Han X. Yang

The mechanism of oxygen reduction on MnO2-catalyzed air cathode in alkaline solution

Abstract The mechanism of oxygen reduction at MnO2-catalyzed air cathode was investigated by measurements of polarization curves in wide OH− concentrations, oxygen pressures and using different crystalline MnO2 catalysts. It is found that the oxygen reduction at MnO2 catalyst is accompanied with the reduction of MnO2 and the catalytic activities of MnO2 is closely related to the electrochemical activities of MnO2. Based on the experimental determination of the reaction orders of OH−, O2 and the intermediate Mn3+ ion, we propose a chemical oxidation mechanism for the catalytic oxygen reduction, in which oxygen reduction proceeds through chemical oxidation of the discharge product of MnO2 rather than through a direct two-electron reduction. The rate expression derived from this mechanism can very well explain the observed polarization properties and the concentration dependence of oxygen reduction.

Improved Electrochemical Performance of Fe-Substituted NaNi0.5Mn0.5O2 Cathode Materials for Sodium-Ion Batteries.

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.

Magnesium-Doped Li1.2[Co0.13Ni0.13Mn0.54]O2 for Lithium-Ion Battery Cathode with Enhanced Cycling Stability and Rate Capability

Mg-doped Li[Li0.2-2xMgxCo0.13Ni0.13Mn0.54]O2 is synthesized by introducing Mg ions into the transition-metal (TM) layer of this layered compound for substituting Li ions through a simple polymer-pyrolysis method. The structural and morphological characterization reveals that the doped Mg ions are uniformly distributed in the bulk lattice, showing an insignificant impact on the layered structure. Electrochemical experiments reveal that, at a Mg doping of 4%, the Li[Li0.16Mg0.04Co0.13Ni0.13Mn0.54]O2 electrode can deliver a larger initial reversible capacity of 272 mAh g(-1), an improved rate capability with 114 mAh g(-1) at 8 C, and an excellent cycling stability with 93.3% capacity retention after 300 cycles. The superior electrochemical performances of the Mg-doped material are possibly due to the enhancement of the structural stability by substitution of Li by Mg in the TM layer, which effectively suppresses the cation mixing arrangement, leading to the alleviation of the phase change during lithium-ion insertion and extraction.

Structural and electrochemical characterization of mechanochemically synthesized calcium zincate as rechargeable anodic materials

Hydrated calcium zincate was synthesized by mechanical ball milling of ZnO and Ca(OH)2 in water at room temperature. The structural and electrochemical properties of this material used as rechargeable anodic material were examined by microelectrode voltammetry, charge–discharge measurements and structural analysis. The results showed that during mechanical milling, ZnO, Ca(OH)2 and H2O reacted rapidly to form Ca[Zn(OH)3]2 · 2H2O which was subsequently transformed to a stable structure CaZn2(OH)6 · 2H2O. Since this composite oxide has lower solubility in KOH solution (<35 wt %) and better electrochemical reversibility than ZnO-based negative materials, the zinc anodes using this material can overcome the problems of shape changes and dendritc formation, and therefore exhibit improved cycling life.