  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.
