A Spidroin‐Inspired Hierarchical‐Structure Binder Achieves Highly Integrated Silicon‐Based Electrodes

As a promising component for next‐generation high‐energy lithium‐ion batteries, silicon‐based electrodes have attracted increasing attention by virtue of their ultrahigh theoretical specific capacities. Nevertheless, fast capacity fading posed by tremendous silicon‐based electrode volume changes during cycling remains a huge challenge before large‐scale applications. In this work, an aqueous–oil binary solution based blend (AOB) binder characterized by a spidroin‐like hierarchical structure for tolerating the huge volume changes of silicon‐based electrodes is developed. In the AOB binder, the polymer, containing hydrophobic tetrazole groups, denoted as PPB, and the water‐soluble amorphous poly(acrylic acid), mimick the β‐sheet and α‐helix structure of spidroin, respectively. Benefitting from such biomimetic design, the AOB binder enables both high tensile strength and elasticity, and strong electrode adhesion, therefore apparently stabilizing the silicon‐based electrode structure and rendering prolonged electrode cycle life. Such a strategy endows 3.3 Ah soft package cells assembled with Si/C composite anode and NCM811 cathode with a discharge specific capacity of 2.92 Ah after 700 cycles. This work marks a milestone in developing state‐of‐the‐art silicon‐based electrodes toward high‐energy‐density lithium‐battery applications.


Introduction
3][4][5] DOI: 10.1002/adma.202303312Thus far, silicon (Si) has been well recognized as the promising active material owing to its high theoretical specific capacity (≈4200 mAh g −1 ) and natural abundance. [6,7]The electrochemical reaction of Si relies on a complex alloying/dealloying process.The Si-Li reaction process is shown in Equations (1-4): [8,9] Si ( crystalline Li 15 Si 4 ( crystalline ) → Li x Si (amorphous) + yLi + + ye − + Li 15 Si 4 ( crystalline, resdual ) Li x Si (amorphous) → Si (amorphous) However, the commercialization of these active materialsbased electrodes is still significantly restricted mainly due to a series of challenging problems below.First, the huge volume expansion (≈300%) and shrinkage during cycling results in the active particle pulverization, exfoliation of electrode films, and uncontrollable growth of electrode/electrolyte interphase; these negative factors render fast decline of battery capacity along with low Coulombic efficiency.Second, poor ionic and electronic conductivity of active materials increases the irreversible degree of lithiation and delithiation, which renders low initial Coulombic efficiency and inferior rate capabilities. [10]All these challenges bring about poor cycling performance and hinder the commercial implementation of high-capacity Si-based anode in rechargeable lithium-ion batteries (LIBs).
[13][14] Among them, the development of advanced binder is deemed as a simple yet powerful way since it bonds active material and conductive additive onto current collector and plays a pivotal role in maintaining the structural integrity of electrodes during charge/discharge processes.[17] Toward this end, rational structure design of polymer binder is of great significance.[20][21] For instance, linear poly(acrylic acid) (PAA), polyacrylamide (PAM), polyvinylidene fluoride (PVDF), polyurethane (PU) and polysaccharides (e.g., sodium carboxymethyl cellulose (CMC) and alginate) have shown relatively improved cycling life of Si-based electrodes. [22,23]However, these polymer binders can hardly effectively address the aforementioned issues confronted by Si-based electrode, mainly owing to their large elongation at break accompanied by low elasticity modulus (e.g., PVDF, PU) or vice versa (e.g., PAA, CMC) when plasticized by liquid electrolyte within LIBs.[26] Nevertheless, the crosslinking reaction generally complicates electrode preparation process, and worse yet, crosslinked binders with excellent mechanical match capabilities remain scarce.Given the drawbacks of the topological structure design mentioned above, it is evident that arousing the hierarchical structure design of polymer materials is helpful to decouple the mechanical tradeoff; however, rare previous reports have focused on this pivotal factor for Si-based anode binder to the best of our knowledge.
