A Modified Natural Polysaccharide as a High-Performance Binder for

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A modified natural polysaccharide as a high-performance binder for silicon anodes in lithium-ion batteries Shanming Hu, Zhixiang Cai, Tao Huang, Hongbin Zhang, and Aishui Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15695 • Publication Date (Web): 02 Jan 2019 Downloaded from http://pubs.acs.org on January 4, 2019

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ACS Applied Materials & Interfaces

A modified natural polysaccharide as a high-performance binder for silicon anodes in lithium-ion batteries Shanming Hu,‡ § Zhixiang Cai,‡ † Tao Huang,* § Hongbin Zhang* † and Aishui Yu*§ §

Laboratory of Advanced Materials, Department of Chemistry, Shanghai Key

Laboratory of Molecular Catalysis and Innovative Materials, Institute of New Energy, Collaborative Innovation Center of Chemistry for Energy Materials, Fudan University, Shanghai 200438, China. †

Department of Polymer Science and Engineering, School of Chemistry and

Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China.

ACS Paragon Plus Environment

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ABSTRACT: A modified natural polysaccharide (carboxymethylated gellan gum) is investigated as a water soluble high-performance binder for silicon anodes in lithium-ion batteries, in order to improve poor cycle life and fast capacity fade of silicon anodes due to dramatic volume expansion during lithiation/delithiation process. Amount of carboxyl groups and acetyl groups distributed homogeneously the modified polysaccharide polymer chain can form strong hydrogen bonds with the surface of Si particle and copper current collector, thus effectively restricting the volume change of Silicon and maintaining electronic integrity of Si electrodes during repeated charge/discharge cycles. As a result, Si anodes with carboxymethylated natural polysaccharide polymer present high capacity performance, excellent rate capability and stable cycling.

KEY

WORDS:

water

soluble

binder,

multiple-carboxyl,

carboxymethylated gellan gum, Silicon anodes, lithium ion batteries 1. INTRODUCTION Lithium ion batteries (LIBs) are widely applied in portable electronic products, electrical vehicles and large-scale energy storage devices, because LIBs possess high energy density, long cycle life and light weight advantages.

1-3

Silicon has been

considered as the most promising next anodes materials with a high theoretical capacity (4200 mA h g-1), compared with traditional commercial graphite (372 mA h g-1).

4

However, a huge volume change (300%) during repetitive charge-discharge

cycles leads to heavy pulverization of Si particle, formation of unstable solid-electrolyte-interphase (SEI) layers on the surface of silicon power and


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continuous consumption of electrolyte, which eventually result in fast capacity fade and short cycle life. These challenges have seriously restricted the commercialization of Si anodes in high energy LIBs.

5-7

Enormous efforts have been dedicated to

overcome the above mentioned challenges of Si anodes, including chemical synthesis of Si composites with all kinds of metals, 8 or carbon materials, 9 ultra-small size Si power and nanostructured Si particles, such as nanowires, 10 nanotubes, 11 nanoporous materials, spheres,

12

15

nanospheres,

13

and yolk-shell

several meso/macroporpus structures, 16

14

core-hollow

have exhibit high performance compared with

unprocessed micro-sized Si particles. However, although these efforts can effectively improve problem of pulverization induced by Silicon materials volume change with lithiation/delithiation processes, the methods with the direct modification of Si particles are not practical to entirely address the problem of mechanical instability between active materials and current collectors because active materials can shed from current collectors and easily lose inter-particle electronic connection due to repeated volume change during long time battery cycling. 17-20 Recently, to improve the problem of this mechanical damage to Si based anodes, many studies have been attempted to develop advanced polymeric binders for Si anodes.

21-24

Electrode binders are considered as play an important role in holding

active materials and conducting additives together, and agglutinating them on the surface of metal current collectors.

25

Traditionally, polymeric binders were only

treated as inactive materials in battery electrodes to conduct some above mentioned simple basic function, for instance, polyvinylidene fluoride (PVDF) as a commercial



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binder has been used in lithium batteries, but it is limited to suppress the volume change of Si materials and uses the toxic N-Methyl pyrrolidone (NMP) as the solvent during the process of electrodes fabrication.

