Structural Origin of Suppressed Voltage Decay in Single-Crystalline Li-Rich Layered Li[Li0.2Ni0.2Mn0.6]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[Li0.2Ni0.2Mn0.6]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 approximate to 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 (approximate to 0.5 V). Such a single-crystalline strategy offers a promising solution to constructing stable high-energy lithium-ion batteries (LIBs).