Highly robust zinc metal anode directed by organic–inorganic synergistic interfaces for wearable aqueous zinc battery

Flexible aqueous zinc batteries (FAZBs) with high safety and environmental friendliness are promising smart power sources for smart wearable electronics. However, the bare zinc anode usually suffers from damnable dendrite growth and rampant side reaction on the surface, greatly impeding practical applications in FAZBs. Herein, a composite polymer interface layer is artificially self‐assembled on the surface of the zinc anode by graft‐modified fluorinated monomer (polyacrylic acid‐2‐(Trifluoromethyl)propenoic acid, PAA‐TFPA), on which an organic–inorganic hybrid (PAA‐Zn/ZnF2) solid electrolyte interface (SEI) with excellent ionic conductivity is formed by interacting with Zn2+. Both the pouch cell and fiber zinc anode exhibit excellent plating/stripping reversibility after protecting by this organic–inorganic SEI, which can be stably cycled more than 3000 h in symmetric Zn||Zn cells or 550 h in fiber Zn||Zn cells. Additionally, this interface layer preserves zinc anode with excellent mechanical durability under various mechanical deformation (stably working for another 1200 h after bending 100 h). The corresponding PAA‐Zn/ZnF2@Zn||MnO2 full cell displays an ultra‐long life span (79% capacity retention after 3000 cycles) and mechanical robustness (85% of the initial capacity for another 3000 cycles after bending 100 times). More importantly, the as‐assembled cells can easily power smart wearable devices to monitor the user's health condition.

The emerging smart wearable electronics with flexible and wearable electronic technology are great prospects for application in aerospace, smart medical monitor, implantable medical devices, etc. [1][2][3] The stability and security of the power sources are crucial for wearable electronic devices, which prompt them to seek a flexible power source without compromising the overall adaptability and wearability. 4,5Recently, aqueous quasi-solid energy storage devices based on hydrogel electrolytes have attracted intense attention due to their high theoretical capacity, high cost-efficiency, and optimal security.Among them, flexible aqueous zinc batteries (FAZBs) based on zinc metal anodes are considered one of the most promising candidates for driving wearable electronic devices because of their low toxicity, environmental friendliness, high energy density, and facilely assembled process.][8] Up to now, many efforts have been devoted to improving the cyclic stability of the zinc anode by inhibiting the dendrite growth, corrosion, and hydrogen evolution reaction (HER) in mild aqueous electrolyte, such as the structural design of anode, artificial protective layer and electrolyte optimization, etc. [9][10][11] Thereinto, artificial solid electrolyte interface (SEI) has been considered as an effective way to protect the zinc anode against them. 12,13In particular, the inorganic passivation layers (i.e., ZnF 2 , ZnS) with high-efficiency zinc ion conductivity have been uniformly coated on the surface of zinc anode by in situ or ex-situ strategies, 14,15 which could have induced uniform deposition of zinc and broaden HER overpotential.For instance, ZnF 2 -based modified layers on the Zn anode surface using electrodeposition and vacuum pyrolysis have effectively inhibited dendrites and interfacial side reactions, extending the life span of the zinc anode to 590 h 16 ; By introducing the weak solvation effect into the electrolyte, the in situ inorganic-rich SEI can significantly improve the interfacial dynamic of the zinc anode by promoting the migration rate of the Zn 2+ . 17However, this kind of rigid inorganic SEI is incompetent to protect the zinc anode from mechanical deformation, as the rigid layer will break during bending or twisting in FAZBs and then the zinc will grow in priority in the crack-forming dendrite.On the contrary, the organic SEI holds sufficient elasticity avoiding the formation of cracks.However, the low mechanical strength is insufficient to suppress zinc dendrite (Figure 1B).Thus the solitary inorganic or organic SEI is inadequate to meet the high demand for electrochemical and mechanical stability as the flexible electrodes.Therefore, it would be an effective strategy to propose a simple method to combine the inorganic and organic SEI into one newly composite layer to improve the cyclic stability and mechanical robustness of metal zinc anodes, which has been proven to be effective in lithium anodes. 18n view of this, here we design a polymer-inorganic SEI (PAA-Zn/ZnF 2 ) on the surface of a zinc anode via fluorinated monomer-modified PAA as the artificial layer.Then an inorganic-organic hybrid PAA-Zn/ZnF 2 SEI is in situ formed on the surface of the zinc anode by interacting with Zn 2+ .This in situ SEI shows favorable mechanical strength and electrochemical performance.As displayed in Figure 1C, the ZnF 2 as a rigid layer efficiently inhibits dendrite growth and by-product formation, and the PAA serves as a flexible layer to prevent the fracture of inorganic ZnF 2 and releases the stresses during bending.Thanks to the enhanced surface architecture, the Zn||Zn symmetric cell exhibits an exceptional cycling performance of more than 3000 h (under 0.5 mA•h/cm 2 , 0.5 mA/cm 2 ).Remarkably, the full cells of PAA-Zn/ZnF 2 @Zn||MnO 2 maintain more than 79% capacity after 3000 cycles at 10 C, which can power the smart wearable devices to continuously detect pulse, temperature, humidity, and pressure signals from the user.This study provides a new insight into FAZBs, which works well not only for FAZBs but probably also for other aqueous flexible batteries.