Spidroin is a natural high-molecular-weight protein (typically 250-400 kDa) secreted by dedicated glands of arthropods, which convert viscous proteinaceous aqueous solutions into a light, solid biofiber.The most attractive point for spidroin is that it can achieve perfect mechanical matching and wonderful shape recovery, which can withstand the impact of prey and entangle flying insects and maintain the structural integrity of the web.These benefits are highly correlated to the unique hierarchical structure of spidroin: 1) In the primary structure, spidroin generally consists of a highly repetitive amino acid sequence segment flanked by amino-and carboxyl-terminal domains (NTD and CTD, respectively), which can form ionic bonding themselves to improve mechanical toughness.[29] Learning from the hierarchical structure of spidroin, we propose a novel aqueous-oil binary solution-based blend (AOB) binder for Si-based electrodes.The as-developed binder is prepared by simply mixing an aqueous poly(acrylic acid) (PAA) solution, and an oily N-methyl pyrrolidone (NMP) solution containing a copolymer (PPB) of polyacrylonitrile (PAN) and poly(ethylene glycol) bisazide (N 3 -PEO-N 3 ).Hydrophobic PPB condensates in the mixture solution to form crystalline regions within sub-micrometer-sized irregular spherical domains as the rigid node of the molecular chain segment similar to -sheet of spidroin; while water-soluble, amorphous PAA mimicks the -helix structure of spidroin, and builds up a network structure by linking with PPB via ionic bonding similar to the interactions between NTD and CTD in the primary structure of spidroin (Figure 1b).The hierarchical structure design endows the as-developed binder with high tensile strength and elasticity, as well as good self-healing abilities.Therefore, compared with traditional binders (e.g., PAA), AOB binder can better accommodate enormous electrode volume deformation and more effectively stabilize the electrode/electrolyte interface upon lithiation/delithiation (Figure 1c-d).As a result, the nano-Si electrode and Si/C composite electrode S600 (a SiOx/graphite electrode specific capacity a specific capacity of 600-650 mAh g −1 at 0.1 C) with the AOB binder exhibit superior long-cycle stability and rate performance to traditional PAA binder.Impressively, in 3.3 Ah soft package cells assembled with S600 anode and NCM811 cathode, a discharge specific capacity of 2.92 Ah after 700 cycles with a capacity loss of 0.013%/cycle can be maintained, undoubtedly confirming the utility of AOB binder.The polymer binder hierarchical structure design philosophy provides an important route to advance practical implementation of high-specific capacity Si-based electrodes.

Hierarchical Structure Design of AOB Binder
To mimick the hierarchical structure of spidroin, it starts with designing the binder topological structure with dynamic ionic bonding.To accomplish this, polymer design based on acidbase interaction is a viable route.As mentioned above, traditional water-soluble binder PAA usually suffers from large elasticity modulus yet low elongation at break at ambient temperature, leading to irreversible electrode film fracture and thus fast degradation of cycling performance. [30,31]To improve the elongation at break of PAA, it is hopeful to introduce other polymers that can construct dynamic ionic bonding with it. [32,33]As a typical nitrogen-rich heterocyclic backbone, tetrazole group is a proton acceptor with multiple coordination sites to form acid-base interactions. [34,35]Another benefit is that the conjugated tetrazole structure can form reversible lithiation sites, beneficial to transport lithium ions and thus to improve rate performance of lithium batteries. [36,37]As a consequence, a robust binder system with a strong dynamic ionic bonding network can be constructed by mixing PAA with a tetrazole-groups-containing polymer.
Therefore, a hydrophobic, tetrazole groups-containing polymer PPB consisting of PAN and PEG segments was designed, both of which help improve elasticity and meanwhile conduct lithium ions. [38,39]A synthesis schematic diagram of PPB is shown in Figure 2a.Via a simple "click" reaction between -CN and -N 3 groups, PPB was obtained via thermally induced cycloaddition of N 3 -PEO-N 3 and PAN under 140 °C. [40]This reaction occurred smoothly, as evidenced by the peak change of -CN and -N 3 groups in 1 H-NMR, 13 C-NMR, and FTIR spectra (Figures S1-S3, Supporting Information).Meanwhile, the N 1 s XPS spectrum further supports this result; four peaks generate corresponding to N=C at 398.7 eV, N-C at 400.4 eV, and + HN-C bonding at 401.5 eV on tetrazole units (Figure S4, Supporting Information). [41,42]n the AOB binder (Figure 2a), the strong ionic bond between tetrazole with carboxyl motifs was confirmed through FTIR spectra (Figure 2b).For PAA, the stretching vibrational absorption peaks of -OH, C=O, C-O are located at 2500-3000 cm −1 , 1708 cm −1 , and 1242 cm −1 , respectively.Tetrazoles in PPB contain four nitrogen atoms as coordination sites, which can all act as carboxylic proton acceptors to form hydrogen bonding.After mixing with PPB, the weakened absorption peak of C=O in PAA accompanied by an obvious redshift (1708→1677 cm −1 ) occurs, indicating that the carboxylic acid in PAA coordinates with nitrogen atoms of tetrazoles in PPB forming tight ionic bonding NH + ••• − O-C=O.Meanwhile, the characteristic