Ultraconformal Horizontal Zinc Deposition toward Dendrite‐Free Anode

Aqueous zinc (Zn)‐ion batteries (ZIBs) have been considered as the most promising candidate for large‐scale energy storage system. However, the severe and uncontrollable dendrite growth of Zn anodes hinders the practical application. Herein, an ultraconformal horizontal Zn deposition is achieved, which is profiting from the epitaxial interface (InGaZn6O9) formed via spontaneously alloying between liquid Ga–In alloy (EGaIn) and Zn. The exposed (0016) plane of InGaZn6O9 matches well with (002) plane of Zn, inducing horizontal and dense Zn deposition. The resultant anode endows with prolonged cycling stability, and the full battery paired with MnO2 exhibits a stable lifespan over 4400 cycles at 5 A g−1. Meanwhile, the self‐formed ultraconformal interface realizes 360° no dead angle protection of anode, which is promising in flexible electronics. And there is no obvious capacity recession of the pouch cell even after bending 180°, demonstrating impressive flexibility. More importantly, the interface can be simply fabricated over a large area, displaying the large‐scale viability. The tailored approach delivers a constructive guideline for dendrite‐free Zn anode, showing great potential in the industrial production.


Introduction
Aqueous zinc (Zn)-ion batteries (ZIBs) have attracted extensive attention on account of their high theoretical capacity, low redox potential, intrinsic safety, nontoxicity, abundant reserves, and low cost. [1][2][3][4][5][6][7][8][9] Nevertheless, the severe dendrite growth of Zn anode leads to poor electrochemical performance during charge and discharge processes. [10][11][12][13] Zn metal exhibits a much higher Young's modulus (108 GPa) than some typical metals as battery anode (e.g., 5 GPa of Li). Once dendrites are formed, they can even easily pierce the separator to cause battery failure. [14][15][16] Unlike the intrinsic fiber-like electrodeposition morphology of Li dendrites, [17] Zn displays a unique structure with high anisotropy platelets owing to the hexagonal close-packed (hcp) structure. [18,19] Thus, precisely regulating the horizontal deposition of Zn is the primary and effective strategy to suppress dendrites. [20][21][22][23] One feasible approach is to design the epitaxial interface which shows the similar lattice arrangement and low lattice mismatch with the Zn (002), [24,25] such as graphene, [14,26] Sn, [27] Cu, [28] and AgZn 3 [29] coating. Although horizontal Zn deposition can be achieved to a certain extent, they all deliberately introduce interface with special texture, resulting in imperfect stitching of hexagonal Zn and inevitable boundary between the epitaxial interface and the Zn plates. This will impede the effect of protection and deteriorate the electrochemical performance during Zn 2þ /Zn stripping/plating. And the complex fabrication process seriously hinders the scaling application of Zn anode. Consequently, to achieve 360°no dead angle horizontal Zn deposition and all-around protection of the anode, it is extremely significant to consider how to construct an ultraconformal epitaxial interface during charge and discharge processes.
Herein, we acquire an ultraconformal epitaxial interface via spontaneous alloying, which promotes the preferred orientation of Zn grains along (002) crystal plane, realizing horizontal, dense, and uniform Zn deposition. Benefited from the excellent deformability, conductivity, and zincophilic of liquid Ga-In alloy (EGaIn), [30][31][32][33][34] the in situ alloying of EGaIn and Zn can spontaneously occur inside the battery, thus 360°no dead angle protecting the anode from dendrite growth. Based on density functional theory (DFT) calculations and in situ characterization, the (0016) plane of alloy displays the same symmetry (P 63 /mmc) and low lattice mismatch with the Zn (002), promoting the preferred growth of Zn (002). The as-prepared anode exhibits a prolonged cycle lifespan of over 600 h with an ultralow overpotential DOI: 10.1002/sstr.202200194 Aqueous zinc (Zn)-ion batteries (ZIBs) have been considered as the most promising candidate for large-scale energy storage system. However, the severe and uncontrollable dendrite growth of Zn anodes hinders the practical application. Herein, an ultraconformal horizontal Zn deposition is achieved, which is profiting from the epitaxial interface (InGaZn 6 O 9 ) formed via spontaneously alloying between liquid Ga-In alloy (EGaIn) and Zn. The exposed (0016) plane of InGaZn 6 O 9 matches well with (002) plane of Zn, inducing horizontal and dense Zn deposition. The resultant anode endows with prolonged cycling stability, and the full battery paired with MnO 2 exhibits a stable lifespan over 4400 cycles at 5 A g À1 . Meanwhile, the self-formed ultraconformal interface realizes 360°no dead angle protection of anode, which is promising in flexible electronics. And there is no obvious capacity recession of the pouch cell even after bending 180°, demonstrating impressive flexibility. More importantly, the interface can be simply fabricated over a large area, displaying the large-scale viability. The tailored approach delivers a constructive guideline for dendrite-free Zn anode, showing great potential in the industrial production.
(12 mV) and excellent rate performance in the symmetric batteries. When the modified anode is coupled with the MnO 2 cathode to assemble full batteries, it could stably retain 4400 cycles at 5 A g À1 . Besides, there is no obvious capacity recession of the pouch cell even bending 180°, implying the extraordinary flexibility of the anode. This tailored method for suppressing Zn dendrites can establish the 360°no dead angle protection by spontaneously ultraconformal Zn deposition, promoting largescale industrialization of ZIBs.

