Non-equilibrium microstructure of Li1.4Al0.4Ti1.6(PO4)3 superionic conductor by spark plasma sintering for enhanced ionic conductivity

In solid-state electrolytes, the large resistance at grain boundaries remains the bottleneck for high ionic conductivity. Here we develop an alternative and somewhat counterintuitive strategy to enhance their ionic conductivity via non-equilibrium microstructure. Using Li1.4Al0.4Ti1.6(PO4)3 as an example, we demonstrate that semi-crystalline interphase between well crystallized ceramic phase and amorphous glass phase can be induced by spark plasma sintering, resulting in total ionic conductivity of 1.3 × 10−3 S cm−1 without any doping, which is 2 orders of magnitude higher than that derived by the conventional method. It is further demonstrated that the non-equilibrium structure is stable in ambient condition, yet can be converted into equilibrium structure by annealing with higher crystallinity but much lower ionic conductivity, proving that the non-equilibrium structure is indeed the key to the high performance. This opens door for its applications in electric vehicles, and the strategy is applicable to other ionic systems as well.