peak intensity of C-O in carboxylic acid of PAA and of C−N in tetrazoles of PPB enhances, accompanied by apparent blueshifts (1242→1265 cm −1 , 1163→1172 cm −1 , respectively).Furthermore, the peak intensity of -OH in PAA becomes weak and a new peak (2700 cm −1 ) generates due to the generation of -NH + on tetrazoles.Temperature-dependent real-time IR analysis was employed to further demonstrate the presence of such ionic bonding in AOB binder.As shown in Figure 2c, with the increase in temperature, the peaks ascribed to C-N, C=O, and C-O stretching vibration gradually shift from ≈1172 to ≈1164 cm −1 , from ≈1670 to ≈1700 cm −1 and from ≈1265 to ≈1225 cm −1 , respectively (Figure 2c), suggesting that the breakage of NH + ••• − O-C=O and hydrogen bonding involving carboxyl, and the generation of a mass of free carboxylic and tetrazole groups.Evidently, the dynamically reversible ionic bond network within AOB binder helps to dissipate stress and thus can accommodate enormous electrode volume deformation. [43]urther theoretical calculation shows that the ionic bond energy between every N of tetrazole with the carboxyl group is similar (ranging from −11 to −14 kcal mol −1 ), but much larger than the hydrogen bonding formed between carboxyl groups themselves (−4.7 kcal mol −1 ) (Figure 2d).Such an ionic bonding helps to reconstruct at shattered interfaces, restoring the mechanical strength and original shape of Si-based electrodes. [44]iven the hierarchical structure of spidroin is highly linked with the assembly of corresponding polyamino acid segments in aqueous solutions, here, a facile aqueous-oil binary mixing process was conducted to regulate the condensed structure of AOB binder.As a fair comparison, the PAA/PPB blend binders in sole water or sole NMP (denoted as PAB-W and PAB-N binder, respectively) were also prepared.To begin with, a PAA/PPB weight ratio of 3/1 was selected in the condensed structure regulation research.As depicted in Figure S5a, Supporting Information, the PAB-N film shows a uniformly dispersed fiber morphology.After addition of water, the sample surface shows many uniformly dispersed or unequally distributed micrometersized or submicron-level irregular rod-or sphere-like clusters (Figure S5b-d, Supporting Information).By adjusting the volume ratio of water/NMP, the condensed structure of AOB binder can be regulated.It is evident that AOB binder films prepared with a water/NMP ratio of 1/5 exhibit the medium uniformly distributed clusters; sub-micrometer-sized irregular spherical domains are observed via higher resolution SEM and top-view atomic force microscopy height sensor imaging analyses (Figures S6b and S7, Supporting Information).Notably, SEM mapping demonstrates that there are much more N elements distributed in the cluster domains than the rest of the film, while C elements are relatively uniform in the AOB binder (Figure S6c-d, Supporting Information).For further microstructure analysis of the AOB binder film, a confocal laser scanning microscopy (CLSM) and AFM were performed.Apparently, distinct distribution of PPB and PAA in the AOB binder film can be observed (Figure S8, Supporting Information).Consistent with the SEM element mapping results (Figure S6c,d, Supporting Information), PAA is uniformly distributed in the whole film (Figure S8c, Supporting Information), while the PPB exists more in the sub-micrometersized irregular spherical domains (Figure S8b, Supporting Information); these results are highly correlated with the ionic bonding-induced coassembly of PAA and PPB.X-ray diffraction demonstrates that there is a strong crystalline peak at 16.78°with d spacing of 5.28 Å in the AOB binder film, in sharp contrast to that of PAB-N films (Figure S9a,b, Supporting Information).This result clearly proves that the aqueousoil binary mixing process renders the generation of crystalline zones in the AOB binder film.Given that PAA is amorphous (Figure S9c, Supporting Information), it can be concluded that the crystallization domains belong to the ordered assembly of PAN segments in PPB.Furthermore, considering the oleophilic PPB distribution, the crystallization domain is prone to exist in the sub-micrometer-sized irregular spherical clusters of AOB binder films.This indicates that sub-micrometer-sized irregular spherical clusters formed by PPB within the AOB binder are, in part, stacked in order.More importantly, the crystallization peak is also present in the Si-based electrode with AOB binder (Figure S9d,e, Supporting Information).
Figure 2e vividly illustrates the condensed structure formation of AOB binder via an aqueous-oil binary mixing process.In the cosolvent, by virtue of strong ionic bonding interactions between PAA and PPB, sub-micrometer-sized irregular spherical clusters can generate through the coassembly of major PPB with minor PAA, and other PPB and PAA synergistically constitute long polymer chains that undergo physical/ionic crosslinking themselves forming the uniform domain in the AOB binder film.During this process, part of PAN segments on PPB can crystallize within the sub-micrometer-sized irregular spherical clusters.Eventually, the spidroin-like hierarchical structure binder with amorphous and crystallization domains is constructed.While the PAB-W film shows clusters with irregular shape, size, and distribution area, which have negative impacts on the electrode particle dispersion (mentioned below).

