Operando monitoring of single-particle kinetic state-of-charge heterogeneities and cracking in high-rate Li-ion anodes.

To rationalize and improve the performance of newly developed high-rate battery electrode materials, it is crucial to understand the ion intercalation and degradation mechanisms occurring during realistic battery operation. Here we apply a laboratory-based operando optical scattering microscopy method to study micrometre-sized rod-like particles of the anode material Nb14W3O44 during high-rate cycling. We directly visualize elongation of the particles, which, by comparison with ensemble X-ray diffraction, allows us to determine changes in the state of charge of individual particles. A continuous change in scattering intensity with state of charge enables the observation of non-equilibrium kinetic phase separations within individual particles. Phase field modelling (informed by pulsed-field-gradient nuclear magnetic resonance and electrochemical experiments) supports the kinetic origin of this separation, which arises from the state-of-charge dependence of the Li-ion diffusion coefficient. The non-equilibrium phase separations lead to particle cracking at high rates of delithiation, particularly in longer particles, with some of the resulting fragments becoming electrically disconnected on subsequent cycling. These results demonstrate the power of optical scattering microscopy to track rapid non-equilibrium processes that would be inaccessible with established characterization techniques.

Forced Disorder in the Solid Solution Li3P-Li2S: A New Class of Fully Reduced Solid Electrolytes for Lithium Metal Anodes.

All-solid-state batteries based on non-combustible solid electrolytes are promising candidates for safe energy storage systems. In addition, they offer the opportunity to utilize metallic lithium as an anode. However, it has proven to be a challenge to design an electrolyte that combines high ionic conductivity and processability with thermodynamic stability toward lithium. Herein, we report a new highly conducting solid solution that offers a route to overcome these challenges. The Li-P-S ternary was first explored via a combination of high-throughput crystal structure predictions and solid-state synthesis (via ball milling) of the most promising compositions, specifically, phases within the Li3P-Li2S tie line. We systematically characterized the structural properties and Li-ion mobility of the resulting materials by X-ray and neutron diffraction, solid-state nuclear magnetic resonance spectroscopy (relaxometry), and electrochemical impedance spectroscopy. A Li3P-Li2S metastable solid solution was identified, with the phases adopting the fluorite (Li2S) structure with P substituting for S and the extra Li+ ions occupying the octahedral voids and contributing to the ionic transport. The analysis of the experimental data is supported by extensive quantum-chemical calculations of both structural stability, diffusivity, and activation barriers for Li+ transport. The new solid electrolytes show Li-ion conductivities in the range of established materials, while their composition guarantees thermodynamic stability toward lithium metal anodes.

Bulk fatigue induced by surface reconstruction in layered Ni-rich cathodes for Li-ion batteries.

Ni-rich layered cathode materials are among the most promising candidates for high-energy-density Li-ion batteries, yet their degradation mechanisms are still poorly understood. We report a structure-driven degradation mechanism for NMC811 (LiNi0.8Mn0.1Co0.1O2), in which a proportion of the material exhibits a lowered accessible state of charge at the end of charging after repetitive cycling and becomes fatigued. Operando synchrotron long-duration X-ray diffraction enabled by a laser-thinned coin cell shows the emergence and growth in the concentration of this fatigued phase with cycle number. This degradation is structure driven and is not solely due to kinetic limitations or intergranular cracking: no bulk phase transformations, no increase in Li/Ni antisite mixing and no notable changes in the local structure or Li-ion mobility of the bulk are seen in aged NMCs. Instead, we propose that this degradation stems from the high interfacial lattice strain between the reconstructed surface and the bulk layered structure that develops when the latter is at states of charge above a distinct threshold of approximately 75%. This mechanism is expected to be universal in Ni-rich layered cathodes. Our findings provide fundamental insights into strategies to help mitigate this degradation process.

Al/Ga-Doped Li7La3Zr2O12 Garnets as Li-Ion Solid-State Battery Electrolytes: Atomistic Insights into Local Coordination Environments and Their Influence on 17O, 27Al, and 71Ga NMR Spectra.

