Interconnected CoFe₂O₄-Polypyrrole Nanotubes as Anode Materials for High Performance Sodium Ion Batteries

CoFe₂O₄-coated polypyrrole (PPy) nanotubes (CFO-PPy-NTs) with three-dimensional (3-D) interconnected networks have been prepared through a simple hydrothermal method. The application has been also studied for sodium ion batteries (SIBs). The finely crystallized CoFe₂O₄ nanoparticles (around 5 nm in size) are uniformly grown on the PPy nanotubes. When tested as anode materials for SIBs, the CFO-PPy-NT electrode maintains a discharge capacity of 400 mA h g^-1 and a stable Coulombic efficiency of 98% after 200 cycles at 100 mA g^-1. Even at a higher current density of 1000 mA g^-1, the composite can still retain a discharge capacity of 220 mA h g^-1 after 2000 cycles. The superior electrochemical performance could be mainly ascribed to the uniform distribution of CoFe₂O₄ on the 3-D matrix of PPy interconnected nanotubes, which favors the diffusion of sodium ions and electronic transportation and also buffers the large volumetric expansion during charge/discharge. Thereby our study suggests that such CFO-PPy-NTs have great potential as an anode material for SIBs.