2.1 Preparation and Structural Features of the Zn Anode

2.2 Theoretical and Experimental Analysis of the Interfacial Reaction of the Zn Anode

Theoretical analysis and DFT calculations were conducted at the atomic level to explore preferable Zn dendrite suppression and CE improvement by driving the exposed (002) plane. The Zn (002) plane with a smooth equipotential surface and compact structure provides more advantageous sites for Zn deposition, effectively delaying the initial dendrite formation point.^[16] DFT results revealed that the (002) plane could afford a special deposition location site 1 at the early stage because of the stronger adsorption energy (−0.87 eV) between the (002) plane surface and Zn, compared to that of site 2 (−0.50 eV) (Figure 2d and Figure S3, Supporting Information). Thus, Zn²⁺ was deposited evenly and parallel to the basal (002) planes, maintaining a horizontal growth on the Zn (002) surface during the subsequent cycling. In this respect, the inhabiting mechanism by changing the surface atomic structure causes the spontaneous reorientation of Zn crystallites from dominantly homeotropic to orientations parallel to the electrode surface;^{[1, 2]} Zn dendrites do not emerge sequentially on the Zn (002) anode. However, the surface of the Zn (100) electrode has an uneven electric field for Zn dendrite aggregation, wherein Zn²⁺ ions tend to initiate individual nucleation points on the top of the Zn (100) layer (site 2, −1.39 eV) and subsequently continue to grow vertically (Figure 2d and Figure S4, Supporting Information).

Apart from Zn dendrites, Zn anodes also suffer from low plating/stripping CEs when an aqueous electrolyte is used. An important reason might be the interfacial side reactions, including the hydrogen evolution (HER) and corrosion reactions. To further understand the mechanism of the enhanced HER performance, the DFT was used to calculate the free energy of H adsorption on Zn anodes based on previous reports.^[17] The thermo-neutral adsorption of H atoms implies a high activity of the HER.^{[18, 19]} The free energy of Pt (111) is −0.09 eV, which is almost thermo-neutral. Notably, the free energy of hydrogen adsorption for the Zn (100) anode is close to thermo-neutral (≈0.7605 eV), as shown in Figure 2e and presented in Table S2 (Supporting Information). Conversely, the Zn (002) peak increased to ≈1.0855 eV (Figure 2e and Table S2, Supporting Information), indicating the suppressed HER by tuning more (002) planes exposed on the Zn anode surface. The corrosion resistance of the Zn (002) anode can also be improved, accompanied by the suppression of the HER. The calculation of Zn tripping off each anode surface offers further insight into the favorable corrosion resistance of the Zn (002) anode. A higher wasted energy of 1.847 eV for Zn (002) compared to 1.651 eV for Zn (100) indicates a possible pronounced interionic attraction between Zn; therefore, the (002) plane possesses a stronger chemical bond to prevent the corrosion of Zn (Figure 2f and Figure S5, Supporting Information). The charge distribution modeling and DFT simulation results collectively corroborate the advanced features of the Zn (002) anode toward dictating uniform Zn deposition along the ab-plane and alleviating side reactions (H₂ evolution, corrosion, and by-products).