26

To overcome the defect of PVDF

binder, a serials of high performance water-soluble binders have emerged, such as sodium carboxymethylated cellulose (CMC), poly(acrylic acid) (PAA).

29

27

alginate sodium (SA),

28

and

At the same time, several novel concepts of polymeric

binders have been introduced to further improve the performance of lithium battery, including self-healing binders,

30

mussel-inspired adhesion binders

conducting binders, 23

31

and natural polysaccharide binders have been

applied to Si anodes. Especially, natural binders such as guar gum, xanthan gum,

34

gum karaya,

35

32

3D network binders,

and natural agarose

36

22

gum arabic,

33

binders exhibit remarkable

performance and many advantages compared with traditional binders. Apart from the abovementioned biopolymers, Gellan gum (GG) has also been used to increase cycling stability of Silicon anodes at elevated temperatures.

37

GG is a linear anionic

microbial polysaccharide consisted of a tetrasaccharide repeating units of (1, 3)-β-D-glucose,

(1,

4)-β-D-glucuronic

acid,

(1,

4)-β-D-glucose,

and

(1,

4)-α-L-rhamnose, reported for wide applications in food, drug delivery and tissue engineering.

38

This biopolymer has also been used to construct bio-inspired GG–

graphene oxide composite films with good biocompatibility and high mechanical performance as an effective adhesive.39 However, the GG exhibits poor water solubility at room temperature,40 which greatly limits its practical use in LIBs. Many studies have been employed to improve the water solubility of gellan gum by


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chemical modifications, among which carboxymethylation has been used as an effective method.

41

The carboxymethylated gellan gum not only exhibits

the high

water solubility, but also shows 2.71-fold higher adhesive strength than gellan gum.42 In addition, carboxymethylated gellan gum has been widely utilized in a various field such as food industry, drug delivery and cosmetic industry because of its advantage of no-irritant, bioadhesive and biocompatible.43 Hence, in the current work we discover a new biopolymer (carboxymethylated gellan gum) with superb water solubility and high adhesive strength as a new green binder to improve the electrochemical stability of Si electrodes. The structure of this new binder is a kind of a rigid skeleton composed of six-membered pyranose contained abundant of carboxyl functional and acetyl groups.37 Numerous carboxylic groups in the structure of modified natural polysaccharide can form many strong hydrogen bond with SiO2 surface of Silicon, and amount of acetyl groups distributed homogeneously in the polymer chain facilitating a higher interfacial interaction between the binder and the active material particles, which can effectively restrain the volume expansion of Si materials and improve the cycling stability of LIBs. Furthermore, the polymer rigid backbone can endure the huge volume change of active materials during charge-discharge and abundant carboxyl groups on the structure of modified polymer not only can strongly enhance adhesion force, but promote the absorption of electrolyte.

44

Thus, the

outstanding advantages of new water-soluble biopolymer can tolerate dramatic volume changes of Si materials to achieve impressive stable cycling performance.

2. EXPERIMENTAL SECTION



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Materials and methods Silicon nano-particles (SiNPs, 99.9%) with an average particle size of 300 nm were purchased from Shanghai ST-nano Science & Technology Co., Ltd. CMC was purchased from Hercules Chemicals Co., Ltd (China). Alginate sodium (SA) was purchased from Sinopharm Chemical Reagent Co., Ltd (China). Super P carbon black was obtained from Hefei Ke Jing Materials Technology Co., Ltd. Gellan gum sample was

acquired

from

Zhejiang

DSM

Zhongken

Biotechnology

Co.,

Ltd.

