Hard carbon nanoparticles (HCNP) were synthesized by the pyrolysis of a polyaniline precursor. The measured Na+ cation diffusion coefficient (10−13–10−15 cm2 s−1) in the HCNP obtained at 1150°C is two orders of magnitude lower than that of Li+ in graphite (10−10–10−13 cm2 s−1), indicating that reducing the carbon particle size is very important for improving electrochemical performance. These measurements also enable a clear visualization of the stepwise reaction phases and rate changes which occur throughout the insertion/extraction processes in HCNP, The electrochemical measurements also show that the nano-sized HCNP obtained at 1150°C exhibited higher practical capacity at voltages lower than 1.2V (vs. Na/Na+), as well as a prolonged cycling stability, which is attributed to an optimum spacing of 0.366nm between the graphitic layers and the nano particular size resulting in a low-barrier Na+ cation insertion. These results suggest that HCNP is a very promising high-capacity/stability anode for low cost sodium-ion batteries (SIBs).