Fluorinated Copolymer Functionalized with Ethylene Oxide as Novel

Subscriber access provided by University of Winnipeg Library

Article

Fluorinated Copolymer Functionalized with Ethylene Oxide as Novel Water–Borne Binder for a High Power Lithium Ion Battery: Synthesis, Mechanism, and Application Chih-Hao Tsao, E-Ting Wu, Wei-Hsun Lee, Chi-Cheng Chiu, and Ping-Lin Kuo ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00335 • Publication Date (Web): 25 Jun 2018 Downloaded from http://pubs.acs.org on June 26, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Energy Materials

Fluorinated Copolymer Functionalized with Ethylene Oxide as Novel Water– Borne Binder for a High Power Lithium Ion Battery: Synthesis, Mechanism, and Application

Chih-Hao Tsao1,2, E-Ting Wu1, Wei-Hsun Lee1, Chi-cheng Chiu1,3*, and Ping-Lin Kuo1,3*

1

Department of Chemical Engineering, National Cheng Kung University, Tainan,

70101, Taiwan 2

Rising Chemical Ltd. Co., Xiaogang Dist., Kaohsiung, 81264, Taiwan

3

Hierarchical Green-Energy Materials (Hi-GEM) Research Center, National Cheng

Kung University, Tainan, 70101, Taiwan

*Author to whom all correspondence should be addressed P. L. Kuo (E-mail: [email protected]) C.C. Chiu (E-mail: [email protected])

ACS Paragon Plus Environment

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract A novel water-borne fluorinated binder is synthesized via copolymerizing 2-(perfluorohexyl) ethyl methacrylate (PFHEMA) and poly(ethylene glycol) methacrylate (PEGMA) to improve the performance of lithium ion battery with LiFePO4-based cathode materials. The resulting copolymer binders can self-assemble into 150-220 nm particles stably dispersed in aqueous solution. The self-dispersed fluorinated binder (SF binders) with the PFHEMA to PEGMA ratio of 3:1 effectively reduce the over-potential during the high discharge current density compared with the conventional PVDF cathode binder. Further increasing the PEGMA amount yet decreases the electrochemical performance of SF binders, inconsistent with the expected Li+ conduction of PEO moiety. Molecular dynamics (MD) simulations show that the PEO segments reduce the Li+ and PF6- interaction and increase the amount of unpaired Li+. In contrast, the PEO moiety wrapping around the Li+ ion can decrease its mobility. These competing effects lead to the observed optimum ratio of PEO to fluorinated moieties. The novel SF binders are fully compatible with LiFePO4-based cathode materials and feature small impedance after charging and discharging. Coin cells assembled with the SF cathode binder demonstrated excellent cyclic performance after 150 cycles with negligible decay and near-100% column efficiency. The superior performance of the novel water-borne SF binders makes them excellent


Page 2 of 42

Page 3 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


candidates for the environmentally-friendly production of high-power lithium ion batteries. Keywords: lithium ion battery; waterborne binder; fluorinated copolymer; self-assembly; molecular dynamics simulations.



Page 4 of 42

Introduction Lithium ion batteries (LIBs) are common energy storage devices for electronic systems owing to their high power and energy densities. Nowadays, secondary lithium batteries are widely used in mobile digital devices and are gaining interest for applications in electric vehicles (EVs) and energy storage systems in smart grid.1,2 To improve the performance of LIBs, current studies are mainly focusing on the new material developments to increase the capacity, operating voltage, energy density, and heat stability.3-5 In particular, the development of functional binders for cathodes and anodes has drawn much attention for high-power LIB capacity enhancement due to their low overall content yet critical role at the electrode interface.6-9 One of the most widely used binders for LIBs is polyvinylidenedifluoride (PVDF), which has good electrochemical stability and adhesion due to the polar C-F bonds.10

However,

the

slurry

fabrication

process

of

PVDF

requires

N-methyl-2-pyrrolidone (NMP) as a solvent, which increases manufacturing costs and has inherent environmental, safety and health (ESH) issues. Therefore, the development of aqueous polymer binders has attracted many interests in the pursuit of switching from the solvent-based to an aqueous-based fabrication process, e.g. carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), and polyacrylic acid (PAA).11-15 The conventional PVDF binder is an inert conductor of electrons and


Page 5 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


lithium ions (Li+) leading to higher polarization resistance at high rate performance. Consequently, introducing ion-conducting materials into polymer binders is one of the strategies to enhance the overall performance of LIBs.16-20 One example is polyethylene oxide (PEO), a common polymer electrolyte known for its high Li+ conductivity, electrochemical stability, and interfacial stability between electrode and electrolytes. Also, hydrophilic PEO can act as surfactants to stably disperse hydrophobic polymer moieties in aqueous solution.21-25 The Li+ conduction mechanism of PEO polymer electrolyte has been comprehensively characterized via molecular dynamics (MD) simulations in combination with experimental data.25-28 Borodin and Smith utilized MD simulation to

investigate

the

polymer

electrolyte

systems

of

PEO

and

lithium

bis(trifluoromethane)-sulfonimide (LiTFSI) salts. Three Li+ transport mechanisms of PEO have been proposed: (1) intra-chain Li+ conduction along the polymer chain; (2) cooperative motion of Li+ coordinated with the PEO segments; and, (3) inter-segmental hopping of Li+ between PEO chains.

