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Qiaohui Duan, Kaiming Xue, Xin Yin, [Denis Y.W. Yu](https://orcid.org/0000-0002-5883-7087)

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[A cationic polymeric interface enabling dendrite-free and highly stable aqueous Zn-metal batteries](https://mdr.nims.go.jp/datasets/10c28506-8d1e-4b83-b926-83657dfb4e7b)

## Fulltext

A Cationic Polymeric Interface Enabling Dendrite-free and Highly Stable Aqueous Zn-metal Batteries Qiaohui Duana,1, Kaiming Xuea,1, Xin Yina,1, Denis Y. W. Yua,b,*a School of Energy and Environment, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong S.A.Rb Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong S.A.R.* Corresponding author1 These authors contributed equally to this workEmail: denisyu@cityu.edu.hkAddress: YEUNG G5702, Tat Chee Ave, Kowloon, Hong Kong SAR, People’s Republic of ChinaABSTRACTRecently, aqueous rechargeable zinc-ion batteries (AZIBs) have attracted much attention owing to their low cost and intrinsic safety. However, the reversibility of AZIBs is limited by dendrite growth on the Zn anode, which leads to low Coulombic efficiency and potential short circuit during cycling. Herein, we develop a new strategy using electrostatic shielding to enable a highly reversible Zn anode. Specifically, a cationic polymeric ionic liquid (PIL) coating layer with a strong positive charge evens out the charge distribution on the surface of the Zn electrode, enabling uniform stripping and deposition of Zn. As a result, a symmetric Zn-Zn cell can sustain stripping-plating for over 2000 h at 1 mA cm-2 with a capacity limit of 1 mAh cm-2, far exceeding the performance of bare Zn electrode. A highly reversible Zn stripping-plating on Cu substrate is also achieved with an average coulombic efficiency of 99.5% over 1100 cycles. PILZ layer can even allow a uniform Zn deposition of up to 30 m thick, allowing large utilization of the Zn foil. Moreover, the coating layer enables the stable cycling of a MnO2-Zn full cell with a high areal capacity of 1 mAh cm-2.  Key words: Aqueous Zn ion batteries; Zn anode; polymeric ionic liquid coating; dendrite suppression; electrostatic shielding effect 1. IntroductionRecently, emerging rechargeable aqueous batteries have attracted much attention as their high safety and low cost are attractive for large-scale energy storage applications.[1-3] Especially, aqueous rechargeable zinc-ion batteries (AZIBs) are promising owing to the high theoretical capacity (5845 mAh cm−3) of metallic Zn anode, low Zn/Zn2+ redox potential (−0.76 V versus SHE) and abundant resources.[4] However, severe dendrite growth of Zn anode usually leads to low Coulombic efficiency and limits the cycle life via dendrite penetration and short circuit, especially for large areal capacity of 0.5 mAh cm-2 or more, which greatly hinders the development of AZIBs.[5-8] So far, various strategies such as electrode engineering,[9] electrolyte control[10] and surface modification[11] were developed to enhance the reversibility of Zn anode. For example, a 3D zincophilic ZnOHF nanowire array was built to enable reversible Zn plating/stripping with low Zn nucleation overpotential, high Zn storage capacity and excellent Coulombic efficiency.[12] Optimizations of electrolyte with highly concentrated electrolytes, deep eutectic electrolytes and ionic liquids, etc. were found to be effective approaches to manipulate the solvation structure of Zn2+ to enhance uniform deposition of Zn.[13-15] However, these approaches in general increases the cost of the electrolytes. Meanwhile, Zn anode surface modifications were also reported to improve Zn deposition. For example, researchers have shown that Zn deposition can be controlled with the use of a graphene interlayer and polyamide (PA) layer.[16, 17] Nevertheless, a rational design to overcome the uneven surface charge distribution that causes dendrite formation and a systematic investigation on the effect of surface modification effect are still lacking. It is well known that the growth of Zn dendrites is driven by tip effect, as Zn tends to be plated on electrode protrusions due to the uneven electric field.