Elucidating the Transport of Electrons and Molecules in a Solid Electrolyte Interphase Close to Battery Operation Potentials Using a Four-Electrode-Based Generator-Collector Setup.

In lithium-ion batteries, the solid electrolyte interphase (SEI) passivates the anode against reductive decomposition of the electrolyte but allows for electron transfer reactions between anode and redox shuttle molecules, which are added to the electrolyte as an internal overcharge protection. In order to elucidate the origin of these poorly understood passivation properties of the SEI with regard to different molecules, we used a four-electrode-based generator-collector setup to distinguish between electrolyte reduction current and the redox molecule (ferrocenium ion Fc+) reduction current at an SEI-covered glassy carbon electrode. The experiments were carried out in situ during potentiostatic SEI formation close to battery operation potentials. The measured generator and collector currents were used to calculate passivation factors of the SEI with regard to electrolyte reduction and with regard to Fc+ reduction. These passivation factors show huge differences in their absolute values and in their temporal evolution. By making simple assumptions about molecule transport, electron transport, and charge transfer reaction rates in the SEI, distinct passivation mechanisms are identified, strong indication is found for a transition during SEI growth from redox molecule reduction at the electrode | SEI interface to reduction at the SEI | electrolyte interface, and good estimates for the transport coefficients of both electrons and redox molecules are derived. The approach presented here is applicable to any type of electrochemical interphase and should thus also be of interest for interphase characterization in the fields of electrocatalysis and corrosion.

Different Chemical Environments of [Ge4Se10]4- in the Li+ Compounds [Li4(H2O)16][Ge4Se10]·4.33H2O, [{Li4(thf)12}Ge4Se10], and [Li2(H2O)8][MnGe4Se10], and Ionic Conductivity of Underlying "Li4Ge4Se10".

The crystalline selenido germanates [Li4(H2O)16][Ge4Se10]·4.3H2O (1), [{Li4(thf)12}Ge4Se10] (2), and [Li2(H2O)8][MnGe4Se10] (3) (thf = THF = tetrahydrofuran) were obtained by an extraction of a glassy ternary phase of the nominal composition Li4Ge4Se10 (=Li2S·2GeSe2) with water (1) or THF (2) and slow evaporation of the solvent or by being layered with MnBr2 in H2O/MeOH (3), respectively. The compounds contain known selenido germanate anions, however, for the first time with Li+ counterions. This is especially remarkable for the prominent ∞3{[MnGe4Se10]2-} open-framework structure, which was reported to crystallize with (NMe4)+, Cs+, Rb+, and K+ counterions, but it has not yet been realized with the smallest alkali metal cation. Impedance spectroscopic studies on Li4Ge4Se10 classify the glassy solid as a moderate Li+ ion conductor.

Quantification of cation-cation, anion-anion and cation-anion correlations in Li salt/glyme mixtures by combining very-low-frequency impedance spectroscopy with diffusion and electrophoretic NMR.

Directional correlations between the movements of cations and anions exert a strong influence on the charge and mass transport properties of concentrated battery electrolytes. Here, we combine, for the first time, very-low-frequency impedance spectroscopy on symmetrical Li|electrolyte|Li cells with diffusion and electrophoretic NMR in order to quantify cation-cation, anion-anion and cation-anion correlations in Li salt/tetraglyme (G4) mixtures with Li salt to G4 ratios between 1 : 1 and 1 : 2. We find that all correlations are negative, with like-ion anticorrelations (cation-cation and anion-anion) being generally stronger than cation-anion anticorrelations. In addition, we observe that like-ion anticorrelations are stronger for the heavier type of ion and that all anticorrelations become weaker with decreasing Li salt to G4 ratio. These findings are in contrast to theories considering exclusively anion-cation correlations in form of ion pairs, as the latter imply positive cation-anion correlations. We analyze in detail the influence of anticorrelations on Li+ transference numbers and on the Haven ratio. In order to rationalize our results, we derive linear response theory expressions for all ion correlations. These expressions show that the Li+ ion transport under anion-blocking conditions in a battery is governed by equilibrium center-of-mass fluctuations in the electrolytes. This suggests that in future electrolyte theories and computer simulations, more attention should be paid to equilibrium center-of-mass fluctuations.

K2Hg2Te3: Straightforward and Large-Scale Mercury-Flux Synthesis of a Small-Band-Gap Photoconducting Material.

