Preparation And Electrochemical Properties Of Spinel AB₂O₄Nanophase Cathodematerials For Lithium Ion Batteries

Nowadays, lithium-ion batteries (LIBs) have become extremely important power sourcesfor use in portable electronic and high energy areas including electric vehicles, hybrid electricvehicles and plug-in hybrid electric vehicles. To meet the requirements, further improvementsin terms of energy density, power density, lifespan are required. The performance of LIBs isdetermined mainly by the choice of anode and cathode materials with the specific morphology,size. Nanostructured transition metal oxides, such as Co3O4、Fe3O4、MnO2、Mn3O4, show atempting prospect as alternative cathode electrode materials for lithium secondary batteriesbecause nanostructures usually provide large surface area, short distance for charge transport,exibility for volume change and improved kinetic properties. Compared to conventionalcarbon cathode electrode materials, they have higher discharge capacities. However, duringthe cycle of lithium batteries, these simple metal oxides have some drawbacks like largechanges in volume during Li+insertion and extraction and the poor cycling performance.Recently, combinations of two transition-metal oxides in spinel-like structures, such as AB2O4,can overcome the above problems to some extent and have shown the excellent cyclingperformance. Therefore, this dissertation focuses on the preparation and electrochemicalinvestigation of hollow CoMn2O4, hollow ZnFe2O4, hollow core-shell ZnCo2O4and hollowcore-shell ZnMn2O4. The main content is as follows:(1) CoMn2O4microspheres were fabricated by one-step solvothermal route usingCo(Ac)2·4H2O and Mn(Ac)2·4H2O. The crystalline phase and the morphology werecharacterized by X-ray diffraction and transmission electron microscopy. The electrochemicalproperties of the composites were evaluated by the LAND testing system. When the composites were tested as cathode materials for lithium ion batteries, the results displayed thatthe CoMn2O4samples exhibit a first discharge capacity as1500mAh/g at a high currentdensity of100mA/g, and it remains at863mAh/g after50cycles and tends to be stable.Therefore, it is confirmed that the as-resultant products display excellent cyclingperformances.(2) Hollow ZnFe2O4microspheres were successfully fabricated by one-stepsolvothermal route using Zn(Ac)2·4H2O and FeCl3·6H2O as raw material. The crystallinephase, morphology and the composition were characterized by X-ray diffraction、Scanningelectron microscopy、Transmission electron microscopy and Thermogravimetric Analysis,respectively. The results showed that the diameter of hollow ZnFe2O4microspheres is300nmand the shell thickness is25nm, respectively. The electrochemical tests show when thecomposites were tested as cathode materials for lithium ion batteries, the results displayed thatthe ZnFe2O4samples exhibit the first discharge capacity of1546mAh/g at a high currentdensity of100mA/g, and it remains at826mAh/g after50cycles and tends to be stable.Therefore, it is confirmed that the as-resultant products display excellent cycling performance.(3) Yolk-shelled ZnCo2O4microspheres were prepared through a novel two-step method.The first step includes the facile method synthesis of metal glycolate microspheres using PVPas complexing agent. In the second step, the metal glycolate microspheres are annealed in airat500℃for2h with a ramp rate of1℃/min to generate the yolk-shelled ZnCo2O4microspheres. The crystalline phase and the morphology were characterized by XRD andTEM, respectively. The data indicates that the outer shell of yolk-shelled ZnCo2O4microspheres is approximately55nm in thickness and the inner yolk is about600nm indiameter. When the composites were tested as cathode materials for lithium ion batteries, theresults displayed that the ZnCo2O4samples exhibit a high discharge capacity as1484mAh/gat a current density of100mA/g, and it remains at1052mAh/g after50cycles. Therefore, itis confirmed yolk-shelled ZnCo2O4microspheres with excellent electrochemical performance are promising cathode materials for high performance lithium-ion batteries.(4) The hollow core-shell ZnMn2O4microspheres are successfully prepared by asolvothermal carbon templating method and then a annealing process. The crystal phase andparticle morphology of resultant ZnMn2O4microspheres are characterized by XRD and TEM.The electrochemical properties of the ZnMn2O4microspheres as cathode materials areinvestigated for lithium ion batteries. The results show that the ZnMn2O4microspheres exhibita reversible capacity of855.8mAh/g at a current density of200mA/g after50cycles. Even at1000mA/g, the reversible capacity of the ZnMn2O4microspheres is still kept at724.4mAh/gafter60cycles. The enhanced electrochemical performance suggests the promising potentialof the hollow core-shell ZnMn2O4microspheres in lithium-ion batteries.