A key cause of chemo-mechanical degradation in battery electrodes is that they undergo abrupt phase transformation during the charging/discharging cycle. This phase transformation is accompanied by lattice misfit strains that nucleate microcracks, induce fracture and, in extreme cases, amorphize the intercalation electrode. In this work, we propose a strategy to prevent the chemo-mechanical degradation of intercalation electrodes: we show that by engineering suitable film strains we can regulate the phase transformations in thin-film intercalation electrodes and circumvent the large volume changes. We test this strategy using a combination of theory and experiment: we first analytically derive the effect of film strain on the electrochemical response of a thin-film intercalation electrode and next apply our analytical model to a representative example (LixV2O5 with multiple phase transformations). We then test our theoretical predictions experimentally. Specifically, we electrochemically cycle thin-film V2O5 electrodes with different film strains and measure their structure, voltage, and stress responses. Our findings show that tensile film strains lower the voltage for phase transformations in thin-film V2O5 electrodes and facilitate their reversible cycling across a wider voltage window without chemo-mechanical degradation. These results suggest that film strain engineering is an alternative approach to preventing chemo-mechanical degradation in intercalation electrodes. Beyond thin-film electrodes, our findings from this study are applicable to the study of stress-induced phase transformations in particle-based electrodes and the thin surface layers forming on cathode particles.

In constrained geometries and in varying oxygen partial pressures and operating temperatures, exchange of oxygen ions between non-stoichiometric oxide thin films (for example, doped and undoped ceria systems) and the gas phase can lead to stresses. In this study, these compositional stresses were investigated in thin films of nanocrystalline 10% praseodymium doped ceria (PCO), as a function of average grain size. In situ wafer curvature measurements, along with High Temperature X-Ray Diffraction (HTXRD), were employed to measure stresses and strains, respectively on the PCO films during oxidation-reduction cycling, over the pO2 range of 10-1-10-5 atm at 750 °C. For relatively large grain sizes, the stress values agree well with the amount of expansion induced by oxygen non-stoichiometry (chemical expansion) predicted by a thin film defect equilibria model that was developed previously. The compositional stresses were found to increase with decreasing grain size. The origin of this effect, including the role of space charge effects near surfaces and interfaces are discussed in this paper. To our knowledge, this is the first time that such comparisons are reported by simultaneously employing high temperature in situ wafer curvature and HTXRD measurements on doped ceria systems.

Stress and strain in thin films of Pr0.1Ce0.9O2-δ, supported on yttria stabilized zirconia (YSZ) and sapphire substrates, induced by large deviations from oxygen stoichiometry (δ = 0) were investigated by in situ high temperature X-ray diffraction and wafer curvature studies. The measured stresses and strains were correlated with change in δ, measured in situ using optical transmission spectroscopy of defect centers in the films and compared with prior chemical capacitance studies. The coefficient of chemical expansion and elastic modulus values for the films were found to be 18% less than, and 16% greater than in the bulk, respectively. Irreproducible stress and strain during cycling on YSZ substrates was observed and related to microstructural changes as observed by TEM. The enthalpy of defect formation was found to be similar for films supported on sapphire and YSZ, and appeared to decrease with tensile stress, and increase with compressive stress. Larger stresses observed for YSZ supported films as compared to sapphire supported films were found and accounted for by the difference in film orientations.

Electrochemical cycling induced mechanical damage of electrode materials actively contributes towards the performance degradation of lithium-ion batteries. The correlation between mechanical damage and performance degradation in anode materials that show large volume changes, such as silicon, graphite and tin, has been amply demonstrated. On the hand, typical oxide electrodes undergo only a few % volume changes, and the non-reversible nature of the crystal structure evolution as a function of lithium concentration in such electrodes is, in general, believed to be the limiting factor for performance degradation. For example, cycling in Li x CoO 2 is generally limited to 0≤x≤0.5, mainly, due to the irreversibility associated with the crystal structure changes beyond further Li extraction/re-insertion. However, due to their brittle nature only a few % volume changes can have significant implication on the mechanical damage leading to performance degradation for such ceramic oxide electrodes. Thus, the issue of electrochemical cycling induced mechanical degradation in oxide electrodes is being actively explored in recent years [1-2]. In this work, we present in-situ stress evolution of i) two canonical cathode systems, namely layered LiCoO 2 and spinel LiMn 2 O 4 and ii) one conversion electrode system, Co 3 O 4 in thin film configuration to quantify the driving force leading to the mechanical degradation. In-situ stress evolution in thin film electrodes was measured by monitoring the change in the elastic substrate curvature during electrochemical cycling in a suitably designed beaker cell using multiple-beam optical sensing (MOS) method. Thin films of the electrodes were prepared using solution deposition technique. Structural characterizations using XRD and Raman spectroscopy showed predominant presence of desired (poly)crystalline phases in the as prepared samples. In addition, SEM images also revealed the presence of dense microstructural features in the as prepared films. During Li-extraction from layered Li x CoO 2 , there was almost linear increase in compressive stress up to ~50% Li removal, which is consistent with its lattice parameter evolution during Li removal [3], and a maximum compressive stress of ~0.35 GPa was observed for x~0.5. Upon lithiation there was almost reversible stress evolution in Li x CoO 2 . Similar behavior was also observed for subsequent cycles as well, while limiting the upper charging cut-off voltage to 4.3V. On the other hand, initial delithiation from spinel Li x Mn 2 O 4 induces tensile stress up to ~4.1V, beyond which the induced stress reverses direction (termed as “compressive drop”) with further delithiation (up to 4.3 V). This reversal of stress evolution in the later stages of delithiation from spinel Li x Mn 2 O 4 is in apparent contradiction with the lattice parameter evolution of spinel Li x Mn 2 O 4 during lithium extraction [4]. Upon lithium re-insertion (up to 3.5V), induced compressive stress increases linearly. The subsequent cycles (in the 4V region), however, did not show any “compressive drop” during later stages of delithiation and the induced stress evolved reversibly during delithiation-lithiation. The origin of this first cycle “compressive drop” in spinel LiMn 2 O 4 is not known at present. In an attempt to establish the origin of the observed first cycle “compressive drop” in spinel LiMn 2 O 4 thin films, stress measurement data varying multiple parameters including cathode film thickness, reannealing a cycled electrode will be presented. The effect of stress evolution in these thin film electrodes during cycling as a function of cycling voltage window and current density will also be presented and discussed in the light of their crystal structural changes. References D. J. Miller, C. Proff, J. G. Wen, D. P. Abraham and J. Bareno, Adv. Energy Mater., 3 , 1098 (2013). W. H. Woodford, W. C. Carter and Y. M. Chiang, Energy Environ. Sci., 5 , 8014 (2012). J. N. Reimers and J. R. Dahn, J. Electrochem. Soc., 139 , 2091 (1992). Y. Xia and M. Yoshio, J. Electrochem. Soc., 143 , 825 (1996).

