Fate of Tungsten-Coated NMC Cathodes in Li-Ion Cells

To achieve high energy density in medium-nickel-layered oxide (NMC) cathodes, the most straightforward approach is to increase the upper cutoff voltage. However, this reduces the cycle life due to enhanced electrolyte oxidation. Many coating elements have been introduced to improve the lifetime, and tungsten (W) is a common vendor choice. We find that surface tungsten compounds dissolve during electrochemical cycling, with the rate increasing at higher cutoff voltages. X-ray photoelectron spectroscopy (XPS) confirms W deposition in the form of metallic W and tungsten oxides on the graphite negative electrode, and X-ray fluorescence (XRF) quantified W content. Surprisingly, a significant amount of W dissolves under high-voltage operation, a problem that becomes more severe because vendors generally employ only trace amounts of coating, which leaves the surface increasingly exposed as the coatings dissolve. This work also investigates the impact of deposited W on the lithiated graphite negative electrode through simulated storage experiments.