Bulk Phase Transformations Research Articles

Two-step nucleation and crystal growth in a metastable solution

This study is concerned with a theory of two-step nucleation and growth of crystals in a metastable liquid. This mechanism is that crystalline nuclei formation occurs in dense liquid clusters suspended in the solution. These clusters contain higher solution concentration and viscosity, leading to a lower surface free energy barrier and faster phase transition route. The theory is based on growth laws of crystals during the two-step bulk phase transformation. At the initial stage, the crystals evolve in a diffusion-limited environment with almost unchanged supersaturation. At the second stage, they become larger, move beyond these clusters, and evolve in accordance with a hyperbolic tangent law. A generalized particle growth law joining the first and second stages is obtained by stitching the diffusion limited and hyperbolic tangent laws. On this basis, an integrodifferential model of the evolution of a polydisperse ensemble of crystals was formulated and solved. The crystal-size distribution function increases and the solution supersaturation remains practically unchanged until the particle size corresponds to a transition in the particle growth rate from a diffusion-limited branch to a hyperbolic tangent branch. This is followed by an increase in the crystal growth rate, a decrease in the distribution function and solution supersaturation. Then the distribution function increases up to the maximum size of crystals grown in the solution. A sufficiently long time interval of almost constant supersaturation and the N-shaped behavior of the distribution function are the consequences of a two-step nucleation and growth mechanisms.

Parasitic-Reaction-Triggered Performance Deterioration of Long-Term Cycling Nickel-Rich Cathodes

Ni-rich LiNi1-x-yMnxCoyO2 (NMC, x+y < 0.5) materials are widely regarded as the promising cathodes for high-energy-density and cost-effective lithium-ion batteries. Cycling performance deterioration is a long-standing challenge for Ni-rich NMC, especially at high potentials (>4.3V vs. Li/Li+). The fundamental understanding of failure mechanisms is crucial for the development of long-life lithium-ion technologies. In our investigation, full cells cycled to 4.35 V for 1000 cycles lose about 49% of their initial capacity, while the counterparts cycled to 4.15V lose only 27% of their initial capacity after 1000 cycles. The relatively low capacity loss of the low voltage cells (4.15 V) is dominated by the impedance hike, while the drastically increased (more than half of) capacity loss of high voltage cells (4.35 V) is contributed from the irreversible degradation of the electrode materials. The post-mortem diagnosis suggests that parasitic reactions are the primary driving force for severe deteriorations, including the irreversible phase transformation. The generally concerned transition metal dissolution and bulk phase transformation negligibly make direct contributions to the severe capacity loss, but the loss of active cathode materials has some substantially detrimental impacts on irreversible capacity loss. Our findings emphasize that mitigating parasitic reactions is crucial for enabling long life of Ni-rich cathodes. Figure 1

Creation of an extrinsic pseudocapacitive material presenting extraordinary cycling-life with the battery-type material Co(OH)2 by S2− doping for application in supercapacitors

Some battery-type materials show the potential for storing high density energy in supercapacitors because of their high theoretical capacity. So far, the very unsatisfied electrochemical performance is still the label of these battery-type materials, which is attributed to their diffusion-controlled kinetics of energy storage. Some efforts try to transform their electrochemical behavior to extrinsic pseudocapacitive behavior through nanostructuring. However, the extrinsic pseudocapacitive materials are rare due to the restriction of no phase transformation. In this work, we propose an alternative strategy to create an extrinsic pseudocapacitive material from the typical battery-type material Co(OH)2 by S2− doping. The doped S2− ions further increase energybarrierfor the intercalation of OH− ions into the interlayer space of Co(OH)2, thus suppressing its bulk phase transformation. Also, the decreased OH− ions-Co(OH)2 interaction plus the surface reaction feature induced by S2− doping results in the pseudocapacitive behavior. Thus, the S2− doped Co(OH)2 presents the comprehensive electrochemical signatures of pseudocapacitive materials and an extraordinary cycling life of 200,000 charge-discharge cycles. This work initiates a new way for acquiring electrode materials with high electrochemical performance for application in supercapacitors.

