Dynamic Patterning in PEO-Based Single Ion Conductors for Li Ion Batteries

Study of the Segmental Dynamics and Ion Transport of Solid Polymer Electrolytes in the Semi-crystalline State

Solid polymer electrolytes are promising in fulfilling the requirements for a stable lithium metal anode toward higher energy and power densities. In this work, we investigate the segmental dynamics, ionic conductivity, and crystallinity of a polymer electrolyte consisting of poly(ethylene oxide) (PEO) and lithium triflate salt, in the semi-crystalline state. Using quasi-elastic neutron scattering, the segmental dynamics of PEO chains confined between the crystalline lamellae is quantified, using Cole-Cole analysis. We show that the structural relaxation time, τ0, of PEO equilibrated near room temperature is six-fold longer than the same sample that had just cooled down to room temperature. This corresponds to a three-fold smaller ionic conductivity in the equilibrated condition. This work reveals that the segmental dynamics of semi-crystalline polymer electrolytes is very sensitive to thermal history. We demonstrate that quasi-elastic neutron scattering can be used to characterize the ion transport and segmental dynamics in the semi-crystalline state.

Ion transport in small-molecule and polymer electrolytes

Solid-state polymer electrolytes and high-concentration liquid electrolytes, such as water-in-salt electrolytes and ionic liquids, are emerging materials to replace the flammable organic electrolytes widely used in industrial lithium-ion batteries. Extensive efforts have been made to understand the ion transport mechanisms and optimize the ion transport properties. This perspective reviews the current understanding of the ion transport and polymer dynamics in liquid and polymer electrolytes, comparing the similarities and differences in the two types of electrolytes. Combining recent experimental and theoretical findings, we attempt to connect and explain ion transport mechanisms in different types of small-molecule and polymer electrolytes from a theoretical perspective, linking the macroscopic transport coefficients to the microscopic, molecular properties such as the solvation environment of the ions, salt concentration, solvent/polymer molecular weight, ion pairing, and correlated ion motion. We emphasize universal features in the ion transport and polymer dynamics by highlighting the relevant time and length scales. Several outstanding questions and anticipated developments for electrolyte design are discussed, including the negative transference number, control of ion transport through precision synthesis, and development of predictive multiscale modeling approaches.

Development of the PEO Based Solid Polymer Electrolytes for All-Solid State Lithium Ion Batteries

Solid polymer electrolytes (SPEs) have attracted considerable attention due to the rapid development of the need for more safety and powerful lithium ion batteries. The prime requirements of solid polymer electrolytes are high ion conductivity, low glass transition temperature, excellent solubility to the conductive lithium salt, and good interface stability against Li anode, which makes PEO and its derivatives potential candidate polymer matrixes. This review mainly encompasses on the synthetic development of PEO-based SPEs (PSPEs), and the potential application of the resulting PSPEs for high performance, all-solid-state lithium ion batteries.

Quasielastic neutron scattering study of microscopic dynamics in polybutadiene reinforced with an unsaturated carboxylate

We studied the dynamics of zinc diacrylate (ZDA) reinforced polybutadiene rubber (BR) (ZDA/BR) using the quasielastic neutron scattering technique to determine the effect of concentration of ZDA on polymer dynamics. First, we evaluated the temperature dependence of mean square displacements (〈u²〉) for ZDA/BR with different ZDA volume fractions. 〈u²〉 increased with temperature below 170 K, and we observed no significant ZDA volume fraction dependence. However, it increased more steeply above 170 K, and the value of 〈u²〉 was smaller for the samples with increasing ZDA fraction. To elucidate the origin of the decrease in 〈u²〉 with increasing ZDA content, dynamic scattering laws (S(Q,ω)) were analyzed. An increase in the elastic component, an increase in the mean relaxation time, and a broadening of distribution of relaxation time were observed with the increasing volume fraction of ZDA. In addition, the ZDA volume fraction dependence of the elastic component roughly corresponded to that of elastic modulus, indicating that the elastic component is related to its mechanical strength. Referring to the previously reported static structure of the present ZDA/BR system, a model for the heterogeneous BR dynamics was proposed. This model assumes the coexistence of immobile, mobile, and interfacial constrained mobile regions. It was found to be appropriate for the explanation of the observed dynamics. We proposed that a network-like structure of the BR having a high crosslinking density around ZDA aggregates is mainly responsible for the high elastic modulus of ZDA/BR.

Thiol-ene synthesis and characterization of lithium bis(malonato)borate single-ion conducting gel polymer electrolytes

The development of high capacity anodes and high voltage cathodes for advanced lithium-ion batteries motivates the search for new polymer electrolytes that exhibit superior electrochemical stabilities and high ionic conductivities. We report a convenient, three-step synthesis of lithium bis(non-8-enyl-malonato)borate (LiBNMB) as a α,ω-diene monomer, which undergoes thermally initiated thiol-ene crosslinking polymerizations in propylene carbonate to yield gel polymer electrolytes with high lithium ion concentrations (∼0.9 M). By conducting these crosslinking polymerizations using mixtures of di- and tri-thiols and LiBNMB with [thiol] : [ene] = 1 : 1, we synthesized a series of gel networks with dynamic elastic moduli ranging from G' = 40-79 kPa that increase monotonically with trifunctional crosslinker content. While ionic conductivities for these polymer gels measured by electrochemical impedance spectroscopy at 22 °C are σ = 0.82-2.5 × 10^-6 S cm^-1, we show that the conductivity of propylene carbonate-solvated lithium ions though the bulk of these gel electrolytes is 8.5 × 10^-5 S cm^-1 independent of crosslinker density. However, the conductivities of the gel interfaces depend sensitively on crosslinker content, suggesting the importance of segmental rearrangement dynamics at the electrode interface in limiting the rate of ion motion. Thus, the design of highly conductive polymer electrolytes for advanced batteries demands careful design of both the internal and interfacial properties of these new materials.

