Irreversible Rearrangements Research Articles

Clustering of negative topological charges precedes plastic failure in 3D glasses.

The deformation mechanism in amorphous solids subjected to external shear remains poorly understood because of the absence of well-defined topological defects mediating the plastic deformation. The notion of soft spots has emerged as a useful tool to characterize the onset of irreversible rearrangements and plastic flow, but these entities are not clearly defined in terms of geometry and topology. In this study, we unveil the phenomenology of recently discovered, precisely defined topological defects governing the microscopic mechanical and yielding behavior of a model 3D glass under shear deformation. We identify the existence of vortex-like and antivortex-like topological defects within the 3D nonaffine displacement field. The number density of these defects exhibits a significant anticorrelation with the plastic events, with defect proliferation-annihilation cycles matching the alternation of elastic-like segments and catastrophic plastic drops, respectively. Furthermore, we observe collective annihilation of these point-like defects via plastic events, with large local topological charge fluctuations in the vicinity of regions that feature strong nonaffine displacements. We reveal that plastic yielding is driven by several large sized clusters of net negative topological charge, the massive annihilation of which triggers the onset of plastic flow. These findings suggest a geometric and topological characterization of soft spots and pave the way for the mechanistic understanding of topological defects as mediators of plastic deformation in glassy materials.

Understanding slow compression of frictional granular particles by network analysis.

We consider frictional granular packings exposed to quasi-static compression rates, with a focus on systems above the jamming transition. For frictionless packings, earlier work (S. Luding et al., Soft Matter, 2022, 18(9), 1868-1884) has uncovered that the system evolution/response involves smooth evolution phases, interrupted by fast transitions (events). The general finding is that the force networks' static quantities correlate closely with the pressure, while their evolution resembles the kinetic energy for both frictionless and frictional packings. The former represents reversible (elastic) particle deformations with affine and non-affine components, whereas the latter also involves much stronger, irreversible (plastic) rearrangements of the packings. Events are associated with jumps in the overall kinetic energy as well as dramatic changes in the force networks describing the particle micro-structure. The frictional nature of particle interactions affects both their frequency and the relevant time scale magnitude. For intermediate friction, events are often followed by an unexpected slow-down during which the kinetic energy drops below its average value. We find that these slow-downs are associated with a significant decrease in the non-affine dynamics of the particles, and are strongly influenced by friction. Friction modifies the structure of the networks, both through the typical number of contacts of a particle, and by influencing topological features of the resulting networks. Furthermore, friction modifies the dynamics of the networks, with larger values of friction leading to smaller changes of the more stable networks.

Theoretical Study on Inhibition of Oxygen Evolution in Layered Cathode Materials during Charging

In recent years, Ni, Mn-based layered cathode materials (Li1.2Ni0.2Mn0.6O2, LiNi1-x-yCoxMnyO2) have shown great promise for high energy density LIBs. Research has shown that nickel-rich materials are ideal candidates for high-capacity batteries. The valence state from Ni2+ to Ni4+ allows more Li+ to transfer between the cathode and anode, providing excellent electrochemical performance. However, the oxidation of oxygen atoms triggers irreversible structural rearrangements on the surface, while Ni3+ introduces Jahn-Teller distortion. At high voltages, the stability of layered cathode materials is severely hampered. We present theoretical work on Li1.2Ni0.2Mn0.6O2 and LiNi1-x-yCoxMnyO2 using density functional theory, considering cation- and anion-doped surface treatments. We found that doped layered cathode materials successfully suppressed O2 formation at highly delithiated levels. Through DFT calculations, we understand the mechanism of oxygen evolution from layered cathode materials during the charging process and discover how to suppress oxygen evolution strategies.

Fatigue failure of amorphous alloys under cyclic shear deformation

The accumulation of plastic deformation and flow localization in amorphous alloys under periodic shear are investigated using molecular dynamics simulations. We study a well-annealed binary mixture of one million atoms subjected to oscillatory shear deformation with strain amplitudes slightly above a critical value. We find that upon approaching a critical strain amplitude from above, the number of shear cycles until the yielding transition is well described by a power-law function. Remarkably, the potential energy at the end of each cycle as a function of the normalized number of cycles is nearly independent of the strain amplitude, which allows for estimation of the fatigue lifetime at a given strain amplitude. The analysis of nonaffine displacements of atoms elucidates the process of strain localization, including irreversible rearrangements of small clusters until the formation of a system-spanning shear band.

