Chitosan-grafted-polyaniline copolymer as an electrically conductive and mechanically stable binder for high-performance Si anodes in Li-ion batteries

Journal Pre-proof Chitosan-grafted-polyaniline copolymer as an electrically conductive and mechanically stable binder for high-performance Si anodes in Li-ion batteries K.K. Rajeev, Eunsoo Kim, Jaebin Nam, Suhyun Lee, Junyoung Mun, Tae-Hyun Kim PII:

S0013-4686(19)32404-1

DOI:

https://doi.org/10.1016/j.electacta.2019.135532

Reference:

EA 135532

To appear in:

Electrochimica Acta

Received Date: 3 September 2019 Revised Date:

13 December 2019

Accepted Date: 14 December 2019

Please cite this article as: K.K. Rajeev, E. Kim, J. Nam, S. Lee, J. Mun, T.-H. Kim, Chitosan-graftedpolyaniline copolymer as an electrically conductive and mechanically stable binder for highperformance Si anodes in Li-ion batteries, Electrochimica Acta (2020), doi: https://doi.org/10.1016/ j.electacta.2019.135532. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

CRediT author statement

Rajeev K.K.: Conceptualization, Methodology, Investigation, Software, Data curation, WritingOriginal draft preparation Eunsoo Kim: Conceptualization, Methodology, Investigation, Data curation, Writing- Original draft preparation. Jaebin Nam: Methodology, Software, Data curation, Investigation. Suhyun Lee: Software, Data curation, Investigation Junyoung Mun: Conceptualization, Supervision, Writing- Reviewing and Editing Tae-Hyun Kim: Conceptualization, Supervision, Writing- Reviewing and Editing

I hereby confirm that the above roles for each author are correct.

2019.11.21 Tae-Hyun Kim, Professor Incheon National University

Graphical Abstract

Chitosan-grafted-polyaniline copolymer as an electrically conductive and mechanically stable binder for high-performance Si anodes in Li-ion batteries Rajeev K.K.,a,b,1 Eunsoo Kim,a,b,1 Jaebin Nam,a,b Suhyun Lee,c Junyoung Munc,* and Tae-Hyun Kima,b,*

3500 CS

Specific Capacity (mAh/g)

3000

CS-g-PANI-0.3 CS-g-PANI-0.5

2500

CS-g-PANI-0.9 PVdF

2000

1500

1000

500

0 0

50

100

Cycle Number

150

200

Chitosan-grafted-polyaniline copolymer as an electrically conductive and mechanically stable binder for high-performance Si anodes in Li-ion batteries Rajeev K.K.,a,b,1 Eunsoo Kim,a,b,1 Jaebin Nam,a,b , Suhyun Leec, Junyoung Munc,* and Tae-Hyun Kima,b,* a

Organic Material Synthesis Lab. Department of Chemistry, Incheon National

University b

Research Institute of Basic Sciences, Incheon National University, 119 Academy-ro,

Songdo-dong, Yeonsu-gu, Incheon 406-772, Korea. c

Department of Energy and Chemical Engineering, Incheon National University, 119

Academy-ro, Songdo-dong, Yeonsu-gu, Incheon 406-772, Korea ∗

Corresponding author.

Tel: +82-32-835-8876; E-mail: [email protected] (J. Mun)

Tel: +82-32-835-8232; Fax: +82-32-835-0762; E-mail: [email protected] (T.-H. Kim) 1

These authors contributed equally to this work.

KEYWORDS: Polymer binder; Si anode; Electrical conductivity; Mechanical property; Chitosan-grafted-poly(aniline)

ABSTRACT: We developed chitosan-grafted-polyaniline copolymers (CS-g-PANIs) as new water-soluble electrically conductive and mechanically robust polymer binder materials for silicon anodes. Unlike most other electrically conductive polymer binders, the CS-g-PANIs were easily prepared by grafting electrically conductive PANI onto the chitosan, which adheres well to Si. Various CS-g-PANI copolymers were prepared with different grafting degrees of PANI to investigate the effect of PANI on the electrochemical and physical properties of the prepared electrodes. The copolymer whose composition was 50% PANI and 50% chitosan showed the best cell performance, due to the well-balanced electrical conductivity and mechanical properties. It exhibited a specific capacity of 1091 mAh g-1 at the 200th cycle with 99.4% Coulombic efficiency.

1. INTRODUCTION With the rapid adoption of lithium-ion batteries (LIBs) in large scale devices such as electric vehicles and energy-storage systems, the development of new 1

electrode materials with higher capacity and longer lifetime is more urgently required than ever, because the energy density of a cell is directly related to the specific capacity of the electrode material. Silicon, with its high theoretical capacity (4200 mAh g-1) and relatively low Li+ insertion potential, has attracted attention as an anode material for next-generation LIBs to replace the currently used graphite (with a theoretical capacity of 372 mAh g1

