Volume 224, Issue 10-12 -

An overview is given on polymer electrolytes based on organo-functionalized polyphosphazenes and polysiloxanes. Chemical and electrochemical properties are discussed with respect to the synthesis, the choice of side groups and the goal of obtaining membranes and thin films that combine high ionic conductivity and mechanical stability. Electrochemical stability, concentration polarization and the role of transference numbers are discussed with respect to possible applications in lithium batteries. It is shown that the ionic conductivities of salt-in-polymer membranes without additives and plasticizers are limited to maximum conductivities around 10 -4 S/cm. Nevertheless, a straightforward strategy based on additives can increase the conductivities to at least 10 -3 S/cm and maybe further. In this context, the future role of polymers for safe, alternative electrolytes in lithium batteries will benefit from concepts based on polymeric gels, composites and hybrid materials. Presently developed polymer electrolytes with oligoether sidechains are electrochemically stable in the potential range 0–4.5V ( vs . Li/Li + reference).

Lithium-transition metal (T)-tetrelides (tetr.= C, Si, Ge, Sn, Pb) are an interesting class of materials with greatly differing crystal structures. The transition metal and tetrel atoms build up covalently bonded networks which leave cavities or channels for the lithium atoms. Depending on the bonding of the lithium atoms to the polyanionic network one observes mobility of the lithium atoms. The crystal chemistry, chemical bonding, 7 Li solid state NMR, and the electrochemical behavior of the tetrelides are reviewed herein.

Materials with high dynamics in the solid state are of potential interest in semiconductor science and for a great variety of energy applications. In most of the cases the majority of physical properties of such compounds are directly related to their electronic structure. Optimization of properties is therefore correlated with direct or indirect control of this parameter. Compounds with highly mobile ions like the coinage metal cations and the heavy chalcogenide anions can easily be modified chemically or physically to fine tune their electronic structures. The range of adjustable properties lasts from ion conductivity, magneto resistance, thermoelectricity to a reversible redox-driven switch of semiconductivity. Today, the strong demand on clean energy production, the efficient energy transport and energy storage is a major goal for present and oncoming generations. Stable, mixed-conducting materials will play a major role in this process. The ongoing development of coinage metal chalcogenide halides during the last 50 years reflects the fundamental interest in this field. Many interesting and sometimes unexpected properties have been determined recently which substantiates the great potential of this class of materials. Especially the new class of coinage metal polychalcogenide halides is of potential interest due to their drastic modulations of physical properties driven by a fundamental tuning of its electronic structures. Thermopower and thermal diffusivity, two major properties to tune thermoelectricity can be varied in such a way that drastic changes can be addressed in very small temperature ranges close to room temperature. Herein we report on the recent progress of polychalcogenide and chalcogenide halides with the mobile d 10 ions Ag and Cu putting the main focus on silver compounds.

We show that NMR multi-time correlation functions provide interesting new insights into the nature of lithium and silver ion dynamics in solids. For solid ion conductors, they usually probe the elementary jumps of the long-range charge transport. NMR two-time correlation functions yield rates and activation energies of the ionic hopping motion. Moreover, they reveal that the ionic relaxation exhibits a strong nonexponentiality in the studied crystals and glasses. NMR three-time correlation functions enable quantitative determination of the origin of the nonexponentiality. For various solid-state electrolytes, the analysis shows that pronounced dynamical heterogeneities govern the ionic hopping motion. If dynamical heterogeneities exist, NMR four-time correlation functions allow one to measure the time scale of exchange processes within the rate distributions. For silver ion dynamics in a crystalline and a glassy material, the results indicate that initially slow ions exhibit random new rates from broad rate distributions after very few jump events.

This paper reviews the progress made in understanding of the mechanisms of ion conduction in polyelectrolyte multilayers (PEM) and polyelectrolyte complexes (PEC). The basis are experimental conductivity data obtained by impedance spectroscopy as a function of relative humidity and temperature, respectively. Mechanically stable thin films of PEM have interesting perspectives as ion conductors, however, being prepared by self-assembly, their stoichiometry and content of ionic charge carriers is unknown. Therefore PEC act as a model material with a variable stoichiometry and known ion content. Employing poly(sodium 4-styrene sulfonate) (NaPSS) and poly(diallyldimethyl ammoniumchloride) (PDADMAC), we present conductivity spectra of dried polyelectrolyte complexes of type x NaPSS·(1- x )PDADMAC as a function of temperature and composition, respectively. The dependence of the dc conductivity is discussed along with scaling properties of the spectra. The results show that the conductivity is always determined by the sodium ions, even in PEC with an excess of PDADMAC. The ion dynamics and transport mechanisms are, however, different in PDADMAC-rich than in NaPSS-rich PEC. PEM of different polyionic compounds are investigated in dependence on relative humidity. A general law of an exponential increase of the dc conductivity with relative humidity is found. Absolute values of the conductivity and the strength of the humidity dependence are different for different polyion materials, however, they do not depend on the type of small counterion employed in layer formation. Therefore, it is concluded that in hydrated PEM, protons are the dominant charge carriers. For both PEM and PEC we show that the MIGRATION concept developed by Funke and co-workers can be used for describing the experimental spectra over wide ranges in frequency. This implies that forward-backward hopping motions of small ions play a vital role in solid polyelectrolyte materials. Apart from these potentially successful hops, localized motions of charged particles are found to influence the conductivity spectra as well.

