Jordi Jacas Biendicho

Investigation of Antisite Defect Formation and Chemical Expansion in LiNiPO4 by in Situ Neutron Diffraction

In situ neutron diffraction was used to characterize the effect of temperature on the crystal structure of LiNiPO4. LiNiPO4 adopts an ordered olivine structure at room temperature, but, with increasing temperature, this work shows that a significant amount of Li and Ni cation exchange occurs, for example, ∼15% at 900 °C. The antisite disorder is detected by residual nuclear densities on the M1 and M2 octahedral sites in the olivine structure using difference Fourier maps and by changes in cation site occupancies, lattice parameters, and mean ⟨M-O⟩ bond distances. The antisite disorder is also responsible for chemical expansion of the crystal lattice in addition to thermal expansion. Antisite defect formation at high temperature and its reversibility on cooling can be understood as an entropically driven feature of the crystal structure of LiNiPO4. The lithium ion diffusion pathway, that follows a curved trajectory along the b axis in the olivine structure, is, therefore, susceptible to be blocked if synthesis conditions are not carefully controlled and should also be influenced by the chemically expanded lattice of the disordered structure if this is preserved to ambient temperature by rapid cooling.

Static and Dynamic Studies on LiNi1/3Co1/3Mn1/3O2‐Based Suspensions for Semi‐Solid Flow Batteries

Abstract LiNi1/3Co1/3Mn1/3O2 (LNCM)‐based suspensions for semi‐solid flow batteries (SSFB) have been investigated by galvanostatic charge/discharge an electrochemical impedance spectroscopy (EIS). The resistance and electrochemical performance of half cells (vs. Li/Li+) as well as the rheological properties are affected by the content of a commercially available electroconductive carbon black [KetjenBlack (KB), AkzoNobel] in the suspensions. In static conditions, a cell with 11.87 and 13.97 % by volume of KB and LNCM delivers high capacity 130 mA h g−1 at 5 mA cm−2, respectively, and a coulombic efficiency of 90 % over 10 injections. The impedance of half cells is dominated by a contact resistance fitted with a resistor and a constant phase element (CPE) in parallel. In flow conditions, cell potential depends on applied current density and measured over potentials are ∼0.3 and 0.7 V for 0.33 and 1 mA cm−2, respectively, for a cell containing a suspension with 9.53 % in volume of KB and 13.90 % in volume of LNCM. The effect of the cell contact resistance on the electrochemical performance is discussed.

A structural study of Ruddlesden–Popper phases Sr3−xYx(Fe1.25Ni0.75)O7−δ with x ≤ 0.75 by neutron powder diffraction and EXAFS/XANES spectroscopy

The structures of Ruddlesden–Popper n = 2 member phases Sr3−xYxFe1.25Ni0.75O7−δ with 0 ≤ x ≤ 0.75 have been investigated using neutron powder diffraction and K-edge Fe and Ni EXAFS/XANES spectroscopy in order to gain information about the evolution of the oxygen vacancy distribution and Fe/Ni oxidation state with x. Both samples prepared at 1300 °C under a flow of N2(g), with δ = 1.41–1.00, and samples subsequently annealed in air at 900 °C, with δ = 0.44–0.59, were characterized. The as-prepared x = 0.75 phase has δ = 1, the O1 atom site is vacant, and the Fe3+/Ni2+ ions have a square pyramidal coordination. With decreasing x the O3 occupancy decreases nearly linearly to 81% for x = 0, while the O1 occupancy increases from 0 for x = 0.4 to 33% for x = 0. The air-annealed x = 0.75 sample has a δ value of 0.59 and the Fe3+/Fe4+/Ni2+/Ni3+ ions have both square pyramidal and octahedral coordination. With decreasing x, the δ value decreases to 0.45 for x = 0, implying an increase in the oxidation states of Fe/Ni ions. EXAFS/XANES data show that for the as-prepared samples the coordination changes are predominantly for Ni2+ ions and that the air-annealed samples contain both Fe3+/Fe4+ and Ni2+/Ni3+ ions.

