Solvent-in-Salt Electrolytes for Fluoride Ion Batteries

Abstract

The fluoride ion battery (FIB) is a promising post-lithium ion battery chemistry owing to its high theoretical energy density and the large elemental abundance of its active materials. Nevertheless, its utilization for room-temperature cycling has been impeded by the inability to find sufficiently stable and conductive electrolytes at room temperature. In this work, we report the use of solvent-in-salt electrolytes for FIBs, exploring multiple solvents to show that aqueous cesium fluoride exhibited sufficiently high solubility to achieve an enhanced (electro)chemical stability window (3.1 V) that could enable high operating voltage electrodes, in addition to a suppression of active material dissolution that allows for an improved cycling stability. The solvation structure and transport properties of the electrolyte are also investigated using spectroscopic and computational methods.

Scheme 1. Representation of the Evolution of the Ionic Species in the Bulk and the Electrode–Electrolyte Interface as the Concentration Is Increased from Salt-in-Solvent to Solvent-in-Salt Electrolytes

Figure 1

Solubility of CsF determined using ICP-MS in common protic solvents (a), solvent-to-salt molar ratio showing <2 coordination number for CsF in water (b), salt-to-solvent weight and volume ratios showing the concentration range of “solvent-in-salt” (c), and electrochemical stability window expansion in the water-in-salt electrolyte (this work) compared to pure water and other aqueous electrolytes (d).

Figure 2

Ionic conductivity (a) and diffusion coefficient for the fluoride and cesium ions and the fluoride transport number (b) as a function of concentration.

Figure 3

Solvation properties for the water-in-salt electrolyte. ¹⁷O NMR spectroscopy showing the peak broadening and peak shift as the water molecules become less mobile (a), fraction of free water molecules calculated from the MD simulation (b), and MD relaxed structure at 1 and 25 m showing the formation of contact ion pairs and aggregates and elimination of free water molecules (c).

Figure 4

Fluoride ion chemical species and electronic environment. pH as a function of concentration indicating the decay of the HF content (a) and ¹⁹F NMR spectroscopy (b).

Figure 5

Galvanostatic cycling of symmetric Pb|PbF₂ at 1 and 25 m showing improved capacity retention in WiSE (a). Cyclic voltammetry of CuF₂ in the 25 m electrolyte (b) and of AgF₂ and ZnF₂ near the oxidative and reductive limits of WiSE (c). Performance comparison with selected fluoride ion battery electrolytes based on ESW and conductivity (d).