arXiv subjects

Clare P. Grey

Publications and source records attributed to Clare P. Grey.

34 records · Page 2Linked to original sources

arXiv2018

Cycling Li-O2 Batteries via LiOH Formation and Decomposition

The rechargeable aprotic Li-air (O2) battery is a promising potential technology for next generation energy storage, but its practical realization still faces many challenges. In contrast to the standard Li-O2 cells, which cycle via the formation of Li2O2, we use a reduced graphene oxide electrode, the additive LiI, and the solvent dimethoxyethane to reversibly form/remove crystalline LiOH with particle sizes > 15 um during discharge/charge. This leads to high specific capacities, excellent energy efficiency (93.2%) with a voltage gap of only 0.2 V, and impressive rechargeability. The cells tolerate high concentrations of water, water being the dominant proton source for the LiOH; together with LiI it has a decisive impact on the chemical nature of the discharge product and battery performance.

physics.chem-ph↗

arXiv2018

Mechanistic Insights into the Challenges of Cycling a Non-aqueous Na-O2 Battery

Superoxide-based non-aqueous metal oxygen batteries have received considerable research attention, as they exhibit high energy densities and round-trip efficiencies. The cycling performance, however, is still poor. Here we study the cycling characteristic of a Na-O2 battery using solid-state nuclear magnetic resonance, Raman spectroscopy and scanning electron microscopy. We found that the poor cycling performance is primarily caused by the considerable side reactions stemming from the chemical aggressiveness of NaO2 both as a solid phase and dissolved species in the electrolyte. The side reaction products cover electrode surfaces and hinder electron transfer across the electrode-electrolyte interface, being a major reason for cell failure. In addition, the available electrode surface and porosity change considerably during cell discharging and charging, affecting the diffusion of soluble species (superoxide and water) and resulting in inhomogeneous reactions across the electrode. This study provides insights into the challenges associated with achieving long-lived superoxide based metal-O2 batteries.

physics.chem-ph↗

arXiv2018

Surface Sensitive NMR Detection of the SEI Layer on Reduced Graphene Oxide

The solid electrolyte interphase (SEI) is detrimental for rechargeable batteries performance and lifetime. Understanding its formation requires analytical techniques that provide molecular level insight. Here dynamic nuclear polarization (DNP) is utilized for the first time for enhancing the sensitivity of solid state NMR (ssNMR) spectroscopy to the SEI. The approach is demonstrated on reduced-graphene oxide (rGO) cycled in Li-ion cells in natural abundance and 13C-enriched electrolyte solvents. Our results indicate that DNP enhances the signal of outer SEI layers, enabling detection of natural abundance 13C spectra from this component of the SEI at reasonable timeframes. Furthermore, 13C- enriched electrolytes measurements at 100K provide ample sensitivity without DNP due to the vast amount of SEI filling the rGO pores, thereby allowing differentiating the inner and outer SEI layers composition. Developing this approach further will benefit the study of many electrode materials, equipping ssNMR with the needed sensitivity to efficiently probe the SEI.

physics.chem-ph↗

arXiv2018

The Effect of Water on Quinone Redox Mediators in Non- aqueous Li-O2 Batteries

The parasitic reactions associated with reduced oxygen species and the difficulty in achieving the high theoretical capacity have been major issues plaguing development of practical non-aqueous Li-O2 batteries. We hereby address the above issues by exploring the synergistic effect of 2,5-di-tert-butyl-1,4- benzoquinone and H2O on the oxygen chemistry in a non-aqueous Li-O2 battery. Water stabilizes the quinone monoanion and dianion, shifting the reduction potentials of the quinone and monoanion to more positive values (vs. Li+). When water and the quinone are used together in a (largely) non-aqueous Li-O2 battery, the cell discharge operates via a two-electron oxygen reduction reaction to form Li2O2, the battery discharge voltage, rate, capacity all being considerably increased and fewer side reactions being detected; Li2O2 crystals can grow up to 30 um, more than an order of magnitude larger than cases with the quinone alone or without any additives, suggesting that water is essential to promoting a solution dominated process with the quinone on discharging. The catalytic reduction of O2 by the quinone monoanion is predominantly responsible for the attractive features mentioned above. Water stabilizes the quinone monoanion via hydrogen bond formation and by coordination of the Li+ ions, and it also helps increase the solvation, concentration, life time and diffusion length of reduced oxygen species that dictate the discharge voltage, rate and capacity of the battery. When a redox mediator is also used to aid the charging process, a high-power, high energy- density, rechargeable Li-O2 battery is obtained.

