Phase Transformations In Solid Solutions Research Articles

LiFe0.3Mn0.7PO4-on-MXene heterostructures as highly reversible cathode materials for Lithium-ion batteries

Two-dimensional (2D) heterostructure materials, incorporating the collective strengths and synergetic properties of individual building blocks, have attracted great interest as a novel paradigm in electrode materials science. The family of 2D transition metal carbides and nitrides (e.g., MXenes) has become an appealing platform for fabricating functional materials with strong application performance. Herein, a 2D LiFe0.3Mn0.7PO4 (LFMP)–on-MXene heterostructure composite is prepared through an electrostatic self-assembly procedure. The functional groups on the surface of MXenes possess highly electronegative properties that facilitate the incorporation of LFMPs into MXenes to construct heterostructure composites. The special heterostructure of nanosized-LiFe0.3Mn0.7PO4 and MXene provides rapid Li+ and electron transport in the cathodes. This LiFe0.3Mn0.7PO4-3.0 wt% MXene composite can exhibit an excellent rate capability of 98.3 mAh g−1 at 50C and a very stable cycling performance with a capacity retention of 94.3 % at 5C after 1000 cycles. Furthermore, NaFe0.3Mn0.7PO4-3.0 wt% MXene with stable cyclability can be obtained by an electrochemical conversion method with LiFe0.3Mn0.7PO4-3.0 wt% MXene. Ex-situ XRD suggests that LiFe0.3Mn0.7PO4-on-MXene achieves a highly reversible structural evolution with a solid solution phase transformation (LFMP→LixFe0.3Mn0.7PO4 (LxFMP), LxFMP→LFMP) and a two-phase reaction (LxFMP←→Fe0.3Mn0.7PO4 (FMP)). This work provides a new direction for the use of MXenes to fabricate 2D heterostructures for lithium-ion batteries.

Stability properties and microstructure properties of microwave-sintered CeO2 doped zirconia ceramics

Nanosized CeO2–ZrO2 powders prepared by atmospheric pressure pyrolysis were used as raw materials to prepare CeO2–ZrO2 ceramics using microwave sintering. The samples were characterised using bulk density measurements, X-ray diffraction (XRD), Fourier Transform Infrared Spectrometer (FT-IR), Raman, and scanning electron microscopy (SEM). The purpose was to determine the optimised microwave sintering process for CeO2–ZrO2 ceramics and reveal the corresponding mechanism. The results show that with a CeO2 addition content above 5 mol%, the tetragonal phase peak appeared obviously in the sample. The results show that the tetragonal phase peak appears when the CeO2 content is more than 5 mol%. The dopants, namely CeO2, have reduced the solid solution's phase transformation temperature with the assistance of microwave heating. Additionally, the grain size of the CeO2–ZrO2 ceramics has shown a negative relationship with Ce content at a temperature of 900 °C. The reason is that the rapid sintering due to microwave sintering and the oxygen vacancies generated by CeO2 can effectively inhibit grain growth. The regulation mechanism on microwave sintering of CeO2–ZrO2 ceramic was clarified, and the technical prototype of controlled prepared CeO2–ZrO2 ceramics by microwave sintering was constructed.

Suppressing High Current Induced Local Compositional Heterogeneity in LiNi0.6Co0.2Mn0.2O2 By Controlling the Kinetic Parameters

Understanding the cycling rate-dependent kinetics is crucial for engineering batteries with better performance in high power applications. Although high cycling rate induces phase heterogeneity which is responsible for life-time and rate capability, such dynamics are poorly understood and uncontrollable. In this study, operando X-ray diffraction analysis with high temporal resolution was utilized to investigate phase transition kinetics of LiNi0.6Co0.2Mn0.2O2 at a high cycling rate. Due to the sluggish Li diffusion at high lithiation levels, highly asymmetric phase transition was observed in charge and discharge. While relatively homogeneous phase transition took place during discharging, strong phase separation was observed during charging. By taking advantage of the dependence of diffusion and redox kinetics on Li composition and tuning the initial lithiation distribution, we were able to manipulate the overall phase transformation kinetics and further induce solid-solution phase transformation at the high C-rate charging. The finite element analysis elucidated the effect of Li content-dependent diffusion kinetics on the phase transition pathway. Our findings suggest a new direction of optimizing fast cycling protocols based on the fundamental understanding of the material properties.

