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Abstract: SnO2 nanoparticles (SNPs) entrapped in a graphene framework are synthesized for use as an anode material in Li ion batteries. A framework is prepared by covalently linking SNPs-anchored graphene oxide layers with diboronic acids. The framework provides the SNPs with more effective buffering than thermally reduced graphene oxide. SNPs in a graphene framework maintain the initial particle size and morphology after repeated charge-discharge cycles, with no inter-particle aggregation. The volume increase of the composite, accompanied by Li+ insertion into SNPs, is also significantly suppressed. The isolation of an individual nanoparticle and the firmness of a framework, which are ascribed to densely cross-linked graphene layers, results in better cyclability and rate performance by comparison with thermally reduced SNPs-anchored graphene oxide. [Copyright &y& Elsevier]

Na x Ca y CoO 2 (0.45 ≤ x ≤ 0.64, 0.02 ≤ y ≤ 0.10) is synthesized via a solid-state method and examined as a cathode material to improve the cyclability of sodium ion batteries (SIBs). The substitution of one Ca 2+ for two Na + does not alter the P2-phase of undoped Na 0.73 CoO 2 , accompanying the continuous contraction of the unit cell volume with Ca-doping. The existence of Ca 2+ in the prismatic sites between CoO 6 slabs also affects the phase transition behaviors during charge–discharge (C–D). The suppression of Na + -vacancy ordering, ascribed to the immobility of Ca 2+ in Na x Ca y CoO 2 , leads to continuous changes of the cell voltages with no abrupt voltage drops during C–D. The reduced cell dimension and the mitigation of distinctiveness between biphasic and solid-solution domains during C-D improve the cyclability of Na x Ca y CoO 2 in SIBs. As an example, the negligible capacity fading in Na 0.60 Ca 0.07 CoO 2 during 60 cycles (0.07 mAh g −1 cycle −1 ) is contrasted with a substantial decrease in the reversible capacity of Na 0.73 CoO 2 (0.56 mAh g −1 cycle −1 ). Furthermore, a slower Na + diffusion of Na x Ca y CoO 2 in Na + -vacancy ordering regions, is sufficiently offset by a more rapid diffusion in all the other regions, which results in an improved rate performance. The platform presented here (multi-valent cation substitution for Na + ) could be utilized in other layered cathode materials to improve the electrochemical performance of SIBs. [ABSTRACT FROM AUTHOR]

Abstract: W-doped LiW x Ni0.5Mn1.5−x O4 (x =0.00–0.10) are synthesized via a sol–gel method, and their electrochemical properties are investigated. During synthesis, W4+ is converted to W6+, the amount of which significantly affects the charge–discharge behaviors of LiW x Ni0.5Mn1.5−x O4. When limited to x ≤0.005, W-doping enhances the electrochemical activity of cathodes, leading to a greater discharge capacity and less capacity fading than LiNi0.5Mn1.5O4 at high C-rates. This is interesting since lowering the average valence state of Mn ions by incorporating W6+ can introduce structural instability, extending a 4V plateau (Mn3+/4+). The reasons for such behavior associated with W-doping are examined by electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and impedance spectroscopy. It is disclosed that the simultaneous increase in Lingth and decrease in transition metal-oxide bond length, while the unit cell volume is maintained almost invariant, provides LiW0.005Ni0.5Mn1.495O4 with optimal high rate performance. When W-doping exceeds x =0.01, 3 factors (intrinsically low electronic conductivity of W6+, the presence of tungsten oxide impurities, and an increase in the inter-metallic distance) aggravates electrochemical performance of LiW x Ni0.5Mn1.5−x O4. The last factor also induces structural instability during repeated cycling because of the expansion of a unit cell volume. [Copyright &y& Elsevier]