Stripy zinc array with preferential crystal plane for the ultra‐long lifespan of zinc metal anodes for zinc ion batteries

Abstract

Rechargeable zinc (Zn) batteries have been regarded as a potential alternative for energy storage instrument, but the poor life‐span of Zn metal anodes restrain their commercial application. In this work, stripy Zn array (ZnSA) with preferential (002) crystal plane was designed and fabricated through the facile treatment with concentrated phosphoric acid. The depth of stripy array and the content of (002) plane were controlled through the optimization of etching time. The synergistic effect of stripy morphology and preferential crystal plane can increase the electroactive surface area, suppress the corrosion and hydrogen evolution of aqueous electrolyte, induce the horizontal growth along with basal (002) plane, and inhibit the formation of Zn dendrites. The as‐synthesized ZnSA electrodes can achieve ultra‐long lifespan of 2500 h at the current density of 2 mA cm⁻² with a constant capacity of 2 mAh cm⁻² and still maintain stable cycling of 1200 and 1400 h even at the higher current density of 10 mA cm⁻² and plating/stripping depth of 5 mAh cm⁻², respectively. This work proposes a facile and effective strategy to improve electrochemical performance of metallic Zn anodes and contributes to the commercial application of Zn ion batteries.

INTRODUCTION

The considerable desire of safety and reliability for energy storage drives the aqueous rechargeable batteries to develop rapidly.¹ Rechargeable zinc (Zn) ion batteries are one of the best choices benefiting from their high safety, natural abundance, and environmental friendliness.² Zn metal is an ideal material for Zn batteries comparing with the reported anode materials, because its relatively high theoretical capacity and low electrochemical potential.³ Nevertheless, the poor cycling life of Zn metal anodes hinders Zn ion batteries from commercial application. The production of Zn dendrites, the corrosion of aqueous electrolyte, and the evolution of hydrogen would induce the short circuit, increase the ion transportation resistant at electrode/electrolyte interface, and cause battery bulge, which deteriorate the electrochemical properties of Zn metal anodes.⁴ Various strategies, including the optimization of electrolyte and additives,^5–12 the preparation of Zn metal electrodes with special morphology and structure,^13,14 the construction of conductive scaffold,^15–17 the surface modification on Zn metal anodes,^18–26 and the fabrication of special separator,^27,28 have been adopted to improve Zn metal anodes to achieve excellent electrochemical performance.

Among these strategies, surface modification on Zn metal is a remarkable approach to enhance the overall performance of Zn metal anodes. The construction of artificial protective layers on the surface of Zn anodes can interdict the direct contact between aqueous electrolyte and Zn metal and suppress Zn dendrites, which improve the stability of interface between electrode and electrolyte and enhance the electrochemical properties of Zn metal anodes.^29–32 The thickness of protective layers can significantly influence the deposition processes of Zn²⁺ ions on Zn metal anodes. On the one hand, the nanosized protective layers are often carried out through chemical vapor deposition and magnetron sputtering,^33,34 which are expensive and time consuming and insufficient to restrain the generation of Zn dendrites because of poor mechanical performance. On the other hand, the microsized passivation layers can increase the interfacial resistance and nucleation overpotential during the plating and stripping processes, which would deteriorate the rate performance and hinder Zn ion batteries from practical application.³⁵ For example, He et al. fabricated Al₂O₃ coating by atomic layer deposition technique to enhance the electrochemical performance of Zn metal anodes.³³ Cui et al. constructed nano-Au particles via ion beam sputtering method as heterogeneous seeds to deposit Zn to get better cyclic stability of Zn anodes.³⁴ The above-mentioned surface modification possesses the complicated preparation process and the expensive raw materials. Therefore, the morphology and crystal plane control of Zn metal have been adopted to perfect the cyclic properties of Zn anodes. For instance, Kim et al. prepared ZnO layers coated Zn anodes with hexagonal pyramid Zn array, which made the electroactive surface area increase and the local current density decrease and induced the selective deposition of Zn²⁺ ions.³⁶ Zhou et al. reported that surface-preferred (002) crystal plane of Zn metal anodes displayed the advantages of no dendrites, weak hydrogen evolution, and almost no by-products as compared with (100) crystal plane, resulting from their relatively smooth arrangement and even interfacial charge density, which contributes to the uniform deposition of Zn²⁺ ions.³⁷ Xie et al. fabricated three-dimensional nanoporous ZnO architecture on the surface of Zn anodes to accelerate the kinetics of Zn²⁺ transfer and deposition.³⁸ Zhao et al. constructed porous covalent organic framework films on the surface of Zn metal to induce the growth of horizontally arranged platelet Zn with preferred (002) orientations during the plating processes.³² Hao et al. prepared gel electrolyte with multifunctional sulfonate and imidazole to control Zn nucleation and grow (002) plane.³⁹ The above-mentioned articles confirm that the presence of special surface morphology and (002) crystal plane can stimulate Zn metal anodes to optimize electrochemical performance.

