Towards a Mechanistic Understanding of Transition-Metal Dissolution Phenomenon in Lithium-Ion Batteries

Dissolution of transition metals (TMs) from the layered NMC-type cathodes (LiNi x Mn y Co z O 2 ) occurs during cycling of lithium-ion full cells and is known to accelerate their capacity fade. This is because the dissolved metals migrate to the graphite anode and deposit there, where they accelerate irreversible active-lithium loss to reductive side reactions. The dissolution is known to get aggravated by cycling the cells to a greater upper cut-off voltage (UCV), where larger amounts of metals are typically found to be accumulated at the anode after long term cycling. At higher UCV oxygen loss from the cathode lattice also gets accelerated causing changes in the layered crystal structure at the surface level to a rock-salt and spinel mixture which has a very high impedance for Li-ion transport. Several other phenomena get accelerated as well, such as proton generation, and solvent decomposition etc. However, a complete mechanistic understanding of the how these phenomena impact the TM dissolution is lacking. For example, it is unclear whether the dissolution can occur in the charged state when the metals are in the oxidized state despite the presence of large number of acidic species, especially since most of the dissolved metal species typically detected in the electrolyte are in lower oxidation state. In this study, this question was targeted by aging the full graphite//NMC cells via long potentiostatic holds at UCV with/without any discharge steps, and recording the dissolved metals deposited at the graphite after aging. It is shown that a much greater amounts are deposited at the anode when discharge steps are included. Atomic structural changes from TM dissolution at the cathode surface corresponding to the two aging protocols in the charged/discharged state will also be presented. This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy (DOE) through Cathode-Electrolyte Interphase (CEI) Consortium. Part of the measurements was performed at the Center for Nanophase Materials Sciences (CNMS), which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences.