Accepted Manuscript Title: Novel Polymer Li-ion Binder Carboxymethyl Cellulose Derivative Enhanced Electrochemical Performance for Li-ion Batteries Author: Lei Qiu Ziqiang Shao Daxiong Wang Feijun Wang Wenjun Wang Jianquan Wang PII: DOI: Reference:
S0144-8617(14)00610-9 http://dx.doi.org/doi:10.1016/j.carbpol.2014.06.034 CARP 8999
To appear in: Received date: Revised date: Accepted date:
9-5-2014 11-6-2014 13-6-2014
Please cite this article as: Qiu, L., Wang, D., Wang, F., Wang, W., and Wang, J.,Novel Polymer Li-ion Binder Carboxymethyl Cellulose Derivative Enhanced Electrochemical Performance for Li-ion Batteries, Carbohydrate Polymers (2014), http://dx.doi.org/10.1016/j.carbpol.2014.06.034 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Highlights
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● Novel water-soluble binder CMC-Li is applied in lithium-ion battery for the first
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time. ● CMC-Li ionize and increase the contents of freely moving lithium ions in
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batteries.
● The good electrochemical property using CMC-Li with high DS as a binder.
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● Three type of binders CMC-Li, CMC-Na, and PVDF are investigated for LFP.
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● CMC-Li can be the next generation cheap and green water binder for LFP
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cathode.
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Novel Polymer Li-ion Binder Carboxymethyl Cellulose
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Derivative Enhanced Electrochemical Performance for Li-ion
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Batteries
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Lei Qiu, Ziqiang Shao, Daxiong Wang, Feijun Wang, Wenjun Wang, Jianquan Wang
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College of Material Science and Engineering, Beijing Institute of technology, Beijing
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Engineering Technology Research Centre for Cellulose and Its Derivative Materials,
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Beijing 100081, P. R. China
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Abstract:
Novel water-based binder lithium carboxymethyl cellulose (CMC-Li) is
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synthesized by cotton as raw material. The mechanism of the CMC-Li as a binder is
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reported. Electrochemical properties of batteries cathodes based on commercially
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available lithium iron phosphate (LiFePO4, LFP) and water-soluble binder are
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investigated. Sodium carboxymethyl cellulose (CMC-Na, CMC) and CMC-Li are
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used as the binder. After 200 cycles, compare with conventional poly (vinylidene
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spectroscopy test results show that the electrode using CMC-Li as the binder has
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lower charge transfer resistance than the electrodes using CMC and PVDF as the
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fluoride) (PVDF) binder, and the CMC-Li binder significantly improves cycling performance of the LFP cathode 96.7 % of initial reversible capacity achieve at 175 mAh g-1. Constant current charge-discharge test results demonstrate that the LFP electrode using CMC-Li as the binder has the highest rate capability, follow closely by those using CMC and PVDF binders, respectively. Electrochemical impedance
Corresponding authors at: College of Material Science and Engineering, Beijing Institute of technology, Beijing 100081, P. R. China. Tel.: +86 10-68940942; fax: +86 10-68941797. E-mail addresses: [email protected] (L.Qiu), [email protected] (Z. Shao) 2
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binders.
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Keywords: Lithium battery; Lithium iron phosphate; Water-based binder; Sodium
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carboxymethyl cellulose; Lithium carboxymethyl cellulose.
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1. Introduction
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The lithium-ion battery (LIB) is one of the most promising energy storage
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systems for mobile devices, portable computers, plug-in hybrid-electric vehicles, and
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hybrid-electric vehicles (HEVs) (Fei et al., 2013; Lu, Huang, Mou, Wang & Xu, 2010;
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Wang et al., 2014; Zhang, Uchaker, Candelaria & Cao, 2013). There have been
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extensive studies on electrode materials (Yang & Leung, 2013), electrolytes
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(Masquelier & Croguennec, 2013), additives (Seh, Zhang, Li, Zheng, Yao & Cui,
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2013), membranes (Zhao et al., 2014), and binders to attain improvement of the
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battery performance (Qiu et al., 2014b). Compared with the significant progress on
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most of these components for the LIB, however, recent works show that the choice of
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binder is very important to stabilize the cycling performance of electrodes for Li-ion
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batteries, which display large-volume changes (Li, Yang & Qin, 2013; Lu et al., 2014; Xun, Song, Battaglia & Liu, 2013). To make electrode sheets for lithium-ion (Li-ion) anodes or cathodes with appropriate mechanical properties, generally have to be blended with specific polymeric binders (Lestrie, Bahri, Sandu, Roue & Guyomard,
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2007; Li et al., 2011). Binders are electrochemically inactive materials which,
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however, can have an important influence on the electrode performance (Mancini,
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Nobili, Tossici & Marassi, 2012). In commercial Li-ion batteries, PVDF has been
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widely used as the binder for both the negative and the positive electrodes in current 3
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collectors and commercial LIBs, because of its good electrochemical stability and
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high binder to electrode materials (Lee & Oh, 2013; Xu, Chou, Gu, Liu & Dou, 2013).
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Nonetheless, PVDF binder is expensive, not easy to recycle, and involved the use of
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volatile organic compounds such as the environmentally harmful and toxic
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N-methyl-2-pyrrolidone (NMP) for its processing (Sethuraman et al., 2013).
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Moreover, accidents that caused by low energy efficiency of lithium battery and NMP
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were often reported. In order to improve these problems and make lithium battery
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more efficient, some scholars proposed the method of substituting water-based binder
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for PVDF in batteries. Nevertheless, the need for eco-friendliness, low cost, better
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electrode performance and solvent recycling has led to recent investigations aiming at
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switching from a non-aqueous-based to an aqueous-based system (Courtemanche,
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Pinter & Hof, 2011; Lacey, Jeschull, Edstrom & Brandell, 2013).
