A Redox Mediator Containing Reversible Dynamic Boron–Oxygen Bonds to Construct an Adaptive SEI Layer for Advanced Li–O2 Batteries

Lithium-oxygen (Li-O₂) batteries have high theoretical energy density, but the discharge product Li₂O₂ of Li-O₂ batteries is difficult to decompose, resulting in the undesirably high charging potential. The use of soluble redox mediators (RMs) can usually reduce the high charging potential of Li-O₂ batteries, but the RM on the cathode side can diffuse to the Li metal anode and react with it, leading to continuous loss of the RM and causing damage to the fragile Li anode interface. So, it is necessary to develop a bifunctional redox mediator (BRM) that can simultaneously reduce the charging potential and protect the Li anode. Herein, we introduced 4-bromomethyl-phenylboronic acid (BPLA) as a BRM. The Br^- ions can be dissociated from BPLA during cycling and serve as an effective component of RM, thereby significantly facilitating the reduction of charging potential of Li-O₂ batteries. Meanwhile, the boronic acid groups in BPLA have the capability to engage in cross-linking reactions on the Li-metal surface, forming a flexible and continuous solid-electrolyte interphase (SEI) layer. More importantly, the SEI layer contains the reversible dynamic B-O covalent bond, which possesses a characteristic of continuous dissociation and rearrangement. Thereby the SEI layer possesses the shape adaptability, inhibits the growth of Li dendrites, and suppresses the reaction between RM and Li. Consequently, our BPLA, serving as the BRM, can enable Li-O₂ batteries to achieve a stable cycle life of 180 cycles under the low charge potential up to 4.0 V.