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ScienceDirect Polymer binder: a key component in negative electrodes for high-energy Na-ion batteries Wanjie Zhang1, Mouad Dahbi1,2 and Shinichi Komaba1,2 In the hope of finding potential replacements of Li-ion batteries, in which lithium, a minor-metal is used as ion carrier, Na-ion batteries that are operable at high voltage, higher than 3 volts, under ambient condition are attractive to develop much lowpriced energy systems. However, owing to the larger atomic/ ionic volume of sodium than that of lithium, severer volume change upon sodiation/desodiation inevitably deteriorates the performance of Na-ion batteries. Polymer binder, as a key component in maintaining the structural integrity during the volume change with pre-formed SEI effect, has not been thoroughly discussed for negative electrode in Na-ion batteries. In this review, we summarized several advanced polymer binders that have already been reported, and introduced their recent advances for high capacity electrode in Na cells. Addresses 1 Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan 2 Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, Kyoto 615-8245, Japan Corresponding author: Komaba, Shinichi ([email protected]
Current Opinion in Chemical Engineering 2016, 13:36–44 This review comes from a themed issue on Energy and environmental engineering Edited by Vilas Pol and Vasilios Manousiouthakis
http://dx.doi.org/10.1016/j.coche.2016.08.001 2211-3398/# 2016 Elsevier Ltd. All rights reserved.
Introduction Over the past decades, portable power applications continue to drive research and development of advanced battery systems. It did not take long for researchers to realize practical lithium-based cells as promising candidates for secondary batteries as lithium has the lightest weight, highest voltage and greatest energy density. After first commercialization in 1991 by Sony Corp., rechargeable Li-ion batteries, due to their outstanding energy densities and power densities, have been extensively used for various applications. Nowadays, Li-ion batteries are found not only in portable devices, such as cellphones, cameras, and laptops etc., but also in electric vehicles and Current Opinion in Chemical Engineering 2016, 13:36–44
large-scale stationary electrical energy storage. To meet the swift growth of portable electronics market requirements, the performance of rechargeable Li-ion batteries needs to be improved. However, the limited resource of lithium awakes the urge of finding potential alternative elements . Therefore, sodium, as one of the most abundant elements in the Earth’s crust and sea water, has revived considerable interest in Na-ion batteries since 2010. Both Li-ion and Na-ion batteries are composed of positive and negative electrodes, current collectors, as well as separators that contains electrolyte solution, as shown in Figure 1a). Powder electrode materials and current collectors are bonded together by polymeric binders. Because of the formation of Li–Al alloy at lower potential (<0.5 V versus Li), costly copper is selected as current collector for graphite negative electrode in Li-ion batteries. In Na-ion batteries, however, Al foil can be used as the current collector for negative electrode because Al does not alloy with Na, which provides a significant advantage of Na system in reducing the total cost of battery materials. Ideal binder materials are expected to form a quality network between the active material and the conductive additive that is stable and facilitates the electronic and ionic transport. Therefore, binder plays an important role in maintaining the structural integrity of the electrodes and thus influences the capacity and the cycleability of batteries. Moreover, due to the larger atomic/ionic size volume of sodium than that of lithium, greater volume change upon sodiation/desodiation has detrimental effects on the performance of Na-ion batteries, particularly for alloy or conversion materials-based electrode that possesses high capacity and lower potential operation, as shown in Figure 1b. To realize higher energy density of Na-ion battery, we essentially need high capacity negative electrode operable at lower potential such as P, phosphides, Sn, and so on. Simultaneously, composite electrodes based on these active materials suffer from severe volume change by sodiation, resulting in unsatisfactory cycle life which is because of unstable passivation and insufficient mechanical stability of the electrodes. To overcome these drawbacks, functional binders is one of the most efficient and simple way to achieve satisfactory cycle life and reversibility by improving passivation and mechanical strength for the composite electrodes, as seen in Figure 1b. In this article, we describe several main binding materials that have already been applied in the negative electrodes www.sciencedirect.com
Binder for high-energy Na-ion batteries Zhang, Dahbi and Komaba 37
(a) Current Collector
Battery charger or Operating Device
Binder PVdF, CMC, PAA, PANa
Ave. E / V (vs. Na+ /Na)
Energy Density /Wh (kg–1 of positive & negative) vs. Na2/3Ni1/3Mn1/2Ti1/6O2 as positive
(125 mAh g–1, Eave.= 3.70 V vs. Na)
Li4Ti5O12 1.0 TiO2-NT
Na2C8H4O4 Na2Ti6O13 Graphite 0.5 Hard carbon Na2Ti3O7
Performance greatly depends on polymer binder
Q / mAh g (of negative) –1
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(a) Schematic illustration of Na-ion batteries. (b) Average voltage and energy density versus gravimetric capacity for various negative electrodes materials for Na-ion batteries, carbonaceous materials (black), oxides and phosphates as sodium insertion materials (red), alloy (blue), phosphorus and metal phosphides (green), oxides and sulfides with conversion reaction (gray). Gravimetric energy density (Wh kg 1), based on the weight of active materials in optimally balanced positive and negative electrodes, is calculated by the difference of average potential and reversible capacity of active materials examined in Na half cells according to previous reports. Red region corresponding higher capacity materials requires good polymer binder to achieve acceptable performance.