) [1-5]. In practice, the use of silicon, however, causes a rapid fading of the cycle

performance of the cell, resulting in poor long-term reliability. This is caused by a drastic volume change in the course of lithiation/delithiation (4.4Li+ + Si <-> Li4.4Si). A number of studies have been conducted in attempts to minimize the capacity fading of Si anode materials. One approach is to use a modified Si, such as silicon nanoparticles, nanowires, nanotubes [6-13], or porous silicon spheres [14-17], which would allow the Si to accommodate the volume change, mitigating the problems mentioned above. These methods, however, increase the cost of fabricating the otherwise low-cost silicon, and also reduce cell efficiency due to the low volume energy density. Alternatively, polymeric binder materials with high mechanical strength have been utilized in recent years to ensure the overall uniformity of Si electrodes, and achieve high electrochemical performance [18-20, 21-22]. For a Si anode, the desired binder must hold the active materials and conductive agents together to accommodate the drastic volume changes of Si during the Li+ charge/discharge process, to provide electrical pathways to the current collector. The most widely used polymer binders in typical LIBs, such as poly(vinylidene fluoride) (PVdF) and styrene-butadiene rubber (SBR) [23-25], are not applicable to Si anodes because their interactions with Si are not strong enough to accommodate large volume changes [2,22,26]. Recently, polymers containing hydroxyl (-OH), carboxylic (-COOH) or amino (-NH2) groups have drawn growing interest as binders for Si anodes because these polymers bind strongly to Si particles through H-bonding with the OH groups on the Si surface. As a result, they significantly reduce the damage caused by volume expansion, thereby ensuring improved electrochemical performance [20,21]. These polymers include not only synthetic polymers such as poly(acrylic acid) [21,26-30], poly(vinyl alcohol) [21,26,31] and carboxylic acid derivative-functionalized poly(phenylene) [32,33] but also natural polymers such as sodium alginate [34,35] xanthan gum [21,36,37], guar gum [21,38,39], chitosan [40-43], cyclodextrin [20, 2

44,45] and carboxymethyl cellulose [20,24,28,29] as well as conjugates of synthetic and natural polymers [20,21,24,29,41,46,47]. The use of nature-derived biopolymers as binders for Si anodes has great practical potential because they are abundant, have environmentally benign characters and are low cost. Among them, chitosan (CS), a linear polysaccharide composed of N-acetyl-Dglucosamine (acetylated unit) and β-(1,4)-linked 2-deoxy-2-amino-D-glucosamine (deacetylated unit) in random distribution, is easily obtainable from shrimp, crab and other crustacean shells [48,49]. In recent years, CS has been investigated as an alternative binder material for Si electrodes, mainly by the formation of cross-linked structures with other polymers that have reactive functional groups such as aldehyde [41,42,49]. These crosslinked CS binder systems have enhanced mechanical robustness and have been successfully demonstrated for application to Si anodes. Meanwhile, some efforts have been also made to provide additional functionalities to polymer binders with ionic and/or electrical conductivities. For example, poly(ethylene glycol) (PEG) [20,21,50] and Nafion® [21,51] in Li+conducting

polymers,

and

the

electrically

conductive

polymers

such

as

poly(dioctylfluorene-co-fluorenone) (PFFO) [20,21,52], polyphenylene (PP) [32,33] and poly(3,4-ethylenedioxythiophene) (PEDOT) [21,53,54] have been employed as binder materials for Si. However, the preparation of these conducting polymers are very costly and their interaction with Si is very poor as well. We have previously used the electrically conductive and easily prepared polyaniline (PANI) as a crosslinking additive to prepare a physically crosslinked binder system, using an acid-base interaction, by blending PANI with the poly(acrylic acid) (PAA) [55]. This imparts the electrical conductivity of PANI to PAA, and enhances the mechanical robustness of the PAA binder, and as a result the system has achieved high-level cell performances [55]. However, PANI - despite the advantages of this PANI-PAA blend system- has very low solubility and is difficult to employ in practical applications such as silicon electrodes. We now report a new multifunctional binder system based on chitosan-PANI copolymer which has both electrical conductivity and mechanical integrity, due to its strong adhesion to Si, leading to improved cycle performance of the Si anode (Figure 1). In this work, electrically conductive PANI was grafted onto chitosan, which has good Si adhesion properties. Different degrees of PANI grafting, that is, different 3

PANI compositions, were investigated, and used as aqueous binders for Si anodes. The structure and the effects of grafting degree on the thermomechanical, morphological and electrochemical properties of these chitosan-grafted-PANI (CS-gPANI) copolymer binder systems for Si anodes were systematically investigated. There have been several reports where a copolymer was grafted onto the polymer backbone and this was used as a polymeric binder material for Si anode, including glycol chitosan-grafted-lithium polyacrylate (GC-g-LiPAA) [43], PAA grafted polyvinylidene fluoride (PVdF) [30], and poly (acrylic acid sodium)-graftedcarboxymethyl cellulose (NaPAA-g-CMC) [29]. Although a much improved cell performance was obtained for these grafted copolymer systems, compared to the linear-type polymers, most of these works were focussed on the mechanial properties of the polymer binder. It has been reported that not only the adhesion property, but also mechanical robustness and the formation of homogeneous mixtures are all very important in developing high performance polymer binders [56]. However, we focus in this work on the combined effects of adhesion and electrical conductivity of the binder. This was achieved by grafting the electrically conductive PANI onto the chitosan with good adhesion property. Therefore, our chitosan-grafted-PANI (CS-g-PANI) copolymer binder systems differ in that not only the mechanical property but also the electrical conductivity were given.

