Enhanced Ion Conductivity in Conducting Polymer Binder for … · 2018-04-27 · in isopropanol (IPA) (binder 2 denoted as c-PEO-PEDOT:PSS/ PEI). Electrostatic and chemical reduction[38]

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Enhanced Ion Conductivity in Conducting Polymer Binder for High-Performance Silicon Anodes in Advanced Lithium-Ion Batteries

Wenwu Zeng, Lei Wang, Xiang Peng, Tiefeng Liu, Youyu Jiang, Fei Qin, Lin Hu, Paul K. Chu,* Kaifu Huo,* and Yinhua Zhou*

DOI: 10.1002/aenm.201702314

the conductive additives may lose electric contact with Si. To address this issue, con-ducting polymers, which have dual func-tions as a binder and conducting additive, have been developed for Si anodes. The conducting polymers provide good electron transport during cycling[13–20] and volume contraction in the Si-based electrode. For example, Liu et al.[13] reported that a poly-fluorene-based conducting polymer with improved electron conductivity and robust mechanical binding force contributes to remarkable rate performance and good cycling stability of the Si anode. Among the conducting polymers, poly(3,4-ethylen-edioxythiophene) (PEDOT) can have a high

conductivity up to 1000 S cm−1 and is easy to processing.[21,22] The PEDOT has been adopted as a part of the Si anode compos-ites to enhance the electronic conductivity and cycling perfor-mance by in situ polymerization of 3,4-ethylenedioxythiophene (EDOT) with the Si[23–26] or mixing the water-based PEDOT:PSS dispersion with the Si and other components (where PSS is poly (styrenesulfonate)).[18,27–30] Beside the electron transport, ion conductivity in the binder also significantly influences the per-formance of the Si anode.[31–33] The binder is required to provide rapid access for lithium-ion (Li-ion) to transport between the Si surface and the binder to achieve high-performance Si anode. Recently, Salem et al.[33] reported that they could improve the rate performance of anodes via enhancing the ion conductivity of the poly(thiophene) conductive polymer binder by attaching ionic alkyl carboxylate groups. However, the electron con-ductivity of the poly(thiophene) is still limited (<10−2 S cm−1). Therefore, increasing the ion conductivity of the highly electron-conductive (up to 103 S cm−1) PEDOT:PSS will be a promising strategy to achieve efficient conductive polymer binders for high-performance silicon anodes.

Herein, a novel polymer binder that possesses high ion as well as electron conductivities suitable for high-performance Si anodes is designed and demonstrated. The binder is prepared by assembling ion-conductive polyethylene oxide (PEO)[34] and polyethylenimine (PEI)[35] onto the electron-conductive PEDOT:PSS chains via chemical crosslinking, chemical reduc-tion, and electrostatic self-assembly. The polymer binder pos-sesses superior lithium-ion and electron transport properties that are 14 and 90 times higher than those of the widely used carboxymethyl cellulose (CMC) (with acetylene black) binder

Polymer binders with high ion and electron conductivities are prepared by assembling ionic polymers (polyethylene oxide and polyethylenimine) onto the electrically conducting polymer poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) chains. Crosslinking, chemical reductions, and electro-statics increase the modulus of the binders and maintain the integrity of the anode. The polymer binder shows lithium-ion diffusivity and electron conduc-tivity that are 14 and 90 times higher than those of the widely used carboxym-ethyl cellulose (with acetylene black) binder, respectively. The silicon anode with the polymer binder has a high reversible capacity of over 2000 mA h g−1 after 500 cycles at a current density of 1.0 A g−1 and maintains a superior capacity of 1500 mA h g−1 at a high current density of 8.0 A g−1.

W. W. Zeng, L. Wang, T. F. Liu, Dr. Y. Y. Jiang, F. Qin, L. Hu, Prof. K. F. Huo, Prof. Y. H. ZhouWuhan National Laboratory for OptoelectronicsSchool of Optical and Electronic InformationHuazhong University of Science and TechnologyWuhan 430074, ChinaE-mail: [email protected]; [email protected]. X. Peng, Prof. P. K. Chu, Prof. K. F. Huo, Prof. Y. H. ZhouDepartment of Physics and Department of Materials Science and EngineeringCity University of Hong KongTat Chee Avenue, Kowloon, Hong Kong, ChinaE-mail: [email protected]

Lithium-Ion Batteries

Lithium-ion batteries (LIBs) become widely used in consumer electronics, hybrid and electric vehicles, and renewable energy storage grids.[1,2] High energy density and long cycle life are highly desirable for the applications of LIBs. Silicon (Si) anodes have been attracting great interest due to its high theoretical specific capacity over 3000 mA h g−1.[3] It is still challenging to achieve good cycling stability for Si anode because of its massive volume change of about 300% in Si during the charge–discharge cycles.[3–11] The typical electrodes of the LIBs are prepared by mixing the electroactive materials, conductive additives such as acetylene black (AB), and polymer binder dissolved in a solvent and then depositing the slurry on a metal current collector.[12] However, this tricomponent electrode generally shows poor cycle life and rate capability with materials undergoing severe volume changes such as Si. The large volume change during insertion and extraction of lithium undermines the electrode integrity and

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system, respectively. Mechanically, the crosslinking and electro-static interactions between the PEDOT:PSS and ionic polymers yield a high modulus that helps to maintain the integrity of the Si anode during volume contraction. The Si anode composed of the polymer binder shows enhanced delithiation rate capability, cycling stability, reversible capacity, and initial Coulombic effi-ciency (ICE).

