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Effect of Binder Architecture on the Performance of Silicon/ Graphite Composite Anodes for Lithium-ion Batteries Peng-Fei Cao, Michael Naguib, Zhijia Du, Eric W. Stacy, Bingrui Li, Tao Hong, Kunyue Xing, Dmitry Voylov, Jianlin Li, David Lee Wood, Alexei P. Sokolov, Jagjit Nanda, and Tomonori Saito ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13205 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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ACS Applied Materials & Interfaces

Effect of Binder Architecture on the Performance of Silicon/Graphite Composite Anodes for Lithium-ion Batteries

Peng-Fei Cao,* a Michael Naguib, b Zhijia Du,c Eric Stacy,d Bingrui Li,a Tao Hong,e Kunyue Xing,e Dmitry N. Voylov,e Jianlin Li,c David L. Wood, III, c Alexei P. Sokolov,a,e Jagjit Nanda,f and Tomonori Saito* a a

Chemical Sciences Division, c Energy & Transportation Science Division, and f Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, USA b Department of Physics and Engineering Physics, Tulane University, New Orleans, LA 70118, USA d Department of Physics and Astronomy, and e Department of Chemistry, University of Tennessee, Knoxville, TN 37996, USA [*] Dr. Cao ([email protected]) and Dr. Saito ([email protected]) This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-publicaccess-plan).

Keywords: Polymer Binder, Graft Copolymer, Silicon/Graphite anode, Grafting Density, Side Chain Length

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Abstract Although significant progress has been made in improving cycling performance of silicon-based electrode, few studies have been performed on the architecture effect on polymer binder performance for lithium-ion batteries. A systematic study on the relationship between polymer architectures and binder performance is especially useful in designing synthetic polymer binders. Herein, a graft block copolymer with readily tunable architecture parameters is synthesized and tested as the polymer binder for the high-mass loading silicon(15wt%)/graphite(73wt%) composite electrode (active materials > 2.5 mg/cm2). With the same chemical composition and functional-group ratio, the graft block copolymer reveals improved cycling performance in both capacity retention (495mAh/g vs 356 mAh/g at 100th cycle) and coulombic efficiency (90.3% vs 88.1% at 1st cycle) than the physical mixing of glycol chitosan (GC) and lithium polyacrylate (LiPAA). Galvanostatic results also demonstrate the significant impacts of different architecture parameters of graft copolymers, including grafting density and side chain length, on their ultimate binder performance. By simply changing the side chain length of GC-g-LiPAA, the retaining de-lithiation capacity after 100 cycles varies from 347 mAh/g to 495 mAh/g.


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Introduction Increasing the energy density of the lithium-ion battery (LIB) is necessary to meet the demands for their expanding applications from portable electronics to large-scale emerging applications, such as renewable energy storage grids and electric vehicles that requires acceptable driving distance upon a single charging.1-4 Among many candidates that can increase the energy density of anode, silicon (Si) is extremely compelling due to its high theoretical capacity (3579 mAh/g for Si compared to 372 mAh/g for commercial graphite anode), low operating potential, nontoxicity and worldwide abundance.5-9 However, the high specific capacity of the silicon-based electrode is typically observed only at the initial cycles, and cannot meet the long cycle life required for typical electric vehicle application. At a fundamental level, the origin of fast capacity fade is mainly due to two main factors: (i) huge volume change (about 320%) during the lithiation and delithiation process that causes the fracturing, pulverization and electrical isolation of silicon particles from the electrode matrix; (ii) continuous formation and reformation of the solid electrolyte interfaces (SEI) during charge-discharge which consumes extra lithium and results in lower coulombic efficiency.10-14 Utilization of polymeric material to hold active materials is a conventional approach, and the polymer binder plays even more significant role in the cell performance of the silicon-based electrodes because of their enormous volume changes during electrochemical cycling.5, 11, 15 The high-performance polymer binder can mitigate the volume expansion stress and maintain the electrical contact with conductive additive and current collector to achieve long-term cycling capacity of the LIB.11, 16-19 Recent progress in the field of polymer binders for silicon-based anode has clearly demonstrated the importance of adhesion strength and mechanical property of the polymeric materials.18, 20-22 Magasinski et al. reported for the first time the improved binder


