Impact of Different Graphite Anodes on the Performance of Lithium-Ion Cells with Ni-Rich NMC Cathodes

The development of nickel-rich NMC cathodes (LiNixMnyCo1-x-yO2 with x > 0.5) is among the most promising routes to deliver lithium-ion cells with energy densities exceeding 250 Wh/kg. Achieving this ambitious target will require cycling cells to higher voltage (>4.35 V) and developing new strategies to mitigate undesirable side-reactions like electrolyte oxidation and cathode surface reconstruction. Since the cathode has long been considered the limiting factor in developing high-energy, high-voltage cells, the impact of the graphite anode has been largely ignored. In this contribution, we show that the choice of graphite significantly impacts both the rate capability and long-term cycling stability of cells with Ni-rich NMC cathodes. Six commercially-available natural and synthetic graphites were tested in full cells with NMC 811 cathodes (LiNi0.8Mn0.1Co0.1O2). The variations in cell performance were correlated with the chemical, morphological, and mechanical properties of the different graphite anodes. Surface chemistry was systematically investigated using X-ray photoelectron spectroscopy (XPS), while electron microscopy and mercury porosimetry were used to understand how electrode structure impacts the electrochemical performance. Peel tests were used to identify the importance of mechanical adhesion of different graphites to the current collector. In-depth electrochemical studies including impedance spectroscopy further shed light on the nature of capacity fading. Our study illustrates the important role of the graphite anode in developing high-energy, high-voltage Li-ion batteries. This work serves as a guide to select the appropriate graphite to optimize performance of Ni-rich NMC cells. Acknowledgement: 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).