Structural Origin of Suppressed Voltage Decay in Single‐Crystalline Li‐Rich Layered Li[Li$_{0.2}$Ni$_{0.2}$Mn$_{0.6}$]O$_{2}$ Cathodes

Abstract

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$_{0.2}$Ni$_{0.2}$Mn$_{0.6}$]O$_{2}$ (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)