Characterisation of Molten Lithium Carbonate Corrosion on SiC Heating Elements Using Raman Spectroscopy

Abstract

The synthesis of lithium battery materials involves the use of muffle furnaces operating with SiC type heating elements. Usually, the industrial SiC heating element is protected by passive oxidation forming a protective silica film on the surface of the element. However, this strategy is not suitable for heating elements that operate in a Li-rich environment. A preliminary study has recently been completed to develop an understanding on the degradation of silicon carbide heating elements under the exposure to lithia. After basic characterisation of the SiC rod and its oxidation in air, its reaction products in the presence of Li was studied. The SiC rod was reacted with a likely Li source, Li2CO3, in three different % Li concentration environments through vapour-phase, wetting and full-immersion studies, particularly at the temperature just above the Li2CO3 melting point in delivering accelerated ageing. The characterisation was achieved via an integrative data analysis through the coordination of Raman, XRD, and Energy dispersive X-ray analysis (EDX) techniques. We found that molten Li2CO3 reacts with the silica surface layer of the element forming three main lithium silicates (LixSiyOx/2+2y). The degradation of surface silica into non-adherent lithium silicate leads to a speeding-up of the SiC oxidation process. Both processes eventually lead to a complete structural failure of the SiC rod. We performed a long-term vapour phase lithium attack experiment characterising the SiC after regular time intervals solely by Raman spectroscopy. Initially, a library of Raman spectra for the commonly encountered compounds in the Si-Li-O system was obtained from specifically synthesised stoichiometric compounds. In doing so, the reaction products at different reaction time intervals can be clearly identified, demonstrating the utility of Raman characterisation in corrosion studies.