Design of the polymer binder with high ion and electron conductivities. PEDOT:PSS is a common electrically conducting polymer in organic electronics,[36] whereas PEDOT and PSS are Coulombically bound (Figure 1a). It has an electron conduc-tivity of up to 103 S cm−1.[36] The sulfonate and sulfonic acid groups in the PEDOT:PSS can be modified to provide high ion conductivity. PEO and PEI (Figure 1a) have abundant lone elec-tron pairs in the polymer chains[6,34,35,37] that facilitate lithium-ion transport (Figure 1b). Preparation of the Si anode is illus-trated in Figure 1c. In the first step, PEDOT:PSS, PEO, and Si nanoparticles (NPs) were mixed and annealed. The PEDOT:PSS and PEO were crosslinked as binder 1 and coated onto the Si nanoparticles. Binder 1 is denoted as c-PEO-PEDOT:PSS. In the second step, the electrode was dipped in diluted (0.1 wt%) PEI in isopropanol (IPA) (binder 2 denoted as c-PEO-PEDOT:PSS/PEI). Electrostatic and chemical reduction[38] between the PEDOT:PSS and PEI enable the PEI chains to permeate the pores and bind to binder 1. The PEI is expected to increase the ion conductivity and improve the integrity of the binder when

the Si nanoparticles contract. The Si nanoparticles are covered by a thin layer of SiO2 (Figure S1, Supporting Information) that bonds covalently with the PEDOT:PSS as shown in Figure S2 (Supporting Information). The chemical bonding restricts the movement of Si nanoparticles,[39–41] thereby improving the cycling stability.

To investigate the interactions among the polymer compo-nents inside the polymer binders, a series of experiments are performed. To confirm crosslinking between PEDOT:PSS and PEO, X-ray diffraction (XRD) was carried out on the mixture of PEDOT:PSS and PEO before and after annealing. Two sharp crystalline peaks[42] at 19° and 23° appear before annealing arising from PEO. After annealing at 180 °C for 6 h, the two crystalline peaks disappear (Figure 2b). For comparison, XRD is also performed on the bare PEO films after annealing under the same conditions and the peaks are visible after annealing (Figure S3, Supporting Information). The change in the PEO crystallinity[42] indicates esterification between PEO and PEDOT:PSS. To further verify the crosslinking between PEDOT:PSS and PEO, Fourier-transform infrared spectros-copy measurement was conducted. As shown in Figure S4 (Supporting Information), after the crosslinking of PEDOT:PSS and PEO, the stretching vibration peak (about 3435 cm−1) of OH in the PEDOT:PSS (SOH) and PEO (COH) is decreased. Moreover, a new vibration peak (about 793 cm−1) of COS appears in the annealed PEO and PEDOT:PSS


Figure 1. a) Chemical structure of the polymers: PEDOT:PSS, PEO, and PEI used to prepare the binders. b) Schematic diagram of the interactions between the polymer components (PEI and PEO) and lithium-ions that improve the lithium-ion transport. c) Schematic diagram of the interactions inside the binders with high ion and electron conductivities. (1) Crosslinking between PEO and PSS; electrostatic interaction between PEI and PSS. (3) PEI chemically reducing PEDOT.



mixture. These results confirm the esterification between PEO and PEDOT:PSS.

To study the interactions between PEDOT:PSS and PEI, a few drops of PEDOT:PSS are added to the PEI solution. Figure 2c shows that when the vials are flipped, PEDOT:PSS inside the PEI solution becomes gel-like and stays on the bottom of the vial, while the PEI solution and PEDOT:PSS dis-persion move to the cap side. The zeta potentials are monitored and Figure 2d shows that PEDOT:PSS is negatively charged in the aqueous solution (zeta potential of about −90 mV), whereas PEI is positively charged in the IPA solution (zeta potential of about +16 mV). When PEDOT:PSS is added to PEI (0.1 wt%) in the IPA solution, PEDOT:PSS rapidly precipitates and the zeta potential shifts to the neutral state indicating electrostatic

interaction between PEDOT:PSS and PEI. PEDOT:PSS and PEI exhibit similar electrostatic interaction in water (Figure S5, Supporting Information). Chemical reduction is also observed between PEI and PEDOT:PSS. PEI changes the color of the PEDOT:PSS film from transparent to dark blue (Figure 2e) that is also observed from the absorbance of the films (Figure 2f). As a reference, IPA does not change the absorbance of the PEDOT:PSS film (Figure S6, Supporting Information). X-ray photoelectron spectroscopy measurement was conducted to confirm the interaction between the PEDOT:PSS and PEI (Figure S7, Supporting Information). Compared with the pris-tine PEI, a new peak of of N1s with higher binding energy (400.3 eV) appears when the PEI is deposited on top of the PEDOT:PSS. This indicates the presence of positively charged


Figure 2. a) Crosslinking between PSS and PEO. b) XRD patterns of the mixture of PEO and PEDOT:PSS before and after annealing at 180 °C for 6 h. c) Pictures of the PEI solution, PEDOT:PSS dispersion, and PEDOT:PSS dropped into the PEI solution (gel form). d) Zeta potential of PEDOT:PSS, PEI, and PEI + PEDOT:PSS. e) Schematic diagram of chemical reduction (electron transfer) from PEI to PEDOT. The inset is a picture with the right half of the PEDOT:PSS film chemically reduced by PEI. f) Absorbance of the PEDOT:PSS films with and without PEI reduction.



nitrogen atoms in the PEI that confirms the reduction between PEI and PEDOT:PSS. These interactions, namely crosslinking, electrostatic interaction, and chemical reduction between the components improve the integrity of the polymer binders.

