Dense Freeze‐cast Li_7La_3Zr_2O_(12) Solid Electrolytes with Oriented Open Porosity and Contiguous Ceramic Scaffold

Freeze casting is used for the first time to prepare solid electrolyte scaffolds with oriented porosity and dense ceramic walls made of Li_7La_3Zr_2O_(12) (LLZO), one of the most promising candidates for solid state battery electrolytes. Processing parameters ‐ such as solvent solidification rate, solvent type, and ceramic particle size ‐ are investigated, focusing on their influence on porosity and ceramic wall density. Dendrite‐like porosity is obtained when using cyclohexane and dioxane as solvents. Lamellar porosity is observed in aqueous slurries resulting in a structure with the highest apparent porosity and densest ceramic scaffold but weakest mechanical properties due to the lack of interlamellar support. The use of smaller LLZO particle size in the slurries resulted in lower porosity and denser ceramic walls. The intrinsic ionic conductivity of the oriented LLZ ceramic scaffold is unaffected by the freeze casting technique, providing a promising ceramic scaffold for polymer infill in view of designing new types of ceramic‐polymer composites

Proton distribution in Sc-doped BaZrO3: a solid state NMR and first principle calculations analysis

Perovskite-based material Sc-doped BaZrO3 is a promising protonic conductor but with substantially lower conductivities than its Y-doped counterpart. H-1 solid-state NMR spectroscopy in combination with DFT modelling was used to analyze the protonic distribution in BaZr1-xScxO3-x/(2-y)(OH)(2y) and its effect on charge carrier mobility. 1H single pulse and H-1-Sc-45 TRAPDOR MAS NMR experiments highlighted the mobile character of the proton charge carriers at room temperature, giving rise to a single broad resonance, protons hopping between multiple sites on the NMR timescale. At low temperatures, the protonic motion was successfully slowed down allowing direct observation of the various proton environments present in the structure. For x <= 0.15, DFT modelling suggested a tendency for strong dopant-proton association leading to Sc-OH-Zr environments with H-1 NMR shifts of 4.8 ppm. The Zr-OH-Zr environment, H-bonded to a Sc-O-Zr, lies 32 kJ mol(-1) higher in energy than the Sc-OH-Zr environment, suggesting that the Sc-OH-Zr environment is trapped. However, even at these low concentrations, Sc-Sc clustering could not be ruled out as additional proton environments with stronger H-1-Sc-45 dipolar couplings were observed (at 4.2 and 2.8 ppm). For x = 0.25, DFT modelling on the dry material predicted that Sc-&-Sc environments were extremely stable, again highlighting the likelihood of dopant clustering. A large number of possible configurations exists in the hydrated material, giving rise to a large distribution in H-1 chemical shifts and multiple conduction pathways. The H-1 shift was found to be strongly related to the length of the O-H bond and, in turn, to the hydrogen bonding and O center dot center dot center dot OH distances. The breadth of the NMR signal observed at low temperature for x = 0.30 indicated a large range of different OH environments, those with lower shifts being generally closer to more than one Sc dopant. Lower DFT energy structures were generally associated with weaker H-bonding environments. Both the calculations and the DFT modelling indicated that the protons tend to strongly bond to the Sc clusters, which, in conjunction with the higher energies of configurations containing Zr-OH-Zr groups, could help explain the lower conductivities recorded for the Sc-substituted BaZrO3 in comparison to its yttrium counterpart

Proton trapping in yttrium-doped barium zirconate

The environmental benefits of fuel cells have been increasingly appreciated in recent years. Among candidate electrolytes for solid-oxide fuel cells, yttrium-doped barium zirconate has garnered attention because of its high proton conductivity, particularly in the intermediate-temperature region targeted for cost-effective solid-oxide fuel cell operation, and its excellent chemical stability. However, fundamental questions surrounding the defect chemistry and macroscopic proton transport mechanism of this material remain, especially in regard to the possible role of proton trapping. Here we show, through a combined thermogravimetric and a.c. impedance study, that macroscopic proton transport in yttrium-doped barium zirconate is limited by proton–dopant association (proton trapping). Protons must overcome the association energy, 29 kJ mol^(−1), as well as the general activation energy, 16 kJ mol^(−1), to achieve long-range transport. Proton nuclear magnetic resonance studies show the presence of two types of proton environment above room temperature, reflecting differences in proton–dopant configurations. This insight motivates efforts to identify suitable alternative dopants with reduced association energies as a route to higher conductivities

Dense Freeze‐cast Li_7La_3Zr_2O_(12) Solid Electrolytes with Oriented Open Porosity and Contiguous Ceramic Scaffold