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performance of pure polyacrylic acid (PAA) compared with conventional polyvinylidene fluoride (PVDF) due to hydrogen bonding or/and covalent connection of the carboxylic groups with the hydroxyl groups on the silicon surface.23 Direct evidence to the formation of covalent bond between the SiNPs and PAA was provided by the Koo et al.24 The mechanical strength of the polymeric materials was also demonstrated important in retaining the capacity of siliconbased anode. Li et al. reported that the improved binder performance of a stiff binder, i.e., sodium carboxymethoxy cellulose (CMC) compared with the elastic PVDF and CMC/styrenebutadiene rubber (SBR).25 Bao and coworkers also demonstrated significant role of the elastic mechanical strength in the polymer binders for micro-size silicon anode.26-27 Recently, Choi et al. also incorporated polyrotaxane in the conventional PAA binder to impart the mechanical elasticity to the obtained polymer network.28 Despite these progresses in the natural/synthetic polymer binder development, few studies have been investigated on the macromolecular architecture effect on their polymer binder performance for silicon based electrodes. Among the polymers with different architectures,29-33 the graft polymers, which are called either combs or bottlebrushes depending on grafting density, have been attracting significant attentions due to their efficient control over the structure and the corresponding properties of obtained materials.32, 34-36 The graft copolymer with chemically distinguished grafted side chains and polymer backbones can retain their desired properties from both polymer components, and enable efficient control of both chemical/physical interaction and mechanical performance,34, 36 which are extremely important for polymer binder applications. The PAA grafted polyvinylidene fluoride (PVDF) and NaPAA grafted CMC are reported to exhibit improved cycling performance compared with linear analogues, while their architecture effect on polymer binder performance were not investigated due to the limitation of synthetic strategy which do not allow a defined


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polymer architecture.37-38 Conventional knowledge of “structure-property” relationships in polymers indicates that the structural parameters, such as the side chain length and grafting density, significantly affect the property of graft polymers.34-36 Another important issue in the real application of typically reported silicon based anode materials is their low mass loading of active materials (usually lower than 0.5 mg/cm2) that renders the low areal capacity and energy density despite of the high specific capacity.37, 39 Utilization of silicon/graphite composite anode materials instead of pure silicon nanoparticles (SiNPs) is one of the solutions to increase the mass loading for practical applications, and this approach is especially popular among the manufactures because the electrode made of graphite has mature manufacturing process and relatively low price.40-42 Herein, to elucidate the architecture effect of synthetic polymers on the polymer binder performance for silicon-based anode, the graft block copolymers with welldefined architecture were synthesized by Reversible Addition-Fragmentation chain Transfer (RAFT) polymerization,43-45 and tested as the polymer binder for the high-mass loading silicon/graphite composite electrode.

Experimental Section Chemicals and Materials: Glycol chitosan (≥60% titration, Degree of Polymerization/DPn≥400, see compound 1 in Scheme 1), N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC), 4,4'-azobis(4-cyanopentanoic acid) (ACPA), lithium hydroxide (LiOH) and Nhydroxysuccinimide (NHS) were obtained from Sigma-Aldrich and used directly. 2(((Butylthio)carbonothioyl)thio)propanoic acid (compound 2) was synthesized following previous literature.44 Acrylic acid was passed through the column with alternative inhibitor remover and aluminum oxide (basic) to remove any inhibitor before the polymerization. The