Li7La3Zr2O12 (LLZO) garnets are among the most promising solid electrolytes for next-generation all-solid-state Li-ion battery applications due to their high stabilities and ionic conductivities. To help determine the influence of different supervalent dopants on the crystal structure and site preferences, we combine solid-state 17O, 27Al, and 71Ga magic angle spinning (MAS) NMR spectroscopy and density-functional theory (DFT) calculations. DFT-based defect configuration analysis for the undoped and Al and/or Ga-doped LLZO variants uncovers an interplay between the local network of atoms and the observed NMR signals. Specifically, the two characteristic features observed in both 27Al and 71Ga NMR spectra result from both the deviations in the polyhedral coordination/site-symmetry within the 4-fold coordinated Li1/24d sites (rather than the doping of the other Li2/96h or La sites) and with the number of occupied adjacent Li2 sites that share oxygen atoms with these dopant sites. The sharp 27Al and 71Ga resonances arise from dopants located at a highly symmetric tetrahedral 24d site with four corner-sharing LiO4 neighbors, whereas the broader features originate from highly distorted dopant sites with fewer or no immediate LiO4 neighbors. A correlation between the size of the 27Al/71Ga quadrupolar coupling and the distortion of the doping sites (viz. XO4/XO5/XO6 with X = {Al/Ga}) is established. 17O MAS NMR spectra for these systems provide insights into the oxygen connectivity network: 17O signals originating from the dopant-coordinating oxygens are resolved and used for further characterization of the microenvironments at the dopant and other sites.

Under Pressure: Mechanochemical Effects on Structure and Ion Conduction in the Sodium-Ion Solid Electrolyte Na3PS4.

Fast-ion conductors are critical to the development of solid-state batteries. The effects of mechanochemical synthesis that lead to increased ionic conductivity in an archetypical sodium-ion conductor Na3PS4 are not fully understood. We present here a comprehensive analysis based on diffraction (Bragg and pair distribution function), spectroscopy (impedance, Raman, NMR and INS), and ab initio simulations aimed at elucidating the synthesis-property relationships in Na3PS4. We consolidate previously reported interpretations regarding the local structure of ball-milled samples, underlining the sodium disorder and showing that a local tetragonal framework more accurately describes the structure than the originally proposed cubic one. Through variable-pressure impedance spectroscopy measurements, we report for the first time the activation volume for Na+ migration in Na3PS4, which is ∼30% higher for the ball-milled samples. Moreover, we show that the effect of ball-milling on increasing the ionic conductivity of Na3PS4 to ∼10-4 S/cm can be reproduced by applying external pressure on a sample from conventional high-temperature ceramic synthesis. We conclude that the key effects of mechanochemical synthesis on the properties of solid electrolytes can be analyzed and understood in terms of pressure, strain, and activation volume.

Modelling amorphous materials via a joint solid-state NMR and X-ray absorption spectroscopy and DFT approach: application to alumina.

Understanding a material's electronic structure is crucial to the development of many functional devices from semiconductors to solar cells and Li-ion batteries. A material's properties, including electronic structure, are dependent on the arrangement of its atoms. However, structure determination (the process of uncovering the atomic arrangement), is impeded, both experimentally and computationally, by disorder. The lack of a verifiable atomic model presents a huge challenge when designing functional amorphous materials. Such materials may be characterised through their local atomic environments using, for example, solid-state NMR and XAS. By using these two spectroscopy methods to inform the sampling of configurations from ab initio molecular dynamics we devise and validate an amorphous model, choosing amorphous alumina to illustrate the approach due to its wide range of technological uses. Our model predicts two distinct geometric environments of AlO5 coordination polyhedra and determines the origin of the pre-edge features in the Al K-edge XAS. From our model we construct an average electronic density of states for amorphous alumina, and identify localized states at the conduction band minimum (CBM). We show that the presence of a pre-edge peak in the XAS is a result of transitions from the Al 1s to Al 3s states at the CBM. Deconvoluting this XAS by coordination geometry reveals contributions from both AlO4 and AlO5 geometries at the CBM give rise to the pre-edge, which provides insight into the role of AlO5 in the electronic structure of alumina. This work represents an important advance within the field of solid-state amorphous modelling, providing a method for developing amorphous models through the comparison of experimental and computationally derived spectra, which may then be used to determine the electronic structure of amorphous materials.

