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Effect of Molecular Weight and Degree of Substitution of a SodiumCarboxymethyl Cellulose Binder on Li4Ti5O12 Anodic Performance Bo-Ram Lee and Eun-Suok Oh* School of Chemical Engineering, University of Ulsan, 93 Daehakro, Nam-gu, Ulsan 680-749, Korea ABSTRACT: This paper presents a detailed investigation of the influence of molecular weight (MW) and degree of substitution (DS) of sodium-carboxymethyl cellulose (CMC) used as binder of Li4Ti5O12 (LTO) anodes. CMC MW and DS were in the range of 90 000 to 700 000 and 0.7 to 1.2, respectively. As demonstrated by coin cell tests, the LTO electrode that contained relatively low MW and high DS CMC showed excellent cyclic performance above 150 mAh g−1 even after 100 cycles. Unlike typical electrodes, the adhesion strength of the LTO electrodes was not a critical factor for cell performance. On the contrary, low coverage of CMC on the surface of LTO, which leads to weak adhesion, was more favorable to the cell performance than strong adhesion caused by high coverage. Both an increase in the DS and a decrease in the MW of CMC gave rise to low charge transfer resistance, high ion conductivity, good wettability with the electrolyte, and good lithium ion mobility owing to a relatively large amount of LTO exposed to directly the electrolyte so long as critical adhesion is guaranteed in the LTO electrode.

1. INTRODUCTION Spinel-structured lithium titanium oxide (Li4Ti5O12, LTO) has attracted great interest recently because of its potential applications in lithium ion battery (LIB) anodes for electric vehicles. LTO has an extremely low strain (∼0.2%) during its charge/discharge process. Additionally, it has a relatively high lithium ion insertion potential at approximately 1.55 V vs Li+/ Li, leading to no electrolyte decomposition and the formation of a solid electrolyte interface on the surface of the anode. Therefore, LTO can provide a long cyclic life, low irreversible capacity, and high rate capability in spite of its low energy density and low voltage.1−4 While many studies have sought to improve the electrochemical properties of LTO, less attention has been paid to the development of an appropriate binder for LTO materials. Most of the recent studies on binders have focused on developing a new kind of polymer suitable for high-capacity LIB anodes such as silicon, tin, and their respective oxides.5−13 The binder for high-capacity anode materials should possess extremely high adhesive qualities in order to endure substantial volume changes during the charge/discharge process. The adhesion strength of the polymer is the most important factor that should be considered when choosing the binder in the case of high-capacity materials. On the other hand, LTO experiences no volume changes during use, indicating that other factors, rather than the adhesive property, could be considered for a polymer to be used as an LTO binder. For instance, low charge transfer resistive polymers with weak adhesion could be more suitable for LTO than those with strong adhesion in highpower applications such as hybrid electric vehicles. © 2013 American Chemical Society

To date, only a few studies regarding binders for LTO, including TiO2, have been published. Chou et al.4 reported that a LTO electrode containing CMC binder showed a better rate capability than that containing polyvinylidene fluoride (PVDF), owing to the low charge transfer resistance and diffusion activation energy of the CMC-containing electrode. Compared to the PVDF binder, the superiority of CMC as a binder of titanium oxides has also been demonstrated due to the suitable compactness and electrolyte wettability of the CMC-containing electrode.14,15 CMC has been commercially used in LIB as a thickening agent for low viscous styrene butadiene rubber (SBR) binder.16−19 However, after Li et al.20 reported that the CMC use of binder led to better capacity retention in a highcapacity Si electrode than the combination of SBR and CMC, the studies using CMC as the sole binder have been performed for various types of active materials: carbon,21−23 silicon including silicon/carbon composite,24−28 tin oxide,29 iron oxide, 30 lithium iron phosphate, 31 lithium manganese oxide,32,33 and anthraquinone.34 These studies found that an electrode using water-soluble CMC binder is cheaper, more environmentally friendly, and more electrochemically stable than electrodes using conventional PVDF binder. Additionally, the electrochemical performance can be improved by the modification of CMC, such as the amount or type of metal salts, degree of substitution (DS) of carboxymethyl groups, and the use of other types of celluloses. Received: November 27, 2012 Revised: January 31, 2013 Published: February 11, 2013 4404

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Lite 100, KSV Instrument Ltd., Finland) in order to investigate the wettability of CMC. The ionic conductivities of a 1 wt % CMC solution containing electrolyte were also measured using a conductivity meter (S230−K, Mettler-Toledo).