Vital Role of Synthesis Temperature in Co–Cu Layered Hydroxides and Their Fenton-like Activity for RhB Degradation

Cu and Co have shown superior catalytic performance to other transitional elements, and layered double hydroxides (LDHs) have presented advantages over other heterogeneous Fenton catalysts. However, there have been few studies about Co–Cu LDHs as catalysts for organic degradation via the Fenton reaction. Here, we prepared a series of Co–Cu LDH catalysts by a co-precipitation method under different synthesis temperatures and set Rhodamine B (RhB) as the target compound. The structure-performance relationship and the influence of reaction parameters were explored. A study of the Fenton-like reaction was conducted over Co–Cu layered hydroxide catalysts, and the variation of synthesis temperature greatly influenced their Fenton-like catalytic performance. The Co–Cut=65°C catalyst with the strongest LDH structure showed the highest RhB removal efficiency (99.3% within 30 min). The change of synthesis temperature induced bulk-phase transformation, structural distortion, and metal–oxygen (M–O) modification. An appropriate temperature improved LDH formation with defect sites and lengthened M–O bonds. Co–Cu LDH catalysts with a higher concentration of defect sites promoted surface hydroxide formation for H2O2 adsorption. These oxygen vacancies (Ovs) promoted electron transfer and H2O2 dissociation. Thus, the Co–Cu LDH catalyst is an attractive alternative organic pollutants treatment.

Enhanced Activity and Reversibility of Anionic Redox by Tuning Lithium Vacancies in Li-Rich Cathode Materials.

Li-rich Mn-based layered oxide cathodes (LLOs) are considered to be the most promising cathode candidates for lithium-ion batteries owing to their high-voltage platform and ultrahigh specific capacity originating from anionic redox. However, anionic redox results in many problems including irreversible oxygen release, voltage hysteresis, and so on. Although many efforts have been made to regulate anionic redox, a fundamental issue, the effect of lithium vacancies on anionic redox, is still unclear. Herein, we synthesized a series of LLO materials with different lithium vacancy contents by controlling the amount of lithium salt. Specifically, lithium-vacancy-type LLOs Li1.11Ni0.18Co0.18Mn0.53O2 with a pompon morphology exhibit an ultrahigh specific capacity (293.9 mA h g-1 at 0.1 C), an outstanding long-term cycling stability (173.5 mA h g-1 after 300 cycles at 1 C), and an excellent rate performance (106 mA h g-1 at 10 C). It reveals that lithium vacancy is a key factor to enhance anionic redox activity and reversibility. Lithium vacancies exhibit different inductive effects on the structure of the surface and bulk. Abundant surface oxygen vacancies and a surface spinel phase layer induced by lithium vacancies suppress irreversible oxygen release, while the bulk phase transformation and cation disorder combined with sufficient lithium vacancies in the bulk stabilize structure and improve anionic redox kinetic. The findings offer a significant theoretical guidance for the practical application of LLO materials.

An Investigation into the Bulk and Surface Phase Transformations of Bimetallic Pd-In/Al2O3 Catalyst during Reductive and Oxidative Treatments In Situ

A series of oxidative treatments of PdIn-supported intermetallic nanoparticles at different temperatures were performed. The bulk and surface structure of catalyst during phase transformation was investigated by bulk- and surface-sensitive techniques (in situ XAFS, DRIFTS of adsorbed CO). It was found that comparison of palladium and indium fractions in bulk and on the surface suggests the formation of a «core-shell» structure. According to obtained results, the core consists of In-depleted intermetallic compound or inhomogeneous bimetallic phase with the inner core of metallic Pd, when a mixture of indium oxide, metallic palladium and small part of PdIn is present on the surface.

From material properties to multiscale modeling to improve lithium-ion energy storage safety

Energy storage using lithium-ion cells dominates consumer electronics and is rapidly becoming predominant in electric vehicles and grid-scale energy storage, but the high energy densities attained lead to the potential for release of this stored chemical energy. This article introduces some of the paths by which this energy might be unintentionally released, relating cell material properties to the physical processes associated with this potential release. The selected paths focus on the anode–electrolyte and cathode–electrolyte interactions that are of typical concern for current and near-future systems. Relevant material processes include bulk phase transformations, bulk diffusion, surface reactions, transport limitations across insulating passivation layers, and the potential for more complex material structures to enhance safety. We also discuss the development, parameterization, and application of predictive models for this energy release and give examples of the application of these models to gain further insight into the development of safer energy storage systems.

