A Conductive Binder for High-Performance Sn Electrodes in Lithium

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A Conductive Binder for High-Performance Sn Electrodes in Lithium-ion Batteries Yan Zhao, Luyi Yang, Dong Liu, Jiangtao Hu, Lei Han, Zijian Wang, and Feng Pan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13692 • Publication Date (Web): 21 Dec 2017 Downloaded from http://pubs.acs.org on December 21, 2017

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ACS Applied Materials & Interfaces

A Conductive Binder for High-Performance Sn Electrodes in Lithium-ion Batteries Yan Zhaoa, Luyi Yanga*, Dong Liub, Jiangtao Hua, Lei Hana, Zijian Wanga and Feng Pana* a

School of Advanced Materials, Peking University, Shenzhen Graduate School, Shenzhen

518055, People’s Republic of China b

BUCT-CWRU International Joint Laboratory, College of Energy, Beijing University of

Chemical Technology, Beijing 100029, People’s Republic of China

KEYWORDS

Tin anode, Li-ion batteries, conductive binder, solid electrolyte interface, tin pulverization.

ABSTRACT

Tin (Sn) has been widely studied as a promising anode material for high energy and power density Li-ion batteries owing to its high specific capacity. In this work, a water-soluble conductive polymer is studied as a binder for nano-sized Sn anodes. Unlike conventional binders, this conductive polymer formed a conductive network, which maintained the mechanical integrity during the repeated charge and discharge processes despite the inevitable Sn particle pulverization. The resultant Sn anode without conductive additives showed a specific capacity of 593 mA h g-1 after 600 cycles at the current density of 500 mA g-1, exhibiting better cycling stability as well as rate performance compared to the Sn anodes with conventional

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binders. Furthermore, it was also found that the conductive binder enhanced the formation of stable solid electrolyte interphase (SEI) layers.

1. INTRODUCTION

Improving the energy density of lithium ion batteries (LIBs) is crucial to the development of electronic vehicles and consumer electronics.1–4 The application of commercially available graphite anode is limited by its low theoretical capacity (372 mA h g-1) and poor rate performance.5 These drawbacks have limited the application of LIBs in electric vehicles. Therefore, alternative anode materials with higher theoretical capacities are highly desired. As a non-toxic and abundant element, tin anode has attracted much attention because of its appealing theoretical capacity. In theory, one tin atom can store 4.4 lithium atoms to form Li22Sn5, resulting in a capacity of 992 mA h g-1.6 However, similar to silicon anode, tin anode suffers from massive volume change due to the large amount of lithium insertion and extraction, which leads to pulverization of electrode and loss of active material.7,8

In order to improve the cycling stability of tin anode, some strategies have been implemented: (1) reduce the size of Sn particles to nanoscale to endure the high degree of volume change,6,9–11 (2) introduce Sn into a conductive matrix (e.g. carbon) to cope with volume change and maintain the integrity of the electrode.12–17 However, these mentioned methods emphasis on the design of Sn or Sn composite materials, the volume expansion is still inevitable. By using conductive binder instead of conventional binders such as polyacrylate acid (PAA),18 poly(vinyldene difluoride) (PVDF)19 and caboxymethyl cellulose (CMC)20, good electrical contact can be maintained despite the volume expansion.21 It has been previously reported that the conductive binder significantly improved the electrochemical performance of Sn anodes.22,23


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Sodium poly(9,9-bis(3-propanoate)fluorine) (PF-COONa) has been successfully applied in Si anodes as a conductive binder.24 In the previous work, PF-COONa exhibited good mechanical adhesion, high electrolyte uptake and certain electric conductivity, and at the same time the polar groups of the binder could form strong chemical bonds with the hydroxyls on nanosized silicon particles. By employing it in Si anode, superior electrochemical performance was obtained, suggesting that PF-COONa is a promising binder for various anode materials that could be affected by the volume expansion.

