Effects of conductive binder on the electrochemical performance of lithium titanate anodes

Solid State Ionics 333 (2019) 18–29

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Effects of conductive binder on the electrochemical performance of lithium titanate anodes

T

S.N. Eliseevaa, E.V. Shkrebaa, M.A. Kamenskiia, E.G. Tolstopjatovaa, R. Holzea,b,c, ⁎ V.V. Kondratieva, a

Department of Electrochemistry, Institute of Chemistry, Saint Petersburg State University, 7/9 Universitetskaya nab., St. Petersburg 199034, Russian Federation Chemnitz University of Technology, Institute of Chemistry, AG Elektrochemie, 62 Strasse der Nationen, Chemnitz D-09111, Germany c State Key Laboratory of Materials-Oriented Chemical Engineering, College of Energy Science and Technology and Institute of Advanced Materials, Nanjing Tech University, Nanjing 211816, Jiangsu Province, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Lithium titanate Conductive binder Intrinsically conducting polymer Poly-3,4-ethylenedioxythiopene Polystyrene sulfonate Carboxymethylcellulose Li-ion batteries

An eco-friendly water-based binder consisting of a combination of intrinsically conducting polymer poly-3,4ethylenedioxythiopene:polystyrene sulfonate (PEDOT:PSS) dispersion and carboxymethylcellulose (СМС) proposed as component of Li4Ti5O12-based negative electrode has been studied at different compositions and compared with conventional PVDF binder. Morphology and structure of the composite materials were investigated by X-ray diffraction, scanning electron microscopy and EDX analysis. Electrochemical characterization was performed by galvanostatic charge-discharge experiments, cyclic voltammetry and impedance spectroscopy. The electrode with combined PEDOT:PSS/CMC binder has superior properties, in particular increased specific capacity and improved C-rate performance during charge-discharge. By using PEDOT:PSS/CMC binder instead of PVDF, the practical specific capacity was increased up to 14% (157 mAh g−1 at 0.2 °C, normalized to total electrode mass). Highest stability during long cycling was observed for Li4Ti5O12-electrode with this binder at < 1% decay after 100 cycles at 1 °C. Electrochemical impedance spectra reveal a significant decrease of interfacial resistance and an increase of apparent diffusion coefficients for Li4Ti5O12 anode material with this binder, which supports improved functional characteristics of the electrode. As combined polyelectrolyte dispersion, the proposed conductive binder is an efficient alternative to the non-conductive PVDF binder for commercial lithium ion batteries.

1. Introduction Among the negative electrode materials for lithium-ion batteries, lithium titanium oxide Li4Ti5O12 (LTO) is one of the most promising alternative materials for practical applications when compared to commonly used graphite [1–8]. Despite of its moderate specific capacity (175 mAh∙g−1) as compared with graphite, Li4Ti5O12 has zero volume change during cycling (zero strain material) and excellent safety as particular advantages [2,4,9]. However, at LTO electrodes with PVDF binder irreversible surface morphology changes were observed resulting in stability problems, corresponding capacity loss and short cycle life [10,11]. Like other inorganic materials with low electronic and ionic conductivity (10−13 Ω−1∙cm−1) [10,12] it requires nanostructuring [13,14] and the introduction of conductive additives to improve high rate performance and efficient material utilization [6,15,16].

⁎

Over the past decades, several approaches have been proposed to improve the rate capability and cycling stability of LTO materials, in particular surface modification of LTO grains with TiO2 layers [17,18], use of graphene-decoration [12,19–21], modification with intrinsically conducting polymers (ICPs) such as polyaniline [22,23], polypyrrole [20], polythiophenes [24–28], and especially PEDOT [26,27] and PEDOT:PSS [28]. The use of an ICP has resulted in improved performance of LTO, but after surface modification with ICPs, electrode slurries were prepared still using conventional PVDF binder in N-methylpyrrolidinone solvent. The values of specific capacity reported (normalized to the weight of the whole electrode) for LTO electrodes modified using these approaches were 140 [24], 143 [25], 136 [28], 134 [21], and 135 mAh∙g−1 [27]. The proposed approaches for surface modification are complex and sophisticated; they can hardly be considered as a practical solution due to the high cost of application on an industrial scale.

Corresponding author. E-mail address: [email protected] (V.V. Kondratiev).

https://doi.org/10.1016/j.ssi.2019.01.011 Received 2 November 2018; Received in revised form 26 December 2018; Accepted 10 January 2019 0167-2738/ © 2019 Elsevier B.V. All rights reserved.

