Novel multi-block conductive binder with polybutadiene for Si anodes in lithium-ion batteries

Electrochimica Acta 315 (2019) 58e66

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Novel multi-block conductive binder with polybutadiene for Si anodes in lithium-ion batteries Qingqin Ye a, 1, Peitao Zheng b, 1, Xiaohu Ao a, 1, Dahua Yao a, Zhiwen Lei a, Yonghong Deng b, **, Chaoyang Wang a, * a b

Research Institute of Materials Science, South China University of Technology, Guangzhou, 510640, China Department of Materials Science & Engineering, Southern University of Science and Technology, Shenzhen, 518055, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 March 2019 Received in revised form 8 May 2019 Accepted 17 May 2019 Available online 20 May 2019

Using conductive binders, instead of the traditional binders and conductive additives, is an effective way to increase the specific capacity and long cycle life of the silicon (Si) anode. However, the rigid conductive binders of Si anodes tend to form cracks during the repetitive huge volume changes of Si. Here, a novel ntype conductive binder, multi-block polymer of poly(1-pyrenemethyl methacrylate) and polybutadiene (PPy-b-PB) was designed. The polybutadiene (PB) rubber was incorporated into the conductive binder by combining condensation and reversible addition-fragmentation chain transfer polymerization. With high initial coulombic efficiency of 77.9%, the PPy-b-PB/Si anode possesses high reversible capacity (2274 mAh g
1) and capacity retention (87.1%) at the rate of 0.2 C (0.84 A g
1) after 200 cycles. The superior cyclic performance of the PPy-b-PB can be ascribed to the PB rubber, which enhanced the reversible deformation ability, flexibility, and adhesion length scale of the binder, without with a negligible change of the charge-transfer resistance for the anode. The original structure and the preparation method of the multi-block conductive binder provide a new way to manufacture multifunctional binders. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Conductive binder Polybutadiene Silicon anode Lithium-ion battery Multi-block

1. Introduction Lithium-ion batteries (LiBs) have been widely applied in portable electronic products and electric vehicles due to their higher energy density, slower self-discharge and more negligible memory effect than other secondary batteries [1e3]. However, to meet the need of rapid development of electric vehicles, the energy density and the long cycle life of LiBs should be further enhanced [4e6], which can be realized by enhancing the specific capacity of anode. Among the negative active materials of LiBs, silicon (Si) is promising because of its high theoretical specific capacity (4200 mAh g
1) and natural abundance. In order to ensure the electrical conductivity between the Si particles, conductive additive is an indispensable component in Si anode. However, the conductive additive, without electrochemical

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Deng), (C. Wang). 1 YQ, ZP and AX contributed equally to this work. https://doi.org/10.1016/j.electacta.2019.05.093 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

[email protected]

activity, decreases the specific capacity of the anode. In addition, the binding force between conductive additive and Si is so poor that the contact between the conductive additive and the Si particles tends to be ineffective during the repetitive huge volume changes (300%) of Si particles, which causes the broken of the conductive network in Si anode [7,8]. To solve this problem, conductive binders, with both good electrical conductivity and binding force, have been designed. The conductive binders can not only serve as conductive additive to keep the contact among Si particle effective, but also act as binders, such as the derivative of polyvinylidene fluoride [9], sodium alginate [10e12], carboxymethyl cellulose [13e15], polyacrylic acid [6,16,17], and polyimide [18,19], to promote the formation of stable solid electrolyte interphase (SEI), restrain the pulverization of Si, retard the decay of the specific capacity of the anode, and enhance the life time of Libs [19,20]. Liu et al. [21] first reported a conductive polymer with both satisfactory specific capacity and long cycle life for the Si anode. So far, more and more kinds of conductive binders with different structures have been synthesized [2,7,8,20e28]. They can be divided into two categories according to their doping type: p-type and n-type conductive binders. The p-type conductive

