Facile Synthesis of FeP-Decorated Heteroatomic-Doped Onion-like Carbon Nanospheres for Pseudocapacitance Enhanced Sodium Storage

Transition-metal phosphides (TMPs), such as iron phosphide (FeP), are currently being contemplated as favorable high-performance anode materials for sodium-ion batteries (SIBs) because of their high theoretical capacity, cost-effectiveness, and availability. However, the inadequate electrochemical reaction dynamics and abrupt volumetric expansion of TMPs during cycling related to inferior conductivity constrain their commercial applications. Herein, we synthesized N,P-codoped onion-like carbon (NP-OLC)-encapsulated FeP (FeP@NP-OLC) using a simple injection pyrolysis method, followed by a successive low-temperature phosphidation treatment. NP-OLC, apart from constraining the volume variation of FeP, also encourages electron transfer to enhance the reversibility of FeP@C during the repeated cycling processes. Advancing from the distinctive onion-like structure, the FeP@NP-OLC nanospheres demonstrate outstanding sodium storage capability in terms of high capacity (543 mAh g –1 at 0.5 A g –1 at over 500 cycles), superb rate capacity (544 mAh g –1 at 2 A g –1 ), and long cyclic life (560 mAh g –1 at 1 A g –1 over 1100 cycles). The exceptional electrochemical performance is correlated to the meaningful involvement of pseudocapacitive behavior through the charge–discharge procedure, specifically at a high rate. Experimental results and theoretical calculations showed that the FeP and carbon interface with a defect can improve charge transfer and strengthen collaboration regarding active nanoparticles and sodium atoms, thereby boosting sodium storage. This facile strategy can be used to prepare other onion-like carbon-coated TMPs for future energy storage of commercial SIBs.