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. Using a water-based coating technology, researchers can now turn carbon nanotubes into coaxial cable-like cathodes for sodium batteries. Sodium ion batteries are cheaper alternatives to widely used lithium ion batteries, but they require conductive cathodes capable of storing and releasing sodium ions without changing their shape. An-Min Cao and co-workers from the Chinese Academy of Sciences used nanometer-thin layers of iron phosphate to achieve this goal. By controlling the pH of an aqueous precursor, they developed a way to slowly solidify a non-crystalline shell of iron phosphate — a substance with a high, reversible sodium ion capacity — around polystyrene seed particles. This strategy proved versatile and successfully coated other seed substrates, such as silicon, gold and conductive carbon nanotubes. Sodium battery cathodes made from the iron phosphate-wrapped nanotubes delivered significantly higher discharge capacity than control samples. We developed a simple but effective protocol to construct uniform FePO4 coating layer on various substrates. By controlling the precipitation kinetics, we were able to form uniform FePO4 nanoshells with its thickness precisely defined in nanometer accuracy. Specifically, a core-shell structured electrode material of MWCNTs@FePO4 was constructed, which showed promising potential as a cathode material for sodium ion battery as revealed by its high discharge capacity as well as the much improved rate capability.