Monochloroacetic acid was purchased from Aladdin Industrial Inc. (Shanghai, China) and was analytical grade. Carboxymethylated gellan gum synthesis and electrode fabrication 1.0 g of gellan gum was uniformly dispersed in 20 mL of the cold sodium hydroxide solution (20%, 4 ℃ ) and stirred for 30 min at room temperature, then the reaction system was warmed to the room temperature and 1.5 g of monochloroacetic acid was dissolved in a little of isopropanol, this solution was slowly added to the reaction system in three times, finally the whole reaction was carried out at room temperature for 5 h. The resulting crude reaction product was washed with ethanol and acetone for several times to remove the crude product and was filtered again to remove impurities in distilled water. The product was purified by exhaustive dialysis in the deionized water at room temperature for 4 days; the final product was freeze-dried for 4 days. 0.9 g of white and flocculent carboxymethylated gellan gum (CGG) was obtained. The coin cells (CR2016) were assembled to test electrochemical performance. SiNPs, super P and binder in the weight ratio of 60:20:20 were stirred and mixed in deionized


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to form homogeneous slurry, which were coated on the copper foils. Three different binders (CMC, SA, and CGG) were used. The loading mass of SiNPs was about 0.80-1.31 mg cm-2 on the copper foils, followed by drying for 12 h in a vacuum oven. The half cells were assembled in the argon-filled glove box, the lithium metals were used as the counter electrode. The electrolyte was LiFP6 mixed ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 v/v) and added 10 wt% fluoroethylene carbonate (FEC). Measurements. Fourier transform infrared (FTIR) spectroscopy was measured by a Shimadzu Corporation (IR Affinity-1, Japan), X-ray photoelectron spectroscopy (XPS) was recorded by on PHI5000C&PHI5300 (American), Nuclear magnetic resonance spectra (NMR) were recorded on 400WB AVANCE III (Bruker, Switzerland), The solvent was the (Methyl sulfoxide)-d6 (DMSO-d6). 180°peel test was recorded on a universal machine (CMT4101, MTS System Corporation). Morphologies of the electrodes were observed by field emission scanning electron microscopy (SEM, S-4800, Hitachi Ltd., Janpan). Galvanostatic charge and discharge cycling test was recorded on a LAND battery test system (Wuhan, China) in a voltage range of 0.01-1.20 V at room temperature. Electrochemical impedance spectroscopy (EIS) was recorded on VSP-300 (Biologic Science Instruments, China) by using a 5 mV of alternating voltage and the frequency ranging from 10 mHz to 100 KHz. The thermal stability of the CGG was measured by TG (209 F1 NETZSCH) at nitrogen flow (50 mL min-1). Cyclic voltammetry (CV) was measured by an electrochemical



workstation (CH Instruments, Inc., U.S.A.).

RESULTS AND DISCUSSION To verify the carboxymethylation of GG, the 1H-NMR spectra of both native GG and carboxymethylated GG (CGG) were determined as shown in Fig. 1. The 1H-NMR spectrum of GG in Figure 1a shows the presence of characteristic peaks that correspond to –CH of rhamnose (δ 5.67 ppm), –CH of glucuronic acid (δ 5.24 ppm), – CH glucose (δ 5.07 ppm) and –CH3 of rhamnose (δ 1.84 ppm), respectively. Remaining signals detected at 3.50~4.60 ppm belong to the skeletal signals of GG. In particular, the signals at 4.40 and 4.58 ppm are attributed to the protons at C-1 position. Figure 1b shows the typical

1H-NMR

spectrum of CGG. After

carboxymethylation, the above signals of the backbone chain were still detectable, indicating that the structure of the original polysaccharide is remained. In addition, compared to the 1H-NMR spectrum of GG, intense peak resonances at 4.30-4.60 ppm were observed in the NMR spectrum of CGG, which were attributed to the protons at C-1 positions and protons of –CH2 groups. Peaks assigned to protons at C-1 positions are clearly seen at 4.40 and 4.58 ppm, whereas the signals detected at 4.54, 4.52, 4.48, 4.45 ppm are assigned to the protons of carboxymethyl groups at C-2, C-3 (or C-4), C-6 positions. The 1H-NMR results indicated that the carboxymethylated GG was prepared successfully.


