Rational Design of a Multifunctional Binder for High-Capacity Silicon

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Letter

Rational Design of Multifunctional Binder for High-Capacity Silicon Based Anodes Peng-Fei Cao, Guang Yang, Bingrui Li, Yiman Zhang, Sheng Zhao, Shuo Zhang, Andrew Erwin, Zhengcheng Zhang, Alexei P. Sokolov, Jagjit Nanda, and Tomonori Saito ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b00815 • Publication Date (Web): 22 Apr 2019 Downloaded from http://pubs.acs.org on April 22, 2019

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ACS Energy Letters

Rational Design of Multifunctional Binder for HighCapacity Silicon Based Anodes Peng-Fei Cao,*† a Guang Yang,†a Bingrui Li,b Yiman Zhang,a Sheng Zhao,b Shuo Zhang,c Andrew Erwin a,d Zhengcheng Zhang,c Alexei P. Sokolov,a,b Jagjit Nanda,* a and Tomonori Saito* a

* Dr. Cao ([email protected]), Dr. Nanda ([email protected]), and Dr. Saito ([email protected])

†The two

authors contribute equally

a Chemical

Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee

37830, USA

b Department

of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, USA

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c

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Chemical Sciences and Engineering Division, Argonne National

Laboratory, Lemont, Illinois 60439, USA

d

School of Material Science and Engineering, Georgia Tech, Atlanta, Georgia 30332,

USA


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ABSTRACT. Although several principles have been recognized to fabricate a nominal “better” binder, there continues to be a lack of a rational design and synthesis approach that would meet the robust criteria required for silicon (Si) anodes. Herein, we report a synthetic polymer binder, i.e., catechol-functionalized chitosan cross-linked by glutaraldehyde (CS-CG+GA), that serves dual functionalities: a) wetness-resistant adhesion capability via catechol grafting and b) mechanical robustness via in-situ formation of a three-dimensional (3D) network. SiNPs based anode with the designed functional polymer network (CS-CG10%+6%GA) exhibits a capacity retention of 91.5% after 100 cycles (2144 ± 14 mAh/g). Properties which are traditionally considered to be advantageous, including stronger adhesion strength and higher mechanical robustness, do not always improve the binder performance. A clear relationship between these properties and ultimate electrochemical performance is established by assessing the rheological behavior, mechanical property, adhesion force, peel stress, morphology evolution, and semi-quantitative evaluation. This study provides a clear path for the rational design of high-performance functional polymer binders for not only Si-based electrodes, but also other types of alloy and conversion-based electrodes. 3 ACS Paragon Plus Environment


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The role of binder is critical to the electrochemical performance and cycle life of the nextgeneration batteries that can fulfill the requirement of various devices for future consumer electronics and electric vehicles.1-9 The critical role of binders is especially true for highcapacity anodes like silicon (Si) that undergoes extraordinary volume expansion during alloying process (up to 280% for Li15Si4).6, 10-11 Rapid capacity fade of Si anode occurs in several modes including pulverization of Si anode, disconnection of active materials with conducting network or current collector, and instability of solid electrolyte interface (SEI).1, 12-19

The traditional role of a binder includes providing an adhesive network to hold the

active material and electronically conducting diluents such as carbon black on the current collector.1-2 With the advent of high capacity alloy and conversion electrodes,4, 20-22 there is a greater necessity to design polymer binders that serve multifunctionalities. The commonly used polymeric binder for lithium-ion battery is polyvinylidene fluoride (PVDF), which performs well for a majority of intercalation-based electrode materials, such as graphite. However, PVDF does not work for alloy and conversion-based electrodes that undergo severe volume changes which are usually accompanied by irreversible structural 5 ACS Paragon Plus Environment


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transitions.23 Furthermore, PVDF is incorporated into the electrode fabrication process through a randomized slurry coating method which allows for fewer controls on aspects like electrode-binder interactions, surface functionality and other relevant attributes.

