I11-High Resolution Powder Diffraction
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Alexandra
Morscher
,
Matthew S.
Dyer
,
Benjamin B.
Duff
,
Guopeng
Han
,
Jacinthe
Gamon
,
Luke
Daniels
,
Yun
Dang
,
T. Wesley
Surta
,
Craig M.
Robertson
,
Frédéric
Blanc
,
John B.
Claridge
,
Matthew J.
Rosseinsky
Diamond Proposal Number(s):
[23666]
Open Access
Abstract: A hexagonal analogue, Li6SiO4Cl2, of the cubic lithium argyrodite family of solid electrolytes is isolated by a computation–experiment approach. We show that the argyrodite structure is equivalent to the cubic antiperovskite solid electrolyte structure through anion site and vacancy ordering within a cubic stacking of two close-packed layers. Construction of models that assemble these layers with the combination of hexagonal and cubic stacking motifs, both well known in the large family of perovskite structural variants, followed by energy minimization identifies Li6SiO4Cl2 as a stable candidate composition. Synthesis and structure determination demonstrate that the material adopts the predicted lithium site-ordered structure with a low lithium conductivity of ∼10–10 S cm–1 at room temperature and the predicted hexagonal argyrodite structure above an order–disorder transition at 469.3(1) K. This transition establishes dynamic Li site disorder analogous to that of cubic argyrodite solid electrolytes in hexagonal argyrodite Li6SiO4Cl2 and increases Li-ion mobility observed via NMR and AC impedance spectroscopy. The compositional flexibility of both argyrodite and perovskite alongside this newly established structural connection, which enables the use of hexagonal and cubic stacking motifs, identifies a wealth of unexplored chemistry significant to the field of solid electrolytes.
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Mar 2021
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[18786]
Abstract: Neutron powder diffraction data indicate that Li2La(TaTi)O7 adopts a polar, a–a–c+/a–a–c+ distorted, n = 2 Ruddlesden–Popper structure described in space group A21am (#36), consistent with a hybrid-improper stabilization of the polar structure via a trilinear coupling between the X3–, X2+, and Γ5– distortion modes. In contrast, Na2La(TaTi)O7 adopts a polar, a–a–c–/a–a–(-c–) distorted, n = 2 Ruddlesden–Popper structure described in space group Pna21 (#33) with the polar Γ3– distortion mode apparently stabilized by a second-order Jahn–Teller distortion of the d0 Ta5+/Ti4+ cations. The change in class of polar distortion of the A2La(TaTi)O7 framework, from hybrid improper (trilinearly coupled) to proper (second-order Jahn–Teller stabilized), on A-cation substitution suggests that the two stabilization mechanisms are in competition in these two materials and many other hybrid-improper ferroelectric phases.
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Mar 2021
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[22322]
Abstract: The lithium-exchanged form of a merlinoite zeolite (MER) with Si/Al = 4.2 (unit cell composition Li6.2Al6.2Si25.8O64) possesses a strongly contracted framework when dehydrated (the unit cell volume decreases by 12.9% from the hydrated “wide-pore” form to the dehydrated “narrow-pore” form). It shows cooperative adsorption behavior for CO2, leading to two-step isotherms with the second step at elevated pressure (>2.5 bar at 298 K). Partially exchanging Na and K cations to give single-phase Li,Na- and Li,K-MER materials reduces the pressure of this second adsorption step because the transition from narrow- to wide-pore forms upon CO2 adsorption occurs at lower partial pressures compared to that in Li-MER: partial exchange with Cs does not reduce the pressure of this transition. Exsolution effects are also seen at K cation contents >2.2 per unit cell. The phase transitions proceed via intermediate structures by complex phase behavior rarely seen for zeolitic materials. The strongly distorted narrow-pore structures adopted upon dehydration give one-dimensional channel structures in which the percolation of CO2 through the material requires cation migration from their locations in ste sites. This is slow in Li3.4Cs2.8-MER where Cs cations occupy these critical ste cavities in the channels, causing very slow adsorption kinetics. As the partial pressure of CO2 increases, a threshold pressure is reached where cooperative adsorption and Cs cation migration occur and the wide-pore form results, with a three-dimensionally connected pore system, leading to a sharp increase in uptake. This is far in excess of the increase of unit cell volume because more of the pore space becomes accessible. Strong hysteretic effects occur upon desorption, leading to CO2 encapsulation. CO2 remaining within the material after repeated adsorption/desorption cycles without heated activation improves sorption kinetics and modifies the stepped isotherms.
