I09-Surface and Interface Structural Analysis
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Zachary W.
Lebens-higgins
,
David M.
Halat
,
Nicholas V.
Faenza
,
Matthew J.
Wahila
,
Manfred
Mascheck
,
Tomas
Wiell
,
Susanna K.
Eriksson
,
Paul
Palmgren
,
Jose
Rodriguez
,
Fadwa
Badway
,
Nathalie
Pereira
,
Glenn G.
Amatucci
,
Tien-lin
Lee
,
Clare P.
Grey
,
Louis F. J.
Piper
Diamond Proposal Number(s):
[22250, 22148]
Open Access
Abstract: Aluminum is a common dopant across oxide cathodes for improving the bulk and cathode-electrolyte interface (CEI) stability. Aluminum in the bulk is known to enhance structural and thermal stability, yet the exact influence of aluminum at the CEI remains unclear. To address this, we utilized a combination of X-ray photoelectron and absorption spectroscopy to identify aluminum surface environments and extent of transition metal reduction for Ni-rich LiNi0.8Co0.2−yAlyO2 (0%, 5%, or 20% Al) layered oxide cathodes tested at 4.75 V under thermal stress (60 °C). For these tests, we compared the conventional LiPF6 salt with the more thermally stable LiBF4 salt. The CEI layers are inherently different between these two electrolyte salts, particularly for the highest level of Al-doping (20%) where a thicker (thinner) CEI layer is found for LiPF6 (LiBF4). Focusing on the aluminum environment, we reveal the type of surface aluminum species are dependent on the electrolyte salt, as Al-O-F- and Al-F-like species form when using LiPF6 and LiBF4, respectively. In both cases, we find cathode-electrolyte reactions drive the formation of a protective Al-F-like barrier at the CEI in Al-doped oxide cathodes.
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Dec 2019
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I12-JEEP: Joint Engineering, Environmental and Processing
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Diamond Proposal Number(s):
[13911]
Abstract: Materials used as electrodes in energy storage devices have been extensively studied with solid-state NMR spectroscopy. Due to the almost ubiquitous presence of transition metals, these systems are also often magnetic. While it is well known that the presence of anisotropic bulk magnetic susceptibility (ABMS) leads to broadening of resonances under MAS, we show that for mono-disperse and non-spherical particle morphologies, the ABMS can also lead to considerable shifts, which vary substantially as a function of particle shape. This, on one hand, complicates the interpretation of the NMR spectrum and the ability to compare the measured shift of different samples of the same system. On the other hand the ABMS shift provides a mechanism with which to derive the particle shape from the NMR spectrum. In this work, we present a methodology to model the ABMS shift, and relate it to the shape of the studied particles. The approach is tested on the $^7$Li NMR spectra of single crystals and powders of LiFePO$_4$. The results show that the ABMS shift can be a major contribution to the total NMR shift in systems with large magnetic anisotropies and small hyperfine shifts, $^7$Li shifts for typical LiFePO$_4$ morphologies varying by as much as 100 ppm. The results are generalised to demonstrate that the approach can be used as a means with which to probe the aspect ratio of particles. The work has implications for the analysis of NMR spectra of all materials with anisotropic magnetic susceptibilities, including diamagnetic materials such as graphite.
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Jul 2019
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I09-Surface and Interface Structural Analysis
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Zachary W.
Lebens-higgins
,
Nicholas V.
Faenza
,
Maxwell D.
Radin
,
Hao
Liu
,
Shawn
Sallis
,
Jatinkumar
Rana
,
Julija
Vinckeviciute
,
Philip J.
Reeves
,
Mateusz
Zuba
,
Fadwa
Badway
,
Nathalie
Pereira
,
Karena W.
Chapman
,
Tien-lin
Lee
,
Tianpin
Wu
,
Clare P.
Grey
,
Brent
Melot
,
Anton
Van Der Ven
,
Glenn G.
Amatucci
,
Wanli
Yang
,
Louis F. J.
