UCTEA Chamber of Metallurgical & Materials Engineers’s Training Center Proceedings Book

956 IMMC 2018 | 19th International Metallurgy & Materials Congress

Th e Eff ect of Binders on the Electrochemical Properties If NMC Based Cathode Electrodes

Lütfullah Özdoğan, Aslıhan Güler, Hatice Güngör, Şeyma Özcan Duman, Deniz Kuruahmet, Aslan Çoban, Mustafa Mahmut Singil, Engin Alkan, Mehmet Oğuz Güler, Hatem Akbulut

Sakarya University, Faculty of Engineering, Department of Metallurgical and Materials Engineering, Esentepe Campus, TR-54187, Sakarya, Turkey

Abstract

LiNi1/3Mn1/3Co1/3O2 (NMC) as a cathode material for Li-ion batteries has been synthesized by the sol-gel method. The X-ray diffraction results indicated that high purity NMC with hexagonal layered structure was obtained. Field emission scanning electron microscope images revealed well crystallized NMC with uniform particle size in the range of 0.8-1 m. The performance of the NMC electrodes with sodium carboxylmethyl cellulose (CMC), poly(vinylidene fluoride) (PVDF), and LA 133 (latex) as binders was compared. Constant current charge discharge test results demonstrated that the NMC electrode using CMC as binder had the highest rate capability, followed by those using LA133 and PVDF binders, respectively.

1. Introduction

Lithium-ion battery is one of the most important energy storage systems for portable computers, mobile devices, hybrid electric vehicles and plug-in hybrid electric vehicles [1]. Comprehensive studies have been carried out on electrode materials, electrolytes, additives, membranes and binders to improve battery performance [2]. When these materials are compared to the most important developments for lithium-ion batteries, the binders for the lithium-ion battery have not yet been sufficiently investigated [3-5]. Although the binders are electrochemically inactive, they can have a significant effect on electrode performance.

Nowadays, organic solvent based PVDF is widely used as a binder for both negative and positive electrodes in commercial lithium ion batteries due to its good electrochemical stability and high adhesion to the electrode materials and current collectors. However, the PVDF binder is expensive, including the use of harmful and toxic environmentally harmful organic compounds such as N-methyl-2-pyrrolidone (NMP) that are not easily recycled and processed. For this reason, it is important to find cheap, environmentally friendly binders to get the current commercial PVDF binder material improve

electrochemical performance and reduce the production costs of lithium ion batteries [6].

Carboxymethyl cellulose (CMC), which is produced from the introduction of natural cellulose into carboxymethyl groups, has received more attention in recent times due to its much easier dissolution in water and lower cost compared to PVDF binder. Due to both distinct advantages, reports that the CMC has implemented as a binding agent for lithium-ion batteries have demonstrated promising properties such as bike stability, improved electrochemical capacity and environmental friendliness [7]. Recently, alginate, a cheap, highly modular natural polysaccharide derived from brown algae, has been used as a binder mixed with Si anodes, which offers more remarkable electrochemical capacity development and bike stability than CMC or PVDF binders [8]. For this reason, the variety of binders and advantages for lithium ion batteries can provide more opportunities for improving the energy densities of these batteries for successful commercial applications.

Although promising research on CMC binder in anodic materials such as natural graphite, LiTi4O12, SnO2, Fe2O3 and Si [9], promising CMC binder for use with cathode materials in lithium ion batteries have also matured for investment. To date, very limited investigations of the effects of CMC on cathode materials have been made when compared to different binders. Zaghib et al. (WSB) 2% water-soluble elastomer binder and 2% weighted CMC with LiFePO4 cathode material, irreversibly with low capacity loss and stable cycle life [10-11]. NMC, one of the most promising large-scale commercial cathodes for lithium-ion batteries, has demonstrated superior advantages such as high operating voltage, high specific capacity, cyclic stability and structural stability [12-14]. However, there is no report on the use of CMC as binder in the NMC cathode materials. Here, we examine the effects of CMC binder on the electrochemical performance of the NMC cathode material prepared by sol-gel method for the first time.

