Conducting Polymer Coating on High-voltage Cathode Based on Soft

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 29457−29466

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Conducting Polymer Coating on a High-Voltage Cathode Based on Soft Chemistry Approach toward Improving Battery Performance Yonguk Kwon,†,‡ Yongho Lee,†,§ Sang-Ok Kim,† Hyung-Seok Kim,† Ki Jae Kim,∥ Dongjin Byun,‡ and Wonchang Choi*,†,⊥

ACS Appl. Mater. Interfaces 2018.10:29457-29466. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 09/05/18. For personal use only.

†

Center for Energy Storage, Korea Institute of Science and Technology, 5, Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea ‡ Department of Materials Science and Engineering and §Department of Chemical and Biological Engineering, Korea University, 145, Anam-ro, Sungbuk-gu, Seoul 02841, Republic of Korea ∥ Department of Energy Engineering, Konkuk University, 120, Neungdong-ro, Gwangjin-gu, Seoul 05029, Republic of Korea ⊥ Division of Energy & Environment Technology, KIST School, Korea University of Science and Technology, Seoul 02792, Republic of Korea S Supporting Information *

ABSTRACT: The surface of a 5 V class LiNi0.5Mn1.5O4 particle is modified with poly(3,4-ethylenedioxythiophene) (PEDOT) conducting polymer by utilizing the hydrophobic characteristics of the 3,4-ethylenedioxythiophene (EDOT) monomer and the tail group of cetyl trimethyl ammonium bromide (CTAB) surfactants, in addition to the electrostatic attraction between cationic CTAB surfactant and cathode materials with a negative ζ potential in aqueous solution. With this novel concept, we design and prepare a uniform EDOT monomer layer on the cathode materials, and chemical polymerization of the EDOT coating layer is then carried out to achieve PEDOT-coated cathode materials via a simple one-pot preparation process. This uniform conducting polymer layer provides notable improvement in the power characteristics of electrodes, and stable electrochemical performance can be obtained especially at severe operating conditions such as the fully charged state and elevated temperatures owing to the successful suppression of the side reaction between the oxide particle and the electrolyte as well as the suppression of Mn dissolution from the oxide material. KEYWORDS: lithium-ion batteries, spinel cathodes, conducting polymer, PEDOT, surface modification, surfactants fluoride (HF), which can attack the active materials and cause Mn dissolution, deteriorating cell performance severely by the polarization increase.8,9 To overcome these drawbacks, various surface modifications of LNMO with a coating layer have been introduced. Metal oxide materials such as ZnO,10 Al2O3,11 SnO2,12 ZnAl2O4,13 and Bi2O314 have been explored because of the excellent stability of these materials in the high-voltage region. However, as the metal oxides exhibit relatively low conductivity, the charge transfer resistance is increased under high current density operation.15 For this reason, carbon-based materials have been considered and investigated to provide the better conductivity of LNMO as well as to protect from side reactions because carbon is highly conductive.16,17 Unfortunately, the utilization of carbon coating is limited because the conventional carbonization coating in an inert atmosphere reduces

1. INTRODUCTION Concerns about air pollution and the exhaustion of fossil fuel resources have attracted more attention toward the use of lithium-ion batteries for hybrid electric vehicles (HEVs), plugin hybrid electric vehicles, and electric vehicles. The batteries for vehicles require high power density and low cost in addition to the high energy density. In this regard, nickelsubstituted LiNi0.5Mn1.5O4 (LNMO) with a spinel structure has been intensively investigated owing to its high working voltage in addition to its use of low-cost and nontoxic manganese.1 However, LNMO cathode materials experience the undesirable side reaction between the active materials and electrolyte during the 5 V class high-voltage operation, which results in the formation of an unwanted cathode electrolyte interphase (CEI) layer.2,3 The thick CEI is accompanied by capacity fading, and these unwanted side reactions are accelerated at elevated temperatures.4,5 Also, at especially elevated temperature environment, the trace water in the electrolyte can lead to the decomposition of the LiPF6 salt,6,7 and this decomposition leads to the generation of hydrogen © 2018 American Chemical Society

Received: May 18, 2018 Accepted: August 13, 2018 Published: August 13, 2018 29457

DOI: 10.1021/acsami.8b08200 ACS Appl. Mater. Interfaces 2018, 10, 29457−29466

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic illustration of the synthesis procedure for PEDOT-coated LNMO; the ζ potential of LNMO (b) before the incorporation of CTAB and (c) after the adsorption of CTAB (step 1).

