Electrode and electrolyte materials for the development of high voltage lithium-ion batteries and secondary batteries based on alkali and alkaline-earth ions

The research activity described in this thesis has been focused on the development and study of novel electrolyte and electrode materials for application in Lithium and Magnesium secondary batteries. The proposed materials belong to the “beyond Li-ion” class of compounds, where systems exceeding the energy density values of classic Li-ion batteries or completely innovative chemistries are presented. Three different classes of electrolytes have been prepared and studied. A solid polymer electrolyte has been obtained by a lithium functionalization of a poly(vinyl alcohol-co-vinyl acetate), forming lithium alkoxide functional groups. In this way, the counter anion of Li+ was the overall polymer chain, giving rise to a single lithium ion conductivity. However, the room-temperature conductivity value observed for this material was quite low (4.6·10-10 S·cm-1). By ionic liquid (IL) doping of the solid polymer electrolyte, we have obtained a double effect: i) lithium cations have been exchanged by the cations of IL, enhancing the mobility of the active species; and ii) the flexibility of polymer chains has been increased by the plasticizing effect of the IL. Thus, a room temperature conductivity of 1.3·10-5 S·cm-1 has been reached, maintaining a high value of Li transference number (0.59). By reacting glycerol with different quantities of lithium hydride, a new family of lithium-ion conducting electrolytes has been synthetized. In these electrolytes the lithium glycerolate component acts as a large and flexible macro-anion which is able to provide a singleion conductivity to the material (2.0∙10-4 at 30 °C and 1.6∙10-2 S∙cm-1 at 150 °C). In the last class of electrolytes, ionic liquid-based materials for magnesium batteries, the cation and anion replacement effect on the structure, conductivity mechanism, and electrochemical performances has been studied. The proposed materials have exhibited a conductivity value between 10-3 and 10-4 S∙cm-1, an overpotential in the magnesium deposition lower than 50 mV vs. Mg/Mg2+, an anodic stability up to +2.35 V vs. Mg/Mg2+, and a coulombic efficiency up to 99.94 %. In the second part of this Ph.D. project, the improvement of the electrochemical features of various cathode materials has been studied. In the first case, it has been found that, by adding CuCO3 to the precursors, segregated CuO particles have been formed. The presence of these particles has improved the charge-transfer kinetics during the charge/discharge processes of the cathode material. On the other hand, graphite addition to the precursors has been found to improve the elasticity of the 3D structure of the cathode backbone. Thus, an increased structural flexibility that facilitates the percolation of lithium ions along the 1D channels of the cathode material has been observed. In the second approach, the improvement of the electron conductivity of a high-voltage cathode has been gauged by V, Nb, or Ta insertion within its olivine structure. This approach has allowed for an improved kinetic and reversibility of Li+ insertion reaction. The specific capacity reached by these cathodes was equal to 149 mAh∙g-1. The last cathode material has been implemented in a magnesium secondary battery device. A graphene oxide surface functionalization of vanadium-based nanoparticles has been obtained thanks to electrostatic interactions through ammonium bridges. This functionalization has allowed for the obtaining of a material able to: a) sustain extremely high current rates (1000 mA∙g-1, 1700 mW∙g-1 of specific power); and b) give reasonable specific capacity values (72 mAh∙g-1).