New Phosphate Framework for Facile Reversible Alkali Metal Ion De/Insertion
Authors: Fedotov Stanislav S. , Nikitina Victoria A. , Aksyonov Dmitry A , Samarin Aleksandr Sh. , Luchinin Nikita D. , Stevenson Keith J. , Abakumov Artem M. , Antipov Evgeny V. 
Published: February 2020

Phosphate-based cathode materials are considered safe, reliable and commercially viable for perfecting the next generation of metal-ion batteries for grid applications. Contrary to oxides, phosphates tend to form more sophisticated and diverse crystal structures thus expanding playground for designing and tuning electrochemical properties. Coupled with fluoride anions phosphates might offer not only even higher electrode potentials but also a richer crystal chemistry and then ampler opportunities in developing new cathode materials.
Recently we proposed a novel series of vanadium-based AVPO4F (A = Li, Na, K, Rb) cathode materials crystallizing in a unique KTiOPO4 (KTP)-type structure. The target materials were synthesized via different soft-chemistry routes including hydrothermal method and freeze-drying.
The KTP-type “VPO4F” framework featured outstanding rate capability and capacity retention especially in Li cells: the material maintained more than 75% of the initial specific capacity at the 40C rate with an average potential of 4.0 V vs. Li/Li+ [1]. The structural evolution during Li de/insertion was examined by operando X-ray diffraction and ex situ electron tomography. The material experienced a series of phase transitions characterized by a volume variation just within 2%.
Another quality of this framework is supporting reversible Na+, K+ and even Rb+ ions de/insertion sustaining the host structure [2]. The experimental electrochemical performance and ion transport properties were different for various alkali ions. The ion diffusion coefficients obtained by PITT were the lowest for Li+ (10-12 – 10-14 cm2/s) and the highest for K+ (10-11 – 10-12 cm2/s), with the latter anticipating high-power applications of KVPO4F in K-ion batteries [3]. According to DFT calculations, the “VPO4F” framework manifested extremely low energy barriers for K+ and Rb+ migration of less than 0.2 eV in accordance with experimental transport properties.
Furthermore, the full extraction of K+ in KVPO4F occurred at the highest potential of ~4.6 V vs. K/K+ compared to Li+, Na+ and Rb+. At the same time the average potential of the material (4.3 V vs. K/K+) lay within a stability window of commercial electrolytes that validates KVPO4F as the most prospective cathode material for high-voltage K-ion batteries [4].
Additionally, the “VPO4F” framework demonstrated reversible electrochemical activity in the low-voltage domain (below 1.8 V vs. A/A+) in Li, Na and K cells thus opening prospects for creating anode materials for metal-ion batteries.
In this talk we will focus on our recent activities on the KTP-type fluoride-phosphates as promising electrode materials for metal-ion rechargeable batteries.

References:

S.S. Fedotov et al. Chem. Mater. 28 (2016) 411-415.

S. S. Fedotov et al. J. Mater. Chem. A. 6 (2018) 14420-14430

V.A. Nikitina et al. J. Electrochem. Soc. 164 (2017) A6373-A6380.

V. A. Nikitina et al. Electrochim Acta. 230 (2017) 479-491