Electron and Li-ion transport properties of the polymer binder. Figure 3a shows the measured ion and electron con-ductivities of the binders [CMC/AB, PEDOT:PSS, binder 1 (c-PEO-PEDOT:PSS), and binder 2 (c-PEO-PEDOT:PSS/PEI)]. The Li-ion diffusion coefficients of the binders are determined by the galvanostatic intermittent titration technique (GITT).[43] Figure S8 (Supporting Information) presents the GITT curve of binder 2 and the detailed procedures of GITT are described in the Experimental Section. As expected, binder 2 (c-PEO-PEDOT/PSS/PEI) exhibits the highest Li-ion diffusion coeffi-cient (4.0 × 10−8 cm2 s−1) which is about 14 times larger than that of the common CMC/AB binder system (2.8 × 10−9 cm2 s−1) and 11 times larger than that of PEDOT:PSS (3.7 × 10−9 cm2 s−1). The enhanced Li-ion diffusion coefficient is attributed to the introduction of ion-conductive PEO and PEI. As for the elec-tron conductivity, since the binders are based on the highly con-ductive PEDOT:PSS high electron conductivity is expected. The pristine PEDOT:PSS has an electron conductivity of 895 S cm−1. After incorporating PEO and PEI into PEDOT:PSS, the electron conductivity decreases (271 S cm−1) but is still about 90 times

larger than that of the common CMC/AB (about 3 S cm−1) polymer binder and high enough for electron transport. The c-PEO-PEDOT:PSS/PEI (binder 2) polymer binder has larger Li-ion diffusion coefficient and electron conductivity than the CMC/AB system for Si anodes.

Li-ion transport in Si electrodes with different binders. The Si anodes are prepared by assembling the binders and Si nano-particles (diameter: 180 nm, Figure S1, Supporting Informa-tion). Figure 3b shows the cyclic voltammetry (CV) curves of the c-PEO-PEDOT:PSS/PEI/Si electrode at different scanning rates from 0.03 to 0.1 mV s−1 between 0.01 and 1.0 V (vs Li/Li+). The wide cathodic peak at 0.17 V (IC) corresponds to the alloying process between Si and Li and the two anodic peaks at 0.35 V (IA1) and 0.55 V (IA2) in the anodic sweep are ascribed to dealloying of Si-Li alloys. The CV curves of the c-PEO-PEDOT:PSS/Si and PEDOT:PSS/Si electrodes are depicted in Figure S9a,b (Supporting Information), which exhibit the three similar alloying/dealloying peaks. The cathodic and anodic current peaks (IC, IA1, IA2) of the three Si-based anodes are shown in Figure 3c. The linear relationship of the peak currents with the square root of scanning rates indicates a diffusion-limited process. Therefore, the classical Randles–Sevcik equation is used to describe the lithium diffusion pro-cess:[44,45] I n AD C(2.69 10 ) ,p

5 1.5Li

0.5Li

0.5ν= × + where IP is the peak


Figure 3. Lithium-ion diffusion and electron conductivity. a) Lithium-ion diffusion coefficient and electron conductivity of various binders. b) CV char-acteristics of the c-PEO-PEDOT:PSS/PEI/Si electrode at various scanning rates after five precycles at a current density of 0.2 A g−1. Plots of the CV peak current versus square root of the scanning rates. c) Cathodic reduction (IC: Si → Li15Si4) and anodic oxidation (IA1 and IA2: Li15Si4 → Si). d) Nyquist plots of Si electrodes with PEDOT:PSS-based binders.



current, n is the charge transfer number, A is the geometric area of the active electrode, DLi+ is the lithium-ion diffusion coefficient, CLi is the concentration of lithium-ions in the anode, and ν is the potential scan rate. The slope of the curve (Ip/ν0.5) reflects the Li-ion diffusion rate (DLi+) since n, A, and CLi are considered to be unchanged for the same electrodes during cycling. Figure 3c indicates that the c-PEO-PEDOT:PSS/PEI/Si electrode exhibits faster Li-ion diffusion than the PEDOT:PSS/Si and c-PEO-PEDOT:PSS/Si electrodes and the PEDOT:PSS/Si electrode shows the lowest Li-ion diffusivity. The results are consistent with Figure 3a. Figure 3d compares the Nyquist plots[6] of the three anodes. The c-PEO-PEDOT:PSS/PEI/Si electrode has the smallest charge-transfer resistance (Rct), which is demonstrated by the diameter of the semicircle in the medium-to-high frequency region. Although PEDOT:PSS has higher electron conductivity (Figure 3a), the higher ion conduc-tivity of c-PEO-PEDOT:PSS/PEI enables faster Li-ion diffusion and smaller charge-transfer resistance in the corresponding Si anodes. These results indicate that assembling ion-conductive PEO and PEI onto the highly electron-conductive PEDOT:PSS chains is an effective approach to improve the Li-ion transport and reduce the overall charge-transfer resistance of Si anodes.

Mechanical properties. Figure S10 (Supporting Informa-tion) shows the elastic moduli of the polymer binders versus indentation depth. Binder 2 (c-PEO-PEDOT:PSS/PEI) shows the largest modulus among the polymer binders. The crosslinking and electrostatic interaction increase the rigidity of the binders.[12,39] The increased modulus helps to main-tain the integrity of the electrodes during volume contraction. Table S1 (Supporting Information) summarizes the thickness

variation of the Si electrodes with binders. After five cycles, the smallest thickness variation (4.9%) is observed from the c-PEO-PEDOT:PSS/PEI/Si, compared to 34.7% for poly(vinylidene fluoride) (PVDF)/AB/Si and 18.2% for CMC/AB/Si. Binder 2 also shows the highest adhesion force to Cu foil (Figure S11, Supporting Information) that contributes to the improved elec-trode integrity. Figure S12 (Supporting Information) shows the morphology changes of the Si anodes consisting of Si nano-particles with polymer binders after five cycles. There are no obvious cracks in the PEDOT:PSS-based binders confirming that the polymer binder of c-PEO-PEDOT:PSS/PEI has robust mechanical properties.

