Substrate Curvature Technique Research Articles

Degradation Mechanisms of Positive Electrode Materials for Sodium-Ion Batteries

Sodium-ion batteries (SIB) require less critical materials and lead to a significant cost reduction compared to state-of-the-art lithium-ion batteries [1]. Although a large-scale commercialization of SIBs is within reach [2], the understanding of the degradation mechanisms of these batteries is still incomplete. The larger ionic radius of sodium in comparison to that of lithium can induce larger stress changes in the host material causing poor long-term stability. Minimizing degradation of positive electrode materials is of high importance for the advancement of SIB technology. Here, we report on degradation of the important structure types: layered oxides, polyanionic compounds, and Prussian blue analogues. Our results show that the polyanionic Na3V2(PO4)3 is a highly reliable cathode material, which can be used as a benchmark or reference material.During battery operation, mechanical stresses are caused by volume changes of the active materials, when sodium ions are inserted or extracted into/from them. We use operando methods such as the substrate curvature technique to measure stresses in the electrode and draw conclusions about electrochemical and physical processes in the materials during cycling [3]. To investigate phenomena like volume changes, crack formation and overall morphology, ex situ scanning electron microscopy (SEM) is used intermittently. Here, we compare the same site on the electrode in different states of charge and for different cycle numbers. Additionally, X-ray diffraction (XRD) measurements are conducted to study phase transitions.Na2/3Fe1/3Mn2/3O2 is a layered P2-type oxide that offers a high specific capacity and is composed of abundant elements. Due to its intrinsic sodium deficiency, diverse presodiation methods become important [4]. We investigate Na2C4O4 as sacrificial salt and its effects on electrode morphology during decomposition. Our goal is to understand the resulting reaction pathways and optimize the full cell performance of layered oxides.Sodium hexacyanoferrate NaxFe[Fe(CN)6] is a Prussian blue analogue and one of the most prominent candidates for commercial SIBs [2]. It also offers high specific capacities and a long cycle life, but its high moisture sensitivity and interstitial water demand appropriate electrode drying procedures to avoid side reactions during cell operation. Crack formation and growth were studied with SEM after and during cycling. Phase transitions are observed by operando XRD and operando substrate curvature measurements.Sodium vanadium phosphate Na3V2(PO4)3 (NVP) with its highly stable three-dimensional network of a NASICON-type structure and its large diffusion channels for sodium ions is another promising electrode material that deserves attention. NVP can be sodiated and desodiated at two different voltage plateaus and therefore can serve either as cathode or anode material. SEM observations at different states of charge show how preexisting cracks open/close during desodiation/sodiation due to volume shrinkage/expansion. We compare symmetric NVP−NVP cells to full cells with NVP cathodes and anodes of Na metal, hard carbon, Sb/C, and SnSb/C. Our data suggests that problems of Na and Na-ion cells frequently result from the anode. NVP itself is extremely reliable and balanced symmetric NVP−NVP cells exhibit very high cycling stability. More than one thousand cycles can be easily achieved. Due to its distinct plateaus and high stability, we suggest NVP as a reference material for cathode and anode materials. For example, NVP can serve as a reliable anode material for testing the stability of different cathode materials. NVP as a cathode may be used to test anode materials or as a benchmark for other cathode materials [5].Acknowledgements:This work contributes to the research performed at CELEST (Center for Electrochemical Energy Storage Ulm-Karlsruhe) and was funded by the German Research Foundation (DFG) under Project ID 390874152 (POLiS Cluster of Excellence, EXC 2154).Literature:[1] C. Vaalma et al., Nature Reviews Materials 2018, 3, 4, 1-11.[2] M. A. Sawhney et al., ChemPhysChem 2022, 23, 5, 1-19.[3] Z. Choi et al., Journal of Power Sources 2013, 240, 245-251.[4] J. M. De Ilarduya et al. Electrochimica Acta 2019, 321, 134693.[5] T. Akçay et al., ACS Applied Energy Materials 2021, 4, 11, 12688-12695.