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Water soluble or water-based binders have received great attention for application in lithium ion batteries due to their environmental friendliness and possibly improved safety of the obtained batteries. Different types of water soluble binders improving the electrochemical performance of anode and cathode materials have been [29,30]. Among water soluble binders used for LTO based electrodes are sodium carboxymethyl cellulose (CMC) [31–33], sodium carboxymethyl cellulose combined with styrene butadiene rubber [34], poly(acrylic acid) [33], commercial Acryl S020 binder [35], and sodium alginate [36]. The values of specific capacity reported (normalized to the weight of the whole electrode) for LTO electrodes with water-based binders at 1 °C discharge rate were 136 [31], 120 [32], 145 [33], 146 [34], 147 [35], and 152 mAh∙g−1 [36]. PEDOT:PSS aqueous dispersion can be successfully applied not only for surface modification of electroactive materials, but also as an important component for slurry preparation of different electrodes: solely as a conductive and binding additive for LiCoO2 [37] and Si [38], along with carbon black for LiFePO4 [39], and mixed with CMC for Si anodes [40], for LiFePO4 [41], LiFe0.4Mn0.6PO4 [42], and LiMn2O4 [43] cathodes. In the case of cathode materials [41–43] use of the polymer dispersion PEDOT:PSS combined with CMC as binder results in enhanced specific capacity and C-rate capability. This was explained by several factors: PEDOT, as p-doped ICP, has good electronic conductivity and stability in a wide electrode potential range. CMC is an effective ionic conductor and thickening agent. Combination of these two components provides better properties. In these studies we have optimized the PEDOT:PSS/CMC binder compositions; their efficiency has been proven for electrode materials with high weight fraction of other types of active grains LiFePO4 [41], LiFe0.4Mn0.6PO4 [42], LiMn2O4 [43]. Combined conductive polymer binder provides partial or complete wrapping of LTO grains, that can more effectively inhibit the interaction of the active material with the electrolyte and side reactions without the hindering of lithium transport, diminishing the degradation [41]. On the other hand, if we used higher weight fraction of active material (from 80 wt% to 90 wt%) in combination with PVDF, the electrochemical performance was poor. This approach was applied here to the LTO anode material. We demonstrate for the first time the superior electrochemical performance of LTO-based anodes with a conductive binder PEDOT:PSS/CMC at much lower mass fraction in the electrode than has been reported before (for instance, 30 wt% in [40]). We discuss the advantages of new LTO-based compositions in term of the practical capacity (normalized to total electrode mass) and the enhanced C-rate capability. Special attention was paid to the study of kinetics of Li+ intercalation/deintercalation processes depending on the type and composition of binder by electrochemical impedance spectroscopy and cyclic voltammetry. The research was directed on better understanding of reasons of binder's influence on the improved electrochemical characteristics of LTO anodes with conducting PEDOT:PSS/CMC binder.

Table 1 Compositions of LTO-based electrodes. Sample S1 S2 S3 S4 S5 S6 S7

LTO, wt%

СB, wt%

PVDF, wt%

PEDOT:PSS, wt%

CMC, wt%

80 90 90 90 95 80 70

10 6 6 6 – 10 20

10 – – – – – –

– 2 4 – 5 – 5

– 2 – 4 – 10 5

commonly employed fraction of LTO (80 wt%) based on literature data [44,45]. The electrodes will be further denoted according to the component ratio as S1–S7. Electrodes were prepared by mechanical mixing of LTO powder, carbon black, and aqueous dispersion of binder for 1 h until the slurry was homogeneous. The resulting viscous slurry was cast with doctor blade on an aluminum foil, dried at 80 °C in a vacuum oven and roll-pressed. All electrodes were cut into disks with 1.75 cm2 area and an average mass material loading of about 4–6 mg cm−2. The electrochemical characterization of the materials was conducted in standard two-electrode coin cells (CR2032). The cells were assembled in an argon-filled glove box Unilab (USA) using Celgard 2325 membrane as separator, Li foil as counter electrode, and the battery electrolyte solution. Electrochemical performance tests were carried out on an automatic galvanostatic charge-discharge battery cell test instrument CT-3008W-5V 10 mA (Neware Co., China) in the potential range 1.0 to 2.5 V vs. Li/Li+ at different rates from 0.2 С to 30 С at room temperature (20 ± 2 °C). We assume that the theoretical charge-discharge capacity of the active material in the examined electrodes is 175 mAh∙g−1. All capacity values presented here were normalized to the total mass of electrodes excluding current collector if not stated otherwise. The full cell was assembled using the Li4Ti5O12 anode (S2) with conductive binder and LiFe0.4Mn0.6PO4 cathode with the same binder composition (2 wt% of PEDOT:PSS and 2 wt% of CMC). Cyclic voltammetry and electrochemical impedance spectroscopy (EIS) were performed with an Autolab PGSTAT 30 potentiostat/galvanostat (Eco-Chemie, Netherlands) equipped with FRA2 module. Cyclic voltammograms were recorded in the potential range from 0.9 to 2.5 V vs. Li/Li+ at potential scan rates 0.5–0.1 mV s−1. The EIS measurements were performed in the frequency range 100 kHz to 0.1 Hz with an applied amplitude of 5 mV in a fully charged state of the battery, at a potential of E = 1.0 V vs. Li/Li+. To achieve a steady state, the batteries were conditioned for 1 h at a given cell voltage before the impedance measurements. The parameters of the equivalent circuits were fitted by computer simulations using the Nova 1.11 software. The morphology and structure of prepared composites were characterized by X-ray diffraction (XRD, Bruker-AXS D8 DISCOVER, Germany) using Cu Kα radiation, and scanning electron microscopy (SEM, SUPRA 40VP Carl Zeiss, Germany).

