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 m 2 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 m 2 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.