article · Advanced Materials
Light-assisted lithium-oxygen batteries can achieve high round-trip efficiency by using light-generated charge carriers to drive oxygen reduction and evolution reactions. However, strong attraction between electrons and holes, known as the excitonic effect, often prevents these carriers from separating, limiting the battery performance. To address this issue, porphyrinic metal-organic frameworks containing iron-nickel oxide clusters were employed as battery photocathodes. Electronic coupling between the nickel and iron orbitals enhances charge transfer from the organic ligands to the metal clusters. This mechanism accelerates the dissociation of excitons into free charge carriers and promotes the formation of superoxide radicals instead of singlet oxygen during photoexcitation. As a result, the light-assisted lithium-oxygen cell achieved a low total overvoltage of 0.28 volts and a 92 percent round-trip efficiency under light irradiation.
Developing highly efficient rechargeable batteries is essential for clean energy systems. Light-assisted lithium-oxygen batteries hold significant promise, but internal energy losses from trapped charges have limited their operation. Demonstrating that tailored metal-organic frameworks can overcome these charge-separation barriers offers a clearer chemical strategy for designing photocathodes that convert light into electrochemical energy with minimal voltage loss.
The material concepts explored here could eventually support developers of next-generation photo-assisted batteries and specialized energy-storage devices. However, the reported data reflects early-stage laboratory research focused on material mechanisms and electrochemical performance under controlled illumination. Substantial development, including long-term cycling assessments, scale-up synthesis of the metal-organic framework, and practical device integration, is required before commercial pathways can be established.
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Light-assisted Li-O<sub>2</sub> batteries exhibit a high round-trip efficiency attributable to the assistance of light-generated electrons and holes in oxygen reduction and evolution reactions. Nonetheless, the excitonic effect arising from Coulomb interaction between electrons and holes impedes carrier separation, thus hindering efficient utilization of photo-energy. Herein, porphyrinic metal-organic frameworks with (Fe<sub>2</sub>Ni)O(COO)<sub>6</sub> clusters are used as photocathodes to accelerate exciton dissociation into charge carriers for light-assisted Li-O<sub>2</sub> batteries. The coupling of Ni 3d and Fe 3d orbitals boosts ligand-to-metal cluster charge transfer, and hence drives exciton dissociation and activates O<sub>2</sub> for superoxide (<sup>•</sup>O<sub>2</sub> <sup>-</sup>) radicals, rather than singlet oxygen (<sup>1</sup>O<sub>2</sub>) under photoexcitation. These enable the light-assisted Li-O<sub>2</sub> batteries with a low total overvoltage of 0.28 V and round-trip efficiency of 92% under light irradiation of 100 mW cm<sup>-2</sup>. This work highlights the excitonic effect in photoelectrochemical processes and provides insights into photocathode design for light-assisted Li-O<sub>2</sub> batteries.
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DOI: 10.1002/adma.202405440
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