In this paper, the impact of the conductive binder on the electrochemical performance of Sn electrode is investigated. PF-COONa can firmly adhere to both Sn particles and Cu current collector to form an integrated structure. Above all, owing to the good conductivity of the binder, a carbon-free conductive network was formed. As a result, the electrode could still maintain good electric contact despite the significant volume changes after repeated cycling. The conductive binder can accommodate the huge volume change of tin particles as well as contribute to the formation of stable SEI films, hence greatly improved the electrochemical properties of Sn electrode. Using this conductive binder, the Sn electrode exhibited excellent long-term cycling capacity, stability as well as rate capability, which outperformed other conventional binders for pure Sn anode (see Table S1), indicating its great potential for highcapacity anode material with large volume changes.

2. RESULTS AND DISSCUSSION

Sn nanoparticles are used as-purchased without further treatments. Figure S1 displays the X-ray diffraction pattern of Sn nanoparticle with average size of 100 nm, where the major peaks can be well indexed to crystalline tin. The weak peaks of tetragonal SnO indicate the presence of a very


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small amount of oxidized impurities. PF-COONa is prepared using the same method as described in the previous work. The infrared spectrum of Sn/PF-COONa in Figure S2 shows that different from the Si/PF-COONa electrodes, where the binder chemically bonds with Si particles, there is no chemical bond formed between Sn and PF-COONa. Therefore, the Sn particles are physically cohered by PF-COONa.The Sn/PF-COONa electrode consist of 80 wt% of Sn particles and 20 wt% of PF-COONa. PF-COONa electrodes with different average Sn loading were prepared for different testing purposes.

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1st 100th 200th 600th

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Figure 1. The 1st, 2nd, 100th and 200th cycle voltage profiles of the cells at the current densities of (a) 100 mA g-1 and (c) 500 mA g-1; cycling capacities (red-discharge, black-charge) of Sn/PFCOONa electrode at current densities of (b) 100 mA g-1 and (d) 500 mA g-1. The loading of active material is approximately 0.6 mg cm-2.


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Figure 1 shows The cycling performances of Sn/PFCOO-Na electrode (with Sn areal loading of approximately 0.6 mg cm-2) at different cycling current densities. The voltage profiles of Sn/PFCOONa are shown in Figure 1a and Figure 1c, it can be seen that as the current density increased, very little overpotential due to DC polarization can be observed, which is an indication of good electronic contact in the electrode. For both cells, an irreversible discharge capacity of approximately 1500 mA h g-1 was obtained at the first cycle, this phenomenon will be discussed in the following content. Figure 1b and Figure 1d demonstrate the cycling capacities of Sn/PF-COONa electrodes at current densities of 100 mA g-1 and 500 mA g-1, respectively. After long cycling, stable capacities of 762 mA h g-1 (200 cycles, 100 mA g-1) and 593 mA h g-1 (600 cycles, 500 mA g-1) have been delivered. The stable capacities showed that the integrity of the electrodes was well maintained after long cycling, suggesting good adhesive property of PFCOONa for Sn nanoparticles. In addition, it can be observed that both cells exhibit excellent coulombic efficiency during long cycling, indicating good electrochemical stability of the binder.

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Figure 2. Cycling performances (a) and rate performances (b) of Sn/PF-COONa, Sn/AB/CMCNa and Sn/AB/PVDF electrode. SEM images of Sn/PF-COONa electrode (c), Sn/AB/CMC-Na electrode (d) and Sn/AB/PVDF electrode (e) after cycling. The loading of active material is approximately 1 mg cm-2.

In order to prove the advantage of using this conductive binder over traditional binder, Sn/acetylene black (AB)/CMC-Na and Sn/AB/PVDF (8:1:1 in weight) electrodes were prepared and compared with Sn/PF-COONa electrode. In this case, all electrodes were prepared with higher loading of active material (approximately 1 mg cm-1). From the long cycling performances shown in Figure 2a, it can be seen that the first cycle coulombic efficiency of Sn/PF-COONa is lower than that of Sn/AB/CMC-Na and Sn/AB/PVDF, which could be attributed to several factors: 1. the n-type doping of PF-COONa; 2. the formation of more stable SEI layer. After 50 cycles, the capacity of Sn/PF-COONa trended to become stable and a capacity of 518 mA h g-1 was achieved after 500 cycles; in contrast, the capacity of Sn/AB/CMC-Na and Sn/AB/PVDF showed rapid declining trends after initial cycles, especially for PVDF, the capacity dropped to less than 100 mA h g-1 after 10 cycles, which can be attributed to the swelling property of PVDF18,26. This wide difference indicates that PF-COONa has superior cycling stability over the traditional combination of AB/CMC-Na and AB/PVDF. Figure 2b compares the rate performances of Sn/PF-COONa, Sn/AB/CMC-Na and Sn/AB/PVDF electrodes. It can be seen that at 100 mA g-1, 200 mA g-1, 500 mA g-1 and 1000 mA g-1, similar capacity variation trends were exhibited by Sn/PF-COONa (1087, 1036, 883 and 767 mA h g-1) and Sn/AB/CMC-Na (892, 793, 679 and 592 mA h g-1), indicating similar rate capabilities of two electrodes. Similar to the results from Figure 2a, Sn/AB/PVDF anode resulted in the poorest capacities at all currents. This result shows that without conductive additives, PF-