Unfortunately, there have been no detailed studies on the relationship between the basic polymer properties of CMC binder and the electrochemical performance of LTO electrodes, with the exception of a study that reported that conventional CMC as the LTO binder was more effective than the conventional PVDF binder.4,14,15 In this study, we conduct a thorough investigation of the effect of molecular weight (MW) and DS of CMC binder on the electrochemical performance of LTO electrodes and propose the optimum CMC binder suitable for the LTO electrode.

3. RESULTS AND DISCUSSION As shown in Figure 1, CMC is a derivative of cellulose with carboxymethyl groups (−CH2COOH) substituting some of the

2. EXPERIMENTAL SECTION LTO active materials were acquired from POSCO Energy Storage Materials Co., Ltd., Korea. The average particle size was 2 μm. Five types of CMC were purchased from Sigma-Aldrich. Their MWs and DS are listed in Table 1. As a simplification, Table 1. MWs and DSs of the CMCs and the Viscosities of the CMCs Dissolved in Deionized Water average molecular weight

degree of substitution

name of CMC

90 000 250 000

0.7 1.2 0.9 0.7 0.9

CMC90-L CMC250-H CMC250-M CMC250-L CMC700-M

700 000

viscosity (cP)

Figure 1. Structure of carboxymethyl cellulose.

63.8 (3% sol.) 643.8 (3% sol.) 1141 (3% sol.) 497.5 (2% sol.) 1000−1100 (1% sol.)

hydroxyl groups (−OH) of cellulose. The DS indicates the average number of carboxymethyl groups in a monomer unit, which significantly affects polymeric properties of CMC such as its solubility in water and the structure of the main chains. As seen in Table 1, an increase in DS decreases the viscosity of the aqueous CMC solution and makes it easy to handle. The DS can vary from 0 to 3, but is commonly in the range of 0.6−0.95 for commercial applications, including LIB.16,36 The hydrogen ions in the carboxymethyl groups are generally substituted by the alkali ions used as catalysts during the synthesis of CMC from cellulose. Sodium ions are used commercially and thus CMC is generally referred to as a cellulose material containing sodium-substituted carboxymethyl groups (−CH2COONa). The cyclic performances of LTO electrodes containing CMC binders of different MWs and DS are shown in Figure 2.

these will be referred to CMC (MW) -L, -M, or -H where L, M, and H indicate a DS of 0.7, 0.9, and 1.2, respectively. CMC was prepared by dissolving in deionized water. The viscosities of the CMC solutions are also listed in Table 1. To make the electrode slurry, 90 wt % LTO, the active material, was put into a zirconia jar with 5 wt % super-p and 5 wt % CMC binder and was mixed for 1 h using a planetary ball mill (Pulverisette 7, Fritsch; speed 380 rpm). The slurry was coated on Al foil and dried in a convection oven at 60 °C for 30 min and subsequently placed in a vacuum oven overnight at 80 °C. To fabricate CR2016 coin cells, the dried electrode was pressed using a mass loading of up to 1.6 g cm−3 and was assembled using lithium foil as a counter electrode in an argonfilled glovebox. The electrolyte used was 1 M LiPF 6 EC:EMC:DMC = 1:1:1 (vol.) (Panaxetec Co., Korea). The electrochemical performance of the coin cells was measured by galvanostatic charge/discharge between 1 and 2.6 V in a battery cycler (PNE solution Co., Korea) with a 0.1 C rate for the first 2 cycles and a 1 C rate for the next 100 cycles. The rate capability of the LTO electrodes was also tested at various charge/discharge current rates. Additionally, electrochemical impedance (VSP, BioLogic Science Instruments) was measured at a frequency range of 0.01 Hz to 100 kHz. A texture analyzer (TA-PLUS, Lloyd Instruments Ltd.) and a balanced beam scrape adhesion and mar tester (PE-5780, BYK) were used to measure 180° peel strength and scratch resistance, respectively, of the LTO electrodes. The procedure for measuring scratch resistance was performed as described previously.35 The surface elements of the electrodes were determined by X-ray photoelectron spectroscopy (ESCALAB 250, Thermo Fisher Sci.) using an Al Ka X-ray source scanning from 0 to 1200 eV. The contact angles of the thin CMC films coated on Al foil were measured as a function of time exposed to an electrolyte droplet using a video-connected device (Theta

