New, Effective, and Low-Cost Dual-Functional ... - ACS Publications

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A New, Effective and Low Cost Dual-Functional Binder for Porous Silicon Anodes in Lithium-Ion Batteries Rongnan Guo, Shunlong Zhang, Hangjun Ying, Wentao Yang, Jianli Wang, and Wei-Qiang Han ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21936 • Publication Date (Web): 22 Mar 2019 Downloaded from http://pubs.acs.org on March 23, 2019

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ACS Applied Materials & Interfaces

A New, Effective and Low Cost Dual-Functional Binder for Porous Silicon Anodes in Lithium-Ion Batteries Rongnan Guo, Shunlong Zhang, Hangjun Ying, Wentao Yang, Jianli Wang, Wei-Qiang Han * School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, P.R. China. E-mail: [email protected] Abstract: In this work, a new effective and low-cost binder applied in porous silicon anode is designed through blending of low cost poly(acrylic acid) (PAA) and poly(ethylene-co-vinyl acetate) (EVA) latex (PAA/EVA) to avoid pulverization of electrodes and loss of electronic contact because of huge volume changes during repeated charge/discharge cycles. PAA with a large number of carboxyl groups offers strong binding strength among porous silicon particles. EVA with the high elastic property enhances the ductility of PAA/EVA binder. The high ductility PAA/EVA binder tolerates the huge silicon volume variations and keeps the electrode integrity during charge/discharge cycle process. EVA colloids act as hosts of material for electrolyte increase the electrolyte uptake of electrodes. The porous silicon electrode with PAA/EVA binder exhibits reversible capacity of 2120 mAh g–1 at 500 mA g–1 after 140 cycles because of the excellent ductility and lithium-ion transport properties of PAA/EVA binder.

Keywords: ductility, low-cost binders, high electrolyte uptake, porous silicon, lithium-ion batteries

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Introduction Silicon (Si) has been considered as the most promising candidate for anodes because of its low cost and high theoretical capacity of 3579 mAh g–1, almost ten times equal to that of conventional graphite anodes (372 mAh g–1).1-3 Nevertheless, Si suffers from huge volume changes (over 300%) during lithiaton/delithiation processes.4, 5 The substantial volume change smashes the active materials on electrodes and breaks the electric contact between Si particles, resulting in degradation of electrodes, and rapidly capacity fading.5 To address this problem, various forms of Si based materials (e.g. nanoporous, nanoparticles and nanowires) have been used.6-10 Besides, polymer binders also play an essential role in overcoming this issue. Unfortunately, commercial binder, poly(vinylidene fluoride) (PVDF), is not suitable for Si electrodes owing to its weak van de Waals force between binder and Si particles, which cannot prevent the cracking

of

electrodes.11

Furthermore,

volatile

and

toxic

organic

N-methyl-2-pyrrolidone is applied for solvent of PVDF, which will contaminate the environment.12 Compared with PVDF, robust and eco-friendly water soluble binders with carboxylic and/or hydroxyl have attracted increasing attention. These functional binders (e.g. sodium carboxylmethyl cellulose (CMC).13,

14

sodium alginate15 and

poly(acrylic acid) (PAA)16) are better than PVDF in improving electrochemical performance of lithium-ion batteries.17 One reason is that carboxylic and/or hydroxyl groups interact with Si particles mainly via covalent and/or hydrogen bonds which are

2


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proposed favor the long cycle stability of Si electrodes.16, 18 Moreover, most of these water soluble binders with high modulus and rigid polymers can suppress Si volume expansion during charge/discharge cycles.19 However, the rigid binders cannot completely prevent the volume expansion of Si and relieve the stress development. The expansion of Si leads to the chains of binders rupture, resulting in the cracks of electrodes during lithiation/delithiation processes. On the contrary, the flexible polymers, such as styrene butadiene rubber,20 with high extensibility and low modulus, are not usable as binders for electrodes. One reason is that the chains of polymers slide easily with active particles and conductive carbon black additives, which brings on the agglomeration of Si particles and conductive agent.21, 22 It’s vital to find or develop a binder with suitable mechanical properties (stiffness/toughness balance) to retain the structural integrity of electrodes.23 Li-ion transport in the electrode, including active materials and binder, is also important for achieving the full capacity.24, 25 High Li-ion transport will reduce the polarization of electrode and the irreversible capacity loss during cycling process. For the binder system, Li-ion transport is mainly affected by the electrolyte uptake.24, 26 Therefore, a binder with suitable mechanical properties and high electrolyte uptake for Si anodes is supposed to improve the electrochemical performance of Si anodes. A water soluble polymer PAA shows reasonable performance for Si anodes, because the rich carboxyl groups and high modulus can reduce the volume change level of Si during cycling process.27 Additionally, the rigid property of PAA keeps the active materials and conductive agent uniformly distributed in electrode.28 However, 3



