Polyvinyl alcohol grafted poly (acrylic acid) as water-soluble binder with enhanced adhesion capability and electrochemical performances for Si anode

Accepted Manuscript Polyvinyl alcohol grafted poly (acrylic acid) as water-soluble binder with enhanced adhesion capability and electrochemical performances for Si anode Jiarong He, Lingzhi Zhang PII:

S0925-8388(18)32019-X

DOI:

10.1016/j.jallcom.2018.05.286

Reference:

JALCOM 46262

To appear in:

Journal of Alloys and Compounds

Received Date: 18 February 2018 Revised Date:

23 April 2018

Accepted Date: 24 May 2018

Please cite this article as: J. He, L. Zhang, Polyvinyl alcohol grafted poly (acrylic acid) as water-soluble binder with enhanced adhesion capability and electrochemical performances for Si anode, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.05.286. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT Polyvinyl alcohol grafted poly (acrylic acid) as water-soluble binder with enhanced adhesion capability and electrochemical performances for Si anode Jiarong He a, b, Lingzhi Zhang a, * a

Key Laboratory of Renewable Energy, Guangdong Key Laboratory of New and

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Renewable Energy Research and Development, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, No.2 Nengyuan Rd., Guangzhou, Guangdong 510640, China

University of Chinese Academy of Sciences, Beijing 100049, China

Corresponding author：

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b

E-mail address: [email protected] (L.Z. Zhang)

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Tel.: +86-20-37246025; Fax: +86-20-37246026.

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Abstract

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Polyvinyl alcohol grafted poly (acrylic acid) (PVA-g-PAA) is synthesized through graft polymerization of acrylic acid (AA) onto PVA backbone via a free radical reaction. PVA-g-PAA is used as a water-soluble binder for silicon (Si) anodes

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in lithium-ion batteries (LIBs). The enhanced adhesion strength, excellent flexibility and high electrolyte uptake after grafting reaction render PVA-g-PAA a robust binder

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for Si anodes. Compared to linear PVA, PAA and CMC, optimal Si-PVA-g-10PAA electrode exhibits better cycle stability, higher Coulombic efficiency and more excellent rate capability, possessing a high electrical conductivity, low SEI/charge transfer resistance and fast lithium-ion diffusion coefficient. PVA-g-PAA binder not

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only maintains the electrode’s mechanical and electrical integrity, facilitates a favorable electrochemical kinetics, but also assists in forming a stable SEI layer on Si surface upon long-term cycling. Such a strategy sheds light on the design of novel

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polymer binders for practical applications of high-capacity active materials with great

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volume change.

Keywords

Graft polymerization, Polyvinyl alcohol-g-poly (acrylic acid), Water-soluble binder, Si anode, Lithium-ion battery

ACCEPTED MANUSCRIPT 1. Introduction Exploring promising lithium-ion batteries (LIBs) with high energy density and excellent cycle life is of critical significance to meet the ever-increasing energy storage demand in portable electronic devices, pure/hybrid electric vehicles

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(EVs/HEVs) and smart grids [1, 2]. Although graphite anode has been widely used in commercial LIBs, it suffers from low theoretical capacity (372 mAh g-1) and many safety problems, which impedes its further practical application in LIBs with the

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above-mentioned technological devices [3]. Silicon (Si), one of the most potential candidates for anode, has possessed many advantages such as high theoretical

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capacity (4200 mAh g-1), low charge-discharge potential (~0.4 V vs. Li/Li+), natural abundance and environmental benignity, which has aroused more and more attentions in the high energy-density LIBs as a promising anode with ultrahigh specific capacity [4]. However, the dramatic volume variations (expansion/contraction) of Si particles

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during the repeated charge-discharge process are always accompanied by large internal stress and freshly exposed Si surfaces, which leads to severe pulverization of Si particles, continuous growth of SEI layer and severe loss of electrical contact in the

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electrode network, thus giving rise to rapid electrode deterioration and rather low

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Coulombic efficiency upon cycling [5, 6]. Tremendous approaches have been explored to address the rapid failure of Si

anodes related to its large volume changes and to improve its cycling stability through well-designed nanostructured Si anodes, such as Si nanowire [7], nanotubes [8, 9], core-shell nanofibers [10, 11], porous nanomaterial [12, 13] and Si/C nanocomposites [14-16]. Although long cycle stability and high Columbic efficiency can be achieved by these methods, they are usually complicated, high-cost and non-environmentally benign, requiring a sophisticated synthesis process and remaining a huge challenge in

ACCEPTED MANUSCRIPT their scalability as well as compatibility with the existing battery manufacture process. Polymeric binder, an inactive but critical component in electrode, has played a critical role in anchoring Si particles/conducting agents onto current collector, maintaining mechanical/electrical integrity of the electrode and alleviating various capacity fading

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of the battery [17]. Carboxymethyl cellulose (CMC) [18], polyacrylic acid (PAA) [5], carboxymethyl chitosan (CCTS) [19] and sodium alginate (SA) [20] have been investigated as binders for Si anodes with enhanced electrochemical properties.

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Recently, many research groups have dedicated to exploring natural and abundant polysaccharide as functional binders for Si anodes to enhance the adhesion capability

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between the binder and Si particles, such as lignin [21], gum arabic [22], gum guar [23] and xanthium gum (XG) [24], each of which showed stronger binding capability than weak van der Waals forces polyvinylidene fluoride (PVDF, non-functional). These natural polymers showed strong interactions with Si particles through hydrogen

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bonding or covalent chemical bonds between their polar carboxylic or hydroxyl functional groups and the partially hydrolyzed SiO2 layer on Si surface, preventing Si particles from disintegrating. Nonetheless, the linear chain nature (one-dimensional,

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1D structure) of these binders is susceptible to sliding upon the continuous volume

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variation (repeated expansion and contraction) of Si particles upon cycling. Therefore, modified or three-dimensional (3D, interconnected network) polymer binders are still required to provide robust mechanical adhesion with Si particles for excellent cycle life and high Columbic efficiency. Choi et al. explored hyperbranched β cyclodextrin (β CDp) polymer as an effective binder for Si anodes with multidimensional hydrogen-bonding interactions with Si particles thus giving markedly improved cycling performance [25]. Besides, they developed a mussel-inspired binder, conjugating dopamine hydrochloride to PAA and SA backbones with high Young’s

ACCEPTED MANUSCRIPT moduli to achieve high-capacity Si anode [26]. Song et al. reported an in-situ cross-linked PAA with PVA as polymer binder in the period of the Si electrode preparation for Si anode [27]. Wei et al. introduced poly (acrylic acid sodium)-grafted-carboxymethyl cellulose (NaPAA-g-CMC) as a binder with enhanced

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binding ability for Si anode, forming stable SEI layer on Si surface upon cycling [28]. However, Wei et al. did not give out systematical investigations on the synthesized NaPAA-g-CMC of its 1H NMR structural details, molecular weight and the adhesion

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capability based on the different grafted lengths of PAA side-chain. We have been dedicating to developing natural and water-soluble polymer binders and their

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derivatives such as XG, CCTS, CN-CCTS and conducting CCTS/PEDOT: PSS not only for anodes (Si, SnS2) [19, 29], but also for cathodes (LFP) [30-33]. The linear chain nature (one-dimensional structure) of natural binders, susceptible to sliding upon cycling, prompted us to develop modified polymers such as grafting side-chains

