Molecular Alignment and Ion Transport in Rigid Rod Polyelectrolyte Solutions

Achieving High-Performance Lithium-Sulfur Batteries by Modulating Li+ Desolvation Barrier with Liquid Crystal Polymers

Lithium-sulfur (Li-S) batteries offer high theoretical capacity but are hindered by poor rate capability and cycling stability due to sluggish Li2S precipitation kinetics. Here a sulfonate-group-rich liquid crystal polymer (poly-2,2'-disulfonyl-4,4'-benzidine terephthalamide, PBDT) is designed and fabricated to accelerate Li2S precipitation by promoting the desolvation of Li+ from electrolyte. PBDT-modified separators are employed to assemble Li-S batteries, which deliver a remarkable rate capacity (761 mAh g-1 at 4 C) and cycling stability (500 cycles with an average decay rate of 0.088% per cycle at 0.5 C). A PBDT-based pouch cell even delivers an exceptional capacity of ≈1400 mAh g-1 and an areal capacity of ≈11 mAh cm-2 under lean-electrolyte and high-sulfur-loading condition, demonstrating promise for practical applications. Results of Raman spectra, molecular dynamic (MD) and density functional theory (DFT) calculations reveal that the abundant anionic sulfonate groups of PBDT aid in Li+ desolvation by attenuating Li+-solvent interactions and lowering the desolvation energy barrier. Plus, the polysulfide adsorption/catalysis is also excluded via electrostatic repulsion. This work elucidates the critical impact of Li+ desolvation on Li-S batteries and provides a new design direction for advanced Li-S batteries.

Mechanically and Thermally Robust Gel Electrolytes Built from A Charged Double Helical Polymer

Polymer electrolytes have received tremendous interest in the development of solid-state batteries, but often fall short in one or more key properties required for practical applications. Herein, a rigid gel polymer electrolyte prepared by immobilizing a liquid mixture of a lithium salt and poly(ethylene glycol) dimethyl ether with only 8 wt% poly(2,2'-disulfonyl-4,4'-benzidine terephthalamide) (PBDT) is reported. The high charge density and rigid double helical structure of PBDT lead to formation of a nanofibrillar structure that endows this electrolyte with stronger mechanical properties, wider temperature window, and higher battery rate capability compared to all other poly(ethylene oxide) (PEO)-based electrolytes. The ion transport mechanism in this rigid polymer electrolyte is systematically studied using multiple complementary techniques. Li/LiFePO4 cells show excellent capacity retention over long-term cycling, with thermal cycling reversibility between ambient temperature and elevated temperatures, demonstrating compelling potential for solid-state batteries targeting fast charging at high temperatures and slower discharging at ambient temperature.

Machine learning-guided discovery of ionic polymer electrolytes for lithium metal batteries

As essential components of ionic polymer electrolytes (IPEs), ionic liquids (ILs) with high ionic conductivity and wide electrochemical window are promising candidates to enable safe and high-energy-density lithium metal batteries (LMBs). Here, we describe a machine learning workflow embedded with quantum calculation and graph convolutional neural network to discover potential ILs for IPEs. By selecting subsets of the recommended ILs, combining with a rigid-rod polyelectrolyte and a lithium salt, we develop a series of thin (~50 μm) and robust (>200 MPa) IPE membranes. The Li|IPEs|Li cells exhibit ultrahigh critical-current-density (6 mA cm^-2) at 80 °C. The Li|IPEs|LiFePO₄ (10.3 mg cm^-2) cells deliver outstanding capacity retention in 350 cycles (>96% at 0.5C; >80% at 2C), fast charge/discharge capability (146 mAh g^-1 at 3C) and excellent efficiency (>99.92%). This performance is rarely reported by other single-layer polymer electrolytes without any flammable organics for LMBs.

