Structural Origin of Suppressed Voltage Decay in Single‐Crystalline Li‐Rich Layered Li[Li_0.2Ni_0.2Mn_0.6]O₂ Cathodes

Lithium‐ and manganese‐rich layered oxides (LMLOs, ≥ 250 mAh g⁻¹) with polycrystalline morphology always suffer from severe voltage decay upon cycling because of the anisotropic lattice strain and oxygen release induced chemo‐mechanical breakdown. Herein, a Co‐free single‐crystalline LMLO, that is, Li[Li_0.2Ni_0.2Mn_0.6]O₂ (LLNMO‐SC), is prepared via a Li⁺/Na⁺ ion‐exchange reaction. In situ synchrotron‐based X‐ray diffraction (sXRD) results demonstrate that relatively small changes in lattice parameters and reduced average micro‐strain are observed in LLNMO‐SC compared to its polycrystalline counterpart (LLNMO‐PC) during the charge–discharge process. Specifically, the as‐synthesized LLNMO‐SC exhibits a unit cell volume change as low as 1.1% during electrochemical cycling. Such low strain characteristics ensure a stable framework for Li‐ion insertion/extraction, which considerably enhances the structural stability of LLNMO during long‐term cycling. Due to these peculiar benefits, the average discharge voltage of LLNMO‐SC decreases by only ≈0.2 V after 100 cycles at 28 mA g^‐1 between 2.0 and 4.8 V, which is much lower than that of LLNMO‐PC (≈0.5 V). Such a single‐crystalline strategy offers a promising solution to constructing stable high‐energy lithium‐ion batteries (LIBs).