Herein, in this work, we designed and developed a facile approach for the fabrication of stripy Zn array (ZnSA) with surface-preferred (002) crystal plane through the treatment with concentrated phosphoric acid. The acid etching times were adjusted to optimize the width and depth of Zn array. On the one hand, the stripy array can increase the electroactive surface area for the selective deposition of Zn²⁺ ions and decrease the regional current density for the restriction of dendrite growth. On the other hand, surface-preferred (002) crystal plane can induce the parallel growth of Zn metal and resist the corrosion of aqueous electrolyte for effectively suppressing the Zn dendrites and stabilizing electrode/electrolyte interface. The as-synthesized ZnSA anodes exhibit ultra-long lifespan of 2500 h at 2 mA cm⁻² with 2 mAh cm⁻².

RESULTS AND DISCUSSION

Scheme 1 presents the fabrication processes of stripy Zn array (ZnSA) electrodes. The commercial Zn metal was cut into circular disks and then immersed in the concentrated phosphoric acid (H₃PO₄) for different times. The hydrogen gas (H₂) and zinc phosphate (Zn₃[PO₄]₂) were produced according the following reaction equation (2H₃PO₄ + 3Zn → Zn₃[PO₄]₂ + 3H₂). The etched Zn disks were washed by water (H₂O) to remove Zn₃(PO₄)₂ by-products and extra H₃PO₄, finally the ZnSA electrodes were obtained after the drying processes.

SCHEME 1. The schematic diagram of the fabrication processes of ZnSA electrodes.

Figure 1 presents surface morphology of bare Zn and ZnSA electrodes. For bare Zn sample, there is a smooth surface and clear boundary, as shown in Figure 1A and Figure S1A. The corresponding element mapping images (Figure S1B,C) of bare Zn exhibit the uniform distribution of Zn element and a small quantity of O element from the oxidation of Zn metal in air. After concentrated phosphoric acid treatment, stripy array is formed due to the selective etch on Zn disks. The depth of stripy array increases from 5 min (Figure 1B) to 10 min (Figure 1C). Lower magnification and larger view images of ZnSA-10 electrode in Figure S2 exhibit the presence of stripy array morphology on the whole Zn disks derived from the homogeneous etch with concentrated phosphoric acid. SEM and element mapping images (Figures S3 and S4) of the surface and cross-section of ZnSA-10 electrode present that ZnSA electrodes mainly consist of Zn and O elements. However, when the etching time increase to 15 min (Figure 1D), the corrosion of the passive plane reduces the height of stripy array. The cross-sectional SEM images of ZnSA electrodes with different etch times were measured and are collected in Figure S5. The thickness of fresh Zn disks (178 μm) was etched to 169 μm for ZnSA-5, 158 μm for ZnSA-10, and 147 μm for ZnSA-15. XPS spectra of Zn 2p and O 1s (Figure S6) demonstrate that ZnSA-10 electrodes possess stronger peak of O element comparing with bare Zn, because the stripy array can increase contact area of Zn with air to form more ZnO layers on surface. The stripy morphology can increase the electroactive surface areas among Zn array and reduce the local current density during the processes of plating and stripping.

