Radially Oriented Single‐Crystal Primary Nanosheets Enable Ultrahigh Rate and Cycling Properties of LiNi0.8Co0.1Mn0.1O2 Cathode Material for Lithium‐Ion Batteries

Abstract

DOI: 10.1002/aenm.201803963 3.8 V (vs Li+/Li), making them a class of promising cathode material and attracting considerable attention. Nevertheless, the inferior cycling stability and poor rate capability of Ni-rich oxide cathode materials have to be overcome before they can compete in practical implementation.[2,4] These drawbacks are largely attributed to their spherical micrometer-sized secondary particles aggregated densely by many randomly oriented primary nanoparticles,[4–6] as shown in Figure 1a. On the one hand, concomitant with this structure, the surface of secondary particles is terminated with random crystal planes. As Li+ can only diffuse along the 2D {010} plane in the hexagonal-layer structure of NCM materials,[4,7] the randomly exposed crystal planes (not solely the active {010} plane) may substantially hinder the Li+ exchange at the electrode/electrolyte interface. Meanwhile, the randomly oriented primary nanoparticles induce a prolonged and mazy Li+ diffusion pathway inside the secondary particles, because Li+ ions have to migrate across the grain boundaries, especially between the grains with inconsistent crystal planes. On the other hand, the successive phase transition accompanied by repeated Li+ insertion/extraction would result in anisotropic variation of the lattice parameters, and such variation is severely aggravated with the increase of Ni content.[8] Accordingly, in the Ni-rich oxide cathode materials, the substantial anisotropic lattice expansion/contraction would result in drastic microstrains at the boundaries of randomly oriented primary particles due to the asynchronous volume Ni-rich Li[NixCoyMn1−x−y]O2 (x ≥ 0.8) layered oxides are the most promising cathode materials for lithium-ion batteries due to their high reversible capacity of over 200 mAh g−1. Unfortunately, the anisotropic properties associated with the α-NaFeO2 structured crystal grains result in poor rate capability and insufficient cycle life. To address these issues, a micrometersized Ni-rich LiNi0.8Co0.1Mn0.1O2 secondary cathode material consisting of radially aligned single-crystal primary particles is proposed and synthesized. Concomitant with this unique crystallographic texture, all the exposed surfaces are active {010} facets, and 3D Li+ ion diffusion channels penetrate straightforwardly from surface to center, remarkably improving the Li+ diffusion coefficient. Moreover, coordinated charge–discharge volume change upon cycling is achieved by the consistent crystal orientation, significantly alleviating the volume-change-induced intergrain stress. Accordingly, this material delivers superior reversible capacity (203.4 mAh g−1 at 3.0–4.3 V) and rate capability (152.7 mAh g−1 at a current density of 1000 mA g−1). Further, this structure demonstrates excellent cycling stability without any degradation after 300 cycles. The anisotropic morphology modulation provides a simple, efficient, and scalable way to boost the performance and applicability of Ni-rich layered oxide cathode materials.