Nanostructured Materials for secondary alkaline ion batteries

Thanks to their superior energy and power density, lithium-ion batteries (LIBs) currently dominate the market of power sources for portable devices. The economy of scale and engineering optimizations have driven the cost of LIBs below the 200 $/KWh at the pack level. This catalyzed the market penetration of electric vehicles and made them a viable candidate for stationary energy storage. However, the rapid market expansion of LIBs raised growing concerns about the future sustainability of this technology. In particular, lithium and cobalt supplies are considered vulnerable, primarily because of the geopolitical implications of their high concentration in only a few countries. In the search for the next generation secondary batteries, known as post-lithium ion batteries, candidates that do not use rare metals have been extensively investigated in the last 10 years. Sodium-ion batteries (SIBs) attracted considerable attention thanks to the high abundance of the precursors and wide distribution of sodium on the earth's crust. As a matter of fact, as it will be pointed out during the dissertation, it is not straightforward to allocate the reduction of the price of the alkaline ion precursors to the reduction of the battery price. However, the difficulties in the supply of raw materials for LIBs, such as shortages in lithium carbonates and cobalt ores, could make lithium and cobalt-free systems, such as SIBs, attractive and cost-competitive alternatives. Compared to other, more exotic chemistries including Ca2+, Mg2+ and Al3+ batteries, SIBs are nowadays considered one as the most promising alternative to LIBs. Despite the extensive research, anode materials for SIBs still represent a serious problem for the commercial exploitation of this technology. Accordingly, the doctoral research on SIBs has been focused on anode materials. In particular, the attention was directed towards conversion oxides. Compared to intercalation materials, conversion-based ones have higher capacities but are more challenging to deal with because of the high volume variation during cycling. This challenge was addressed by material's nanostructuring and morphology control which proved to significantly reduce the pulverization of the active material. Different anode candidates have been studied during the doctoral work. Cobalt oxide nanofibers have been here explored as a first prototype for conversion materials in sodium ion batteries. The sodiation-desodiation mechanism is analyzed by means of ex situ XRD which led to a deeper understanding of the conversion reaction in SIBs. A cost-effective and environmentally benign alternative based on iron oxide is then considered. The limits of iron (III) oxide are tackled by combining the advantages of the nanostructuring and the doping with an aliovalent element. Si-doped Fe2O3 nanofibers are synthesized via an easy scalable process based on the electrospinning method. It is found that Si-addition improves the transport properties as well as induces changes in the crystal structure and morphology. In the final section of the thesis, potassium-ion batteries (KIBs) are examined as a promising alternative to sodium ion batteries. KIBs exhibit all the benefits of SIBs, with the additional advantage that graphite, can reversibly accommodate K-ions. On the positive side, Potassium manganese hexacyanoferrate (KMnHCF), has been reported to provide high operating voltages and satisfactory capacity retention. The proposed research activity presents the use of an ionic liquid based electrolyte compatible with the most promising anode and cathode for KIBs. In addition, a high-throughput optimization of the KMnHCF synthesis is reported. The selected candidates are then fully characterized, and their electrochemical properties investigated. The optimized material exhibits the highest ever reported coulombic efficiency for the KMHCF. This find, opens up the possibility of highly efficient, high energy potassium ion batteries.