Search

For the development of safe and long-lasting lithium-ion batteries we need electrolytes with excellent ionic transport properties. Argyrodite-type Li6PS5X (X: Cl, Br, I) belongs to a family of such a class of materials offering ionic conductivities, at least if Li6PS5Br and Li6PS5Cl are considered, in the mS cm−1 range at room temperature. Although already tested as ceramic electrolytes in battery cells, a comprehensive picture about the ion dynamics is still missing. While Li6PS5Br and Li6PS5Cl show an exceptionally high Li ion conductivity, that of Li6PS5I with its polarizable I anions is by some orders of magnitude lower. This astonishing effect has not been satisfactorily understood so far. Studying the ion dynamics over a broad time and length scale is expected to help shed light on this aspect. Here, we used broadband impedance spectroscopy and 7Li NMR relaxation measurements and show that very fast local Li ion exchange processes are taking place in all three compounds. Most importantly, the diffusion-induced NMR spin–lattice relaxation in Li6PS5I is almost identical to that of its relatives. Considering the substitutional disorder effects in Li6PS5X (X = Br, Cl), we conclude that in structurally ordered Li6PS5I the important inter-cage jump processes are switched off, hindering the ions from taking part in long-range ion transport. [ABSTRACT FROM AUTHOR]

Coarse grained, well crystalline γ-LiAlO2 (P43212) is known as an electronic insulator and a very poor ion conductor with the lithium ions occupying tetrahedral voids in the oxide structure. The introduction of structural disorder such as point defects or higher-dimensional defects, however, may greatly affect ionic conduction on both short-range as well as long-range length scales. In the present study, we used high-energy ball milling to prepare defect-rich, nanocrystalline LiAlO2 that was characterized from a structural point of view by powder X-ray diffraction, TEM as well as small angle X-ray scattering (SAXS). Temperature-dependent conductivity spectroscopy revealed an increase of the room-temperature ionic conduction by several orders of magnitude when going from microcrystalline γ-LiAlO2 to its nanocrystalline form. The enhanced ion transport found is ascribed to the increase of Li ions near defective sites both in the bulk as well as in the large volume fraction of interfacial regions in nano-LiAlO2. The nanocrystalline ceramic prepared at long milling times is a mixture of γ-LiAlO2 and the high-pressure phase δ-LiAlO2; it adapts an amorphous like structure after it has been treated in a planetary mill under extremely harsh conditions. [ABSTRACT FROM AUTHOR]

Synthesizing Li-ion-conducting solid electrolytes with application-relevant properties for new energy storage devices is a challenging task that relies on a few design principles to tune ionic conductivity. When starting with originally poor ionic compounds, in many cases, a combination of several strategies, such as doping or substitution, is needed to achieve sufficiently high ionic conductivities. For nanostructured materials, the introduction of conductor-insulator interfacial regions represents another important design strategy. Unfortunately, for most of the two-phase nanostructured ceramics studied so far, the lower limiting conductivity values needed for applications could not be reached. Here, we show that in nanoconfined LiBH 4 /Al 2 O 3 prepared by melt infiltration, a percolating network of fast conductor-insulator Li + diffusion pathways could be realized. These heterocontacts provide regions with extremely rapid 7 Li NMR spin fluctuations giving direct evidence for very fast Li + jump processes in both nanoconfined LiBH 4 /Al 2 O 3 and LiBH 4 -LiI/Al 2 O 3 . Compared to the nanocrystalline, Al 2 O 3 -free reference system LiBH 4 -LiI, nanoconfinement leads to a strongly enhanced recovery of the 7 Li NMR longitudinal magnetization. The fact that almost no difference is seen between LiBH 4 -LiI/Al 2 O 3 and LiBH 4 /Al 2 O 3 unequivocally reveals that the overall 7 Li NMR spin-lattice relaxation rates are solely controlled by the spin fluctuations near or in the conductor-insulator interfacial regions. Thus, the conductor-insulator nanoeffect, which in the ideal case relies on a percolation network of space charge regions, is independent of the choice of the bulk crystal structure of LiBH 4 , either being orthorhombic (LiBH 4 /Al 2 O 3 ) or hexagonal (LiBH 4 -LiI/Al 2 O 3 ). 7 Li (and 1 H) NMR shows that rapid local interfacial Li-ion dynamics is corroborated by rather small activation energies on the order of only 0.1 eV. In addition, the LiI-stabilized layer-structured form of LiBH 4 guarantees fast two-dimensional (2D) bulk ion dynamics and contributes to facilitating fast long-range ion transport., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

