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

Hard carbon nanoparticles (HCNP) were synthesized by the pyrolysis of a polyaniline precursor. The measured Na+ cation diffusion coefficient (10−13–10−15cm2s−1) in the HCNP obtained at 1150°C is two orders of magnitude lower than that of Li+ in graphite (10−10–10−13cm2s−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). Hard carbon nanoparticles (HCNPs) synthesized by the pyrolysis of a polyaniline precursor display an optimum electrochemical performance. Particularly, the first measurements of the Na cations diffusion variation throughout the insertion/extraction processes in HCNP provide helpful insight into the stepwise Na cation insertion/extraction phases and rates in hard carbon materials. [Display omitted] •Hard carbon nanoparticles are synthesized by pyrolysis of a polyaniline precursor.•The measured DNa+ in the HCNP obtained at 1150°C is 10−13–10−15cm2s−1.•The nano-sized HCNP obtained at 1150°C exhibits higher electrochemical performance.