Browsing by Author "He, Y"

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    An electron energy loss spectroscopy and electron diffraction study of the Pmnb polymorph of Li2MnSiO4
    (Elsevier, 2013-02-25) Gummow, RJ; Blackford, MG; Lumpkin, GR; He, Y
    The Mn valency and the crystallinity of Li2MnSiO4 cathodes (Pmnb form) were examined with electron energy-loss spectroscopy (EELS) and selected area electron diffraction (SAED) both before and after electrochemical lithium extraction. A decrease in the crystallinity of the delithiated charged cathode particles compared to the as-prepared material was observed. The decrease in crystallinity varied from particle to particle. EELS analysis showed that the non-uniform decrease in crystallinity was due to a non-uniform extraction of lithium from the particles. The observed decrease in discharge capacity of the Pmnb polymorph of Li2MnSiO4 with cycling was attributed to the progressive loss of crystallinity and the structural collapse of Li diffusion pathways. © 2012, Elsevier B.V.
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    Synthesis, structure, and electrochemical performance of magnesium-substituted lithium manganese orthosilicate cathode materials for lithium-ion batteries
    (Elsevier, 2012-01-01) Gummow, RJ; Sharma, N; Peterson, VK; He, Y
    Magnesium-substituted lithium manganese orthosilicate (Li2MnSiO4) cathode materials with a nominal composition of Li2MgxMn1−xSiO4, for x = 0.4 and 0.5 are synthesized by a solid-state route, at 700 Â°C in argon. The samples are characterized using powder X-ray and neutron diffraction, scanning electron microscopy, and galvanostatic cell-cycling. Rietveld analyses of the powder X-ray and neutron diffraction data show the formation of a monoclinic P21/n structure related to gamma lithium phosphate with no significant impurity peaks. This structure of the Mg-substituted samples is in contrast to the unsubstituted Li2MnSiO4 compound that has a Pmn21 structure when synthesized under the same conditions. Unit-cell volumes of the Mg-substituted materials are intermediate between those of the P21/n structure of Li2MnSiO4 and the isostructural low-temperature form of Li2MgSiO4, indicating the formation of a solid solution. The Mg-substituted materials feature mixed Mg/Mn cation sites, although no evidence of Li/Mn, Li/Mg or Li/Mg/Mn mixed sites are found. The Li2MgxMn1−xSiO4 cathodes show improved electrochemical performance over that reported for the unsubstituted Li2MnSiO4 P21/n phase. The Li2MgxMn1−xSiO4 cathode performance remains limited by its poor electronic properties and the large particle size of the solid-state synthesized products. Optimization of the synthesis conditions is likely to lead to enhanced electrochemical performance. (C) 2011 Elsevier B.V. All rights reserved.