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binders, such as polyanilines [2,29] and poly(3, 4ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) [7,22,23], needs to be doped with strong acid, which increases the requirement in corrosion resistance of the equipment for processing the electrode. Moreover, the p-type conductive binders may not stay p-doped in the Si anode during the charge-discharge procedure ranging between 0.01 and 1 V versus Li/Liþ, which will result in the poor electrical conductivity of the polymer and the rapid decays capacity of the anode [21,25]. On the contrary, the n-type conductive binders could be cathodically doped to be highly conductive in the Si anode below 1 V [21]. Another difference between p-type and n-type conductive binders is that the former are usually multicomponent before thermal treatment of the anode slurry, whereas the latter are single-component, which can make it easier to prepare anode. Besides, it also means that the performance of the n-type conductive binders, such as adhesion ability and electrical conductivity, can be further enhanced through blending with other small molecules or polymers, which shows the worth of further research of n-type conductive binders [20]. The n-type conductive binders can be classified into main-chain [8,20,21,24,25] and side-chain conductive binders [26e28]. Different from the main-chain conductive binders which are both complicated and time-consuming to prepare [26], the side-chain conductive binders that have been reported are the homopolymer or copolymer which can be easily prepared by free radical polymerization of 1-pyrenemethyl methacrylate (PyMA) (and other vinyl monomers). Consequently, the scale-up potential of the sidechain conductive binders could be greater than that of the mainchain conductive binders. The works of the side-chain conductive binders are mainly focused on the electrical conductivity, stronger adhesion, and swelling performance of the binder. Besides the performances above, the flexibility of poly(1-pyrenemethyl methacrylate) (PPy) should be concerned because PPy is too rigid to totally relieve the stress under the high-stress condition, which will promote the destruction of the conductive network formed via p-p stacking interaction of pyrene rings during the repetitive huge volume changes of Si particles (Fig. 1A) [24]. To this problem, one advisable method is incorporation of rubber into the conductive binder since reversible extension and contraction of the rubber part

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contribute to the superior cyclic performance of the binder used in Si anode [30]. Among all kinds of rubbers, the purchasable hydroxyl-terminated polybutadiene (HTPB), a kind of low molecular weight liquid polybutadiene (PB) terminated with a hydroxyl group on both ends, has been widely used in PB-based polyurethane elastomer for its low-temperature and chemical resistance [31]. In this work, a n-type conductive binder consisting of multiblock copolymer of Poly(1-pyrenemethyl methacrylate) and polybutadiene (PPy-b-PB), was designed. The PPy-b-PB was prepared by combining condensation (Fig. 2C) and reversible additionfragmentation chain transfer (RAFT) (Fig. 2D) polymerization. The PPy-b-PB binder outperforms the PPy binder in three aspects, as demonstrated below. Firstly, with great reversible deformation ability, the PPy-b-PB will hold Si particles together, acting like a spring (Fig. 1B), to reduce the change of spacing among the particles during the delithiation process, which contribute to the integrity of the electronic conduction pathways. Secondly, the flexible PB blocks in PPy-b-PB can not only extended without destruction to accommodate the volume change of Si particles, but also make it easy for the rigid PPy blocks to self-assemble to reform conductive network between Si particles by the p
p stacking force of the pyrene ring [26]. Thirdly, the electrode using the PPy-b-PB as conductive binder possesses better mechanical stability than the electrode with the PPy in spite of the absence of polar groups and self-assembling groups in the PB, which is due to the larger adhesion length scale and the relief of the stress concentration of the PPy-b-PB. The PPy-b-PB/Si anode possesses high initial coulombic efficiency (77.9%), superior reversible capacity (2274 mAh g
1) and high capacity retention (87.1%) at the rate of 0.2 C (0.84 A g
1) after 200 cycles. 2. Experimental 2.1. Materials Pyridine (99.5%, super dry) and triethylamine (99.5%, super dry) were purchased from J&K Scientific Ltd. Tetrahydrofuran (99.0%), purchased from Aladdin Industrial Corporation, was distilled from

Fig. 1. Schematic illustration of the Si electrodes employing PPy (A) or PPy-b-PB (B) as conductive binders during delithiation/lithiation process.