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with abundant polar functional groups (-COOH, -CN) to increase the intrinsic adhesion strength of the binder itself, which can ensure a robust electrode architecture and achieve an excellent cycle life in LIBs. Here, we proposed a novel water-soluble

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polymer binder for Si anodes by a facile free radical graft polymerization, along with

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systematical characterizations on the binder’s structure, the adhesion capability and the electrochemical properties with different grafted lengths of the side-chain. Polyvinyl alcohol grafted poly (acrylic acid) (PVA-g-PAA) was synthesized as a

novel and robust polymer binder for Si anode in LIBs by graft polymerization of acrylic acid monomer onto PVA backbone via a free radical reaction. The as-prepared PVA-g-PAA was characterized with Fourier transform infrared spectroscopy (FT-IR), 1

H NMR spectra, gel permeation chromatography (GPC) and thermogravimetric

analysis (TGA) measurements. The strong interactions between PVA-g-PAA and Si

ACCEPTED MANUSCRIPT were characterized by adhesion capability test, folding experiment, X-ray photoelectron spectroscopy (XPS) and transmission electron microscope (TEM). The electrochemical performances and kinetic properties were studied by deep charge-discharge test with various current densities, cyclic voltammetry (CV) and

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electrochemical impedance spectroscopy (EIS) measurements.

2. Experimental

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2.1 Materials and Equipment

Si nanoparticles (30-80 nm) were provided by Xuzhou Jiechuang New Material

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Technology Co. (China). Carbon black (CB) was achieved from Guangzhou Lithium Force Energy Co. (China). The electrolyte of 1 M lithium hexafluorophosphate (LiPF6) with 10% fluoroethylene carbonate (FEC) dissolved in ethylene carbonate (EC) /diethylene carbonate (DEC) /dimethyl carbonate (DMC) (v/v/v=1/1/1) was obtained

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from Zhangjiagang Guotai-Huarong New Chemical Materials Co. (China) with water content less than 10 ppm. Carboxymethyl cellulose (CMC) (viscosity=800-1200 mPa s) was purchased from Sigma-Aldrich. Poly (vinyl alcohol) (PVA, 1797, degree of

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saponification, 96.0~98.0%, average degree of polymerization, ~1700), poly (acrylic

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acid) (PAA, Mw ~1,250,000), acrylic acid monomer (AA, >99%), sodium persulfate (Na2S2O8, 99.99%) and sodium bisulfite (NaHSO3, 99.99%) were acquired from Aladdin Chemistry Co. (China). AA monomer was reflux distilled to remove the polymerization inhibitors before use. Except for AA, all the reagents were used as received. The deionized water was used throughout the whole grafting reactions. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were recorded to characterize the thermal stability of the as-prepared PVA-g-PAA binder

on

a

STA409C/PC-PFEIFFER

VACUUMTGA-7

analyzer

ACCEPTED MANUSCRIPT (NETZSCH-Gertebau GmbH, Germary) in Ar atmosphere with a flow rate of 30 ml min-1 from 30 °C to 750 °C. Fourier Transform Infrared spectroscopy (FT-IR) measurements were conducted to identify the functional groups of synthesized PVA-g-PAA binders on a TENSOR 27 spectrometer (Bruker, Germany) from 4,000 to

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400 cm-1 at the resolution of 4 cm-1. Gel Permeation Chromatography (GPC) analysis of PVA-g-PAA was carried out on a Breeze HPLC system (Waters), equipped with a Refractive Index Detector at room temperature using potassium phosphate monobasic

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solution (0.02 mol/L) as a mobile phase. Average molecular weights of the polymers were calculated using a universal calibration from polystyrene standards and

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multiplied by 0.77 in order to obtain accurate values, along with polydispersity index (PDI) for analysis. The chemical structure of PVA-g-PAA polymer was characterized by Nuclear Magnetic Resonance Spectroscopy (NMR).

1

H NMR spectra was

performed on a Bruker AVANCE 600 spectrometer using D2O as a solvent (D2O: δ

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4.79). Rheological properties of different binders (PVA, PVA-g-PAA, PAA and CMC) with specified concentrations were analyzed by rotational coaxial cylindrical system (SNB-2, NiRun Co. China). The electrical conductivity of Si electrode with various

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binders was measured by the four-probe method (Guangzhou, RTS-9, China) at room

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temperature and 60% relative humidity. The interactions between PVA-g-PAA binder and Si particles were investigated by X-ray photoelectron spectroscopy (XPS) spectra using a ThermoFisher Scientific instrument (ESCALAB 250Xi). Visualization of material morphologies of Si electrode with various binders was characterized using scanning electron microscopy (SEM, Hitachi S-4800, Japan) operating at 2 kV and a field-emission transmission electron microscope (TEM; JEOL JEM-2100F) operating at 200 kV. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were obtained using Zennium-IM6 Zahner electrochemical workstation

ACCEPTED MANUSCRIPT (Germany). 2.2 Synthesis and characterizations of PVA-g-PAA The graft polymerization was conducted in a 250 ml three-necked flask, equipping with a dropping funnel, a reflux condenser and a magnetic stirrer under Ar

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atmosphere. In typical process: 1 g of PVA was firstly dissolved in 100 ml deionized water and continuously stirred in Ar atmosphere for 5 h to prepare a transparent oxygen-degassed solution. Then 0.1 g Na2S2O8/ 0.03 g NaHSO3 initiator was added to

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the PVA solution and stirred for 10 min to generate alkoxy radicals. Different amounts of AA monomer (2.5, 5, 10 and 15 g) were transferred to the funnel under Ar flux and

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then dropwisely introduced into the flask. After finish adding AA monomer, the mixture solution was heated and vigorously stirred for 2.5 h at 60 °C with a thermostatic oil bath for complete graft polymerization. The solubility of the as-prepared PVA-g-PAA, PVA and PAA in ethanol and acetone was investigated and

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shown in Fig.S1. The obtained solution was added dropwisely into ethanol to remove the PVA reactant with vigorous stirring. The formed homopolymer of poly (acrylic acid) (PAA) was washed out by extracting with acetone, and then extensively

H NMR in D2O solution. The residual grafted polymer PVA-g-PAA was obtained as a

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1

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extracted in a Soxhlet apparatus with acetone for 5 h until no PAA can be detected by

light white solid after dried under vacuum condition. The grafting percentage (G%) of AA monomer onto PVA backbone was

calculated based on the following equation (1): G%=

Wg -W0 W0

×100%

(1)

where Wg and W0 were the weight of grafted polymer PVA-g-PAA and PVA, respectively.

ACCEPTED MANUSCRIPT PVA-g-PAA: 1H NMR (400 MHz, D2O); δ=3.97 (1H, -CH-O-), 3.58 (2H, -O-CH2-),

2.17

(1H,

-CH-COOH),

1.55-1.64

(2H,

-CH2-),

1.11

(1H,

-O-CH2-CH-COOH). FT-IR: 3450 cm-1 (O-H stretching), 1700 cm-1 (COO- asymmetric stretching),

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1452 cm-1 (COO- symmetric stretching) and 1250 cm-1 (C-O stretching).

For comparison, 1H NMR and FT-IR for PVA are also listed as following.

PVA: 1H NMR (400 MHz, D2O); δ=3.98 (1H, -CH-OH), 1.58-1.65 (2H, -CH2-).

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FT-IR: 3450 cm-1 (O-H stretching), 1085-1250 cm-1 (C-O stretching).