| Preparation PAA@Zn
A total of 400 mg PAA was dissolved in the mixed solution of 20 mL DI water and 20 mL ethanol via continuous stirring.The zinc foils (1 cm × 3 cm, 50 μm) were immersed into the above solution for 5 min, then taken out and rinsed with ethanol and DI water, and dried in an oven to obtain PAA@Zn.

| Preparation PAA-TFPA@Zn
A total of 400 mg PAA and 200 mg TFPA were dissolved in the mixed solution of 20 mL DI water and 20 mL ethanol via continuous stirring.Then 30 mg ammonium persulfate (APS) was added into the above solution under continuous stirring, which acts as an initiator.The zinc foils (1 cm × 3 cm, 50 μm) were immersed into the above solution for 5 min, then taken out and rinsed with ethanol and DI water, and dried in an oven to obtain PAA-TFPA@Zn.

| Preparation MnO 2
A total of 3.24 mmol MnSO 4 •H 2 O and 12 mmol H 2 SO 4 were dissolved in 360 mL DI water denoted as solution A. 24 mmol KMnO 4 was dissolved in 80 mL of DI water denoted as solution B. Then dropwise added B to A under vigorously stirring for 2 h.And then transfer it into stainless steel autoclave maintaining it at 120 °C for 12 h.After the reaction was completed, the precipitate was rinsed with DI water and absolute ethanol several times under centrifugation, and dried in air at 60 °C overnight.

| Preparation PAM gel
The PAM hydrogel electrolyte was prepared by adding 1.90 g acrylamide monomer into 20 mL electrolyte solution (1 mol/L ZnSO 4 ) and stirring for half an hour.Then, 0.094 g 2-hydroxy-4′-(2-hydroxyethoxy)-2methylphenylacetone (UV-I 2 ) functioning and 4 mg of the N,N′-methylenebis(acrylamide) (MBAA) were added into the above solution under magnetic stirring for 1 h at 25 °C to obtain a transparent solution.After deoxygenation and nitrogen filling, the solution was transferred to the UV light box for photopolymerization for an hour.

| Electrochemical testing
The pouch cells of Zn||Zn, PAA@Zn||PAA@Zn, and PAA-TFPA@Zn||PAA-TFPA@Zn were assembled with sandwich configuration, in which PAM gel (1 mol/L ZnSO 4 ) was used as both the electrolyte and the separator.For the MnO 2 cathode, MnO 2 powder, acetylene black, and polyvinylidene difluoride were mixed in a mass ratio of 7:2:1 to prepare a slurry, and then the slurry was coated on the carbon fiber as the electrode, after that the electrode was dried at 60 °C for 12 h.The collected electrodes are cut into 1 cm × 3 cm.The pouch cells were packaged with plastic wrap.For the fiber cells, the MnO 2 powder was coated on the surface of the conductive carbon fibers as the cathode.And then the PAM hydrogel electrolyte was coated on the surface of the MnO 2 cathode and PAA-Zn/ZnF 2 @Zn fiber anode, respectively.Then, the positive and negative electrodes are placed in a mold, injected with a hydrogel solution, and polymerized by using an ultraviolet lamp.The LAND CT2001A battery test systems were used to test the electrochemical performance of the symmetric cell and Zn||MnO 2 full cell.Electrochemical impedance spectroscopy (EIS) was performed on AUTOLAB electrochemical workstation with a frequency ranging from 0.1 MHz to 10 MHz.The organohydrogel was sandwiched between two stainless steel disks for the measurement.Ionic conductivity is determined by EIS.
