Promoted de-solvation effect and dendrite-free Zn deposition enabled by in-situ formed interphase layer for high-performance zinc-ion batteries

INTRODUCTION

With the large-scale and efficient use of renewable energy such as solar and wind energy, there is an urgent need to develop low-cost, high-safety and high-performance energy storage devices as intermediate media for power transmission^[1-3]. Due to the use of aqueous electrolytes, aqueous rechargeable batteries have the benefits of being inexpensive, safe, non-flammable, and environmentally friendly^[4-7]. Among them, aqueous Zn-ion batteries (AZIBs) are thought to be a promising kind of rechargeable battery for large-scale applications because of their affordable water electrolyte and manufacturing process, safety, and plentiful Zn resources^[8,9]. Unfortunately, the following factors are the principal hindrances to the practical application of Zn metal anodes: growth of Zn dendrites, parasitic reactions, and sluggish transport kinetics^[10-12]. Numerous pits on the rough surface of commercial Zn foil result in the unequal distribution of the electric field^[13,14]. Zn²⁺ ions preferentially nucleate and deposit inhomogeneously in the convex pits due to the tip effect^[15-17]. With repetitive plating/stripping, Zn dendrites develop, resulting in rapid electrolyte consumption and decreased Coulombic efficiency (CE)^[18,19]. Furthermore, the dendrites can grow over the separator onto the cathode side, potentially generating an internal short circuit^[20-22]. In addition, the chemically active Zn anode reacts with the mild aqueous solution to form the by-product [i.e., Zn₄SO₄(OH)₆·nH₂O], and releases hydrogen (H₂), which leads to rapid capacity decay and cycle instability of AZIBs^[23-26].

To address the aforementioned issues, various strategies have been developed to stabilize the Zn anode, including electrolyte modification, structural design, and interface layer construction^[27-29]. Among these, the introduction of an interface layer is considered more promising than the other two methods^[30-32]. The interface layer not only physically isolates Zn metal from the aqueous electrolyte to inhibit side reactions, but also offers great possibilities for regulating Zn deposition behavior due to its diversity of materials and functions^[33-35]. For example, different oxides including ZnO^[36], CeO₂^[37], WO₃^[38], and MoO₃^[39] have been used as interfacial materials and have been shown to be effective in Zn anode stabilization. However, it is important to note that most reported interfacial protective layers are quite thick, typically ranging from 5 to 25 μm, to maintain membrane integrity and ensure complete isolation of Zn from the electrolyte [Supplementary Table 1]. For example, Lu et al. constructed a 10 μm-thick Layered Double Oxide (LDO) layer to regulate the deposition of Zn^[40]. In addition, Zhang et al. found that the TiO₂ interface layer with a thickness of 20 μm coupled with optimized plane adjustment could guarantee the uniform deposition of Zn^[41]. Considering the poor ion conductivity of these interfacial materials, such a thick coating will inevitably lead to a large internal resistance (R_ct) of the battery, resulting in sluggish Zn ion transport kinetics^[42,43]. The thin interface layer can alleviate the issue of sluggish transport kinetics, but it may be difficult to resist the drastic volume changes during plating and stripping at high current density^[44-46]. In addition, when the interface layer is too thin, it may become nonuniform, which would compromise the effect of Zn metal stabilization^[47-49]. Therefore, it is critical to obtain an interface layer with appropriate thickness while minimizing factors that reduce Zn ion transport kinetics^[50,51].

Inspired by this, a thin V₂O_5-x protective layer was constructed on the surface of the Zn anode by a simple chemical reduction method (designated as V@Zn). By altering the reaction time, the thickness of the protective layer (220 nm, 320 nm, 615 nm) can be precisely controlled. The optimized thickness of the protective layer can prevent direct contact between the electrolyte and Zn metal anode, and demonstrated a de-solvation effect, which effectively promotes the uniform deposition of Zn²⁺ and inhibits dendrite formation. In addition, the low-valent V element in the protective layer promotes the combination of oxygen anion and Zn²⁺, forming an efficient Zn²⁺ transport channel and accelerating the ion transport process. As a result, the symmetrical V@Zn//V@Zn cell demonstrates an ultra-long life of 1400 h at a current density of 1 mA cm^-2 with a capacity of 1 mA h cm^-2. Moreover, a full cell consisting of a V@Zn anode and a V₂O₅ cathode shows improved capability and cycling stability.

