K₂[(VO)₂(HPO₄)₂(C₂O₄)] and K _x VOPO₄ as 4 V-Class Positive Electrode Materials for K-Ion Batteries

Abstract

K-ion batteries (KIBs) potentially operate at high-voltage and high-current thanks to the lower standard electrode potential of K/K+ and the lower solvation energy than those of Li/Li+ in organic electrolytes and fast K-ion diffusion.[1] To realize practical applications, high energy density positive electrode materials are, however, indispensable. In this study, two vanadium based positive electrode materials with layered structure, namely, K2[(VO)2(HPO4)2(C2O4)] and and K x VOPO4 are synthesized and their electrode performances are examined in potassium cells. Highly reversible Li insertion/extraction oxalatophosphate material was reported with reversible capacities of >100 mAh g-1 and with a high voltage operation of ~3.8 V vs. Li/Li+. [2, 3] Hence, reversible K extraction/insertion and high voltage operation are expected in K cells. K2[(VO)2(HPO4)2(C2O4)] was prepared by a simple precipitation method at room temperature. As shown in Fig. 1, K2[(VO)2(HPO4)2(C2O4)] delivers a reversible capacity of ~90 mAh g-1 in the K-half cell at 0.1 C in the voltage range of 2.5 - 4.5 V vs. K/K+ and excellent cycle stability is demonstrated. Furthermore, high rate capability with 76 mAh g-1 at 10 C is achieved by optimization of the electrolyte composition. Another vanadium-based layered positive electrode material, K x VOPO4 was also prepared by a precipitation method at room temperature. By optimization with carbon-coating and ball-milling, the material delivers a reversible capacity of 80 mAh g-1 at 3.7 V for over 200 cycles. Fig.1: Voltage profiles and capacity retention plots of K2[(VO)2(HPO4)2(C2O4)] in a K-half cell. References [1] K. Kubota, S. Komaba, et al., Chem. Rec., 2018, 18, 459-479. [2] A.S. Hameed et al., Sci. Rep., 2015, 5, 16270. [3] A.S. Hameed et al., J. Mater. Chem. A, 2013, 1, 5721-5726. Figure 1