2. Experimental 3. Results and discussion Li4Ti5O12 (LTO) powder (< 200 nm), poly-3,4-ethylenedioxythiophene/polystyrene sulfonate (PEDOT:PSS) 1.3 wt% aqueous dispersion, polyvinylidene fluoride (PVDF) and N-methylpyrrolidone were purchased from Aldrich. Conducting carbon black «Super P» (CB) was from Timcal Inc. (Belgium). Сarboxymethylcellulose (CMC) was from MTI Corp. (USA). Carbon-coated LiFe0.4Mn0.6PO4 (C-LFMP) was obtained from Clariant Produkte GmbH (Germany). Commercial battery electrolyte solution TCE918 was from Tinci Materials Technology Co. Ltd. (China). All materials were used as received. Seven types of electrodes with different compositions of single (PVDF, PEDOT:PSS or CMC) and combined (PEDOT:PSS/CMC) binders and carbon additives were prepared. The ratios of components are given in Table 1. In the samples with PVDF binder we used the

3.1. Sample characterization Typical SEM images of the electrode material surface of conventional (with PVDF) and modified composition (with PEDOT:PSS/CMC binder) are presented in Figs. 1 and 2 with main element distribution data of the (Ti, F, C) over the same sample surface. A compact layer of about 12 μm thickness on the current collector was observed for asprepared S1 material (Fig. 1). S1 appears highly homogeneous; the main elements are well distributed in the electrode material. Similar data for sample S2 are presented in Fig. 2 (15 μm thickness). The electrode layer of S2 composite material has a dense compact structure with sufficient adhesion to the current-collector, and maintained layer 19

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Fig. 1. SEM and EDX elemental mapping of thick electrode coating of S1 LTO-composite on Al foil current collector before charge-discharge.

based active material. SEM-images at higher magnification are presented in Fig. 3. Two types of particles are observed - larger particles (200–300 nm) corresponding to LTO grains, whereas the smaller ones are carbon black particles. The morphology of the surface of prepared materials depends on the binder. PVDF-bound sample (Fig. 3a) is inhomogeneous and has a rather uneven surface; the electrode material modified with

integrity after cyclic tests. The distribution of the main elements (Ti, S, C) was mapped using locally-resolved EDX analysis (Fig. 2). A relatively uniform distribution of sulfur was observed for sample S2, indicating a uniform distribution of the sulfur-containing PEDOT:PSS component. It shows a good mixing of the electrode material components during its preparation. These results demonstrate the possibility of using a mixture of polymer dispersion PEDOT:PSS and CMC as a binder in the LTO20

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Fig. 2. SEM and EDX elemental mapping of thick electrode coating with sample S2 on Al foil current collector before charge-discharge.

the phase composition of the material and allow monitoring changes in the structure of the material as well as the appearance of new impurity phases. Fig. 4 shows the X-ray diffraction patterns of S1 and S2 before and after 100 charge-discharge cycles. No significant differences in phase composition were observed in the diffractograms of as-prepared modified LTO-based material compared to the pristine LTO powder, confirming that material processing with the use of aqueous binder does not change the phase composition of the LTO material. The XRD

PEDOT:PSS and CMC (Fig. 3c) has a more dense, smooth surface structure. The gaps between the LTO grains are filled with the conductive binder, which provides a tight contact of the active grains with the conductive additives. After the electrochemical tests in coin cells the surface morphology of both types of materials becomes slightly different: the shape of LTO grains is more smooth, sharp edges of grains visible in Fig. 3a, c disappeared in Fig. 3b, d. X-ray diffraction measurements provide additional information on 21

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Fig. 3. SEM images of samples as prepared S1 (a) and S2 (c) and after long cycling S1 (b) and S2 (d).

Fig. 4. XRD patterns of a) S1 and b) S2 on Al before and after 100 charge-discharge cycles.

whereas the sample S1 cycled in commercial electrolyte TCE918 (curve 2), shows the band at 169.0 eV, corresponding to S2p binding energies of O]S]O groups of sulfur-containing compounds in the commercial electrolyte solution TCE918. The S2p band observed for the as-prepared sample S2 (curve 3) at the binding energy of about 168 eV corresponds to the sulfur signal from SO3− groups of PSS; the spin-split doublet S2p1/2 and S2p3/2 (at 164.0 and 165.0 eV) corresponds to the sulfur signal from thiophene rings of PEDOT. For the sample S2 cycled in commercial electrolyte solution TCE918 (curve 4), the band at 168 eV corresponding to the sulfur signal from SO3− of PSS overlaps with the band at 169.0 eV of TCE918. The initial peak at around 688 eV (Fig. 5c, curve 1) corresponds to the F1s in PVDF, whereas no appreciable quantity of F was detected for sample S2 (curve 2). The appearance of well-expressed peaks of F1s at 685.0 eV for both samples after 100 charge-discharge cycles (curves 2,4) indicates the formation of metal fluorides (probably, lithium fluoride) on the surface. The Li1s spectrum (Fig. 5d) exhibits a band at around 55.6 eV which is typical for the Li4Ti5O12 structure.

