The interdisciplinary group for methodology at Helmholtz Institute Münster explores concepts and analytical approaches for the comprehensive characterization and further development of materials in the field of electrolyte research. The focus is on polymer and liquid electrolytes as well as hybrid systems resulting from combination with ceramic components, but also electrodes. The tools of analysis include common and more recent electrochemical and spectroscopic method. In cooperation with partners from industry and academia computer-aided techniques are used.
The researchers analyse the charge transport in electrochemical components, including interfaces and interphases, such as the diffusion of complexed ions in liquid or polymer electrolytes or mechanisms of ion conduction within (solid) electrolytes and electrodes. Further electrochemical methods like field gradient NMR as well as impedance spectroscopy are applied. Molecular dynamics simulations can provide impulses for the evaluation of experimental data or check possible strategies of targeted material optimization, particularly with respect to suitable structural motifs and the presence of functional groups that facilitate optimal charge transport in electrolyte systems.
Analysis of Capacity Loss
Due to element selectivity and in principle quantitative measurements, NMR spectroscopy can be exploited for the characterization and operando determination of lithium metal deposits occurring in electrochemical cells. For example, a recently developed NMR-based protocol allows for a detection of contributions of irreversible capacity losses of cells due to possible formation of electrochemically inactive lithium deposits or interfacial layers on (particulate) active materials. In addition, NMR measurements with pouch-type thin-film cells provide information on the homogeneity and reversibility of the lithium deposition depending on the treatment of the electrode surfaces, including the introduction of possible functional/protective layers, variations in electrolyte formulations, and possible adjustments of the electrochemical operating parameters of the cells.
Magnetic Resonance Imaging
The operando acquisition of spatially resolved electrochemical reactions is in principle made possible by magnetic resonance imaging (MRI) techniques. Current work of the methodology group is particularly focused on the development of suitable MRI model cells and experimental adaptation of respective MRI methods for the suppression of possible interference signals and optimization of one-dimensional ion concentration profiles. Future steps of HI MS research include the implementation of two-dimensional chemical shift imaging (CSI) and three-dimensional imaging of metallic deposits in electrochemical cells as well as the targeted customization of electrochemical methods to investigate irreversible processes in electrolytes.
Atomic Force Microscopy-based Measurement Methods
With the help of various measurement methods based on atomic force microscopy (AFM), it is possible to measure the sample topography as well as different chemical and physical properties with high spatial resolution. One focus is on Kelvin Probe Force Microscopy (KPFM), which can be used to analyse the local surface potential of solids. The method allows, for example, detailed measurements of the local potential distribution at boundary layers, for example at phase boundaries in hybrid electrolytes.
In parallel, other properties such as local elasticity, roughness, chemical diffusion coefficients or electron conductivity of the sample surface (Current Sensing AFM, C-AFM) can be analysed. An encapsulated system allows measurements in different gas atmospheres as well as in liquid electrolytes and in a variable temperature range of 0 - 150 °C.
Journal of The Electrochemical Society 2021, 168, 010531, DOI: 10.1149/1945-7111/abda59
Beilstein Journal of Nanotechnology 2021, 12, 1380–1391, DOI: 10.3762/bjnano.12.102
Cell Reports Physical Science 2020, 1, 100139, DOI: 10.1016/j.xcrp.2020.100139
Physical Chemistry Chemical Physics 2019, 21, 26084-26094, DOI: 10.1039/C9CP05334D
Journal of Power Sources 2018, 378, 522-526, DOI: 10.1016/j.jpowsour.2017.12.069