Reversible and Irreversible Deformation Mechanisms of Composite Graphite Electrodes in Lithium-Ion Batteries

Abstract

Repeated charge and discharge of graphite composite electrodes in lithium-ion batteries cause cyclic volumetric changes in the electrodes, which lead to electrode degradation and capacity fade. In this work, we measure in situ the electrochemically-induced deformation of graphite composite electrodes. The deformation is divided into a reversible component and an irreversible component. Reversible expansion/contraction of the composite electrodes is correlated with localized changes in graphite layer spacing associated with different graphite-lithium intercalation compounds. Phase transitions between different intercalation compounds are manifested during galvanostatic cycling as peaks in the derivative of capacity with respect to voltage; these peaks correspond remarkably well with peaks in the derivative of strain with respect to voltage. Irreversible electrode deformation is correlated with deposition of electrolyte decomposition products on graphite particles during the formation and growth of the solid electrolyte interphase (SEI). Both the irreversible capacity and the irreversible strain developed during galvanostatic cycling increase with increasing electrode surface area and increasing cycling time. During a potentiostatic voltage hold at 0.5 V vs Li+/0, in which electrolyte decomposition is the dominating electrochemical reaction, both the capacity and the electrode strain increase proportional to the square root of time. Interestingly, the choice of polymer binder, either carboxymethyl cellulose (CMC) or polyvinylidene fluoride (PVdF), has a significant influence on the irreversible electrode deformation, suggesting that the formation and growth of the SEI layer is influenced by the polymer binder.