2. EXPERIMETAL SECTION 2.1. Materials. Chitosan (CS) was purchased from TCI, aniline and ammonium persulfate (APS) were obtained from Sigma-Aldrich. Silicon nanoparticles (SiNPs) (50 nm) were purchased from Alfa-Aesar. Hydrochloric acid and N-methyl-2pyrrolidone (NMP) were purchased from Daejung (Korea). The electrolyte used for all electrode preparations consisted of 1 M LiPF6 in a combination of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (volume ratio of EC/ EMC=1/2 (v/v)) with 10% fluoroethylene carbonate (FEC), and was

purchased from

PanaxEtec. 2.2. Synthesis of the chitosan-grafted-polyaniline (CS-g-PANI). The chitosangrafted-polyaniline copolymer was synthesized by radical polymerization, by modifying the procedure reported in the literature [57]. A solution of chitosan (CS) 4

was prepared by dissolving 2.0 g of CS in 200 mL of a 1 M HCl solution. 0.98 mL of aniline monomer was then added dropwise at room temperature to this CS solution to keep the 1:1 molar ratios between the two monomers. After fully dissolving the aniline, 2.24 g of ammonium persulphate aqueous solution prepared in a 1 M HCl solution was added dropwise into the chitosan-aniline solution. The mixture was left to stir overnight under N2 atmosphere and a crude product was obtained, dark green in color. The reaction mixture was neutralized with 5 M NaOH solution, and the obtained polymer was precipitated into excess ethanol. The resultant precipitate was washed with NMP to remove any PANI which was not grafted onto chitosan, from the copolymer. Finally, washing with acetone, and drying in vacuum at 40 oC produced the desired chitosan-grafted-PANI copolymer. 2.3. Fabrication of the Si nanoparticle (SiNP) electrode. To fabricate the silicon nanoparticle-based electrode, chitosan-grafted PANI was first dissolved in a 1 M acetic acid solution with sonication for 45 minutes. The silicon electrode consisted of SiNPs, acetylene black and binder in 60:20:20 ratio by weight. To prepare the silicon electrode without a conducting agent (acetylene black), a silicon nanoparticle : polymer binder was used, with a 75:25 ratio by weight. The SiNPs and acetylene black were first ground together, and then the polymer binder solution was added into it to form a homogeneous slurry in the mortar with controlled viscosity. Before being transferred into the glove box for cell assembly, the working electrodes were prepared by spreading the above slurry onto Cu foil using a doctor blade and dried at 120 ºC for 3 h under vacuum to remove water. For the electrochemical measurements, the mass loading of all the electrodes was fixed at 0.8-0.9 mg cm-2. Other electrodes using unmodified chitosan (CS), PVdF were prepared using practically the same method, except that NMP was used as a solvent for the PVdF electrode. 2.4. Characterization methods and measurement of electrical properties of the cells. To characterize the polymers and obtain electrochemical and mechanical measurements of the electrodes, we followed the previous report [57,58], and the methods are detailed in the supporting information. 2.5. Full cell fabrications. The composite positive electrode was prepared with LiCoO2 (JES-Echem), PVdF and super-P in the weight ratio of 90:5:5 by using the same method for the NP-Si electrode, except the 16 ㎛ thick Al substrate as a current collector. To control the N/P ratio as 1.2, the weight loading of LiCoO2 on the 1 cm2 5

of Al foil was controlled to have 120% of the discharge capacity of the counter negative electrode, SiNPs, in the same area of 1 cm-2. A round LiCoO2 electrode of a 1.2 cm diameter and a 1.3 cm dimeter round SiNPs composite electrode with different binder were assembled in the 2032 coin cell with a polypropylene separator and the same electrolyte. The full cells were changed and discharged in the potential range between 3.0 and 4.2 V under 0.1 C for the initial three cycles and 1 C for the subsequent cycles at the room temperature.