for Na cells, as shown in Figure 2. Poly(vinylidene fluoride) (PVdF) is a conventional binder for Li-ion batteries due to its good electrochemical stability and adhesion to the electrode materials and current collector. However PVdF binder-containing electrode suffers from large www.sciencedirect.com
volume expansion materials upon cycling and its high cost. As a result, non-fluorinated binders that favor more stable passivation, superior cycleability, high tolerance against the internal mechanical stress by the volume change, less costly and more environmentally friendly, Current Opinion in Chemical Engineering 2016, 13:36–44
38 Energy and environmental engineering
R = H or
n Current Opinion in Chemical Engineering
Typical binders and their molecular structures, (a) PVdF, (b) Na-CMC, (c) PAA, and (d) PANa.
have been taken into account to overcome the disadvantages of PVdF. In this context, considerable interest has been revived in water-soluble polymer binders, such as sodium carboxymethyl cellulose (Na-CMC), polyacrylic acid (PAA), and sodium polyacrylate (PANa).
Binder materials for composite negative electrode Poly(vinylidene fluoride) (PVdF)
As a typical binder used in Li-ion batteries, PVdF has been widely applied for the graphite negative electrode because of its good electrochemical stability and adhesion to the electrode materials and current collector, as well as the ability to absorb electrolyte for facile transport of Li+ to the active material surface [2–7]. Nevertheless, PVdF reacts with lithium metal and lithiated graphite (LixC6) at elevated temperatures to form LiF . Moreover, PVdF is wettable in non-aqueous liquid electrolytes, which sometimes deteriorates the adhesion between electrode material and current collector, finally leading to increases of contact resistance. Indeed, graphite has been extensively used as negative electrode material in Li-ion batteries in comparison with other carbon materials because of its high gravimetric and volumetric capacity [9–11]. However, in Na cell, Na insertion amount is considerably smaller due to the fact that Na atoms seems to be inserted into the graphite by heating with a Na metal under helium or vacuum atmosphere and by electrochemical reduction, forming NaC64 Current Opinion in Chemical Engineering 2016, 13:36–44
[12,13]. Very recently, it has been demonstrated by Jache et al. that graphite can be activated for Na-ion storage in cells using diglyme as electrolyte solvent, exhibiting an excellent capacity retention for over 1000 cycles, nonetheless, the reversible capacity was limited to less than 150 mAh g 1 . The storage mechanism of graphite is based on co-intercalation of solvent molecules along with the Na-ions, forming ternary graphite intercalation compounds (t-GICs) . Therefore, the research interest on carbonaceous materials has been extended to lower crystalline carbons, that is, soft-carbon and hard-carbon. The electrochemical reversibility of sodium intercalation into hard-carbon was reported by Stevens et al. in 2000 , the early studies showed that the hardcarbon electrodes deliver a reversible capacity of ca. 300 mAh g 1 with acceptable capacity retentions [16– 18]. We reported that the defluorination of PVdF binder inevitably occurs during sodium insertion, which is due to insufficient passivation in Na cell compared to that of Li cell . When the hard-carbon composite electrode is easily damaged by the defluorination, hard-carbon particles should be partly loosened and electrically isolated, resulting in the lager polarization, capacity degradation and lower efficiency . As broadly known, PVdF shows very poor performance as binder for large volume expansion alloy materials such as Sn–Co–C , a-SiSn  and TiPx  in Li cells. As well as in Na cells, as reported by Li et al., PVdF binder for FeP composite electrode exhibit a severe capacity fading . www.sciencedirect.com
Binder for high-energy Na-ion batteries Zhang, Dahbi and Komaba 39
Therefore, PVdF is likely attached to activated materials particles via weak Van der Waals forces between its fluorine atoms and hydrogen atoms. It fails easily to accommodate large changes in spacing between the particles during battery cycling and becomes quickly unable in keeping the particles together and maintaining electrical conductivity . Sodium carboxymethyl cellulose (Na-CMC)
