Disordered Rocksalt Phase Research Articles

Highly stable operation of LiCoO2 at cut-off ≥ 4.6 V enabled by synergistic structural and interfacial manipulation

Elevating the charge cut-off voltage is the most effective approach for boosting the energy density of LiCoO2 (LCO), which however is hindered by accelerated structural devastation and interfacial degradation at high voltages, e.g. ≥ 4.6 V vs. Li/Li+. In this work, we propose a synergistic strategy by designing a Mg doped and Se coated LCO (LCO-Mg@Se). The strong Mg-O bond greatly stabilizes the layered structure of LCO, to alleviate the lattice parameter variation during deep delithiation. Se surface treatment, acting as an artificial CEI layer, interacts with O2−: 2p during deep stage of charge, effectively weakening the Co: 3d-t2g – O2−: 2p hybridization. Moreover, the Se coating layer could inhibit the parasitic side reactions and refrains the electrolyte corrosion toward the electrolyte-cathode interface, further mitigating the phase transformation to disordered rock salt phase at LCO particle surface and reducing the generation of resistive byproducts. These cooperative efforts enable the substantially enhanced high-voltage electrochemical performance of LCO-Mg@Se, achieving 147 mAh g−1 after 1000 cycles (72.9% retention) at charge cut-off 4.6 V and 148 mAh g−1 after 400 cycles at charge cut-off 4.65 V. The high-temperature cycling performance and thermal stability are greatly enhanced as well. This bifunctional structure/interface engineering approach provides scalable design ideas for developing cathode materials for high energy density batteries.

Single‐crystalline Ni‐rich LiNi 0 . 91 Co 0 . 06 Mn 0 . 03 O 2 cathode enables durable interfacial stability for high electrochemical performances

Despite the commercialization and tremendous attention of the Ni-rich LiNiCoMnO2 cathode due to high energy and working voltage, there are still hurdles to be improved. The conventional polycrystalline Ni-rich LiNiCoMnO2 suffers from decays of specific capacity and energy during long-term cycling. It originates from more deficiency of oxygen and microcracking, resulting in severe phase transition toward disordered rock salt phase. Herein, single-crystalline Ni-rich LiNi0.91Co0.06Mn0.03O2 is successfully prepared to overcome these inherent drawbacks of polycrystalline. Benefiting from reduced specific surface area and grain boundaries, single-crystalline Ni-rich LiNi0.91Co0.06Mn0.03O2 outperforms polycrystalline in all electrochemical performances. The single-crystalline LiNi0.91Co0.06Mn0.03O2 exhibits not only superior cycle stability of 84.7% at 40°C after 100 cycles but also high-capacity retention of 76.2% under 2 C via improving structural integrity. Therefore, this work reveals that single crystalline can be a breakthrough for stable and high electrochemical performances.

Disordering process of GeSb2Te4 induced by ion irradiation

The disordering process in crystalline GeSb2Te4 films has been studied by means of ion irradiation with 150 keV Ar+ ions. The effect of the interfaces and the role of the crystal microstructure has been investigated. The disordering path observed in a randomly oriented polycrystalline material with trigonal structure involves the transition to the disordered rocksalt structure (at fluence 7 × 1013 cm−2) and then to the amorphous phase (at 1.5 × 1014 cm−2). In GeSb2Te4 epitaxially grown on Si(1 1 1) the formation of the disordered rocksalt phase (DRS) occurs at much higher fluence (3 × 1014 cm−2) and it is preceded by the conversion of the stable phase into the ordered rocksalt structure (at 5 × 1013 cm−2), with the formation of ordered vacancy layers, associated to a local variation of the stoichiometry. Even by increasing the fluence up to 3.5 × 1014 cm−2, the films remains mainly crystalline.The observed behaviour has been attributed to the effect of the interfaces and suggests the possibility to promote switching between the crystalline phases by interface engineering.