25-27

Subsequently, Marita and

Heuer extended the work of Borodin and Smith and devised an analytical expression between Li+ diffusivity and the polymer-chain length based on Rouse modes.28 However, the molecular mechanisms of the PEO segment as binder material remain elusive.



In this work, fluorinated acrylate is copolymerized with poly(ethylene glycol) methacrylate (PEGMA) to form a Li+ conducting water-borne binder for LIBs. The fluorinated moiety provides good electrochemical stability and adhesive properties; yet it is highly hydrophobic and incompatible with aqueous solutions. In contrast, PEO segements of PEGMA improves its hydrophilicity and provides the Li+ conducting ability. Hence, PEGMA is expected to simultaneously improve the Li+ transport and stabilize the fluorinated copolymer in aqueous systems. These fluorinated binders have an optimum electrochemical performance when the molar ratio of fluorinated acrylate to PEGMA is 3 to 1. The PEO segment can interact with the Li+ ions and enlarge the distance between the Li+ and PF6- anions, thereby enhancing the performance of the lithium battery. However, the electrochemical performance decreases when the ratio of PEO is increased. The strong affinity between PEO segment and Li+ ions decreases the diffusion coefficients compared with liquid electrolytes. The competition between unpair Li+ enhancements and Li+ mobility reductions leads to the optimum PEGMA content of ionic conducting systems. As a result, the fluorinated water-borne copolymer with suitable PEO content can be used as environmental-friendly cathode binders for high-power LIB applications.


Page 6 of 42

Page 7 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Experiment Synthesis of self-assembled fluorinated (SF) copolymer 2-(Perfluorohexyl)

ethyl

methacrylate

(PFHEMA,

Capstone™

62-MA,

Chemours Company) was distilled at 60 ⁰C under vacuum of 10-12 Torr. Poly(ethylene glycol) methacrylate (PEGMA, Mn ~ 360 mol g-1, Sigma-Aldrich) was purified by inhibitor removers before use. 2,2’-Azobis(2-methylpropionamidine) dihydrochloride (AIBA, Sigma-Aldrich) and acetone (Sigma-Aldrich) were used without additional purifications. Different amounts (as outlined below) of PFHEMA and PEGMA were mixed in a water/acetone solution under a nitrogen atmosphere using a four-neck round-bottomed flask equipped with a thermometer, a mechanical stirrer, and a reflux condenser. AIBA was then added under intense stirring until dissolved in a water solution. Then, the mixture was heated to 60 ⁰C for 12 hours under a nitrogen atmosphere to produce a series of SF copolymer solutions. The three prepared samples were identified as SFx, where x = 2, 3, or 4 denotes the feeding molar ratio of PFHEMA to PEGMA. Characterization methods



1

H NMR (600 MHz) spectra were measured on the Bruker AMX600MHz Digital

spectrometer. FT-IR spectra were recorded with a Nicolet Magna II 550 spectrometer. Size and structure characterization were performed by transmission electron microscopy (TEM) using the Hitachi H-7500 microscope operating at 80 kV. The morphologies of the LiFePO4 electrodes with various binders were characterized with the JEOL JEM6700 field-emission scanning electron microscope (FE-SEM) operating at 10 kV. The size distribution was obtained using the Malvern Nano-ZS dynamic light scattering. Electrochemical measurements Electrodes and coin cells (CR2032) were prepared as follows. The LiFePO4 powder (Aleees, Taiwan), Super P, and polymer binder (weight ratio of 8:1:1) were mixed with deionized (DI) water. The slurries were coated onto aluminum foil with an automatic coating machine (PI-1210 AUTO FILM APPLICATION, TESTER SANGYO CO. JAPAN). The LiFePO4 cathode was then dried at 120 ⁰C for 48 hr under vacuum and roll-pressed before use. Finally, the coin cells were assembled with a lithium-metal anode, LiFePO4 cathode, and liquid electrolytes composed of ethylene carbonate and diethyl carbonate (1M LiPF6, EC/DEC (v/v=1/1)) in an Ar-filled glove box.


Page 8 of 42

Page 9 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Electrochemical instruments (CHI604A, CH Instrument, Inc.) were used to examine

the

electrochemical

impedance

spectra

and

cyclic

voltammetry.