[6, 8] Hence in this work, we report a facile polymer coating with a cationic shielding layer consisting of a polymeric ionic liquid (PIL) and zinc trifluoromethanesulfonate (Zn(OTf)2), denoted as PILZ layer, on Zn anode to inhibit the dendrite growth. Specifically, poly(diallyldimethylammonim bis(trifluoromethylsulfonyl) imide) (PDADMA-TFSI) is used as the PIL layer because it features a strong positive charge with a polymer backbone (PDADMA+). Zn(OTf)2 is added to improve the ionic conductivity of the coating layer. Comparing with a bare Zn electrode with a neutral surface charge and polyvinylidene fluoride (PVDF)-coated Zn electrode with a negative surface charge, the positively-charged PILZ layer is effective in enabling uniform Zn stripping-plating, which is attributed to the ‘shielding’ of the Zn surface protrusions by the surface layer. As a result, steady Zn stripping/plating is maintained for over 2000 h at a current of 1 mA cm-2 with a capacity limit of 1 mAh cm-2. In addition, a flat and compact Zn deposition up to 20 mAh cm-2 (30 mm) is also demonstrated. Moreover, a highly reversible Zn stripping/plating on a Cu substrate is also realized with an average Coulombic efficiency of 99.5% for over 1100 cycles. In addition, the PILZ@Zn electrode extends the cycle life of a MnO2-Zn full cell to 100 cycles under 1 mA cm-2 and 4000 cycles under 2.5 mA cm-2.  2. Experimental2.1 Preparation of electrodesPIL powder was fabricated using a previously reported method.[18] Specifically, 3M LiTFSI (Dodo Chem) aqueous solution was added dropwise into PDADMA-chloride solution with stirring for 1 h (Sigma Aldrich, 20% in H2O) in a molar ratio of 1.1:1 to ensure complete replacement of Cl- by TFSI-. During the process, white PDADMA-TFSI (PIL) powder was precipitated, which was then collected by vacuum filtration, thoroughly washed with de-ionized water and dried at 60 C for 24 h. 2 g of the as-prepared PIL powder and 1g of Zn(OTf)2 were then dissolved in 7 ml NMP to make a viscous and clear solution. The solution was then tape casted onto Zn or Cu foil using a doctor blade with a controllable thickness to form PILZ@Zn or PILZ@Cu, respectively. The solvent was removed by drying at 80℃ for 4 h. For comparison, PVDF is also coated on Zn foil using the same procedure to form the PVDF@Zn electrode. 2.2 CharacterizationsThe morphology of the electrodes was characterized by a scanning electron microscope (SEM, QUATTRO S), combined with energy dispersive X-ray (EDX) for the determination of the composition of different elements. Fourier transform infrared spectroscopy (FTIR) measurements were performed with an IR spectrometer (Shimadzu, IRAffinity-1). To measure the electrode surface charge, zeta potential tests of the Zn foils with or without coating were conducted on Malvern Zetasizer Nano-ZS in an aqueous solution containing diluted tracer beads, of which pH was adjusted to 4.7, same as the electrolyte. 10% of Malvern transfer standard DTS1235, 10% or 2% of Downy fabric softener were used as cationic and anionic tracers for testing the PVDF@Zn, PILZ@Zn and bare Zn electrodes, respectively.[19]2.3 Electrochemical testsFor the Zn stripping-plating tests, symmetric 2032 coin cells were assembled with Zn foils (99.9%, 0.04 mm thick, 16 mm diameter) or coated Zn foils as both the working and counter electrodes, 1M aqueous ZnSO4 as electrolyte and glass fiber (Advantec #GD-120) as separator. Chronoamperometry (CA) measurements were conducted in a three-electrode configuration, in which Zn or PILZ@Zn foil, stainless steel plate and saturated calomel electrode (SCE) were used as working electrode, counter electrode, and reference electrode, respectively. A constant potential of -1.15 V or -1.30 V vs. SCE was applied, and the current was recorded over 150 s. In the in-situ optical microscope tests, an electrochemical cell was assembled with Cu or PILZ@Cu (1 cm by 1 cm) and Zn foil (1 cm by 5 cm) as electrodes in a transparent cuvette, which was connected to a portable potentiostat (Biologic SP-50). A constant current discharge of 10 mA cm-2 was applied for 2 h, and the cross-section images of the Cu or PILZ@Cu foil were recorded by a ZWC200 optical microscope every 30 min. In the full cell tests, cathode electrodes are comprised of ball-milled commercial MnO2 (Xiangtan Electrochemical Scientific Ltd., 60%), Super P carbon (29%), single-wall carbon nanotube (Sigma Aldrich, 1%) and PVDF (10%) coated on graphite paper (50 μm thick, Chenxin-Induction). The ball-milled MnO2 was prepared through ball-milling the as-received commercial MnO2 for 12 h in ZrO2 bowls at 200 rpm with ethanol as dispersant. The MnO2 cathode (diameter of 12 mm) was then assembled with Zn anode in 2032 coin cells. For high loading tests, the MnO2 electrodes with a loading of ~4 mg cm-2, a thickness of 50 m and a packing density of ~1.3 g cm-3 were used. High-rate tests were conducted with MnO2 electrodes with a loading of ~1 mg cm-2. 