Solid State Sodium Batteries: Synthesis and characterization of a sodium based composite electrode, in combination with NaSICON and glass-ceramic solid electrolyte

With the current energy needs and depletion of fossil-fuel resources, the exploitation of renewable technologies is an essential task. Their intermittence requires reliable and efficient storage systems, as the electrochemical batteries. In the last years, the liquid electrolyte Li-ion battery was considered one of the most important storage technologies, providing high voltage, high energy density and stable cycling performance. Nonetheless, the liquid electrolyte exposes the devices to wear, risks of leakage, dendritic growth and thermal runaway. The scarce availability of Li in the Earth crust is an additional problem. To overcome these issues, the liquid electrolyte can be substituted with a solid one, which guarantees high gravimetric and volumetric energy density, easy transfer of ions, and provides structural and mechanical stability. Lithium can be replaced by Na, because of its low cost, natural abundance and the higher redox potential (2.7 V Na/Na+) after Li (3.03 Li/Li+). Among several cathode materials, the most interesting are the polyanions, in particular phosphates, ensuring rapid conduction of alkali ions in the framework formed by MeOx polyhedral (with Me=Fe, Mn, Co) and tetrahedral (PO4)n-. The following study aims at realising a sodium-iron-phosphate cathode active material, starting from glass powder precursors co-fired via sintering process on a solid electrolyte substrate. Precedent studies claim that it is possible to create a new candidate with triclinic P1 structure as Na2FeP2O7 (theoretical capacity of 97 mAhg-1, associated with the redox couple Fe3+/Fe2+). After several analyses, the adapt sintering procedure is defined: the batch is fired in Varigon atmosphere, at 5 K/min up to 560 °C and held for 3 hours; it is composed by NFP glassy powders, mixed with 5 %wt. of graphite to promote iron reduction and electronic conductivity. The mainly prevalent crystalline phase is the triclinic Na3.12Fe2.44(P2O7)2, characterised by electrochemical reaction mechanisms similar to those of Na2FeP2O7. The solid electrolytes investigated are a NaSICON material (Na3Zr2Si2PO12, with σionic=10^-3 Scm-1 at 25°C) and a glass-ceramic structure labelled as Na07 (57.1 %mol. SiO2, 35.7 %mol. Na2O, 7.2 %mol. Y2O3, with σionic of 10^-4-10^-3 Scm-1 at 25°C). Blended with a polymeric binder (PPC) and a solvent (MEK), the amorphous powders with desired composition are reduced to a slurry, applied to the surface of solid electrolytes and co-fired with the defined sintering procedure. The stack is coupled with metallic sodium as anode and submitted to EIS analyses and charge-discharge measurements. The results reveal a strong interface resistance, mainly because of high impedance at interface between electrolyte and cathode material. The charge-discharge measurements (1/50 C, at 30 °C) show irrelevant capacity values, even at the first cycle. Nonetheless, charge-discharge measurements with liquid electrolyte report curves similar to the ones of previous studies, with initial discharge capacity as about 30 mAhg-1: those evidences confirm the occurrence of an electrochemical reaction and the realisation of a cathode active material. It is concluded that sintering procedure is an effective way to produce crystalline and conductive cathode elements. The association with solid electrolyte in a solid state battery is possible, but it is fundamental to perform further investigations on the interface between the two materials.