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CMC, which is produced from the insertion of carboxymethyl groups into natural
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cellulose, is the commonly used binder for anodes and cathodes of Li-ion batteries
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(Sivasankaran et al., 2011). CMC has a strong shear-thinning behavior that adjusts the slurry rheology(Qiu, Shao, Liu, Wang, Li & Zhao, 2014). CMC is a weak polyacid that dissociates to form carboxylate anionic functional groups. Because CMC is very stiff, having a small elongation at break (< 8%), it could not function as the
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elastomeric binder (Qiu et al., 2013; Wach, Kudoh, Zhai, Muroya & Katsumura,
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2005). Thus, the reasons why a brittle polymer like CMC can give good results for
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electrodes must be explored. Nevertheless, the need for providing the lithium ion,
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eco-friendliness, low cost, better electrode performance and solvent recycling has led 4
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to designing new binder as switching from a no-provide the lithium-ion to provide the
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lithium-ion (Qiu et al., 2014a). Novel water-based binder CMC-Li with a unique molecular structure is a
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polyhydroxy water soluble cellulose derivative polymer as CMC (Qiu, Shao, Wang,
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Zhang & Cao, 2013). When such a solution reaches a certain concentration, the
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adhesiveness of CMC-Li is strong enough to act as a binder. The studied show that
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except some advantages of other water-based binders, CMC-Li was also an effective
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ion-conductive polymer, and ionized to obtain lithium ions, which could increase the
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contents of freely moving lithium ions in lithium-ion batteries, shorten the diffusion
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pathway to the cathode particle surface (Augustyn et al., 2013; Gibot et al., 2008). It
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could also improve the efficiency of extraction and insertion of lithium ions between
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the cathode and anode electrode, and improving the charge and discharge capacity of
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the whole LIB. Meanwhile, excepting the advantages of CMC, this method, by
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substituting lithium for sodium, can avoid the decline of charge and discharge
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efficiency, as well as the cycle capacity performance of the battery, with CMC for a binder. Moreover, CMC-Li can prevent an exchange reaction between the Na-ion deposited on the carbon anode surface and the Li-ion, and the decomposition of the electrolyte solution. Since the conductive efficiency of CMC was weak, it was
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difficult for the lithium battery to exhibit its merits, such as small volume, light
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quality and extraordinary energy (Qiu, Shao, Liu, Wang, Li & Zhao, 2014; Qiu et al.,
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2014b). Therefore, a current development allows applying the CMC-Li binder directly
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to the lithium battery, with a very broad range of potential applications. 5
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In this investigation, water-soluble binder CMC-Li and CMC are applied to
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prepare the LFP cathode. Compared with PVDF, CMC-Li and CMC, which have two
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functional groups, carboxylate anion and hydroxyl, are well-known as an effective
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dispersion and thickener agent for aqueous suspension. CMC-Li as a binder greatly
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increased the actual specific capacity of batteries. The first charge and discharge
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specific capacity achieved a maximum value 183 mAh g-1. After 200 cycles, the
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maximum charge and discharge capacity loss were merely 3.31 %. It can conclude
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that CMC-Li as the binder increased the contents of lithium ions in the entire battery,
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improved the efficiency of extraction and insertion of lithium ions between cathode
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and anode electrode, as well as the diffusion coefficient. Furthermore, it can also
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enhance the charge and discharge platform of battery and conductive efficiency of
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ions, and reduce the degree of polarization between electrode materials as well as film
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impedance on the surface of electrode material. All the above excellent properties
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showed that CMC-Li as a new efficient binder can be applied in lithium batteries and
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further extended to preparation of other electrode materials. 2. Experimental
2.1 Powder preparations
The preparation process of CMC and CMC-Li can be clarified by the slurry
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process in a solution of isopropyl alcohol in our laboratory (Qiu, Shao, Wang, Zhang
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& Cao, 2013). CMC-Lis with the different degree of substitution (DS) were prepared
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by selecting the DS of CMC and the amount of LiOH.H2O. In this paper, CMC and
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CMC-Lis with two different DS values were prepared, respectively. CMC- Na-1 6
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(DS=0.68, Mw = 110,000 g/mol), CMC- Na-2 (DS=1.22, Mw = 370,000 g/mol),
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CMC-Li-1 (DS=0.65, Mw = 90,000 g/mol), CMC-Li-2 (DS=1.25, Mw = 350,000
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g/mol).
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2.2 Structural characterization and electrochemical measurements
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XRD diffraction experiments were performed with an Inel Cps 300 diffract meters using the Cu K radiation (-1.54056Å).
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Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS)
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results were obtained with electrochemical workstation CHI660E from Shanghai
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Chen Hua Instruments Co., Ltd. The coin cells (CR2025) were assembled to test the
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electrochemical performance of the as-prepared LFP: acetylene black: binder is 80: 10:
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10, using 1.0 M LiPF6 ethylene carbonate (EC): diethyl carbonate (DEC): dimethyl
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carbonate (DMC) =1:1:1 by volume as the electrolyte and a Li foil as the counter
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electrode. The cells were charged and discharged galvanostatically in the fixed
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potential window from 2.0 to 4.2 V on a Shenzhen Neware battery cycle (China) at 25
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C. The calculated capacity was based on the entire mass of the cathode electrodes.
EIS was measured by applying an alternating potential of 5 mV over the frequency ranging from 10−1 to 105 Hz after 5 cycles. CV was conducted in a cell at 0.1 mV s−1 and performed from 4.2 V to 2.0 V.
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3. Results and discussion
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3.1 Electrode characterization
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Fig. 1 XRD patterns for the electrode slices with CMC-Li as a binder (PDF#40-1499
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is standard XRD of LiFePO4, “♥” is standard XRD of Li3P (114) and “◆” is
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standard XRD of Li2O2 (114))
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XRD patterns obtain from the LFP electrode using CMC-Li as the binder (Fig.1).
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Typical LFP peaks are observed for these samples. In addition, CMC-Li as the binder
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samples presents typical diffraction peaks at 2 theta of about 65.7o, 78.6o,
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corresponding to the (114) planes of Li3P and Li2O2 crystals, respectively. All of these show that CMC-Li as a binder maybe increases the contents of lithium ions in the entire battery, and improves the effectiveness of extraction and insertion of lithium ions between Li-ions, thus, improving the diffusion efficiency and specific capacity. 3.2 Charge and discharge curves of LFP electrode using different binders.
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Fig. 2 (a) Initial charge and discharge curves of LFP electrode using different binders.
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(b) Specific capacity versus the cycle number for the LFP electrodes with CMC-Li,
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CMC and PVDF binder at 0.1C charge and discharge at room temperature.
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The first discharge specific capacity of battery with CMC-Li as the binder was
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higher than that with CMC and PVDF as the binder when electricity density was 0.2
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mA cm−2, respectively. LFP as cathode material, which prove that the utilization rate
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of LFP in battery with a water-soluble binder is increased (Fig. 2a). Moreover, the
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first charge specific capacity of battery with high DS of CMC-Li as the binder reaches
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181 mAh g-1, which increase by 19.3 % relative to the battery with PVDF as the binder. After 200 cycles, the specific capacity of all batteries with CMC-Li as the binder is higher than that with CMC and PVDF as the binder, respectively (Fig.2b). The capacity loss of the PVDF as a binder is up to 19.4 %, and the discharge capacity reaches 141 mAh g-1. However, the capacity loss of battery with CMC-Li-2 as the
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binder is merely 3.31 %, and discharge specific capacity reached 175 mAh g-1. All of
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these showed that CMC-Li as a water-based binder can significantly improve the
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performance of electrode material and reduce the degree of polarization, and it can
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also promote the efficiency and the diffusion rate of Li-ions (Armstrong, Lyness, 9
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Panchmatia, Islam & Bruce, 2011; Hu, Wu, Lin, Khlobystov & Li, 2013). Meanwhile,
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the study of the molecular structure of CMC-Li shows that the battery with high DS
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CMC-Li as a binder has the superior performance than that with low DS CMC-Li.
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3.3 Electrical and electrochemical properties
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Fig. 3 Nyquist plots of LFP electrodes using CMC-Li, CMC, and PVDF as the binder
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at the discharged state of 3.50 V (vs. Li/Lit) at frequencies from 100 KHz to 100 mHz.
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The insets show the equivalent circuit.
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To further investigate the electrode electrochemical impedance spectra (EIS), Fig.
