The further development of lithium-ion batteries leads to an improvement in power densities as well as safety and thus requires an optimization of the materials used. For this purpose, among other approaches, the anode materials are doped with silicon oxide or metallic silicon is used as the anode. However, silicon is a semimetal and is known to lead to the formation of jelly-like fluids in hydrometallurgical processes under certain conditions. This publication evaluates which parameters are responsible for this viscosity change in the leaching solutions during the recycling of lithium-ion batteries and examines the corresponding reaction mechanism behind this phenomenon. Furthermore, the leaching efficiency for the valuable metals nickel, cobalt, lithium and manganese is evaluated and the influence of different silicon contents in the solution is investigated. It could be shown that, especially the simultaneous presence of H2SO4, H2O2 and Si or SiO2, lead to a significant viscosity increase due to the formation of metasilicic acid and, accordingly, the leaching efficiencies of the valuable metals are negatively influenced.