Research Progress of Iron Fluorides in Lithium/Sodium Ion Batteries

Research Progress of Iron Fluorides in Lithium/Sodium Ion Batteries

Research Progress in Lithium-Ion Batteries

• Innovations in Preparation Methods:
• Fluorocarbon Shear Fluorination: The team led by Researcher Li Chilin from the Shanghai Institute of Ceramics, Chinese Academy of Sciences, used ferric chloride hexahydrate (FeCl₃·6H₂O) as the iron source and fluorocarbon (CFₓ) as the fluorine source. Through the proton-coupled electron transfer (PCET) hydrogenation defluorination reaction between benzylamine and CFₓ, an intermediate (NH₄)₃FeF₆ was formed at 95°C. After subsequent thermal treatment to remove ammonium and gas, hexagonal tungsten bronze-structured iron trifluoride (HTB-FeF₃) with a porous cubic cage morphology was obtained. This method not only enhanced the safety and controllability of the preparation process but also avoided the introduction of additional conductive agents. The porous cubic cage structure and one-dimensional open tunnel structure of the final product significantly enhanced the electrochemical reaction kinetics.
• Deep Eutectic Solvent Method: The team also proposed a synthesis method based on deep eutectic solvents. Ferric nitrate nonahydrate (Fe(NO₃)₃·9H₂O) and dimethyl sulfoxide form a transparent liquid through a Lewis acid-base interaction, serving as the iron source and solvent, with ammonium hydrogen fluoride (NH₄HF₂) as the fluorine source. The reaction occurs at 60°C. The addition of cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O) helps to control the morphology of the iron fluoride product, resulting in a three-dimensional porous brick-like structure. This structure increases the contact area between the electrode and the electrolyte and enhances the active sites for electrochemical reactions.
• Optimization of Structural Design:
• Heterostructure Design: An FeF₂/FeF₃ iron fluoride heterojunction was constructed. FeF₂ maintains the topotactic conversion mechanism of the anion framework, acting as a buffer and confinement for the surrounding FeF₃ conversion reaction. The checkerboard-like structure of LiF/Fe domains ensures the interconnection of the built-in conductive network and accelerates interfacial mass transport, promoting interfacial charge transfer and topotactic conversion reactions. This design achieves a synergistic improvement in the cycling stability and reversible capacity of iron fluoride cathodes.
• Porous Morphology Design: The porous structures obtained through the aforementioned preparation methods, such as the porous cubic cage morphology of HTB-FeF₃ and the three-dimensional porous brick-like morphology of iron fluoride, effectively increase the contact area between the electrode and the electrolyte and enhance the active sites for electrochemical reactions, thereby improving the kinetics of the electrode reactions.
• Optimization of Electrolyte Systems:
• Local High-Concentration Electrolyte: A local high-concentration electrolyte (LHCE) was developed, dissolving lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in diglyme (G₂) solvent and using 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether (OFE) as a diluent with weak solvation ability to form a local high-concentration environment. The Li⁺ in the electrolyte forms a solvation sheath by sharing solvent molecules with TFSI⁻. The OFE diluent reduces the viscosity of the electrolyte, achieving a lithium ion transference number of up to 0.74 and extending the oxidation stability potential to 5V. Additionally, lithium difluoro(oxalato)borate (LiDFOB) was introduced as a film-forming lithium salt additive to construct robust interfacial membranes (CEI/SEI) on the cathode/anode side, effectively suppressing the dissolution of active materials from the FeF₃ cathode and the cross-interference with the lithium anode.
• Flame-Retardant Electrolyte: A functional flame-retardant electrolyte (FRE-TD82) based on a binary flame-retardant solvent of triethyl phosphate (TEP) and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) was designed, with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium difluoro(oxalato)borate (LiDFOB) added as the main salt and film-forming lithium salt additives, respectively. This electrolyte has an oxidation stability potential exceeding 4.5V. The non-polar TTE diluent improves the wettability of the electrolyte on the separator and electrodes, inducing the formation of a rich anionic solvation structure. Through the anionic reduction decomposition reaction, a CEI film rich in inorganic lithium-containing components is formed on the surface of the iron fluoride cathode, enhancing the interfacial stability and reaction reversibility of the cathode.
• Achievements in Performance Enhancement:
• High Capacity and Long Cycle Stability: By using fluorocarbon shear fluorination synthesis methods and electrolyte solvation structure regulation strategies, the reversible capacity of the Li-FeF₃ battery remains at 335mAh/g after 130 cycles. The Li-FeF₃ battery based on the flame-retardant electrolyte has an average cycle capacity decay rate of only 0.16% per cycle, achieving high areal capacity and excellent cycle stability under lean electrolyte conditions.
• Wide-Temperature Operation: The iron fluoride cathode material prepared by the deep eutectic solvent method enables the Li-FeF₃ battery to operate efficiently over a wide temperature range, with reversible specific capacities of up to 486mAh/g at 25°C and 235mAh/g at -20°C.

Research Progress in Sodium-Ion Batteries