Glassy ionic conductors have attained considerable importance in the solid state battery technology field. To understand the influence of composition on ionic mobility and transport, information about the local and dynamic environments of the mobile ions in glasses is required. Solid state NMR spectroscopy is one of the most powerful tools for addressing these questions. Our research effort in this area comprises (1) the development of efficient techniques and strategies for the study of disordered materials, (2) the detailed application of optimized measurement and data analysis protocols to different glass systems under systematic compositional variation, and (3) the comprehensive interpretation of the results obtained in the overall literature context. This article attempts to convey a concise overview of our contribution to this field over the period from 2004 to 2010, to describe the most significant insights obtained and to place them into the overall field of Solid State Ionics .

Frequency-dependent third-order conductivity spectra σ 3 ´ ( ν ) of various ion conducting glasses and glass ceramics were obtained by applying sinusoidal electric fields with high amplitudes and by analysing the resulting higher-harmonic currents. In the DC conductivity regime, the third-order conductivity σ 3, dc was found to be positive for all materials and at all temperatures. From the ratio of the third-order conductivity to the low-field conductivity, σ 3, dc / σ 1, dc , apparent jump distances were calculated. These apparent jump distances are much larger than jump distances between neighbouring sites in the glasses and decrease with increasing temperature. In (Li 2 O) 1- x ·(Na 2 O) x ·Al 2 O 3 ·(SiO 2 ) 4 glasses, the mixed alkali effect leads to a minimum in the apparent jump distance, while partial crystallisation of Li 2 O·Al 2 O 3 ·(SiO 2 ) 2 glasses leads to an increase of the apparent jump distance. In the dispersive regime, the third-order conductivity σ 3 ´ ( ν ) of all glasses and glass ceramics is negative and exhibits an approximate power-law dependence, however with a larger exponent than the dispersive low-field conductivity σ 1 ´ ( ν ). For a given material, the third-order conductivity spectra σ 3 ´ ( ν ) obey the time–temperature superposition principle and can be superimposed by using the Summerfield scaling. Remarkably, the shift between the σ 3 ´ ( ν ) master curves of different materials is much stronger than the shift between the σ 1 ´ ( ν ) master curves. In order to rationalize this effect, we calculate the nonlinear dispersive hopping conductivity in a double minimum potential approximation.

Conductivity measurements and diffusion studies with radioactive and stable isotopes were performed to investigate the diffusion of both network modifiers (alkali (A) and alkaline-earth (M) ions) and network formers (oxygen (O) and silicon (Si)) in various mixed-cation silicate glasses below the respective glass transition temperatures. The dynamics of the network modifiers A and M were studied in bulk glasses prepared from the melt and in sol–gel derived glass films. The diffusion anneals were performed under air and under reducing conditions in forming gas. No impact of the glass preparation on the diffusion of the network modifiers was found. However, the diffusion of the alkaline-earth ions is affected by additional defects formed under reducing conditions. The dynamics of the network formers O and Si was investigated against the type of alkali and alkaline-earth ions, their composition ratio, and the number of non-bridging oxygens. We found that Si diffusion is directly related to the rigidity of the glass. Combining the data on O diffusion with previous results on the diffusion of alkaline-earth ions M in glasses of the same composition, we conclude that the diffusion of O is likely assisted by M via the formation of O–M pairs.

The paper highlights some remarkable results obtained by applying the radiotracer diffusion (RTD) technique to the study of ionic transport in salt-in-polymer electrolytes. The technique is based on the determination of a radiotracer depth profile by serial sectioning following isothermal diffusion annealing. Unlike alternative methods, RTD is able to measure the self-cation and self-anion diffusivity even for systems dilute in salt. Another unique feature is the capability to investigate foreign-ion diffusion at extremely low concentration levels. Combined with DC conductivity data, RTD may provide a virtually complete picture of mass and charge transport in solid-like polymer electrolytes (SPEs). The paper describes the special SPE-related procedures used in the RTD experiments and their analysis. The advantages of the method will be demonstrated with selected examples of self- and foreign-ion diffusion in prototype SPE systems. We also present prominent examples of RTD dealing with the effects of salt precipitation and oxide nano-particles used as dispersed filler material.

