A Versatile Polyol Synthesis for Layered, Spinel, and Olivine-Type Cathode Material

Abstract

One of the most pressing challenges modern society faces is how to provide an energy source for a variety of applications, ranging from small portable devices to electric vehicles (EVs) and a large grid-scale system to store energy from intermittent solar or wind-driven devices. Recently, enough technological advancement has been made that Li-ion batteries (LIBs) are considered as the most promising solution to solve this problem. Over the years, diverse synthetic methods have been developed to synthesize battery cathode materials, such as: solid-state, co-precipitation, sol-gel, hydrothermal, and molten salt syntheses. Each of the synthesis process used to make the cathode oxide material proved to be useful, but simultaneously accompanies some major drawbacks. In the traditional solid-state method, the mixing of multiple metal sources is done by grinding or ball milling. This results in a microcrystalline product with long Li diffusion pathways as well as inhomogeneous morphology and metal distribution. Co-precipitation compensates for some of these shortcomings, but it also requires a careful control of pH in the carbonate method or an inert atmosphere to minimize the undesired impurities in the hydroxide method. Previous studies show that the hydrothermal method is an effective way to synthesize cathode material with high crystallinity, but it requires a complex experimental set up which utilizes a stainless steel autoclave to withstand high vapor pressure during its lengthy reaction time. Furthermore, all the methods require a long post annealing process, typically lasting more than 12 hours at high temperature (>800ºC). Herein, we developed a novel polyol synthetic process to prepare the most promising cathode materials in layered, spinel, and olivine structure. Developed by Fievet group in 1985, polyol-mediated synthesis has been widely used for the past decades, but its scope has been mostly limited to the synthesis of metals and metal oxides. In this work, our group has extended this synthesis method to develop more complex metal oxide used for battery cathode materials. All three of the synthesized materials are monodispersed nanoparticles with dispersive morphology and competitive electrochemical performance. Using a combination of powder x-ray diffraction (XRD), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and energy dispersive spectroscopy (EDS), we also confirmed its high crystallinity and uniform elemental distribution for all three polyol-synthesized cathode materials. Finally, HAADF-STEM image and electron energy loss spectroscopy (EELS) for cycled NMC material was analyzed to confirm its structural stability even after charge-discharge process. We anticipate that these findings lead to the plethora of new cathode materials that can be explored using this synthetic method. Figure 1