ABSTRACT
Layered cathodes have been recognized as potential advanced candidates for sodium-ion batteries (SIBs), but the poor electrochemical performance has seriously hindered their further development. Herein, an ordered Na2/3[Ni2/9Mg1/9Mn5/9Ti1/9]O2 (NMMT) is designed and investigated as a high-performance cathode for SIBs through the synergistic effect of Mg and Ti codoping. Compared to the single Mg- or Ti-doped materials, NMMT clearly exhibits superstructure ordering diffraction peaks, and neutron diffraction further confirms that the diffraction peaks can be well indexed by a larger supercell P63, rather than the common unit cell P63/mmc by X-ray diffraction (XRD). High-resolution transmission electron microscopy also approves the ordering arrangement. This material shows an obvious capacity activation process during the first cycles, thus delivering 113 mA h g-1 specific capacity at 0.1 C (close to the theoretical value). Excellent rate capability even at 15 C and cycling stability after 500 cycles between 2.0 and 4.3 V can also be achieved, indicating that an ordered cathode is still promising. Besides, a single-phase reaction mechanism is revealed by ex situ/in situ XRD experiments. This study offers some insights into the material design and characterization of layered oxide cathodes for high-performance SIBs in the future.
ABSTRACT
The electronic structures of V-intercalated TiSe2 and substitutionally doped dichalcogenides Ti1-xVxSe2 have been studied using soft X-ray photoelectron, resonant photoelectron, and absorption spectroscopies. In the case of the substitution of Ti by V, the formation of coherently oriented structural fragments VSe2 and TiSe2 is observed and a small charge transfer between these fragments is found. Intercalation of the V atoms into TiSe2 leads to charge transfer from the V atoms to the Ti atoms with the formation of covalent complexes Ti-Se3-V-Se3-Ti.
ABSTRACT
Structural features and kinetics of the transition between ordered metastable b.c.c.-derived D03 and equilibrium f.c.c.-derived L12 phases of Fe-xGa alloys (x = 27.2% and 28.0%) have been analyzed by in situ real-time neutron diffraction during isothermal annealing in the temperature range 405-470°C. It has been revealed that the transition proceeds with alternation of the first- and second-order phase transformations according to a D03 â A2 â A1 â L12 scheme, where A2 and A1 are disordered b.c.c. and f.c.c. structures. Deformations of the crystal lattice that arise due to these transitions are determined. The kinetics of the L12 phase nucleation and growth were analyzed in the frame of the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model; however, only the early stage of the D03 â L12 transition is well described by the JMAK equation. The value of the Avrami exponent corresponds to the constant growth rate of the new L12 phase and decreasing nucleation rate in the Fe-27.2Ga alloy and indicates the presence of pre-existing nucleation centres of the L12 phase in the Fe-28.0Ga alloy.