Several key functionalities have been identified towards the design of high-performance polymer binders: (1) adhesion capability with active materials,24-26 (2) mechanical robustness,27-30 (3) forming a homogenous mixture (with active materials and conducting additive) with suitable solution viscosity.2, 31 Adhesion capability of polymer binders to Si can contribute to a better anode performance in two aspects: generates a protective layer on the surface of Si and suppresses the decomposition of electrolyte; adheres Si with conducting additives and maintains the electrical contact of active materials.16,

25

For

example, by implementing binders capable of forming covalent bonds and/or facilitating physical interactions with Si, the batteries fabricated using polyacrylic acid (PAA),24 guar gum (GG),32 gum arabic,33 alginate,34 sodium poly[9,9-bis(3-propanoate)ﬂuorine] (PFCOONa),26 sodium carboxylmethoxy cellulose (CMC),23 cyclodextrin (-CD),35-37 selfhealing

polymers,28

poly(3,4-ethylenedioxythiophene):polystyrene

sulfonate


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(PEDOT:PSS),38 and their combinations (for example, PAA-CMC)39 exhibited much better capacity retention of Si anodes than that delivered by PVDF binders,25 which only have weak van der Waals interactions with Si. Recently, Sottos and coworkers reported a Si composite electrode with increased cell lifetime by exploiting covalent attachment of amines on SiNP surface and dynamic ionic bonding between carboxylic acid and amines that is stronger than typical hydrogen bonding.40 On the other hand, mechanical degradation of composite electrode is demonstrated to be associated with a decreased Young’s modulus and hardness during the cycling process, which is also responsible for the rapid capacity fade of Si anode.27 Mechanical robustness of a polymer binder helps maintain the integrity of electrode morphology and minimizing the destruction of electric pathways of active materials to the current collectors. Bao and coworkers demonstrated that the viscoelasticity of self-healing polymers can accommodate the stress built up during the volume expansion of Si.28, 41 In-situ crosslinking to form a three dimensional (3-D) network around the active material was demonstrated to be especially useful in preserving the integrity of Si anode during cycling.29, 42 Forming a homogenous mixture and hence uniform anode coating on the current collector usually requires compatible 7 ACS Paragon Plus Environment


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chemical composition and optimized slurry viscosity over a specific range of solution concentrations.31 Instead of continuously exploring different types of polymeric materials by Edisonian approach, rational design principles need to be established for developing binders with the optimal electrochemical performance. Moreover, the effect of certain properties normally considered to be advantageous, including adhesion capability and mechanical robustness, on the ultimate electrochemical performance of SiNPs-based anode are yet to be well investigated.

Herein, we report the design and fabrication of a crosslinked catechol-functionalized chitosan (CS-CG+GA) network, which was evaluated as a binder material for Si electrode in lithium-ion batteries. Catechol functionality was selected as adhesion groups due to its mussel-inspired

wetness-resistant

adhesion

capability

that

was

demonstrated

advantageous for battery applications considering the liquid environment inside the battery.25 Atomic force microscopy (AFM) pulling tests indicate that the catechol-Si interaction is stronger than typical hydrogen bonding.25 Furthermore, additional in-situ crosslinking reaction was used to form 3-D network, which provides mechanical


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robustness.29, 42 We also performed a semi-quantitative evaluation on the influence of the catechol grafting density and the crosslinking density to the main-chain stiffness for insightful binder performance analysis. The correlation between polymer binder properties and electrochemical performance is anticipated to shed light on the rational design of polymeric materials with superior binder performance for Si anodes.

Chitosan (CS) is a linear polysaccharide composed of randomly distributed -(1,4)-linked D-glucosamine (de-acetylated unit) and N-acetyl-D-glucosamine (acetylate units) that can be easily obtained from the chitin, the second most abundant polymer in nature.43 Chitosan is a low-cost and environmentally benign water-soluble polymer. The presence of large number of free amines on the backbones makes CS an attractive polymer for further functionalization while typically maintaining solubility in aqueous solution. Catechol groups have been reported to play a significant role in the exceptional wetnessresistant adhesion capability of the mussels, thus having been demonstrated as useful adhesion groups in binder materials.25, 44 Instead of using amide coupling reaction which involves multi-step reaction/purification and difficulty in achieving high degree of