Mixing enthalpies of alloys with dynamical instability: bcc Ti-V system

Enthalpy of mixing is among the key materials parameters to determine phase stability and phase transformations in solid solutions. The possibility to predict it from first principles in the framework of the density functional theory is one of the corner stones of the modern materials modeling and the future data-driven materials design. Here we have considered body-centered cubic (bcc) Ti-V alloys, a system with high potential for aerospace, automotive biomedical and energy applications, which is known to exhibit the dynamical instability of the crystal lattice for Ti-rich alloys at low temperature. We have calculated the mixing enthalpies ΔH of bcc Ti-V alloys in the whole interval of concentration at high temperature using ab initio molecular dynamics (AIMD) simulations. A comparison with state-of-the-art static calculations at temperature 0 K shows drastic difference between the two methods: while AIMD predicts positive values of ΔH in the whole range of concentrations, the static zero-temperature simulations result in negative values of ΔH for Ti-rich alloys. We have measured the mixing enthalpy of bcc Ti-V alloys experimentally at 1073 K using an isoperibol high temperature Tian-Calvet calorimeter and found that the enthalpies are positive, in agreement with our finite temperature AIMD calculations. We attribute the failure of the standard static calculations of ΔH to lattice distortions associated with the dynamical instability of bcc Ti-V alloys at zero temperature and argue that the effect should be generally important in theoretical predictions of thermodynamic properties, especially for systems with dynamical instability.

Solid-solution transformation and photoluminescence control in Ce3+-doped Ln4Si2-xMxO7+xN2-x (Ln = Y, Lu; M = B, Al, P) oxonitridosilicate phosphors

By designing diverse composition substitutions in phosphor materials to construct solid solution phase transformation are effective strategies to adjust the luminescence properties of phosphor materials. In this study, the Ce3+-doped Y4Si2O7N2 is selected as template material to design a series of composition substitutions including [Y3+-Lu3+] cation substitution, and [SiN+-AlO+] cation-anion cosubstitution, [B3+/P5+-Si4+] substitutions for tuning photoluminescence property and thermal stability of the as-prepared solid solution phosphors. According to XRD and Rietveld refinement results, the Ln4Si2-xAlxO7+xN2-x:Ce3+ (Ln = Y, Lu) could form perfect solid solution phases at 0 ≤ x ≤ 1 in terms of the same monoclinic structure of Ln4Si2O7N2-Ln4Al2O9 with space group P21/a (14). While B- and P-doping only remain the solid solution phase at a low substitution ratio. Continuous spectral blue shift in the blue-green region and improved thermal stability are observed in Ln4Si2-xAlxO7+xN2-x:Ce3+ (Ln = Y, Lu) and B/P-doped Y4Si2O7N2:Ce3+ systems, which are possibly attributed to the enlarged crystal lattice environment around Ce3+ ions with the [SiN-MO] (M = B, Al, O) cosubstitutions. In addition, the red-shift emission of Y4Si2O7N2:Ce3+ with the increase of Ce3+ concentrations are also discussed. In summary, this study systematically discussed the effect of the diverse composition substitutions on the luminescence properties, which could serve as a guide in developing novel oxonitridosilicate luminescent materials with controllable optical properties.