Physical Property Evaluations of AOB Binder
Via stress-strain curves analyses, the water/NMP ratio of 1/5 in the aqueous-oil binary mixing process of AOB binder samples, which renders superior mechanical properties, was cho-sen as the optimum one for the following research (Figure S10, Supporting Information).Additionally, it is worth mentioning that compared with PAB-N and PAB-W binder, the AOB binder renders enhanced mechanical and adhesion properties (Figure S10-S11, Supporting Information), clearly proving the significance of binder hierarchical structure design.Compared with the traditional PAA binder film, AOB binder achieves evidently enhanced tensile strength (68 vs 21 MPa) and higher elongation at break (22.1% vs 5.5%) (Figure 3a).Noting that as the strain increases, the corresponding stress of AOB binder films steadily increases at the early stage, and then abruptly rises.At the early stage, the extension of entangled polymeric chains, especially in the micro-nano cluster could account for the small stress with straining.After that, since the initial deformation process gives rise to a high orientation degree of intertwined polymer chains, the AOB binder film can experience higher stress with stretching.To reflect the practical mechanical properties of binders within cells, the AOB binder films were soaked in electrolytes for 24 h. Figure 3b and Figure S12, Supporting Information, show the sequential loading-unloading test of the asprepared electrolyte-soaked AOB binder film at a strain limit of 30%.The stress values remain approximately constant for ten cycles, indicative of a decent recoverable behavior of AOB binder films (Figure 3b).Superior mechanical properties of AOB binder films are also reflected in the nanoindentation test.At a given nanoindentation force (a maximum load of 500 μN), AOB binder films exhibit an evidently smaller indentation depth (479 vs 738 nm) than PAA binder (Figure S13, Supporting Information).Moreover, the nano-Si electrode with AOB binder shows much higher reduced modulus (1.08 vs 0.6 GPa) and hardness (0.08 vs 0.03 GPa) than those of PAA binder (Figure 3c).Enhanced mechanical properties of the as-developed binder are more favorable to decrease excessive Si-based electrode volume expansion. [45]n a 180°peeling test, the average peel strengths of the AOBand PAA-binders-based nano-Si electrodes are 2.56 N and 0.64 N, respectively (Figure 3d), showing the enhanced adhesion to Cu current collector.Additionally, the wettability test on Si surface was conducted to provide good insight into the affinity of binder to Si. Polymer solutions containing 1 wt% of PAA or AOB were dropped onto the surface of the monocrystalline Si wafer to conduct contact angle measurements.After standing for 2 min, the contact angles of the PAA and AOB solutions are 50.6°and39.1°, respectively (Figure S14, Supporting Information), indicative of the improved wettability of the AOB binder solution.Enhanced wettability can prevent stress concentration at the interface between binder and Si-based electrode from forming a large number of defects; this effect is effective to stabilize the Si-based electrode structure. [46]This observation can be explained by the higher affinity of AOB binder to the Si surface due to abundant polar groups such as carboxyl and tetrazole.Furthermore, the AOB binder film exhibits a lower swelling ratio (4% vs 6%) of electrolytes than the PAA counterpart (Figure S15, Supporting Information), which helps retain mechanical strength and high adhesion during battery cycling.Moreover, we found that AOB binder shows an enhanced self-healing ability than PAA binder (Figures S16 and S17, Supporting Information).Superior mechanical, adhesive, and self-healing abilities of AOB binder are anticipated to better withstand the huge volume change of Si particles and suppress excessive volume expansion of Si-based electrodes.

Evolution of Electrodes During Cycling
SEM imaging was used to explore the surface morphology evolution of nano-Si electrodes with varied binders before and after 50 cycles.As shown in Figure 4a,b, for the pristine nano-Si electrode, AOB binder renders more uniformly dispersed electrode particles than PAA binder, mainly due to the superior wettability of AOB binder to nano-Si.In sharp contrast with AOB binder, PAB-W binder incurs large cluster and obvious cracks on the pristine nano-Si electrode surface (Figure S18, Supporting Information), which is highly correlated with its clusters with irregular shape, size and distribution area.After 50 cycles, surface SEM images show more cracks on PAA-based nano-Si electrodes, compared with the AOB binder-based one (Figure 4c,d).Furthermore, cross-sectional ion milling-scanning electron microscopy (IM-SEM) analyses present the thickness variation of nano-Si electrodes with different binders before and after 50 cycles (Figure 4e-h).The AOB binder-based nano-Si electrode shows a thickness enhancement of 22% (18→22 μm), much smaller than that (50%, 20→30 μm) of PAA counterparts.Notably, there were several cracks in the cycled PAA-based electrode, in sharp contrast to the well integrity of the one with AOB binder.In addition, the thickness change of electrode during the first lithiation and delithiation process was observed through the in situ optical microscopy (OM) in real time.It is evident that the PAA-based nano-Si electrode thickens severely after first lithiation (Movie S1, Supporting Information).In sharp contrast, AOB binder can better suppress electrode expansion (Movie S2, Supporting Information).For understanding the effect of varied binders on electrode evolution during cycling, finite element simulation was carried out to investigate the contact stress on the surface of Si particles at the phase of Li 15 Si 4 (Figure 4i-k and S19, Supporting Information).It can be seen that the von Mises stress between adjacent Si particles in the presence of PAA binder is much higher than that (≈2000 vs ≈800 MPa) of AOB binder.These results demonstrate that the AOB binder can better buffer the volume stress and thus maintain the integrity of Si-based anode.This is mainly ascribed to its improved mechanical properties benefited by the rational hierarchical structure design.