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substitution, aldehyde terminated catechol, i.e., di-hydroxybenzaldehyde, was utilized to couple with the primary amine groups on CS via a Schiff reaction followed by reduction of imine bond as shown in Scheme 1 and Figure 1 (B).45 In the Fourier-Transform InfraRed (FT-IR) spectrum of CS and catechol functional chitosan (CS-CG), the appearance of absorption bands at 787 cm-1 and 1347 cm-1 corresponds to the aromatic bending and phenyl O-H bending, indicating successful grafting of catechol groups on the CS (Figure 1(A)). The 1H NMR spectrum of CS-CG confirmed the presence of aromatic protons corresponding to the catechol moieties, and the grafting ratio of catechol group on the chitosan is calculated to be 10% from the comparative peak integration of the aromatic protons and the methine next to the amine groups (See Supporting Information for details).

Formation of 3-D polymer network was proved useful in improving the mechanical robustness of polymer binder materials, and in-situ crosslinking with the active materials is especially advantageous on the binder performance in a cell.42 Glutaraldehyde (GA) was demonstrated as an efficient cross-linker for CS via the Schiff mechanism.29 A significant rise in viscosity following the reaction between GA and CS-CG is attributed to


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the crosslinking reaction (Figure S1), and the peak at 1660 cm-1 in the FT-IR spectrum of CS-CG+GA (Figure 1 (A)) confirms the formation of imine bond.29 With high rigidity of the crosslink network, it was difficult to perform the rheological measurement on the CSCG+GA to estimate the crosslinking density.46 Therefore, we performed Soxhlet extraction on dried sample CS-CG+GA to determine their gel fractions (ratio of crosslinked materials). After 24 hours’ extraction at 130 ºC, the gel fractions of CSCG10%+1%GA, CS-CG10%+6%GA and CS-CG10%+20%GA are calculated to be 0.41%, 16.4% and 47.8%, respectively. These results demonstrate that GA is an efficient cross-linker for CS-CG in forming a functional polymer network, and high feed ratio of GA afford a polymer network with higher crosslinking density. In aqueous solution, GA (6 wt% to that of CS-CG) was added to the catechol functionalized chitosan (CS-CG10%) to form a pre-crosslinked polymer solution (CS-CG10%+6%GA), which was then mixed with silicon nanoparticles (SiNPs, active materials, see Figure S2, S3 for SEM images and XRD patterns) and carbon black (CB, conductive agent). The SiNPs based composite film was dried at 80 °C under vacuum to remove solvent and facilitate the in-situ crosslinking reaction. Illustrated by the scanning electron microscopy (SEM) image in 11 ACS Paragon Plus Environment


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Figure 1 (C I), the pristine composite film exhibits porous architectures (the porosity was calculated to be 73.1±1.0%). Energy-dispersive X-ray spectroscopy (EDX) mapping of carbon, oxygen and especially silicon confirms the homogeneous distribution of active materials in the composite anode film (Figure 1(C III, IV and V) and Figure S4). The resulting homogenous dispersion is achieved due to the suitable solution viscosity and the controlled substitution of adhesion groups that prevents aggregation of active components during the electrode fabrication process.31, 47

Scheme 1. Illustration on the synthesis of crosslinked catechol-rich chitosan network (top), and charging/discharging process of SiNPs with polymer binder (bottom)


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The adhesion capability of the polymer binders was initially tailored according to the galvanostatic test of Si-based anode. Depending on different types of SiNPs, the cycling performances with PVDF binder are slightly different with each other.11, 15, 32 However, all of them (including those reported in literature and the one shown in Figure 2) exhibited rapid capacity fading. The primary reason for the poor binder performance of PVDF for Si electrode is its weak adhesion capability (see Figure 3(A)), although the cycling performance of the Si-PVDF electrode may be improved by appropriate thermal treatment.37,

