| With the developments of economy and battery technology, it should meet the needs of the application for electric automobile and intelligent energy storage power grid. LiMn2O4is a promising cathode of lithium ion battery because of its significant advantages, such as excellent rate capability, environmental friendliness and low cost. Ionic conductivity and electronic conductivity are two important parameters of influencing power density and energy density of Li-ion battery. The goal of this paper is to prepare LiMn2O4that shows the high power density and high energy density. Compared with doped LiMn2O4and LiMn2O4/G composites, the influence of ionic conductivity and electronic conductivity is researched.The LiM0.1Mn1.9O4(M=Mn, Al, Fe) were prepared by the sol-gel method using citric acid and glycol as the chelating agent. The XRD pattern indicates that the samples could be indexed as a spinel structure without any impurities such as Al2O3and Fe2O3. The doped Al and Fe can be clearly observed on the surface of the samples from the energy dispersive spectrometer (EDS) images. The TEM images show that the particle size is about-100nm. The SEM images reveal the particles aggregated into porous structures. Textural analysis was carried out by measuring the N2adsorption/desorption isotherms. The relatively big surface area such as LiMn2O49.317m2g-1and pore diameter distributions in range of3-6nm are obtained from adsorption/desorption isotherms. Among the samples, the galvanostatic charge-discharge testing shows that LiAl0.1Mn1.9O4sample exhibits the best electrochemical performance at high rate and at high temperature. A typical Li/LiAl0.1Mn1.9O4cell shows a discharge capacity of95.8mAh g-1at50C discharge rate (charged at5C) at25℃. The specific power and the specific energy of LiAl0.1Mn1.9O4are26.6kW kg-1and359Wh kg-1at discharge rate50C, respectively. The LiAl0.1Mn1.9O4obtained100.6mAh g-1after500cycles with a current of5C at25℃. Even at55℃,86.8mAh g-1of the discharge capacity and78%of the capacity retention ratio are maintained at a rate of5C after500cycles. The diffusion coefficient of Li+(DLi) calculated from cyclic voltammograms (CVs) is in the range of10-8-10-7cm2s-1. The Rct of LiAl0.1Mn1.9O4(89.3Ωcm-2) is lowest among the three half-cells from the EIS.LiMn2O4on the surface of graphene nanosheets composites are synthetized using graphene nanosheets and LiMn2O4prepared above. The XRD pattern can be ascribed to LiMn2O4with except the peak at20=26.6°that can be assigned to the (002) plane of graphene sheets. The intensity of peak at20=26.6°grows with the increment of G content. TEM shows that LiMn2O4is on the surface of graphene nanosheets. The LiMn2O4/G composite could improve the cycle performance at55℃. Among the three electrodes, LiMn2O4/5%G sample exhibits the best rate properties. It shows the discharge capacity about108.3mAh g-1at5C charge rate and50C discharge rate. The discharge capacity of101.6mAh g-1is obtained at5C charge rate and60C discharge rate. The specific capacities of104.3mAh g-1are obtained at20C current rate. It is shortened the time of charge to about2min at current rate20C. The specific power and the specific energy of LiMn2O4/5%G are11.4kW kg-1and400Wh kg-1at current rate20C, respectively. The diffusion coefficient of Li+(DLi) calculated from CVs is10-7-10-8cm2s-1. It shows that the graphene nanosheets could not improve the diffusion of Li’. The Rct is gradually reduced as increasing the content of the graphene nanosheets.The results show that electronic conductivity is to be dominant in the conductivity of LiMn2O4at room temperature. The Al doped LiMn2O4maybe stabilize the structure of LiMn2O4then improve the electronic conductivity. The LiMn2O4/G composite improves the electronic conductivity between particles and particles and obtains the high power density and high energy density. |
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