Proton conductivity of hexagonal and cubic BaTi1-XScxO3-delta (0.1 l= x l= 0.8)

BaTi1-xScxO3-δ (x = 0.1-0.8) was prepared via solid state reaction. High resolution X-ray powder diffraction was used to characterise the synthesised materials. It was found that low substitution (x = 0.1 and 0.2) of Ti(4+) for Sc(3+) gives a hexagonal perovskite structure, whereas high substitution (x = 0.5-0.7) results in a cubic perovskite structure. Thermogravimetric analysis revealed significant levels of protons in both as-prepared and hydrated samples. Electrical conductivity was measured by AC impedance methods under oxygen, argon and under dry and humid, both H2O and D2O, conditions for BaTi1-xScxO3-δ (x = 0.2, 0.6 and 0.7). In the temperature range of 150-600 °C, under humid conditions, the conductivity is significantly higher than that under the dry conditions. The increase in conductivity is especially prominent for the cubic phases, indicating that protons are the dominant charge carriers. The proton conductivity of hexagonal BaTi0.8Sc0.2O3-δ is approx. two orders of magnitude lower than that of the more heavily substituted cubic phases. Conductivity is also found to be higher in dry O2 than in Ar in the whole temperature range of 150-1000 °C, characteristic of a significant contribution from p-type charge carriers under oxidising atmospheres. Greater Sc(3+) substitution leads to a higher proton concentration and the highest proton conductivity (σ∼ 2 × 10(-3) S cm(-1) at 600 °C) is found for the BaTi0.3Sc0.7O3-δ composition.

Insights into the Performance of CoxNi1–xTiO3 Solid Solutions as Photocatalysts for Sun-Driven Water Oxidation

CoxNi1-xTiO3 systems evaluated as photo- and electrocatalytic materials for oxygen evolution reaction (OER) from water have been studied. These materials have shown promising properties for this half-reaction both under (unbiased) visible-light photocatalytic approach in the presence of an electron scavenger and as electrocatalysts in dark conditions in basic media. In both situations, Co0.8Ni0.2TiO3 exhibits the best performance and is proved to display high faradaic efficiency. A synergetic effect between Co and Ni is established, improving the physicochemical properties such as surface area and pore size distribution, besides affecting the donor density and the charge carrier separation. At higher Ni content, the materials exhibit behavior more similar to that of NiTiO3, which is a less suitable material for OER than CoTiO3.

Synthesis, structure and electrical properties of N-doped Li3VO4

N-doped Li3VO4 of general formula Li3+xVO4−xNx was prepared by solid state reaction of Li3N, V2O5 and either Li2CO3 or LiOH·H2O. A solid solution based on the low temperature β polymorph of Li3VO4 was obtained with composition 0 ≤ x ≤ 0.2. Structural studies by X-ray and neutron powder diffraction confirmed the partial replacement of oxygen on the O(1) sites by N together with creation of an equal number of Li+ ions which are located off-centre in adjacent octahedral Li(3) sites. Electrical property measurements on sintered pellets using impedance spectroscopy showed that the solid solutions are modest conductors of Li+ ions, consistent with the partial occupancy of Li+ ions in the interstitial octahedral sites. The activation energy for conduction of samples prepared using LiOH·H2O, ∼1.91 eV, is much greater than for samples prepared at higher temperature, using Li2CO3, ∼0.78 eV; it is speculated that this is due to ion trapping in Lii˙/N0′ defect clusters. This study represents a relatively new method for doping Li+ ions into a structure by aliovalent anion doping: partial replacement of O by N is compensated by creation of interstitial Li+ ions.

