High‐Density and Freestanding Porous Carbon Film for Compact Sodium‐Ion Storage

Abstract: Porous carbon materials are often difficult to achieve high density while possessing high porosity, which limits their application in compact energy storage. Here, a design of freestanding porous‐yet‐dense carbon films with a tunable density (1.08–1.33 g cm−3) and porosity (specific surface area of 0–423.8 m2 g−1) is presented through an assembly of porous carbon nanosheet with graphene oxide under vacuum filtration. The typical freestanding carbon films simultaneously deliver a high density of 1.08 g cm−3 and a high specific surface area of 423.8 m2 g−1 when the porous carbon nanosheet content is 75 wt.%. As anode materials for sodium‐ion batteries, the optimized freestanding carbon films deliver high volumetric capacity (270 mAh cm−3 at 20 mA g−1), high initial capacity efficiency (81%) and superior long‐term cycling stability (1300 cycles with a capacity decay rate of 0.012% per cycle). This study provides a promising direction for creating freestanding electrodes that meet both high‐porosity and high‐density requirements for compact sodium‐ion batteries.