The hACF ! Q i measures the probability of finding a Liion at time t = t m in a position with an equivalent ! Q as it had at time t = 0. Thus, in materials in which the Li atoms visit sites with different ! Q , the hACF ! Q i in Equation 4 typically behaves as a decaying exponential function and one can extract the decay time ⌧ SAE directly using a stretched exponential form of the Lipari Szabo relation ,