Influence of Morphology and Compositional Mixing on the Electrochemical Performance of Li-Rich Layered Oxides Derived from Nanoplatelet-Shaped Transition Metal Oxide–Hydroxide Precursors

Abstract

The influence of morphology and compositional mixing on the electrochemical performances of Li-rich layered oxides (LLOs), specifically to address the high rate capability, is investigated. LLOs of composition xLi2MnO3·(1 – x)LiMn0.25Ni0.38Co0.37O2 (LMNC, x = 0, 0.2, 0.4, and 0.6), lying in the plane NMC(640)–LCO–LMO, are synthesized in nanoplatelet morphology, and the results are compared to the same compounds prepared by a conventional solid-state reaction (SSR). Hexagonal-shaped thin (∼50 nm) flakes of transition metal oxide–hydroxide [TMO(OH)], prepared by the hydrothermal process, are reacted with Li carbonate to derive nanoplatelet morphology of LMNC by topotactic conversion. Structural and compositional evolutions of LLOs are analyzed with Rietveld refinement. The composite nature of LMNC comprising of monoclinic Li2MnO3 and rhombohedral LiMO2 phases is evidenced. High-resolution transmission electron microscopy studies show the existence of a monoclinic Li2MnO3 phase embedded within the rhombohedral layered oxide phase. A uniform compositional distribution of all elements is discerned from EDS mapping, strongly suggesting that metal cations in both TMO/OH and LMNC are highly intermixed. Electrochemical properties become better with the larger fraction of the Li2MnO3 phase in LiMO2. Among four compositions examined, LMNC (x = 0.6) shows the best electrochemical performance, with a capacity of ∼240 mAh g–1 (∼173 mAh g–1) at 0.1 C (1 C) current rate. Cycling stability studies, carried out at 1 C rate for 100 cycles, show a high capacity retention of 86%. Capacity at 3 C (5 C) is ∼140 mAh g–1 (∼80 mAh g–1) in LMNC (x = 0.6). LMNC (x = 0 and 0.6) prepared by SSR show inferior properties, suggesting that morphology and thorough intermixing of monoclinic Li2MnO3 and rhombohedral LiMO2 phases are shown to play a significant role. Although enhanced performance is generally attributed to the extra capacity contribution from the Li2MnO3 phase, this study unequivocally brings out the influence of morphology on the electrochemical properties.