High Capacity Co₂VO₄ Anodes For Lithium Ion Batteries

Lithium ion batteries are one of the most important energy storage devices in everyday life.The use of lithium ion batteries pushes forward the development of human society and green energy industry.With the fast development of electrical cars and mobile electrical devices,traditional lithium ion battery technologies can not meet the ever-increasing demands in terms of energy density or cycling life.It is urgent to develop the high-performance lithium ion batteries with higher energy density,better rate capability and longer cycling life.One important approach is to develop new anode materials or to improve the electrochemical properties of anode materials.Vanadates,for examples,cobalt vanadate,zinc vanadate,are a class of new oxides that can be used to replace graphite as the anode material of lithium ion batteries because of it high specific capacity and good cyclability.Thus,vanadates become one hot research area in the field of Li-ion anode materials.In this thesis,two novel spinel C02VO4 with different morphologies and microstructure were synthesized via the glycine-combustion and hydrothermal routes.The electrochemical properties of the prepared anode materials were critcallly measured.We drew the conclusion as follows:(1)Porous C02VO4 powder synthesized via a glycine combustion route.Porous C02VO4 powder with fluffy microstructure in nanoscales was synthesized as the anode material of lithium ion batteries by glycine combustion.The obtained powder was then mixed with binder and conductive agents to form a slurry and cast on the surface of copper foil.The resulting anode of C02VO4 powder shows high specific capacity,over 730 mAh/g,at the current density of 0.1 A/g and relatively stable cycling properties.(2)C02VO4 nanosheets on the surface of 3D ultralight porous Ni foamAlthough fluffy porous Co2VO4 powder shows the good electrochemical properties as the anode material of lithium ion batteries,its electrochemical performance was not fully unleashed because of its poor electron conductivity.To further improve the electrochemical performance of Co2VO4,a hydrothermal method was used to anchor Co2VO4 nanosheets on the surface of 3D ultralight porous Ni foam.A thin amorphous carbon layer was then coated onto the surface of C02VO4 nanosheets to form a sandwich structure(PCC@Co2VO4@C).As a result,the unique anode had the rapid electron pathway and stable mechanical properties,which led to the excellent rate capabilities and cycling properties.At a current density of 1 A/g,the PCC@Co2VO4@C anode was able to deliver a stable reversible capacity of about 706.8 mAh/g even after 1000 cycles.Because the porous Ni foam was ultralight,the specific capacity of PCC@Co2VO4@C electrode based on the whole electrode was 2-5 times higher than those of 2D C02VO4 electrode at varied current densities.(3)Mechanism studies of the lithiation and delithiation of Co2VO4.Transition electron microscopy and X-ray diffraction techniques were employed to study the lithiation and delithiation process at different voltages.In combination with g the experimental results and relatd publications,a possible two-step route was proposed:First,C02VO4 was converted to Co and Lix+yVO2 during the first lithiation.Second,the intercalation and conversion reactions were combined as follows:CoO + LixVO2 +(y +2)Li+ +(y + 2)e-(?)Co + Li2O +Lix+yVO2.At last,the thesis developed a novel materials of Co2VO4 which shows high capacity and long cyclability as the anode of lithium ion batteries.With 3D structural design,the 3D sandwiched C02VO4 delivered a stable reversible capacity of about 706.8 mAh/g after 1000 cycles at the current density of 1 A/g,which is higher than that of graphite used in commercial lithium ion batteries now.