Figure 2. Cycling performance of the Li4Ti5O12 electrodes.

Compared to typical graphite and silicon electrodes, LTO electrodes have very high initial Coulombic efficiencies due to the high charge/discharge potentials of LTO. All of the CMCcontaining LTO electrodes show high initial efficiency of greater than 93%, while a previous study showed 80% initial Coulombic efficiency.4 As mentioned in the literature, this difference may be attributed to the existence of impurities in LTO materials rather than to the CMC binder used. As expected, the CMC-containing electrodes also show high 4405

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Figure 3. Cycling behavior of the Li4Ti5O12 electrodes at C rates of 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C, 5 C, 10 C. (a) discharge capacity vs cycle number and (b) discharge capacity vs current density.

Figure 4. Adhesion and scratch strength of the Li4Ti5O12 electrodes. The pictures were taken after the LTO electrodes were immersed in electrolyte for 1 day.

reversible capacities with excellent cyclic retention. The LTO electrode containing CMC250-H maintains a reversible capacity of 153 mAh g−1 after 100 cycles, whereas the CMC700-M electrode shows a reversible capacity of 144 mAh g−1. Even though this difference is not large, the electrochemical performance of CMC binders can be ordered based on the cyclic reversible capacity as follows: CMC250-H > CMC90-L > CMC250-M > CMC700-M ≈ CMC250-L. By comparing CMC250 binders with the same MW, the cyclic performance improves as the DS in CMC increases. Additionally, it is clear from the observation of CMC90-L and CMC250-L or CMC250-M and CMC700-M that an increase in the MW of CMC with the same DS decreases the cyclic capacity of the LTO electrodes. A very similar result was obtained from the rate capability tests, except that the LTO electrode containing CMC700-M shows a slightly higher capacity than that containing CMC250L, as shown in Figure 3. At a low current rate of charge/ discharge, there is not much difference in reversible capacity among the LTO electrodes. However, the difference is clearly seen with an increasing current rate. Meanwhile, the capacities of the electrodes are completely restored to their initial capacities when the rate returns to 0.1 C from high-rate cycles. Various characterization techniques were performed to analyze the results of cell tests. The first property of a binder that is generally considered is adhesion ability, either between active materials or between the active material and the current collector. These can be measured by the 180° peel strength or the scratch resistance of the electrodes, as illustrated in Figure