The compatibility and swelling property of PVA-g-PAA binder with the

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electrolyte were investigated by swelling test and calculated from the weight increment of the binder films. Dry binder films were initially weighed (Winitial) and then immersed into the electrolyte of EC/DEC/DMC (v/v/v=1/1/1) for 48 h. Weight measurements were carried out by blotting the binder films dry and immediately

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weighing them again (Wafter). The swelling ratio (S%) was obtained from the weight increment of the tested binder films absorbed with electrolyte and calculated as Wafter -Winitial × 100% Winitial

(2)

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S%=

2.3 Preparation of electrode and its mechanical property

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PVA-g-PAA and PAA binder were dissolved in deionized water (DI-water) with

5 wt.% concentration while CMC binder was prepared with DI-water in 2 wt.% concentration due to its high viscosity before slurry preparation. Si nanoparticles, conductive carbon black and polymeric binder (PVA, PVA-g-PAA, PAA and CMC, respectively) in a weight ratio of 7:2:1 were mixed in DI-water to obtain a homogeneous slurry. Then the prepared slurry was cast onto a copper foil (20 µm thick) using an automated doctor blade technique and dried in a vacuum oven at 100 °C for 24 h to remove the water thoroughly. Si electrode sheets were punched out

ACCEPTED MANUSCRIPT before the assembly of coin cells, which was conducted in an Ar-filled glove box (<0.1 ppm water). Adhesion strength of Si-PVA-g-PAA electrode with different grafting ratios of AA onto PVA backbone (2.5, 5, 10 and 15 g) were quantitatively measured during the

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peeling-off process of 3M tape (adhered onto the laminate side) using a 180° high-precision micromechanical peel tester from Shenzhen Kaiqiangli testing instruments Co. (China) with the constant displacement rate of 20.0 mm min-1. The

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applied load used to peel off the electrode laminate was constantly recorded with the displacement to investigate the interactions among the PVA-g-PAA chains and the

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hydrogen bonding forces between the hydroxylated Si surface and the free PVA-g-PAA’s carboxylic moieties. The folding experiment was carried out by repeatedly folding Si electrode sheets back and forth for five times to evaluate the flexibility of Si electrode with different binders. All the electrode sheets were

measurement.

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pre-dried in vacuum oven at 70 °C for 12 h before the peeling and folding

2.4 Characterization of electrochemical performances

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The electrochemical performances were examined by deep galvanostatic cycling

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of CR2025 coin half cells, which was assembled with pure lithium metal, 1 M LiPF6 with 10% fluoroethylene carbonate (FEC) in EC/DEC/DMC (v/v/v=1/1/1) and Celgard 2400 as counter electrode, electrolyte and separator, respectively. Circular Si electrode sheets were punched out with a loading amount of 0.5-0.6

mg cm-2 and an area of 1.54 cm2. To evaluate the cycle stability and rate capability, the cells were charged and discharged at 25 °C in a constant potential range from 0.01 V to 1.5 V (vs. Li/Li+) on Shenzhen Neware battery cycler (China). The different charge-discharge rates were calculated by assuming the theoretical capacity of 4200

ACCEPTED MANUSCRIPT mAh g-1 for Si active material. The CE was obtained by the equation (Cdealloy/Calloy) × 100%, where Calloy and Cdealloy correspond to the capacity of lithium insertion and extraction for Si anodes, respectively. CV and EIS measurements were carried out at the electrochemical workstation

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(Zahner, Germany). CV was conducted with 0.2 mV s-1 scan rate for the first 5 cycles of Si electrode at the voltage range between 0.01 V and 1.5 V (vs. Li/Li+) while EIS was obtained using an alternating voltage of 5 mV at the frequency region from 100

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3. Results and discussion

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kHz to 0.01 Hz.

3.1 Synthetic mechanism of PVA-g-PAA

The PVA-g-PAA binder was prepared through free radical graft polymerization in Ar atmosphere with Na2S2O8/NaHSO3 initiator. The specified mechanism for

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grafting AA monomer onto PVA backbone is displayed in Scheme.1. The persulfate group (S2O82-) decomposed upon heating to produce sulfate anion radical (SO4•-), abstracting hydrogen from -OH group of PVA backbone to form alkoxy radicals.

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These new-appeared active centers on PVA backbone initiated AA monomer to

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polymerize onto them, leading to a polymer network with grafted structure. 3.2 Characterizations of PVA-g-PAA 3.2.1 FT-IR and 1H NMR spectra of PVA-g-PAA The FT-IR spectra were measured to identify the grafted polymer PVA-g-PAA

with different grafting lengths of PAA and their interactions with Si active material. As shown in Fig.1a, apart from the absorption bands at 3450 cm-1 due to O-H vibration, PVA-g-PAA displayed three strong new peaks at 1700 cm-1, 1452 cm-1 and 1250 cm-1, which were attributed to the asymmetric, symmetric stretching of

ACCEPTED MANUSCRIPT carboxylate group (COO-) and C-O stretching vibration, respectively. These three new appeared peaks and the varied intensity of O-H vibration indicated the successful grafting polymerization of PAA onto PVA backbone. Moreover, the interactions between PVA-g-10PAA and Si particles was also characterized by FT-IR spectrum in

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Fig.1b. After PVA-g-PAA binder interacted with Si particles, the relative intensity of characteristic bands at 3450, 1700, 1400 and 1250 cm-1 decreased remarkably and the peak width of the above bonds significantly narrowed when compared to the pristine

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PVA-g-PAA binder. The significantly decreased peak intensity (especially the weakened O-H and C-O bond) and the narrowed peak width both indicated the strong

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interactions between PVA-g-PAA and Si particles, arising from the hydrogen bond force between the polar carboxylic group of PVA-g-PAA and the hydroxyl groups on Si surface. The hydroxyl groups on Si surface were manifested by TEM spectra and EDS mapping analysis in Fig.S2, which has been identified to play a crucial role in

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maintaining a superior cycling stability of Si anodes [27].

As shown in Fig.S3, 1H NMR spectrum was displayed to further identity the grafted polymer PVA-g-10PAA binder. Compared to the pristine PVA, 1H NMR

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spectrum of PVA-g-PAA binder exhibited three new peaks at δ 3.58, 2.17 and 1.11

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ppm, which corresponded to the methylene protons (2H, -O-CH2-), (1H, -CH-COOH) and the methenyl protons (1H, -O-CH2-CH-COOH) from the grafted PAA side-chain on PVA backbone, respectively. Calculating from the integral area of protons (1H, -CH-O-), (2H, -O-CH2-) and (2H, -CH2-CHCOOH-), the grafted ratio of -OH on PVA and the average degree of polymerization of PAA side-chain was 15.9% and 16, respectively. As the homopolymer of PAA has been already removed during the extraction process, the detected new protons in 1H NMR, combined with the three new stretching vibrations in FT-IR spectra (1700 cm-1, 1452 cm-1 and 780 cm-1),

ACCEPTED MANUSCRIPT further manifested the successful graft polymerization of PAA onto PVA backbone. 3.2.2 GPC and viscosity measurements of PVA-g-PAA PVA-g-PAA binders with different grafting lengths was achieved by graft polymerizing with different amounts of AA monomer (2.5, 5, 10 and 15 g). Their molecular

weight

were

measured

using

gel

permeation

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corresponding

chromatography (GPC), summarized with polymer dispersity index (PDI) and grafting percentages in Table.1. As exhibited in Table.1, increasing the amount of