physics.chem-ph↗

arXiv2018

Exfoliation of layered Na-ion anode material Na2Ti3O7 for enhanced capacity and cyclability

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, the use of basic conditions can lead to control over the adherence between the exfoliated layers. 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 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.

physics.chem-ph↗

arXiv2018

Identifying the Structural Basis for the Increased Stability of the Solid Electrolyte Interphase Formed on Silicon with the Additive Fluoroethylene Carbonate

To elucidate the role of fluoroethylene carbonate (FEC) as an additive in the standard carbonate-based electrolyte for Li-ion batteries, the solid electrolyte interphase (SEI) formed during electrochemical cycling on silicon anodes was analyzed with a combination of solution and solid-state NMR techniques, including dynamic nuclear polarization. To facilitate characterization via 1D and 2D NMR, we synthesized 13C-enriched FEC, ultimately allowing a detailed structural assignment of the organic SEI. We find that the soluble PEO-like line- ar oligomeric electrolyte breakdown products that are observed after cycling in the standard ethylene carbonate (EC)-based electrolyte are suppressed in the presence of 10 vol % FEC additive. FEC is first defluorinated to form soluble vinylene carbonate and vinoxyl species, which react to form both soluble and insoluble branched ethylene-oxide based polymers. No evidence for branched polymers are observed in the absence of FEC.

physics.chem-ph↗

arXiv2018

Understanding LiOH Chemistry in a Ruthenium Catalyzed Li-O2 Battery

Non-aqueous Li-O2 batteries are promising for next generation energy storage. New battery chemistries based on LiOH, rather than Li2O2, have recently been reported in systems with added water, one using a soluble additive LiI and the other using solid Ru catalysts. Here, we focus on the mechanism of Ru-catalyzed LiOH chemistry. Using nuclear magnetic resonance, operando electrochemical pressure measurements and mass spectrometry, we show that on discharging LiOH forms via a 4 e- oxygen reduction reaction, the H in LiOH coming solely from added H2O and the O from both O2 and H2O. On charging, quantitative LiOH oxidation occurs at 3.1 V, with O being trapped in a form of dimethyl sulfone in the electrolyte. Compared to Li2O2, LiOH formation over Ru incurs hardly any side reactions, a critical advantage for developing a long-lived battery. An optimized metal catalyst-electrolyte couple needs to be sought that aids LiOH oxidation and is able stable towards attack by hydroxyl radicals.

physics.chem-ph↗

arXiv2018

Low-dimensional quantum magnetism in Cu(NCS)$_2$: A molecular framework material

Low-dimensional magnetic materials with spin-$\frac{1}{2}$ moments can host a range of exotic magnetic phenomena due to the intrinsic importance of quantum fluctuations to their behavior. Here, we report the structure, magnetic structure and magnetic properties of copper(II) thiocyanate, Cu(NCS)$_2$, a one-dimensional coordination polymer which displays low-dimensional quantum magnetism. Magnetic susceptibility, electron paramagnetic resonance (EPR) spectroscopy, $^{13}$C magic-angle spinning nuclear magnetic resonance (MASNMR) spectroscopy, and density functional theory (DFT) investigations indicate that Cu(NCS)$_2$ behaves as a two-dimensional array of weakly coupled antiferromagnetic spin chains ($J_2 = 133(1)$ K, $α= J_1/J_2 = 0.08$). Powder neutron-diffraction measurements confirm that Cu(NCS)$_2$ orders as a commensurate antiferromagnet below $T_\mathrm{N} = 12$ K, with a strongly reduced ordered moment (0.3 $μ_\mathrm{B}$) due to quantum fluctuations.