Electrochemical performance. Figure 4a shows the cycling performance of the Si nanoparticles with different binders (about 80 wt% Si content, Figure S13, Supporting Informa-tion). The c-PEO-PEDOT:PSS/PEI/Si electrode delivers an ini-tial delithiation capacity of 2440 mA h g−1 at a current density of 1.0 A g−1 and the capacity remains at 2027 mA h g−1 after 500 cycles. This cycling performance is better than that of previ-ously reported Si anodes (Table S2, Supporting Information). The capacity change in the first few cycles is due to increased electrolyte penetration/electrode activation during battery oper-ation.[46] Figure S14 (Supporting Information) indicates that the contribution of the c-PEO-PEDOT:PSS/PEI polymer binder to the overall capacity of Si anode is negligible. Compared to the c-PEO-PEDOT:PSS/PEI/Si electrode, the c-PEO-PEDOT:PSS/Si, PEDOT:PSS/Si and PEDOT:PSS/PEI/Si (Figure S15, Sup-porting Information) electrodes exhibit inferior cycling sta-bility. The CMC/AB and PVDF/AB/Si electrodes show a poorer cycling performance (Figure 4a). The poor cycling stability of


Figure 4. Electrochemical performance of the Si electrodes with various polymer binders. a) Cycling performance of the Si electrodes containing PVDF, CMC, PEDOT:PSS, c-PEO-PEDOT:PSS, and c-PEO-PEDOT:PSS/PEI as binders at a current density of 1.0 A g−1. b) Rate capability of the Si electrodes (except PVDF/AB/Si) at different current densities between 0.2 and 8.0 A g−1. c) CV curves of the PEDOT:PSS-based Si electrodes at a scanning rate of 0.1 mV s−1 after five precycles at a current density of 0.2 A g−1. d) ICE of the Si electrodes with polymer binders. In all the cases, the areal loading is 1.0 mg cm−2.



CMC/AB/Si and PVDF/AB/Si may be due to breakage of the conductive network caused by the large volume changes of Si nanoparticles during cycling (Figure S12 and Table S1, Sup-porting Information). Figure S16 (Supporting Information) shows the cycling performance of the electrode with carbon coated nano-Si (Si@C NPs) is inferior to that of the electrode with silicon. This indicates the importance of surface functional groups of the nano-Si to form the chemical bonds between the binder and nano-Si. High mass loading is important to realize large energy density. The c-PEO-PEDOT:PSS/PEI/Si electrode with a Si mass loading of 2.2 mg cm−2 (Figure S17, Supporting Information) remains good cycling stability and maintains a high reversible capacity of around 1840 mA h g−1 after 60 cycles that corresponds to the areal capacity of about 4.0 mA h cm−2 of the electrode. The superior cycling stability and high reversible capacity of the c-PEO-PEDOT:PSS/PEI/Si electrode arise from the robust mechanical properties and excellent Li-ion and elec-tron transport network during battery operation.

Figure 4b shows the delithiation rate capability of the Si elec-trodes with the binders. The CMC/AB/Si electrode delivers the worst rate performance. Although the c-PEO-PEDOT:PSS/PEI/Si, c-PEO-PEDOT:PSS/Si, and PEDOT:PSS/Si electrodes have similar capacities at 0.2 A g−1, the c-PEO-PEDOT:PSS/PEI/Si electrode shows higher rate capability with better capacity retention of 50.4% when the current density is increased 40 times from 0.2 to 8.0 A g−1. This rate capability is higher than that of the previously reported Si anodes (Table S2, Sup-porting Information). In contrast, the c-PEO-PEDOT:PSS/Si and PEDOT:PSS/Si electrodes exhibit much lower capacity retention of 35.6% and 16.1%, respectively. The superior rate performance of the c-PEO-PEDOT:PSS/PEI/Si electrode arises from the fast electrode reaction as a result of the excellent Li-ion and electron transport. Figure 4c shows the CV curves of the PEDOT:PSS-based Si electrodes at a scanning rate of 0.1 mV s−1. The difference between the anodic and cathodic peak potentials (ΔE) reflects electrochemical polarization. The smallest ΔE of the c-PEO-PEDOT:PSS/PEI/Si electrode indi-cates the lowest polarization, which further corroborates the highest rate capability. The results indicate that the c-PEO-PEDOT:PSS/PEI polymer binder with high Li-ion and electron transport yields excellent battery performance in the c-PEO-PEDOT:PSS/PEI/Si during battery operation. Figure 4d shows the ICE of the different Si electrodes. The c-PEO-PEDOT:PSS/PEI/Si electrode possesses the highest ICE of 82.0%. The excellent ICE in the c-PEO-PEDOT:PSS/PEI/Si elec-trode stems from the crosslinked polymer chain network and covalent attachment to Si mitigating unwanted side reactions or excessive solid–electrolyte interphase formation during lithiation.

In conclusion, a novel polymer binder is designed and prepared by assembling ion-conductive PEO and PEI poly-mers onto high electron-conductive PEDOT:PSS chains by chemical crosslinking, chemical reduction, and electrostatic self-assembly. The polymer binder has the following advan-tages: (1) The binder has high Li-ion diffusivity and electron conductivity and (2) The crosslinking and electrostatic interac-tions between the PEDOT:PSS and ionic polymers yield high modulus that helps to maintain the mechanical structure of the Si anode during volume contraction. The Si electrode con-sisting of this binder and 180 nm Si nanoparticles exhibits a

high reversible capacity of over 2000 mA h g−1 after 500 cycles at a current density of 1.0 A g−1 and maintains a high capacity of 1500 mA h g−1 at a large current density of 8.0 A g−1. The electrode shows a high ICE of 82.0%. This new binder can be extended to other high-capacity anodes, such as Sn, Ge, and Sb for LIBs and sodium-ion batteries. Furthermore, this innovative binder provides a new perspective to the design of multifunc-tional binders for high-power and high-capacity lithium-ion batteries.