patterns characteristic of Li4Ti5O12 structure were identified on the ICDD card #00-049-0207d. In addition to the LTO peaks, the peaks of aluminium used as current collector were identified in X-ray diffraction patterns. After long charge-discharge cycling, the composites retained a well-crystallized state, as evidenced by sharp diffraction peaks. Neither shifts of the main reflexes nor the appearance of new ones after 100 charge-discharge cycles were detected. It can be concluded that LTO retains its structure and noticeable impurities of new phases are not formed. To determine the chemical state of elements on the surface of electrodes, ex situ XPS measurements were performed for S1 and S2 samples taken in fully charged state. Fig. 5 shows high resolution X-ray photoelectron spectra of Ti2p, F1s, Li1s and S2p for as-prepared and cycled samples. Pronounced surface changes for both S1 and S2 electrodes were observed in the XPS spectra after cycling. The XP-spectrum of Ti2p (Fig. 5a) shows a peak doublet at 464.8 and 459.1 eV, corresponding to the binding energies of Ti2p1/2 and Ti2p3/2 respectively. The split between the Ti2p1/2 and Ti2p3/2 core levels is 5.7 eV, revealing a normal state of Ti4+ in the spinel LTO. It is clear from Fig. 5b, that as-prepared PVDF-based sample S1 contains no sulfur (curve 1), 22

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1

1

2

2

3

3

4

4

Binding Energy

Binding Energy 1 1

2 2

3 3

4

4

Binding Energy

Binding Energy

Fig. 5. XPS spectra of (a) Ti 2p b) S2p c) F1s d) Li1s for LTO-electrodes S1 and S2 1) as prepared S1 and 2) after cycling S1 and 3) as prepared S2 and 4) after cycling S2.

3.2. Electrochemical performance of Li4Ti5O12-electrodes

Table 2 The specific capacities of samples at 0.2 °C.

The galvanostatic charge-discharge curves at 0.2 °С rate for all studied electrodes in coin cells are shown in Fig. 6. Characteristic charge and discharge potential plateaus corresponding to intercalation/ deintercalation of lithium ions into/out of the LTO spinel structure are observed. For the charging curve, the average value of the plateau potential was about 1.6 V and for the discharge curve about 1.5 V. The specific capacities normalized to the total electrode mass (Q) of all studied electrodes with different compositions are shown in Table 2.

16

453 2

7

16

4 53 2

S1 S2 S3 S4 S5 S6 S7

Q, mAh g−1 137 157 155 149 150 138 123

As can be seen from Fig. 6 and Table 2, ICP-modified electrodes show higher discharge capacities, except for sample S7, compared to the conventional PVDF-bound electrode. The best results are achieved with the S2 electrode containing PEDOT:PSS and CMC, which shows a 14% higher specific discharge capacity (157 mAh∙g−1) than the standard electrode composition (137 mAh∙g−1) with PVDF binder. Thus, replacing the binder and tuning the amount of carbon black in the electrode slurry during electrode preparation significantly affect the capacitive characteristics of the LTO material in the electrode. The increase in capacity of an ICP-modified material can be due to various reasons. The first reason is an increase in relative fraction of the mass of the active component in the composition of the electrode material because of the reduced fraction of “dead mass”. This is confirmed by the fact that a pair of electrode compositions S3 and S2, and another pair S1 and S6, containing equal amounts of active material, have very close capacity values. For a better understanding of the magnitude of this effect, it is also worth to compare the capacity values normalized to the mass of LTO (QLTO). In this case, the capacity of a conventional composition

E/

7

Sample

Q Fig. 6. Charge-discharge curves of the samples S1–S7 at 0.2 C. Curve numbers correspond to samples: 1 – S1, 2 – S2, 3 – S3, 4 – S4, 5 – S6, 6 – S6, 7 – S7. 23

