide/hydroxides,which store charges via external and internal redox reactions,can easily achieve high specific energy/powder indicating that transition metal oxide/hydroxides have important application prospect.Nevertheless,the disadvantages such as low conductivity and severe volume expansion during the discharge/charge process put sand on the wheel of its application in high-performance energy storage devices.It is an effective strategy that building composites with nano structured electrode materials and carbon materials with high conductivity and mechanical stability to solve these problems.Nano structuring of electrode materials have many advantages such as shorter ion transport/diffusion path,more active reaction sites,lower resistance between the electrode/electrolyte interfaces.And carbon materials with high conductivity and mechanical stability can increase the conductivity and accommodate the volume change during cycling sustaining the stability of electrode.Based on this idea,the main research contents are listed as follows:1.Uniform ultra-fine single-crystal Co2(CO3)(OH)2 nanowires,with a diameter of 15 – 25 nm,were fabricated via a glycol-assisted hydrothermal reaction.Graphene oxide(GO)sheets were prepared by improved Hummers method.Their water solutions have different Zeta potential in different pH value.Then the positively charged nanowires were attached to negatively charged GO sheets by electrostatic interaction.Then GO was in situ reduced by a high pressure hydrothermal method.Eventually,a unique self-assembled rGO/Co2(CO3)(OH)2 wires/sheets mutual-supporting open architecture was successfully prepared.In a three-electrode configuration,we tested the pseudocapacitances of r GO/Co2(CO3)(OH)2 composite and pure Co2(CO3)(OH)2 and explored the effect of rGO.An asymmetric SC fabricated by using r GO/Co2(CO3)(OH)2 composite as positive electrode and activated carbon as negative electrode has demonstrated high energy/power density(26.7 Wh kg-1 at 751 W kg-1 and 13.1 Wh kg-1 at 15362 W kg-1)and outstanding cycle stability(10000 times with only 5.4% loss).2.rGO/CoO wires/sheets mutual-supporting porous composites were prepared via annealing GO/Co2(CO3)(OH)2 composite in the previous chapter.During the heat treatment,Co2(CO3)(OH)2 nanwires and GO were in situ transformed to CoO and rGO.The procedure is obviously effective to enhance the adhesion force between CoO nanowire and rGO.So electron can freely transfer from CoO to rGO.And rGO sheets are able to act as an expressway for charge transfer.So the conductivity of the electrode is improved.CoO nanowires and rGO can inhibit the aggregation of themselves.In addition,the voids between CoO nanoparticles and flexible rGO sheets can buffer the stress from the volume change during discharge/charge.This series of synergistic effects make rGO/CoO own high electrochemical performance.The reversible specific capacity can reach 504 mA h g-1 at the current density of 5 A g-1;After cycling 750 times at 3 A g-1,the specific capacity still sustained 520 mA h g-1.3.We have successfully inserted a high-quality rGO sheet between two mesoporous NiCoO2 nanosheets(~ 2 nm)to form an ultrathin randomly wrinkled NiCoO2/rGO/NiCoO2 sandwich pieces(~ 6 nm)using a facile and scalable chemical co-precipitation method.As these randomly wrinkled sheets stack together,they mutually brace each other to form a hierarchical porous composite architecture.We simultaneously explored the forming process of NiCoO2 and found generated HCHO can effectively reduce GO in the reaction process.The high quality rGO endows the composite with excellent conductivity.And the firm adhesion between NiCoO2 sheet and flexible rGO also guarantees the integration of the electrode.In a three-electrode configuration,the reversible specific capacitance can reach 784 F?g-1 at the current density of 5 A g-1;After cycling 2000 times at 20 A g-1,the capacitance still sustained87.6%.As an electrode of LIBs,the reversible specific capacity can reach 706 mA h g-1 at the current density of 1.6 A g-1;After cycling 350 times at 1 A g-1,the specific capacity still sustained 595 mA h g-1.4.A facile and scalable in situ sol-gel strategy combined with carbon thermal reduction is developed to fabricate carbon encapsulated extra small Co3O4(average diameter ~ 15.7 nm)homogeneously embedded in porous graphitic carbon(Co3O4@C@PGC)nanosheets of ~ 20 nm thickness.We explored the effect that NaCl played in fabricated Co3O4@C@PGC nanosheets and the lithium storage mechanism of Co3O4@C@PGC nanosheets.Co3O4@C@PGC nanosheets show excellent lithium storage performance.The reversible specific capacity can reach 1413 m A h g-1 after cycling 100 times at 0.1 A g-1.At 20 A g-1,the reversible specific capacity still can maintain 345 mA h g-1.After cycling 1000 times at 5 A g-1,the specific capacity still can sustain 760 mA h g-1.