Mn4+ in the LNMO to Mn3+ and damages the crystal structure of the parent oxide materials during the thermal treatment.15,18 Moreover, other carbon coating methods by mechanical approaches such as ball-milling deteriorate the surface properties or the surface morphology of the parent cathode materials.18−20 In this regard, conducting polymers such as polypyrrole,21 polyaniline, 22 and poly(3,4-ethylenedioxythiophene) (PEDOT)23,24 have been considered as promising coating materials for the parent oxide materials owing to mild synthesis conditions at ambient temperatures, which preserve the crystal structure and morphology of the parent materials.25,26 Also, conducting polymers can play a role as a protective layer against side reactions and HF attack, as well as to improve electrical conductivity.15,27 Among various conducting polymers, PEDOT is known to exhibit high electronic conductivity, electrochemical stability, and chemical stability.28,29 To achieve PEDOT coating on the cathode materials, a conventional mechanical method or wet coating method has been commonly applied, employing PEDOT-dispersed aqueous solution.23,24 However, it is difficult to achieve a uniform layer with PEDOT material via wet coating methods without aggregation or agglomeration because there is no driving force to attach the PEDOT polymer on the surface of inorganicbased parent cathode materials during the coating process. We utilized the hydrophobic features of cetyl trimethyl ammonium bromide (CTAB) surfactant and 3,4-ethylenedioxythiophene (EDOT) monomer in aqueous solution to provide a reasonable driving force to guarantee a uniform coating layer during surface coating with a conducting polymer. This also introduced electrostatic attraction between the cathode materials and the CTAB surfactant during the simple onepot process in this study. We first designed LNMO particles

surrounded by the CTAB surfactant by utilizing the electrostatic force between negatively charged LNMO particles in water and cationic CTAB surfactant.30 Then, the EDOT monomer was added, which is known to exhibit hydrophobic characteristics in aqueous solution, with the intention of selectively surrounding the hydrophobic tail group of the CTAB surfactant to arrange the EDOT monomer on the surface of LNMO materials.31 Finally, a simple chemical polymerization of the EDOT in a one-pot process is expected to give a homogeneous PEDOT coating layer on the surface of the parent materials. The effects of using this conducting polymer as a coating layer for the high-voltage application of cathode materials were also addressed in this study.

2. EXPERIMENTAL SECTION 2.1. Synthesis of PEDOT-Coated LiNi 0.5 Mn 1.5 O 4 . LiNi0.5Mn1.5O4 particles were acquired from the Tanaka Chemical Corporation (Japan). To prepare the PEDOT-coated LiNi0.5Mn1.5O4, LiNi0.5Mn1.5O4 parent material (2.0 g) was dispersed in 200 mL of distilled water, into which 1.2 g of cetyl trimethyl ammonium bromide (CTAB, Sigma-Aldrich) was then dissolved. A stoichiometric amount of 3,4-ethylenedioxythiophene (EDOT, Sigma-Aldrich) was added to the solution and stirred for 2 h. Then, ammonium persulfate (APS, Daejung Chemistry) solution as the initiator for polymerization was added to the solution. At this stage, the mole ratio of APS to EDOT was fixed at 1, and the mixtures were stirred for 48 h at 30 °C. After polymerization, the resulting powder was collected by centrifugation and washed with 50 mL of distilled water and 50 mL of ethanol four times, respectively. The final product was dried at 80 °C. To determine the optimum level of PEDOT coating, 1, 2, and 5 wt % of coating were applied and evaluated. 2.2. Materials’ Characterization. Fourier transform infrared (FT-IR) spectroscopy (iS10, Thermo) was performed to confirm the synthesis of polymer, and X-ray diffraction (XRD, Rigaku) using Cu Kα radiation was carried out to identify the crystal structure of the 29458


