Laser-pyrolysed ZnFe2O4 anode for lithium-ion batteries : understanding of the lithium storage mechanisms

The lithium-ion technology is well implemented for portable electronic devices but suffers from low energy density, safety, cost, cycle life and power density for next-generation large scale applications in electric vehicles and renewable energy storage. Presently, graphite is a widely used anode for commercial LIB but has a relatively low specific capacity (372 mAh.g-1). The use of graphite also induces safety issues due to its very low working voltage vs. Li/Li+ that favours lithium plating at fast charging. Hence there is a need for alternative anode material that can in future replace graphite to address these challenges. Among various candidates, conversion-based materials (mostly binary and mixed-transition metal oxides) are particularly promising owing to their high theoretical capacity (between 800 and 1100 mAh.g-1). This high capacity is counterbalanced by a high working voltage vs. Li/Li+, interesting from a safety point of view to avoid lithium plating. However this high capacity is combined to a volumetric expansion of the material during lithiation leading to the pulverization of the electrode during longterm cycling. ZnFe2O4 is a conversion-based material that delivers a high theoretical capacity of 1001 mAh.g-1 at a relatively low working voltage compared to other oxides (1.5 V vs. Li/Li+). Moreover, as a cheap, abundant, non-toxic and environment-friendly material, it is more interesting than other metal oxides (Co-based ones for instance). To enhance the performances of the material and hinder the drawbacks of volumetric expansion in terms of electrode durability, it is interesting to address nanostructured electrodes using ZnFe2O4 nanoparticles. Indeed, a higher surface area and a smaller grain size provide more contact between the electrode material and the electrolyte and could also enable faster kinetics for lithium, while ensuring a better mechanical stability. To lead such a study, ZnFe2O4 nanoparticles have to be synthesized. Laser pyrolysis is a versatile and up-scalable gas phase process that allows the one step, continuous production of crystalline nanoparticles with controlled chemical composition, size and morphologies. The initial part of this thesis focuses on the syntheses of ZnFe2O4 nanoparticles by laser pyrolysis. Characterizations techniques such as SEM, TEM, XRD are used to analyse the produced materials. The second part is dedicated to the study of the synthesized materials as anode for Li storage and to the related mechanisms. Cyclic voltammetry measurements as well as galvanostatic cyclings were conducted to evaluate the electrochemical performance of the laser-pyrolyzed samples whereas operando measurements were used to understand the related mechanisms.Doctor of Philosoph