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In situ strain and stress measurements are performed on composite electrodes to monitor potentialdependent stiffness changes in lithium manganese oxide (LiMn 2 O 4 ). Lithium insertion and removal results in asynchronous strain and stress generation in the electrode. Electrochemical stiffness changes are calculated by combining coordinated stress and strain measurements. The electrode experiences dramatic changes in electrochemical stiffness due to potential-dependent Li + ion intercalation mechanisms. The development of stress in the early stages of delithiation (at ca. 3.95 V) due to a kinetic barrier at the electrode surface gives rise to stiffness changes in the electrode. Strain generation due to phase transformations reduces stiffness in the electrode at 4.17 V during delithiation and at 4.11 V during lithiation. During lithiation, stress generation due to Coulombic repulsions between occupied and incoming Li + ions leads to stiffening of the electrode at 3.96 V. The electrode also experiences greater changes in stiffness during delithiation compared to lithiation. These changes in electrochemical stiffness provide insight into the interplay between mechanical and electrochemical properties which control electrode response to lithiation and delithiation.

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.

While stress is thought to play an important role in the development of self-organized porous films, mechanisms of stress generation during anodizing are not yet understood. In order to reveal depth distributions of stress in anodic films, phase-shifting curvature interferometry was used to monitor force transients (in-plane stress integrated through the sample thickness) during formation of anodic oxides on aluminum in phosphoric acid, as well as subsequent open-circuit dissolution. The measurements were not influenced significantly by electrostatic stress, internal stress in the metal samples, thermal stress, or stress induced by open-circuit dissolution. At typical current densities, the force became more compressive during anodizing, while a net tensile force change was measured after anodizing followed by complete oxide dissolution. Thus, it was revealed that anodizing generates both compressive stress in the oxide and tensile stress near the metal-oxide interface. Analysis of the open-circuit stress change revealed separate contributions from diffusional stress relaxations, and removal of residual oxide stress by dissolution. Residual stress distributions in the oxide, at nanometer depth resolution, were determined from measurements of dissolution rate and stress at open circuit, and validated through variations of the open-circuit dissolution rate.

Stress measurements yield insight into technologically relevant deformation and failure mechanisms in electrodeposition, battery reactions, corrosion and anodic oxidation. Aluminum anodizing experiments were performed to demonstrate the effectiveness of phase-shifting curvature interferometry as a new technique for high-resolution in situ stress measurement. This method uses interferometry to monitor surface curvature changes, from which stress evolution is inferred. Phase-shifting of the reflected beams enhanced measurement sensitivity, and the separation of the optical path from the electrochemical cell in the present system provided increased stability. Curvature changes as small as 10−3 km−1 were detected, at least comparable to the resolution of state-of-the-art multiple beam deflectometry. It was demonstrated that small curvature change rates of 10−3 km−1s−1 could be reliably measured, indicating that the technique can be applied to bulk samples. The dependence of the stress change during anodizing on current density (tensile at low current density, but increasingly compressive at higher current densities) was quantitatively consistent with earlier multiple-beam deflectometry measurements. The close similarity between the results of these different high-resolution measurements helps to resolve conflicting reports of anodizing-induced stress changes found in the literature.