Electrochemical impedance spectra were obtained with a potential amplitude of 10 mV and a frequency variation from 0.1 Hz to 1 MHz. Cyclic voltammetry was performed at a rate of 0.1 mV s-1 between 2.5 V and 4.2 V. Battery performance tests were conducted in the range of 2.5–4.2 V at 25 ⁰C on a Battery Automatic Test System (Acu Tech Systems, BAT-750B). Molecular Dynamics Simulations In this work, we utilized molecular dynamics (MD) simulations to characterize the molecular mechanisms of SF binders. Since only the PFHEMA to PEGMA ratio is controlled experimentally, we constructed three series of SF copolymer systems with: (1) different molecular weights (MWs) of SF copolymers with the fixed PFHEMA to PEGMA ratio of 3:1 and fixed total amount of monomers; (2) various block sizes of PEGMA and PFHEMA with a fixed polymer MW of approximately 26000; and, (3) various PEGMA to PFHEMA ratios with a fixed polymer MW of around 26000. The electrolyte is composed of 1M LiPF6 in EC / DEC with a 1 to 1 volume ratio. The OPLS-AA force field was applied to describe all molecules, including polymers, organic solvents, and lithium salts.29,30 The OPLS force field has been widely used in MD studies of ionic liquids, polymers, and solid polymer electrolyte systems.30,31 The



parameters for PF6 were taken from updated OPLS potentials developed by Lopes & Pauda.32 The SF polymer/electrolyte systems for MD studies consisted of an SF polymer film solvated with 1034 EC molecules, 534 DEC molecules, and 144 LiPF6 molecules within a 5 x 5 x 15 nm3 simulation box. The box size and the number of solvent molecules were chosen to obtain 1M LiPF6, EC/DEC (v/v = 1/1) mixture. Note that the simulation box size is much smaller than the self-assembled particle size of the SF polymer reported in the Results section. Hence, the polymer/electrolyte interface in the MD simulation was considered as a microscopic representation of the surface of the SF polymer self-assembled particles. We also constructed a simplified polymer / electrolyte interface system for rigorously controlling the chemical properties of the interface, where only the interfacial polymer chains were explicitly represented. Please refer to the supporting information for detailed procedures of constructing the initial polymer/electrolyte configurations. All MD simulations of SF binder / electrolyte systems were conducted via Gromacs 5.0.5 simulation package,33,34 whereas the simplified interface systems were simulated via LAMMPS package.35 Periodic boundary conditions were applied in all three dimensions. Additionally, temperature and pressure were controlled at 298K and 1 bar by Nosé-Hoover and Parrinello-Rahman algorithms, respectively.36-40 Van der


Page 10 of 42

Page 11 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Waals and short-range electrostatic interactions were calculated with a 1.2 nm cut-off, while long-range electrostatic force was evaluated using the particle mesh Ewald method (PME).41 All bonds were constrained through the LINCS algorithm at their equilibrium lengths.42 Motion equations were evaluated with an integration time step of 2 fs; and, each simulation was equilibrated for 250 ns with the system coordinates saved every 10 ps. The last 50 ns trajectories were taken for the analyses of structural and dynamic properties. Result and Discussion SF polymer binder syntheses and characterizations A new type of waterborne fluorinated binder was synthesized via dispersed polymerization of fluorinated and ethylene oxide (EO) acrylate monomer. Figure 1 shows the design concept and schematic illustration of the syntheses of these self-dispersed fluorinated (SF) binders. The hydrophilic EO segments resulted in stable dispersions of the SF polymer particles for storage in the aqueous solution. As illustrated in Figure 1, the self-dispersed fluorinated copolymers with higher ethylene oxide content, i.e. SF2 and SF3, maintained good dispersion after 7 days; in contrast, the SF4 copolymers with low EO content exhibited some precipitation after 7 days. This can be attributed to the SF4 copolymer having fewer hydrophilic PEO segments, leading to poor suspension in the aqueous solution. Figure 2 presents the SEM images



of the as-prepared LiFePO4 electrodes with the SF binders. All electrodes have LiFePO4 and conducting carbon homogeneously dispersed to form complex and porous morphologies, allowing liquid electrolyte to penetrate and contact with the cathode materials without peeling and cracking. The EO segment of these SF copolymers not only supports the suspension stability, but also provides the ionic conducting ability in this system. These results suggest that the SF copolymer binders are fully compatible with commercial cathode materials.

Figure 1.

(a) Scheme of the synthesis of SF copolymers, and (b) stability of SF the copolymer in aqueous solution


Page 12 of 42

Page 13 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. SEM images of the LiFePO4 electrodes with (a) SF2, (b) SF3, and (c) SF4 binders The SF copolymer can self-assemble into spherical nanoparticles in water



solution forming a fluorinated segment core and EO shell. From the TEM images shown in Figure 3, the SF2 copolymer has the smallest particle size; whereas the SF4 copolymer forms the largest nanoparticles. Moreover, dynamic light scattering (DLS) analyses show that the SFx nanoparticles have diameters ranging between 150-220 nm. Similar to the TEM results, DLS results showed that SFx copolymers with higher fluorinated content exhibited larger particle sizes and boarder size distributions. Combined with the dispersion stability test, out results suggest that the lower hydrophilic PEO amount of SFx led to larger hydrophobic aggregates with greater particle sizes and decreased suspension stability.


