Exploration of a Metastable Normal Spinel Phase Diagram for the Quaternary Li–Ni–Mn–Co–O System

In an attempt to enlarge the normal spinel phase diagram for the quaternary Li–Ni–Mn–Co–O system, the transformation at moderate temperatures (150–210 °C) of layered Li_0.5(Ni_{1–<i>y</i>–<i>z</i>}Mn_<i>y</i>Co_<i>z</i>)­O₂ (<i>R</i>3̅<i>m</i>), which were obtained by an ambient-temperature extraction of lithium from Li_0.5(Ni_{1–<i>y</i>–<i>z</i>}Mn_<i>y</i>Co_<i>z</i>)­O₂, into normal spinel-like (<i>Fd</i>3̅<i>m</i>) Li­(Ni_{1–<i>y</i>–<i>z</i>}Mn_<i>y</i>Co_<i>z</i>)₂O₄ has been investigated. The phase-conversion mechanism has been studied by joint time-of-flight (TOF) neutron and X-ray diffractions, thermogravimetric analysis, and bond valence sum map. The ionic diffusion of lithium (3a, 6c) and nickel (3a, 3b) ions has been quantified as a function of temperature. The investigated spinel phases are metastable, and they are subject to change into rock-salt phases at higher temperatures. The phases have been characterized as cathodes in lithium-ion cells. The study may serve as a strategic model to access other metastable phases by low-temperature synthesis approaches

Low-Temperature Synthesis, Structural Characterization, and Electrochemistry of Ni-Rich Spinel-like LiNi_2–<i>y</i>Mn_<i>y</i>O₄ (0.4 ≤ <i>y</i> ≤ 1)

The thermal conversion of chemically delithiated layered Li_0.5Ni_1–<i>y</i>Mn_<i>y</i>O₂ (0.2 ≤ <i>y</i> ≤ 0.5) into spinel-like LiNi_2–<i>y</i>Mn_<i>y</i>O₄ (0.4 ≤ <i>y</i> ≤ 1) has been systematically investigated. The formed spinel-like phases are metastable and cannot be accessed by a conventional high-temperature solid-state method. The layered-to-spinel transformation mechanism has been studied by the Rietveld refinement of <i>in situ</i> neutron diffraction as a function of temperature (25–300 °C). In particular, the ionic diffusion of Li and M ions is quantified at different temperatures. Electrochemistry of the metastable spinel-like phases obtained has been studied in lithium-ion cells. A bond valence sum map has been performed to understand the ionic diffusion of lithium ions in the Ni-rich layered, spinel, and rock-salt structures. The study can aid the understanding of the possible phases that could be formed during the cycling of Ni-rich layered oxide cathodes

β‑NaVOPO₄ Obtained by a Low-Temperature Synthesis Process: A New 3.3 V Cathode for Sodium-Ion Batteries

Vanadyl phosphates (VOPO₄) represent a class of attractive cathodes in lithium-ion batteries. However, the exploration of this type of materials in sodium-ion batteries is rare. Here, we report for the first time the synthesis of orthorhombic β-NaVOPO₄ by first chemically extracting lithium from β-LiVOPO₄ and then inserting sodium into the obtained β-VOPO₄ by a microwave-assisted solvothermal process with NaI, which serves both as a reducing agent and sodium source. Intermediate Na_<i>x</i>VOPO₄ compositions with <i>x</i> = 0.3, 0.5, and 0.8 have also been obtained by controlling the amount of NaI in the reaction mixture. Joint Rietveld refinement of synchrotron X-ray diffraction (XRD) and neutron diffraction confirms that the fully sodiated β-NaVOPO₄ is isostructural with the lithium counterpart β-LiVOPO₄. Bond valence sum maps suggest that sodium ions possibly diffuse along the [010] direction in the lattice, similar to the ionic conduction pathway in β-LiVOPO₄. Although the initial discharge capacity is low due to the protons in the structure, it steadily increases with cycling with a long plateau at 3.3 V. Ex situ XRD data of cycled β-VOPO₄ and β-NaVOPO₄ electrodes confirm the reversible reaction in sodium cells involving the V⁴⁺/V⁵⁺ redox couple

Electronic and Electrochemical Properties of Li_1–<i>x</i>Mn_1.5Ni_0.5O₄ Spinel Cathodes As a Function of Lithium Content and Cation Ordering

