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Water soluble styrene butadiene rubber and sodium carboxyl methyl cellulose binder for ZnFe2O4 anode electrodes in lithium ion batteries
Journal of Power Sources 285 (2015) 227e234
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Water soluble styrene butadiene rubber and sodium carboxyl methyl cellulose binder for ZnFe2O4 anode electrodes in lithium ion batteries Rongyu Zhang a, Xu Yang a, Dong Zhang a, Hailong Qiu a, Qiang Fu a, Hui Na a, b, Zhendong Guo a, Fei Du a, Gang Chen a, c, Yingjin Wei a, * a b c
Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education), College of Physics, Jilin University, Changchun 130012, PR China College of Chemistry, Jilin University, Changchun 130012, PR China State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ZnFe2O4 nano particles were prepared by the glycine-nitrate combustion method. SBR/CMC water soluble binder was used to prepare ZnFe2O4 anode electrode. Excellent cycle stability and rate capability were obtained using SBR/ CMC. SBR/CMC is more promising than PVDF for the ZnFe2O4 anode electrode.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 February 2015 Received in revised form 12 March 2015 Accepted 16 March 2015 Available online 16 March 2015
ZnFe2O4 nano particles as an anode material for lithium ion batteries are prepared by the glycine-nitrate combustion method. The mixture of styrene butadiene rubber and sodium carboxyl methyl cellulose (SBR/CMC) with the weight ratio of 1:1 is used as the binder for ZnFe2O4 electrode. Compared with the conventional polyvinylidene-fluoride (PVDF) binder, the SBR/CMC binder is much cheaper and environment benign. More significantly, this water soluble binder significantly improves the rate capability and cycle stability of ZnFe2O4. A discharge capacity of 873.8 mAh g1 is obtained after 100 cycles at the 0.1C rate, with a very little capacity fading rate of 0.06% per cycle. Studies show that the SBR/CMC binder enhances the adhesion of the electrode film to the current collector, and constructs an effective threedimensional network for electrons transport. In addition, the SBR/CMC binder helps to form a uniform SEI film thus prohibiting the formation of lithium dendrite. Electrochemical impedance spectroscopy shows that the SBR/CMC binder lowers the ohmic resistance of the electrode, depresses the formation of SEI film and facilitates the charge transfer reactions at the electrode/electrolyte interface. These advantages highlight the potential applications of SBR/CMC binder in lithium ion batteries. © 2015 Elsevier B.V. All rights reserved.
Keywords: Lithium-ion battery Zinc iron oxide Binder Styrene butadiene rubber Carboxyl methyl cellulose
1. Introduction
* Corresponding author. E-mail address: [email protected] (Y. Wei). http://dx.doi.org/10.1016/j.jpowsour.2015.03.100 0378-7753/© 2015 Elsevier B.V. All rights reserved.
Lithium ion batteries (LIBs) have been intensively investigated for promoting the developments of portable electronics, electric vehicles and renewable energy storage. Conventional LIBs use graphite as the anode material which is cheap, abundant and stable
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for cycling. However, both the natural graphite and artificial graphite have hindered further developments of LIBs due to their low specific capacities (theoretically 372 mAh g1) and serious safety issues. As a consequence, searching for alternative anode materials is strongly required for the development of advanced LIBs. In numerous new anode materials under consideration, recently spinel ternary iron oxides MFe2O4 (MFO, M ¼ Co, Cu, Ni, Zn) have attracted particular attention [1e7]. Especially, ZnFe2O4 (ZFO) has been regarded as the most promising one because of its high theoretical capacity (1072 mAh g1), low toxicity and resource abundance. However, there are still some issues to be solved for ZFO such as low electronic conductivity, high initial irreversible capacity and large volume variations during repeated cycling. In these regards, it has been engineered into many different nanostructures to enhance its electrode performance. For instance, the ZFO nano particles synthesized by the urea combustion method showed a reversible capacity of 615 mAh g1 [8]. The electrochemical performance was further enhanced by either nanostructuring to form hierarchically hollow microspheres (~900 mAh g1) [9], nanofibers (730 mAh g1) [10], nano particles (800 mAh g1) [11], or by the incorporation of a conductive carbon framework to construct ZFO/carbon hollow spheres (841 mAh g1) [12] and carbon-coated ZFO nano particles (~1000 mAh g1) [13]. In order to enhance the electrochemical performance of LIBs, researchers are not only devoted to searching for new electrode materials, but also to developing new electrode fabrication techniques. Binder is an important component of battery electrodes which acts as an effective dispersion agent to connect the active material and conductive additive together and steadily adhere them to the current collector. The most commonly used binder in LIBs is poly(vinylidene)-fluoride (PVDF) due to its high electrochemical stability and good connection between the electrode films and current collectors. However, PVDF is always dissolved in some organic solvents such as N-methy-l-2-pyrrolidone (NMP). It is known NMP is a volatile and combustible solvent which causes safety problems and severe pollution. Moreover, PVDF is readily swollen, gelled, or dissolved by nonaqueous liquid electrolytes to form a viscous fluid or gel polymer electrolyte, which results in desquamation of electrode particles and hence fast capacity fading of the battery after prolonged cycling. In the last few years, the mixture of sodium carboxyl methyl cellulose (CMC) and styrene butadiene rubber (SBR) has attracted great attention as a new binder for LIBs. Fig. 1 shows the molecular structures of PVDF, CMC and SBR, respectively. CMC is a typical polymeric derivative of cellulose containing carboxylate anion and hydroxyl functional groups. The existing of these two groups makes CMC a water-soluble binder and an effective thickening agent. SBR as an elastomer has higher flexibility, stronger binding force and better heat resistance than PVDF. In addition, it is very attractive that the prices of CMC (1e2 EUR kg1) and SBR (0.2e1 EUR kg1) are much cheaper than that of PVDF (15e18 EUR kg1) [14]. To date, the SBR/CMC binder has shown much effectiveness for a number of electrode materials including graphite [15e17], transition metal
oxides [18e20], sulfur [21,22], and silicon based materials [23e25]. But to the best of our knowledge, most of the studies on the ZFO anode still used PVDF as the binder. The electrodes thus prepared showed unsatisfying cycle stability and rate capability especially for those of ordinary ZFO particles without novel nanostructures and conductive carbon framework. Herein in this work, we synthesized ZFO nano particles by the glycine-nitrate combustion method. Then we prepared ZFO electrode using SBR/CMC as the binder. The electrode showed much better electrochemical properties compared to the traditional PVDF-based ZFO electrode. 2. Experimental section The ZFO nano particles were prepared by the glycine-nitrate combustion method. The chemicals used in the synthesis process were zinc nitrate (Zn(NO3)2$6H2O, Aladdin), ferric nitrate (Fe(NO3)3$9H2O, Aladdin) and glycine (Aladdin). The ratio of glycine (fuel) to metal nitrates (oxidizer) was fixed at 3:5 to allow completion of the combustion reactions. The above mixture was dissolved in 30 mL of deionized water under continuous stirring. Ammonia water was dropped slowly to keep the pH value of the solution at 9e10. Then, the solution was transformed into oven. At a critical temperature of ~200 C, a spark was generated due to enormous heat creation in the solution. This caused an exhaustive heat release through the foam, yielding a brown voluminous fluffy powder in the container. The obtained powder was then calcinated at 350 C for 3 h in air to remove the un-reacted fuel and nitrates. Subsequently, the decomposed powder was grinded and calcined at 700 C for 2 h. The ZnFe2O4 nano particles were obtained after being cooled to room temperature. The crystal structure of the material was studied by X-ray diffraction (XRD) on a Bruker AXS D8 X-ray diffractometer with Cu Ka radiation. The morphologies of the material and the electrodes were studied by JSM-6700F field emission scanning electron microscope (SEM). Transmission electron microscope (TEM) was performed on FEI Tecnai G2 F20 S-TWIN. X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB spectrometer using Mg-Ka light source. The binding energy was corrected using the C 1s peak at 284.6 eV. The resistance of the electrodes were measured by the two-point-probe method. Electrochemical experiments were conducted with CR2032 coin cells between 0.01 V and 3.0 V using metallic lithium foil as the anode. The ZFO electrodes were prepared by mixing 70 wt% of active material, 20 wt% of super P conductive additive and 10 wt% of PVDF (or SBR/CMC) binder. The PVDF binder was dissolved in NMP solvent. The CMC/SBR binder was a mixture of CMC and SBR in an empirically optimized weight ratio of 1:1, which was dissolved in deionized water. Hereafter the electrodes were denoted as ZFOPVDF and ZFO-SBR/CMC accordingly. The slurries were pasted onto a Cu current collector and dried in vacuum oven at 120 C for 12 h. The dried electrodes were cut into 8 8 mm2 for use. The loading density of the ZFO active material in the electrodes is about 3.0 mg/cm2. The electrolyte was a 1 M LiPF6 solution dissolving in
Fig. 1. Molecular structures of (a) PVDF, (b) CMC and (c) SBR.