High temperature nanoindentation as a tool to investigate plasticity upon phase transformations demonstrated on Cobalt

A new field of application for high temperature nanoindentation as a complimentary method to understand the mechanics of plasticity upon bulk phase transformations in thermodynamic equilibrium is introduced. The feasibility is outlined on polycrystalline Cobalt involving a low-temperature hexagonal closed packed phase, and above 700K, a high-temperature face centered cubic phase, which was conventionally characterized by means of differential scanning calorimetry and high temperature X-ray diffraction. Strain rate sensitivity, activation volume and activation energy of plastic deformation were determined up to 873 K to identify the rate-controlling deformation mechanism. From RT to 473 K plasticity was found to be controlled by lattice friction on the basal plane, where dislocations have to overcome the Peierls barrier and deformation twinning was apparent in the hexagonal phase. The thermal activation of cross slip lead to reduced twinning actions when the phase transition temperature is approached. In the high temperature face centered cubic phase deformation was found to be controlled by cutting of forest dislocations and no deformation twinning was observed.

Pop-In Phenomenon as a Fundamental Plasticity Probed by Nanoindentation Technique.

The attractive strain burst phenomenon, so-called “pop-in”, during indentation-induced deformation at a very small scale is discussed as a fundamental deformation behavior in various materials. The nanoindentation technique can probe a mechanical response to a very low applied load, and the behavior can be mechanically and physically analyzed. The pop-in phenomenon can be understood as incipient plasticity under an indentation load, and dislocation nucleation at a small volume is a major mechanism for the event. Experimental and computational studies of the pop-in phenomenon are reviewed in terms of pioneering discovery, experimental clarification, physical modeling in the thermally activated process, crystal plasticity, effects of pre-existing lattice defects including dislocations, in-solution alloying elements, and grain boundaries, as well as atomistic modeling in computational simulation. The related non-dislocation behaviors are also discussed in a shear transformation zone in bulk metallic glass materials and phase transformation in semiconductors and metals. A future perspective from both engineering and scientific views is finally provided for further interpretation of the mechanical behaviors of materials.

Stacking Fault Driven Phase Transformation in CrCoNi Medium Entropy Alloy

Phase transformation is an effective means to increase the ductility of a material. However, even for a commonly observed face-centered-cubic to hexagonal-close-packed (fcc-to-hcp) phase transformation, the underlying mechanisms are far from being settled. In fact, different transformation pathways have been proposed, especially with regard to nucleation of the hcp phase at the nanoscale. In CrCoNi, a so-called medium-entropy alloy, an fcc-to-hcp phase transformation has long been anticipated. Here, we report an in situ loading study with neutron diffraction, which revealed a bulk fcc-to-hcp phase transformation in CrCoNi at 15 K under tensile loading. By correlating deformation characteristics of the fcc phase with the development of the hcp phase, it is shown that the nucleation of the hcp phase was triggered by intrinsic stacking faults. The confirmation of a bulk phase transformation adds to the myriads of deformation mechanisms available in CrCoNi, which together underpin the unusually large ductility at low temperatures.

In-situ neutron diffraction study of wrought and selective laser melted maraging stainless steels

Bulk phase transformations and dislocation density were monitored in a maraging stainless steel on a bulk level using in-situ neutron diffraction up to 1340 °C, i.e. 30 °C below the melting temperature. Three materials with different initial microstructure and/or different composition were studied: wrought, as-built SLM-ed (selective laser melted) and re-austenitized SLM-ed. In contrast to the wrought martensitic steel, the as-built SLM-ed steel was essentially ferritic. However, re-austenitized SLM-ed steel recovered the usual martensitic microstructure. A δ-ferritic domain above 1200 °C was confirmed for all the materials studied. However, the amount of δ-ferrite at the melting point strongly depends upon steel composition. Additionally, it was shown that the initial microstructure of the steel (ferritic or martensitic) has little to no influence on re-austenization. Dislocation densities were estimated from diffraction peak broadening. The ferritic as-built SLM material contains a high dislocation density (~4 × 1014 m−2), which is however far less than in the martensitic materials (~5 × 1015 m−2). Dislocations start to annihilate from 550 °C / 600 °C in all the materials studied, but a measurable dislocation density of ~1013 m−2 is still observed at 950 °C / 1000 °C.