Page 14 of 42

Page 15 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. TEM images of (a) SF2, (b) SF3 and (c) SF4, and (d) DSL curve of SF copolymers Figure 4 presents the cyclic voltammetry (CV) curves of the LiFePO4 electrodes prepared with the SF binders, in which the redox coupling between 3.2 and 3.6 V corresponds to the Fe3+/Fe2+ redox reaction of LiFePO4. In addition, the CV curves of the LiFePO4 electrodes with SF2, SF3, and SF4 indicate good electrochemical stability within the operation voltage window. The difference between the redox peaks were 0.343, 0.282, and 0.287 V for the SF2, SF3, and SF4 systems, respectively. Although both the SF3 and SF4 binders have lower potential differences of ~0.285 V between the oxidation and reduction peaks, the SF3 cell featured a sharper CV curve than the SF4 system, indicating a higher electrochemical activity. This demonstrates that the SF3 binder can more effectively reduce the concentration polarization during the charge–discharge process, thereby improving the rate performance of the battery. However, the superior performance of the SF3 binder implies unexpected drawbacks for increasing the PEO amount of SF copolymer. Therefore, we conducted MD simulations to investigate the molecular mechanism of the SF copolymer at the electrolyte/electrode interface.



3

SF2 SF3 SF4

2

Current (mA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 42

1

0

-1

-2

-3 2.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0

Voltage (V)

Figure 4. Cyclic voltammetry scan of LiFePO4 electrodes with SF binders Molecular mechanism of SF binder To understand the reasons behind the superior performance of the SF3 binder, we conducted series of MD simulations of SF polymers with: (1) different polymer molecular weights; (2) different PEGMA block sizes; and, (3) PFHEMA to PEGMA ratios of 2, 3, and 4, corresponding to SF2, SF3 and SF4, respectively. We examined the effects of these polymer composition variations on the structural properties of the polymer/electrolyte interface and the dynamic properties of lithium ions at the interface. Figure 5 compares the radial distribution functions (RDFs) between Li+ ions and the counter ion PF6-, the oxygen atoms from EC and DEC, and the ether oxygen from


Page 17 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the PEGMA chains in both the bulk electrolyte region and at the polymer/electrolyte interface. We found that in the bulk electrolyte, Li+ ions are mainly coordinated by the solvent EC and DEC, as expected; while at the interface, the Li+ ions are surrounded by the EO segments of PEGMA due the strong interaction between the Li+ ions and the EO segments.

Figure 5. Radial distribution functions between Li+ ions and (a) the carbonyl oxygen of EC, (b) the carbonyl oxygen of DEC, (c) the counter ion PF6-, and (d) the PEO ether oxygen from the PEGMA chains both in the bulk electrolyte region and at polymer/electrolyte interface. (e) Simulation snapshots for Li+ ion



coordination in the bulk electrolyte and at the SF binder surface. Figures were generated with VMD.43 Indeed, as listed in Table S3 in the supplemental information, the total Li+ ion coordination number (CN) is maintained at around 6 in both the bulk region and at the interface for all the SF-binder systems. Yet, the coordination of Li+ ions and EO polymer segments becomes dominated at the polymer/electrolyte interface with a CN of 5, suggesting that Li+ ions are mainly bounded by the PEGMA block of the SF binder, as expected. Figure 6 displays the representative Li+ ion coordination profile along the interface normal, i.e. the Z-direction in our simulations. As Li+ ions approach the interface, the coordination of EC/DEC around a Li+ ion is gradually reduced and replaced by the polymer EO segments coordination. More importantly, the coordination between Li+ ions and PF6- anions is nearly absent at the polymer/electrolyte interface. These structural analyses indicate that the PEGMA of the novel SF binder can reduce the interactions between Li+ and PF6- and release free Li+ ions, which corresponds with the reduced redox potentials in the CV measurements.


Page 18 of 42

Page 19 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. (a) Configuration of SF polymer / electrolyte system aligned with (b) the coordination fraction profile of Li+ ions along the interface normal, (c) the profile of the total coordination number around the Li+, and (d) the number density profiles for the PEO chains, EC, DEC, PF6-, and Li+, respectively.



To characterize the effects of SF binder on the kinetic properties of Li+, we calculated the diffusion coefficients of all the tested SF copolymer systems from the mean square displacement data. Figure 7(a) plots the diffusion coefficient of the Li+ (DLi) and EO oxygen atom (DEO) from all tested systems versus the number of bound Li+ ions. Matching between DLi and DEO suggests the cooperative motions between the EO segments and surface bound Li+ due to the strong coordination of EO around Li+ ions. Furthermore, DLi is negatively correlated with the number of surface bound Li+, i.e. as more Li+ bound with the copolymer, the mobility of Li+ becomes lower. As shown in Figure 7(b), the number of surface bound Li+ increases with the surface density of the EO segments ρEO. This suggests that DLi is mainly affected by ρEO, which can be modulated by adjusting the PFHEMA: PEGMA ratios in the experiments.


Page 20 of 42

Page 21 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. (a) Diffusion coefficients of surface bound Li+ and ether oxygen of PEO and (b) surface EO chain density plotted as function of the number of surface bound Li+ ion. Note that ρEO can be affected by various factors, such as those tested in this MD study, namely polymer molecular weight, polymer block size, and the PFHEMA: PEGMA ratio. In addition, the electrolyte composition, temperature, and processing conditions, among others, can modulate the self-assembled conformation of the SF copolymer, and thus ρEO. To focus on the effects of ρEO, we utilized a simplified interface system to precisely control the number of EO segments, as described in the supplemental information. As shown in Figure 8, we found a negative linear correlation between the DLi and ρEO, indicating that increasing ρEO reduces Li+ mobility. Introducing PEGMA within the SF copolymer can improve the



water-solubility and is expected to provide the same Li+ conductive functions as in the PEO-based polymer electrolyte. Yet, the diffusivity data suggests the opposite Li+ conductive effects from the PEO segments.