Chiral emergence in multistep hierarchical assembly of achiral conjugated polymers

Intimately connected to the rule of life, chirality remains a long-time fascination in biology, chemistry, physics and materials science. Chiral structures, e.g., nucleic acid and cholesteric phase developed from chiral molecules are common in nature and synthetic soft materials. While it was recently discovered that achiral but bent-core mesogens can also form chiral helices, the assembly of chiral microstructures from achiral polymers has rarely been explored. Here, we reveal chiral emergence from achiral conjugated polymers, in which hierarchical helical structures are developed through a multistep assembly pathway. Upon increasing concentration beyond a threshold volume fraction, dispersed polymer nanofibers form lyotropic liquid crystalline (LC) mesophases with complex, chiral morphologies. Combining imaging, X-ray and spectroscopy techniques with molecular simulations, we demonstrate that this structural evolution arises from torsional polymer molecules which induce multiscale helical assembly, progressing from nano- to micron scale helical structures as the solution concentration increases. This study unveils a previously unknown complex state of matter for conjugated polymers that can pave way to a field of chiral (opto)electronics. We anticipate that hierarchical chiral helical structures can profoundly impact how conjugated polymers interact with light, transport charges, and transduce signals from biomolecular interactions and even give rise to properties unimagined before.

High-Strength Liquid Crystal Polymer-Graphene Oxide Nanocomposites from Water

We report on the morphology and mechanical properties of nanocomposite films derived from aqueous, hybrid liquid crystalline mixtures of rodlike aggregates of a sulfonated, all-aromatic polyamide, poly(2,2'-disulfonyl-4,4'-benzidine terephthalamide) (PBDT), and graphene oxide (GO) platelets. An isothermal step at 200 °C facilitates in situ partial thermal reduction of GO to reduced GO (rGO) in nanocomposite films. X-ray scattering studies reveal that PBDT-rGO nanocomposites exhibit both higher in-plane alignment of PBDT (the order parameter increases from 0.79 to 0.9 at 1.8 vol % rGO) and alignment along the casting direction (from 0.1 to 0.6 at 1.8 vol % rGO). From dynamic mechanical thermal analysis, the interaction between PBDT and rGO causes the β-relaxation activation energy for PBDT to increase with rGO concentration. Modulus mapping of nanocomposites using atomic force microscopy demonstrates enhanced local stiffness, indicating reinforcement. From stress-strain analysis, the average Young's modulus increases from 16 to 37 GPa at 1.8 vol % rGO and the average tensile strength increases from 210 to 640 MPa. Despite polymer alignment along the casting direction, an average transverse tensile strength of 230 MPa is obtained.

Solid-state rigid-rod polymer composite electrolytes with nanocrystalline lithium ion pathways

A critical challenge for next-generation lithium-based batteries lies in development of electrolytes that enable thermal safety along with the use of high-energy-density electrodes. We describe molecular ionic composite electrolytes based on an aligned liquid crystalline polymer combined with ionic liquids and concentrated Li salt. This high strength (200 MPa) and non-flammable solid electrolyte possesses outstanding Li⁺ conductivity (1 mS cm^-1 at 25 °C) and electrochemical stability (5.6 V versus Li|Li⁺) while suppressing dendrite growth and exhibiting low interfacial resistance (32 Ω cm²) and overpotentials (≤120 mV at 1 mA cm^-2) during Li symmetric cell cycling. A heterogeneous salt doping process modifies a locally ordered polymer-ion assembly to incorporate an inter-grain network filled with defective LiFSI and LiBF₄ nanocrystals, strongly enhancing Li⁺ conduction. This modular material fabrication platform shows promise for safe and high-energy-density energy storage and conversion applications, incorporating the fast transport of ceramic-like conductors with the superior flexibility of polymer electrolytes.

Irreversible Shear-Activated Gelation of a Liquid Crystalline Polyelectrolyte

We report irreversible, shear-activated gelation in liquid crystalline solutions of a rigid polyelectrolyte that forms rodlike assemblies (rods) in salt-free solution. At rest, the liquid crystalline solutions are kinetically stable against gelation and exhibit low viscosities. Under steady shear at, or above, a critical shear rate, a physically cross-linked, nematic gel network forms due to linear growth and branching of the rods. Above a critical shear rate, the time scale of gelation can be tuned from hours to nearly instantaneously by varying the shear rate and solution concentration. The shear-activated gels are distinct in their structure and rheological properties from thermoreversible gels. At a fixed concentration, the induction time prior to gelation decreases exponentially with the shear rate. This result indicates that shear-activated thermalization of the electrostatically stabilized rods overcomes the energy barrier for rod-rod contact, enabling rod fusion and subsequent irreversible network formation.

