High-performance sodium storage for cobalt phosphide composite array electrodes

Transition metal phosphides hold great potential as sodium-ion batteries anode materials owing to their high theoretical capacity and modest plateau. However, volume changes and low intrinsic conductivity seriously largely hinder the further development of metal phosphide anodes. The design of phosphide anode materials with reasonable structure is conducive to solving the problems of volume expansion and slow reaction kinetics during the reaction. In this work, a composite material integrating zeolite imidazolate backbone (ZIF) and carbon materials was synthesized by the original growth method. Furthermore, by the oxidation-phosphating process, CoP nanoarray composites riveted to carbon fiber (CoP@CF) were obtained. In the CoP@CF, CoP nanoparticles are uniformly distributed on ZIF-derived carbon, reducing agglomeration and volume change during cycling. CF also provides a highly conductive network for the active material, improving the electrode kinetics. Therefore, when evaluated as an anode for sodium-ion batteries, CoP@CF electrode displays enhanced reversible capacity (262 mAh·g⁻¹ at 0.1 A·g⁻¹ after 100 cycles), which is much better than that of pure CF electrode (57 mAh·g⁻¹ at 0.1 A·g⁻¹ after 100 cycles) prepared without the addition of CoP. The rate performance of CoP@CF electrode is also superior to that of pure CF electrode at various current densities from 0.05 to 1 A·g⁻¹. The sodium storage behavior of CoP@CF was revealed by ex-situ X-ray photoelectron spectroscopy, X-ray diffraction, and synchrotron radiation absorption spectroscopy. This method provides a reference for the design and synthesis of anode materials in sodium-ion batteries.