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COONa not only acts as binder, but also exhibits excellent conductivity that could accommodate high-rate tests. Furthermore, the post-mortem SEM images (Figure 2c and Figure 2d) showed that after cycling the Sn/PF-COONa electrode demonstrated a smooth surface while a much more uneven surface is resulted from the Sn/AB/CMC-Na and Sn/AB/PVDF electrodes. For the first time, the conductive polymer has been found to promote the formation of stable SEI layer. This difference in surface topography could be attributed to that the electronic conductivity of PF-COONa facilitated the formation of a homogeneous and stable SEI layer on the Sn electrode.

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Figure 3. Cycling performances of cells using different binders (a). Impedance spectra of cells using PF-COONa (b), CMC-Na (c) and PVDF (d) at different cycle numbers. The loading of active material is approximately 2.5 mg cm-2.


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Furthermore,

the

cycling

performance

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nanotube

(CNT)/PF-COONa,

Sn/CNT/CMC-Na and Sn/CNT/PVDF electrodes with higher Sn loading (2.5 mg cm-2) was also tested. As shown in Figure 3, at the current density of 200 mA g-1, the capacity of Sn/CNT/PFCOONa was retained at around 555 mA h g-1 after 150 cycles. In contrast, capacities of 254 mA h g-1 and 87 mA h g-1 were obtained from Sn/CNT/CMC-Na and Sn/CNT/PVDF respectively. Therefore, the electrochemical performance of Sn electrode can be further improved by adding conductive CNT. In addition, alternative current (AC) impedance spectroscopy was used to measure the impedance of the cells at different cycle number. As shown in Figure 3, three cells showed similar charge transfer impedance initially. However, as cycle number increased, the cell using PF-COONa exhibited much lower impedance compared to the cells using CMC-Na and PVDF. This result not only proved the formation of stable SEI layer, but also indicated that PFCOONa served as a conductive network in the electrode.


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Figure 4. (a) Cyclic voltammogram of single-dispersed Sn/PF-COONa electrode at the scanning rate of 0.1 mV s-1; SEM images of single-dispersed Sn/PF-COONa electrode (b) before and (c) after cycling; SEM image of highly pulverized Sn nano particles after cycling (d).

To observe the morphology, change of Sn nanoparticles after cycling, single-dispersed electrodes were prepared where the Sn weight content is 12.5 %. Figure 4a shows the cyclic voltammogram (CV) of the single-dispersed electrode at the scan rate of 0.1 mV s-1. During the first cathodic sweep, two broad peaks at 1.5 V and 0.75 V vs Li/Li+ can be attributed to the ntype doping of the polyfluorene structure, which enhances the conductivity of PF-COONa;24 while the cathodic peak and the at 0.3 V vs Li/Li+ can be attributed to both Sn reduction and the formation of the solid-electrolyte interface film. This result corresponds to the irreversible