To verify the mechanism of Zn deposition behavior, chronoamperometry (CA) characterization was conducted on symmetric cells (Figure 3a). In a CA result, the variation in current density with time at a constant potential can sensitively reflect the nucleation process and surface change.^[20] During the rampant 2D diffusion process, the absorbed ions laterally diffuse along the surface to find the most energetically favorable sites for charge transfer. 3D diffusion implies that Zn²⁺ ions absorbed on the surface appear to be locally reduced to Zn⁰ with constrained 2D surface diffusion.^[21] Under a constant voltage of −150 mV, the current curve of Zn (002) is almost unchanged and stable at 17 mA cm⁻² after short 2D diffusion (15 s), reflecting a constant 3D diffusion process after nucleation. A linear increase curve of Zn (100) demonstrates a rampant 2D diffusion process and uneven dendrite growth because of the “tip effect” that preferentially accumulate into the higher active sites and grow vertically.^[20] The Zn plating quality can also be determined by the nucleation overpotential.^[22] The typical voltage profiles of Zn deposition (Figure S6, Supporting Information) show that Zn (002) displayed 17.6, 10.2, and 4.6 mV in three repeated experiments, respectively, at 0.5 mA (0.44 mA cm⁻²). When the current increased to 1 (0.88 mA cm⁻²), 2 (1.77 mA cm⁻²), and 5 mA (4.42 mA cm⁻²), the corresponding nucleation overpotential also showed an increasing trend, as shown in Figure 3b. A similar trend was observed for Zn (100), indicating that the nucleation overpotential is proportional to the current density to some extent. The competitive H₂ evolution at the anode side inevitably disturbs the normal recharging process; the resulting change in local pH increases the accumulation of by-products on the anode surface^[23] (Figure S7, Supporting Information). This is in line with the dissolution of Zn and the absorption of H, as determined by DFT calculations. The HER was measured by linear sweep voltammetry (LSV), as the corrosion and deposition of Zn inevitably compete with the H₂ evolution.^[24] Zn (002) obviously lowered the current density of the hydrogen reaction at voltages between −0.6 and −0.9 V (vs reversible hydrogen evolution, RHE), especially before the Zn deposition realm of −0.76 V. This strongly reveals the suppression ability of the preferred orientation crystal endowed by the Zn (002) plane (Figure 3c). Meanwhile, the Zn (002) anode shows a lower slope (b) of 178.9 mV dec⁻¹ than that of the Zn (100) anode (204.0 mV dec⁻¹; Figure 3d). According to the Tafel equation (Equation (S1), Supporting Information), the corresponding exchange current density of hydrogen evaluation ($$) was calculated to be 8.79 × 10⁻⁵ mA cm⁻² for the Zn (002) anode. This is almost nine times lower than that of Zn (100) (7.89 × 10⁻⁴ mA cm⁻²; Figure 3d and Table S3, Supporting Information), which further verifies its ability to avoid the hydrogen adsorption and generate H₂. This is in accordance with the regulation of the Zn (002) surface, which has a stronger corrosion resistance than the Zn (101) and (100) surfaces in alkaline systems.^{[13, 25]} The improved performance of Zn (002) was also verified by the decreased corrosion current, higher ionic conductivity (Table S4, Supporting Information), and increased electroactive surface area (Table S5, Supporting Information), as shown in Figure 3e and Figure S8 (Supporting Information). More direct evidence for modulated Zn plating/stripping enabled by this stable Zn (002) anode was obtained from two-electrode system observations with a constant current charge–discharge. Obviously, many black pits are located on the rough Zn surface, and many hydrogen bubbles surround the Zn (100) anode (Figure 3f). The microstructure from the scanning electron microscopy (SEM) image indicates the corresponding protuberance. Owing to electrostatic attraction, ions (OH⁻, Zn²⁺, SO₄²⁻, and H₂O) reach a limited solubility, leading to the formation of by-products and further reducing the coulombic efficiency.^[26] The Zn (002) anode surface remained smooth after more than ten cycles of Zn stripping/plating and showed almost no bubbles (Figure 3g). This is distinct from the Zn (100) electrode, wherein an uneven structure with a Zn dendrite or by-product is displayed, which is harmful to the reversibility of Zn-based batteries. A comprehensive consideration of the electrochemical properties discussed above indicates that Zn (002) is optimum to inhabit Zn dendrites and improve CEs because of the smoother surface and stronger atomic binding forces of the (002) crystal plane.

