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

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.