A series of O3-phase NaFe(x)(Ni0.5Mn0.5)(1-x)O2 (x = 0, 0.1, 0.2, 0.3, 0.4, and 1) samples with different Fe contents was prepared and investigated as high-capacity cathodic hosts of Na-ion batteries. The partial substitution of Ni and Mn with Fe in the O3-phase lattice can greatly improve the electrochemical performance and the structural stability. A NaFe0.2Mn0.4Ni0.4O2 cathode with an optimized Fe content of x = 0.2 can deliver an initial reversible capacity of 131 mAh g(-1), a reversible capacity greater than 95% over 30 cycles, and a high rate capacity of 86 mAh g(-1) at 10 C in a voltage range of 2.0-4.0 V. The structural characterizations reveal that pristine NaMn0.5Ni0.5O2 and Fe-substituted NaFe0.2Mn0.4Ni0.4O2 lattices underwent different phase transformations from P3 to P3″ and from P3 to OP2 phases, respectively, at high voltage interval. The as-resulted OP2 phase by Fe substitution has smaller interslab distance (5.13 Å) than the P3″ phase (5.72 Å), which suppresses the co-insertion of the solvent molecules, the electrolyte anions, or both and therefore enhances the cycling stability in the high voltage charge. This finding suggests a new strategy for creating cycle-stable transition-metal oxide cathodes for high-performance Na-ion batteries.

Three types of FeF 3 nanocrystals were synthesized by different chemical routes and investigated as a cathode-active material for rechargeable lithium batteries. XRD and TEM analyses revealed that the as-synthesized FeF 3 samples have a pure ReO 3 -type structure with a uniformly distributed crystallite size of ∼10 to 20 nm. Charge-discharge experiments in combination with cyclic voltammetric and XRD evidence demonstrated that the FeF 3 in the nanocomposite electrode can realize a reversible electrochemical conversion reaction from Fe 3+ to Fe 0 and vice versa, enabling a complete utilization of its three-electron redox capacity (∼712 mAh • g -1 ). Particularly, the FeF 3 /C nanocomposites can be well cycled at very high rates of 1000-2000 mA • g -1 , giving a considerably high capacity of ∼500 mAh • g -1 . These results seem to indicate that the electrochemical conversion reaction can not only give a high capacity but also proceed reversibly and rapidly at room temperature as long as the electroactive FeF 3 particles are sufficiently downsized, electrically wired, and well-protected from aggregation. The highrate capability of the FeF 3 /C nanocomposite also suggests its potential applications for high-capacity rechargeable lithium batteries.

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Ultra-fine amorphous alloy particles (UAAP) of Co−B were synthesized and investigated as an anode material in aqueous KOH solution. The experimental results demonstrated that the Co−B particles so prepared show excellent electrochemical reversibility and considerably high charge−discharge capacity. The reversible discharge capacity of the Co−B UAAP electrode is found to exceed 300 mAh/g at a current rate 100 mA/g, similar to values for conventional hydrogen storage alloys. In addition, the cycling ability and high rate capability of the Co−B electrode are fairly good with only 10% capacity decay after 100 cycles at a high rate of 300 mA/g. These exceptional electrochemical performances are suggested to arise from the electrochemical hydrogen storage reaction on the Co−B material, which is brought about by the nanosize effects and special amorphous structure of the Co−B UAAP particles.

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