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Published on: November 11, 2013
Low-spin state Fe(III)-rich surface engineering in Na4Fe3(PO4)2P2O7 cathode for enhanced sodium storage
Yukun Xi1, Xifei Li2, Zongnan Lv2
1Xi'an Key Laboratory of New Energy Materials and Devices, Institute of Advanced Electrochemical Energy & School of Materials Science and Engineering, Xi'an University of Technology, Xi'an, Shaanxi 710048, China; Engineering Research Center of Conducting Materials and Composite Technology, Ministry of Education, PR China; School of Electrical Engineering, Xi'an University of Technology, Xi'an, Shaanxi 710048, China.
None:
To promote sodium storage capability of NASICON-type Na4Fe3(PO4)2P2O7 (NFPP) a NFPP nanoparticle material is successfully synthesized via a sand-milling freeze-drying method. D50 values of nanoparticles could be tuned from 0.278 to 0.304 μm by controlling milling time. This particle-size engineering establishes a radial Fe-valence gradient with Fe3+ rich surface and Fe2+ dominated core. Further investigations reveal that surface Fe3+ adopts a low-spin configuration (t2g5eg0), whereas bulk Fe2+ is high-spin (t2g4eg2). The nanoparticle size decrease not only leads to an increase of surface-to-volume ratio increase, but also promote an intermediate-spin state (t2g5eg1) that narrows the electronic bandgap to 1.66 eV. The optimized NFPP nanoparticle delivers 106.2 mAh g-1 at 0.1C and retains 94.2 % of initial discharge capacity after 1000 cycles at 1C. This study underscores that the surface Fe modification can be an efficient method to improve NFPP sodium storage performance.