Experimental SectionMaterials: The aqueous PEDOT:PSS formulation (Clevios PH1000)

was purchased from Heraeus. PEO (Mw = 100,000 g mol−1) was bought from Alfa Aesar. PEI (MW = 25000 g mol−1) and IPA were obtained from Sigma-Aldrich.

Preparation of Binders, Electrodes, and Batteries: In the electrode preparation, the slurries were prepared by dispersing the Si and polymer binder with a weight ratio of 80: 20 of PEDOT:PSS, c-PEO-PEDOT:PSS (PEO: PEDOT:PSS = 8: 12 wt%), and c-PEO-PEDOT/PEI (PEO: PEDOT:PSS: PEI = 7: 10.5: 2.5 wt%) polymer binders and the Si, polymer binder, and AB were dispersed with a weight ratio of 80: 10: 10 in N-methyl-2-pyrrolidone (Aldrich) for the PVDF polymer binder and distilled water for the CMC polymer binder. A small amount of dimethyl sulfoxide (5 wt% relative to the PEDOT:PSS solution) was added to the PEDOT:PSS-based Si electrodes to optimize the electron conductivity. It evaporated at a high temperature (for example, 180 °C). Each slurry was cast on a copper foil and dried in a vacuum oven at 80 °C overnight. For the electrodes with the PEDOT:PSS and c-PEO-PEDOT:PSS polymer binders, then were predried at 80 °C under vacuum to remove water. As for the PEDOT:PSS/Si electrodes, the temperature was increased to 180 °C for 2 h under vacuum to form covalent bonds between PEDOT:PSS and Si particles. For the c-PEO-PEDOT:PSS/Si electrodes, the temperature was increased to 180 °C for 6 h to form the crosslinked binder (c-PEO-PEDOT:PSS). For the electrodes with the c-PEO-PEDOT:PSS/PEI polymer binder, the crosslinked binder (c-PEO-PEDOT:PSS) was immersed together in a beaker filled with PEI/IPA (1:999 by weight) for a few seconds and the electrodes were dried in a vacuum oven at room temperature for 48 h. The 2025 coin-like cells with metallic Li counter electrodes were fabricated to evaluate the electrochemical performance of all the electrodes. Lithium hexafluorophosphate (1 m LiPF6) in the cosolvent ethylene carbonate and diethyl carbonate (1:1 by weight) with 5% fluoroethylene carbonate additive were used as the electrolyte. The coin cells were assembled in an argon-filled glove box. Charging and discharging of the batteries were conducted on an MTI automatic battery cycler with voltage cutoffs between 0.01 and 1.0 V versus Li/Li+.

Characterization: Thermal gravity analysis was conducted in air at a heating rate of 10 °C min−1. The structure of silicon (Si) was characterized by X-ray diffraction (GAXRD, Philips X′Pert Pro) and high-resolution transmission electron microscopy (JEM-2010F). The nitrogen adsorption and desorption isotherms were acquired on the TriStar II 3020 (V1.03) surface area and porosity measurement system (Micromeritics Inc., USA). SEM was carried out on the Si nanopowder and electrodes on a high-resolution field emission scanning electron microscope (FEI Nova Nano-SEM 450). The MAS NMR 29Si spectra were acquired on the AVANCE III 400MHz wide cavity solid state nuclear magnetic resonance spectrometer. The Young’s moduli of the binders were determined by nanoindentation (Hysitron T1750). Electrochemical impedance spectroscopy was conducted from 10 mHz to 105 Hz with Autolab PGSTAT302N. The lithium-ion diffusion coefficients of the polymer binders were determined by Autolab PGSTAT302N with the GITT. Various binder films with a thickness of ≈20 µm were deposited on Cu substrates forming two-electrode 2025 coin-type half-cells with a Li foil counter electrode for the GITT tests. The GITT procedures consisted of galvanostatic discharging/charging pulses 10 min each



© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim1702314 (7 of 8)Adv. Energy Mater. 2018, 8, 1702314

followed by relaxation for 10 min. A complete discharging/charging cycle by GITT needed about 40 h. The thickness of the polymer films and Si electrodes was measured by surface profilometry (Veeco Dektak 150) and micrometry (TESA, Sweden). The sheet resistance of the polymer films was measured by a four-point probe (RTS-8) and the conductivity was calculated according to the following equation

sr1R tσ ( )= × −

(1)

where σ is the conductivity, Rsr is the sheet resistance, and t is the thickness of the polymer films.

The lithium-ion diffusion coefficient (DLi) of the binders was calculated according to the following equation[43]

4

/,Li

22

2

Li2

Dm VM A

E

dE d

LD

B m

B

s

π τ ττ( )( )

=

∆

τ

(2)

where mB is binder mass in the electrode, Vm is the molar volume of the compound, MB is molar mass of the binder, A is the contact area of electrolyte and binder, τ is the duration of the current pulse (s), L is the thickness of the electrode, ΔEs is the steady-state voltage change, due to the current pulse and ( )/( )dE d ττ is the slope of the linearized plot of the potential E (V) during the current pulse of duration τ (s). The Coulombic efficiency was calculated according to the following equation

100%dealloy

alloyCE C

C= ×

(3)

where Calloy and Cdealloy are the capacity of the anodes for Li insertion and extraction.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsW.W.Z. and L.W. contributed equally to this work. The work is supported by the National Natural Science Foundation of China (Grant Nos. 21474035, 51403071, and 51572100), Recruitment Program of Global Youth Experts, Fundamental Research Funds for the Central Universities, HUST (Grant No. 2016JCTD111), Natural Science Foundation of Hubei Province (2015CFA116), Postdoctoral Science Foundation of China (2016M602289), and City University of Hong Kong Applied Research Grant Nos. 9667122 and 9667144. The authors thank Mr. Tianqi Li and Prof. Jun Zhou for the help of GITT measurement and acknowledge the Nanodevices and Characterization Centre of WNLO-HUST and Analytical and Testing Center of HUST for support.

