Segmented Current Collector for the Locally Resolved Measurement of Current Distribution, Temperature and Impedance in Li-Ion Pouch-Cells

摘要

As a broad market penetration of lithium-ion batteries becomes reality in a multitude of relevant applications, safety and reliability are major issues that have to be ensured. In recent years, especially driven by the demands and rapid growth in the field of electro-mobility, the trend in the development of Li-ion batteries goes towards the manufacturing of large-format cells to increase system energy density. In such large-format cells, thermal gradients and an inhomogeneous state-of-charge (SOC) and current distribution over the cell-stack become more significant. This can present a severe problem regarding cycle stability and safety, especially when high currents are applied. On the one hand, such gradients can lead to a more pronounced aging behavior as function of location in the electrode stack which, in turn, induces performance losses and compromises lifetime. On the other hand, it also constitutes a safety issue as local hot-spots can develop at positions with higher impedance or local current density, encompassing an increase in temperature and the risk of triggering thermal degradation. Accordingly, obtaining information on how material properties, cell engineering and charge/discharge parameters (e.g. C-rate, temperature) influence such gradients is critical to maintain inside a safe operating window, to optimize performance and to facilitate longer lifetime. To gain better insight in the above phenomena, the German Aerospace Center (DLR) is working on a comprehensive characterization approach combining modelling and experimental investigation, complemented by post-mortem analysis of aged cells. This contribution will present how this combination can help in understanding the inhomogeneities in the cell and how they are affected during operation. At the core of this work is the development of a segmented current collector which is coated by the active material and inserted into the pouch-cell as part of the electrode stack. This in-house development facilitates the real-time recording of local current density, temperature and impedance on each segment distributed over the electrode as function of operating conditions. The so-obtained knowledge is of considerable value for scientists, cell manufacturers and users, as it allows e.g. insights into locally resolved aging and the development of critical positions on the electrode surface. The experimental results further serve as validation for impedance-based cell modelling in which the cell is described as a set of hypothetical parallel cells in the vertical plane connected by a current conducting network in the horizontal plane. The single cell behavior is described by an equivalent circuit model derived from fitting relevant impedance spectra. Finally, postmortem analysis of small electrode segments obtained from aged pouch-cells and electrochemically characterized in coin-cells, reveals the actual result of these effects which are reflected in considerable deviations in SOC, SOH and thermal safety over the cell-stack.