Recent Advances in Layered-Layered and Layered-Layered-Spinel Composite Cathode Materials for Lithium-Ion Batteries Produced with Solid State Synthesis: An Experimental and First-Principles Study

Abstract

High voltage cathode materials for rechargeable lithium-ion batteries that can be produced from cheap raw materials through a facile synthetic route are desired for emerging energy applications such as portable electronic devices and electric vehicles. Incorporating the monoclinic C2/m Li2MnO3 to form a “layered-layered” integrated structure cathode with the hexagonal R-3 m LiMO2 (M = Ni, Mn, and Co) has been first proposed by Thackeray and co-workers.1 This layered-layered cathode structure has been proven to be a viable option to achieve higher capacity of >200 mAh g-1 and to operate at a higher voltage above 4.5 V vs. Li/Li+.1 Typically, the layered-layered cathode materials need to be prepared using a co-precipitation process by adding excess Li source to transition metal precursors because this process allows to access a high degree of homogeneous structural integration of layered-layered character at the atomic scale leading to improved electrochemical performances. Recently, an alternate synthetic process route was developed, where Li2MnO3 and LiMO2 are prepared separately and mechanochemically-alloyed with a high-energy ball-milling method.2-5 This synthetic route achieve the same crystallographic features and very similar electrochemical performance with the powders produced with co-precipitation.2-5 Adding a third spinel component, for example, LiNi0.5Mn1.5O4, to this layered-layered composite cathode has been proposed to allow a fast Li+ diffusion and to operate even at a higher voltage (>4.7 V vs. Li/Li+).6-9 Here, we have precisely controlled the mole ratio of C2/m Li2MnO3, R-3 m LiNi0.5Mn0.3Ni0.2O2, and Fd-3 m LiNi0.5Mn1.5O4 to form a closely-connected “layered-layered-spinel” composite cathode materials using a robust and efficient solid state high-energy ball-milling process.10 We have carried out both ex situ and synchrotron-based in situ x-ray diffraction (XRD) analysis and observe that there are less structural distortions during charge/discharge cycling.10 Furthermore, we carry out density functional theory (DFT) calculations with van der Waals (vdW) correction to explain reduced phase transformation and enhanced stabilization in the integrated cathode system that lead to a large and stable capacity of ~200 mAh g-1 operating at high voltages up to 4.9 V vs. Li/Li+.10