FIGURE 1. The surface SEM images and the contact angles of bare Zn (A and E), ZnSA-5 (B and F), ZnSA-10 (C and G), and ZnSA-15 (D and H) (Inserts are the corresponding high magnification images).

The surface wetting ability of bare Zn and ZnSA electrodes with aqueous electrolyte was evaluated by the contact angel meter and is recorded in Figure 1E-H. The contact angle (Figure 1E) of ZnSA (16-40°) is lower than that of bare Zn (88°), resulting from the hydrophilic surface and the stripy array morphology after concentrated phosphoric acid treatment. ZnSA-10 sample (Figure 1G) possesses the lower contact angle at 16° than that of ZnSA-5 (40°) (Figure 1F) and ZnSA-15 (27°) (Figure 1H), demonstrating that the optimal etching time and stripy array structure facilitate the formation of hydrophilic interface from the capillarity effect, which can enhance the surface wettability of aqueous electrolyte on Zn metal. The high surface wetting ability of ZnSA electrodes guarantees the uniform concentration gradient of Zn²⁺ ion flux from electrolyte to electrode, reduce the resistance for charge transfer, and avoid the growth of Zn dendrites during the processes of Zn plating and stripping.

To further observe the stripy morphology of Zn array, three-dimensional (3D) AFM images of bare Zn and ZnSA sheets were measured and collected in Figure 2. The top-view and section-view AFM images (Figure 2A) of bare Zn electrodes exhibit the irregular surface at the micro-scale and the main depth distribution of 0.25 μm. After the treatment of concentrated phosphoric acid at different times, the stripy morphology of Zn array can be seen obviously resulting from the selective etching of Zn metal. ZnSA-10 electrodes (Figure 2C) display the better stripy morphology and higher depth (2.20 μm) of Zn array than that of ZnSA-5 (0.70 μm) (Figure 2B) and ZnSA-15 (0.39 μm) (Figure 2D) electrodes, because of the insufficient etching for 5 min and the excess etching for 15 min. The corresponding cross-sectional AFM images also demonstrate the higher depth of ZnSA-10 as compared with ZnSA-5 and ZnSA-15 electrodes, indicating that the optimal etching time is beneficial to generate the stripy Zn array.

FIGURE 2. The surface and cross-sectional 3D AFM (10 μm × 10 μm) images and depth distributions of bare Zn (A), ZnSA-5 (B), ZnSA-10 (C), and ZnSA-15 (D).

The electrochemical properties of host-less Zn metal anodes can be affected significantly by the surface atomic arrangement with hexagonal close-packed structure, especially for (100) and (002) crystal planes. The heterogeneous interfacial charge density distribution of Zn (100) crystal plane leads to the uneven Zn²⁺ ion flux and the growth of Zn dendrites, while the smooth surface and the even interfacial charge density of Zn (002) crystal plane facilitate to the uniform plating of Zn²⁺ and the formation of surface texture with the crystallographic orientation of (002) plane.³⁷ Acid etching approach has been considered as an effective method to selectively remove (100) and retain (002) crystal planes, due to the preferential corrosion of (100) plane with low binding energy of Zn atoms and high surface energy, since Zn (002) crystal plane coordinated with nine other atoms possesses the compact structure and high binding energy of Zn atoms.⁴⁰

