Research paperMultiple hydrogel alginate binders for Si anodes of lithium-ion battery
Graphical abstract
Introduction
To meet the increasing demand of portable energy storage and electric vehicles, lithium ion batteries (LIBs) are requested to provide higher energy density, lower cost and longer cycle life [1], [2], [3], [4]. Various types of materials have been investigated as LIB anodes for replacing the widely used graphite with a relative low capacity, such as Li-alloys based materials and metal oxides [5], [6]. Among these materials, Si has been considered as a promising candidate anode for next-generation LIB, due to its ultrahigh theoretical capacity, relatively low Li+ insertion potential, low cost and nature abundance [7].
However, Si anodes have less successful commercial applications mainly due to the huge volume change (>300%) during lithium alloying/dealloying processes, which causes anode materials pulverized and peeled off from the electrode [2]. Such degradation of electrode brings about two serious problems for their practical applications in LIB. One is the separation of electrical contact between Si particles and current collector that leads to a rapid capacity loss. The other is the continuous formation of the solid electrolyte interphase (SEI) on the Si particle surface, which causes continuous increasing electric resistance and reducing coulombic efficiency [8]. To tackle these challenges, several strategies have been explored [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], including the change or modification of binders.
Due to the weak Van der Waals force between binder and Si particles, the traditional polyvinylidene fluoride (PVDF) binder is difficult to buffer the huge volume change of Si particles during the cycling. However, the binders with hydroxyl group (OH) or carboxyl group (COOH) could form stronger hydrogen bonds with the silicon oxide thin layer on Si surface. Such kind of hydrogen bonds intensely adhere Si nanoparticles (SiNPs) and the current collector, ensuring an improved electrochemical performance [19]. Along this line, plenty of polymers with OH or COOH were applied as the binders of Si anodes in the past few years.
Polyacrylic acid (PAA) and carboxymethyl cellulose (CMC) were demonstrated to play significant positive roles on the electrochemical performance of Si anodes firstly [20], [21]. Then, some kinds of natural biopolymers have also been reported. Kovalenko et al. developed sodium alginate (Na-alg) as binder for Si anode. The electrode with Na-alg could keep intact better, and result in more stable cycle performance [22]. Ling et al. found that the long chains of Gum Arabic binder could reinforce the Si-based electrode, thus bringing an outstanding cycle performance [23]. Jeong et al. reported that Si particles with Xanthan Gum binder displayed 72.2% capacity retention after 200 cycles [24]. Our research group chose Guar Gum as binder for its high mechanical strength and lithium ion transport ability, and demonstrated that the Si anode with Guar Gum showed low capacity fading even in a lower binder ratio (15 wt%) [25]. Hwang et al. used natural agarose as an alternative binder for both high-performance Si anode and LiMn2O4 cathode. The full cell with agarose binder exhibited a capacity retention of 87% after 50 cycles [26]. Bie et al. demonstrated that the Si anode with Karaya Gum presented a specific capacity of 2421 mAh g−1 at 1.5 A g−1 with a capacity retention of 80% after 150 cycles [27].
To further improve the performances, the modifications of binders have been developed. The polyvinyl alcohol (PVA) and PAA cross-linked binder was synthesized by in-situ esterification reaction. A three-dimensional network could be formed and effectively buffer the volume change of Si anode [28]. Ryou et al. prepared mussel-inspired polymer binders through conjugation of dopamine hydrochloride to Na-alg and PAA to improve the adhesion [29]. Jeong et al. synthesized hyper-branched network structure polymerized β-cyclodextrin, and it presented multidimensional hydrogen-bonding interactions with Si particles and offered robust contacts between both components [30]. We also reported a simple and effective method to synthesize a kind of hydrogel alginate binder, which could greatly enhance Na-alg by only a small amount of CaCl2 (Ca-alg). The Si/C composite electrode with Ca-alg binder could be cycled stably for 200 cycles while the electrode with Na-alg binder showed a rapid capacity fade only after 30 cycles at the same current density [31].
Herein, we choose some cations to synthesize a series of hydrogel alginate binders. Al3+ is used at first to coordinate with alginate due to its higher valence, which was expected to cross-link more alginate chains. Then Ba2+, in the same element group of Ca2+, is applied on the synthesis of hydrogel alginate binder because of its larger ion radius than Ca2+. Also, two common used transition metal cations Zn2+ and Mn2+ with relatively large sizes are introduced to coordinate with alginate respectively. These transition metal cations are expected to interact with alginate differently. The mechanical properties characterization results demonstrate that Al-alg and Ba-alg are more robust, and the Si anodes with Al-alg or Ba-alg binder represent higher capacity retention than the electrodes with other hydrogel alginate binders significantly. Even in a low binder ratio (SiNPs: acetylene black: binder = 8:1:1), the Si anode with Ba-alg can still display a capacity retention of 49.1% after 200 cycles at 840 mA g−1, while the electrodes with other binders show very poor cycle performances at the same condition.
Section snippets
Preparation of binders
All the binder slurries were prepared at room temperature. 0.02 g sodium alginate (Taiming, Shanghai, China) was stirred homogeneous with 1.0 mL 1.8 mmol L−1 MCl2 solutions (M = Ca, Ba, Mn, Zn) respectively to form M-alg binders. The Al-alg binder was prepared with Al2(SO4)3 solution which contained the Al3+ concentration of 1.8 mmol L−1 and Na-alginate by the same method. A 0.02 g mL−1 Na-alg binder solution was also prepared for comparison.
Battery fabrication
The SiNPs (Alfa Aesar, Tianjin, China) and acetylene black were
Results and discussion
Fig. 1 shows the possible molecular structures of Na-alg, M-alg (M = Ca, Ba, Mn, Zn) and Al-alg binders. The Na-alg molecule displays a linear structure, while the M-alg and Al-alg present different cross-linked structures owing to the coordination between multivalent cations and alginate chains [32], [33], [34]. The coordinate interactions in the cross-linked structures are much stronger than the hydrogen bonds between linear alginate molecules, thus M-alg and Al-alg binders are expected to be
Conclusions
In this study, Al3+, Ba2+, Mn2+ and Zn2+ cations have been applied to synthesize new kinds of hydrogel alginate binders for Si anode. Al-alg and Ba-alg binder display superior mechanical properties among these binders which make the active materials more difficult to peel off from the current collector, and the electrode will keep integrity better during the charge/discharge processes. Thus, the Si anodes with the Al-alg and Ba-alg binders exhibit higher initial capacity, longer cycle life and
Acknowledgements
This work was supported by National Natural Science Foundation of China (21373008, 21621091), National Key Research and Development of China (2016YFB0100202), and Natural Science Foundation of Fujian Province of China (2015J01063).
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