Session L43: Focus Session: Polymers for Energy Storage and Conversion -- Emerging Applications

Block copolymers exhibiting simultaneous electronic and ionic conduction for use in lithium battery electrodes

A block copolymer system that can demonstrate both electronic and ionic conductivity is analyzed for its performance in rechargeable lithium batteries. Here, the electrically active polymer is poly(3-hexylthiophene), while poly(ethylene oxide) is used as the lithium ion conductor. This block copolymer is then mixed with LiFePO$_{4}$ and used as the cathode material. Other components in the battery include a lithium metal anode and poly(styrene)-\textit{block}-poly(ethylene oxide) (SEO) as the solid electrolyte. Lithium bis(trifluoromethane)sulfonimide (LiTFSI) is utilized to facilitate ionic conductivity in both the electrolyte and the cathode. The synthesis of the block copolymer and its device performance in rechargeable lithium metal batteries will be presented.   

All Solid State Rechargeable Lithium Batteries using Block Copolymers

The growing need for alternative energy and increased demand for mobile technology require higher density energy storage. Existing battery technologies, such as lithium ion, are limited by theoretical energy density as well as safety issues. Other battery chemistries are promising options for dramatically increasing energy density. Safety can be improved by replacing the flammable, reactive liquids used in existing lithium-ion battery electrolytes with polymer electrolytes. Block copolymers are uniquely suited for this task because ionic conductivity and mechanical strength, both important properties in battery formulation, can be independently controlled. In this study, lithium batteries were assembled using lithium metal as negative electrode, polystyrene-b-poly(ethylene oxide) copolymer with lithium salt as electrolyte, and a positive electrode. The positive electrode consisted of polymer electrolyte for ion conduction, carbon for electron conduction, and an active material. Batteries were charged and discharged over many cycles. The battery cycling results were compared to a conventional battery chemistry.   

Solubility of Lithium Polysulfides in a Block Copolymer Electrolyte for Lithium/Sulfur Batteries

The primary challenges to commercialization of the high-energy-density lithium sulfur battery are dendrite growth of the lithium metal at the anode and capacity fade due to loss of active mass through dissolution at the cathode. Nanostructured solid polymer electrolytes offer one potential solution to reduce the amount of capacity fade seen in lithium metal/sulfur batteries by keeping the active material localized at the cathode and to prevent the growth of dendrites at the anode due to their high shear moduli. The block copolymer electrolyte poly(styrene)-\textit{block}-poly(ethylene oxide) (SEO) has shown acceptable ionic conductivity and sufficient shear modulus to retard lithium dendrite growth. The solubility of the lithium polysulfide reaction intermediates Li$_{2}$S$_{x}$, where 1 $\le $ x $\le $ 8, was studied in SEO copolymers with a range of molecular weights and salt concentrations using small angle X-ray scattering, X-ray diffraction, and differential scanning calorimetery.   

Solution Processable Hybrid Polymer-Inorganic Thermoelectric Materials

In the last decade thermoelectric material improvements have largely been attributed to a reduction in thermal conductivity due to nanostructuring. An alternative approach is to decouple and optimize the power factor using the unique properties of organic-inorganic interfaces. One method to do this could rely on the electrical properties of a conducting polymer in combination with the thermoelectrical proprieties of an inorganic semiconductors. It is expected that the thermal conductivity of this hybrid material would be low due to the inherent phonon mismatch between polymers and inorganics. Recently we have developed a method for producing a solution processable bulk thermoelectric material (ZT$>$0.1) using a hybrid polymer-inorganic systems consisting of crystalline tellurium nanowires coated in a thin layer of PEDOT:PSS. The interface properties of these materials scale and bulk films demonstrate enhanced transport properties beyond those of either component. Here, we present our methodology, theoretical explanation, and experimental transport properties of this new class of materials where the thermal conductivity, electrical conductivity, and thermopower predictably vary as a function of polymer loading in the hybrid composite.   