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is 171 mAh∙g−1, and the highest capacity achieved for the PEDOT:PSS and CMC modified electrode was 175 mAh∙g−1 (175 mAh g−1 being the theoretical capacity of LTO). In this case, the difference in capacities between an electrode of conventional composition and modified electrodes is less noticeable. The capacity decrease for the sample S7 is associated with a decrease in the proportion of active material in the anode composition. For S3 and S4 electrodes, the achieved capacities were slightly lower than for S2, which indicates a decrease in the efficiency of the charge-discharge process when only PEDOT:PSS or CMC is used as a binder. The largest polarization value of about 50 mV is observed for the sample S1 of conventional composition, while the polarization value is reduced to approximately 28 mV for the composition containing PEDOT:PSS and CMC additives. The decrease in polarization can be due to both a decrease in the kinetic limitations of LTO charge-discharge process and to an improvement in the electrical contact between the particles and a decrease in the internal Ohmic resistance of the battery. S5 (no CB) and S3 (CB and PEDOT:PSS only) compositions show the highest polarizations among the polymer-modified compositions probably due to the fact that the single additive PEDOT:PSS as a binder, having a conductivity of about 1 Sm∙cm−1 [37], is not sufficient to ensure the effective performance of the material. Additionally, as can be seen in Fig. 6, electrode S2 apparently shows higher values both of specific energy and specific power in comparison with other samples. This suggests a possibility to obtain material with higher energy and simultaneously power density (in a full cell) by substituting conventional binder with PEDOT:PSS and CMC. LTO-electrodes were cycled from 0.2 to 10 °C within the potential range 2.5 to 1.0 V. The dependencies of the capacity on the charge rate for electrodes of all studied compositions (S1–S7) are presented in Fig. 7. The largest drop in the capacity (60%) at the highest discharge rate (10 °C) is observed for S5 without CB containing only PEDOT:PSS as a conductive additive. However, with increase of current density the specific capacity of S1 electrode dropped more significantly than that of S2 electrode. Moreover, an increased difference between the potential plateaus of charge and discharge was observed for S1 electrode due to higher Ohmic resistance in comparison to S2 electrode. It can be seen that at higher discharge rates, the observed capacity drop and the polarization value for electrode S1 with conventional PVDF binder are higher than the corresponding values for electrodes S2 with PEDOT:PSS/CMC binder. The S2 electrode demonstrates remarkably better rate performance, so that even at 10 °C the capacity still remains at 138 mAh∙g−1.

After measurements at 10 °C rate, an additional repeated measurement was performed at a current of 1 °C. The capacities of all compositions returned to the initial values at the current 1 °C. This indicates the stability of the observed values of discharge capacities; it also suggests that kinetic limitations should be considered as the main reason for the drop in capacity at high discharge rates when recharging LTO-materials. Thus, the data obtained at higher charge rates confirm the assumption of decreased kinetic limitations in the case of LTO modification with PEDOT:PSS and CMC, as compared to the PVDF binder. Smaller capacity drops at higher current densities indicate the possibility of increasing the battery power when using this material. The difference in specific capacities between electrodes employing different binders becomes more noticeable with increasing of charge current density. Charge-discharge characteristics of S1 and S2 electrodes were more thoroughly investigated at higher charge rates, i.e. at higher current densities. Fig. 8a shows the dependence of the capacity value on different charge currents from 0.2 °C to 30 °C (with a constant discharge current of 0.2 °C). In both cases, with an increased charge rate, a gradual decrease in charge capacity is observed. However, with increase of current density the specific capacity of S1 electrode dropped more significantly than that of S2 electrode. Moreover, the increased difference between the potentials of charge and discharge plateaus was observed for S1 electrode due to higher Ohmic resistance in comparison to S2. It can be seen that at higher charge rates the observed capacity drop and the polarization value for electrode S1 with conventional PVDF binder are higher than the corresponding values for electrode S2 with PEDOT:PSS/CMC binder. S2 demonstrates remarkably better rate performance, so that even at 10 °C the capacity still remains at 134 mAh∙g−1, at 20 °C at 110 mAh∙g−1, at 30 °C at 63 mAh∙g−1. A comparison of the electrodes S1 and S2 in terms of their specific energy and specific power is presented in the Ragone plot (Fig. 8b). The electrode S2 has significantly better values of both parameters in comparison with the electrode S1. The cyclic performance was recorded with the electrodes at a constant charge-discharge current density (1 °C) as shown in Fig. 9. All modified electrodes exhibit stable cyclic behavior, in contrast to the PVDF-bound sample, but small capacity fluctuations can be observed for samples S3 and S5. Such behavior is probably due to the inhomogeneity and lower conductivity of the electrodes. The drop in specific capacity for composition S1 after 100 cycles was 8%. The best capacity and stability were observed for composition S2. For materials where only PEDOT:PSS or CMC additives were used as a binder, a faster drop in capacity was observed during prolonged cycling, whereas the polymer combination PEDOT:PSS/CMC seems to allow more efficient binding of components in a composite material and its adhesion to the current collector and conductivity. The Coulombic efficiency of all electrodes remains stable and close to 100% in the initial 100 cycles; among all compositions sample S7 is the least effective. The electrode composition S2 showed the smallest decay in efficiency; it almost did not change at all with cycling, as compared to other samples. Based on the obtained cycling stability data and Coulombic efficiency of all samples (≈100%), it can be concluded that there is no visible degradation of the properties of the electrodes, resulting both from the stability of the material structure during cycling. Based on the results of recent studies of mechanism of LTO degradation [46], we can conclude that the PEDOT:PSS/CMC polymer binder probably more effectively suppresses the structural transformation of the surface of LTO grains and diminishes the interaction with the electrolyte solution without hindering of lithium transport, reducing the degradation processes. To reveal the influence of PEDOT:PSS/CMC on the electrochemical properties of Li4Ti5O12 material and its charge-discharge kinetics, a detailed electrochemical characterization of electrodes S1–S7 was carried out by cyclic voltammetry and electrochemical impedance

