The Synthsis And Electrochemical Performances Of Molybdenum Oxides For Lithium Ion Batteries

Molybdenum dioxide (MoO2) has attracted a lot of attentions in lithium ionbatteries field due to its high electrical conductivity, good structure stabilityand high charge/discharge capacity. However, the specific charge and dischargecapacities for MoO2are generally far below the theoretical value. The specificcapacity decays dramatically due to the formation of Li0.98MoO2with highresitance and the significant volume change during the lithiation and delithiatonprocesses. Moreover, unlike the traditional metal oxides, a charge/dischargecapacity increase of MoO2can be observed during the charge and dischargeprocess at the first30cycles. This uncommon phenomenon has not been wellunderstood and is usually described as the activation of the electrode material.Thus, the main contents of this thesis are summarized as following:(1) Introducing a hierarchical porous structure is an efficient way toimprove the cycling performance and rate capability of MoO2since thehierarchical porous structure is benefit for the transmission of the electrons andlithium ions. Hierarchial porous MoO2nanotubes were obtained through aone-step carbothermal reduction by using CMK-3both as a reducing agent andas a template. The weight ratio of MoO3/C, calcination temperture and time arethe main issues which influence the electrochemical performances of MoO2.The hierarchical MoO2nanotubes obtained at the MoO3/C weight ratio of10after calcination at820°C for90min exhibit the best cycling performances and rate capability. When cycled at a current density of100mA g-1after70cycles, adischarge capacity of720mAh g-1is obtained.(2) Based on the investigation of the pure hierarchical MoO2nanotubes,Mo2C with a high electrical conductivity introduced into the MoO2material forthe first time to improve the cycling performance and rate capability of MoO2.The hierarchical porous MoO2/Mo2C nanotubes are obtained through a one-stepcarbothermal reduction method. The weight ratio of MoO2and Mo2C is themain issue which influences the electrochemical performance of hierarchicalporous MoO2/Mo2C nanotubes. The hierarchical porous MoO2/Mo2C nanotubesshow the best cycling performance and rate capability when the weight of Mo2Cin the composite is30%. When cycled at a current density of200mA g-1, theMoO2/Mo2C composite delivers a discharge capacity of790mAh g-1after140cycles.(3) In situ carbon coating is an efficient way to improve the electricalconductivity and electrochemical performance of MoO2. A simplesoft-templating method is developed for in situ carbon coating. Resorcinol andformaldehyde were used as soft templates to react with AHM (Ammoniummolybdate tetrahydrate) to form a spherical MoO3/RF resin precursor under ahydrothermal condition. The ferriero chocolate like MoO2/C composite wasobtained after the calcination of the precursor at a N2atmosphere. The weightratio of MoO3/RF (resorcinol formaldehyde) resin and the calcinationtemperature influence the morphology and the electrochemical performances ofthe MoO2/C composite. Futher studies reveal that the MoO2/C compositeobtaind by adding3.20g resorcinol,3.09g AHM and7mL formaldehyde at acalcination temperature of600°C shows the best cycling performance. Whencycled at a current density of1000mA g-1after400cycles, a charge anddischarge capacity of520mAh g-1is still retained. (4) Unlike traditional metal oxides, the charge and discharge capacity ofMoO2material gradually increases during the first30cycles. This unusualphenomenon is generally ascribed to the activation of the electrode material.The phase transformation of MoO2during the charge and discharge process isobserved through an in situ XRD method. The increase of the charge anddischarge capacity of MoO2material at the frist30cycles is attributed thetransformation of lithiation mechanism of MoO2material. During the first30cycles, MoO2is first lithiated to Li0.98MoO2through insertion mechanism witha high reversibility. However, a few Li0.98MoO2is further lithiated to Mo andLi2O. After30cycles, the conversion between MoO2, Mo and Li2O is highlyreversible through conversion mechanism.