The surface of the zinc metal anodes was analyzed by using a HITACHI SU8010 field emission scanning electron microscope (SEM).The component analysis was analyzed by using an X-ray diffractometer (XRD).Atomic force microscopy (AFM) was conducted on a Bruker Dimension ICON.Young's modulus was determined by using the quantitative nanomechanical mapping mode in the AFM.X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI) was used to analyze the surface compositions of samples.Thermogravimetry (TG) was tested by TGA5500.

| Density functional theory (DFT) computations
The highest occupied molecular orbital (HOMO) and lower unoccupied molecular orbital (LUMO) calculations of the solvent/anion and cation-solvent/anion complex were performed using Gaussview.The adsorption energies were performed by Vienna Ab initio Simulation Package.

| RESULTS AND DISCUSSION
As shown in Figure 2A, a large amount of carboxyl group originating from the PAA and TFPA will ionize in the solution, and then react with the zinc forming the Zn-PAA/ TFPA layer adsorbing on the surface of zinc. 19Furthermore, the C-F bonds of TFPA will be broken when the zinc anode works at a lower potential, and then react with Zn 2+ .After activation for several cycles, an in situ PAA-Zn/ZnF 2 polymer-inorganic hybrid SEI is obtained.][22] SEM images are gathered to observe the surface morphologies of SEI based on different components.As shown in Figure 2B and Figure S1A, the PAA layer tightly covers the metal zinc surface with some visible cracks.This uneven interface will lead to a nonuniform surface electric field during zinc deposition, resulting in a side reaction and dendrite growth at the cracks after cycling (Figure 2C).In comparison, PAA/TFPA forms a uniform interface layer without cracks on the metal zinc surface (Figure 2D F I G U R E 2 (A) Schematic illustration of organic-inorganic hybrid SEI formed.SEM images of (B) PAA@Zn; (C) PAA@Zn after 20 cycles; (D) PAA-TFPA@Zn; (E) PAA-TFPA@Zn after 20 cycles.(F) The C 1s spectra and (G) F 1s spectra of PAA-TFPA after cycling.SEM, scanning electron microscope.and Figure S1B), which is probably due to the fluorinated group in PAA-TFPA acting as a surfactant to induce the polymer film uniform deposition.Moreover, as depicted in Figure 2E, a uniform and dense interface layer can still be retained on the metal zinc surface after 20 cycles implying that this hybrid layer is tightly adhesive on the zinc surface and greatly suppresses the dendrite and side reactions.
The XPS is used to determine the components of the interface layer of the Zn anode after cycling.In the C 1s spectra (Figure S2 and Figure 2F), the binding energies at 284.8 eV, 285.4,286.2, and 289.7 eV can be assigned to CH 2 , CH-COOH, CH-OH, and COOH, manifesting that this PAA polymer layer is successfully constructed on the surface of the zinc anode. 23A new peak corresponding to ZnCO 3 appears at 290.5 eV after etching 30 s, which is probably due to the decomposition of TFPA. 24The F 1s spectrum of PAA-TFPA@Zn (Figure 2G) after cycling shows that the peaks at binding energies of 688.5 and 685.2 eV correspond to ZnF 2 and CF 3 .The signal of CF 3 disappears and the peak intensities of the ZnF 2 gradually increase as the etching time increases from 0 to 90 s, indicating a ZnF 2 layer is formed in the polymer layer due to the decomposition of TFPA that matches well with the results from C 1s spectra. 25dditionally, previous studies have demonstrated that ZnF 2 is a favorable Zn 2+ conductor, effectively promoting the uniform deposition of Zn 2+ .The XPS results confirm that the PAA-TFPA on the zinc anode surface can be in situ evolved into organic-inorganic hybrid SEI after several cycles in the electrolyte.
