Sol-gel Method To Synthesise And Modified Li₂FeSiO₄

Lithium ion batteries (LIBs) have been a hot spot because of their high specificcapacity, long life, high safety performance and environmently friendly advantages.Hitherto, LIB are not only widely used in mobile phones, laptops and other portableequipment, but also in electric vehicle (EV) or hybrid electrical vehicle (HEV). LIBshave very broad prospects as important energy storage and conversion divices. Thecathode material plays a key role in LIBs and determines the energy density oflithium-ion batteries. So far, the energy density of LIB cathode materials can notreach the requirements of electrical equipment, which needs further improvement.Li2FeSiO4as a new generation cathode material of LIBs, has high theoreticalcapacity (each Li2FeSiO4molecule can possess intercalation/deintercalation of twolithium ions, when a lithium-ion reversible intercalation/deintercalation is completed,its specific capacity is166mAh/g, when two reversible lithium ionsintercalation/deintercalation is carried out, its theoretical specific capacity can reach333mAh/g), a good safety performance and friendly to the environment. Howeverthe poor electronic (about10-14s/cm) and ion conductivity,of Li2FeSiO4hinderits application in LIBs. Currently, carbon coating and metal cation doping areemployed to improve the electrochemical performance of lithium iron silicate. Inthis thesis, titanium doped Li2FeSiO4is prepared and its electrochemical propertiesare investigated as follows:Li2FeSiO4cathode material is prepared by sol-gel method where citric acid isused as the complexing agent, reducing agent and carbon source, FeNO3·9H2O asthe source of iron, and four ethyl orthosilicate (TEOS) as the silica source. Toexplore the effects of carbon content and temperature on Li2FeSiO4, the treatmenttemperature is set at600℃,650℃and650℃, and the molar ratio of citric acid toferric nitrate is1:1,1:1.5,1:2, respectively. Note that under650℃and the moleratio of citric acid to and nitric acid iron of1:1, the pure phase of Li2FeSiO4cathodematerial can be obtained. The electrochemical performance of Li2FeSiO4is improved by doping titaniuminto Li2FeSiO4molecules. Among the Li2Fe1-xTixSiO4samples (x=0.02,0.05,0.08,respectively) with different titanium doping amount, the Li2Fe0.95Ti0.05SiO4(x=0.05)has the best electrochemical performance: its first cycle discharge capacity can reach184.9mAh/g at the rate of0.2C. After five cycles its discharge specific capacityis168.5mAh/g, and the capacity retention is91.13%. Also, Li2Fe0.95Ti0.05SiO4hasa good cycle stability at1C charge and discharge rate.Due to the excellent electronic conductivity, graphene is introduced intoLi2Fe0.95Ti0.05SiO4. The Li2Fe0.95Ti0.05SiO4/GNS (GNS for graphene nano sheets)composite shows a higher discharge capacity (222.1mAh/g) for the first time at0.2C to compare pure Li2Fe0.95Ti0.05SiO4. After five cycles its discharge capacity is220.8mAh/g, and the capacity retention is99.41%. Thus, the electrochemicalperformance of Li2FeSiO4can be greatly improved by titanium doping and graphenecoating of Li2FeSiO4materials the, indicating a promising application in LIBs.