EXPERIMENTAL

RESULTS AND DISCUSSION

The structural and dendritic evolution of the electrode surface following the initial plating and cyclic stages is depicted in the schematic diagram of Figure 1. Due to the uneven deposition of Zn²⁺, some small protrusions formed on the bare Zn surface. After several cycles, the electron attraction is high at the protrusion due to the tip effect, leading to a large amount of Zn²⁺ deposition^[52,53]. After that, the protrusions evolved into Zn dendrites [Figure 1A]. In addition, due to chemical reactions and Zn surface roughness, continuous corrosion begins as soon as the Zn anode comes into contact with the weakly acidic electrolyte. This process consumes both the Zn anode and the electrolyte, producing insulating by-products and hydrogen gas (H₂) [Supplementary Figure 1]. Moreover, the competitive hydrogen evolution reaction (HER) during the recharging process not only directly lowers CE but also accelerates H₂ generation and increases the local pH. These effects lead to elevated internal pressure and intensified parasitic reactions. In contrast, a uniform electric field and homogeneous distribution of Zn²⁺ flux was enabled on the surface because of the introduction of the V₂O_5-x interface interlayer, resulting in uniform deposition of Zn [Figure 1B]. Moreover, the contact between Zn and the electrolyte is effectively blocked, thereby suppressing side reactions.

Figure 1. Diagrammatic representation of the surface-structure differences between the (A) bare Zn electrodes and (B) the V@Zn electrodes.

The V@Zn anode was prepared using a simple in situ chemical reduction method [Figure 2A]. The high-valence vanadium oxide in solution is reduced to low-valent vanadium oxide via redox reaction with Zn, resulting in the growth of a V₂O_5-x layer with yellow color on the surface of bare Zn [Supplementary Figure 2]. The conversion of V₂O₅ to V₂O_5-x during assembly can be proved using XPS. The two peaks of the V 2p spectrum in Supplementary Figure 3 show the coexistence of V⁵⁺ and V⁴⁺. V⁵⁺ is the main valence state of V₂O₅ samples, while the proportion of V⁵⁺ in V@Zn electrodes decreases. This is attributed to the reduction of V₂O₅ by Zn foil, forming V@Zn. Due to the ultrathin nature of the V2O5-x layer relative to the bare Zn substrate, the X-ray powder diffractio (XRD) patterns of the V@Zn electrodes are dominated by the strong Zn peaks, making them appear similar to those of bare Zn [Supplementary Figure 4]. The XRD pattern of V₂O_5-x powder scraped from the V@Zn electrodes aligns well with the standard pattern of VO₂(H₂O)_0.5 [Supplementary Figure 5]. To further characterize the materials, EPR measurements were also performed. As illustrated in Supplementary Figure 6, V₂O_5-x exhibits paramagnetic properties with EPR activity (g=1.96). SEM observation of bare Zn foil and the V@Zn electrode in their original state shows that the surface of bare Zn shows an uneven surface [Figure 2B]. After immersion of Zn in V₂O₅ solution, Zn foil is evenly covered with a porous interface layer [Figure 2C and D, Supplementary Figure 7]. The results from the energy dispersive spectroscopy show a homogeneous distribution of Zn, V, and O elements and the absence of other impurity elements [Figure 2E], demonstrating the uniform V₂O_5-x grown on the bare Zn surface.

Figure 2. (A) Schematic diagram of V@Zn fabricating; Top-view SEM pictures of the bare Zn foil (B) and V@Zn (C and D); (E) SEM image of V@Zn and corresponding elemental maps. SEM: Scanning electron microscopy.