Conflict of InterestThe authors declare no conflict of interest.

Keywordsconducting polymers, ion conductivity, large capacity, lithium-ion batteries, Si anodes

Received: August 22, 2017Revised: October 20, 2017

Published online: January 8, 2018

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Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2018.

Supporting Information

for Adv. Energy Mater., DOI: 10.1002/aenm.201702314

Enhanced Ion Conductivity in Conducting Polymer Binder forHigh-Performance Silicon Anodes in Advanced Lithium-IonBatteries

Wenwu Zeng, Lei Wang, Xiang Peng, Tiefeng Liu, YouyuJiang, Fei Qin, Lin Hu, Paul K. Chu,* Kaifu Huo,* andYinhua Zhou*

Supporting Information

Enhanced Ion Conductivity in Conducting Polymer Binder for High-Performance

Silicon Anodes in Advanced Lithium-Ion Batteries

Wenwu Zeng†, Lei Wang

†, Xiang Peng, Tiefeng Liu, Youyu Jiang, Fei Qin, Lin Hu, Paul K.

Chu*, Kaifu Huo* and Yinhua Zhou

*

W. W. Zeng, L. Wang, T. F. Liu, Y. Y. Jiang, F. Qin, L. Hu, T. Q. Li, Prof. K. F. Huo, Prof. Y.

H. Zhou

Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic

Information, Huazhong University of Science and Technology, Wuhan 430074 (China)

E-mail: [email protected] (K.F. Huo); [email protected] (Y.H. Zhou)

Dr. X. Peng, Prof. P. K. Chu, Prof. K. F. Huo, Prof. Y. H. Zhou

Department of Physics and Department of Materials Science and Engineering, City University

of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong (China)

E-mail: [email protected] (P.K. Chu)

[+] These authors contributed equally to this work

mailto:[email protected]



1

Figure S1. Structure of the nanoSi. a) SEM image of the Si nanopowder. b) Size distribution of the Si

nanopowder. The average diameter of the Si nanoparticles is 180 nm. c) N2 sorption isotherms of Si

nanopowder. d) TEM image of the Si nanoparticle. The Si nanoparticles are surrounded by an

amorphous SiO2 layer with a thickness of 2 nm. e) XRD spectra of the Si nanopowder.

a)

Si

2 nm

SiO2

b)

c)

d)

e)

2

Figure S2. 29

Si MAS NMR spectra of the Si nanopowder and Si electrode with the PEDOT:PSS

binder annealed at 180 ºC for 2 h under vacuum. The peaks corresponding to hydroxyl moieties at the

surface of SiO2 on the Si nanopowder mostly disappear due to a condensation reaction between the

OH groups of Si and the SO3H groups of PEDOT:PSS in the Si electrode.

3

Figure S3. XRD spectra of the pristine and annealed PEO (180 ºC for 6 h).

10 15 20 25 30 35 40 45

Initial PEO

2-Theta (deg)

Inte

nsity (

a.u

.)

Cured PEO

4

Figure S4. FTIR spectra of PEDOT:PSS, PEO, initial and annealed PEO+PEDOT:PSS (180 ºC for 6

h).

4000 3500 3000 2500 2000 1500 1000 500

Tra

nsm

itta

nce (

a.u

.)

Initial PEO+PEDOT:PSS

PEDOT:PSS

PEO

Wavenumber (cm-1)

Cured PEO+PEDOT:PSS

O-H stretching

C-O-S stretching

5

Figure S5. Zeta potential of PEDOT:PSS, PEI in water (0.1 wt.%) and PEDOT:PSS+PEI (in water).

-120 -80 -40 0 400

1

2

3

4

PEDOT:PSS

PEI/H2O

PEDOT:PSS+PEI

To

tal c

ou

nts

/ 1

05

Zeta potential (mV)

6

Figure S6. Absorbance of the PEDOT:PSS films with and without IPA treatment.

400 600 800 1000 1200 1400 16000.0

0.1

0.2

0.3

PEDOT:PSS

PEDOT:PSS/IPA

Ab

so

rba

nce

(a

. u

.)

Wavelength (nm)

7

Figure S7. XPS spectra of N1s for PEI and PEI+PEDOT:PSS.

396 398 400 402 404

N1S

PEI

Inte

nsit

y (

a.u

.)

Binding energy (eV)

PEI +PEDOT:PSS

399.7 eV

400.3 eV

N1S

8

Figure S8. GITT curve of the c-PEO-PEDOT:PSS/PEI binder (voltage vs. time).

Eτ

ΔEs

τ

0 600 1200 1800 24000.74

0.76

0.78

0.80

0.82

0.84

Pote

ntial (V

) vs. Li/Li+

Time (s)

9

Figure S9. Cyclic voltammetry of Si electrodes with two different polymer binders: a)

c-PEO-PEDOT:PSS/Si; b) PEDOT:PSS/Si at different scanning rates. Cyclic voltammetry was

performed after 5 pre-cycles at a current density of 0.2 A g-1

.

a) b)

0.0 0.2 0.4 0.6 0.8 1.0

-0.8

-0.4

0.0

0.4

0.8 0.1 mV/s

0.07 mV/s

0.05 mV/s

0.03 mV/s

Cu

rre

nt

(mA

)

Potential vs. Li/Li+ (V)

IA1

IA2

IC c-PEO-PEDOT:PSS/Si

0.0 0.2 0.4 0.6 0.8 1.0

-0.8

-0.4

0.0

0.4

0.8

PEDOT:PSS/SiIC

IA2

IA1

0.1 mV/s

0.07 mV/s

0.05 mV/s

0.03 mV/s

Cu

rre

nt

(mA

)

Potential vs Li/Li+ (V)

10

Figure S10. Elastic modulus-distance loading curves of polymer films. For instance, the elastic

modulus of c-PEO-PEDOT:PSS/PEI film is relatively higher compared to other polymer binders.