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ethylene carbonate (EC), dimethyl carbonate (DMC) and ethylmethyl carbonate (EMC) with EC:DMC:EMC ¼ 1:1:8 by v/v ratio. The cathode and anode were separated by Celgard 2320 membrane. Galvonostatic chargeedischarge was performed on a Land2010 automatic battery tester. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed on a Bio-Logic VSP multichannel potentiostatic galvanostatic electrochemical workstation. The CV measurements were performed in the voltage window of 0.01e3.0 V at a scan rate of 0.1 mV s1. The EIS data were obtained in a frequency range between 1 MHz and 1 mHz by applying an ac voltage of 5 mV. 3. Results and discussion 3.1. Characterizations of the ZnFe2O4 nano particles The XRD pattern of the as-prepared product is shown in Fig. 2. All the diffraction peaks can be indexed as spinel ZnFe2O4 possessing a face-centered cubic structure. No peaks from any impurities are observed in the diffraction pattern. The diffraction peaks at 18.1, 29.9 , 35.2 , 36.8 , 42.8 , 53.1, 56.5 , 62.1, 65.2 , 70.4 and 73.4 correspond to the (111), (220), (311), (222), (400), (422), (511), (440), (531), (620) and (533) planes of ZnFe2O4, respectively. The lattice constant of the material is calculated to be 8.453 Å which fit well with that recorded in JCPDS No. 22-1012. The strong intensity and narrow peak width indicate good crystallinity of the product. The average crystallite size (D) of the material is calculated using the Scherrer's formula, D ¼ Kl/Bcosq, where l is the wavelength of the X-ray radiation, B is the angular width at half of the maximum intensity of the selected peak, q is the corresponding Bragg angle and K is a constant which is 0.9. The calculated crystallite sizes for the three strongest XRD peaks, (311), (511) and (440) are 100 nm, 91 nm and 95 nm, respectively. Therefore, the average crystallite size of the ZnFe2O4 product is about 95 nm. The morphology of the ZFO powders is characterized by SEM as shown in Fig. 3. The material is composed of sphere-like nano particles with unavoidable agglomeration. The TEM image in Fig. 4a shows that the particle size of the material is in the range of 50e100 nm, which fits well with the crystallite size calculated from the XRD data. Fig. 4b shows the HRTEM image and selected area electron diffraction (SAED) pattern of the material. It is seen that the particle surface is very clean. The lattice fringes with a distance of 4.88 Å correspond to the (111) planes of ZnFe2O4. In addition, the SAED pattern can be indexed well as the cubic structure of spinel ZnFe2O4.
Fig. 2. XRD pattern of the as-prepared ZnFe2O4 nano particles.
Fig. 3. SEM image of the ZnFe2O4 nano particles.
XPS was used to study the chemical properties of the material. The signals of Zn, Fe and O can be identified from the full-scale XPS pattern (Fig. 5a). The presence of C 1s peak at 284.6 eV can be assigned to carbon contamination and CO2 adsorbed on the surface of the product. The O 1s binding energy is observed at 530.1 eV, which is characteristic of oxygen in metal oxides. In Fig. 5b, the XPS peaks at 1045.1 eV and 1022.4 eV can be attributed to Zn 2p1/2 and Zn 2p3/2 of Zn2þ [26]. There are four signals in the Fe 2p XPS as shown in Fig. 5c. The signals centered around 711.6 eV and 726.1 eV are due to the Fe3þ at octahedral sites. The satellite peaks, with binding energies 8.0 eV higher than the main peaks, confirm the þ3 oxidation state of iron ions [27]. 3.2. Cyclic voltammetry Fig. 6a shows the initial four CV cycles of the ZFO-PVDF electrode at a scan rate of 0.1 mV s1. The significant differences between the first and subsequent scans indicate a different Liþ storage reaction taking place in the first scan compared with the following scans. According to the literature [6,12,27], the small current peak at 0.815 V in the first reduction is attributed to Liþ intercalation into ZnFe2O4 (Reaction (1)). The large and spiky current peak centered at 0.591 V corresponds to the conversion reaction of Li2ZnFe2O4 with Liþ to form metallic Zn and Fe (Reaction (2)). This peak could be overlapped with the peak related to the formation of SEI film. Moreover, the weak peak near the cut-off voltage of 0.01 V is associated with the LieZn alloying reaction (Reaction (3)). ZnFe2O4 þ 2Liþ þ 2e / Li2ZnFe2O4
(1)
Li2ZnFe2O4 þ 6Liþ þ 6e / 4Li2O þ 2Fe þ Zn
(2)
Zn þ Liþ þ e / LieZn (alloy)
(3)
LieZn (alloy) / Zn þ Liþ þ e
(4)
Zn þ Li2O / ZnO þ 2Liþ þ 2e
(5)
2Fe þ 3Li2O / Fe2O3 þ 6Liþ þ 6e
(6)
Based on the aforementioned mechanisms, the crystal structure of ZnFe2O4 is essentially destroyed in the first discharge, which
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Fig. 4. (a) TEM and (b) HRTEM image of the ZnFe2O4 nano particles. Inset in (b) shows the SAED pattern of ZnFe2O4 particle.
Fig. 5. XPS spectra of the ZnFe2O4 sample: (a) survey spectrum; (b) Zn 2p spectrum; and (c) Fe 2p spectrum.
Fig. 6. Cyclic voltammograms of the (a) ZFO-PVDF and (b) ZFO-SBR/CMC electrodes.
