K 0.83 V 2 O 5 : A New Layered Compound as a Stable Cathode Material for Potassium-Ion Batteries.

Recently, potassium-ion batteries (PIBs) are being actively investigated. The development of PIBs calls for cathode materials with a rigid framework, reversible electrochemical reactivity, and a high amount of extractable K ions, which is extremely challenging due to the large size of potassium. Herein, a new layered compound K _0.83 V ₂ O ₅ is reported as a potential cathode material for PIBs. It delivers an initial depotassiation capacity of 86 mAh g ^-1 and exhibits a reversible capacity of 90 mAh g ^-1 with a high redox potential of 3.5 V (vs K ⁺ /K) and a capacity retention of more than 80% after 200 cycles. Experimental investigations combined with theoretical calculation indicate that depotassiation-potassiation is accommodated by contraction-expansion of the interlayer spacing along with unpuckering-puckering of the layers. Additionally, the calculated electronic structure suggests the (semi)metallic feature of K _x V ₂ O ₅ (0 < x ≤ 0.875) and K-ion transport in the material is predicted to be one-dimensional with the experimentally estimated chemical diffusion coefficient in the order of 10 ^-15 -10 ^-12 cm ² s ^-1 . Finally, a K-ion full cell consisting of the K _0.83 V ₂ O ₅ cathode and a graphite anode is demonstrated to deliver an energy density of 136 Wh kg ^-1 . This study will provide insights for further designing novel layered cathodes with high K-ion content for PIBs.