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reactive AA monomer resulted in a higher molecular weight of PVA-g-PAA, along with a high grafting percentage. Compared with PVA, PVA-g-10PAA displayed a

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monomodal distribution with low PDI value after the graft polymerization, indicating relative mono-dispersity in molecular weight of the synthesized PVA-g-PAA. With an appropriate length of PAA side chain onto PVA backbone, PVA-g-PAA can not only enhance its binding capability with Si nanoparticles, forming a grafted polymeric

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network anchored on Si surface, but also improve its rheological property in viscosity to disperse the Si particles and conducting additives well during the slurry-preparation process. As shown in Table.1, the high viscosity of PVA-g-10PAA and PVA-g-15PAA

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binder can prevent the Si nanoparticles and conducting additives from sedimentation

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or aggregation during the slurry formation, which played a significant role in achieving homogeneous distribution of Si particles and conducting additives in their electrodes. Moreover, the enhanced molecular weight and the improved rheological property in viscosity further confirmed the successful graft polymerization of PAA onto PVA backbone. 3.2.3 TGA/DSC measurement of PVA-g-PAA The thermal stability and endothermic/exothermic behavior of PVA-g-10PAA binder were investigated by TGA/DSC measurement with the heating rate of 10 °C

ACCEPTED MANUSCRIPT min-1 under Ar atmosphere, along with PVA and PAA for comparison. As shown in Fig.2, the weight loss relevant to the absorbed water was about 2.2% and the PVA-g-10PAA polymer maintained its thermal stability before 200 °C without any additional endothermic/exothermic peaks, indicating its good thermal stability as a

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potential binder for Si anodes during the electrode preparation process (T<120 °C). Therefore, Si-PVA-g-PAA electrode can be dried in vacuum oven around 100 °C before assembling the cell.

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3.2.4 Swelling test of PVA-g-PAA

Si anode material has strongly depended on the Li+ transportation in the binder

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film for achieving high full specific capacity and excellent rate capability. Sufficient electrolyte uptake is critical to facilitate the Li+ transportation through the polymer binder film to the active material. As shown in Fig.S4, the swelling properties of different binders (PVA, PVA-g-PAA, PAA and CMC) was investigated by immersing

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them into the electrolyte of EC/DEC/DMC (v/v/v=1/1/1) for 48 h at room temperature. The electrolyte uptake for PVA-g-10PAA and PVA-g-15PAA was 25.5 wt.% and 28.5 wt.%, respectively, much higher than that of PVA (18.2 wt. %), PAA (14.8 wt. %) and

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CMC (15.9 wt. %), which can be ascribed to the interconnected graft network of

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PVA-g-PAA binder. In addition, the PVA-g-10PAA and PVA-g-15PAA binder films exhibited a faster electrolyte-wetting rate compared to that of PVA, PAA and CMC. The high amount electrolyte uptake and fast wetting rate of PVA-g-PAA binder are beneficial to the rapid electrolyte impregnation through the entire electrode network, thus improving the Li+ diffusion rate and Li+ transportation efficiency, which plays a crucial role in achieving an excellent rate performance as well as a high Columbic efficiency for Si anodes [34]. 3.3 Characterization of Si-PVA-g-PAA electrode

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Si-PVA-g-10PAA displayed significantly improved binding capability (1.14 N cm-1, Table.1) and enhanced flexibility (without any crack, Fig.3b) during the peeling and folding measurements, much higher than that of Si-PVA (0.91 N cm-1), Si-PAA (0.17

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N cm-1) and Si-CMC (0.93 N cm-1). Although Si-PVA-g-15PAA electrode exhibited higher adhesion strength (0.94 N cm-1) than that of Si-PVA after the grafting reaction,

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it suffered from unfavorable brittleness due to the overlong PAA side chain onto PVA backbone. The superior mechanical properties of PVA-g-10PAA binder for Si anode may attribute to its grafted polymeric network, introducing more active interaction sites between the binder and Si nanoparticles, anchoring itself firmly on Si surface

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across the current collector and maintaining a robust mechanical and electrical network with homogeneous Si particles and conducting additives dispersed over the electrode.

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3.3.2 High-resolution TEM measurement

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High-resolution TEM (HRTEM) images were obtained to investigate the electrode’s homogeneity and the mutual interactions between Si nanoparticles and PVA-g-PAA binder. As shown in Fig.4, grafted PVA-g-10PAA polymer could be firmly anchored on the surface of Si particles (red arrow) due to the strong hydrogen bonding between the polar carboxylic (-COOH) / hydroxyl (-OH) functional groups of the binder and the partially hydrolyzed SiO2 layer on Si surface. The anchored PVA-g-PAA binder can ensure a good coverage of Si particles, decreasing the contact area between the Si active particles and the electrolyte, thus resulting in a reduced

ACCEPTED MANUSCRIPT decomposition of electrolyte on Si surface. In addition, the homogeneously attached PVA-g-10PAA layer on Si surface could tightly bind the active material as well as the conducting additives together and adhere them to the current collector to preserve the electrode’s mechanical and electrical integrity during the prolonged cycling. This

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robust electrode architecture with grafted binder network can facilitate both electron and Li+ transportation after being permeated by the electrolyte, which will be discussed below.

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3.3.3 XPS measurement of PVA, PVA-g-PAA and Si-PVA-g-PAA

To identify the structure difference between PVA-g-10PAA and PVA as well as

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evaluate the mutual interactions between PVA-g-10PAA binder and Si nanoparticles, X-ray photoelectron spectroscopy (XPS) measurement was conducted. The Si-PVA-g-PAA composite was washed thoroughly with a large amount of DI-water and then Si particles were filtered, which process was repeated for three times until

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extensive purification (denoted as Si-PVA-g-PAA-w). Compared with PVA, the C1s and O1s spectra of PVA-g-PAA in Fig.5 showed new appeared characteristic peak of -O-C=O bond (288.3 eV) and C=O bond (531.2 eV), respectively, confirming the

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successful graft polymerization of PAA onto PVA backbone after the reaction. After

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extensive purification by washing, XPS spectrum of Si-PVA-g-PAA-w still showed strong C1s and O1s signals, illustrating that a substantial amount of PVA-g-PAA was still remained on the washed Si particles’ surface from the composite. Moreover, compared with PVA-g-PAA, the binding energy of C-C, C-O and -O-C=O (C1s) in Si-PVA-g-PAA-w increased from 284.5 to 284.6 eV, 285.8 to 286.0 eV and 288.3 to 288.5 eV, respectively, while the O1s signals of C-O and C=O shifted from 532.3 to 533.0 eV and 531.2 to 531.8 eV. These evidences indicated that grafted PVA-g-PAA polymer network has a strong interaction with the hydroxylated Si surface, most

ACCEPTED MANUSCRIPT likely by hydrogen bonding (one of the crucial factors for achieving excellent cycle life and high Columbic efficiency in Si anodes [19]), which was consistent with the FT-IR result mentioned above. 3.4 Electrochemical performances

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In terms of the binding capability, flexibility and the electrolyte uptake, PVA-g-10PAA and PVA-g-15PAA were picked out as potential binders for Si anode to evaluate their electrochemical performances. The first charge-discharge profiles at the

