A Layered Inorganic-Organic Open Framework Material as a 4 V Positive Electrode with High-Rate Performance for K-Ion Batteries

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, K-2[(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.