Bulk fatigue induced by surface reconstruction in layered Ni-rich cathodes for Li-ion batteries.

Ni-rich layered cathode materials are among the most promising candidates for high-energy-density Li-ion batteries, yet their degradation mechanisms are still poorly understood. We report a structure-driven degradation mechanism for NMC811 (LiNi0.8Mn0.1Co0.1O2), in which a proportion of the material exhibits a lowered accessible state of charge at the end of charging after repetitive cycling and becomes fatigued. Operando synchrotron long-duration X-ray diffraction enabled by a laser-thinned coin cell shows the emergence and growth in the concentration of this fatigued phase with cycle number. This degradation is structure driven and is not solely due to kinetic limitations or intergranular cracking: no bulk phase transformations, no increase in Li/Ni antisite mixing and no notable changes in the local structure or Li-ion mobility of the bulk are seen in aged NMCs. Instead, we propose that this degradation stems from the high interfacial lattice strain between the reconstructed surface and the bulk layered structure that develops when the latter is at states of charge above a distinct threshold of approximately 75%. This mechanism is expected to be universal in Ni-rich layered cathodes. Our findings provide fundamental insights into strategies to help mitigate this degradation process.

Halide Perovskites – Optoelectronic and Structural Characterization Methods

Halide Perovskites – Optoelectronic and Structural Characterization Methods

Photocatalytic TiO₂ Nanoparticles Activated by Dielectric Barrier Discharge Plasma Assisted Ball Milling.

Developing defects on the surface of TiO₂ nanoparticles by mechanical treatment is a fascinating approach to enhance photocatalytic efficiency. However, it poses risks to producing bulk defects and phase transformation, which seriously deteriorates photocatalytic performance. Here, activating TiO₂ nanoparticles was elaborately fulfilled by using dielectric barrier discharge plasma assisted ball milling (DBDP-milling) as it imposes plasma to nano-scaled particles along with soft mechanical impact. The evolution of surface properties of TiO₂ nanoparticles with DBDP-milling durations was inspected using a Fourier transform infrared (FTIR) and an X-ray photoelectron spectrometer (XPS), while the morphology and structure were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy. Results showed that DBDP-milling developed a considerable number of oxygen vacancies in TiO₂ nanoparticles as well as bulk defects free. These TiO₂ nanoparticles formed agglomerates with BET surface area of 37.40 m2/g and exhibited enhanced photocatalytic efficiency.

Flash sintering activated by bulk phase and grain boundary complexion transformations

A naturally-occurring coupled thermal and electric runaway, resulted from an Arrhenius temperature-dependent specimen conductivity, can trigger flash sintering in many ceramics. This study reveals another possibility to activate flash sintering: a bulk phase transformation or a grain boundary (phase-like) complexion transition can cause an abrupt rise in the specimen conductivity to jump start flash sintering (prior to the occurrence of a natural thermal runaway). In undoped and Al2O3-doped ZnO, the flash sintering is activated by natural thermal runways that can be quantitively predicted from an Arrhenius extrapolation of low-temperature specimen conductivity. In contrast, a bulk eutectic reaction and the associated formation of premelting-like intergranular films (IGFs) in Bi2O3-doped ZnO can lead to a nonlinear rise in the specimen conductivity (above the Arrhenius extrapolation) to trigger flash sintering prior to the occurrence of the predicted natural thermal runaway. Yet, a natural thermal runaway can still take place in Bi2O3-doped ZnO before the occurrence of the interfacial and bulk transformation if the initial electric field is increased to a sufficiently high level. All five cases can be fully explained in a consistent framework so that this set of experiments systematically validate our theory of flash initiation. This work uncovers the roles of the bulk phase and interfacial (phase-like) complexion transformations in initiating flash sintering, thereby suggesting a new direction to understand and tailor the flash sintering process. An observation of ultra-fast field-induced migration of aliovalent cations during the flash sintering of Al2O3-doped ZnO is also reported.

