Bulk Stress Evolution during Intercalation of Lithium in Graphite

Abstract

Common Li-ion battery anodes consist of graphite particles adhered to a copper current collector with polyvinylidene fluoride (PVDF) binder. This work investigates the bulk stress evolution that occurs during reduction and oxidation of lithium at these graphite composite electrodes. The major stress generators are the (de)intercalation of lithium into (from) graphite and the formation of a passivating solid electrolyte interphase (SEI). Intercalation of lithium into graphite occurs in stages and produces anisotropic strains. As battery current collectors constrain the active material size and shape these strains are converted into mechanical stresses. These stresses have been measured in this work to be −3.7 ± 0.4 MPa and +1.9 ± 0.2 MPa for intercalation and extraction of lithium, respectively. The first lithium reduction wave occurs simultaneously with formation of the SEI. Work herein shows SEI formation generates +1.6 ± 0.4 MPa of stress. The charge and stress of SEI formation have been calculated dynamically as a function of potential. Additionally, electrochemically induced staging transitions during Li+ (de)intercalation are detected in the charge and stress profiles. The analysis of mechanical stresses associated with Li+ (de)intercalation and SEI formation provides key information relevant to the cycle life, shelf life, and power density of both primary and secondary lithium-ion batteries.