Observation of transition cascades in sheared liquid crystalline polymers

We report on the shear rheology of liquid crystalline solutions composed of charged, rodlike polymers that form supramolecular assemblies dispersed in water. Under steady shear, we observe shear thickening behavior, followed by a hesitation in the viscosity accompanied by an extremely narrow range of negative first normal stress difference. The Peclet number (Pe, shear rate normalized by rod rotational diffusivity) for the onset of shear thickening is in agreement with previous, high-resolution numerical simulations of the Doi-Edwards-Hess kinetic theory. We interrogate these dynamic responses through shear step-down experiments, revealing a complex evolution of transient responses. Detailed analysis of the stress transients provides compelling evidence that the principal axis of the rod orientational distribution, the nematic director, undergoes a cascade of transitions and coexistence of periodic states known as kayaking, tumbling, and wagging, before transitioning to steady flow alignment above a critical shear rate.

Strong graphene oxide nanocomposites from aqueous hybrid liquid crystals

Combining polymers with small amounts of stiff carbon-based nanofillers such as graphene or graphene oxide is expected to yield low-density nanocomposites with exceptional mechanical properties. However, such nanocomposites have remained elusive because of incompatibilities between fillers and polymers that are further compounded by processing difficulties. Here we report a water-based process to obtain highly reinforced nanocomposite films by simple mixing of two liquid crystalline solutions: a colloidal nematic phase comprised of graphene oxide platelets and a nematic phase formed by a rod-like high-performance aramid. Upon drying the resulting hybrid biaxial nematic phase, we obtain robust, structural nanocomposites reinforced with graphene oxide.

SAXS signature of the lamellar ordering of ionic domains of perfluorinated sulfonic-acid ionomers by electric and magnetic field-assisted casting

The origins of the matrix and ionomer peaks of Nafion SAXS pattern are revisited.

Nanofibrillar Ionic Polymer Composites Enable High-Modulus Ion-Conducting Membranes

Polymer electrolyte membranes (PEMs) with high volume fractions of ionic liquids (IL) and high modulus show promise for enabling next-generation gas separations, and electrochemical energy storage and conversion applications. Herein, we present a conductive polymer-IL composite based on a sulfonated all-aromatic polyamide (sulfo-aramid, PBDT) and a model IL, which we term a PBDT-IL composite. The polymer forms glassy and high-aspect-ratio hierarchical nanofibrils, which enable fabrication of PEMs with both high volume fractions of IL and high elastic modulus. We report direct evidence for nanofibrillar networks that serve as matrices for dispersed ILs using atomic force microscopy and small- and wide-angle X-ray scattering. These supramolecular nanofibrils form through myriad noncovalent interactions to produce a physically cross-linked glassy network, which boasts the best combination of room-temperature modulus (0.1-2 GPa) and ionic conductivity (8-4 mS cm^-1) of any polymer-IL electrolyte reported to date. The ultrahigh thermomechanical properties of our PBDT-IL composites provide high moduli (∼1 GPa) at temperatures up to 200 °C, enabling a wide device operation window with stable mechanical properties. Together, the high-performance nature of sulfo-aramids in concert with the inherent properties of ILs imparts PBDT-IL composites with nonflammability and thermal stability up to 350 °C. Thus, nanofibrillar ionic networks based on sulfo-aramids and ILs represent a new design paradigm affording PEMs with exceptionally high moduli at exceedingly low polymer concentrations. This new design strategy will drive the development of new high-performance conductive membranes that can be used for the design of gas separation membranes and in electrochemical applications, such as fuel cells and Li-metal batteries.

