Structurally identical KVPOF and KVOPO are evaluated as positive electrode materials for non-aqueous potassium-ion batteries. KVPOF and KVOPO show highly reversible potassium extraction/insertion with discharge capacities of ca. 92 mA h g and ca. 84 mA h g, respectively, and their average discharge voltage reaches above 4.0 V with 1 M KPF EC/PC electrolyte at 2.0-5.0 V. Despite the extraction of large potassium-ions, their lattice volume shrinkages after charging to 5.0 V are 5.8% for KVPOF and 3.3% for KVOPO, leading to stable cycle performance. This is the first report to confirm the charge/discharge behaviours of vanadium phosphate electrodes in 4 V-class K cells.

K‐ion batteries (KIBs) are promising for large‐scale energy storage owing to various advantages like the high abundance of potassium resources in the Earth's crust, high operational potentials, and high power due to fast diffusion of K+ ions. However, to realize the practical application of KIBs, electrode materials are needed with high operational voltage, good capacity, long cycle life, and low‐cost. This work reports a layered open framework material, K2[(VOHPO4)2(C2O4)], composited with reduced graphene oxide (rGO) as a 4 V positive electrode material for KIBs. The material is prepared by a simple precipitation reaction at room temperature. The material demonstrates reversible K‐extraction/insertion with conventional carbonate ester KPF6 solutions; however, with low specific capacity and low Coulombic efficiency. A high discharge capacity of >100 mAh g−1 with good cycling stability and higher Coulombic efficiency is achieved in a highly concentrated electrolyte, 7 mol kg−1 of potassium bis(fluorosulfonyl)amide (KFSA) in dimethoxyethane (DME) at 0.1 C rate. Due to the facile migration of K+ ions in the framework, the material exhibits excellent rate capability with a discharge capacity of 80 mAh g−1 at 10 C rate, and a good capacity retention of 67% after 500 cycles at 2 C rate.

Lepidocrocite-like titanate, Na 0.9 [Ti 1.7 Li 0.3 ]O 4 , having a C-base-centered lattice (C-type) is prepared by dehydration of Na 0.9 [Ti 1.7 Li 0.3 ]O 4 •nH 2 O having a primitive lattice (P-type) and influence of the interlayer water on its electrochemical performance is examined in nonaqueous Na cells. Upon heating up to 350 °C, P-type Na 0.9 [Ti 1.7 Li 0.3 ]O 4 •nH 2 O transforms into the anhydrous C-type phase as a result of gradual removal of the interlayer water. The C-type Na 0.9 [Ti 1.7 Li 0.3 ]O 4 delivers reversible capacity of 120 mAh g −1 between 2.0 and 0.1 V vs Na with high Coulombic efficiency at an initial cycle and good capacity retention over 50 cycles. Structural change of the C-type phase during sodiation is investigated by using operando X-ray diffraction measurement.

K-ion batteries (KIBs) are promising for large-scale electrical energy storage owing to the abundant resources and the electrochemical specificity of potassium. Among the positive electrode materials for KIBs, vanadium-based polyanionic...

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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