Thermal stability studies of lithium-intercalated electrode materials with accelerating rate calorimetry (ARC)

Kurzfassung

Lithium-ion battery cells for portable electronic and automotive applications have to be carefully designed and engineered to be safe under different thermal and electrical abuse conditions. Some electrode materials will approach market introduction soon, while others still require some basic groundwork in materials research. In order to produce market readiness battery materials and cells, it is necessary to improve their safety characteristics. In general, it is extremely challenging to design large lithium-ion cells which pass the same safety test criteria that are applied for smaller lab-sized cells. There is often a big lack between research results on smaller lab-sized and large-sized battery cells or even stacks since the thermal behavior can differ fundamentally due to the various cell housings. Therefore, our goal is to provide a method to test the reactivity of samples that containing small amount of electrode material in electrolyte to make quantitative predictions about the safety of larger and more application-oriented lithium-ion battery cells subjected to thermal or electrical abuse. An accelerating rate calorimeter (ARC), which is an adiabatic calorimeter, is a very advantageous method for such studies. The sample size can be considerably lower than 1 g without losing the sensitivity of self-heating detection, if the mass of the sample container is reduced in the same ratio, too. In this case, safety of large-sized cells can be easily studied on the laboratory scale which is less dangerous for the involved persons and additionally safes costs and expensive research materials. In this work, an ARC is used to measure the thermal stability of lithiated electrode materials in standard electrolytes under adiabatic conditions. In this context, the lithium content is varied in a wide range in order to represent different states of charge in the battery. Measurements are carried out to determine the effects of the lithium content of the electrode as well as the initial heating temperature on thermal stability. In the first instance, the study concentrates on anode materials, in particular graphite and silicon-graphite composite electrodes. First results show that the initial self-heating rate depends strongly on the amount of intercalated lithium in the electrode material, whereby the reactivity increases with increasing lithium content.