The ever-growing demand for portable energy in various fields puts forward new challenges to rechargeable battery technologies in improving specific energy and power, life and safety. The battery performance is critically governed by the properties of the cathode material. Fluoride-phosphates with AxMPO4F (A = Li, Na, K; M = V, Fe, Co) formula take several advantages over existing oxide cathode materials: a substantial increase in operating voltages and much faster kinetics. Among them Li2CoPO4F have gained much attention as a promising high-voltage cathode material with high specific capacity (~280 mAh/g) expected for an exchange of two alkali ion per formula unit. A computational study of this material with DFT-NEB method revealed low energy barriers (0.12-0.43 eV) of Li–Li transitions and possible 3D Li-ion migration system anticipating fast transport properties. However, only 1 Li per formula unit can be de/intercalated within the potential stability window of the commercially available electrolytes. Its counterpart, Li2FePO4F, was found to exhibit severe cation intermixing during operation that may limit the possible application. Exploring other compositions we designed and prepared a novel vanadium-based AVPO4F (A = Li, K) cathode material adopting a KTP-type framework with excellent rate capability. The material showed remarkable capacity retention in Li cells at 40C maintaining more than 75% of the initial specific capacity. It can also reversibly incorporate Na+ and K+ ions unveiling rapid kinetics and preserving the host structure. The alkali ion diffusion coefficients measured using PITT and EIS techniques were the lowest for Li+ (10-12 – 10-14 cm2/s) and highest for K+ (10-11 – 10-12 cm2/s) making AVPO4F a promising candidate for potassium-ion batteries. Our current activities on fluoride-phosphate based cathode materials will be reported with a particular focus on the interrelation between crystal structure peculiarities and electrochemical properties.

Alkali and transition metal fluoride-phosphates as cathode materials for metal-ion batteries

253rd National Meeting (2017)

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