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semi-permeable membrane made of regenerated cellulose with molecular weight cut-off equal to 3.5 kDa and 14 kDa were ordered from SpectrumLabs and Ward’s Science, respectively. Synthesis of the macro RAFT-CTA (compound 3): A typical synthesis of GC22%-RAFT was performed as following. 800 mg (3.60 mmol repeating units) of GC was dissolved in 120 mL of mixing solvent (V(H2O) : V(CH3OH) = 2:1) by vigorously stirring overnight at 45 °C. 0.552 g EDC (2.88 mmol) and 0.332 g (2.88 mmol) NHS was added to the solution following by the addition of 0.514 g (2.16 mmol) of carboxylic acid terminated RAFT-CTA (compound 2) in 10 mL mixture solvent (V(H2O) : V(CH3OH) = 1:1). The mixture was stirring at 35 °C for two days before it was purified by dialysis using the semi-permeable membrane (Mw =3.5 kDa) against the mixture solvent (V(H2O) : V(CH3OH) = 3:1). The 1H NMR and IR spectra were shown in Figure 1. Synthesis of the graft block copolymer GC-g-LiPAA (compound 5 in Scheme 1): A typical synthesis of GC22%-g-LiPAA62 was performed as following. 452 mg GC22%-RAFT (0.367 mmol RAFT-CTA) was dissolved in 60 mL mixing solvent (V(H2O) : V(CH3OH) = 3:1). To that solution, 3.97 g acrylic acid (55.1 mmol) and 10.3 mg (3.67×10-4 mol) ACPA in 40 mL of DI water was added. The RAFT polymerization was performed at 70 °C for 10 hours. Further dialysis using the semi-permeable membrane (Mw =14 kDa) against water will afford the purified GC22%-g-PAA62 with the 1H NMR and IR spectra shown in Figure S1 and Figure 1. The GC22%-g-LiPAA62 was obtained by dropping the aqueous solution of LiOH to that of GC22%-gPAA62 until the pH = 7. See Figure S7 and Figure 1 for the 1H NMR and IR spectra. Characterization: 1H Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker 500 MHz NMR spectrometer using chloroform-d or DMSO-d. Infrared (IR) spectra were recorded on a Cary 600 Series FT-IR Spectrometer (Agilent Technologies), and the scanning range was


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4000 - 400 cm-1. The solution viscosity of polymers were measured on AR2000ex (TA instrument) through small amplitude oscillatory shear measurements at the shear rate of 1 s-1. For peer test, a 1-inch-wide and 3-inch-long electrode sample was attached to 3M tape, and the peel strength of the electrode specimens was measured with a high-precision micromechanical test system (FPT-H1 Horizontal Dedicated Friction, Peer and Tear Tester; Mecmesin). By pulling the tape at a constant displacement rate of 100 µm/s, the applied load was continuously monitored and force-displacement plots were made. The atomic force microscopy (AFM) indentation was performed using OmegaScope, AIST-NT (Novato, US) scanning probe microscope. The measurement was performed within indentation depth of up to 200 nm to reduce the surface effect in elastic modulus of the studies polymers.46 A Hertz approach was employed to calculate the Young’s modulus by assuming the Young’s modulus of silicon E=150 GPa and Poisson’s ration ν=0.22.47 Adhesion forces analysis was based on measurement of Force-distance curves using CP-PNPS-SiO-E-5 cantilever with spherical SiO2 tip of 15 µm size (resonance f=67 kHz, spring constant k= 0.32 N/m, length L 100 µm).

Preparation of Electrodes and Assembly of Coin Cells: All of the coin cells (stainless steel CR2032, Hohsen Corp., Osaka, Japan) were prepared and assembled in an argon-filled glovebox. The synthetic polymers (10% in weight) were dissolved in DI water (NMP for PVDF), followed by the addition of SiNPs (15% in weight, Nanostructured & Amorphous Materials Inc, 70 ~ 120 nm), graphite (73% in weight, TIMCAL) and carbon black (2% in weight, TIMICAL SUPER C65). The composite was mixed using a homogenizer for 2 hours, and the slurry was coated on the copper foil by using a doctor blade (8 mil). The coated electrode was placed in the vacuum oven for 18 h at 120 °C before it was transferred into the glove box for coin cell assembly. The coin cells consisted of polyprolylene (Celgard 2400) as separator, 1.2 M lithium