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Figure 2. FT-IR spectra of pristine LNMO and various levels of PEDOT coating on LNMO (a) full scale, (b) magnified scale. (c) XRD patterns of pristine LNMO and various levels of PEDOT coating on LNMO. samples. The morphology was examined using a field emission scanning electron microscope (FE-SEM, Teneo Volume Scope, FEI). Transmission electron microscopy (TEM) images and energydispersive spectroscopy (EDS) elemental maps were obtained using Talos TEM (Talos F200X, FEI), operated at 200 keV and equipped with a high-brightness Schottky field emission electron source and a Super-X EDS detector system. Elemental analysis (EA, CS600, Leco) was performed on the samples, and dynamic light scattering (Litesizer 500, Anton Paar) was conducted to measure the ζ potentials. 2.3. Electrochemical Measurements and Postmortem Analysis. The cathode electrode was fabricated by blending a slurry composed of 92 wt % active material, 4 wt % Denka Black, and 4 wt % poly(vinylidene fluoride) (PVdF, Solef 6020) in N-methyl-2pyrrolidone (NMP).13 The slurry was cast onto an aluminum foil and then dried at 80 °C in a vacuum oven for 24 h to remove residual water and NMP. The loading weight of the electrode was 9−10 mg cm−2. Cointype cells (CR2032, Hohsen) were fabricated with the cathode electrode, lithium metal as the counter electrode, and a polyethylene separator (Tonen, Japan). A 1.3 M LiPF6 salt in a mixture of ethylene carbonate and ethyl methyl carbonate (3:7 v/v, Panax Etec, Korea) was utilized as the electrolyte.32 Galvanostatic charge and discharge tests were carried out in a voltage range between 3.5 and 4.9 V versus Li/Li by a Maccor series 4000 (Thermo-Tech). Every cell was precycled for three formation cycles at 0.1 C for activation. Electrochemical impedance spectroscopy (EIS) was carried out using a VMP3 (Bio-Logic, multichannel potentiostat/galvanostat with EIS). EIS analyses were carried out between 10 mHz and 1 MHz, applying an alternating current voltage of 5 mV. For postmortem X-ray photoelectron spectroscopy (XPS, 5000 VersaProbe, ULVAC-PHI) analyses, the coin cells after storage at high temperature and cycling were disassembled in an Ar-filled glovebox and rinsed with dimethyl carbonate.33 Furthermore, to investigate Mn dissolution from the cathodes after storage and cycling, Li anodes of disassembled coin cells were dissolved in deionized water and measured by inductively coupled plasma-optical emission spectrometry (ICP-OES, Thermo iCAP 6500 DUO).33

study to evaluate the surface charge of pristine LNMO dispersed in distilled water, and the results of ζ potential measurement shown in Figure 1b clearly indicate that pristine LNMO exhibits a negatively charged state after immersion in water. On the basis of this result, we designed the selective arrangement of cationic CTAB surfactant on the parent LNMO particles. As shown in step 1 of Figure 1a, it is expected that the positively charged head of the CTAB surfactant will be homogeneously arranged on the surface of LNMO materials because the surface charge of spinel-based oxides exhibits a negative charge in distilled water, as reported previously.34,35 After step 1, the CTAB surfactants are adsorbed on the surface of the parent materials by electrostatic force,36,37 resulting in micelle formation with LNMO and a hydrophobic region should be partially formed at the interface between the LNMO surface and the solution owing to the alignment of the hydrophobic tail groups of the CTAB surfactant.38 The positive ζ potential value on the surface of inorganic material after the incorporation of the CTAB surfactant, as shown in Figure 1c, also confirms that the adsorption of the CTAB surfactants is effectively carried out on the surface of LNMO. Then, in step 2, the EDOT monomer is added into the aqueous solution. Although EDOT is generally known to exhibit extreme self-aggregation behavior in aqueous solution because of its strong hydrophobic characteristics,39 the addition of EDOT monomer in step 2 leads to the spontaneous and preferential penetration of monomers into the hydrophobic tail region of the CTAB surfactant.31,40,41 Therefore, the hydrophobicity of surfactants near the surface of LNMO powders acts as a driving force to attract and arrange the EDOT monomers uniformly on the surface of LNMO materials.42 Subsequently, the polymerization of EDOT molecules on the surface of oxide materials is carried out by adding an oxidant as the initiator, and the surface of LNMO is effectively surrounded and coated with PEDOT conducting polymer after spontaneous polymerization in step 3. The removal of CTAB surfactants is easily carried out by washing with deionized water and ethanol because the adsorption ability of CTAB requires the critical micelle concentration. Thus, a homogeneous PEDOT coating layer is formed without any exposure of bare LNMO surface or severe aggregation of the PEDOT polymer itself. To confirm the occurrence of successful polymerization of EDOT on the surface of LNMO materials, FT-IR spectroscopy was carried out for the pristine LNMO and various levels of

