A Segmented Cell Measuring Technique for Locally Resolved In-Operando Current Distribution and Impedance Measurements of Lithium-Sulfur Pouch Cells

Kurzfassung

With their unmatched specific energy, lithium-sulfur batteries (LSB) are an outstanding example for the potential of next-generation energy storage. Recently, an organosulfur active material namely sulfurized polyacrylonitrile (SPAN) serving as the cathode showed promising results overcoming LSBs well-known shortcomings, such as polysulfide shuttle, pushing the battery chemistry’s technological maturity towards pre-pilot scale level. However, when scaling up from premature laboratory cells to higher TRL pouch-format, local variations in the cell pose a particular challenge to capacity retention, cycle life and safety. Driven by multiple factors such as inhomogeneous coatings, wetting issues, local depletion, or geometric effects, the cells can experience mechanical stress, temperature gradients and locally enhanced overpotentials along the electrode surface, resulting in differences in current density and resistivity. While modeling and simulation try to provide insight into local conditions, experimental confirmation of simulative findings is difficult to assess and requires considerable effort.

In this work, a promising in-house developed segmented cell measurement technique is presented, which is utilized to experimentally access local behavior of single-layer pouch cells. A segmented current collector is employed, allowing each segment of the SPAN cathode to be contacted individually. The segmented approach enables in-operando recording of the current density distribution as well as local impedance discrepancies of the LSB pouch-cell, giving insight to otherwise hidden inequalities during operation and upon scaling up. Observed current variations of the individual segments during galvanostatic cycling experiments show deviations up to 10%, even for moderate C-rates. Moreover, SOC progression of the individual segments during cycling is monitored and linked to the full cell’s potential plateaus, exhibiting locally different charge/discharge states stemming from coating inhomogeneities or geometrical effects. The spatial differences are further investigated by local impedance measurements, which allow in-depth analysis on the origin of the found variations, such as ohmic losses, kinetic or transport limitations. The implications of the identified local effects to the overall cell performance are discussed in detail.