Experimentation allowed us to synthesise Na0.44MnO2 (NMO), a cathode material that can be potentially used for realising hybrid battery-supercapacitor systems with sodium ions able to operate in the presence of aqueous electrolytes. A supercapacitor is a particular electrochemical storage system characterised by a high power density (W/Kg) and a high number of charge and discharge cycles at constant performance, which normal electrochemical accumulation systems with alkaline ions, such as lithium ion batteries, cannot guarantee. However, supercapacitors have very low energy density compared to batteries. The supercapacitor hybridisation of active materials of alkaline ion batteries aims to optimise both technologies. Compared to lithium ion, the use of sodium ion in storage systems has two main advantages that are the reasons why RSE has chosen it for developing this technology: (i) lithium deposits are highly localised (China and South America), while sodium is distributed in a rather uniform manner across the Earth’s crust and this reduces possible geopolitical criticalities that could arise from the potential development of and demand by the market of sodium ion accumulators; (ii) the extraction costs and the environmental impact of sodium are certainly lower than those of lithium.

The material (NMO) was synthesised using two different experimental techniques: hydrothermal reaction and solid-state reaction with Spark Plasma Sintering (SPS). The two different synthesis techniques are alternatives to the sol-gel synthesis with acetates of the same material (at RSE, this technique is the subject of other experiments), as the latter is not able to produce a morphologically suitable material. Hydrothermal tests were carried out at 205°C and by putting in contact, for 1 to 6 days, an aqueous solution of sodium hydroxide at different concentrations and a solid reagent consisting of oxides and a soluble manganese salt. SPS tests were carried out using solid mixtures consisting of manganese and sodium salts and oxides in different experimental conditions that differ in heating rate, maximum temperature and holding time.

All the materials produced were characterised by X-ray diffraction analysis; samples with an NMO content higher than 96% were subjected to morphological and electrochemical characterisation. Electrochemical measurements were performed with an electrode consisting of the active material supported with a graphite electrode in the presence of an aqueous electrolyte.

All materials synthesised with the hydrothermal technique showed good crystallographic, structural and electrochemical reproducibility. The tests carried out with the SPS technique highlighted high dispersion of data and low purity. The hydrothermal technique, taking place in the absence of oxygen, is also potentially applicable to the hybridisation process of the material with activated carbon.