48

The electrode with CS showed significantly improved electrochemical

performance (in terms of initial de-lithiation capacity, initial coulombic efficiency and cycling performance) compared with that of PVDF, which is most likely due to its promoted adhesion with the SiNPs (34.3 ±1.6 nF for CS vs 10.0±3.3 nF for PVDF, Figure 3(A)). The presence of large amount of polar groups (free amine and hydroxyl groups) in the CS facilitates strong hydrogen bonding with the hydroxylated Si surface,49 while the PVDF can only interact via the Van der Waals forces with the active materials. CS with different



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grafting densities of catechol groups was synthesized and confirmed by the 1H NMR spectra (Figure S5). The CS-CG10% showed higher initial de-lithiation capacity and better cycling performance (1890 ± 17 mAh/g at 100th cycle) than those of CS (1734 ± 12 mAh/g at 100th cycle) due to the addition of wetness-resistant adhesion groups. As illustrated in Figure 3(A), the adhesion capability with SiNPs enhances significantly when the grafting density of catechol groups increased from 10% to 25% (see comparative 1H NMR spectra in Figure S5). However, the SiNPs based anode with the binder possessing higher catechol functionalities, i.e., CS-CG25%, revealed more rapid capacity fade, which dropped below 1,000 mAh/g after 10 charging-discharging cycles. Moreover, the prepared composite slurry with further increased grafting density of catechol groups, i.e., CS-CG56% as the binder could not even form a qualified coating on the copper foil.


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Figure 1. (A): Comparative FT-IR spectra of CS, CS-CG10%, and CS-CG10%+6%GA; dash line from left from right, wavenumber at 1660 cm-1, 1347 cm-1and 787 cm-1; (B): 1H NMR spectra of CS (top) and CS-CG10% (bottom); (C): (I) SEM images of the SiNP based composite electrode with CS-CG10%+6%GA as the binder; (II) zoom-in area for Energy-dispersive X-ray spectroscopy (EDX) mapping; EDX mapping of silicon (III), carbon (IV), and oxygen (V) in the specified area

The significantly improved cycling performance of Si-based electrodes with binders from PVDF to CS and then to CS-CG10% (see initial coulombic efficiencies and capacity



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retention in Figure 2(A) and (B)) can be attributed to their enhanced adhesion capability with SiNPs (see Figure 3(A)). The enhanced adhesion force of polymer binders with SiNPs results into a relatively thinner SEI layer on SiNPs, and the thinner SEI improves the electrochemical stability and capacity retention by lowering the ionic resistance at the interface.11 The relatively lower resistance, measured by electrochemical impedance spectroscopy (EIS) as shown in Figure S6, of Si based electrode with CS-CG10% after 5 and 100 cycles comparing with that of CS is the evidence for thinner and more ionic conductive SEI layer.40 In addition, the polymer binders with polar groups (-OH and -NH2 groups in CS) tend to form a composite coating, possessing higher adhesion strength with copper foil (see Figure 3(B) for comparative peel strength). Further substitution of catechol units provided stronger interaction with the current collector, which ensures the electric pathway to the current collector.50 However, simply enhancing the adhesion force with SiNPs will not improve the cycling performance indefinitely and other parameters, such as chain stiffness and solution viscosity, also become important. Despite the higher adhesion force of CS-CG25% over CS-CG10% with SiNPs (Figure 3(A), the comparative peel test of SiNPs based anodes (Figure S7) demonstrated much lower peel strength of 16 ACS Paragon Plus Environment

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anode coating with CS-CG25% with the current collector, indicating a higher chance of delamination during the cycling process. The lower solution viscosity (See Figure S1) and higher rigidity for the CS with higher degree of catechol substitution may also affect the mechanical robustness of composite coating and adhesion strength with the current collector.