Real time monitoring of stress evolution in electrodes during electrochemical cycling can help quantify the driving forces that dictate their mechanical degradation. In the present work, in-situ stress evolution in thin films of spinel Li1+xMn2O4 (LMO) was measured by monitoring the change in the elastic substrate curvature during electrochemical cycling in a specially designed beaker cell in the 3.5-4.3 V (vs. Li/Li+) voltage range. The LMO thin films were prepared using a solution deposition technique and their structures and morphologies were characterized by X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM). The stress evolution in the early part of the first delithiation cycle (<4.05 V) was consistent with the XRD data. However, stress evolution during later stages of the first delithiation cycle (>4.05 V) was not consistent with the XRD results, and showed irreversible behavior, suggesting irreversible changes in the electrode. Beyond the first delithiation cycle, the stress evolution was reversible, with a steady buildup of compressive and tensile stress during lithium insertion and extraction, respectively. Measurements on LMO films of varying thicknesses suggest that the first cycle irreversibility in stress response arises primarily from the electrode bulk. (C) 2016 The Electrochemical Society. All rights reserved.

Transitioning from the micro to nano morphology scale can result in dramatic changes in electrical and optical properties of films. Space charge effects and creation of a significantly greater fraction of interfaces/grain boundaries are believed to give rise to such size scale effects. While electrical properties have generally dominated research in this area, mechanical properties, such as film stress and strain, also exhibit scaling effects. In-situ wafer curvature measurements are employed to measure these stresses on pure and 10% Praseodymium doped ceria (PCO) thin films during oxidation-reduction cycling and over a range of temperatures and oxygen partial pressures. These measured stress values are compared to the values reported for the bulk PCO and also to the thin film defect equillibria model developed by some of the co-authors and was found to fit well with the predicted values. In both the pure ceria and PCO thin films, the compositional stresses increased with decreasing grain sizes thus corroborating the idea that the space charge effects near surfaces and interfaces has a significant impact on the compositional stresses.

In battery electrode materials, phase transformations during Li insertion and removal generally lead to volume changes. These can lead to localized stresses that contribute directly to the degradation of the active material. We have employed thin V2O5 films as model cathode materials, to systematically investigate relationships between stress, phase transformations, and degradation. To directly probe the impact of stress, processing conditions were controlled to vary the initial stress state over a wide range (while maintaining similar grain structures). All of these investigations employed precise in situ stress measurements, along with x-ray diffraction and detailed electron microscopy studies. The experimental data was then compared to a thermodynamic model that takes into account the impact of initial and lithiation induced stresses. The experiments and analysis reveal that stress in the electrode can significantly alter the voltage plateaus associated with two phase equillibria.

The point defect induced strain and stress in thin films was analyzed using in situ wafer curvature and in-situ XRD measurements. The strains and stresses were modeled using a previously well-developed and experimentally validated point defect equilibria model. Values for the chemical expansion coefficient and Young’s modulus for bulk ceramics were successfully used to predict stress and strain behavior of the corresponding thin films.

The piezoelectric β-phase of poly(vinylidene fluoride) (PVDF) has been synthesized through solution route in presence of varying percentage of silicon carbide (SiC). Various measurements were conducted to analyze the effect of SiC addition on structural, mechanical, and dielectric properties, as also the properties affecting hydrophilicity and morphology of PVDF. The x-ray diffraction, Fourier transform infrared spectroscopic and differential scanning calorimetry analyses confirm the presence of β-phase of PVDF on addition of SiC. The Young’s modulus of the composites increases as compared with pristine PVDF. The hydrophilic nature of the composites improves with increasing SiC content. Dielectric constant increases in composites, especially at lower frequency range and the relaxation pattern in the crystalline phase changes significantly causing reduction of relaxation frequency with increasing SiC concentration.