Impact of composite modes on electrochemical properties of Fe₃O₄/CNTs for Li-ion storage

Transition metal oxides (TMOs, e.g., Fe₃O₄) with high theoretical capacity hold tremendous potential in the field of lithium-ion batteries. Unfortunately, TMOs still suffer from the significant volume expansion and low intrinsic electronic conductivity, which results in irreversible structural collapse, uncontrollable particle aggregation, and sluggish electrochemical reaction kinetics, leading to bad rate performance and cycling stability. Combining TMOs with carbon has become the most effective approach to address the aforementioned issues. However, the interaction mechanism between TMOs and carbon influencing on electrochemical properties has not yet been thoroughly explored and widely recognized. Herein, we proposed a simple and controllable method to synthesize a series of Fe₃O₄ composites, i.e., Fe₃O₄ combined with carbon nanotubes (CNTs), via different composite modes, i.e., mixing, embedding and encapsulation. The mixing one exhibits the highest initial capacity but the poorest cycling stability. Encapsulating Fe₃O₄ into the cavities of CNTs effectively mitigates the volume expansion of Fe₃O₄, thereby achieving satisfactory rate performance. The embedded composite produces ultrafine Fe₃O₄ particles within the walls of CNTs, which not only enhances the dispersion of Fe₃O₄, but also provides an efficient pathway for electronic transport. The electrode based on the embedded composite mode demonstrates excellent cycling stability and an acceptable reversible capacity of 683.7 mAh/g at 1 A/g after 100 cycles. The relationship between structure and electrochemical properties of Fe₃O₄/CNTs composites is elaborated, which provides universal aspect for future design of high-energy–density lithium-ion batteries.