Determining Performance-Limiting Mechanisms in Fluorophosphate Sodium-Ion Battery Cathodes Via Transition-Metal Mixing

Motivated by the potential for new intercalation chemistries, sodium-ion batteries have received renewed interest as a possible replacement technology for lithium-ion batteries. To date, fluorophosphates, particularly Na₃V₂(PO₄)₂F₃, are among the most promising sodium-ion cathodes studied. Fluorophosphates exhibit redox activity near 4V due to the strong inductive effect of phosphates and fluorine, and have been shown to reversibly cycle near their single-electron capacity of 128mAh/g, yielding energy densities close to 480Wh/kg^1,2. However, efforts to further improve the capacity of these materials have achieved very little success.

            This work reports on our efforts to determine the mechanism by which fluorophosphate cathodes appear to be limited to their single-electron capacity. Our method is to examine systems that decouple previously observed Na⁺extraction limits from redox limits. We will discuss the electrochemical characterization of novel mixed-transition metal fluorophosphate systems, from which we can conclude that fluorophosphate cathodes’ capacities are not redox-limited. We demonstrate reversible capacity exceeding the single-electron theoretical capacity during a single charge/discharge. In addition, computational examination of limiting behaviors will be elaborated on. Specifically, possible site-limiting and diffusion-limiting mechanisms in fluorophosphate cathodes will be presented.

References:

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[2]  K. Chihara, A. Kitajou, I. Gocheva, S. Okada, and J. Yamaki. J. Power Soources., 227, 80 – 85 (2013).