Mechanism of Intercalation Processes in Anode and Cathode Materials for Batteries

Li ion batteries represent the state of the art for applications in portable devices like notebook computers and cell phones, and are currently the best choice for applications in electromobility. However, these batteries still suffer from short lifetime, high cost, low energy density and thus high weight or limited driving range, and severe safety issues. A significant breakthrough in the area of batteries would also facilitate the widespread market introduction of electric vehicles.

The project “Mechanisms of intercalation processes in anode and cathode materials of batteries” is part of the research within the Sino-German Network on Electromobility. It takes place in collaboration with the Chair of Technical Chemistry at TU Berlin (Prof. Peter Strasser), with the group of Prof. QIU Xinping at Tsinghua University in Beijing and Prof HUANG Yunhui at HUST in Wuhan. It aims both at improving the fundamental understanding of intercalation and ion insertion processes into electrode materials of batteries, at developing improved electrode materials especially for the positive electrode, and at replacing Lithium-based systems by Mg-based systems.

Mg is much more abundant compared to Li, therefore cost and availability issues as for Li are not expected. However, there are other drawbacks, like the choice of an electrolyte suitable for reversible battery operation and preventing electrode passivation, a sluggish kinetics of the Mg redistribution especially inside the solid phase of positive electrode materials, and an overall lower cell voltage and thus reduced energy density. Some of these issues may be solved by the choice of suitable nanostructured electrode materials. As electrolyte, in literature so far metal-organic compounds dissolved in THF have been successfully applied. Other possible electrolytes are based on ionic liquids and linear ethers like diglyme. Within the project, in addition to Mg, recently also studies on Na ion based batteries are carried out.

Fundamentals Studies with Electrochemical Scanning Tunneling Microscopy

Electrochemical STM permits an in-depth study of local morphological changes at the surface of an electrode material. Within the project, well defined, single-crystalline analogues of typical battery electrode materials are currently under investigation. A large number of studies has already been carried out at single-crystalline carbon, the highly oriented pyrolithic graphite (HOPG) that is a model system for the graphitic carbon particles used in commercial batteries as the negative electrode material. In addition oxidic (cathode) materials are under study. Typical Li ion battery electrolytes, but also ionic liquid based electrolytes and Na electrolytes are studied. Such studies permit a closer understanding of the SEI formation at the electrode surface.

Synthesis and Testing of Battery Materials

A number of different nanoscale oxide materials are synthesized using chemical, sonochemical and electrochemical methods. These materials are then processed using standard methods into electrode layers and tested in three electrode Swagelok cells. Charge-discharge cycles, rate capability, slow scanrate cyclic voltammetry, impedance spectroscopy and PITT are among the electrochemical methods used. Some examples of typical results are given below.