Elucidating the Extraordinary Rate and Cycling Performance of Phenanthrenequinone in Aluminum-Complex-Ion Batteries

Aluminum batteries are of great interest in "beyond-lithium" battery research because of their remarkably high performance in terms of rate capability and cycle life, in addition to the intrinsic advantages of aluminum metal such as its natural abundance and high theoretical capacity of 8056 mAh cm^-3. The electrochemical performance that has been achieved thus far is unusual, as cells usually adopted viscous ionic liquid (IL) electrolytes with bulky complex carrier ions. Herein, we not only demonstrate the excellent rate and cycling performance of phenanthrenequinone (PQ) but also elucidate the origin of this extraordinary performance. Density functional theory (DFT) calculations and experimental analyses jointly revealed that the long-term cyclability of PQ arises from PQ-AlCl₂ complexation, which lessens the effective charge of PQ to mitigate its dissolution into the electrolyte. Moreover, the formation of AlCl₂⁺ without a separate desolvation step allows fast charge transfer, accelerating the AlCl₂⁺ insertion process. This work unveils the importance of aluminum coordination chemistry in determining the key electrochemical properties of aluminum batteries and forms the basis of a new research direction for the development of battery systems based on complex ions.