Piper
Diamond Proposal Number(s):
[19162]
Open Access
Abstract: Oxygen participation, arising from increased transition metal–oxygen covalency during delithiation, is considered essential for the description of charge compensation in conventional layered oxides. The advent of high-resolution mapping of the O K-edge resonant inelastic X-ray scattering (RIXS) provides an opportunity to revisit the onset and extent of oxygen participation. Combining RIXS with an array of structural and electronic probes for the family of Ni-rich LiNi0.8Co0.2−yAlyO2 cathodes, we identify common charge compensation regimes that are assigned to formal transition metal redox (<4.25 V) and oxygen participation through covalency (>4.25 V). From O K-edge RIXS maps, we find the emergence of a sharp RIXS feature in these systems when approaching full delithiation, which has previously been associated with lattice oxidized oxygen in alkali-rich systems. The lack of transition metal redox signatures and strong covalency at these high degrees of delithiation suggest this RIXS feature is similarly attributed to lattice oxygen charge compensation as in the alkali-rich systems. The RIXS feature's evolution with state of charge in conventional layered oxides is evidence that this feature reflects the depopulation of occupied O 2p states associated with oxygen participation.
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Jul 2019
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I11-High Resolution Powder Diffraction
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Simon J.
Cassidy
,
Michael J.
Pitcher
,
Jared J. K.
Lim
,
Joke
Hadermann
,
Jeremy P.
Allen
,
Graeme W.
Watson
,
Sylvia
Britto
,
Elena J.
Chong
,
David G.
Free
,
Clare P.
Grey
,
Simon J.
Clarke
Diamond Proposal Number(s):
[13284, 18786]
Open Access
Abstract: The chemical accessibility of the CeIV oxidation state enables redox chemistry to be performed on the naturally coinage-metal-deficient phases CeM1–xSO (M = Cu, Ag). A metastable black compound with the PbFCl structure type (space group P4/nmm: a = 3.8396(1) Å, c = 6.607(4) Å, V = 97.40(6) Å3) and a composition approaching CeSO is obtained by deintercalation of Ag from CeAg0.8SO. High-resolution transmission electron microscopy reveals the presence of large defect-free regions in CeSO, but stacking faults are also evident which can be incorporated into a quantitative model to account for the severe peak anisotropy evident in all the high-resolution X-ray and neutron diffractograms of bulk CeSO samples; these suggest that a few percent of residual Ag remains. A straw-colored compound with the filled PbFCl (i.e., ZrSiCuAs- or HfCuSi2-type) structure (space group P4/nmm: a = 3.98171(1) Å, c = 8.70913(5) Å, V = 138.075(1) Å3) and a composition close to LiCeSO, but with small amounts of residual Ag, is obtained by direct reductive lithiation of CeAg0.8SO or by insertion of Li into CeSO using chemical or electrochemical means. Computation of the band structure of pure, stoichiometric CeSO predicts it to be a Ce4+ compound with the 4f-states lying approximately 1 eV above the sulfide-dominated valence band maximum. Accordingly, the effective magnetic moment per Ce ion measured in the CeSO samples is much reduced from the value found for the Ce3+-containing LiCeSO, and the residual paramagnetism corresponds to the Ce3+ ions remaining due to the presence of residual Ag, which presumably reflects the difficulty of stabilizing Ce4+ in the presence of sulfide (S2–). Comparison of the behavior of CeCu0.8SO with that of CeAg0.8SO reveals much slower reaction kinetics associated with the Cu1–xS layers, and this enables intermediate CeCu1–xLixSO phases to be isolated.
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Feb 2019
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[15118]
Open Access
Abstract: We report the first examples of thiocyanate-based analogues of the cyanide Prussian blue compounds, MIII[Bi(SCN)6], M = Fe, Cr, Sc. These compounds adopt the primitive cubic pcu topology and show strict cation order. Optical absorption measurements show these compounds have band gaps within the visible and near IR region, suggesting that they may be useful for applications where light harvesting is key, such as photocatalysis. We also show that Cr[Bi(SCN)6] can reversibly uptake water into its framework structure pointing towards the possibility of using these frameworks for host/guest chemistry.
