  In recent years, the mixed phosphates based polyanionic electrode materials
have attracted great attention in sodium-ion batteries due to their structural
stability during cycling and open framework for ion diffusion. Here, we report
the electrochemical performance of
Na$_{4}$Co$_{3}$(PO$_{4}$)$_{2}$P$_{2}$O$_{7}$/nitrogen doped carbon (NCPP/NC)
composite as a negative electrode (anode) for sodium ion batteries in the
working potential range of 0.01--3.0~V. It delivers a reversible discharge
capacity of 250~mAhg$^{-1}$ at 0.5~C current rate, which corresponds to the
insertion/extraction of four sodium ions. The rate capability study indicates
the reversible mechanism and highly stable capacity (61 mAhg$^{-1}$) even at
high rate up to 5~C as compared to pristine NCPP. The incorporation of the N
doped carbon spheres in the composite is expected to enhance the
electronic/ionic conductivity, which plays an important role in improving the
performance and stability up to 400 cycles at 1~C rate. Intriguingly, the
analysis of cyclic voltammetry data measured at different scan rates confirm
the capacitive/diffusive controlled mechanism and the extracted diffusion
coefficient is found to be around 10$^{-10}$ cm$^2$s$^{-1}$. Our results
demonstrate that the NCPP/NC composite is also a potential candidate as anode
in sodium-ion batteries due to its three dimensional framework, cost
effectiveness, enhanced specific capacity as well as further possibility of
improving the stability.