Phase Transitions in Ge-Sb-Te Alloys Induced by Ion Irradiations

ABSTRACTThe variation of the electrical and optical properties under 150 keV Ar+ ion irradiation has been studied in Ge2Sb2Te5 polycrystalline films, either in the rocksalt or in the trigonal structure, by in situ reflectivity measurements and ex situ resistance measurements. As the irradiation dose increases, the disorder introduced in the crystalline films increases and the reflectivity decreases, down to a minimum value that corresponds to complete amorphization. Large differences are found by changing the irradiation temperature, for the two crystalline structures. Indeed, the measured amorphization threshold is the same for the two crystalline phases and equal to 1x1013 cm-2 under irradiation at 77K, whilst at room temperature the trigonal phase requires a dose almost double than the rocksalt phase to be amorphized. By structural analyses we found that, before amorphization, ion irradiation induces a transition from the trigonal to the rocksalt structure. The van der Waals gaps present in the trigonal phase might act as preferential sinks for the displaced and mobile atoms, thus promoting this transition. By further increasing the irradiation dose the formed disordered rocksalt phase converts into the amorphous phase. Ion irradiation also affects the electrical properties of the material: the disorder modifies the temperature dependence of resistance of the trigonal Ge2Sb2Te5 and induces a change of sign (from metallic to insulating behavior) at a dose of 2x1013 cm-2, well below the amorphization threshold.

Vibrational study of HgGa2S4 under high pressure

In this work, we report on high-pressure Raman scattering measurements in mercury digallium sulfide (HgGa2S4) with defect chalcopyrite structure that have been complemented with lattice dynamics ab initio calculations. Our measurements evidence that this semiconductor exhibits a pressure-induced phase transition from the completely ordered defect chalcopyrite structure to a partially disordered defect stannite structure above 18 GPa which is prior to the transition to the completely disordered rocksalt phase above 23 GPa. Furthermore, a completely disordered zincblende phase is observed below 5 GPa after decreasing pressure from 25 GPa. The disordered zincblende phase undergoes a reversible pressure-induced phase transition to the disordered rocksalt phase above 18 GPa. The sequence of phase transitions here reported for HgGa2S4 evidence the existence of an intermediate phase with partial cation-vacancy disorder between the ordered defect chalcopyrite and the disordered rocksalt phases and the irreversibility of the pressure-induced order-disorder processes occurring in ordered-vacancy compounds. The pressure dependence of the Raman modes of all phases, except the Raman-inactive disordered rocksalt phase, have been measured and discussed.

Cation disordered rock salt phase Li2CoTiO4 as a potential cathode material for Li-ion batteries

Li2CoTiO4 as a new titanate cathode material for Li-ion batteries was successfully synthesized by a sol–gel process. It had a cation disordered rock salt structure and showed an excellent cycle stability at room temperature. After coating with carbon, the Li2CoTiO4 possessed a high discharge capacity of 144.3 mAh g−1. The poor conductivity and lithium ion diffusion coefficient of pure Li2CoTiO4 are the main reasons for the limited electrochemical performance of the lithium batteries. The irreversible capacity loss during the first cycle was assigned to the irreversibility of the structural changes.

Synthesis and high-pressure transformation of metastable wurtzite-structured CuGaS2 nanocrystals

The metastable wurtzite nanocrystals of CuGaS(2) have been synthesized through a facile and effective one-pot solvothermal approach. Through the Rietveld refinement on experimental X-ray diffraction patterns, we have unambiguously determined the structural parameters and the disordered nature of this wurtzite phase. The metastability of wurtzite structure with respect to the stable chalcopyrite structure was testified by a precise theoretical total energy calculation. Subsequent high-pressure experiments were performed to establish the isothermal phase stability of this wurtzite phase in the pressure range of 0-15.9 GPa, above which another disordered rock salt phase crystallized and remained stable up to 30.3 GPa, the highest pressure studied. Upon release of pressure, the sample was irreversible and intriguingly converted into the energetically more favorable and ordered chalcopyrite structure as revealed by the synchrotron X-ray diffraction and the high-resolution transmission electron microscopic measurements. The observed phase transitions were rationalized by first-principles calculations. The current research surely establishes a novel phase transition sequence of disorder → disorder → order, where pressure has played a significant role in effectively tuning stabilities of these different phases.