Detection of the Order-to-Disorder Transition in Block Copolymer Electrolytes Using Quadrupolar 7Li NMR Splitting

The order-to-disorder transition temperature (T_ODT) in a series of mixtures of polystyrene-b-poly(ethylene oxide) (SEO) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt is identified by the disappearance of a quadrupolar ⁷Li NMR triplet peak splitting above a critical temperature, where a singlet is observed. The macroscopic alignment of ordered domains required to produce a quadrupolar splitting occurs due to exposure to the NMR magnetic field. Alignment is confirmed using small-angle X-ray scattering (SAXS). The T_ODT determined by NMR is consistent with that determined using SAXS.

Double helical conformation and extreme rigidity in a rodlike polyelectrolyte

The ubiquitous biomacromolecule DNA has an axial rigidity persistence length of ~50 nm, driven by its elegant double helical structure. While double and multiple helix structures appear widely in nature, only rarely are these found in synthetic non-chiral macromolecules. Here we report a double helical conformation in the densely charged aromatic polyamide poly(2,2′-disulfonyl-4,4′-benzidine terephthalamide) or PBDT. This double helix macromolecule represents one of the most rigid simple molecular structures known, exhibiting an extremely high axial persistence length (~1 micrometer). We present X-ray diffraction, NMR spectroscopy, and molecular dynamics (MD) simulations that reveal and confirm the double helical conformation. The discovery of this extreme rigidity in combination with high charge density gives insight into the self-assembly of molecular ionic composites with high mechanical modulus (~ 1 GPa) yet with liquid-like ion motions inside, and provides fodder for formation of other 1D-reinforced composites.

Double helix structures appear widely in nature, but only rarely in synthetic non-chiral macromolecules. Here the authors describe a double helix in a densely charged aromatic polyamide, which exhibits an axial rigidity persistence length of ~ 1 μm, much higher than that of DNA (~ 50 nm).

Double network hydrogels based on semi-rigid polyelectrolyte physical networks

Double network hydrogels are formulated from polyacrylamide hydrogels containing semi-rigid polyelectrolytes through post-polymerization immersion in multi-valent ion solutions.

Molecular Structure and Dynamics of Ionic Liquids in a Rigid-Rod Polyanion-Based Ion Gel

The recent fabrication of liquid crystalline ion gels featuring rigid-rod polyanions aligned within room-temperature ionic liquids (RTILs) opens up exciting new avenues for engineering ion conducting materials. These gels exhibit an unusual combination of properties including high ionic conductivity, distinct transport anisotropy, and widely tunable elastic modulus. Using molecular simulations, we study the structure and dynamics of the ions in an ion gel consisting of rigid-rod polyanions and [C₂mim][TfO] RTILs. We show that the ion distribution in the interstitial space between polymer rods exhibits the hallmarks of the RTIL structure near charged surfaces; i.e., cations (C₂mim⁺) and anions (TfO^-) form alternating layers around the polymer rods and the charge on the rod is overscreened by the ionic layer surrounding it. The distinct ordering of ions suggests the formation of a long-range "electrostatic network" in the ion gel, which may contribute to its mechanical cohesion and high modulus. The dynamics of both C₂mim⁺ and TfO^- ions slow down due to the fact that some C₂mim⁺ ions become associated with the sulfonate groups of the polymer rod on nanosecond time scales, which hinders the dynamics of all ions in the gel. C₂mim⁺ and TfO^- ion diffusion in the gel are only 2-10 times slower than in bulk RTILs, which is still much faster than, e.g., Li ions in typical ion conducting polymers. This fast ion transport combined with strong mechanical cohesion open up exciting opportunities for application of these gels in electrochemical devices including Li-metal batteries.

Highly Conductive and Thermally Stable Ion Gels with Tunable Anisotropy and Modulus

A new liquid-crystalline ion gel exhibits unprecedented properties: conductivity up to 8 mS cm(-1) , thermal stability to 300 °C, and electrochemical window to 6.1 V, as well as adjustable transport anisotropy (up to 3.5×) and elastic modulus (0.03-3 GPa). The combination of ionic liquid and magnetically oriented rigid-rod polyanion provides widely tunable properties for use in diverse electrochemical devices.