Studies On Lithium Storage Properties Of Molybdenum-based Oxide Composite Materials

Metal oxides are considered as potential anode materials for lithium ion batteries due to their abundant resources,low cost and high theoretical specific capacities.Among them,molybdenum-based oxides(molybdenum oxides and molybdate compounds)have been widely investigated and confirmed to have high initial charge-discharge capacity and relatively stable cycle performance.However,the volume expansion effect caused by the irreversible conversion reaction of metal oxide materials at low potentials also leads to huge capacity decay,which limits its large-scale application.Therefore,composite with other materials may be one of the effective strategies to improve the electrochemical performance.Thus,in this paper,we take molybdenum dioxide(MoO2)and lithium molybdate(Li2MoO4)as the research objects,and study their composition properties.The structure and reaction mechanism of the composite materials for lithium storage have been studied.The main work is as follows:1.With molybdate and bismuth nitrate pentahydrate as raw materials,together with anhydrous citric acid as chelating agent and carbon source,Bi/MoO2/C composite materials with Bi/MoO2 nanoparticles embedded in the carbon layer were prepared by sol-gel method and thermal reduction process.Electrochemical test results show that the cycling and rate properties of the composite are obviously better than those of Bi/C and MoO2/C.EIS,CV,GITT and other test results show that the reaction of lithium bismuth alloy improves the migration rate of ions in the material.The supporting effect of carbon layer also inhibits the volume expansion and enables the stability during cycles.2.Li2MoO4 was synthesized by sol-gel method with lithium carbonate and molybdenum trioxide as raw materials.On this basis,carbon nanofibers(CNF)were added for modification(They are denoted LMO and LMO@CNF,respectively).The morphology characterization shows that LMO crystal tends to be in the shape of hexagonal rods,and the morphology of LMO@CNF is carbon fiber wrapped rods,with small particles embedded in the CNF network.The electrochemical performance test results show that LMO@CNF has higher initial coulombic efficiency than LMO.At a current density of 100 mA g-1,it has a reversible specific capacity of up to 831 mAh g-1 from the second cycle.The lithium storage process and mechanism of LMO@CNF were analyzed by in situ XRD,ex situ TEM and ex situ XPS.The results show that the CNF conductive support network alleviates the stress expansion of LMO particles during the conversion reaction process and enables isolated LMO particles to be reaction active.The reversible Li-ion lithiation/delithiation process in LixMoyOz nanocrystalline phase allows LMO@CNF to maintain a high lithium storage capacity at extended long cycles.3.NCM//LMO@CNF full lithium ion battery was fabricated using NCM ternary material(LiNi0.83Co0.12Mn0.05O2)as positive electrode and LMO@CNF as negative electrode,and its electrochemical characteristics were studied.The results show that the prepared lithium-ion battery still has a high specific capacity of 149 mAh g-1 after 150 cycles at a current density of 200 mA g-1.This experiment confirms the feasibility of using LMO@CNF as the anode material of full battery,and provides experimental reference for the practical production and application of metal oxide anode material.