2 3 4 5 61

Q

7

Fig. 7. Dependences of the specific capacities of S1–S7 electrodes with various binders at different current densities. Curve numbers correspond to samples: 1 – S1, 2 – S2, 3 – S3, 4 – S4, 5 – S5, 6 – S6, 7 – S7. 24

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S2 S1

Q

Energy

S2 S1

P Fig. 8. a) C-rate capability of S1 and S2 electrodes; b) Ragone plots for coin cells with S1 and S2 and lithium foil.

2 5 6 7 2 3 4

1

4

I

3

Q

56

1 7

E Fig. 9. Cycling performance curves at 1 °C current for 100 cycles and Coulombic efficiency. Curve numbers correspond to samples: 1 – S1, 2 – S2, 3 – S3, 4 – S4, 5 – S5, 6 – S6, 7 – S7.

Fig. 10. Cyclic voltammograms of coin-cells with S1–S7 LTO-electrodes at 0.1 mV∙s−1. Curve numbers correspond to samples: 1 – S1, 2 – S2, 3 – S3, 4 – S4, 5 – S5, 6 – S6, 7 – S7.

spectroscopy.

Table 3 Cyclic voltammetry data for S1–S7 electrodes.

3.3. Cyclic voltammetry

Sample

Reduction of kinetic limitations of the LTO charge-discharge process for electrode with combined PEDOT:PSS and CMC binder is also confirmed by the results of cyclic voltammetry (Fig. 10). At a potential scan rate of 0.1 mV∙s−1, in the range from 1.0 to 2.5 V, a pair of distinct redox peaks is observed in the potential range 1.43 to 1.83 V. The observed pair of redox peaks is due to the redox transition of Ti4+/Ti3+ in the LTO structure, followed by the intercalation of lithium ions.

Li 4 Ti5O12 + 3e + 3Li+ = Li7 Ti5O12

S1 S2 S3 S4 S5 S6 S7

(1)

Epa, V

Epc, V

Epa-Epc, V

Ipa, mA

Ipc, mA

1.83 1.68 1.69 1.69 1.71 1.67 1.66

1.42 1.45 1.45 1.44 1.43 1.46 1.48

0.41 0.23 0.24 0.25 0.28 0.21 0.18

0.7 3.6 0.9 3.0 0.5 1.8 1.4

1.2 3.1 0.8 2.1 1.2 1.4 1.3

polymer additive is introduced. It can be seen from the voltammetry data, that the peak potential difference for compositions with PEDOT:PSS and CMC is lower in comparison to a conventional electrode. The peak potential difference is determined both by thermodynamic reasons (changes in the formal potential of the redox process in this phase) and by kinetic reasons (slow kinetics of lithium intercalation, which leads to a shift in the peak potential in the direction of the potential scan with an increase in the scan rate, and a delayed nucleation stage) as well as Ohmic voltage drops in the battery. In addition, the cyclic voltammograms of different electrode compositions show different peak shapes: high and sharp redox peaks indicate an improvement in the electrochemical kinetics of the electrode S2 charge-discharge process, while other compositions are characterized by wider and lower peaks, which can be associated with

The cathodic peak located in the range 1.42 to 1.48 V corresponds to the discharge voltage plateau when the lithium ions are intercalated into LTO, and the anodic peak located in the range 1.66 to 1.83 V corresponds to the charge voltage plateau, when lithium ions are deintercalated from LTO. The data on the cathodic and anodic peak potentials and their differences are collected in Table 3. The pronounced difference in the shape of the peaks of samples S1 and S2 is drawing attention. It indirectly indicates different conditions of charge transfer in S1 and S2 (Fig. 10). More symmetric and welldefined peaks were observed for S2, whereas wider peaks with lower peak currents and higher peak potential separation were observed for S1. These results imply a better reversibility of lithium intercalation processes during phase transition between Li4Ti5O12 and Li7Ti5O12 and lower internal resistance in the case of S2 electrode when a conductive 25

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250

S1 S2

200

-ZIm / Ω

150

S1 S5 S3 S2 S4 S6 S7

f = 153 Hz

100 50 0

f = 7 Hz

0 Fig. 11. Dependencies of the anodic and cathodic peak currents on the square root of the scan rate.

50

100

150

ZRe / Ω

200

250

Fig. 13. Nyquist plots of LTO-electrodes in coin-cells at E = 1.0 V and equivalent electrical circuit.

