The conducted experimentation was aimed at optimizing the hybridization process of the anodic electrode material called NTP whose synthesis was developed at the RSE laboratories. The hybridization process is fundamental in order to allow the coupling of this electrode material with the cathode material Na0.44MnO2 (NMO) developed in the previous years. The hybridization process has involved the use of the anodic material synthesized using the solvothermal technique and different types of carbonaceous materials in order to obtain an anodic electrode material able to operate in an aqueous environment with a capacity comparable to that shown by the hybridized cathodic material.

The hybridization process of the NaTi2(PO4)3 (NTP) anodic material synthesized by hydrothermal route was obtained using different carbonaceous sources: Activated Carbon (AC), Conductive Carbon (CC), Carbon Nano Fibers (CNF) and carbon (C) deriving from the decomposition of organic material such as glucose and citric acid. Hybridization tests were conducted using different precursor blending technologies. The hybridization of electrode materials such as the NTP species requires a hybridization process with materials that have a high electronic conductivity, such as carbonaceous materials, in order to be used in electrochemical storage systems.

In fact, this type of electrode material has a conductivity component associated with the ionic mobility process inside the crystalline lattice but on the other hand they show extremely low electronic conductivity values; of the order of 10-6 S/cm which make all the redox reactions associated with the intercalation process of sodium ions particularly slow, leading to an inevitable reduction in capacity (expressed in mAh/g), such as to render the material unusable.All the hybrid materials have been subjected to a structural characterization phase by the X-ray diffraction technique (XRD) which allows the qualitative and semi-quantitative determination of the different crystalline species formed during the synthesis process.

The determination of the electronic conductivity of the synthesized materials has been performed with the measuring cell developed during the previous year. This measurement makes it possible to verify whether the hybridized material has an electrical conductivity comparable to the solid mixtures made for the preparation of the coatings: mixing between the active material and the polytetrafluorethylene (PTFE) binder for the realization of the electrode. The carbonaceous species is not always easily quantifiable from the analysis of the diffraction peaks determined by X-ray diffractometry. Consequently, the measure of the “C” content due to the hybridization procedure, has been performed by the TGA (Thermal Gravimetric Analysis) technique which allowed its correct determination by the evaluation of the material weight loss due to thermal decomposition process of the graphite component (C) when subjected to a high temperature calcination process.

Based on the characterization outcome, the quantitative analyzes performed by X-ray diffraction were corrected according to the “C” content determined by TGA. Following a chemical, physical and structural characterization, some materials, considered interesting, have been used for the production of electrodes and subjected to an electrochemical characterization by cyclic voltammetry (CV) techniques and charge and discharge measurements with different applied current values (GCPL). The experimentation showed that the best results were obtained by using citric acid as a carbon source with a final carbon content of about 16% by weight.

The experimentation also showed that the carbon produced during the hybridization process is not sufficient to support the flow of current applied to the electrode material when operating with current speeds higher than 2C. It was therefore necessary to add an additional carbon content during the electrode manufacturing phase. The best results have been obtained by operating with an overall carbon content of around 25% by weight, of which 10% deriving from activated carbon (AC) and/or carbon nano fibers (CNF) with additives during the realization of the laying of the electrode material.

The Report is available on the Italian site