Stable High-Capacity Organic Aluminum–Porphyrin Batteries

Aluminum-ion batteries (AIBs) attract interest for their promising features of superior safety and long-life energy storage. Organic materials with engineered active groups are considered promising for promoting energy storage capabilities. However, the corresponding energy density (both voltage plateau and sufficient active sites required) and stability are still unexpectedly poor. To address these challenges, here π-conjugated organic porphyrin molecules, that is, 5,10,15,20-tetraphenylporphyrin (H₂TPP) and 5,10,15,20-tetrakis(4-carboxyphenyl) porphyrin (H₂TCPP), are selected as the positive electrode materials for AIBs. Owing to the highly reversible coordination/dissociation with aluminum complex cations, H₂TPP presents long-term cycling stability beyond 5000 cycles at 200 mA g⁻¹. Compared with the specific capacity of H₂TCPP (≈24 mA h g⁻¹ at 100 mA g⁻¹), the enhanced capabilities in H₂TPP (reversible specific capacity of ≈101 mA h g⁻¹ at 100 mA g⁻¹) are attributed to removal of the carboxyl functional groups, which plays a role in reducing the basicity of porphyrin induced via electron withdrawing effects. Additionally, the mechanism of electrochemical reaction between AlCl₂⁺ and porphyrin as well as ionic diffusion behaviors on the surface of the electrode are investigated. The results establish a platform to develop long-term organic aluminum batteries for safe and stable energy storage.