Chemically modified lignin toward high-performance hard carbon anodes in sodium-ion batteries

Lignin, as a sustainable, oxygen-rich natural aromatic polymer, is a promising precursor for carbon electrode materials. However, the abundant aromatic rings in lignin tend to form substantial graphite-like nanodomains during the carbonization process, which hinders the formation of abundant closed pores and consequently restricts the sodium-ion storage capacity of lignin-derived hard carbons. Herein, a chemical modification strategy is proposed to modify the functional groups of lignin by grafting low bond-energy, nitrogen-containing groups (quaternary ammonium and amide groups) onto lignin molecules. These chemical modifications reduce the thermal stability of lignin and promote the release of pyrolysis gas, increasing the short-range disordered graphene nanodomains in the hard carbon. The hard carbons exhibit enhanced closed pore volumes and improved sodium-ion storage capacity. Compared to lignin-derived hard carbon, the ammoxidized/quaternized lignin-derived hard carbons exhibit an increased closed pore volume from 0.225 to 0.259/0.289 cm³ g^-1, resulting in an enhanced sodium-ion storage capacity from 301 to 330/367 mAh g^-1 and a plateau capacity from 205 to 232/262 mAh g^-1. Notably, the ammoniated lignin-derived hard carbon demonstrates excellent rate capability and cycling stability. This strategy provides novel methods for enhancing the sodium-ion storage capacity of biomass-derived hard carbon anodes in sodium-ion batteries.