Lithium-Conducting

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 15681−15690

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Efficient Transport Networks in a Dual Electron/Lithium-Conducting Polymeric Composite for Electrochemical Applications Michael B. McDonald and Paula T. Hammond* Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States S Supporting Information *

ABSTRACT: In this work, an all-functional polymer material composed of the electrically conductive poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonic acid) (PEDOT:PSS) and lithium-conducting poly(ethylene oxide) (PEO) was developed to form a dual conductor for three-dimensional electrodes in electrochemical applications. The composite exhibits enhanced ionic conductivity (∼10−4 S cm−1) and, counterintuitively, electronic conductivity (∼45 S cm−1) with increasing PEO proportion, optimal at a monomer ratio of 20:1 PEO:PEDOT. Microscopy reveals a unique morphology, where PSS interacts favorably with PEO, destabilizing PEDOT to associate into highly branched, interconnected networks that allow for more efficient electronic transport despite relatively low concentrations. Thermal and X-ray techniques affirm that the PSS−PEO domain suppresses crystallinity, explaining the high ionic conductivity. Electrochemical experiments in lithium cell environments indicate stability as a function of cycling and improved overpotential due to dual transport characteristics despite known issues with both individual components. KEYWORDS: mixed conductors, conductive binders, lithium ion batteries, solid-state electrolytes, conducting polymers, PEDOT interface for electron and ion transport.9 Here, this inactive component is essentially a dual electron/proton conductor, without which the electrode kinetics would be severely limited. These dual conductive materials are also of significant interest for solid oxide fuel cell (SOFC) electrodes10 and are also studied more generally in a variety of fields.11 While these interfaces have been extensively innovated to match reactants with both electrons and ions in PEMFCs and SOFCs, three-dimensional electrodes are also desirable for lithium-based battery technology, such as the lithium-ion battery (LIB), which is selected to power the vast majority of emerging portable electronics because of its high energy density and excellent rechargeability properties.12 The active component in an LIB is the storage material (e.g., graphite, LiCoO2, LiFePO4, and LiNixMnyCozO2) in the electrodes, which holds and releases charge via intercalation/deintercalation of Li+. The combination of the anode and cathode storage materials selected will determine performance characteristics; however, LIB storage materials are generally resistive to electron and lithium-ion transport.13 Electrodes composed entirely of storage materials are therefore not possible, and so the storage materials must be in a particulate form in a conductive structural matrix to permit short diffusion lengths for electrons and ions between intercalation/deintercalation and collection

1. INTRODUCTION Electrochemical technologies, such as those for the production of value-added chemicals (e.g., CH3OH from CO2 reduction1 and H2 from electrolysis of H2O2) and especially energy storage (e.g., batteries3 and fuel cells4), require innovations that improve portability, energy efficiency/capability, and cost.5 The usage of electrochemical technology has become increasingly prolific, with mainly batteries utilized in mobile telephones and laptop computers,6 and emerging integration in electric vehicles7 and the renewable energy grid.8 Electrochemical technology is fundamentally distinguished from regular chemical reactions by the spatial mitigation of both electrons and ions from the reactions for control of the flow of energy. Henceforth, there is required advancement in the various material components that facilitate the movement of charges throughout them. In addition to enhancing the intrinsic conductivities of the components contained in the electrode and electrolyte phases of a cell, the number and composition of nonactive components in a cell should be minimized. In particular, an electrode phase that is three-dimensional (thick) requires efficient transport of both electrons and ions, unlike the electrolyte and electric circuit phases. Examples of this requirement are present in such systems as proton exchange membrane fuel cells (PEMFCs), where several different layers compose the electrode and incorporate gas diffusion and catalyst (active) components nanostructured to conductive carbon (CC)−proton exchange polymer (inactive) components at the electrode−electrolyte © 2018 American Chemical Society

Received: January 26, 2018 Accepted: April 16, 2018 Published: April 16, 2018 15681