yields a mixture of Li2O and metallic phases. In the following anodic scan, the broad oxidation peak near 0.106 V corresponds to the dealloying of LieZn phase (Reaction (4)). The peak at approximate 1.604 V is attributed to the oxidation of Fe to Fe3þ and Zn to Zn2þ, along with the decomposition of Li2O (Reactions (5)e(6)). The material thus could not recover to ZnFe2O4 after the first cycle but be replaced by simpler oxides of Fe2O3 and ZnO. The cathodic peak shifts to a higher voltage of 0.924 V in the second cycle due to the permanent phase transformation of the material. Even though the oxidation peak still keeps at 1.604 V, its current intensity becomes weaker and weaker with cycling. In addition, the reduction peak
continuously shifts to lower voltages. This indicates that the ZFOPVDF electrode could not maintain well structure stability and electrochemical reversibility during cycling. In contrast, it is seen from Fig. 6b that the CV curves of the ZFO-SBR/CMC electrode are very stable from the second cycle. Thus, this electrode is expected to show better electrochemical performance than that of ZFO-PVDF. 3.3. Charge-discharge cycling The cycle stability of the ZFO-PVDF and ZFO-SBR/CMC electrodes are studied at a current density of 100 mA g1 (about 0.1C
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rate). Fig. 7a and b compare the chargeedischarge curves of the electrodes. Both electrodes show similar chargeedischarge curves in the first cycle. The small plateau at 0.840 V in the first discharge is due to Liþ intercalation into ZnFe2O4, and the long plateau is due to the conversion reaction of Li2ZnFe2O4 with Liþ to form metallic Zn and Fe. The slope region below 0.656 V is attributed to the formation of SEI film and the LieZn alloying reaction. The first charge process shows an S-shaped voltage profile which is attributed to the de-alloying of LieZn phase and the oxidation of Fe to Fe3þ and Zn to Zn2þ. The initial discharge capacities of the ZFOPVDF and ZFO-SBR/CMC electrodes are 1149.8 mAh g1 and 1053.9 mAh g1, respectively. In the following charge process, the ZFO-PVDF and ZFO-SBR/CMC electrodes exhibit specific capacities of 911.6 mAh g1 and 865.4 mAh g1, respectively. Apparently, the ZFO-SBR/CMC electrode shows a larger initial columbic efficiency (82.1%) than that of ZFO-PVDF (69.8%). The irreversible capacity is due to the formation of SEI film. In addition, a part of the irreversible capacity could be due to the incomplete Liþ extraction from the electrode due to high kinetic barriers of the conversion reactions (Reactions (5)e(6)). Fig. 7c shows the discharge capacities and columbic efficiencies of the electrodes during chargeedischarge cycling. The columbic efficiencies of the ZFO-PVDF electrode increases to ~100% in 15 cycles accompanied with continuous capacity fading. Even though the electrode eventually gets a stable discharge capacity of 461.0 mAh g1 after 15 cycles, it is useless for practical applications. The poor electrochemical performance of the ZFO-PVDF electrode is similar as those reported ZFO electrodes using the PVDF binder [6,28e31]. Contrarily, the columbic efficiencies of the ZFO-SBR/CMC electrode rapidly increase to ~100% in the initial two cycles. In addition, the discharge capacities of this electrode are very stable, which still shows 873.8 mAh g1 after 100 cycles. The capacity fading rate is only 0.06% per cycle with respect to the second cycle. Fig. 7d shows the rate performance of the ZFO-PVDF and ZFO-SBR/CMC electrodes with the cycle rate increasing from 0.1C to 1C. Obviously, ZFO-SBR/ CMC shows much better rate capability than that of ZFO-PVDF. It
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Fig. 8. Nyquist plots of the ZFO-PVDF and ZFO-SBR/CMC electrodes.
still has a high capacity of 627.6 mAh g1 at the 1C rate, which is much larger than that of ZFO-PVDF (144.9 mAh g1). The small discharge capacities of ZFO-PVDF at high current rates indicate slight structure changes of the active material resulting in good capacity retention. One can see that the ZFO-SBR/CMC electrode exhibits relatively bad capacity retention at high current rates. This may be due to the fast volume change of the active material which would decrease the structure stability of the active material and the electrode. S. Passerini et al. have shown that the carbon coated ZnFe2O4 nano material showed high electronic conductivity, and the surface carbon coating could depress the structure changes of the material thus improving the cycle stability of the electrode at high current rates [13]. 3.4. Electrochemical impedance spectroscopy EIS is performed to study the effects of binder on the electrochemical performance of ZFO. Fig. 8 shows the Nyquist plots of the
Fig. 7. Discharge-charge profiles of the (a) ZFO-PVDF and (b) ZFO-SBR/CMC electrodes. (c) cycling performance and (d) rate performance of the electrodes.
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ZFO-PVDF and ZFO-SBR/CMC electrodes after the first discharge. Both electrodes show two well defined semicircles in the high-tomiddle frequency region which are due to the SEI film and the charge transfer process, respectively. The slope line in the low frequency region is due to the lithium diffusion in the electrode bulk. Based on this, the Nyquist plots can be simulated by the equivalent circuit given in the inset of Fig. 8. In this equivalent circuit, Rs represents the ohmic resistance of the cell. Rf and Csl are the resistance and capacitance of the SEI film. Rct and Cdl represent the charge transfer resistance and double layer capacitance. W is the Warburg parameter reflecting the lithium diffusion in the electrode bulk. The simulated Rs, Rct and Rf values for the ZFO-PVDF electrode are 8.0 U, 178.1 U and 216.9 U, respectively, all of which are much larger than those of the ZFO-SBR/CMC electrode, i.e. 4.9 U, 111.2 U, and 148.9 U, respectively. This indicates that the SBR/CMC binder lowers the ohmic resistance of the electrode, depresses the formation of SEI film and facilitates the charge transfer reactions at the electrode/electrolyte interface. Therefore, the electrochemical kinetics of ZFO-SBR/CMC is better than that of ZFO-PVDF, resulting in improved capacity retention and rate capability. 3.5. Morphologies of the ZnFe2O4 electrodes The morphologies of the ZFO electrodes prepared by different binders before and after chargeedischarge cycling are investigated by SEM. Fig. 9a and b shows the SEM images of the fresh ZFO-PVDF and ZFO-SBR/CMC electrodes, respectively. From the first sight, the surface of ZFO-SBR/CMC is very smooth and have a lot of small pores, while ZFO-PVDF has a rough surface and seems less compact than ZFO-SBR/CMC. A larger magnification of the surface morphologies is shown in the inset of Fig. 9a and b. It is seen that the
ZFO-PVDF electrode shows severe agglomeration of the ZFO and super P particles, but these two components are well dispersed in the ZFO-SBR/CMC electrode. Note that it is difficult to discern the ZFO and super P particles from the present SEM images. In the electrode slurries, a three-dimensional network is formed due to bridging of the particles by the binders since the segments of polymer chain adsorb on the ZFO and super P particles and the adsorbed chains form entanglements. Comparing to PVDF, the carboxylate groups of CMC give rise to an effective surface charge on ZFO and super P particles therefore stabilize the particles dispersion through an electrostatic double-layer repulsion mechanism [22]. As a result, a homogeneous dispersion of the ZFO and super P particles could be obtained using the SBR/CMC binder. Fig. 9c and d shows the cross-section SEM images of the ZFOPVDF and ZFO-SBR/CMC electrodes. The electrode film of ZFOSBR/CMC shows good adhesion to the current collector, while some big gaps are found in the ZFO-PVDF electrode. This indicates that the SBR/CMC binder helps to enhance the adhesion of the electrode film to the current collector. This is due to the strong hydrogen bonding of the carboxyl and hydroxyl groups in CMC with the active material and the current collector, whereas the fluorine atoms in PVDF only form very weak hydrogen bonds [32]. However, it should be noted that CMC is extremely stiff and brittle which could easily form cracks or slide off the current collector when used alone as the binder. Addition of SBR to CMC results in electrodes that are less brittle and, compared to PVDF, shows a smaller Yong's modulus, a larger maximum elongation and stronger adhesion strength to the current collector [33]. For the present ZFO nano particles, the empirically optimized weight ratio of SBR:CMC has been determined to be 1:1 as shown in Fig. 10. The unique morphology of the ZFO-SBR/CMC electrode is helpful to form an
Fig. 9. SEM images of the (a, c) ZFO-PVDF and (b, d) ZFO-SBR/CMC electrodes.