XRD patterns of bare Zn and ZnSA disks were detected and are shown in Figure 3A. For bare Zn sample, the peaks observed at 36°, 38°, 43°, 54°, and 70°, corresponding to the characteristic peaks of (002), (100), (101), (102), and (103) planes for crystalline Zn (JCPDS no. 04–0831), in which the peak intensity of (002) is similar to that of (100). After the etching by concentrated phosphoric acid, the peak intensity ratio of I₀₀₂/I₁₀₀ (Figure 3B) increases from 5 min (ZnSA-5) (1.0) to 10 min (ZnSA-10) (1.8), confirming the occurrence of the selective etching reaction as shown in Figure 1B,C. However, when the etching time increases to 15 min, the corrosion reaction of (002) plane for ZnSA-15 electrodes leads to the decrease of I₀₀₂/I₁₀₀ ratio (1.3) as compared with ZnSA-10, consisting with the above SEM images. The etching speed of (002) plane is lower than that of (100) plane, deriving from the higher atomic coordination, tighter atomic bonds, and lower dissolution tendency in concentrated phosphoric acid of (002) plane as compared with loosely packed crystallographic (100) plane, which also leads to the decrease of (002) plane with the increase of the etching time (15 min).³⁷

FIGURE 3. XRD patterns (A) and the peak intensity ratio of I(002)/I(100) (B) of the pristine, immersed, and cycled bare Zn and ZnSA; XRD patterns (C) and SEM photos of the immersed bare Zn (D), ZnSA-5 (E), ZnSA-10 (F), and ZnSA-15 (G) (Inserts are the corresponding high magnification images).

To investigate the effect of stripy array and preferential crystal plane on the anticorrosion of Zn anodes with ZnSO₄ electrolyte, bare Zn and ZnSA electrodes were immersed for 10 days in ZnSO₄ aqueous electrolyte. XRD patterns of the immersed bare Zn and ZnSA electrodes (Figure 3C) show the clearly characteristic peaks of Zn₄SO₄(OH)₆·xH₂O for the immersed bare Zn sample, indicating the serious side-reaction of Zn metal with aqueous electrolyte. Nevertheless, after the etching processes, ZnSA electrodes exhibit lower intensity of Zn₄SO₄(OH)₆·xH₂O peaks and higher I₀₀₂/I₁₀₀ ratio in XRD patterns (Figure 3B,C) and FTIR spectra (Figure S7), demonstrating that the (002) crystal plane and the stripy array can strengthen the corrosion resistance of Zn metal to aqueous electrolyte.

The surface SEM images of the immersed bare Zn, ZnSA-5, ZnSA-10, and ZnSA-15 electrodes were taken and are organized in Figure 3D-G. The obvious flaky morphology of the immersed bare Zn sample (Figure 3D) indicates the existence of Zn₄SO₄(OH)₆·xH₂O, while the relatively smooth surface can be maintained for the immersed ZnSA samples. As compared with ZnSA-5 (Figure 3E) and ZnSA-15 (Figure 3G), ZnSA-10 (Figure 3F) possesses flatter surface without flaky morphology, demonstrating that the synergistic effect of the preferential crystal plane and the stripy array can enhance the anticorrosion ability of Zn metal in aqueous electrolyte. XPS spectra of Zn 2p and S 2p (Figure S8) after immersion display the poorer Zn and S peaks of the immersed ZnSA-10 electrodes than that of bare Zn electrodes, as well as Zn LMM Auger peak (Figure S9) demonstrates that the surface of the immersed ZnSA-10 electrodes mainly consist of ZnO layers,⁴¹ further confirming the enhancement of the anticorrosion ability of stripy Zn array with the preferential crystal plane.

Linear sweep voltammetry (LSV) (Figure 4A) was conducted at the scan speed of 0.2 mV s⁻¹ and voltage range from −1.2 to −0.5 V for analyzing the anticorrosive effect of stripy Zn array with preferential crystal plane. Comparing with bare Zn (−996 mV, 47.3 μA cm⁻²), the increased corrosion potential as well as the reduced current density can be obtained for ZnSA-5 (−988 mV, 4.6 μA cm⁻²), ZnSA-10 (−988 mV, 4.4 μA cm⁻²), and ZnSA-15 (−993 mV, 3.8 μA cm⁻²), demonstrating the slower corrosion reaction rate and the better corrosion resistance resulting from the higher free energy of hydrogen adsorption and stronger chemical bonds of Zn (002) plane.

