Graphene-supported novel chemistries in lithium ion-batteries

Abstract

Among energy storage devices, lithium-ion batteries represent the state-of-the-art technology, being widely employed in portable electronics, stationary storage and even electric mobility. Despite being our best solution, there are numerous problems bound to their employment, such as the still not clear recycling path of exhausted batteries, the scarcity or toxicity of key materials (e.g., nickel, cobalt, manganese etc) that are not sufficient to support an electric fueled mobility and the intrinsic limits of the performance of the commercially available lithium-ion batteries. In fact, commercial cathodes are characterized by relatively low capacities and an unfavorable energy density, because of the presence of heavy metals. Also commercial anodes have limited capacities, if compared with newly proposed materials. Apparently, extensive research can only cut down little of both prices and slightly increase the performance in the foreseeable future, due to the already optimized technology, while only largescale production can moderately inhibit the costs. A viable solution is offered by innovative chemistries of lithium-ion batteries, which employ novel active materials characterized by elevated energy densities and low impact. These technologies are not already widely commercialized because they are still affected by limitations, that so far make these solutions suitable only for niche applications. The current interest is in making these devices robust and versatile, and the ideal path is to create composites with materials that will overcome the limitations of the bare active material. In this work, macroscopic amount of chemically produced graphene was employed as a scaffold for such new chemistries with a twofold role: on the one hand, graphene is able to support the electroactive materials at the nanoscale; on the other hand, it provides a conductive network that promotes the charges extraction from the electrode. In addition, chemically derived graphene is a porous material with a high specific surface area, and can successfully buffer the volumetric swelling of the electroactive materials which happen upon lithium uptake and retain the eventual cracking of the particles due to continuous swelling and shrinkage. For these reasons, graphene is an ideal candidate for these roles, due to its electrical, mechanical and specific surface area characteristics. In this thesis, graphene was combined with three different innovative electroactive materials, namely titanium dioxide, sulfur and silicon. For the case of titanium dioxide, a chemically derived graphene, obtained by the thermal exfoliation of graphite oxide, was used to enhance the performance of the still under study TiO2-based lithiumion batteries. The composite materials were successfully produced, either via decoration of thegraphene during the solvothermal synthesis of TiO2 nanoparticles, or via physical mixing by means of high energy ball milling. The electrochemical performance of the electrodes have been markedly improved by the addition of just a 1% in weight of graphene. In addition, the structural evolution of the electrode upon lithiation and de-lithiation was investigated via operando synchrotron light diffraction, which highlighted the different steps of the Li intercalation process in these materials. These results encouraged the employment of TiO2-graphene composites as anodes in lithium-ion batteries. Subsequently, graphene has been combined with sulfur to produce S-graphene composites, following four different routes: I) physical mixing of the reagents via ball milling, II) liquid-assisted ball milling (or wet ball milling), III) thermal infiltration of sulfur in the graphene matrix and IV) chemical decoration of graphene with sulfur. Specifically, the last synthesis was performed by precipitation of sulfur in a solution containing graphene oxide, which was subsequently chemically reduced. A systematic study of the synthetic route along with the research of the best sulfur-to-carbon ratio was carried out, and the chemically produced sample with 70 % sulfur loading was proven as the best candidate for lithium-sulfur batteries, obtaining a mean reversible capacity of about 500 mAh/g after 100 cycles. Finally, graphene was employed as a conductive agent for silicon-based batteries. In particular, silicon nanoparticles produced by the disproportionation of silicon monoxide achieved remarkable results. The Si nanoparticles obtained by this synthesis measured about 4 nm in size. The challenging etching of the silicon oxides was performed in an ad hoc plastic apparatus, employing ammonium hydrogen difluoride as an etching agent. Unfortunately, the etching step was not successful, but the silicon mixture was electrochemically characterized nonetheless. The electrodes fabricated with disproportionated silicon monoxide were measured having an impressive starting capacity exceeding 7000 mAh/g, with a reversible capacity of 1150 mAh/g retained after more than 70 cycles.