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Figure 2. (A) Specific capacity and (B) coulombic efficiency of SiNPs based electrodes with PVDF, CS, CS-CG10% and CS-CG25% as the binder

To further evaluate the effect of catechol grafting density on the chitosan backbone stiffness, Kratcky-Porod wormlike cylinder model was employed.51



  s

where  stands for main chain stiffness parameter,  is the intrinsic backbone stiffness without side chain grafting, and s relates to excess free energy brought by the side chain to minimize the main chain bending. Herein,  can be estimated to be the same value of persistence length (lp) with equation of lp=lk/2, and Kuhn length (lk) can be calculated via equation lk=C∞×lb, where C∞ is the Flory characteristic ratio, and lb is estimated at 5.15Å, the length of the anhydroglucose unit (Figure S13). Since the reference value of C∞ of the chitosan backbone in carbonaceous electrolyte (LiPF6 in EC-EMC solution) is difficult to find out, we here resort to using C∞=19 for uncharged chitosan in the theta condition for the lower limit of the chain stiffness.52 The increase of both catechol grafting and ion-concentration in the solvent may increase the Flory characteristic ratio due to the steric hindrance to chain rotation and the main chain stiffness resulted from the electrostatic interaction of charged groups.52-53 From


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the two equations, the  and lp can be estimated to be 4.9 nm, in accordance to the lower limit that reported for chitosan-water solution. s is a molecular weight dependent term that can be estimated from a simple yet general scaling law via equation (2),54

s1 

NS lg

(2)

where Ns is the degree of polymerization (here Ns=1), and lg is the contour length of the backbone between two side chain that can be estimated by equation (3).

lg=lM (1+n)

(3)

with lM the dimension of the catechol group (estimated at 5.8 Å, see SI) and n the number of the repeating units along chitosan backbone between the two adjacent catechol side groups. Here, n is 9 for CS-CG10%, 3 for CS-CS25%, and 1 for CS-CG56%. Combination of equation (1)-(3) results in the plot of stiffness parameter,  v.s. grafting density of CG (Figure 4(B)). It should be noted that the slope of the linear relation between s and lg in equation (2) is not specified for current study. However, it can be clearly seen the backbone chain stiffness increases with the graft density of CG. With slope = 2, the value of  is estimated to be 6.62 nm for CS-CG56%, around 50% stiffer than that of the CS-CG10%. The significantly increased glass transition temperature with the increase



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grafting density of CG verified the calculated results (Figure 4(A)). These results demonstrate that catechol grafting leads to the steric hindrance to chitosan chain bending. This further results in a stiffened main chain backbone, and the stiff backbone with the higher grafting density of CG will adversely affect the efficient “binder” effect on the active materials by limiting the coverage of binders on the surface of SiNPs as shown in Scheme 2.

Figure 3. (A) Adhesion force (normalized by probe diameter) of different polymer binders measured by AFM with a colloidal silica tip (15 m diameter); (B) Peel test results of 20 ACS Paragon Plus Environment

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SiNPs based anode with different polymer binders; (C) SEM images of SiNPs-based electrodes with polymer binder (I) PDVF, (II) CS, (III) CS-CG10%+6%GA, and (IV) CSCG10%+20%GA before (top) and after 100 lithiation/de-lithiation cycles (bottom); all of the scale bars are in the same dimension.

With CS-CG10% as the functional polymer binder, the cross-link density of functional polymer network is further tuned by varying the feed ratio of GA to CS-CG10%. With 1 wt% GA, the SiNPs based electrode exhibited improved cycling performance (2030 ± 7.9

v.s. 1890 ± 9.3 mAh/g at 100th cycle) due to the formation of 3-D network. Slightly improved capacity retention was observed when increasing the crosslinking density of functional polymer network (addition of GA: from 1wt% to 3wt% and to 6wt%). The SiNPs based anode with CS-CG10%+6%GA as binder exhibited the highest initial de-lithiation capacity (2345 ± 19 mAh/g) and improved cycling performance with 91.5% capacity retention after 100 cycles. However, when further increasing the crosslinking density, the SiNPs based anode with CS-CG10%+10%GA only had a specific capacity of 1246 mAh/g after 100 cycles, although its initial specific capacity is not significantly lower than that of CS-CG10%+6%GA (2317± 21 vs 2345 ± 19 mAh/g). The electrode with CS-



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CG10%+20%GA binder showed even worse electrochemical performance in both initial specific capacity and cycling stability.

