Investigating the dendritic growth during full cell cycling of garnet electrolyte in direct contact with Li metal.

All-solid-state batteries including a garnet ceramic as electrolyte are potential candidates to replace the currently used Li-ion technology, as they offer safer operation and higher energy storage performances. However, the development of ceramic electrolyte batteries faces several challenges at the electrode/electrolyte interfaces, which need to withstand high current densities to enable competing C-rates. In this work, we investigate the limits of the anode/electrolyte interface in a full cell that includes a Li-metal anode, LiFePO4 cathode, and garnet ceramic electrolyte. The addition of a liquid interfacial layer between the cathode and the ceramic electrolyte is found to be a prerequisite to achieve low interfacial resistance and to enable full use of the active material contained in the porous electrode. Reproducible and constant discharge capacities are extracted from the cathode active material during the first 20 cycles, revealing high efficiency of the garnet as electrolyte and the interfaces, but prolonged cycling leads to abrupt cell failure. By using a combination of structural and chemical characterization techniques, such as SEM and solid-state NMR, as well as electrochemical and impedance spectroscopy, it is demonstrated that a sudden impedance drop occurs in the cell due to the formation of metallic Li and its propagation within the ceramic electrolyte. This degradation process is originated at the interface between the Li-metal anode and the ceramic electrolyte layer and leads to electromechanical failure and cell short-circuit. Improvement of the performances is observed when cycling the full cell at 55 °C, as the Li-metal softening favors the interfacial contact. Various degradation mechanisms are proposed to explain this behavior

Thermal phase transformations in LaGaO3 and LaAlO3 perovskites: An experimental and computational solid-state NMR study

Multinuclear 71Ga, 69Ga, 27Al and 17O NMR parameters of various polymorphs of LaGaO3 and LaAlO3 perovskites were obtained from the combination of solid-state MAS NMR with solid-state DFT calculations. Some of the materials studied are potential candidate electrolyte materials with applications in intermediate temperature solid oxide fuel cells (ITSOFCs). Small variations in the local distortions of the subject phases are experimentally observed by 71Ga (and 69Ga) and 27Al NMR in the LaGaO3 and LaAlO3 phases, respectively, with heating to 1400 K. The orthorhombic-to-rhombohedral phase transformation occurring in LaGaO3 at approximately 416 K is clearly observed in the 71Ga/69Ga NMR spectra and is associated with a significant increase in the quadrupolar coupling constant (QCC). Thereafter a gradual decrease in QCC is observed, consistent with increased motion of the GaO6 octahedral units and a reduction in the degree of octahedral tilting. The experimental and theoretical 71Ga, 69Ga, 27Al and 17O NMR parameters (including isotropic and anisotropic chemical shift parameters, quadrupolar coupling constants, and associated asymmetries) of the low and high temperature polymorphs are compared. In general, the calculated values display good agreement with experimental data, although some significant deviations are identified and discussed

Mechanical failure of garnet electrolytes during Li electrodeposition observed by in-operando microscopy

Metallic Li anodes are key to reaching high energy densities in next-generation solid-state batteries, however, major problems are the non-uniform deposition of Li at the interface and the penetrative power of Li metal during operation, which cause failure of the ceramic electrolyte, internal short-circuits and a premature end of battery life. In this work, we explore the anode-electrolyte interface instability of a Li metal-garnet electrolyte system during Li electrodeposition, and its implications for mechanical fracture, Li metal propagation, and electrolyte failure. The degradation mechanism was followed step-by-step during in-operando electrochemical cycling using optical and scanning electron microscopy. High amounts of Li electrodeposition in a localized zone of the interface lead to ceramic fracture followed by an electrode-to-electrode electrical connection via a conductor Li metal filament. This work enables deeper understanding of battery failure modes in all-solid-state batteries containing a ceramic electrolyte membrane

Efficient amplitude-modulated pulses for triple- to single-quantum coherence conversion in MQMAS NMR