3. RESULTS AND DISCUSSION 3.1. Preparation of the Chitosan-grafted-polyaniline (CS-g-PANI). Polyaniline was grafted onto chitosan by the oxidative polymerization of aniline in a 1 M HCl solution of chitosan in the presence of APS (ammonium persulphate) as an initiator to produce the chitosan-grafted-PANI copolymer (CS-g-PANI) following the modified procedure (Figure S1). CS-g-PANIs with various compositions (or grafting degree) of polyaniline were prepared by changing the feed ratios of aniline to CS by 30%, 50% and 90% by weight, and they are designated CS-g-PANI-0.3, CS-g-PANI-0.5, and CS-g-PANI-0.9, respectively. The structures of the CS-g-PANI copolymers with different PANI contents were confirmed by both 1H NMR and FT-IR spectroscopic analyses. New peaks between 6.9 and 7.2 ppm, corresponding to the aromatic protons (Ha), appeared after the incorporation of PANI (Figure 2). With increasing aniline content, the integrals of the aromatic proton peaks increased, confirming the incorporation of a higher degree of PANI. The structures of the CS-g-PANI copolymers with various PANI compositions were further confirmed by comparative spectroscopic methods, using FT IR (Figure S2). New peaks at 1582 cm-1 (due to the quinoid-benzenoid moieties), 1500 cm-1 (due to the C=C stretching of the aromatic ring) and 1298 cm-1 (due to the C-N stretching of the secondary amine from PANI) appeared for the CS-g-PANI copolymers, while the peaks at 2874 cm-1 and 1654 cm-1 corresponding to the C-H stretching and carbonyl stretching (-C=O) of the amide moiety in chitosan remained the same. Moreover, the above-mentioned characteristic peaks of the PANI moiety (at 1582, 1500 and 1298 cm-1) increased, with a decrease in the relative intensity of the peaks for the amide group at 2874 and 1654 cm-1, as the aniline content was increased, indicating the successful incorporation of the PANI onto the chitosan unit. 6

3.2. Thermal, mechanical and electrical properties of the CS-g-PANI binders. The thermal stability levels of the prepared CS-g-PANI copolymers were measured using thermogravimetric analysis (TGA), and the results were compared with that of pristine CS and PANI (Figure 3). The initial weight loss below ca. 120 o

C was attributed to the evaporation of water and was observed for all the samples

measured. The three chitosan-grafted-PANI samples (CS-g-PANI-0.3, CS-g-PANI0.5, and CS-g-PANI-0.9) then experienced a gradual degradation from ca. 130 oC to 330 oC due to the degradation of both the PANI unit and the decomposition of the chitosan (CS) structure. Compared to the pristine chitosan, the CS-g-PANI copolymers showed lower onset temperature degradation because of the incorporated PANI, which decomposed at a lower degradation temperature. The residue of the CSg-PANIs increased in the order of PANI-g-CS-0.3, PANI-g-CS-0.5 and PANI-g-CS0.9 due to the higher contents of thermally stable PANI. The electrical conductivity of the chitosan-grafted-PANI binders was measured using the four-probe method, and the results were compared with chitosan (CS). As expected, chitosan did not show any meaningful conductivity value, and this value increased with the increase in the degree of PANI grafted onto chitosan (CS < CS-gPANI-0.3 < CS-g-PANI-0.5 < CS-g-PANI-0.9) (Figure S3 in the ESI). The three CS-g-PANI copolymers prepared (CS-g-PANI-0.3, CS-g-PANI-0.5, and CS-g-PANI-0.9), each consisting of a different density of PANI, were employed as a binder in a Si electrode. The weight ratio of the active material to the polymer binder and conducting agent was 60:20:20. The binding affinities (or adhesive strength) of each chitosan-grafted-PANI (CS-g-PANI) to Si were then measured using the conventional 180º peel-off test, and the results were compared with that of a CSbased electrode and a PVdF-based electrode, as reference, prepared under the same conditions as those used for the CS-g-PANI-based electrodes (Figure 4). As expected, the pristine chitosan (CS) showed the strongest adhesion due to the presence of large numbers of free hydroxyl and amino groups, and the lowest adhesive force was obtained for the PVdF binder. As for the CS-g-PANI binders, the polymers which had a higher degree of PANI (in other words less CS) adhered less strongly to the Si electrode. This is because PANI has no functional groups in its structure to induce hydrogen-bond interactions between the polymer (PANI) and various Si groups such as SiO2, Si-OH. Nevertheless, it was observed that the CS-g7

PANI-0.9 copolymer, despite its relatively high PANI content, still displayed much stronger adhesion to the Si than the PVdF binder, and CS-g-PANI-0.3 showed very little reduction in adhesion strength compared to the CS binder (0.7 N for CS and 0.6 N for CS-g-PANI-0.3) (Figure 4). These results suggest that our CS-g-PANI binders, despite the incorporation of a conductive structure (PANI), still provide good mechanical property.

3.3. Electrochemical properties. The electrochemical behaviors of the CS-gPANI polymer binders were evaluated using 2032 type coin half cells with the Si composite electrode as a working electrode, and a lithium metal disk both as a counter and reference electrode. The cell performances were then compared with those of CS and PVdF-based electrodes as references (Figure 5a). Firstly, the electrode with the PVdF binder exhibited very poor electrochemical performance in terms of Coulombic efficiency and cycleability. The PVdF-electrode presented a reasonable specific lithiation capacity of 3950 mAh g-1, which is close to the theoretical capacity of 4200 mAh g-1 for the Si active material, in the voltage curves at the initial lithiation for charging (Figure S4). For the rechargeable batteries, Coulombic effiecieny is a very essential requirement for high energy density to preserve redox lithium ions delivering electrons between positive and negative electrodes. However, the initial Coulombic efficiency of the cell having PVdF was crucially poor as 42.2 %, by seriously decreasing the initial delithiation capacity to 1666 mAh g-1. It is thought that the PVdF binder having poor physical adhesion force was not able to effectively preserve the electron pathway among particles in the electrode due to the large volume change of the Si active material even at the 1st charge and discharge cycle. In detail, the subsequent 2nd lithiation specific capacity of 1496 mAh g-1 was close to the 1st delithiation specific capacity of 1666 mAh g1

