A Conductive Binder for High-Performance Sn Electrodes in Lithium

School of Advanced Materials, Peking University, Shenzhen Graduate School, Shenzhen. 518055, People's Republic of China b. BUCT-CWRU International ...
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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 1672−1677

A Conductive Binder for High-Performance Sn Electrodes in LithiumIon Batteries Yan Zhao,† Luyi Yang,*,† Dong Liu,‡ Jiangtao Hu,† Lei Han,† Zijian Wang,† and Feng Pan*,† †

School of Advanced Materials, Peking University, Shenzhen Graduate School, Shenzhen 518055, People’s Republic of China BUCT-CWRU International Joint Laboratory, College of Energy, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: Tin (Sn) has been widely studied as a promising anode material for high-energy and high-power-density Li-ion batteries owing to its high specific capacity. In this work, a watersoluble conductive polymer is studied as a binder for nanosized 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 Sn anodes with conventional binders. Furthermore, it was also found that the conductive binder enhanced the formation of stable solid electrolyte interphase (SEI) layers. KEYWORDS: tin anode, Li-ion batteries, conductive binder, solid electrolyte interface, tin pulverization

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 the 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 nontoxic and abundant element, the tin (Sn) anode has attracted much attention because of its appealing theoretical capacity. In theory, one Sn atom can store 4.4 lithium atoms to form Li22Sn5, resulting in a capacity of 992 mA h g−1.6 However, similar to the silicon anode, the Sn anode suffers from a massive volume change due to the large amount of lithium insertion and extraction, which leads to pulverization of the electrode and loss of active material.7,8 In order to improve the cycling stability of the Sn 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 place emphasis on the design of Sn or Sn composite materials, and 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 © 2017 American Chemical Society

previously reported that the conductive binder significantly improved the electrochemical performance of Sn anodes.22,23 Sodium poly(9,9-bis(3-propanoate)fluorine) (PF-COONa) has been successfully applied in Si anodes as a conductive binder.24 In 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 an 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 the Sn electrode is investigated. PF-COONa can firmly adhere to both Sn particles and the 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 Sn particles as well as contribute to the formation of stable SEI films, hence greatly improving the electrochemical properties of the Sn electrode. With use of this conductive binder, the Sn electrode exhibited Received: September 9, 2017 Accepted: December 21, 2017 Published: December 21, 2017 1672

DOI: 10.1021/acsami.7b13692 ACS Appl. Mater. Interfaces 2018, 10, 1672−1677

Research Article

ACS Applied Materials & Interfaces

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

Figure 2. Cycling performances (a) and rate performances (b) of Sn/PF-COONa, Sn/AB/CMC-Na, and Sn/AB/PVDF electrodes. 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.

peaks can be well indexed to crystalline Sn. The weak peaks of tetragonal SnO indicate the presence of a very small amount of oxidized impurities. PF-COONa is prepared using the same method as described in the previous work. The IR 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 consists of 80 wt

excellent long-term cycling capacity, stability, as well as rate capability, outperforming other conventional binders for the pure Sn anode (see Table S1) and indicating its great potential for high-capacity anode material with large volume changes.

2. RESULTS AND DISCUSSION Sn nanoparticles were used as-purchased without further treatments. Figure S1 displays the X-ray diffraction pattern of Sn nanoparticles with an average size of 100 nm, where the major 1673

DOI: 10.1021/acsami.7b13692 ACS Appl. Mater. Interfaces 2018, 10, 1672−1677

Research Article

ACS Applied Materials & Interfaces

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.

factors: (1) the n-type doping of PF-COONa and (2) the formation of a more stable SEI layer. After 50 cycles, the capacity of Sn/PF-COONa tended 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 PVDF.18,25 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 the Sn/ PF-COONa, Sn/AB/CMC-Na, and Sn/AB/PVDF electrodes. It can be seen that at 100, 200, 500, 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, the Sn/AB/PVDF anode resulted in the poorest capacities at all currents. This result shows that without conductive additives, PF-COONa not only acts as binder but also exhibits excellent conductivity that could accommodate high-rate tests. Furthermore, the postmortem SEM images (Figure 2c,d) showed that after cycling, the Sn/PF-COONa electrode demonstrated a smooth surface, while a much more uneven surface results 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 the fact that the electronic conductivity of PF-COONa facilitated the formation of a homogeneous and stable SEI layer on the Sn electrode. Furthermore, the cycling performance of the Sn/carbon nanotube (CNT)/PF-COONa, Sn/CNT/CMC-Na, and Sn/

% of Sn particles and 20 wt % of PF-COONa. PF-COONa electrodes with different average Sn loading were prepared for different testing purposes. Figure 1 shows the cycling performances of the Sn/PFCOONa electrode (with Sn areal loading of approximately 0.6 mg cm−2) at different cycling current densities. The voltage profiles of Sn/PF-COONa are shown in Figure 1a,c, and 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,d demonstrates the cycling capacities of Sn/PF-COONa electrodes at current densities of 100 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 a good adhesive property of PF-COONa 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. 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 the Sn/PF-COONa electrode. In this case, all electrodes were prepared with a 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/CMCNa and Sn/AB/PVDF, which could be attributed to several 1674

DOI: 10.1021/acsami.7b13692 ACS Appl. Mater. Interfaces 2018, 10, 1672−1677

Research Article

ACS Applied Materials & Interfaces

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

Scheme 1. Schematic Illustration of the Sn Anodes in the Processes of Lithiation/Delithiationa

a

Through use of AB and nonconductive polymer (upper) and conductive binder (lower).

