Evaluating lithium diffusion mechanisms in the complex spinel Li2NiGe3O8

A review on TiO2-x-based materials for photocatalytic CO2 reduction

Photocatalytic CO₂ reduction technology has a broad potential for dealing with the issues of energy shortage and global warming. As a widely studied material used in the photocatalytic process, titanium dioxide (TiO₂) has been continuously modified and tailored for more desirable application. Recently, the defective/reduced titanium dioxide (TiO_2-x) catalyst has attracted broad attention due to its excellent photocatalytic performance for CO₂ reduction. In this perspective review, we comprehensively present the recent progress in TiO_2-x-based materials for photocatalytic CO₂ reduction. In detail, the review starts with the fundamentals of CO₂ photocatalytic reduction. Then, the synthesis of a defective TiO₂ structure is introduced for the regulation of its photocatalytic performance, especially its optical properties and dissociative adsorption properties. In addition, the current application of TiO_2-x-based photocatalysts for CO₂ reduction is also highlighted, such as metal-TiO_2-x, oxide-TiO_2-x and TiO_2-x-carbon-based photocatalysts. Finally, the existing challenges and possible scope of photocatalytic CO₂ reduction over TiO_2-x-based materials are discussed. We hope that this review can provide an effective reference for the development of more efficient and reasonable photocatalysts based on TiO_2-x.

Defect Properties of Li2NiGe3O8

There is a growing interest in finding a suitable electrolyte material for the construction of rechargeable Li-ion batteries. Li2NiGe3O8 is a material of interest with modest Li-ionic conductivity. The atomistic simulation technique was applied to understand the defect processes and Li-ion diffusion pathways, together with the activation energies and promising dopants on the Li, Ni, and Ge sites. The Li-Ni anti-site defect cluster was found to be the dominant defect in this material, showing the presence of cation mixing, which can influence the properties of this material. Li-ion diffusion pathways were constructed, and it was found that the activation energy for a three-dimensional Li-ion migration pathway is 0.57 eV, which is in good agreement with the values reported in the experiment. The low activation energy indicated that Li-ion conductivity in Li2NiGe3O8 is fast. The isovalent doping of Na, Fe and Si on the Li, Ni and Ge sites is energetically favorable. Both Al and Ga are candidate dopants for the formation of Li-interstitials and oxygen vacancies on the Ge site. While Li-interstitials can improve the capacity of batteries, oxygen vacancies can promote Li-ion diffusion.

Extremely Fast Interfacial Li Ion Dynamics in Crystalline LiTFSI Combined with EMIM-TFSI

Materials providing fast transport pathways for ionic charge carriers are at the heart of future all-solid state batteries that completely rely on sustainable, nonflammable solid electrolytes. The mobile ions in fast ion conductors may take benefit from structural disorder, cation and anion substitution, or dimensionality effects. While these effects concern the bulk regions of a given material, one may also manipulate the surface or interfacial regions of a polycrystalline poorly conducting electrolyte to enhance its transport properties. Here, we used ⁷Li NMR to characterize interfacial effects in crystalline lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) to which a small amount of ionic liquid EMIM-TFSI (EMIM: 1-ethyl-3-methylimidazolium cation, C₆H₁₁N₂ ⁺) was added. We recorded longitudinal spin-lattice relaxation (SLR) curves M _z (t _d) that directly mirror the ⁷Li spin-fluctuations controlled by motional processes in such ionic-liquids-in-salt composites. Already at room temperature we observe strongly bimodal buildup curves M _z (t _d) leading to two distinct SLR rates differing by a factor of 100. While the slower rate does exactly reflect the temperature behavior expected for poorly conducting LiTFSI, the faster rate mirrors rapid motional processes that are governed by an activation energy as low as 73 meV. We attribute these fast processes to highly mobile Li⁺ ions in or near the contact area of crystalline LiTFSI and EMIM-TFSI. By using a method that characterizes motional processes from the atomic-scale point of view, we emphasize the importance of interfacial regions as reservoirs for fast Li⁺ ions in such solid composite electrolytes.

