Hard carbon nanoparticles as high-capacity, high-stability anodic materials for Na-ion batteries

Highlights: • Hard carbon nanoparticles are synthesized by pyrolysis of a polyaniline precursor. • The measured D_Na+ in the HCNP obtained at 1150 °C is 10⁻¹³–10⁻¹⁵ cm² s⁻¹. • The nano-sized HCNP obtained at 1150 °C exhibits higher electrochemical performance. 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 1.2 V (vs. Na/Na⁺), as well as a prolonged cycling stability, which is attributed to an optimum spacing of 0.366 nm 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).