image: Figure 1 illustrates the molecular design for high-energy organic cathodes in AIBs, detailing their charge storage mechanism, the relationship between open-circuit voltage and cathode potential, the structures and electrostatic potential distributions of key molecules, the correlation between calculated energy levels and redox potentials, and a performance comparison of newly designed materials with existing cathodes.
Credit: ©Science China Press
Researchers have developed a novel dual-strategy approach to design organic cathode materials that simultaneously achieve high energy density and long cycle life in rechargeable aluminum-ion batteries (AIBs), a promising technology for large-scale energy storage. This work overcomes a critical bottleneck that has hampered the progress of aluminum-organic batteries.
Rechargeable AIBs are attractive due to aluminum’s abundance, low cost, and high theoretical capacity. However, their development has been persistently challenged by the lack of suitable cathode materials capable of efficiently storing the large chloroaluminate anions (like AlCl4−). While graphite cathodes are stable, their capacity is limited, and many inorganic alternatives suffer from low voltage or poor cycling. Organic cathode materials offer structural flexibility but have typically struggled to combine high operating voltage with high specific capacity, resulting in low energy density. Issues like material dissolution have further compromised stability.
To solve this, a research team led by Prof. Shuqiang Jiao and Prof. Wei Wang from University of Science and Technology Beijing and Yanli Zhu from Beijing Institute of Technology employed a rational molecular design strategy. Their work is based on the principle that p-type organic cathodes operate by losing electrons from their highest occupied molecular orbital (HOMO) during oxidation. Strategically lowering the HOMO energy level should therefore increase the battery’s output voltage. The researchers first used heteroatom substitution to engineer this property. By comparing a series of conjugated heterocyclic molecules, they identified thianthrene (TT), featuring dual sulfur atoms, as the most electronegative unit with the lowest HOMO energy level, promising high redox potential. To concurrently boost the material’s capacity, the team developed a second strategy focusing on polymer linkage patterns. They designed four sulfur-heterocyclic polymers (PTT-1 to PTT-4), systematically increasing the linkage of active TT units from dimer to hexamer. This design successfully increased the maximum number of electrons transferred per unit from 4 to 12, directly enhancing the theoretical capacity.
The synergy of these two strategies proved highly successful. The optimized polymer, PTT-4, functioned as an exceptional cathode material. It delivered a high average discharge voltage of approximately 1.7 V and a specific capacity of 150 mAh g−1. This combination resulted in a record-breaking energy density of 255 Wh kg−1 for aluminum-organic batteries, decisively surpassing the performance ceiling of typical graphite cathodes. Beyond energy density, the PTT-4 cathode demonstrated remarkable durability, a key requirement for practical applications. It exhibited outstanding cycling stability even under extreme conditions, retaining nearly 100% of its capacity after 12,000 charge-discharge cycles at a low temperature of −20 °C.
This research demonstrates that targeted molecular engineering can overcome fundamental limitations in next-generation battery chemistries. By independently tailoring voltage through heteroatom substitution and capacity through polymer unit linkage, the study provides a powerful design blueprint. The achieved milestone in energy density, coupled with exceptional low-temperature cycle life, validates the high technological relevance of molecularly engineered organic polymers for advancing high-performance, sustainable energy storage systems.
Journal
National Science Review