Nanoscale-Resolved STXM Reveals Shell π-Type Superexchange for Enhancing High-Voltage Stability in Surface Al-Doped LMR Cathodes

Lithium- and manganese-rich layered oxides (LMR) are promising high-capacity cathodes for next-generation lithium-ion batteries, yet their practical use is hindered by severe surface degradation and irreversible oxygen release under high-voltage operation. These surface instabilities not only accelerate voltage decay but also limit the structural integrity and cycle life. Here, we employ synchrotron-based scanning transmission X-ray microscopy (STXM), complemented by X-ray absorption fine structure (XAFS), to directly visualize shell-core-specific (surface-bulk) electronic reconstruction in LMR cathodes and establish its correlation with local atomic structures. We show that surface Al incorporation generates a controlled density of oxygen vacancies in the shell, which weakens 3d transition metal (TM)-O covalency while activating Ni t_2g-O 2p-Mn t_2g π-type superexchange (SE) interactions. This orbital reconfiguration stabilizes Ni in a high-spin Ni⁴⁺ configuration even at 4.6 V, allowing Ni t_2g orbitals to buffer oxygen oxidation charges and thereby mitigate excessive oxygen anionic redox (OAR). As a result, irreversible O₂ evolution and lattice collapse are noticeably suppressed, while XAFS analysis confirms shortened Ni-O bonds and reduced coordination loss in Al-modified LMR. Importantly, the Mn valence and 3d configuration remain unchanged, highlighting the fact that the stabilization is dominantly Ni-O driven. These findings demonstrate how nanoscale orbital engineering at the particle shell can suppress the level of OAR driven surface degradation, offering a practical pathway to improve the cycling durability and high-voltage stability of advanced LMR cathodes.