Stability of FeF₃-Based Sodium-Ion Batteries in Nonflammable Ionic Liquid Electrolytes at Room and Elevated Temperatures

Abstract

Iron trifluoride (FeF3), a conversion-type cathode for sodium-ion batteries (SIBs), is based on cheap and abundant Fe and provides high theoretical capacity. However, the applications of FeF3-based SIBs have been hindered by their low-capacity utilization and poor cycling stability. Herein, we report greatly enhanced performance of FeF3 in multiple types of ionic liquid (IL) electrolytes at both room temperature (RT) and elevated temperatures. The Pyr1,4FSI electrolyte demonstrated the best cycling stability with an unprecedented decay rate of only ∼0.023% per cycle after the initial stabilization and an average coulombic efficiency of ∼99.5% for over 1000 cycles at RT. The Pyr1,3FSI electrolyte demonstrated the best cycling stability with a capacity decay rate of only ∼0.25% per cycle at 60 °C. Cells using Pyr1,3FSI and EMIMFSI electrolytes also showed promising cycling stability with capacity decay rates of ∼0.039% and ∼0.030% per cycle over 1000 cycles, respectively. A protective and ionically conductive cathode electrolyte interphase (CEI) layer is formed during cycling in ILs, diminishing side reactions that commonly lead to gassing and excessive CEI growth in organic electrolytes, especially at elevated temperatures. Furthermore, the increased ionic conductivity and decreased viscosity of ILs at elevated temperatures help attain higher accessible capacity. The application of ILs sheds light on designing a protective CEI for its use in stable SIBs.

Ionic liquids as potential electrolytes for sodium-ion batteries: an overview

“…Alternatively, Sun et al showed that pyrrolidinium-based ILs can sustain the full reduction of FeF 3 to Fe + NaF and deliver a capacity of close to 150 mA h g −1 (at 50 mA g −1 ) using 1.0 mol L −1 NaFSI in BMPyr-FSI. Na–FeF 3 retained this capacity over 1000 cycles at 25 °C, 150 which is a promising cycle life for a conversion-type reaction. The cell also showed a stable cycle life when operated at 60 °C, especially when the MPPyr-FSI IL was used.…”

Toward High Energy Density FeFx (x=2 and 3) Cathodes for Lithium‐Ion Batteries

“…Carbon nanotubes (CNTs) and nanofibers (CNFs) have similar physicochemical properties and 1D morphological characteristics [95,96] . The conductive 1D carbon network not only distinctly restricts the FeF x nanoparticles within a conductive matrix but also minimizes adverse reactions with liquid electrolytes.…”

“…ILs electrolytes have a more desirable safety than organic electrolytes due to their low vapor pressure and no gas generation [124,125] . For the FeF x cathodes, there is no systematic study based on ILs electrolytes.…”

“…Carbon nanotubes (CNTs) and nanofibers (CNFs) have similar physicochemical properties and 1D morphological characteristics. [95,96] The conductive 1D carbon network not only distinctly restricts the FeF x nanoparticles within a conductive matrix but also minimizes adverse reactions with liquid electrolytes. Thanks to this, a 3D conductive network structure FeF 3 • 0.33H 2 O/CNTs/GO composite was fabricated using 1D CNTs and 2D porous GO as a conductive carbon matrix.…”

“…ILs electrolytes have a more desirable safety than organic electrolytes due to their low vapor pressure and no gas generation. [124,125] For the FeF x cathodes, there is no systematic study based on ILs electrolytes. In addition, the viscosity of ILs is higher than that of ordinary liquid organic electrolytes, which significantly impacts the charge transfer process during multielectron conversion reactions.…”