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hexafluorophosphate (LiPF6) in ethylene carbonate (EC), ethyl methyl carbonate (EMC) (EC:EMC=3:7 wt) with 10wt% of fluoroethylene carbonate (FEC) as electrolyte, and Li metal as counter electrode. Galvanostatic cycling between 50 mV and 1.5 V was conducted using Series 4000 MACCOR batteries cycler (MACCOR Inc. Tulsa OK, USA). The cycling was carried out at 25 °C by keeping the cells in a temperature control chamber while testing. Cyclic voltammetry (CV) between 50 mV and 3V vs Li/Li+ was executed by using a VSP300 potentiostat (Biologic, Claix, France) with a fixed voltage rate (10mV/s) at room temperature. Three coin cells were made from each of the composite anode, and average specific capacity was reported in the manuscript. Error analysis was included in Table 1. All of the specific capacities reported in this study are per gram of active materials (SiNPs+Graphite). The C rate was also determined based on the theoretical capacity of active materials (SiNPs+Graphite).

Result and Discussion Synthesis of Graft copolymer Systematic investigation on the architecture effect of synthetic polymer on binder performance requires the synthesis of graft copolymers with well-controlled structure parameters, such as side chain length and grafting density. “Graft from” strategy was selected here instead of “graft to” method because it can overcome the steric hindrance and provide a controlled grafting density and side chain length.48-49 A hydrophilic derivative of chitosan, glycol chitosan (GC) was selected as the polymer backbone due to its high-modulus nature, good water solubility and abundant reactive groups for facile modification.50 The macro RAFT-CTA (Scheme 1, compound 3) was synthesized via the amidation reaction between the GC and carboxylic acid terminated RAFT-CTA (compound 2) in the aqueous solution. The comparative integration of


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peak e, f and h in Figure 1(A) allows the calculation of the average grafting density of the RAFT-CTA on the GC backbone: 22.1% of the repeating units have been functionalized which corresponds to 88 arms per graft copolymer.

Scheme

1. Synthesis

of graft copolymer GC-g-LiPAA via

RAFT polymerization

RAFT polymerization of acrylic acid from the macro GC22%-RAFT (3) allows the growth of polymer side chains. The significant absorption peak lying at 1692 cm-1 attributed to the typical stretching mode of C=O bond in the carboxylic acid suggests the successful grafting of PAA side chains. Moreover, comparative integration of the peak from terminal methyl groups in the RAFT-CTA compared with that from the repeating units in the PAA provides the degree of polymerization (DPn) of the PAA being 62 (See Figure S1). Neutralization of the obtained GC22%-g-PAA62 by addition of LiOH produces the GC22%-g-LiPAA62, which is confirmed by the


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significant growth of the carboxylate peak at 1544 cm-1 at the expense of the carboxylic acid peak as shown in Figure 1(B).

Figure 1. (A) Comparative 1H NMR spectra of GC (1) and GC22%-RAFT (3); (B) IR spectra of GC (1), GC22%-RAFT (3), GC22%-g-PAA62 (4), and GC22%-g-LiPAA62 (5).

Comparative Binder Performance with Linear Analogue The composite electrode with 73% graphite and 15% silicon as the active materials at a relatively higher mass loading (loading of active materials > 2.5 mg/cm2; thickness > 30 µm) than the typical silicon based electrode (< 0.5 mg/cm2) was fabricated (Figure 2). The theoretical specific capacity is calculated to be 875 mAh/g. Moreover, the good water solubility of the obtained graft copolymer GC22%-g-LiPAA62 allows to use water as the solvent, which possesses significant advantages over the traditional organic solvent, i.e.,1-methyl-2-pyrrolidine (NMP), such as low cost and minimized environmental effect.51 Energy-dispersive X-ray spectroscopy (EDX) mapping was also applied to scan the obtained composite film as shown in Figure 2 (C). In addition to forming a porous architecture, the homogenous distribution of silicon suggests their