DOI: 10.1021/acsami.8b01519 ACS Appl. Mater. Interfaces 2018, 10, 15681−15690

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Conventional LIB electrode containing storage material particles (purple) and CC particles (black) in a polymer binder. Electrons and Li ions move between the storage material, and collector and electrolyte, respectively. Electron transport relies on pathways randomly formed between CC particles; (b) chemical structure of PEDOT:PSS; and (c) chemical structure of PEO with oxygen lone electron pairs interacting with Li ions.

outperformed the conventional CC/polyvinylidene fluoride (PVDF), a result of the longer-range, continuous conductive networks formed from a polymer. Unlike CC particles that are incorporated based on percolation thresholds, conductive polymers make up most of the bulk of conductive binders, leading to continuous charge pathways throughout the material. Specific Li+ transport is typically ignored in both battery electrode binders as well as dual conductors as a whole. This can also be specifically addressed in a functional binder using polymers. Polyethylene oxide (PEO, Figure 1c) is the most widely studied Li+-conductive polymer for lithium battery solid electrolytes.24 Its conductivity in the amorphous state arises from ion−dipole interactions between Li+ and lone pair electrons on PEO oxygen, along with its flexible chains that allow ion mobility.25 PEO has previously been combined with PEDOT with the intent to enhance electronic conductivity both in small quantities26 and in large quantities to, for example, improve mechanical properties for electrospinning applications27 and form a mixed conductor for electrochemical supercapacitors.28 In addition, PEO has also been combined previously with another conducting polymer, poly(pyrrole), as a coating for LIB cathode storage materials to successfully access greater capacity.29 Exploiting the greater mechanical and transport properties of PEDOT:PSS longer-chain PEO in a bulk matrix compared to a coating approach is the next logical progression. In this work, quantities of PEO are combined with PEDOT:PSS to form a dual-conductive, all-polymer composite for applications requiring both electronic and Li+ conductivity, as well as mechanical properties including adhesion, structure, and flexibility, such as a highly functional binder in lithiumbased battery electrodes. Furthermore, the processing is waterbased, eliminating toxic, high-cost organic solvents.30 The scope of this work involves the thorough materials characterization of the composite, followed by an electrochemical analysis of the composite, its parent materials, and conventional binder material under vigorous LIB cell conditions to evaluate its potential for lithium cell electrode/binder applications (in the absence of storage materials). The development of this material concept (“PEDOT:PEO”) has potential to minimize the binder-active material ratio while also improving electron and ion transport properties simultaneously to enhance energy density, power density, and cyclability in electrochemical systems. In addition to conductivity optimization, the resulting morphologies are thoroughly characterized to unravel the mechanism of dual charge transport. This novel all-polymer

(Figure 1a).14 Because the binder phase is necessary for effective transport of charge, it therefore determines power density but also adds inactive mass and so also controls energy density (capacity). In LIBs and other similar electrode systems, electronic conductivity is usually achieved through CC additives which, at sufficient concentrations (5−30 wt %), form a a critical number of percolative pathways.15 These pathways, along with the storage material particles, must themselves be suspended in a structural binder, often an inert polymer.16 This polymer binder adds inactive mass and also blocks CC transport pathways. Because the pathways are limited by length, efficient transport is ensured by casting thin layers17 of slurry containing all components on metal foil current collectors.18 Ionic conductivity is gained via the wetting of porous regions of the electrode with the liquid electrolyte and is often limited by the electrode morphology. The limitations of this conventional approach to access the active portion of electrodes present an opportunity to integrate new electron and ion-conducting materials in place of CC− inert polymer composites. To reduce the number of components, an ideal material will possess dual conductivity (e.g., transporting both electrons and Li+), as well as adhesive properties to bind active material particulates, in a single material. Although analogous to modern PEMFC electrodes that interface the active component to the electrolyte and the circuit, bulk carbon fiber is not practical or efficient to form the electron transport network, and lithium conductivity has not been specifically considered for such materials. One method to replace CC percolation is with structural binders that are also conductive. This “functional binder” approach attempts to combine adhesion and conductivity,19,20 and so in principle, the inactive material can be reduced and energy density increased. One class of material that has recently been investigated for this approach is electronically conducting polymers. Of these, poly(3,4-ethylenedioxythiophene) (PEDOT) is the most attractive because of its excellent conductivity, chemical stability, and processability.21 PEDOT is commercially available in an aqueous dispersion with poly(4styrenesulfonic acid) (PSS), which helps solubilize PEDOT and compensates its charged backbone (Figure 1b). PEDOT:PSS has recently been studied as a functional binder in LIB electrodes22 and was shown to decrease the porosity of the electrode (inactive space between storage material particles) and the cell overpotential for lithium intercalation/deintercalation, enhancing rate capability (∼80% at 5C).23 This system 15682


