Engineering crystal-facet modulation to obtain stable Mn-based P2-layered oxide cathodes for sodium-ion batteries.

The undesirable phase transformation of Mn-based P2-layered oxide cathodes is a tremendous challenge in commercializing Mn-based oxide cathodes for sodium-ion batteries. In this work, Na0.67MnO2 cathode with stable P2-type structure was successfully synthesized by modulating its coordination numbers to suppress the preferred orientation growth of (001) crystal plane, which was realized to maintain a stable P2-type structure in the whole state of charging and discharging. Specifically, designing Mn2+ six coordination sites to lower the high surface energy of (001) crystal plane is an effective way to reduce nucleation rates, which leads to the production of few grain boundaries and the suppression of layer-to-layer stacking in the crystal growth stage. Due to their fewer grain boundaries and skeleton structure with layer-to-layer stacking, the interlaminar stress and intragranular fatigue cracks can be alleviated in the long-life cycling performance of Na0.67MnO2 cathode. Na0.67MnO2 cathodes derived from the precursor of Mn2+ six coordination sites (C-Na0.67MnO2) have more exposed {010} crystal face and enlarged sodium-ion diffusion channels and structure integrity compared to Na0.67MnO2 cathode prepared by the precursor of Mn2+ four coordination sites (O-Na0.67MnO2). Therefore, C-Na0.67MnO2 cathode delivers an initial capacity of 106.8 mAh/g and has excellent capacity retention of 94.8 % after 150 cycles at 80 mAh/g. The rational design strategy endows Mn-based P2-layered oxide cathodes with stable sodium-ion diffusion channels and lamellar structure.

Two-Step Process of a Crystal Facet-Modulated BiVO4 Photoanode for Efficiency Improvement in Photoelectrochemical Hydrogen Evolution.

The photoactivity of nanoporous bismuth vanadate (BiVO4, BVO) photoanodes that were fabricated by a two-step process (electrodeposition and then thermal conversion) in photoelectrochemical (PEC) hydrogen (H2) evolution can be enhanced about 1.44-fold by improving the constitutive ratio of (111̅), (061), and (242̅) crystal facets. The PEC characterization was carried out to investigate the factors altering the performance, which revealed that the crystal facet modulation could improve the photoactivity of the BVO photoanodes. In addition, the orientation-controlled BVO thin-film electrodes are introduced as evidence that the present crystal facet modulation is the positive effect for BVO photoanodes in PEC. The investigation of energy band structures and interfacial charge carrier dynamics of the BVO photoanodes reveals that the crystal facet modulation could result in a shorter lifetime of charge carrier recombination and larger band bending at the interface between BVO and electrolytes. This outcome could improve the charge separation and charge transfer efficiencies of BVO photoanodes, promoting the efficiency of PEC H2 evolution. Moreover, this crystal facet modulation can combine with co-catalyst decoration to further improve the solar-to-hydrogen efficiency of BVO photoanodes in PEC. This study presents a potential strategy to promote the PEC activity by crystal facet modulation and important insights into the interfacial charge transfer properties of semiconductor photoelectrodes for the application in solar fuel generation.