Engineers tune hard carbon nanopores to boost sodium-ion battery kinetics

Researchers from Guangdong University of Technology have established a direct link between the pore architecture of hard carbon anodes and the kinetics of sodium-ion storage, according to a study published on dx.doi.org. By modifying lignin precursors, the team synthesized hard carbon materials featuring distinct nanoscale pore dimensions to examine how structural parameters govern charging rates and electrode stability springer.comdx.doi.orgspringer.comOpen source ↗.

During testing, precursors such as alkaline lignin, phenolic resin, lignin-phenolic resin, and sodium lignosulfonate were carbonized into hard carbons with pore diameters ranging from 1.5 nm to roughly 2 nm. The authors observed that hard carbons with smaller, highly dense closed pores lower the formation energy of sodium clusters inside the voids while providing richer diffusion pathways for sodium ions. These structural features led to accelerated charge transfer, improved rate capability, and sustained sodium-ion storage performance at low operating temperatures.

Nanopore geometry and sodium cluster formation

The authors monitored the sodium-ion storage process using methods including small-angle X-ray scattering combined with theoretical calculations. In the plateau-potential region of the discharge profile, sodium ions aggregate into metallic sodium clusters within the closed pores. In hard carbons with smaller pore sizes of around 1.5 nm, this clustering occurs at lower formation energy and with reduced charge-transfer resistance, yielding a higher sodium-ion diffusion coefficient.

Conversely, materials with larger pores of approximately 1.7 to 2 nm exhibited larger sodium clusters that require higher formation energy. Under high operating currents, electrochemical polarization lowers the formation potential of these clusters toward 0 V, raising the likelihood of uncontrolled sodium metal deposition on the anode surface springer.comdx.doi.orgspringer.comOpen source ↗.

Degradation mechanics under high-current cycling

Structural analyses revealed that hard carbon domains characterized by large pores and elevated proportions of sp3-hybridized carbon are susceptible to mechanical breakdown during extended cycling. High-current cycling causes graphene nanodomains in these structures to lose structural order, leading to a contraction of the active pore volume and an accelerated loss of plateau capacity.

The resulting structural disorder between graphene sheets obstructs the interlayer diffusion of sodium ions. This kinetic barrier reduces the operating formation potential of sodium clusters and increases the risk of sodium dendrite growth, which can induce internal short circuits.