CO2 Etching Modulates Lithium and Sodium Storage Performance of Hard–Soft Carbon Composite-Based Freestanding Thick Electrodes

Carbon-based materials are the most prospective anodes. Typically, a single carbon-based material is applied to different energy storage systems (EESs) without modification. However, the microcrystal structure of carbon plays a decisive role in the energy storage performance, and therefore, it should be adjusted when applied to different EESs. Here, a hierarchical porous carbon monomer monolith (HPCM) embedded with carbon nanotubes blooming on ZIF-67 was designed as a soft–hard carbon-based freestanding thick electrode for achieving high-energy lithium-ion and sodium-ion batteries. HPCM is resorcinol–formaldehyde (RF) resin-derived carbon, mainly composed of hard carbon, which has outstanding mechanical properties, a high surface area, and high porosity. Carbon nanotubes (CNTs) derived from ZIF-67 have extraordinary electronic conductivity, which provides soft carbon. High-temperature CO2 etching was performed to adjust the microcrystal structure, and the lithium/sodium storage performance of the electrode was evaluated. After CO2 etching, the materials lose almost half their weight (mainly hard carbon), and pseudocapacitive contribution decreases for both lithium-ion and sodium-ion batteries, whereas the specific capacity increases for lithium-ion batteries and decreases for sodium-ion batteries. Capacities of 5.96 mAh cm–2 (areal) and 132.48 mAh cm–3 (volumetric) were achieved for lithium storage, and those for sodium storage were 2.31 and 51.24 mAh cm–3, respectively. In summary, it is significant to adjust the microcrystal structure of carbon-based electrodes, and this study provides related experience for lithium and sodium storage.