High-Voltage Cathode Materials for Li-Ion Batteries; LiNi0.5Mn1.5O4 Stabilized By Surface Coating
Authors: Oestli Elise Ramleth , Wagner Nils P. , Svensson Ann Mari , Vullum-Bruer Fride 
Published: February 2020

The implementation of cathode materials with a high operating potential vs. Li/Li+ has the potential of increasing the energy density in state-of-the-art Li-ion batteries [1]. LiNi0.5Mn1.5O4 (LNMO) is a promising candidate due to its high operating voltage of 4.7 V vs Li/Li+, high safety, and economical and environmental advantages compared to other materials [2]. However, Ni and Mn dissolution and migration from the cathode to the anode during cycling due to the lack of a stable cathode/electrolyte interface reduces the lifetime and durability of the battery, and thus limits the commercialization of this material [3].

Surface coating of the cathode material can be an effective way to stabilize the cathode/electrolyte interface, and consequently increase the cyclability of the battery by reducing the transition metal ion dissolution [4]. Several authors have obtained quite promising results from thin surface coating of LNMO with Al2O3 and TiO2 by using coating techniques such as atomic layer deposition (ALD) [5,6]. However, most of the studies have only done electrochemical characterization in half-cell configuration, losing a lot of information of the possible gain in cycling stability. One of the main issues with transition metal ion dissolution is the destabilization of the SEI-layer on the anode, in particular for graphite [7]. Thus, full-cell measurements followed by post mortem investigation of the electrodes, e.g. by the use of X-ray photoelectron spectroscopy (XPS), is essential to properly evaluate the effect of the surface coating. In addition, alternative coating techniques may improve the cost and ease of the coating process. Spray drying is a particularly promising coating technique as this is an already commercialized process and can, with small modifications, be used for production of large amounts of coated cathode material [8].

In this work, high quality LNMO has been coated with thin Al2O3 and TiO2 layers. The effect of powder morphology is systematically investigated by using ALD and spray drying as coating techniques, and comparing the morphology and electrochemical properties. The coated and uncoated LNMO have been tested in LNMO|graphite full-cells, and the stabilizing effect of the surface coating is evaluated by post mortem XPS analysis of the graphite anodes.

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