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Dimension 3100 D3005-1 detector using a Bruker cantilever (k = 40 N m−1) in a tapping mode at a 4 μm s−1 scan rate. Differential scanning calorimetry (DSC) thermograms were collected on a TA Instruments discovery calorimeter from 25 to 225 °C in aluminum T-zero pans containing drop-cast samples removed from glass substrates and cut into small pieces with a razor blade. Wide-angle X-ray diffraction (WAXD) samples were prepared by drop-casting materials on glass pieces and were measured on a Bruker D8 General Area Detector Diffraction System with a 0.5 mm collimator. 2.3. Electrochemical Characterization. Samples were assembled into cathodes using CR2016 coin cell parts (Pred Materials International, Inc. New York) by drop-casting 0.5 mL sample solutions onto coin cell spacers directly. Spacers were dried overnight at room temperature, followed by drying overnight at 100 °C under vacuum to remove all water possible. Spacers were assembled into coin cells in an argon glovebox (MBRAUN) (water-free) with a Celgard 2400 polyethylene separator and 1 M LiPF6 in 1:1 ethylene carbonate/ dimethyl carbonate solvent electrolyte system (BASF), in the order of: 20 μL electrolyte, separator, 20 μL electrolyte, separator, 20 μL electrolyte, followed by a lithium metal counter electrode (0.75 mm thickness, 99.9%, Alfa Aesar). Galvanostatic cycling was carried out on a Solartron 1470E battery cycler, allowing charging for 12 h/5 V and indefinite discharge time to 1.5 V. Cyclic voltammetry (CV) was performed on the coin cells using a EG&G Princeton Applied Research 263A potentiostat at a scan rate of 0.1 mV s−1. All potentials reported herein are relative to Li/Li+.

electrode binder is (elemental) carbon-free and is shown to have special dual-conductive properties.

2. METHODS 2.1. Synthesis of Materials. PEDOT:PSS was purchased from Ossila Ltd. under the trade name Heraeus Clevios PH 1000, which came as a 1.0−1.3 wt % (1:2.5 PEDOT:PSS ratio) aqueous dispersion. PEO (MW = 5 MDa, “PEO-5M”) was purchased from Sigma-Aldrich and was dissolved in distilled water by heating and stirring to make a 0.01 g mL−1 50/50 v/v water/methanol solution. PEOs of other molecular weights were also purchased from Sigma-Aldrich and were used in select confirmation experiments (e.g., Figure S2). Desired proportions of the PEDOT:PSS and PEO stock solutions were combined in sample vials and mixed vigorously for ∼3 min using a vortex mixer. Solutions could be drop-cast onto the desired substrate (glass, conductive glass, and metal spacers) and dried at room temperature overnight, followed by an additional 4 h at 100 °C under vacuum. Solutions were always drop-cast immediately after mixing. The conventional electrode matrix consisting of the CC-binder was fabricated by combining Super P (Alfa Aesar) with PVDF (Alfa Aesar) in a 2:1 ratio with N-methylpyrrolidone (NMP) to form a solution of the same weight percent as the polymer mixtures herein, followed by 20 min pulsed ultrasonication (Misonix). 2.2. Physical Characterization. Electronic conductivity was measured on samples drop-cast on glass pieces using a Signatone S3042 four-point probe with a Keithley SCS4200 current source and voltage measurement digital interface and calculated using the standard eq 1 σe = 1/[(π /ln(2)) × (V /I ) × t ]

