Na₄Co₃(PO₄)₂P₂O₇/NC Composite as a Negative Electrode for Sodium-Ion Batteries

In recent years, the mixed phosphate-based polyanionic electrode materials have attracted great attention in sodium-ion batteries (SIBs) due to their structural stability during cycling and open framework for ion diffusion. Here, we report the electrochemical performance of a Na4Co3(PO4)2P2O7/nitrogen-doped carbon (NCPP/NC) composite as a negative electrode (anode) for SIBs in the working potential range of 0.01–3.0 V. It delivers a reversible discharge capacity of 250 mA h g–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 mA h g–1) even at a high rate of 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 confirms the capacitive/diffusive-controlled mechanism, and the extracted diffusion coefficient is found to be around 10–10 cm2 s–1. Our results demonstrate that the NCPP/NC composite is also a potential candidate as an anode in SIBs due to its three-dimensional framework, cost effectiveness, enhanced specific capacity, and further possibility of improving the stability.