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Fig. 2. Synthesis routes of PPy-b-PB.

sodium/benzophenone before used. HTPB (Mn ¼ 2700e3300 Da) was purchased from Yingkou Tanyun Chemical Research Institute Co., Ltd. Dialysis tube (molecular weight cut off is 7000 Da) was purchased from Guangzhou Yingmao Analytical Technology Services Ltd. Silicon powder was purchased from Shanghai Naiou Nano Technology Co., Ltd. All the other reagents (analytical grade) were purchased from Aladdin Industrial Corporation and used as received. PyMA (Fig. 2A) [26], S,S0 -bis(R,R0 -dimethyl-R00 -acetic acid)-trithiocarbonate (CTA) (Fig. 2B) [32], and PPy [26], were synthesized following the previous literature. 2.2. Synthesis of multi-block polymer of CTA and PB (CTA-b-PB) (Fig. 2C) HTPB of 10 g was dissolved in cyclohexane (100 mL) in a threenecked flask. After the solution of CTA (3.7 mmol) and p-toluenesulfonic acid (1.5 mmol) in 1, 4-dioxane (25 mL) was added to the flash, Dean-Stark trap and reflux condenser were installed to separate the water produced during the reaction. Then the mixture was refluxed at 100  C under a nitrogen atmosphere for 36 h. After the mixture was cooled to room temperature, 1, 4-dioxane (25 mL) was added to dissolve the unreacted reactants. Then the solution was dialyzed against 1, 4-dioxane for 3 days. The resultant solution was freeze-dried and further dried in vacuum oven to afford a yellow viscous liquid. 2.3. Synthesis of multi-block polymer of PPy and PB (PPy-b-PB) (Fig. 2D) CTA-b-PB (0.126 g), PyMA (2.400 g) and azobisisobutyronitrile (AIBN) (0.016 g) were dissolved in tetrahydrofuran (8 mL). After three freeze-vacuum-thaw cycles, the solution was immersed into oil bath at 60  C. One day later, the reaction was terminated by cooling in ice-water bath. Then the solution was precipitated in diethyl ether. Finally, the yellowish powder was obtained by drying under vacuum. The result of gel permeation chromatography (GPC,

CHCl3, polystyrene standards) of PPy-b-PB: Mn ¼ 65000, PDI ¼ 1.24. 2.4. Preparation of electrodes and assembly of coin batteries Firstly, all of the conductive binders in this work were dissolved in N-Methyl pyrrolidone (NMP) to attain yellowish solution. Then the Si nanoparticles, with diameter between 79 and 122 nm (obtained by dynamic light scattering test with Malvern Mastersizer 2000S) according to Fig. S1, were dispersed into the solution of conductive binders by ball mill to make homogeneous anode slurry which was cast on a Cu foil using doctor blade method. The weight ratio of conductive binders to Si nanoparticles is 0.5 according to the previous literature [26]. NMP was removed by using the vacuum oven for 24 h at 90  C. The CR2025 type coin cells were assembled using Li metal as counter electrode and Celgard 2400 as separator in an Ar-filled glovebox. The electrolyte used in the batteries consists of 1 M LiPF6 in Propylene carbonate/Diethyl carbonate (PC/DEC) (1:4 by volume) and 16% by volume of fluoroethylene carbonate (FEC). 2.5. Characterization of PPy-b-PB Nuclear magnetic resonance (NMR) spectra were recorded in CDCl3 or DMSO‑d6 with Bruker AVANCE III HD 600 spectrometer at 600.17 MHz (1H) or 150.93 MHz (13C). Fourier transform infrared (FTIR) spectra were recorded with Bruker VERTEX 33 from 400 to 4000 cm
1. The thermo stability of PPy and PPy-b-PB were characterized by with Netsch TG 209 F3 Tarsus at the heating rate of 10  C min
1 under a nitrogen atmosphere. The temperature in the Thermalgravimetric analysis (TGA) experiment ranged from 30 to 480  C. With a three-sided pyramidal tip whose radius is around 150 nm, Keysight UTM150 nanoindentation system was used to carry out the nanoindentation test. The mode of the experiment was selected as load-controlled mode, in which the load was fixed at 200 mN. The time load time was 10 s, while the hold time at the peak (200 mN) of the Load-Indentation Depth curves is 5 s.