3. RESULTS AND DISCUSSION 3.1. Charge Transport Properties. It is expected that blending a conductor and an insulator for a given charge will lower the conduction of the other charge. PEO and PEDOT:PSS were combined in varying ratios from 0.5 to 300 PEO:PEDOT by monomer from their aqueous solutions. Upon vigorous mixing, the homogeneous aqueous solutions could be easily cast onto the desired substrate under ambient conditions. A library of varying PEO:PEDOT composites was cast on conductive and nonconductive glass to measure the ionic conductivity by electrochemical impedance spectroscopy (EIS) and electronic conductivity by four-point-probe, respectively. The values obtained are shown in Figure 2. The ionic conductivity was calculated from the resulting Nyquist plots (Figure S1) using an established EIS model for mixed ionic−electronic conductors, which assumes that the electronic conductivity is significantly greater than the ionic conductivity.33−35 According to Figure 2, this assumption is valid, and therefore, it is reasonable to consider that the values calculated

(1)

where σ is the conductivity; V/I is the inverse slope of the resulting I− V curve by applying current, I, across the outer two probes and measuring the voltage drop, V, between the inner two probes; and t is the film thickness. Ionic conductivity was measured on samples dropcast on a 1 cm2 masked area of conductive glass electrode pieces (“TEC 15”, Hartford Glass Inc., Hartford, IN). Sample electrodes were placed in an open electrochemical cell containing 0.5 M LiClO4 in propylene carbonate to mimic the inert electrolyte used in LIB cells, with the working electrode lead attached to the exposed conductive glass and reference/counter electrode lead attached to an epoxy-sealed platinum foil in a 2-electrode configuration. The cell was connected to a Solartron 1255B frequency response analyzer and was oscillated with a 10 mV ac perturbation (no dc control) from 105 to 10−1 Hz. The ionic resistance was deduced from the width of the 45° high-frequency region of the resulting Nyquist plots (Figure S1) between the highand (extrapolated) low-frequency intercepts (Zreal) and accounting for electrode area (A) and thickness (t), which were calculated from eq 2.31

σi = t /(3(Zreal(low) − Zreal(high)) × A)

(2)

The distance on the impedance plane between the origin and the high-frequency intercept is commonly attributed to the solution resistance, which is not included in the calculation. Film thicknesses were measured using a Dektak 150 Surface Profiler. Transmission electron microscopy (TEM) images were taken from FEI Tecnai G2 Spirit Twin at 120 keV accelerating voltage with a Gatan CCD camera. TEM samples were prepared by drop-casting thick films of polymers and cutting them into small pieces with a razor blade, which were then glued onto a stub head. The stub was placed in a Leica UC7 ultramicrotome with FC7 cryochamber accessory. Glass knives were cut with a Leica EM KMR3 Knife Maker. The chamber, knife, and sample were allowed to equilibrate to −45 °C (near the glass transition temperature for PEO31) for 15 min before advancing the knife stage at the sample. The sample was rocked at a rate of 0.1 cm s−1, and the knife was advanced to cut 40 nm thick slices. Sample flakes were collected from the knife edge using a loop with water, followed by subsequent deposition onto lacey carbon/copper grids (Ted Pella). Atomic force microscopy (AFM) images of materials drop-cast on glass were gathered on a Veeco NanoScope V with a

Figure 2. Electronic (squares) and ionic (triangles) conductivities of varying PEDOT−PEO polymer composite ratios (light blue), PEDOT:PSS (dark blue), and 2:1 Super P−PVDF (black). 15683