Na-CMC is a linear polymeric derivative of cellulose with varying levels of carboxymethyl substitution. The carboxymethyl groups that dissociate to form carboxylate anionic functional groups are responsible for the aqueous solubility of the CMC relative to the insoluble cellulose, which makes it possible to be used as a binder of Li-ion batteries to be processed in water, promoting less expensive electrode fabrication process . In addition, it has been demonstrated that calcination of Na-CMC at 700 8C yields Na2CO3, a naturally occurring mineral deposit , which reveals that Na-CMC is more environmentally friendly than the conventional PVdF binder. The application of Na-CMC binder for hard-carbon electrode in Na cells was reported by our group in 2014, where we discussed the synergetic effects of binders and electrolyte additives on the electrochemical performance . The decomposition of electrolyte on hard-carbon during the initial cycle was sufficiently suppressed by NaCMC binder, which implies that the role of Na-CMC for pre-formed SEI is similar to the carboxyl polyanion binder in lithium insertion electrodes . Moreover, the uniform coverage with Na-CMC was proposed to be advantageous to stabilize the electrochemical performance, leading to excellent capacity retention with the higher efficiency compared to PVdF . During the recent developments for Na-ion batteries, materials that electrochemically alloy with Na have been studied as potential replacement of carbonaceous materials [27–30]. Qian et al. reported that Sb–C nanocomposite electrode with Na-CMC can deliver a reversible capacity of 610 mAh g 1 with good rate capability at a very high current density of 2000 mA g 1 and long-term cycling stability with 94% capacity retention over 100 cycles . Commercial micro-sized Sb electrode investigated by Darwiche et al. revealed a good cycling performance with a capacity of 600 mAh g 1 up to 160 cycles and a Coulombic efficiency of 98% when using Na-CMC as binder and fluoroethylene carbonate (FEC) as electrolyte additive . Qian et al.  reported an amorphous phosphorus composite electrode with carbon can deliver a high capacity of 1764 mAh g 1 at a current density of 250 mA g 1 using Na-CMC as binder. Furthermore, conversion materials have also been extensively studied in the research field of Li-ion batteries as potential replacements of insertion materials. Compared www.sciencedirect.com
to the insertion mechanism, conversion reaction allows more electrons to participate in the electrochemical reaction, resulting in much higher reversible capacities. In the meantime, large volume change occurring during conversion reaction becomes problematic, which makes the selection of polymer binder crucial to achieve satisfying cycleability for such materials. Among conversion materials, metal phosphides exhibit electrochemical reactivity with Na, leading to the formation of Na3P and nanosized metallic particles, creating a conductive network which supports good electrical conduction inside the electrode upon cycling . Owing to the high tolerance of using water-based polymer binders against the internal mechanical stress caused by the volume change , NaCMC has been extensively applied in metal phosphide electrodes. Fullenwarth and coworkers reported that NiP3 composite electrode with Na-CMC shows a reversible storage capacity for 900 mAh g 1 after 15 cycles , as shown in Figure 3a. Tin phosphide Sn4P3 was studied by Qian et al., as given in Figure 3b, the composite electrode delivered a high reversible capacity of 850 mAh g 1 with a remarkable rate capability, where 50% capacity was remained at a current density of 500 mA g 1 . As well as for CoP composite electrode developed by Li et al., which exhibited a high initial specific capacity of 770 mAh g 1 using Na-CMC binder , as shown in Figure 3c. However, none of the metal phosphide electrode survived long-term cycling stability over 100 cycles except for Sn4P3. The composite electrode containing 80% active material (Sn4P3:carbon black = 7:3) with 10 wt% Na-CMC and 10% Super P was able to reach a capacity retention of 86% over 150 cycles . Polyacrylic acid (PAA)
Similar to CMC, PAA and its derivatives are often used as dispersants, thickeners. The effects of PAA on suspension stability and the resulting mechanical properties of the electrodes in Li-ion batteries were investigated by Lee et al. . The adhesion strength of graphite sheet prepared with PAA/CMC was three times greater than that of CMC alone. Moreover, the electrochemical performance indicated that better adhesion strength with PAA binder correlates with better capacity retention during cycling. Recently, PAA has also been reported in the developments in Na cells, especially for metal phosphide electrodes. As reported by Kim et al., Sn4P3 composite electrode with PAA delivers a high reversible capacity of 718 mAh g 1, and shows very stable cycle performance with negligible capacity fading over 100 cycles . CuP2/ C composite electrode developed by Zhao et al., with the presence of PAA binder, was capable to show a large capacity over 500 mAh g 1 with high rate capability and decent short-term cycling stability , as shown in Figure 3d. Li et al. investigated the binder dependency Current Opinion in Chemical Engineering 2016, 13:36–44
