Synthesis and characterization of ZnSe overlayer.
Fig. 1a schematically depicts the synthetic set-up for the in-situ growth of ZnSe overlayer on commercial Zn foil by CVD. The Zn foil serving as the growth substrate is placed at the downstream of the Ar/H2 gas flow, which assists to carry the sublimed Se powders coming from the upstream (See Methods). This ambient-pressure process is facile, simple, economical and potentially scalable. It also bypasses the lengthy high-temperature annealing and tedious vacuum operations. The size of the product, ZnSe@Zn, is merely limited by the furnace dimension. Fig. 1b presents a digital photograph of a 20 cm × 10 cm sized ZnSe@Zn foil with perfect film homogeneity produced in a 4-inch tube furnace, demonstrating the viability of large-scale synthesis toward practical applications. The surface morphology of ZnSe overlayer was examined by scanning electron microscopy (SEM) and atomic force microscopy (AFM). As shown in Fig. 1c, the full coverage of ZnSe layer upon CVD reaction is helpful to even out a plethora of sharp protuberances on bare Zn surface (Supplementary Fig. 1). Close-up view further suggests that thus-grown ZnSe exists in the form of nanoparticles with diameters of 30-50 nm (Fig. 1c inset; Supplementary Fig. 2). Such a three-dimensional porous texture would be beneficial to Zn2+ transport. Notably, the thickness of ZnSe overlayer can be simply dictated by controlling the CVD synthetic parameters, i.e., altering the temperature ramping rate and dwelling duration (Fig. 1d; Supplementary Fig. 3). Fig. 1e presents a side-view SEM image of ZnSe overlayer (affording a thickness of 0.75 μm), where corresponding elemental maps manifest uniform distribution of detected elements.
The wettability to electrolyte is one of the key factors for reversible Zn stripping/plating as it exerts influence upon interfacial ion-transfer resistance8. In this respect, bare Zn foil possesses a fairly poor wettability by 2 M ZnSO4 electrolyte, displaying a contact angle of 82° based on static contact angle measurement (Fig. 1f). In contrast, the contact angle sharply declines to 14° on ZnSe@Zn foil (ZnSe thickness: 0.75 μm), implying markedly enhanced wettability (Fig. 1g). The contact angle values in the case of ZnSe@Zn foil with different ZnSe thickness are further compared (Supplementary Fig. 4), amongst which the 0.75 μm-thick ZnSe harvests the smallest contact angle. Collected XRD patterns of ZnSe@Zn and bare Zn indicate that the ZnSe grows primarily along (111) direction (Fig. 1h; Supplementary Fig. 5)29. Recognizable Raman signals further verify the successful preparation of ZnSe overlayer (Supplementary Fig. 6).
Electrochemical performance of ZnSe@Zn
To evaluate the plating/stripping reversibility and Zn utilization of Zn anode with/without ZnSe protection, Coulombic efficiency (CE) measurements were carried out in two-electrode cells (ZnSe@Ti–Zn and Ti–Zn) with a fixed capacity of 0.5 mAh cm−2 at a current density of 2.0 mA cm−2. The ZnSe coating on Ti foil was obtained by CVD to derive the ZnSe@Ti electrode (Supplementary Fig. 7). In terms of the galvanostatic cycling performance, ZnSe@Ti–Zn cell presents an initial plating-stripping voltage hysteresis of 87 mV, which is obviously lower than that (133 mV) of bare Ti–Zn cell (Fig. 2a inset; Supplementary Fig. 8). Encouragingly, ZnSe@Ti–Zn cell could maintain 400 cycles with an average CE of 99.2%, indicating favorable reversibility and excellent durability (Fig. 2a). In stark contrast, bare Ti–Zn cell merely sustains for 50 cycles with drastic fluctuations of CE values, suggesting the presence of side reactions and dendrite growth. It is safe to conclude that the ZnSe overlayer would well manipulate the nucleation and growth of Zn toward a highly reversible anode.
