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

Abstract

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.