Bioinspired Nanocomposites with Self‐Adaptive Stress Dispersion for Super‐Foldable Electrodes

Abstract In flexible electronics, appropriate inlaid structures for stress dispersion to avoid excessive deformation that can break chemical bonds are lacking, which greatly hinders the fabrication of super‐foldable composite materials capable of sustaining numerous times of true‐folding. Here, mimicking the microstructures of both cuit cocoon possessing super‐flexible property and Mimosa leaf featuring reversible scatheless folding, super‐foldable C‐web/FeOOH‐nanocone (SFCFe) conductive nanocomposites are prepared, which display cone‐arrays on fiber structures similar to Mimosa leaf, as well as non‐crosslinked junctions, slidable nanofibers, separable layers, and compressible network like cuit cocoon. Remarkably, the SFCFe can undergo over 100 000 times of repeated true‐folding without structural damage or electrical conductivity degradation. The mechanism underlying this super‐foldable performance is further investigated by real‐time scanning electron microscopy folding characterization and finite‐element simulations. The results indicate its self‐adaptive stress‐dispersion mechanism originating from multilevel biomimetic structures. Notably, the SFCFe demonstrates its prospect as a super‐foldable anode electrode for aqueous batteries, which shows not only high capacities and satisfactory cycling stability, but also completely coincident cyclic voltammetry and galvanostatic charge–discharge curves throughout the 100 000 times of true‐folding. This work reports a mechanical design considering the self‐adaptive stress dispersion mechanism, which can realize a scatheless super‐foldable electrode for soft‐matter electronics.


Supporting Information
Bioinspired Nanocomposites with Self-adaptive Stress Dispersion for Super-Foldable Electrodes  Table S1 Notes S1 and S2 Caption for Videos S1

Other Supplementary Materials for this manuscript include the following:
Videos S1

Experimental section
Preparation of the carbon web substrates. The super-flexible carbon web substrates were prepared by improved electrospinning /carbonization method. First, a certain polyacrylonitrile (PAN) (Mw=150,000) powder was dissolved in DMF to form a homogeneous solution with 10% concentration. Then, the above solution was put into a 20 mL plastic syringe which was connected to a stainless-steel needle with 0.6 mm internal diameter. The electrospinning of PAN was operated under the electrospinning voltages of 12.5 kV, the solution flow rate of 0.5 mL h -1 , and collecting distance of 16 cm, and the temperature and humidity were respectively controlled at ~25 ℃ and ~35% RH. Afterwards, the electrospun PAN film was peeled off from the Al foil collector, and dried at 60 ℃ in an oven before the following carbonization transformation process. During the carbonization process in a tube furnace, the PAN film underwent gradient temperature heating in air for stabilizing.
It was firstly heated at 105 ℃ for 30 min with a heating rate of 2 ℃ min -1 , and then heated at 160 ℃ for 30 min with a rate of 1 ℃ min -1 , and further heated at 270 ℃ for 2 h with a rate of 1 ℃ min -1 .
Subsequently, the stabilized products were carbonized at 800 ℃ for 2 h in N 2 atmosphere with a heating rate of 2 ℃ min -1 to obtain free-standing carbon substrates.

Preparation of super-foldable C-web/FeOOH-nanocone (SFCFe).
In a typical synthesis, the above carbon substrates was firstly pretreated by soaking in the 50 mL aqueous solution containing 20 mL HNO 3 for 12h, which favors inward diffusion of solution into carbon web and increases the interfacial combination between nanofibers and functional matters. Then 0.1 g of FeCl 3 ·6H 2 O, 0.25 g of Na 2 SO 4 and 0.2 g of CO(NH 2 ) 2 were dissolved in 50 mL of distilled water to form homogenous reaction solution. Afterwards, five pieces of the pretreated carbon substrates were added into the reaction solution. The solution was heated at 50°C for 6 hours. After cooled to room temperature naturally, the products were separated, washed and dried at 60 °C overnight to obtain super-foldable SFCFe anode materials. For the inlaid quantity control of FeOOH nanocones, the reaction times are set as 3h and 9h with other synthesis conditions are the same as above. To explore the role of Na 2 SO 4 , the reaction solution for depositing FeOOH was prepared without Na 2 SO 4 , and other reaction conditions were the same as the above SFCFe preparation process.  Mechanical simulations. Finite element method was applied for the mechanical simulations of material folding process from three different structural levels. At 1D nanofiber level, three composite nanofiber models of different FeOOH morphologies (nanocones, nanopillars, and densely wrapped structures) loaded on carbon nanofiber were constructed. And the stress distributions of the above three fiber models were calculated by three-point bending simulations to the same bending degree, and the maximum stress values are obtained from the simulated results. At 2D layer level, we built two representative layer models, which are crosslinked layer and non-crosslinked layer, and calculated their stress distributions during bending. In this process, the carbon/FeOOH composite nanofibers were equivalently simplified as simple nanofibers. At 3D body level, two kinds of 3D folding structures, the super-foldable stereostructure and brittle stereostructure, were established and simulated Electrochemical measurements. Electrochemical properties were measured on a CHI660E electrochemical workstation (Shanghai Chenhua Instruments Co.). The electrochemical performance of the freestanding C-web/FeOOH-nanocones materials was evaluated in a three-electrode system in 6M KOH aqueous electrolyte at room temperature, and they were directly used as working electrode and their a platinum foil and an Ag/AgCl electrode acted as the counter electrode and reference electrode, respectively. For the investigation of the electrochemical property during cyclic folding, the electrode materials were taken down from electrode holder, and folded for different times on the folding machine, and placed back on electrode holder for tests. The specific capacities (C s , mAh/g) were calculated from galvanostatic charge-discharge (GCD) curves by the formula of C s = IΔt/m, where I is the constant current (mA), m is the mass (g) of electrode material, and Δt is the discharge time (h) during the discharge process.

