Peanut-like hierarchical carbon nanofibers encapsulated carbon-coated NiCo/NiCoO hollow spheres as high-performance anode materials for Li-ion batteries and theoretical insights

Transition metal oxides (TMOs) are considered favorable anode materials for lithium-ion batteries (LIBs); however, their poor conductivity, low-rate performance, and abrupt lithiated volumetric expansion make them unviable materials for a practical Li-ion full cell. As a prospective solution, a facile method of fabricating peanutlike porous carbon fibers (CFs) consisting of hollow carbon-coated NiCo-metal/oxide (NiCo-MO) spheres as high-performance anode (NiCo-MO@CFs) is reported. This synthesis route is simple and eco-friendly compared with the complicated synthesis of hollow TMOs, such as the hard template method, multistep route, or use of toxic reagents. The hollow structure can accommodate the abrupt lithiated volume variation of NiCo-MO, and the CFs can ensure structural durability during the discharge/charge process, offering a double buffering for the stress during volumetric fluctuation. A mechanistic study suggests that the hollow structure is formed by the typical Kirkendall diffusion, where encapsulation by CFs is a prerequisite. Outstanding Li-ion storage performance is supported by good rate capability and prolonged durability for 500 cycles at 1 A g- 1 of NiCo-MO@CFs electrode. Remarkably, it shows a reversible discharge capacity of 894 mA h g- 1 after the consecutive rate capability study and a successive discharge-charge process for 92 cycles at 0.1 A g- 1. The computational study confirms that the most favorable Li adsorption sites for hybrid NiCo-MO@CFs surface are CoO-bridges and the hybrid surface provides enhanced Li adsorption energies and conductivity. Furthermore, a full cell was constructed using LiFePO4 (cathode) and NiCo-MO@CFs (anode), which exhibits a discharge capacity of 616 mA h g- 1 (anode-based) at 0.1 A g- 1 with 66 % capacity retention after 100 cycles.