Research on solid ionic conductors for use as electrolytes in all solid state batteries still constitutes a rather vivid branch of today´s materials science. Despite enormous efforts, neither the development of a solid electrolyte fulfilling the key requirements such as mechanical stability and high ionic conductivity at ambient temperature has been successful nor has an extended understanding of the local Li coordination motifs in the often amorphous systems been obtained. In this contribution, recent progress both in the development of novel solid state electrolytes with high ionic conductivity and mechanical stability and in the characterization of the local Li coordination motifs in these electrolytes from our laboratory is presented. The work was performed as a project within the framework of the Collaborative Research Centre SFB 458 “Ionic Motion in Materials with Disordered Structures — From Elementary Steps to Macroscopic Transport”. Results will be given for polymer electrolytes based on polyethylene oxide (PEO), polyphosphazene (PPZ) and polyacrylonitrile (PAN) with various Li salts, nano-composites of these polymer electrolytes and Al 2 O 3 as a ceramic filler, novel inorganic/organic hybrid electrolytes, in which a mixture of an ionic liquid and Li salt is confined within the pore system of a SiO 2 glass, and a crystalline electrolyte, Li 5 La 3 Nb 2 O 12 . Employing a range of advanced solid state NMR methodologies including dipolar based NMR techniques and pulsed field gradient (PFG) NMR and impedance spectroscopy we were able to obtain a detailed knowledge about the local Li cation coordination motifs and the mechanism of Li transport in these electrolytes. Especially the hybrid electrolytes and the salt rich PAN based polymer electrolytes were identified as rather promising materials which combine a high ionic conductivity and mechanical stability.

Salt-in-polymer electrolytes based on graft copolymers with oligoether side chains and added LiCF 3 SO 3 (LiTf) are investigated concerning the transport and dynamics of the ionic species with respect to applications as Li ion conductors. Polymer architectures are based on polysiloxane or polyphosphazene backbones with one or two side chains per monomer, respectively. NMR methods provide information about molecular dynamics on different length scales: The mechanisms governing local dynamics and long range mass transport are studied on the basis of temperature dependent spin-lattice relaxation rates and pulsed field gradient diffusion measurements for 7 Li, 19 F and 1 H, respectively. The correlation times characterizing local ion dynamics reflect the complexation of the cations by the oligoether side chains of the polymer. 7 Li and 19 F diffusion coefficients and their activation energies are rather similar, suggesting the formation of ion pairs and clusters with similar activation barriers for cation and/or anion long-range transport. Activation energies of local reorientations are generally significantly smaller than activation energies of long range diffusion. Long range transport is affected by (1) the coupling of conformational side chain reorientations to the cation movement, and (2) the correlated diffusion of cations and anions within ion pairs. Ion pairs and their dissociation play a major role in controlling the resulting conductivity of the material. Guidelines for material optimization in terms of a maximized conductivity can thus be derived by identifying a compromise between high ionic mobility and good Li complexation by the coordinating side chains.

Thin film ion-conductive materials may represent essential components of future all-solid-state batteries. Besides, they are also of interest from a fundamental point of view. In this article, the deposition of thin films of complex oxides and amorphous glasses is discussed. Methods of local chemical analysis by analytical electron microscopy and atom probe tomography are described and studies of atomic transport in sputtered network glasses are presented. As experimental examples, thin films of Li borate and silicate glasses, LiCoO 2 , V 2 O 5 , and Li 4 Ti 5 O 12 are addressed.

A nanoscopic technique based on electrostatic force spectroscopy in the time domain is introduced. This technique is used to characterize the ion dynamics in nanoscale subvolumes of solid electrolytes. Nanoscopic polarization spots can be created and directly visualized, and their time evolution can be studied. In the case of partially crystallized glass ceramics, the dynamic processes in different phases (glassy, crystalline and interface) can be distinguished and their activation energies and pre-exponential factors can be quantified. By applying grid-type spectroscopic measurements, maps of the local relaxation strength are obtained, giving information about the spatial distribution of the glassy and crystalline phase.

The modelling of the ion conduction in disordered systems is analysed from two different perspectives. First, molecular dynamics simulations are employed to extract some basic properties of the hopping dynamics. It turns out that the dynamical processes can be described to a very good approximation as vacancy hopping processes. Second, the information content of nonlinear conductivity experiments, using high electric fields, is elucidated. For this purpose the single-particle dynamics on 1D and 2D model energy landscapes is elucidated numerically and partly analytically. The approaches encompass discrete as well as continuous energy landscapes, yielding complementary results about the dynamics. The impact for the interpretation of experimental data is discussed.

Understanding the mechanisms of translational and localised ionic movements in disordered materials has seen intense activity spanning several decades. This article attempts to convey a concise overview of our contribution to this field over the period from 2005 to 2010 and to place it in its broad context.