In Situ Observation of Strains during Lithiation of a Graphite Electrode

The mechanics and microstructure of electrodes are critical in determining the performance and durability of lithium-ion batteries, especially the new large format cells and packs developed for transportation applications. During battery operation, Li diffuses into and out of the electrode particles, causing microstructural changes and deformation-induced degradation. A variety of models have been proposed to interpret these mechanical and microstructural changes, but they have no direct experimental support. We report direct in situ measurements of the microstructural strain in a composite electrode during lithium insertion using a side-by-side cell geometry. The color variation in graphite with Li concentration creates lithium spatial maps. A digital image correlation analysis provides corresponding deformation and strain fields, displaying both dilation and contraction. Through a combination of experimental measurement and theoretical analysis, the unexpected contraction during lithiation is explained by the stiffening of graphite upon lithiation. The result confirms the change in modulus that we recently predicted. Quantification of local strains shows that an increased graphite crystallite volume during lithiation is accommodated primarily by a decrease in the composite (or particle) porosity. The change in porosity can substantially impact battery power; however, this effect has generally been ignored in cell performance models for lithium-ion batteries.