Fuzzy logic: about the origins of fast ion dynamics in crystalline solids

Nuclear magnetic resonance offers a wide range of tools to analyse ionic jump processes in crystalline and amorphous solids. Both high-resolution and time-domain [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] NMR helps throw light on the origins of rapid self-diffusion in materials being relevant for energy storage. It is well accepted that [Formula: see text] ions are subjected to extremely slow exchange processes in compounds with strong site preferences. The loss of this site preference may lead to rapid cation diffusion, as is also well known for glassy materials. Further examples that benefit from this effect include, e.g. cation-mixed, high-entropy fluorides [Formula: see text], Li-bearing garnets ([Formula: see text]) and thiophosphates such as [Formula: see text]. In non-equilibrium phases site disorder, polyhedra distortions, strain and the various types of defects will affect both the activation energy and the corresponding attempt frequencies. Whereas in [Formula: see text] ([Formula: see text]) cation mixing influences F anion dynamics, in [Formula: see text] ([Formula: see text]) the potential landscape can be manipulated by anion site disorder. On the other hand, in the mixed conductor [Formula: see text] cation-cation repulsions immediately lead to a boost in [Formula: see text] diffusivity at the early stages of chemical lithiation. Finally, rapid diffusion is also expected for materials that are able to guide the ions along (macroscopic) pathways with confined (or low-dimensional) dimensions, as is the case in layer-structured [Formula: see text] or [Formula: see text]. Diffusion on fractal systems complements this type of diffusion. This article is part of the Theo Murphy meeting issue 'Understanding fast-ion conduction in solid electrolytes'.

In Situ Diffusion Measurements of a NASICON-Structured All-Solid-State Battery Using Muon Spin Relaxation

In situ muon spin relaxation is demonstrated as an emerging technique that can provide a volume-averaged local probe of the ionic diffusion processes occurring within electrochemical energy storage devices as a function of state of charge. Herein, we present work on the conceptually interesting NASICON-type all-solid-state battery LiM2(PO4)3, using M = Ti in the cathode, M = Zr in the electrolyte, and a Li metal anode. The pristine materials are studied individually and found to possess low ionic hopping activation energies of ∼50-60 meV and competitive Li+ self-diffusion coefficients of ∼10-10-10-9 cm2 s-1 at 336 K. Lattice matching of the cathode and electrolyte crystal structures is employed for the all-solid-state battery to enhance Li+ diffusion between the components in an attempt to minimize interfacial resistance. The cell is examined by in situ muon spin relaxation, providing the first example of such ionic diffusion measurements. This technique presents an opportunity to the materials community to observe intrinsic ionic dynamics and electrochemical behavior simultaneously in a nondestructive manner.

Recent Developments of Nanomaterials and Nanostructures for High-Rate Lithium Ion Batteries

Lithium ion batteries have been considered as a promising energy-storage solution, the performance of which depends on the electrochemical properties of each component, including cathode, anode, electrolyte and separator. Currently, fast charging is becoming an attractive research field due to the widespread application of batteries in electric vehicles, which are designated to replace conventional diesel automobiles in the future. In these batteries, rate capability, which is closely linked to the topology and morphology of electrode materials, is one of the determining parameters of interest. It has been revealed that nanotechnology is an exceptional tool in designing and preparing cathodes and anodes with outstanding electrochemical kinetics due to the well-known nanosizing effect. Nevertheless, the negative effects of applying nanomaterials in electrodes sometimes outweigh the benefits. To better understand the exact function of nanostructures in solid-state electrodes, herein, a comprehensive review is provided beginning with the fundamental theory of lithium ion transport in solids, which is then followed by a detailed analysis of several major factors affecting the migration of lithium ions in solid-state electrodes. The latest developments in characterisation techniques, based on either electrochemical or radiology methodologies, are covered as well. In addition, state-of-the-art research findings are provided to illustrate the effect of nanomaterials and nanostructures in promoting the rate performance of lithium ion batteries. Finally, several challenges and shortcomings of applying nanotechnology in fabricating high-rate lithium ion batteries are summarised.

Lithium-Ion Transport in Nanocrystalline Spinel-Type Li[InxLiy]Br4 as Seen by Conductivity Spectroscopy and NMR

Currently, a variety of solid Li+ conductors are being discussed that could potentially serve as electrolytes in all-solid-state Li-ion batteries and batteries using metallic Li as the anode. Besides oxides, sulfides and thioposphates, and also halogenides, such as Li3YBr6, belong to the group of such promising materials. Here, we report on the mechanosynthesis of ternary, nanocrystalline (defect-rich) Li[In x Li y ]Br4, which crystallizes with a spinel structure. We took advantage of a soft mechanochemical synthesis route that overcomes the limitations of classical solid-state routes, which usually require high temperatures to prepare the product. X-ray powder diffraction, combined with Rietveld analysis, was used to collect initial information about the crystal structure; it turned out that the lithium indium bromide prepared adopts cubic symmetry ( Fd 3 ¯ m ). The overall and electronic conductivity were examined via broadband conductivity spectroscopy and electrical polarization measurements. While electric modulus spectroscopy yielded information on long-range ion transport, 7Li nuclear magnetic resonance (NMR) spin-lattice relaxation measurements revealed rapid, localized ionic hopping processes in the ternary bromide. Finally, we studied the influence of thermal treatment on overall conductivity, as the indium bromide might find applications in cells that are operated at high temperatures (330 K and above).