4. The adhesion values of the LTO electrode made using PVDF (Sigma-Aldrich, MW = 180 000) as binder are also shown in the figure for comparison. The adhesion of the electrode containing CMC binders is much stronger than that containing the PVDF binder. This is due to the strong hydrogen bonding of the carboxyl and hydroxyl groups in CMC with the active material and the current collector, whereas the fluorine atoms in PVDF form very weak hydrogen bonds.37 This weak adhesion of the PVDF-containing electrode leads to the delamination of the electrode after being exposed to the electrolyte for one day, as shown in Figure 4. In the case of the CMC-containing electrodes, the adhesion strength was in the order of CMC700-M > CMC250-L > CMC250-M > CMC90-L > CMC250-H. As expected, an increase in the MW of CMC enhances the adhesion capability both between LTO particles and between the LTO and Al foil. This may be attributed to the formation of a more flocculated structure in the electrode by longer polymer chains. On the other hand, it is obvious from the results of the CMC250containing electrodes that an increase in the DS of CMC weakens the adhesion of the binder. High DS CMC introduces a greater amount of carboxymethyl groups substituting for the hydroxyl groups. From our previous results,9 the hydroxyl groups in polyvinyl alcohol form very strong hydrogen bonds with the active materials and the current collector compared to carboxyl groups in poly(acrylic acid) . Thus the substitution of hydroxyl groups with carboxymethyl groups causes weakness in adhesion. 4406

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performance. That is, as DS increases and MW decreases, the interaction between LTO and CMC becomes weak due to short chain length and reduction in the amount of hydroxyl groups in the CMC polymer. This leads to a lower area of Ti that is covered by the CMC binder and ultimately weakens the adhesion. On the basis of the results of the adsorption isotherms of CMC on graphite, Lee et al.36 reported a similar result where CMC with a lower DS had more hydrophobicity and thus covered more of the graphite surface. Greater surface coverage by the binder is unfavorable for lithium ion mobility in electrodes since the ionic conductivity of the polymer is significantly lower than that of the electrolyte. Kovalenko et al.11 reported that the ionic conductivity of alginate, whose structure is very similar to CMC, was approximately ∼10−8 S cm−1, while normal liquid electrolyte has an ionic conductivity of ∼10−3 S cm−1. In order to examine the effect of ionic conductivity on electrochemical performance, EIS spectra were obtained and are shown in Figure 6. The data indicate that the DS and MW of the CMC binders both play an important role in the charge transfer resistance (Rct) of the LTO electrodes, as estimated from the size of the semicircle in the medium frequency region. Figure 6a shows that the first two-cycled electrodes reflect resistances that can be attributed to scarce electrolyte wetting and impurities in the electrodes. These transient resistances are stabilized after tens of charge/ discharge cycles so that the effect of the binders can be clearly compared. As shown in Figure 6b, the CMC250-H and CMC90-L electrodes covered with a lesser amount of CMC has a smaller Rct than the LTO electrodes that had broader surfaces. The Rct is mainly affected by lithium ion mobility and electronic conductivity. Since the electronic conductivity is not distinguishable among the electrodes, the lower coverage by CMC contributes to the improvement in Rct. Consequently, a CMC binder with low MW and high DS is favorable to the LTO electrodes if critical adhesion is guaranteed due to lower surface coverage and Rct, thus leading to higher cell performance. The inherent properties of the binder can be estimated using the measurements of both the contact angle of pure CMC film and the ionic conductivity of CMC solution. The electrolyte wettability of the CMC is represented by the contact angle as shown in Figure 7. As seen in Figure 7a, the contact angles of the samples are stabilized within 10 s after a drop of electrolyte falls on the surface of the CMC film. From the pictures shown in Figure 7b, it can be clearly observed that at a given MW, an increase in DS decreases the contact angle. The increase in

Surprisingly, the order of the binders in terms of adhesion is exactly the opposite of the order of the cell performance displayed in Figures 3 and 4. In general, strong adhesion strength contributes to stable cyclic capacity retention, especially for high-capacity anodes that experience significant volume change over the course of use. Thus, adhesion is one of the key factors in the choice of a binder. In contrast, the volume change of the LTO during the charge/discharge process is nearly zero, and thus no stress is generated either inside the electrode or at the interface between the electrode and the current collector. This implies that the need for adhesion as a mitigator of volume change is not a factor if an electrode possesses the critical adhesion required for the electrode manufacturing process. Factors other than adhesion may affect the cell performance of the LTO electrodes. To investigate the surface coverage of the LTO, XPS measurement of the electrodes was performed, and the simple Ti 2p spectra with Ti/C atomic ratios are shown in Figure 5. The peak intensity quantitatively indicates the Ti

