Design And Properties Of Transition Metal Oxide-based Anode Materials For Lithium Ion Batteries

Developing renewable energy and reducing dependence on fossil fuels have become the consensus of the international community.Many renewable energy sources are intermittent in nature,so storage must be incorporated as part of any energy solution.Lithium-ion batteries?LIBs?now are the fastest growing energy storage system as compared to all other technologies.Thus improving their energy storage capacity is the subject of ongoing research for inclusion in many modem applications.The anode material usually plays a key role in the determination of the safety and cycling stability of LIBs.Among anode materials,transition metal oxide has been considered as one of the most promising anode candidates because it has high theoretical capacity,low working plateau,and excellent thermal and structure stability.However,transition metal oxide-based batteries always suffer from some major problems,including inherent poor electrical conductivity and large volume change during the charge/discharge process,leading to serious capacity decay,which severely restricts their practical application.To solve this problem,many different ways have been developed.One of the ways is to prepare transition metal oxide with different morphologies.And the other way is to develop a composite electrode with carbon-based materials or other transition metal oxide.In this thesis,the structural design of anode materials for LIBs was studied in detail and several transition metal oxide-based carbon composites were prepared to obtain excellent electrochemical properties.The detailed research contents mainly include the following four works:1.By preparing transition metal oxide with different morphologies,benefiting from rationally designed structures with more active sites and superior mechanical stability,the rapid-rate capability and cycling stability can be improved.Dandelion-like mesoporous Co₃O₄ nanomaterials with good morphology were prepared by simple hydrothermal method and followed by calcination at 400oC in air.Dandelion-like Co₃O₄ material is composed of many secondary nanoneedle structures.These needles are composed of small nanoparticles which form porous structure between these nanoparticles,which can effectively alleviate the volume expansion during charging/discharging process.The results show that the first discharge capacity of the prepared electrode material can reach 1430.0 mA h g^-1.At the same time,high discharge capacity of 1013.4 mA h g^-1can be obtained after 100 cycles at 0.2 A g^-1.In addition,the nanomaterials also show satisfactory rate performance.The preparation method is simple and the electrochemical performance is excellent.The dandelion-like mesoporous Co₃O₄ material is expected to be a candidate material for the next generation of lithium-ion battery anode materials.2.Heteroatom doping is another important way to improve the electrochemical properties of electrode materials,which can obviously enhance the electrical conductivity.Based on the typical cubic structure of MOF-5,a series of cobalt-doped bimetallic oxides Co_xZn_1-x-x O?x=0,0.05,0.10,0.50?were successfully synthesized by adjusting the molar ratio of cobalt to zinc by solvothermal method and subsequent calcination treatment.The morphology,microstructure,specific surface area and electrochemical properties of the obtained materials were systematically studied.X-ray diffraction analysis showed that the Co_xZn_1-xO material had pure ZnO phase.Scanning electron microscopy?SEM?and transmission electron microscopy?TEM?images confirm that the porosity and specific surface area of the material increase with the appropriate doping amount of cobalt.Compared with single metal material,cobalt doping improves the conductivity of the material,which benefits from the special cubic porous structure.It can effectively adapt to the volume expansion of ZnO during charging/discharging,facilitate charge transfer and enhance the stability of electrode structure.In addition,owing to the synergistic effect of cobalt and zinc,the electrochemical performance of Co_0.10Zn_0.90O products as anode materials of LIBs is significantly enhanced.After 100 cycles,the reversible capacity of Co_0.10Zn_0.90O composites reaches 949.5 mA h g^-11 at 0.2 A g^-11 with excellent rate performance and cycle stability.This work provides a method for the production of transition metal oxide electrodes with excellent electrochemical performance and industrial potential.3.Other than structure design,the introduction of biomass derived carbon materials can not only reduce production costs,but also overcome the inherent low conductivity of transition metal oxide materials.A novel and simple method for preparing G-Co/CoO-3D SPDCF composites using shaddock peel derived carbon as carbon source was proposed.Due to the large specific surface area of graphene 3-dimensional shaddock peel derived carbon foam?G-3D SPDCF?,Co/CoO nanoparticles are easier to be immobilized and uniformly dispersed on the surface of G-3D SPDCF.G-Co/CoO-3D SPDCF as anode material of LIBs shows good lithium ion storage capacity,improves rate performance(average reversible capacity is about 405 mA h g^-1at 2 A g^-1high current density),and enhances cycle stability(capacity can reach 600 mA h g^-11 after 80 cycles at 0.2 A g^-1current density).The electrochemical properties of G-Co/CoO-3D SPDCF composites have been improved mainly because the surface area of the composites has been increased by the nanosheets and porous structure of G-3D SPDCF so that a large number of Co/CoO nanoparticles are evenly dispersed on G-3D SPDCF.The porous carbon material is also conducive to the diffusion and electron transfer of lithium ions.In addition,G-3D SPDCF has good mechanical ductility.The G-3D SPDCF can be used as a buffer matrix to reduce the huge volume expansion during charging/discharging.The synthesis method is simple,low cost and high efficiency,and may become a promising large-scale production method.Importantly,as a kind of biomass energy,pomelo peel provides a direction for the preparation of other carbon-based composite materials.4.By screening the thermal conditions,ZIFs can be selectively transformed into highly electro-conductive carbons or metal oxides.Composite of ZIFs-derived functional materials with other materials can improve electrochemical performance.An unique Co₃O₄ coated with CNT composites?Co₃O₄@CNT?were designed and fabricated using novel Co-based zeolite imidazole frameworks?ZIFs?as precursor.ZIF-67@CNT composite was synthesized by coprecipitation of cobalt ions and 2-methylimidazolium salts and then CNT.The composite was composed of polyhedron with smooth surface coated by uniform CNT,and its size was about 1?m.ZIF-67@CNT nanocomposites were converted into Co@CNT nanocomposites after pyrolysis in nitrogen at 500°C.Then Co₃O₄@CNT nanocomposites were obtained by air calcination at 300°C after cooling and good size distribution and morphology were maintained.The most significant difference between ZIF-67 and Co₃O₄polyhedrons is that the latter form mesoporous structure,which is due to the release of CO₂ and H₂O during the first calcination.TEM characterization further confirmed the existence of pore and intertwined and interpenetrated carbon nanotubes in the Co₃O₄polyhedron.As a negative material for lithium ion batteries,Co₃O₄@CNT nanocomposites exhibit good cyclic stability(1014.2 mA h g^-11 capacity after 150cycles at 0.1 A g^-1)and good rate performance(625 mA h g^-1capacity at 1.0 A g^-1).Notably,The in-situ synthesis of Co₃O₄@CNT from ZIF-67@CNT improves the combination between Co₃O₄ nanoparticles and CNT substrate,ensures high structure stability and facilitates the electron and ion transport upon cycling.This method can also be extended to the preparation of ZnCo₂O₄@CNT composites using heterogeneous bimetallic ZIF-67@CNT?Zn,Co?as precursor.