Preparation and Morphology of the Zn@LM Anode
We present an ultraconformal epitaxial interface to protect Zn anode from dendrite growth. The corresponding mechanism is depicted in Figure 1. The ultraconformal epitaxial interface is prepared by coating liquid EGaIn on the surface of Ti foil by simply scraping, which can be fabricated over a large area and exhibits huge viability in large-scale preparation ( Figure S1, Supporting Information). Moreover, the commercial carbon nanotube (CNT) that is conductive and zincophobic is carried out to prevent the flow of liquid EGaIn. As shown in Figure 1a, Zn randomly nucleates on Ti foil, and preferentially deposits along Zn (100) crystal plane, leading to severe dendrite growth after further plating. Conversely, dendrites are effectively inhibited when Ti foil is coated with EGaIn because the coating can 360°no dead angle protect electrode and promote the horizontal Zn deposition during the process of Zn plating (Figure 1b). The initial morphology and element distribution of the coating are displayed in Figure 1c and S2, Supporting Information, indicating the flat surface of the electrode and uniform distribution of Ga and In before Zn plating process. Then, Zn (hcp), which can provide hexagonal crystal nucleus, induces the formation of InGaZn 6 O 9 alloy with matched (0016) plane. The spontaneous alloying promotes the ultraconformal horizontal Zn deposition. The corresponding galvanostatic chargedischarge (GCD) curves of predeposition process are shown in Figure S3, Supporting Information. To verify the gradual stitching of hexagonal Zn, a series of scanning electron microscope (SEM) measurements were carried out with the capacity increasing ( Figure 1d and S4, Supporting Information). As shown in Figure 1e, Zn keeps growing laterally and merges together with the capacity increasing to 5 mAh cm À2 , bringing about a relatively smooth and compact deposition over the electrode. As to Zn anode, many vertical flakes in various sizes are irregularly distributed on its surface ( Figure S5, Supporting Information). Therefore, the proposed method obtains the stable anode with an ultraconformal epitaxial interface to regulate the morphology of Zn deposition. Impressively, the interface is gained by a simple fabrication process, showing great potential in large-scale production. Herein, Zn is predeposited onto liquid EGaIn-modified Ti foil with the capacity of 5 mAh cm À2 , and then the predeposited Ti@LM anode acts as Zn electrode (named as Zn@LM). Similarly, we prepare the predeposited Ti (named as Zn) as contrast to the Zn@LM.