K2Hg2Te3 was synthesized via a mercury-flux synthesis pathway. Single-crystal and powder X-ray diffraction reveal the compound to be isostructural to its lighter congener K2Hg2Se3, yet exhibiting enhanced photoconductivity and electrical conductivity of (several) orders of magnitude and a decreased thermal conductivity and band gap. In this report, we elaborate on the synthesis and properties of the novel ternary compound.

Investigation of Bistetramethylammonium Hydrogencyclotriphosphate-A Molecular Rotor?

The crystalline phase ß-[N(CH3 )4 ]2 HP3 O9 undergoes a reversible phase transition to γ-[N(CH3 )4 ]2 HP3 O9 , which was studied by dynamic scanning calorimetry and X-ray diffraction. The rotational dynamics of the anion [P3 O9 ]3- were evident from variable temperature 31 P magic angle spinning (MAS) NMR spectroscopy. The rotational dynamics could be simulated with a 3-site jump model, which yields spectra in good agreement with experiment. An activation energy of 0.6 eV could be estimated from line shape analysis. Impedance spectra reflect a bulk proton conductivity of γ-[N(CH3 )4 ]2 HP3 O9 of 6.9×10-5  S cm-1 at 240 °C and an activation energy of approximately 1.0 eV. Thus this salt features bulk protonic motion, while local rotational anionic motion happens with activation energies of the same order, as suggested by the paddle-wheel mechanism.

How efficient is Li+ ion transport in solvate ionic liquids under anion-blocking conditions in a battery?

An experimental analysis based on very-low-frequency (VLF) impedance spectra and the Onsager reciprocal relations is combined with advanced analysis of dynamic correlations in atomistic molecular simulations in order to investigate Li+ transport in solvate ionic liquids (SILs). SILs comprised of an equimolar mixture of a lithium salt with glyme molecules are considered as a promising class of highly concentrated electrolytes for future Li-ion batteries. Both simulations and experiments on a prototypical Li-bis(trifluoromethanesulfonyl)imide (TFSI) salt/tetraglyme mixture show that while the ionic conductivity and the Li+ transport number are quite high, the Li+ transference number under 'anion-blocking conditions' is extremely low, making these electrolytes rather inefficient for battery applications. The contribution of cation-anion correlation to the total ionic conductivity has been extracted from both studies, revealing a highly positive contribution due to strongly anti-correlated cation-anion motions. Such cation-anion anti-correlations have also been found in standard ionic liquids and are a consequence of the constraint of momentum conservation. The molecular origin of low Li+ transference number and the influence of anti-correlated motions on Li+ transport efficiency have been investigated as a function of solvent composition. We demonstrate that Li+ transference number can be increased either by reducing the residence time between Li+ and solvent molecules or by adding excessive solvent molecules that are not complexing with Li+.

Time-resolved determination of the potential of zero charge at polycrystalline Au/ionic liquid interfaces.

The potential of zero charge (PZC) is a fundamental property that describes the electrode/electrolyte interface. The determination of the PZC at electrode/ionic liquid interfaces has been challenging due to the lack of models that fully describe these complex interfaces as well as the non-standardized approaches used to characterize them. In this work, we present a method that combines electrode immersion transient and impedance measurements for the determination of the PZC. This combined approach allows the distinction of the potential of zero free charge (pzfc), related to fast double layer charging on a millisecond timescale, from a potential of zero charge on a timescale of tens of seconds related to slower ion transport processes at the interface. Our method highlights the complementarity of these electrochemical techniques and the importance of selecting the correct timescale to execute experiments and interpret the results.

Vacancy-Controlled Na+ Superion Conduction in Na11 Sn2 PS12.

Highly conductive solid electrolytes are crucial to the development of efficient all-solid-state batteries. Meanwhile, the ion conductivities of lithium solid electrolytes match those of liquid electrolytes used in commercial Li+ ion batteries. However, concerns about the future availability and the price of lithium made Na+ ion conductors come into the spotlight in recent years. Here we present the superionic conductor Na11 Sn2 PS12 , which possesses a room temperature Na+ conductivity close to 4 mS cm-1 , thus the highest value known to date for sulfide-based solids. Structure determination based on synchrotron X-ray powder diffraction data proves the existence of Na+ vacancies. As confirmed by bond valence site energy calculations, the vacancies interconnect ion migration pathways in a 3D manner, hence enabling high Na+ conductivity. The results indicate that sodium electrolytes are about to equal the performance of their lithium counterparts.

Synthesis and characterization of 5-cyanotetrazolide-based ionic liquids.