restrictions on charge transport in the bulk material and ohmic losses. The obtained data on the peak potential difference, shape and height of the peaks correlate with the data obtained from the charge-discharge curves. The anodic and cathodic peaks of both S1 and S2 electrodes showed a linear dependence of the peak current on the square root of the various potential scan rates from 0.1 to 0.5 mV s−1 (Fig. 11). The value of the slope of the plot lg Ip vs. lg ν close to 0.5 indicates the predominance of diffusion limitations of the intercalation/deintercalation process of lithium ions (Fig. 12).

circuit for all LTO-electrodes in coin-cells are shown in Fig. 13. The measurement was carried out in a fully discharged state at a potential of 1.0 V. Impedance spectra for all coin-cells have a similar form. The choice of an electrical equivalent circuit for simulation of experimental impedance spectra was based on the consistence of fitting results with experimental spectra. The most adequate equivalent circuit reflecting the nature of physicochemical processes proposed for modeling the electrode processes contains the following elements: Rs, Rct, W and CPE. The intercept on the ZRe axis in the high-frequency region was attributed to the ohmic resistance of the cell (equivalent series resistance ESR, Rs) representing the resistance of the electrolyte solution, the resistance at the contacts of the current collectors and the electronic resistance of the electrode material. The semicircle in the mid-frequency range is associated with the charge transfer resistance (Rct), it passes into a line with a slope of ~45° in the low-frequency region, which is the Warburg impedance region associated with the slow diffusion of lithium ions in the bulk of the material. The constant phase element CPE was used in order to take into account the capacitance vs. frequency dependence, apparently connected with inhomogeneous bulk and surface composition of the electrode material and its porosity. The parameters obtained from the analysis of the impedance spectra are collected in Table 4. The ohmic resistance of the cell (Rs) for all compositions is not higher than 12 Ω. The charge transfer resistance (Rct) for the conventional electrode S1 is 178 Ω and for S6 only 119 Ω. Smaller semicircles and hence Rct are observed for all modified electrodes. The composition S5 without carbon black containing only PEDOT:PSS is characterized by the relatively highest Rct value among the modified electrodes. The sample S2 demonstrates the smallest Rct

3.4. Electrochemical impedance spectroscopy

I

I

As impedance spectra of electrode materials are sensitive to the battery history and testing duration (cycle number, preliminary treatment) EIS measurement were performed only for batteries with similar history and electrochemical perturbations applied during testing. The investigation of influence of electrode composition on electrochemical performance was conducted by measuring EIS spectra in two electrode cell configuration. The obtained responses mainly reflect the properties of LTO-based electrodes due to the fact that the contribution of Li/Li+ electrode to the overall impedance of battery cell can be neglected. This was checked by measurements in 3-electrode cells, using metallic Li as counter and reference electrodes, which have shown minor discrepancies between 2- and 3-electrode cell impedances. Therefore, the overall impedance of a battery cell can be interpreted as the impedance of the LTO-based electrode. Nyquist plots of impedance spectra and the equivalent electrical

Fig. 12. Bilogarithmic dependencies of the anodic peak currents of a) S1 and b) S2 on the potential scan rate. 26

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characteristics of LTO particles can be significantly improved due to low charge transfer resistance (Rct) and a high lithium diffusion coefficient. Here, it should be mentioned that the obtained data clearly demonstrate the complex nature of the experimentally determined values of Warburg constants (and corresponding values of the Warburg diffusion resistance), which cannot be unambiguously associated only with the diffusion of lithium ions in the active grains. The intergranular space between LTO active grains, consisting of the binder and conductive additive, cannot affect the intrinsic conductivity of the bulk of grains. Probably, the influence of the type of binder originates from the difference in the lithium ion transport from electrolyte to the surface of active grains before lithium insertion into the LTO, which proceeds in media with different porosity and conductivity surrounding the active grains. The obtained results show that the use of conductive binder in composite LTO-based materials can be considered as an alternative to conventional binders. Summarizing the results of the electrochemical performance of Li4Ti5O12-electrodes we can conclude that the modified Li4Ti5O12-based electrode with conducting polymer PEDOT:PSS in combination with CMC demonstrated the highest specific capacity at 1 °C (152 mAh∙g−1) in comparison with 142.4 mAh∙g−1 [24] and 143.5 mAh∙g−1 [25], related to the total electrode mass. This advantage is growing with the increase of current density. We believe that an optimized composition of LTO/PEDOT:PSS/CMC negative electrode material can be applied for fabrication in commercial lithium ion batteries. We have tested a coin cell composed of a positive LiFe0.4Mn0.6PO4 electrode and a negative Li4Ti5O12 with the same conducting polymer binder for both electrodes. The mass loading ratio of LiFe0.4Mn0.6PO4 positive electrode and Li4Ti5O12 negative electrode was calculated before battery assembly to be 0.879. Cycle life stability of this battery is shown in Fig. 15. The discharge capacity, referred to the mass of cathode material, and coulombic efficiency is displayed versus cycle number. The good cycling performance observed with capacity fading of only 16% over 1000 cycles is remarkable.