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Oct 2018
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B18-Core EXAFS
I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[15309, 16387]
Abstract: All-solid-state batteries (ASSBs) based on non-combustible solid electrolytes are promising candidates for safe and high energy storage systems, but it remains a challenge to prepare systems with stable interfaces between the various solid components that survive both the synthesis conditions and electrochemical cycling. We have investigated cathode mixtures based on a carbon-coated LiFePO4 active material and Li3+xP1−xSixO4 solid electrolyte for potential use in all-solid-state batteries. Half-cells were constructed by combining both compounds into pellets by spark plasma sintering (SPS). We report the fast and quantitative formation of two solid solutions (LiFePO4−Fe2SiO4 and Li3PO4−Li2FeSiO4) for different compositions and ratios of the pristine compounds, as tracked by powder X-ray diffraction and solid-state nuclear magnetic resonance; X-ray absorption near edge spectroscopy confirms the formation of iron silicates similar to Fe2SiO4. Scanning electron microscopy and energy dispersive X-ray spectroscopy reveals diffusion of iron cations up to 40 µm into the solid electrolyte even in the short processing times accessible by SPS. Electrochemical cycling of the SPS treated cathode mixtures demonstrates a substantial decrease in capacity following the formation of the solid solutions during sintering. Consequently, all-solid-state batteries based on LiFePO4 and Li3+xP1−xSixO4 would necessitate iron ion blocking layers. More generally, this study highlights the importance of systematic studies on the fundamental reactions at the active material–solid electrolyte interfaces to enable the introduction of protective layers for commercially successful ASSBs.
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Jul 2018
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B18-Core EXAFS
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Diamond Proposal Number(s):
[11433, 14956, 16387, 17913]
Open Access
Abstract: The maximum power output and minimum charging time of a lithium-ion battery depend on both ionic and electronic transport. Ionic diffusion within the electrochemically active particles generally represents a fundamental limitation to the rate at which a battery can be charged and discharged. To compensate for the relatively slow solid-state ionic diffusion and to enable high power and rapid charging, the active particles are frequently reduced to nanometre dimensions, to the detriment of volumetric packing density, cost, stability and sustainability. As an alternative to nanoscaling, here we show that two complex niobium tungsten oxides—Nb16W5O55 and Nb18W16O93, which adopt crystallographic shear and bronze-like structures, respectively—can intercalate large quantities of lithium at high rates, even when the sizes of the niobium tungsten oxide particles are of the order of micrometres. Measurements of lithium-ion diffusion coefficients in both structures reveal room-temperature values that are several orders of magnitude higher than those in typical electrode materials such as Li4Ti5O12 and LiMn2O4. Multielectron redox, buffered volume expansion, topologically frustrated niobium/tungsten polyhedral arrangements and rapid solid-state lithium transport lead to extremely high volumetric capacities and rate performance. Unconventional materials and mechanisms that enable lithiation of micrometre-sized particles in minutes have implications for high-power applications, fast-charging devices, all-solid-state energy storage systems, electrode design and material discovery.