2.3 Electrochemical Performance and Mechanism Analysis of Zn/Zn Symmetric Batteries

The superior performance of the Zn (002) anode was demonstrated under galvanostatic conditions in a Zn/Zn symmetric configuration. The cell with the Zn (002) anode exhibited a much lower polarization of 71 mV at the start under a current density of 1 mA cm⁻², in contrast to 107 mV of Zn (100). Zn (002) can maintain 1000 cycling plating/stripping processes with extremely stable voltage–time curves with 38 mV voltage hysteresis at the end of 500 h, whereas Zn (100) exhibits large fluctuations and gradual short-circuits (Figure 4a). The good cycle life of the Zn (002) anode was also observed when cycled at elevated rates of 2 and 5 mA cm⁻² (Figure S9, Supporting Information). Furthermore, A-Zn (002)-#1 and A-Zn (002)-#2 anodes with different levels of (002) plane exposure also exhibited better cycling life than that of the commercial Zn (100) anode (Figure S2c, Supporting Information). Notably, in the early stage, Zn (100) and Zn (002) showed similar overpotentials at each current density. However, during long-term cycling, the overpotential of Zn (100) was much smaller than that of Zn (002). This may be ascribed to the formation of Zn tips on the surface of Zn (100) after initial cycling, which is favorable for the formation of more active sites and an ion flow concentration. However, this aggravates the uneven deposition of Zn (100) and deteriorates its electrochemical performance. Conversely, even increasing the capacity of the charging–discharging process can maintain the cycle life of Zn/Zn symmetrical batteries for more than 200 h, and there is little influence on the interfacial structure and morphology of the Zn (002) anode (Figure S10, Supporting Information). Further investigation of the Zn (002) anode was conducted at 0.2–5 mA cm⁻². The cell with the Zn (002) anode exhibited voltage hysteresis of 42 and 62 mV at 2 and 5 mA cm⁻², respectively, whereas a terrible short-circuit was observed for those with the (100) plane (Figure 4b), which could be ascribed to the severe side reactions occurring simultaneously with the Zn deposition.^[27] Direct support for our hypothesis comes from the field emission SEM observation, electron probe microanalysis (EPMA), and XRD spectrum analysis (Figure 4c–e). Unlike the even surface of Zn (002), it is apparent that a mass of inhomogeneous deposit with obvious tips was formed on the Zn (100) anode during charging–discharging, and highly accumulated Zn can be easily detected in those tips from the color index section in the EPMA mapping. This was further confirmed by the wavelength-dispersive X-ray (WDS) results, where the Zn distribution ratio ranges from 91.32 to 97.64%. Such different behaviors indicate the decisive role of the surface atomic structure in inhibiting Zn dendrites. The XRD patterns (Figure 4e) of the Zn (002) anode matched well with the Zn metal phase (PDF no. 04-0831) after 50 cycles. Conversely, with the exception of the Zn metal phase, several obvious peaks arise on the Zn (100) anode after 50 cycles, which correspond to the irreversible Zn₄SO₄(OH)₆·H₂O phase (PDF No. 39-0690, Figure 4e inset). This suggests a highly reversible reaction occurring at the surface and thus, an excellent interface stability of the Zn (002) anode. Raman spectra further verified the cycling stability of the metallic Zn (002) anode (Figure S11, Supporting Information). The CEs of Zn plating/stripping, the most critical parameters responsible for the redox reversibility, were investigated in the Zn/Cu coin cell configuration (Figure 4f).^[1c^] Notably, the cell using the Zn (002) anode exhibited a superior cycling performance by maintaining an average CE of over 97.71% after ten cycles; alternatively, the Zn (100) cell exhibited a lower average CE of 95.21%. The increased CEs are mainly attributed to the (002) plane that inhibits the side reactions and facilitates the formation of even nuclei and reversible Zn plating/stripping.^{[27, 28]} As another reliable method,^[3] Zn-Cu unsymmetrical cells after long-term cycling were also applied to evaluate the superior CEs of the Zn (002) anode (Figures S12 and S13, Supporting Information). In situ optical visualization observations of the Zn deposition under a high current of 5 mA and 240 s of intermittence were obtained to record the growth process of Zn dendrites. After 120 s of plating, nuclei or protrusions were observed at the edges and on the surface, which evidences inhomogeneous Zn plating on the Zn (100) anode (Figure 4g). Under further plating, some nuclei evolve into Zn dendrites on the edge, eventually inducing a short-circuit.^[29] For the cell using Zn (002), uniform and compact Zn deposits can be achieved at a current of 5 mA, which further confirms the availability of the (002) basal plane in the aqueous electrolyte. Thus, it can provide a better interface for the nucleation and simultaneously suppress side reactions by regulating the surface atomic structure through a preferred orientation crystal for the Zn anode.^{[12, 30]}