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Fig. 1. (a) 1H-NMR spectra of GG. (b) carboxymethylated GG (CGG)

Fourier transform infrared (FTIR) spectroscopy studies show the strong bonding between the CGG and SiNPs (Fig. 2a). The FT-IR spectrum of the CGG exhibits several bands peaks around 3143, 1622, 1407 and 1040 cm-1, a peak of 3143 cm-1 related to the stretching of O-H, a peak of 1622 cm-1 related COO- (asymmetric), a peak of 1407 cm-1 related to COO- (symmetric) and a peak of 1040 cm-1 corresponding to C-O-C (asymmetric), at the same time the spectrum of Si@CGG shows characteristic bands at 3134 cm-1 (O-H stretching), 1612 cm-1 and 1400 cm-1 (COO- stretching), 1037 cm-1 (C-O-C asymmetric). After CGG being mixed with Si, the bands of O-H, and COO- obviously shift to lower wavenumbers, demonstrating that an interaction occurs between the CGG binder and Silicon. The shift of bands had been related to the formation of hydrogen bonding and a condensation reaction between carboxyl groups of CGG and hydroxyl groups of SiO2 on the surface of SINPs. So the shift of bands assigned to O-H and COO- shows the chemical bonds between CGG and Si.22,

35

X-ray photoelectron spectroscopy (XPS) spectra studies



further confirm the strong chemical interaction between the CGG and SiNPs (Fig. 2b and 2c). The C1s XPS spectrum of the CGG shows three characteristic peaks related to C-C and C-H bonds (284.6 eV), C-O- bond (286.4 eV) and -O-C=O bond (288.0 eV) and the SiNPs doesn’t show any signs of C atom. A comparison of the pristine CGG and Si@CGG composites reveals the formation of strong chemical bonds between the SiNPs surface and CGG. The spectrum of Si2p shows two peaks of the pure SiNPs corresponding to Si-Si bond at 98.6 eV and Si-O-H at 100.9 eV. Compared the two binding energy of pure SiNPs, SiNPs mixing with CGG shows the two peaks corresponding to Si-Si at 99.9 eV and Si-O-O (R) at 104 eV, the change of binding energy of Si atom further indicates the formation of strong bonds between the CGG and the SiO2 surface of SiNPs. 28, 44

Fig. 2. (a) FT-IR spectra of the polymer CGG, Si nanoparticle and Si@CGG electrode. (b) and (c) XPS spectra of the polymer of CGG, Si nanoparcitle and Si@CGG electrode.

The electrochemical performance of SiNPs electrodes with different binders were shown in Fig. 3. The charge-discharge test of all cells was cycled at 100 mA g-1 for two cycles in a voltage range of 0.01-1.20 V. Fig. 3a shows the Si@CGG electrode (Si mass loading = 0.80 mg-1) maintaining the Li-extraction capacity of 1138 mA h g-1 at a 1000 mA g-1 current density after 200 cycles, compared with


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Si@CMC, Si@SA electrodes, indicating a higher cycling stability of the Si@CGG electrode. Rate capacities of Si electrodes with different binders were shows in Fig. 3b. The reversible delithiation capacities of Si@CGG electrode is 2491, 2123, 1849, 1390, 1078, 865, 2125 mA h g-1 at current densities of 100, 500, 1000, 2000, 3000, 4000, 100 mA g-1. Furthermore, Si@CGG electrode exhibits much higher capacities more than Si electrodes with CMC and SA binder at different current densities, especially, Si@CGG electrode retains specific capacity of 865 mA h g-1 at current densities of 4000 mA h g-1, compared with 474 mA h g-1 of Si@CMC electrode and 463 mA h g-1 of Si@SA electrode. The excellent cycle and rate performance of Si@CGG electrode compared to commonly used SA and CMC binders, should be attributed to CGG contains more polar functional groups including carboxyl groups and acetyl groups which are distributed homogeneously the structure of polymer chain leading to a more strong interaction with the surface of Si particle, robust adhesion and maintained mechanical and electrical integrity at high current densities. Fig. 3c shows the voltage profile of a SiNPs electrode with CGG binder after different cycles, the electrode was tested at current density of 100 mA g-1 two cycles and 1000 mA g-1 current density in subsequent cycles. The first discharge curve exhibits a long plateau, corresponding to crystalline Si reacts with electrolyte to form a solid electrolyte interphase (SEI) and an amorphous Lix Si.

