A Kinetic Study of Silicate Formation in Mg-Doped SiOx for Lithium-ion Battery Anodes

Sammanfattning

As the world endeavours to transition to a sustainable society, efficient energy storage is a key factor to enable real-time reliable access to sustainable energy and reduce transportation related pollution. For several years, lithium-ion batteries with graphite anodes have been used to power all manner of electronic devices. However, as the electrification of the transportation sector progresses, the demand for high-performance batteries is increasing. Better cycle life, electrochemical performance and energy density are required for applications in electrical vehicles and the battery performance must thus be improved. SiOx has shown potential to outperform graphite as anode material in lithium-ion batteries due to its high energy density. Its commercialisation is, however, hindered by its relatively low conductivity and significant volume change during cycling, two issues which may be overcome through doping with elements such as magnesium. To aid the development of this anode material, the purpose of the thesis work was to investigate the kinetics and mechanisms of silicate formation in a SiOx anode material upon 4 wt% and 20 wt% Mg-doping at 850 ℃. High temperature experiments were conducted where samples of powdered SiOx and Mg were mixed with the compositions 0, 4 and 20 wt% Mg and heat treated in sealed graphite crucibles at 850 ℃, in an Ar-atmosphere in a vertical tube furnace for holding times ranging between 2 and 24 hours. The samples were then quenched, mounted, polished and analysed using sweeping electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Gradients in magnesium content were clearly visible in all doped samples, signalling that equilibrium was not obtained for any of the holding times at 850 ℃ and indicating slow reaction kinetics for the silicate formation. Six different types of grains were found in all doped samples located in different zones over the Mg-gradients, and studying the evolution of these, reaction mechanisms for the silicate formation were suggested. In addition to identifying the phases present in in the samples, the main conclusions of the study were that i) a holding time of 24 hours at 850 ℃ was not sufficient to reach equilibrium, ii) that MgSiO3 forms at lower Mg contents than Mg2SiO4, and iii) that Mg2SiO4 appears to form from MgSiO3 and Mg. It was noted that the method used was repeatable and that grain size was not significantly affected by doping whereas the grain shape was altered due to silicate formation. For future studies it is recommended to investigate longer holding times, higher reaction temperatures and to investigate other doping elements besides Mg as well. It is also suggested that Mg-powder of finer particle size may be used to promote fast kinetics and that it may be of interest to use X-ray diffraction analysis to analyse the phases in the samples.