Atomistic Insights of Irreversible Li+ Intercalation in MnO2 Electrode

Tunnel-structured MnO2 represents open-framed electrode materials for reversible energy storage. Its wide application is limited by its poor cycling stability, whose structural origin is unclear. We tracked the structure evolution of β-MnO2 upon Li+ ion insertion/extraction by combining advanced in situ diagnostic tools at both electrode level (synchrotron X-ray scattering) and single-particle level (transmission electron microscopy). The instability is found to originate from a partially reversible phase transition between β-MnO2 and orthorhombic LiMnO2 upon lithiation, causing cycling capacity decay. Moreover, the MnO2/LiMnO2 interface exhibits multiple arrow-headed disordered regions, which severely chop into the host and undermine its structural integrity. Our findings could account for the cycling instability of tunnel-structured materials, based on which future strategies should focus on tuning the charge transport kinetics toward performance enhancement.