Unraveling the Chemical/Structural Degradation Mechanisms of Graphite/SEI in the Presence of Dissolved Manganese Ions in Lithium-Ion Batteries

Abstract

Understanding the degradation mechanisms of a Li-ion battery is of considerable importance for achieving a longer battery lifetime. In a Li-ion battery, the degradation process involves several physical, mechanical, and chemical processes that are interdependent. The complexity of these degradation mechanisms makes it difficult to improve the performance degradation of the battery over time, especially at elevated temperatures. Among various other reasons, the dissolution of active materials at the cathode and the resultant presence of transition metal ions in the vicinity of the anode/SEI are two of the key phenomena responsible for the degradation. While the dissolution at the cathode is effectively suppressed by the surface coating and the doping of the active materials, the degradation that occurs at the anode/SEI still remains the main challenge for Li-ion batteries due to the presence of transition metal ions (especially manganese ions). Thus, it is essential to understand how dissolved manganese ions contribute to the degradation of the anode/SEI. Although it is obvious that the deposition of dissolved manganese ions occurs on the anode side, the nature of the precise mechanism of the interaction between the dissolved manganese ions and the anode/SEI is still being debated. In addition, the effect of dissolved manganese ions on the structural degradation of graphitic anode has not yet been systematically investigated, even if recent studies have reported the presence of manganese compounds near the graphite surface and within the graphite due to cracks or defects. In this work, we first show how the chemical degradation of the SEI is caused by deposited manganese compounds and where the manganese is significantly distributed at the graphite/SEI. We also demonstrate that the ion-exchange reaction mechanism between inorganic SEI species and manganese ions occurs and that the identified manganese compounds at the SEI are the result of the reaction. In addition, we reveal that the surface structural disordering of the graphitic anode can be caused by dissolved manganese ions that are diffused through cracks or defects in the graphite/SEI. Density function theory (DFT) calculations are used to explain the structural effect of co-intercalated manganese ions into the graphite anode. This work sheds further light on the mechanism of capacity fading as driven by deposited manganese compounds at the anode/SEI.