DFT calculations are conducted to investigate the evolution mechanism of the hybrid SEI.As recorded in Figure 3A, the LUMO of Zn 2+ -TFPA (−2.2 eV) is much lower than those of Zn 2+ -H 2 O (−0.75 eV) and Zn 2+ -PAA (−1.72 eV).This result indicates that the Zn 2+ -TFPA will be reduced preferentially to produce a ZnF 2 inorganic layer, which is consistent with the results of XPS.As observed in Figure 3B, the binding energy between Zn 2+ and TFPA − (−11.56 eV) is significantly stronger than that of Zn 2+ coordinated with H 2 O (−1.5 eV), suggesting that the TFPA is prior to absorbing the Zn 2+ from solvation configuration with H 2 O.These results imply that the Zn 2+ will prefer to absorb onto TFPA, and is then reduced and decomposed into the ZnF 2 layer, which is further confirmed by the electrolyte affinity test of the zinc anode through the contact angle measurement.As illustrated in Figure S3A,B, the contact angle of the PAA@Zn anode is reduced from 69.7°to 49.9°after the highly hydrophilic PAA interface in situ formed.In comparison, the contact angle of PAA-Zn/ ZnF 2 @Zn is slightly increased to 58.6°(Figure S3C), which is mainly owing to the formation of a hydrophobic ZnF 2 layer. 26The impact of the PAA-Zn/ZnF 2 on the hydrogen evolution side-reaction was evaluated by the LSV measurement for the PAA-Zn/ZnF 2 @Zn and pure zinc in the 1 mol/ L Na 2 SO 4 solution.As shown in Figure S4, the PAA-Zn/ ZnF 2 @Zn exhibits a more negative HER potential, suggesting the introduction of PAA-Zn/ZnF 2 significantly inhibits the HER side-reaction of zinc anode.
Generally, a lower Zn 2+ transference number (t Zn2+ ) means a larger Zn 2+ concentration gradient on the electrode/ electrolyte interface, leading to a strong interfacial electric field and aggravating the dendrite propagation.Herein, the t Zn2+ is evaluated in the symmetrical Zn cell via chronoamperometry and EIS tests (Figure S5 and Table S1). 27,28As shown in Figure 3C, the PAA-Zn/ ZnF 2 @Zn has higher t Zn2+ (0.507) than that of bare Zn (0.309) and PAA@Zn (0.381), indicating that the organic-inorganic hybrid SEI possesses high Zn 2+ conductivity. 29AFM is carried out to investigate the mechanical strength and roughness of different Zn anodes. 30As illustrated in Figure 3D, the surface of the PAA-Zn/ZnF 2 layer is considerably flat compared to a solitary organic PAA interface, from which the surface roughness of PAA@Zn (~120 nm) is much larger than PAA-Zn/ZnF 2 @Zn (~58 nm) (Figure 3E).The smaller interface roughness is conducive to avoiding the tip effect in the zinc deposition process, thus inhibiting dendritic growth. 31Furthermore, to verify the application potential of PAA-Zn/ZnF 2 @Zn anode in flexible devices, the mechanical indentation test was performed to evaluate the mechanical strength (Figure 3F).The average Young's modulus of PAA-Zn/ZnF 2 @Zn is up to 2000 MPa, much higher than the 800 MPa of PAA@Zn, indicating that the PAA-Zn/ZnF 2 @Zn holds the stronger anti-deformation ability, which can effectively inhibit dendrite growth. 32onsidering the actual function of SEI, in addition to higher mechanical stability, the high ionic conductivity of the SEI layer needs to be guaranteed.The ionic conductivity of the PAA-Zn/ZnF 2 was evaluated in Figures S6 and S7; A high ionic conductivity of 2.5 × 10 −8 S/cm was realized. 33he interfacial compatibility between electrodes and electrolytes is a key factor for flexible energy storage devices. 30Here, a PAM-ZnSO 4 gel serves as the electrolyte.The adhesive ability of PAM gel for different interfaces is exhibited in Figure S8, where the PAM has a stronger interaction with the PAA-TFPA@Zn interface compared to pure Zn and PAA@Zn.These results are further confirmed by the gel adhesion test as shown in Figure S9A.The maximum peel force between PAM and PAA-Zn/ZnF 2 @Zn is reached as high as 10.8 N. In contrast, the physical adhesion of PAM hydrogels to Zn and PAA@Zn is merely 1.8 and 3.2 N, respectively (Figure S9B).Besides, the hydrogel shows excellent physical and electrochemical properties.As shown in Figure S10, the ionic conductivity of PAM gel was evaluated, and a high ionic conductivity of 3.1 × 10 −2 S/ cm is realized.At the same time, compared with the traditional liquid electrolyte, hydrogel shows obvious inhibition of hydrogen evolution (shown in Figure S11).As shown in Figure S12, the synthetic PAM hydrogel can be easily stretched to 225% strain without any fracture or even visible cracks.The thermal stability of PAM hydrogel was investigated by TG.As shown in Figure S13, PAM hydrogel experiences three weight loss stages, with weight loss rates of 1.7%, 25.3%, and 29.3%, respectively, indicating that PAM hydrogel has excellent thermodynamic properties. 