CNT/PVDF electrodes with higher Sn loading (2.5 mg cm−2) was also tested. As shown in Figure 3, at a current density of 200 mA g−1, the capacity of Sn/CNT/PF-COONa was retained at around 555 mA h g−1 after 150 cycles. In contrast, capacities of 254 and 87 mA h g−1 were obtained from Sn/CNT/CMC-Na and Sn/CNT/PVDF, respectively. Therefore, the electrochemical performance of the 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 numbers. 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 the stable SEI layer but also indicated that PFCOONa served as a conductive network in the electrode. To observe the morphology and 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 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 the cathodic peak and the peak at 1675

DOI: 10.1021/acsami.7b13692 ACS Appl. Mater. Interfaces 2018, 10, 1672−1677

Research Article

ACS Applied Materials & Interfaces 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 discharge capacity during the first galvanostatic cycling. From the second cycle, three reproducible cathodic peaks can be found at 0.66, 0.62, and 0.41 V vs Li/Li+, which can be explained by the formation of the LixSn alloy.8 As for the anodic sweep, four oxidation peaks can be found at 0.45, 0.60, 0.72, 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/PFCOONa electrodes before and after cycling in Figure 4b,c, 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 inevitably result, and the use of conductive binder is a practical solution. As shown in Scheme 1, when the conventional binder/ conductive material combination is used, 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.

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 (400 M Hz, CDCl3), δ (ppm): 7.68−7.50 (m, 6H); 2.30 (t, 4H); 1.46 (t, 4H); 1.33 (s, 18H). 13C 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 of anhydrous DMF, and subsequently, 300 mg of Pd(dppf)2Cl2 was added quickly under a nitrogen atmosphere. The reaction was conducted in the dark 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)] (PFCOONa): A mixture containing 1.741 g (3 mmol) of M1, 2.023 g (3 mmol) of M2, 35 mg of Pd(PPh3)4, and several drops of Aliquat 336 was added to a flask. Then, 12 mL of 2 M Na2CO3 solution and 36 mL of THF was added, and the flask was degassed by three freeze−pump− thaw cycles. The mixture was heated to 85 °C 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 of dichloromethane containing 15% trifluoroacetic acid. The mixture was stirred at room temperature for 24 h. And after the solvent removed under reduced pressure, 200 mL of 0.5 M aqueous Na2CO3 solution was added, stirred for 6 h, and dialyzed against water several times. The product was obtained via freeze drying 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 the Sn nanoparticles was analyzed by XRD using a Bruker D8-Advantage powder diffractometer (Cu Kα radiation) with 2θ from 20 to 90° at 0.2 s per step. Electrochemical Measurements. All electrodes in this work were prepared using a casting method. First, Sn nanoparticles (Aladdin, 99.99% metals basis ≤100 nm) were dispersed in the 2% PF-COONa solution and stirred vigorously for 24 h. Then, the slurry is casted onto the Cu foil current collector and dried naturally at room temperature. After being cut into pieces, the electrodes are dried at 110 °C under vacuum to remove the remaining H2O content. The electrodes with PVDF, CMC-Na binders, and CNT additives were prepared by using the same procedures. All of the coin cells were fabricated in an Ar-filled drybox. Coin cells (2032) were used as to assemble half cells. Li foil (99.9%) was used as the negative electrode. LiPF6 (1.2 M) in ethylene carbonate (EC)/ diethylene carbonate (DEC) (1:1 w/w) with an additive of 10 wt % fluoroethylene carbonate (FEC) was used as the electrolyte. Galvanostatic cycling was conducted in the voltage range between 0.01 and 1 V at room temperature using a battery test system (Newell, China). The CV measurements were performed at a scan rate of 0.1 mV s−1 from 0.01 to 2 V on an electrochemical workstation (CHI 604E, CH Instruments).

3. CONCLUSION In summary, a water-soluble conductive polymer (PF-COONa) is investigated as a promising binder for the nanosized Sn anode. The Sn/PF-COONa electrodes demonstrated excellent reversible capacities of 762 and 593 mA g−1 at current densities of 100 and 500 mA h g−1 after long cycling, respectively. By comparing the electrochemical performances of the Sn electrode using PFCOONa as binder and those using conventional binder, PFCOONa delivered significantly better cycling capacities and a 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 fact that 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 a stable SEI layer, which can also be 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 the Sn anode with great commercialization potential but also provide guidance on designing Sn-based electrodes. 4. EXPERIMENTAL PROCEDURES Preparation and Characterization of Materials. Preparation of 2,7-dibromo-9,9-bis(3-tert-butyl propanoate)fluorine (M1): First, 5 g (76.1 mmol) of 2,7-dibromofluorene and 300 mg (0.94 mmol) of tetrabutylammonium bromide (TBAB) were mixed in 35 mL of 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 12 h. 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

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b13692. Performance comparison of PF-COONa with other reported binders; XRD pattern of Sn; IR spectra of Sn, 1676

DOI: 10.1021/acsami.7b13692 ACS Appl. Mater. Interfaces 2018, 10, 1672−1677

Research Article

ACS Applied Materials & Interfaces

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Sn/PF-COONa, and PF-COONa; cycling capacities of Sn/SWCNT/PF-COONa electrode (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (L.Y.) *E-mail: [email protected] (F.P.) ORCID

Feng Pan: 0000-0002-8216-1339 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.

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ACKNOWLEDGMENTS 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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DOI: 10.1021/acsami.7b13692 ACS Appl. Mater. Interfaces 2018, 10, 1672−1677