Decreased nano-Si electrode expansion in the presence of AOB binder helps to maintain solid electrolyte interphase (SEI) stability.This can be reflected by surface chemical components of cycled nano-Si electrodes characterized through X-ray photoelectron spectroscopy (XPS) measurements.In the XPS C1s spectra, one of the most noticeable differences is the intensity of the peak at 289.6 eV corresponding to electrolyte decomposition side products ROCO 2 Li /Li 2 CO 3 (Figure S20a,b, Supporting Information); [45] the AOB binder based nano-Si electrode shows a weaker signal intensity ratio than that of the one with PAA binder, demonstrating that less electrolyte decomposition occurs at the interface of the cycled AOB binder based nano-Si electrode.Moreover, in the F 1s spectra, there are three typical peaks, LiF (685 eV), Li x PO y F z (687 eV), and Li x PF y (689 eV) (Figure S20c,d, Supporting Information). [47,48]Apparently, the nano-Si electrode with AOB binder shows higher LiF, which is a crucial component of SEI offering preferable mechanical stability.The N 1s spectrum of the AOB-based anode surface after 50 cycles show a strong N-Li peak (Figure S21, Supporting Information), corresponding to lithiation of tetrazole motifs, which could be beneficial to transport lithium ions. [36,37]This finding proves that AOB binder tends to participate in SEI formation.To elucidate the mechanism behind this, density functional theory (DFT) calculations were performed.As shown in Figure S22, Supporting Information, AOB binder has a lowerlying LUMO level (−0.97 eV) than those of PAA, PPB, and carbonate solvents (i.e., DMC and EC).This finding demonstrates that AOB binder would be subjected to be reduced prior to the electrolyte solvents and participate in the formation of polymerreinforced SEI; such a function of AOB binder toughens SEI and effectively decreases electrolyte decomposition, beneficial to electrochemical performance of Si-based electrode.In addition, the evolution of H 2 and CO 2 was analyzed through the online DEMS system.As shown in Figure S23, Supporting Information, the amount of H 2 and CO 2 generated in the nano-Si/Li half-cell with AOB binder is relatively lower than the PAA counterpart.The main reason for this is that AOB binder inhibits excessive expansion of nano-Si electrodes and reduces decomposition side reactions of electrolyte at electrode/electrolyte interface.

Si-Based Electrode Performance Evaluations
As plotted in Figure S24, Supporting Information, the electrochemical stability of nano-Si/Li half-cells with AOB binder was measured via cyclic voltammetry (CV) analysis at a scan rate of 0.5 mV s −1 .Two broad peaks around ≈0.37 and ≈0.51 V correspond to the delithiation process of Li-Si phases. [49]Moreover, a reduction peak at ≈0.19 V appears, assigned to the reversible lithiation process of amorphous Si domains to Li-Si phase.The intensity of these peaks increases with cycling, attributed to the formation of ion-conducting SEI, and more electrolyte permeation into the electrode facilitating ionic transport (deduced via battery cycling test below).These results confirm the high electrochemical stability of AOB binder.To verify the utility of AOB binder, Si-based electrode assembled cell performance was evaluated.By measuring nano-Si/Li half battery performance, the AOB binder with a PAA/PPB weight ratio of 3:1 was selected as the optimal component ratio for further cell performance evaluations (Figure S25, Supporting Information).Noting that compared with PAB-N and PAB-W, AOB binder shows improved battery cycle performance attributable to its enhanced mechanical and adhesion abilities; this clearly proves hierarchical structure design significance of AOB binder.
