In situ X-ray absorption spectroscopic investigation of the electrochemical conversion reactions of CuF2–MoO3 nanocomposite

Metal Fluoride Cathode Materials for Lithium Rechargeable Batteries: Focus on Iron Fluorides

Exploring prospective rechargeable batteries with high energy densities is urgently needed on a worldwide scale to address the needs of the large-scale electric vehicle market. Conversion-type metal fluorides (MFs) are attractive cathodes for next-generation rechargeable batteries because of their high theoretical potential and capacities and provide new perspectives for developing novel battery systems that satisfy energy density requirements. However, some critical issues, such as high voltage hysteresis and poor cycling stability must be solved to further enhance MF cathode materials. In this review, the recent advances in mechanisms focused on FeF₃ cathodes under lithiation/delithiation processes are discussed in detail. Then, the classifications and advantages of various synthesis methods to prepare MF-based materials are first minutely discussed. Moreover, the performance attenuation mechanisms of MFs and the effort in the development of mitigation strategies are comprehensively reviewed. Finally, prospects for the current obstacles and possible research directions, with the aim to provide some inspiration for the development of MF cathode-based batteries are presented.

Super-Reversible CuF2 Cathodes Enabled by Cu2+ -Coordinated Alginate

Copper fluoride (CuF₂ ) has the highest energy density among all metal fluoride cathodes owing to its high theoretical potential (3.55 V) and high capacity (528 mAh g^-1 ). However, CuF₂  can only survive for less than five cycles, mainly due to serious Cu-ion dissolution during charge/discharge cycles. Herein, copper dissolution is successfully suppressed by forming Cu²⁺ -coordinated sodium alginate (Cu-SA) on the surface of CuF₂  particles during the electrode fabrication process, by using water as a slurry solvent and sodium alginate (SA) as a binder. The trace dissolved Cu²⁺ in water from CuF₂  can in situ cross-link with SA binder forming a conformal Cu-SA layer on CuF₂  surface. After water evaporation during the electrode dry process, the Cu-SA layer is Li-ion conductor but Cu²⁺ insulator, which can effectively suppress the dissolution of Cu-ions in the organic 4 m LiClO₄ /ethylene carbonate/propylene carbonate electrolyte, enhancing the reversibility of CuF₂ . CuF₂  electrode with SA binder delivers a reversible capacity of 420.4 mAh g^-1  after 50 cycles at 0.05 C, reaching an energy density of 1009.1 Wh kg^-1 . Cu²⁺ cross-link polymer coating on CuF₂  opens the door for stabilizing the high-energy and low-cost CuF₂  cathode for next-generation Li-ion batteries.

Cycle stability of conversion-type iron fluoride lithium battery cathode at elevated temperatures in polymer electrolyte composites

Metal fluoride conversion cathodes offer a pathway towards developing lower-cost Li-ion batteries. Unfortunately, such cathodes suffer from extremely poor performance at elevated temperatures, which may prevent their use in large-scale energy storage applications. Here we report that replacing commonly used organic electrolytes with solid polymer electrolytes may overcome this hurdle. We demonstrate long-cycle stability for over 300 cycles at 50 °C attained in high-capacity (>450 mAh g^-1) FeF₂ cathodes. The absence of liquid solvents reduced electrolyte decomposition, while mechanical properties of the solid polymer electrolyte enhanced cathode structural stability. Our findings suggest that the formation of an elastic, thin and homogeneous cathode electrolyte interphase layer on active particles is a key for stable performance. The successful operation of metal fluorides at elevated temperatures opens a new avenue for their practical applications and future successful commercialization.

Ammonium Fluoride Mediated Synthesis of Anhydrous Metal Fluoride-Mesoporous Carbon Nanocomposites for High-Performance Lithium Ion Battery Cathodes

Metal fluorides (MF_x) are one of the most attractive cathode candidates for Li ion batteries (LIBs) due to their high conversion potentials with large capacities. However, only a limited number of synthetic methods, generally involving highly toxic or inaccessible reagents, currently exist, which has made it difficult to produce well-designed nanostructures suitable for cathodes; consequently, harnessing their potential cathodic properties has been a challenge. Herein, we report a new bottom-up synthetic method utilizing ammonium fluoride (NH₄F) for the preparation of anhydrous MF_x (CuF₂, FeF₃, and CoF₂)/mesoporous carbon (MSU-F-C) nanocomposites, whereby a series of metal precursor nanoparticles preconfined in mesoporous carbon were readily converted to anhydrous MF_x through simple heat treatment with NH₄F under solventless conditions. We demonstrate the versatility, lower toxicity, and efficiency of this synthetic method and, using XRD analysis, propose a mechanism for the reaction. All MF_x/MSU-F-C prepared in this study exhibited superior electrochemical performances, through conversion reactions, as the cathode for LIBs. In particular, FeF₃/MSU-F-C maintained a capacity of 650 mAh g^-1_FeF3 across 50 cycles, which is ∼90% of its initial capacity. We expect that this facile synthesis method will trigger further research into the development of various nanostructured MF_x for use in energy storage and other applications.

In situ TEM studies of micron-sized all-solid-state fluoride ion batteries: Preparation, prospects, and challenges

Trustworthy preparation and contacting of micron-sized batteries is an essential task to enable reliable in situ TEM studies during electrochemical biasing. Some of the challenges and solutions for the preparation of all-solid-state batteries for in situ TEM electrochemical studies are discussed using an optimized focused ion beam (FIB) approach. In particular redeposition, resistivity, porosity of the electrodes/electrolyte and leakage current are addressed. Overcoming these challenges, an all-solid-state fluoride ion battery has been prepared as a model system for in situ TEM electrochemical biasing studies and first results on a Bi/La0.9 Ba0.1 F2.9 half-cell are presented. Microsc. Res. Tech. 79:615-624, 2016. © 2016 Wiley Periodicals, Inc.