KVPOF and KVOPO toward 4 volt-class potassium-ion batteries.

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.