Green-produced Ferrous and Manganese Oxalates as Negative Battery Electrodes

Abstract

Energy storage is an important area of research as demand for reliable energy grows, and greater emphasis is placed on producing electrical energy from intermittent renewable sources. One aspect of developing energy storage technologies, in particular lithium-ion batteries, is green electrode materials. These materials need to be capable of providing high capacities and possess a long cycle life. Additionally, the synthesis of the materials and the materials themselves should be green. This thesis focusses on the green synthesis, characterisation and electrochemical properties of ferrous and manganese oxalates as negative electrode materials in lithium-ion batteries. Different synthesis methods were explored, with the aim of using waste materials and applying the principles of Green Chemistry. The prepared materials were heat-treated to dehydrate the oxalate and form metal oxide. Products varied depending on the temperature and atmosphere. Since transition metal oxalates are used as precursors to metal oxides, the electrochemical behaviour and performance of oxalates were assessed and compared against oxides prepared from oxalates. In the first study, the feasibility of using transition metal oxalates to store carbon and energy simultaneously was investigated using FeC2O4.2H2O synthesised electrochemically from carbon dioxide and scrap mild steel. The product, irregularly shaped sub-micrometre particles, was heat-treated under nitrogen at 200, 300, 400C and at 300C in air, to yield FeC2O4 and iron oxides. Compared to iron oxides, FeC2O4 exhibited the highest gravimetric discharge capacities whilst retaining the largest proportion of carbon derived from carbon dioxide. This study shows that transition metal oxalates should be considered as an energy and carbon storage material. Furthermore, oxalates should be directly applied rather than used as precursors to metal oxides, to fulfil this dual functionality. The second study is based on FeC2O4.2H2O synthesised from rust. Rust was dissolved in aqueous oxalic acid. The resulting solution was either irradiated, aged for at least 24 hours, hydrothermally treated, or dried through solvent evaporation. The yield of FeC2O4.2H2O, product morphology, and processing times varied depending on the method used. Irradiation was the fastest process and had the highest yield of FeC2O4.2H2O, in the form of micro-particles. Particle size was reduced by ball milling. While increasing the milling time from 1 to 6 and 12 hours resulted in a more uniform reduction in particle size, the products were less crystalline, and milling fragments were introduced. Discharge capacities of the dehydrated samples were highest after 1 hour of milling. In the third study, nanostructured manganese oxalates fibres were prepared by immersing electrospun polymer fibres, containing manganese cations, in a solution of oxalic acid. Although commercially available metal salts and oxalic acid were used, electrospinning and immersion solutions were designed based on using green solvents and maximising atom economy. Product morphology and ability to form transition metal oxalates were influenced by the composition of the electrospinning solution and immersion solution. Compared to Mn3O4 obtained via thermal decomposition of manganese oxalates, MnC2O4 exhibited higher gravimetric discharge capacities attributed to the extensive formation of a polymer-gel layer. However, both MnC2O4 and Mn3O4 had poor long-term stability and slow reaction kinetics. This thesis shows that transition metal oxalate can be synthesised from waste materials and are attractive green energy storage materials. It also shows that transition metal oxalates can be better utilised if applied directly as energy storage materials than as precursors to oxides. For future research on transition metal oxalates, it is recommended that methods of improving their electrochemical cycling stability and synthesis methods using other waste materials be explored. Show more