FIGURE 4. Linear polarization curves (A), chronoamperometry tests (B), the galvanostatic cycling (C, D, E, and F) of bare Zn and ZnSA at various current densities and fixed capacities, the comparison diagram (G) of cyclic life of Zn-Zn symmetric batteries with the report articles, and the stability of cycling and the according coulombic efficiency curves (H) of Zn-MnO2 full batteries.

Chronoamperometry (CA) (Figure 4B) was conducted at −150 mV overpotential for 200 s to indirectly demonstrate the nucleation processes of Zn²⁺ ions on the Zn metal surface. The decrease of current density for bare Zn electrodes from 0 to 200 s indicates a sustaining and rampant two-dimensional (2D) diffusion process of Zn²⁺ laterally along the surface to seek out favorable nucleation sites and minimize the surface energy, causing Zn aggregation and dendrites. The nucleation and 2D diffusion of Zn²⁺ ions of ZnSA-5, ZnSA-10, and ZnSA-15 electrodes shorten within 30, 20, and 50 s, respectively, and then stable and continuous 3D diffusion proceeds with a constant current density for the directly reduction of the absorbed Zn²⁺ ions without any lateral transfer on the surface, which can evolve into a uniform and dense Zn-plated layer.

Zn-Zn symmetric cells were assembled to explore the effect of stripy array and preferential crystal plane on the electrochemical properties of Zn metal anodes and measured at 2, 5, and 10 mA cm⁻² with 2 and 5 mAh cm⁻². In Figure 4C, the voltage-time curves of bare Zn show the obvious increase (~140 h) of the stripping and plating voltage resulting from the formation of passive layers of Zn₄SO₄(OH)₆·xH₂O. After the acid-etching processes, ZnSA-5, ZnSA-10, and ZnSA-15 electrodes (Figure 4C) exhibit better life-span than bare Zn anodes, indicating that the formation of stripy array and preferentially exposed (002) plane benefit to restrain the Zn dendrites and avoid the adverse-reaction. In addition, ZnSA-15 electrode exhibits poor cycling life and short circuit after 860 h, indicating that the long etching time can dissolve (002) plane and destroy the stripy array (Figure 1D). However, ZnSA-10 electrode (Figure 4C) can achieve stable cycling life of 2000 h, better than that of ZnSA-5, illustrating the optimal etching time is conducive to the preferred exposure of (002) plane and the generation of stripy array. The cyclic performance of ZnSA-15 is poorer than that of ZnSA-10, mainly because the decrease of (002) plane and the reduction of the height and regularity of Zn arrays can weaken the anticorrosion ability, decrease the electroactive area, and produce the ununiform deposition sites of Zn²⁺ ions. Therefore, the electrochemical properties of Zn metal anodes can be improved. The detailed voltage-capacity profiles in Figure 4C are presented in Figure S10. ZnSA-10 (69 mV) electrode possesses the lowest nucleation overpotential than that of bare Zn (78 mV), ZnSA-5 (70 mV), and ZnSA-15 (105 mV) electrodes, which contribute to achieve the uniform deposition and nucleation of Zn²⁺ ions and avoid the side-reaction of Zn electrode with aqueous electrolyte. At the current density of 2 mA cm⁻², ZnSA-10 electrode (Figure 4D) can obtain the significantly longer life-span of 2500 h than bare Zn (270 h). Increasing the current density to 10 mA cm⁻², ZnSA-10 electrode (Figure 4E) also displays the much more excellent cycling lifespan of 1200 h comparing with that of bare Zn electrode (70 h). Importantly, the stable stripping and plating processes of ZnSA-10 electrodes (Figure 4F) can be sustained for 1400 h even with the deep discharge/charge capacity of 5 mAh cm⁻² at the current density of 5 mA cm⁻², much better than of bare Zn (33 h). The cycling life of Zn metal anodes reported in recent years is summarized and compared in Figure 4G and Table S1, indicating this work takes the lead with the ultra-long cycling lifespan. The synergistic effect of regular stripy array and optimal (002) plane for ZnSA anodes can be effectively conductive to suppress Zn dendrites and the corrosion of aqueous electrolyte and enhance the cyclic stability.

