Browsing by Author "Gummow, RJ"
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- ItemAn 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, YThe 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.
- ItemHigh Performance Composite Lithium-Rich Nickel Manganese Oxide Cathodes for Lithium-Ion Batteries(2013-01-01) Gummow, RJ; Sharma, N; Feng, RS; Han, GH; He, YHLi-1.0[Li1/7Mn4/7 Ni-2/7]O-2 cathode material was prepared by a facile, one-pot synthesis method. The structure of the material was determined by Rietveld refinement of structural models using high-resolution synchrotron X-ray and neutron powder diffraction data and was found to consist of two distinct phases. The major phase, with composition Li1.25(3)Ni0.17(1) Mn0.61(1)O2, close to the well-characterized L1.2Ni0.2Mn0.6O2 composition can be described as an intergrowth structure of Li2MnO3 and LiMn0.5Ni0.5O2 domains and the second phase is a lithium-deficient layered structure with refined composition Li0.85(1)Ni0.57(1)Mn0.55(1)O2. The composite cathode has a high initial discharge capacity of 250 mAh/g which drops to 225 mAh/g on the 2nd discharge cycle. This capacity is maintained on subsequent cycles. Time-resolved in-situ synchrotron XRD data was used to study the changes in the lattice parameters and phase evolution of the two phases during Li-insertion and extraction. © 2013, Electrochemical Society Inc.
- ItemSynthesis, 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, YMagnesium-substituted lithium manganese orthosilicate (Li2MnSiO4) cathode materials with a nominal composition of Li2MgxMn1âˆ’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 Li2MgxMn1âˆ’xSiO4 cathodes show improved electrochemical performance over that reported for the unsubstituted Li2MnSiO4 P21/n phase. The Li2MgxMn1âˆ’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.