In coin-type half-cells, AOB binder renders much superior electrochemical performance to those of the PAA binder (Figure 5a-d).Specifically, the AOB binder based nano-Si electrodes can retain a delithiation capacity of 1617 mAh g −1 in halfcells after 1000 cycles at 0.4 C under 0.005-1.5V (Figure 5a).The average Coulombic efficiency (CE) of the AOB and PAA binders in this half-cell are 99.7% and 98.9%, respectively.Higher CE represent fewer electrolyte decomposition side reactions in the presence of AOB binder, mainly ascribed to that its superior mechanical and adhesive properties can maintain electrode interface stability.Additionally, the AOB binder-based nano-Si electrode shows good rate performance (Figure 5b), achieving a delithiation capacity of 1350 mAh g −1 at 2 C, evidently exceeding the PAA-based one (980 mAh g −1 ).[38][39] Application superiority of AOB binder was further evidenced by stable cycling of commercial-level S600 electrode with limited binder dosage (7 wt% of the whole electrode film weight).As shown in Figure 5c, S600 electrodes based on AOB binder deliver an initial delithiation capacity of 650 mAh g −1 at 0.1 C and of 539 mAh g −1 at 0.5 C, and then stabilize at 464 mAh g −1 in half-cells after 300 cycles at 0.5 C, much superior to that (230 mAh g −1 after 300 cycles at 0.5 C) of the PAA counterpart.At various Crates from 0.1 C to 2 C, AOB binder-based S600 electrodes also show apparently enhanced delithiation capacity when compared with PAA (Figure 5d). Figure S27, Supporting Information.exhibits the cyclability results of μm-Si electrodes with different binders.The AOB binder based μm-Si electrode shows improved cycling stability with a discharge capacity of 2767 mAh g −1 after 100 cycles at 0.2 C, compared with that of the one with PAA binder (1800 mAh g −1 ).Moreover, typical galvanostatic voltage profiles of nano-Si, S600, and μm-Si electrodes in half-cells present that AOB binder renders slighter battery polarization than PAA binder upon long cycling (Figures S28-S30, Supporting Information); this can be supported by lower impedance evolution (Figure S31 and Table S1, Supporting Information), which is highly correlated with stable SEI enabled by AOB binder.assembled coin-type half-cells under 0.005-1.5V. c) Long-term cycling performance at 0.5 C and d) rate capability of various binder-based S600 electrodes (a mass loading of 4 mg cm −2 ) in coin-type half-cells under 0.005-1.5V. e) Cycling performance of NCM811/AOB binder based S600 coin-type full cell (a NCM811 loading of 16 mg cm −2 ) under 3-4.2V and then under 2.7-4.25 V at 0.2 C. f) Cycling performance of NCM811/AOB binder based nano-Si coin-type full cells (a NCM811 loading of 16 mg cm −2 ) under 3-4.2[52][53][54][55][56][57][58][59][60][61][62][63][64][65] More impressively, in practical LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811)-based full cells, the as-developed binder enables good cyclabilities (Figure 5e-j).The AOB binder-based NCM811/S600 coin-type full cell (a NCM811 mass loading of 16 mg cm −2 , a S600 loading of 5.72 mg cm −2 ) displays an areal capacity of 3.01 mAh cm −2 at the initial 3 cycles of 0.1 C followed by stable cycling at 0.2 C in the voltage range of 3.0-4.2V (Figure 5e and S32, Supporting Information).After 100 cycles at 0.2 C, the full cell can still maintain an areal capacity of 2.44 mAh cm −2 .Then, by extending the voltage range from 2.7 to 4.25 V, the battery can still stably cycle in the following cycle.As for NCM811/nano-Si coin-type full cells (a NCM811 loading of 16 mg cm −2 ; a nano-Si loading of 1.1 mg cm −2 ), a capacity retention of 80% after 200 cycles is achieved (Figure 5f and S33, Supporting Information).In order to further prove the practicability of AOB binder, the practical 3.3 Ah soft package cell (Figure 5g) was prepared by pairing the AOB binder-based S600 anode with a commercial-level NCM811 cathode (see detailed information in Table S2, Supporting Information).The full cells were activated for 3 cycles at 0.1 C followed by charging and discharging at 1C (1C = 600 mA g −1 ) between 2.8 and 4.25 V.When cycling at 1 C, the NCM811/S600 full cell delivers an initial capacity of 3.22 Ah and still preserves a discharge capacity of 2.92 Ah after 700 cycles (capacity loss: 0.013% per cycle, Figure 5g and Figure S34, Supporting Information).Undoubtedly, electrochemical performance evaluations fully confirm that AOB binder is very valuable in the practical implementation of Si-based batteries.
Furthermore, we compared the Si-based electrode cycle performance of previously reported typical binders with AOB binder in half-cells and soft package full cells (Figure 5h,i and Table S3, Supporting Information).Obviously, AOB binder has the leading cycling performance of Si-based electrodes, to the best of our knowledge.These results clearly verify the hierarchical structure design rationality of the AOB binder.

Conclusion
We have developed a spidroin-inspired hierarchical structure binder to unlock the stiff challenges faced by Si-based electrode.In the structure of AOB binder, hydrophobic PPB polymer condensates with a minor part of PAA in the mixture solution to form crystalline regions within sub-micrometer-sized irregular spherical domains, functioning as the rigid node of binder mimicking -sheet of spidroin; while amorphous PAA imitates the -helix structure of spidroin, and builds an ionic bonding network structure with PPB, similar to the interactions between NTD and CTD in the primary structure of spidroin.Benefited from such bionics design, the as-developed AOB binder has the two following competitive edges: 1) High mechanical and adhesive capabilities were achieved via the hierarchical structure design including the dynamically reversible ionic bonding network.These factors help achieve energy dissipation and thus can accommodate enormous electrode volume deformation.2) Low-lying LUMO leads to preferential reduction, which helps form a polymer-reinforced SEI layer.As a result, AOB binder endows nano-Si and S600 electrodes with improved electrochemical performance to traditional PAA binder.Impressively, in 3.3 Ah soft package cells assembled with S600 anode and NCM811 cathode, a discharge specific ca-pacity of 2.92 Ah after 700 cycles with a capacity loss of 0.013%/cycle can be maintained, showing good potential for practical implementation of Si-based electrodes that meet the requirement of commercial-level high-energy-density LIBs.This work marks a milestone in achieving advanced silicon-based electrodes toward high-energy lithium-battery applications.