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current density of 400 mA g-1 in the voltage range of 0.01-1.5 V (vs. Li/Li+) of Si electrode with various binders (PVA, PVA-g-10PAA, PVA-g-15PAA, PAA and CMC)

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are presented in Fig.6a. All five samples exhibited similar characteristic plateau of Si with lithiation at 0.1 V and delithiation at 0.45 V. In the first discharge process, the sloping region (dark cycle) between 1.0 V and 0.4 V was ascribed to the formation of SEI layer on the Si/electrolyte interface while the low voltage plateau (around 0.1 V)

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corresponded to the lithiation of amorphous silicon in α-LixSi phase. As to the charge process, the sloping plateaus indicated the two-step delithiation (phase transition) of α-LixSi into amorphous α-Si, which can be further identified by the two anodic peaks

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at 0.32 V and 0.49 V during the first anodic potential scanning in the subsequent CV measurement (Fig.S5). In addition, Si-PVA-g-10PAA and Si-PVA-g-15PAA

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electrodes exhibited the first charge capacity of 3260.5 mAh g-1 and 3216.8 mAh g-1 with high initial Coulombic efficiency (ICE) of 82.4% and 81.4%, respectively, compared with Si-PVA (3142.3/79.6%), Si-PAA (2924.8/74.1%) and Si-CMC (3163.1/79.9%). The distinct ICE values represented the different SEI formation process and the varying reversibility during the first charge-discharge test. The superior charge capacity and high ICE of Si-PVA-g-PAA electrode indicated that grafted PVA-g-PAA network can promote the reversibility of Si electrode during the

ACCEPTED MANUSCRIPT first charge-discharge process and form a stable SEI layer on Si surface without severe pulverization or facture of Si particles, which will be further confirmed in subsequent galvanostatic cycling test. The cycling performance of Si electrode with various binders (PVA,

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PVA-g-10PAA, PVA-g-15PAA, PAA and CMC) was investigated by deep galvanostatic charge-discharge process from 0.01 V to 1.5 V (vs. Li/Li+) at a current density of 400 mA g-1 for 150 cycles. As displayed in Fig.6b, after 150 cycles test,

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Si-PVA-g-10PAA electrode remained a much higher charge capacity of 2001.3 mAh g-1, corresponding to a capacity retention of 61.4%, than that of Si-PVA

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(1210.3/38.5%), Si-PVA-g-15PAA (1576.8/49.0%), Si-PAA (933.6/31.9%) and Si-CMC (1362.2/43.1%). The Coulombic efficiency (CE) of Si-PVA-g-10PAA electrode can reach as high as 99.5% at 150th cycle and the average CE between 2nd cycle and 150th cycle was 98.7%. The increasing CE values with the cycle numbers at

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the initial charge-discharge process may ascribe to the unavoidable SEI fracture and continuous electrolyte decomposition on the freshly generated Si surface until a stable SEI layer was achieved. Moreover, after a long-term charge-discharge test for 1000

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cycles at 400 mA g-1 (Fig.6c), Si-PVA-g-10PAA can still retain a charge capacity of

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1315.8 mAh g-1 and a capacity retention of 40.3%, corresponding to a very small capacity fading of 0.06% per cycle. The superior charge capacity, the high capacity retention and the superb average CE value indicated the crucial role of PVA-g-10PAA in forming a stable SEI layer on Si surface with enhanced binding capability and appropriate flexibility based on its grafted polymeric network. The robust grafted PVA-g-10PAA can firmly anchor on Si surface, restrain any large movement of Si nanoparticles and maintain the whole mechanical and electrical network of Si electrode for long-term cycle life. By contrast, the relatively inferior cycle stability of

ACCEPTED MANUSCRIPT Si-PVA-g-15PAA electrode may attribute to the unfavorable brittleness due to the overlong PAA side chain onto PVA backbone thus causing a fast capacity fading and an inferior cycle life. As to the linear PVA, PAA and CMC binders, they are susceptible to slide during the large volume variation of Si nanoparticles, leading to

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continuous disconnection of the conducting electrical network, unfavorable isolation of Si particles and severe cracks over the entire laminate thus leading to a fast capacity deterioration.

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To identify the robustness of grafted PVA-g-PAA polymer, rate capability of Si electrode with various binders (PVA, PVA-g-10PAA, PVA-g-15PAA, PAA and

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CMC) was investigated at different charge-discharge current density from 0.2 A g-1 to 10 A g-1. As displayed in Fig.7, Si-PVA-g-10PAA electrode delivered a high reversible capacity of 1525.5 mAh g-1 at current density of 10 A g-1, retaining 47.9% of that obtained at 0.2 A g-1, much higher than that of PVA (5.7 mAh g-1, 0.2 %),

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PVA-g-15PAA (1092.6 mAh g-1, 34.4%), PAA (754.1 mAh g-1, 23.6%) and CMC (49.5 mAh g-1, 1.6%). The capacity can quickly increase to 2574.7 mAh g-1 when the current density was changed back to 0.2 A g-1. The significantly superior rate

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capability of Si-PVA-g-10PAA electrode can be attributed to the grafted PVA-g-PAA

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network with robust mechanical capability and good rheological property, maintaining the entire mechanical and electrical integrity of the laminate and facilitating the lithium-ion transportation upon high current density charge-discharge process. By contrast, the inferior rate performances of Si-PVA-g-15PAA electrode at high current densities of 6, 8 and 10 A g-1 may ascribe to the brittleness of its polymeric network, which was insufficient to accommodate the large mechanical stress and hold the electrical integrity of the laminate. As to the linear PVA, PAA and CMC binders, they were susceptible to slide at the high current density, leading to fast

ACCEPTED MANUSCRIPT capacity fading and severe loss of the electrical pathways. The rate capability of Si anode is mainly determined by the electrical conductivity, the interfacial characteristics and the lithium-ion diffusion rate of the entire electrode. The electrical conductivity of Si electrode sheet with different was

investigated

by four-probe

method.

As

shown in

Table.1,

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binders

Si-PVA-g-10PAA electrode has a higher electrical conductivity of 2.15*10-2 S cm-1 than that of Si-PVA (3.8*10-3 S cm-1), Si-PVA-g-15PAA (1.25*10-2 S cm-1), Si-PAA

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(1.00*10-2 S cm-1) and Si-CMC (1.11*10-2 S cm-1), indicating a more homogenous dispersion of Si nanoparticles and conducting additives during the slurry-preparation

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process when using grafted PVA-g-PAA polymer as novel binder for Si anodes. To further investigate the interfacial characteristics and lithium-ion diffusion rate of Si electrode with different binders (PVA, PVA-g-10PAA, PVA-g-15PAA, PAA and CMC), electrochemical impedance spectroscopy (EIS) was carried out in the

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discharged state of 0.01 V (vs. Li/Li+) after 150 charge-discharge cycles at room temperature. Fig.8a shows the Nyquist plots of Si electrode with different binders (symbol line: the tested impedances; solid lines: the fitted impedances) and the inset

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graph corresponds to the equivalent circuit for analysis. All the impedance spectra

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consist of two parts: one is an overlapped depressed semicircle (high to intermediate frequency range) relating to the SEI resistance (RSEI) and the charge transfer resistance (Rct) at the active material’s interface, and the other is a straight line (low frequency region) corresponding to Warburg impedance (Zw) caused by semi-infinite Li+ diffusion in electrode bulk [35, 36]. The Re, RSEI, Rct, CPE and Zw in the equivalent circuit are representative of the electrolyte resistance, the SEI resistance, the charge transfer resistance, the constant phase element (the double layer capacitance and passivation film capacitance) and the Warburg impedance,

