Lattice Compatibility and Energy Barriers in Intercalation Compounds

We present a continuum model for symmetry-breaking phase transformations in intercalation compounds, based on Ericksen’s multiwell energy formulation. The model predicts the nucleation and growth of crystallographic microstructures in <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msub><a:mi>Li</a:mi><a:mn>2</a:mn></a:msub><a:msub><a:mi>Mn</a:mi><a:mn>2</a:mn></a:msub><a:msub><a:mrow><a:mi mathvariant="normal">O</a:mi></a:mrow><a:mn>4</a:mn></a:msub></a:math>—a representative intercalation compound—with twin boundary orientations and volume fractions that closely match experimental observations. Our chemomechanically coupled model not only generates geometrically accurate microstructures through energy minimization, but also reveals a subtle interplay between twinned domains and electrochemomechanical behavior. A key finding is that intercalation compounds satisfying specific compatibility conditions (e.g., <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"><d:msub><d:mi>λ</d:mi><d:mn>2</d:mn></d:msub><d:mo>=</d:mo><d:mn>1</d:mn></d:math> or <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"><f:mrow><f:mo stretchy="false">|</f:mo></f:mrow><f:mi>det</f:mi><f:mspace width="0.1em"/><f:mrow><f:mi mathvariant="bold">U</f:mi></f:mrow><f:mo>−</f:mo><f:mn>1</f:mn><f:mrow><f:mo stretchy="false">|</f:mo></f:mrow><f:mo>=</f:mo><f:mn>0</f:mn></f:math>) show lower elastic energy barriers, require smaller driving forces, and display narrower voltage hysteresis loops. Furthermore, we show that twinned domains act as conduits for fast <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"><l:mi>Li</l:mi></l:math> diffusion. These results establish quantitative design guidelines for intercalation compounds, which focuses on tailoring lattice deformations (rather than suppressing them) and reducing energy barriers to mitigate structural degradation and enhance the electrochemical performance of battery electrodes.