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even distribution of the active materials in the composite anode film. The high viscosity of the polymer binder solution (Figure 3(A)) can prevent sedimentation and aggregation of the active materials during the electrode fabrication process, and contribute to the formation of uniform film on the current collector.21

Figure 2. (A) Photograph of the silicon/graphite/GC22%-g-LiPAA62 electrode on copper foil (film width is 10 cm), (B) SEM image, (C) Energy-dispersive X-ray spectroscopy (EDX) mapping and (D) Silicon distribution of the same electrode in A

The architecture effect of synthetic polymer binders on the electrochemical performance of silicon/graphite electrode was firstly investigated by comparing the synthesized graft copolymer with the linear analogues. Pure GC and physical mixture of GC and LiPAA (GC21%m-LiPAA76, 11.8wt% of GC and 88.2wt% of LiPAA, same with that of GC22%-g-LiPAA62) were


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utilized as the linear analogues for comparison purpose. The linear LiPAA with the comparable DPn (76 vs 62) was also synthesized by RAFT polymerization of the acrylic acid and subsequent neutralization by LiOH (see Scheme S1 for chemical structure). As illustrated in Figure 4(A), the electrodes based on PVDF and GC showed acceptable de-lithiation capacities, i.e., 445 mAh/g and 558 mAh/g, at the first cycle. However, a rapid capacity loss was observed for the subsequent cycles, and only less than 300 mAh/g capacity was retaining, which is mostly contributed by the graphite. The poor cyclability of the electrodes with PVDF and GC binders indicates the isolation of silicon nanoparticles from the conduction network (or called “dead” silicon) after initial cycles.19, 52-54

Table 1. Composite anodes with different polymer binders th

th

Anodes with different Polymer Binders

Porosity (%)

Initial De-lithiation capacity (mAh/g)

100 De-lithiation capacity (mAh/g)

Initial Columnbic Efficiency (%)

20 Columnbic Efficiency (%)

PVDF GC

63.8±0.5 62.7±0.9 59.2±6.3

445±2.7 558±18 729±13

278±1.6 155±5.6 356±3.6

63.8±0.9 76.1±0.1 88.1±0.3

99.7±0.1 99.7±0.0 99.1±0.1

56.1±7.0 73.2±1.5

723±17 745±12

411±8.9 495±2.9

65.3±0.4 90.3±0.1

99.3±0.1 99.4±0.0

64.5±0.9 47.7±1.2 68.7±0.7

771±10 671±21 758±6.4

425±3.2 346±4.7 400±1.2

92.9±0.0 92.8±0.2 91.1±0.1

99.2±0.1 99.2±0.0 99.1±0.0

a

GC21%-m-LiPAA76 LiPAA76 b

GC22%-g-LiPAA62 GC22%-g-LiPAA27 GC22%-g-LiPAA17 GC12%-g-LiPAA76 a

21% means the molar ratio of LiPAA to the monomer of GC; m means physical mixing; 76 is the DPn of LiPAA. b 22% means the grafting molar ratio of LiPAA to the monomer of GC; g means chemical grafting; 62 is the DPn of grafted LiPAA.

Addition of neutralized linear LiPAA to the GC can promote the physical interaction with the silicon materials and provide extra lithium-ion access to mitigate the de-activation of silicon particles.23, 53, 55-56 As expected, the electrodes with the physical mixture of LiPAA and GC (GC21%-m-LiPAA76) as the binder showed higher initial de-lithiation capacity, i.e., 725 mA/g and better cycling performance (356 mAh/g after 100 cycles) than GC and PVDF. The electrodes