10, 45

Respectively, the charge

plateau of the Si@CGG electrode are almost unchanged after 50 cycles, indicating it can keep electrochemical stability during the charge-discharge cycling. Furthermore, the loading mass of active material and specific capacity are equally crucial for



achieving high-performance LIBs in commercial application. However, most reported Si electrodes with low mass loading (< 1.0 mg Si/cm-2) are less meaningful for practical application in lithium batteries and specific capacity will decline with high loading mass due to higher polarization and the decrease of structural stability caused by poor adhesion of the thick electrode. 32, 46 Fig. 3d shows good cycle performance of a high mass loading of the Si@CGG electrode (1.31 mg Si/cm2), the charge capacity retains 900 mA h g-1 at current densities of 1000 mA g-1 after 100 cycles, which is only slightly lower than the low loading mass electrode shown in Fig. 3a. At the same time, the initial Coulombic efficiency (ICE) of high mass loading Si anode achieves 82.5% (Fig. 3e). The superior ICE of Si@CGG electrode suggests that amount of carboxyl groups of CGG form much hydrogen bonding with the surface of Si particle and CGG well-covered with Si decreases the decomposition of the electrolyte. Fig. 3f exhibits the first ten cyclic voltammetry (CV) curves of a Si@CGG electrode at a scan rate of 0.2 mV s-1. The first CV curve presents a cathodic peak around 1.0 V, which is attributed to the formation of SEI on the surface of the amorphous Si. Subsequently, beginning at second CV curves, the Si electrode is stable and the cathodic peak at 0.25 V suggests the formation of Li-Si alloy. The two anodic peaks at 0.36 and 0.52 V are ascribed to the procession of de-lithiation. 45, 47 The increase of peak current is caused by a gradual activation that more SiNPs particles can touch and react with the electrolyte. The plenty of hydroxyl groups and carboxyl groups of CGG, SA, and CMC binders can produce the heavy interaction such as hydrogen bonding or covalent bonding with the hydroxyl groups on the surface of Si. 28


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Fig. 3. Electrochemical performance of Si anodes. (a) Cycling performance of Si@CGG, Si@SA and Si@CMC electrodes between a cycling voltage of 0.01 and 1.2 V for 200 cycles at current density of 1000 mA g-1. (b) Rate performance of Si@CGG, Si@SA and Si@CMC electrode. (c) 1st, 2nd, 5th, 10th, 50th cycle curves of Si@CGG electrode. (d) Cycling performance of 1.31 mg of Si mass loading of Si@CGG electrode at current density of 1000 mA g-1. (e) Initial cycle curve of 1.31 mg of Si mass loading of Si@CGG electrode at current density of 100 mA g-1. (f) Cyclic voltammetry profile of Si@CGG electrode at scanning rate of 0.2 mV s-1 between 0.01 and 1.2 V (vs Li/Li+).

To compare the strong adhesion between CGG and copper foils, we conducted the 180 ° peel test to evaluate the mechanical property. Fig. 4 shows the force-displacement curves of Si electrodes with different binders, CMC and SA as the reference. Si@CGG electrode exhibits the highest average peeling force (2.5 N), compared with Si@SA (1.8 N), Si@CMC (1.5 N) electrodes. The result suggests that the CGG presents more robust binding ability, Although CGG, CMC, and SA all



contain many carboxyl groups for binding, the higher concentration and a more uniform distribution of the carboxylic groups along the chain in CGG, which exhibits stronger adhesion and guarantees the integrity of electrical contacts between Si particles and current collectors. As a result, Si@CGG electrodes compared to Si@CMC and Si@SA electrodes present a better electrochemical performance.

Fig. 4. Force-displacement curves of the Si@CGG, Si@SA and Si@CMC electrodes.

The thermal gravimetric analysis (TGA) result of CGG (Fig. S1.) presents a good thermal stability below 200 ℃. The weight loss in first stage, observed up to 100 ℃, was related to the loss of adsorbed and bound water. The second stage was ascribed to the decrosslinking of polymer networks and formation of a carbonaceous residue.43 Electrochemical impedance spectroscopy (EIS) was further used to investigate the electrochemical performance of Si electrode with different binders. Fig. 5 shows


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the Nyquist plots of different Si electrodes, compared the impedance properties of the Si@CMC, Si@SA, and Si@CGG electrodes after 2 cycles (Fig.5a) and 50 cycles (Fig. 5b). A straight sloping line of the low frequency regions is related to Warburg impedance. The middle frequency region reflects the charge transfer resistance (Rct). The high frequency semicircle reflects the resistance of the SEI film (RSEI).

