Recent years have seen an increasing demand for high-density and price-competitive energy storage devices, mostly pushed by the electric automotive industry and the need for back-up storage for renewable energies. Lithium-ion batteries, the technology currently being the most used for high-density applications, uses lithium intercalation compounds to store ions at the anode during the charging process. While this chemistry owes its success to its high stability and safety, intercalation compounds possess a relatively limited theoretical capacity: storing an alkali metal in its elemental metallic state would allow to significantly reduce the weight of “inactive” material. Moreover, even though lithium possesses higher capacity than its neighboring elements thanks to its smaller atomic ra...[ Read more ]

Recent years have seen an increasing demand for high-density and price-competitive energy storage devices, mostly pushed by the electric automotive industry and the need for back-up storage for renewable energies. Lithium-ion batteries, the technology currently being the most used for high-density applications, uses lithium intercalation compounds to store ions at the anode during the charging process. While this chemistry owes its success to its high stability and safety, intercalation compounds possess a relatively limited theoretical capacity: storing an alkali metal in its elemental metallic state would allow to significantly reduce the weight of “inactive” material. Moreover, even though lithium possesses higher capacity than its neighboring elements thanks to its smaller atomic radius, its limited availability suggests that sodium might become a more competitive alternative in the near future. While research on metallic lithium as anode is slowly enabling its use in next-generation batteries, the use of metallic sodium as a room-temperature anode is still taunted by several issues. Most importantly, the high reactivity of sodium to the electrolyte leads to a poor plating homogeneity, with the risk of permanent loss capacity and dendrite-caused short circuit: to this day, no set strategy has been established that was able to fully understand and patch the inhomogeneity of sodium electrodeposition in a sodium battery.

In this thesis a host-material approach to the issue is presented: a scaffold made of electrospun carbon nanofibers (CNF), free-standing and of facile production, which acts as a 3D current collector reducing the local current density, impeding the growth of dendrites and mechanically stabilizing the volume change inside of the cell during sodium plating. While sodium plated on a simple copper or aluminum foil showed instability from the very first test cycles, the as-produced CNF host could achieve up to 1600 stable cycles at 1 mA cm^-2. However, increasing the current was found to lead to early failure of the CNF anode. In search of appropriate sodiophilic surface for stable sodium plating, the affinities between sodium and different substrate materials were analyzed by measuring overpotentials, Coulombic efficiencies, and through density functional theory calculations. Zinc oxide was the final candidate selected to be incorporated into the fibers as an additive. By reacting and alloying with sodium during discharge, zinc oxide functions as effective high-affinity nucleation sites for the depositing sodium. CNF with embedded ZnO nanoparticles were produced and tested, proving that the additive component was electrochemically active and it successfully extended the efficacy range of the nanofibers to currents up to 3 mA cm^-2. Ex-situ microscopy was used to analyze the morphology of the deposited sodium, finding an enhanced homogeneity of the deposition on the fibers surface for the ZnO@CNF host. Moreover, the plating thickness of sodium was predicted based on simple electrochemical principle and geometric considerations, corroborating experimental measurements. After proving that the fibers scaffold could accommodate the plating of large amounts of sodium (up to 6 mAh/cm²), the material was tested in a symmetric setup, achieving more than 1000 hours of stable charge-discharge cycles versus the quick performance degradation of a pure sodium plate (<250 hours at 1 mA/cm², <50 hours at 3 mA/cm²). Finally, it was proven that the produced material was able to improve the battery stability even when using a commercial carbonate-based electrode, which is known to be unstable with metallic sodium.