Pomegranate-Structured Conversion-Reaction Cathode with a Built-in Li Source for High-Energy Li-Ion Batteries

Transition metal fluorides (such as FeF3 or CoF2) promise significantly higher theoretical capacities (>571 mAh g(-1)) than the cathode materials currently used in Li-ion batteries. However, their practical application faces major challenges that include poor electrochemical reversibility induced by the repeated bond-breaking and formation and the accompanied volume changes and the difficulty of building an internal Li source within the material so that a full Li-ion cell could be assembled at a discharged state without inducing further technical risk and cost issues. In this work, we effectively addressed these challenges by designing and synthesizing, via an aerosol-spray pyrolysis technique, a pomegranate-structured nanocomposite FeM/LiF/C (M = Co, Ni), in which 2-3 nm carbon-coated FeM nanoparticles (∼10 nm in diameter) and LiF nanoparticles (∼20 nm) are uniformly embedded in a porous carbon sphere matrix (100-1000 nm). This uniquely architectured nanocomposite was made possible by the extremely short pyrolysis time (∼1 s) and carbon coating in a high-temperature furnace, which prevented the overgrowth of FeM and LiF in the primordial droplet that serves as the carbon source. The presence of Ni or Co in FeM/LiF/C effectively suppresses the formation of Fe3C and further reduces the metallic particle size. The pomegranate architecture ensures the intimate contact among FeM, LiF, and C, thus significantly enhancing the conversion-reaction kinetics, while the nanopores inside the pomegranate-like carbon matrix, left by solvent evaporation during the pyrolysis, effectively accommodate the volume change of FeM/LiF during charge/discharge. Thus, the FeM/LiF/C nanocomposite shows a high specific capacity of >300 mAh g(-1) for more than 100 charge/discharge cycles, which is one of the best performances among all of the prelithiated metal fluoride cathodes ever reported. The pomegranate-structured FeM/LiF/C with its built-in Li source provides an inspiration to the practical application of conversion-reaction-type chemistries as next-generation cathode materials for high-energy density Li-ion batteries.