Freeze casting is used for the first time to prepare solid electrolyte scaffolds with oriented porosity and dense ceramic walls made of Li_7La_3Zr_2O_(12) (LLZO), one of the most promising candidates for solid state battery electrolytes. Processing parameters ‐ such as solvent solidification rate, solvent type, and ceramic particle size ‐ are investigated, focusing on their influence on porosity and ceramic wall density. Dendrite‐like porosity is obtained when using cyclohexane and dioxane as solvents. Lamellar porosity is observed in aqueous slurries resulting in a structure with the highest apparent porosity and densest ceramic scaffold but weakest mechanical properties due to the lack of interlamellar support. The use of smaller LLZO particle size in the slurries resulted in lower porosity and denser ceramic walls. The intrinsic ionic conductivity of the oriented LLZ ceramic scaffold is unaffected by the freeze casting technique, providing a promising ceramic scaffold for polymer infill in view of designing new types of ceramic‐polymer composites

Crystal structure and proton conductivity of BaSn0.6Sc0.4O3-delta: insights from neutron powder diffraction and solid-state NMR spectroscopy

The solid-state synthesis and structural characterisation of perovskite BaSn(1–x)Sc(x)O(3–δ) (x = 0.0, 0.1, 0.2, 0.3, 0.4) and its corresponding hydrated ceramics are reported. Powder and neutron X-ray diffractions reveal the presence of cubic perovskites (space group Pm3m) with an increasing cell parameter as a function of scandium concentration along with some indication of phase segregation. (119)Sn and (45)Sc solid-state NMR spectroscopy data highlight the existence of oxygen vacancies in the dry materials, and their filling upon hydrothermal treatment with D(2)O. It also indicates that the Sn(4+) and Sc(3+) local distribution at the B-site of the perovskite is inhomogeneous and suggests that the oxygen vacancies are located in the scandium dopant coordination shell at low concentrations (x ≤ 0.2) and in the tin coordination shell at high concentrations (x ≥ 0.3). (17)O NMR spectra on (17)O enriched BaSn(1–x)Sc(x)O(3–δ) materials show the existence of Sn–O–Sn, Sn–O–Sc and Sc–O–Sc bridging oxygen environments. A further room temperature neutron powder diffraction study on deuterated BaSn(0.6)Sc(0.4)O(3–δ) refines the deuteron position at the 24k crystallographic site (x, y, 0) with x = 0.579(3) and y = 0.217(3) which leads to an O–D bond distance of 0.96(1) Å and suggests tilting of the proton towards the next nearest oxygen. Proton conduction was found to dominate in wet argon below 700 °C with total conductivity values in the range 1.8 × 10(–4) to 1.1 × 10(–3) S cm(–1) between 300 and 600 °C. Electron holes govern the conduction process in dry oxidizing conditions, whilst in wet oxygen they compete with protonic defects leading to a wide mixed conduction region in the 200 to 600 °C temperature region, and a suppression of the conductivity at higher temperature

Probing Cation and Vacancy Ordering in the Dry and Hydrated Yttrium-Substituted BaSnO₃ Perovskite by NMR Spectroscopy and First Principles Calculations: Implications for Proton Mobility

Hydrated BaSn_1–<i>x</i>Y_<i>x</i>O_{3–<i>x</i>/2} is a protonic conductor that, unlike many other related perovskites, shows high conductivity even at high substitution levels. A joint multinuclear NMR spectroscopy and density functional theory (total energy and GIPAW NMR calculations) investigation of BaSn_1–<i>x</i>Y_<i>x</i>O_{3–<i>x</i>/2} (0.10 ≤ <i>x</i> ≤ 0.50) was performed to investigate cation ordering and the location of the oxygen vacancies in the dry material. The DFT energetics show that Y doping on the Sn site is favored over doping on the Ba site. The ¹¹⁹Sn chemical shifts are sensitive to the number of neighboring Sn and Y cations, an experimental observation that is supported by the GIPAW calculations and that allows clustering to be monitored: Y substitution on the Sn sublattice is close to random up to <i>x</i> = 0.20, while at higher substitution levels, Y–O–Y linkages are avoided, leading, at <i>x</i> = 0.50, to strict Y–O–Sn alternation of B-site cations. These results are confirmed by the absence of a “Y–O–Y” ¹⁷O resonance and supported by the ¹⁷O NMR shift calculations. Although resonances due to six-coordinate Y cations were observed by ⁸⁹Y NMR, the agreement between the experimental and calculated shifts was poor. Five-coordinate Sn and Y sites (i.e., sites next to the vacancy) were observed by ¹¹⁹Sn and ⁸⁹Y NMR, respectively, these sites disappearing on hydration. More five-coordinated Sn than five-coordinated Y sites are seen, even at <i>x</i> = 0.50, which is ascribed to the presence of residual Sn–O–Sn defects in the cation-ordered material and their ability to accommodate O vacancies. High-temperature ¹¹⁹Sn NMR reveals that the O ions are mobile above 400 °C, oxygen mobility being required to hydrate these materials. The high protonic mobility, even in the high Y-content materials, is ascribed to the Y–O–Sn cation ordering, which prevents proton trapping on the more basic Y–O–Y sites

Thermal phase transformations in LaGaO3 and LaAlO3 perovskites: An experimental and computational solid-state NMR study