V@Zn electrodes were obtained by immersing Zn in 0.5 mg mL^-1 V₂O₅ solution for 1 min, 5 min and 10 min, respectively, named V@Zn-1, V@Zn-5, and V@Zn-10, respectively. By varying the reaction time, the thickness of the V₂O_5-x layer can be precisely controlled. SEM results clearly show that the thickness of the layers varies from 220 to 615 nm as the reaction time increases [Figure 3A-C]. As shown in Figure 3D, the polarization voltages of the V₂O_5-X layer at 220 nm and 320 nm thickness are lower than those of bare Zn, indicating that V₂O_5-x layers promote Zn²⁺ conductivity. However, the polarization voltage of the V₂O_5-X layer at 615 nm thickness is significantly higher than that of bare Zn, indicating that the thicker vanadium oxide layer has an inhibitory effect on ion migration. The polarization voltage of plating/stripping increases with the film thickness, indicating that the film thickness increases the barrier to Zn²⁺ migration. Subsequently, the cycling stability of the V@Zn electrodes and bare Zn was evaluated with symmetrical cells at 1 mA cm^-2 and 1 mA h cm^-2. The sudden drop in voltage at 300 h in bare Zn symmetrical cells is probably due to the formation of dendrites [Figure 3E]. The abrupt failure of the V@Zn-1 symmetrical cell after ~800 h may be due to its weak mechanical strength causing it to fracture during long-term repetitive plating/stripping. In addition, the V@Zn-10 symmetrical battery only maintains a lifespan of 700 h. This may be due to dendrite formation, which causes a short circuit in the battery. In contrast, a V@Zn-5 symmetrical battery can be stable for more than 1,400 h of cycling, indicating its long-term effectiveness in inhibiting dendrite growth and side reactions.

Figure 3. Bare Zn and V₂O_5-x protective layer V@Zn after 1, 5 and 10 min reaction, respectively. (A-C) Thickness of the V₂O_5-x interface layer; (D) the magnified voltage-time profiles from 200 h to 210 h; (E) long cycle stability testing; (F) contact angle measurement.

The compatibility between the electrode surface and the electrolyte was investigated by a contact angle experiment. The contact angles of the 3 M Zn(CF₃SO₃)₂ solution with bare Zn and different V@Zn anodes were 83.7°, 45.1°, 26°, and 25.6°, respectively [Figure 3F]. This indicates that the V₂O_5-x film improves the wettability of the electrolyte, facilitating Zn-ion transport due to the easy access of the Zn²⁺ in the electrolyte. In summary, the 320 nm-thick V₂O_5-x interface layer shows the best stabilization effect and was used for further battery performance evaluation.

In addition to enhancing the plating/stripping kinetics of Zn²⁺, the V₂O_5-x layer is also crucial for adjusting Zn nucleation and enabling uniform Zn deposition. The nucleation overpotential of V@Zn was measured at various current densities in order to demonstrate the function of the V₂O_5-x layer in controlling Zn nucleation behavior [Supplementary Figure 8]. Apparently, the nucleation overpotential of the V@Zn at 0.5 mA cm^-2 is only 20.1 mV, which is even smaller than the 66.1 mV of bare Zn [Supplementary Figure 8A and B]. In addition, this significant difference can still be observed even at higher current densities [Supplementary Figure 8D and E]. This is because when Zn²⁺ passes through the V₂O_5-x interface layer, the V element in the protective layer is in a low-valent state, and the oxygen anion in the crystal structure is more likely to bind to Zn²⁺, so that a Zn²⁺ fast transport channel can be formed. The V₂O_5-x layer effectively reduces the nucleation barrier. In addition, it is worth noting that even at the stable Zn deposition stage, the deposition potential of the V@Zn electrode is significantly lower than that of the bare Zn electrode. The V@Zn electrode deposition overpotential (42.2 mV) is significantly less than that of the bare Zn electrode (100.1 mV) at a current density of 0.5 mA cm^-2. This pattern is also found at other higher current densities [Supplementary Figure 8C-E].