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Fig. 10. Cycling performance of the ZFO-SBR/CMC electrodes with different SBR/CMC ratios.
efficient conductive network. Conductivity measurement shows that the electrical resistance of the ZFO-SBR/CMC electrode is 2.1 U, which is smaller than that of ZFO-PVDF (6.7 U). Charge-discharge experiments show that the ZFO-PVDF electrode undergoes severe capacity fading in the initial 15 cycles while the ZFO-SBR/CMC electrode shows perfect capacity retention. In order to know what happens during cycling, we collect the SEM images of the electrodes after 15 cycles as shown in Fig. 11. The electrodes have been completely washed off by DMC before SEM measurement. Therefore, only solid compounds are observed in the SEM images. A dense surface film could be seen in the cycled ZFOSBR/CMC electrode which could be due to the SEI film and swelling of the binder. As a result, some of the small pores are closed and the electrode becomes more compact comparing to the fresh electrode. In addition, the electrode is still very smooth indicating its good maintainability during cycling. The cycled ZFO-PVDF electrode, on the contrary, exhibits a totally different morphology. The relatively smooth electrode is severely destroyed after cycling. Especially, a number of spikes which could be due to lithium dendrites are observed on the electrode. The lithium dendrite has been regarded as the most detrimental to the cycling efficiency and battery safety since dendrite could accelerate the capacity fading due to the formation of electrochemical isolated regions, or even trigger the internal short when piercing through the battery separator [34]. In LIBs, the Li ions could be reduced to metallic lithium and deposit on the electrode when the working voltage is close to 0 V. It has shown that the deposition of lithium is mainly dependent on
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the properties of SEI film, rather than the initially prepared microstructure of the electrode surface. A uniform texture of the SEI film could generate uniform deposition, which depresses the formation of lithium dendrite. On the contrary, The SEI film with multiple types of defects, such as pits, cracks, crystalline defects and grain boundaries, will result in non-uniform lithium deposition thus facilitates dendrite formation [35]. Fig. 12 shows the TEM images of the cycled ZFO-PVDF and ZFO-SBR/CMC electrodes. It is seen that the ZFO-PVDF electrode shows non-uniform microstructures. The dark-black region in the center of the image could be assigned to the active material. Around the active material, the amorphous phase with an anisotropy growth behavior is attributed to the SEI film. The SEI film shows a tendency to spread in the whole electrode. In addition, the nano crystallites with metallic textures dispersing in the SEI film may be attributed to the segments of lithium dendrite. On the contrary, the cycled ZFO-SBR/CMC electrode shows a very uniform SEI film with thickness about 15 nm. The active material could be effectively protected by this SEI film from attacking by the electrolyte. In addition, this uniform SEI film also prohibits the formation of lithium dendrite. Therefore, SBR/ CMC shows more effectiveness than PVDF in maintaining the stability of the ZFO electrode thus resulting in good rate capability and excellent capacity retention.
4. Conclusions In summary, the electrochemical performance of the ZnFe2O4 electrodes prepared by the PVDF binder and the SBR/CMC binder has been investigated. Comparing with the traditional PVDF binder, the water soluble SBR/CMC binder could enhance the adhesion of the electrode film to the current collector. In addition, it could provide an effective three-dimensional network with uniform distribution of the ZnFe2O4 active material and super P conductive additive. During chargeedischarge cycling, the SBR/CMC binder lowers the ohmic resistance of the electrode, depresses the formation of SEI film and facilitates the charge transfer reactions at the electrode/electrolyte interface. In addition, a uniform SEI film forms on the surface of the active material thus prohibiting the formation of lithium dendrite. Due to these advantages, the ZnFe2O4 electrode prepared by the SBR/CMC binder exhibits high specific capacities, good capacity retention and excellent rate capability. This work shows the possibility of manufacturing electrodes using the cheap water soluble SBR/CMC binder, instead of expensive PVDF and hazardous NMP solvents, thereby improving battery performance, reducing cost and protecting the environment.
Fig. 11. SEM images of the ZFO-PVDF and ZFO-SBR/CMC electrodes after 15 cycles.
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Fig. 12. TEM images of the ZFO-PVDF and ZFO-SBR/CMC electrodes after 15 cycles.
Acknowledgments This work was supported by the 973 Program (No. 2015CB251103), National Natural Science Foundation of China (No. 51472104, 21473075, 51272088), National Found for Fostering Talents of Basic Science (No. J1103202); References [1] C.T. Cherian, J. Sundaramurthy, M.V. Reddy, P.S. Kumar, K. Mani, D. Pliszka, C.H. Sow, S. Ramakrishna, B.V.R. Chowdari, ACS Appl. Mater. Interfaces 5 (2013) 9957e9963. [2] P. Lavela, J.L. Tirado, M. Womes, J.C. Jumas, J. Phys. Chem. C 113 (2009) 20081e20087. [3] L.M. Jin, Y.C. Qiu, H. Deng, W.S. Li, H. Li, S.H. Yang, Electrochim. Acta 56 (2011) 9127e9132. [4] Y.L. Xiao, J.T. Zai, X.M. Li, Y. Gong, B. Li, Q.Y. Han, X.F. Qian, Nano Energy 6 (2014) 51e58. [5] Z.L. Zhang, Y.H. Wang, M.J. Zhang, Q.Q. Tan, X. Lv, Z.Y. Zhong, F.B. Su, J. Mater. Chem. A 1 (2013) 7444e7450. [6] A.S. Hameed, H. Bahiraei, M.V. Reddy, M.Z. Shoushtari, J.J. Vittal, C.K. Ong, B.V.R. Chowdari, ACS Appl. Mater. Interfaces 6 (2014) 10744e10753. [7] F. Martinez-Julian, A. Guerrero, M. Haro, J. Bisquert, D. Bresser, E. Paillard, S. Passerini, G. Garcia-Belmonte, J. Phys. Chem. C 118 (2014) 6069e6076. [8] Y. Sharma, N. Sharma, G.V. Subba Rao, B.V.R. Chowdari, Electrochim. Acta 53 (2008) 2380e2385. [9] X.W. Guo, X. Lu, X.P. Fang, Y. Mao, Z.X. Wang, L.Q. Chen, X.X. Xu, H. Yang, Y.N. Liu, Electrochem. Commun. 12 (2010) 847e850. [10] P.F. Teh, Y. Sharma, S.S. Pramana, M. Srinivasan, J. Mater. Chem. 21 (2011) 14999e15008. [11] Y. Ding, Y.F. Yang, H.X. Shao, Electrochim. Acta 56 (2011) 9433e9438. [12] Y.F. Deng, Q.M. Zhang, S.D. Tang, L.T. Zhang, S.N. Deng, Z.C. Shi, G.H. Chen, Chem. Commun. 47 (2011) 6828e6830. [13] D. Bresser, E. Paillard, R. Kloepsch, S. Krueger, M. Fiedler, R. Schmitz, D. Baither, M. Winter, S. Passerini, Adv. Energy Mater. 3 (2013) 513e523.
