. Conclusions
We created a unique low surface area hard carbon sphere (PF-HCS) that was derived from Phenolic Resin by a facile, sustainable hydrothermal method. The effect of carbonization temperature on the structural evolution and sodium insertion behavior of PF-HCS is meticulously studied. The best electrochemical performances with a high reversible capacity of 311 mAh g− 1 and good cycling stability are obtained for the sample heat treated at 1250 °C. We believe that PF-HCS can be a promising candidate for low-cost sodium ion battery systems by further modification of material parameters such as morphology, size and pore structure, which is an effective strategy to improve the rate capacity by boosting ion transport and storage.