34e cycling durability and reversibility of the zinc anode are evaluated via constant current charge-discharge for the symmetrical Zn||Zn cells at the current density of 0.5 mA/ cm 2 and areal capacity of 0.5 mA•h/cm 2 .The Zn||Zn cell coated with PAA exhibits an enhanced life span of 323 h far higher than the bare Zn||Zn battery (Figure 4A).The sharply increased voltage hysteresis of symmetrical PAA@Zn|| PAA@Zn cell is attributed to the cell polarization caused by the accumulation of dead zinc and by-products. 35,36In contrast, the cyclic life of the PAA-Zn/ZnF 2 @Zn||PAA-Zn/ ZnF 2 @Zn cell is improved to 3060 h, and the voltage hysteresis is lower than those of the bare zinc and PAA@Zn||PAA@Zn symmetric cells in the whole charge/ discharge process.The low polarization voltage and ultralong cycle life demonstrate that the organic-inorganic hybrid SEI effectively inhibits the growth of zinc dendrite and side reactions, and accelerates the interfacial transference kinetics of zinc ions as well.Een at a considerable depth of discharge of 5 mA•h/cm (43% utilization rate of zinc), the PAA-Zn/ ZnF 2 @Zn||PAA-Zn/ZnF 2 @Zn cell still demonstrates low overpotentials (around 30 mV) and high reversibility for at least 300 h (Figure S14).
Besides that, mechanical stability is also a key point for flexible energy storage devices.Herein, a PAA-Zn/ ZnF 2 @Zn||PAA-Zn/ZnF 2 @Zn symmetrical pouch cell with the size of 1 cm × 3 cm was assembled to measure the mechanical flexibility under different bending angles ranging from 0°-180°with the bending radius of 1 cm.As delivered in Figure 4B, the overpotential of the PAA-Zn/ZnF 2 @Zn is consistently steady during bending to different angles even at 180°, in which the high adhesion of the gel PAM electrolyte endows the PAA-Zn/ZnF 2 @Zn cell with remarkable mechanical stability.It is noteworthy that the PAA-Zn/ ZnF 2 @Zn pouch cell can still operate stably for more than 1200 h benefiting from the interfacial compatibility bringing outstanding mechanical stability (Figure 4C and Figure S15).The overpotential of bare Zn is suddenly increased after bending to 90°, possibly as the electrolyte peels off from the electrode.Although the PAA@Zn survives bending, the overpotential clearly increases after bending and then soon dies.
The surface morphologies of zinc anode intuitively illustrate the formation of dendrites and by-products after plating/stripping several cycles.The severe dendrites and byproducts cover all over the surface of bare zinc merely after cycling 50 h (Figure 4D).Thanks to the protection of the PAA layer, although there are obvious cracks on the surface of the PAA@Zn anode after cycling 50 h, the surface is still uniform.However, massive dendrites pierce out from the PAA layer with continued cycling, which is mainly due to the PAA layer lacking rigidity to suppress the growth of dendrites.By comparison, the PAA-Zn/ZnF 2 anode is rather even after cycling without no obvious dendrites or by-products on the surface.These results reveal that the PAA-Zn/ZnF 2 hybrid SEI has coupled rigidity and flexibility to effectively inhibit dendrite growth.
The results are also confirmed by XRD patterns.As shown in Figure 4E and Figure S16A, the peaks at 16.The formation of by-products will result in irreversible consumption of Zn 2+ and increase the interface phase impedance between the zinc metal electrode and electrolyte, seriously affecting the diffusion of electrons/ions in the interface phase. 37The Zn 4 SO 4 (OH) 6 •5H 2 O by-product on the surface of PAA@Zn is substantially reduced after cycling, as Cycle performance of Zn, PAA@Zn, PAA-Zn/ZnF 2 @Zn.(B) Cycle performance of Zn, PAA@Zn, PAA-Zn/ZnF 2 @Zn during bending from 0°-180°.(C) Cycle performance of Zn, PAA@Zn, PAA-Zn/ZnF 2 @Zn after bending 100 h.(D) SEM images of Zn, PAA@Zn, and PAA-Zn/ZnF 2 @Zn after cycling at different times.XRD spectra of (E) Zn, (F) PAA@Zn, (G) PAA-Zn/ZnF 2 @Zn.SEM, scanning electron microscope.
