A grand challenge facing society today is the need to develop green energy technologies. For instance, as we increase our use of renewable energy sources to cut our consumption of CO2-producing fossil fuels, we must be capable of efficiently storing the energy such sources provide for when demand is high. Li-ion batteries, for example, are widely employed as portable energy storage devices. However, safety concerns surround the liquid electrolytes currently utilised in commercially available Li-ion batteries. An important question, therefore, is whether new solid electrolyte materials can improve the safety and performance of solid-state batteries. Here, we propose to study the candidate solid electrolyte material, Li5La3Ta2O12, and explore how doping its structure with sodium ions affects its electrolyte function using muon spin relaxation measurements.