(Invited) In-Situ Measurements of Stress Evolution in Composite Sulfur Cathodes

Owing to its enormous capacity, sulfur has emerged as a promising cathode material. Recent studies have identified and begun to mitigate a number of key electrochemical issues in sulfur-based cathodes, including poor conductivity and polysulfide dissolution/shuttling. However, the mechanics of lithiation/de-lithiation of sulfur are much more complicated than in other systems due to solid-to-liquid, liquid-to-liquid, and liquid-to-solid phase transformations, and thus remain poorly understood. To this end, this talk will discuss our recent measurements of the evolution of stresses in composite sulfur-carbon cathodes during electrochemical cycling, performed using an in-situ substrate curvature technique. This mechanical behavior is connected to electrochemical and structural evolution via combined scanning electron microscopy, energy dispersive spectroscopy, and x-ray diffraction during cycling. Overall, these studies provide fundamental insight into the practical design of mechanically robust sulfur cathodes, thereby paving the way toward realizing new high-capacity systems.

In-Situ Monitoring of Stress Development and Electrochemical Behaviour of Metallic Thin Film Anode Materials for Li-Ion Batteries

Due to their high specific capacities and higher (safer) operating potentials, metallic materials such as Sn and Al are among the promising alternatives to graphitic carbon as anode materials for Li-ion batteries. However, the metallic anode materials undergo huge volumetric changes during lithiation/delithiation which leads to the development of stresses that result in severe mechanical degradation and concomitant drastic capacity fade. Furthermore, the stresses affect the thermodynamics of the lithiation/delithiation ‘reactions’ and exert influences on the various electrochemical behaviour. In order to develop better understanding of the origins, magnitudes and resulting effects of such stress developments, in correlation with the initial stress states, crystallographic orientations, stages of lithiation/delithiation, associated phase transformations and surface reactions, we have monitored the stress developments in-situ during lithiation/delithiation. Sn and Al have been selected for the present studies, with the concerned electrodes possessing simple thin film architectures, without binder/additive/porosity. The active electrode films of ~100 nm in thickness have been deposited via e-beam deposition on different current collector films (~100 nm thick), such as Cu (for both Sn and Al), Ni (for Sn) or Ti (for Sn) on thicker (~0.5 mm) quartz substrates (stiff substrate) for allowing monitoring of the in-plane stress developments via substrate curvature technique using multi-beam optical stress sensor (MOSS) in-situ during electrochemical cycling against Li metal in custom-made electrochemical cell [1]. In addition to the expected developments of compressive stresses during lithiation and reverse during delithiation, we focus on some of the finer features of the stress developments, which involves the stress evolution during the first order phase transformations that take place during the Li-alloying reactions (i.e. formation of the various Li-Sn and Li-Al intermetallic) [2]. To develop better understanding, attempt has also been made to mathematically model the observed in-plane stress developments at the different stages (single and two-phase regimes). As will be demonstrated, our observations suggest occurrences of mechanical degradation mainly during the phase transformations, with the electrode materials being fairly stable during the single phase (solid solution) regimes. In order to explore a possible route towards improving the mechanical integrity and cyclic stability upon lithiation/delithiation, the effects of inducing pre-cycling residual stresses via simple annealing treatments have been investigated. Carefully planned experiments with films possessing different residual stress states, aided with in-situ monitoring of the stress developments during lithiation/delithiation, have highlighted the importance of such initial conditions on aspects related to cyclic stability and overpotentials. The use of Sn and Al based electrodes, where the molar volume change associated with the phase transformations are considerably higher compared to other electrode materials, have further strengthened the understanding. On a different note, the influences of different current collectors (Cu, Ni and Ti) on the crystallographic orientations, and concomitantly on the various electrochemical behaviour and stress developments will also be highlighted. Our in-situ observations also suggest the development of stresses due to SEI formation, similar to the case of graphitic carbon [3]. It is believed that the understandings developed as part of the present work will contribute towards successful developments of electrode materials based on alloying reactions. Keywords: anode materials, Sn, Al, stress development

A simple technique for measuring the fracture energy of lithiated thin-film silicon electrodes at various lithium concentrations