Dendrite-Inhibiting Behavior at the Ultraconformal Epitaxial Interface
The ultraconformal epitaxial interface plays an essential role in achieving dendrite-free Zn anode. To verify the dendriteinhibiting behavior, SEM and atomic force microscopy (AFM) measurements were performed to investigate the morphology of Zn deposits after 30 cycles of stripping/plating under different current densities and capacities (Figure 2). At a current density of 0.5 mA cm À2 and a capacity of 0.5 mAh cm À2 , the newly generated Zn deposits horizontally on Zn@LM anode, maintaining a compact deposition with hexagonal morphology (Figure 2a). This derives from the effective induction of ultraconformal epitaxial interface. As the current density and the areal capacity increase to 1 mA cm À2 and 1 mAh cm À2 , respectively, the Zn@LM electrode displays a smooth interface (Figure 2b). The hexagonal Zn can grow layer-by-layer and form a dense structure even when the current density is expanded to 5 mA cm À2 (Figure 2c), which facilitates uniform ion transport. The SEM image of deposited Zn still exhibits a very dense surface even after cycling for 50 times ( Figure S6, Supporting Information). In contrast, the symmetric battery (Zn@LM without CNT) displays an uneven surface, for there is no CNT to prevent the flow of liquid metal ( Figure S7, Supporting Information). The dendrite of Zn@LM anode is effectively inhibited, which could keep a stable plating/stripping performance. As to Zn anode, due to the random nucleation, Zn tends to accumulate at the initial nucleation sites and constantly gets thickening. Therefore, the Zn anode shows a highly disordered morphology with tremendous dendrites under the same conditions as Zn@LM anode  (Figure 2d-f ), which may reduce stability drastically. The obvious morphology differences manifest the key role of the ultraconformal epitaxial interface generated by spontaneous alloying.
To precisely determine the surface of Zn@LM anode is smoother than Zn anode, AFM was applied to measure accurate height changes. After cycling for 30 times at a current density of 1 mA cm À2 with the capacity of 1 mAh cm À2 , the Zn@LM anode still shows a highly uniform surface (Figure 2g), while the surface of Zn anode is irregular (Figure 2i). The corresponding relative height information of the two kinds of surfaces along the dashed line is displayed in Figure 2h. Apparently, the relative height change of Zn anode is 16 times larger than that of Zn@LM anode. The uniform morphology and low relative height change of Zn@LM anode surface can be allied to the ultraconformal epitaxial interface which will induce the horizontal Zn deposition. However, the nonuniform Zn distribution of Zn anode will get worse during the stripping/ plating processes, forming dendrites and ultimately piercing the separator. Therefore, the self-formed epitaxial interface can drive the horizontal Zn deposition, which establishes a dendrite-free anode.

Mechanism Analysis of Ultraconformal Horizontal Zn Deposition
To thoroughly explore the mechanism of Zn deposition, DFT calculations were performed. The spontaneously formed InGaZn 6 O 9 (0016) plane displays the same symmetry (P 63 /mmc) and low lattice mismatch (18.6%) with the Zn (002), which could be arranged parallel on Ti foil to work as the epitaxial interface for Zn deposition (Figure 3a and S8, Supporting Information). Then, we constructed the models of Zn atom attached to different sites of Zn and InGaZn 6 O 9 ( Figure S9 and S10, Supporting Information), and the interaction of Zn with InGaZn 6 O 9 was evaluated by the adsorption energy. As shown in Figure 3b, the exposed O-edge of InGaZn 6 O 9 (0016) plane exhibits the strong adsorption of Zn atom owing to negative adsorption energy (À2.36 eV) between O-edge plane surface and Zn. And Zn (002) is easier to form in Zn-rich solution ( Figure 3c). Therefore, the InGaZn 6 O 9 (0016) plane can induce epitaxial growth of Zn along (002) plane, achieving horizontal Zn deposition.
Furthermore, we performed in situ X-ray diffraction (XRD) to explore the electrodeposition process for clarifying the generation of ultraconformal interface and the spontaneous behavior of horizontal Zn deposition (Figure 3d,e). In the beginning, only the characteristic peak of EGaIn appears (Figure S11, Supporting Information). As the deposition proceeds, Zn effectively deposits on the interface. Subsequently, the additional peak located at 33.3°begins to appear, which is indexed to the (0016) plane of InGaZn 6 O 9 (PDF # 40-0256) and proves the in situ alloying of Zn and EGaIn. The diffraction peak located at 33.9°is related to the excess of Ga (PDF # 27-0224), which is used to fill gaps in the process of hexagonal lattice stitching ( Figure S12, Supporting Information). Simultaneously, three sets of peaks appeared at 36.4°, 39.2°, and 43.4°are ascribed to the (002), (100), and (101) reflections of Zn (PDF # 04-0831), respectively. It is significant that the intensity of Zn (002) plane is further stronger than that of (100) and (101) plane, indicating the preferred orientation of Zn (002) plane. These results are well matched with the ex situ characterizations (Figure 3f,g). When the areal capacity of Zn predeposition is up to 5 mAh cm À2 , the XRD results further convince the existence of InGaZn 6 O 9 (Figure 3f ). After 30 cycles, the intensity of Zn (002) plane is the strongest for Zn@LM anode, while the Zn (101) plane and Zn (100) plane dominate among all the peaks in Zn anode ( Figure 3g). As summarized in Figure S13, Supporting Information, the peak intensity ratio of I (002) /I (100) and I (002) / I (101) further confirms that more (002) plane is exposed under the influence of InGaZn 6 O 9 . Moreover, the XRD results also exhibit the same trend after predepositing Zn with the capacity of 5 mAh cm À2 ( Figure S14, Supporting Information). As to the weak intensity of InGaZn 6 O 9 , it derives from the existence of additional Zn layers. The cross-sectional SEM and corresponding energy-dispersive spectrometer (EDS) measurements were also performed to show the ultroconformal interface to introduce horizontal zinc deposition (Figure 3h). The Zn and InGaZn 6 O 9 are distributed hierarchically and the Zn layer is uniform, reflecting the horizontal Zn deposition on the epitaxial interface. Also, it is apparent that there is no inevitable boundary between the epitaxial substrate and the Zn plates. The interfacial characteristics, along with the horizontal alignment of the hexagonal Zn, illustrate that the spontaneous interface possesses ultraconformal contact with the Zn plates, achieving 360°no dead angle protection of the anode from dendrite growth. Moreover, X-ray photoelectron spectroscopy (XPS) patterns also support the existence of the InGaZn 6 O 9 layer ( Figure S15, Supporting Information).