0 100 200 300 400 500

2

4

6

8

10

12 c-PEO-PEDOT:PSS/PEI

c-PEO-PEDOT:PSS

PEDOT:PSS

CMC

PVDF

Ela

stic m

od

ulu

s (

GP

a)

Indentation depth (nm)

11

Figure S11. The adhesion force between the Cu foil and films consisting of Si with different binders

(PVDF, CMC, PEDOT:PSS, binder 1: c-PEO-PEDOT:PSS, binder 2: c-PEO-PEDOT:PSS/PEI).

0.0

0.2

0.4

0.6

binder 2

binder 1

PEDOT:PSS

CMC

Adh

esio

n forc

e (

N m

m-2)

PVDF

12

Figure S12. SEM images of Si electrodes with a) c-PEO-PEDOT:PSS/PEI, b) c-PEO-PEDOT:PSS, c)

PEDOT:PSS, d) CMC and e) PVDF before cycling and after 5 cycles. The current density is 1.0 A g-1

and potential range is 0.01-1.0 V. Obvious morphological changes are observed from the Si electrodes

with PEDOT:PSS, c-PEO-PEDOT:PSS, particularly, CMC and PVDF binders. The cycled electrodes

with the binders show a smooth topography on some areas of the electrode surface. The smooth

topography is considered to be the thick SEI on the Si surface. Furthermore, large cracks and dents are

shown in Si electrodes with CMC and PVDF, indicating they cannot endure the internal stress

generated by the huge volume change of Si during charge/discharge. The minimum morphology

changes is observed in the cycled c-PEO-PEDOT:PSS/PEI/Si electrode. This indicates that the

c-PEO-PEDOT:PSS/PEI binder contributes to the stable SEI layer and can accommodate the volume

changes in Si during cycling.

5

cycles

c-PEO-PEDOT:PSS/PEI c-PEO-PEDOT:PSS/PEI

c-PEO-PEDOT:PSS c-PEO-PEDOT:PSS

PEDOT:PSS PEDOT:PSS

CMC/AB CMC/AB

PVDF/ABPVDF/AB

13

Figure S13. Thermogravimetic analysis (TGA) curves of the pure Si nanopowder, the

c-PEO-PEDOT:PSS/PEI binder and the anode of nano-Si with the c-PEO-PEDOT:PSS/PEI binder.

This indicates the Si electrode containing about 80 wt. % of Si. Thermal gravity analysis (TGA) was

conducted in air at a heating rate of 10 ºC/min.

200 400 6000

20

40

60

80

100

c-PEO-PEDOT:PSS/PEI/Si

c-PEO-PEDOT:PSS/PEI

Si

Ma

ss

Ch

an

ge

s (

wt.

%)

Temperature (ºC)

14

Figure S14. Cyclic voltammetry (CV) of the c-PEO-PDOT:PSS/PEI/Si electrode and

c-PEO-PEDOT:PSS/PEI binder. a) The cathodic peak near 0.7 V in the first cycle indicates SEI

formation. The increase in current indicates that there is increased electrolyte penetration/electrode

activation in the electrode during first few cycles. b) This indicates that the contribution of the

c-PEO-PEDOT:PSS/PEI polymer binder to the overall capacity of Si electrodes is negligible.

0.0 0.5 1.0 1.5 2.0 2.5-0.012

-0.008

-0.004

0.000

0.004

1st Cycle

2nd

Cycle

Cu

rre

nt (m

A)

Potential vs. Li/Li+

scanning rate: 0.1mV/s

c-PEO-PEDOT:PSS/PEI

0.0 0.2 0.4 0.6 0.8 1.0

-0.8

-0.4

0.0

0.4

0.8

Cu

rre

nt (m

A)

Potential vs. Li/Li+ (V)

scanning rate: 0.1mV/s

1st- 6

th cycle


a) b)

15

Figure S15. Cycling stability of Silicon anode with PEDOT:PSS/PEI binder. The current density is

1.0 A g-1

, the potential range is 0.01 – 1.0 V.

0 30 60 90 120 1500

1000

2000

3000

4000

Sp

ecif

ic c

ap

acit

y (

mA

h g

-1)

Cycle

PEDOT:PSS/PEI/Si

16

Figure S16. Cycling performance of the electrodes of Si@C NPs and nano-Si mixed with the binder

(c-PEO-PEDOT:PSS/PEI). The current density is 1.0 A g-1

. The potential range is 0.01 – 1.0 V. The

fabrication process for Si@C NPs is included as follows: 0.2g Si NPs (diameter: ~180 nm ) were

dispersed in 100 ml of the Tris-HCl buffer solution (pH = 8.5) and ultrasonically agitated for 20 min,

and 0.2 g of dopamine chloride were added with a concentration of 2 mg ml−1

. Self-polymerization of

dopamine on Si NPs was conducted at room temperature under continuous stirring for 4 h, resulting in

the formation of polydopamine (PDA) coated Si NPs (Si@PDA). The Si@PDA NPs were carbonized

at 850 ºC in Ar to produce Si@C NPs.

0 20 40 60 80 1000

1000

2000

3000

4000

c-PEO-PEDOT:PSS/PEI/Si@C

Sp

ecif

ic c

ap

acit

y (

mA

h g

-1)

Cycle


17

Figure S17. Cycling stability of c-PEO-PEDOT:PSS/PEI/Si electrode with a high mass loading of 2.2

mg-Si cm-2

. The current density is 1.0 A g-1

, the potential range is 0.01 – 1.0 V.