Lithium polyacrylate (LiPAA) is considered to be one of the state-of-the-art nonconductive (electrically) polymer binders for Si anode, and the high molecular weight LiPAA has been employed as a control binder for SiNPs-based anode due to its strong interaction with SiNPs and capability to provide an additional source of lithium ions.38, 55 As illustrated in Figure 5 (B) and Figure S8, the SiNPs based anode with LiPAA exhibited better cycling performance (1884±11 mA/g at 100th cycle) than that of CS (1734 ± 12 mAh/g at 100th cycle) and comparable with that of CS-CG10% (1890 ± 17 mAh/g at 100th cycle). The functional polymer network (CS-CG10%+6%GA) fabricated by rational design in this study demonstrates better binder performance than that of LiPAA (around 14% capacity improvement at 100th cycle). Table S1 shows the comparative cycling performance for all the polymer binders discussed in the manuscript, and the data of some other representative non-conductive polymer binders has also been included. The functional polymer network in this study (CS-CG10%+6%GA) shows advantageous


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cycling performance over the other binders, although the de-lithiation capacity and coulombic efficiency of SiNPs based anode varies with Si source, mass loading and other parameters. As illustrated in the dQ/dV profile of SiNPs electrode with CSCG10%+6%GA as a binder (Figure S9), the sharp cathodic peak lying close to 0.01 V at the first cycle corresponding to the lithiation of crystalline SiNPs. The two anodic peaks (0.34 V and 0.44 V) at the following delithiation scan transfer the LixSi to amorphous Si, and following cathodic peaks (0.24 V and 0.09 V) represent typical lithiation of amorphous Si. The first voltage v.s. capacity profile (Figure S14) shows a slightly lower initial lithiation capacity of Si (3159 mAh/g) compared with the theoretical capacity (3759 mAh/g for Li15Si4), which is probably due to the incomplete lithiation of crystalline Si and the presence of native SiO2. The broad peak lying between 965 cm-1 and 1260 cm-1 in the infra-red (IR) spectrum of the pristine SiNPs based electrode are attributed to the Si-O bond on the SiNPs and C-O bond in the polymer binders (See Figure S10). After chargingdischarging process, appearance of the peaks lying at 1410 cm-1 and 832 cm-1 associated with Li2CO3 and LixPFyOz demonstrated the formation of SEI layer on the SiNPs.



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For practical application, electrodes with high mass loading of active materials are required for high areal capacity. The electrode (mass loading of active materials = 1.1 mg/cm2) with only SiNPs as active materials exhibits rapid capacity decay (from 1958 mAh/g to 840 mAh/g after 100 cycles), which is not suitable in practical applications. For that matter, the functional polymer network (CS-CG10%+6%GA) was tested as the binder of composite electrode containing 50 wt% of graphite and 20 wt% of SiNPs as shown in Figure 5(C) and Figure S11. The CS-CG10%+6%GA exhibited excellent binder performance for high-mass loading composite electrode (mass loading of active materials = 2.5 mg/cm2) with retention capacity of 750 mAh/g after 100 cycles. Carbon coated SiNPs (C-SiNPs) was also fabricated with polyvinylchloride as the carbon precursor. Appearance of the broad peak around 24 degree comparing with the original XRD patterns with only sharp peaks from the crystalline silicon demonstrates the successful coating of amorphous carbon on the SiNPs (See Figure S3).56-57 The carbon coating was also observed from the SEM images of C-SiNPs by comparing with those of bare SiNPs (Figure S2). Compared to the composite electrode with SiNPs, the composite electrode with C-SiNPs (comparable mass loading of active materials) showed lower specific 24 ACS Paragon Plus Environment

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capacity but exhibited improved electrochemical stability, with a capacity retention of nearly 100% from 30th cycle to 100th cycles as shown in Figure 5(D).

Figure 4. The plot of (A) glass transition temperature, Tg (obtained from temperaturemodulated DSC) and (B) main chain stiffness parameter  versus the grafting density of CG (%); The plot of (C) Tg and (D) root mean square end-to-end distance, Re, of the polymer main chain segment between the two adjacent crosslink points with respect to feed ratio of GA(%).

