Metal-Organic Framework-Derived Nanoconfinements of CoF₂ and Mixed-Conducting Wiring for High-Performance Metal Fluoride-Lithium Battery.

Abstract

Metal fluoride (MF) conversion cathodes theoretically show higher gravimetric and volumetric capacities than Ni- or Co-based intercalation oxide cathodes, which makes metal fluoride-lithium batteries promising candidates for next-generation high-energy-density batteries. However, their high-energy characteristics are clouded by low-capacity utilization, large voltage hysteresis, and poor cycling stability of transition MF cathodes. A variety of reasons is responsible for this: poor reaction kinetics, low conductivities, unstable MF/electrolyte interfaces and dissolution of active species upon cycling. Herein, we combine the synthesis of the metal-organic-framework (MOF) with the low-temperature fluorination to prepare MOF-shaped CoF₂@C nanocomposites that exhibit confinement of the CoF₂ nanoparticles and efficient mixed-conducting wiring in the produced architecture. The ultrasmall CoF₂ nanoparticles (5-20 nm on average) are uniformly covered by graphitic carbon walls and embedded in the porous carbon framework. Within the CoF₂@C nanocomposite, the cross-linked carbon wall and interconnected nanopores serve as electron- and ion-conducting pathways, respectively, enabling a highly reversible conversion reaction of CoF₂. As a result, the produced CoF₂@C composite cathodes successfully restrain the above-mentioned challenges and demonstrate high-capacity utilization of ∼500 mAh g^-1 at 0.2C, good rate capability (up to 2C), and long-term cycle stability over 400 cycles. Overall, the presented study not only reports on a simple composite design to achieve high-energy characteristics in CoF₂-Li batteries but also may provide a general solution for many other metal fluoride-lithium batteries.