Real Time Analysis of Dissolution of Active Materials and Current Collectors of Lithium Ion Batteries

Abstract

In this talk we will address the problem of dissolution of transition metals of cathode active materials (CAMs) and the current collector aluminum from the long list of degradation processes of lithium ion batteries affecting their safety, cycling stability and lifetime.The problem of transition metal dissolution has been evidenced already in the early 1990’s and since then has been investigated by many groups. While the loss of the active material itself plays only a minor role, the deposition of released metal ions on the graphite anode with the associated increased impedance and loss of active lithium impairs mainly the specific capacity retention. Although essential for effective prevention, the underlying mechanism of this process is not fully understood. The vast majority of investigations is based on ex situ and post mortem analysis of cycled battery cells or on simple tests probing the effect of various extra-electrochemical experimental conditions on the stability of CAMs. These type of studies can, however, not give a thorough insight into the potential dependent dissolution mechanisms and cannot judge the effect of chemical modifications on the stability of cells rapidly. There exists only few in situ methods for such investigations, which require the use of huge research facilities (like synchrotron radiation).Aluminum is the choice of material in state-of-the-art batteries for cathode current collector. This metals can undergo severe dissolution processes as well, and therefore, it is essential to understand the underlying mechanisms in order to propose reliable solutions to mitigate this problem. The consequences of these processes are not just the passivation of active material areas due to the loss of electrical contact but higher self-discharge rate through the ion contaminations in the electrolytes and impedance growth due to the deposition of dissolved ions on the anode SEI altering its structure and functioning, similarly to the effect of transition metals. These consequences all lead to capacity fade and decrease of specific energy.We have developed an electroanalytical flow cell (EFC) coupled to an ICP-MS, which is suitable for the use in aggressive nonaqueous electrolytes. Furthermore, the EFC is operated in an argon filled glovebox, which allows otherwise unattainably strict control of experimental conditions, like the water content of electrolytes [1,2]. In this contribution we give insights in the features of the EFC-ICP-MS method. Furthermore, the intrinsic stability of a state-of-the-art layered transition metal oxide cathode material LiNi0.8Co0.1Mn0.1O2 and of aluminum is characterized by on-line monitored time- and potential-resolved dissolution profiles. Special attention is paid to the influence of varying test procedures such as upper and lower cut off potentials and scan / C-rates on the onset potential of the dissolution and curve progression. In addition, we relate the amount of dissolved transition metal ions measured with the EFC-ICP-MS setup to data reported in the literature and own ex-situ analysis results. In case of Al, we give an insight into the effect of LiPF6 on the stability of the metal based on the analysis of data from single potential cycles.Literature:[1] J. Ranninger et al., Electrochem. Comm., 2020, 114, 106702[2] J. Ranninger et al., J. Electrochem. Soc., 2020, 167, 121507