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Feb 2021
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I14-Hard X-ray Nanoprobe
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Diamond Proposal Number(s):
[22264]
Abstract: The high-voltage (4.7 V vs Li+/Li) spinel lithium nickel manganese oxide (LiNi0.5Mn1.5O4, LNMO) is a promising candidate for the next generation of lithium-ion batteries due to its high energy density, low cost, and low environmental impact. However, poor cycling performance at high cutoff potentials limits its commercialization. Herein, hollow-structured LNMO is synergistically paired with an ionic liquid electrolyte, 1 M lithium bis(fluorosulfonyl)imide (LiFSI) in N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide (Pyr1,3FSI), to achieve stable cycling performance and improve the rate capability. The optimized cathode–electrolyte system exhibits extended cycling performance (>85% capacity retention after 300 cycles) and high rate performance (106.2 mAh g–1 at 5C) even at an elevated temperature of 65 °C. X-ray photoelectron spectroscopy and spatially resolved X-ray fluorescence analyses confirm the formation of a robust, LiF-rich cathode–electrolyte interphase. This study presents a comprehensive design strategy to improve the electrochemical performance of high-voltage cathode materials.
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Feb 2021
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Abstract: A geometric analysis of the cubic A2BX6 structure commonly formed by metal halides is presented. Using the “hard-sphere” approximation, where the ions are represented by spheres of a fixed radius, we derive four limiting models that each constrain the distances between constituent ions in different ways. We compare the lattice parameters predicted by these four models with experimental data from the Inorganic Crystal Structure Database (ICSD). For the fluorides, the maintenance of the AX bond length at the sum of the A and X radii gives the best approximation of the lattice parameter, leading to structures with widely separated BX6 octahedra. For the heavier halides, a balance between forming an A-site cavity of the correct size and maintaining suitable anion–anion distances determines the lattice parameter. It is found that in many A2BX6 compounds of heavier halides, the neighboring octahedra show very significant anion–anion overlap. We use these models to predict a compound with A-site rattling and use density functional theory (DFT) to confirm this prediction. Finally, we use the geometric models to derive formability criteria for vacancy-ordered double perovskites.
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Nov 2020
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I15-1-X-ray Pair Distribution Function (XPDF)
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Diamond Proposal Number(s):
[21467, 22209]
Open Access
Abstract: Tetrel (Tt = Si, Ge, and Sn) clathrates have highly tunable host–guest structures and have been investigated as novel electrode materials for Li-ion batteries. However, there is little understanding of how the clathrate structure affects the lithiation processes and phase evolution. Herein, the electrochemical lithiation pathway of type I clathrate Ba8Ge43 is investigated with synchrotron X-ray diffraction (XRD) and pair distribution function (PDF) analyses and compared to the lithiation of germanium with a diamond cubic structure (α-Ge). The results confirm previous laboratory XRD studies showing that Ba8Ge43 goes through a solely amorphous phase transformation, which contrasts with the crystalline phase transformations that take place during lithiation of micrometer-sized α-Ge particles. The local structure of framework-substituted clathrate Ba8Al16Ge30 after lithiation is found to proceed through an amorphous phase transformation similar to that in Ba8Ge43. In situ PDF and XRD during heating show that the amorphous phases derived from lithiation of Ba8Ge43 are structurally related to various Li–Ge phases and crystallize at low temperatures (350–420 K). We conclude that the Ba atoms inside the clathrate structure act to break up the long-range ordering of Li–Ge clusters and kinetically prevent the nucleation and growth of bulk crystalline phases. The amorphous phase evolution of the clathrate structure during lithiation results in electrochemical properties distinct from those in α-Ge, such as a single-phase reaction mechanism and lower voltage, suggesting possible advantages of clathrates over elemental phases for use as anodes in Li-ion batteries.
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Oct 2020
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I15-1-X-ray Pair Distribution Function (XPDF)
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Abstract: The La1+xAE1–xGa3O7+x/2 melilite family (AE = Ca, Sr, and Ba and 0 ≤ x ≤ 0.64) demonstrates remarkable oxide ion conductivity due to the ability of its layered tetrahedral [Ga3O7+x/2] network to accommodate and transport interstitial oxide ions (Oint). Compositions of x > 0.65 with very high Oint concentrations (referred to here as “super-excess” compositions) have the potential to support correspondingly high ionic conductivities but have never before been accessed due to the limitations of conventional solid-state ceramic synthesis. Here, we report that fully substituted La2Ga3O7.5 (x = 1) melilite ceramics can be synthesized by direct crystallization of an under-cooled melt, demonstrating that super-excess compositions are accessible under suitable nonequilibrium reaction conditions. La2Ga3O7.5 is stable up to 830 °C and exhibits an ionic conductivity of 0.01 S·cm–1 at 800 °C, 3 orders of magnitude higher than the corresponding x = 0 end-member LaSrGa3O7 and close to the range exhibited by the current best-in-class La1.54Sr0.46Ga3O7.23 (0.1 S·cm–1). It crystallizes in an orthorhombic √2a × √2a × 2c expansion of the parent melilite cell in the space group Ima2 with full long-range ordering of Oint into chains within the [Ga3O7.5] layers. The emergence of this chain-like (1D) ordering within the 2D melilite framework, which appears to be an incipient feature of previously reported partially ordered melilites, is explained in terms of the underlying hexagonal topology of the structure. These results will enable the exploration of extended compositional ranges for the development of new solid oxide ion electrolytes with high concentrations of interstitial oxide charge carriers.
