Thermodynamic and Kinetic Barriers Limiting Solid-State Reactions Resolved through In Situ Synchrotron Studies of Lithium Halide Salts

Although halide salts such as LiCl and LiBr are routinely used as a source of Li ions during ion exchange reactions, a detailed understanding of the processes controlling the rates of these reactions is presently lacking. Recently, we discovered that the rate-limiting barriers for ion exchange are commonly associated with these salts rather than the ceramic target of ion exchange, making it important to quantitatively understand salt processes. Here, it is demonstrated that in situ synchrotron studies of ion exchange reactions can be used to precisely quantify the thermodynamic activation energies associated with these solid-state reactions in a manner that can be directly compared with predictions from density functional theory (DFT). While the temperature dependence of the LiCl reaction rate is found to be set by a barrier associated with ion hopping, it was discovered that for LiBr, the rate is also affected by the defect formation energy─an energy found to be substantially lower than predicted by DFT. Furthermore, it is shown that by varying the relative amounts of reactants, the resulting change in reaction rate can be used to identify the rate-limiting reagent and to elucidate an overall scaling relationship that controls the concentration dependence of the reaction rate. Also, it is demonstrated that global fits across doped and undoped salts can be used to probe both intrinsic and extrinsic vacancy concentrations. This improved understanding of ion exchange mechanisms can be used to predict reaction conditions that can accelerate ion exchange reaction rates by orders of magnitude. The techniques demonstrated here can be broadly applied to probe the kinetics and thermodynamics of solid-state reactions.