3 shows the Nyquist plots of the electrodes with different binders in the discharged state of 3.50 V (vs. Li/Lit) at room temperatures after charge-discharge for 5 cycles. All the impedance curves of LFP with different binders show one semicircles in the
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medium frequency and the low frequency regions, which could be assigned to the
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lithium ion diffusion through the solid electrolyte interphase (SEI) film (Rg) and the
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charge transfer resistance (Rct), respectively, and an unclear ～110o inclined line in
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the low frequency range with CMC-Li as the binder, which could be considered to be 10
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a Warburg impedance (Zw). The Rct is calculated using the equivalent circuit shown
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in the inset of Fig. 3. The equivalent circuit model also includes electrolyte resistance
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(Rs), a constant phase element (CPE1) and a non-ideal constant phase element
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(CPE2). LFP electrode with CMC-Li binder had the smallest Rct values, below those
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of CMC binder and PVDF binder, respectively, indicating the enhancement of the
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kinetics and the consequent improvement in the rate capability of LFP using CMC-Li
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binder. These results indicate that the interfacial properties between LFP electrode
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and electrolyte are improved by using CMC-Li as the binder to increase the
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electrochemical reaction and dynamic performance. CMC-Li is played a positive role
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in the charge transfer process. For CMC-Li with different DS, the polar group
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numbers carry by unit mass of CMC-Li with high DS are greater than those carry by
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CMC-Li with low DS. The repellence of CMC-Li with high DS is better, and the LFP
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layer form on the active grain surface is more stable. The charge-transfer resistance of
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the cell using CMC-Li with high DS is obviously lower than that with low DS.
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Fig. 4 (a) CV curve of battery with pure CMC-Li as cathode material and binder. (b∼
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d) CV curves of the LFP cathodes at a scanning rate of 0.1mV s-1 using different
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binders: (b) CMC-Li, (c) PVDF, and (d) CMC. Different electrodes are different in the position of the oxidation reduction peak
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(Fig. 4). The CV curves of battery with CMC-Li as cathode material and binder show
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the weaker electricity, which proves that CMC-Li has some electrochemical
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performances and thus laying the foundation for applying it to batteries (Fig. 4a). In
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addition, the difference in voltage of the oxidation reduction peak of the electrode
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with water-based binder is lower than that with PVDF as binder, and the CV curves of
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electrodes with the water-based binder are sharper than those with PVDF binder. The
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distance between the discharge and charge curves of the CMC-Li as the binder are
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smaller than that of the CMC and PVDF as the binder, and minimum value reaches
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0.22 V (Fig.4b∼d), which show that the polarization of the former is weaker than the
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latter and increase the activity of electrochemical reaction. This may be explained by
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the fact that most of the active materials are coated with PVDF binder, causing poor ionic conductivity of PVDF, so that PVDF hinders the diffusion of lithium in the charge and discharge process of the battery, and reduces the electrochemical activity of electrode material. While the active material is coated with CMC-Li binder to form
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an effective ionic conductivity layer, which provides a diffusion pathway for Li+ to
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reach the active particle surface and enhance the activity of electrochemical reaction,
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reversibility of Li-ions and cycle performance.
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Fig. 5 Rate performance of LFP electrodes with different binders at 0.1C to 5C.
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It can be seen from Fig. 5 that the rate performance and cycle property of
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electrode material with CMC-Li-2 as the binder is excellent. At discharge rate of 0.2
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C, the discharge capacity is up to 171.5 mAh g-1. When discharge rate increases to 0.5
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C, and discharge capacity decreases to 158.2 mAh g-1. Even at a rate of 1C, the
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capacity is still up to 139.8 mAh g-1, which showed that the discharge capacity present
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linear variation with the increment of rate performance. This depends on the diffusion
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of Li-ions in material. It is because CMC-Li as the binder can be uniformly distributed in the surface of LFP material, the ionized Li-ions can short its transmission distance between cathode and anode electrode, and enhance the transmission efficiency and conductivity, which cause the material to have the
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relatively good rate performance and cycle property.
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3.4 SEM images of LFP cathode materials with different binders
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Fig. 6 shows that we successfully synthesize a new binder CMC-Li (Fig. 6a).
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Related product characterization is presented in our previously published articles (Qiu, 13
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Shao, Liu, Wang, Li & Zhao, 2014; Qiu et al., 2014a; Qiu et al., 2014b). SEM images
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of electrode prepare by different binder and LFP as cathode material after 200 cycles
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(Fig. 6b∼f). SEM photo of electrode slice of PVDF as binder (Fig. 6 b), as shows the
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image we can see that the particle of electrode material is big and distributed non
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uniformly, and the gathering phenomenon among particles is more serious. SEM
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images of the electrode slice of CMC as a binder (Fig. 6 c), and we can see that the
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particle is bigger and distribution is more uniform than that with PVDF as binder, and
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the immersion process of electrolyte is better. SEM images of electrode slice of
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CMC-Li as binder (Fig. 6d∼f). It presents that the particle is the smallest and no
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serious aggregation of particles, and the distribution is uniform and the immersion
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process of electrolyte is best. Moreover, there exist many “black holes” in the surface
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of electrode slice and the crystal structure in inside presents the regular change. “black
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hole” phenomenon may be the reason for improving the charge and discharge specific
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capacity of battery. The formation of the "black hole" maybe benefit the extraction
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and insertion efficiency of Li-ions between anode and cathode, as well as the extraction and insertion time of Li-ions to form fast lanes, thus increasing the ionic conductive efficiency and electronic conductive efficiency in order to improve the performance of the whole battery.
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Fig. 6. SEM images of LFP cathode materials with different binders at the end of the
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200th cycles charge-discharge. (a) Digital pictures of CMC-Li, (b) PVDF as binder, (c)
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CMC as binder, (d∼f) CMC-Li as binder.
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3.5 The investigation of the mechanism of the CMC-Li as the binder in LFP battery
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The preparation process of CMC-Li can be clarified by the following schematic equation:
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R
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R
R
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O
HO
R
R
R O
O
O
R
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266 267
O
H
+
OH R
R (n-2)
Cellulose (R=OH, Cell-(OH)3)
Cell-(OH)3
NaOH ClCH 2 COOH
Cell-CH2COONa(CMC-Na)
HCl
Cell-(OH)3-x(ONa)x-n(OCH2COO-Na+)n (Cell-CH2COONa)
Cell-CH2COOH(CMC-H)
LiOH
Cell-CH2COOLi(CMC-Li)
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HCl
Cell-(OH)3-x(ONa)x-n(OCH2COO-Na+)n (Cell-CH2COONa) Cell-(OH)3-x(OH)x-n(OCH2COO-H+)n (Cell-CH2COOH) Cell-(OH)3-x(OLi)x-n(OCH2COO-Li+)n (Cell-CH2COOLi)
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LiOH.H2O
CMC is prepared by the slurry process in a solution of isopropyl alcohol in our
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laboratory, after alkalization, etherification, neutralization, washing on cellulose, we
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can get the CMC for this article, and DS is measured by the method of Grey alkali.
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DS of CMC-Li and CMC-H depends on the DS of raw materials CMC.
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Fig. 7 Preliminary investigation of the mechanism of the CMC-Li binder in LFP battery.