2M ZnSO4 + 0.1M MnSO4 was used as the electrolyte for the full cell. 3. Results and discussionOne of the main reasons for Zn dendrite formation during deposition is the uneven distribute of charge on the surface. As illustrated in Figure 1a, protrusions can act as sites for Zn deposition as the higher charge density will attract more Zn2+ to be reduced and deposited there, facilitating the growth of dendrites.[6, 8] To suppress such process, here, we design a cationic polymer coating in order to shield the Zn surface from the Zn ions in the electrolyte. With a net positive charge, the PILZ coating layer can reduce the effect of uneven charge distribution on the Zn surface. Meanwhile, the charge repulsion between the Zn ions and the positive layer can prevent the Zn ions from its preferential growth on the protrusions and promoting uniform deposition of Zn on the anode surface. [11, 20]PILZ was first coated on Zn foil using a tape casting method (Figure 1b). Figure 1c shows the top-view SEM image of the surface of PILZ@Zn and also the molecular structure of PIL The PILZ@Zn surface is smooth and dense, exhibiting completely different surface morphology compared with the bare Zn foil (Figure S1a). The cross-sectional SEM image of the Zn foil coated with PILZ is further shown in Figure 1d. The thickness of the Zn foil and the PILZ coating on its surface is about 40 m and 13 m, respectively. The insets in Figure 1d are the EDX mappings of the sample (overall EDX spectrum is shown in Figure S1b). The Zn element is clearly originated from the Zn foil. The S and F elements observed in EDX are from the TFSI- and triflate (OTf)- anions inside the PIL (i.e. PDADMA-TFSI and the Zn(OTf)2) while the C element is attributed maintly to carbon atoms in TFSI- anion and the polymer backbone. FTIR experiments were further utilized to analyze the functional groups of the PILZ coating. Compared to bare Zn foil, there are several new peaks in the spectrum of PILZ@Zn, which are labelled in Figure 1e. δ(CH3) and νas(CH3) originate from the -CH3 functional group of the PDADMA+ polymer backbone. The remaining peaks (νas(SO2), νs(SO2), δ(CF3), ν(SN), νs(SNS) and δ(SNS)) are attributed to the stretching or bending of the functional groups in TFSI- anions.[21] Figure 1. (a) illustration showing the proposed Zn deposition process on the bare and PILZ@Zn electrodes; (b) Schematic illustration of the PILZ@Zn preparation process; (c) SEM image (top view) of PILZ@Zn,electrode (inset is the molecular structure of PIL); (d) cross-sectional SEM image of PILZ@Zn and the corresponding EDX mappings of Zn, S, F, and C elements; (e) FTIR spectra of bare Zn and PILZ@Zn electrodes.To verify the electrostatic shielding effect on Zn, we have measured the surface zeta potential of the bare Zn, PVDF@Zn and PILZ@Zn electrodes (Figure 2a). The measured data are shown in Figure S2 and Table S1. The results suggest that the original surface charge of the bare Zn electrode is close to zero in a mild aqueous environment, while the PILZ coating introduces a strong positive charge of 28.5 mV on the surface of Zn. Meanwhile, PVDF@Zn shows a negative surface charge of -42.8 mV due to the fluoride groups on PVDF. The measured zeta potentials are close to the previous reported values.[22-24] The Zn foils with and without coating are made into three-electrode cells with ZnSO4 electrolyte and subjected to CA test with a constant potential of -1.15 V vs. SCE for 150 s (Figure 2b). During the test, Zn ions in the electrolyte will be deposited onto the Zn working electrode, and the applied current is measured to understand the deposition behaviour.