ACCEPTED MANUSCRIPT respectively. Quantitatively, according to the fitted equivalent circuit (the inset of Fig.8a), the Rct value (54.66 Ω) of Si-PVA-g-10PAA electrode in Table.2 was much lower than that of Si-PVA (70.64 Ω), Si-PVA-g-15PAA (60.49 Ω), Si-PAA (418 Ω) and Si-CMC (209.1 Ω), which indicated that grafted PVA-g-10PAA network with

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improved adhesion capability and high electrolyte uptake was favorable to facilitate the Li+ transportation and enhance the charge transfer rate across the electrode-electrolyte interface, thus possessing a superior high charge-discharge rate

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performance in Si electrode. Moreover, the RSEI values of Si-PVA-g-10PAA and Si-PVA-g-15PAA electrodes were much lower than that of PVA, PAA and CMC

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systems, further confirming that grafted PVA-g-PAA network anchored on Si surface can effectively decrease the contact area between Si active particles and the electrolyte, thus resulting in a reduced decomposition of electrolyte on Si surface and a stable SEI layer for excellent long-term cycle life. Besides, the Bode diagram of

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Si-PVA-g-10PAA electrode in Fig.8b (inset graph) kept consistent with its Nyquist plots, consisting of two time-constants, Re, RSEI, Rct, CPESEI, CPEct and Zw. The lithium-ion diffusion coefficient (D) can be calculated from the plots at the

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low frequency range of the straight lines (Fig.8b), which corresponds to the

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lithium-ion diffusion into the bulk of the electrode materials (Warburg diffusion, Zw). The parameters of impedance spectra are simulated and fitted with an equivalent circuit by ZSimpWin software. The Warburg coefficient (σw) and lithium-ion diffusion efficient (D) can be calculated from the equation (3) and (4), respectively. Zre =R s +R ct +σ w ω-1/2

D=0.5(

RT 2 ) AFσ w C

(3)

(4)

where w, σw, D, R, T, F, A and C correspond to angular frequency (at low frequency

ACCEPTED MANUSCRIPT range), Warburg impedance coefficient, diffusion coefficient, gas constant, absolute temperature, Faraday’s constant, area of electrode surface and molar Li+ concentration (mole cm-3), respectively. The impedance parameters and diffusion coefficient are given in Table.2. Si-PVA-g-10PAA electrode possessed the highest lithium-ion

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diffusion coefficient of 4.70*10-10 cm2 s-1, much higher than that of PVA (7.80*10-11 cm2 s-1), PVA-g-15PAA (2.41*10-10 cm2 s-1), PAA (1.00*10-12 cm2 s-1) and CMC (8.40*10-11 cm2 s-1), which was favorable to the rapid Li+ diffusion and charge

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transportation, indicating that grafted PVA-g-10PAA network was robust enough to adhere Si particles onto the current collector, preserve the porous architecture and

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provide the rapid Li+ diffusion channels during high C-rate charge-discharge process. The small SEI/charge transfer resistance, the fast lithium-ion diffusion coefficient and the afore-mentioned high electrical conductivity of Si-PVA-g-10PAA electrode together contributed to its excellent cycle stability and rate capability, which was

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consistent with the superior electrochemical properties mentioned above. In term of the superior cycle stability and rate performances, Si-PVA-g-10PAA electrode was picked out for further comparative study with linear PVA, PAA and

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CMC. The CV profiles of Si electrode with different binders (PVA, PVA-g-10PAA,

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PAA and CMC) for the first 5 cycles at a scan rate of 0.2 mV s-1 are shown in Fig.S5. During the cathodic potential scanning, all the electrodes showed a broad peak and sharp peak at the voltage of 0.19 V and 0.02 V, respectively, which corresponded to the alloy process (lithiation) of Si into α-LixSi phase. For the anodic potential scanning, all the samples exhibited two broad peaks at 0.36 V and 0.40 V due to the two-step dealloying process (delithiation, phase transition) of α-LixSi into amorphous α-Si. The intensity of above-mentioned peaks in Si-PVA-g-10PAA electrode keep a slight variation during the first 5 cycles due to the moderate activation of Si

ACCEPTED MANUSCRIPT nanoparticles with the cycle numbers, which indicated that Si-PVA-g-10PAA electrode showed a good reversibility during the first 5 cycles and maintained a relatively stable SEI layer on Si surface, which was in good agreement with the electrochemical performances above.

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To investigate the mechanical stability of Si electrodes upon the long-term deep galvanostatic cycling and further understand their electrochemical performances, surface morphologies of Si electrode with different binders (PVA, PVA-g-10PAA,

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PAA and CMC) before/after 150 cycles at the current density of 400 mA g-1 were characterized by SEM. As shown in Fig.9 (a, c, e and g), Si electrode with various

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binders (PVA, PVA-g-10PAA, PAA and CMC) all displayed homogenous particles distributions in their pristine state before cycling. The differences between these electrodes (Fig.9b, d, f and h), however, became obvious after 150 charge-discharge deep galvanostatic cycling such that Si-PVA-g-10PAA electrode maintained

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relatively clear morphology of Si nanoparticles and kept the porous architecture without any severe pulverization/agglomerate of Si particles or micro-cracks over the entire laminate, whereas Si-PVA and Si-PAA electrodes showed severe pulverization

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of Si nanoparticles with bulk aggregations of them (Fig.9b and f). Moreover, Si-CMC

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electrode clearly lost the electrical contacts between Si nanoparticles and the conducting additives, showing micro-cracks on its electrode’s surface (Fig.9h). The well-preserved original Si morphology, the remained porous electrode architecture and the relatively uniform particles distribution after cycling indicated that PVA-g-10PAA binder was robust enough to accommodate the Si particles’ large volume variations, keep the original electrode morphology (forming a thin and stable SEI layer) and maintain the conducting electrical network as well as mechanical integrity of Si electrode upon the deep charge-discharge cycling.

ACCEPTED MANUSCRIPT Fig.10 displays the schematic illustration of Si-PVA-g-10PAA electrode during the repeated large volume variations of Si particles upon cycling. In a conventional Si electrode with a linear binder, such as PVA, the polymer binder holds the active material and conducting carbon black particles together and adheres them to the

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current collector before cycling to maintain the whole electrical integrity. However, with the prolonged cycles proceeding, the internal stress generated by large volume variations of Si particles during the repeated charge-discharge process causes

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pulverization and fracture of Si particles while the linear-chain binder slides upon the continuous volume expansion or contraction of Si particles, leading to severe loss of

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electrical contact, excessive solid-electrolyte-interphase (SEI) growth and continuous deterioration of cycle performance. By contrast, grafted PVA-g-10PAA binder with strong adhesion strength and appropriate flexibility can maintain the interconnected network among the Si particles and preserve the original electrode architecture upon

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cycling, resulting in a robust mechanical/electrical network and excellent cycle life of Si electrode upon cycling.

