3 research outputs found
Probing Cation and Vacancy Ordering in the Dry and Hydrated Yttrium-Substituted BaSnO<sub>3</sub> Perovskite by NMR Spectroscopy and First Principles Calculations: Implications for Proton Mobility
Hydrated BaSn<sub>1ā<i>x</i></sub>Y<sub><i>x</i></sub>O<sub>3ā<i>x</i>/2</sub> 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<sub>1ā<i>x</i></sub>Y<sub><i>x</i></sub>O<sub>3ā<i>x</i>/2</sub> (0.10 ā¤ <i>x</i> ā¤ 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 <sup>119</sup>Sn
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, YāOāY linkages
are avoided, leading, at <i>x</i> = 0.50, to strict YāOāSn
alternation of B-site cations. These results are confirmed by the
absence of a āYāOāYā <sup>17</sup>O resonance
and supported by the <sup>17</sup>O NMR shift calculations. Although
resonances due to six-coordinate Y cations were observed by <sup>89</sup>Y 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 <sup>119</sup>Sn and <sup>89</sup>Y 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 SnāOāSn
defects in the cation-ordered material and their ability to accommodate
O vacancies. High-temperature <sup>119</sup>Sn NMR reveals that the
O ions are mobile above 400 Ā°C, oxygen mobility being required
to hydrate these materials. The high protonic mobility, even in the
high Y-content materials, is ascribed to the YāOāSn
cation ordering, which prevents proton trapping on the more basic
YāOāY sites
Li-Rich Mn/Ni Layered Oxide as Electrode Material for Lithium Batteries: A <sup>7</sup>Li MAS NMR Study Revealing Segregation into (Nanoscale) Domains with Highly Different Electrochemical Behaviors
We
present a <sup>7</sup>Li MAS NMR study carried out before (pristine
material) and during the first cycle of charge/discharge of LiĀ[Li<sub>0.2</sub>Mn<sub>0.61</sub>Ni<sub>0.18</sub>Mg<sub>0.01</sub>]ĀO<sub>2</sub> 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<sup>+</sup>, Mn<sup>4+</sup>/Ni<sup>2+</sup>) 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 Li<sub>2</sub>MnO<sub>3</sub> cores (volume ā¼58%),
while the second more disordered type concentrates most of the Ni
and is labeled LiMO<sub>2</sub>-like (volume ā¼20%) where M
= Mn<sub>1/2</sub>Ni<sub>1/2</sub>. Finally, at the interphase of
these two Ni-free and Ni-rich domains, there are slightly Ni-contaminated
Li<sub>2</sub>MnO<sub>3</sub>-like regions, most probably surrounding
the Li<sub>2</sub>MnO<sub>3</sub> domains and thus labeled āNi-poor
boundariesā (volume ā¼21%). This partition is confirmed
by the behavior of the NMR signals during the first electrochemical
cycle. At the initial state of charge (ā¤4.3 V), Li-ion extraction
occurs mainly from the (Ni-rich) Li<sub>1ā<i>x</i></sub>MO<sub>2</sub>-like domains via Ni<sup>2+</sup> oxidation.
At higher states of charge (ā„4.5 V), the Li<sub>2</sub>MnO<sub>3</sub>-like domains become highly involved via oxygen-based (ir)Āreversible
oxidation processes, leading to significant structural transformations.
During discharge, only ā¼60% of the initial lithium is reinserted
into the structure. The (Ni-rich) LiMO<sub>2</sub>-like domains are
fully refilled (via reversible Ni<sup>4+</sup> reduction into Ni<sup>2+</sup>), while the ordered Li<sub>2</sub>MnO<sub>3</sub>-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<sup>+</sup> Ionic Conductivity in Li<sub>7</sub>La<sub>3</sub>Zr<sub>2</sub>O<sub>12</sub> 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 Li<sub>7</sub>La<sub>3</sub>Zr<sub>2</sub>O<sub>12</sub> 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, Ga<sup>3+</sup>, is introduced on the Li sites
to stabilize the fast-conducting cubic phase. Simultaneously, a second
cation, Sc<sup>3+</sup>, 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 Li<sub>7</sub>La<sub>3</sub>Zr<sub>2</sub>O<sub>12</sub> according to the resulting stoichiometry,
Li<sub>7ā3<i>x</i>+y</sub>Ga<sub><i>x</i></sub>La<sub>3</sub>Zr<sub>2ā<i>y</i></sub>Sc<sub><i>y</i></sub>O<sub>12</sub>. 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<sub>6.65</sub>Ga<sub>0.15</sub>La<sub>3</sub>Zr<sub>1.90</sub>Sc<sub>0.10</sub>O<sub>12</sub>, with
preferential Ga<sup>3+</sup> occupation of tetrahedral Li<sub>24<i>d</i></sub> sites over the distorted octahedral Li<sub>96<i>h</i></sub> sites. <sup>7</sup>Li 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 Ć 10<sup>ā3</sup> S cm<sup>ā1</sup> at 300 K and 0.29 eV in the temperature
range of 180 to 340 K, respectively