0 10 20 30 40 50 600

500

1000

1500

2000

2500

Are

al c

ap

ac

ity (m

Ah

cm

-2)

Sp

ec

ific

ca

pa

cit

y (

mA

h g

-1)

Cycle

~ 4.0 mAh cm-2

0.0

1.1

2.2

3.3

4.4

5.5

18

Table S1. Thickness of the Si electrodes before cycling, at the end of the 5th

discharge, and at the end

of the 5th charge with thickness variation of discharging/charging vs. initial Si electrodes.

Sample

Avg. electrode thickness (μm) Avg. electrode thickness

variation (%)

Initiala) Dischargeb) Chargec) Dischargeb)

vs. initiala)

Chargec) vs.

initiala)

c-PEO-PEDOT:PSS/PEI/Si 30.5 40.5 32 32.8 4.9

c-PEO-PEDOT:PSS/Si 24 32.5 26 35.4 8.3

PEDOT:PSS/Si 26.5 36 30.5 35.8 15.1

CMC/AB/Si 27.5 38 32.5 38.2 18.2

PVDF/AB/Si 24.5 35 33 42.9 34.7

a),

b) and

c)Characterization of the Si electrodes before cycling, at the end of the 5

th discharge, and at the

end of the 5th charge, respectively. The current density is 1.0 A g

-1 and the potential range is 0.01-1.0 V.

19

Table S2. Comparison of the electrochemical performance of Si-based anodes.

Binder

Diameter of Si

nanoparticles

(nm)

Electrode composition

Electrode mass

loading

/ mg cm-2

Electrode performance Ref.

c-PEO-PEDOT:PSS/PEI ~ 180 Si : c-PEO-PEDOT:PSS/PEI

=80 : 20 1

1500 mAhg-1 @ 8.0 A g-1

2027 mAhg-1 @ 1.0 A g-1, 500th

Our

work

PEDOT:PSS 50-70 Si : PEDOT:PSS = 80 : 20 1 ~ 750 mAhg-1 @ 7.0 A g-1

1950 mAhg-1 @ 1.0 A g-1, 100th [2]

PEDOT:PSS

conducting agent +

CMC binder

50 - 100 Si : CB : PEDOT:PSS : CMC

= 70 : 10 : 10 : 10 ~ 1

871 mAhg-1 @ 10.0 A g-1

1834 mAhg-1 @ 0.2 A g-1, 100th [3]

PPyE - Si: PPyE = 66.6 : 33.3 0.22 ~ 1500 mAhg-1 @ 8.4 A g-1 [4]

PAA-PVA < 100 Si : CB : PAA-PVA = 60 : 20 : 20 2.4 ~ 1800 mAhg-1 @ 4.0 A g-1, 50th [5]

PANi ~ 60 Si : PANi = 75 : 25 0.3 ~ 0.4 1100 mAhg-1 @ 3.0 A g-1

~1200 mAhg-1 @ 1.0 A g-1, 1000th [6]

PFFOMB - Si: PFFOMB = 66.6 : 33.3 ~ 0.3 2050 mAhg-1 @ 4.2 A g-1

~2100 mAhg-1 @ 0.42 A g-1, 650th [7]

Meldrum’s acid

incorporated polymer 90 Si : CB : binder = 60 : 20 : 20 1 ~ 1400 mAhg-1 @ 3.0 A g-1, 50th [8]

CNT and PEDOT:PSS 50 - 200 Si : CNT/PEDOT:PSS = 57 : 43 ~ 2 1802 mAhg-1 @ 0.42 A g-1, 100th [9]

PAA 100 - 400 Si : CB : PAA = 43 : 42 : 15 - ~ 2000 mAhg-1 @ 2.1 A g-1, 100th [10]

Mussel-inspired binder - Si : CB : binder = 60 : 20 : 20 0.2 ~ 0.3 ~2000 mAhg-1 @ 2.1 A g-1, 400th [11]

PEFM < 100 Si : PEFM = 66.6 : 33.3 ~ 0.2 ~ 1800 mAhg-1 @ 7.5 A g-1

~ 3000 mAhg-1 @ 0.375 A g-1, 50th [12]

Native xanthan gum ~ 60 Si : CB : gum binder = 60 : 20 :

20 ~ 0.3

~ 1600 mAhg-1 @ 7.0 A g-1

2150 mAhg-1 @ 3.5 A g-1, 200th [13]

PVDF Si : CB : PVDF = 80 : 12 : 8 2.6 ~ 600 mAhg-1 @ 0.15 A g-1, 50th [14]

c-PAA-CMC < 100 Si : CB : c-PAA-CMC = 60 : 20 :

20 -

~ 1200 mAhg-1 @ 3A g-1

~ 2000 mAhg-1 @ 0.3 A g-1, 100th [15]

PAI - Si : CB : PAI = 84: 6: 10 - ~1700 mAhg-1 @ 0.42 A g-1, 20th [16]

PAA-co-PVA 100 Si : CB : PAA-co-PVA

= 60 : 20 : 20 -

~ 1500 mAhg-1 @ 2.0 A g-1

1250 mAhg-1 @ 0.2 A g-1, 80th [17]

Na-CMC 50 Si : CB : Na-CMC = 40: 40: 20 - ~1000 mAhg-1 @ 2.1 A g-1, 100th [18]

20

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Documents

Enhanced Ion Conductivity in Conducting Polymer Binder for … · 2018-04-27 · in isopropanol (IPA) (binder 2 denoted as c-PEO-PEDOT:PSS/ PEI). Electrostatic and chemical reduction[38]