Elucidating Surface and Bulk Phase Transformation in Fischer–Tropsch Synthesis Catalysts and Their Influences on Catalytic Performance

Understanding the structure–performance relationship for Fe-based Fischer–Tropsch synthesis (FTS) catalysts is crucial for the design of improved FTS catalysts. Herein, catalysts derived from four ...

Three-Dimensional Compositional and Charge Heterogeneities in Sodium Layered Oxide Cathode Materials

Rechargeable energy storage devices have revolutionized the modern electronic industries and electric vehicles. Li-ion batteries with lithium layered oxide cathode materials have mainly dominated the market so far. However, energy storage devices based on sodium layered oxide cathode materials have drawn renewed attention because of the wide availability and cheap sodium resources. These devices are also particularly suitable for some applications such as grid energy storage, which require load leveling and adjusting to intermittent energy production from the renewable energy resources. To date, sodium-ion batteries based on layered oxide cathode materials are far from commercialization because of the low power and energy density as well as low round-trip efficiency because of the bulk phase transformation and surface degradation. Thus, novel design strategies must be undertaken to improve the electrochemical performance of these sodium layered oxide cathode materials. We have designed a highly compositionally heterogeneous Na0.9Cu0.2Fe0.28Mn0.52O2 cathode material for sodium-ion batteries. The material delivers a capacity of 125 mAh/g at C/10 with no capacity fading after 100 cycles and 75 mAh/g at 1C with negligible capacity fading after 200 cycles. In addition to the compositional heterogeneity, we have further demonstrated the heterogeneous nature of redox chemistry of the cathode material. The surface redox behavior of the transition metals is different than that in the bulk. In the bulk, both Cu2+ and Fe3+ play an active role in the redox chemistry of the cathode material whereas the surface redox chemistry is mainly dominated by Mn4+ ion. Furthermore, we also demonstrate that Mn segregation on long term cycling may play a pivotal role for the performance degradation. Through this study, we demonstrate that there is further room for improvement for sodium layered oxide cathode materials by tuning the nano/mesoscale elemental distribution. Tunable elemental distribution may also enable favorable surface chemistry of cathode materials that can account for the Mn segregation on long term cycling and enhance the cycle life of sodium-ion batteries.

First-principles calculations of high pressure phase transition of Ce-La-Th alloy

The lanthanide and actinide metals and alloys are of great interest in experimental and theoretical high-pressure research, because of the unique behavior of the f electrons under pressure and their delocalization and participation in bonding. Cerium (Ce) metal is the first lanthanide element with a 4f electron. It has a very complex phase diagram and displays intriguing physical and chemical properties. In addition, it is expected to be an excellent surrogate candidate for plutonium (Pu), one of the radioactive transuranic actinides with a 5f electron. The bulk properties and phase transformation characteristics of Ce-based alloys are similar to those of Pu and its compounds. Thus, the investigations of Ce-based alloys are necessary and can potentially advance the understanding of the behavior of Pu. In this work, the equation of state, phase transition, elastic and thermodynamic properties of Ce&lt;sub&gt;0.8&lt;/sub&gt;La&lt;sub&gt;0.1&lt;/sub&gt;Th&lt;sub&gt;0.1&lt;/sub&gt; alloy at high pressure are investigated by using first-principles calculations based on the density-functional theory. The structural properties of the Ce&lt;sub&gt;0.8&lt;/sub&gt;La&lt;sub&gt;0.1&lt;/sub&gt;Th&lt;sub&gt;0.1&lt;/sub&gt; alloy are in good agreement with the available experimental and theoretical data. The lattice constant &lt;i&gt;a&lt;/i&gt; decreases with pressure increasing, while &lt;i&gt;c&lt;/i&gt; shows an opposite variation. It is found that the lattice parameter &lt;i&gt;c&lt;/i&gt; shows abnormal jump. And the critical volume is located at 20.1 Å&lt;sup&gt;3&lt;/sup&gt;. The axial ratio jumps from a value of about &lt;inline-formula&gt;&lt;tex-math id="M90"&gt;\begin{document}$\sqrt 2 $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20182128_M90.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20182128_M90.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; (corresponding to the fcc structure) to a higher value, which indicates that the fcc-bct transition occurs. And the corresponding transition pressure is located at ~31.6 GPa. When the pressure rises to 34.9 GPa, the bct structure displays a saturated &lt;i&gt;c/a&lt;/i&gt; axial ratio close to about 1.67. The Young's modulus &lt;i&gt;E&lt;/i&gt;, shear modulus &lt;i&gt;G&lt;/i&gt; and the Debye temperature of the fcc phase tend to be " softened” around the phase transition pressure. The vibrational free energy is obtained by using the quasi-harmonic Debye model. And then the thermodynamic properties including the thermal equation of state, heat capacity and entropy under high pressure and high temperature are also predicted successfully. The results show that the heat capacity and entropy increase rapidly with temperature increasing, and decrease with pressure increasing. The high pressure can suppress part of the anharmonicity caused by temperature.

