Facilitating Sodium-Ion Diffusion in Fe-Doped Co₃O₄ for High-Rate Performance

Due to its high theoretical capacity, cobalt oxide (Co₃O₄) has attracted attention to sodium-ion battery (SIB) anodes. However, its low conductivity and poor rate performance have limited its practical application. This work proposes a co-precipitation doping strategy to synthesize iron-doped Co₃O₄ nanoparticles (Fe_xCo_3-xO₄ NPs). Both experimental and theoretical results confirm that iron (Fe) doping at octahedral sites within spinel structures is a critical factor in enhancing rate performance. The decreased bandgap and enlarged ion transport spacing originate in Fe doping. This effectively facilitates the electron and Na-ion (Na⁺) transport during discharge/charge processes, delivering an impressive rate capability of 402.9 mAh g^-¹ at 3 A g^-¹. The Fe_xCo_3-xO₄ NPs demonstrate remarkable cycling stability. They maintain a high specific capacity of 786.2 mAh g^-¹ even after 500 cycles at 0.5 A g^-¹, with no noticeable capacity fading. When assembled into a Na-ion full cell, a remarkable discharge capacity of 105 mAh g^-1 with stable cycling performance is attained. This work provides valuable insights into the functional design of high-rate electrodes, offering a promising approach to addressing the critical challenges faced by sodium anodes.