3. RESULTS AND DISCUSSION Figure 1a illustrates the synthetic process of PEDOT-coated LNMO cathode materials, which utilizes the electronegativity of the LNMO particles and the hydrophobicity of the EDOT monomer to achieve the effective coating with PEDOT polymer on the surface of lithium transition metal oxides. Recently, Manthiram et al. proposed a coating method using the electrostatic force between cathode materials and coating materials and confirmed the surface modification of the spinel cathode LiNi0.42Mn1.42Co0.16O4 with inorganic materials such as Al2O3, ZnO, Bi2O3, and AlPO4.34 First, we carried out preliminary experiments on the parent LNMO materials in this 29459


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of pristine LNMO can be observed in Figure S1a. Although the clear evidence of a coating layer is not seen in the case of 1 wt % PEDOT-coated LNMO, as shown in Figure S1b, the surfaces of 2 and 5 wt % PEDOT-coated LNMO powders exhibit a homogeneously rough morphology without severe agglomeration or aggregation of materials. This overall roughness of surface suggests that a uniform PEDOT layer was formed homogeneously on the surface of LNMO materials. Although the surface morphology of pristine LNMO particles beneath the coating layer is not confirmed owing to the uniform coating materials, it is evident that the PEDOT coating by chemical polymerization does not influence the morphology of LNMO material at ambient temperature, as compared to the conventional ball-milling method in previous reports. The comparison of TEM images in Figure 3 confirms the homogeneous and thin coating layer on the surface of pristine

PEDOT coating on LNMO, as shown in Figure 2a. Pure PEDOT material was also prepared by employing the same polymerization process, and its FT-IR spectrum is included in Figure 2a. Although several peaks are not clearly observed because of the small portion of coating layer,43 the enlarged spectra in Figure 2b indicate that all of the PEDOT-coated LNMO powders exhibit the most significant peaks related to PEDOT materials. The peaks at 840, 930, and 983 cm−1 correspond to C−S bonds,41,44−47 which originate from the thiophene ring of the PEDOT polymer. The peaks observed at 1080 and 1470 cm−1 are related to C−O−C and CC bonds, respectively, which correspond to the ethylenedioxy group of PEDOT.44,48−50 Also, the peaks observed at 2850 and 2920 cm−1 indicate the −CH2-related bond from the ethylenedioxy bridge and support that the PEDOT conducting polymer was effectively formed by the one-pot synthesis process.51 Figure 2c compares the XRD patterns obtained from pristine and PEDOT-coated LNMO materials, and the profiles of all of the powders clearly indicate the typical patterns of the spinel structure, LiNi0.5Mn1.5O4 (Fd3m, JCPDS #80-2162).5,11,15 Although the presence of PEDOT is not detected because of the amorphous characteristics of PEDOT conducting layer by XRD analysis, the XRD patterns imply that the coating process carried out in this study does not influence crystallographic degradation.24,25 Elemental analysis was carried out, and the results are shown in Table 1. The pure PEDOT conducting polymer consists of Table 1. Elemental Analysis of Pure PEDOT, Pristine LNMO, and Various Levels of PEDOT Coating on LNMO element (wt %) sample

C

H

S

PEDOT ratio

pure PEDOT pristine LNMO 1 wt % PEDOT−LNMO 2 wt % PEDOT−LNMO 5 wt % PEDOT−LNMO

46.72 0 0.44 0.88 2.32

4.93 0 0.04 0.09 0.19

19.07 0 0.00 0.05 0.18

100a 0 0.93b 1.89b 4.96b

a We assumed that there is no impurity in pure PEDOT. bPEDOT ratio is determined on the basis of experimental carbon content with respect to that in pure PEDOT.