Table 4 The parameters obtained from fitting of impedance spectra. Sample S1 S2 S3 S4 S5 S6 S7

Rs, Ohm

Rct, Ohm

σw, Ohms−0.5

Dapp, cm2 s−1

5.7 11.4 7.6 8.4 6.9 9.9 7.1

178.1 26.1 82.0 35.0 110.3 119.0 43.3

42.6 24.3 68.1 32.1 29.9 65.1 52.2

3.3 × 10−13 1.0 × 10−12 1.3 × 10−13 5.7 × 10−13 6.6 × 10−13 1.4 × 10−13 2.2 × 10−13

4. Conclusions In this work, we present a simple and cost-effective approach to fabricate Li4Ti5O12 electrodes with enhanced functional properties by using eco-friendly water-based binder, which is an alternative to fluorine-containing binders. The morphology and the structure of the composite materials were investigated by X-ray diffraction, scanning electron microscopy and EDX analysis. The obtained results show that the introduction of PEDOT:PSS/СМС binder maintains the good integrity of material and adhesion to current collector. Among the seven

S1 S5 S3 S2 S4 S6 S7

Q

Zre

value 26 Ω, which is about 7 times lower than that of the conventional electrode, and indicates the decrease in kinetic limitations on interfacial charge transfer. It should be noted that two samples S1 and S6 show close results in Rct. These samples contain equal amounts of LTO (80 wt %) and CB (10 wt%), and 10 wt% of non-electronically conductive binder PVDF (S1) and CMC (S6). They have slightly higher values of Rct, compared to the composition with PEDOT:PSS (S5), containing only 5% of PEDOT:PSS as binding and conducting additive, which has Rct = 110.3 Ω. The results show that very proper balance of components should be used for achievement of optimized composition of electrode material. In general, a lower value of charge transfer resistance is favorable from the point of kinetics of electrochemical charge-discharge process. Thus, the electrode material with PEDOT:PSS and CMC shows an increase in the electronic and ionic conductivity of the active material, which leads to a significant decrease of Rct and an increase in energy and power density. The Warburg constants (σw) and the values of the apparent diffusion coefficients of lithium ions presented in Table 4 were calculated from the slopes of the low-frequency linear parts of the ZRe vs. ω−0.5 plots (Fig. 14). The apparent diffusion coefficient of Li ions in an electrode of conventional composition was 3.3 × 10−13 cm2∙s−1. Similar values of the apparent diffusion coefficients (1.4–6.6 × 10−13 cm2∙s−1) were observed for the electrodes with polymer-modified compositions. The largest Dapp value 1.0 × 10−12∙cm2∙s−1, three times higher than that of the conventional electrode, was found for the sample S2 with PEDOT:PSS and CMC. The data obtained from the analysis of the impedance spectra correlate with the cyclic voltammetry data. Actually, the Rct value depends on the electronic and ionic conductivity of the electrode. During the electrochemical lithiation/delithiation reaction, the electron and a lithium ion have to reach or leave the reaction site in the electrode simultaneously. Thus, the electrode material with improved high-speed characteristics should have both high electronic and ionic conductivity. As a result, the electrochemical

Fig. 15. Cycling performance and Coulombic efficiency for the LiFe0.4Mn0.6PO4 cathode vs. Li4Ti5O12 anode (S2) at 1 °C.

Fig. 14. ZRe-ω−0.5 plots of the impedance in low frequency region (Е = 1.0 V). 27

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different electrodes, an optimized composition of LTO material, containing conductive binder (composition 90 wt% of LTO, 6 wt% of CB, 2 wt% of PEDOT:PSS and 2 wt% of CMC), shows the best rate capability with discharge capacity 157 mAh∙g−1 at 0.2 °C (or 174 mAh∙g−1normalized to Li4Ti5O12 mass) and 63 mAh∙g−1 at 30 °C as well as good cycling stability at 1 °C (< 1% decay after 100 cycles). These characteristics are markedly better than those of PVDF-bound Li4Ti5O12 material. The kinetic parameters for modified and conventional electrode compositions were obtained by cyclic voltammetry and electrochemical impedance spectroscopy. The LTO electrodes with combined conducting binder PEDOT:PSS/CMC (composition S2) demonstrate the smallest charge transfer resistance and the highest apparent diffusion coefficients of lithium ions, which favor enhanced kinetics of chargedischarge processes. The electrochemical performance of optimal Li4Ti5O12 composition with conductive binder (S2) was tested in the full coin cell with LiFe0.4Mn0.6PO4 cathode with the same binder composition (2 wt% of PEDOT:PSS and 2 wt% of CMC). The test confirmed the good cycling performance of a battery with an environmentally friendly conductive binder with capacity fading of only 16% over 1000 cycles. We propose this simple and cost-effective approach to fabricate Li4Ti5O12 electrodes for use in the production of commercial batteries.

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