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.