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The layered transition metal oxide NMC is a promising cathode material due to its relatively low thermal stability, lower cost, and relatively less toxicity than LiCoO2, with relatively low reversibility capacity [15]. Water resistant connectors for the NMC electrode were used due to the stronger basicity. Recently, it has been reported that NMC electrode with CMC connector exhibits better electrochemical properties in PVDF and alginate [16]. However, it is difficult for the electrode slurry using the CMC connector to distribute the electrode plate homogeneously on an Al foil during manufacture. In addition, the CMC is also prone to bacterial growth, limiting shelf life [17]. Polyacrylic latex (LA132) is a triblock copolymer of acrylamide (AM), lithium methacrylate (LiMAA) and acrylonitrile (AN) with good stability and high adhesion properties. Recently, LA132 has been reported as a binding agent for electrode materials such as LFP and LCO [18] and has shown promising electrochemical performances. Until now, it has not yet been used as a binder for the NCM cathode electrodes. In this study, we compare our research results on the electrochemical properties of LA132, water-soluble CMC binders and commercial non-aqueous PVDF binders with the NCM cathode materials. 2. Experimental Procedure NMC based cathode electrodes used in this study were prepared as follow; a slurry containing 80 wt.% cathode active electrodes, 10 wt.% conductive carbon and 10 wt.% PvDF binder dissolved in a N‐methyl‐ 2‐ pyrrolidinone solution, a slurry containing 85 wt.% cathode active electrodes, 5 wt.% conductive carbon and 10 wt.% LA-133 binder and a slurry containing 85 wt.% cathode active electrodes, 10 wt.% conductive carbon, 2.5 wt.% CMC and 2.5 wt.% SBR binder were prepared. The as-prepared slurries were then casted onto the Aluminum foil by Doctor Blade method with a thickness of 30 m. The resulting electrodes were dried at 60 °C in a vacum oven for 12h. Samples for coin cells were then cut from these foils at a diameter of 2.54 cm2. The interrupted electrodes were assembled to test the electrochemical performance in the battery cell (2032) using metallic lithium foil as counter and reference electrode in the Ar filled glovebox. Cells were galvanostatically charged and discharged at a room temperature (25 ° C) voltage range of 2.4-4.3 V. 3. Results and Discussion Fig. 1 displays the XRD patterns of NMC powders. As can be concluded from the XRD results, NMC peaks are quite sharp, clear and, well-defined. All NMC peaks are consistent with the hexagonal -NaFeO2 structure with a space group of R-3m without any impurity phases (JCPDS 44-0145) [18]. The lattice constants for NMC are a = 2.851

Å and c = 14.219 Å, while the lattice constants for NMC/graphene are a=2.857 Å and c=14.227 Å.

Figure 1. The XRD patterns of NMC powders

Figure 2. FESEM images of a) NMC b) NMC- PVDF, c) NMC- LA133, d) NMC-SBR CMC electrodes. Fig. 2 presents FESEM images of a) NMC b) NMC- PvDF, c) NMC- LA133, d) NMC-SBR CMC electrodes. As can be seen from fig, NMC powders has polyhedron structure and powder size ranges from 0.8-1 m. The NMC electrode prepared with PvDF binder forms a conductive layer by covering a very thin layer of NMC grains. However, no conductive layer is obtained in LA 133 binder. Samples

UCTEA Chamber of Metallurgical & Materials Engineers’s Training Center Proceedings Book

958 IMMC 2018 | 19th International Metallurgy & Materials Congress

prepared with SBR-CMC binder, a thicker conductive layer is obtained over the surfaces of NMC particles.

Figure 3. Cyclic voltammometry tests of NMC electrodes prepared with (a) PvDF, (b) LA-133 and (c) SBR-CMC binders.

Fig 3 shows cyclic voltammometry tests of NMC electrodes prepared with PvDF, LA-133 and SBR-CMC binders. The LA 133 NMC has a higher cathodic potential of 4.401 V than the PvDF and CMC binders with cathodic potentials of 4.3 and 4.296 V, respectively. The lower polarization for the NMC electrode prepared using the CMC binder, is attributed to the higher ionic conductivity of the samples.

The galvanostatic charge-discharge curves of NMC electrodes prepared with LA 133, PvDF and SBR-CMC binders at a potential range of 2.2 to 4.3V and at a speed of 1C are shown in Fig 4. The electrode prepared with the LA 133 binder showed a specific capacity of 121 mAh/g after 100 cycles, resulting in a capacity loss of 22.9%. The NMC electrode prepared with PvDF showed a specific capacity value of 154 mAh /g after 100 cycles and a capacity loss of 5.7%. The NMC electrode prepared with CMC-SBR connectors showed a capacity value of 162 mAh / g even after 100 cycles, resulting in a capacity loss of only 1.22%.CMC-SBR binder showed better electrochemical performance than PvDF and LA 133 binder due to its high ionic conductivity.

Figure 4. Galvanostatic charge/discharge tests of NMC electrodes prepared with (a) LA-133, (b) PvDF, and (c) SBR-CMC binders

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Conclusion

The electrochemical results revealed that, although the NMC electrodes blended with wt. 2.5% CMC-SBR binders presented higher discharge capacity at low rate (0.1 C) than the electrode using PvDF and LA133 binders, the NMC electrode blended with CMC binder presented better cycling performance and rate capability than those with LA133 and PVDF binders.

5. Acknowledgment

This work is supported by the Sakarya University Scientific Projects Coordinator (BAPK) under the “Sakarya University National Battery Development” project. The authors thank the Sakarya University BAPK workers for their kind support.

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