, indicating that lithiation behaviour did not critically deteriorate the cell-

performance. On the other hand, subsequently, the 2nd de-lithiation specific capacity rapidly dropped to 577.9 mAh g-1 because the volume shrinkage occurred in the de-lithiation process caused a severe particle pulverization and isolation raising internal resistance. Therefore, the kinetic hindrance produced by this unfavourable volume changing behaviour of the Si accelerates the serious capacity drops. Even worse, the PVdF cell continues to drop capacity 8

from the subsequent cycle. The performance of the PVdF-based electrode indicated that PVdF was not proper to accommodate the volume changes of Si (Figure 5a). For the cells made of chitosan (CS) and the chitosan-grafted-PANI binders (CS-g-PANI-0.3, CS-g-PANI-0.5, and CS-g-PANI-0.9), their initial capacities as well as cycleabilities were much superior to that of the PVdF-electrode (Figure 5a). They had a greatly better cycle life than the cell with PVdF. Initially, the capacities from the group of the cell having the controlled binders except PVdF decreases slightly, but after 50th cycle, it is well maintained. In addition, the galvanostatic charge-discharge voltage profiles at the 1st cycle and 200th cycle for the CS-g-PANI electrodes were further investigated and the results were compared with those of the CS and PVdF electrode (Figure 5b and Figure S4). Like the electrode with the PVdF binder, the initial lithiation capacities of the electrodes containing CS-g-PANI-0.3, CS-g-PANI-0.5, CS-g-PANI-0.9, and CS were 4275, 4417, 4457 and 4118 mAh g-1, respectively, and they were all close to the theoretical capacity of Si active material as well. However, their subsequent delithiation capacities were greatly superior to the PVdF electrode (1666 mAh g-1), with delithiation capacities of 3092, 3198, 3180 and 2801 mAh g-1, respectively. Therefore, their Coulombic efficiencies were high, at 71.9%, 72.4%, 71.3% and 68.0% (Figure 5c). These results indicate that the new binders with CS groups can improve the physical dissociation problems of Si-based electrodes during the electrochemical cycling. More specifically, the group of the electrode with CS-g-PANI displayed higher initial specific discharge capacities than the CS electrode, possibly due to the lower resistance of the conductive PANI-grafted electrodes (CS-g-PANIs) (Figure 5a and 5b). For the same reason, the CS-g-PANI-0.9 electrode, which had the highest number of polyaniline units, showed a very high initial capacity, while the CS-gPANI-0.3 electrode, with the smallest composition of PANI in the CS-g-PANI group, showed the lowest capacity at the 1st cycle among the three CS-g-PANI electrodes. It is correspondent that higher amounts of PANI reduced electrical resistance, however, the small amount of PANI in CS-g-PANI-0.3 was not enough to solve the problem. In terms of cycleability, the pristine CS-electrode exhibited 554 mAh g-1 at the 200th cycle with 27% capacity retention of initial capacity. Despite its good adhesion 9

property, chitosan has limited usefulness as a binder for Si because it lacks conductivity, thus lowering the initial capacity of the CS-based electrode. The cycleability of the electrode with too much PANI, such as CS-g-PANI-0.9, however, deteriorated dramatically during the initial 40 cycles, and showed the lowest overall capacity among the CS-containing electrodes (CS and CS-g-PANIs) (Figure 5a). This can be explained by the low CS content and relatively lower adhesion force of this electrode, which made it more mechanically unstable, like the sample with the PVdF binder (see Figure 4). The CS-g-PANI-0.3 electrode, with the smallest amount of PANI, on the other hand, still preserved its capacity, and was higher than the pristine CS-electrode. Overall, a reversible capacity of 677 mAh g-1 with a retention of 30% of the initial capacity was obtained at the 200th cycle for the CS-g-PANI-0.3 electrode. Finally, at the 200th cycle, the CS-g-PANI-0.5 composite electrode, which had the optimized level of both adhesion property and polyaniline-grafting degree among the three CS-g-PANI electrodes, exhibited the highest specific capacity of 1087 mAh g-1 , which is approximately three times higher than the theoretical capacity of conventional graphite active material, with a retention of 42% of the initial capacity (after three charge-discharge cycles for electrode stabilization) (Figure 5a). Overall, the CS-g-PANI-0.5 electrode showed the best cyclability over the course of 200 cycles of charging and discharging due to the balance between electrical conductivity and the mechanical property of the binder. Moreover, all three CS-g-PANI electrodes showed a long and flat charge/discharge plateau at approximately 1.5 to 0.01 V (vs Li/Li+), indicating lower resistance levels for the CS-g-PANI binder electrodes compared with the CS and PVdF binder electrodes (Figure 5b). Compared with the PVdF electrode, the chitosan-based electrodes (CS and CS-g-PANIs) exhibited significantly higher initial Coulombic efficiencies, and these values were remained above 97% (after the initial three cycles) over 200 cycles (Figure 5c). The introduction of conductive PANI further enhanced the initial Coulombic efficiencies of the corresponding electrodes (CS-g-PANI-0.3, CS-g-PANI-0.5, and CS-g-PANI-0.9). The rate capability of the CS-g-PANI electrodes was also investigated to further investigate how incorporating the polyaniline unit onto the chitosan affected the electrochemical performances of the corresponding electrodes (Figure 5d). The rate performances were evaluated at various discharge current densities ranging from 0.1 C to 3 C, at which lithiation proceeds with a constant 10