We thank EPSRC (EP/E041825/1 and EP/J501542/1) for support, for the award of a studentship to H.C. We also thank the ERC (EU FP7 Consolidator Grant 614290 “EXONMR”).The conversion between multiple- and single-quantum coherences is integral to many nuclear magnetic resonance (NMR) experiments of quadrupolar nuclei. This conversion is relatively inefficient when effected by a single pulse, and many composite pulse schemes have been developed to improve this efficiency. To provide the maximum improvement, such schemes typically require time-consuming experimental optimization. Here, we demonstrate an approach for generating amplitude-modulated pulses to enhance the efficiency of the triple- to single-quantum conversion. The optimization is performed using the SIMPSON and MATLAB packages and results in efficient pulses that can be used without experimental reoptimisation. Most significant signal enhancements are obtained when good estimates of the inherent radio-frequency nutation rate and the magnitude of the quadrupolar coupling are used as input to the optimization, but the pulses appear robust to reasonable variations in either parameter, producing significant enhancements compared to a single-pulse conversion, and also comparable or improved efficiency over other commonly used approaches. In all cases, the ease of implementation of our method is advantageous, particularly for cases with low sensitivity, where the improvement is most needed (e.g., low gyromagnetic ratio or high quadrupolar coupling). Our approach offers the potential to routinely improve the sensitivity of high-resolution NMR spectra of nuclei and systems that would, perhaps, otherwise be deemed "too challenging".Publisher PDFPeer reviewe

Crystal structure and proton conductivity of BaSn0.6Sc0.4O3-delta: insights from neutron powder diffraction and solid-state NMR spectroscopy

The solid-state synthesis and structural characterisation of perovskite BaSn(1-x)ScxO(3-delta)(x = 0.0, 0.1, 0.2, 0.3, 0.4) and its corresponding hydrated ceramics are reported. Powder and neutron X-ray diffractions reveal the presence of cubic perovskites (space group Pm (3) over barm) with an increasing cell parameter as a function of scandium concentration along with some indication of phase segregation. Sn-119 and Sc-45 solid-state NMR spectroscopy data highlight the existence of oxygen vacancies in the dry materials, and their filling upon hydrothermal treatment with D2O. It also indicates that the Sn4+ and Sc3+ local distribution at the B-site of the perovskite is inhomogeneous and suggests that the oxygen vacancies are located in the scandium dopant coordination shell at low concentrations (x = 0.3). O-17 NMR spectra on O-17 enriched BaSn1-xScxO3-delta materials show the existence of Sn-O-Sn, Sn-O-Sc and Sc-O-Sc bridging oxygen environments. A further room temperature neutron powder diffraction study on deuterated BaSn0.6Sc0.4O3-delta refines the deuteron position at the 24k crystallographic site (x, y, 0) with x = 0.579(3) and y = 0.217(3) which leads to an O-D bond distance of 0.96(1) angstrom and suggests tilting of the proton towards the next nearest oxygen. Proton conduction was found to dominate in wet argon below 700 degrees C with total conductivity values in the range 1.8 x 10(-4) to 1.1 x 10(-3) S cm(-1) between 300 and 600 degrees C. Electron holes govern the conduction process in dry oxidizing conditions, whilst in wet oxygen they compete with protonic defects leading to a wide mixed conduction region in the 200 to 600 degrees C temperature region, and a suppression of the conductivity at higher temperature

Crystal structure and proton conductivity of BaSn0.6Sc0.4O3-delta: insights from neutron powder diffraction and solid-state NMR spectroscopy

The solid-state synthesis and structural characterisation of perovskite BaSn(1-x)ScxO(3-delta)(x = 0.0, 0.1, 0.2, 0.3, 0.4) and its corresponding hydrated ceramics are reported. Powder and neutron X-ray diffractions reveal the presence of cubic perovskites (space group Pm (3) over barm) with an increasing cell parameter as a function of scandium concentration along with some indication of phase segregation. Sn-119 and Sc-45 solid-state NMR spectroscopy data highlight the existence of oxygen vacancies in the dry materials, and their filling upon hydrothermal treatment with D2O. It also indicates that the Sn4+ and Sc3+ local distribution at the B-site of the perovskite is inhomogeneous and suggests that the oxygen vacancies are located in the scandium dopant coordination shell at low concentrations (x = 0.3). O-17 NMR spectra on O-17 enriched BaSn1-xScxO3-delta materials show the existence of Sn-O-Sn, Sn-O-Sc and Sc-O-Sc bridging oxygen environments. A further room temperature neutron powder diffraction study on deuterated BaSn0.6Sc0.4O3-delta refines the deuteron position at the 24k crystallographic site (x, y, 0) with x = 0.579(3) and y = 0.217(3) which leads to an O-D bond distance of 0.96(1) angstrom and suggests tilting of the proton towards the next nearest oxygen. Proton conduction was found to dominate in wet argon below 700 degrees C with total conductivity values in the range 1.8 x 10(-4) to 1.1 x 10(-3) S cm(-1) between 300 and 600 degrees C. Electron holes govern the conduction process in dry oxidizing conditions, whilst in wet oxygen they compete with protonic defects leading to a wide mixed conduction region in the 200 to 600 degrees C temperature region, and a suppression of the conductivity at higher temperature