40 Energy and environmental engineering
HT R S
84 82 80 20
Coulombic efficiency (%) Capacity (mAh g–1)
1600 Sn4P3/C P/C Sn/C
800 600 400 200 0 0
Cycle number (d)
2.0 1.5 1.0 0.5 0.0
Capacity (mAh g–1)
1000 900 800 700 600 500 400 300 200 100 0
60 40 20 0
Coulombic Efficiency (%)
Specific Capacity (mAh/g)
Cycle number Current Opinion in Chemical Engineering
Electrochemical properties of metal phosphides composite electrodes. (a) NiP3, reprinted with permission from Fullenwarth et al. , copyright (2013) Royal Society of Chemistry. (b) Sn4P3, reprinted with permission from Qian et al. , copyright (2014) American Chemical Society. (c) CoP, reprinted with permission from Li et al. , copyright (2015) Elsevier. (d) CuP2, reprinted with permission from Zhao et al. , copyright (2015) Royal Society of Chemistry.
on the electrochemical performance of FeP composite electrode, PVdF binder presented a severe capacity fading during 5 cycles, and CMC alone suggested a better capacity retention during the initial cycles, while a high capacity of 764.7 mAh g 1 and an improved capacity retention to 20 cycles was obtained using a combined binder of Na-CMC/PAA . Sodium polyacrylate (PANa)
The preparation of PANa was described previously by our group, a PANa aqueous solution was obtained by neutralization of PAA by adding sodium hydroxide aqueous solution . We also demonstrated that sodium or lithium polyacrylate binder is able to suppress the co-intercalation of Li+ and propylene carbonate into the graphite electrodes, and suggested that polyacrylate is an effective SEI layer modifier . It was also reported that the viscosity of PAA solution increases with neutralization due to different polymer conformation [41,43,44], Current Opinion in Chemical Engineering 2016, 13:36–44
indicating that polyacrylate leads to a more homogeneous layer of electrode material during electrode preparation. Consequently, it is believed that polyacrylate can form a better passivation layer on the surface of active materials. During the first cycle, this layer will help to form a more effective SEI layer to alleviate the capacity loss and improve the cycling performance [41,43]. Therefore, polyacrylate binder provides high adhesive strength, uniform dispersion of active materials, and pre-formed SEI effect, which suppresses the electrolyte decomposition at the initial cycle [45–47]. Given that polyacrylate has carboxylic acid sites similar to that of CMC, it may also has chemical interaction with alloy materials particles that usually have hydroxyl groups on the surface, therefore resulting in better capacity retention . Indeed, as previously reported in Li cells, negative electrode material Sn–Co–C exhibited an excellent capacity retention using PALi as polymer binder www.sciencedirect.com
Binder for high-energy Na-ion batteries Zhang, Dahbi and Komaba 41
compared to PVdF or Na-CMC , as well as the application of PANa for Si electrode , both indicating that polyacrylate is a promising binder for materials with large-volume expansion during cycling. In the hope of finding high-energy electrode material for Na-ion batteries, elemental phosphorus, which enables three-electron reversible redox couple of P/PIII with sodium uptake, has been reported to present a high reversible capacity more than 2000 mAh g 1 [30,49]. In 2012, our group reported redox process of red phosphorus electrode and its potential application in rechargeable Na batteries . Binder dependency of its electrochemical properties was also of quite importance as we discussed from comparison between PVdF and PANa binder , as shown in Figure 4a. However, the low electrical conductivity of red phosphorus, <1 10 14 S cm 1 , has detrimental effects on its electrochemical performance . Since black phosphorus has higher
