Influence of Binder Coverage on Interfacial Chemistry of Thin Film LiNi0.6Mn0.2Co0.2O2 Cathodes

Abstract

Given the pervasive implementation of lithium ion batteries in portable electronics, focus has increased on improving energy density for transportation applications. One route to accomplish this is through the development of high capacity cathodes such as Ni-rich LiNixMnyCo1-x-yO2 (x > 0.5). When charged beyond 4.3 V vs. Li/Li+, however, these materials suffer from irreversible structural changes and gas evolution, accelerating electrolyte decomposition at the cathode/electrolyte interface (CEI). These reactions are convoluted in composite electrodes due to binder and carbon additives as well as porosity creating a poorly defined cathode/electrolyte contact area. To reduce this complexity, thin films comprised solely of active material can be used as a model system to study the interface with controlled quantities and chemistries of coatings such as binders found in standard slurry cast electrodes. In this study, thin films of LiNi0.6Mn0.2Co0.2O2 were prepared by radio frequency magnetron sputtering to examine the effects of binder coverage morphology and chemistry on the CEI. Binder films were deposited atop the active material by spin coating, which provides a reliable means to cast uniform films down to a few nanometers. Spray coating was used to deposit a conglomerated binder morphology for comparison to the spun coating. These morphologies were repeated for three binders: polyvinylidene difluoride (PVDF), carboxymethyl cellulose (CMC), and lithium polyacrylate (LiPAA) for subsequent characterization. Scanning electron microscopy and atomic force microscopy examined the morphology and topology of the binder films, and samples were weighed with a microbalance to verify equivalent binder loadings across each sample. X-ray photoelectron spectroscopy results indicated that the amount of insulating species present at the interface varied with binder morphology and chemistry. The interfacial species evolved at different rates for each system during cycling, suggesting that binder selection has implications on the initial surface environment as well as over the lifetime of the cathode. These data will be presented alongside Raman, FTIR, and AC impedance spectroscopy to examine how the composition of the interface evolves with cycling and between systems. These results provide valuable insight into the influences of binders on the CEI of Ni-rich cathode materials. Figure 1