Lithium-ion battery technology has improved tremendously since it was first commercialized in the 1990s, and today lithium-ion batteries are one of the lightest and smallest rechargeablebatteries commercially available.While lithium-ion batteries are ubiquitous in portable electronics, limited lifetime and reliability hinder widespread adoption of the batteries inlarge-scale energy storage devices such as in electric vehicles. Key areas of active research that offer potential to improve lithium-ion batteries include the development of new anodeand cathode materials and new electrolytes, a more complete understanding of in situ and artificial interfacial stabilization processes at the electrode / electrolyte interface, and strategies to mitigate combined electro-chemo-mechanical degradation of the batteries resulting from extended electrochemical cycling.The present work touches on all three of these areas by investigating the electrochemically-induced mechanical response of lithium-ion battery anodes. First, an experimental protocol was developed to measure the strains induced in free-standing battery electrodes (i.e. unconstrained electrodes not adhered to a substrate) due to electrochemical processes. Graphite composite electrodes, similar to commercial battery anodes, were investigatedfirst to garner baseline results. The electrodes were comprised of particles of graphite as the active material, carbon black for a conductive additive, and a polymer binder. Reversible macroscale electrode deformation was traced to nanoscale changes in graphite layer spacing as lithium was inserted into and removed from the electrodes. Irreversible electrode deformation wascorrelated with accumulation of electrolyte decomposition products on the surface of the electrode. Many parameters were varied, including the electrode composition (ratio of graphite to carbon black as well as the choice of polymer binder), electrolyte composition, and cycling rate. The effect of the variation of these parameters on the strain response of graphite composite electrodes was investigated.In situ strain measurements of free-standing graphite composite electrodes were combined with in situ stress measurements of electrodes adhered to a substrate. A new electrochemo-mechanical property of the electrodes, the "electrochemical stiffness," was defined as a measure of the relative effects of stress compared to strain at any point during electrochemical cycling. Changes in the electrochemical stiffness as a function of electrode potential or capacity provided new insights into the mechanisms governing electrochemically-induced stress and strain development in graphitic electrodes.Finally, the strain response of high-capacity silicon composite electrodes was investigated and compared to the strain response of composite graphite electrodes. The large volumetricexpansion of silicon during lithiation (ca. 300 - 400 %) caused the electrodes to fracture and pulverize where they were attached to a substrate, which frustrated in situ strain measurements.Therefore, strain was measured only during the initial, partial lithiation of silicon-based electrodes, before the electrodes fractured. The macroscopic strain developed in the electrodes was dominated by the total amount of lithium inserted into the electrode.The type of active material (either graphite or silicon) and the ratio of electrode components (active material, conductive additive, and polymer binder) were found to be secondaryinfluences, while the size of the active material particles had insignificant influence on the average strain response of the electrodes.
PDF
download
878987797
028-85220240
