Dopant-Induced Bond Strength Polarization Drives High Bidirectional Catalytic Activity in Li–CO2 Batteries

Spinel oxides have attracted increasing attention as cathode in Li–CO 2 batteries because of their cost–effectiveness and facile synthesis. Unfortunately, their catalytic activity for CO 2 reduction reaction and CO 2 evolution reaction remains below the necessary for the implementation at scale. Here, the introduction of first–row 3d transition metals (TMs) in Fe 3 O 4 can construct an asymmetric M–O–Fe backbone, which is defined as the bond strength polarization and leads to continuous modulation in the electron activity (TM used in this work including Mn, Co, Ni, and Cu). Theoretical calculations confirm that the bond strength polarization can optimize the adsorption behavior of the reactants (CO 2 , Li + , and Li 2 CO 3 ), promoting the catalytic activity toward CO 2 reduction and CO 2 evolution. Experimental and theoretical results display a volcano–shaped relationship between the bond strength polarization and the catalytic activity in the TM–doped Fe 3 O 4 cathodes. The decisive role of the discharge product morphology in battery performance is also delineated. These findings enable the design of an efficient catalyst, Mn–Fe 3 O 4 , which can run stably up to 2000 h with a low voltage gap of ∼0.8 V. This work offers a rational viewpoint for tuning the catalytic activities of TM oxides in Li–CO 2 batteries.