46

It is

clearly seen that the Si@CGG electrode exhibits a smaller semicircle diameter at the high frequency regions and no semicircle diameter occurs in middle frequency regions after 2nd cycle, compared Si electrodes with other binders. It could be observed that the Rct and RSEI of Si@CGG electrode are the smallest among Si@CMC, and Si@SA electrodes in the high and middle frequency regions even after 50 cycles in the Nyquist plot. The excellent impedance performance of Si@CGG may be attributed to CGG monomer containing a higher degree of polar groups including amount of carboxyl groups and acetyl groups distributed homogeneously in the polymer chain, which are responsible for the better transport of Li ions in the vicinity of Si particles and the formation of a more efficient SEI film compared with common CMC and SA materials. This result suggests that the procession of charge transfer and polarization can be improved in electrode reactions and the CGG binder can well accommodate the volume change of Si power.



Fig. 5. Nyquist plots of Si@CGG, Si@SA, Si@CMC electrodes between 100 mHz and 100 kHz. (a) after 2 cycles. (b) after 50 cycles.

Since it has been found that the effects of the binder were reflected in the surface morphology, we used scanning electron microscopy (SEM) to image the surface morphology of Si@CGG and Si@SA electrodes at before and after 200 cycles. Although Si@CGG (Fig. 6a) and Si@SA electrodes (Fig. 6c) possess a smooth surface before cycling, Si@SA electrode (Fig. 6d) presents serious cracks compared with Si@CGG (Fig. 6c) with micro-crack and integrated surface morphology after 200 cycling. The emergence of these cracks is ascribed to the stress release of electrodes produced by the volume expansion of Si materials during charge-discharge cycling. The result of SEM images suggesting CGG binder can preserve mechanical integrity and accommodate the volume changes of Si electrodes more than SA binder.


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Fig. 6. SEM images of Si electrodes with CGG (a, b) and SA (c, d), before (left) and after 200 cycles (right).

CONCLUSIONS In summary, we discovered a modified natural polysaccharides polymer (carboxymethylated gellan gum) as a new water soluble binder for Si anodes of lithium-ion batteries. The Si@CGG anodes have exhibited superior rate capacity, reversible capacity, and prolonged cycle life. The excellent performance of Si@CGG electrodes is mainly ascribed to CGG containing a high degree of polar functional groups including amount of acetyl groups and carboxyl groups which are distributed homogeneously the structure of polymer chain leading to a more strong interaction with the surface of Si particle, robust adhesion and a more stable SEI film compared to commonly used SA and CMC binders. Special structure significantly increases the binding force and maintains the electrical and mechanical integrity between Si



particle and current collectors. As a result, Si@CGG electrode containing 0.8 mg of active material shows a specific capacity of 1138 mA h g-1 at 1000 mA g-1 current density after 200 cycles. Moreover, Si@CGG electrode containing 1.31 mg of active material exhibits a reversible capacity of 900 mA h g-1 at 1000 mA g-1 current density after 100 cycles and presents an excellent as nature raw materials, low cost, mature industrial process and eco-friendliness, make it has a great potential as a binder to be used for Silicon anodes in next generation lithium-ion batteries.

ASSOCIATED CONTENT Supporting Information Additional data from TG of CGG

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. * E-mail: [email protected]. * E-mail: [email protected]. ORCID Hongbin Zhang: 0000-0002-4419-4818 Aishui Yu: 0000-0002-8135-5123 Author Contributions ‡ Shanming Hu and Zhixiang Cai contributed equally to this work . Notes The authors declare no competing financial interest.


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ACKNOWLEDGEMENTS The authors acknowledge funding support from 973 program (2014CB932301), the National Natural Science Foundation (No. 21473040).

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