Milica Vujković

This study presents a comprehensive investigation of the electrical, structural, and electrochemical properties of graphite flake (GF)-reinforced epoxy composites for energy storage applications. Epoxy/GF composites with filler loadings of 10, 30, 50, 70, and 80% wt. were fabricated to evaluate the effect of graphite concentration on conductivity, charge storage, and structural integrity. Impedance spectroscopy demonstrated that quantum-mechanical tunneling, consistent with fluctuation-induced tunneling transport, predominates charge transfer over a wide temperature range, ensuring strong electrical performance. The results show that at 10–30% wt.% GFs, incomplete conductive networks and limited electron and ion transport reduce electrochemical performance. At 50–70% wt.% GFs, the composites exhibited the highest specific capacitance and excellent cyclic stability due to the formation of well-connected three-dimensional conductive networks with sufficient porosity for efficient ion diffusion and charge transport. At filler loadings above 70 wt.%, graphite agglomeration, pore blockage, and microstructural defects were observed, resulting in reduced conductivity and capacitance. SEM, FTIR, and XRD analyses confirmed optimal chemical and morphological interactions at moderate filler contents, highlighting structural degradation at excessive loadings. These results indicate that an optimal graphite content of 50–70% by weight balances conductive pathways, mechanical stability, and electrolyte accessibility, providing a blueprint for designing epoxy/graphite composites that are robust and efficient for next-generation energy storage devices.

A new concept for affordable supercapacitors based on Al aqueous electrolytes was proposed recently. This study provides a deeper insight into the cyclic performance of vine shoots-derived activated carbon using three different 1 M aqueous electrolytes: Al2(SO4)3, Al(NO3)3 and AlCl3. Cyclic voltammetry (CV), galvanostatic cycling and impedance measurements have shown that the type of anion causes the differences in the rate capability and long-term cyclability. Although CV deviation is provoked by aggravated sulphate penetration into pores upon switching at higher currents, Al2(SO4)3 emerged as the most promising electrolyte solution due to the best cycling stability of the 1.5 V full cell over 15,000 cycles. Intensive oxidation of the positive electrode during initial cycling, induced by nitrates reduction, is the main reason for the fastest capacitance drop observed in Al(NO3)3. Therefore, the capacitance values of the carbon cell measured after 5,000 cycles in Al(NO3)3 (75 F g-1) are two times lower than the corresponding values in Al2(SO4)3 and AlCl3 (131 and 127 F g-1, respectively). The oxidation becomes more pro­nounced only after 10,000 cycles in AlCl3, thus causing a notable capacitance drop, which is not evidenced in Al2(SO4)3. Al2(SO4)3-based cell can withstand 15,000 with good specific capacitance/energy retention.

Since LiFePO₄ batteries play a major role in the transition to safe, more affordable and sustainable energy production, numerous strategies have been applied to modify LFP cathode, with the aim of improving its electrochemistry. In this contribution, a highly vanadium-doped LiFe_0.9V_0.1PO₄/C composite (LFP/C-10V) is synthesized using the glycine combustion method and characterized by x-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Thermogravimetry Differential Thermal Analysis (TGDTA) and Cyclic Voltammetry (CV). It is shown that 10wt.% of vanadium can substitute Fe positions, thus decreasing unit cell volume, which is followed by generation of Li₃V₂PO₄ traces, as detected by CV. High vanadium doping does not change the carbon content in the composite (≈13 wt%) but improves its electronic conductivity and electrochemical performance in both aqueous and organic electrolytes. The reversibility and current response are increasing following the trend: LFP/C, LFP/C -3mol%V, LFP/C - 5 mol % and LFP/C-10 mol %. The best specific capacity is obtained for the most highly doped olivine, which exhibits a reversible process at 1 mV s⁻¹ in an aqueous electrolyte, thus showing a peak-to-peak distance of 56 mV. The high capacity of LFPC-10V is measured in both LiNO₃ and NaNO₃ electrolytes amounting to around 100 mAh g⁻¹ at 20 mV s⁻¹. Still, the material is only stable in LiNO₃ electrolyte, making it more suitable for Li than Na-ion aqueous rechargeable batteries.

Layered VOPO₄·2H₂O is synthesized by the sonochemical method. An X-ray powder diffraction is used to examine the crystal structure, while scanning electron microscopy is used to reveal the morphology of the powder. The crystal structure refinement is performed in the P4/nmmZ space group. The electrochemical intercalation of several cations (Na⁺, Mg²⁺, Ca²⁺, and Al³⁺) in saturated nitrate aqueous solutions is investigated. The most notable reversible activity is found for the cycling in aluminium nitrate aqueous solution in the voltage range from −0.1 to 0.8 V vs. SCE. During the preparation of the electrode, it is observed that the structure is prone to changes that have not been recorded in the literature so far. Namely, the use of conventional binder PVDF in NMP solution deteriorates the structure and lowers the powder’s crystallinity, while the use of Nafion solution causes the rearrangement of the atoms in a new crystal form that can be described in the monoclinic P2₁/c space group. Consequently, these structural changes affect electrochemical performances. The observed differences in electrochemical performances are a result of structural rearrangements.