Abstract
Interface resistances between the different components of battery cells limit their fast charge and discharge capability which is required for different applications such as electromobility. To decrease interface resistances, it is necessary to understand which individual interface they arise at and how they can be controlled. Electrochemical impedance spectroscopy is a well-established technique for the distinction of different contributions to the internal cell resistance and allows the characterization of interface resistances. Especially the use of suitable cell setups allows one to attribute the measured resistances to specific interfaces. In this contribution, we investigate the impedance of dry polymer full cells containing a lithium iron phosphate/ poly(ethylene oxide)-lithium bis(trifluoromethanesulfonyl)imide composite cathode, a solid polymer electrolyte separator and a lithium-metal anode. Based on the results on different cell setups, we are able to reliably determine the planar resistances between the components as well as the charge transfer resistance inside the composite cathode. For unoptimized systems, we find high planar resistances, which can be significantly reduced by coating and processing strategies. For the charge transfer resistance, we find a dependence on the SOC as well as on the charging direction. Possible mechanisms for the evolution of interface resistances are discussed also based on chemical analysis performed by photoelectron spectroscopy (XPS).
Highlights
The resistances of different interfaces in cells with LFP/PEO composite cathodes were characterized by electrochemical impedance spectroscopy
Information about the several resistances was gained by the implementation of different cell designs
Significant reductions of the interface resistances to the aluminum current collector, as well as to the polymer electrolyte separator were achieved by a carbon coated current collector and a better contact was achieved by a calendering step
Summary
To cite this article: Verena Wurster et al 2019 J. Cells with a reference electrode (Figure 4e) enable the separation of the cathode and the anode spectra in a working full cell These full cells can be charged and discharged, so the cathode spectra of different SOC are measured in situ. This work investigates the resistance contributions of poly(ethylene oxide) (PEO)-based solid electrolyte full cells with a LixFePO4 (LFP) composite cathode and a planar Li-metal anode using EIS analysis on full and symmetric cells. Hanai et al. characterized partly charged cathodes by building up cathode symmetric cells and charging them by a stainless steel mesh This mesh is placed between two polymer electrolyte layers, comparable to the reference electrode in our study. This cell setup enabled a charge of the cathodes until Li0.6FePO4 (SOC 40) by Li deposition on the mesh at 50◦C.
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