Electrochemical impedance spectra (EIS) of Zn-Zn symmetric batteries using bare Zn or ZnSA as the working and counter electrodes before and after 50 cycles were measured and are recorded in Figure S11. The resistance of ZnSA electrodes is lower than that of bare Zn electrode, suggesting that the stripy array morphology increase the active area of Zn anodes and facilitate the electrochemical reaction. After 50 cycles, the resistance of ZnSA electrodes is still lower than that of bare Zn electrode and the initial state, indicating that the stripy morphology and (002) plane contribute to stabilize the electrode/electrolyte interface and uniform the electrochemical deposition processes.

To investigate the applicability of stripy Zn array in real Zn ion batteries, MnO₂-Zn full cells were assembled and are recorded in Figure 4H. XRD patterns (Figure S12) of MnO₂ cathode materials can be assigned to the standard peaks of β-MnO₂, confirming the formation of pure MnO₂ after ball-milling processes. SEM images (Figure S13) of MnO₂, AB, and MnO₂/C display the uniform mixing and the nano-sized granular morphology of MnO₂/C cathode materials. The charge/discharge curves (Figure S14) of MnO₂-Zn full batteries exhibit that the charge platforms at ~1.56 and ~1.66 V as well as the discharge platforms at ~1.40 and ~1.24 V correspond to the intercalation and de-intercalation processes of Zn²⁺ ions and H⁺, indicating the stripy morphology and (002) plane of Zn anodes cannot influence the electrochemical behavior of MnO₂ cathodes. The capacity of MnO₂-ZnSA full batteries increase gradually to 100 mAh g⁻¹ at 36^th cycle resulting from the electrode activation of typical behavior for MnO₂-Zn batteries⁴² and still achieve 65 mAh g⁻¹ capacity after 1600 cycles at 1 A g⁻¹, better than that of MnO₂-bare Zn (35 mAh g⁻¹) full batteries, demonstrating the good reversibility due to the stripy array morphology and the preferential crystal plane of ZnSA electrodes.

The in-situ optical microscope was applied for dynamic observation of the plating processes of bare Zn and ZnSA-10 electrodes via the symmetric transparent cells (Figure S15) after different plating times of 0, 10, 20, 30, and 40 min at the current of 1 mA. In Figure 5A, the generation of bubble (marked by red circle) for bare Zn electrodes after 10 min demonstrates the occurrence of hydrogen evolution on bare Zn surface. The formation of irregular bulges from 20 to 40 min results from the uneven plating of Zn, which can ultimately evolve into dendric morphology at the high current (Figure S16). In sharp contrast, ZnSA-10 electrodes (Figure 5B) treated by concentrated phosphoric acid, exhibit smooth and compact surface, no dendritic morphology and hydrogen bubbles, originating from the formation of stripy array and preferential (002) plane. Comparing with Zn (100) crystal plane, Zn (002) crystal plane has higher free energy of hydrogen adsorption, which is helpful to suppress the hydrogen evolution and corrosion reaction for ZnSA-10 electrodes. In the meantime, Zn (002) crystal plane can provide more advantageous sites with flat equipotential surface as well as the compact structure, inducing parallel deposition of Zn metal on the surface of ZnSA-10 electrodes.³⁷ In addition, the stripy array morphology of ZnSA-10 electrode can offer more deposition sites and boost the homogeneous plating of Zn²⁺ ions,³⁶ which contribute to control the homogeneous Zn plating processes and restrain the Zn dendrites.

FIGURE 5. The operando optical microscope photos of bare Zn (A) and ZnSA-10 (B) electrodes via the symmetric transparent cells at the plating times of 0, 10, 20, 30, and 40 min.