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2.6. Characterization of PPy-b-PB/Si anode The galvanostatic charge-discharge cycling performance test was conducted from a potential range from 0.01 to 1.0 V versus Li/ Liþ by using a Neware Battery Test System. Rate performance was tested on the Neware Battery Test System at the current density of 0.1 C, 0.2 C, 0.5 C, 1 C, and 0.1C after 15 pre-cycles at a current density of 0.05 C. Cyclic voltammetry (CV) curves were measured with a Solartron analytical electrochemical workstation at a scanning rate of 0.05 mV s
1 from 0.01 to 2.5 V. Three cycles have been run for the cells before the EIS test. Firstly, the cells were chargeddischarged for 2 cycles between 0.01 and 1 V versus Li/Liþ. Then the cells were charged to 0.01 V, followed by discharged to 60% state of charge (SOC). The rate of charge-discharge cycles is 0.05 C. Finally, EIS tests were performed on the Solartron analytical electrochemical workstation from 0.01 to 106 Hz. The samples for the peeling test was prepared according to the following steps: cutting the electrode into rectangle (30  12 mm), sticking the side of the electrode without anode materials to a glass slide with doublesided tape, sticking one end of 3 M Scitch Tape to the side of the electrode with anode materials, and sticking the other end of the 3 M Scitch Tape to another glass slide. Then the 180 peeling tests of samples were measured with a universal testing machine at the speed of 5 mm min
1. All the morphologies were characterized by field-emission scanning electron microscopy (Tescan MIRA3) under high vacuum. 3. Results and discussion 3.1. Preparation of PPy-b-PB To incorporate the PB rubber into the multi-block polymer, two methods are available. One is combination of the polymerization of butadiene and the reactions which are used to prepare the block copolymer. The other is the reaction between two terminal hydroxyl groups of HTPB and two functional end groups of PPy. The latter is preferable since the HTPB is purchasable and the polymerization of butadiene is complicated. Among the ways to prepare the polymer with two functional end groups, RAFT polymerization deserves choosing. It is because RAFT polymerization is suitable for various kinds of monomers and the polymerization conditions of the RAFT polymerization are not too strict [33]. The RAFT agent (CTA) was designed to contain two carboxyl groups, which can react with two terminal hydroxyl groups of HTPB by polycondensation (Fig. 2C). In consideration of steric effect, the reaction between a small molecule and a polymer is easier than the reaction between polymers. Thus, the RAFT polymerization (Fig. 2D) was conducted after the polycondensation (Fig. 2C). Fig. 3A,B shows the IR spectra of the reactants (CTA and HTPB) and product (CTA-b-PB) of the polycondensation (Fig. 2C). The formation of the stretching vibrations peak of C]O in the ester group (1732 cm
1) proves the successful preparation of CTA-b-PB. There are some experimental details supportive to the conclusion. First of all, the product was thoroughly dialyzed in a dialysis bag with a Molecular Weight Cut Off of 7000 Da, ensuring the smaller molecules of HTPB (2700e3300 Da) and CTA (282.39 Da) to be eliminated. The CTA-b-PB (viscous liquid), soluble in hexane which can't dissolve CTA (solid), is yellow, while the HTPB is colorless. The RAFT polymerization (Fig. 2D) was conducted to prepare the conductive binder PPy-b-PB. As is depicted in Fig. 3C, the peak at 7.45 ppm of PPy-b-PB is in accord with the characteristic peak of pyrene ring in PPy, which proves the existence of PPy blocks in PPyb-PB. In addition, the characteristic peak of eCH2- in CTA-b-PB at 2.08 ppm also appears in the 1H NMR spectrum of PPy-b-PB instead of PPy. So, 1H NMR tests of PPy, CTA-b-PB, and PPy-b-PB verify the