Revealing Operando Transformation Dynamics in Individual Li-ion Electrode Crystallites Using X-Ray Microbeam Diffraction

For the development of next-generation batteries it is important to understand the structural changes in electrodes under realistic non-equilibrium conditions. With microbeam X-ray diffraction it is possible to probe many individual electrode grains concurrently under non-equilibrium conditions in realistic battery systems. This makes it possible to capture phase transformation behavior that is difficult or even impossible with powder diffraction. By decreasing the X-ray beam size, the diffraction powder rings fall apart in the (hkl) reflections belonging to individual electrode crystallites. Monitoring these reflections during (dis)charging provides direct insight in the transformation mechanism and kinetics of individual crystallite grains. Here operando microbeam diffraction is applied on two different cathode materials, LiFePO4 (LFP) displaying a first-order phase transformation and LiNi1/3Co1/3Mn1/3O2 (NCM) displaying a solid solution transformation. For LFP four different phase transformation mechanisms are distinguished within a single crystallite: (1) A first-order phase transformation without phase coexistence, (2) with phase coexistence, (3) a homogeneous solid solution phase transformation and (4) an inhomogeneous solid solution crystal transformation, whereas for NCM only type (3) is observed. From the phase transformation times of individual crystallites, the local current density is determined as well as the active particle fractions during (dis)charge. For LFP the active particle fraction increases with higher cycling rates. At low cycling rates the active particle fraction in NCM is much larger compared to LFP which appears to be related to the nature of the phase transition. In particular for LFP the grains are observed to rotate during (dis)charging, which can be quantified by microbeam diffraction. It brings forward the mechanical working of the electrodes due to the volumetric changes of the electrode material possibly affecting electronic contacts to the carbon black conducting matrix. These results demonstrate the structural information that can be obtained under realistic non-equilibrium conditions, combining local information on single electrode crystallites, as well as global information through the observation in many crystallites concurrently. This provides new and complementary possibilities in operando battery research, which can contribute to fundamental understanding as well as the development of electrodes and electrode materials.

High electrochemical performance of high-voltage LiNi0.5Mn1.5O4 by decoupling the Ni/Mn disordering from the presence of Mn3+ ions

Electrochemical activity in high-voltage spinel LiNi0.5Mn1.5O4 (LNMO) is strongly affected by the disordering of Ni/Mn and the presence of Mn3+ ions. However, understanding the effect of the Ni/Mn disordering or the presence of Mn3+ ions on electrochemical properties is not trivial because disordering is typically coupled with the presence of Mn3+ ions. Here, we demonstrate for the first time that the doping of Li instead of Ni increases Ni/Mn disordering, which is decoupled from the presence of Mn3+ ions. The resultant material has a particle size of ~1–2 μm and can achieve 120 mAh g−1 at 10 C for 50 cycles and further deliver about 60 mAh g−1 even at a rate of ~60 C (1 min discharge). Superior electrochemical performance is achieved by increased solid-solution phase transition behavior, which is caused by increased Ni/Mn disordering during delithiation. By decoupling, we find that the electrochemical properties in LNMO strongly depend on the phase transformation behavior and that the Ni/Mn disordering, rather than Mn3+ ions, affects the phase transformation by increasing the solid-solution reaction. The fundamental understanding gained from this work could be applied to the development of other phase-separating compounds to improve their electrochemical performance. Doping LiNi0.5Mn1.5O4 (LNMO) with Li instead of Ni significantly improves its electrochemical performance, find scientists in Korea. Cathode materials with high redox potentials are needed to realize lithium-ion batteries with high energy densities. The high-voltage spinel LNMO is a promising cathode material, but its electrochemical performance is strongly affected by the disordering of Ni/Mn in the presence of Mn3+ ions. Now, Junghwa Lee, Chaeah Kim and Byoungwoo Kang at Pohang University of Science and Technology in Korea have found that doping LNMO with Li decouples Ni/Mn disordering from the presence of Mn3+ ions. This significantly improves its electrochemical performance through enhanced solid-solution phase transition. The researchers anticipate that the results of the current study can be used to improve the electrochemical performance of other phase-separating compounds. We at the first time demonstrate that the Ni/Mn disordering in LNMO spinel is decoupled from the presence of Mn3+ ions by doping Li and strongly affects phase transformation resulting in the increase of solid-solution phase transformation. The resulting material with 1–2 μm achieves superior rate capability, 120 mAh g−1 at ~38 C and 60 mAh g−1 at 60 C.

