The experimentation conducted was directed toward optimizing the hybridization process of the anode electrode material NaTi2(PO4)3 (referred to as NTP), the synthesis of which was developed at the RSE laboratories. The hybridization process is of fundamental importance to enable the coupling of this electrode material with the cathode material Na0.44MnO2 (referred to as NMO) developed in previous years. In the hybridization process, the anode material, synthesized using the solvothermal method, was mixed with different types of carbon materials in order to be able to operate in an aqueous environment with a capacity comparable to that exhibited by the hybridized cathode material.

The hybridization of the hydrothermally synthesized anode material NaTi2(PO4)3 (NTP) was obtained using different carbon sources: Active Carbon (AC), Conductive Carbon (CC), Nano Carbon Fibers (CNF), and carbon (C) derived from decomposition of organic material such as glucose and citric acid. Hybridization tests were conducted using different precursor mixing technologies.

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The hybridization of electrode materials, such as the NTP species, requires materials that exhibit high electronic conductivity, such as carbon materials, in order to be employed in electrochemical storage systems. This type of electrode material, in fact, has a conductivity component associated with ion mobility process within the crystal lattice, but on the other hand it shows extremely low electronic conductivity values, on the order of 10-6 S/cm, which make all the oxidation-reduction reactions associated with the sodium ion intercalation process particularly slow, leading to an inevitable reduction in capacity (expressed in mAh/g), such that the material becomes unusable.

All hybrid materials were subjected to a structural characterization step using the X-ray diffraction (XRD) technique, which allows the qualitative and semi-quantitative determination of the different crystalline species formed during the synthesis process. The measurement of the electronic conductivity of the synthesized materials was carried out to check whether the hybridized material has comparable values with the solid mixtures made for their preparation: mixing between active material and polytetrafluorethylene (PTFE) binder for making the electrode.

The carbonaceous species is not always easily detectable from the analysis of diffraction peaks determined by using X-ray diffractometry. Therefore, the measurement of the “C” content, which is present as a result of the hybridization process, was carried out using the TGA (Thermal Gravimetric Analysis) method, which allowed its correct determination through the evaluation of the weight loss of the material resulting from the thermal decomposition of the graphitic component (C) when subjected to calcination at high temperature. Quantitative analyses carried out by X-ray diffraction were corrected according to the “C” content determined by TGA.

Following chemical-physical and structural characterization, the materials deemed most interesting were used to make electrodes and subjected to electrochemical characterization using cyclic voltammetry (CV) techniques and charge-discharge measurements with different values of applied current (GCPL).

The experimentation showed that the best results were obtained using citric acid as the carbon source with a final carbon content of about 16% by weight. The experimentation also showed that the carbon produced during the hybridization procedure is not sufficient to support the current flow applied to the electrode material when operating at current velocities above 2C. Therefore, it was necessary to add additional carbon content during the electrode fabrication stage.

The best results were obtained by operating with a total carbon content in the order of about 25 wt%, 10 wt% of which was derived from activated carbon (AC) and/or carbon nano fibers (CNF) added during the fabrication of the electrode material.