The improved cycling performance of silicon electrode with crosslinked functional polymer binders (from CS-CG10% to +1%GA, +3%GA and +6%GA) can be explained by the 25 ACS Paragon Plus Environment


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improved mechanical robustness of the composite electrodes and stability of the conductive network (polymer network with attached carbon black) around the SiNPs. As illustrated in Figure 3(A) (see CS-CG10% and CS-CG10%+6%GA), no significant difference was observed regarding the adhesion forces with SiNPs upon the addition of cross-linkers. However, increased adhesion strength of functional polymer network was observed in a peel test demonstrating a mechanically robust composite coating by crosslinking, that is important in retaining electron transport during galvanostatic cycling. SEM micrographs collected before and after cycling provide additional evidence as illustrated in Figure 3(C). After 100 lithiation-delithiation cycles, significant cracks were observed for the SiNPs based anode with PVDF as the binder due to their poor capability to hold the composite materials, whereas the electrode with CS as a binder exhibited a slightly lower density of cracks. A rougher smooth surface was observed for the pristine composite anode with CS-CG10%+6%GA as a binder, which is probably due to the formation of 3-D network and higher porosity in the as-cast electrode coating. However, after 100 lithiation-delithiation cycles, no significant crack was observed on the SiNPsbased anode with CS-CG10%+6%GA. The comparative SEM results are consistent with 26 ACS Paragon Plus Environment

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Liu’s results that the SiNPs based anode with mechanically robust binder formed by crosslinking can maintain the integrity of composite electrode after cycling,58 which is vital for retaining the conductive network and minimizing the de-activation of the active materials during cycling. The relatively stable resistance of the SiNPs based anode with CS-CG10%+6%GA (more stable than those with CS or CS-CG10% as binders as illustrated in Figure S6) confirms the robust conductive network with a stable SEI layer.40 These results emphasize the role of both enhanced adhesion capability and mechanical robustness of the polymer binders in the ultimate cycling performance of Si anodes.



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Scheme 2. Illustration of the effect of grafting density of adhesion group and degree of crosslinking density on backbone stiffness of polymer binder

Chemical crosslinking not only provides mechanical robustness of polymer binder but also enhances the overall stiffness of the resulting polymer network. As evidenced in Figure 4(C), with increasing amount of cross-linker, GA restricts the chain segmental motion, thus increasing Tg value, consistent with a previous report.59 When the crosslinking density is low, it exhibits a linear relationship with Tg as assumed by Fox and others.60 The crosslink density exhibits an exponential relationship with Tg with a broader range of values according to DiBenedetto’ equation.61

Tg  Tgo o g

T

K

Xc 1 Xc

(4)

where Tog is the glass transition temperature of the uncrosslinked polymer as a reference, K is a constant estimated to be 4.3 for the CS-CG10%. Xc is the crosslink density characterizing the mole fraction of monomer units that function as cross-linking units, which is further related to the root mean square end-to-end distance of the polymer segment between two crosslink points, i.e., Re. The Xc was calculated to be 0.013, 0.04, 0.08 and 0.27 for CS-


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ACS Energy Letters

CG10%+1%GA, CS-CG10%+3%GA, CS-CG10%+6%GA, and CS-CG10%+20%GA, respectively. Re is calculated from the average repeating units per crosslink point and the average length of each repeating unit is calculated to be 0.44 nm.

Figure 5. (A) cycling performance of SiNPs-based electrodes from the polymer binder with different crosslinking densities: CS-CG10%, CS-CG10%+1%GA, CS-CG10%+3%GA, CSCG10%+6%GA, CS-CG10%+10%GA, CS-CG10%+20%GA; (B) cycling performance of SiNPs based electrodes with different polymer binders: PVDF, CS, LiPAA, CS-CG10% and CSCG10%+6%GA; (C) and (D) cycling performance of composite electrode (SiNPs for C and carbon coated SiNPs for D and graphite) with polymer binder of CS-CG10%+6%GA.

