Pyrolysis of Prussian blue for lignin-derived nitrogen-doped biocarbon to boost sodium storage

Hard carbons are the potential high-capacity anode candidates for sodium-ion batteries (SIBs). However, the commercialization of hard carbons is significantly limited by high cost and uncontrollable complicated microstructure. Herein, a novel nitrogen-doped carbon composite with unique micro/mesopores has been fabricated based on both low-cost Prussian blue and biomass enzymatic hydrolysis lignin as nitrogen-containing carbon sources. In particular, Prussian blue-derived hierarchical carbon spheres with abundant mesopores and a relatively high graphitization degree possess good compatibility with enzymatic hydrolysis lignin-derived hard carbons with rich micropores. The resultant nitrogen-doped porous carbon composite anode with the enlarged interlayer distance of 0.392 nm and improved N amount of 6.74 wt% delivers a large reversible specific capacity of 293 mAh g−1 at 0.02 A g−1 and 193 mAh g−1 at 0.1 A g−1 with an extremely high capacity retention of 98.1% after 200 cycles. Such superior rate performance and excellent cycling stability are beneficial to stable chemical functionalities originating from effectively N atom doping in the carbon structure, unique micro/mesopores and dilated interlayer spacing distance, which provides abundant active sites, the electrolyte of permeation and low Na+ insertion strain, respectively, and further accelerate sodium ions transport and electrons transfer for high kinetics and maintain the stable microstructure. In addition, the pseudocapacitive behavior has substantial contribution towards the overall capacity at high rate, closely approaching solvated sodium ions adsorption at the disorder sites/defect sites/chemical functionalities. This strategy provides novel insights for the design of high-performance nitrogen-doped hard carbons for practical SIBs.