We have measured the fracture energy of lithiated silicon thin-film electrodes as a function of lithium concentration using a bending test. First, silicon thin-films on copper substrates were lithiated to various states of charge. Then, bending tests were performed by deforming the substrate to a pre-defined shape, producing a variation of the curvature along the length of the electrode. The bending tests allow determination of the critical strains at which cracks initiate in the lithiated silicon. Using the substrate curvature technique, we also measured the elastic moduli and the stresses that develop in the electrodes during electrochemical lithiation. From these measurements, the fracture energy was calculated as a function of lithium concentration using a finite element simulation of fracture of an elastic film on an elastic–plastic substrate. The fracture energy was determined to be Γ = 12.0 ± 3.0 J m−2 for amorphous silicon and Γ = 10.0 ± 3.6 J m−2 for Li3.28Si, with little variation in the fracture energy for intermediate Li concentrations. These results provide a guideline for the practical design of high-capacity lithium ion batteries to avoid fracture. The experimental technique described in this paper also provides a simple means of measuring the fracture energy of brittle thin-films.

Stress Evolution in Lithium-Ion Composite Electrodes during Electrochemical Cycling and Resulting Internal Pressures on the Cell Casing

Composite cathode coatings made of a high energy density layered oxide (Li1.2Ni0.15Mn0.55Co0.1O2, theoretical capacity ∼377 mAh-g−1), polyvinylidene fluoride (PVdF) binder, and electron-conduction additives, were bonded to an elastic substrate. An electrochemical cell, built by pairing the cathode with a capacity-matched graphite anode, was electrochemically cycled and the real-time average stress evolution in the cathode coating was measured using a substrate-curvature technique. Features in the stress evolution profile showed correlations with phase changes in the oxide, thus yielding data complementary to in situ XRD studies on this material. The stress evolution showed a complex variation with lithium concentration suggesting that the volume changes associated with phase transformations in the oxide are not monotonically varying functions of lithium concentration. The peak tensile stress in the cathode during oxide delithiation was approximately 1.5 MPa and the peak compressive stress during oxide lithiation was about 6 MPa. Stress evolution in the anode coating was also measured separately using the same technique. The measured stresses are used to estimate the internal pressures that develop in a cylindrical lithium-ion cell with jelly-roll electrodes.

Stress release and defect occurrence in V1−xFex films upon hydrogen loading: H-induced superabundant vacancies, movement and creation of dislocations

Hydrogen-induced elastic/plastic deformation was studied in V1−xFex (x=0.02–0.08) films with thicknesses between 10 and 400nm and prepared at different temperatures. The combination of several in situ techniques such as X-ray diffraction, acoustic emission, electromotive force and substrate curvature techniques allows sensitive studies of defects generated in these thin films. As well as conventional out-of-plane linear elastic film expansion and in-plane compressive stress increase during hydrogen absorption, the investigations uncovered new details: as soon as hydrogen predominately solved in interstitial lattice sites, discrete stress relaxation (DSR) events were detected, after which the film continued to behave in a linear elastic manner. DSRs were interpreted by uncorrelated movement of pre-existing dislocations. Particularly in the case of films deposited at higher temperatures, in-plane tensile stress was found at very small H concentrations of less than 0.005 H/V. Upon further H uptake, this turned into compressive stress. However, this stress increase differed from theoretical predictions. This behavior is explained by the generation of superabundant vacancies. Dislocation emission and plastic deformation are linked to the formation of the hydride phase in the V1−xFex films.

Measurements of the Fracture Energy of Lithiated Silicon Electrodes of Li-Ion Batteries

We have measured the fracture energy of lithiated silicon thin-film electrodes as a function of lithium concentration. To this end, we have constructed an electrochemical cell capable of testing multiple thin-film electrodes in parallel. The stress in the electrodes is measured during electrochemical cycling by the substrate curvature technique. The electrodes are disconnected one by one after delithiating to various states of charge, that is, to various concentrations of lithium. The electrodes are then examined by optical microscopy to determine when cracks first form. All of the observed cracks appear brittle in nature. By determining the condition for crack initiation, the fracture energy is calculated using an analysis from fracture mechanics. In the same set of experiments, the fracture energy at a second state of charge (at small concentrations of lithium) is measured by determining the maximum value of the stress during delithiation. The fracture energy was determined to be Γ = 8.5 ± 4.3 J/m(2) at small concentrations of lithium (~Li0.7Si) and have bounds of Γ = 5.4 ± 2.2 J/m(2) to Γ = 6.9 ± 1.9 J/m(2) at larger concentrations of lithium (~Li2.8Si). These values indicate that the fracture energy of lithiated silicon is similar to that of pure silicon and is essentially independent of the concentration of lithium. Thus, lithiated silicon demonstrates a unique ability to flow plastically and fracture in a brittle manner.