46.72% carbon with a small trace of hydrogen and sulfur, whereas no trace of carbon, hydrogen, or sulfur is detected in the case of pristine LNMO powder. For the PEDOT-coated LNMO powders, elemental analysis indicates that the amount of carbon qualitatively increases with an increase in PEDOT coating amount while hydrogen and sulfur are scarcely observed because of the small portion of PEDOT material present. The ratio of PEDOT material to parent LNMO powder was calculated on the basis of carbon contents. The determined values for the amount of PEDOT polymer as a coating material correspond to 0.93, 1.89, and 4.96 wt %, respectively, for the nominal compositions of 1, 2, and 5 wt % PEDOT-coated LNMO, indicating that the preferential arrangement of EDOT monomer by employing CTAB surfactant is effective for the control of the amount of PEDOT polymer coating during the surface coating process. To observe the morphology and confirm the effect of the coating concept in this study, SEM and TEM analyses were carried out for the pristine and various levels of PEDOT coating on LNMO. Figure S1 shows typical SEM images of pristine and PEDOT-coated LNMO, and the smooth surface

Figure 3. TEM images of (a) pristine, (b) 1 wt %, (c) 2 wt %, and (d) 5 wt % PEDOT-coated LNMO, and the elemental mapping distribution of Ni, Mn, C, and S in (e) pristine and (f) 5 wt % PEDOT-coated LNMO.

LNMO powders after the coating process. While the pristine LNMO exhibits a flat and smooth surface in TEM images, all of the PEDOT-coated LNMO particles contain the amorphous coating layer less than 10 nm in thickness without severe agglomeration of layers on the surface of LNMO. The EDS mapping image concerning C and S components in Figure 3e,f reveals that the PEDOT layer is effectively constructed on the LNMO material,26,52 and this result confirms that the coating 29460


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Figure 4. (a) Initial charge−discharge voltage profiles and (b) dQ/dV plots for pristine LNMO and various levels of PEDOT coating on LNMO.

concept in this study is effective, which is designed to arrange the EDOT monomer effectively by employing the hydrophobic property of EDOT monomer and tail group of the CTAB surfactant in addition to the utilization of electrostatic force between the cathode materials and the CTAB surfactant during the simple one-pot synthesis process. Various electrochemical tests were performed to examine the effect of the thin PEDOT polymer coating on the surface of the cathodes and to understand the major factors in controlling the electrochemical performance of cathode electrodes during the high-voltage operation. Figure 4a shows the initial charge−discharge profiles between 3.5 and 4.9 V at a 0.1 C rate, and the differential discharge capacity (dQ/dV) plots obtained from Figure 4a are shown in Figure 4b. All of the electrodes in Figure 4a represent the voltage plateau corresponding to the Ni2+/Ni4+ redox reaction at 4.7 V as well as the small portion of plateau concerning the Mn3+/ Mn4+ redox reaction in the 4.0 V region.12,53,54 Pristine LNMO delivers the initial discharge capacity around 125.0 mA h g−1, whereas all of the PEDOT-coated LNMO electrodes exhibit a slightly increased capacity at the first cycle. As the initial Coulombic efficiencies of pristine LNMO, 1, 2, and 5 wt % PEDOT−LNMO are 96.2, 97.4, 97.2, and 96.8%, respectively, this increase in discharge capacity suggests that surface modification with a PEDOT coating layer contributes to the effective construction of a cathode electrolyte interface (CEI) layer, suppressing the consumption of lithium ion and consequent decrease in irreversible capacity during battery operation. However, the slightly decreased efficiency in the 5 wt % PEDOT-coated LNMO electrode implies that excessive coating layer, in turn, may hinder the electrochemical reaction of lithium during the battery operation in practical full cell system. The dQ/dV plots in Figure 4b indicate that all of the electrodes exhibit the typical peaks concerning the Ni2+/3+ and Ni3+/4+ redox reactions, whereas the pristine LNMO electrode experiences an additional oxidative reaction at around 4.75 V,55 implying that the introduction of the PEDOT coating layer effectively leads to the suppression of undesirable side reaction and results in an enhanced initial Coulombic efficiency during the initial charge−discharge process.15,56−58 As the PEDOT conducting polymer material is expected to increase the electronic conductivity of electrodes during battery operation, the rate capability characteristics were evaluated at discharge C rates ranging from 0.2 to 10 C, while the charge current was fixed at a 0.2 C rate. Although all of the electrodes in Figure 5 maintain similar discharge