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successful preparation of the PPy-b-PB. The number average molar mass of the PPy-b-PB (65000) is approximately three times as large as that of the PPy (21000) [26], which shows that the average number of PPy blocks in PPy-b-PB is 3. Thermalgravimetric analysis curves (Fig. S5) of PPy and PPy-b-PB indicate that both PPy and PPyb-PB are stable below 300  C. 3.2. Electrochemical performances of PPy-b-PB/Si anode The mass loading of Si of all the anode in this section is ~0.24 mg cm
2. Fig. 4A shows the galvanostatic cycling performances of the Si electrodes employing PPy or PPy-b-PB as conductive binders. The initial discharge specific capacity of the PPy-b-PB/Si anode (3393 mAh g
1) is 21% higher than that of PPy/Si anodes (2808 mAh g
1), indicating the SEI formed in the PPy-b-PB/ Si anode was more stable. Compared with the specific capacities of the PPy/Si anode (1752 mAh g
1) after 200 charge-discharge cycles, the PPy-b-PB/Si anode possesses higher reversible specific capacity (2274 mAh g
1). In addition, the capacity retention of the PPy-b-PB/ Si anode is 87.1%, which is calculated according to the discharge specific capacity at the 9th and 200th cycle. Thus, the average capacity losses per cycle during the entire process of galvanostatic cycling test of PPy-b-PB/Si anodes is as low as 0.067%. The excellent performance may be explained by the hypothesis that the integrity of the electronic conduction pathways in the PPy-b-PB/Si anode were well maintained. As is depicted in Fig. 4B, both the initial coulombic efficiency and the average coulombic efficiency of the PPy-b-PB/Si anode (77.9%, 99.3%) are higher than those of the PPy/Si anode (75.9%, 98.9%), which indicates that the SEI formed in the PPy-b-PB/Si anode is more stable than that of the PPy/Si anode [7]. After the analysis of the coulombic efficiency during the whole process of galvanostatic charge-discharge test, we find that the coulombic efficiency of the PPy-b-PB/Si anode keeps over 93.4% after the first cycle and 99.0% after the 60th cycle in the test, which suggests that PPy-b-PB can function well in transporting Liþ and electronic for long-term cycling. The rate performances of the PPy/Si and PPy-b-PB/Si anodes are shown in Fig. 4C. The average specific capacities of the PPy-b-PB/Si anode at the rate of 0.1 C, 0.2 C, 0.5 C, and 1 C are 2398, 1918, 1196, and 570 mAh g
1, respectively, which are significantly higher than those of the PPy/Si anode (1629, 1110, 557, and 145 mAh g
1, respectively). The capacity retention of the PPy-b-PB/Si anode (22.7%), which is equal to the ratio of specific capacity at 16th cycle (0.1 C) to 35th cycle (1 C), is 3 times as high as that of the PPy/Si anode (7.8%). The preferable rate performance of the PPy-b-PB/Si anode may be contributed to the integrity of the Liþ transport and the electronic conductive network [19,22,25]. In addition, the specific capacity of the PPy-b-PB/Si anode increases quickly with the decrease of current density from 1 C to 0.1 C, and even exceeds the specific capacity at 0.1 C from 16th to 20th cycle. This phenomenon indicates that the conductive network of the PPy-b-PB/Si anode preserves intact even though the current density is as high as 1 C. Besides, the reason why the specific capacity of both PPy/Si and PPy-b-PB/Si anodes at 36th cycle are higher than those of 16th cycle could be the improved infiltration of electrolytes in the electrode or the gradual activation of Si [2,19,34]. The reaction of Si and Li to form LiySi ranges from 0.15 to 0.01 V are shown in the first charge curve of the potential profiles of PPy-b-PB/Si anodes (Fig. 4D) [2]. The percentage of discharge specific capacity decay of PPy-b-PB/Si anodes from 50th to 200th cycle is as low as 0.12% per cycle, which suggests the superior performance of PPy-b-PB/anode. To research whether PB blocks have adverse effects on the PPyb-PB as conductive binders, CV and EIS test were conducted. Both the cyclic voltammetry curves of the PPy/Si (Fig. S6) and PPy-b-PB/

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Fig. 3. Global (A) and local enlarged (B) FTIR spectra for CTA, HTPB, and CTA-b-PB. (C) 1H NMR spectra for PPy, CTA-b-PB, and PPy-b-PB.

