Blood clot-inspired viscoelastic fibrin gel: New aqueous binder for silicon anodes in lithium ion batteries
Graphical abstract
Introduction
Efforts aimed at making electronics smaller have led to a demand for lithium-ion batteries (LIBs) with higher energy density. Si has long been considered attractive as an anode material because of its exceptional storage capacity (4200 mA h g−1) relative to commercial graphite (372 mA h g−1) [1]. However, the rapid failure induced by the swelling of Si during cycling has limited its practical utility [2]. Many pathways based on Si manipulation have been proposed to address this by offering available free volume through nanostructuring [3] and pore engineering [4], or by adding buffer matrices through surface coating to fabricate composites [[5], [6]]. It remains challenging to produce these materials at a low cost or on a massive scale.
The technology that is now emerging involves exploiting new binders that are compatible with Si powder [7]. The main functions of binders are to keep (i) integrity of the electrode components such as active materials and conductive additives and (ii) adhesion of them to a current collector [7]. Therefore, engineering the binder chemistry to form stable interfaces with the active material like Si, electrolyte and current collector is particularly important [7,8]. Weak van der Waals interactions between the hydroxylated polar Si surface (Si-OH) and a non-polar binder (e.g., polyvinylidene fluoride) are one of the factors limiting the cycle life [9]. Many polar binders, including polysaccharides [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], vinyl polymers [22], [23], [24], self-healing polymers [25], [26], [27], and their combinations [28,29], have proven effective for enhancing cycling stability. The polar hydroxyl and/or carboxylate functional groups present in these binders form strong hydrogen bonds with the Si surface [7,30]. This enables stable linkages between Si and the conductive network that survive the huge volume changes that occur in Si during cycling.
In addition to controlling the intermolecular interactions, the mechanical properties of the binders need to be carefully designed for enhanced electrochemical performance. In general, stiff cross-linked binders show better capacity retention relative to soft non-cross-linked binders because appropriate levels of stiffness can maintain electrode integrity and electrical connectivity [[11], [12], [13],23,28]. However, excessive stiffness has proven to be no longer advantageous for cycling performance [14,25,31,32], as stiff binders cannot effectively relax the stress generated by the volume changes. Therefore, it is important to determine the appropriate balance between the stiffness and stress relaxation by controlling the time-dependent mechanical responses of the binders. Generalized but effective approaches to modulating mechanical behavior include blending two different binders [23,28,29], adjusting the cross-linking density [11,13,14,25], or changing the flexibility of polymer chains [9,[33], [34], [35]].
Fibrin is a biopolymer hydrogel composed of a three-dimensional (3D) network of amino acid chains [36]. When a blood vessel is injured or cut, fibrin serves as a wound healing matrix for a blood clot (called thrombus) that drives the regenerative process in the presence of red blood cells and platelets (schematic in Fig. 1a). The feature that makes fibrin an effective healing agent is its viscoelasticity. Constituent fibers in fibrin have a large persistence length (∼500 nm) that is comparable with the fiber contour length, rendering it semi-flexible [37]. Semi-flexible fibers are covalently cross-linked to form a 3D viscoelastic network with a large mesh size (1–10 μm) and mechanical deformability [37,38]. Factor XIII present in fibrinogen is activated by thrombin to form an active transglutaminase enzyme (factor XIIIa) [39]. Catalyzed by factor XIIIa, the chemical reaction between γ406 lysine residue of one γ-chain and γ398/399 glutamine residue of another γ-chain yields the formation of isopeptide ε-(γ-glutamyl)-lysyl bonds in the C-terminal regions (Fig. S1). These bonds are the origin of covalent cross-links in fibrin. It has been proved that the same isopeptide bonds formed between α- and α-chains or those between α- and γ-chains also contribute to fibrin cross-linking [39]. As a result, other components of a blood clot, such as red blood cells and platelets [39], can stay in place under external stress [40]. Because of this mechanical integrity, the blood clot sometimes causes a circulatory disease, such as thrombosis, when it forms within one of the major veins deep inside the human body [41].
