Cracking resistance and electrochemical performance of silicon anode on binders with different mechanical characteristics

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Abstract

Silicon anodes in lithium-ion batteries have attracted attention for their exceptionally high theoretical capacity. However, their practical application is hindered as the capacity fades quickly due to huge volume changes of silicon particles during the lithiation/delithiation process. In order to reduce volume changes, polyvinylidene fluoride, gellan gum, and pullulan modified gellan gum were introduced in the electrode as a binder. This work focuses on understanding the role of these binders on the mechanical behavior and cracking resistance of silicon electrodes during electrochemical cycling. A binder with moderate elasticity can be used to extend the capacity lifetime of a silicon anode.

Introduction

Lithium-ion batteries (LIBs) are considered a promising energy storage technology for portable electronics, electric vehicles, and renewable energy systems operating on green energy sources, such as wind and solar [1]. Among the candidate anode materials for large-scale application in LIBs, silicon is particularly attractive due to its exceptionally high theoretical gravimetric capacity (Li15Si4 = 3579 mAh g−1 at room temperature) and low charge/discharge potential (˜0.4 V vs Li/Li+) [2]. Nevertheless, the use of silicon thus far is limited due to its short cycle life. This fatal shortcoming in silicon anodes originates from a well-known large volume change (˜300%) between the charge and discharge states, which triggers various fading mechanisms, including pulverization of the active material, destabilization of the solid electrolyte interphase (SEI) layer, loss of electrical connections, and electrode delamination [3]. One approach to resolving these problems is optimization of the chemical structure of a polymeric binder in a silicon anode. A polymeric binder allows an electrode film to adhere to a current collector and facilitates the coherence of silicon nanoparticles, which is crucial for good anode performance [4]. Apart from maintaining chemical and electrochemical stability during electrode/electrolyte interactions, a polymeric binder must withstand large, repeated volume changes of the silicon anode during cycling [5]. In addition to the binder properties that prior works to enhance electric conductivity, cyanoethyl polyvinyl alcohol (PVA-CN)-coated surface is better than bare surface of silicon and also significantly more advantageous than the surface coated by poly(acrylic acid)/carboxylic methyl cellulose (PAA/CMC) binder [6].

For commercial LIBs, poly(vinylidene fluoride) (PVdF) is a linear-type homopolymer binder which is commonly used in LIBs due to its strong adhesion to active materials and current collectors, reasonable electrochemical stability, and good wettability toward polar electrolyte solutions for facile Li ion transport [7]. However, PVdF does not support stable cycling of silicon anodes due to its weak van der Waals interactions, which are insufficient for maintaining the structural integrity of the electrode during large volume change of silicon [8], [9]. More importantly, the low tensile strength of the PVdF can be attributed to its high swellability value [9]. For this reason, adhesive and/or mechanically robust polymeric binders have been studied because they can accommodate severe volume changes and maintain high electrical conduction in the electrode. To overcome mechanical fracturing of electrodes, various polymer binder architectures for silicon anodes have been investigated, such as linear (weak interaction) [10], branched (strong interaction) [11], cross-linking (covalent bonding) [12], [13], [14], and self-healing (recovering) [15] architectures. However, most discussions have been confined to bonding mechanisms (adhesion strength) and their interaction between binders and silicon particles.

Meanwhile, binders with the proper mechanical strength minimize pulverization of silicon particles and disintegration of silicon mass in the electric pathway, even if an intrinsic volumetric change of silicon is inevitable [16], [17]. Specifically, gellan gum, a high-modulus natural anionic polysaccharide, progresses in capacity retention during repeated cycles compared with polymeric binders such as CMC [18]. Gellan gum contains more functional groups (Fig. 1), including a large number of acetyl groups that are distributed homogeneously within the polymer, leading to higher reactivity [18], [19]. It is likely that the presence of a variety of polar functional groups leads to efficient coverage of the silicon particles and improves the cycling stability. The tensile strength of the gellan gum film was a factor ˜2 larger than the value of dry PVdF [20], [21]. This mechanical characteristic may allow this material to survive the large volumetric changes that occur in silicon particles. However, rigid or stiff polymers cannot completely relieve stress that develops due to intrinsic volume changes. For this reason, Yoon et al. emphasized that the mechanical properties between two elastic polymers and two rigid polymers should be optimized [16]. More importantly, the swellability of the binder with the electrolyte affects the mechanical strength of the binder. Therefore, the mechanical strength of a wet binder should be considered when developing the appropriate binder for a silicon anode [22], [23], [24]. In this work, three binders with different mechanical properties (gellan gum, pullulan modified gellan gum, and PVdF) were chosen to investigate the effects of the binders on the electrochemical performance of a composite electrode. In addition, electrolyte uptake and the tensile strength of a wet binder were studied using electrolyte absorption measurements and strain-stress curves in order to determine the origin of the improved cycle performance.

Section snippets

Materials and synthesis

Silicon nanoparticles (APS = 50 nm, Alfa Aesar; Fig. S1) and carbon black (Denka black, Li-250, Singapore) were used as the active material and conductive carbon material for the Si electrode, respectively. Gellan gum (Phytagel) and pullulan (from Aureobasdium pullulans) were purchased from Sigma-Aldrich. An aqueous solution of gellan gum (denoted Gell) and pullulan modified gellan gum (denoted Gell/Pull; gellan:pullulan = 7:3 w/w) binders with 2 wt% total solid content was prepared in

Results and discussion

FTIR spectroscopy studies provide evidence for the strong interaction between the binder and Si nanoparticles. The broad band located at ˜3400 cm−1 can be attributed to Osingle bondH stretching, the peak at ˜2900 cm−1 corresponds to Csingle bondH stretching, and the peaks at ˜1600 and ˜1405 cm−1 correspond to COOsingle bond stretching (Fig. 2a) [26]. The addition of pullulan caused the intensity of the bands at 1600 and 1405 cm−1 to decrease, and the intermolecular bonds between the two polysaccharides could be observed as the

Conclusion

Four crucial properties of binders required for Si anodes to maintain their structure and thereby achieve repeatable LIB operation were investigated: the molecular-level bonding configuration of polysaccharides, adhesion between the electrode and current collector, electrolyte uptake, and mechanical strength. The cycle performance of the Si-Gell cell is poor compared to those of the Si-Gell/Pull, even though it exhibits similar bonding configuration, adhesion strength, and electrolyte uptake.

Acknowledgment

This research was supported by the Technology Development Program to Solve Climate Changes through the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT (NRF-2018M1A2A2063349).

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