New Template Synthesis of Anomalously Large Capacity Hard Carbon for Na‐ and K‐Ion Batteries

Hard carbon (HC) is a promising negative‐electrode material for Na‐ion batteries. HC electrochemically stores Na⁺ ions, resulting in a non‐stoichiometric chemical composition depending on their nanoscale structure, including the carbon framework, and interstitial pores. Therefore, optimizing these structures for Na storage by altering the synthesis conditions can enhance the capacity of Na‐ion batteries. In this study, HCs using MgO, ZnO, and CaCO₃ as nanopore templates are systematically investigated, and the ZnO template is found to be particularly effective. By optimizing the concentration of ZnO embedded in the carbon matrix, utilizing a blend of zinc gluconate, and zinc acetate as starting materials, the optimal ZnO‐template HC demonstrates a reversible capacity of 464 mAh g⁻¹ (corresponding to NaC_4.8) with high initial coulombic efficiency of 91.7% and low average potential of 0.18 V versus Na⁺/Na. Thus, a Na‐ion battery full cell consisting of Na_5/6Ni_1/3Fe_1/6Mn_1/6Ti_1/3O₂ and the optimized ZnO‐template HC demonstrates a remarkable energy density of 312 Wh kg⁻¹, comparable to that of a Li‐ion battery with LiFePO₄ and graphite. Moreover, the ZnO‐template HC in a K half‐cell also displays a significant capacity of 381 mAh g⁻¹, that is, KC_5.8 where the alkali content is higher than stage‐1 graphite intercalation compounds, LiC₆ and KC₈.