Honeycomb-like Hard Carbon Derived from Pine Pollen as High-Performance Anode Material for Sodium-Ion Batteries

Sodium-ion batteries are regarded as one of the most promising energy storage systems, but the choice of anode material is still facing great challenges. Biomass carbon materials were explored for their low cost and wide range of sources. Here, a hard carbon material with a "honeycomb" structure using pine pollen (PP) as a precursor was successfully prepared and applied as an anode. The initial discharge capacity can reach 370 mA h g^-1 at a current density of 0.1 A g^-1. After cycling 200 times, the reversible capacity also stabled at 203.3 mA h g^-1 with the retention rate of 98%. We further studied the sodium storage mechanism by different methods, especially the Na⁺ diffusivity coefficient ( D_Na) calculated by galvanostatic intermittent titration technique, which was more accurate. Interestingly, the trend of D_Na coincides with cyclic voltammetry curves. Carbonized PP exhibited excellent electrochemical properties because of its three-dimensional structure and larger layer spacing (∼0.41 nm), which reduces the resistance of sodium ions to intercalation and deintercalation.