Alternate Storage of Opposite Charges in Multisites for High-Energy-Density Al-MOF Batteries

The limited active sites of cathode materials in aluminum-ion batteries restrict the storage of more large-sized Al-complex ions, leading to a low celling of theoretical capacity. To make the utmost of active sites, an alternate storage mechanism of opposite charges (AlCl4- anions and AlCl2+ cations) in multisites is proposed herein to achieve an ultrahigh capacity in Al-metal-organic framework (MOF) battery. The bipolar ligands (oxidized from 18 pi to 16 pi electrons and reduced from 18 pi to 20 pi electrons in a planar cyclic conjugated system) can alternately uptake and release AlCl4- anions and AlCl2+ cations in charge/discharge processes, which can double the capacity of unipolar ligands. Moreover, the high-density active Cu sites (Cu nodes) in the 2D Cu-based MOF can also store AlCl2+ cations for a higher capacity. The rigid and extended MOF structure can address the problems of high solubility and poor stability of small organic molecules. As a result, three-step redox reactions with two-electron transfer in each step are demonstrated in charge/discharge processes, achieving high reversible capacity (184 mAh g(-1)) and energy density (177 Wh kg(-1)) of the optimized cathode in an Al-MOF battery. The findings provide a new insight for the rational design of stable high-energy Al-MOF batteries.

Automated summary

This study proposes a new mechanism for utilizing active sites in aluminum-metal organic framework batteries, resulting in increased capacity and energy density. By using bipolar ligands and high-density active sites, the researchers were able to achieve higher reversible capacity and energy density in the battery.