To study the effect of stripy array and preferential crystal plane on the construction and surface morphology of Zn metal anodes during the charge/discharge processes, bare Zn and ZnSA electrodes were cycled for 10 cycles and characterized by XRD, FTIR, and SEM. The new diffraction peaks of the cycled bare Zn electrodes (Figure 6A) at 9.5°, 32.1°, and 33.9° are assigned to Zn₄SO₄(OH)₆·xH₂O, due to the reaction of Zn²⁺ with OH⁻ from the hydrogen evolution reaction.⁴³ Nevertheless, the characteristic peaks of Zn₄SO₄(OH)₆·xH₂O for the cycled ZnSA electrodes (Figure 6A) are much weaker than that of bare Zn, revealing that the stripy array and preferential crystal plane can resist the corrosion reaction of aqueous electrolyte. Surprisingly, as shown in Figure 3B, the peak intensity ratio of I₍₀₀₂₎/I₍₁₀₀₎ of ZnSA (1.8–3.4) is stronger than that of untreated pristine Zn (0.7), revealing that the stripy array and preferential (002) crystal plane can induce the preferred orientation crystal of Zn metal during the processes of Zn plating and stripping. The cycled ZnSA-10 electrode (3.4) possess higher peak intensity ratio (I₍₀₀₂₎/I₍₁₀₀₎) than that of ZnSA-5 (1.0) and ZnSA-15 (1.8) electrodes, indicating that the better stripy array and stronger (002) peak intensity of pristine ZnSA-10 contribute to the nucleation of (002) crystal plane during the plating processes. The peak intensity ratio of I₍₀₀₂₎/I₍₁₀₀₎ for ZnSA-10 electrode increase from 1.8 before cycling to 3.4 after 10 cycles, indicating that the preferential (002) plane with smooth equipotential surface and compact structure on the surface of Zn metal can provide more advantageous sites for Zn deposition and induce to form plate-like Zn with parallel to the [002] direction of Zn crystal.^37,44,45 FTIR peaks (Figure 6B) of stretching vibration at 1162, 1104, and 1037 cm⁻¹ and bending vibration at 960 cm⁻¹ of SO₄²⁻ can be distinctly observed for the cycled bare Zn electrodes.¹¹ Nevertheless, the cycled ZnSA electrodes possess much weaker peaks of by-products (Zn₄SO₄(OH)₆·xH₂O) because the passive (002) crystal plane and stripy array can avoid the occurrence of corrosion reaction.

FIGURE 6. XRD patterns (A), FTIR spectra (B) and SEM images of bare Zn (C and G), ZnSA-5 (D), ZnSA-10 (E and H), and ZnSA-15 (F) after 10 cycles (Inserts are the corresponding high magnification images).

The top-view and section-view of SEM photos and the corresponding element mapping images of the cycled electrodes can be observed in Figure 6C-F, Figures S17, and S18. The rugged surface with dendritic morphology stacked by large numbers of flakes and more S and O elements can be observed for the cycled bare Zn electrodes (Figure 6C and Figure S17), demonstrating the uneven plating of Zn²⁺ and the serious side-reaction with aqueous electrolyte, which would cause short circuit in the battery system. The cycled ZnSA electrodes (Figure 6D-F and Figure S18) possess flat surface, clear boundary, and less S and O elements after repeated plating and stripping processes, indicating that their preferential (002) crystal plane with the stronger adsorption energy with Zn induce the parallel deposition on basal (002) plane and sustain the horizontal growth of Zn (002) surface during the subsequent cycling,³⁷ which confirms the above XRD results. Besides, the stripy array of ZnSA electrodes can provide more deposition sites and result in the homogeneous plating of Zn metal. ZnSA-10 electrode (Figure 6E) displays smoother surface than that of ZnSA-5 (Figure 6D) and ZnSA-15 (Figure 6F) electrodes, demonstrating that more (002) crystal plane and better stripy array are helpful for the horizontal growth of Zn metal during the plating processes. XPS spectra of Zn 2p and S 2p of the bare Zn and ZnSA-10 electrodes after cycling were tested and are presented in Figure S19. The stronger Zn 2p and poorer S 2p peaks of the cycled ZnSA electrodes as compared with that of the cycled electrodes of bare Zn demonstrate the infinitesimal Zn₄SO₄(OH)₆·xH₂O and stable electrode/electrolyte interface on ZnSA electrodes benefiting from synergistic effect of the preferential (002) plane and stripy array. Zn LMM Auger peak (Figure S20) of ZnSA-10 after 10 cycles exhibits the presence of ZnO layers on the surface of Zn metal,⁴¹ indicating the stable electrode/electrolyte interface can be well maintained.