Preparation of PPB: Typically, the tetrazole-based polymer PPB was obtained by dissolving 2.0 g PAN powder and 0.08 g poly(ethylene glycol) bisazide in a solution comprising 9.4 g DMF and 1.9 g acetone in an oil bath under 140 °C for 4 h.By adding the mixture into ethanol solution dropwise, the precipitate was formed.The crude product was collected through centrifugation for 20 min at a speed of 4000 r min −1 .Then by dissolving the crude product in DMF again and repeating this dissolution/precipitation process three times, the final precipitate was dried in an oven at 60 °C for 24 h to give the pure polymer PPB.
Preparation of AOB, PAB-N, and PAB-W Binders: AOB binder was prepared by an aqueous-oil binary mixing process.Specifically, AOB binder solution (5 wt%) was obtained by mixing preprepared aqueous PAA solution and oily NMP solution containing PPB in a given ratio.While PPB and PAA were mixed in sole water and sole NMP to form 5 wt% PAB-W binder and PAB-N binder solutions, respectively.
Preparation of Electrodes: The Si-based-anode slurry contains active material (nano-Si and μm-Si), Super P, and binder (PAA, AOB binder) with a mass ratio of 8:1:1, which was blade-coated on a Cu current collector.After drying at 60 °C for 2 h and 120 °C for 12 h under vacuum, the anodes were cut into wafers with a 14 mm diameter.The S600 anode slurry for coin-type cells contains corresponding active material, Super P, and AOB binder with a mass ratio of 90:3:7, which was blade-coated on a Cu current collector and dried under vacuum.
Sample Characterization: 1 H-NMR and 13 C-NMR spectroscopies were performed to analyze the polymer structure on a Varian 600 MHz Gemini, and the DMSO-d6 was used as the solvent.In addition, the Fourier transform infrared (FT-IR, Bruker VERTEX 70) spectrum was also used to characterize the structure of different polymers.Temperature-dependent IR spectroscopy was measured on Thermo iS10 and the sample was prepared by spin-coating the AOB binder solution on KBr, while the corresponding data was collected upon heating with the temperature ranging from 30 to 100 °C.Nano-indentation test was carried out using Bruker Hysitron TI980 nanoindentation system with normal Berkovich indenter.The tensile and peeling tests of AOB binder films were analyzed by using a universal testing machine (MTS, E43).For tensile testing analysis, the polymer films were prepared by drying the polymer solutions at 120 °C overnight and then cut into 1 cm × 5 cm.Using the same MTS, a 180°peeling test was conducted to evaluate the peel strength of each electrode.The electrode was cut into a rectangular shape with 2 cm × 7 cm.The active material side was adhered to a wood bar by using 3M double-sided adhesive tapes, and meanwhile, the collector side was adhered by using traditional 3M tapes.The peel strength value would be output directly to a computer.The swelling of the binder film in liquid electrolyte was examined gravimetrically by soaking the binder film into the liquid electrolyte before and after 24 h.Contact angles of different binder solutions on monocrystalline Si were measured by using DSA100.The in situ optical microscopy (Cossim CMY-400Z optical microscope) was used to observe the cross-section of the Si electrode with different binders in real time in order to study the electrode deformation process during battery cycling.Top-viewed and cross-sectional images of the electrodes were obtained by using scanning electron microscopy (SEM, Hitachi S-4800) and cross-sectional ion milling scanning electron microscopy (IM-SEM, Hitachi IM4000PLUS), respectively.X-ray photoelectron spectroscopy (XPS) analysis was carried out using Thermo Fisher ESCALAB XI+ spectrometer equipped with an Al K source.