Figure 5. Rate capability test for the pristine LNMO and various levels of PEDOT coating on LNMO.

capacities up to a discharge C rate of 2 C, the pristine LNMO electrode delivers a smaller discharge capacity than that of the PEDOT-coated electrodes. At a high C rate condition of 10 C, which corresponds to a fast discharging operation to insert all the lithium ions in 6 min, the 2 wt % PEDOT-coated LNMO electrode still delivers a discharge capacity around 92.4 mA h g−1, whereas the 1 and 5 wt % PEDOT-coated LNMO electrodes only maintain a capacity below 60 mA h g−1. The superior performance for the 2 wt % PEDOT-coated electrode, as compared to the 1 wt % PEDOT-coated electrode, indicates that the PEDOT-based layer clearly improves the conductivity and electrochemical performance of electrodes. This result also implies that the PEDOT conducting polymer coating layer may improve Li ion accessibility because of the enhancement of electronic conductivity, although an excessively thick coating layer may impede the diffusion of lithium ions during a fast discharging process, as reported in the previous literature.25,27,59,60 29461


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Figure 6. EIS spectra of pristine LNMO and various levels of PEDOT coating on LNMO after (a) three formation cycles and (b) 200 cycles at room temperature.

Table 2. Fitting Results after Three Formation Cycles and 200 Cycles of EIS for the Pristine LNMO and Various Levels of PEDOT Coating on LNMO, As Shown in Figure 6 after three formation cycles

after 200 cycles

sample

Re (Ω)

RCEI (Ω)

Rct (Ω)

Re (Ω)

RCEI (Ω)

Rct (Ω)

pristine LNMO 1 wt % PEDOT−LNMO 2 wt % PEDOT−LNMO 5 wt % PEDOT−LNMO

1.258 1.395 1.225 1.204

13.65 8.044 8.586 7.719

89.76 61.93 60.01 55.28

1.528 1.349 1.392 1.425

44.31 32.83 30.68 31.73

141.4 107.9 89.35 117.5

cycles. However, after 200 cycles, the pristine electrode experiences the largest increase in Rct value of 141.4 Ω, compared to those of the PEDOT-coated LNMO electrodes. For the RCEI values, the PEDOT-coated electrodes experience more increase in RCEI values as compared to the pristine electrode although all of the PEDOT-coated electrodes still exhibit the smaller RCEI values than those of pristine electrode after 200 cycles. This confirms that the PEDOT coating layer is effective at providing better electronic conductivity during the charge transfer process, whereas the pristine electrode without a coating layer experiences an increase in polarization probably because of an unwanted side reaction with the electrolyte and consequent formation of a resistive CEI film on the cathode material during cycling. Also, the large increase in charge transfer resistance for the 5 wt % PEDOT-coated LNMO electrodes confirms that an excessively thick coating layer increases the resistance of Li ion diffusion and leads to capacity fading during the cycle test, which is in good agreement with the cycle data in Figure S2.4,23,62 In addition to its contribution to electrochemical performance as a conducting polymer, surface modification with PEDOT polymer coating is designed and expected to protect the cathode materials from the side reaction between cathode and electrolyte, especially at high-voltage environment. As degradation in the highly oxidizing environment is especially accelerated at elevated temperatures during charging,5,63 the storage test at 60 °C and in a fully charged state for 3 days was conducted in the middle of the cyclability test conducted at ambient temperature. As shown in Figure 7, after the first storage test at elevated temperature, pristine LNMO shows unstable feature during the early stages of subsequent cycling at ambient temperature, while the PEDOT-coated LNMO

