Estany Sancho, Eloi
Solid state electrolytes have gained a lot of attention recently as potential substitutes for commercial liquid electrolytes as they offer improved stability and safety. However, the ionic conductivity in solid-state electrolytes is different from that of liquid electrolytes. The ionic conductivity in a crystalline solid depends on its structure. For ions to diffuse through different mechanisms, there must be space in the crystal structure, which is generated by point defects. Ionic conductivity can be improved by increasing ionic disorder. This can be achieved by thermally inducing or doping intrinsically or extrinsically to form non-stoichiometric compounds. Typical families used as lithium solid state electrolytes are perovskite type, LISICON and thio-LISICON type, garnet type and NASICON type.
In this work, the research is focused on the improvements in ionic conductivity that have been reported in recent years in garnet and perovskite electrolytes. These improvements include structural and morphological modifications. In reference to structural modifications, the changes have been made through homovalent or aliovalent doping. Homovalent doping changes the diffusion channels of lithium ion in the structure, a factor that clearly influences ionic conductivity. On the other hand, allovalent doping changes the Li / vacancy ratio and the diffusion channels of the lithium ion. In reference to morphological modifications, changes have been made in order to obtain a higher relative density and a lower grain boundary strength. This can be achieved by doping with a cation that promotes sinterability or by changing the conditions of the sinter.
Some studies have also focused on minimizing the loss of lithium due to the evaporation of Li2O, as it causes a decrease in the conductivity of the lithium ion. In order to prevent loss of lithium during the synthesis and sintering processes, changes have been made to conventional solid state synthesis and sintering methods. These changes include the addition of an excess lithium precursor to compensate for the loss of lithium during synthesis and the use of mother powder to cover the electrolyte pellets during sintering. In addition, alternative methods such as sol-gel synthesis or spark plasma sintering have been studied.