A Mechanistic Understanding to Aging at High Voltage and Elevated Temperature in Lithium-Ion Batteries

Join us for a seminar by Lennart Reuter, a PhD candidate from Technical University of Munich, on "A Mechanistic Understanding to Aging at High Voltage and Elevated Temperature in Lithium-Ion Batteries".

Abstract: 

Lithium-ion batteries using Ni-rich layered transition metal oxides (NCMs, NCAs) are widely used CAMs for automotive cells. However, with increasing Ni-content, their stability is compromised at elevated temperatures and high SOC, due to chemical electrolyte oxidation by lattice oxygen release of the CAM and/or by electrochemical electrolyte oxidation at high potentials. Amongst the products of these reactions are protic species (H₂O, HF, and H⁺), which can lead to LiPF₆ salt decomposition in the electrolyte, active lithium loss via cross-over reactions at the anode, and transition metal dissolution. Furthermore, lattice oxygen release leads to the formation of highly resistive oxygen-depleted surface layers on the CAM particles. For these reasons, significant aging of lithium-ion batteries with Ni-rich NCMs/NCAs is not only observed upon extended charge/discharge cycling but also when storing the batteries at high SOC and elevated temperatures.

First, we will discuss the possible reaction mechanisms that lead to these degradation phenomena, making use of conventional battery testing, chemical analysis of the electrolyte as well as more sophisticated techniques such on-line electrochemical mass spectrometry (OEMS), and operando hard X ray absorption spectroscopy (XAS).

Second, we will also present a simplified method that can detect and quantify released O₂ and TM^II+ ions. It is based on a setup consisting of a 3-electrode setup consisting of an NCA working electrode (WE), a lithium iron phosphate (LFP) counter electrode (CE), and a Vulcan carbon sense electrode (SE). When polarizing the SE to a constant potential of 2.2 V vs. Li⁺/Li, evolved O₂ and TM^II+ ions upon charging the NCA are reduced at that potential allowing for the detection of a reductive current. By the precise knowledge of the number of electrons involved in the conversion, we are able to convert the reductive current into a potential-dependent amount of released O₂ and TM^II+ ions.