Multinuclear 71Ga, 69Ga, 27Al and 17O NMR parameters of various polymorphs of LaGaO3 and LaAlO3 perovskites were obtained from the combination of solid-state MAS NMR with solid-state DFT calculations. Some of the materials studied are potential candidate electrolyte materials with applications in intermediate temperature solid oxide fuel cells (ITSOFCs). Small variations in the local distortions of the subject phases are experimentally observed by 71Ga (and 69Ga) and 27Al NMR in the LaGaO3 and LaAlO3 phases, respectively, with heating to 1400&#xa0;K. The orthorhombic-to-rhombohedral phase transformation occurring in LaGaO3 at approximately 416&#xa0;K is clearly observed in the 71Ga/69Ga NMR spectra and is associated with a significant increase in the quadrupolar coupling constant (QCC). Thereafter a gradual decrease in QCC is observed, consistent with increased motion of the GaO6 octahedral units and a reduction in the degree of octahedral tilting. The experimental and theoretical 71Ga, 69Ga, 27Al and 17O NMR parameters (including isotropic and anisotropic chemical shift parameters, quadrupolar coupling constants, and associated asymmetries) of the low and high temperature polymorphs are compared. In general, the calculated values display good agreement with experimental data, although some significant deviations are identified and discussed

Li-Rich Mn/Ni Layered Oxide as Electrode Material for Lithium Batteries: A Li-7 MAS NMR Study Revealing Segregation into (Nanoscale) Domains with Highly Different Electrochemical Behaviors

International audienceWe present a Li-7 MAS NMR study carried out before (pristine material) and during the first cycle of charge/discharge of Li[Li0.2Mn0.61Ni0.18Mg0.01]O-2 layered oxide, a promising active material for positive electrode in Li-ion batteries. For the pristine material, at least five NMR signals were observed. To analyze these results, we developed an 18 cation local model (first and second spheres): aiming at identifying very precise cationic (Li+, Mn4+/Ni2+) configurations compatible with all our NMR data while satisfying local electroneutrality constraints (the key ingredient of our approach). Our results strongly suggest that the material presents two types of coexisting nanoscale domains. The first type is highly ordered and consists of pure Li2MnO, cores (volume similar to 58%), while the second more disordered type concentrates most of the Ni and is labeled LiMO2-like (volume similar to 20%) where M = Mn1/2Ni1/2. Finally, at the interphase of these two Ni-free and Ni-rich domains, there are slightly Ni-contaminated Li2MnO3-like regions, most probably surrounding the Li2MnO3 domains and thus labeled "Ni-poor boundaries" (volume similar to 21%). This partition is confirmed by the behavior of the NMR signals during the first electrochemical cycle. At the initial state of charge (= 4.5 V), the Li2MnO3-like domains become highly involved via oxygen-based (ir)reversible oxidation processes, leading to significant structural transformations. During discharge, only similar to 60% of the initial lithium is reinserted into the structure. The (Ni-rich) LiMQ(2)-like domains are fully refilled (via reversible Ni4+ reduction into Ni2+), while the ordered Li2MnO3-like domains experience a Significant size decrease after the first cycle of charge/discharge. The originality of the present approach consists of analyzing NMR data with a new model that includes at its heart local electroneutrality constraints. This model allowed us to shed light on the processes Occurring in the Li-rich Mn/Ni layered oxide compound during the first electrochemical cycle on the microscopic level

Dual Substitution Strategy to Enhance Li+ Ionic Conductivity in Li7La3Zr2O12 Solid Electrolyte

Solid state electrolytes could address the current safety concerns of lithium-ion batteries as well as provide higher electrochemical stability and energy density. Among solid electrolyte contenders, garnet-structured Li7La3Zr2O12 appears as a particularly promising material owing to its wide electrochemical stability window; however, its ionic conductivity remains an order of magnitude below that of ubiquitous liquid electrolytes. Here, we present an innovative dual substitution strategy developed to enhance Li-ion mobility in garnet-structured solid electrolytes. A first dopant cation, Ga3+, is introduced on the Li sites to stabilize the fast-conducting cubic phase. Simultaneously, a second cation, Sc3+, is used to partially populate the Zr sites, which consequently increases the concentration of Li ions by charge compensation. This aliovalent dual substitution strategy allows fine-tuning of the number of charge carriers in the cubic Li7La3Zr2O12 according to the resulting stoichiometry, Li7–3x+yGaxLa3Zr2–yScyO12. The coexistence of Ga and Sc cations in the garnet structure is confirmed by a set of simulation and experimental techniques: DFT calculations, XRD, ICP, SEM, STEM, EDS, solid state NMR, and EIS. This thorough characterization highlights a particular cationic distribution in Li6.65Ga0.15La3Zr1.90Sc0.10O12, with preferential Ga3+ occupation of tetrahedral Li24d sites over the distorted octahedral Li96h sites. 7Li NMR reveals a heterogeneous distribution of Li charge carriers with distinct mobilities. This unique Li local structure has a beneficial effect on the transport properties of the garnet, enhancing the ionic conductivity and lowering the activation energy, with values of 1.8 × 10–3 S cm–1 at 300 K and 0.29 eV in the temperature range of 180 to 340 K, respectively