Aqueous Binders for Stable Cycling of Iron Fluoride Conversion Cathodes
Authors: Green Tianna , Lopez Jeffrey 
Published: December 2024

To meet the demand of the rapidly growing electric vehicle (EV) market, we must develop low cost, safe, and high energy density batteries. Exploring cathode materials composed of abundant and cheap elements, like iron, can replace the critical minerals used in commercial cathodes today. Transition metal fluoride conversion cathodes are excellent candidates, boasting a 200-300% higher energy density than traditional intercalation cathodes due to a 2-3 electron transfer reaction mechanism [1,2]. Iron fluorides (FeF2, FeF3) are among the most studied conversion cathodes. However, they suffer from active material dissolution and structural damage upon cycling that result in poor performance and low reversibility [3].

To increase stability, we are investigating aqueous binders with physical properties to promote electrode cohesion, increase elasticity, and better accommodate large volume changes upon cycling [4,5]. The choice of binders was inspired by polymers used to mitigate extreme volume changes in Si-based anodes. Alternative binders with functional groups that interact with Si particles demonstrate better electrochemical performance compared to electrodes made using the conventional polyvinylidene fluoride binder (PVDF), which is attributed to improved mechanical properties of the binder and the composite electrode [6,7]. With these properties in mind, we aim to address the abovementioned challenges of FeF2 conversion cathodes through binder design. FeF2/Li cells with our alternative binder exhibit an initial specific capacity of 357 mAh/g and maintain 272 mAh/g after 50 cycles.Meanwhile, electrodes with PVDF binder exhibit an initial specific capacity of 354 mAh/g and maintain only 161 mAh/g after 50 cycles. Fourier-transform infrared spectroscopy (FTIR) reveals the presence of weak bonds between the aqueous binders and FeF2. This interaction improves the structural integrity of FeF2 electrodes. Post-mortem X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) of our alternative binder electrodes shows increased contact between the conversion reaction products and the formation of favorable interphases, which results in high reversibility. This work supports the use of alternative binders in conversion cathodes for better long-term stability and capacity retention. Additionally, aqueous binders will eliminate the need for the toxic solvent in PVDF electrodes, N-methyl-2-pyrrolidone (NMP).

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