Synthesis and Mechanism of High Structural Stability of Nickel-Rich Cathode Materials by Adjusting Li-Excess

Abstract

It has been a long-term challenge to improve the phase stability of Ni-rich LiNi<i>_x</i>Mn<i>_y</i>Co_{1-<i>x</i>-<i>y</i>}O₂ (<i>x</i> ≥ 0.6) transition metal (TM) oxides for large-scale applications. Herein, a new structure engineering strategy is utilized to optimize the structural arrangement of Li_1+<i>x</i>(Ni_0.88Mn_0.06Co_0.06)_1-<i>x</i>O₂ (NMC88) with a different Li-excess content. It was found that structure stability and particle sizes can be tuned with suitable Li-excess contents. NMC88 with an actual Li-excess of 2.7% (<i>x</i> = 0.027, Li/TM = 1.055) exhibits a high discharge capacity (209.1 mAh g^-1 at 3.0-4.3 V, 0.1 C) and maintains 91.7% after the 100th cycle at 1 C compared with the NMC88 sample free of Li-excess. It also performs a delayed voltage decay and a good rate capacity, delivering 145.8 mAh g^-1 at a high rate of 10 C. Multiscale characterization technologies including ex/in situ X-ray diffraction (XRD), focused ion beam (FIB) cutting-scanning electronic microscopy (SEM), and transmission electron microscopy (TEM) results show that a proper Li-excess (2.7%) content contributes to the formation of a broader Li slab, optimized cation mixing ratio, and even particle sizes. Therefore, NMC88 with a proper Li-excess is a good choice for next-generation cathode materials.