What is 'Battery Science'? (Interplay of Bulk and Surface)  Seminar

 

The next generation of high-end rechargeable batteries will still rely on lithium-ion host materials. Later, post-lithium-ion systems, such as Li/S and eventually Li/air, are claimed to enter the market. Independently of the technology, understanding the fundamental properties of battery materials and the interactions of these materials with their environment will be the key to further im¬provements in energy density, safety, and life time of batteries. Our approach to answer the related scientific questions starts with the development of various in situ methods for use mainly in the field of nonaqueous solid-state electrochemistry. Then, the physi¬cal and electro¬chemical proper¬ties of host materials and electrochemical interfaces are investigated in situ. Wherever possible, the combina¬tion of two in situ methods in a single electrochemical cell is a highly promising approach. In the talk, an overview of the most promising methods will be given.

(i) Differential Electrochemical Mass Spectrometry (DEMS) can detect qualitatively and quantitatively volatile reaction products that are evolved during cycling of an electrochemical cell. The evolution of CO₂ due to the decomposition of carbonate electrolyte solvents was investigated for anodes as well as for cathodes. For cathodes we demonstrated, e.g., that the extra charge observed in the first oxidation of the overlithiated NCM material is due to the release of oxygen from the bulk structure. Furthermore, using the DEMS technique we follow the reversibility of the oxygen electrode in lithium/air test cells.

(ii) Spectroscopic techniques are established for investigations of surface related electrochemical processes, with the comprehensive understanding of the SEI film formation as a center of attention. In situ Raman spectroscopy allows following the intercalation and deintercalation of lithium and other ions into/from graphite, and the lithiation /delithiation as well as other structural changes of layered materials, such as NCM. It is possible to measure on multiple spots on the surface in order to get statistics about the homogeneity of the electrochemical processes at the electrode. Special electrochemical cells have been developed also for in situ FTIR microscopy, and finally for the combined Raman / FTIR microscopic techniques to collect spectral information in situ from the same spot at an electrode under current flow.

(iii) In contrast to XRD, neutrons are sensitive to lithium. Therefore, the combination of results from synchrotron based in situ X-ray diffraction methods and in situ neutron diffraction is essential for understanding the reactions of battery materials. In situ XRD is used to monitor structural changes of battery materials with high resolution in a short time scale. Structural changes of alloys, graphite, and oxide materials (e.g., overlithiated transition metal oxides) were followed and the corresponding reaction mechanisms evaluated. For in situ neutron diffraction experiments a sophisticated electrochemical cell was constructed allowing quantitative analysis using the Rietveld method. Investigations of various materials, especially overlithiated NCM, were successfully performed and changes of the cell parameters as well as oxygen release were determined during cycling.