Fig. 7 shows the distribution of CMC-Li in the LFP-based electrode. The
performance of CMC-Li can be ascribed to strong hydrogen bonds resulting from the action of the –OH groups, which contribute to form an efficient network. In addition,
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the hydrophilic CMC-Li binder is insoluble in organic electrolytes and does not swell
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in a cell. It has strong adhesion, thus maintaining the stability of the electrode
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structure (Ryou et al., 2013). Furthermore, CMC-Li is a good lithium-ion conductive
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binder as there are various functional groups on the macromolecular chain of CMC-Li. 16
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During the discharge process, the active substances react with lithium-ions on the
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molecular chain of CMC-Li adjacent to the active center of active substances (Liang,
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Zhang & Chen, 2013). The reactions between carboxymethyl and hydroxyl groups from the CMC-Li
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binder and the lithium-ions will form the coordination bonds (Qiu, Shao, Wang,
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Zhang & Cao, 2013). Under an electric field force, lithium-ions can be transmitted
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along the same molecular chain or adjacent molecular chains, without destroying the
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structure
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charge-discharge of lithium-ion batteries, and lithium-ions will be combined with the
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LFP particles and the carboxymethyl and hydroxyl groups from the CMC-Li. So the
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CMC-Li binder raises the transport efficiency of lithium-ions, the utilization rate and
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discharge capacity of LFP (Armand et al., 2009; Young, Ransil, Amin, Li & Chiang,
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2013). In addition, the -CH2COOH and - OH groups of CMC-Li can easily react with
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anode lithium metal and form the -CH2COOLi and -OLi, respectively. This reaction is
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chain. Eventually,
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processing on
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irreversible so as to reduce the initial coulomb efficiency. Therefore, the initial discharge capacity of LFP electrode with CMC-Li for binder is higher than that with CMC and PVDF as the binder. Moreover, the increase in content of -CH2COOLi equates to improvement of the DS of CMC-Li (Gibot et al., 2008). Adhesive coating
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layer form on the surface will be more stable. In addition, the increase in the
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carboxymethyl anion also contributes to the transport of Li+.
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4. Conclusion
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In summary, we are using water-soluble CMC-Li, CMC instead of traditional 17
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PVDF as a binder for the LFP cathode. The CMC-Li cathode exhibits a reversible
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capacity of 175 mAh g-1 after 200 cycles, the specific capacity of the battery is still
307
higher than the theoretical specific capacity of LFP and the loss is very little, showing
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remarkably improved cyclability as compared with the traditional PVDF cathode. The
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battery delivers high capacity which consists with the results of EIS measurement
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shows a low internal resistance and low charge transfer impedance for the CMC-Li
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cathode, due to an efficient electronic conductivity. It indicates that the CMC-Li
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binder can provide an effective electronically conductive network and a stable
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interface structure for the cathode. Our results clearly show that the cheap and
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environmentally friendly CMC-Li as a highly efficient binder for the LFP cathode can
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improve the electrochemical performance. The present findings open new prospects
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for the electrode performance optimization via better understanding the role of the
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binder.
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Acknowledgement
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Financial support from the special programs of graduate students' scientific and
technological
innovation
activities
of
Beijing
Institute
of
Technology
(3090012241302) and Youth Science and Technology Innovation Projects of North Chemical Industry CO., LTD (QKCZ-BIT-04).
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Lu, X. J., Huang, F. Q., Mou, X. L., Wang, Y. M., & Xu, F. F. (2010). A General Preparation Strategy for Hybrid TiO2 Hierarchical Spheres and Their Enhanced Solar Energy Utilization Efficiency. Advanced Materials, 22(33), 3719-+. Mancini, M., Nobili, F., Tossici, R., & Marassi, R. (2012). Study of the electrochemical behavior at low temperatures of green anodes for Lithium ion batteries prepared with anatase TiO2 and water soluble sodium carboxymethyl cellulose binder. Electrochimica Acta, 85, 566-571. Masquelier, C., & Croguennec, L. (2013). Polyanionic (Phosphates, Silicates, Sulfates) Frameworks as Electrode Materials for Rechargeable Li (or Na) Batteries. Chemical Reviews, 113(8), 6552-6591.
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Qiu, L., Shao, Z., Liu, M., Wang, J., Li, P., & Zhao, M. (2014). Synthesis and Electrospinning Carboxymethyl Cellulose Lithium (CMC-Li) modified 9, 10-anthraquinone (AQ) High-Rate Lithium-Ion Battery. Carbohydrate Polymers, 102, 986-992. Qiu, L., Shao, Z., Wang, J., Zhang, D., & Cao, J. (2013). Study on Synthesis, Rheological and Electrospinning Functional Materials of Carboxymethyl Cellulose Lithium (CMC-Li). Acta Chimica Sinica, 71(11), 1521-1526.
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cathodes through strong affinity with a bifunctional binder. Chemical Science, 4(9), 3673-3677. Sethuraman, V. A., Nguyen, A., Chon, M. J., Nadimpalli, S. P. V., Wang, H., Abraham, D. P., Bower, A. F., Shenoy, V. B., & Guduru, P. R. (2013). Stress Evolution in Composite Silicon Electrodes
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Wang, W., Ruiz, I., Guo, S., Favors, Z., Bay, H. H., Ozkan, M., & Ozkan, C. S. (2014). Hybrid carbon nanotube and graphene nanostructures for lithium ion battery anodes. Nano Energy, 3, 113-118.
Xu, J. T., Chou, S. L., Gu, Q. F., Liu, H. K., & Dou, S. X. (2013). The effect of different binders on electrochemical properties of LiNi1/3Mn1/3C1/3O2 cathode material in lithium ion batteries. Journal of Power Sources, 225, 172-178.
Xun, S., Song, X., Battaglia, V., & Liu, G. (2013). Conductive Polymer Binder-Enabled Cycling of Pure Tin Nanoparticle Composite Anode Electrodes for a Lithium-Ion Battery. Journal of the Electrochemical Society, 160(6), A849-A855. Yang, L., & Leung, W. W.-F. (2013). Electrospun TiO2 Nanorods with Carbon Nanotubes for Efficient Electron Collection in Dye-Sensitized Solar Cells. Advanced Materials, 25(12), 1792-1795. Young, D., Ransil, A., Amin, R., Li, Z., & Chiang, Y.-M. (2013). Electronic Conductivity in the Li4/3Ti5/3O4-Li7/3Ti5/3O4 System and Variation with State-of-Charge as a Li Battery Anode. Advanced Energy Materials, 3(9), 1125-1129.
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Zhang, Q., Uchaker, E., Candelaria, S. L., & Cao, G. (2013). Nanomaterials for energy conversion and storage. Chemical Society Reviews, 42(7), 3127-3171. Zhao, C., Liu, L., Zhao, H., Krall, A., Wen, Z., Chen, J., Hurley, P., Jiang, J., & Li, Y. (2014). Sulfur-infiltrated porous carbon microspheres with controllable multi-modal pore size distribution for high energy lithium-sulfur batteries. Nanoscale, 6(2), 882-888.
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Figure captions
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Fig. 1. XRD patterns for the electrode slices with CMC-Li as a binder (PDF#40-1499
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is standard XRD of LiFePO4, “♥” is standard XRD of Li3P (114) and “◆” is
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standard XRD of Li2O2 (114)) It shows that CMC-Li as a binder maybe
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increases the contents of lithium ions in the entire battery.
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Fig. 2. (a) Initial charge and discharge curves of LFP electrode using different binders.
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(b) Specific capacity versus the cycle number for the LFP electrodes with
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CMC-Li, CMC and PVDF binder at 0.1C charge and discharge at room
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temperature. CMC-Li as a water-based binder can significantly improve the
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performance of electrode material and reduce the degree of polarization, and it
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can also promote the efficiency and the diffusion rate of Li-ions.
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Fig. 3. Nyquist plots of LFP electrodes using CMC-Li, CMC, and PVDF as the binder
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at the discharged state of 3.50 V (vs. Li/Lit) at frequencies from 100 KHz to
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100 mHz.