The electronic and electrochemical properties of the high-voltage spinel LiMn_1.5Ni_0.5O₄ as a function of cation ordering and lithium content have been investigated. Conductivity and activation energy measurements confirm that charge transfer occurs by small polaron hopping, and the charge carrier conduction is easier in the Ni:3d band than in the in Mn:3d band. Seebeck coefficient data reveal that the Ni^2+/3+ and Ni^3+/4+ redox couples are combined in a single 3d band, and that maximum charge carrier concentration occurs where the average Ni oxidation state is close to 3+, corresponding to <i>x</i> = 0.5 in Li Li_1–<i>x</i>Mn_1.5Ni_0.5O₄. Accordingly, maximum electronic conductivity is found at <i>x</i> = 0.5, regardless of cation ordering. The thermodynamically stable phases formed during cycling were investigated by recording the X-ray diffraction (XRD) of chemically delithiated powders. The more ordered spinels maintained two separate two-phase regions upon lithium extraction, while the more disordered samples exhibited a solid-solubility region from LiMn_1.5Ni_0.5O₄ to Li_0.5Mn_1.5Ni_0.5O₄. The conductivity and phase-transformation data of four samples with varying degrees of cation ordering were compared to the electrochemical data collected with lithium cells. Only the most ordered spinel showed inferior rate performance, while the sample annealed for a shorter time performed comparable to the unannealed or disordered samples. The results presented here challenge the most common beliefs about high-voltage spinel: (i) low Mn³⁺ content is responsible for poor rate performance and (ii) thermodynamically stable solid-solubility is critical for fast kinetics

High-Voltage, High-Energy Layered-Spinel Composite Cathodes with Superior Cycle Life for Lithium-Ion Batteries

The unique structural characteristics and their effect on the electrochemical performances of the layered-spinel composite cathode system <i>x</i>Li­[Li_0.2Mn_0.6Ni_0.17Co_0.03]­O₂–(1 – <i>x</i>)­Li­[Mn_1.5Ni_0.425Co_0.075]­O₄ (0 ≤ <i>x</i> ≤ 1) have been investigated by a systematic analysis of the X-ray diffraction (XRD) data, neutron diffraction data (ND), electrochemical charge–discharge profiles, and electrochemical differential–capacity measurements. In the 0.5 ≤ <i>x</i> < 1 samples, the capacity and energy density of the composite cathodes gradually increase during 50 cycles with a change in the shape of the charge–discharge profiles. Ex situ X-ray diffraction data reveal two important findings, which account for the superior cycle performance: (i) the layered phase in the composite cathodes (<i>x</i> = 0.5 and 0.75) undergoes an irreversible phase transformation to a cubic spinel phase during extended electrochemical cycling, and the newly formed spinel phase exhibits only a 3 V plateau without any 4 or 4.7 V plateau as both Mn and Ni are present in the 4+ state; (ii) the parent 5 V cubic spinel phase undergoes a cubic to tetragonal transition during discharge, but the volume change is small (∼5%) for the <i>x</i> = 0.5 and 0.75 compositions. Both the small volume change associated with the cubic to tetragonal transition and the excellent stability of the newly evolved 3 V spinel-like phase lead to remarkable cycle life despite a wide voltage range (2–5 V) involving phase transitions

Nanoscale Ni/Mn Ordering in the High Voltage Spinel Cathode LiNi_0.5Mn_1.5O₄

Nanoscale Ni/Mn Ordering in the High Voltage Spinel Cathode LiNi_0.5Mn_1.5O<sub>4</sub

Insights into B‑Site Ordering in Double Perovskite-Type Ba₃Ca_1+<i>x</i>Nb_2–<i>x</i>O_9‑δ (0 ≤ <i>x</i> ≤ 0.45): Combined Synchrotron and Neutron Diffraction and Electrical Transport Analyses

Perovskite-type metal oxides are being used in a wide range of technologies, including fuel cells, batteries, electrolyzers, dielectric capacitors, and sensors. One of their remarkable structural properties is cationic ordering in A or B sites, which affects electrical transport properties under different gaseous atmospheres, and chemical stability under CO₂ and humid conditions. For example, a simple-perovskite-type Y-doped BaCeO₃ forms BaCO₃ and ((Ce,Y)­O_2−δ) under CO₂ at elevated temperature, while B-site-ordered double-perovskite-type Ba₃Ca_1.18Nb_1.82O_9−δ remains chemically stable under the same conditions. Early structural studies on Ba₃Ca_1+<i>x</i>Nb_2–<i>x</i>O_9−δ (BCN) showed that the B-site ordering (1:1) is sensitive to the Ca content. However, ambiguity rises, as 1:2 B-site ordering was not observed in the parent and doped analogues when <i>x</i> was varied, which motivated us to revisit the complex oxides BCN (<i>x</i> = 0–0.45) to determine the atomic structure by a mean of combined synchrotron X-ray and neutron diffraction methods. Surprisingly, the B-site ordering increases with increasing Ca/Nb mixing in the B-sites in BCN. In addition, the electrical conductivity of BCN was found to be the highest at <i>x</i> = ∼0.18, and it decreased as the Ca/Nb ratio further increased in BCN. Such a result was very similar to that for the Y-doped BaZrO₃, where the mobility of proton carriers was found to decrease as the dopant (Y) increased. A higher Ca/Nb ratio also promotes the growth of grain size, as Ca ions could serve as a sintering aid, improving the structural integrity