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Jul 2018
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B18-Core EXAFS
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Abstract: The properties of mixed ionic–electronic conductors (MIECs) are most conveniently controlled through site-specific aliovalent substitution, yet few techniques can report directly on the local structure and defect chemistry underpinning changes in ionic and electronic conductivity. In this work, we perform high-resolution 17O (I = 5/2) solid-state NMR spectroscopy of La2-xSrxNiO4+δ, a MIEC and prospective solid oxide fuel cell (SOFC) cathode material, showing the sensitivity of 17O hyperfine (Fermi contact) shifts and quadrupolar coupling constants due to local structural changes arising from Sr substitution (x). Previously, we resolved resonances from three distinct oxygen sites (interstitial, axial, and equatorial) in the unsubstituted x = 0 material (Halat et al., J. Am. Chem. Soc. 2016, 138, 11958). Here, substitution-induced changes in these three spectral features indirectly report on the ionic conductivity, local octahedral tilting, and electronic conductivity, respectively, of the (substituted) materials. In particular, the intensity of the 17O resonance arising from mobile interstitial defects decreases, and then disappears, at x = 0.5, consistent with reports of lower bulk ionic conductivity in Sr-substituted phases. Secondly, local distortions among the split axial oxygen sites diminish, even on modest incorporation of Sr (x < 0.1), which is also accompanied by faster spin-lattice (T1) relaxation of the interstitial 17O resonances, indicating increased mobility of the associated sites. Finally, the hyperfine shift of the equatorial oxygen resonance decreases due to conversion of Ni2+ (d8) to Ni3+ (d7) by charge compensation, a mechanism associated with improved electronic conductivity in the Sr-substituted phases. Valence and coordination changes of the Ni cations are further supported by Ni K-edge X-ray absorption near edge structure (XANES) measurements, which show a decrease in the Jahn-Teller distortion of the Ni3+ sites and a Ni coordination change consistent with the formation of oxygen vacancies. Ultimately, these insights into local atomic and electronic structure that rely on 17O solid-state NMR spectroscopy should prove relevant for a broad range of aliovalently-substituted functional paramagnetic oxides.
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Jun 2018
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I15-Extreme Conditions
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Diamond Proposal Number(s):
[14177]
Abstract: We report the exfoliation of layered Na2Ti3O7, a promising anode material for Na-ion batteries, and restacking using HNO3 and NaOH to form H-[Ti3O7] and Na(x)-[Ti3O7] compositions, respectively. The materials were characterised by a range of techniques (SEM, TEM, solid state NMR, XRD, PDF). Although the formation of aggregated nanoparticles is favoured under acidic restacking conditions, control over the adherence between the exfoliated layers can be achieved in basic conditions. Pair distribution function (PDF) analysis confirms that the local TiO6 connectivity of the pristine material is maintained. The lowest sodium-containing Na(1)-[Ti3O7] phase, which is the stable product upon Na+ leaching after consecutive washing steps, displays the best performance among the compositions studied, affording a stable reversible capacity of about 200 mAh.g-1 for 20 cycles at a C/20 rate. Washing removes the excess of ‘free/reactive’ Na+, which otherwise forms inactive Na2CO3 in the insufficiently-washed compositions.
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Feb 2018
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I09-Surface and Interface Structural Analysis
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Zachary W.
Lebens-higgins
,
Shawn
Sallis
,
Nicholas V.
Faenza
,
Fadwa
Badway
,
Nathalie
Pereira
,
David M.
Halat
,
Matthew
Wahila
,
Christoph
Schlueter
,
Tien-lin
Lee
,
Wanli
Yang
,
Clare P.
Grey
,
Glenn G.
Amatucci
,
Louis F. J.
Piper
Diamond Proposal Number(s):
[12764, 16005]
Abstract: For layered oxide cathodes, impedance growth and capacity fade related to reactions at the cathode-electrolyte interface (CEI) are particularly prevalent at high voltage and high temperatures. At a minimum, the CEI layer consists of Li2CO3, LiF, reduced (relative to the bulk) metal-ion species, and salt decomposition species but conflicting reports exist regarding their progression during (dis)charging. Utilizing transport measurements in combination with x-ray and nuclear magnetic resonance spectroscopy techniques, we study the evolution of these CEI species as a function of electrochemical and thermal stress for LiNi0.8Co0.15Al0.05O2 (NCA) particle electrodes using a LiPF6 ethylene carbonate: dimethyl carbonate (1:1 volume ratio) electrolyte. Although initial surface metal reduction does correlate with surface Li2CO3 and LiF, these species are found to decompose upon charging and are absent above 4.25 V. While there is trace LiPF6 breakdown at room temperature above 4.25 V, thermal aggravation is found to strongly promote salt breakdown and contributes to surface degradation even at lower voltages (4.1 V). An interesting finding of our work was the partial reformation of LiF upon discharge which warrants further consideration for understanding CEI stability during cycling.
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Jan 2018
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