The Secret Life of LiFePO4 Particles

In a recent issue of Nature Materials, Yiyang Li, Saiful Islam, Martin Bazant, William Chueh, and colleagues identify the major role of solvent-assisted lithium migration in LiFePO4 particles along the solid/liquid interface using a combination of X-ray diffraction, microscopy experiments, and ab initio molecular dynamics simulations. This finding suggests that at the particle scale LiFePO4 effectively becomes a three-dimensional Li conductor, and follow-up phase-field simulations suggest that lowering surface diffusivity is a predominant factor in determining the bulk phase transformation behavior during cycling.

Theoretical Understanding and Experimental Suppression of the Capacity Degradation of Ni-Rich Layered Oxide Cathode Materials for Li Ion Battery

Ni-rich layered oxides LiNi1-x M x O2 (x = 0.1-0.2, M = Ni, Co, Mn, Al, etc.) have been widely viewed as the next generation cathode materials for Li ion battery, due to their high capacities (>200 Ah/kg), high energy densities (>800 Wh/kg), high ionic diffusivity (10-8 cm2/s) and high electronic conductivity (10-1 S/cm).[1] However, these oxides have faced a big challenge of severe structural instability. Depending on different particle sizes, morphologies and doping/alloying treatments, the materials normally have 40%-80% capacity degradation after ~100 charging/discharging cycles. Based on traditional understanding of layered oxides, three main hypothesis[2-3] have been proposed within battery research community: 1) surface phase transition and interaction, 2) bulk phase transformation, and 3) mechanical degradation involving inter-grain crack formation. However, it is still not clear which mechanism is playing the dominating role during degradation of Ni-rich layered oxides. Unfortunately, due to lack of systematic theoretical study on the thermodynamic and kinetic process of degradation upon delithiation, the fundamental driving forces and the relative kinetics are poorly understood. Therefore, in this work, we report our recent results from atomic scale Density Functional Theory modeling, which carefully simulates the processes relevant to degradation of Ni-rich layered oxides, from which we have delivered significant theoretical understanding of the above mentioned three hypothesis. Furthermore, based on these theoretical understandings, we have performed experiments to validate the proposed surface modifications through both conventional wet chemistry coating and advanced particle Atomic Layer Deposition (ALD). These computational and experimental combined studies could provide us deeper understanding of degradation of Ni-rich layered oxides, and shed light on the future direction towards stabilized high energy cathode materials. [1] J. Xu, F. Lin, M. M. Doeff, W. Tong, J. Mater. Chem. A, 2017, 5, 874 [2] M. D. Radin, Y. S. Meng, A. Van der Ven, etc., Adv. Energy Mater. 2017, 1602888 [3] W. Li, B. Song, A. Manthiram, Chem. Soc. Rev., 2017, 46, 3006