Symmetric Battery Performance
The positive effect of the self-formed ultraconformal epitaxial interface was further evaluated by testing the long-term cycling stability and rate performances in symmetric batteries. As exhibited in Figure 4a, a prolonged cycle lifespan of over 600 h has been achieved by the as-prepared Zn@LM anode at 0.5 mA cm À2 for 0.5 mAh cm À2 , in contrast to the sudden voltage drop of Zn anode with the shorter lifespan of 125 h. Besides, Zn@LM anode displays a more stable voltage plateau and a much lower overpotential at the first cycle (Zn@LM anode: 12 mV vs Zn anode: 70 mV), indicating the enhanced Zn transport kinetics, which is ascribed to the ultraconformal contact and stable interface. When the current density expands to 1 mA cm À2 and the capacity increases to 1 mAh cm À2 , a similar trend of electrochemistry profiles can be noticed. In Figure 4b, Zn@LM anode can still cycle for over 360 h with a much lower overpotential, which is superior to that of Zn anode (30 h). Under elevated current density of 5 mA cm À2 , the Zn@LM symmetric battery shows a long cycle life of 270 h without obvious fluctuation at the capacity of 1 mAh cm À2 , which is approximately 17 times better than that of the Zn anode (16 h) (Figure 4c). Furthermore, even when the current density is increased to 10 mA cm À2 , the Zn@LM anode can also keep stable for 170 h ( Figure S16, Supporting Information). The outstanding electrochemical performance of Zn@LM electrode is acquired with the perfect stitching of hexagonal Zn and the 360°protection from dendrite growth without dead angle. Besides, the batteries without LM coating display a shorter lifespan than batteries without CNT coating, further revealing excellent dendrite inhibition behavior of EGaIn ( Figure S17, Supporting Information). Additionally, the rate performance of symmetric batteries is further compared under various current densities at a fixed plating/stripping capacity of 1.0 mAh cm À2 (Figure 4d). As illustrated, the symmetric battery assembled with the Zn@LM electrode presents a lower mass-transfer-controlled overpotential than that with the Zn electrode when the current density is increased from 0.25, 0.5, 1, 2 to 5 mA cm À2 . Meanwhile, the cyclic stability of the Zn@LM symmetric battery is more extraordinary in different current densities, and the battery still retains stable even when current density returns to 0.5 mA cm À2 . In contrast, Zn symmetric battery displays the terrible rate performance and biggish voltage fluctuation, resulting in battery failure at 2 mA cm À2 . It is caused by the random Zn deposition and severe dendrite growth. The high stability and long cycling life of the as-proposed Zn@LM anode manifest the effectiveness of this strategy for dendrite suppression. As the electrochemical impedance spectroscopy (EIS) plots shown in Figure S18, Supporting Information, the interfacial resistance of the Zn@LM symmetric battery is lower than Zn symmetric battery, demonstrating that the ultraconformal interface can facilitate the transport of electron and Zn 2þ .

