Molecular Surface Modification of NCM622 Cathode Material Using Organophosphates for Improved Li-Ion Battery Full-Cells.

Abstract

Surface coating is a viable strategy for improving the cyclability of Li1+ x(Ni1- y- zCo yMn z)1- xO2 (NCM) cathode active materials for lithium-ion battery cells. However, both gaining synthetic control over thickness and accurate characterization of the surface shell, which is typically only a few nm thick, are considerably challenging. Here, we report on a new molecular surface modification route for NCM622 (60% Ni) using organophosphates, specifically tris(4-nitrophenyl) phosphate (TNPP) and tris(trimethylsilyl) phosphate. The functionalized NCM622 was thoroughly characterized by state-of-the-art surface and bulk techniques, such as attenuated total reflection infrared spectroscopy, X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry (ToF-SIMS), to name a few. The comprehensive ToF-SIMS-based study comprised surface imaging, depth profiling, and three-dimensional visualization. In particular, tomography is a powerful tool to analyze the nature and morphology of thin coatings and is applied, to our knowledge, for the first time, to a practical cathode active material. It provides valuable information about relatively large areas (over several secondary particles) at high lateral and mass resolution. The electrochemical performance of the different NCM622 materials was evaluated in long-term cycling experiments of full-cells with a graphite anode. The effect of surface modification on the transition-metal leaching was studied ex situ via inductively coupled plasma optical emission spectroscopy. TNPP@NCM622 showed reduced transition-metal dissolution and much improved cycling performance. Taken together, with this study, we contribute to optimization of an industrially relevant cathode active material for application in high-energy-density lithium-ion batteries.