Microscopic mechanism of energy storage in PVDF-CTFE from ab-initio calculations

Polypropelene is most used capacitor dielectric for high energy density storage. However, exotic materials such as copolymerized Polyvinylidene fluoride (PVDF) could potentially lead to an order of magnitude increase in the stored energy density [1,2]. In contrast to linear dielectric properties of polypropelene, several polymers in the PVDF family display nonlinear dielectric properties under electric field. The nonlinearity was postulated to be due to a phase transition from non-polar to a polar structure, whose energy is lowered by an electric field [2]. Our calculations map out the atomistic details of phase transformations for both pure PVDF and PVDF-CTFE. Interestingly, admixture of a small amount of copolymer lowers both the polarization and the energy barriers for the transformation. The barrier lowering facilitates the transformation and may result in reduced loss in the charge-discharge cycle, enabling tuning of material properties for energy storage applications. \\[4pt] [1] B. Chun et al, Science \textbf{313}, 334 (2006).\\[0pt] [2] V. Ranjan et al, Phys. Rev. Lett. \textbf{99}, 047801 (2007).   

Nanotube Forests for Electrochemical Energy Storage from Electrostatic Assembly

With increasing global energy consumption, efficient energy storage sytems are urgently needed. Currently, lithium-ion batteries are prevalent in many of these applications because of their established reliability and superior performance relative to older technologies; however, Li-ion batteries can be limited by mass transfer and safety concerns. Here, we present nanostructured polymer-based electrodes that potentially address these limitations. We apply layer-by-layer (LbL) assembly and nanotemplating to realize LbL-nanotube cathode arrays containing vanadium pentoxide and polyaniline. Both polyaniline and V2O5 store charge via doping/undoping and intercalation/deintercalation, respectively. The aim is to create high surface area electrodes that minimize the diffusion resistance of reactants, which could boost power density. The (LbL) growth profile was monitored using UV-Vis spectroscopy and profilometery. Electrochemical properties were characterized using cyclic voltammetery. Scanning electron microscopy images confirm that large areas of LbL nanotubes can be made. Future work will assess how nanostructured cathodes will behave electrochemically as nanotube aspect ratio is varied.   

Block-Copolymer Lithium Battery Electrolytes

With high energy density at low cost, Li ion has become the most prevalent portable rechargeable battery chemistry in the world. As demand for smaller and lighter batteries grows, the energy density limitation of Li ion batteries presents a significant hurdle. Pushing the existing Li ion platform to higher energy densities compromises lifetime and safety, and these have emerged as the most pressing challenges in today's industry. The weakest link in terms of safety and stability of Li ion batteries is the organic liquid electrolyte that facilitates ionic transport between the electrodes. The continuous electrochemical degradation of the electrolyte at the electrodes causes poor cycle life of the batteries, and in some cases, runaway reactions that lead to explosions. Dry polymer electrolytes coupled to Li metal anodes had been considered a high energy alternative to liquid-based systems, as the solid-solid interface promised to alleviate the stability problems of the liquid electrolyte. However, repeated cycling of Li metal anodes leads to dendrite formation, reducing battery life and compromising safety. Recent theoretical work indicates that dendrite growth can be stopped if the shear modulus of current polymer electrolytes can be increased by three orders of magnitude without a significant decrease in ionic conductivity. Thus, the mechanical properties of polymer electrolytes are particularly important in rechargeable solid-state lithium batteries. Because ion transport in polymers is coupled to the motion of the molecules that are solvating the ions, the presence of mobile molecules is essential to allow for a conductive medium. However, the same mobility of molecules is detrimental to the polymer's structural integrity. There is, thus, a clear need to develop methodologies for decoupling the conductive and mechanical properties of polymer electrolytes. Electrolytes comprised of self-assembled block-copolymer nanostructures overcome this principal constraint.   

Effect of Nanoscale Morphology on Selective Ethanol Transport through Block Copolymer Membranes

We have examined the possibility of using A-B block copolymers for selective separation of alcohols from aqueous mixtures. The A block is not soluble in the liquids of interest and serves as the structural block while B serves as the transporting block. The size of the transporting channels has been controlled by varying the molecular weight, and the geometry has been controlled by varying the composition of the copolymer. Experimental results that reveal the dependence of membrane transport on the size and geometry of the transporting domains will be presented.   

Simulation study of charge distribution near an ionomer-electrode interface

Molecular dynamics simulations have been used to investigate the nature of the electrostatic field and of the proton density distribution in a Nafion-like ionomer in contact with an electrode. We compare our results for a heterogeneous ionomer, in which a partial phase separation has resulted in separate nanoscopic regions of hydrophobic and hydrophilic material, with those predicted by one-dimensional theoretical models in which Poisson-Boltzmann techniques are used to derive self-consistent potentials and concentration distributions. We further examine the effects of the strong inhomogeneous electrostatic fields in changing the morphology of the ionomer in the vicinity of the electrode from its original form in the bulk material.