Figure 8. Diffusion coefficient of surface bound Li+ ions plotted as a function of surface EO chain density ρEO from all-atom MD simulations and simplified interface model. The Li+ transport within the PEO-based polymer electrolyte includes three main mechanisms: (1) the intra-chain Li+ transportation as Li+ diffusing along the PEO chain; (2) the cooperative motion between Li+ and the polymer; and, (3) the inter-chain Li+ hopping between polymer chains. Note that the PEGMA of the SF copolymer had short PEO chains with only 6 to 7 EO segments, indicating the absence of intra-chain conductions, as revealed by the trajectory of the coordinated


Page 22 of 42

Page 23 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


oxygen index around the surface bound Li+ ions shown in Figure S5 in the supporting information. Furthermore, the EO segments were grafted to the acrylate backbone in the SF copolymer, suggesting limited cooperative motion of the EO segments and Li+ ions. Finally, from our simulation results, the occurrence of inter-chain hopping of Li+ increased with ρEO, as summarized in Table S4 of the supporting information. However, the hopping events are not frequent enough to achieve fast ion conduction. Indeed, as suggested by Webb et al., the major ion conduction mechanism of PEO-based polymers is the intra-chain Li+ conduction rather than inter-chain Li+ hopping.27 These results suggest that the SF copolymer provides only a limited Li+ conduction ability. Therefore, increasing ρEO leads to a reduction of DLi, according to MD data. Combining the structural and dynamic analyses, we found that the PEGMA blocks within the SF polymer can reduce the coordination of Li+ by EC, DEC, and PF6- and release Li+ ions for further redox reactions. Yet, increasing the PEGMA amount, i.e. increasing the EO segments, can reduce the Li+ ion mobility near the polymer/electrolyte interface. These two competing effects thus provide a molecular rationale for the superior performance of SF3. Effects of SF binder on cell performance To characterize the battery performance of the coin cells with various SFx



binders, we carried out a series of galvanostatic cycling tests. Figure 9 illustrates the galvanostatic charge/discharge cycling profiles of the LiFePO4 electrode with the SF and PVDF binders at various C-rates. Under the charge-discharge process, these profiles have flat-shaped voltages due to electrochemical lithiation. Moreover, the discharge plateau shifts to lower voltages at higher C-rates because of the large current density induced by the concentration polarization. Specifically, the potential differences between the charge and discharge plateaus at the 1C discharge rate are 0.114, 0.097, and 0.104 V for the SF2, SF3, and SF4 systems, respectively. Note that the polarization of SF3 is lower than the others, implying the best specific capacity at high C rates. The discharge capacities of the SF3 cell are 138, 113, and 97 mAh g-1 at the rates of 1C, 5C, and 10C, respectively, all of which are higher than those of the SF2 and SF4 cells. Further, compared to SF3 binder, the cell with the PVDF at 1C, 5C and 10C produced discharge capacities of 142 mAh g-1, 116 mAh g-1 and 95 mAh g-1, respectively, which is similar to SF binder. However, at high current density (20 C), the SF3 copolymer provides the discharge capacity of 63 mAh g-1, higher than that of 44 mAh g-1 for the PVDF binder. This result indicates that an adequate EO amount of the SF copolymer enhances the overall battery performance. The electrochemical performance can correlate with the MD results where the EO segments increase free Li+ ions but reduce Li+ ion mobility. These competitive effects result in the optimum


Page 24 of 42

Page 25 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


EO-moiety content of the SF binder for lithium battery performance.



4500

(d) PVDF

0.1C

4000

Voltage(mV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 42

3500

3000

2500 10C

20C

5C

3C

1C

0.1C

2000 0

20

40

60

80

100

120

140

160

-1

Capacity(mAhg

)

Figure 9. Charge–discharge profiles of the LiFePO4 electrodes (a) with SF2 binder, (b) SF3 binder, (c) SF4 binder, and (d) PVDF at different current rates To characterize the interfacial barrier during Li-ion charge transfer, we measured the electrochemical impedance spectra (EIS) of the LiFePO4 electrodes after different charge–discharge cycles, as shown in Figure 10. All SF cell samples show typical EIS


Page 27 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


plots, where the intercept of the real axis and the diameter of the semicircle are assigned to the bulk (Rb) and interfacial resistance (Ri), respectively. At the initial state of the 1st cycle, the lithium metal has not yet formed a stable passivation layer, and the Ri is mainly controlled by the cathode side. Therefore, the Ri of EIS spectra reflects the binder effects of the lithium charge transfer. As shown in Figure 10 (a), the Ri values of the SF2, SF3, and SF4 binders at the initial state are 44, 83, and 98 Ω, respectively. It can be inferred that the SF3 binder has an adequate amount of EO segments to improve the electrode wettability and repel the PF6- anions from LiFePO4 surface, resulting in reduced Ri. The large amount EO moieties in the SF2 binder decreases the Li+ mobility, which enlarges the charge transfer resistance of the electrode, thereby yielding a higher Ri. As for the SF4 system, the smaller amount of EO causes poor wettability of the electrode surface, resulting in a larger resistance at the initial state. After the 1st cycle, the liquid electrolytes penetrate into the electrode and reduce the Ri.