Full Battery Performance
To prove the feasibility of such an ultraconformal epitaxial surface in practical devices, the Zn@LM and Zn anode were coupled with the MnO 2 cathode [35] to assemble full batteries. As the cyclic voltammetry (CV) curves shown in Figure 5a, the Zn@LM|| MnO 2 battery displays the similar redox peaks to Zn||MnO 2 battery. However, Zn@LM||MnO 2 battery possesses a much smaller polarization than Zn||MnO 2 , indicating the improved redox kinetics. Besides, Zn@LM||MnO 2 diaplays a larger area of CV profiles, which is consistent with the better rate performance of full batteries (Figure 5b and S19, Supporting Information).
With the current density increasing from 0.2 to 5 A g À1 , the Zn@LM||MnO 2 battery exhibits a much higher capacity than that of Zn||MnO 2 battery. And the severe capacity fluctuation of the Zn||MnO 2 battery is ascribed to the growth of dendrites. When the current density returns to 0.5 A g À1 , Zn@LM||MnO 2 battery regains a quite high average capacity of 257 mAh g À1 , which is well ahead of Zn||MnO 2 battery (159 mAh g À1 ). Here, this capacity is calculated from the cathode mass. As shown in Figure 5c, Zn@LM||MnO 2 battery offers a smaller charge-transfer resistance, manifesting apparent enhancement of the ion conductivity. The cycling stability of both batteries is further evaluated (Figure 5d,e). As for Zn@LM||MnO 2 battery, a high specific capacity (260 mAh g À1 at 0.5 A g À1 ) with stable coulombic efficiency (CE) of nearly 100% over 240 cycles has been achieved. Moreover, it can also retain 4400 cycles with the capacity of 80 mAh g À1 at 5 A g À1 . As to the fast capacity drop of Zn@LM||MnO 2 during the early cycles of stability test at 5 A g À1 , it is owing to the fact that the MnO 2 cathode is not very stable initially at large current densities. To verify it, we assembled the full batteries using the same MnO 2 as cathode and pure Zn foil as anode (named as Zn||MnO 2 ). As displayed in Figure S20, Supporting Information, the same phenomenon of the dramatically decreased capacity in initial cycles at 5 A g À1 is observed. The above results indicate that InGaZn 6 O 9 epitaxial interface stabilizes Zn anode and improves electrochemical performance by guiding the horizontal Zn electrodeposition. Furthermore, considering the growing pursuit of wearable and flexible electronics, the pouch cell was assembled to further explore the flexibility of Zn@LM (Figure 5f ). The pouch cell manages to deliver a special capacity of 180 mAh g À1 after 260 cycles under 0.5 A g À1 ( Figure S21, Supporting Information). www.advancedsciencenews.com www.small-structures.com As a proof-of-concept exhibition, the pouch cell can stably power a red light-emitting diode (LED) indicator under diverse bending angles of 45°, 90°, and 180°in working states, implying the 360°n o dead angle protection of ultraconformal interface ( Figure 5g). As shown in Figure 5h and S22, Supporting Information, the GCD profiles reveal that the pouch cell maintains an eminent stability without capacity recession even bending 180°, and still possesses 97.1% of its initial capacity at 0.2 A g À1 . These results corroborate the excellent mechanical stability of Zn@LM electrode and its promising prospect in flexible electronics.

Conclusion
We have developed an effective strategy to modify Zn anode with liquid EGaIn, which can form an ultraconformal epitaxial interface and dendrite-free Zn anode. The ultraconformal interface which derives from spontaneous alloying between EGaIn and Zn serves as an epitaxial surface, inducing the growth of Zn grains along Zn (002). Moreover, profiting from the good deformability of liquid EGaIn, this strategy can achieve 360°n o dead angle protection to avoid dendrite growth. As a result, the Zn@LM symmetric battery displays the excellent Zn stripping/plating performance with a long lifespan (600 h at 0.5 mA cm À2 for 0.5 mAh cm À2 ) and an ultralow overpotential (12 mV). Based on the dendrite-free electrode, the Zn@LM|| MnO 2 battery exhibits a long cycle lifespan, excellent rate performance, and specific capacity when compared with Zn||MnO 2 battery. This study clarifies an effective approach to achieve ultraconformal horizontal Zn deposition toward dendrite-free anode, which may greatly facilitate practical applications in industrial production.

Experimental Methods
Experimental Material Preparation: The liquid Ga-In alloy (EGaIn) was prepared by physically mixing 78.4 wt% gallium and 21.6 wt% indium (both purchased from Shanghai Minor Metals Co. Ltd.) at 200°C for Figure 5. Electrochemical performance of Zn||MnO 2 batteries with Zn and Zn@LM anode. a) CV curves at a scan rate of 0.1 mV s À1 . b) Rate performance. c) EIS plots after ten cycles at 0.5 A g À1 . Long-term cycling performance of Zn@LM||MnO 2 at d) 0.5 A g À1 and e) 5 A g À1 . f ) Schematic illustration of the pouch cell. g) Digital photographs to show the working states of the pouch cell to power an LED indicator under different bending angles. h) GCD profiles of the pouch cell tested at 0.2 A g À1 under different bending angles.