The insets show the equivalent circuit.
The interfacial properties between LFP electrode and electrolyte are improved by using CMC-Li as the binder to increase the electrochemical reaction and dynamic performance.
Fig.4. (a) CV curves of battery with pure CMC-Li as cathode material and binder.
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(b∼d) CV curves of the LFP cathodes at a scanning rate of 0.1mV s-1 using
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different binders: (b) CMC-Li, (c) PVDF, and (d) CMC.
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While the active material is coated with CMC-Li binder to form an effective
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ionic conductivity layer, which provides a diffusion pathway for Li+ to reach the 22
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active particle surface and enhance the activity of electrochemical reaction,
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reversibility of li-ions and cycle performance.
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Fig. 5. Rate performance of LFP electrodes with different binders at 0.1C to 5C. It is because CMC-Li as the binder can be uniformly distributed in the
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surface of LFP material, the ionized li-ions can short its transmission distance
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between cathode and anode electrode, and enhance the transmission efficiency
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and conductivity, which cause the material to have the relatively good rate
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performance and cycle property.
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Fig.6. SEM images of LFP cathode materials with different binders at the end of the
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200th cycles charge-discharge. (a) Digital pictures of CMC-Li, (b) PVDF as
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binder, (c) CMC as binder, (d～f) CMC-Li as binder.
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“Black hole” phenomenon may be the reason for improving the charge and discharge specific capacity of battery. Fig.7. Preliminary investigation of the mechanism of the CMC-Li binder in LFP
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battery. Active substances react with lithium-ions on the molecular chain of CMC-Li adjacent to the active center of active substances.
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S0144-8617(14)00610-9 http://dx.doi.org/doi:10.1016/j.carbpol.2014.06.034 CARP 8999
To appear in: Received date: Revised date: Accepted date:
9-5-2014 11-6-2014 13-6-2014
Please cite this article as: Qiu, L., Wang, D., Wang, F., Wang, W., and Wang, J.,Novel Polymer Li-ion Binder Carboxymethyl Cellulose Derivative Enhanced Electrochemical Performance for Li-ion Batteries, Carbohydrate Polymers (2014), http://dx.doi.org/10.1016/j.carbpol.2014.06.034 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Highlights
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● Novel water-soluble binder CMC-Li is applied in lithium-ion battery for the first
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time. ● CMC-Li ionize and increase the contents of freely moving lithium ions in
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batteries.
● The good electrochemical property using CMC-Li with high DS as a binder.
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● Three type of binders CMC-Li, CMC-Na, and PVDF are investigated for LFP.
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● CMC-Li can be the next generation cheap and green water binder for LFP
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cathode.
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Novel Polymer Li-ion Binder Carboxymethyl Cellulose
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Derivative Enhanced Electrochemical Performance for Li-ion
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Batteries
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Lei Qiu, Ziqiang Shao, Daxiong Wang, Feijun Wang, Wenjun Wang, Jianquan Wang
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College of Material Science and Engineering, Beijing Institute of technology, Beijing
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Engineering Technology Research Centre for Cellulose and Its Derivative Materials,
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Beijing 100081, P. R. China
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Abstract:
Novel water-based binder lithium carboxymethyl cellulose (CMC-Li) is
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synthesized by cotton as raw material. The mechanism of the CMC-Li as a binder is
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reported. Electrochemical properties of batteries cathodes based on commercially
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available lithium iron phosphate (LiFePO4, LFP) and water-soluble binder are
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investigated. Sodium carboxymethyl cellulose (CMC-Na, CMC) and CMC-Li are
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used as the binder. After 200 cycles, compare with conventional poly (vinylidene
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spectroscopy test results show that the electrode using CMC-Li as the binder has
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lower charge transfer resistance than the electrodes using CMC and PVDF as the
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fluoride) (PVDF) binder, and the CMC-Li binder significantly improves cycling performance of the LFP cathode 96.7 % of initial reversible capacity achieve at 175 mAh g-1. Constant current charge-discharge test results demonstrate that the LFP electrode using CMC-Li as the binder has the highest rate capability, follow closely by those using CMC and PVDF binders, respectively. Electrochemical impedance
Corresponding authors at: College of Material Science and Engineering, Beijing Institute of technology, Beijing 100081, P. R. China. Tel.: +86 10-68940942; fax: +86 10-68941797. E-mail addresses: [email protected] (L.Qiu), [email protected] (Z. Shao) 2
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binders.
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Keywords: Lithium battery; Lithium iron phosphate; Water-based binder; Sodium
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carboxymethyl cellulose; Lithium carboxymethyl cellulose.
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1. Introduction
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The lithium-ion battery (LIB) is one of the most promising energy storage
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systems for mobile devices, portable computers, plug-in hybrid-electric vehicles, and
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hybrid-electric vehicles (HEVs) (Fei et al., 2013; Lu, Huang, Mou, Wang & Xu, 2010;
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Wang et al., 2014; Zhang, Uchaker, Candelaria & Cao, 2013). There have been
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extensive studies on electrode materials (Yang & Leung, 2013), electrolytes
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(Masquelier & Croguennec, 2013), additives (Seh, Zhang, Li, Zheng, Yao & Cui,
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2013), membranes (Zhao et al., 2014), and binders to attain improvement of the
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battery performance (Qiu et al., 2014b). Compared with the significant progress on
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most of these components for the LIB, however, recent works show that the choice of
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binder is very important to stabilize the cycling performance of electrodes for Li-ion
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batteries, which display large-volume changes (Li, Yang & Qin, 2013; Lu et al., 2014; Xun, Song, Battaglia & Liu, 2013). To make electrode sheets for lithium-ion (Li-ion) anodes or cathodes with appropriate mechanical properties, generally have to be blended with specific polymeric binders (Lestrie, Bahri, Sandu, Roue & Guyomard,
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2007; Li et al., 2011). Binders are electrochemically inactive materials which,
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however, can have an important influence on the electrode performance (Mancini,
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Nobili, Tossici & Marassi, 2012). In commercial Li-ion batteries, PVDF has been
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widely used as the binder for both the negative and the positive electrodes in current 3
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collectors and commercial LIBs, because of its good electrochemical stability and
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high binder to electrode materials (Lee & Oh, 2013; Xu, Chou, Gu, Liu & Dou, 2013).
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Nonetheless, PVDF binder is expensive, not easy to recycle, and involved the use of
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volatile organic compounds such as the environmentally harmful and toxic
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N-methyl-2-pyrrolidone (NMP) for its processing (Sethuraman et al., 2013).
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Moreover, accidents that caused by low energy efficiency of lithium battery and NMP
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were often reported. In order to improve these problems and make lithium battery
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more efficient, some scholars proposed the method of substituting water-based binder
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for PVDF in batteries. Nevertheless, the need for eco-friendliness, low cost, better
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electrode performance and solvent recycling has led to recent investigations aiming at
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switching from a non-aqueous-based to an aqueous-based system (Courtemanche,
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Pinter & Hof, 2011; Lacey, Jeschull, Edstrom & Brandell, 2013).