Effect of substrate bias voltage on the properties of CrCN and CrN coatings deposited by cathodic arc evaporation

The substrate bias voltage is one of the significant parameters of technology controlling the mechanical properties of PVD coatings. Chromium nitride and chromium carbonitride coatings were deposited on HS6-5-2 steel substrates by cathodic arc evaporation technique. The relationships between one of deposition parameter, negative substrate bias voltages Vs (10 V–300 V), and the coatings characterization such as microhardness, stress, adhesion of the coatings, elastic strain to failure related to H/E ratio were investigated. On SEM photos one can observe a dense polycrystalline structure with columnar grains for the coatings deposited at low bias voltage and homogenous, fine-grained morphology obtained at higher bias voltage. Residual stresses were measured by substrate curvature technique. The CrN coatings show a compressive residual stress that increases from 1.0 (Vs = −10 V) to 2.1 GPa (Vs = −70 V). At higher bias voltages a decrease of the compressive residual stresses is apparent. Similar relation but for higher stress values is observed for CrCN coatings. Nanoindentation showed a maximum hardness of 25 GPa for CrN and 26 GPa for CrCN coatings deposited at −150 V of bias voltage. The critical loads of CrN coatings in a scratch test decreased monotonically from 95 N to 78 N with increasing negative substrate bias voltage. The critical load of CrCN coatings was nearly constant, about 78 N.

Application of substrate curvature method to differentiate drying stresses in topical coatings and human stratum corneum

Despite the extensive use of topical coatings in cosmetics, their effect on the mechanical properties of human skin and the perception of skin tightness in the form of drying stresses is not well understood. We describe the application of a recently developed substrate curvature technique to characterize stresses in drying and non-drying occlusive topical coatings. We then extend the technique to measure the combined effects of the coating applied to human stratum corneum (SC) where the overall drying stresses may have contributions from the coating, the SC and the interaction of the coating with the SC. We show how these separate contributions in the coating and SC layers can be differentiated.

Drying stress and damage processes in human stratum corneum

The drying stresses that develop in stratum corneum (SC) are crucial for its mechanical and biophysical function, its cosmetic feel and appearance, and play a central role in processes of dry skin damage. However, quantitative methods to characterize these stresses are lacking and little understanding exists regarding the effects of drying environment, chemical exposures and moisturizing treatments. We describe the application of a substrate curvature technique adapted for biological tissue to accurately characterize SC drying stresses as a function of time following environmental pre-conditioning and chemical treatment in a range of drying environments. SC stresses were observed to increase to stress levels of up to approximately 3 MPa over periods of 8 h depending on pretreatment and drying environment. A unique relationship between the SC stress and water in the drying environment was established. The effect of glycerol on lowering SC stresses and damaging surfactants on elevating SC stresses were quantified. Extensions of the method to continuous monitoring of SC stresses in response to changes in environmental moisture content and temperature are reported. Finally, a biomechanics framework to account for the SC drying stress as a mechanical driving force for dry skin damage is presented.

Effects of nitrogen pressure and pulse bias voltage on the properties of Cr–N coatings deposited by arc ion plating

Cr–N coatings were deposited on 1Cr18Ni9Ti stainless steel in the pure N 2 atmosphere by arc ion plating (AIP). The relationships between deposition parameters and coating properties were investigated. X-ray diffraction showed a phase transformation from CrN + Cr 2N + Cr → CrN + Cr → CrN and the CrN preferred orientation changed from (200) to (220) as N 2 pressure increased. Increasing bias voltage led to CrN preferred orientation changed from (200) to (220) and the formation of Cr 2N. XPS results indicated that chemical composition of the coatings changed as N 2 pressure increased but it changed little with bias voltage. The lower melting point of chromium nitride formed on target surface induced the increase of macroparticles and deposition rate with increasing N 2 pressure; and bias voltage had an obvious effect on reducing macroparticles of the Cr–N coatings. Residual stresses were measured by substrate curvature technique, and the changing tendency coincided with the microhardness of the coatings.