The special structure of as-synthesized stripy Zn array with preferential (002) crystal plane endows Zn metal anodes with excellent cycling stability. The higher atomic coordination and tighter atomic bonds of Zn (002) plane endow its stronger resistance of acid etching as compared with Zn (100) plane, as shown in Scheme 2A. The selective etching of Zn (100) plane occurs at the interface of Zn metal and concentrated phosphoric acid (H₃PO₄) to form hydrogen (H₂) and zinc phosphate (Zn₃(PO₄)₂), while Zn (002) plane cannot be corroded at short time because of its higher atomic coordination and tighter atomic bonds. The formed Zn₃(PO₄)₂ can dissolve in H₃PO₄ solution and be removed through the water wash processes. The hydrogen evolution and corrosive reactions and the dendrite growing of bare Zn (Scheme 2B) during the plating process deteriorate the electrochemical properties of Zn metal anodes. Nevertheless, ZnSA electrodes (Scheme 2B) exhibit better anticorrosion ability and no hydrogen evolution, resulting from high electroactive surface area and low local current density of stripy array and high hydrogen adsorption free energy and strong chemical bonds of (002) plane.^36,37 In addition, the preferential (002) crystal plane with the stronger atomic packing density and adsorption energy with Zn induce the parallel deposition on basal (002) plane and sustain the horizontal growth of Zn (002) surface.³⁷

SCHEME 2. The schematic diagram of the selective etching (A) and the plating (B) processes of bare Zn and ZnSA electrodes.

CONCLUSIONS

In summary, we have successfully fabricated stripy Zn array (ZnSA) with the preferential crystal plane through the treatment with concentrated phosphoric acid. The depth of stripy morphology of Zn array and the content of (002) plane on Zn surface are adjusted via the optimalization of etching time. The stripy Zn array can increase the electroactive surface area and thus decrease the local current density, which provide the uniform nucleation sites, boost the homogeneous Zn plating, and restrain the Zn dendrites. In addition, the preferential (002) crystal plane with high atomic coordination and free energy of hydrogen adsorption, tight atomic bonds and packing density, and strong chemical bond and adsorption energy with Zn can resist the acid etch, suppress the hydrogen evolution and the corrosion reaction triggered by aqueous electrolyte, and induce the parallel deposition on basal (002) plane and the horizontal growth of Zn (002) surface. The synergistic effect of stripy array and preferential (002) plane endow the ultra-long lifespan of 2500 h for ZnSA electrodes with 2 mA cm⁻² current density and 2 mAh cm⁻² fixed capacity, better than the previous reported works. An effective strategy is provided by this work for suppressing the corrosion reaction of aqueous electrolyte, restraining the Zn dendrites, and enhancing the cycling stabilization of Zn metal anodes through the synergistic effect of stripy array and preferential crystal plane.

ACKNOWLEDGMENTS

This work was supported by grants from NSFC-CONICFT Joint Project (51961125207), Innovation Support Program for High-level Talents of Dalian (Top and Leading Talents) (201913), Liaoning Province “Xingliao Talent Plan” Outstanding Talent Project (XLYC1901004), Natural Science Foundation of Liaoning Province (2021-MS-301), Basic Scientific Research Project of Education Department of Liaoning Province (LJKZ0546), and Scientific Research Foundation of Dalian Polytechnic University (6102072111).

CONFLICT OF INTEREST

The authors declare no conflict of interest.