Heat Generation during the First Activation Cycle of Li-Ion Batteries with Li- and Mn-Rich Layered Oxides Measured by Isothermal Micro-Calorimetry

Using isothermal micro-calorimetry, we investigate the heat generation of lithium- and manganese-rich layered oxides (LMR-NCMs) during the first cycle in which LMR-NCM exhibits a pronounced voltage hysteresis leading to a low energy efficiency (≈73%). In the first charge, LMR-NCM shows a unique voltage plateau at ≈4.5 V where irreversible structural rearrangements lead to an activation of the material as well as a large voltage hysteresis. We found that only a fraction of the lost electrical work (≈43%) is converted into waste heat. Thereby, the heat flow profile of the first charge is unique and shows considerable heat generation during the voltage plateau. With complementary electrochemical methods, contributions of conventional sources of heat, i.e., because of polarization and entropy, are determined. However, they do not cause the considerable generation of heat during the voltage plateau. Our results therefore suggest that the structural rearrangements during activation lead to a significant generation of heat. In window-opening experiments, we demonstrate that the activation is a gradual process and that the heat generated during the first discharge is directly linked to the extent of activation during the preceding charge. We also investigate the effect of the degree of overlithiation on the heat generated during activation.

π-type orbital hybridization and reactive oxygen quenching induced by Se-doping for Li-rich Mn-based oxide cathode

Li-rich Mn-based oxide cathodes for next generation high-energy-density batteries are unprecedentedly enticing; however, its implementation has been largely plagued by capacity fading and potential decline, mainly associated with the irreversible lattice oxygen redox and structure rearrangements. Hereby, electrochemically stable Li-rich Mn-based oxide cathode is successfully designed by manipulating molecular polarity within host structure through the introduction of Se. Notably, the restructured electronic distribution around lattice oxygen is aroused from weak electronegativity of Se in the bulk. It is beneficial for enhancing the π -type orbital hybridization between O 2p and Mn 3d(t 2g ) due to the lowered energy level of O 2p states, resulting in the mitigation of lattice oxygen loss, which is strongly validated by ex-situ soft/hard X-ray absorption spectroscopy coupled with density functional theory calculations. Concomitantly, reactive oxygen species is deactivated with anti-aging effects in the primary/second particle sub-surface, considerably suppressing the S N 2 attack related to electrolyte decomposition and subsequent transition metals dissolution to render a well-knit cathode electrolyte interface, intensively verified by time-off light secondary-ion mass spectrometry. Greatly, the as-designed Se-LRM delivers excellent long cycling stability after 400 loops with only a 0.029% capacity fading and 1.37 mV potential decline per cycle. Given this, this elaborate work might inaugurate a potential avenue for rationally tuning the structure/interface evolution towards advanced electrodes in lithium-ion batteries.

Building interface bonding and shield for stable Li-rich Mn-based oxide cathode

Implementation of Li-rich Mn-based oxide cathode with high-energy-density has been restrained by capacity/voltage degradation that results from irreversible lattice oxygen loss and structure rearrangements. To resolve these challenges, in this work, Li1.2Mn0.54Ni0.13Co0.13O2 encapsulated by amorphous CoxB (CB-LRM) is rationally designed via autocatalytic plating for highly reversible cationic/anionic hybrid cathode material. Band coherency is ingeniously evoked by interface-reconstruction between bulk structure and amorphous coating layer, which lower the energy of O 2p states. This is associated with strengthened orbital hybridization of O 2p-Mn 3d and increased formation energy of oxygen vacancy, which mitigates the lattice oxygen loss considerably. Additionally, interface shielding effects that protect electrode against electrolyte corrosion and the reduction of surface oxygen are also present with fully coverage of amorphous CoxB coating layer. As a result, the as-designed CB-LRM cathode exhibits excellent cycle stability after 100 loops with only 0.154% per cycle capacity fade and improved voltage degradation. Given this, this work provides a potential avenue for rational design of lattice oxygen-based electrode materials with high-energy-density.

Superior Multielectron-Transferring Energy Storage by π-d Conjugated Frameworks.

