The effect of the cutoff voltages on the working voltage decay and cyclability of the lithium-rich manganese-based layered cathode (LRMO) was investigated by electrochemical measurements, electrochemical impedance spectroscopy, ex situ X-ray diffraction, transmission electron microscopy, and energy dispersive spectroscopy line scan technologies. It was found that both lower (2.0 V) and upper (4.8 V) cutoff voltages cause severe voltage decay with cycling due to formation of the spinel phase and migration of the transition metals inside the particles. Appropriate cutoff voltage between 2.8 and 4.4 V can effectively inhibit structural variation as the electrode demonstrates 92% capacity retention and indiscernible working voltage decay over 430 cycles. The results also show that phase transformation not only on high charge voltage but also on low discharge voltage should be addressed to obtain highly stable LRMO materials.
The effects of contamination overlayer and density as well as surface and interface roughnesses on the x-ray reflectivity of a SiO2 ultrathin film are discussed from viewpoints of experiment and theory. Grazing incidence x-ray reflectivity (GIXRR) is used to accurately measure physical structures of SiO2 ultrathin films grown on Si substrate by effectively resolving deviations caused by a contamination overlayer (i.e., H2O and carbonaceous compounds). GIXRR results indicate that only the thickness accuracy of a SiO2 film is largely affected by the overlayer. The thickness of a SiO2 film obtained from GIXRR peak extrema and theoretical fitting reveals that if a SiO2 film with the thickness of 2.64nm is considered as a single layer, the H2O overlayer with a thickness of 0.55nm makes the thickness of the SiO2 film increase to 3.09nm, and the deviation is about 17% from its corrected thickness. By evaluating the GIXRR results of three repeating measurements of a nominal 4-nm SiO2 film, its density, thickness, and surface and interface roughnesses are 2.43±0.01g∕cm3, 3.99±0.03nm, and 0.40±0.02nm and 0.25±0.02nm, respectively.
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