Study on sodium ion supplementation performance of CNT-coated sodium oxalate in sodium ion batteries

Low initial coulombic efficiency of hard carbon anodes and irreversible sodium loss during cycling in sodium-ion batteries (SIBs) lead to reduced specific capacity and shortened cycle life. Introducing sodium supplementation can effectively enhance the specific capacity and cycle life of SIBs. Na₂C₂O₄ was selected as the sodium compensation agent owing to its high theoretical capacity, excellent chemical stability, and environmental friendliness. To improve conductivity and reduce decomposition voltage of Na₂C₂O₄, carbon nanotubes (CNTs) were employed for coating. The optimized 10% CNT-coated sodium oxalate (CNT-10@Na₂C₂O₄) exhibited a reduced initial decomposition voltage of 3.82 V (vs. Na⁺/Na) and delivered an irreversible (sodium compensation) capacity of 392.65 mAh g^-1. NNFMO-CN||HC full cells were assembled using NaNi_1/3Fe_1/3Mn_1/3O₂ cathode containing 10% CNT-10@Na₂C₂O₄ and hard carbon anode, demonstrating a discharge retention capacity of 38.35 mAh g^-1 after 100 cycles, higher than the retained capacity of only 15.16 mAh g^-1 of the NNFMO||Na cell without CNT-10@Na₂C₂O₄. These results indicate that CNT-10@Na₂C₂O₄ significantly improves the specific capacity and cycling performance of SIBs.