100

(b) 1 cycle

75

-Z" (Ω)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 42

50

25

SF2 SF3 SF4

0 0

50

100

Z' (Ω)


150

200

Page 29 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 10. Electrochemical impedance spectra of the cells at the (a) initial state, (b) first cycle, and (c) after 100 cycles The cycling performance tests were conducted with the SF binders and conventional PVDF binder at the constant charge/discharge current density of 1/1 C, as shown in Figure 11. The SF cells feature cyclic stability with a columbic efficiency closed to 99 to 99.1 % after 150 cycles, showing a satisfactory long-term stability compared to PVDF. The high capacity retention is likely to be originated from the good adhesion of the active materials on the aluminum current collector and the good electrochemical stability during the charge-discharge cycles. As shown in Figure 10 (c), the SF binder cells started to consistently exhibit small Ri values after 100 cycles, suggesting the formation of stable passivation layers on the lithium surface. Moreover, similar to the rate performance test, the SF3 cell featured the highest reversible capacity during cyclic testing among the investigated SF systems. These results



demonstrate that the LIB with the SF3 binder has excellent reversible charge– discharge cycling performance and a higher discharge capacity due to the reduced polarization and charge transfer resistance.


Page 30 of 42

Page 31 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60




Figure 11. Cyclic performance of the cells with (a) SF2 binder, (b) SF3 binder, (c) SF4 binder and (d) PVDF.

Conclusion A series of self-assembled fluorinated binders (SF binders) was synthesized by the copolymerization of hydrophilic PEMGA and hydrophobic fluorinated PFHEMA monomers. The SF binders formed spherical polymer particles dispersed in water and featured long-term stability. The PEO segments of PEGMA acted as Li+ binding moieties and dispersants of the SF binders. Interestingly, the SF3 binder with the PFHEMA to PEGMA ratio of 1:3 showed the optimum electrochemical performance. The MD simulation results revealed that the main function of the PEO segments was


Page 32 of 42

Page 33 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


to reduce the Li+ - PF6- interactions and increase the amount of unpaired Li+ ions. However, increasing PEO amount decreased the mobility of bound Li+ ions and hindered the Li+ transportation at the electrode/electrolyte interface. These competing effects led to the optimum ratio of SF3 with the best electrochemical performance as a LIB binder. The SF3 binder exhibited the satisfactory rate performance of 138 and 97 mAh g-1 at the rates of 1C and 10C, respectively. In addition, these SF binders provided excellent cyclic durability after 150 cycles with small interfacial resistances. These allowing to be a good candidate of substitution of PVDF for lithium batteries. The combine results illustrate the superior performance of LIBs with SF binders, rendering the SF binders as good candidates for environmentally-friendly PVDF substitutes for high-power lithium ion batteries.



Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Supporting Information provides characterization of SF copolymer, such as IR, NMR, and solvent effect. Moreover, we also provide simulation details of system setup, simplified interface model, and the result for MD simulation of SF polymer. ACKNOWLEDGMENTS This work was financially supported by the Hierarchical Green-Energy Materials (Hi-GEM) Research Center, from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan. The authors also thank the Ministry of Science and Technology, Taipei, R.O.C. for supporting this research under the grants of MOST 106-2923-E-006-007, and 107-3113-E-006-006.


Page 34 of 42

Page 35 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Reference 1.

Lee, H.; Yanilmaz, M.; Toprakci, O.; Fu, K.; Zhang, X. A Review of Recent Developments in Membrane Separators for Rechargeable Lithium-ion Batteries. Energy Environ. Sci., 2014, 7, 3857–3886

2.

Tsao, C. H.; Kuo, P. L. Poly(dimethylsiloxane) Hybrid Gel Polymer Electrolytes of a Porous Structure for Lithium Ion Battery. J. Membr. Sci., 2015, 489, 36−42.

3.

Mauger, A.; Armand, M.; Julien, C. M.; Zaghib, K. Challenges and Issues Facing Lithium Metal for Solid-state Rechargeable Batteries. J. Power Sources., 2017, 353, 333-342.

4.

Armand, M.; Tarascon, J.-M. Building Better Batteries. Nature, 2008, 451, 652−657.

5.

Choi, J. W.; Aurbach, D. Promise and Reality of Post-Lithium-Ion Batteries with High Energy Densities. Nat. Rev. Mater., 2016, 1, 16013.

6.

Chou, S. L.; Pan, Y.; Wang, J. Z.; Liu, H. K.; Dou, S. X. Small Things Make a Big Difference: Binder Effects on The Performance of Li and Na Batteries. Phys. Chem. Chem. Phys., 2014, 16, 20347—20359.

