Hyperaccumulation Route to Ca-Rich Hard Carbon Materials with Cation Self-Incorporation and Interlayer Spacing Optimization for High-Performance Sodium-Ion Batteries

The hard carbon (HC) has been emerging as one of the most promising anode materials for sodium-ion batteries (SIBs). Incorporation of cations into the HC lattice proved to be effective to regulate their d-interlayer spacing with a modified SIB performance. However, the complexity and high cost of current synthetic processes limited its large-scale application in SIBs. Through the natural hyperaccumulation process, a cost-effective and scale-up-driven procedure to produce Ca-ion self-incorporated HC materials was proposed by applying tamarind fruits as the precursor with the enrichment of Ca ions. In virtue of one-step pyrolysis, the self-incorporated and well-distributed Ca ions in tamarind fruits had successfully served as the buffer layer to expand the d-interlayer spacing of HC materials. Furthermore, the natural porosity hierarchy could be largely preserved by the optimization of calcination temperature. As a result, the Ca-rich HC material had exhibited the optimized cycling performance (326.7 mA h g^-1 at 50 mA g^-1 and capacity retention rate of 89.40% after 250 cycles) with a high initial Coulombic efficiency of 70.39%. This work provided insight into applying the hyperaccumulation effect of biomass precursors to produce doped HC materials with ion self-incorporation and the optimized d-interlayer spacing, navigating its large-scale application for high-performance SIBs.