Cell Assembly and Electrochemical Measurements: Coin-cells (S600/Li, nano-Si/Li, μm-Si/Li, NCM811/nano-Si, and NCM811/S600) were assembled in a 2032-coin cell device within a glove box containing less than 0.1 ppm water and O 2 , and measured on Land battery test system (Land CT2001A, Wuhan Land Electronic Co. Ltd., China).The microporous polypropylene film (Celgard 2500) was performed as the separator.Commercially available 1.0 m LiPF 6 in EC/DMC/FEC (4.5/4.5/1,v/v/v) was used as the electrolyte.The nano-Si/Li and S600/Li half cells were measured under 0.005-1.5V.The μm-Si/Li half cells were measured under 0.005-1.0V. To assemble NCM811/S600 and NCM811/nano-Si full-cells, the n/p ratio was 1.15.The Si-based anodes were precharged and predischarged for three cycles at a current density of 0.1 C. Soft package cells' fabrication was conducted in a dry room with a dew point of −40 °C.The details of the soft package cell were shown in Table S3, Supporting Information.Soft package cells used commercial NCM811 cathode (a mass loading of 16 mg cm −2 on each side) and S600 anode (a mass loading of 5.5 mg cm −2 on each side), the n/p ratio was 1.15 and the electrolyte employed commercially available 1 m LiPF 6 in EC/DMC/FEC (4.5/4.5/1,v/v/v) with a dosage of 2.7 g Ah −1 .Cyclic voltammetry (CV) measurements for nano-Si anode were conducted on cells using BioLogic VSP-300 conducted at a scan rate of 0.5 mV s −1 at room temperature.Electrochemical impedance spectroscopy (EIS) measurements in the frequency range from 100 mHz to 7 MHz were carried out on BioLogic VSP-300 to compare the impedance.The ionic conductivity of binder films plasticized by liquid electrolyte was calculated according to the following formula: where  denotes the ionic conductivity, L represents the thickness of the binder film, R stands for the bulk resistance, and S is the surface area of the binder film.
Online Differential Electrochemical Mass Spectrometry Analysis: The evolution of H 2 and CO 2 was analyzed through the online DEMS system.According to the previous work, [66] online differential electrochemical mass spectrometry (DEMS, EL-CELL GmbH, Germany) analysis was used to monitor the gas evolution in the nano-Si (a mass loading of 1 mg cm −2 )/Li half-cells during cycling under 0.005-1.5V at 0.2 C and 25 °C.The electrolyte 1 m LiPF 6 in EC/DMC/FEC and GF/D separators were used in these half-cells.In situ DEMS cells were assembled in an argon-filled glove box that contained <0.1 ppm of both oxygen and H 2 O.
Calculations: DFT calculations were conducted using the Gaussian09 software.All the molecules were pre-optimized at the B3LYP levels of theory with the 6-311+G(d,p) basis set.The single point energy was calculated based on the same function with 6-311+G(d,p).
The bonding energies (ΔE) were calculated using the following formula: where E A+B is the total energy of the system, E A and E B are the energy of the isolated molecules.
In addition, the finite element analysis (FEA) simulation was conducted by COMSOL Multiphysics 5.6.The Si-based anode was simplified as a typical two-phase composite material with Si particles embedding in the binder.The stress distribution induced by the Li insertion process follows   2 L t 2 = ∇ ⋅ (C :  el ) , where C = C(E,v) is the fourth-order elastic tensor, E where L is the displacement,  h is the expansion coefficients, M is the molar mass, c ef is the reference concentration, ∇ is the divergence, and Δ is the Laplace operator.In the static stress analysis, the mechanical properties of different binders were obtained from experimental results.The corresponding parameters are shown in Table 1.

Figure 1 .
Figure 1.a-d) Schematic illustrations of: a) the hierarchical structure of spidroin, b) the hierarchical structure of AOB binder, and c,d) Si-based electrode evolution using the AOB binder (c) and the traditional binder (d) during long cycling.

Figure 2 .
Figure 2. a) Schematic diagrams of PPB synthesis, PAA structure, and AOB binder constitution.b) FTIR spectra of PAA, PPB, and AOB binder.c) Temperature-dependent FTIR spectrum of AOB binder with a wavenumber range from 1800 to 1150 cm −1 .d) Bonding types and bond energies calculated by DFT simulations.e) The condensed structure formation of AOB binder via an aqueous-oil binary mixing process.

Figure 3 .
Figure 3. a) Stress-strain curves of PAA and AOB binder membranes.b) Stress of the as-prepared electrolyte-soaked AOB binder films in the successive loading-unloading test.c) Reduced modulus and hardness of nano-Si electrodes with varied binders obtained from nanoindentation tests.d) Peeling force-displacement curves of nano-Si electrodes with PAA and AOB binders.

Figure 4 .
Figure 4. a-d) Top-viewed SEM images of the pristine nano-Si electrodes (a,b) and the ones after 50 cycles with PAA and AOB binders (c,d).e-h) Crosssectional IM-SEM images of nano-Si electrodes before (e,f) and after (g,h) 50 cycles with PAA binder and AOB binder.i) The regular symmetry finite element model between adjacent Si particles.j,k) Contour plots of von Mises stress between adjacent nano-Si particles in the presence of PAA binder (j) and AOB binder (k) at Li 15 Si 4 phase in regular symmetry finite element model.

Table 1 .
Parameters of different binders for FEA simulation.Young's modulus, and v is the Poisson's ratio; ɛ el is the elastic strain which can be described by the following equation is