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with graft copolymer GC22%-g-LiPAA62 showed comparable initial de-lithiation capacity (745 mAh/g) but significantly improved cycling performance with a de-lithiation capacity of 495 mAh/g after 100 cycles. Moreover, as illustrated in Figure 4(C), the graft copolymer GC22%-gLiPAA62 exhibited higher initial coulombic efficiency than its linear analogues (90.3% vs 62.8%, 76.1%, and 88.1%). The electrode with GC22%-g-LiPAA62 as the binder also showed higher coulombic efficiency for the following 30 cycles than that of GC22%-m-LiPAA76 (inset of Figure 4(C)). The cycling performance of electrodes with high charging rate (C/1) suggested even more significant advantageous of binder GC22%-g-LiPAA62 compared with GC22%-m-LiPAA76 (221 mAh/g compared with 64 mAh/g after 100 cycles, Figure 4(B)). The comparatively improved cycling performance of the graft copolymer compared with linear analogue is consistent with previous publications.37-38 The interesting phenomenon that worth our attention is the significantly improved cycling performance of GC22%-g-LiPAA62 compared with that of GC21%m-LiPAA76 in both retaining capacity and coulombic efficiency. With the same chemical composition and functional-group ratio, the significantly improved cycling performance clearly suggests the advantage of a multi-grafted architecture over the physical mixture which may be contributed by the formation of a better interfacial architecture.


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Figure 3. (A).Solution viscosity of different polymer binders at the same polymer concentration (1 mg/mL), shear rate (10 s-1) and temperature (20 oC): (I) GC22%-g-LiPAA16, (II) GC22%-gLiPAA27, (III) GC22%-g-LiPAA62, (IV) GC12%-g-LiPAA76, (V) GC21%-m-LiPAA76, (VI) GC; (B) CV curves of Si/graphite electrode with GC22%-g-LiPAA62 as polymer binder, inset is the zoomin image to show the small peak in the first cathodic scan. (C) Peer test result of anode films with different polymer binders; (D) Related adhesion force of different polymer binders measured by AFM; the curve with lower relative force value (more negative) means the higher adhesion force

The improved binder performance of graft copolymer GC22%-g-LiPAA62 can be explained as follows. First, the incorporation of numerous LiPAA side chains in the graft copolymer allows more efficient interaction with the SiNPs which can efficiently prevent the isolation of SiNPs from the surrounding conductive network. The enhanced interaction was supported by the higher adhesion force of GC22%-g-LiPAA62 with silicon tip compared to that of GC (Figure 3 (D)). Comparing with the GC-m-LiPAA, the covalent connection of LiPAA on GC


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may contribute to the formation of a three-dimensional network around the SiNPs.21,

54

The

significantly higher peeling force of the electrode made from GC22%-g-LiPAA62 (~0.75 N/cm) compared with that from GC22%-m-LiPAA76 (< 0.1 N/cm) as illustrated in bulk-scale peeling mechanics (Figure 3(C)) confirms improved adhesion strength of composite electrode using GC22%-g-LiPAA62 with the current collector (See Figure S4 for setup image).27, 57 Secondly, the enhanced interaction and extra lithium-ion source should help forming a stable solid electrolyte interphase (SEI) layer on SiNPs. The small irreversible peak at about 1.25 V in the first cathodic scan, which disappeared in the subsequent scans, can be associated with the formation of SEI film (Figure 3(B)).19, 58 The sharp peak below 0.3 V during the first lithiation scan corresponds to lithiation in graphite and the phase transition of Si (from crystalline Si to amorphous lithium silicide).8 The two oxidation peaks, i.e., 0.32 V and 0.51 V, during the de-lithiation scans are ascribed to the de-lithiation of LixC and LixSi, which transform to graphite and amorphous silicon (See Figure S5 for peak assignment).59 The significantly higher coulombic efficiency of the electrode from the GC22%-g-LiPAA62 also suggests the formation of a stable SEI layer on SiNPs. The comparatively lower level of polymer/electrolyte interactions of GC22%-g-LiPAA62 compared with PVDF should be another reason to its improved binder performance at the current rate of C/10. The atomic force microscopy indentations studies (Figure S6) shows that the stiffness of the film made from GC22%-g-LiPAA62 did not change significantly after immersed into the electrolyte solvent (2.46 GPa vs 3.99 GPa), whereas the film from PVDF became more than 8 times softer (0.09 GPa vs 0.74 GPa). This significantly lower polymer/electrolyte interaction may prevent the undesirable access of electrolyte liquid to the binder/Si interface and cause deformation of the electrode.22 Lastly, the comparative higher solution viscosity of the