Si (Fig. S7) anode are similar to the results in previous literature [26,28], which shows that PB blocks are scarcely unfavorable to the electrochemical stability of the PPy-b-PB/Si anode. Although there are non-conducting parts (PB blocks) in PPy-b-PB, the chargetransfer resistance (Rct) of PPy-b-PB/Si anode (188.8 U), obtaining from the electrochemical impedance spectra (Fig. S8), is only slightly higher to that of the PPy/Si anode (182.7 U). Besides, the resistance of SEI (RSEI) of the PPy-b-PB/Si anode (7.5 U) is 70.1% as high as of that of the PPy/Si anode (10.7 U). The EIS test shows that the incorporation of PB can significantly improve the stability of SEI without apparent change of the Rct. 3.3. Advantages of PPy-b-PB To prove the importance of PB blocks in the PPy-b-PB, nanoindentation test of pure conductive binders without Si was firstly carried out. As is depicted in Fig. 5A (solid line), the maximum depths (dmax) of the Load-Indentation Depth curve of the pure PPy-

b-PB (6641.8 nm) is 158.2 nm higher than that of the pure PPy, which show that only 5 wt% PB can make it better for PPy-b-PB to adapt the expansion of Si in spite of the brittleness of PPy blocks. Since the nanoindentation system we used to carry out the nanoindentation test held the indenter at 10% of the maximum load for a dwell time to determine the influence of thermal drift on the measured displacement before unloading to 0 N, we cannot directly obtain the residual depth (dres) at the situation when the load decreased to 0 N during the unloading procedure from the unloading curves. Thus, forth-order polynomial fit of the unloading curves (solid line in Fig. 5A) was performed to obtain the dres (the Indentation Depth axis intercept of dotted line in Fig. 5A). The elastic recovery rate (wrev) was calculated according to the equation wrev ¼ ðdmax
dres Þ=dmax [35]. With higher wrev (41.4%) than that of the PPy (39.2%), the PPy-b-PB possesses stronger reversible deformation capability, which means that the PPy-b-PB does better in maintaining the integrity of the network of electronic conduction pathways of electrode than the PPy during the delithiation/

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Fig. 4. The discharge specific capacity (A) and coulombic efficiency (B) of PPy/Si and PPy-b-PB/Si anodes at a current density of 0.05 C (0.21 A g
1) for first 4 pre-cycles, 0.1 C for 5th to 8th pre-cycles, and 0.2 C for subsequent cycles. (C) Rate performance of PPy/Si and PPy-b-PB/Si anodes at the rate of 0.1 C, 0.2 C, 0.5 C, 1 C, and 0.1C after 15 pre-cycles at a rate of 0.05 C. (D) The charge-discharge profiles of PPy-b-PB/Si anodes at 1st, 50th, 100th and 200th cycle at the rate of 0.2 C. The mass loading of Si is ~0.24 mg cm
2.

lithiation process. The elastic modulus and hardness calculated from the nanoindentation test are shown in Fig. 5B. Because both of the elastic modulus and hardness of the PPy-b-PB (5.137 and 0.252 GPa) are lower than those of the PPy (5.045 and 0.237 GPa), the PPy-b-PB are more flexible than that of the PPy, which shows two merits of PPy-b-PB. Firstly, the flexible PB blocks of the PPy-bPB can extend to accommodate the increase of volume of the Si particles. Secondly, the mobility of the rigid PPy blocks of the PPyb-PB are enhanced, which benefits to easily self-assemble to reform conductive network between Si particles by the p
p stacking force of the pyrene ring. All results in the nanoindentation test prove the advantages of the PB blocks in reversible deformation ability and flexibility, which makes for the maintenance or regeneration of the conductive network of the PPy-b-PB/Si anode during the lithiation and delithiation process (Fig. 1B). Because neither polar groups (like carboxyl groups) nor selfassembling groups (like pyrene rings with p-p stacking force) exist in PB, the binding force among PPy-b-PB, Si and copper foil in PPy-b-PB/Si anode seems to be weaker than that of the PB/Si anode. However, the result of the peeling test of these two anodes negates from this thought. The average peeling force of PPy-b-PB/Si anode (4.02 N) is slightly larger than that of the PPy/Si anode (3.92 N). This phenomenon may be explained by the relation between adhesion length scale of a binder and the intimate contact between the binder and the adherend [30,36]. The adhesion length scale of a binder means deformability in terms of length when the binder gets in touch with the adherend. The higher adhesion length scale the binder possesses, the more intimate the contact among the binder, Si and copper will be. Since the adhesion length scale of a binder is inversely proportional to the modulus of the binder