The Fluorite-Like Phase Nd5Mo3O16±δ in the MoO3–Nd2O3 System: Synthesis, Crystal Structure, and Conducting Properties

This paper describes a study of the system MoO3-Nd2O3 using a combination of X-ray powder diffraction (XRD), neutron powder diffraction (NPD), thermogravimetric analysis (TGA), and ac impedance spectroscopy (IS). A phase-pure material is observed at a composition of 45.5 mol % Nd2O3, which corresponds to an ideal stoichiometry of Nd5Mo3O16.5. XRD and NPD show that the crystal structure is a superstructure of the fluorite arrangement, with long-range ordering of the two cation species leading to a doubled unit cell parameter. The sample is found to be significantly oxygen deficient, i.e. Nd5Mo3O15.63(4), when it is prepared by a solid-state reaction at 1473 K in air. TGA measurements indicate that the sample loses only minimal mass on heating to 1273 K in O2. IS studies of the mean conductivity under different atmospheres show that the sample is a mixed conductor between ambient temperature and 873 K, with a dominant electronic component at higher temperatures, as demonstrated by measurements under inert atmosphere. NPD measurements indicate that the anion vacancies are preferentially located on the O2 sites, while studies of the temperature dependence performed under an O2 atmosphere to 1273 K show significantly anisotropic thermal parameters of the anions. Together with analysis of the total neutron scattering data, this supports a model of oxygen ions hopping between O2 positions, with a vacancy, rather than interstitial, mechanism for the anion diffusion.

New Opportunities for Air Cathode Batteries; in-Situ Neutron Diffraction Measurements

Batteries with air electrodes are gaining interest as Energy Storage Systems (ESSs) for Electrical Vehicles (EVs) because of their high specific energy density. The electrochemical performance of these batteries is limited by the metallic electrode, which suffers structural transformations and corrosion during cycling that reduces the cycle life of the battery. In this context, relevant information on the discharge products may be obtained by in-situ neutron diffraction, a suitable technique to study electrodes that contain light elements or near neighbor elements in the periodic table. Case studies of MH-air and Fe-air batteries are highlighted.

Tuning the high-temperature properties of Pr2NiO4+δ by simultaneous Pr- and Ni-cation replacement

Novel Pr2−xSrxNi1−xCoxO4±δ (x = 0.25; 0.5; 0.75) oxides with the tetragonal K2NiF4-type structure have been prepared. Room-temperature neutron powder diffraction (NPD) study of x = 0.25 and 0.75 phases together with iodometric titration results have shown the formation of hyperstoichiometric oxide for x = 0.25 (δ = 0.09(2)) and a stoichiometric one for x = 0.75. High-temperature X-ray powder diffraction (HT XRPD) showed substantial anisotropy of the thermal expansion coefficient (TEC) along the a- and c-axis of the crystal structure, which increases with increasing the Co content from TEC(c)/TEC(a) = 2.4 (x = 0.25) to 4.3 (x = 0.75). High-temperature NPD (HT NPD) study of the x = 0.75 sample reveals that a very high expansion of the axial (Ni/Co)–O bonds (75.7 ppm K−1 in comparison with 9.1 ppm K−1 for equatorial ones) is responsible for such behaviour, and is caused by a temperature-induced transition between low- and high-spin states of Co3+. This scenario has been confirmed by high-temperature magnetization measurements on a series of Pr2−xSrxNi1−xCoxO4±δ samples. For compositions with high Ni content (x = 0.25 and 0.5) we synthesised K2NiF4-type oxides Pr2−x−ySrx+y(Ni1−xCox)O4±δ, y = 0.0–0.75 (x = 0.25); y = 0.0–0.5 (x = 0.5). The studies of the TEC, high-temperature electrical conductivity in air, chemical stability of the prepared compounds in oxygen and toward interaction with Ce2−xGdxO2−x/2 (GDC) at high temperatures reveal optimal behaviour of Pr1.35Sr0.65Ni0.75Co0.25O4+δ. This compound shows stability in oxygen at 900 °C and does not react with GDC at least up to 1200 °C. It features low TEC of 13 ppm K−1 and high-temperature electrical conductivity in air of 280 S cm−1 at 900 °C, thus representing a promising composition for use as a cathode material in intermediate temperature solid oxide fuel cells (IT-SOFC).