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3.2. Morphological and Structural Analysis. The 20:1 polymer composite possesses attractive but unexpected properties. Therefore, it is critical to understand the resulting structural features underlying the enhanced functionality; specifically, why Li+ transport is higher in PEDOT−PEO than pure PEO and why electron transport is improved when less electronic conductor is present. Figure 3 shows TEM and

from the EIS method reflect only ion transport. In addition, it can be assumed that the observed impedance is due exclusively to the working electrode, which is relatively thick and porous compared to the planar platinum counter electrode. While ClO4− is also present in the test electrolyte, it is expected to be repelled by the large amount of negatively charged PSS immobilized in the polymer matrix, and so it is assumed that Li+ will be the dominant charge transported in this material.36 Here, acidic PSS charge-compensates PEDOT in its native, positively charged conductive state. Cations (Li+) compensating the excess sulfonate groups on PSS are driven out of the composite matrix when the material (PEDOT) is positively polarized because PSS is immobilize, and are likewise drawn into the matrix upon negative polarization (transport of anions is excluded). The ionic conductivity of pure PEDOT:PSS is measured to be 1 × 10−5 S cm−1, which is comparable to known Li+ conductors.37 No other studies were found that analyzed Li+ transport in PEDOT. When PEO is incorporated, the ionic conductivity steadily increases and peaks at 8 × 10−5 S cm−1 for the 20:1 PEO:PEDOT monomer combination. This ratio amounts to 64 wt % PEO and is in good agreement with the reported values for pure PEO in its conductive state.25 However, beyond this point, the conductivity is not proportional to the amount of PEO present, as the value declines for higher loadings. The 300:1 PEO:PEDOT composite is 97 wt % PEO, and thus, the ionic conductivity with a large presence of PEDOT:PSS is greater than nearly pure PEO in these conditions. The electronic conductivity of pure PEDOT:PSS was found to be 4.2 × 10−1 S cm−1, which is in good agreement with literature reports.21 This also indicates that the as-purchased PEDOT:PSS is in its conductive state, validating the assumption of large electronic conductivity versus ionic conductivity for the EIS model (above). A standard conductive binder mixture, Super P CC and PVDF structural polymer, was cast from a 2:1 ratio in NMP, and the material was found to have a lower conductivity than pure PEDOT:PSS. As PEO is incorporated with PEDOT:PSS, it is logical that the electronic conductivity will decrease as the concentration of PEDOT decreases, diminishing continuous transport pathways. However, the conductivity is practically unaffected when PEO:PEDOT < 10:1. When the ratio increases to 20:1, the electronic conductivity surges to nearly 50 S cm−1, an increase of more than 2 orders of magnitude. The conductivity decreases by half but remains within this range when the ratio is yet again increased to 85:1, which yields a material that contains 89 wt % insulating PEO. It is not until the PEO loading is increased to 97 wt % (only trace amounts of PEDOT) that the conductivity decreases by 5 orders of magnitude (4 V,52 and so, electrochemically induced reactions between the electrode components, electrolyte, and perhaps unwanted residual water may occur that cause spikes in current in this region. This was not observed in Figure 5 and so might be the result of rearrangement to the polymer matrix rather than side reactions. The discharge peak is not observable in this voltage window and was found to occur at an even lower potential than 1.5 V (Figure S4), signifying that the superior ionic and electronic transport pathways of the composite result in lower discharge overpotentials. This suggests that this dual conductor will result in improved power density in Li-based electrochemical devices because more facile movement of charges throughout the electrode will allow for greater currents.

4. CONCLUSIONS The electronically conductive polymer system PEDOT:PSS was combined with the lithium-ion-conductive polymer PEO by mixing and casting aqueous solutions at ambient conditions to form composites of varying PEDOT:PEO ratios. For the first time, such a composite of polymers is optimized and examined in terms of application to lithium-based cells as an alternative, multifunctional binder that is (elemental) carbon-free and considers Li+ transport. The dual conductor was found to have greatly enhanced transport properties, both electrically and ionically, with the addition of large loadings (up to 89 wt %) of PEO. This system is optimal at the 20:1 PEO:PEDOT monomer ratio (64 wt % PEO), where electronic conductivity is ∼45 S cm−1 and Li+ conductivity is ∼10−4 S cm−1 (increases of 2 orders of magnitude and 8 fold, respectively, compared to pristine PEDOT:PSS). The Li+ conductivity is comparable to 15687


