The Effect of Mechanical Strain on Lithium Staging in Graphene

Lithium intercalation into graphite is the foundation for the lithium-ion battery, and the thermodynamics of the lithiation of graphitic electrodes have been heavily investigated. Intercalated lithium in bulk graphite undergoes structural ordering known as staging to minimize electrostatic repulsions within the crystal lattice. While this process is well-understood for bulk graphite, confinement effects become important at the nanoscale, which can significantly impact the electrochemistry of nanostructured electrodes. Therefore, graphene offers a model platform to study intercalation dynamics at the nanoscale by combining on-chip device fabrication and electrochemical intercalation with in situ characterization. We show that microscale mechanical strain significantly affects the formation of ordered lithium phases in graphene. In situ Raman spectroscopy of graphene microflakes mechanically constrained at the edge during lithium intercalation reveals a thickness-dependent increase of up to 1.26 V in the electrochemical potential that induces lithium staging. While the induced mechanical strain energy increases with graphene thickness to the fourth power, its magnitude is small compared to the observed increase in electrochemical energy. We hypothesize that the mechanical strain energy increases a nucleation barrier for lithium staging, greatly delaying the formation of ordered lithium phases. Our results indicate that electrode assembly can critically impact lithium staging dynamics important for cycling rates and power generation for batteries. We demonstrate strain engineering in two-dimensional nanomaterials as an approach to manipulate phase transitions and chemical reactivity.