Depth Distribution of Residual Stress in Graded (Ti, Al) N Coating and Fatigue Properties of Coated Stainless Steel

The depth distribution of the residual stress in graded (Ti, Al) N coating deposited on steel by arc ion plating was measured by the Stripping Layer Substrate Curvature Technique, and the effect of graded (Ti, Al) N and mono-layered (Ti, Al)N coating on the fatigue properties of 1Cr11Ni2W2MoV stainless steel were investigated. The depth distribution of the residual stress in mono-layered Ti70Al30N and TiN were also measured for comparison. The results show that the residual stresses in the coatings are compressive, which increase gradually from the coating/substrate interface and reach a maximum value at the middle region, then decrease until the surface. Compared with TiN and Ti70Al30N, the stress maximum value in the graded coating is nearer to the coating surface. It is also shown that the fatigue strength of the graded (Ti, Al)N and Ti70Al30N coated samples are superior to that of the uncoated substrate. The improvement of the fatigue properties for the coated samples is thought to be attributed to the hard coatings with high compressive stress.

Micro-mechanical testing of SiLK by nanoindentation and substrate curvature techniques

Advanced micro-mechanical characterization methods provide material properties of thin films for modeling thermo-mechanical behavior of thin films for micro-electronic applications. Here, we focus on the local measurement method of nanoindentation for finding visco-elastic properties, and a global method of substrate curvature testing that provides linear elastic properties. Our specimen SiLK, Dow chemicals, is a low- k dielectric thin polymer film with a thickness of 400 nm, 6 and 8 μm, deposited on Si substrate. Our results show temperature dependent linear elastic and linear visco-elastic material properties for thin film materials.

Combined elastic strain and macroscopic stress characterization in polycrystalline Cu thin films

This work introduces a new simple approach to determine experimental X-ray elastic constants (XECs) of thin films by coupling the sin2ψ method and the substrate curvature technique. The approach is demonstrated on polycrystalline Cu thin films with the thickness 200, 800, and 2400 nm deposited on Si(100) substrates. Applying synchrotron radiation, the elastic strains in the films are determined using sin2ψ method while the macroscopic stresses are assessed by measuring the substrate curvature. The stresses are calculated using the Stoney formula from the radius of substrate curvature determined by the rocking curve measurement of substrate 400 reflection at different sample positions. Results show that the magnitude of the macroscopic stress in the films is proportional to the magnitude of the slope in the sin2ψ plots. On the basis of this observation, XECs of the films were calculated showing no dependence on the film thickness. The characterization of the samples was performed at the synchrotron source Hasylab.

Comparison of stress in single and multiple layer depositions of plasma-deposited amorphous silicon dioxide

The stress in a single-layer continuous deposition of amorphous silicon dioxide (SiO2) film is compared with the stress within multiple-layer intermittent or ‘stop–start’ depositions. The films were deposited by helicon activated reactive evaporation (plasma assisted deposition with electron beam evaporation source) to a 1 µm total film thickness. The relationships for stress as a function of film thickness for single, two, four and eight layer depositions have been obtained by employing the substrate curvature technique on a post-deposition etch-back of the SiO2 film. At film thicknesses of less than 300 nm, the stress–thickness relationships clearly show an increase in stress in the multiple-layer samples compared with the relationship for the single-layer film. By comparison, there is little variation in the film stress between the samples when it is measured at 1 µm film thickness. Localized variations in stress were not observed in the regions where the ‘stop–start’ depositions occurred. The experimental results are interpreted as a possible indication of the presence of unstable, strained Si–O–Si bonds in the amorphous SiO2 film. It is proposed that the subsequent introduction of a ‘stop–start’ deposition process places additional strain on these bonds to affect the film structure. The experimental stress–thickness relationships were reproduced independently by assuming a linear relationship between the measured bow and film thickness. The constants of the linear model are interpreted as an indication of the density of the amorphous film structure.