Hydrogen trapping at dislocation cores at room temperature in deformed Pd

Abstract Small-angle neutron scattering (SANS) measurements of hydrogen segregation at dislocations in heavily deformed single crystal Pd have been performed at very low solute concentration (PdH0.0016) at equilibrium with respect to hydrogen gas at 295 K. The net (the without-hydrogen measurement subtracted from with-hydrogen measurement) absolute differential macroscopic scattering cross section has been fit with a cylindrical form factor to represent the Cottrell atmosphere, yielding local trapped concentration δ ∼ 0.06 [H]/[Pd], local volumetric dilatation f ∼ 1.01, and trapping radius R ∼ 4 A of the segregated hydrogen. This measurement augments SANS results below ambient temperature [B.J. Heuser, H. Ju, Phys. Rev. B 83 (2011) 094103]. The temperature dependence of the measured radius is confirmed by a Fourier transform of hydrogen occupation at dislocations based on an elastic continuum treatment [Trinkle et al., Phys. Rev. B 83 (2011) 174116]. The measured trapping parameters are consistent with a depopulation of weak long-range dislocation strain fields at ambient temperature; hydrogen binding to stronger core dislocation sites persists at 295 K and results in the measured net scattering. The local solute concentration and trapping radius (less than two Burgers vectors in Pd), are both too small to support optic mode dispersion due to inter-hydrogen interactions. This result supports the conclusion that trapped hydrogen undergoes a hydride to solid solution phase transformation between 200 and 300 K based on hydrogen vibration density of states measurements using incoherent inelastic neutron scattering [Trinkle et al., Phys. Rev. B 83 (2011) 174116, Ju et al., Nucl. Instr. Meth. Phys. Res. A 654, (2011) 522, and Heuser et al., Phys. Rev. B 78 (2008) 214101].

Microstructure of Fe–ZrSiO 4 solid solutions prepared from gels

Microstructural changes associated with chemical and structural evolution from gels to Fe x –ZrSiO 4 solid solutions are reported. Mineralizer-free Fe x –ZrSiO 4 gels in the compositional range 0 ≤ x ≤ 0.15 were prepared by sol–gel liquid-phase route from mixtures of alkoxides of silicon and zirconium, and iron (III) acetylacetonate, and annealed at different temperatures and/or times. The first step on the whole process to the final Fe x –ZrSiO 4 solid solutions was the formation of aggregated of tetragonal Fe-doped ZrO 2 nanocrystals with diameters smaller than 50 nm. At this stage the tetragonal Fe–ZrO 2 were embedded in amorphous silica resulting nanocomposite materials. The formation of the final Fe x –ZrSiO 4 solid solution nanocrystals at around 1100 °C occurred almost simultaneously to the Fe–ZrO 2 solid solution phase transformation from the tetragonal to the monoclinic form. The microstructural examination of specimens after annealing at 1200 °C revealed the development of Fe-doped zircon nanoparticles smaller than 100 nm.

The phase diagram of the Al2O3-ZrO2-Sm2O3 system. IV. Polythermal sections of the phase diagram

Three polythermal sections have been constructed to give a fuller description of the phase diagram structure for the Al2O3-ZrO2-Sm2O3 system over wide ranges in temperature and concentration, which indicate the interactions in the ternary system. The bisector of the angle Sm2O3-(50 mole% Al2O3-50 mole% ZrO2) indicate the structure of the phase diagram in the region rich in Sm2O3, and also the mechanisms of the phase transformations of solid solutions based on Sm2O3: X ⇔ H ⇔ A ⇔ B. The 20 mole% ZrO2 isoconcentrate (20Z) indicates the phase diagram structure in the region adjoining the binary bounding system Al2O3-Sm2O3. The 40 mole% isoconcentrate (40Z) describes the phase diagram in regions rich in ZrO2 as well as the mechanism for the solid-solution phase transformation F ⇔ T based on ZrO2. These sections intersect three nominally quasibinary sections in the ternary system and indicate the dynamic of the width change in the two-phase regions.