Figure S2 compares the cyclability of electrodes evaluated at 1 C rate over 200 cycles. All of the electrodes exhibit the slightly unstable behavior in Coulombic efficiency, especially during the initial cycles possibly because the pristine LNMO material requires a few cycles to achieve the full activation of electrochemical reaction. Although the pristine LNMO electrode maintains only 83.4% of its initial discharging capacity after 200 cycles, PEDOT-coated LNMO electrodes retain 91.0, 92.8, and 85.3% of their initial discharging capacity, respectively. Interestingly, the 5 wt % PEDOT-coated LNMO electrode exhibits clear capacity fading after the 110th cycle, although it delivers the highest discharging capacity during the initial cycles. This result also implies that an excessively thick coating layer impedes the electrochemical reaction of the electrode by interrupting the movement of lithium ions, resulting in a gradual impedance increase of the electrode during the prolonged cycle test.60,61 The evaluation of EIS was carried out before and after 200 cycles to elucidate the impedance behavior of electrodes during the cycle test, and the Nyquist plots obtained are shown in Figure 6. The results are fitted by employing the equivalent circuit (the inset of Figure 6), and resistance data obtained from fitting the results are given in Table 2. The Re corresponds to the electrolyte resistance, the first semicircle of RCEI at high-frequency region exhibits the CEI resistance, the second semicircle of Rct at medium frequency is related to charge transfer resistance, and Zw at low frequency is the Warburg resistance from solid-state diffusion.33 As shown in Table 2, the pristine electrode shows the slightly larger Rct value after three formation cycles than that for PEDOT-coated LNMO electrodes, and the 5 wt % PEDOT-coated LNMO exhibits the smallest charge transfer resistance before 200 29462


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electrodes maintain stable electrochemical performance during the cycle test. More importantly, the pristine LNMO electrode exhibits severe capacity fading after the second storage test in the subsequent 50 cycles and retains only 56.3% of initial discharge capacity at the end of the cycling test at ambient temperature, while the PEDOT-coated LNMO electrodes show stable and outstanding cyclability with capacity retention above 90% of initial capacity during the room temperature cycling after the storage test. This result implies that the PEDOT-based coating layer inhibits the side reaction between the cathode and the electrolyte or suppresses the Mn dissolution from HF attack, which has been addressed in the previous literature.13,15,27,64 The electrodes before and after the cycling performance combined with the storage test were analyzed by XPS surface analysis and peak fitting, as shown in Figure 8, to clarify the effect of PEDOT-based coating layer and elucidate the capacity fading mechanism for the pristine LNMO electrode. In the C 1s spectra displayed in Figure 8a, the peaks for C−C bonds

Figure 7. Comparison of cycle performances combined with the storage test for pristine LNMO and various levels of PEDOT coating on LNMO. The storage test at 60 °C for 3 days was performed after the charging process up to 4.9 V.

Figure 8. XPS spectra of C 1s and O 1s for pristine LNMO and various levels of PEDOT coating on LNMO after (a) three formation cycles (25 °C) and (b) cycling performance combined with the storage test. 29463


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results indicate that the PEDOT-based coating layer protects the cathode material from HF attack and effectively suppresses the dissolution of Mn from the cathode materials.15,27,68