Reversible multielectron-transfer materials are of considerable interest because of the potential impact to advance present electrochemical energy storage technology by boosting energy density. To date, a few oxide-based materials can reach an electron-transfer number per metal-cation (eM ) larger than 2 upon a (de)intercalation mechanism. However, these materials suffer from degradation due to irreversible rearrangements of the cation-oxygen bonds, and are based on precious metals, for example, Ir and Ru. Hence, a design of the non-oxide-based reversible multielectron-transfer materials with abundant elements can provide a promising alternative. Herein, it is demonstrated that the bis(diimino)copper framework can show eM = 3.5 with cation/anion co-redox mechanism together with a dual-ion mechanism. In this study, the role of the cation-anion interactions is unveiled by using an experiment/theory collaboration applied to a series of the model non-oxide abundant electrode systems based on different metal-nitrogen bonds. These models provide designer multielectron-transfer due to the tunable π-d conjugated electronic structures. It is found that the Cu-nitrogen bonds show a unique reversible rearrangement upon Li-intercalation, and this process responds to acquire a significant reversible multielectron-transfer. This work provides new insights into the affordable multielectron-transfer electrodes and uncovers an alternative strategy to advance the electrochemical energy storage reactions.

Effect of adhesive interaction on strain stiffening and dissipation in granular gels undergoing yielding

Shear induced yielding in disordered solids, characterized by irreversibility and enhanced dissipation, is important for a wide range of industrial and geological processes. Although such phenomena in thermal systems have been extensively studied, they remain poorly understood for granular solids. Here, using oscillatory shear rheology we study energy dissipation in a disordered solid formed by dense granular suspensions of adhesive frictional particles. We find non-linear flow regimes showing intra-cycle strain stiffening and plasticity that strongly depend on both the applied strain amplitude and particle volume fraction, which can be captured by the normalized energy dissipation. Furthermore, in-situ optical imaging reveals irreversible particle rearrangements correlating with the spatio-temporal fluctuations in local velocity across the yielding transition. By directly measuring the critical jamming packing fraction using particle settling experiments, we propose a phase diagram that unravels the effect of inter-particle interactions on flow properties of the system for a large parameter space.

How vertical oscillatory motion above a saturated sand bed leads to heap formation.

We show how oscillations in fluid flow over a fluid-saturated and porous sediment bed leads to the development of a bedform. To understand the role of pressure fluctuations on the bed associated with flow oscillations, we analyze how the flow penetrates into and through the bed. We then calculate the corresponding vertical pressure gradients within the bed that tend to expand the bed along the vertical direction. When these pressure gradients are large enough, they facilitate small irreversible rearrangements of the grains within the bed, and so cause granular creep. We conjecture that this granular creep alternates with jamming to produce a granular ratchet that slowly lifts the surface of the bed locally where pressure gradients dominate, and depresses the surface where shear stresses dominate. We observe that the shape of the resulting heap exhibits a constant characteristic width. The height of this heap evolves approximately as the square root of time, in agreement with dimensional arguments predicated on a coarse-grained viscous deformation of the bed. The surface of the heap contracts initially with the square root of time, consistent with an incompressible analysis of the flow of grains within the heap. Near its peak the heap grows due to a dilatation of the bed, to inward radial flux, or to a combination of the two.

Reversible motions and disordered structure of soft particles in amorphous solids

In amorphous solids, soft vibrational modes derived from normal mode analysis can be utilized to identify the soft particles that are prone to irreversible rearrangements. However, the normal mode analysis approach cannot explain why the spatial distributions of clustered soft particles do not change over time. We define a softness parameter based on the vibrational density of states calculated directly from molecular dynamics simulations with both the harmonic vibrations and anharmonic relaxations being properly captured at finite temperatures. This parameter spontaneously correlates with the real space atomic motions and the dynamics heterogeneity. Using the softness parameter, we show that the softest particles are confined within rigid cages. These particles keep rearranging reversibly without long-range diffusion. The moderately soft particles rearrange irreversibly, and the hard particles mainly participate in vibrations without rearrangement. We also show that the soft particles form locally disordered structures, while the hard particles present strong ordering. These findings confirm the defective nature of soft particles, and provide insights on the nature of softness as the ability to rearrange, but not necessarily irreversibly.

Glycosaminoglycans and glycoproteins influence the elastic response of synovial fluid nanofilms on model oxide surfaces

Synovial fluid (SF) is the natural lubricant found in articulated joints, providing unique cartilage surface protecting films under confinement and relative motion. While it is known that the synergistic interactions of the macromolecular constituents provide its unique load-bearing and tribological performance, it is not fully understood how two of the main constituents, glycosaminoglycans (GAGs) and glycoproteins, regulate the formation and mechanics of robust load-bearing films. Here, we present evidence that the load-bearing capabilities, rather than the tribological performance, of the formed SF films depend strongly on its components' integrity. For this purpose, we used a combination of enzymatic treatments, quartz crystal microbalance with dissipation (QCM-D), and the surface forces apparatus (SFA) to characterize the formation and load-bearing capabilities of SF films on model oxide (i.e., silicates) surfaces. We find that, upon cleavage of proteins, the elasticity of the films is reduced and that cleaving GAGs results in irreversible (plastic) molecular re-arrangements of the film constituents when subjected to confinement. Understanding thin film mechanics of SF can provide insight into the progression of diseases, such as arthritis, but may also be applicable to the development of new implant surface treatments or new biomimetic lubricants.

