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 bondbreaking 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 aerosolspray pyrolysis technique, a pomegranate structured nanocomposite FeM/LiF/C (M = Co, Ni), in which 2-3 mn carbon-coated FeM nanoparticles (similar to 10 nm in diameter) and LiF nanoparticles (similar to 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 (similar to 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.