Highly Reversible Solid‐State Lithium‐Oxygen Batteries by Size‐Matching Between Fe‐Fe Cluster and Li2‐xO2

Abstract

AbstractSolid‐state lithium‐oxygen batteries (SSLOBs) offer high energy density with enhanced safety. However, the challenge of energy loss induced by the high polarization of the solid‐state air electrode has become a major bottleneck for its further development. Here, a homonuclear Fe‐Fe catalyst (Fe2‐N‐C) matched to reactants (Li2‐xO2, 0 ≤ x ≤ 2) size to alleviate the redox polarization in solid‐state electrodes is designed. Atomically resolved transmission electron microscopy reveals that the spatial size of Fe‐Fe clusters (1.5–2 Å) is commensurate with Li2‐xO2 (1.2–2.2 Å), which can overcome the challenges associated with catalyst/reactant size mismatch. In‐depth theoretical analyses show that orbital coupling and spin state jumping between Fe‐Fe sites allows the Fe centers to exhibit d‐orbital delocalization and high electronic spin states. These features enhance the activation of paramagnetic oxygen species, optimize the binding strength to LiO2 and reduce the charge/discharge voltage gap. Meanwhile, the size‐matching effect induces the ductile growth of the discharge product into a highly reversible film‐like morphology. The Fe2‐N‐C cathode with low overpotential exhibits significantly improved round‐trip efficiency (84.9%). The concept of this work opens up new opportunities for designing high‐performance SSLOBs.