Investigating the Electrochemical Performance of MnFe 2 O 4 @xC Nanocomposites as Anode Materials for Sodium-Ion Batteries.

Transition metal oxides (TMOs) are important anode materials in sodium-ion batteries (SIBs) due to their high theoretical capacities, abundant resources, and cost-effectiveness. However, issues such as the low conductivity and large volume variation of TMO bulk materials during the cycling process result in poor electrochemical performance. Nanosizing and compositing with carbon materials are two effective strategies to overcome these issues. In this study, spherical MnFe 2 O 4 @xC nanocomposites composed of MnFe 2 O 4 inner cores and tunable carbon shell thicknesses were successfully prepared and utilized as anode materials for SIBs. It was found that the property of the carbon shell plays a crucial role in tuning the electrochemical performance of MnFe 2 O 4 @xC nanocomposites and an appropriate carbon shell thickness (content) leads to the optimal battery performance. Thus, compared to MnFe 2 O 4 @1C and MnFe 2 O 4 @8C, MnFe 2 O 4 @4C nanocomposite exhibits optimal electrochemical performance by releasing a reversible specific capacity of around 308 mAh·g -1 at 0.1 A·g -1 with 93% capacity retention after 100 cycles, 250 mAh·g -1 at 1.0 A g -1 with 73% capacity retention after 300 cycles in a half cell, and around 111 mAh·g -1 at 1.0 C when coupled with a Na 3 V 2 (PO 4 ) 3 (NVP) cathode in a full SIB cell.