Inspired by the mechanically robust and injury-healing aspects of the blood clot, we report new high-performance fibrin-based binders for Si nanopowder (Si NP) anodes in LIBs (schematics in Fig. 1b and c). In addition to its mechanical properties, fibrin exhibits an ideal molecular structure as a Si binder. It consists of two peripheral carboxyl-terminal (C-terminal) D regions connected to a central amino-terminal (N-terminal) E region by a pair of stranded coiled-coils of three polypeptide chains (αA, βB, and γ) (schematic in Fig. 1d) [36,39]. Each polypeptide chain is a linear combination of various kinds of amino acids including glycine, arginine, proline, and etc., which are linked by amide bonds (zoom-in schematic in Fig. 1d) [36,39]. Therefore, there are many polar functional groups in fibrin that can form hydrogen bonds with Si, including amide, hydroxyl, and carboxylate groups. A fibrin fiber also has a branched architecture, in which a polypeptide backbone has side chains of amino acids [42] that help maintain electrode integrity. The benefits of fibrin-based binders are illustrated in Fig. 1e. We show that the electrochemical and mechanical properties of a Si electrode based on the fibrin binder are superior to those based on an alginate binder, a representative polysaccharide with a small persistence length (∼12 nm) and high flexibility [43]. We present systematic modulation of the mechanical properties by varying the mixing ratio of fibrin to alginate and by feeding various amounts of Ca2+ cross-linkers to the mixtures. A binder composed of a 10 μl Ca2+-added 4:1 (w/w) fibrin and alginate mixture demonstrated the best electrochemical performance. We discuss the correlation among the electrochemical performance, the adhesion strength of the electrode, and the stiffness of the binder.
Section snippets
Materials
Si NPs (∼100 nm size, 99%) and CaCl2 (97%) were purchased from Alfa Aesar (USA). Alginic acid sodium salt (Alg) was purchased from Sigma-Aldrich (USA). Bovine fibrinogen (free of plasminogen and fibronectin) and thrombin were obtained commercially from Enzyme Research Laboratories (USA) and stored at −80 °C to preserve activity. Before use, both samples were thawed at ∼37 °C.
Binders fabrication
To prepare the fibrin binder, we followed previously reported procedures with minor modifications [44]. Fibrinogen and
Results and discussion
FTIR spectroscopy was conducted to study the intermolecular interactions between Si and the binders. The IR absorbance spectra of fibrin only (Fib) and Si/fibrin (Si/Fib) samples (lower part of Fig. 2a) both show characteristic peaks at ∼3264 (N-H stretch: amide A and hydrogen-bonded O-H stretch), ∼2943 (C-H stretch), ∼1620 (C-=-O stretch: amide I), ∼1521 (N-H bend: amide II), and ∼1392 cm−1 (C-H bend) [43,46]. One difference between the two samples is that two peaks corresponding to C=O
Conclusion
In conclusion, we developed a new route for designing high-performance binders for Si anodes for the first time using a viscoelastic fibrin natural polymer. Optimization of the stiffness and stress relaxation by mixing fibrin with alginate followed by ionic cross-linking led to an advanced binder with outstanding cycling stability. The mechanical and electrochemical properties can be further enhanced by controlling the fibrinogen concentration to tune the mesh sizes of the fibrin network or by
CRediT authorship contribution statement
Woong-Ju Kim: Data curation, Formal analysis, Methodology, Visualization, Writing – original draft. Jin Gu Kang: Conceptualization, Validation, Methodology, Visualization, Writing – original draft. Dong-Wan Kim: Conceptualization, Investigation, Supervision, Writing – review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Ministry of Science and ICT (2020R1A6A1A03045059), the Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (2018M3D1A1058744), and by the Institutional Program (2E31171) of the Korea Institute of Science and Technology (KIST).
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