Kinetically controlled formation of uniform FePO4 shells and their potential for use in high-performance sodium ion batteries

Amorphous iron phosphates are potential cathode materials for sodium ion batteries. The amorphous FePO4 matrix is able to insert/extract sodium ions reversibly without apparent structural degradation, resulting in stable performance during the charge/discharge process. However, the extremely low electronic conductivity of FePO4 itself becomes a formidable obstacle for its application as a high-performance cathode material. Here, by tuning the growth kinetics of FePO4 in an aqueous solution, we were able to control its formation onto a large variety of substrates, forming uniform core-shell structures. Specifically, the use of multiwalled carbon nanotubes as the core material together with the growth control of FePO4 produced the core-shell structure of MWCNTs@FePO4 with a delicately controlled shell thicknesses. We confirmed that such a nanocomposite can act as an effective cathode material by taking advantage of both the highly conductive core and the electrochemically active shell, leading to improved battery performance as revealed by the high discharge capacity and the greatly improved rate capability. We anticipate that our progress in FePO4 control offers new potential in different research fields, such as materials chemistry, catalysis and energy storage devices. NPG Asia Materials (2017) 9, e414; doi:10.1038/am.2017.136; published online 28 July 2017