Impact of Orientational Glass Formation and Local Strain on Photo-Induced Halide Segregation in Hybrid Metal-Halide Perovskites.

Band gap tuning of hybrid metal-halide perovskites by halide substitution holds promise for tailored light absorption in tandem solar cells and emission in light-emitting diodes. However, the impact of halide substitution on the crystal structure and the fundamental mechanism of photo-induced halide segregation remain open questions. Here, using a combination of temperature-dependent X-ray diffraction and calorimetry measurements, we report the emergence of a disorder- and frustration-driven orientational glass for a wide range of compositions in CH3NH3Pb(Cl x Br1-x )3. Using temperature-dependent photoluminescence measurements, we find a correlation between halide segregation under illumination and local strains from the orientational glass. We observe no glassy behavior in CsPb(Cl x Br1-x )3, highlighting the importance of the A-site cation for the structure and optoelectronic properties. Using first-principles calculations, we identify the local preferential alignment of the organic cations as the glass formation mechanism. Our findings rationalize the superior photostability of mixed-cation metal-halide perovskites and provide guidelines for further stabilization strategies.

Establishing Ultralow Activation Energies for Lithium Transport in Garnet Electrolytes.

Garnet-type structured lithium ion conducting ceramics represent a promising alternative to liquid-based electrolytes for all-solid-state batteries. However, their performance is limited by their polycrystalline nature and inherent inhomogeneous current distribution due to different ion dynamics at grains, grain boundaries, and interfaces. In this study, we use a combination of electrochemical impedance spectroscopy, distribution of relaxation time analysis, and solid-state nuclear magnetic resonance (NMR), in order to understand the role that bulk, grain boundary, and interfacial processes play in the ionic transport and electrochemical performance of garnet-based cells. Variable temperature impedance analysis reveals the lowest activation energy for Li transport in the bulk of the garnet electrolyte (0.15 eV), consistent with pulsed field gradient NMR spectroscopy measurements (0.14 eV). We also show a decrease in grain boundary activation energy at temperatures below 0 °C, that is followed by the total conductivity, suggesting that the bottleneck to ionic transport resides in the grain boundaries. We reveal that the grain boundary activation energy is heavily affected by its composition that, in turn, is mainly affected by the segregation of dopants and Li. We suggest that by controlling the grain boundary composition, it would be possible to pave the way toward targeted engineering of garnet-type electrolytes and ameliorate their electrochemical performance in order to enable their use in commercial devices.

7Li NMR Chemical Shift Imaging To Detect Microstructural Growth of Lithium in All-Solid-State Batteries.

All-solid-state batteries potentially offer safe, high-energy-density electrochemical energy storage, yet are plagued with issues surrounding Li microstructural growth and subsequent cell death. We use 7Li NMR chemical shift imaging and electron microscopy to track Li microstructural growth in the garnet-type solid electrolyte, Li6.5La3Zr1.5Ta0.5O12. Here, we follow the early stages of Li microstructural growth during galvanostatic cycling, from the formation of Li on the electrode surface to dendritic Li connecting both electrodes in symmetrical cells, and correlate these changes with alterations observed in the voltage profiles during cycling and impedance measurements. During these experiments, we observe transformations at both the stripping and plating interfaces, indicating heterogeneities in both Li removal and deposition. At low current densities, 7Li magnetic resonance imaging detects the formation of Li microstructures in cells before short-circuits are observed and allows changes in the electrochemical profiles to be rationalized.