Interfacial Anionic Competition‑Driven Electrochemical Evolution in FeF₃ Conversion Electrodes

Introduction

FeF₃ has emerged as a promising high‑energy conversion electrode for next‑generation lithium‑ion batteries. Yet its practical performance is limited by sluggish reaction kinetics and rapid capacity fading. Recent research reveals that the electrochemical evolution at the electrode/electrolyte interface is governed by anionic competition—particularly between fluoride, carbonate, and solvent‑derived anions. Understanding this competition unlocks pathways to engineer more robust FeF₃ electrodes.

Why Anionic Competition Matters

During discharge, FeF₃ undergoes a conversion reaction that produces metallic Fe and LiF. The newly formed LiF forms a passivating solid‑electrolyte interphase (SEI) that can either facilitate ion transport or block it, depending on its composition and morphology. Competing anions from the electrolyte (e.g., PF₆⁻, BF₄⁻, carbonate anions) can incorporate into the SEI, altering its ionic conductivity, mechanical stability, and electronic insulation.

Key Anionic Players

• Fluoride (F⁻): Generates LiF, the primary SEI component on FeF₃.
• Carbonate (CO₃²⁻) and Alkyl‑Carbonate Fragments: Result from electrolyte decomposition, can form mixed‑anion Li₂CO₃/LiF layers.
• PF₆⁻ / BF₄⁻: Decompose to produce POₓ/F⁻ or BFₓ species that compete with LiF formation.

Electrochemical Evolution Pathways

The competition unfolds in three distinct stages:

1. Initial Reduction (0–0.8 V vs. Li⁺/Li): Solvent molecules and salt anions are reduced, generating a thin, organic‑rich SEI.
2. Conversion Onset (0.8–1.5 V): FeF₃ begins to convert to Fe⁰ and LiF. Fluoride anions dominate, but any residual carbonate anions can be incorporated, creating a mixed LiF/Li₂CO₃ matrix.
3. SEI Maturation (below 0.8 V): Continued cycling leads to SEI thickening. If competing anions persist, the SEI becomes heterogeneous, increasing impedance.

Strategies to Control Anionic Competition

Researchers have demonstrated several effective approaches:

• Fluoride‑Rich Electrolytes: Adding LiF or F‑containing additives (e.g., fluoroethylene carbonate) shifts the competition toward LiF formation, yielding a more uniform SEI.
• High‑Donor‑Number Solvents: Solvents like dimethyl sulfoxide (DMSO) stabilize PF₆⁻, reducing its decomposition and limiting unwanted POₓ species.
• Surface Coatings: Atomic‑layer‑deposited Al₂O₃ or carbonaceous layers act as diffusion barriers, suppressing electrolyte‑anion infiltration while still allowing Li⁺ transport.
• Pre‑lithiation Techniques: Introducing a thin LiF seed layer before cycling encourages homogeneous LiF growth during conversion.