recorded in Figure 4F and Figure S16B, indicating that the by-products have been slightly inhibited by the PAA layer.Comparatively, there are no additional peaks in the XRD indexed to any by-products on the surface of the PAA-Zn/ ZnF 2 @Zn anode (Figure 4G and Figure S16C) confirming that the organic-inorganic hybrid SEI greatly suppresses the dendrite growth and by-products formation, 18 which is well consistent with the SEM results.The EIS in Figure S17 indicates that the resistance of the bare Zn anode is much higher than those of PAA@Zn and PAA-ZnF 2 @Zn before and after cycling.It is probably due to the lower hydrophilicity and by-products hindered ion transport, 38,39 which is confirmed by significantly increasing resistance after cycling.In contrast, the resistance of the PAA-ZnF 2 @Zn anode is reduced accompanied by the cycle continually due to the gradual formation of high ionic conductivity ZnF 2 layer and finally approaches a fixed value.
In addition, it also remains valid when the in situ construction of hybrid SEI further extends to the fiber batteries.The PAA-Zn/ZnF 2 symmetric fiber batteries stably cycle for more than 550 h, as shown in Figure S18, meaning a great prospect for the use of building stable fiber batteries.
All the above results verify that the PAA-Zn/ZnF 2 organic-inorganic hybrid SEI has favorable mechanical stability and effectively inhibits Zn dendrite and side reactions, which endow it with a promising potential application in the flexible zinc ion battery.Herein, a piece of PAM-gel is served as both electrolyte and separator to prepare a flexible Zn||MnO 2 full battery.As shown in Figure S19, the battery shows two pairs of redox peaks at a scan rate of 0.1-1 mV/s, which is a typical Zn 2+ and H + co-intercalation behavior. 40,41As illustrated in Figure 5A, the specific capacity of PAA-Zn/ ZnF 2 @Zn||MnO 2 full cell is about 248 mA•h/g at the F I G U R E 5 (A) Rate performance of PAA-Zn/ZnF 2 @Zn||MnO 2 from 1 to 10 C (1 C = 308 mA•h/g).(B) Corresponding GCD curves of PAA-Zn/ZnF 2 @Zn||MnO 2 from 1 to 10 C. (C) Cycle performance of Zn||MnO 2 , PAA@Zn||MnO 2 , PAA-Zn/ZnF 2 @Zn||MnO 2 .(D) GCD curves of PAA@Zn||MnO 2 before and after 100 times.(E) EIS curves of PAA-Zn/ZnF 2 @Zn||MnO 2 before and after 100 times.(F) Cycle performance of Zn||MnO 2 , PAA@Zn||MnO 2 , PAA-Zn/ZnF 2 @Zn||MnO 2 after 100 times.current density of 1 C.The voltage plateaus will remain as the current density increases, and the specific capacity slightly decreases to 142 mA•h/g at 10 C (Figure 5B) with ~59% capacity retention.Additionally, the capacity of the PAA-Zn/ZnF 2 @Zn||MnO 2 cell soon recovers when the current density returns to 10 C. In contrast, the capacities of PAA@Zn||MnO 2 and Zn||MnO 2 cells from 1 to 10 C fast drop to 54% and 28% of initial capacity, respectively.The reason for this is that the by-products of the PAA@Zn anode and Zn anode greatly hinder the ion transport for one thing, and the favorable ionic conductivity of the PAA-Zn/ZnF 2 hybrid SEI layer for another. 42he cycling performance of the full cell is investigated at a current density of 10 C (Figure 5C).Remarkably, the reversible capacity of PAA-Zn/ZnF 2 @Zn||MnO 2 cell is 103 mA•h/g after 3000 cycles with 79% of the initial capacity, which is far higher than those of PAA@Zn|| MnO 2 and Zn||MnO 2 .