2.4 Highly Stable Zn-Ion Full Batteries

Manganese-based and vanadium-based cathodes have attracted great attention as promising choices for aqueous ZIBs.^[31] In support of the advanced effect of Zn (002) in aqueous ZIBs, the Zn (002) anodes were matched with MnO₂ and NH₄V₄O₁₀ cathodes. Detailed characterizations of the two cathodes are shown in Figures S14–S16 (Supporting Information). As expected, the cyclic stability of the Zn (002) full cells outperformed their Zn (100) counterparts at each current density. By virtue of the low polarization and high reactivity of the Zn (002)/MnO₂ cell (observed from the CV profiles and electrochemical impedance spectroscopy (EIS) test; Figure S17 and Table S6, Supporting Information), the Zn (002) anode exhibited a long cycle life of up to 1800 cycles along with stable CEs of ≈100% at 0.5 A g⁻¹. However, the Zn (100)/MnO₂ battery failed to maintain stable cycling and CEs (Figure 5a). The corresponding selected discharge and charge curves for the different cycles are shown in Figure S18 (Supporting Information). A similar phenomenon was observed at 0.2 A g⁻¹ (Figure S19, Supporting Information), which could be ascribed to the severe parasitic reactions occurring simultaneously during the Zn deposition,^[32] along with the uncontrolled dendrites.^[33] The effect of the (002) plane on parasitic reactions was evaluated by monitoring the open-circuit voltage decay of fully charged Zn/MnO₂ batteries with 2 m ZnSO₄ + 0.1 m MnSO₄ and subsequently discharging after 60 h of storage. 96.61% of the original capacity was retained (Figure 5e) in the cell using the Zn anode with more (002) planes exposed and that of the Zn (100) anode exceeded 92.35% (Figure 5f). Consequently, the long cycling at low current density and rate capacity (Figure S20, Supporting Information) also exhibits a much better cyclic stability of the Zn (002) anode. A good application prospect for Zn (002) anodes is also evident when matched with vanadium-based cathodes. Compared with the long-term cycling performance of the Zn/NH₄V₄O₁₀ full batteries at 5 A g⁻¹ (Figure 5d), the cycling stability of Zn (002) improved significantly, as demonstrated by the corresponding discharge and charge curves (Figure S21, Supporting Information). Even after rest for 48 h, a highly reversible specific capacity of nearly 116.3 mAh g⁻¹ was observed; 96.65% of the initial capacity could be retained after 1000 cycles with a high average CE (≈100%). However, the capacity of the cell using the Zn (100) anode rapidly decayed to 88.9 mAh g⁻¹ (capacity retention < 30.75%) after 1000 cycles and a CE fluctuation occurred after rest, which is mainly ascribed to the formation of an insulating passivation layer on the Zn anode (Figure S22, Supporting Information) that blocks the interfacial transport of Zn²⁺, resulting in an increase in the polarization. The CE of Zn (002) still approached 83.46% after discharging for 60 h (Figure 5e); conversely, an inferior CE of 82.08% was obtained for Zn (100) (Figure 5f) under identical conditions. The cycling stability of Zn (002) was also verified by charging–discharging cycling at low current densities and rate capacity tests (Figure S23, Supporting Information). To verify the availability of the exposed (002) planes of the Zn anode, we further investigated the digital (Figure 5g) and SEM images (Figure 5h,i) for the separators and Zn surfaces of Zn/MnO₂, as representative after 50 charging–discharging cycles. Unsurprisingly, an obvious protuberance was observed on the Zn (100) anode surface, and some black products accumulated on its corresponding separator surface, whereas a flat surface accompanied by a clear separation was obtained for the Zn (002) anode. A large amount of leakage hangs at the edge of the Zn (100) anode battery matches well with the uneven Zn (100) surface, as mentioned previously (Figure S24, Supporting Information). In the SEM images of the anodes, a well-preserved morphology was observed on the Zn (002) electrode surface after long-term cycling. Conversely, the Zn (100) electrode was covered with numerous vertical tips and extrusions with micrometer-level lateral sizes, which could be attributed to the inhomogeneous plating and corrosion of Zn (100). Atomic force microscopy (AFM) analysis also showed a tiny and uniform Zn deposits arranged on the Zn (002) surface, and protuberant and uneven ones on Zn (100) surfaces (Figure S25, Supporting Information). Similar to the function of the anode obtained in surface-modified cells,^{[1, 3]} this rich (002) plane on the Zn surface can induce regular Zn deposition and eliminate the dissolution of atoms in contact with the electrolyte. Therefore, this effectively inhibits the Zn dendrites and interface side reactions (such as corrosion and H₂ evolution) during storage. The comparison of typical parameters and cycling performance of this work with recently reported Zn-based symmetric cells are presented in Table S7 (Supporting Information). The Zn (002) metal delivers a superior electrochemical behavior compared to other common Zn, indicating a better resistance to the Zn dendrites and by-products. The comparison of the Zn-based full cell (Table S8, Supporting Information) also reveals improved electrochemical properties by optimizing the surface structure by exposing more (002) basal planes.

2.5 Summary

In summary, we have demonstrated that exposing the preferred crystal plane, for example, the (002) plane, on the Zn anode surface is a feasible strategy for highly stable and reversible Zn anodes. Owing to the even interfacial charge density and stronger adsorption energies in the parallel direction rather than the vertical direction on the Zn (002) anode surface, Zn was deposited uniformly along the (002) crystal plane during the initial cycling process and subsequently grew laterally. The side reactions of the Zn (002) anode can be effectively prevented owing to the strong catching ability of Zn atoms and the high free energy for hydrogen evolution. With this structural modulation, dendrite-free and intrinsically reversible Zn plating/stripping can be realized at an areal capacity of 1 mAh cm⁻² for more than 500 h in Zn/Zn symmetric batteries using an aqueous ZnSO₄ + MnSO₄ electrolyte with a CE of 97.71%. This significantly improves the electrochemical performance of the full-cell Zn/MnO₂ and Zn/NH₄V₄O₁₀ batteries. In particular, Zn/MnO₂ batteries deliver long cycling stability and reversibility with life spans of over 2000 cycles. The strategy of regulating the preferred orientation crystal of Zn anodes could open research avenues for the development of highly stable and reversible metallic anodes for next-generation secondary batteries.