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Oct 2020
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I09-Surface and Interface Structural Analysis
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Jack E. N.
Swallow
,
Christian
Vorwerk
,
Piero
Mazzolini
,
Patrick
Vogt
,
Oliver
Bierwagen
,
Alexander
Karg
,
Martin
Eickhoff
,
Jörg
Schörmann
,
Markus R.
Wagner
,
Joseph William
Roberts
,
Paul R.
Chalker
,
Matthew J.
Smiles
,
Philip
Murgatroyd
,
Sara
Mohamed
,
Zachary W.
Lebens-higgins
,
Louis F. J.
Piper
,
Leanne A. H.
Jones
,
Pardeep K.
Thakur
,
Tien-lin
Lee
,
Joel B.
Varley
,
Juergen
Furthmüller
,
Claudia
Draxl
,
Tim D.
Veal
,
Anna
Regoutz
Diamond Proposal Number(s):
[21430, 24670]
Abstract: The search for new wide band gap materials is intensifying to satisfy the need for more advanced and energy effcient power electronic devices. Ga2O3 has emerged as an alternative to SiC and GaN, sparking a renewed interest in its fundamental properties beyond the main β-phase. Here, three polymorphs of Ga2O3, α, β, and ε, are investigated using X-ray diffraction, X-ray photoelectron and absorption spectroscopy, and ab initio theoretical approaches to gain insights into their structure - electronic structure relationships. Valence and conduction electronic structure as well as semi-core and core states are probed, providing a complete picture of the influence of local coordination environments on the electronic structure. State-of-the-art electronic structure theory, including all-electron density functional theory and many-body perturbation theory, provide detailed understanding of the spectroscopic results. The calculated spectra provide very accurate descriptions of all experimental spectra and additionally illuminate the origin of observed spectral features. This work provides a strong basis for the exploration of the Ga2O3 polymorphs as materials at the heart of future electronic device generations.
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Sep 2020
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I15-1-X-ray Pair Distribution Function (XPDF)
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Diamond Proposal Number(s):
[22774]
Abstract: To understand the importance of the crystallite size on the stabilization of metastable tetragonal ZrO2, ultra-fine ZrO2 nanocrystallites were synthesized via: (i) the precipitation method in supercritical water using nitrate precursors, (ii) the sol-gel method in a supercritical ethanol-water mixture and (iii) the borderline non-hydrolytic sol-gel route in supercritical ethanol using propoxide precursors. The obtained nanocrystals displayed a variation of the monoclinic versus tetragonal molar frac-tions from 100 wt. % down to ≈ 10 wt. % of monoclinic. This variation was concomitant with an overall size decrease of the nanocrystals, ranging from 7 to 2 nm depending on the synthesis procedures. Phase contents were quantified by refinement analysis of X-ray scattering datasets, and crosschecked with Raman spectroscopy. Our results suggest that an upper limit of ≈ 90 wt. %, of tetragonal ZrO2 phase is possible, even for ultra-fine nanoparticles (2 nm). These findings thus question the exist-ence of any critical size limit below which stabilization of pure t-ZrO2 is attainable at low temperatures.
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Sep 2020
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[14809]
Abstract: Potassium-ion batteries (KIB) are a promising complementary technology to lithium-ion batteries because of the comparative abundance and affordability of potassium. Currently, the most promising KIB chemistry consists of a potassium manganese hexacyanoferrate (KMF) cathode, a Prussian blue analog, and a graphite anode (723Whl−1 and 359Whkg−1 at 3.6V). No electrolyte has yet been formulated that is concurrently stable at the high operating potential of KMF (4.02V vs K+/K) and compatible with K+ intercalation into graphite, currently the most critical hurdle to adoption. Here we combine a KMF cathode and a graphite anode with a KFSI in Pyr1,3FSI ionic liquid electrolyte for the first time and show unprecedented performance. We use high-throughput techniques to optimize the KMF morphology for operation in this electrolyte system, achieving 119 mA h g−1 at 4 V vs K+/K and a coulombic efficiency >99.3%. In the same ionic liquid electrolyte graphite shows excellent electrochemical performance and we demonstrate reversible cycling by operando XRD. These results are a significant and essential step forward towards viable potassium-ion batteries.
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Aug 2020
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