(284.6 eV), C−H bonds (285.0 eV), and C−F bonds (290.7 eV) are assigned to the PVdF binder from the electrode.56,59 The peaks observed at 284.6 eV are C−C bonds resulting from the conductive materials (Super P) or the PEDOT conducting polymer.65 Also, the C−O bond peak around 286 eV corresponds to the PEDOT coating layer or RCO2M formation from the decomposition of the electrolytes,13,33 and the CO peak at 289 eV is related to Li2CO3, which is formed from the decomposition of the carbonate-based electrolyte components.33,59 In the O 1s spectra shown in Figure 8a, the peaks assigned at around 531.5 and 533.2 eV generally correspond to Li2CO3 components and OC bonds,33,56 respectively, which originate from CEI formation, while the peak at around 529.6 eV is related to Me−O bonds derived from the metal oxide component of cathode materials.33,56 Initially, the PEDOT-coated LNMO electrodes exhibit a lower intensity peak for Me−O bonds than that of pristine LNMO because of the PEDOT layer on the LNMO particle. After the storage test at 60 °C combined with the cycling test, all electrodes show similar intensities of C−O bonds without any distinguishable difference, as shown in Figure 8b. Also, C−H bonds concerning PVdF binder component are not observed after cycles because of the formation and gradual growth of CEI film. In contrast, the Li2CO3-related peaks of pristine LNMO at around 531.5 and 533.2 eV in the O 1s spectra shown in Figure 8b become larger than those for the PEDOT-coated LNMO electrodes. Furthermore, for pristine LNMO, the intensity of the peak related to Me−O bonds significantly decreases, whereas the 1 and 2 wt % PEDOT-coated LNMO electrodes retain the peak intensity for Me−O bonds. This reduction of intensity is mainly attributable to the growth of CEI film derived from electrolyte decomposition during cycling.64,66 Therefore, it confirms that the PEDOT conducting polymer successfully suppresses the side reaction with electrolytes during the highvoltage operation of spinel-based cathode materials. At elevated temperature, Mn-based cathode materials are known to experience Mn dissolution from HF.6,7 If the dissolved Mn ions migrate through the electrolyte and subsequently reduce to Mn metal on the anode surface, the impedance of the cell increases because of the insulating characteristics of Mn metal and consequently leads to the degradation of capacity during the operation.67 To confirm the occurrence of Mn dissolution, we conducted ICP analysis for the Li metal anode electrode after the electrochemical test, as shown in in Figure 7, and the results are demonstrated in Table 3. The Li metal anode of the pristine LNMO shows the highest amounts of Mn, while the PEDOT-coated LNMO cathodes show considerably less-dissolved Mn. Specifically, the Mn concentration for pristine LNMO is 7.74 ppm, whereas 1, 2, and 5 wt % PEDOT-coated LNMO electrodes give concentrations of 4.61, 2.35, and 1.22, respectively. These

4. CONCLUSIONS In summary, we prepared a PEDOT conducting polymercoated LNMO employing surfactants. The surfactants adsorbed on the cathode materials via electrostatic forces and acted as a driving force to aid the placement of the EDOT monomer on the surface of the cathode material, and we confirmed that a uniform PEDOT coating layer was formed without aggregation of the conducting polymer. Electrochemical evaluation demonstrated that the PEDOT-coated LNMO shows better rate capability and cyclability, and the 2 wt % PEDOT-coated LNMO electrode exhibits the superior power characteristics. Furthermore, EIS and XPS analyses supported that the PEDOT coating layer worked as a protective layer and effectively suppressed the side reaction between the cathode and the electrolyte, resulting in superior capacity retention, especially at high temperature. Our future work will focus on the precise control and optimization of the amount of coating layer for next step to improve the materials in this study.

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* Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b08200.

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deposited Mn on a Li metal anode (ppm)

pristine LNMO 1 wt % PEDOT−LNMO 2 wt % PEDOT−LNMO 5 wt % PEDOT−LNMO

7.74 4.61 2.35 1.22

SEM images; comparison of the cycle performance (Figures S1 and S2) (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +82 2 958 5253. Fax: +82 2 958 5229. ORCID

Sang-Ok Kim: 0000-0001-5628-9331 Ki Jae Kim: 0000-0002-2166-7467 Wonchang Choi: 0000-0002-8463-1517 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (No. 2018R1A2B2007081) and the Korea Institute of Science and Technology (KIST) institutional Program (Project No. 2E28142). The ζ potential experiment was carried out with the help of Anton Paar.

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Table 3. Result of Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) of Li Metal Anodes after Cycling Performance Combined with the Storage Test cathode

ASSOCIATED CONTENT

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REFERENCES

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Conducting Polymer Coating on High-voltage Cathode Based on Soft

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