Alternative Li-ion Cathodes based on Transition Metal Orthosilicates

Abstract

The aspiration of today’s society to become independent from crude oil and nuclear power led to a rapid growth of renewable energy production. Unfortunately, the majority of these energy sources are discontinuous and require energy storage. Alongside this, particularly in Norway, a trend towards electric transportation is observed. Even if details differ, both applications require advanced battery technologies to store energy and facilitate it with a high conversion rate when needed.

Li-ion batteries outperform other battery concepts, but the technology has to be advanced in order to meet the demands of specific applications. In particular, the common positive Li-ion electrode (LiCoO2) has safety and pricing issues to be overcome. Not to mention that it also limits the battery capacity. Both safety and moderate cost are of major importance for the large-scale batteries required in electric vehicles and in stationary energy storage.

This study concerns an alternative positive electrode material class, namely the lithium transition metal orthosilicates (Li2MSiO4). These materials consist of abundant elements Fe, Mn and Si and could hence considerably reduce the cost of Li-ion electrodes. In addition, these materials could theoretically double the capacity by a two-electron reaction. Unfortunately, the orthosilicates are poor conductors and require nanoarchitectures and conductive coatings to be applicable as an electrode. In this study, the impact of morphology, conductive coating and the incorporation of foreign elements into the structure are examined and discussed. Nanostructured orthosilicates were synthesised by wet chemical and flame assisted processes. A small particle size (< 50 nm) in combination with a conductive carbon coating and a high level of porosity was found to be crucial for the electrode performance. Furthermore, it was shown that replacing silicon with small amounts of vanadium had a huge, positive impact on the performance. Finally, a time-efficient flame method for high throughput synthesis of high quality nanosized orthosilicates was developed.