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

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

We present a ⁷Li MAS NMR study carried out before (pristine material) and during the first cycle of charge/discharge of Li­[Li_0.2Mn_0.61Ni_0.18Mg_0.01]­O₂ 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 Li₂MnO₃ cores (volume ∼58%), while the second more disordered type concentrates most of the Ni and is labeled LiMO₂-like (volume ∼20%) where M = Mn_1/2Ni_1/2. 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 ∼21%). This partition is confirmed by the behavior of the NMR signals during the first electrochemical cycle. At the initial state of charge (≤4.3 V), Li-ion extraction occurs mainly from the (Ni-rich) Li_1–<i>x</i>MO₂-like domains via Ni²⁺ oxidation. At higher states 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 ∼60% of the initial lithium is reinserted into the structure. The (Ni-rich) LiMO₂-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, Li_{7–3<i>x</i>+y}Ga_<i>x</i>La₃Zr_2–<i>y</i>Sc_<i>y</i>O₁₂. 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 Li_6.65Ga_0.15La₃Zr_1.90Sc_0.10O₁₂, with preferential Ga³⁺ occupation of tetrahedral Li_24<i>d</i> sites over the distorted octahedral Li_96<i>h</i> 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