[30,36], the PPy-b-PB binder, with lower modulus (Fig. 5B) according to the nanoindentation test, possesses higher adhesion length scale than PPy. Therefore, although the existence of the PB blocks decreases the proportion of the p-p stacking interaction of PPy-b-PB in comparison to PPy, lower modulus PB blocks are beneficial to the intimate contact among PPy-b-PB, Si and copper. Since both of the PPy-b-PB/Si and PPy/Si anodes possess the similar average peeling force, the fluctuation degree of the forcedisplacement curves becomes more important to the mechanical stability of the electrodes. It can be explained that under this premise, the electrode tends to deteriorate from where the binding force is the weakest regardless of the part with larger binding force, which can be proved by the digital photograph of the sample after the peeling test (Fig. 5D). The standard deviation of peeling force during the increase of displacement could be used as quantitative description of the fluctuation degree of peeling force. The standard deviation of peeling force of PPy-b-PB/Si anode is 0.17 N, which is only 62.5% of that of the PPy/Si anode (0.26 N). This phenomenon may be explained by the relief of the stress concentration in PPy-bPB/Si anode due to the flexible PB blocks (Fig. 1A). Taking the average peeling forces, the standard deviation of peeling force, and photographs of the sample after peeling test into account, a conclusion could be drawn that three most vital factors for the mechanical stability of the electrode may be the amount of polar groups and self-assembling groups in the binder, the adhesion length scale of the binder, and the stress concentration in PPyb-PB/Si anode, respectively. Thus, the electrode with the PPy-b-PB as conductive binder possesses better mechanical stability than the electrode with the PPy in spite of the absence of polar groups and self-assembling groups in the PB.

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Fig. 5. Nanoindentation test of different pure binders PPy and PPy-b-PB. Load-Indentation Depth curves (A) and elastic modulus and hardness (B) of different binders. Peeling test of different electrodes employing PPy or PPy-b-PB as conductive binders. Force-Displacement curves (C) and digital photograph after peeling test (D) of different electrodes.

SEM experiment was chosen to characterize the morphologies of PPy/Si and PPy-b-PB/Si anodes before and after the long-term cycling test. Although the morphologies of PPy/Si and PPy-b-PB/Si anodes are similar before charge-discharge cycles (Fig. 6A and B),

the number and width of cracks on the two anodes generated after 200 charge-discharge cycles are different. As is depicted in Fig. 6C and D, the largest width of the crack in the PPy-b-PB/Si anode is approximate to 0.34 mm after 200 cycles, which is only 18% as wide

Fig. 6. SEM images of PPy (A,C) and PPy-b-PB (B,D) based electrode before (A,B) and after (C,D) 200 cycles at the rate of 0.2 C.

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Fig. 7. The galvanostatic cycling performances of PPy-b-PB/Si anodes with higher Si mass loading (0.2 C, 0.60 mg cm
2) (A) and at higher current density (0.5 C, 0.24 mg cm
2) (B). The pre-cycles procedure in the cycling test with higher Si mass loading was the same as that in the cycling test described in Fig. 4A, while another pre-cycles from 9th to 12th cycle at the rate of 0.2 C existed in the cycling test at higher current density.