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separation for enhanced conductivity here can be better understood using conductive AFM. Polymers are already studied heavily for the LIB electrolyte phase to produce all-solid systems,59 so it is reasonable that polymer-based electrodes will form favorable interfaces for charge transport. Additionally, other desirable features of polymers such as flexibility may make for the ability to adapt to many modules and configurations (e.g., wearables and unique folding for improved packing efficiency).60 Lastly, the improved transport may enable thicker architectures, which would further increase energy density without loss to power density.

other lithium-based polymer electrolytes at room temperature.24 These conditions are also improved by 2.5 orders of magnitude compared to a conventional binder system for electron transport. TEM, AFM, DSC, and WAXD were used to expose the structure−function relationship, and it was found that PEO and PSS interact strongly and drive the formation of better-aligned and grouped PEDOT networks such that they form more efficient pathways throughout the bulk material despite their decreasing concentration as PEO is added, giving to the enhanced electronic conductivity. PSS in turn suppresses PEO crystallization, leading to enhanced Li+ conductivity. The system is optimal near the saturation point, where excess PEO begins to crystallize, disrupting this unique morphology. The electrochemical properties of this composite were investigated in lithium cells from 1.5 to 5 V, and it was found that electrochemical processes that occur to polymers in high-potential environments are not detrimental to transport functions, with high Faradaic efficiencies stabilizing over cycling and CV showing lower charge transfer overpotential because of improved ion and electron transport properties. While the evidence from experiments performed in this work support this conclusion, more rigorous electrochemical testing with emphasis on mechanisms is required in future work to fully understand the electrochemical capabilities of this material. We note that it will be important to ensure the complete removal of water post-casting to decrease electrochemical side reactions between the electrode and the electrolyte. Because this preliminary testing is performed under purposefully vigorous conditions to explore the full potential of the material, it is expected that the voltages of storage materials to be incorporated will be smaller and therefore the material will be more stable yet. In terms of electrochemical device applications, the better ionic and electronic conductivity can potentially increase power density and energy density. With more efficient charge transport compared to conventional binder materials, and all functionality combined into a single material, presumably less of the binder phase would be needed to achieve the required conductivity, and so, the proportion of active mass can be increased. Future work should include monitoring of the conductive properties as a function of cycling in an electrochemical cell environment, and especially the incorporation of active materials with the polymer composite for applications of interest such as LIBs. This will also provide information regarding the ability of the material to retain its unique morphology in real conditions. In addition to the importance of ionic and electronic conductivity to binder functionality, it should be emphasized that a detailed understanding of the mechanical properties, including adhesion, flexibility, and strength, should be established in future work. Because the fabrication is a simple addition of components into a processable, aqueous solution, this is an attractive alternative to toxic processing solvents such as NMP and vigorous dispersion procedures that are costly and more timeconsuming. When combined with active materials, the presence of PSS may also have dispersion qualities because of its surfactant nature to prevent agglomeration and maximize surface area in three-dimensional electrodes.58 It is also important to note that the composite morphology will likely be dependent on the fabrication procedure, and it is possible that other interesting, higher functioning morphologies are attainable by deviating from the simple drop-casting and curing in ambient conditions used in this work. The role of phase

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b01519. Nyquist plots from impedance spectroscopy, conductivity as a function of PEO:PEDOT content for PEO of varying molecular weight, galvanostatic charge−discharge curves, and CV of PEDOT−PEO in a larger voltage window (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Michael B. McDonald: 0000-0002-3629-7526 Paula T. Hammond: 0000-0002-9835-192X Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This publication is based on work funded by the Skolkovo Institute of Science and Technology (Skoltech), program name “Center for Research, Education and Innovation for Electrochemical Energy Storage” under contract number 186-MRA. The authors wish to acknowledge infrastructure support through the Koch Institute for Integrative Cancer Research (MIT), the Center for Materials Science and Engineering (MIT), and the Institute for Soldier Nanotechnologies (MIT).

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REFERENCES

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