Flouride Ion Batteries Could Be Ten Times Better and Cheaper Than Lithium Ion

Researchers at Caltech, the Jet Propulsion Laboratory, the Honda Research Institute, and Lawrence Berkeley National Laboratory and others are working together to develop rechargeable batteries based on fluoride, the anion (negatively charged form) of elemental fluorine.

Fluoride batteries can have a higher energy density, which means that they may last longer—up to eight times longer than batteries in use today. But fluoride is corrosive and reactive. Iron fluorides already have more than double the lithium capacity of traditional cobalt- or nickel-based cathodes. Iron is 300 times less expensive than cobalt and 150 times less costly than nickel.

Researcher team made a new type of cathode from iron fluoride active material and a solid polymer electrolyte nanocomposite. They tested several variations of the new solid-state batteries to analyze their performance over more than 300 cycles of charging and discharging at elevated temperature of 122 degrees Fahrenheit, noting that they outperformed previous designs using metal fluoride even when these were kept cool at room temperatures.

They will make new and improved solid electrolytes to enable fast charging and also to combine solid and liquid electrolytes in new designs that are fully compatible with conventional cell manufacturing technologies employed in large battery factories.

Nature Materials – 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 (over 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.

Science – Room-temperature cycling of metal fluoride electrodes: Liquid electrolytes for high-energy fluoride ion cells (Dec 2018)

Abstract
Fluoride ion batteries are potential “next-generation” electrochemical storage devices that offer high energy density. At present, such batteries are limited to operation at high temperatures because suitable fluoride ion–conducting electrolytes are known only in the solid state. We report a liquid fluoride ion–conducting electrolyte with high ionic conductivity, wide operating voltage, and robust chemical stability based on dry tetraalkylammonium fluoride salts in ether solvents. Pairing this liquid electrolyte with a copper–lanthanum trifluoride (Cu@LaF3) core-shell cathode, we demonstrate reversible fluorination and defluorination reactions in a fluoride ion electrochemical cell cycled at room temperature. Fluoride ion–mediated electrochemistry offers a pathway toward developing capacities beyond that of lithium ion technology.

Journal of the Electrochemical Society – An Aqueous Rechargeable Fluoride Ion Battery with Dual Fluoride Electrodes (2019)

An anion flow battery has recently emerged as an option to store electricity with high volumetric energy densities. In particular, fluoride ions are attractive for these batteries because they have the smallest size among anions, which is beneficial for charge transport. To date, reported fluoride ion batteries either operate with an ionic liquid, organic electrolyte or solid-state electrolyte at high temperatures. Herein, an aqueous fluoride ion flow battery is proposed that consists of bismuth fluoride as the anode, 4-hydroxy-TEMPO (TEMPO) as the cathode, and NaF salt solution as the aqueous electrolyte. During the charging process, bismuth fluoride electrochemically releases fluoride ions with the formation of bismuth metal, while TEMPO captures the fluoride ions. A reversible and stable discharge capacity of 89.5 mAh g−1 was achieved at 1000 mA g−1 after 85 cycles. The fluoride ion battery possesses excellent rate performance. To the best of our knowledge, this is the earliest demonstration that fluoride ion batteries can work in aqueous solutions, which can be used for future clean energy applications.

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