as that of the PPy/Si anode (1.85 mm). Two reasons may explain why the PPy-b-PB/Si anode possesses narrower and fewer cracks after 200 cycles. First of all, the better mechanical stability of the PPy-bPB/Si anode, which has been testified in the part of discussion of the peeling test, benefits to the maintenance of integrity of electrode. Secondly, the PB blocks enhance the reversible deformation ability of the PPy-b-PB, which is beneficial to keep the conductive network of the PPy-b-PB/Si anode intact, despite the repeated huge volume changes of Si particles. Thus, the results of the SEM test are consistent with that of the electrochemistry (Fig. 4) and mechanical (Fig. 5) properties mentioned above. 3.4. Cyclic performances of the PPy-b-PB/Si anode with higher Si mass loading or at higher current density To meet the needs of practical application, the high mass loading and charge-discharge rate cannot be ignored. As is shown in Fig. 7A, with the Si mass loading of 0.6 mg cm
2 at the rate of 0.2 C, the PPyb-PB/Si anode possesses high initial discharge specific capacity (2805 mAh g
1) and high the reversible specific capacity (2052 mAh g
1) at 200th cycle, which is better than most of the ntype conductive binders used in Si anode (Table S1). In addition, the initial coulombic efficiency and the average coulombic efficiency of the PPy-b-PB/Si anode are 76.6% and 98.8%, respectively, revealing the formation of the stable SEI in the PPy-b-PB/Si anode even though the Si mass loading is as high as 0.6 mg cm
2. The PPy-b-PB/ Si anode exhibits satisfactory reversible specific capacity (1752 mAh g
1), although the current density increases from 0.2 C (Fig. 4A) to 0.5 C (Fig. 7B). In addition, the PPy-b-PB/Si anode, with high initial coulombic efficiency (79.5%), possesses steady the coulombic efficiency, which maintains over 98.9% after the 9th cycle in the test. This phenomenon indicates that PPy-b-PB can maintain the transport of Liþ and electronic effective for long-term cycling even though the rate is as high as 0.5 C. It is worth noting that different from the Capacity-Cycle Number curve of the PPy-bPB/Si anode in Fig. 4A, the curves in Fig. 7A and B of the PPy-b-PB/Si anode show an extreme value at 13th and 17th cycle, respectively. The explanation for this phenomenon is as follows. With increases of the Si mass loading and the current density of charge-discharge test, the infiltration of electrolyte becomes more difficult at the beginning, but it will be gradually improved during the next several cycles [2], so the extreme value occurred in Fig. 7A,C. 4. Conclusions In summary, a novel n-type multi-block conductive binder, PPy

modified with PB rubber (PPy-b-PB), was prepared by combining condensation and RAFT polymerization. According to the nanoindentation test, peeling test and SEM images, the reversible deformation ability, flexibility, and adhesion length scale of the conductive binder are enhanced by incorporation of PB, which contributes to the superior reversible specific capacity and the long cycle life of the PPy-b-PB/Si anode. The PPy-b-PB/Si anode possesses high initial coulombic efficiency (77.9%), excellent reversible capacity (2274 mAh g
1) and superior capacity retention (87.1%) at the rate of 0.2 C (0.84 A g
1) after 200 cycles. The anode also exhibits satisfying cyclic performance with higher Si mass loading or at higher current density, which is superior to most of the n-type conductive binders. Since PPy-b-PB is single-component, further research of blending PPy-b-PB with other materials can be conducted to obtain the Si anode with more superior performance. What's more, the original structure of the multi-block conductive binder and the unique synthesis method provide a new idea to prepare multifunctional binders. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (21805127), the Postdoctoral Research Foundation of China (2018M640778), and the State Key Program of National Natural Science of China (51732005). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.05.093. References [1] M. Ishikawa, T. Sugimoto, M. Kikuta, E. Ishiko, M. Kono, Pure ionic liquid electrolytes compatible with a graphitized carbon negative electrode in rechargeable lithium-ion batteries, J. Power Sources 162 (2006) 658. [2] H. Wu, G. Yu, L. Pan, N. Liu, M.T. McDowell, Z. Bao, Y. Cui, Stable Li-ion Battery anodes by in-situ polymerization of conducting hydrogel to conformally coat silicon nanoparticles, Nat. Commun. 4 (2013) 1943. [3] B. Han, Y. Yang, X. Shi, G. Zhang, L. Gong, D. Xu, H. Zeng, C. Wang, M. Gu, Y. Deng, Spontaneous repairing liquid metal/Si nanocomposite as a smart conductive-additive-free anode for lithium-ion Battery, Nano Energy 50 (2018) 359. [4] C.Y. Wu, J.G. Duh, Ionic network for aqueous-polymer binders to enhance the electrochemical performance of Li-ion batteries, Electrochim. Acta 294 (2019) 22. [5] Y. Zhao, L.Y. Yang, D. Liu, J.T. Hu, L. Han, Z.J. Wang, F. Pan, A conductive binder for high-performance Sn electrodes in lithium-ion batteries, Acs Appl. Mater. Inter. 10 (2018) 1672. [6] G. Zhang, Y. Yang, Y. Chen, J. Huang, T. Zhang, H. Zeng, C. Wang, G. Liu, Y. Deng,

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