A topological view on microscopic structural evolution for granular material under loading and unloading path

Granular materials demonstrate complex mechanical behaviors due to the irreversible rearrangements at particle scale. The irreversible behaviors of granular materials can be observed after unloading the granular assembly. In this study, biaxial tests of dense and loose granular assemblies are conducted using discrete element method (2D DEM). We investigate the multiscale behaviors of granular materials under loading–unloading-reloading path. Both dense and loose granular assemblies show hysteresis loops, but their underlying mechanisms and meso-structural evolutions are different. The hysteretic behavior of the dense sample is relevant to the dynamics of contact force network and the evolution of contact loops. The approximate linearity of stress–strain relation during unloading and reloading stages is attributed to the robustness of strong contact network, while the nonlinearity of stress–strain relation is related to the evolution of contact topology. Specifically, loops of 4-sides within the contact network play an important role by transforming into smaller or larger loops by losing and joining particles during the unloading and reloading path. The loose compactness of granular media results in the unapparent evolution of structures at the mesoscale. Besides, the roles of strong and weak subnetworks are not distinguished. The fewer changes in contact loops may cause unconspicuous nonlinear deformation.

(Invited) O-Redox Cathodes the Role of Trapped O2

The energy density of Li-ion batteries can be improved by storing charge at high voltages through the oxidation of oxide ions in the cathode material. However, oxidation of O2− triggers irreversible structural rearrangements in the bulk and an associated loss of the high voltage plateau, which is replaced by a lower discharge voltage, and a loss of O2 accompanied by densification at the surface. Understanding the O-redox process and the nature of oxidised oxygen has proved very challenging.Using a range of techniques including XAS, RIXS, STEM, NMR, diffraction and DFT, applied across a wide range of alkali metal rich transition metal oxides, reveals that O2- is oxidised to O2.1-4 The O2 is either evolved from the surface or trapped in voids formed in the bulk by reorganisation of vacancies within the structure. Although O2 can be reduced back to O2-, the process is not energetically reversible, explaining the much lower voltage on discharge compare with the first charge (approx. 1eV less), the phenomenon of so-called voltage hysteresis. Furthermore, it is possible to suppress O2 formation, trapping hole states on O2- and obtaining energetic (voltage) and structural reversibility.4 Such behaviour points the way towards high energy density cathodes for Li-ion batteries. House, R. A. et al. First cycle voltage hysteresis in Li-rich 3d cathodes associated with molecular O2 trapped in the bulk. Nature Energy 8, 777–785 (2020).House, R. A. et al. The role of O2 in O-redox cathodes for Li-ion batteries. Nature Energy 1–9 (2021).Boivin, E. et al. The Role of Ni and Co in Suppressing O‐Loss in Li‐Rich Layered Cathodes. Advanced Functional Materials 2003660 (2020).House, R. A. et al. Superstructure control of first-cycle voltage hysteresis in oxygen-redox cathodes. Nature 577, 502–508 (2020).

Un-jamming due to energetic instability: statics to dynamics

Jamming/un-jamming, the transition between solid- and fluid-like behavior in granular matter, is an ubiquitous phenomenon in need of a sound understanding. As argued here, in addition to the usual un-jamming by vanishing pressure due to a decrease of density, there is also yield (plastic rearrangements and un-jamming that occur) if, e.g., for given pressure, the shear stress becomes too large. Similar to the van der Waals transition between vapor and water, or the critical current in superconductors, we believe that one mechanism causing yield is by the loss of the energy’s convexity (causing irreversible re-arrangements of the micro-structure, either locally or globally). We focus on this mechanism in the context of granular solid hydrodynamics (GSH), generalized for very soft materials, i.e., large elastic deformations, employing it in an over-simplified (bottom-up) fashion by setting as many parameters as possible to constant. Also, we complemented/completed GSH by using various insights/observations from particle simulations and calibrating some of the theoretical parameters—both continuum and particle points of view are reviewed in the context of the research developments during the last few years. Any other energy-based elastic-plastic theory that is properly calibrated (top-down), by experimental or numerical data, would describe granular solids. But only if it would cover granular gas, fluid, and solid states simultaneously (as GSH does) could it follow the system transitions and evolution through all states into un-jammed, possibly dynamic/collisional states—and back to elastically stable ones. We show how the un-jamming dynamics starts off, unfolds, develops, and ends. We follow the system through various deformation modes: transitions, yielding, un-jamming and jamming, both analytically and numerically and bring together the material point continuum model with particle simulations, quantitatively.Graphic abstract