[17] As shown in the results, both bare Zn and PVDF@Zn exhibit a large current density, reflecting a rampant diffusion of Zn ions to the surface and a fast deposition rate onto the most energetically favorable sites on the surface, resulting in nucleation and growth of dendrites. In contrast, the current density is much smaller for the PILZ@Zn, suggesting that PILZ layer regulates the nucleation of Zn islands.[25] Similar behaviour is also observed if the CA potential is changed to -1.30 V (Figure 2c). The stripping-plating stability of the Zn electrodes is further evaluated in a Zn-Zn symmetric cell with a current density of 1 mA cm-2 and an areal capacity of 1 mAh cm-2 in Figure 2d with extended cycling. The inset of Figure 2d reveals the details of the voltage profiles of the electrodes during different periods. During the stripping-plating tests, the polarizations in general decrease gradually over cycles. The higher initial overpotentials can be caused by the surface oxide layer on the Zn foil at the beginning.[26, 27] Upon cycling, fresh Zn layer is deposited on the surface, leading to a decrease in overpotential. Comparing the three electrodes, bare Zn and PVDF@Zn stripping-plating process only last for about 167 h and 162 h, respectively, when the charge-discharge voltage starts to fluctuate and suddenly changes. In comparison, PILZ@Zn electrode shows long-term stability while cycling for more than 2000 h, which exceeds that of most of the recently reported works (Table S2).   Cycle stability of the PILZ@Zn electrodes with PILZ layer thickness of 30 mm and 50 mm are also demonstrated in Figure S3. We found that the long-term cycle stability of Zn can be achieved with different PILZ layer thicknesses, though a thicker layer will give rise to a larger voltage polarization resulting from increased resistance to ion transfer.  To understand the difference in the observed cycle stability, the surface morphologies of the different Zn electrodes after 100 h cycling were studied by SEM (Figure 2e and Figure S4). Note that the PILZ and PVDF coatings were first removed from the Zn electrode before observations. As shown in the surface view in Figure 2e, while the pristine Zn foil before cycling is smooth, roughened surface with Zn protrusions are evident on the bare Zn electrode after cycling (note that the fibers observed in the image are originated from the glass fiber separator and not part of Zn). For the Zn anode coated with PVDF, it also shows a roughened surface similar to that of the bare Zn, suggesting that PVDF coating cannot suppress dendrite formation. On the contrary, the surface of Zn foil coated with PILZ layer features a compact, dense and dendrite-free surface, indicating that the PILZ facilitates a uniform deposition of Zn. The cross-sectional views of the cycled electrodes are shown in Figure S4. We can see uneven surface deposits of up to 46 m and 56 m on the bare Zn and PVDF@Zn electrodes, respectively, after cycling, while the deposit is only about 5 mm on the PILZ@Zn electrode. The results are in line with their morphologies as observed in the top-view SEM images in Figure 2e. Figure 2. (a) Surface zeta potentials, CA curves with constant potential applied with (b) -1.15 V and (c) -1.30 V, (d) galvanostatic cycling of symmetrical Zn-Zn stripping-plating at a current density of 1 mA cm-2 and areal capacity of 1 mAh cm-2 of bare Zn, PVDF@Zn and PILZ@Zn electrodes, (e) top-view SEM images of pristine Zn and the electrodes after cycling for 100 h in (d) (the coating layers are peeled off).The improvement in the stability of the Zn stripping-plating process can be attributed to electrostatic shielding effect of the PILZ layer.[11,20] During the Zn plating process, there is an uneven distribution of electric field on the rough surface. In particular, electric field is stronger on a curved surface of a conductor, so the small protrusions would attract Zn ions to deposit on the tips to form larger dendrites. The PILZ layer effectively shields the Zn ions from the uneven negative surface charge distributions, thus preventing the Zn ions from aggregating to the Zn tips and induces an even deposition of Zn. In addtion, the PILZ polymer structure is stable in the system, as the zeta potential of the PILZ@Zn electrode remains the same after different hours of testing (Figure S5). Hence the positive shielding effect can be maintained upon cycling.   