Perovskite framework NH4FeF3/carbon composite nanosheets as a potential anode material for Li and Na ion storage

Transition metal fluorides (TMFs) have received increasing attention as promising electrode materials for lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) due to their resource abundance, low-cost and high specific capacities. However, TMFs usually suffer from low electron conductivity and high Li/Na ion diffusion resistance, which lead to rapid capacity fading. In order to further improve their electrochemical performance, designs of carbon-based TMFs with crystal topologies favorable for Li/Na ion diffusion are greatly needed. Here, NH4FeF3/carbon nanosheet (NH4FeF3/CNS) composites were prepared via a facile co-pyrolysis of ferric acetylacetonate and NH4F. NH4FeF3 has an open framework structure with a perovskite topology, in which FeF6 octahedral monomers are connected with each other via F− anions to form cavities, and NH4+ cations reside inside the cavities. The interesting perovskite structure is favorable for Li/Na ion storage. When used as an anode for LIBs, the NH4FeF3/CNS exhibits a specific capacity of 1000 mA h g−1 at 200 mA g−1 after 300 cycles, which is much higher than those of other reported TMFs. When used as an anode for SIBs, the NH4FeF3/CNS also exhibits a high specific capacity of 504 mA h g−1 as well as better rate-performance and cycling stability. The better electrochemical performance of NH4FeF3/CNS composites for both LIBs and SIBs should be ascribed to, on one hand, the fact that the perovskite framework structure of NH4FeF3 with NH4+ fillers has kinetically favorable Li/Na ion channels so as to be helpful to alleviate volume expansion during the cycling process and, on the other hand, the fact that carbon nanosheets can act as a conductive network to improve the conductivity of NH4FeF3 nanoparticles.