current at 0.1 C. The results were also compared with the reference electrodes (CS and PVdF). As expected, the CS-g-PANI-0.5 electrode, which showed the best cyclability characteristics due to the balanced conductivity and mechanical properties, also showed the best rate performance. The specific capacity of this electrode was retained at around 2200 mAh g-1 at a high current density of 3 C. Furthermore, its initial specific capacity was nearly recovered when the cycle was subsequently run at 0.1 C after cycling at 3 C. In contrast, the rate performance of CS-g-PANI-0.9 was the worst, other than PVdF, due to the relatively lower adhesion property of this polymer binder compared with the other CS-based binders (CS, CS-g-PANI-0.3, and CS-g-PANI-0.5). It was, however, notable that the capacity of this electrode was recovered higher than that of the pristine CS at 0.1 C after running at higher current density (3 C). This feature is attributed to the higher conductivity provided by the high polyaniline contents of this binder. Other electrodes, including CS-g-PANI-0.3 and PVdF, also showed almost the same tendency as expected from the cyclability data. The findings indicates that the CS-g-PANI binder improves the rate capability as well. By electrochemical impedance spectroscopy (EIS), we further investigated to better understand the electrochemical behaviour of the CS-g-PANI type electrodes. The results were also compared with the pristine CS- and PVdFbased reference electrodes (Figure 6b, Figure S5 and Figure S6). In Figure S6, the Nyquist plot from the PVdF electrode has two distinct semi-circles. The diameters of the two semi-circles observed in the high and low frequency domains can be assigned to internal contact resistance and charge transfer resistance, respectively [59,60]. It was clearly observed that both semi-circles grew severely as the cycles proceeded, due to particle isolations induced by the large volume change in the active material and the weak adhesive force of the PVdF binder. The poor electrochemical performances in Figure S3 are consistent with these EIS results, showing increasing resistance as the cycles proceeded. On the other hand, all the electrodes containing the CS component had much smaller resistances than those with the PVdF binder. Among them, the pristine CSreference electrode had the largest semi-circle, showing 138, 52 and 61 Ohms for the 1st, 25th and 50th cycles. Considering the frequency on the knee point, the observed semi-circle in Figure 6d was considered to be contact resistance. Even though the 11

discharge sequences were performed before the EIS measurement, the state of charge (SOC) influencing the charge transfer resistance was not perfectly controlled to 100% due to various polarization behaviors in the samples. In the middle of the SOC, it reduced the charge transfer resistance, therefore the semi-circle of the charge transfer resistance was not clearly observed. The CS-g-PANI binder electrodes exhibited greatly reduced semi-circles compared to those of the CS and PVdF binder electrodes. Interestingly, the diameter of the semi-circle decreased as the content of PANI in the binder increased, indicating PANI reduced internal resistance due to its high electrical conducting behavior [58]. Considering the cycleability of the CS-g-PANI binder electrodes, the electrodes with the CS-g-PANI binders were not totally the same regarding initial status because of different physical deteriorations. Nevertheless, the electrode with more PANI presented less resistance because of the electrically conductive PANI, even though it had less physical adhesion because it had less CS content. This EIS behavior strongly supported the conclusion that the PANI contributes to the electrical conductivity in the electrode. Overall, the PANI exhibits an electron conducting process via conjugated structures in the polymer structure and the chitosan having polar groups of hydroxide and amine performs a strong interaction through dipole-dipole interaction between Si and binder. In addition, the detrimental behaviour of Si active material having large volume changes was believed to be highly relieved by multi-functional binder having high electrical conductivity as well as high physical adhesion. The internal resistance of the electrode consists of various parameters such as particle pulverization, SEI formation, and inactive components having poor conductivities. During repeating charge and discharge, the severe volume changes cause resistance growth. This effective binder of CS-g-PANI-0.5 is believed to have a beneficial role on the relief of the polarization by additional electron channels and strong adhesion forces among Si particles.