To further demonstrate the effectiveness of the PILZ layer, Zn is deposited onto and stripped away from a Cu substrate with or without PILZ coating in a Cu-Zn cell. The areal capacity is set to 0.5 mAh cm-2 under a current density of 1 mA cm-2. As shown in Figure 3a, the cell with bare Cu shows an average Coulombic efficiency (CE) of 98.7% (from 2nd cycle onwards) and lasts for only 95 cycles before a sudden failure. The low CE of the bare Cu indicates that after every cycle, there is a certain amount of Zn left on the surface of the bare Cu. The Zn will form into dendrites and eventually penetrate through the separator after cycling, leading to the observed sudden failure. In contrast, the PILZ@Cu can be stably cycled for over 1100 cycles with an average CE as high as 99.5%. The PILZ layer allows more efficient plating and stripping of Zn from Cu, thus extending the cycle performance. In addition, from the corresponding voltage profiles of the bare Cu and the PILZ@Cu as shown in Figure 3b and 3c, respectively, it can be seen that the voltage hysteresis between charge and discharge of the PILZ-coated Cu electrode is merely 39 mV, lower than that of the bare Cu electrode of 51 mV. The PILZ layer also reduces the voltage polarization on Cu.To clarify the deposition process of Zn on Cu substrate, a thick layer of Zn with an areal capacity of 10 mAh cm-2 (equivalent to a thickness of 17 m if evenly deposited) was deposited onto a bare Cu substrate and a PILZ@Cu substrate and then taken out for SEM observations. The cross-section SEM image of the bare Cu substrate (Figure 3d) shows a rough surface with significant variation in the thickness of the deposited Zn, i.e. some places show thick deposition of Zn while other places have little amount of Zn due to an uneven deposition of Zn on its surface. In contrast, the PILZ@Cu substrate shows a flat and compact Zn deposition layer with a thickness of about 17 m between the PILZ coating layer and the Cu foil (Figure 3e), again showing the effectiveness of the PILZ layer to reduce dendrite growth. A direct observation of the Zn deposition process on the surface of bare Cu and PILZ@Cu was carried out by in-situ optical microscope with a current density of 10 mA cm-2. As shown in the optical microscope images in Figure 3f, a rough surface on top of the bare Cu foil is already observed after 30 mins of deposition, and the roughness grows with deposition time. After 120 mins, the difference between the thickest and the thinnest part of the Zn layer is as large as 25 m (Figure 3g). For PILZ@Cu, while the thickness of Zn layer grows with increasing deposition time, the Zn layer remains flat throughout the process, as shown in Figure 3f. The thicknesses of the deposited Zn layer on the 2 substrates were measured and plotted in Figure 3g. We can see that the average thicknesses of Zn on both substrates are close to the calculated theoretical value based on the applied current of 10 mA cm-2. Bare Cu shows a large variation of thickness throughout the electrode with deposition. On the other hand, PILZ layer can significantly reduce the roughness of Zn deposition. After 2 hours deposition, a uniform layer of Zn with a thickness of 32 m is achieved with PILZ coating. As far as we know, this is the thickest uniform deposition of Zn ever reported (Table S3). PILZ layer can allow large utilization of Zn, which could further increase energy density of the battery in the future.  Figure 3. (a) CE of Zn plating/stripping on bare Cu and PILZ@Cu foil substrates at a current density of 1 mA cm-2 with a capacity limitation of 0.5 mAh cm-2; the corresponding voltage profiles at various cycles of (b) bare Cu and (c) PILZ@Cu; cross-sectional SEM images of Zn deposition on (d) bare Cu and (e) PILZ@Cu at a current density of 1 mA cm-2 and an areal capacity of 10 mAh cm-2; (f) cross-sectional Zn deposition morphology on bare Cu and PILZ@Cu observed by an in-situ optical microscope; (g) average thickness of the Zn deposition over time derived from (f).Figure 4. (a) Cycle performance of high loading MnO2//Zn full cell with bare Zn and PILZ@Zn (~4 mg cm-2 MnO2) at 100 mA g-1; corresponding voltage profiles of (b) MnO2//bare Zn and (c) MnO2//PILZ@Zn of (a); (d) optical photograph and (e) SEM image of the cycled bare Zn electrode in (b); (f) optical photograph and (g) SEM image of the cycled PILZ@Zn