[14] X. Yan, Y.Q. Zhang, K. Zhu, Y. Gao, D. Zhang, G. Chen, C.Z. Wang, Y.J. Wei, J. Power Sources 246 (2014) 95e102. [15] J.H. Lee, U. Paik, V.A. Hackley, Y.M. Choi, J. Electrochem. Soc. 152 (2005) A1763eA1769. [16] S.S. Zhang, K. Xu, T.R. Jow, J. Power Sources 138 (2004) 226e231. [17] J.P. Yen, C.M. Lee, T.L. Wu, H.C. Wu, C.Y. Su, N.L. Wu, J.L. Hong, ECS Electrochem. Lett. 1 (2012) A80eA82. [18] M. He, L.X. Yuan, X.L. Hu, W.X. Zhang, J. Shu, Y.H. Huang, Nanoscale 5 (2013) 3298e3305. [19] M. Mancini, F. Nobili, R. Tossici, M. Wohlfahrt-Mehrens, R. Marassi, J. Power Sources 196 (2011) 9665e9671. [20] C. Zhong, J.Z. Wang, S.L. Chou, K. Konstantinov, M. Rahman, H.K. Liu, J. Appl. Electrochem. 40 (2010) 1415e1419. [21] Z.A. Zhang, W.Z. Bao, H. Lu, M. Jia, K.Y. Xie, Y.Q. Lai, J. Li, ECS Electrochem. Lett. 1 (2012) A34eA37. [22] M. He, L.X. Yuan, W.X. Zhang, X.L. Hu, Y.H. Huang, J. Phys. Chem. C 115 (2011) 15703e15709. , D. Guyomard, Electrochem. Commun. 9 [23] B. Lestriez, S. Bahri, I. Sandu, L. Roue (2007) 2801e2806. [24] J. Li, R.B. Lewis, J.R. Dahn, Electrochem. Solid St. 10 (2007) A17eA20. [25] S. Komaba, K. Shimomura, N. Yabuuchi, T. Ozeki, H. Yui, K. Konno, J. Phys. Chem. C 115 (2011) 13487e13495. [26] M. Wang, Z.H. Ai, L.Z. Zhang, J. Phys. Chem. C 112 (2008) 13163e13170. [27] Z. Xing, Z.C. Ju, J. Yang, H.Y. Xu, Y.T. Qian, Nano Res. 5 (2012) 477e485. [28] A.K. Rai, S. Kim, J. Gim, M.H. Alfaruqi, V. Mathew, J. Kim, RSC Adv. 4 (2014) 47087e47095. [29] X.B. Zhong, B. Jin, Z.Z. Yang, C. Wang, H.Y. Wang, RSC Adv. 4 (2014) 55173e55178. [30] W.T. Song, J. Xie, S.Y. Liu, G.S. Cao, T.J. Zhu, X.B. Zhao, New. J. Chem. 36 (2012) 2236e2241. [31] H. Xia, Y.Y. Qian, Y.S. Fu, X. Wang, Solid State Sci. 17 (2013) 67e71. [32] B.R. Lee, E.S. Oh, J. Phys. Chem. C 117 (2013) 4404e4409. [33] J. Chong, S.D. Xun, H.H. Zheng, X.Y. Song, G. Liu, P. Ridgway, J.Q. Wang, V.S. Battaglia, J. Power Sources 196 (2011) 7707e7714. [34] O. Crowther, A.C. West, J. Electrochem. Soc. 155 (2008) A806eA811. [35] Z. Li, J. Huang, B.Y. Liaw, V. Metzler, J.B. Zhang, J. Power Sources 254 (2014) 168e182.
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Water soluble styrene butadiene rubber and sodium carboxyl methyl cellulose binder for ZnFe2O4 anode electrodes in lithium ion batteries Rongyu Zhang a, Xu Yang a, Dong Zhang a, Hailong Qiu a, Qiang Fu a, Hui Na a, b, Zhendong Guo a, Fei Du a, Gang Chen a, c, Yingjin Wei a, * a b c
Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education), College of Physics, Jilin University, Changchun 130012, PR China College of Chemistry, Jilin University, Changchun 130012, PR China State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ZnFe2O4 nano particles were prepared by the glycine-nitrate combustion method. SBR/CMC water soluble binder was used to prepare ZnFe2O4 anode electrode. Excellent cycle stability and rate capability were obtained using SBR/ CMC. SBR/CMC is more promising than PVDF for the ZnFe2O4 anode electrode.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 February 2015 Received in revised form 12 March 2015 Accepted 16 March 2015 Available online 16 March 2015
ZnFe2O4 nano particles as an anode material for lithium ion batteries are prepared by the glycine-nitrate combustion method. The mixture of styrene butadiene rubber and sodium carboxyl methyl cellulose (SBR/CMC) with the weight ratio of 1:1 is used as the binder for ZnFe2O4 electrode. Compared with the conventional polyvinylidene-fluoride (PVDF) binder, the SBR/CMC binder is much cheaper and environment benign. More significantly, this water soluble binder significantly improves the rate capability and cycle stability of ZnFe2O4. A discharge capacity of 873.8 mAh g1 is obtained after 100 cycles at the 0.1C rate, with a very little capacity fading rate of 0.06% per cycle. Studies show that the SBR/CMC binder enhances the adhesion of the electrode film to the current collector, and constructs an effective threedimensional network for electrons transport. In addition, the SBR/CMC binder helps to form a uniform SEI film thus prohibiting the formation of lithium dendrite. Electrochemical impedance spectroscopy shows that the SBR/CMC binder lowers the ohmic resistance of the electrode, depresses the formation of SEI film and facilitates the charge transfer reactions at the electrode/electrolyte interface. These advantages highlight the potential applications of SBR/CMC binder in lithium ion batteries. © 2015 Elsevier B.V. All rights reserved.
Keywords: Lithium-ion battery Zinc iron oxide Binder Styrene butadiene rubber Carboxyl methyl cellulose
1. Introduction
* Corresponding author. E-mail address: [email protected] (Y. Wei). http://dx.doi.org/10.1016/j.jpowsour.2015.03.100 0378-7753/© 2015 Elsevier B.V. All rights reserved.
Lithium ion batteries (LIBs) have been intensively investigated for promoting the developments of portable electronics, electric vehicles and renewable energy storage. Conventional LIBs use graphite as the anode material which is cheap, abundant and stable
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for cycling. However, both the natural graphite and artificial graphite have hindered further developments of LIBs due to their low specific capacities (theoretically 372 mAh g1) and serious safety issues. As a consequence, searching for alternative anode materials is strongly required for the development of advanced LIBs. In numerous new anode materials under consideration, recently spinel ternary iron oxides MFe2O4 (MFO, M ¼ Co, Cu, Ni, Zn) have attracted particular attention [1e7]. Especially, ZnFe2O4 (ZFO) has been regarded as the most promising one because of its high theoretical capacity (1072 mAh g1), low toxicity and resource abundance. However, there are still some issues to be solved for ZFO such as low electronic conductivity, high initial irreversible capacity and large volume variations during repeated cycling. In these regards, it has been engineered into many different nanostructures to enhance its electrode performance. For instance, the ZFO nano particles synthesized by the urea combustion method showed a reversible capacity of 615 mAh g1 [8]. The electrochemical performance was further enhanced by either nanostructuring to form hierarchically hollow microspheres (~900 mAh g1) [9], nanofibers (730 mAh g1) [10], nano particles (800 mAh g1) [11], or by the incorporation of a conductive carbon framework to construct ZFO/carbon hollow spheres (841 mAh g1) [12] and carbon-coated ZFO nano particles (~1000 mAh g1) [13]. In order to enhance the electrochemical performance of LIBs, researchers are not only devoted to searching for new electrode materials, but also to developing new electrode fabrication techniques. Binder is an important component of battery electrodes which acts as an effective dispersion agent to connect the active material and conductive additive together and steadily adhere them to the current collector. The most commonly used binder in LIBs is poly(vinylidene)-fluoride (PVDF) due to its high electrochemical stability and good connection between the electrode films and current collectors. However, PVDF is always dissolved in some organic solvents such as N-methy-l-2-pyrrolidone (NMP). It is known NMP is a volatile and combustible solvent which causes safety problems and severe pollution. Moreover, PVDF is readily swollen, gelled, or dissolved by nonaqueous liquid electrolytes to form a viscous fluid or gel polymer electrolyte, which results in desquamation of electrode particles and hence fast capacity fading of the battery after prolonged cycling. In the last few years, the mixture of sodium carboxyl methyl cellulose (CMC) and styrene butadiene rubber (SBR) has attracted great attention as a new binder for LIBs. Fig. 1 shows the molecular structures of PVDF, CMC and SBR, respectively. CMC is a typical polymeric derivative of cellulose containing carboxylate anion and hydroxyl functional groups. The existing of these two groups makes CMC a water-soluble binder and an effective thickening agent. SBR as an elastomer has higher flexibility, stronger binding force and better heat resistance than PVDF. In addition, it is very attractive that the prices of CMC (1e2 EUR kg1) and SBR (0.2e1 EUR kg1) are much cheaper than that of PVDF (15e18 EUR kg1) [14]. To date, the SBR/CMC binder has shown much effectiveness for a number of electrode materials including graphite [15e17], transition metal
