Tin-based Glasses: Structure and Electrochemical Properties

Doktorsavhandling, 2003

This thesis presents an investigation of microscopic structure and electrochemical properties, and their relationship, for various tin-based borate, phosphate and borophosphate glasses. Tin-based glasses have attracted a lot of scientific interest as anode materials in Li-ion batteries. The battery performance for a glass is promising, but the large capacity loss occurring during the first electrochemical cycle, is a negative feature. Furthermore, the mechanism for the lithium insertion and extraction process of the glasses is not well understood and it is a prerequisite for a rational optimisation of the properties of the materials. A cornerstone for the investigations is to understand the structure of the pristine glasses. Detailed structural characterisation of the complex glassy materials is, however, difficult because of the lack of long-range translational order. Various local molecular arrangements do exist and these have been probed using infrared and Raman spectroscopy. By combining this information with that obtained from neutron diffraction in conjunction with Reverse Monte Carlo modelling, a picture of the glass structures has been made on a length scale corresponding to molecular arrangements as chains and clusters. In glasses, tin is able to act both as a glass former and as a network modifier. Its function is dependent on the glass composition. The present structural studies show that, in a borate glass, tin acts similar as a glass former, whereas in a phosphate dominated glass, tin instead behaves more like a network modifier. In order to understand the mechanism of the lithiation process and the cause of the capacity loss, the structural changes of the glass materials that occur during electrochemical cycling have been probed using infrared, Raman and in situ Mössbauer spectroscopy. The results show that, during the first cycle, lithium reacts with the phosphate part of the network to form lithium orthophosphate. This is the irreversible process, which constitutes the major component of the capacity loss of the first cycle. However, in the absence of phosphate, the borate part of the network is attacked instead by the lithium but to a lesser extent. Thus, the structural role of tin influences the cycling performance, which is also dependent on the upper potential; tin and lithium form aggregates which lead to capacity fading on cycling above 0.8 V.