Preparation And Energy Storage Properties Of LiMn₂O₄ Electrode Materials

Increasing demands for energy are pushing the research on rechargeable energy-storage devices with high-energy and high-power densities.Asymmetric supercapacitors?ASCS?with a battery-type electrode and a capacitor-type electrode have been regarded as the most promising energy-storage devices to meet the need.Two of LiMn₂O₄ microspheres with similar sizes but adjustable hollow structures have been synthesized using a new and simple approach.Their performance as cathode materials for lithium ion battery applications has been studied and correlated with the hollow structural parameters.Through a solid state reaction between LiOH and mesoporous MnO₂ obtained by the complete decomposition of MnCO₃,LiMn₂O₄ hollow spheres with a robust wall and small cavity size are prepared?LiMn₂O₄-porous?.The fusion of the mesopores and the Kirkendall effect during the lithiation process are responsible for the hollow interior formation.In another approach through partial decomposition and selective etching,LiMn₂O₄hollow spheres with thin walls are prepared?LiMn₂O₄-hollow?.It is demonstrated that LiMn₂O₄-porous shows larger capacity,superior rate capability,and better cycling stability compared to LiMn₂O₄-hollow.The former exhibits an initial discharge capacity of 149 mAh·g^-1,maintains 96.7%of the initial capacity after 200 cycles at1C and 84%of the capacity at 1C when discharged at 10C.It is the first time to use LiMn₂O₄ microspheres with porous and hollow microstructures as cathode materials and activated carbon?AC?as the anode electrode for AC//LiMn₂O₄ supercapacitors.Cyclic voltammetry and capacity measurements were used to investigate their electrochemical behaviors.The test results demonstrated that porous LiMn₂O₄ microspheres showed a higher reversible capacity and rate capability compared to hollow LiMn₂O₄ microspheres,which could deliver a specific capacitances of 536 F·g^-1at 1mV·s^-1.The AC//porous LiMn₂O₄supercapacitor showed better electrochemical performance,which delivered 33.12Wh·kg^-1at the power density of 90 W·kg^-1.Porous LiMn₂O₄ hollow microspheres were facilely prepared by incorporation of Li and Mn elements into a spherical polymeric precursor through copolymerization of lithium and manganese acetates with resorcinol and hexamethylenetetramine and then burning off the organic matrix at appropriate temperatures in air.The sample inherited the spherical morphology of the polymeric precursor but showed hollow porous structure assembled by nanocrystals of about 50-100 nm in size.When tested as cathode of Li-ion batteries,the LiMn₂O₄ hollow spheres exhibited excellent rate capability and cycle stability.A discharge capacity of above 101 mAh g^-11 was maintained at 10C(1 C=148 mAh g^-1),and the cells can still maintains 92%of the initial capacity after 1000 cycles at 0.5C.With such excellent electrochemical properties,the prepared LiMn₂O₄ hollow microspheres could be promising cathode of Li-ion batteries for long term and high power applications.