3.4. Morphological analysis. The CS-g-PANI-0.5 electrode had the best cell performance, and its morphology was analyzed by taking top view images of this electrode using FE-SEM, before and after 50 cycles. The results were compared with those of the pristine CS and PVdF electrodes (Figure 7). It was found that the 12

electrodes prepared from the CS-g-PANI-0.5 displayed a homogeneous surface before cycling (Figure 7a). The EDS mapping image further confirmed that the electrode materials, including Si nanoparticles, the conducting agent and binder material (CS), were distributed uniformly on the surface (Figure S7). Moreover, the CS-g-PANI-0.5 electrode maintained its smooth surface and the electrical contact of the electrode materials, with much fewer and narrower cracks after 50 cycles (Figure 7a’). In contrast, the pristine CS-based electrode showed an uneven morphology, with some dense domains caused by the formation a thick SEI layer before cycling. Furthermore, several cracks formed islands of the electrode materials after 50 cycles (Figure 7b’). Similarly, the PVdF electrode did not maintain its original uniformity and revealed huge cracks after 50 cycles due to large volume changes during the process of charge and discharge (Figure 8c). The electrode materials were even found to have detached from the current collector when the cell prepared with the PVdF binder was disassembled after 50 cycles, while the CS- and CS-g-PANI-0.5 electrodes maintained their original status even after 50 cycles (Figure S8). These results confirm that our CS-g-PANI-0.5 electrode was able to exhibit more stable cell performance than the pristine CS and PVdF binders, because of the balanced mechanical strength and electrical conductivity of the binder material. A similar work to show the balanced mechanical property and conductivity was also reported recently using the stretchable conductive glue polymer [61].

3.5. Further electrochemical properties. To focus just on the electrochemical impact of the binder, composite electrodes were prepared with binders without the conducting agent, Super-P, and evaluated for cycle life. The cycle life characteristics of the CS-g-PANI0.5, chitosan, and PVdF electrodes without the conducting agent were evaluated to demonstrate the conductive behaviour of the binder (Figure S9). The CS-gPANI-0.5 electrode showed a higher capacity of 1087 mAh g-1 at the 100th cycle than the electrodes with the chitosan and PVdF binder. The capacity of the cell having the CS-g-PANI-0.5 binder was higher than that having the reference binder and it maintained its high capacity even without conductive carbon black. This suggests that incorporating aniline into the chitosan enhanced the electrical conductivity of the copolymers. This result confirms 13

that the CS-g-PANI-0.5 electrode developed here should be highly promising for the development of high-energy-density LIBs based on Si. Finally, the full cell configuration with the combinations of LiCoO2 and SiNPs electrode with different binders were investigated (Figure S10). The full cell with the negative electrode with PVdF exhibited the lowest initial capacity. In the full cell system, where the rocking chair mechanism works with reversible lithium ions, the capacity is highly affected by the initial Coulombic efficiency. Due to the poor Coulombic efficiency of the negative electrode with PVdF binder caused a poor capacity even for the 1st cycle by losing lithium ions from the LiCoO2 as well as forming the SEI film. In terms of cycleability, the severe capacity drop was also highly correspondent with the result shown in the cycleability of the half-cell (Figure S4). In contrast, the full cell with SiNPs electrode with CS and CS-g-PANI-0.5 displayed relatively higher capacities than that the full cell with a PVdF binder. Especially for the cell with CS-gPANI-0.5, it exhibited a higher initial capacity than the full cell with CS binder. Although they showed higher Coulombic efficiencies in the half cell results, the initial severe polarization of CS restricts the reversible capacity in its full cell. Therefore, the CS-g-PANI-0.5 showed a higher capacity because of low polarization. Also, the capacity from the full cell at the 200th cycle was slightly further preserved even though the SOC of the negative electrode with CS-gPANI-0.5 was deeper than that of the electrode with CS. Considering the volume changes from the electrode with CS-g-PANI-0.5 was thought to be larger than that with CS, the efficient binder characteristics of CS-g-PANI-0.5 also worked in the full cell system. Based on these full cell results, the proposed binder with CS and PANI has a practical benefit in the full cell as well as in the half cell. 4. CONCLUSION In conclusion, we successfully synthesized water-soluble electrically conductive and mechanically robust binder materials for silicon anodes by grafting electrically conductive PANI onto chitosan, which has good adhesion with Si. Chitosan-grafted-PANI copolymers (CS-g-PANIs) were prepared with different grafting degrees of PANI, and the effect of PANI composition on the electrical and mechanical properties of the corresponding electrodes were investigated. The 14

copolymer with 50% chitosan and 50% aniline, designated CS-g-PANI-0.5, exhibited the best cyclability and rate performance due to its balanced electrical conductivity and mechanical properties. The electrode made of CS-g-PANI-0.5 achieved a specific capacity of 1091 mAh g-1 at 200th cycle with 99.4% Coulombic efficiency. The excellent electrochemical properties of the cells prepared with CS-g-PANI polymer binders, as well as the low cost of their synthesis, which involves simply grafting the conductive PANI onto the environmentally benign chitosan, are expected to advance the commercialization of lithium-ion batteries fabricated with silicon anodes, for applications requiring high energy density levels.

Acknowledgments. This work was supported by the National Research Foundation of Korea

(NRF)

grant

funded

by

the

Korea

government

(MEST)

(NRF-

2018R1D1A1B07048006). Part of this work was also supported by the Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education (NRF-2017R1A6A1A06015181).