High pressure X-ray diffraction study of CdAl2Se4 and Raman study of AAl2Se4 (A=Hg, Zn) and CdAl2X4 (X=Se, S)

We report the results of an X-ray diffraction study of CdAl2Se4 and of Raman studies of HgAl2Se4 and ZnAl2Se4 at room temperature, and of CdAl2S4 and CdAl2Se4 at 80 K at high pressure. The ambient pressure phase of CdAl2Se4 is stable up to a pressure of 9.1 GPa above which a phase transition to a disordered rock salt phase is observed. A fit of the volume pressure data to a Birch–Murnaghan type equation of state yields a bulk modulus of 52.1 GPa. The relative volume change at the phase transition at ∼9 GPa is about 10%. The analysis of the Raman data of HgAl2Se4 and ZnAl2Se4 reveals a general trend observed for different defect chalcopyrite materials. The line widths of the Raman peaks change at intermediate pressures between 4 and 6 GPa as an indication of the pressure induced two stage order–disorder transition observed in these materials. In addition, we include results of a low temperature Raman study of CdAl2S4 and CdAl2Se4, which shows a very weak temperature dependence of the Raman-active phonon modes.

Phase relations of AgI under high pressure and high temperature

Using a multi-anvil high-pressure device and synchrotron radiation, X-ray in situ observations of AgI under high pressure and high temperature up to 6.0 GPa and 1000 K have been performed to investigate the phase relations between α-AgI, rocksalt-type, disordered rocksalt-type, and liquid phases. The melting curve of α-AgI has a temperature maximum at about 1.0 GPa, which suggests the existence of a dense liquid phase above this pressure. The three phases of α-AgI, disordered rocksalt-type, and liquid meet in a triple point in the vicinity of 823 K and 1.3 GPa. The melting curve of the disordered rocksalt phase is linear with a positive slope ( dT/ dP=40 deg/ GPa). The transition between the rocksalt and the disordered rocksalt phases is a broad and diffuse disordering type within the same structure. The formation of the disordered rocksalt phase has been hence estimated with the variation of a relative intensity ratio of (111) and (200) diffractions from the rocksalt phase. With increasing pressure the rocksalt phase becomes stable at higher temperature. At 6.0 GPa the rocksalt phase directly melts without the transition to the disordered rocksalt phase.

A New Conducting Ternary Nitride: Na xTa 3N 5(0≤ x≤1.4)

Topotactic insertion of sodium into the channel structure of Ta 3N 5has been accomplished by the reaction between Na and Ta 3N 5at temperatures between 450 and 600°C in sealed Ni tubes. The limiting stoichiometry of Na x Ta 3N 5obtained is x≈1.4. The structure of Na 1.0Ta 3N 5has been refined using Rietveld full-profile refinement of X-ray powder diffraction data using the Ta 3N 5structure as the starting model. The space group remains Cmcm(No. 63) with parameters a=3.995(1), b=10.197(2), and c=10.331(2) Å. The Na atoms are shown to occupy 7-coordinate sites in the holes present in the Ta 3N 5framework. The weak temperature dependence of the magnetic susceptibility between 100 and 320 K of Na 1.0Ta 3N 5and its small value ( χ mol(300 K)=6.5×10 −4emu mol −1) suggest that the compound is a metallic conductor. This is supported by the moderate conduction of an unsintered pellet of Na 1.0Ta 3N 5powder. We also report the identification of the poorly crystalline disordered rock salt phase Na 2Ta 3N 5( a=4.47(4) Å) which is obtained when the reaction temperature is between 600 and 800°C and a partly characterized hexagonal phase that results from nitrogen loss, Na 1− x Ta 3+ x N 4( x≈0), which forms at temperatures above 800°C. The latter compound crystallizes in P6 3/ mcm(No. 193) with a=5.1784(1) and c=10.3650(3) Å and is analogous to previously reported Li and Mg compounds.