Charge Transport of Polyester Ether Ionomers in Unidirectional Silica Nanopores

Dielectric relaxation spectroscopy is employed to investigate charge transport properties of two polyester ether ionomers in the bulk state and when confined in unidirectional nanoporous membranes (average pore diameter = 7.5 nm). Under nanometric confinement in nonsilanized pores, the macroscopic transport quantities (dc conductivity and characteristic frequency rate) are lower by about 1.4 decades compared to the bulk. The remarkable decrease of transport quantities in nonsilanized nanoporous membranes can be quantitatively explained by considering the temperature dependence of the interfacial layer between the ionomer and the silica membrane surfaces. On the other hand, an enhancement of dc conductivity is observed when the surfaces of the pores are treated with a nonpolar organosilane. This effect becomes more pronounced at lower temperatures and is attributed to slight changes in molecular packing density caused by the two-dimensional geometrical constraint.

Computer Simulations of Ion Transport in Polymer Electrolyte Membranes

Understanding the mechanisms and optimizing ion transport in polymer membranes have been the subject of active research for more than three decades. We present an overview of the progress and challenges involved with the modeling and simulation aspects of the ion transport properties of polymer membranes. We are concerned mainly with atomistic and coarser level simulation studies and discuss some salient work in the context of pure binary and single ion conducting polymer electrolytes, polymer nanocomposites, block copolymers, and ionic liquid-based hybrid electrolytes. We conclude with an outlook highlighting future directions.

Anisotropic ion transport in nanostructured solid polymer electrolytes

We discuss recent progresses on anisotropic ion transport in solid polymer electrolytes.

Silica nanoparticles densely grafted with PEO for ionomer plasticization

PEO-grafted nanoparticles and hydroxylated nanoparticles demonstrate different ionic conductivity–viscosity temperature dependence in nanocomposite ionomers.

Efficient charge transport of a radical polyether/SWCNT composite electrode for an organic radical battery with high charge-storage density

A fast and reversible charge storage capability was established for the radical polyether/SWCNT composite layer with a large layer thickness of several tens of micrometres despite the low SWCNT content of 10%.

Single-ion polymer electrolyte membranes enable lithium-ion batteries with a broad operating temperature range

Conductive processes involving lithium ions are analyzed in detail from a mechanistic perspective, and demonstrate that single ion polymeric electrolyte (SIPE) membranes can be used in lithium-ion batteries with a wide operating temperature range (25-80 °C) through systematic optimization of electrodes and electrode/electrolyte interfaces, in sharp contrast to other batteries equipped with SIPE membranes that display appreciable operability only at elevated temperatures (>60 °C). The performance is comparable to that of batteries using liquid electrolyte of inorganic salt, and the batteries exhibit excellent cycle life and rate performance. This significant widening of battery operation temperatures coupled with the inherent flexibility and robustness of the SIPE membranes makes it possible to develop thin and flexible Li-ion batteries for a broad range of applications.

Superionic behavior in polyethylene-oxide-based single-ion conductors

We demonstrate superionic ion conduction in simulations of a poly(ethylene oxide)-based polymer electrolyte. The superionic conduction uses cation hopping via chain-like ion aggregates, enabling long-range charge transfer while ions only move locally. The Na single-ion conductor achieves two essential features of superionic metal ion conductors: one-dimensional ion structure and immobile anions. The superionic conduction depends on the number and length of conduction pathways, the conduction pathway lifetime, and the rate at which end ions join and leave the pathway.

A physical organogel electrolyte: characterized by in situ thermo-irreversible gelation and single-ion-predominent conduction

Electrolytes are characterized by their ionic conductivity (σ_i). It is desirable that overall σ_i results from the dominant contribution of the ions of interest (e.g. Li⁺ in lithium ion batteries or LIB). However, high values of cationic transference number (t₊) achieved by solid or gel electrolytes have resulted in low σ_i leading to inferior cell performances. Here we present an organogel polymer electrolyte characterized by a high liquid-electrolyte-level σ_i (~10¹ mS cm⁻¹) with high t₊ of Li⁺ (>0.8) for LIB. A conventional liquid electrolyte in presence of a cyano resin was physically and irreversibly gelated at 60°C without any initiators and crosslinkers, showing the behavior of lower critical solution temperature. During gelation, σ_i of the electrolyte followed a typical Arrhenius-type temperature dependency, even if its viscosity increased dramatically with temperature. Based on the Li⁺-driven ion conduction, LIB using the organogel electrolyte delivered significantly enhanced cyclability and thermal stability.