Figure 5. X-ray photoelectron spectra for Ti 2p states of the Li4Ti5O12 electrodes. The values in parentheses indicate the ratios of Ti atoms to carbon atoms obtained from XPS results.

atoms exposed to electrolyte and not covered by CMC binder. Using the Ti/C atomic ratios, an approximation on the amount of CMC coverage can be also made since carbon is originated from binder. As seen in the figure, the trend in the amount of exposed Ti atoms is opposite that of trend in adhesion strength shown in Figure 4 but corresponds to the trend in cell

Figure 6. EIS spectra of the Li4Ti5O12 electrodes using CMC binder with different MW and DS (a) after 2 cycles at 0.1 C and (b) after 2 cycles at 0.1 C and 100 cycles at 1 C. 4407

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(1) The electrolyte wettability and ionic conductivity of CMC increases due to easier vibration of the polymer segments. (2) The interaction between CMC and LTO decreases and leads to lower surface coverage of CMC on the surface of LTO materials. This indicates that the broader surface of LTO is more exposed to electrolyte. (3) Both the charge transfer resistance and the adhesion strength decrease in relation to the surface coverage of CMC. Owing to these positive effects, an LTO electrode containing a relatively high DS and low MW CMC, CMC250-H, showed the best cell performance even though its adhesion capability was not as good as that of the other four CMCs.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +82 52 2592783. Fax: +82 52 2591689. E-mail: esoh1@ ulsan.ac.kr.

Figure 7. (a) Contact angle measurement of an electrolyte drop fallen on the CMC films. (b) Photographs of the contact angle after 10 s.

Notes

The authors declare no competing financial interest.

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carboxymethyl groups weakens the hydrogen bonds between the hydroxyl groups in CMC and helps carbonate solvents to be absorbed more quickly. Additionally, the affinity between the carboxymethyl groups and the carbonate electrolyte may be higher than that between the hydroxyl groups and the carbonate since both carboxymethyl and carbonate contain carbonyl groups. On the other hand, the observation of CMC90-L and CMC250-L or CMC250-M and CMC700-M indicates that an increase in MW increases the contact angle due to an increased entanglement of the longer polymer chains. In order to make a relative comparison of ionic conductivity among CMC samples, a small amount of electrolyte (1 M LiPF6 in EC:EMC:DMC = 1:1:1 vol) is added to an aqueous solution of 1 wt % CMC, and the ionic conductivity is measured. The result is listed in Table 2. By comparing the

ACKNOWLEDGMENTS This research was financially supported by the Ministry of Knowledge Economy (MKE), Korea Institute for Advancement of Technology (KIAT) through the Inter-ER Cooperation Projects (R0000497). The work was also supported by the National Research Foundation of Korea Grant funded by the Korean Government (MEST) (NRF-2009-C1AAA001-20090093307) and the Basic Science Research Program funded by the Ministry of Education, Science and Technology (20100024077).

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Table 2. Ionic Conductivities of CMC Binder Solution binder solution

ionic conductivity of binder solution (mS cm−1)

ionic conductivity by adding electrolyte (mS cm−1)

CMC90-L CMC250-L CMC250-M CMC250-H CMC700-M

1.90 1.88 2.07 2.15 1.84

2.19 2.11 2.31 2.44 2.20

REFERENCES

CMC250 samples, it can be seen that an increase in DS increases the ionic conductivity, indicating that carboxymethyl groups are more favorable to lithium ion transfer than hydroxyl groups. In addition, low MW CMC vibrates with greater freedom and thus transfers ions more easily than high MW CMC. This also contributes the low Rct’s of the high DS and low MW CMCs CMC250-H and CMC90-L, as shown in Figure 5.

4. CONCLUSIONS Five different kinds of CMCs were used as a binder for LTO electrodes to determine optimum MW and DS of the CMC binder. The following conclusions can be drawn as the DS increases and MW decreases: 4408

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