Polymer-Based HardCarbons with Enhanced Initial CoulombicEfficiency for Practical Sodium Storage

Hard carbon (HC) is a cost-effective, chemically stable material which is deemed as the most promising anode material for sodium-ion batteries (SIBs). However, its practical application is severely limited by the poor initial Coulombic efficiency, long-term cycling stability, and scale production difficulty. Here, we developed a sodium involved process for preparing low-cost polymer-based hard carbon microspheres (HCMSs) with controllable morphology and properties. First, phenolics were dissolved in water with the assistance of sodium hydroxide by forming a sodium salt solution, and PF microspheres with adjustable sodium contents and sizes were gathered by a spray drying method. Then, sodium elements were removed during the carbonization process, which led to HCMSs with low surface area, abundant defect sites, and controllable interlayer spacing. Benefiting from the sodium involved process, the HCMS-1400-40 sample exhibits a high reversible specific capacity of 326.1 mAh/g in SIBs and remarkable initial Coulombic efficiency of 91.8%. Besides, the HCMS-1400-40 sample demonstrates excellent stability at large current of 1 C and retained 95.7% of its initial capacity even after 6000 cycles.