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discharge capacity during the first galvanostatic cycling. From the second cycle, three reproducible cathodic peaks can be found at 0.66 V, 0.62 V and 0.41 V vs Li/Li+, which can be explained by the formation of LixSn alloy.8 As for the anodic sweep, four oxidation peaks can be found at 0.45 V, 0.60 V, 0.72 V and 0.78 V vs Li/Li+, which can be assigned to the dealloying of LixSn.8 As a result, Sn particles underwent a full lithiation/delithiation process in this system. Owing to the low thickness of the electrode, the morphology of Sn particles can be observed using SEM. By comparing the SEM images of Sn/PF-COONa electrodes before and after cycling in Figure 4b and Figure 4c, it can be seen that after cycling, the surface of Sn nanoparticles has become much rougher, suggesting the start of pulverization. It is also observed for some particles, smaller subgrains are formed on the surface, indicating a higher degree of pulverization (see Figure 4d). Therefore, it can be inferred that after long-term galvanostatic cycling, highly pulverized nanoparticles will be inevitably resulted and the use of conductive binder is a practical solution.


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Scheme 1. Schematic illustration of the Sn anodes in the processes of lithiation/delithiation using (upper) AB and non-conductive polymer and (lower) conductive binder.

As shown in Scheme 1, when using the conventional binder/conductive material combination, the pulverized particles may will lose electronic contact with the conducting network, leading to capacity fading. By contrast, the conductive material will provide a conducting network, which facilitates electron transfer pathways for pulverized Sn particles, hence greatly reducing the capacity loss.

3. CONCLUSION

In summary, a water-soluble conductive polymer (PF-COONa) is investigated as a promising binder for nano-sized Sn anode. The Sn/PF-COONa electrodes demonstrated excellent reversible capacities of 762 mA g-1and 593 mA g-1at the current density of 100 mA h g-1 and 500 mA h g1

after long cycling, respectively. By comparing the electrochemical performances of Sn electrode


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using PF-COONa as binder and those using conventional binder, PF-COONa delivered significantly better cycling capacities and very similar rate capability. After examining the cycled electrodes, it can be seen that pulverization of Sn particles generally exists during the cycling. It is believed that the superior electrochemical performance is due to the conductive binder provides a conductive network, which will keep the pulverized Sn particles from losing electronic contact. Furthermore, the use of PF-COONa promotes the formation of stable SEI layer, which can be also attributed to its excellent performance. In this case, it is reasonable to speculate that this type of binder can generally boost the performance of anode materials with large volume changes. Therefore, this work not only demonstrates a polymeric binder for Sn anode with great commercialization potential, but also provide guidance on designing Sn-based electrodes.

4. EXPERIMETAL PROCEDURES

Preparation and characterization of materials:

Preparation of 2,7-Dibromo-9,9-bis(3-tert-butyl propanoate)fluorine (M1): Firstly, 5 g (76.1mmol) of 2,7-dibromofluorene and 300mg (0.94 mmol) of tetrabutylammonium bromide (TBAB) were mixed in 35 mL toluene solution. Then 8 mL of 50 wt% NaOH aqueous solution was injected dropwise into the above solution under N2 atmosphere. After half an hour, 8 g (62.5 mmol) of tert-butyl acrylate was slowly added. The solution was vigorously stirred at RT for 12h. After the reaction completed, the products were extracted by dichloromethane and washed with water three times. The organic solution was dried over aqueous Na2SO4 and further concentrated under reduced pressure. The crude product was purified by column chromatography using ethyl acetate: petroleum ether (1:50) in order to obtain a white solid (M1) with a 60% yield. 1H NMR


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(400 M Hz, CDCl3), δ(ppm): 7.68-7.50 (m, 6H); 2.30 (t, 4H); 1.46 (t, 4H); 1.33 (s, 18H).

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C

NMR (400 MHz, CDCl3), δ(ppm): 172.2, 150.1, 139.2, 131.2, 126.2, 122.2, 121.6, 80.5, 54.2, 34.5, 29.9, 28.

