Sodium Vanadium Fluorophosphates (NVOPF) Array Cathode Designed for High‐Rate Full Sodium Ion Storage Device

Abstract 3D batteries continue to be of widespread interest for flexible energy storage where the 3D nanostructured cathode is the key component to achieve both high energy and power densities. While current work on flexible cathodes tends to emphasize the use of flexible scaffolds such as graphene and/or carbon nanotubes, this approach is often limited by poor electrical contact and structural stability. This communication presents a novel synthetic approach to form 3D array cathode for the first time, the single‐crystalline Na 3 (VO) 2 (PO 4 ) 2 F (NVOPF) by using VO 2 array as a seed layer. The NVOPF cathode exhibits both high‐rate capability (charge/discharge in 60 s) and long‐term durability (10,000 cycles at 50 C) for Na ion storage. Utilizing in situ X‐ray diffraction and first principles calculations, the high‐rate properties are correlated with the small volume change, 2D fast ion transport, and the array morphology. A novel all‐array flexible Na + hybrid energy storage device based on pairing the intercalation‐type NVOPF array cathode with a cogenetic pseudocapacitive VO 2 nanosheet array anode is demonstrated.