Investigation On The Electrochemical Performances Of Flower-like NiCo₂O₄ Anode Material With Surface Modification

To meet the increasing reqirements for high-energy density of lithium ion batteries,binary transition metal oxide NiCo2O4 has been paid much attention due to its high specific capacity,which is considered as a promising altemtive for commercial carbon anodes.Unfortunately,large volume expansion of NiCo2O4 occurs during the charge/discharge process,resulting in the destroy of crystal structure and consequently poor cycling stability.On the other hand,NiCo2O4 also suffers poor rate performances and large initial irreversible capacity loss.In terms of its poor rate and cycling performances mentioned above,surface-modification with TiO2,Fe2O3 or Al2O3,has been carried out to improve the electrochemical performances of NiCo2O4 anodes.The enhanced mechanism of improved performances is investigated using electrochemical measurements and Kevin probe atomic force microscope so on.The main work in this thesis is divided into three parts:Firstly,porous flower-like NiCo2O4 precursors are synthesized via hydrothermal method using PVP as template and surfactant,and TiO2 nanoparticles are deposited on NiCo2O4 surface by low-temperature hydrolysis.The obtained results reveal that surface modification with TiO2 significantly improves the cycling stability and rate performance of NiCo2O4 anodes.The enhanced performances would be attributed to the suppression of HF-attacking in the electrolyte and the volume expansion of NiCo2O4 during the charge/discharge process.Analysis from Kelvin probe force microscopies indicates that TiO2-coating would reduce workfunction,low interfacial charge transfer resistance and enhance lithium-ion diffusion.Secondly,Fe2O3 nano-layer is coated on the surface of the as-prepared NiCo2O4 by hydrothermal process.It is found that Fe2O3 nanoparticles are uniformly deposited on NiCo2O4 surface and improve the electrochemical performances.It is expected that the formation of metal Fe in Fe2O3 in the lithium-ion insertion/extraction process,would facilitate electron-transfer in composites.Furthermore,surface-modification with Fe2O3 could also enhance structure stability,resulting in improved cycling stability and rate performances.Finally,Al2O3-layer is deposited on NiCo2O4 surface by chemical deposition.The results show that Al2O3-coateing significantly improves the cycling stability and rate performance of NiCo2O4 anodes.Analysis from electrochemical measurements reveals that surface-modification of Al2O3 layer would efficiently suppress HF-attacking in the electrolyte and the volume expansion of NiCo2O4 during the charge/discharge process.On the other hand,Al2O3-coateing wolud reduce workfunction,low interfacial charge transfer resistance and enhance lithium-ion diffusion.