Revealing the Effect of Nanopores in Biomass-Derived Carbon on its Sodium-Ion Storage Behavior

The sodium storage mechanism in biomass-derived carbon is composed of absorption/desorption from defects, insertion/extraction between layers in pseudographitic domains, and nanopores filling. Nanopores are produced to provide spaces for sodium-ion storage and diffusion, which attracts more attention. However, roles of nanopores in biomass-derived carbon microstructure and their influences on electrochemical behavior of sodium-ion batteries still need to be further explored. Therefore, we control nanopore size of biomass carbon by three different methods. Results show the size and distribution of nanopores have effect on uniformity of interlayer spacing in pseudographitic domains around pores. After cycling, the stability of nanopore structure plays an important role on sodium storage behavior. Without nanopores as buffer spaces, the diffusion resistance of sodium ions increases because of decreased interlayer spacing from outside to inside. When more micropores are formed with uniform interlayer spacing distribution, the structure of electrode is well-maintained and higher specific capacity is achieved after sodium-ion inserting/extracting. The formation of mesopores accompanies with nonuniform interlayer spacing. Mesopore structure tends to be destroyed easily by large amount of sodium ions inserting/extracting. Cracked parts provide unstable buffer space for sodium ions. The interlayer spacing in meso-porous carbon tube increases rapidly after cycling, which leads to capacity loss.