Coupled Carbonization Strategy toward Advanced Hard Carbon for High-Energy Sodium-Ion Battery

Sodium-ion batteries (SIBs) are expected to be a promising commercial alternative to lithium-ion batteries for grid electricity storage due to their potential low cost in the near future. Up to the present, the anode material still remains a great challenge for the application of SIBs, especially at room temperature. Graphite has an obvious limitation to store larger radius sodium ions (Na⁺) in comparison with lithium ions (Li⁺), while the hard carbon with large interlayer distance can demonstrate a relatively high storage capability and durable cycle life. However, the disadvantages of low initial Coulombic efficiency (ICE) mainly caused by large surface area and high cost synthetic approach hinder its practical applications. Herein, a new coupled carbonization strategy is presented to prepare a cost-effective hard carbon material by pyrolyzing and carbonizing the mixture of abundant sucrose and phenolic resin. Benefiting from the specialized pyrolysis reaction process and optimized conditions as studied in detail, the hard carbon has an extremely low surface area of 1.54 m² g^-1 and high initial Coulombic efficiency of 87%, which have been rarely reported before and enhance the utilization efficiency of Na⁺ consumption within the cathode in the future. More importantly, the hard carbon, with a high interlayer distance 3.95 Å, can deliver a higher capacity of 319 mAh g^-1 and maintain a finer capacity retention of 90% over 150 cycles. Besides, a full cell with the configuration of as-prepared hard carbon anode versus an air-stable O3-Na_0.9[Cu_0.22Fe_0.30Mn_0.48]O₂ cathode is further presented, and it has a high ICE of 80% and energy density of 256 Wh kg_anode^-1 (vs hard carbon) with reliable cycle performance. The results demonstrate that our synthetic strategy is feasible and extendable, while the tunable carbon-based materials should have wider applications in addition to the attractive properties in Na-ion batteries.