Preparation

of

2,7-bis(4,4,5,5-tetrameth-yl-1,3,2-dioxaborolan-2-yl)-9,9-bis

(3-tert-

butyl

propanoate)fluorine (M2): 6.912 g (12 mmol) of M1, 6.0 g (60 mmol) of anhydrous KOAc and 6.4 g (25.2 mmol) of bis(pinacolato)diboron were added into 80 mL anhydrous DMF, subsequently 300 mg of Pd(dppf)2Cl2 was added quickly under a nitrogen atmosphere. The reaction was conducted in the dark condition at 90 °C for 10 h. The completed mixture was poured into deionized water and extracted with dichloromethane. The obtained organic solution was washed with deionized water seven times and then dried with aqueous MgSO4. After concentration under reduced pressure, the product was purified via column chromatography (ethyl acetate: hexane = 1: 20) to obtain a white product (M2) with a 75% yield. 1H NMR (400 M Hz, CDCl3), δ(ppm): 7.84-7.72 (m, 6H); 2.39 (t, 4H); 1.44-1.39 (m, 28H); 1.31 (s, 18H). 13C NMR (400 MHz, CDCl3), δ(ppm): 172.8, 147.8, 143.8, 134.3, 129.0, 119.6, 83.8, 76.68, 53.5, 34.4, 29.92, 28.0, 25.0.

Synthesis of sodium Poly[9,9-bis(3-propanoate)fluorine)] (PF-COONa): A mixture containing 1.741 g (3mmol) of M1 and 2.023 g (3 mmol) of M2, 35 mg Pd(PPh3)4, and several drops of Aliquat 336 was added to a flask. Then 12 mL 2 M Na2CO3 solution and 36 mL THF was added and the flask was degassed by three freeze–pump–thaw cycles. The mixture was heated to 85 oC for 72 h under Ar and after cooling down to RT, the crude product was precipitated from methanol and dried under vacuum. Then the obtained material was dissolved in 200 mL dichloromethane containing 15% trifluoroacetic acid. The mixture was stirred at room


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temperature for 24h. And after the solvent removed under reduced pressure, 200 mL 0.5 M aqueous Na2CO3 solution was added, stirred for 6h and dialyzed against water for several times. The product was obtained via freeze-dry with a 75% yield. 1H NMR (400 M Hz, CDCl3) δ (ppm): 7.92-7.76 (br, 6H); 2.45 (br, 4H); 1.45 (br, 4H).

Fourier transform infrared (FTIR) spectroscopy was performed on a Nicolet Avatar 360 spectrophotometer. The surface morphology of the electrodes was characterized by a scanning electron microscope (SEM, ZEISS Supra 55). The crystal structure of Sn nanoparticles was analyzed by XRD using a Bruker D8-Advantage powder diffractometer (Cu-Kradiation) with 2θ from 20° to 90° at 0.2 s per step.

Electrochemical measurements:

All electrodes in this work were prepared using casting method. Firstly, Sn nanoparticles (Aladdin, 99.99% metals basis ≤100nm) was dispersed in the 2% PF-COONa solution and stirred vigorously for 24 hours. Then slurry is casted onto the Cu foil current collector and dried naturally at room temperature. After cut into pieces, the electrodes are dried at 110 °C under vacuum for removing remained H2O content. The electrodes with PVDF, CMC-Na binders and CNT additives were prepared by using same procedures.

All of coin cells were fabricated in an Ar-filled dry-box. Coin cells (2032) were used as to assemble half cells. Li foil (99.9%) was used as the negative electrode. 1.2 M LiPF6 in ethylene carbonate (EC): diethylene carbonate (DEC) (1:1 w/w) with additive of 10 wt% fluoroethylene carbonate (FEC) was used as electrolyte. Galvanostatic cycling was conducted in the voltage range between 0.01 V and 1 V at room temperature using a battery test system (Newell, China).


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The CV measurements were performed at the scan rate of 0.1 mV s-1 from 0.01 V to 2 V on an electrochemical workstation (CHI 604E, CH Instruments).

ASSOCIATED CONTENT

Supporting Information

Performance comparison of PF-COONa with other reported binders; XRD pattern of Sn; infrared spectra of Sn, Sn/PF-COONa and PF-COONa; cycling capacities of Sn/SWCNT/PF-COONa electrode.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]

*E-mail: [email protected]

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT


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The research was financially supported by National Materials Genome Project (2016YFB0700600), Guangdong Innovation Team Project (No. 2013N080) and Shenzhen Science and Technology Research Grant (peacock plan KYPT20141016105435850, No. JCYJ20151015162256516, JCYJ20150729111733470).

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Conventional binder 0 0

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A Conductive Binder for High-Performance Sn Electrodes in Lithium

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