Over the past years, ceramic fluorine ion conductors with high ionic conductivity have stepped into the limelight of materials research, as they may act as solid-state electrolytes in fluorine-ion batteries (FIBs). A factor of utmost importance, which has been left aside so far, is the electrochemical stability of these conductors with respect to both the voltage window and the active materials used. The compatibility with different current collector materials is important as well. In the course of this study, tysonite-type La 0.9 Ba 0.1 F 2.9 , which is one of the most important electrolyte in first-generation FIBs, was chosen as model substance to study its electrochemical stability against a series of metal electrodes viz. Pt, Au, Ni, Cu and Ag. To test anodic or cathodic degradation processes we carried out cyclic voltammetry (CV) measurements using a two-electrode set-up. We covered a voltage window ranging from -1 to 4 V, which is typical for FIBs, and investigated the change of the response of the CVs as a function of scan rate (2 mV/s to 0.1 V/s). It turned out that Cu is unstable in combination with La 0.9 Ba 0.1 F 2.9 , even before voltage was applied. The cells with Au and Pt electrodes show reactions during the CV scans; in the case of Au the irreversible changes seen in CV are accompanied by a change in color of the electrode as investigated by light microscopy. Ag and Ni electrodes seem to suffer from contact issues which, most likely, also originate from side reactions with the electrode material. The experiments show that the choice of current collectors in future FIBs will become an important topic if we are to develop long-lasting FIBs. Most likely, protecting layers between the composite electrode material and the metal current collector have to be developed to prevent any interdiffusion or electrochemical degradation processes.

Operando magnetic susceptibility measurements of sodium ion cathode materials during repetitive electrochemical cycling enable a continuous and bulk sensitive monitoring of the transition metal oxidation states. Such measurements on NaxV2(PO4)3 identified vanadium to be the only ion undergoing oxidation/reduction processes upon battery operation. For the initial battery charging-discharging cycle as well as for the first cycle after prolonged room temperature storage, however, peculiarities within the magnetic susceptibility measurements indicate parasitic side reactions, likely on the cathode surface.

For the development of safe and long-lasting lithium-ion batteries we need electrolytes with excellent ionic transport properties. Argyrodite-type Li6PS5X (X: Cl, Br, I) belongs to a family of such a class of materials offering ionic conductivities, at least if Li6PS5Br and Li6PS5Cl are considered, in the mS cm-1 range at room temperature. Although already tested as ceramic electrolytes in battery cells, a comprehensive picture about the ion dynamics is still missing. While Li6PS5Br and Li6PS5Cl show an exceptionally high Li ion conductivity, that of Li6PS5I with its polarizable I anions is by some orders of magnitude lower. This astonishing effect has not been satisfactorily understood so far. Studying the ion dynamics over a broad time and length scale is expected to help shed light on this aspect. Here, we used broadband impedance spectroscopy and 7Li NMR relaxation measurements and show that very fast local Li ion exchange processes are taking place in all three compounds. Most importantly, the diffusion-induced NMR spin-lattice relaxation in Li6PS5I is almost identical to that of its relatives. Considering the substitutional disorder effects in Li6PS5X (X = Br, Cl), we conclude that in structurally ordered Li6PS5I the important inter-cage jump processes are switched off, hindering the ions from taking part in long-range ion transport.

Diffusion of small ions in materials with confined space for translational dynamics can be quite different to isotropic (3D) diffusion, which is found in the majority of solids. Finding credible indications for 2D diffusion is not as easy as it looks at first glance, especially if only powder samples are available. Here we chose the ternary fluoride RbSn2F5 as a new model system to seek out low-dimensional anion diffusion in a nanocrystalline material. We prepared RbSn2F5via mechanochemically-assisted solid state synthesis and used both ac conductivity spectroscopy and spin-lock NMR relaxation measurements to find evidence that the fluorine ions preferably diffuse between the Rb-rich layers. In both cases the diffusion induced spin-lock NMR rates are only consistent with conductivity data if they are analyzed with the semi-empirical spectral density function for 2D jump diffusion as introduced by P. M. Richards [Solid State Commun., 1978, 25, 1019].

Herein, we report the synthesis of a novel, tetrazine-based conjugated polymer. Tetrazines have the benefit of being strong electron acceptors, while little steric hindrance is imposed on the flanking thiophene rings. Conversion of a suitably substituted nitrile precursor led to 3,6-bis(5-bromo-4-(2-octyldodecyl)thiophen-2-yl)-1,2,4,5-tetrazine (2OD-TTz). Palladium-catalyzed copolymerization of 2OD-TTz with a bithiophene monomer yielded an alternating tetrazine-quaterthiophene copolymer (PTz4T-2OD). The polymer PTz4T-2OD showed an optical band gap of 1.8 eV, a deep HOMO energy level of - 5.58 eV and good solubility. In combination with the non-fullerene acceptor ITIC-F, solar cells with power conversion efficiencies of up to 2.6% were obtained., Competing Interests: Compliance with ethical standardsThe authors declare that they have no conflict of interest.