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CMC, which is produced from the insertion of carboxymethyl groups into natural
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cellulose, is the commonly used binder for anodes and cathodes of Li-ion batteries
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(Sivasankaran et al., 2011). CMC has a strong shear-thinning behavior that adjusts the slurry rheology(Qiu, Shao, Liu, Wang, Li & Zhao, 2014). CMC is a weak polyacid that dissociates to form carboxylate anionic functional groups. Because CMC is very stiff, having a small elongation at break (< 8%), it could not function as the
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elastomeric binder (Qiu et al., 2013; Wach, Kudoh, Zhai, Muroya & Katsumura,
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2005). Thus, the reasons why a brittle polymer like CMC can give good results for
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electrodes must be explored. Nevertheless, the need for providing the lithium ion,
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eco-friendliness, low cost, better electrode performance and solvent recycling has led 4
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to designing new binder as switching from a no-provide the lithium-ion to provide the
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lithium-ion (Qiu et al., 2014a). Novel water-based binder CMC-Li with a unique molecular structure is a
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polyhydroxy water soluble cellulose derivative polymer as CMC (Qiu, Shao, Wang,
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Zhang & Cao, 2013). When such a solution reaches a certain concentration, the
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adhesiveness of CMC-Li is strong enough to act as a binder. The studied show that
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except some advantages of other water-based binders, CMC-Li was also an effective
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ion-conductive polymer, and ionized to obtain lithium ions, which could increase the
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contents of freely moving lithium ions in lithium-ion batteries, shorten the diffusion
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pathway to the cathode particle surface (Augustyn et al., 2013; Gibot et al., 2008). It
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could also improve the efficiency of extraction and insertion of lithium ions between
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the cathode and anode electrode, and improving the charge and discharge capacity of
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the whole LIB. Meanwhile, excepting the advantages of CMC, this method, by
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substituting lithium for sodium, can avoid the decline of charge and discharge
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efficiency, as well as the cycle capacity performance of the battery, with CMC for a binder. Moreover, CMC-Li can prevent an exchange reaction between the Na-ion deposited on the carbon anode surface and the Li-ion, and the decomposition of the electrolyte solution. Since the conductive efficiency of CMC was weak, it was
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difficult for the lithium battery to exhibit its merits, such as small volume, light
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quality and extraordinary energy (Qiu, Shao, Liu, Wang, Li & Zhao, 2014; Qiu et al.,
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2014b). Therefore, a current development allows applying the CMC-Li binder directly
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to the lithium battery, with a very broad range of potential applications. 5
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In this investigation, water-soluble binder CMC-Li and CMC are applied to
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prepare the LFP cathode. Compared with PVDF, CMC-Li and CMC, which have two
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functional groups, carboxylate anion and hydroxyl, are well-known as an effective
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dispersion and thickener agent for aqueous suspension. CMC-Li as a binder greatly
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increased the actual specific capacity of batteries. The first charge and discharge
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specific capacity achieved a maximum value 183 mAh g-1. After 200 cycles, the
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maximum charge and discharge capacity loss were merely 3.31 %. It can conclude
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that CMC-Li as the binder increased the contents of lithium ions in the entire battery,
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improved the efficiency of extraction and insertion of lithium ions between cathode
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and anode electrode, as well as the diffusion coefficient. Furthermore, it can also
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enhance the charge and discharge platform of battery and conductive efficiency of
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ions, and reduce the degree of polarization between electrode materials as well as film
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impedance on the surface of electrode material. All the above excellent properties
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showed that CMC-Li as a new efficient binder can be applied in lithium batteries and
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further extended to preparation of other electrode materials. 2. Experimental
2.1 Powder preparations
The preparation process of CMC and CMC-Li can be clarified by the slurry
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process in a solution of isopropyl alcohol in our laboratory (Qiu, Shao, Wang, Zhang
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& Cao, 2013). CMC-Lis with the different degree of substitution (DS) were prepared
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by selecting the DS of CMC and the amount of LiOH.H2O. In this paper, CMC and
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CMC-Lis with two different DS values were prepared, respectively. CMC- Na-1 6
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(DS=0.68, Mw = 110,000 g/mol), CMC- Na-2 (DS=1.22, Mw = 370,000 g/mol),
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CMC-Li-1 (DS=0.65, Mw = 90,000 g/mol), CMC-Li-2 (DS=1.25, Mw = 350,000
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g/mol).
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2.2 Structural characterization and electrochemical measurements
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XRD diffraction experiments were performed with an Inel Cps 300 diffract meters using the Cu K radiation (-1.54056Å).
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Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS)
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results were obtained with electrochemical workstation CHI660E from Shanghai
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Chen Hua Instruments Co., Ltd. The coin cells (CR2025) were assembled to test the
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electrochemical performance of the as-prepared LFP: acetylene black: binder is 80: 10:
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10, using 1.0 M LiPF6 ethylene carbonate (EC): diethyl carbonate (DEC): dimethyl
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carbonate (DMC) =1:1:1 by volume as the electrolyte and a Li foil as the counter
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electrode. The cells were charged and discharged galvanostatically in the fixed
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potential window from 2.0 to 4.2 V on a Shenzhen Neware battery cycle (China) at 25
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C. The calculated capacity was based on the entire mass of the cathode electrodes.
EIS was measured by applying an alternating potential of 5 mV over the frequency ranging from 10−1 to 105 Hz after 5 cycles. CV was conducted in a cell at 0.1 mV s−1 and performed from 4.2 V to 2.0 V.
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3. Results and discussion
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3.1 Electrode characterization
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Fig. 1 XRD patterns for the electrode slices with CMC-Li as a binder (PDF#40-1499
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is standard XRD of LiFePO4, “♥” is standard XRD of Li3P (114) and “◆” is
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standard XRD of Li2O2 (114))
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XRD patterns obtain from the LFP electrode using CMC-Li as the binder (Fig.1).
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Typical LFP peaks are observed for these samples. In addition, CMC-Li as the binder
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samples presents typical diffraction peaks at 2 theta of about 65.7o, 78.6o,
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corresponding to the (114) planes of Li3P and Li2O2 crystals, respectively. All of these show that CMC-Li as a binder maybe increases the contents of lithium ions in the entire battery, and improves the effectiveness of extraction and insertion of lithium ions between Li-ions, thus, improving the diffusion efficiency and specific capacity. 3.2 Charge and discharge curves of LFP electrode using different binders.
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Fig. 2 (a) Initial charge and discharge curves of LFP electrode using different binders.
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(b) Specific capacity versus the cycle number for the LFP electrodes with CMC-Li,
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CMC and PVDF binder at 0.1C charge and discharge at room temperature.
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The first discharge specific capacity of battery with CMC-Li as the binder was
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higher than that with CMC and PVDF as the binder when electricity density was 0.2
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mA cm−2, respectively. LFP as cathode material, which prove that the utilization rate
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of LFP in battery with a water-soluble binder is increased (Fig. 2a). Moreover, the
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first charge specific capacity of battery with high DS of CMC-Li as the binder reaches
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181 mAh g-1, which increase by 19.3 % relative to the battery with PVDF as the binder. After 200 cycles, the specific capacity of all batteries with CMC-Li as the binder is higher than that with CMC and PVDF as the binder, respectively (Fig.2b). The capacity loss of the PVDF as a binder is up to 19.4 %, and the discharge capacity reaches 141 mAh g-1. However, the capacity loss of battery with CMC-Li-2 as the
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binder is merely 3.31 %, and discharge specific capacity reached 175 mAh g-1. All of
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these showed that CMC-Li as a water-based binder can significantly improve the
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performance of electrode material and reduce the degree of polarization, and it can
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also promote the efficiency and the diffusion rate of Li-ions (Armstrong, Lyness, 9
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Panchmatia, Islam & Bruce, 2011; Hu, Wu, Lin, Khlobystov & Li, 2013). Meanwhile,
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the study of the molecular structure of CMC-Li shows that the battery with high DS
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CMC-Li as a binder has the superior performance than that with low DS CMC-Li.
