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

Abstract

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.

Computationally Guided Synthesis of Battery Materials

“…ML potentials have also been shown to be effective for computing the thermodynamics of surfaces and for probing the crystallization of phases from amorphous structures . Nonetheless, these approaches are still a long way from providing a full description of the kinetics involved in solid-state transformations, which remains challenging to obtain even using in situ experimental characterization. , …”

“…66 ML potentials have also been shown to be effective for computing the thermodynamics of surfaces 69 and for probing the crystallization of phases from amorphous structures. 70 Nonetheless, these approaches are still a long way from providing a full description of the kinetics involved in solid-state transformations, which remains challenging to obtain even using in situ experimental characterization. 53,71 In addition to reaction kinetics involving diffusion and nucleation, melting and volatility are difficult to model but can be highly influential in the synthesis of battery materials.…”

A high-temperature sample changer for parallelized in situ X-ray studies to efficiently explore reaction space

“…backgrounds, individual precursors, among others) and replicates to evaluate reproducibility. As such, systematic studies of the mechanism and kinetics and how these depend on the system chemistry are enabled (Cosby et al, 2020(Cosby et al, , 2023Mathiesen et al, 2023). These can be valuable in disentangling causal relationships and interdependencies in reactions, which may not be apparent from analysis of the X-ray scattering data alone.…”