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The O3-type layered Na(NixFeyMnz)O2 (0 < x, y, z < 1) cathode materials have attracted great interest in sodium ion batteries due to the abundance and cost of raw materials and their high specific capacities. However, the cycling stability and rate capability at high voltages (> 4.0 V) of these materials remain an issue. In this work, we successfully synthesized a Li-substituted layered-tunneled (O3-spinel) intergrowth cathode (LS-NFM) to address these issues. The remarkable structural compatibility and connectivity of the two phases were confirmed by X-ray diffraction (XRD), selected area electron diffraction (SAED), and high-resolution transmission electron microscopy (HRTEM). The LS-NFM electrode reached a discharge capacity of 96 mAh g-1 with a capacity retention of 86% after 100 cycles at a current rate of 100 mA g-1 in a voltage window of 2.0-4.2 V. Moreover, the LS-NFM cathode exhibited an enhanced rate capability in comparison to the undoped single-phased layered cathode (NFM). The enhanced rate capability of LS-NFM can be explained by the significantly increased effective Na+ diffusivity measured by the galvanostatic intermittent titration technique (GITT) compared to the undoped control NFM cathode, which can be ascribed to the improved charge transport kinetics through shortened diffusion path by direct connection between the 3D channels in the spinel phase and 2D channels in the layered phase. The results from ex situ hard/soft X-ray adsorption spectroscopy (XAS) suggest that the capacity of the LS-NFM cathode is mainly associated with the Ni2+/Ni4+ redox couple and slightly from the Fe3+/Fe4+ redox couple. The voltage profile of the LS-NFM cathode exhibits a reversible plateau above 4.0 V, indicating great stability at high voltages and structural stabilization by the spinel phase. In addition to the substitution of various transition metals, or the modification of the stoichiometry of each transition metal, this study provides a new strategy to improve electrochemical performance of layered cathode materials for sodium ion batteries.

Nanoscale oxide-based negative electrodes are of great interest for lithium ion batteries due to their high energy density, power density and enhanced safety. In this work, we conducted a case study on mesoporous TiO2 nanoparticle negative electrodes with uniform size and varying crystallinity in order to investigate the trend in the electrochemical properties of oxide-based nanoscale negative electrodes with varying crystallinity. Mesoporous solid spherical TiO2 nanoparticles with a uniform particle size and varying crystallinity, i.e., amorphous TiO2 (A-TiO2), partially crystalline TiO2 (PC-TiO2) and fully crystalline TiO2 (FC-TiO2) nanoparticles were studied. At low current rate (quasi steady-state), the specific capacity of the samples drops with the decrease of crystallinity. Ex situ synchrotron pair distribution function analysis reveals that the 1D zigzag Li ion diffusion pathway becomes expanded with the increase of crystallinity, which promotes ion mobility and charge storage. At high current rates (away from equilibrium states), however, the A-TiO2 sample demonstrates slightly larger capacity than the FC-TiO2 sample, both of which show larger capacities than that of the PC-TiO2 sample. Both A-TiO2 and FC-TiO2 samples exhibit higher capacitive contribution to the charge storage and larger Li+ diffusivity than those of the PC-TiO2 sample, which explains their better rate capability. Moreover, the larger Li+ diffusivity of the A-TiO2 sample leads to the slightly larger specific capacity than the FC-TiO2 sample at the highest current rate. [ABSTRACT FROM AUTHOR]

The article features Donna Pierce, director of the Pierce Planetarium located in Dallas, Texas. Topics include her 35-year stint as the planetarium's director, why she considers the planetarium as an extension of the classroom, and the 2011 decision of Highland Park Integrated School District to name the planetarium after her.

Layered NaNi x Fe y Mn z O 2 cathode (NFM) is of great interest in sodium ion batteries because of its high theoretical capacity and utilization of abundant, low-cost, environmentally friendly raw materials. Nevertheless, there remains insufficient understanding on the concurrent local environment evolution in each transition metal (TM) that largely influences the reversibility of the cathode materials upon cycling. In this work, we investigate the reversibility of TM ions in layered NFMs with varying Fe contents and potential windows. Utilizing ex situ synchrotron X-ray absorption near-edge spectroscopy and extended X-ray absorption fine structure of precycled samples, the valence and bonding evolution of the TMs are elucidated. It is found that Mn is electrochemically inactive, as indicated by the insignificant change of Mn valence and the Mn-O bonding distance. Fe is electrochemically inactive after the first five cycles. The Ni redox couple contributes most of the charge compensation for NFMs. Ni redox is quite reversible in the cathodes with less Fe content. However, the Ni redox couple shows significant irreversibility with a high Fe content of 0.8. The electrochemical reversibility of the NFM cathode becomes increasingly enhanced with the decrease of either Fe content or with lower upper charge cutoff potential.