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4. Conclusions

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Grafted PVA-g-PAA polymer was synthesized through graft polymerization of AA monomer onto PVA backbone via a free radical reaction. PVA-g-PAA was used as water-soluble binder for Si anodes in LIBs. The enhanced adhesion strength, excellent flexibility and high electrolyte uptake after grafting reaction rendered PVA-g-PAA a robust binder for high-capacity Si anodes. Compared to linear PVA, PAA and CMC, optimal Si-PVA-g-10PAA electrode exhibited better cycle stability, higher Coulombic efficiency and more excellent rate capability, possessing a high electrical conductivity, low SEI/charge transfer resistance and fast lithium-ion diffusion coefficient.

ACCEPTED MANUSCRIPT PVA-g-PAA binder not only maintained the electrode’s mechanical and electrical integrity, but also assisted in forming a stable SEI layer on Si surface upon long-term charge-discharge cycling without excess growth of it. Such a strategy shed light on

materials with great volume change in LIBs.

Acknowledgments

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the design of novel polymer binders for practical applications of high-capacity active

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This work was supported by the K.C. Wong Education Foundation, National Natural Science Foundation of China (21573239), Guangdong Province for

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High-Level Talents (2014TX01N014), Science & Technology project of Guangdong province (2015B010135008/2016B010114003), Guangzhou Municipal Project for Science & Technology (201509010018), and the Natural Science Foundation of

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Guangdong Province (2015A030313721).

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ACCEPTED MANUSCRIPT Table Caption Table.1. The intrinsic properties of synthesized graft polymer binders in viscosity and molecular weight, and the characterizations of their corresponding electrode sheets in

Va Graft polymer (mPa s)

GPC Mpb

Gd

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electrical conductivity and adhesion strength.

Ee

ASf

PDIc

(%)

(*10-2 S cm-1)

(N cm-1)

0.38

0.91

(kDa) 18.2

75

3.7

—

PVA-g-2.5PAA

37.6

332

5.7

242.5

0.91

0.74

PVA-g-5PAA

47.0

1100

4.7

433.6

1.10

0.80

PVA-g-10PAA

2460.9

1660

PVA-g-15PAA

6021.7

2138

PAA

4380.7

1250

CMC

4500

700

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820.9

2.15

1.14

8.3

1313.6

1.25

0.94

—

—

1.00

0.17

—

—

1.11

0.93

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a.

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PVA

Viscosity; b. Peak average molecular weight; c. Polymer dispersity index; d. Grafting

percentage;

e.

Electrical conductivity of electrode sheet;

f.

Adhesion strength of

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electrode sheet. * The viscosity of PVA-g-PAA binders was measured at 5% concentration while PAA and CMC was prepared with 1% and 1.5% concentration

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due to their high viscosity.

ACCEPTED MANUSCRIPT

Table.2. The impedance parameters of Si-PVA, Si-PVA-g-10PAA, Si-PVA-g-15PAA, Si-PAA and Si-CMC electrodes after 150 cycles. Rct

σw

D

(Ω)

(Ω)

(Ω cm2 s1/2)

PVA

297.3

70.64

21.3

PVA-g-10PAA

2.37

54.66

8.685

PVA-g-15PAA

7.63

60.49

12.13

2.41

PAA

1126

418

182.1

0.01

CMC

64.62

209.1

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RSEI

(*10-10 cm2 s-1)

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Si electrode

20.57

0.78

4.70

0.84

RSEI: the SEI resistance; Rct: the charge transfer resistance; σw: Warburg impedance

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coefficient; D: lithium-ion diffusion coefficient.

ACCEPTED MANUSCRIPT Figure captions Fig.1. FT-IR spectra of PVA-g-PAA polymers with different grafting lengths of PAA (a) and the interactions between PVA-g-10PAA binder and the hydroxylated Si surface (b).

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Fig.2. TGA (a) and DSC (b) curves of PVA-g-10PAA binder at Ar atmosphere, along with PVA and PAA for comparison.

Fig.3. Peeling profiles (a) of Si electrode with different binders (PVA, PVA-g-2.5PAA,

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PVA-g-5PAA, PVA-g-10PAA and PVA-g-15PAA, PAA and CMC) and their corresponding photographs (b) after repeated folding test.

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Fig.4. High-resolution TEM images with different magnification levels (a and b) of a fresh Si electrode with PVA-g-10PAA binder and conducting additives. Fig.5. XPS measurement: (a) C1s and (b) O1s spectra of pristine Si nanoparticles, as-prepared PVA-g-10PAA and Si particles extracted from the Si-PVA-g-10PAA

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electrode after extensive purification.

Fig.6. First galvanostatic charge/discharge profiles (a) in the voltage range of 0.01-1.5 V (vs. Li/Li+) and cycle performances (b, c) of Si electrode with different binders

EP

(PVA, PVA-g-10PAA, PVA-g-15PAA, PAA and CMC) for 150/1000 cycles at a

AC C

current density of 400 mA g-1.

Fig.7. Rate performances (a) of Si electrode with PVA, PVA-g-10PAA, PVA-g-15PAA, PAA and CMC binder and charge/discharge voltage profiles (b) of Si-PVA-g-10PAA electrode at various current density of 0.2, 0.4, 0.6, 0.8, 1, 2, 4, 6, 8, 10 A g-1. Fig.8. (a) Nyquist plots of Si electrode with different binders (PVA, PVA-g-10PAA, PVA-g-15PAA, PAA and CMC) in the discharged state of 0.01 V (vs. Li/Li+) after 150 cycles at room temperature. The inset graph corresponds to the equivalent circuit used

ACCEPTED MANUSCRIPT to model the impedance spectra; (b) the plot between Zre and w-1/2, the inset graph corresponds to the Bode diagram of Si-PVA-g-PAA electrode. Fig.9. SEM images of Si electrode with different binders (a, c, e, g) before and (b, d, f,

and (g, h) CMC, respectively.

RI PT

h) after 150 cycles charge/discharge test: (a, b) PVA, (c, d) PVA-g-10PAA, (e, f) PAA

Fig.10. Schematic illustration of grafted PVA-g-PAA binder in forming strong interactions between its carboxylic/hydroxyl groups and the hydroxylated Si surface

AC C

EP

TE D

M AN U

SC

upon cycling, along with linear PVA for comparison.

ACCEPTED MANUSCRIPT

PVA

-1

PVA-g-2.5PAA 1700cm-1

-1

3450cm

-1

780cm

PVA-g-10PAA

3500

3000

2500

2000

1500

1000

4000

500

3500

3000

2500

2000

780cm-1

1400cm-1

PVA-g-10PAA

1250cm

Wavenumbers (cm-1)

1700cm-1

PVA-g-10PAA-Si

-1

PVA-g-15PAA 4000

Si 3450cm-1

SC

PVA-g-5PAA

(b)

RI PT

1452cm

Transmittance (a.u.)

Transmittance (a.u.)

(a)

1250cm-1

1500

Wavenumbers (cm-1)

1000

500

M AN U

Fig.1. FT-IR spectra of PVA-g-PAA polymers with different grafting lengths of PAA (a) and the interactions between PVA-g-10PAA binder and the hydroxylated Si surface (b).

TE D

2

(a)

EP

60 40

0

0

100

200

300

-2

-4

PVA PVA-g-PAA PAA

20

(b)

0

DSC / mW/mg

80

AC C

Mass Retained / wt.%

100

PVA PVA-g-PAA PAA

-6

-8 400

500

Temperature/ oC

600

700

0

100

200

300

400

500

Temperature/ oC

600

700

Fig.2. TGA (a) and DSC (b) curves of PVA-g-10PAA binder at Ar atmosphere, along with PVA and PAA for comparison.