oxides [18e20], sulfur [21,22], and silicon based materials [23e25]. But to the best of our knowledge, most of the studies on the ZFO anode still used PVDF as the binder. The electrodes thus prepared showed unsatisfying cycle stability and rate capability especially for those of ordinary ZFO particles without novel nanostructures and conductive carbon framework. Herein in this work, we synthesized ZFO nano particles by the glycine-nitrate combustion method. Then we prepared ZFO electrode using SBR/CMC as the binder. The electrode showed much better electrochemical properties compared to the traditional PVDF-based ZFO electrode. 2. Experimental section The ZFO nano particles were prepared by the glycine-nitrate combustion method. The chemicals used in the synthesis process were zinc nitrate (Zn(NO3)2$6H2O, Aladdin), ferric nitrate (Fe(NO3)3$9H2O, Aladdin) and glycine (Aladdin). The ratio of glycine (fuel) to metal nitrates (oxidizer) was fixed at 3:5 to allow completion of the combustion reactions. The above mixture was dissolved in 30 mL of deionized water under continuous stirring. Ammonia water was dropped slowly to keep the pH value of the solution at 9e10. Then, the solution was transformed into oven. At a critical temperature of ~200 C, a spark was generated due to enormous heat creation in the solution. This caused an exhaustive heat release through the foam, yielding a brown voluminous fluffy powder in the container. The obtained powder was then calcinated at 350 C for 3 h in air to remove the un-reacted fuel and nitrates. Subsequently, the decomposed powder was grinded and calcined at 700 C for 2 h. The ZnFe2O4 nano particles were obtained after being cooled to room temperature. The crystal structure of the material was studied by X-ray diffraction (XRD) on a Bruker AXS D8 X-ray diffractometer with Cu Ka radiation. The morphologies of the material and the electrodes were studied by JSM-6700F field emission scanning electron microscope (SEM). Transmission electron microscope (TEM) was performed on FEI Tecnai G2 F20 S-TWIN. X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB spectrometer using Mg-Ka light source. The binding energy was corrected using the C 1s peak at 284.6 eV. The resistance of the electrodes were measured by the two-point-probe method. Electrochemical experiments were conducted with CR2032 coin cells between 0.01 V and 3.0 V using metallic lithium foil as the anode. The ZFO electrodes were prepared by mixing 70 wt% of active material, 20 wt% of super P conductive additive and 10 wt% of PVDF (or SBR/CMC) binder. The PVDF binder was dissolved in NMP solvent. The CMC/SBR binder was a mixture of CMC and SBR in an empirically optimized weight ratio of 1:1, which was dissolved in deionized water. Hereafter the electrodes were denoted as ZFOPVDF and ZFO-SBR/CMC accordingly. The slurries were pasted onto a Cu current collector and dried in vacuum oven at 120 C for 12 h. The dried electrodes were cut into 8 8 mm2 for use. The loading density of the ZFO active material in the electrodes is about 3.0 mg/cm2. The electrolyte was a 1 M LiPF6 solution dissolving in
Fig. 1. Molecular structures of (a) PVDF, (b) CMC and (c) SBR.
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229
ethylene carbonate (EC), dimethyl carbonate (DMC) and ethylmethyl carbonate (EMC) with EC:DMC:EMC ¼ 1:1:8 by v/v ratio. The cathode and anode were separated by Celgard 2320 membrane. Galvonostatic chargeedischarge was performed on a Land2010 automatic battery tester. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed on a Bio-Logic VSP multichannel potentiostatic galvanostatic electrochemical workstation. The CV measurements were performed in the voltage window of 0.01e3.0 V at a scan rate of 0.1 mV s1. The EIS data were obtained in a frequency range between 1 MHz and 1 mHz by applying an ac voltage of 5 mV. 3. Results and discussion 3.1. Characterizations of the ZnFe2O4 nano particles The XRD pattern of the as-prepared product is shown in Fig. 2. All the diffraction peaks can be indexed as spinel ZnFe2O4 possessing a face-centered cubic structure. No peaks from any impurities are observed in the diffraction pattern. The diffraction peaks at 18.1, 29.9 , 35.2 , 36.8 , 42.8 , 53.1, 56.5 , 62.1, 65.2 , 70.4 and 73.4 correspond to the (111), (220), (311), (222), (400), (422), (511), (440), (531), (620) and (533) planes of ZnFe2O4, respectively. The lattice constant of the material is calculated to be 8.453 Å which fit well with that recorded in JCPDS No. 22-1012. The strong intensity and narrow peak width indicate good crystallinity of the product. The average crystallite size (D) of the material is calculated using the Scherrer's formula, D ¼ Kl/Bcosq, where l is the wavelength of the X-ray radiation, B is the angular width at half of the maximum intensity of the selected peak, q is the corresponding Bragg angle and K is a constant which is 0.9. The calculated crystallite sizes for the three strongest XRD peaks, (311), (511) and (440) are 100 nm, 91 nm and 95 nm, respectively. Therefore, the average crystallite size of the ZnFe2O4 product is about 95 nm. The morphology of the ZFO powders is characterized by SEM as shown in Fig. 3. The material is composed of sphere-like nano particles with unavoidable agglomeration. The TEM image in Fig. 4a shows that the particle size of the material is in the range of 50e100 nm, which fits well with the crystallite size calculated from the XRD data. Fig. 4b shows the HRTEM image and selected area electron diffraction (SAED) pattern of the material. It is seen that the particle surface is very clean. The lattice fringes with a distance of 4.88 Å correspond to the (111) planes of ZnFe2O4. In addition, the SAED pattern can be indexed well as the cubic structure of spinel ZnFe2O4.
Fig. 2. XRD pattern of the as-prepared ZnFe2O4 nano particles.
Fig. 3. SEM image of the ZnFe2O4 nano particles.
XPS was used to study the chemical properties of the material. The signals of Zn, Fe and O can be identified from the full-scale XPS pattern (Fig. 5a). The presence of C 1s peak at 284.6 eV can be assigned to carbon contamination and CO2 adsorbed on the surface of the product. The O 1s binding energy is observed at 530.1 eV, which is characteristic of oxygen in metal oxides. In Fig. 5b, the XPS peaks at 1045.1 eV and 1022.4 eV can be attributed to Zn 2p1/2 and Zn 2p3/2 of Zn2þ [26]. There are four signals in the Fe 2p XPS as shown in Fig. 5c. The signals centered around 711.6 eV and 726.1 eV are due to the Fe3þ at octahedral sites. The satellite peaks, with binding energies 8.0 eV higher than the main peaks, confirm the þ3 oxidation state of iron ions [27]. 3.2. Cyclic voltammetry Fig. 6a shows the initial four CV cycles of the ZFO-PVDF electrode at a scan rate of 0.1 mV s1. The significant differences between the first and subsequent scans indicate a different Liþ storage reaction taking place in the first scan compared with the following scans. According to the literature [6,12,27], the small current peak at 0.815 V in the first reduction is attributed to Liþ intercalation into ZnFe2O4 (Reaction (1)). The large and spiky current peak centered at 0.591 V corresponds to the conversion reaction of Li2ZnFe2O4 with Liþ to form metallic Zn and Fe (Reaction (2)). This peak could be overlapped with the peak related to the formation of SEI film. Moreover, the weak peak near the cut-off voltage of 0.01 V is associated with the LieZn alloying reaction (Reaction (3)). ZnFe2O4 þ 2Liþ þ 2e / Li2ZnFe2O4
(1)
Li2ZnFe2O4 þ 6Liþ þ 6e / 4Li2O þ 2Fe þ Zn
(2)
Zn þ Liþ þ e / LieZn (alloy)
(3)
LieZn (alloy) / Zn þ Liþ þ e
(4)
Zn þ Li2O / ZnO þ 2Liþ þ 2e
(5)
2Fe þ 3Li2O / Fe2O3 þ 6Liþ þ 6e
(6)
Based on the aforementioned mechanisms, the crystal structure of ZnFe2O4 is essentially destroyed in the first discharge, which
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Fig. 4. (a) TEM and (b) HRTEM image of the ZnFe2O4 nano particles. Inset in (b) shows the SAED pattern of ZnFe2O4 particle.