cond-mat.str-el↗

arXiv2018

Three dimensional localization of nanoscale battery reactions using soft X-ray tomography

Battery function is determined by the efficiency and reversibility of the electrochemical phase transformations at solid electrodes. The microscopic tools available to study the chemical states of matter with the required spatial resolution and chemical specificity are intrinsically limited when studying complex architectures by their reliance on two dimensional projections of thick material. Here, we report the development of soft X-ray ptychographic tomography, which resolves chemical states in three dimensions at 11-nm spatial resolution. We study an ensemble of nano-plates of lithium iron phosphate (LixFePO4) extracted from a battery electrode at 50% state of charge. Using a set of nanoscale tomograms, we quantify the electrochemical state and resolve phase boundaries throughout the volume of individual nano-particles. These observations reveal multiple reaction points and intra-particle heterogeneity that highlights the importance of electrical connectivity, providing novel insight to the design of the next generation of high-performance devices.

cond-mat.mtrl-sci↗

arXiv2018

Realistic atomistic structure of amorphous silicon from machine-learning-driven molecular dynamics

Amorphous silicon (a-Si) is a widely studied non-crystalline material, and yet the subtle details of its atomistic structure are still unclear. Here, we show that accurate structural models of a-Si can be obtained by harnessing the power of machine-learning algorithms to create interatomic potentials. Our best a-Si network is obtained by cooling from the melt in molecular-dynamics simulations, at a rate of 10$^{11}$ K/s (that is, on the 10 ns timescale). This structure shows a defect concentration of below 2% and agrees with experiments regarding excess energies, diffraction data, as well as $^{29}$Si solid-state NMR chemical shifts. We show that this level of quality is impossible to achieve with faster quench simulations. We then generate a 4,096-atom system which correctly reproduces the magnitude of the first sharp diffraction peak (FSDP) in the structure factor, achieving the closest agreement with experiments to date. Our study demonstrates the broader impact of machine-learning interatomic potentials for elucidating accurate structures and properties of amorphous functional materials.

cond-mat.mtrl-sci↗

arXiv2017

Structural Simplicity as a Restraint on the Structure of Amorphous Silicon

Understanding the structural origins of the properties of amorphous materials remains one of the most important challenges in structural science. In this study we demonstrate that local 'structural simplicity', embodied by the degree to which atomic environments within a material are similar to each other, is powerful concept for rationalising the structure of canonical amorphous material amorphous silicon (a-Si). We show, by restraining a reverse Monte Carlo refinement against pair distribution function (PDF) data to be simpler, that the simplest model consistent with the PDF is a continuous random network (CRN). A further effect of producing a simple model of a-Si is the generation of a (pseudo)gap in the electronic density of states, suggesting that structural homogeneity drives electronic homogeneity. That this method produces models of a-Si that approach the state-of-the-art without the need for chemically specific restraints (beyond the assumption of homogeneity) suggests that simplicity-based refinement approaches may allow experiment-driven structural modelling techniques to be developed for the wide variety of amorphous semiconductors with strong local order.

cond-mat.mtrl-sci↗

arXiv2016

Floating zone growth of α-Na$_{0.90}$MnO$_2$ single crystals

Single crystal growth of α-Na$_x$MnO$_2$ (x = 0.90) is reported via the floating zone technique. The conditions required for stable growth and intergrowth-free crystals are described along with the results of trials under alternate growth atmospheres. Chemical and structural characterizations of the resulting α-Na$_{0.90}$MnO$_2$ crystals are performed using ICP-AES, NMR, XANES, XPS, and neutron diffraction measurements. As a layered transition metal oxide with large ionic mobility and strong correlation effects, α-Na$_x$MnO$_2$ is of interest to many communities, and the implications of large volume, high purity, single crystal growth are discussed.