ACCEPTED MANUSCRIPT

2.0

PVA PVA-g-2.5PAA PVA-g-5PAA PVA-g-10PAA PVA-g-15PAA PAA CMC

(a)

1.0

RI PT

Load / N/cm

1.5

0.5

0.0 0

5

10

15

20

25

30

SC

Distance/mm

Fig.3. Peeling profiles (a) of Si electrode with different binders (PVA, PVA-g-2.5PAA,

M AN U

PVA-g-5PAA, PVA-g-10PAA and PVA-g-15PAA, PAA and CMC) and their

AC C

EP

TE D

corresponding photographs (b) after repeated folding test.

Fig.4. High-resolution TEM images with different magnification levels (a and b) of a fresh Si electrode with PVA-g-10PAA binder and conducting additives.

ACCEPTED MANUSCRIPT

Si-PVA-g-PAA-w 284.5

PVA-g-PAA

288.3 -O-C=O 288.5

PVA

284

286

288

290

532.3

292

PVA-g-PAA

SiO2

532.0

M AN U

282

Si-PVA-g-PAA-w

533.0 C-O

532.2

O1s

nano Si 280

C=O

531.8

SC

285.8 C-O 286.0

C1s

531.2

(b) Intensity/ a.u.

Intensity/ a.u.

(a)

284.6

284.3

RI PT

C-C

294

526

528

530

532

534

536

PVA

nano Si

538

540

542

Binding Energy/eV

Binding Energy/eV

Fig.5. XPS measurement: (a) C1s and (b) O1s spectra of pristine Si nanoparticles, as-prepared PVA-g-10PAA and Si particles extracted from the Si-PVA-g-10PAA

0.8 0.6

1.5

AC C

Voltage / V vs. Li/Li+

1.0

3600

PVA PVA-g-10PAA PVA-g-15PAA PAA CMC

(a)

0.4 0.2

1.0

0.0

0

80

160

240

320

400

0.5

0.0

Charge Capacity/mAh g -1

1.2

2.0

100

3200 2800

(b)

2400 2000

90 80

1600 1200

70

PVA PVA-g-10PAA PVA-g-15PAA PAA CMC

800 400 0

60 50

0

1000

2000

3000

Specific capacity / mAh g

-1

4000

0

20

40

60

80

100

Cycle number

120

140

Coulombic efficiency/%

EP

TE D

electrode after extensive purification.

ACCEPTED MANUSCRIPT

100

2800

(c)

90

400 mA g-1

80

Si-PVA-g-10PAA

2400

RI PT

2000

Coulombic efficiency/%

1600

70

1200 800

60

400

50

0

SC

Charge Capacity/mAh g-1

3200

-400

0

100

200

300

M AN U

-800 400

500

600

700

800

40

30 900 1000

Cycle number

Fig.6. First galvanostatic charge/discharge profiles (a) in the voltage range of 0.01-1.5 V (vs. Li/Li+) and cycle performances (b, c) of Si electrode with different binders

TE D

(PVA, PVA-g-10PAA, PVA-g-15PAA, PAA and CMC) for 150/1000 cycles at a

AC C

EP

current density of 400 mA g-1.

ACCEPTED MANUSCRIPT

0.2 A/g

(a)

0.2 A/g

1 A/g 2 A/g 4 A/g 6 A/g

60

8 A/g 10 A/g

1500

40

PVA PVA-g-10PAA PVA-g-15PAA PAA CMC

500 0

80

20

(b) 1.0

0.5

RI PT

2000

1.5

Voltage / V vs. Li/Li+

2500

1000

0.2, 0.4, 0.6, 0.8, 1, 2, 4, 6, 8, 10 A/g

100

0.4 A/g 0.6 A/g 0.8 A/g

3000

Coulombic Efficiency/%

Charge capacity /(mAh g -1 )

3500

Si-PVA-g-10PAA

0.0

0

-500 0

10

20

30

40

50

60

0

1000

2000

3000

4000

Specific capacity / mAh g-1

SC

Cycle number

Fig.7. Rate performances (a) of Si electrode with PVA, PVA-g-10PAA,

M AN U

PVA-g-15PAA, PAA and CMC binder and charge/discharge voltage profiles (b) of Si-PVA-g-10PAA electrode at various current density of 0.2, 0.4, 0.6, 0.8, 1, 2, 4, 6, 8,

450

2800

Z im / Ω cm 2

300 250

200

200

160 120

100

80

EP

150

(b)

2400

2.2

Zw

2.0

1600

PVA PVA-g-10PAA PVA-g-15PAA PAA CMC

1200 800

-35

Si-PVA-g-10PAA

2000 log |Z |

350

(a)

TE D

400

Z re / Ω cm 2

PVA PVA-g-10PAA PVA-g-15PAA PAA 240 CMC

R

log|Z| Theta

ct CPE

R

1.8

-20

SEI

Bode plot

1.6

-30 -25

ct

-15 CPE

SEI

1.4

R

-10

T heta / deg ree

10 A g-1.

-5 e

1-st time constant 0 2-nd time constant capacitance of charge transfer capacitance of SEI

1.2

5 -2

-1

0

1

2

3

4

5

log (f)

40

50

400

0

0

0

250

500

750

AC C

0

75

150

1000

225

Zre / Ω cm2

300

1250

375

450

1500

525

600

1750

0 1.0

1.5

2.0

2.5

3.0

3.5

4.0

w-1/2/s-1/2

Fig.8. (a) Nyquist plots of Si electrode with different binders (PVA, PVA-g-10PAA, PVA-g-15PAA, PAA and CMC) in the discharged state of 0.01 V (vs. Li/Li+) after 150 cycles at room temperature. The inset graph corresponds to the equivalent circuit used to model the impedance spectra; (b) the plot between Zre and w-1/2, the inset graph corresponds to the Bode diagram of Si-PVA-g-PAA electrode.

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig.9. SEM images of Si electrode with different binders (a, c, e, g) before and (b, d, f, h) after 150 cycles charge/discharge test: (a, b) PVA, (c, d) PVA-g-10PAA, (e, f) PAA and (g, h) CMC, respectively.

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig.10. Schematic illustration of grafted PVA-g-PAA binder in forming strong

EP

interactions between its carboxylic/hydroxyl groups and the hydroxylated Si surface

AC C

upon cycling, along with linear PVA for comparison.

ACCEPTED MANUSCRIPT Scheme captions

.-

.2SO4 OH

CH2 CH

. O CH2 CH

n COOH

n

CH2

O

.

CH2 CH

CH

n

SC

SO4

Heating

COOH

O CH2 CH

M AN U

S2O82-

RI PT

Scheme.1. The synthetic mechanism of PVA-g-PAA binder.

CH2 CH

n

AC C

EP

TE D

Scheme.1. The synthetic mechanism of PVA-g-PAA binder.

m+1

ACCEPTED MANUSCRIPT Highlights:

1. Polyvinyl alcohol grafted poly (acrylic acid) (PVA-g-PAA) was synthesized through a free radical reaction.

electrolyte uptake.

RI PT

2. PVA-g-PAA exhibited enhanced adhesion strength, excellent flexibility and high

3. Si-PVA-g-PAA electrode possessed better cycle stability and higher Coulombic

SC

efficiency than PVA, PAA and CMC.

AC C

EP

TE D

M AN U

4. Si-PVA-g-PAA electrode had favorable electrochemical kinetics.