New salts based on imidazolium, pyrrolidinium, phosphonium, guanidinium, and ammonium cations together with the 5-cyanotetrazolide anion [C2 N5 ](-) are reported. Depending on the nature of cation-anion interactions, characterized by XRD, the ionic liquids (ILs) have a low viscosity and are liquid at room temperature or have higher melting temperatures. Thermogravimetric analysis, cyclic voltammetry, viscosimetry, and impedance spectroscopy display a thermal stability up to 230 °C, an electrochemical window of 4.5 V, a viscosity of 25 mPa s at 20 °C, and an ionic conductivity of 5.4 mS cm(-1) at 20 °C for the IL 1-butyl-1-methylpyrrolidinium 5-cyanotetrazolide [BMPyr][C2 N5 ]. On the basis of these results, the synthesized compounds are promising electrolytes for lithium-ion batteries.

LiNi(0.5)Mn(1.5)O4 high-voltage cathode coated with Li4Ti5O12: a hard X-ray photoelectron spectroscopy (HAXPES) study.

A Li4Ti5O12 (LTO) film was coated as buffer layer onto a LiNi0.5Mn1.5O4 (LNMO) high-voltage cathode, and after cycling of the cathode in a battery electrolyte, the LTO film was investigated by means of synchrotron radiation based hard X-ray photoelectron spectroscopy (HAXPES). By tuning the photon energy between 2 keV and 6 keV, we obtained non-destructive depth profiles of the coating material with probing depths ranging from 6 nm to 20 nm. The coating was found to be covered by a few nanometers thin surface layer resulting from electrolyte decomposition. This layer consisted predominantly of organic polymers as well as metal fluorides and fluorophosphates. A positive influence of the Li4Ti5O12 coating with regard to the size and stability of the surface layer was found. The coating itself consisted of a uniform mixture of Li(I), Ti(IV), Ni(II) and Mn(IV) oxides that most likely adopted a spinel structure by forming a solid solution of the two spinels LiNi0.5Mn1.5O4 and Li4Ti5O12 with Li, Mn, Ni and Ti cations mixing on the spinel octahedral sites. The diffusion of Ni and Mn ions into the Li4Ti5O12 lattice occurred during the heat treatment when preparing the cathode. The doping of Li4Ti5O12 with the open d-shell ions Ni(2+) (d(8)) and Mn(4+) (d(3)) should increase the electronic conductivity of the coating significantly, as was found in previous studies. The complex signal structure of the Ti 2p, Ni 2p and Mn 2p core levels provides insight into the chemical nature of the transition metal ions.

Li10SnP2S12: an affordable lithium superionic conductor.

The reaction of Li2S and P2S5 with Li4[SnS4], a recently discovered, good Li(+) ion conductor, yields Li10SnP2S12, the thiostannate analogue of the record holder Li10GeP2S12 and the second compound of this class of superionic conductors with very high values of 7 mS/cm for the grain conductivity and 4 mS/cm for the total conductivity at 27 °C. The replacement of Ge by Sn should reduce the raw material cost by a factor of ~3.

Challenging Prevalent Solid Electrolyte Interphase (SEI) Models: An Atom Probe Tomography Study on a Commercial Graphite Electrode.

Lithium-ion batteries (LIBs) are the dominating energy storage technology for electric vehicles and portable electronic devices. Since the resources of raw materials for LIBs are limited and recycling technologies for LIBs are still under development, improvements in the long-term stability of LIBs are of paramount importance and, in addition, would lead to a reduction in the levelized cost of storage (LCOS). A crucial limiting factor is the aging of the solid electrolyte interphase (SEI) on the active material particles in the anode. Here, we demonstrate the potential of atom probe tomography for elucidating the complex mosaic-type structure of the SEI in a graphite composite anode. Our 3D reconstruction shows unseen details and reveals the existence of an apolar organic microphase pervading the SEI over its entire thickness. This finding is in stark contrast to the prevalent two-layer SEI model, in which organic compounds are the dominating species only in the outer SEI layer being in contact with the liquid electrolyte. The observed spatial arrangement of the apolar organic microphase promises a better understanding of the passivation capability of the SEI, which is necessary to expand the battery lifetime.

Heterobimetallic chalcogenidometallate strands: synthesis, structure, magnetism, and conductivity.

Two salts with one-dimensional, SiS(2)-type telluridostannate chain anions {[MSnTe(4)](2-)}(n), Rb(2)[HgSnTe(4)] (2) and (NMe(4))(2)[MnSnTe(4)] (3), were prepared by the reactions of [SnTe](4-) anions with Hg(2+) or Mn(2+) ions in solution. We present the crystal structures of 2 and 3, as well as the magnetic properties of the previously reported Cs(+) analogue Cs(2)[MnSnTe(4)] (1).