To verify the advanced effect of anode protection via in-situ formed ZnSe overlayer, galvanostatic cyclic stability of symmetric cells was evaluated under various current densities and areal capacities. Fig. 2b shows the long-term cycling performance of bare Zn and ZnSe@Zn symmetric cells at 1.0 mA cm−2 with a capacity of 1.0 mAh cm−2. Upon cycling for ~80 h, a sudden and irreversible voltage rise occurs for bare Zn cell, which could be attributed to the accumulation of adverse “dead Zn” and by-products. The presence of these species is detrimental throughout occluding ion transport pathways, resulting in a large voltage hysteresis of 128 mV. In contrast, ZnSe@Zn symmetric cell readily displays a far more stable voltage profile with much declined voltage hysteresis of 30 mV that sustains more than 860 h, approximately 10 times longer in comparison with bare Zn cell. Note that our CVD route is versatile enough to enable the delicate control over the thickness of ZnSe layer, where an optimized thickness at 0.75 μm could be gained in response to generating the smallest polarization (Supplementary Fig. 9). This result echoes well with the observation from electrolyte wettability tests. The refinement of electrolyte/ZnSe@Zn interface, which expedites the electrokinetics of Zn deposition, can further be confirmed throughout electrochemical impedance spectroscopy (EIS) analysis. As disclosed in the Nyquist plots in Fig. 2c, the impedance of the bare Zn cell manifests a remarkable increase (from 0.7 to 7.7 kΩ) after 10 cycles, whereas the impedance of the ZnSe@Zn cell shows a slight decrease from 0.4 to 0.25 kΩ, revealing that the ZnSe overlayer is competent in declining charge transfer resistance in aqueous ZnSO4 electrolyte. According to Sand’s model30, the formation time of dendrite is inversely proportional to current density. In addition, the interfacial electric field becomes rather uneven under elevated current densities, inducing less yet adverse nucleation sites. Areal specific capacity is also a nontrivial factor, where a higher one would augment the dendrite size. Therefore, Zn plating/stripping behavior quickly deteriorates under high current densities/capacities due to rampant dendrite formation7. Impressively, the ZnSe@Zn cell still remains highly stable for more than 250 h even under the high current density/capacity at 10.0 mA cm−2/10.0 mAh cm−2 (Fig. 2d; Supplementary Fig. 10), representing one of the best performances achieved by far in such stringent conditions (Supplementary Table 1). Fig. 2e displays the rate performances of symmetric cells with a fixed capacity of 1.0 mAh cm−2 under varied current densities from 1.0 to 10.0 mA cm−2. Apparently, ZnSe@Zn cell harvests a much lower voltage hysteresis at all current densities in comparison with bare Zn counterpart, demonstrating superior stability and high reversibility. In addition, dramatic reduction of nucleation overpotential tested under various conditions was witnessed upon the introduction of ZnSe overlayer (Supplementary Fig. 11), which is conducive to uniform deposition of Zn.
Mechanism analysis on ZnSe protection effect
To probe the electrochemical protection mechanism of ZnSe overlayer in ZnSO4 electrolyte system, in-situ/ex-situ scrutinization in combination with electroanalytic characterization were carried out. First of all, real-time Zn plating/stripping process in transparent cell configuration was visualized with the aid of operando optical microscopy. A current density of 5.0 mA cm−2 was employed for the electrodeposition. As depicted in Fig. 3a, a number of randomly distributed protrusions commence to appear on the surface of bare Zn anode after 30 min deposition. When the deposition time reaches 60 min, the growth of dendrites is evident. In comparison, ZnSe@Zn anode retains a smooth surface texture during the entire plating process with no discernible dendritic formation (Fig. 3b), intuitively reflecting the capability of ZnSe overlayer in restraining the dendrite growth. Post-mortem SEM characterization of bare Zn and ZnSe@Zn anodes in transparent cells was conducted after 40-cycled plating/stripping process at 1.0 mA cm–2 (Fig. 3c-f; Supplementary Fig. 12). As for the bare Zn anode, along with the mossy Zn generated on the surface, noticeable pits (marked by dotted cycle in Fig. 3c) are also produced owing to the uneven plating/stripping and severe side reactions31, 32. By contrast, the ZnSe@Zn anode affords a compact, uniform and dendrite-free morphology with no observed protuberances and/or pits (Fig. 3e). Moreover, the thickness of deposited Zn on ZnSe@Zn anode (15 μm) stays much smaller as compared to that on bare Zn (136 μm), indicative of effective mitigation of dendrite formation with the ZnSe overlayer. Supplementary results for electrodes cycled at 5.0 mA cm–2 in transparent cells and at 1.0 mA cm–2 in coin cells further corroborate the positive impact of ZnSe protection (Supplementary Fig. 13 and 14).
