Stability of Ni-Rich NMC Cathode Materials in Water and Electrolyte Solutions during Aqueous Electrode Processing

Abstract

Despite recent advances in lithium-ion battery technology, further energy density improvements and cost reductions are necessary to achieve widespread adoption of electric vehicles. Nickel-rich formulations of lithium nickel manganese cobalt oxide (NMC) have been developed as promising high-energy, high-voltage cathode materials capable of significantly increasing the energy density of lithium-ion batteries. Aqueous processing of these cathodes offers significant environmental benefits and lower production expenses compared to traditional processing using N-methylpyrrolidone (NMP). However, Ni-rich NMC materials can react with water during electrode preparation, and with electrolyte during battery assembly and cycling, resulting in metal leaching that can cause structural changes and performance degradation. We investigated the effect of aqueous processing on the structure and electrochemical behavior of these cathode materials by exposing four NMC powders with varying nickel contents (NMC 333, NMC 532, NMC 622, NMC 811) to water and electrolyte for different time durations meant to simulate electrode processing and battery assembly conditions. The resulting metal dissolution was then measured using inductively coupled plasma mass spectrometry (ICP-MS). Since the pH of aqueous slurries can differ depending on composition, and pH variation may lead to additional reactions, the effect of pH was also explored. We found that lithium dissolution increases with increasing nickel content at all pH values, and slurries made with all four powders become basic during typical processing times. Dissolution of Ni, Mn, and Co in water was minimal for all formulations. In order to gain a better fundamental understanding of the structural changes taking place during these reactions, a subset of the NMC powders was characterized by SEM and X-ray photoelectron spectroscopy (XPS) before and after exposure to water and electrolyte. In addition, the effect of aqueous processing on battery performance was evaluated by comparing the rate performance of pouch cells made using cathodes processed with different water exposure durations. Correlating processing conditions with performance could lead to the design of new approaches to mitigate metal leaching from Ni-rich NMC materials and enable the use of these cathodes for high energy density lithium-ion batteries. Acknowledgment This research at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE) under contract DE-AC05-00OR22725, was sponsored by the Office of Energy Efficiency and Renewable Energy (EERE) Vehicle Technologies Office (VTO) (Deputy Director: David Howell) Applied Battery Research subprogram (Program Manager: Peter Faguy). Figure 1