Although the chain conformation differs between a linear system and crosslinked counter parts, the scaling behavior of the segment between the two adjacent crosslink points in



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the CS-CG10%+GA can still be evaluated using the linear polymer as a reference. As illustrated in Figure 4(D), when feed ratio of GA is lower than 6%, the distance between the two adjacent crosslink point (Re, 5.5 nm) is larger than persistence length (lp, 4.9 nm), indicating a flexible polymer chain. An increased feed ratio of GA to 10%, lead to Re (3.3 nm) smaller than the persistence length, indicative of a stiff rod for the polymer segment between the two adjacent crosslink points as shown in Scheme 2.

The balance between stress relaxation and polymer stiffness should be responsible for the unique relationship between the crosslinking density of functional polymer network and cycling performance of the resulting Si electrodes.28, 62 Upon further increasing the crosslinking density from 6% to 10% and 20% (Figure 5(A)), significantly higher network stiffness dominates the ultimate cycling performance. Peel test of silicon anode with CSCG10%+20%GA as a binder (Figure S12) demonstrated the weak adhesion strength of anode coating with the current collector, and significant force drop (almost to 0) was frequently observed during the peeling process. SEM images of silicon anode with CSCG10%+20%GA revealed slight cracks even before the galvanostatic test (Figure 3(C-


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ACS Energy Letters

IV) top and Figure S15), and more significant cracks were observed after the lithiation/delithiation process (Figure 3(C-IV) bottom). These results suggest that the brittle nature of composite coating causes adverse effect, which should be attributed to the high stiffness of highly crosslinked polymer network. Although both adhesion capability and mechanical robustness are crucial to the electrochemical performance of polymer binders, this study clearly indicates that the cycling performance is controlled by balancing rheological properties of the composite slurry, backbone stiffness of binder materials, mechanical property of anode coating, and adhesion capability with both an active material and a current collector

In conclusion, the rational design of a functional polymer network was created by tailoring the grafting density of adhesion functionalities and the degree of crosslinking density as the binder of Si electrodes. Catechol functionalized chitosan can be synthesized via a Schiff reaction, and the remaining primary amines can be readily reacted with glutaraldehyde for an in-situ formation of a 3-D network around the SiNPs. The degree of wetness-resistant adhesion functionality and crosslinking density were tailored



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independently for the optimized electrochemical performance of functional polymer network. The results here demonstrated that neither of these two well-reported “better” properties alone do not necessarily improve the binder performance in an absolute manner. Rather, the resulting binder performance is significantly influenced by the viscosity of slurry, stiffness of polymer network, mechanical robustness of composite coating, and adhesion strength with a current collector. A semi-quantitative analysis on the main-chain scaling behavior and stiffness also demonstrated that both catechol grafting and crosslinking lead to the reduced flexibility of polymer backbone for the resultant binder system. It turns out that the end-to-end distance of the two adjacent crosslinking points (Re) should be no less than the persistence length (lp) of the chitosan backbone for an efficient binding performance. In the current system, the CSCG10%+6%GA, i.e., 10% of catechol functionalization and 6wt% cross-linker, is the polymer binder with the best cycling performance, and shows better cycling performance than the state-of-the-art non-conductive polymer binder (2144 ± 14 mAh/g for electrode with CS-CG10%+6%GA vs 1884±11 mA/g for that with LiPAA as the binder). The excellent cycling performance of the composite electrode (SiNPs/graphite or C-

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ACS Energy Letters

SiNPs/graphite) with CS-CG10%+6%GA as binder also demonstrated its great potential for practical applications in lithium-ion batteries. The relationship established between binder properties and electrochemical performance of consequent Si electrode is imperative for designing polymer binders with optimized electrochemical performance. The strategy of in-situ construction of a functional polymer network is instructive for the design of high-performance polymer binders not only for Si based anodes, but also for a wide variety of other types of alloy and conversion-based electrodes.

Acknowledgement

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). A. Erwin also acknowledges partial financial support for adhesion force measurements by the National Science Foundation DMR 1505234. APS acknowledge partial financial support for the polymer characterization by the U.S. Department of


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Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.

Supplementary Information

Experimental section, solution viscosity, 1H NMR spectrum of polymer binder; EDX mapping, peel test and SEM image of SiNPs based anode are provided.

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Rational Design of a Multifunctional Binder for High-Capacity Silicon

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