Effects of Extended Aqueous Processing on Structure, Chemistry, and Performance of Polycrystalline LiNi_xMn_yCo_zO₂ Cathode Powders

Abstract

The prospect of aqueous processing of Li-Ni x Mn y Co z O 2 (NMC) cathodes has significant appeal to battery manufacturers for the reduction in materials cost, toxicological risk, and environmental impact compared to conventional N-methyl-2pyrrolidone (NMP)-based processing. However, the effects of aqueous processing of NMC powders at industrial timescales are not well studied, with prior studies mostly focusing on relatively brief water washing processes. In this work, we investigate the bulk and surface impacts of extended aqueous processing of polycrystalline NMC powders with different compositions. We demonstrate that at timescales of several hours, polycrystalline NMC is susceptible to intergranular fracture, with the severity of fracture scaling with the NMC nickel content. While bulk crystallinity and composition are unchanged, surface sensitive techniques such as X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) indicate that the exposure of water leads to a level of delithiation, nickel reduction, and reconstruction from the layered to rock-salt structure at the surface of individual grains. Dynamic single NMC microparticle compression testing suggests that the resulting mechanical stresses weaken the integrity of the polycrystalline particle and increases susceptibility of intergranular fracture. The initially degraded surfaces along with the increased surface area lead to faster capacity fade and impedance growth during electrochemical cycling. From this work, it is demonstrated that NMC powders require surface or grain boundary modifications to make industrial-scale aqueous cathode processing viable, especially for next-generation nickel-rich NMC chemistries.

From Materials to Cell: State-of-the-Art and Prospective Technologies for Lithium-Ion Battery Electrode Processing

“…While the advantages of water-based cathode processing are apparent, there are six primary technical barriers restricting it from commercialization. These concerns are as follows: Replacing PVDF with water-soluble binders; Removing high residual moisture in electrodes processed in water due to hydrophilic binders; Agglomeration of active material and conductive-additive particles in water; Inferior wetting on the aluminum current collector due to the high surface tension of water; Corrosion on the aluminum current collector due to the basicity of the aqueous slurries; and Leaching of lithium and transition metal from cathode active materials when exposed to water, which may lead to surface reconstruction. ,, …”

“…Leaching of lithium and transition metal from cathode active materials when exposed to water, which may lead to surface reconstruction. ,, …”

Effect of precursor particle size and morphology on lithiation of Ni0.6Mn0.2Co0.2(OH)2

“…Water having detrimental effect on the cathode surface structure has been shown to be the cause for decreased cycle retention after washing. [14][15][16] It is logical to assume that materials resistance to the effects caused by washing is related to the materials surface area and how deep the water can penetrate inside the particles. According to XRD results (Fig.…”

“…Residual surface lithium may also lower first cycle capacity [13], so to compare the material properties, a complete removal of excess surface lithium was deemed necessary. Exposure of cathode material to water will, however, lead to surface layer structural changes which form a resistance layer [14][15][16]. Because of this the cycling performance of water exposed sample will be worse than unwashed sample.…”

“…Because of this the cycling performance of water exposed sample will be worse than unwashed sample. Washing with ethanol does not cause similar structural changes [14]. However, because water is occasionally used in industry for washing, we wanted to compare if there is any difference in how the materials react to water washing.…”