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3.3 Electrical and electrochemical properties
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Fig. 3 Nyquist plots of LFP electrodes using CMC-Li, CMC, and PVDF as the binder
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at the discharged state of 3.50 V (vs. Li/Lit) at frequencies from 100 KHz to 100 mHz.
177
The insets show the equivalent circuit.
179 180 181
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d
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To further investigate the electrode electrochemical impedance spectra (EIS), Fig.
3 shows the Nyquist plots of the electrodes with different binders in the discharged state of 3.50 V (vs. Li/Lit) at room temperatures after charge-discharge for 5 cycles. All the impedance curves of LFP with different binders show one semicircles in the
182
medium frequency and the low frequency regions, which could be assigned to the
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lithium ion diffusion through the solid electrolyte interphase (SEI) film (Rg) and the
184
charge transfer resistance (Rct), respectively, and an unclear ～110o inclined line in
185
the low frequency range with CMC-Li as the binder, which could be considered to be 10
Page 10 of 23
a Warburg impedance (Zw). The Rct is calculated using the equivalent circuit shown
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in the inset of Fig. 3. The equivalent circuit model also includes electrolyte resistance
188
(Rs), a constant phase element (CPE1) and a non-ideal constant phase element
189
(CPE2). LFP electrode with CMC-Li binder had the smallest Rct values, below those
190
of CMC binder and PVDF binder, respectively, indicating the enhancement of the
191
kinetics and the consequent improvement in the rate capability of LFP using CMC-Li
192
binder. These results indicate that the interfacial properties between LFP electrode
193
and electrolyte are improved by using CMC-Li as the binder to increase the
194
electrochemical reaction and dynamic performance. CMC-Li is played a positive role
195
in the charge transfer process. For CMC-Li with different DS, the polar group
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numbers carry by unit mass of CMC-Li with high DS are greater than those carry by
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CMC-Li with low DS. The repellence of CMC-Li with high DS is better, and the LFP
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layer form on the active grain surface is more stable. The charge-transfer resistance of
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the cell using CMC-Li with high DS is obviously lower than that with low DS.
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Fig. 4 (a) CV curve of battery with pure CMC-Li as cathode material and binder. (b∼
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d) CV curves of the LFP cathodes at a scanning rate of 0.1mV s-1 using different
203
binders: (b) CMC-Li, (c) PVDF, and (d) CMC. Different electrodes are different in the position of the oxidation reduction peak
205
(Fig. 4). The CV curves of battery with CMC-Li as cathode material and binder show
206
the weaker electricity, which proves that CMC-Li has some electrochemical
207
performances and thus laying the foundation for applying it to batteries (Fig. 4a). In
208
addition, the difference in voltage of the oxidation reduction peak of the electrode
209
with water-based binder is lower than that with PVDF as binder, and the CV curves of
210
electrodes with the water-based binder are sharper than those with PVDF binder. The
211
distance between the discharge and charge curves of the CMC-Li as the binder are
212
smaller than that of the CMC and PVDF as the binder, and minimum value reaches
213
0.22 V (Fig.4b∼d), which show that the polarization of the former is weaker than the
214
latter and increase the activity of electrochemical reaction. This may be explained by
216 217 218
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the fact that most of the active materials are coated with PVDF binder, causing poor ionic conductivity of PVDF, so that PVDF hinders the diffusion of lithium in the charge and discharge process of the battery, and reduces the electrochemical activity of electrode material. While the active material is coated with CMC-Li binder to form
219
an effective ionic conductivity layer, which provides a diffusion pathway for Li+ to
220
reach the active particle surface and enhance the activity of electrochemical reaction,
221
reversibility of Li-ions and cycle performance.
12
Page 12 of 23
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Fig. 5 Rate performance of LFP electrodes with different binders at 0.1C to 5C.
an
223
It can be seen from Fig. 5 that the rate performance and cycle property of
225
electrode material with CMC-Li-2 as the binder is excellent. At discharge rate of 0.2
226
C, the discharge capacity is up to 171.5 mAh g-1. When discharge rate increases to 0.5
227
C, and discharge capacity decreases to 158.2 mAh g-1. Even at a rate of 1C, the
228
capacity is still up to 139.8 mAh g-1, which showed that the discharge capacity present
229
linear variation with the increment of rate performance. This depends on the diffusion
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of Li-ions in material. It is because CMC-Li as the binder can be uniformly distributed in the surface of LFP material, the ionized Li-ions can short its transmission distance between cathode and anode electrode, and enhance the transmission efficiency and conductivity, which cause the material to have the
234
relatively good rate performance and cycle property.
235
3.4 SEM images of LFP cathode materials with different binders
236
Fig. 6 shows that we successfully synthesize a new binder CMC-Li (Fig. 6a).
237
Related product characterization is presented in our previously published articles (Qiu, 13
Page 13 of 23
Shao, Liu, Wang, Li & Zhao, 2014; Qiu et al., 2014a; Qiu et al., 2014b). SEM images
239
of electrode prepare by different binder and LFP as cathode material after 200 cycles
240
(Fig. 6b∼f). SEM photo of electrode slice of PVDF as binder (Fig. 6 b), as shows the
241
image we can see that the particle of electrode material is big and distributed non
242
uniformly, and the gathering phenomenon among particles is more serious. SEM
243
images of the electrode slice of CMC as a binder (Fig. 6 c), and we can see that the
244
particle is bigger and distribution is more uniform than that with PVDF as binder, and
245
the immersion process of electrolyte is better. SEM images of electrode slice of
246
CMC-Li as binder (Fig. 6d∼f). It presents that the particle is the smallest and no
247
serious aggregation of particles, and the distribution is uniform and the immersion
248
process of electrolyte is best. Moreover, there exist many “black holes” in the surface
249
of electrode slice and the crystal structure in inside presents the regular change. “black
250
hole” phenomenon may be the reason for improving the charge and discharge specific
251
capacity of battery. The formation of the "black hole" maybe benefit the extraction
253 254 255
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and insertion efficiency of Li-ions between anode and cathode, as well as the extraction and insertion time of Li-ions to form fast lanes, thus increasing the ionic conductive efficiency and electronic conductive efficiency in order to improve the performance of the whole battery.
14
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Fig. 6. SEM images of LFP cathode materials with different binders at the end of the
258
200th cycles charge-discharge. (a) Digital pictures of CMC-Li, (b) PVDF as binder, (c)
259
CMC as binder, (d∼f) CMC-Li as binder.