Post-mortem XRD was performed to identify the composition change of cycled electrodes. As shown in Fig. 3g, Zn4SO4(OH)6•4H2O by-product is proven to be generated on bare Zn upon cycling. This insoluble species would inevitably induce high overpotential of Zn symmetric cell due to its impenetrable nature for Zn ion. In contrast, there is no trace of by-product diffraction peak for ZnSe@Zn electrode, implying high reversibility of Zn dissolution/deposition reaction by virtue of ZnSe overlayer. Note further that the peak intensity ratio of Zn (002) to Zn (101) plane for ZnSe@Zn after cycling significantly augments about 12 times as compared to that for bare Zn. The existing similarity of lattice structure between ZnSe (111) and Zn (002) plane possibly guide an oriented Zn formation (Supplementary Fig. 15 and 16), where ZnSe layer can act as a template to induce (002) monocrystal Zn. Such a Zn growth behavior is desirable for anode protection17, 33. More intriguingly, there is no sign of ZnSe signals whatsoever for the ZnSe@Zn electrode after cycling, indicating the fall-off of ZnSe coating from the Zn foil, in good agreement with visualized characterizations (Supplementary Fig. 16; Supplementary Video 1). This phenomenon stems primarily from the fact that Zn is deposited at the interface between ZnSe overlayer and underlying Zn foil, where a flat and oriented Zn film is readily formed prior to the ZnSe detachment. Thus-produced Zn film enables to continuously guide a uniform Zn deposition even in the absence of ZnSe coating. Collectively, our results demonstrate that ZnSe overlayer is crucial to achieve optimized interfacial manipulation targeting highly reversible Zn anode.
The suppression effect on Zn corrosion via ZnSe overlayer was analyzed by linear polarization tests in 2 M ZnSO4 electrolyte (Fig. 3h). In comparison with bare Zn, the corrosion potential of the ZnSe@Zn reaches −1.013 V (−1.017 V for bare Zn), suggesting that it is less prone to corrosion. Meanwhile, the declined corrosion current by ca. 300 μA cm–2 again represents a retarded corrosion rate34. It is well received that the corrosion mainly originates from hydrogen evolution reaction (HER) accompanied by Zn dissolution/deposition reaction in weakly acidic ZnSO4 electrolyte11. Along this line, HER activity was additionally evaluated by linear sweep voltammetry (LSV) measurements24. As displayed in Fig. 3i, it is striking to find that ZnSe@Zn electrode harvests a depressed HER capability as compared to that of bare Zn electrode.
The solvation of Zn2+ is a major obstacle for circumventing rapid transport of Zn ions throughout the interface between electrolyte and Zn anode24, 35. The activation energy (Ea), which represents the energy required for de-solvation36, can be quantitatively derived using the Arrhenius equation: 1/Rct =Aexp (−Ea/RT), where Rct is the charge transfer resistance and R is the ideal gas constant. Herein, Rct (Supplementary Table 2) was fitted based on the variable-temperature EIS curves of Zn-Zn and ZnSe@Zn-ZnSe@Zn symmetric cells from 15 to 60 ℃ (Fig. 3j-k). It is evident that all Rct values of ZnSe@Zn cell (Fig. 3k) are much lower than those of bare Zn cell (Fig. 3j). Accordingly, Ea of ZnSe@Zn can be calculated to be ~44.0 kJ mol−1 (Fig. 3l), in contrast to that of bare Zn cell (~67.5 kJ mol−1). This result implies the superior kinetics of Zn2+ transfer throughout the interface between electrolyte and anode.
The nucleation and deposition of Zn relies heavily upon the electric field distributions at the anode/electrolyte interface7, 16. To investigate the role of in-situ grown ZnSe overlayer played in regulating interfacial electric field, finite element method carried out by COMSOL Multiphysics was employed. As depicted in Fig. 4a, the electric field distribution is homogeneous on bare Zn anode harnessing an ideally smooth surface. Nevertheless, the presence of micro-protrusions would strengthen the surrounding field intensity, which is expected to guide uneven deposition of Zn and lead to the dendrite formation (Fig. 4b). Encouragingly, these protuberances could be well eliminated by in-situ selenation process. In response, the peak value of field intensity sharply decreases from 2.2×105 to 1.6×104 V m−1 with the aid of ZnSe overlayer (Fig. 4c). Such a textured ZnSe coating further helps homogenize the electric field on Zn surface, followed by building up uniform charge flux.
