Restricted lithium ion dynamics in PEO-based block copolymer electrolytes measured by high-field nuclear magnetic resonance relaxation

The intrinsic ionic conductivity of polyethylene oxide (PEO)-based block copolymer electrolytes is often assumed to be identical to the conductivity of the PEO homopolymer. Here, we use high-field ⁷Li nuclear magnetic resonance (NMR) relaxation and pulsed-field-gradient (PFG) NMR diffusion measurements to probe lithium ion dynamics over nanosecond and millisecond time scales in PEO and polystyrene (PS)-b-PEO-b-PS electrolytes containing the lithium salt LiTFSI. Variable-temperature longitudinal (T₁) and transverse (T₂) ⁷Li NMR relaxation rates were acquired at three magnetic field strengths and quantitatively analyzed for the first time at such fields, enabling us to distinguish two characteristic time scales that describe fluctuations of the ⁷Li nuclear electric quadrupolar interaction. Fast lithium motions [up to O(ns)] are essentially identical between the two polymer electrolytes, including sub-nanosecond vibrations and local fluctuations of the coordination polyhedra between lithium and nearby oxygen atoms. However, lithium dynamics over longer time scales [O(10 ns) and greater] are slower in the block copolymer compared to the homopolymer, as manifested experimentally by their different transverse ⁷Li NMR relaxation rates. Restricted dynamics and altered thermodynamic behavior of PEO chains anchored near PS domains likely explain these results.

Negative effective Li transference numbers in Li salt/ionic liquid mixtures: does Li drift in the “Wrong” direction?

Due to association with anions and an inverted drift direction in an electric field, Li⁺ cations have negative effective transference numbers.

Spin relaxation studies of Li+ ion dynamics in polymer gel electrolytes

Two ternary polymer gel electrolyte systems are compared, containing either polyethylene oxide (PEO) or the poly-ionic liquid poly(diallyldimethylammonium) bis(trifluoromethyl sulfonyl)imide (PDADMA-TFSI). Both gel types are based on the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethyl sulfonyl)imide (P₁₄TFSI) and LiTFSI. We study the influence of the polymers on the local lithium ion dynamics at different polymer concentrations using ⁷Li spin-lattice relaxation data in dependence on frequency and temperature. In all cases the relaxation rates are well described by the Cole-Davidson motional model with Arrhenius dependence of the correlation time and a temperature dependent quadrupole coupling constant. For both polymers the correlation times are found to increase with polymer concentration. The activation energy of local motions slightly increases with increasing PEO concentration, and slightly decreases with increasing PDADMA-TFSI concentration. Thus the local Li⁺ motion is reduced by the presence of either polymer; however, the reduction is less effective in the PDADMA⁺ samples. We thus conclude that mechanical stabilization of a liquid electrolyte by a polymer can be achieved at a lower decrease of Li⁺ motion when a cationic polymer is used instead of PEO.

A single cation or anion dendrimer-based liquid electrolyte

We propose here a novel liquid dendrimer-based single ion conductor as a potential alternative to conventional molecular liquid solvent-salt solutions in rechargeable batteries, sensors and actuators. A specific change from ester (-COOR) to cyano (-CN) terminated peripheral groups in generation-one poly(propyl ether imine) (G1-PETIM)-lithium salt complexes results in a remarkable switchover from a high cation (tLi+ = 0.9 for -COOR) to a high anion (tPF6- = 0.8 for -CN) transference number. This observed switchover draws an interesting analogy with the concept of heterogeneous doping, applied successfully to account for similar changes in ionic conductivity arising out of dispersion of insulator particle inclusions in weak inorganic solid electrolytes. The change in peripheral group simultaneously affects the effective ionic conductivity, with the room temperature ionic conductivity of PETIM-CN (1.9 × 10-5 Ω-1 cm-1) being an order of magnitude higher than PETIM-COOR (1.9 × 10-6 Ω-1 cm-1). Notably, no significant changes are observed in the lithium mobility even following changes in viscosity due to the change in the peripheral group. Changes in the peripheral chemical functionality directly influence the anion mobility, being lower in PETIM-COOR than in PETIM-CN, which ultimately becomes the sole parameter controlling the effective transport and electrochemical properties of the dendrimer electrolytes.

A new organic–inorganic hybrid electrolyte based on polyacrylonitrile, polyether diamine and alkoxysilanes for lithium ion batteries: synthesis, structural properties, and electrochemical characterization

The plasticized hybrid electrolyte exhibits a maximum ionic conductivity of 6.4 mS cm⁻¹ at 30 °C and an initial discharge capacity of 123 mA h g⁻¹ in battery testing.

Local Li Cation Coordination and Dynamics in Novel Solid Electrolytes

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 Al2O3 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 SiO2 glass, and a crystalline electrolyte, Li5La3Nb2O12. 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.

Transport Mechanisms of Ions in Graft-Copolymer Based Salt-in-Polymer Electrolytes

Salt-in-polymer electrolytes based on graft copolymers with oligoether side chains and added LiCF3SO3 (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 7Li, 19F and 1H, respectively. The correlation times characterizing local ion dynamics reflect the complexation of the cations by the oligoether side chains of the polymer. 7Li and 19F 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.

Salt-in-Polymer Electrolytes for Lithium Ion Batteries Based on Organo-Functionalized Polyphosphazenes and Polysiloxanes

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 10-4S/cm. Nevertheless, a straightforward strategy based on additives can increase the conductivities to at least 10-3S/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).