cond-mat.str-el↗

arXiv2014

Lattice simulation method to model diffusion and NMR spectra in porous materials

A coarse-grained simulation method to predict NMR spectra of ions diffusing in porous carbons is proposed. The coarse-grained model uses input from molecular dynamics simulations such as the free-energy profile for ionic adsorption, and density-functional theory calculations are used to predict the NMR chemical shift of the diffusing ions. The approach is used to compute NMR spectra of ions in slit pores with pore widths ranging from 2 to 10 nm. As diffusion inside pores is fast, the NMR spectrum of an ion trapped in a single mesopore will be a sharp peak with a pore size dependent chemical shift. To account for the experimentally observed NMR line shapes, our simulations must model the relatively slow exchange between different pores. We show that the computed NMR line shapes depend on both the pore size distribution and the spatial arrangement of the pores. The technique presented in this work provides a tool to extract information about the spatial distribution of pore sizes from NMR spectra. Such information is diffcult to obtain from other characterisation techniques.

cond-mat.mtrl-sci↗

arXiv2014

Visualizing Skin Effects in Conductors with MRI: ${}^7$Li MRI Experiments and Calculations

While experiments on metals have been performed since the early days of NMR (and DNP), the use of bulk metal is normally avoided. Instead, often powders have been used in combination with low fields, so that skin depth effects could be neglected. Another complicating factor of acquiring NMR spectra or MRI images of bulk metal is the strong signal dependence on the orientation between the sample and the radio frequency (RF) coil, leading to non-intuitive image distortions and inaccurate quantification. Such factors are particularly important for NMR and MRI of batteries and other electrochemical devices. Here, we show results from a systematic study combining RF field calculations with experimental MRI of $^7$Li metal to visualize skin depth effects directly and to analyze the RF field orientation effect on MRI of bulk metal. It is shown that a certain degree of selectivity can be achieved for particular faces of the metal, simply based on the orientation of the sample. By combining RF field calculations with bulk magnetic susceptibility calculations accurate NMR spectra can be obtained from first principles. Such analyses will become valuable in many applications involving battery systems, but also metals, in general.

cond-mat.mtrl-sci↗

arXiv2005

Octahedral Tilt Instability of ReO_3-type Crystals

The octahedron tilt transitions of ABX_3 perovskite-structure materials lead to an anti-polar (or antiferroelectric) arrangement of dipoles, with the low temperature structure having six sublattices polarized along various crystallographic directions. It is shown that an important mechanism driving the transition is long range dipole-dipole forces acting on both displacive and induced parts of the anion dipole. This acts in concert with short range repulsion, allowing a gain of electrostatic (Madelung) energy, both dipole-dipole and charge-charge, because the unit cell shrinks when the hard ionic spheres of the rigid octahedron tilt out of linear alignment.

cond-mat.str-el↗

arXiv2003

Rapid Acquisition Pair Distribution Function (RA-PDF) Analysis

An image plate (IP) detector coupled with high energy synchrotron radiation was used for atomic pair distribution function (PDF) analysis, with high probed momentum transfer \Qmax $\leq 28.5$ \RAA from crystalline materials. Materials with different structural complexities were measured to test the validity of the quantitative data analysis. Experimental results are presented here for crystalline Ni, crystalline \alf, and the layered Aurivillius type oxides \bivo and \bivtio . Overall, the diffraction patterns show good counting statistics with measuring time from one to tens of seconds. The PDFs obtained are of high quality. Structures may be refined from these PDFs, and the structural models are consistent with the published literature. Data sets from similar samples are highly reproducible.

cond-mat↗