Theoretical simulation based on density functional theory (DFT) route was performed to gain insight into the interaction between Zn and ZnSe. The calculated adsorption energy of a Zn atom on ZnSe support is apparently higher than that on bare Zn support (Fig. 4d; Supplementary Fig. 17). The strong affinity of Zn with ZnSe would be of benefit to suppressing two-dimensional (2D) diffusion of Zn ions. Chronoamperometry was accordingly employed to probe the Zn2+ diffusion dynamics at the anode/electrolyte interface (Supplementary Fig. 18). The current density of bare Zn symmetric cell continues to increase beyond 140 s under 150 mV, implying a violent 2D diffusion process. In turn, Zn2+ and hydrated Zn2+ ions tend to aggregate and grow into dendrites to minimize the surface energy (Fig. 4e). As for the ZnSe@Zn electrode, the 2D diffusion only occurs within the initial 20 s, after which a stable three-dimensional (3D) diffusion pattern becomes predominant. As illustrated in Fig. 4f, a desolvation process proceeds rapidly for hydrated Zn2+ ions upon their arrival at the ZnSe layer owing to low activation energy. Benefiting from a favorable 3D diffusion, Zn2+ could then pass through the ZnSe layer rapidly under the potential gradient resulting from the high electron resistance of ZnSe37 (Supplementary Fig. 19). This fast 3D diffusion process was confirmed by the high ionic conductivity of ZnSe layer reaching ~1.7 × 10–5 S cm–1 (Supplementary Fig. 20). Finally, these Zn ions are reduced to Zn0 and start to grow along (002) plane on Zn metallic surface.
Electrochemical performance of AZIB full cells
To demonstrate the feasibility of thus-designed ZnSe@Zn anode in practical devices, AZIB full cells comprising KV12O30-y·nH2O (KVOH)38 cathode (see methods) and ZnSe@Zn anode were assembled employing 2 M ZnSO4 electrolyte (Supplementary Fig. 21). Fig. 5a records the CV profiles of ZnSe@Zn−KVOH and bare Zn−KVOH cells at a scan rate of 0.1 mV s–1 in a voltage window between 0.2 and 1.6 V. Both cells manifest two main pairs of redox signals corresponding to two-step redox reactions of V3+/V4+ and V4+/V5+. The higher current response of full cell with ZnSe@Zn anode as compared to bare Zn anode implies a higher capacity value, which can be confirmed by galvanostatic charge/discharge (GCD) curves in Fig. 5b. Fig. 5c draws a comparison of rate performances of both cells. As expected, ZnSe@Zn−KVOH harvests a capacity of 294.2, 259.2, 232.9, 193.4 and 155.1 mAh g−1 at 0.5, 1.0, 2.0, 5.0 and 10.0 A g−1, respectively. When the current density returns to 0.5 A g–1, the device still retains a capacity of 253.1 mAh g–1. This readily outperforms the bare Zn−KVOH cell under identical conditions. Such outstanding reversibility and rate capability could be attributed to the inhibition of dendrite formation/side reactions and the maintenance of a homogeneous interface via the versatile ZnSe coating, which functions as a high-performance artificial SEI layer. Nyquist plots before and after cycling also reveal that the ZnSe@Zn−KVOH cell exhibits lower charge-transfer resistance and promoted ion diffusion kinetics in comparison with the Zn−KVOH counterpart (Fig. 5d). The long-term cyclic stability of both cells was further evaluated (Fig. 5e). As for ZnSe@Zn−KVOH full cell, it manages to deliver an initial capacity of 194.5 mAh g−1 and stabilizes at 163.9 mAh g−1 after 1000 cycles with a retention rate of 84% at 5.0 A g−1. In contrast, the capacity of bare Zn−KVOH cell sharply drops to 47.2 mAh g−1 after 1000 cycles.
More impressively, ZnSe@Zn anode harnessing mechanical robustness and large-scale availability enlists the construction of flexible AZIB full cells (see methods) toward practical applications (Supplementary Fig. 22). Fig. 5f presents the GCD profiles of assembled flexible AZIB at 2.0 mA cm−2 under various bending angles of 0°, 90°, 135°, and 180°. Notably, 97.8% of the initial capacity could be retained upon bending at 180°, showing excellent mechanical flexibility. As a proof-of-concept demonstration, Fig. 5g displays digital photos of the working states of two flexible AZIBs in tandem configuration, enabling to continuously powering a light emitting diode (LED) indicator under different bending angles, showing its application prospect in wearable electronics. Taken together, these results corroborate that the ZnSe overlayers can effectively inhibit the parasitic reactions at the anode/electrolyte interface and guide uniform Zn deposition in favor of advanced stability of AZIB.