Supplementary Note 1
Metals are made up of metallic bonds that are non-directional, thus most of them are flexible and can be bent or even folded for a certain times. After all, metallic bonds belong to chemical bonds which are short-range force, so they can't resist large bending deformation. As a result, when they undergo 180° true-folding, their structures will be damaged. Eventually, material fracture will occur due to cumulative damage caused by repeated folding. Conductive polymers consist of conjugated Π bonds, which have the nature of double bonds and thus are more rigid than single bonds, so they can hardly bear large bending deformation, not to mention repeated true-folding. For the abundant carbon materials, the flexible conductive ones mainly include graphene, carbon nanotubes and partially graphited carbon. Monolayer graphene is an ultrathin plane structure and shows some flexibility. While its constructions of sp2 hybrid conjugate Π bonds have the property of double bonds, so single-layered graphene also can't sustain repeated true-folding. With regard to a singlewalled carbon nanotube, it is actually equivalent to the rolled monolayer graphene, and of course can't bear numerously repeated true-folding. Thus it can be concluded that none of the intrinsic conductive materials can sustain numerous repeated true-folding when they directly cope with folding using chemical bonds due to the limit of short-range force of chemical bonds.

Supplementary Note 2
As a source of inspiration for innovation, the cuit silkworm cocoon with super-flexible feature is investigated in detail from processing to structures. The starting materials for preparing cuit cocoon, raw cocoon, are actually stiff and difficult to be folded, but they become flexible and foldable just after simply boiling in alkaline aqueous (reeling cocoon process). The SEM observation shows that the completely folded cuit cocoon can form a 3D "ε"-like structure at the crease due to redistribution of space, which means some parts are compressed and some are loosened around the crease. The "ε"-like structures contains bulged layers and two dispersed arcs, and those structural changes can effectively disperse stress through avoiding the formation of 0° folding angle. To unraveling the mysteries of above property variation, the material constructions before and after the reeling process are investigated. Results indicate the cross-linked network structures of raw cocoon, whose fibers are wrapped by the sericin and glued tightly at their intersections. While for cuit cocoon, their intersections are completely unfastened, and the fibers becomes porous and fluffy. The above analysis give us new inspiration to obtain similar structures. The silkworm constructs cocoon by spinning, and it reminds us of the polymer nanofibers electrospinning, which can produce abundant transformable hierarchical structures, such as adjustable pores, detachable intersections, separable layers, and nimble nanofibers. However, the electrospinning polymer with satisfying transformation can only be obtained by parameter optimization, just like that the structure of raw cocoon requires the control of silkworm spinning. Even though obtaining such materials, it is a pity they are still not conductive. At that aspect, the precisely controlled carbonization process provides an efficient way to simultaneously realize material conductivity and structural maintaining.    Result shows that the C-web substrate can hang on a hair through electrostatic adherence, while the hair has no bending deformantion, which indicates its ultralight feature.

Figure S6. SEM images of super-flexible C-web substrate at top view (a) and cross section (b).
Results indicate the C-web substrate has network structures stacked by layers of carbon nanofibers. Those nanofibers are straight and smooth with large aspect ratio. Notably, they are stacked in order and have no adhesion between each other.

Figure S7. TEM and HRTEM images of super-flexible C-web substrate.
Results show that the nanofiber is porous. And the interplanar spacing of 0.34 nm can be indexed to (002) crystal face of graphite, indicating its partially graphitized structure. Such porous and partially graphitized structures endow the carbon nanofibers with both high flexibility and good conductivity.  Results indicate that the raw carbon substrate is hydrophobic with a contact angle of 119.6°, while the pretreated carbon substrate becomes hydrophilic with a contact angle of 25.3°, which may be due to the introduction of oxygen-containing functional groups on the fiber surface during pretreated process. The hydrophilic property makes the FeOOH pass through the mesh and grow on each fiber inside the film.           It shows that the products with lower loading amount and density of FeOOH also have super-foldable property, and they don't have microstructural damage or detachment.  The SFCFe has no structure damage during the severe folding process and quickly recovers to its initial status once unfolded, indicating their outstanding undamaged foldability. Results manifest that its unfolding process is just opposite to the folding process, and eventually, the SFCFe can nearly recover to its initial state. It is reasonable that the unfolded structures are not completely the same as their initial states, because the unfolded structures have the lowest energy and most stable state through nanofibers' sliding and adaptive adjustment in folding process. By calculation, the outer plane only has a largest elongation of 2.3% when completely folded. Generally, for unfoldable materials, only one inner acute angle forms during folding, which results in the materials damage until fracture due to too large stress. By contrast, our SFCFe will generate two dispersed arcs to avoid the formation of inner acute angle to decrease the stress and realize superfolable property.   Result shows that without the C-web substrate, the FeOOH nanocones will assemble into spheres, which may be not favorable for their full use for electrochemical reactions. Video S1.
Strict standard of repeated true-folding test using folding machine. This video shows the strict repeated true-folding process: the two folded parts of the sample are folded to 180° and completely cling.