Performance and Failure Mechanisms of Si Based Li-Ion Full Cells
Authors: Wagner Nils P. , Asheim Karina , Svensson Ann Mari , Vullum-Bruer Fride 
Published: February 2020

Lithium based chemistry is on the forefront of battery technology and many efforts are done to optimise Li-ion batteries in terms of lifetime and energy density. Many alternative Li chemistries arose during the last decade, which could potentially replace classical rocking chair batteries. Today, most of these technologies are still in their infancy and it is difficult to foresee if the corresponding technology can be commercialised. Current Li-ion batteries rely on intercalation host materials and the capacity is limited by the positive host materials delivering a specific capacity of around 120-200 mAh/g. Graphite, the state of the art negative electrode in commercial cells, delivers a moderate capacity of 320-350 mAh/g. A two to three fold increase in specific capacity of the negative electrode would have a profound effect on the specific capacity of a full Li-ion cell.
Si is a possible contender to substitute graphite in the next generation of Li-ion batteries. It undergoes an alloying reaction with Li at low potentials resulting in a high theoretical capacity of about 3600 mAh/g or 8500 mAh/cm3. This high level of lithiation causes enormous volume changes. It comes with the price of electrode detachment or pulverisation and an instable SEI layer, which cannot keep intimate contact with the Si surface upon these volume changes. Hence, Si negative electrodes suffer from severe inefficiencies and charge fade.1,2

Various morphologies have been synthesised with the aim to better accommodate the volume expansions. Furthermore, several binding agents, electrolyte additives and salts have been studied to increase the cyclability of Si based electrodes.3,4 Most of the synthesised Si structures are very costly and would result in a low tap density and hence a low volumetric capacity. In addition, the vast majority of studies have been performed on half-cells, where a Li counter electrode acts as a sheer endless Li reservoir. Unfortunately, this Li reservoir can mask several issues of the Si electrode and it has been shown that the performance and failure mechanisms are different in full cells.5,6

This study concerns the performance and failure mechanism of full cells with layered oxide positive electrodes and µ-Si negative electrodes. The Si electrodes consisted of commercial available battery grade silicon from Elkem (Silgrain ®, e-Si 400), graphite and carbon black and an aqueous Na-CMC binder. Layered oxides such as LCO and NCA were synthesised by a citric acid aided wet chemical route. Performance and failure mechanisms of the cells were analysed by a toolbox of electrochemical methods and post mortem analysis using electron microscopy coupled with energy dispersive X-ray spectroscopy and electron energy loss spectroscopy.
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