The role of O2 in O-redox cathodes for Li-ion batteries

The energy density of Li-ion batteries can be improved by storing charge at high voltages through the oxidation of oxide ions in the cathode material. However, oxidation of O2− triggers irreversible structural rearrangements in the bulk and an associated loss of the high voltage plateau, which is replaced by a lower discharge voltage, and a loss of O2 accompanied by densification at the surface. Here we consider various models for oxygen redox that are proposed in the literature and then describe a single unified model involving O2− oxidation to form O2, most of which is trapped in the bulk and the remainder of which evolves from the surface. The model extends the O2 formation and evolution at the surface, which is well known and well characterized, into the electrode particle bulk as caged O2 that can be reversibly reduced and oxidized. This converged understanding enables us to propose practical strategies to avoid oxygen-redox-induced instability and provide potential routes towards more reversible, high energy density Li-ion cathodes.

Shear Banding in a Contact Problem between Metallic Glasses

The case of a frictionless contact between a spherical body and a flat metallic glass is studied using a mesoscopic description of plasticity combined with a semi-analytical description of the elastic deformation in a contact geometry (code ISAAC). Plasticity is described by irreversible strain rearrangements in the maximum deviatoric strain direction, above some random strain threshold. In the absence of adhesion or friction, the plastic deformation is initiated below the surface. To represent the singularities due to adhesion, initial rearrangements are forced at the boundary of the contact. Then, the structural disorder is introduced in two different levels: either in the local strain thresholds for plasticity or in the residual plastic strains. It is shown that the spatial organization of plastic rearrangements is not universal, but it is very dependent on the choice of disorder and external loading conditions. Spatial curved shear bands may appear below the contact but only for a very specific set of parameters, especially those characterizing the random thresholds compared to externally induced strain gradients.

Accelerated rejuvenation in metallic glasses subjected to elastostatic compression along alternating directions

The influence of static stress and alternating loading direction on the potential energy and mechanical properties of amorphous alloys is investigated using molecular dynamics simulations. The model glass is represented via a binary mixture which is first slowly annealed well below the glass transition temperature and then subjected to elastostatic loading either along a single direction or along two and three alternating directions. We find that at sufficiently large values of the static stress, the binary glass becomes rejuvenated via collective, irreversible rearrangements of atoms. Upon including additional orientation of the static stress in the loading protocol, the rejuvenation effect is amplified and the typical size of clusters of atoms with large nonaffine displacements increases. As a result of prolonged mechanical loading, the elastic modulus and the peak value of the stress overshoot during startup continuous compression become significantly reduced, especially for loading protocols with alternating stress orientation. These findings are important for the design of novel processing methods to improve mechanical properties of metallic glasses.

Origin of reversible and irreversible atomic-scale rearrangements in a model two-dimensional network glass.

In this contribution, we investigate the fundamental mechanism of plasticity in a model two-dimensional network glass. The glass is generated by using a Monte Carlo bond-switching algorithm and subjected to athermal simple shear deformation, followed by subsequent unloading at selected deformation states. This enables us to investigate the topological origin of reversible and irreversible atomic-scale rearrangements. It is shown that some events that are triggered during loading recover during unloading, while some do not. Thus, two kinds of elementary plastic events are observed, which can be linked to the network topology of the model glass.

Non-local cooperative atomic motions that govern dissipation in amorphous tantala unveiled by dynamical mechanical spectroscopy

The mechanisms governing mechanical dissipation in amorphous tantala are studied at microscopic scale via Molecular Dynamics simulations, namely by mechanical spectroscopy in a wide range of temperature and frequency. We find that dissipation is associated with irreversible atomic rearrangements with a sharp cooperative character, involving tens to hundreds of atoms arranged in spatially extended clusters of polyhedra. Remarkably, at low temperature we observe an excess of plastically rearranging oxygen atoms which correlates with the experimental peak in the macroscopic mechanical losses. A detailed structural analysis reveals preferential connections of the irreversibly rearranging polyhedra, corresponding to edge and face sharing. These results might lead to microscopically informed design rules for reducing mechanical losses in relevant materials for structural, optical, and sensing applications.