Furthermore, the mechanical flexibility of the PAA-Zn/ZnF 2 @Zn||MnO 2 cell is detected via bending to different angles from 0°-180°under the CV test, as shown in Figure S20.The CV curves of PAA-Zn/ ZnF 2 @Zn||MnO 2 well overlap each other during bending implying superior capacity retention, by virtue of the excellent interface adhesion between PAA-Zn/ZnF 2 SEI and PAM electrolyte. 43On the contrary, the closed areas of Zn||MnO 2 and PAA@Zn||MnO 2 cells apparently decrease with the bending angle increased especially for Zn||MnO 2 cell.This capacity decay during bending is probably caused by the separation of the electrode and electrolyte.And the mechanical durability has been attested through continuous bending after 100 times (Figure 5D and Figure S21).The GCD voltage profiles of the PAA-Zn/ZnF 2 ||MnO 2 cell well preserve the shape during bending compared to the Zn||MnO 2 and PAA@Zn||MnO 2 cell after bending 100 times, showing good mechanical properties.The R ct and R s of the PAA-Zn/ZnF 2 ||MnO 2 cell are almost unchanged after bending 100 times, as illustrated in Figure 5E.It is further demonstrated that this PAA-Zn/ZnF 2 SEI possesses high robust and excellent interface adhesion between PAA-Zn/ ZnF 2 SEI and PAM electrolyte endowing PAA-Zn/ZnF 2 || MnO 2 cell with remarkable mechanical durability. 44urthermore, the PAA-Zn/ZnF 2 @Zn||MnO 2 cell can steadily work another 3000 cycles with 85% capacity retention after bending 100 times (Figure 5F).As shown in Figures S22-S24, fiber cell has been successfully assembled to verify the feasibility of flexibility power in smart wearable devices. 45,46The shape of CV curves is well consistent with the pouch cell.Moreover, the F I G U R E 6 (A, B) Schematic illustrations of the wearable integration system.(C) Digital graph of the wearable integration system.(D) Charging via wireless with the PAA-Zn/ZnF 2 @Zn||MnO 2 as the power source.(E) The application of a smart integrated system.PAA-Zn/ZnF 2 @Zn||MnO 2 fiber cell has a larger closed area of CV curves, indicating the PAA-Zn/ZnF 2 @Zn|| MnO 2 fiber cell has higher capacity and better electrochemical performance.][49][50][51][52][53][54][55] The pouch cell or fiber cell as the core components couples with solar panels, wireless charging, and battery indicators (Figure 6A).The "smart watch," driven by PAA-Zn/ZnF 2 @Zn||MnO 2 pouch cell or fiber cell, is used to collect the corresponding information including temperature, humidity, and pressure sensors modular integrated system, and then send the collected data to the mobile phone through the Bluetooth protocol. 56,57Through the graphical support interface of the smartphone, the collected pulse, breath, and body movement data can be displayed at the same time.As the schematic diagram, shown in Figure 6B, the pouch cell or fiber cell is used to power the mobile phones and bracelets through wireless charging.We detected the performance of the constructed system in the outdoor environment. 58,59As shown in Figure 6C, the system can distinctly detect the wearer's heart rate, ambient temperature, and humidity in real-time, and display them on the phone.Figure 6D shows that the mobile phone is powered with a zinc ion battery through the wireless charging module.And the current situation can be detected in real-time through the motion system (as shown in Figure 6E).The current ambient temperature is 27 °C, the heartbeat is 115 times per minute, and the ambient humidity is 62%.

| CONCLUSIONS
In conclusion, here, an organic-inorganic hybrid PAA-ZnF 2 SEI is fabricated to enhance the electrochemical stability by inhibiting the dendrite growth and byproducts of zinc anode, improving the mechanical durability via favorable electrode/electrolyte compatibility.Therefore, the PAA-Zn/ZnF 2 @Zn||PAA-Zn/ ZnF 2 @Zn symmetric cell exhibits excellent cycling stability (cycling more than 3000 h) with good flexibility (stably cycling another 1200 h after bending 100 h).And the PAA-Zn/ZnF 2 @Zn||MnO 2 full cell can steadily work another 3000 cycles with 85% capacity retention after bending 100 times.What's more, the PAA-Zn/ ZnF 2 @Zn||MnO 2 full cell (both pouch cell and fiber cell) works well on the smart integration system.This organic-inorganic hybrid SEI provides a convenient and efficient strategy for next-generation flexible Zn-metal batteries with better safety and stability.
2°and 24.4°are attributed to the by-products of Zn 4 SO 4 (OH) 6 •5H 2 O (JCPDS card No. 39-0688).Moreover, the peak intensities of Zn 4 SO 4 (OH) 6 •5H 2 O are stronger with the cycling time increased, which means that the bare zinc anode goes through serious continuous side reactions during cycling.