Na battery chemistries show poor passivation behavior of low voltage Na storage compounds and Na metal with organic carbonate-based electrolytes adopted from Li-ion batteries. Therefore, a suitable electrolyte remains a major challenge for establishing Na batteries. Here we report highly concentrated sodium bis(fluorosulfonyl)imide (NaFSI) in dimethoxyethane (DME) electrolytes and investigate them for Na metal and hard carbon anodes and intercalation cathodes. For a DME/NaFSI ratio of 2, a stable passivation of anode materials was found owing to the formation of a stable solid electrolyte interface, which was characterized spectroscopically. This permitted non-dentritic Na metal cycling with approximately 98 % coulombic efficiency as shown for up to 300 cycles. The NaFSI/DME electrolyte may enable Na-metal anodes and allows for more reliable assessment of electrode materials in Na-ion half-cells, as is demonstrated by comparing half-cell cycling of hard carbon anodes and Na 3 V 2 (PO 4 ) 3 cathodes with a widely used carbonate and the NaFSI/DME electrolyte., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Amorphous self-assembled titania nanotube layers are fabricated by anodization in ethylene glycol based baths. The nanotubes having diameters between 70-130 nm and lengths between 4.5-17 μm are assembled in Na-ion test cells. Their sodium insertion properties and electrochemical behavior with respect to sodium insertion is studied by galvanostatic cycling with potential limitation and cyclic voltammetry. It is found that these materials are very resilient to cycling, some being able to withstand more than 300 cycles without significant loss of capacity. The mechanism of electrochemical storage of Na(+) in the investigated titania nanotubes is found to present significant particularities and differences from a classical insertion reaction. It appears that the interfacial region between titania and the liquid electrolyte is hosting the majority of Na(+) ions and that this interfacial layer has a pseudocapacitive behavior. Also, for the first time, the chemical diffusion coefficients of Na(+) into the amorphous titania nanotubes is determined at various electrode potentials. The low values of diffusion coefficients, ranging between 4 × 10(-20) to 1 × 10(-21) cm(2)/s, support the interfacial Na(+) storage mechanism.

A novel electrolyte concept for lithium-ion batteries, termed "ionic liquid-in-salt", is introduced. Our feasibility study on (1 - x)EMIMTFSI:(x)LiTFSI, 0.66 ≤ x ≤ 0.97, showed that at elevated temperatures the various dual liquid and solid phase regions are characterized by a wide thermal stability window, high ionic conductivities and appreciable mechanical integrity. The highest conductivity values are obtained for the compositions x = 0.70 and x = 0.75 (σ ≈ 6 × 10(-3) S cm(-1)) and are related to the final melting of the materials. Overall, high conductivities are observed for 0.70 < x < 0.90 while low ones are found for x > 0.90. Raman and NMR spectroscopies reveal the presence of highly mobile Li-containing species, partly identified as [Li(TFSI)2](-), albeit rather unexpected for these high x values, which might explain the high ionic conductivities observed. To prove the general value of our concept in more detail, some first results on BMIMTFSI and PY13TFSI based systems are also presented.

Ex situ Nuclear Magnetic Resonance (NMR) measurements were carried out to study lithium ion dynamics in lithium intercalated mesoporous anatase (LixTiO2) serving as an anode material for rechargeable lithium-ion batteries. As has been shown recently, hierarchically ordered TiO2 shows excellent cycling performance and ensures a high lithium storage capacity. (7)Li spin-lattice relaxation NMR and stimulated echo NMR serve as a powerful combination to shed light on the Li hopping processes from an atomic-scale point of view. To determine atomic Li jump rates and microscopic activation energies temperature-variable SLR NMR measurements, in both the laboratory and rotating frame of reference, as well as mixing-time dependent spin-alignment echo NMR measurements were carried out. The results point to moderate Li diffusivities; however, in a lithium-ion cell this is compensated for by taking advantage of nm-structured materials with greatly reduced diffusion lengths. Importantly, although a phase transition from tetragonal symmetry to orthorhombic symmetry takes place at increased states of charge, the diffusion parameters and activation energies probed (0.4 to 0.5 eV) do depend weaker on Li content x than expected. Thus, despite the increased value of x, the evolution of the orthorhombic phase seems to support Li diffusivity rather than to affect the transport properties in a negative way. This interesting feature might be highly beneficial for the excellent cycling behavior observed recently.

We describe a novel approach for the fabrication of tailored nanowires using a two-step electrochemical process. It is demonstrated that self-organized TiO(2) nanotubes can be used to activate and guide the electrochemical growth of Sn crystallites, leading to the formation of vertical features with a high aspect ratio. We show that the dimensions and the density of Sn crystallites depend on the electrodeposition parameters.