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Figure and Scheme captions Figure 1. Design of the chitosan-grafted-polyaniline (CS-g-PANI) binder. Figure 2. 1H NMR spectra of the CS-g-PANI copolymers with different PANI compositions (CS-g-PANI-0.3, CS-g-PANI-0.5 and CS-g-PANI-0.9) and the pristine PANI in DMSO-d6. Figure 3. TGA curves of various CS-g-PANI copolymer binders containing different aniline contents in N2 atmosphere, showing CS and PANI for comparison. Figure 4. (a) Adhesion graph and (b) peel strength test results of Si electrodes prepared with CS, CS-g-PANI-0.3, CS-g-PANI-0.5, CS-g-PANI-0.9 and PVdF binders.

Figure 5. (a) The cycling performances of electrodes with various Si@CS-g-PANI, Si@CS and Si@PVdF binders at a charge-discharge current density of 1C for 200 cycles (b) The 1st and 200th galvanostatic discharge-charge profiles of the various Si@CS-g-PANI ratios, Si@CS and Si@PVdF electrodes (c) Coulombic efficiency of the electrodes prepared with different polymer binders and(d) rate performance of Si electrodes with various CS-g-PANI ratios, CS and PVdF binders at the chargedischarge at various current densities.

Figure 5. Electrochemical impedance spectra of (a) [email protected], (b) [email protected], (c) [email protected] and (d) Si@CS electrodes after 1st, 25th, and 50th cycles.

Figure 6. FE-SEM images of SiNP electrodes employing a) CS-g-PANI-0.5 b) CS c) PVdF before cycling and, a’), b’) and c’) after 50 cycles.

22

Figure 1.

Si

\

23

Figure 2.

24

Figure 3.

100 90 80

Weight (%)

70 60 50 40

CS

30

PANI

20

CS-g-PANI-0.3 CS-g-PANI-0.5

10

CS-g-PANI-0.9

0 100

200

300

400

500 o

Temperature ( C)

25

600

700

Figure 4. a)

b) 0.9 1.0

CS

Adhesson Strength N/mm

0.7 0.6

CS-g-PANI-0.3

CS-g-PANI-0.5

0.5 CS-g-PANI-0.9

0.4

PVdF

0.3 0.2

Adhesion Strength (Nm)

0.8 0.8

0.6

0.4

CS

0.2

CS-g-PANI-0.3 CS-g-PANI-0.5

0.1

CS-g-PANI-0.9

0.0

PVdF

0 0

5

10

15

20

Displacement (mm)

26

25

30

Figure 5.

a)

b)

3500

CS CS-g-PANI-0.3 CS-g-PANI-0.5 CS-g-PANI-0.9 PVdF

2500

CS Cycle 1 CS Cycle 200 CS-g-PANI-0.3 Cycle 1 CS-g-PANI-0.3 Cycle 200 CS-g-PANI-0.5 Cycle 1 CS-g-PANI-0.5 Cycle 200 CS-g-PANI-0.9 Cycle 1 CS-g-PANI-0.9 Cycle 200

3.0

2.5

Potential (V vs Li/Li+)

Specific Capacity (mAh/g)

3000

2000

1500

1000

2.0 200 Cycle

1 Cycle

1.5 Charging 1.0

0.5

500

Discharging 0.0

0 0

50

100

150

200

Cycle Number

c)

0

500

1000

3500

2500

1C

40

Specific Capacity (mAh/g)

CS CS-g-PANI-0.3 CS-g-PANI-0.5 CS-g-PANI-0.9

3500

CS CS-g-PANI-0.3 CS-g-PANI-0.5 CS-g-PANI-0.9 PVdF

0.5C

80

60

3000

0.1C

3000

Coulombic efficiency (%)

2000

4000

4500

Specific Capacity (mAh/g)

d)

100

1500

2C

2500

0.1C

3C

2000

1500

1000

20

500

0

0 0

50

100

150

200

3

6

9

Cycle Number

Cycle Number

27

12

15

18

Figure 6.

a)

200 190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0

b)

0

20

40

60

CS-g-PANI-0.5 cycle 1 CS-g-PANI-0.5 cycle 25 CS-g-PANI-0.3 cycle 50

c) 190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0

0

80 100 120 140 160 180 200 220 240 260 280

Re(Z) (Ohm)

20

40

60

40

60

80 100 120 140 160 180 200 220 240 260 280 Re(Z) (Ohm)

CS-g-PANI-0.9 cycle 1 CS-g-PANI-0.9 cycle 25 CS-g-PANI-0.9 cycle 50

0

20

d)

R im(Z) (Ohm)

R im(Z) (Ohm)

190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0

R im(Z) (Ohm)

R im(Z) (Ohm)

CS-g-PANI-0.3 Cycle 1 CS-g-PANI-0.3 Cycle 25 CS-g-PANI-0.3 Cycle 50

80 100 120 140 160 180 200 220 240 260 280

Re(Z) (Ohm)

190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0

CS cycle 1 CS cycle 25 CS cycle 50

0

20

40

60

80

100 120 140 160 180 200 220 240 260 280

Re(Z) (Ohm)

28

Figure 7.

a

Pristine

b

Pristine

10µm

10µm

a’

50th Cycle

c

b’

50th Cycle

10µm

10µm

29

Pristine

10µm

c’

50th Cycle

10µm

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: