Session C11: Polymers for Energy Storage and Conversion I

Ohm's Law, Batteries, and the Clean Energy Landscape

The need for creating safe electrolytes for lithium batteries is significant given the continued safety problems associated with current lithium-ion batteries. Nonflammable polymer electrolytes offer a possible solution but the rate of lithium ion transport is too low for practical applications. In this talk, I will discuss some of the fundamental factors that limit ion transport in polymers. Polymer electrolytes obey Ohm's Law, i.e. in the limit of small applied potentials, the current generated at steady state is proportional to the applied potential. Factors that determine the current generated will be determined using the continuum theory of Newman. Independent measurements of ion diffusion by pulsed-field gradient NMR will also be presented. The talk will end with a discussion of the possibility of commercializing all-solid batteries with polymer electrolytes.   

Elucidating the effects of blending and salt-doping in A-B/A polymer blends for lithium-ion battery electrolytes

Block polymer (BP) electrolytes are promising materials for improving lithium-ion battery performance and stability by decoupling ionic conductivity, modulus, and thermal properties. To potentially increase ion mobility in the conducting domains, A-B block polymers were blended with A homopolymers and doped with a series of lithium salts. The homopolymer distribution in the BP electrolyte was determined via neutron reflectivity, leveraging the contrast between deuterated homopolymer and non-deuterated BP; a series of homopolymer molecular weights was employed to access both wet brush and dry brush regimes. The homopolymer distributions were correlated to the conductivity (measured by AC impedance spectroscopy) and glass transition temperature (determined by differential scanning calorimetry) to elucidate the effects of the blended homopolymer on physical and transport properties. Various lithium salts were used to establish the effect of the counterion on both the homopolymer and lithium ion distribution. These combined efforts allow us to tease out the complex interplay between lithium salt counterions, homopolymers, BPs, and their relative distributions in BP electrolytes.   

Salt Distribution, Domain Spacing, and Interfacial Characteristics in Lithium Ion-Doped Block Polymer Electrolyte Films

Block polymer (BP) electrolytes have significant potential for use as battery membranes; however, to enable the design of efficient and reliable battery materials, open questions must be answered about the effects of lithium ion dopants on BP microstructure (including domain spacing and mixing near the interface) and the distribution of lithium ions in the BP domains. In this work, X-ray and neutron reflectometry (XRR and NR, respectively) revealed the morphological changes introduced by doping lamellar polystyrene-$b$-poly(\textit{oligo}(oxyethylene methacrylate)) (PS-POEM) block polymer films with various lithium salts, as well as the lithium ion distribution in the ion-conducting POEM domain. XRR indicated swelling of both the POEM and PS domains with increasing salt content, with a corresponding decrease in the interfacial width as the salt increased the segregation strength of the BP. However, at very high salt concentrations ([EO]:[Li] $=$ 6:1), roughening of the film caused a slight increase in the interfacial width. NR showed similar trends in domain spacing with salt content, and fits to the NR allow for determination of the lithium salt distribution across the POEM domains. These results help identify the implications of doping lithium salts into BP battery membranes and inform the design of BP electrolyte materials with controlled structure and properties.   

Electrochemical Characterization of Ion Transport Properties of Poly(ethylene oxide)- and Poly(diethylene oxide-methylene oxide)-LiTFSI Electrolytes

Polymer electrolytes may enable the next generation of lithium-ion batteries with improved energy density and safety. Improving battery electrolyte performance requires the optimization of three independent transport properties: ionic conductivity, diffusion coefficient, and transference number. To gain a fundamental understanding of the relationship between monomer structure and ion transport, we compare the electrolyte properties of two linear polyethers, poly(ethylene oxide) (PEO) and poly(diethylene oxide-methylene oxide) (2EO-MO), mixed with bis(trifluoromethane)sulfonimide lithium salt (LiTSFI). We characterize the ion transport properties through potentiostatic methods including ac impedance spectroscopy, restricted diffusion, and steady-state current measurements as a function of temperature and salt concentration. Results indicate that PEO and 2EO-MO have comparable ionic conductivities, and 2EO-MO has a higher transference number throughout the entire concentration range. Impedance measurements also suggest that 2EO-MO has a lower interfacial resistance, indicating that charge transfer at the electrode surface occurs more rapidly. Our results suggest that monomer structure can be tuned in order to optimize ion transport properties of polymer electrolytes.   

Elucidating the Charge Transfer Mechanism in Radical Polymer Thin Films

The active role of polymers in organic electronics has attracted significant attention in recent decades. Beyond conventional conjugated polymers, recently radical polymers have received a great deal of consideration by the community. Radical polymers are redox-active macromolecules with non-conjugated backbones functionalized with persistent radical sites. Because of their nascent nature, many open questions regarding the physics of their solid-state charge transfer mechanism still exist. In order to address these questions, well-defined radical polymers were synthesized and blended in a manner such that there was tight control over the radical density within the conducting thin films. We demonstrate that the systematic manipulation of the radical-to-radical spacing in open-shell macromolecules leads to exponential changes in the macroscopic electrical conductivity, and temperature-independent charge transport behaviour. Thus, a clear picture emerges that charge transfer in radical polymers is dictated by a tunnelling mechanism between proximal sites. This behavior is consistent with a distinct mechanism similar to redox reactions in biological media, but is unique relative to transport in common conjugated polymers. These results constitute the first experimental insight into the mechanism of solid-state electrical conduction in radical polymers.   

Impact of Cation form on Structure/Function Relationships of Perflurosulfonic Acid Ionomers.

Perfluorosulfonic-acid (PFSA) ionomers are widely used as ion-exchange solid-electrolytes in electrochemical devices, where their behavior are influenced by the interactions among its sulfonate groups, mobile cations, and water. The properties of a PFSA depends on its hydration, which drives its phase-separated morphology and controls the extent of sulfonate-cation interaction. Thus, cation-form and hydration collectively affect the structure/transport relationship, yet their interplay is still not well known. To elucidate this interplay, water uptake and conductivity of cation-exchanged PFSA are studied at various relative humidities (RHs) and in water, which are then correlated with mechanical properties and nanostructure. With increasing cation size and valence, the modulus increases, while swelling and conductivity decreases. The extent to which the cations impact the conductivity depends on hydration; at low RH the controlling factor is the cation (interactions), while with increasing RH, the key factor becomes water (swelling), although it is also controlled by the cations. Changes in conductivity with cations and RH are analyzed to establish a universal conductivity-hydration correlation, by accounting for charge density and water content.   

Probing nanoscale ion dynamics in ultrathin films of polymerized ionic liquids by broadband dielectric spectroscopy

Continuous progress in energy storage and conversion technologies necessitates novel experimental approaches that can provide fundamental insights regarding the impact of reduced dimensions on the functional properties of materials. In this talk, a nondestructive experimental approach to probe nanoscale ion dynamics in ultrathin films of polymerized ionic liquids over a broad frequency range spanning over six orders of magnitude by broadband dielectric spectroscopy will be presented. The approach involves using an electrode configuration with lithographically patterned silica nanostructures, which allow for an air gap between the confined ion conductor and one of the electrodes. It is observed that the characteristic ion dynamics rates significantly slow down with decreasing film thicknesses above the calorimetric glass transition of the bulk polymer. However, the mean rates remain bulk-like at lower temperatures. These results highlight the increasing influence of the polymer/substrate interactions with decreasing film thickness on ion dynamics.   

Dynamics of Lithium Polymer Electrolytes using X-ray Photon Correlation Spectroscopy and Rheology

Polymer electrolytes are promising materials for high energy density rechargeable batteries. Battery fade can be caused by structural evolution in the battery electrode and loss of electrode/electrolyte adhesion during cycling. Both of these effects are dependent on polymer mechanical properties. In addition, cycling rate is dictated by the ion mobility of the polymer electrolyte. Lithium ion mobility is expected to be strongly coupled to polymer dynamics. Therefore, we investigate polymer dynamics as a function of salt concentration using X-ray Photon Correlation Spectroscopy (XPCS) and rheology. We report the influence of lithium salt concentration on the structural relaxation time (XPCS) and stress relaxation time (rheology) of high molecular weight poly(styrene -- ethylene oxide) block copolymer membranes.   

Electrostatic Network Ion Gels Formed from Rigid-Rod Polyelectrolytes and Ionic Liquids

Imagine a non-flammable solid with the modulus of bulk PMMA (plexiglass), but where a high density of ions inside move as if they were in a liquid. We will describe such a solid in a new class of ion gels formed using an interfacial ion exchange process between a seed solution of a rigid-rod polyanion (a sulfonated aramid) and an ionic liquid. This gel electrolyte provides an unprecedented combination of tunable properties: transport anisotropy up to 3.5X, ionic conductivity up to 8 mS/cm, widely tunable modulus (0.003$-$3 GPa) and thermal stability up to 300 deg. C. This material breaks the usual tradeoff between ionic conductivity and modulus in solid-polymer or composite electrolytes, demonstrating its potential to resolve current limitations in Li metal batteries and allow for 2-3X higher energy density than existing Li batteries. This material also promises to enable batteries that can operate over a wide temperature range and are immune to fire. We will describe comprehensive studies of ion transport, morphology, thermal and mechanical properties, and battery testing. We will also discuss our fundamental understanding of the electrostatic network that gives rise to mechanical strength in this completely new type of gel.   

Dynamic and Structure of Polymer-Cellulose Composite Electrolyte for Li-ion Battery

Crystalline PEO$_{\mathrm{6}}$LiX complex is a tunnel-like polymer/salt structure that promotes fast Li motion. The application is limited because high ion conductivity is only observed with short molecular weight PEO, as the molecular weight increase, tunnels are misaligned and the conductivity is decreased. High aspect ratio nanofillers based on cellulose nanowhiskers are hypothesized to promote the formation of tunnel structures. Compared with unfilled electrolyte, the room temperature ion conductivity increased as much as 1100{\%} in filled electrolyte. With wide angle x-ray scattering (WAXS), we observe that the structure transitions from amorphous phase to crystalline phase as we add cellulose nanowhiskers and this is because the interaction between cellulose surface and polymer chain enhances the crystallization. From the temperature dependence of conductivity, the calculated Li$^{\mathrm{+}}$ hopping activation energy is shown to be lower in acidic cellulose nanowhisker filled samples. Our quasi-elastic neutron scattering (QENS) indicates with acidic surface, the rotation of PEO$_{\mathrm{6}}$ channels are more stabilized and this could be the origin of the low activation energy and high conductivity   

Self-Doped Conjugated Polymer as Binders for Lithium-ion Battery Cathode

Water-soluble, self-doped conjugated polymers have been reported to have good electrical conductivity, making them potentially strong candidates for energy storage and organic solar cell applications. In this work, two types of self-doped polymers with different pi-conjugated backbones were developed and studied systematically as organic multi-functional polymeric binders for V$_{2}$O$_{5}$ cathode in lithium-ion batteries: PFP with fluorene-phenol backbone, and PCPDTBTSO3K with cyclopenta-[2,1-b;3,4-b']-dithiophene-alt-4,7-(2,1,3-benzothiadiazole) backbone. A series of ex-stu thermal annealing experiment was carried out to investigate the structural impacts of incorporating self-doped polymers into V$_{2}$O$_{5}$ electrode at high temperature. X-ray powder diffraction (XRD) and grazing-incidence wide-angle x-ray scattering (GIWAXS) showed clear evidence that addition of only 5wt{\%} polymer can suppress V$_{2}$O$_{5}$ crystallization up to 450$^{\circ}$ C. Electrochemical tests of V$_{2}$O$_{5}$/polymer hybrid electrodes showed best capacity improvement at 250$^{\circ}$ C (\textasciitilde 190 mAh/g for 5wt{\%} PFP addition), alongside with enhancement in rate performance and charge-transport in thicker electrodes. Peel test was conducted with varying polymer content to show how these polymeric binders improve electrode adhesion.   

Conducting Block Copolymer Battery Electrode Binders Compared Against Their Homopolymer Blend Analogs

Electron- and ion-conducting block copolymers have been explored as battery electrode binders as a means to enhance both electrochemical and mechanical performance. The question remains as to whether the block copolymer architecture is truly necessary or how the block copolymer compares against an analogous homopolymer blend. Here, we explore this question by blending a diblock copolymer bearing electron- and ion-conducting blocks, poly(3-hexylthiophene)-\textit{block}-poly(ethyleneoxide) (P3HT-$b$-PEO), with V$_{\mathrm{2}}$O$_{\mathrm{5}}$ to form a electro-mechanically stable hybrid electrode. These are compared against similar electrodes that contain P3HT and PEO homopolymers of similar molar mass. It is found that the homopolymer blends suffer from poor electrode morphology, leading to subpar performance. In contrast the diblock copolymer binder is superior as phase separation is discouraged and the electrode exhibits a more homogeneous structure. The electrode with P3HT-$b$-PEO has the highest capacity of 190 mAh/g, whereas V$_{\mathrm{2}}$O$_{\mathrm{5}}$ is only 77 mAh/g at a C rate of 0.1 after over 200 cycles. P3HT, PEO, and the blend have capacities of 139, 130, and 70 mAh/g, which are not nearly as impressive as the block copolymer binder.   

Charge transport kinetics in a robust radical-substituted polymer/nanocarbon composite electrode

We have reported a series of organic radical-substituted polymers as new-type charge storage and transport materials which could be used for energy related devices such as batteries and solar cells. Redox-active radical moieties introduced to the non-conjugated polymer backbones enable the rapid electron transfer among the adjacent radical sites, and thus large diffusive flux of electrical charge at a bulk scale. Here we present the elucidated charge transport kinetics in a radical polymer/single-walled carbon nanotube (SWNT) composite electrode. The synergetic effect of electrical conduction by a three-dimensional SWNT network and electron self-exchange reaction by radical polymers contributed to the 10$^{\mathrm{5}}$-fold (per 1 g of added SWNT) boosting of electrochemical reactions and exceptionally large current density (greater than 1 A/cm$^{\mathrm{2}})$ as a rechargeable electrode. A totally organic-based secondary battery with a submicron thickness was fabricated to demonstrate the splendid electrochemical performances.