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3.5 The investigation of the mechanism of the CMC-Li as the binder in LFP battery
261
The preparation process of CMC-Li can be clarified by the following schematic equation:
d
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R
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263
R
R
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O
HO
R
R
R O
O
O
R
264 265
266 267
O
H
+
OH R
R (n-2)
Cellulose (R=OH, Cell-(OH)3)
Cell-(OH)3
NaOH ClCH 2 COOH
Cell-CH2COONa(CMC-Na)
HCl
Cell-(OH)3-x(ONa)x-n(OCH2COO-Na+)n (Cell-CH2COONa)
Cell-CH2COOH(CMC-H)
LiOH
Cell-CH2COOLi(CMC-Li)
15
Page 15 of 23
HCl
Cell-(OH)3-x(ONa)x-n(OCH2COO-Na+)n (Cell-CH2COONa) Cell-(OH)3-x(OH)x-n(OCH2COO-H+)n (Cell-CH2COOH) Cell-(OH)3-x(OLi)x-n(OCH2COO-Li+)n (Cell-CH2COOLi)
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268
LiOH.H2O
CMC is prepared by the slurry process in a solution of isopropyl alcohol in our
270
laboratory, after alkalization, etherification, neutralization, washing on cellulose, we
271
can get the CMC for this article, and DS is measured by the method of Grey alkali.
272
DS of CMC-Li and CMC-H depends on the DS of raw materials CMC.
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cr
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274 275 276 277 278
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Fig. 7 Preliminary investigation of the mechanism of the CMC-Li binder in LFP battery.
Fig. 7 shows the distribution of CMC-Li in the LFP-based electrode. The
performance of CMC-Li can be ascribed to strong hydrogen bonds resulting from the action of the –OH groups, which contribute to form an efficient network. In addition,
279
the hydrophilic CMC-Li binder is insoluble in organic electrolytes and does not swell
280
in a cell. It has strong adhesion, thus maintaining the stability of the electrode
281
structure (Ryou et al., 2013). Furthermore, CMC-Li is a good lithium-ion conductive
282
binder as there are various functional groups on the macromolecular chain of CMC-Li. 16
Page 16 of 23
283
During the discharge process, the active substances react with lithium-ions on the
284
molecular chain of CMC-Li adjacent to the active center of active substances (Liang,
285
Zhang & Chen, 2013). The reactions between carboxymethyl and hydroxyl groups from the CMC-Li
287
binder and the lithium-ions will form the coordination bonds (Qiu, Shao, Wang,
288
Zhang & Cao, 2013). Under an electric field force, lithium-ions can be transmitted
289
along the same molecular chain or adjacent molecular chains, without destroying the
290
structure
291
charge-discharge of lithium-ion batteries, and lithium-ions will be combined with the
292
LFP particles and the carboxymethyl and hydroxyl groups from the CMC-Li. So the
293
CMC-Li binder raises the transport efficiency of lithium-ions, the utilization rate and
294
discharge capacity of LFP (Armand et al., 2009; Young, Ransil, Amin, Li & Chiang,
295
2013). In addition, the -CH2COOH and - OH groups of CMC-Li can easily react with
296
anode lithium metal and form the -CH2COOLi and -OLi, respectively. This reaction is
298 299 300
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chain. Eventually,
us
molecular
during the
processing on
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of the
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irreversible so as to reduce the initial coulomb efficiency. Therefore, the initial discharge capacity of LFP electrode with CMC-Li for binder is higher than that with CMC and PVDF as the binder. Moreover, the increase in content of -CH2COOLi equates to improvement of the DS of CMC-Li (Gibot et al., 2008). Adhesive coating
301
layer form on the surface will be more stable. In addition, the increase in the
302
carboxymethyl anion also contributes to the transport of Li+.
303
4. Conclusion
304
In summary, we are using water-soluble CMC-Li, CMC instead of traditional 17
Page 17 of 23
PVDF as a binder for the LFP cathode. The CMC-Li cathode exhibits a reversible
306
capacity of 175 mAh g-1 after 200 cycles, the specific capacity of the battery is still
307
higher than the theoretical specific capacity of LFP and the loss is very little, showing
308
remarkably improved cyclability as compared with the traditional PVDF cathode. The
309
battery delivers high capacity which consists with the results of EIS measurement
310
shows a low internal resistance and low charge transfer impedance for the CMC-Li
311
cathode, due to an efficient electronic conductivity. It indicates that the CMC-Li
312
binder can provide an effective electronically conductive network and a stable
313
interface structure for the cathode. Our results clearly show that the cheap and
314
environmentally friendly CMC-Li as a highly efficient binder for the LFP cathode can
315
improve the electrochemical performance. The present findings open new prospects
316
for the electrode performance optimization via better understanding the role of the
317
binder.
318
Acknowledgement
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Financial support from the special programs of graduate students' scientific and
technological
innovation
activities
of
Beijing
Institute
of
Technology
(3090012241302) and Youth Science and Technology Innovation Projects of North Chemical Industry CO., LTD (QKCZ-BIT-04).
323
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Figure captions
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Fig. 1. XRD patterns for the electrode slices with CMC-Li as a binder (PDF#40-1499
422
is standard XRD of LiFePO4, “♥” is standard XRD of Li3P (114) and “◆” is
423
standard XRD of Li2O2 (114)) It shows that CMC-Li as a binder maybe
424
increases the contents of lithium ions in the entire battery.
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Fig. 2. (a) Initial charge and discharge curves of LFP electrode using different binders.
426
(b) Specific capacity versus the cycle number for the LFP electrodes with
427
CMC-Li, CMC and PVDF binder at 0.1C charge and discharge at room
428
temperature. CMC-Li as a water-based binder can significantly improve the
429
performance of electrode material and reduce the degree of polarization, and it
430
can also promote the efficiency and the diffusion rate of Li-ions.
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Fig. 3. Nyquist plots of LFP electrodes using CMC-Li, CMC, and PVDF as the binder
432
at the discharged state of 3.50 V (vs. Li/Lit) at frequencies from 100 KHz to
434 435 436 437
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433
te
431
100 mHz.
The insets show the equivalent circuit.
The interfacial properties between LFP electrode and electrolyte are improved by using CMC-Li as the binder to increase the electrochemical reaction and dynamic performance.
Fig.4. (a) CV curves of battery with pure CMC-Li as cathode material and binder.
438
(b∼d) CV curves of the LFP cathodes at a scanning rate of 0.1mV s-1 using
439
different binders: (b) CMC-Li, (c) PVDF, and (d) CMC.
440
While the active material is coated with CMC-Li binder to form an effective
441
ionic conductivity layer, which provides a diffusion pathway for Li+ to reach the 22
Page 22 of 23
442
active particle surface and enhance the activity of electrochemical reaction,
443
reversibility of li-ions and cycle performance.
444
Fig. 5. Rate performance of LFP electrodes with different binders at 0.1C to 5C. It is because CMC-Li as the binder can be uniformly distributed in the
446
surface of LFP material, the ionized li-ions can short its transmission distance
447
between cathode and anode electrode, and enhance the transmission efficiency
448
and conductivity, which cause the material to have the relatively good rate
449
performance and cycle property.
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Fig.6. SEM images of LFP cathode materials with different binders at the end of the
451
200th cycles charge-discharge. (a) Digital pictures of CMC-Li, (b) PVDF as
452
binder, (c) CMC as binder, (d～f) CMC-Li as binder.
455 456 457
d
te
454
“Black hole” phenomenon may be the reason for improving the charge and discharge specific capacity of battery. Fig.7. Preliminary investigation of the mechanism of the CMC-Li binder in LFP
Ac ce p
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M
450
battery. Active substances react with lithium-ions on the molecular chain of CMC-Li adjacent to the active center of active substances.
23
Page 23 of 23