Charge Disproportionation in Tetragonal La₂MoO₅, a Small Band Gap Semiconductor Influenced by Direct Mo–Mo Bonding

The structure of the novel compound La₂MoO₅ has been solved from powder X-ray and neutron diffraction data and belongs to the tetragonal space group <i>P</i>4/<i>m</i> (no. 83) with <i>a</i> = 12.6847(3) Å and <i>c</i> = 6.0568(2) Å and with <i>Z</i> = 8. It consists of equal proportions of bioctahedral (Mo₂O₁₀) and square prismatic (Mo₂O₈) dimers, both of which contain direct Mo–Mo bonds and are arranged in 1D chains. The Mo–Mo bond length in the Mo₂O₁₀ dimers is 2.684(8) Å, while there are two types of Mo₂O₈ dimers with Mo–Mo bonds lengths of 2.22(2) and 2.28(2) Å. Although the average Mo oxidation state in La₂MoO₅ is 4+, the very different Mo–Mo distances reflect the fact that the Mo₂O₁₀ dimers contain only Mo⁵⁺ (d¹), while the prismatic Mo₂O₈ dimers only contain Mo³⁺ (d³), a result directly confirmed by density function theory calculations. This is due to the complete disproportionation of Mo⁴⁺, a phenomenon which has not previously been observed in solid-state compounds. La₂MoO₅ is diamagnetic, behavior which is not expected for a nonmetallic transition-metal oxide whose cation sites have an odd number of d-electrons. The resistivity displays the Arrhenius-type activated behavior expected for a semiconductor with a band gap of 0.5 eV, exhibiting an unusually small transport gap relative to other diamagnetic oxides. Diffuse reflectance studies indicate that La₂MoO₅ is a rare example of a stable oxide semiconductor with strong infrared absorbance. It is shown that the d-orbital splitting associated with the Mo₂O₈ and Mo₂O₁₀ dimeric units can be rationalized using simple molecular orbital bonding concepts

Role of Cation Ordering and Surface Segregation in High-Voltage Spinel LiMn_1.5Ni_{0.5–<i>x</i>}M_<i>x</i>O₄ (M = Cr, Fe, and Ga) Cathodes for Lithium-Ion Batteries

The high-voltage doped spinel oxides LiMn_1.5Ni_{0.5–<i>x</i>}M_<i>x</i>O₄ (M = Cr, Fe, and Ga; 0 ≤ <i>x</i> ≤ 0.08) synthesized at 900 °C have been investigated systematically before and after postannealing at 700 °C. Neutron diffraction studies reveal that the cation-ordered domain size tends to increase upon annealing at 700 °C. Time-of-flight secondary-ion mass spectroscopy data reveal that the dopant cations M = Cr, Fe, and Ga segregate preferentially to the surface, resulting in a more stable cathode–electrolyte interface and superior cyclability at both room temperature and 55 °C with conventional electrolytes. The doping with Cr and Fe stabilizes the structure with a significant disordering of the cations in the 16d sites even after postannealing at 700 °C, resulting in high rate capability due to low charge-transfer resistance and polarization loss. In contrast, the Ga-doped and undoped LiMn_1.5Ni_0.5O₄ samples experience an increase in cation ordering upon postannealing at 700 °C, resulting in degradation in the rate capability due to an increase in the charge-transfer resistance and polarization loss

Postsynthetic Functionalization of Mg-MOF-74 with Tetraethylenepentamine: Structural Characterization and Enhanced CO₂ Adsorption

Postsynthetic functionalization of magnesium 2,5-dihydroxyterephthalate (Mg-MOF-74) with tetraethylenepentamine (TEPA) resulted in improved CO₂ adsorption performance under dry and humid conditions. XPS, elemental analysis, and neutron powder diffraction studies indicated that TEPA was incorporated throughout the MOF particle, although it coordinated preferentially with the unsaturated metal sites located in the immediate proximity to the surface. Neutron and X-ray powder diffraction analyses showed that the MOF structure was preserved after amine incorporation, with slight changes in the lattice parameters. The adsorption capacity of the functionalized amino-Mg-MOF-74 (TEPA-MOF) for CO₂ was as high as 26.9 wt % versus 23.4 wt % for the original MOF due to the extra binding sites provided by the multiunit amines. The degree of functionalization with the amines was found to be important in enhancing CO₂ adsorption, as the optimal surface coverage improved performance and stability under both pure CO₂ and CO₂/H₂O coadsorption, and with partially saturated surface coverage, optimal CO₂ capacity could be achieved under both wet and dry conditions by a synergistic binding of CO₂ to the amines as well as metal centers