electrode in (c); (h) rate performance and (i) cycle performance of MnO2//bare Zn and MnO2//PILZ@Zn full cells (~1 mg cm-2 MnO2). To further evaluate the practicality of the PILZ@Zn in ZIBs, full cells using ball-milled commercial MnO2 (Figure S6) as cathode and bare Zn or PILZ@Zn anode were assembled. First, a high active mass loading of MnO2 of ~4 mg cm-2 is applied, corresponding to an areal capacity of close to 1 mAh cm-2 with a capacity of ~230 mAh g-1. As shown in Figure 4a, the MnO2//Zn full cell with bare Zn anode can cycle for only 52 cycles before failure. During the 53rd charge process, the cell cannot be charged to the target potential (Figure 4b). In addition, the cell voltage will continue to drop after stopping the charging process, indicating a short circuit in the cell. [28] Figure 4d and 4e shows the optical photograph and SEM image, respectively, of the bare Zn electrode after the 53rd cycle. Uneven coloration of the Zn foil and also roughened Zn deposition are observed. Short circuiting of the cell is likely due to the even deposition of Zn.  In comparison, the MnO2//PILZ@Zn cell shows excellent cycle stability (Figure 4a and 4c), with a similar initial capacity of 234 mAh g-1 as MnO2//Zn and lasts for over 100 cycles with a high capacity retention of 91% (Figure 4a) without short circuiting. The corresponding charge-discharge profiles are also stable with cycling (Figure 4c), indicating excellent stability for the full cell with PILZ@Zn electrode. The optical photograph and SEM image of the PILZ@Zn electrode after 100 cycles are shown in Figure 4f and 4g, respectively. No significant coloration of the Zn foil is observed, and the Zn surface remains compact and dendrite-free. In addition, the MnO2//Zn full cell performances with higher current densities are compared using MnO2 electrodes with a loading of 1 mg cm-2. In Figure 4h, cell with PILZ@Zn electrode gives a capacity of 285, 266, 182, 137 and 110 mAh (gMnO2)-1 under a current rate of 1C, 2C, 4C, 8C and 16C, respectively, while that with bare Zn electrode shows a capacity of 283, 261, 197, 164 and 140 mAh (gMnO2)-1 at the respective current rates [1C is defined as 300 mA (gMnO2)-1]. It is noted that with current rate exceeding 4C, the capacities obtained from battery with PILZ@Zn electrode are slightly lower than those of the bare Zn cell, which is consistently with the bigger polarization with the PILZ layer. Nevertheless, the cycle stability of the MnO2//PILZ@Zn cell far exceeds that of the MnO2//bare Zn cell (Figure 4i). Specifically, under a current of 8C (~2.5 mA cm-2), the capacity of the MnO2//bare Zn cell decays fast to only 52 mAh (gMnO2)-1 after 4000 cycles, corresponding to a capacity retention of 32%. In contrast, MnO2//PILZ@Zn cell maintains a high capacity of 124 mAh (gMnO2)-1 for over 4000 cycles, with a capacity retention of 96%. The excellent cycle performance is attributed to the uniform Zn stripping/plating processes facilitated by the PILZ layer.  4. ConclusionsIn brief, we proposed an interface engineering approach to tune the Zn surface charge with a PILZ layer to achieve highly reversible Zn anode. The PILZ layer bears a strong positive charge, which can shield the protrusions and even out the charge distribution on the Zn electrode, leading to uniform deposition of Zn. This allows uniform deposition of Zn even up to 30 m thick. This facile and effective approach of Zn surface engineering can further improve stability of aqueous ZIBs, making them possible for large-scale applications.     Declaration of Competing InterestThe 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.AcknowledgementThe work described in this paper was supported by the Strategic Research Grant  (PJ7005725) and HKTech300 funding of City University of Hong Kong. Appendix A. Supplementary dataSupplementary data related to this article is available online. Correspondence and requests for materials should be addressed to D.Y.W.Y. References[1] J. Hao, X. Li, X. Zeng, D. Li, J. Mao, Z. Guo, Deeply understanding the Zn anode behaviour and corresponding improvement strategies in different aqueous Zn-based batteries, Energy & Environmental Science, 13 (2020) 3917-3949.[2] D. Chao, W. Zhou, F. Xie, C. Ye, H. Li, M. Jaroniec, S.-Z. 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