  We report in situ measurements of stress evolution in a silicon thin-film
electrode during electrochemical lithiation and delithiation by using the
Multi-beam Optical Sensor (MOS) technique. Upon lithiation, due to substrate
constraint, the silicon electrode initially undergoes elastic deformation,
resulting in rapid rise of compressive stress. The electrode begins to deform
plastically at a compressive stress of ca. -1.75 GPa; subsequent lithiation
results in continued plastic strain, dissipating mechanical energy. Upon
delithiation, the electrode first undergoes elastic straining in the opposite
direction, leading to a tensile stress of ca. 1 GPa; subsequently, it deforms
plastically during the rest of delithiation. The plastic flow stress evolves
continuously with lithium concentration. Thus, mechanical energy is dissipated
in plastic deformation during both lithiation and delithiation, and it can be
calculated from the stress measurements; we show that it is comparable to the
polarization loss. Upon current interrupt, both the film stress and the
electrode potential relax with similar time-constants, suggesting that stress
contributes significantly to the chemical potential of lithiated-silicon.
