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Departments

Techniques

Our research group focuses on developing and understanding a variety of electrochemical systems with the overall aim to design a well-performing system for energy storage. Electrochemical systems of interest are investigated with a variety of instrumental methods. Firstly, their suitability for electrochemical energy storage is probed with different types of electrochemical methods such as galvanostatic cycling. This is then complemented by several additional methods which provide with information on the processes occurring in the investigated system.

The commonly employed methods comprise electrochemical impedance spectroscopy (EIS), photoelectron spectroscopy (PES), focused ion beam microscopy (FIB), transmission electron microscopy (TEM), scanning electron microscopy (SEM), nuclear magnetic resonance (NMR) and so on. Further insight into the systems of interested is obtained through computational methods (e.g. DFT). A few of the frequently used techniques are briefly described in the text below.

Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) is an electrochemical technique where a small amplitude alternating current or voltage is applied to the system. Measurement is conducted at multiple different frequencies, where each frequency results in a single point in the impedance spectrum. The technique is unique in its ability of separation of processes according to their time constants. This allows for separation between contributions due to migration, reaction and diffusion. Spectra can be fitted with equivalent circuits composed of resistors and capacitors. Our laboratory has developed physics-based transmission line models capable of simulating impedance spectra and allowing for determination of important parameters (conductivity, diffusional coefficient, transport number) and prediction of spectra change for various degradation scenarios (electrolyte dry out, passive film growth, etc.).

Photoelectron spectroscopy

Photoelectron spectroscopy (PES) is a spectroscopic technique based on photoelectric effect. Samples are irradiated with X- or UV-rays and the interaction between photons and matter results in photoelectron emission. The emitted photoelectrons are collected at the detector which measures their kinetic energy. Kinetic energy of photoelectrons is then used for calculating their binding energy. In other words, PES allows for determining the electronic structure of the sample.

Knowing the sample electronic structure allows for extracting a variety of information. PES is frequently employed for qualitative and quantitative analysis. A particular strength of the method is the ability to infer not only on the elemental composition but also on the chemical environment (i.e. oxidation states) of the sample species. PES is a surface sensitive technique with a probing depth of a few nanometers. It is therefore employed to investigate the composition and processes occurring at interfaces or within surface layers.

 

FIB-SEM

Focused Ion Beam – Scanning Electron Microscopy (FIB-SEM) is used for site specific sub-surface analysis of materials. FIB-SEM allows for deeper understanding of morphological and chemical information along ion beam polished cross-sections and direct observation of freshly exposed surfaces as well as interfaces. It can also be used for nano-patterning, TEM sample preparation, 3D tomography and deposition of thin conductive or dielectric films via ion-beam induced deposition.

For FIB-SEM analysis of battery materials we use dedicated VAC transfer holder which enables sample preparation within argon filled glow box and direct transfer into FIB-SEM chamber. Due to e-beam sensitive nature of battery materials we developed special techniques for cross-section and TEM lamella preparation suitable for soft materials. Freshly exposed cross-sections are imaged at low kV SEM with pre-monochromated electron beam to enable high resolution imaging and detection of nano-compositional differences. For chemical analysis we use state-of-the-art EDX techniques capable of quantitative analysis as well as quantitative elemental mapping of sub-nanometer features.

 

Uv-Vis

UV/Vis spectrophotometry is demonstrated as a powerful analytical method for the operando study of polysulfides. Through the interactions that occur between different chain‐length polysulfide molecules and the UV/Vis radiation, quantitative and qualitative determination of the polysulfides formed during Li–S battery operation can be achieved. The cell design with the quartz window allows us to probe the color changes into the separator.

Synchrotron experiments (XANES, EXAFS)

Developed in-situ electrochemical cell with beryllium window allows structural investigation of electrode materials while applying an electrochemical experiment. This setup is used for different synchrotron experiments (synchrotron X-ray absorption (XANES, EXAFS)). With those techniques we study the structural changes of the materials during cycling.

IR

We have developed a new operando technique based on infrared spectroscopy to probe the electrochemical changes in the organic cathodes. The cell was designed with a silicon wafer window to allow simultaneous IR measurements. With this operando technique we can monitor the electrochemical mechanism of metal-organic batteries and the degradation processes during prologue cycling.

 

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