The Research Of Preparation And Electrochemical Properties Of NiCo₂O₄Nanomaterials

With the advent of the energy crisis and the environmental pollution, thedevelopment of clean and renewable and new energy technology is one of the mostdecisive influence in the future world economy. Because of high working voltage,large energy density, long cycle life, light weight, low self-discharge andpollution-free, lithium ion battery have been widely applied in mobile phones, laptopsand digital camera and is developing towards the fields of electronic vehicle, spacetechnology and national defense industry. The main attack of lithium ion battery hasimproved on performances and decreased costs and the key lies in developing the highperformance and low cost electrode materials. Electrochemical capacitor is a new typeof energy devices with high power density and good cycle life, which can be used asnot only the backup power sources for computers, but also the driving power systemsfor electric vehicle combined with lithium ion batteries. Therefore, electrochemicalcapacitor is also the hotspot in new type of chemical energy device studies.The present thesis reviewed the newest development in research of electrodematerials for electrochemical capacitors and lithium ion battery, then prepared theNiCo2O4nanosheet arrays by electrodeposition way. The micro-structures andmorphologies of the electrode materials were characterized by scanning electronmicroscopy (SEM) transmission electron microscopy (TEM) and X-ray diffraction(XRD) tests. The electrochemical performances of the electrode materials wereevaluated by cyclic voltammetry (CV), galvanostatic charge-discharge capability andelectrochemical impedance spectra (EIS). The main content is the following:(1) The nickel cobaltite nanosheet arrays were prepared by electrodeposition oncarbon cloth in both Ni(NO3)2and Co(NO3)2mixed aqueous solution. The structuresand morphologies of the electrode materials were characterized by SEM, TEM andXRD tests. The results of XRD show the diffraction peaks of the materials can beindexed as the crystal planes of NiCo2O4(JCPDS No.73-1702), respectively. TheSEM results reveal that the electrode materials display a mutual crisscross nanosheetarrays, which have a thickness of ca.10nm. The unique structure can provide thechannels not only on the surface but also in the solid bulk, which is conductive tomaking full use of the electroactive sites to take place the redox reaction. The TEMfurther observations suggest that the nanosheets are constructed by multiple particles (diameter ca.5nm).The electrochemical performances of the electrode materials wereevaluated by CV, galvanostatic charge-discharge capability and EIS measurement.The results show that as-obtained NiCo2O4electrode materials have excellentelectrochemical performances and its specific capacitance reach2506F/g at a currentdensity of2A/g. The specific capacitance of the electrode materials can keep80%initial capacity at the current density of10A/g after3000cycles. This indicate thatthe electrode materials with high specific capacity and good cycle stability is apromising electrode material for electrochemical capacitors.(2) The above-mentioned NiCo2O4electrode materials were assembled intolithium ion battery and evaluated the performances. Its electrochemical performanceswere measured by cyclic voltammograms, charge-discharge performance and cycleperformance test. The experimental results show its excellent cycle performance andhigh rate performance. The first discharge capacity is2147mAh/g at the currentdensity of200mAh/g and the coulombic efficiency is99%. The reversible capacityretained1502mAh/g after50cycles. It is thought that for the one hand NiCo2O4nanosheets can provide more Li-intercalation sites, for the other hand its uniquestructure facilitates the Li ion penetration in the active material and cycling stability.(3) The graphene/NiCo2O4composite materials were prepared byelectrochemical deposition method on carbon cloth. The materials were characterizedand measured the electrochemical performance. The results show that the morphologyof the materials is a kind of unique, which nanosheets and grapheme layer wereconnected into the net structure. Thus, it is advantageous to the electronictransmission and maintain the stability of the structure, and will be a promisingelectrode materials of electrochemical capacitor.