New insights into the interface between a single-crystalline metal electrode and an extremely pure ionic liquid: slow interfacial processes and the influence of temperature on interfacial dynamics.

Ionic liquids are of high interest for the development of safe electrolytes in modern electrochemical cells, such as batteries, supercapacitors and dye-sensitised solar cells. However, electrochemical applications of ionic liquids are still hindered by the limited understanding of the interface between electrode materials and ionic liquids. In this article, we first review the state of the art in both experiment and theory. Then we illustrate some general trends by taking the interface between the extremely pure ionic liquid 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate and an Au(111) electrode as an example. For the study of this interface, electrochemical impedance spectroscopy was combined with in situ STM and in situ AFM techniques. In addition, we present new results for the temperature dependence of the interfacial capacitance and dynamics. Since the interfacial dynamics are characterised by different processes taking place on different time scales, the temperature dependence of the dynamics can only be reliably studied by recording and carefully analysing broadband capacitance spectra. Single-frequency experiments may lead to artefacts in the temperature dependence of the interfacial capacitance. We demonstrate that the fast capacitive process exhibits a Vogel-Fulcher-Tamman temperature dependence, since its time scale is governed by the ionic conductivity of the ionic liquid. In contrast, the slower capacitive process appears to be Arrhenius activated. This suggests that the time scale of this process is determined by a temperature-independent barrier, which may be related to structural reorganisations of the Au surface and/or to charge redistributions in the strongly bound innermost ion layer.

Which Exchange Current Densities Can Be Achieved in Composite Cathodes of Bulk-Type All-Solid-State Batteries? A Comparative Case Study.

The performance of bulk-type all-solid-state Li batteries (ASSBs) depends critically on the contacts between cathode active material (CAM) particles and solid electrolyte (SE) particles inside the composite cathodes. These contacts determine the Li+ exchange current density at the CAM | SE interfaces. Nevertheless, there is a lack of experimental studies on Li+ exchange current densities, which may be caused by the poor understanding of the impedance spectra of ASSBs. We have carried out a comparative case study using two different active materials, namely, single-crystalline LiCoO2 particles and single-crystalline LiNi0.83Mn0.06Co0.11O2 particles. Amorphous 0.67 Li3PS4 + 0.33 LiI particles act as a solid electrolyte within the cathode and separator, and lithiated indium acts as the anode. The determination of the cathode exchange current density is based on (i) impedance measurements on In-Li | SE | In-Li symmetric cells in order to determine the anode impedance together with the anode | separator interfacial impedance and (ii) variation in the composite cathode thickness in order to differentiate between the ion transport resistance and the charge transfer resistance of the composite cathode. We show that under the application of stack pressures in the range of 400 MPa, the Li+ exchange current densities can compete with or even exceed those obtained for CAM | liquid electrolyte interfaces.

Dihalo bismuth cations: unusual coordination properties and inverse solvent effects in Lewis acidity.

A series of well-defined cationic hepta-coordinate bismuth halides [BiX2(py)5][B(3,5-(CF3)2-C6H3)4] (X = Cl, Br, I), stabilized only by substitutionally labile solvent molecules, were synthesized and fully characterized. Their apparent D5h symmetry with a lone pair at the central atom is unprecedented for main group compounds. The potential of BiX3 to show unexpectedly high Lewis acidities in moderately polar solvents is likely due to the formation of [BiX2(solv)5]+ and related ionic species.

An in situ STM/AFM and impedance spectroscopy study of the extremely pure 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate/Au(111) interface: potential dependent solvation layers and the herringbone reconstruction.

The structure and dynamics of the interfacial layers between the extremely pure air- and water-stable ionic liquid 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate and Au(111) has been investigated using in situ scanning tunneling microscopy, cyclic voltammetry, electrochemical impedance spectroscopy, and atomic force microscopy measurements. The in situ scanning tunnelling microscopy measurements reveal that the Au(111) surface undergoes a reconstruction, and at -1.2 V versus Pt quasi-reference the famous (22 × âˆš3) herringbone superstructure is probed. Atomic force microscopy measurements show that multiple ion pair layers are present at the ionic liquid/Au interface which are dependent on the electrode potential. Upon applying cathodic electrode potentials, stronger ionic liquid near surface structure is detected: both the number of near surface layers and the force required to rupture these layers increases. The electrochemical impedance spectroscopy results reveal that three distinct processes take place at the interface. The fastest process is capacitive in its low-frequency limit and is identified with electrochemical double layer formation. The differential electrochemical double layer capacitance exhibits a local maximum at -0.2 V versus Pt quasi-reference, which is most likely caused by changes in the orientation of cations in the innermost layer. In the potential range between -0.84 V and -1.04 V, a second capacitive process is observed which is slower than electrochemical double layer formation. This process seems to be related to the herringbone reconstruction. In the frequency range below 1 Hz, the onset of an ultraslow faradaic process is found. This process becomes faster when the electrode potential is shifted to more negative potentials.