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graft copolymer may also contribute to the formation of a homogenous electrode film due to the reason pointed out above.21

Figure 4. Cycling performance of silicon/graphite electrodes from the polymer binder of PVDF, GC, LiPAA76, GC21%-m-LiPAA76 and GC22%-g-LiPAA62 at a current rate of C/10 (A) and C/1 (B); (C) Coulombic efficiency of the electrodes from different polymer binders, inset is the zoom-in area showing the comparative efficiency of electrodes from GC21%-m-LiPAA76 and GC22%-g-LiPAA62 in the first 30 cycles; (D) is the voltage profiles of the electrodes with GC22%g-LiPAA62 binders, respectively. Effect of Architecture Parameters on Binder Performance The architecture parameters, such as the side chain length and grafting density, significantly impact the property of graft copolymer, and numerous studies including experimental,60-61 theoretical,62 and computational63-64 are devoted to establish correlations between the


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architecture parameters and physical properties of the graft polymers. Herein, the architecture effect on binder performance was investigated using the graft polymer GC22%-g-LiPAA62 with variable side chain length and grafting density. The side chain length of grafted copolymer is adjusted by changing the feeding ratio of monomer (acrylic acid) to macro RAFT-CTA (GC22%-RAFT, 4). After neutralization by LiOH, the GC22%-g-LiPAAx with different DPns were obtained (Figure S7 and S8). The galvanostatic results of the graft copolymer GC22%-g-LiPAAx with fixed grafting density and different side chain lengths, i.e., DPn =17, 27, and 62, are shown in Figure 5. Significant improvement was observed when the side chain length of GC22%-g-LiPAAx increased from DPn=17 to DPn=27 in terms of both first-cycle de-lithiation capacity (771 mAh/g vs 656 mAh/g) and long cycling performance (425 mAh/g vs 347 mAh/g at 100th cycle). Comparable initial discharge capacity was obtained when further increasing the side chain length from DPn=27 to DPn=62 (746 mAh/g for DPn=62 vs 771 mAh/g for DPn=27). In terms of long-term cycling performance, the electrode made of GC22%-g-LiPAA62 showed better performance (495 mAh/g at 100th cycle) than that with the GC22%-g-LiPAA27 (425 mAh/g at 100th cycle), while this difference is much less significant comparing with that when the DPn of side chain increased from 17 to 27 (See Figure 5). The significantly improved long-term cycling performance of the electrode made from the graft copolymer with increased side chain length may be explained as follows. The graft block copolymer with longer side chains possess higher degree of intermolecular entanglement/interaction and hence also more interactions with active materials and current collector. The significantly higher adhesion force of GC22%-g-LiPAA62 with silicon tip (Figure 3(D)) and higher peeling strength of the electrode made from the GC22%-g-LiPAA62 (Figure 3(C)) compared with those from GC22%-g-LiPAA17 provide solid support for the interaction


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enhancement with longer side chains.19,

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Moreover, the higher solution viscosity of graft

copolymer with longer side chains (26.3 mPa.s to 56.0 mPa.s and 195 mPa.s when DPn of side chain increased from 17 to 27 and 62, as shown in Figure 3(A)) may provide additional evidence for their higher degree of intermolecular entanglement/interaction and hence also more interactions with the electrode composite.65 This interaction enhancement will definitely reduce the possibility of SiNPs being disconnected from the conductive network during cycling process.

