The introduction of Lithium-ion batteries (LIBs) in the 1990s by Sony company to the market has revolutionized energy transition globally. Because of their massive consumption in both stationary and mobile sections, the market has encountered the exponential growth of the price of LIBs. This challenge is the outcome of demand and limitation of the primary applied materials in these batteries, such as transition metal oxides (e.g., cobalt (Co), nickel (Ni) and manganese (Mn)), Lithium (Li) as well as graphite (G) [1]. To solve the issue associated with the shrinkage of resources, one may consider alternative elements. Hence, researchers propose potassium (K) ion batteries (PIBs) as the alternative to LIBs since K is a more abundant element, has a higher operating potential, a faster diffusion rate, and the lowest stokes radius in comparison to Li and Na [2].

Polyanionic compounds as active materials are applied as either cathode or anode. Such compounds possess the general formula as AMXO₄L [A = Li, Na, K; M = Fe, Ti, V; X = P, S, Si; L= O, F, OH] with the three-dimensional arrangement of A ions while separated by ML₆ octahedra and XL₄ tetrahedra, which endow the benefit of the reduction of A⁺-A⁺ repulsion [3]. KVPO₄F and KTiPO₄F (KTiOPO₄ (KTP) structure-type material) are manifest paradigms of such compounds, which our group has previously reported as promising cathodes for PIBs with decent electrochemical properties [4].

Furthermore, solution-based pre-alkaliation of the composite electrode or active material is considered an effective strategy to boost capacity of batteries [5]. Researchers have reported pre-alkaliation of some materials for negative electrode materials such as Hard carbon (HC), Phosphorous (P), Silicon (Si), Graphite (G), and so on [6]. However, there is no report on materials with a polyhedral framework structure.

In this talk, we discuss analyzing and comparing crystal structure, chemical composition, morphology, and K-ion storage properties of pristine and pre-potassiated KTP-type KMPO₄F (M=Ti, V) anode materials for PIBs. Firstly, we have synthesized KVPO₄F with a novel and facile hydrothermal method, which has not been reported yet. In addition, the electrochemical properties of KVPO₄F and pre-potassiated (K-rich) KVPF (K_1.1VPO₄F) have been studied in detail. Our investigation demonstrated that the pre-potassiation procedure considerably influences boosting capacity and cyclability. As a result, the composite electrode containing 85 Wt% of carbon-coated K_1.1VPO₄F (C/K_1.1VPF) demonstrated a specific discharge capacity of more than 150 mAh g^-1 at 26.6 mA g^-1 (C/5 rate), while the pristine one exhibited only 70 mAh g^-1. Surprisingly, as for the long cycling performance, C/K_1.1VPF delivers over 100 mAh g^-1 at the current density of 130 mA g^-1 for 400 charge and discharge cycles. we have also suggested and characterized KTiPO₄F as a novel and promising anode material for potassium batteries. Our investigation reveals that pristine and K-rich KTiPO₄F composite electrodes deliver similar discharge capacities > 150 mAh g^-1 at 26.6 mA g^-1 (C/5 rate) in the potential window of 0.001-3 V vs. K⁺/K. The long cycling performance (at least 1000 charging-discharging cycles) of KTiPO₄F is achieved at 130 mAh g^-1 (C rate), delivering discharge capacity > 130 mAh g^-1. To verify the application of KTiPO₄F in a full symmetric battery, we assemble full symmetric batteries, demonstrating >70 mAh g^-1 in the voltage range of 0.001-4.2V.

Acknowledgment

This work was supported by RFBR and DFG, project # 21-53-12039.

References

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(2) Xu, Y. S.; Duan, S. Y.; Sun, Y. G.; Bin, D. S.; Sen Tao, X.; Zhang, D.; Liu, Y.; Cao, A. M.; Wan, L. J.; J. Mater. Chem. A, 2019, 7, 4334–4352.

(3) Kumar, P. R.; Kubota, K.; Igarashi, D.; Komaba, S.; J. Phys. Chem. C, 2021, 45, 24823-24830.

(4) Fedotov, S. S.; Luchinin, N. D.; Aksyonov, D. A.; Morozov, A. V.; Ryazantsev, S. V.; Gaboardi, M.

Plaisier, J. R.; Stevenson, K. J.; Abakumov, A. M.; Antipov, E. V. ; Nat. Commun., 2020, 11, 1–11.

(5) Zhan, R.; Wang, X.; Chen, Z.; Seh, Z. W.; Wang, L.; Sun, Y. ; Adv. Energy Mater., 2021, 11, 2101565.

(6) Kapaev, R. R.; Stevenson, K. J.; J. Mater. Chem. A, 2021, 9, 11771–11777.