Browsing by Author "Du, XW"

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    An ordered P2/P3 composite layered oxide cathode with long cycle life in sodium-ion batteries
    (American Chemical Society, 2019-10-16) Rahman, MM; Mao, J; Kan, WH; Sun, CJ; Li, LX; Zhang, Y; Avdeev, M; Du, XW; Lin, F
    Developing stable cathode materials represents a crucial step toward long-life sodium-ion batteries. P2-type layered oxides are important as cathodes for their reversibility, but their long-term performance in full cells remains a key challenge. Herein, we report Na0.75Co0.125Cu0.125Fe0.125Ni0.125Mn0.5O2 with an intergrowth of ordered P2 and P3 phases, studied by neutron diffraction and Rietveld refinement. A stable electrochemical performance is achieved in Na half cells with 100% capacity retention at a rate of C/10 after 100 cycles (initial capacity of 90 mAh/g), 96% capacity retention at a rate of 1 C after 500 cycles (initial capacity of 70 mAh/g), and 85% capacity retention at a rate of 5 C after 1000 cycles (initial capacity of 55 mAh/g). Stable full cell performance is achieved with 84.2% capacity retention after 1000 cycles at a rate of 1 C. Synchrotron X-ray diffraction, spectroscopy, and imaging are applied to elucidate the relationship between chemical/structural evolution and battery performance. A reversible local and global structural evolution is observed during initial cycles. Meanwhile, the challenges with enabling prolonged cycling (beyond 1000 cycles) may be associated with Fe dissolution and formation of a copper oxide phase. This study implies that cathodes with complex chemical and structural formations may stabilize electrochemical performance and highlights the importance of decoupling the contribution of each transition metal to performance degradation. © 2019 American Chemical Society
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    Surface characterization of Li-substituted compositionally heterogeneous NaLi0.045Cu0.185Fe0.265Mn0.505O2 sodium-ion cathode material
    (American Chemical Society, 2019-04-11) Rahman, MM; Zhang, Y; Xia, S; Kan, WH; Avdeev, M; Mu, L; Sokaras, D; Kroll, T; Du, XW; Nordlund, D; Liu, Y; Lin, F
    The understanding of surface chemical and structural processes can provide some insights into designing stable sodium cathode materials. Herein, Li-substituted and compositionally heterogeneous NaLi0.045Cu0.185Fe0.265Mn0.505O2 is used as a platform to investigate the interplay between Li substitution, surface chemistry, and battery performance. Li substitution improves the initial discharge capacity and energy density. However, there is no noticeable benefit in the long-term cycling stability of this material. The Li substitution in the transition-metal (TM) layer also seems to influence the transition-metal (TM) 3d–oxygen (O) 2p hybridization. Upon desodiation, the surface of active particles undergoes significant transition-metal reduction, especially Mn. Furthermore, the presence of electrolyte drastically accelerates such surface degradation. In general, the Li-substituted material experiences severe surface degradation, which is partially responsible for the performance degradation upon long-term cycling. While some studies have reported the benefits of Li substitution, the present study suggests that the effectiveness of the Li substitution strategy depends on the TM compositional distribution. More efforts are needed to improve the surface chemistry of Li-substituted sodium cathode materials. © 2019 American Chemical Society