7.

Lee, J. H.; Lee, S.; Paik U.; Choi, Y. M. Aqueous Processing of Natural Graphite

Particulates

for

Lithium-ion

Battery

Anodes

Electrochemical Performance. J. Power Sources, 2005, 147, 249.


and

Their


8.

Zaghib, K.; Striebel, K.; Guerfi, A.; Shim, J.; Armand, M.; Gauthier, M.; LiFePO4/Polymer/Natural Graphite: Low Cost Li-ion Batteries, Electrochim. Acta., 2004, 50, 263-270.

9.

Li, J.; Christensen, L.; Obrovac, M. N.; Hewitt, K. C.; Dahn, J. R. Effect of Heat Treatment on Si Electrodes Using Polyvinylidene Fluoride Binder. J. Electrochem. Soc., 2008, 155, A234- A238.

10. Zhang, Z.; Zeng, T.; Lai, Y.; Jia, M.; Li, J. A Comparative Study of Different Binders and Their Effects on Electrochemical Properties of LiMn2O4 Cathode in Lithium Ion Batteries. J. Power Sources, 2014, 247, 1−8. 11. Lee, J. H.; Paik, U.; Hackley, V. A.; Choi, Y. M. Effect of Carboxymethyl Cellulose on Aqueous Processing of Natural Graphite Negative Electrodes and their Electrochemical Performance for Lithium Batteries J. Electrochem. Soc., 2005, 152, A1763. 12. Li, J.; Lewis, R. B.; Dahn, J. R. Sodium Carboxymethyl Cellulose A Potential Binder for Si Negative Electrodes for Li-Ion Batteries. Electrochem. Solid-State Lett., 2007, 10, A17. 13. Lux, S. F.; Schappacher, F.; Balducci, A.; Passerini, S.; Winter, M. Low Cost, Environmentally Benign Binders for Lithium-ion Batteries. J. Electrochem. Soc. 2010, 157, A320–A325.


Page 36 of 42

Page 37 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


14. Li, J.; Kloepsch, R.; Kunze, M.; Winter, M.; Passerini, S. Investigations on Cellulose-based High Voltage Composite Cathodes for Lithium ion Batteries. J. Power Sources, 2011, 196, 7687−7691. 15. Wang, A.; Dupre, N.; Gaillot, A.-C.; Lestriez, B.; Martin, J.-F.; Daniel, L.; Patoux, S.; Guyomard, D. CMC as a Binder in LiNi0.4Mn1.6O4 5V Cathodes and Their Electrochemical Performance for Li-ion Batteries. Electrochim. Acta, 2012, 62, 77−83. 16. Li, J.; Le, D. B.; Ferguson, P. P.; Dahn, J. R. Lithium Polyacrylate as a Binder for Tin–cobalt–carbon Negative Electrodes in Lithium-ion Batteries. Electrochim. Acta, 2010, 55, 2991−2995. 17. Tsao, C. H.; Hsu, C. H.; Kuo, P. L. Ionic Conducting and Surface Active Binder of Poly(ethylene oxide)-block-poly(acrylonitrile) for High Power Lithium-ion Battery. Electrochim. Acta., 2016, 196, 41–47. 18. Wei, Z.; Xue, L.; Nie, F.; Sheng, J.; Shi, Q.; Zhao, X. Study of Sulfonated Polyether Ether Ketone With Pendant Lithiated Fluorinated Sulfonic Groups as Ion Conductive Binder in Lithium-ion Batteries. J. Power Sources, 2014, 256, 28-31. 19. Shi, Q.; Xue, L.; Wei, Z.; Liu, F.; Du, X.; DesMarteau, D. D. Improvement in LiFePO4–Li Battery Performance via Poly(perfluoroalkylsulfonyl)imide (PFSI)



Based Ionene Composite Binder. J. Mater. Chem. A, 2013, 1, 15016–15021. 20. Chiu, K. F.; Su, S. H.; Leu, H. J.; Chen, Y. S. Application of lithiated perfluorosulfonate ionomer binders to enhance high rate capability in LiMn2O4 cathodes for lithium ion batteries, Electrochim. Acta, 2014, 117, 134-138 21. Tran, B.; Oladeji, I. O.; Wang, Z.; Calderon, J.; Chai, G.; Atherton, D.; Zhai, L.; Thick LiCoO2/Nickel Foam Cathode Prepared by an Adhesive and Water-Soluble PEG-Based Copolymer Binder. J. Electrochem. Soc., 2012, 159, A1928. 22. Tsao, C. H.; Hsiao, Y. H.; Hsu, C. H.; Kuo, P. L. Stable Lithium Deposition Generated from Ceramic-Cross-Linked Gel Polymer Electrolytes for Lithium Anode. ACS Appl. Mater. Interfaces, 2016, 8, 15216−15224. 23. Kuo, P.L.; Wu, C.A.; Lu, C.Y.; Tsao, C.H.; Hsu, C.H.; Hou, S.S. High Performance of Transferring Lithium Ion for Polyacrylonitrile-Interpenetrating Crosslinked Polyoxyethylene Network as Gel Polymer Electrolyte. ACS Appl. Mater. Interfaces, 2014,6 , 3156–3162. 24. Huang , C.W.; Wu, C.A.; Hou, S.S.; Kuo, P.L.; Hsieh, C.T.; Teng, H.S. Gel Electrolyte Derived from Poly(ethylene glycol) Blending Poly(acrylonitrile) Applicable to Roll-to-Roll Assembly of Electric Double Layer Capacitors Adv. Funct. Mater. 2012, 22, 4677–4685.