owing to the brittleness of PAA film, the volume change causes the fracture of PAA film, and renders the pulverization of Si electrodes during lithiation/delithiation processes. Some of PAA-based binders, such as PAA/PVA,29-31 PAA-grafted CMC32 and glycinamide modified PAA,33 have developed to enhance the cycling performance of Si, but they are still too rigid to accommodate the volume change of active materials. Poly(ethylene-co-vinyl acetate) (EVA) has drawn much attention because of its tunable physical and chemical characteristics. The mechanical properties of EVA are controlled by ethylene/vinyl acetate (VA) weight composition.34-36 EVA with higher VA unites, shows rubber property,37 is always applied to improve the fracture toughness of rigid substrates.36, 38-40 Furthermore, the EVA colloids act as hosts of material for electrolyte due to much of electrolyte permeating into the colloids. Inspired by the merits of PAA and EVA, we report a new binder PAA/EVA prepared by simply mixing low cost PAA and EVA. It is well established that synergy between PAA and EVA prominently increase the ductility and electrolyte uptake of PAA/EVA film. The electrochemical properties of Si porous electrodes with PAA/EVA demonstrate a significant improvement in comparison with PAA binder based electrodes. PAA/EVA keeps the electrode laminates integrity after long-term cycle process. Experimental Electrode preparation: The PAA/EVA was prepared by mixing 10 wt% PAA (Mw=450 000, Sigma-Aldrich) aqueous solution and 10 wt% EVA emulsion (84 mol% VA units, Beijing Oriental Petroleum Organic Chemical Plant) with weigh ratio of 7:3. 4


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PAA, CMC/SBR and poly(vinyl alcohol) (PVA) (PVA-1788, Aladdin) were used as control binders. Control binder PVA is the same as the emulsifier PVA in EVA emulsion. The porous Si was prepared based on the previous report.41 Fe-Si scraps, obtained from a metallurgical factory, were crushed by ball-milling with a rotation speed of 350 rmp for 12h. After that, the ball-milled Fe-Si alloy was slowly added into 1 M HCl solution with continuously stirring for 12 h to remove the Fe in the alloy. Then, the SiO2 on the surface of Si powder was removed by immersed the material in a 5 wt% HF for 8 h. The obtained Si powder was washed by ethanol and deionized water and dried in vacuum at 60 ℃ for 12 h. To prepare the working electrodes, the casting aqueous slurry method was used. The slurry was prepared by mixing porous Si, carbon black (Super P) and binder with weight ratio of 7:1:2 in deionized water and stirring for over 5 h. Then the homogeneous slurry was cast onto a thin copper foil, followed by vacuum drying at 60 ℃ for 12 h. The mass loading of all electrodes are about 1 mg cm–2. Materials characterization: The morphologies of binders and electrodes were obtained on field-emission scanning electron microscope (SEM, SU-80, Hitachi Ltd, Japan). The morphology of PAA/EVA was also performed using transmission electron microscopy (TEM, JEM-1200EX, Japan) after negative staining with phosphotungstic acid. Mechanical properties of binders were performed on a UTM2102 electric universal testing machine (SUNS, China) under ambient atmospheric conditions. The thickness of films was measured on a CHY-CA Thickness Tester (Labthink, China). For the 5



tensile strength and elongation at break tests, binder films were prepared by solution-cast method. The PAA aqueous solution was casting on the glass mould, meanwhile the PAA/EVA and the EVA latex were casting on Teflon moulds. The moulds filled with binders were dried at room temperature for 12 h, subsequently at 60 ℃ for 12 h. The dry films were cut into 3 cm length and 2 cm wide strips, and the thickness ranged from 0.01 to 0.03 mm. The tensile strength and elongation at break tests were carried out at a rate of 10 mm min–1. The toughness is the area blow the tress-strain curve. Young’s modulus was calculated from the slope of the stress-strain curve in the elastic deformation region during the initial stretching period. Adhesion experiments were performed on both a dry state and a wet state. Electrode strips (2.5 cm × 1.27 cm) were prepared. Scotch magic tape (3M) was affixed to the surface of electrode laminates side and was peeled off at a peel angle of 180° and at a rate of 50 mm min–1. For the wet state electrodes, the laminates were immersed in salt-free electrolyte for 48 h in a glove box filled with argon gas followed by surface completely dry at room temperature. The compatibility of the binder with the electrolyte solvent was examined by a swelling test. The binder sheets and electrodes with different binders were immersed in salt-free electrolyte at room temperature for 48 h, following by removing the electrolyte on the surface of sheets and electrodes via filter paper. The swelling ratio was defined as the weight ratio of the amount of solvent absorbed to the dry weight of the tested binder sheet or the electrodes without Cu foil. Electrochemical measurements: The electrochemical behavior of porous Si 6


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electrodes with different binders was carried out in CR2032 coin cells with metallic lithium wafer as counter and reference electrode. The coin cells were assembled in an argon-filled glove box (

New, Effective, and Low-Cost Dual-Functional ... - ACS Publications

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