Characterization of Vegard strain related to exceptionally fast Cu-chemical diffusion in Cu[Formula: see text]Mo[Formula: see text]S[Formula: see text] by an advanced electrochemical strain microscopy method.

Electrochemical strain microscopy (ESM) has been developed with the aim of measuring Vegard strains in mixed ionic-electronic conductors (MIECs), such as electrode materials for Li-ion batteries, caused by local changes in the chemical composition. In this technique, a voltage-biased AFM tip is used in contact resonance mode. However, extracting quantitative strain information from ESM experiments is highly challenging due to the complexity of the signal generation process. In particular, electrostatic interactions between tip and sample contribute significantly to the measured ESM signals, and the separation of Vegard strain-induced signal contributions from electrostatically induced signal contributions is by no means a trivial task. Recently, we have published a compensation method for eliminating frequency-independent electrostatic contributions in ESM measurements. Here, we demonstrate the potential of this method for detecting Vegard strain in MIECs by choosing Cu[Formula: see text]Mo[Formula: see text]S[Formula: see text] as a model-type MIEC with an exceptionally high Cu chemical diffusion coefficient. Even for this material, Vegard strains are only measurable around and above room-temperature and with proper elimination of electrostatics. The analyis of the measured Vegards strains gives strong indication that due to a high charge transfer resistance at the tip/interface, the local Cu concentration variations are much smaller than predicted by the local Nernst equation. This suggests that charge transfer resistances have to be analyzed in more detail in future ESM studies.

Understanding the Lifetime of Battery Cells Based on Solid-State Li6PS5Cl Electrolyte Paired with Lithium Metal Electrode.

All-solid-state batteries with solid electrolytes having ionic conductivities in the range of those of liquid electrolytes have gained much interest as safety is still a major issue for applications. Meanwhile, lithium metal seems to be the anode material of choice to face the demand for higher capacities. Still, the main challenges that come with the use of a lithium metal anode, i.e., formation and growth of lithium dendrites, are still not understood very well. This work focuses on the reasons of the lifetime behavior of lithium symmetric cells with the solid electrolyte Li6PS5Cl and lithium electrode. In particular, the voltage increases during the application of a constant current density are investigated. The interface between the lithium metal electrode and the solid electrolyte is analyzed by X-ray photoelectron spectroscopy, and the resistance changes of each electrode during stripping and plating are investigated by impedance spectroscopy on a three-electrode cell. A main factor for the lifetime influenced by lithium dendrite formation and growth is the buildup of a lithium vacancy gradient, leading to voids which decrease the interface area and therefore increase the local current density. Additionally, those lithium vacancies in lithium metal represent a limitation for conductivity rather than migration in solid electrolyte. Further experiments indicate that the seedlike plating behavior of lithium also plays a key role in increased local current density and therefore decreased lifetime. Plating of only a small amount of lithium leads to small areas of well-connected interfaces, resulting in high local current density. A medium amount of plated lithium leads to larger areas of interface between lithium and electrolyte, balancing the current density distribution. In contrast, a high amount of repeatedly deposited lithium leads to lithium seed plating on top of already plated lithium. Those seed spots grown on top represent a better interface connection, which again leads to higher local current densities at those spots and therefore results in shorter lifetimes due to short circuits caused by lithium dendrites.

Inorganic frameworks from selenidotetrelate anions [T2Se6]4- (T = Ge, Sn): synthesis, structures, and ionic conductivity of [K2(H2O)3][MnGe4Se10] and (NMe4)2[MSn4Se10] (M = Mn, Fe).

Syntheses, structures, and physical properties of three inorganic framework compounds [K(2)(H(2)O)(3)][MnGe(4)Se(10)] (1), (NMe(4))(2)[MnSn(4)Se(10)] (2), and (NMe(4))(2)[FeSn(4)Se(10)] (3) are presented. The title compounds are based on a prominent open framework anionic structure; in these cases, however, they contain K(+), the smallest type of counterion to be included so far (1), or represent Sn analogues (2, 3). Both changes with respect to related compounds are reflected in peculiar physical properties, such as ion conductivity or relatively small band gaps.