Page 38 of 42

Page 39 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


25. Mogurampelly, S.; Borodin, O.; Ganesan, V. Computer Simulations of Ion Transport in Polymer Electrolyte Membranes. Annu. Rev. Chem. Biomol. Eng. 2016, 7, 349−371. 26. Borodin, O.; Smith, G. D. Mechanism of Ion Transport in Amorphous Poly(Ethylene

Oxide)/LiTFSI

From

Molecular

Dynamics

Simulations.

Macromolecules 2006, 39, 1620–1629. 27. Webb, M. A.; Jung, Y.; Pesko, D. M.; Savoie, B. M.; Yamamoto, U.; Coates, G. W.; Balsara, N. P.; Wang, Z.-G.; Miller, T. F., III. Systematic Computational and Experimental

Investigation

of

Lithium-Ion

Transport

Mechanisms

in

Polyester-Based Polymer Electrolytes. ACS Central Science 2015, 1, 198–205. 28. Maitra, A.; Heuer, A. Cation Transport in Polymer Electrolytes: a Microscopic Approach. Phys. Rev. Lett. 2007, 98, 227802. 29. Jorgensen, W. L.; Maxwell, D. S.; Tirado-Rives, J. Development and Testing of the OPLS All-Atom Force Field on Conformational Energetics and Properties of Organic Liquids. J. Am. Chem. Soc. 1996, 118, 11225–11236. 30. Sambasivarao, S. V.; Acevedo, O. Development of OPLS-AA Force Field Parameters for 68 Unique Ionic Liquids. J. Chem. Theory Comput. 2009, 5, 1038–1050. 31. Mogurampelly, S.; Ganesan, V. Structure and Mechanisms Underlying Ion



Transport in Ternary Polymer Electrolytes Containing Ionic Liquids. J. Chem. Phys. 2017, 146, 074902. 32. Canongia Lopes, J. N.; Pádua, A. A. H. Molecular Force Field for Ionic Liquids Composed of Triflate or Bistriflylimide Anions. J. Phys. Chem. B 2004, 108, 16893–16898. 33. Hess, B.; Kutzner, C.; van der Spoel, D.; Lindahl, E. GROMACS 4: Algorithms for Highly Efficient, Load-Balanced, and Scalable Molecular Simulation. J. Chem. Theory Comput. 2008, 4, 435–447. 34. Pronk, S.; Páll, S.; Schulz, R.; Larsson, P.; Bjelkmar, P.; Apostolov, R.; Shirts, M. R.; Smith, J. C.; Kasson, P. M.; van der Spoel, D.; Hess, B.; Lindahl, E. GROMACS 4.5: a High-Throughput and Highly Parallel Open Source Molecular Simulation Toolkit. Bioinformatics 2013, 29, 845–854. 35. Plimpton, S. Fast Parallel Algorithms for Short-Range Molecular Dynamics. J. Comput. Phys. 1995, 117, 1–19. 36. Nose, S. A Molecular-Dynamics Method for Simulations in the Canonical Ensemble. Mol. Phys. 1984, 52, 255–268. 37. Hoover, W. G. Canonical Dynamics: Equilibrium Phase-Space Distributions. Phys. Rev., A 1985, 31, 1695–1697. 38. Martyna, G. J.; Klein, M. L.; Tuckerman, M. Nosé–Hoover Chains: the


Page 40 of 42

Page 41 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Canonical Ensemble via Continuous Dynamics. J. Chem. Phys. 1992, 97, 2635– 2643. 39. Parrinello, M.; Rahman, A. Polymorphic Transitions in Single-Crystals - a New Molecular-Dynamics Method. J. Appl. Phys. 1981, 52, 7182–7190. 40. Martyna, G. J.; Tobias, D. J.; Klein, M. L. Constant Pressure Molecular Dynamics Algorithms. J. Chem. Phys. 1994, 101, 4177–4189. 41. Essmann, U.; Perera, L.; Berkowitz, M. L.; Darden, T.; Lee, H.; Pedersen, L. G. A Smooth Particle Mesh Ewald Method. J. Chem. Phys. 1995, 103, 8577–8593. 42. Hess, B.; Bekker, H.; Berendsen, H.; Fraaije, J. LINCS: a Linear Constraint Solver for Molecular Simulations. J. Comput. Chem. 1997, 18, 1463–1472. 43. Humphrey, W.; Dalke, A.; Schulten, K. VMD: Visual Molecular Dynamics. J. Mol. Graph. 1996, 14, 33-38.



TOC


Page 42 of 42

Fluorinated Copolymer Functionalized with Ethylene Oxide as Novel

Recommend Documents