Coatings with intrinsic stress profile: Refined creep analysis of (Ti,Al)N and cracking due to cyclic laser heating

A refined non-linear creep analysis of ceramic coatings has been performed by means of an improved substrate curvature technique in vacuum facilitating increased sensitivity by noise reduction. In this way the intrinsic stress profile may be derived from the time-dependent substrate curvature. Creep and stress parameters of 0.5 μm (Ti,Al)N-coatings have been found. At 500 °C the stress varies between − 1 GPa at the surface and + 0.2 GPa at the interface with the silicon substrate. From the onset of crack formation as observed in cyclic laser shock experiments a coating tensile strength of about 0.7 GPa is deduced. The number of cycles to cracking and delamination/spalling vs. maximum coating temperature as derived from creep simulation agrees quantitatively rather well with the experimental data. A rough estimate of the interface fracture toughness on WC/Co substrates yields 11 N/m as a lower bound.

Indentation method to measure the residual stress induced by ion implantation

A depth-sensing indentation system was used to measure the residual stress in a stainless steel plate implanted by Fe2+ ions with an energy of 3MeV at a dose of 3×1016cm−2. Unlike other measurement methods, such as substrate curvature and X-ray diffraction techniques, depth-sensing indentation provides an accurate measurement of local residual surface stresses in the thin ion-implanted layer. Micromechanical analysis was carried out based on the premise that elastic unloading responses during indentation are fully independent of any pre-existing residual stresses at the indented surface. The correctness of this premise was verified by FEM simulation. It is found that the surface stress is proportional to the load shift induced by the surface stress. By using the energy method, a formula was derived for determining the surface stress by depth-sensing indentation studies. The residual surface stress induced by ion implantation was evaluated by using this formula.

Investigation of the fracture toughness of radio frequency magnetron sputtered Al–Cu–Fe films via white-beam synchrotron radiography/topography

A novel white-beam synchrotron radiography/topography substrate curvature technique has been used to study stress development in situ during annealing of Al–Cu–Fe quasicrystalline and approximant coatings, as well as to image their failure modes in real time. Single crystal Si and sapphire substrates were coated with a 2.55 µm precursor coating by RF sputtering from an Al65Cu23Fe12 powder composite target and subsequently annealed at 585°C while stress and imaging data were acquired. After the initial ramp to the annealing temperature, a stress plateau was reached for coatings on both Si and sapphire substrates, although the magnitude of the stress plateau was different in each case. A tensile stress developed in the coatings during cooling due to differential thermal expansion between the coating and substrate, allowing for calculation of both the coefficient of thermal expansion and elastic modulus of the film. During cooling, the films exhibited different stress evolution above and below 470°C, a temperature of interest in Al–Cu–Fe quasicrystal and approximant phase development. The Al–Cu–Fe coating on the Si substrate fractured at approximately 954 MPa, while the coating on the sapphire substrate fractured at approximately 431 MPa. From these values the fracture toughness was calculated to be 1.9 MPa m1/2 and 0.76 MPa m1/2 for the coatings on Si and sapphire, respectively.

Influence of Gas Atmosphere on the Plasticity of Metal Thin Films

AbstractStresses in thin films are routinely measured by the so-called substrate curvature technique. These experiments are usually carried out in air or under a protective gas atmosphere. In this contribution we describe a new set-up capable of performing substrate curvature measurements under ultra-high vacuum conditions. The advantages are the absence of possible artifacts due to gas/film interactions, better control of gas composition, and the possibility to measure chemical effects on mechanical properties in a controlled way. We present first results that indicate an unexpected sensitivity even of polycrystalline Cu films to the gas environment.

Residual stresses in sputter-deposited copper/330 stainless steel multilayers

The evolution of residual stresses as a function of bilayer period from 10nmto1μm in sputter-deposited Cu/330 stainless-steel (SS) multilayered films is evaluated by the substrate curvature technique. The multilayer stress evolution is compared with residual stresses in single layer Cu films and single layer 330 SS films, also measured by substrate curvature technique, with respective film thicknesses varying from 5to500nm. Both single layer and multilayer films exhibit high tensile residual stresses that increase with decreasing layer thickness, but are found to be lower than the respective yield strengths. The intrinsic tensile residual stress evolution with film thickness is explained using the island coalescence model. The difference between the multilayer residual stress and the average residual stresses in single-layered Cu and 330 SS films is interpreted in terms of interface stress.