Figure 5. Cycling performance of silicon/graphite electrodes from the polymer binder of GC22%g-LiPAA17, GC22%-g-LiPAA27, GC22%-g-LiPAA62, and GC12%-g-LiPAA76 at a current rate of 0.1 C. To further investigate the effect of grafting density on the cycling performance, a graft copolymer with lower grafting density and comparable side chain length was also synthesized. The 1H NMR spectra analysis (Figure S10) of the obtained graft block copolymer suggested the grafting ratio and side chain length are 12% and DPn=76, respectively. The galvanostatic result showed that the electrode with GC12%-g-LiPAA76 exhibited comparable initial delithiation capacity with that from GC22%-g-LiPAA62 (758 mAh/g vs 745 mAh/g). However, after 10 cycles,


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the graft copolymers with higher grafting density start showing the advantage in the long-term cycling performance (67% retaining capacity for GC22%-g-LiPAA62 vs 54% retaining capacity for GC22%-g-LiPAA62 at 100th cycle). The better cycling performance of the graft copolymer with higher grafting density should be mainly attributed to the increased number of anchoring points (88 arms vs 48 arms) that interact with the active material and conduction network. Similar to the explanation above, the higher peeling strength (Figure 3(C)) and increased solution viscosity (23.1 mPa.S to 195 mPa.S) suggest that the enhanced interaction of the high grafting-density graft copolymer with electrode composite is responsible for the improved binder performance.

Conclusion In summary, using a well-defined multi-grafting block copolymer as the binder for silicon/graphite electrode, this study demonstrated the significant impact of polymer architectures on their property and ultimate battery performance. The graft copolymer GC-gLiPAA with GC as backbone and LiPAA as side chains was obtained by amidation reaction and RAFT polymerization. The side chain length and degree of grafting density are tunable by changing the feed ratio during the reaction. Using the environmentally friendly and economically saving solvent, i.e., water, a high mass-loading silicon/graphite composite electrode (active materials > 2.5 mg/cm2), based on graft copolymer (GC-g-LiPAA), was fabricated and tested by galvanostatic measurement. Compared with the linear analogues, such as GC and physical mixing of GC and LiPAA, the silicon/graphite composite electrode made with the graft copolymer GC-g-LiPAA reveals significantly improved initial discharge capacity, long-term retaining capacity and coulombic efficiency. With the same chemical composition and functional-group ratio, the improved cycling performance of graft block copolymer GC-g-


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LiPAA in both retaining capacity and coulombic efficiency demonstrated the importance of polymer binders’ architecture. Significant improvement in binder performance was observed when the DPn of side chains in GC-g-LiPAA increased from 17 to 27, and slightly improvement was observed when the DPn increased from 27 to 62. Moreover, significant advantage in the long-term cycling performance was also observed when grafting density of the polymer binders increased from 11% to 22%. The preferred longer side-chain length and higher grafting density as the polymer binder for silicon/graphite composite electrode is due to the enhanced interaction with the anode composite, which was supported by the higher peeling strength and increased solution viscosity. Our work demonstrates the importance of polymer-binder morphology and architecture as a key component for mitigating capacity loss and degradation in high capacity electrode materials that undergoes severe volume expansion under electrochemical cycling. This is not limited to the silicon based anode but also for other anodes beyond Li-ion batteries, such as phosphorus anode for sodium ion batteries.66 The concept and conclusion here may contribute to the design of polymeric materials with optimized architecture parameters for improved electrochemical performance, including the modification of current start-of-art polymer binders.

Supporting Information Supporting Information is available free of charge on the ACS Publication website. 1H NMR and IR spectra of synthesized grafted copolymer, EDX mapping of electrode, and Young’s Modulus of grafted copolymer and PVDF.

Acknowledgements This research at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE) under contract DE-AC05-00OR22725, was sponsored by the Office of Energy Efficiency and Renewable Energy (EERE) Vehicle Technologies Office (VTO) (Acting Program Deputy Director: David Howell; Applied Battery Research Program Manager: Peter Faguy). APS acknowledge partial financial support for the polymer characterization by the


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U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. KX acknowledges financial support for the rheology measurement from NSF Polymer program (DMR-1408811).


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66. Song, J.; Yu, Z.; Gordin, M. L.; Li, X.; Peng, H.; Wang, D. Advanced Sodium Ion Battery Anode Constructed via Chemical Bonding between Phosphorus, Carbon Nanotube, and Cross-Linked Polymer Binder. ACS Nano 2015, 9, 11933-11941.


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