electrical conductivity compared with the other two allotropes, white and red phosphorus [52,53], it has been revealed to show high capacity as negative electrodes for Li cells [54–56], as well as for Na cells [57,58]. Our group also reported that a black phosphorus electrode with the PANa binder shows high specific capacity and excellent cycling performance in Na cells [59,60]. The composite electrode showed high initial charge capacities of 2050 mAh g 1 and discharge capacities over 1610 mAh g 1 1 at a rate of 125 mA g 1 using 1 mol dm 3 NaPF6 EC:DEC. The averaged Coulombic efficiency was maintained at 97% from the second cycle, as presented in Figure 4b. Nano-sized Sn electrode with PANa binder was also investigated by our group, and the composite electrode delivered a reversible capacity of ca. 600 mAh g 1 with a good capacity retention in a voltage range of 0.00–0.65 V, in despite of the volume change of during cycling , as given in Figure 4c. Moreover, very recently our group has also developed iron phosphide FeP4
Voltage / V
P:AB:PVdF = 1:1:1
P:AB:PANa = 1:1:1
Voltage / V
P:AB:PVdF = 8:1:1
Capacity / mAh g–1
Capacity / mAh·g–1
Voltage / V
Coulombic Efficiency / %
Voltage / V
Capacity / mAh g–1
Sodiation Desodiatioin Coulombic Efficiency 10
Capacity / mAh g–1
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Electrochemical properties of electrodes prepared with PANa binder. (a) Red phosphorus, reprinted with permission from Yabuuchi et al. , copyright (2014) John Wiley and Sons. (b) Black phosphorus, reprinted with permission from Dahbi et al. , copyright (2016) American Chemical Society. (c) Sn-based electrode, reprinted with permission from Fukunishi et al. , copyright (2016) American Chemical Society. (d) FeP4, reprinted with permission from Zhang et al. , copyright (2016) Elsevier. www.sciencedirect.com
Current Opinion in Chemical Engineering 2016, 13:36–44
42 Energy and environmental engineering
electrode incorporating PANa binder . FeP4 composite electrode delivered a large reversible capacity of 1137 mAh g 1 and a Coulombic efficiency of 84.0% during the first cycle under a current density of 89 mA g 1, as shown in Figure 4d. The presence of PANa binder resulted in the formation of a relatively stable SEI layer on electrode surface, and thus ensured a high capacity over 1000 mAh g 1 for 30 cycles, which was a much enhanced performance compared to the other reported transition metal phosphides [23,36,37,40].
Concluding remarks In summary, binder plays a key role in maintaining the structural integrity of electrode materials; therefore, electrochemical performance of Na cell is highly influenced by the choice of binders. Especially for high-energy electrode materials, such as phosphorus, Sb, Sn, and metal phosphides, requirements of binder, such as high adhesive strength, uniform dispersion of active materials, preformed SEI effect that suppresses the electrolyte decomposition at the initial cycle have to be fulfilled. Moreover, in order to avoid using toxic and costly organic solvents for slurry during electrode preparation, less costly and more environmentally friendly water-soluble binders are recommended as potential replacements for conventional PVdF binder, for instance Na-CMC, PAA, and PANa. Furthermore, it must be taken into consideration that the binder also reduces the gravimetric capacity because of the extra inactive binder mass, and thus not only binder itself but also its content in the electrode should additionally be optimized. We believe that engineering binder having functionality of surface modification and mechanical durability to improve passivation and reversibility of powdery active materials will play a more important role to realize higher capacity electrode for energetic Li-ion, Na-ion, and K-ion batteries.
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This study was in part supported by Ministry of Education, Culture, Sports, Science and Technology (MEXT) under the ‘Elemental Strategy Initiative for Catalysis and Batteries (ESICB), and JSPS KAKENHI Grant Number JP16K14103 and JP16H04225.
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Papers of particular interest, published within the period of review, have been highlighted as: of special interest of outstanding interest 1.
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Current Opinion in Chemical Engineering 2016, 13:36–44
61. Fukunishi M, Yabuuchi N, Dahbi M, Son JY, Cui Y, Oji H, Komaba S: Impact of the cut-off voltage on cyclability and passive interphase of Sn–polyacrylate composite electrodes for sodium-ion batteries. J Phys Chem 2016, 120:15017-15026. 62. Zhang W, Dahbi M, Amagasa S, Yamada Y, Komaba S: Iron phosphide as negative electrode material for Na-ion batteries. Electrochem Commun 2016, 69:11-14.