Fig. 5. XPS spectra of the ZnFe2O4 sample: (a) survey spectrum; (b) Zn 2p spectrum; and (c) Fe 2p spectrum.
Fig. 6. Cyclic voltammograms of the (a) ZFO-PVDF and (b) ZFO-SBR/CMC electrodes.
yields a mixture of Li2O and metallic phases. In the following anodic scan, the broad oxidation peak near 0.106 V corresponds to the dealloying of LieZn phase (Reaction (4)). The peak at approximate 1.604 V is attributed to the oxidation of Fe to Fe3þ and Zn to Zn2þ, along with the decomposition of Li2O (Reactions (5)e(6)). The material thus could not recover to ZnFe2O4 after the first cycle but be replaced by simpler oxides of Fe2O3 and ZnO. The cathodic peak shifts to a higher voltage of 0.924 V in the second cycle due to the permanent phase transformation of the material. Even though the oxidation peak still keeps at 1.604 V, its current intensity becomes weaker and weaker with cycling. In addition, the reduction peak
continuously shifts to lower voltages. This indicates that the ZFOPVDF electrode could not maintain well structure stability and electrochemical reversibility during cycling. In contrast, it is seen from Fig. 6b that the CV curves of the ZFO-SBR/CMC electrode are very stable from the second cycle. Thus, this electrode is expected to show better electrochemical performance than that of ZFO-PVDF. 3.3. Charge-discharge cycling The cycle stability of the ZFO-PVDF and ZFO-SBR/CMC electrodes are studied at a current density of 100 mA g1 (about 0.1C
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rate). Fig. 7a and b compare the chargeedischarge curves of the electrodes. Both electrodes show similar chargeedischarge curves in the first cycle. The small plateau at 0.840 V in the first discharge is due to Liþ intercalation into ZnFe2O4, and the long plateau is due to the conversion reaction of Li2ZnFe2O4 with Liþ to form metallic Zn and Fe. The slope region below 0.656 V is attributed to the formation of SEI film and the LieZn alloying reaction. The first charge process shows an S-shaped voltage profile which is attributed to the de-alloying of LieZn phase and the oxidation of Fe to Fe3þ and Zn to Zn2þ. The initial discharge capacities of the ZFOPVDF and ZFO-SBR/CMC electrodes are 1149.8 mAh g1 and 1053.9 mAh g1, respectively. In the following charge process, the ZFO-PVDF and ZFO-SBR/CMC electrodes exhibit specific capacities of 911.6 mAh g1 and 865.4 mAh g1, respectively. Apparently, the ZFO-SBR/CMC electrode shows a larger initial columbic efficiency (82.1%) than that of ZFO-PVDF (69.8%). The irreversible capacity is due to the formation of SEI film. In addition, a part of the irreversible capacity could be due to the incomplete Liþ extraction from the electrode due to high kinetic barriers of the conversion reactions (Reactions (5)e(6)). Fig. 7c shows the discharge capacities and columbic efficiencies of the electrodes during chargeedischarge cycling. The columbic efficiencies of the ZFO-PVDF electrode increases to ~100% in 15 cycles accompanied with continuous capacity fading. Even though the electrode eventually gets a stable discharge capacity of 461.0 mAh g1 after 15 cycles, it is useless for practical applications. The poor electrochemical performance of the ZFO-PVDF electrode is similar as those reported ZFO electrodes using the PVDF binder [6,28e31]. Contrarily, the columbic efficiencies of the ZFO-SBR/CMC electrode rapidly increase to ~100% in the initial two cycles. In addition, the discharge capacities of this electrode are very stable, which still shows 873.8 mAh g1 after 100 cycles. The capacity fading rate is only 0.06% per cycle with respect to the second cycle. Fig. 7d shows the rate performance of the ZFO-PVDF and ZFO-SBR/CMC electrodes with the cycle rate increasing from 0.1C to 1C. Obviously, ZFO-SBR/ CMC shows much better rate capability than that of ZFO-PVDF. It
231
Fig. 8. Nyquist plots of the ZFO-PVDF and ZFO-SBR/CMC electrodes.
still has a high capacity of 627.6 mAh g1 at the 1C rate, which is much larger than that of ZFO-PVDF (144.9 mAh g1). The small discharge capacities of ZFO-PVDF at high current rates indicate slight structure changes of the active material resulting in good capacity retention. One can see that the ZFO-SBR/CMC electrode exhibits relatively bad capacity retention at high current rates. This may be due to the fast volume change of the active material which would decrease the structure stability of the active material and the electrode. S. Passerini et al. have shown that the carbon coated ZnFe2O4 nano material showed high electronic conductivity, and the surface carbon coating could depress the structure changes of the material thus improving the cycle stability of the electrode at high current rates [13]. 3.4. Electrochemical impedance spectroscopy EIS is performed to study the effects of binder on the electrochemical performance of ZFO. Fig. 8 shows the Nyquist plots of the
Fig. 7. Discharge-charge profiles of the (a) ZFO-PVDF and (b) ZFO-SBR/CMC electrodes. (c) cycling performance and (d) rate performance of the electrodes.
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ZFO-PVDF and ZFO-SBR/CMC electrodes after the first discharge. Both electrodes show two well defined semicircles in the high-tomiddle frequency region which are due to the SEI film and the charge transfer process, respectively. The slope line in the low frequency region is due to the lithium diffusion in the electrode bulk. Based on this, the Nyquist plots can be simulated by the equivalent circuit given in the inset of Fig. 8. In this equivalent circuit, Rs represents the ohmic resistance of the cell. Rf and Csl are the resistance and capacitance of the SEI film. Rct and Cdl represent the charge transfer resistance and double layer capacitance. W is the Warburg parameter reflecting the lithium diffusion in the electrode bulk. The simulated Rs, Rct and Rf values for the ZFO-PVDF electrode are 8.0 U, 178.1 U and 216.9 U, respectively, all of which are much larger than those of the ZFO-SBR/CMC electrode, i.e. 4.9 U, 111.2 U, and 148.9 U, respectively. This indicates that the SBR/CMC binder lowers the ohmic resistance of the electrode, depresses the formation of SEI film and facilitates the charge transfer reactions at the electrode/electrolyte interface. Therefore, the electrochemical kinetics of ZFO-SBR/CMC is better than that of ZFO-PVDF, resulting in improved capacity retention and rate capability. 3.5. Morphologies of the ZnFe2O4 electrodes The morphologies of the ZFO electrodes prepared by different binders before and after chargeedischarge cycling are investigated by SEM. Fig. 9a and b shows the SEM images of the fresh ZFO-PVDF and ZFO-SBR/CMC electrodes, respectively. From the first sight, the surface of ZFO-SBR/CMC is very smooth and have a lot of small pores, while ZFO-PVDF has a rough surface and seems less compact than ZFO-SBR/CMC. A larger magnification of the surface morphologies is shown in the inset of Fig. 9a and b. It is seen that the
ZFO-PVDF electrode shows severe agglomeration of the ZFO and super P particles, but these two components are well dispersed in the ZFO-SBR/CMC electrode. Note that it is difficult to discern the ZFO and super P particles from the present SEM images. In the electrode slurries, a three-dimensional network is formed due to bridging of the particles by the binders since the segments of polymer chain adsorb on the ZFO and super P particles and the adsorbed chains form entanglements. Comparing to PVDF, the carboxylate groups of CMC give rise to an effective surface charge on ZFO and super P particles therefore stabilize the particles dispersion through an electrostatic double-layer repulsion mechanism [22]. As a result, a homogeneous dispersion of the ZFO and super P particles could be obtained using the SBR/CMC binder. Fig. 9c and d shows the cross-section SEM images of the ZFOPVDF and ZFO-SBR/CMC electrodes. The electrode film of ZFOSBR/CMC shows good adhesion to the current collector, while some big gaps are found in the ZFO-PVDF electrode. This indicates that the SBR/CMC binder helps to enhance the adhesion of the electrode film to the current collector. This is due to the strong hydrogen bonding of the carboxyl and hydroxyl groups in CMC with the active material and the current collector, whereas the fluorine atoms in PVDF only form very weak hydrogen bonds [32]. However, it should be noted that CMC is extremely stiff and brittle which could easily form cracks or slide off the current collector when used alone as the binder. Addition of SBR to CMC results in electrodes that are less brittle and, compared to PVDF, shows a smaller Yong's modulus, a larger maximum elongation and stronger adhesion strength to the current collector [33]. For the present ZFO nano particles, the empirically optimized weight ratio of SBR:CMC has been determined to be 1:1 as shown in Fig. 10. The unique morphology of the ZFO-SBR/CMC electrode is helpful to form an
Fig. 9. SEM images of the (a, c) ZFO-PVDF and (b, d) ZFO-SBR/CMC electrodes.
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Fig. 10. Cycling performance of the ZFO-SBR/CMC electrodes with different SBR/CMC ratios.
efficient conductive network. Conductivity measurement shows that the electrical resistance of the ZFO-SBR/CMC electrode is 2.1 U, which is smaller than that of ZFO-PVDF (6.7 U). Charge-discharge experiments show that the ZFO-PVDF electrode undergoes severe capacity fading in the initial 15 cycles while the ZFO-SBR/CMC electrode shows perfect capacity retention. In order to know what happens during cycling, we collect the SEM images of the electrodes after 15 cycles as shown in Fig. 11. The electrodes have been completely washed off by DMC before SEM measurement. Therefore, only solid compounds are observed in the SEM images. A dense surface film could be seen in the cycled ZFOSBR/CMC electrode which could be due to the SEI film and swelling of the binder. As a result, some of the small pores are closed and the electrode becomes more compact comparing to the fresh electrode. In addition, the electrode is still very smooth indicating its good maintainability during cycling. The cycled ZFO-PVDF electrode, on the contrary, exhibits a totally different morphology. The relatively smooth electrode is severely destroyed after cycling. Especially, a number of spikes which could be due to lithium dendrites are observed on the electrode. The lithium dendrite has been regarded as the most detrimental to the cycling efficiency and battery safety since dendrite could accelerate the capacity fading due to the formation of electrochemical isolated regions, or even trigger the internal short when piercing through the battery separator [34]. In LIBs, the Li ions could be reduced to metallic lithium and deposit on the electrode when the working voltage is close to 0 V. It has shown that the deposition of lithium is mainly dependent on
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the properties of SEI film, rather than the initially prepared microstructure of the electrode surface. A uniform texture of the SEI film could generate uniform deposition, which depresses the formation of lithium dendrite. On the contrary, The SEI film with multiple types of defects, such as pits, cracks, crystalline defects and grain boundaries, will result in non-uniform lithium deposition thus facilitates dendrite formation [35]. Fig. 12 shows the TEM images of the cycled ZFO-PVDF and ZFO-SBR/CMC electrodes. It is seen that the ZFO-PVDF electrode shows non-uniform microstructures. The dark-black region in the center of the image could be assigned to the active material. Around the active material, the amorphous phase with an anisotropy growth behavior is attributed to the SEI film. The SEI film shows a tendency to spread in the whole electrode. In addition, the nano crystallites with metallic textures dispersing in the SEI film may be attributed to the segments of lithium dendrite. On the contrary, the cycled ZFO-SBR/CMC electrode shows a very uniform SEI film with thickness about 15 nm. The active material could be effectively protected by this SEI film from attacking by the electrolyte. In addition, this uniform SEI film also prohibits the formation of lithium dendrite. Therefore, SBR/ CMC shows more effectiveness than PVDF in maintaining the stability of the ZFO electrode thus resulting in good rate capability and excellent capacity retention.
4. Conclusions In summary, the electrochemical performance of the ZnFe2O4 electrodes prepared by the PVDF binder and the SBR/CMC binder has been investigated. Comparing with the traditional PVDF binder, the water soluble SBR/CMC binder could enhance the adhesion of the electrode film to the current collector. In addition, it could provide an effective three-dimensional network with uniform distribution of the ZnFe2O4 active material and super P conductive additive. During chargeedischarge cycling, the SBR/CMC binder lowers the ohmic resistance of the electrode, depresses the formation of SEI film and facilitates the charge transfer reactions at the electrode/electrolyte interface. In addition, a uniform SEI film forms on the surface of the active material thus prohibiting the formation of lithium dendrite. Due to these advantages, the ZnFe2O4 electrode prepared by the SBR/CMC binder exhibits high specific capacities, good capacity retention and excellent rate capability. This work shows the possibility of manufacturing electrodes using the cheap water soluble SBR/CMC binder, instead of expensive PVDF and hazardous NMP solvents, thereby improving battery performance, reducing cost and protecting the environment.
Fig. 11. SEM images of the ZFO-PVDF and ZFO-SBR/CMC electrodes after 15 cycles.
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Fig. 12. TEM images of the ZFO-PVDF and ZFO-SBR/CMC electrodes after 15 cycles.
Acknowledgments This work was supported by the 973 Program (No. 2015CB251103), National Natural Science Foundation of China (No. 51472104, 21473075, 51272088), National Found for Fostering Talents of Basic Science (No. J1103202); References [1] C.T. Cherian, J. Sundaramurthy, M.V. Reddy, P.S. Kumar, K. Mani, D. Pliszka, C.H. Sow, S. Ramakrishna, B.V.R. Chowdari, ACS Appl. Mater. Interfaces 5 (2013) 9957e9963. [2] P. Lavela, J.L. Tirado, M. Womes, J.C. Jumas, J. Phys. Chem. C 113 (2009) 20081e20087. [3] L.M. Jin, Y.C. Qiu, H. Deng, W.S. Li, H. Li, S.H. Yang, Electrochim. Acta 56 (2011) 9127e9132. [4] Y.L. Xiao, J.T. Zai, X.M. Li, Y. Gong, B. Li, Q.Y. Han, X.F. Qian, Nano Energy 6 (2014) 51e58. [5] Z.L. Zhang, Y.H. Wang, M.J. Zhang, Q.Q. Tan, X. Lv, Z.Y. Zhong, F.B. Su, J. Mater. Chem. A 1 (2013) 7444e7450. [6] A.S. Hameed, H. Bahiraei, M.V. Reddy, M.Z. Shoushtari, J.J. Vittal, C.K. Ong, B.V.R. Chowdari, ACS Appl. Mater. Interfaces 6 (2014) 10744e10753. [7] F. Martinez-Julian, A. Guerrero, M. Haro, J. Bisquert, D. Bresser, E. Paillard, S. Passerini, G. Garcia-Belmonte, J. Phys. Chem. C 118 (2014) 6069e6076. [8] Y. Sharma, N. Sharma, G.V. Subba Rao, B.V.R. Chowdari, Electrochim. Acta 53 (2008) 2380e2385. [9] X.W. Guo, X. Lu, X.P. Fang, Y. Mao, Z.X. Wang, L.Q. Chen, X.X. Xu, H. Yang, Y.N. Liu, Electrochem. Commun. 12 (2010) 847e850. [10] P.F. Teh, Y. Sharma, S.S. Pramana, M. Srinivasan, J. Mater. Chem. 21 (2011) 14999e15008. [11] Y. Ding, Y.F. Yang, H.X. Shao, Electrochim. Acta 56 (2011) 9433e9438. [12] Y.F. Deng, Q.M. Zhang, S.D. Tang, L.T. Zhang, S.N. Deng, Z.C. Shi, G.H. Chen, Chem. Commun. 47 (2011) 6828e6830. [13] D. Bresser, E. Paillard, R. Kloepsch, S. Krueger, M. Fiedler, R. Schmitz, D. Baither, M. Winter, S. Passerini, Adv. Energy Mater. 3 (2013) 513e523.
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