The current-time curve, as presented in Supplementary Figure 9, shows both the nucleation process and surface variations of the Zn anode^[54]. When the -150 mV overpotential is applied, the current of bare Zn continues to decrease for 150 s, showing that the two-dimensional diffusion process is prolonged and Zn deposition remains uncontrolled. During charging and discharging, Zn²⁺ tends to concentrate at the electrode tip, reducing surface energy and speeding dendrite formation on the electrode surface. In contrast, the initial Zn nucleation and two-dimensional diffusion process of the V@Zn reached a stable three-dimensional diffusion after 42 s. Furthermore, the Tafel experiment was conducted to evaluate the corrosion behavior of Zn and V@Zn [Supplementary Figure 10]. Obviously, compared with bare Zn, a V@Zn electrode has a higher corrosion potential, demonstrating that the V₂O_5-x protective layer can effectively prevent Zn corrosion by separating Zn from its surrounding electrolyte.

To further investigate cycling performance, symmetrical cells with bare Zn or V@Zn electrodes were built, and charged/discharged tests at various current densities were conducted. Figure 4A and B shows that at a current density of 3 mA cm^-2, the symmetrical cell V@Zn//V@Zn has a stable cycle life and voltage distribution over 700 h. On the other hand, the initial polarization voltage of a bare Zn//Zn symmetrical battery is around 55 mV at the start of the cycle, and severe polarization occurs at 120 h. Even at 5 mA cm^-2 current density, V@Zn//V@Zn cells can sustain nearly 320 h [Figure 4C]. In contrast, Zn//Zn cells undergo a short circuit after 175 h. Different current densities were used to investigate the magnification capabilities of V@Zn and bare Zn [Figure 4D]. At varying current densities, the V@Zn clearly shows lower polarization voltages than bare Zn. Therefore, it was proved that the V₂O_5-x layer could lower the polarization voltage of the cell and effectively induce uniform Zn²⁺ deposition and inhibit the growth of Zn dendrites^[55,56].

Figure 4. Electrochemical characteristics of symmetric Zn and V@Zn cells. (A) cycling performance at 3 mA cm^-2; (B) the magnified graph of the long-cycling voltage profiles, which spans 40 h to 50 h; (C) cycling performance at 5 mA cm^-2; (D) rate curves.

In order to further illustrate the effect of V₂O_5-x layer on the reversibility of Zn plating/stripping, the CE of Zn//Ti asymmetric cells was also studied. As shown in Figure 5A-C, the CE and constant current charge-discharge [galvanostatic charge/discharge (GCD)] curves of a Zn//Ti asymmetric cell with or without a V₂O_5-x layer are measured at a current density of 3 mA cm^-2 and 2 mA cm^-2. The initial CE of Zn//Tihalf-cells is low (79.9%). Zn//Ti has a limited cycle life of 170 cycles, and after 50 cycles, the overpotential of Zn//Ti is as high as 175.8 mV [Figure 5A and B]. In contrast, the V@Zn//Ti half-cell can be stably plated/stripped over 600 cycles, and the overpotential (108.8 mV) is less than that of the bare Zn//Ti cell (175.8 mV) after 50 cycles. At a current density of 2 mA cm^-2, the reversibility of plating/stripping of V@Zn//Ti cells is also better than that of bare Zn//Ti cells [Figure 5C]. Furthermore, oxidation/reduction currents in V@Zn//Ti cells are higher than in bare Zn//Ti [Figure 5D]. When the potential negative sweep portion increases, V@Zn//Ti cells (A: -50 mV) are more likely to nucleate than bare Zn//Ti cells (B: -93.2 mV). EIS experiments further verified the kinetic performance of the reaction. Figure 5E and F shows the EIS of bare Zn and V@Zn electrodes at various temperatures between 35 °C and 65 °C. At different temperatures, the R_ct of the V@Zn electrode is smaller than that of the bare Zn, which confirms that V₂O_5-x can increase the charge transfer kinetics. As shown in Figure 5G, the de-solvation activation energy E_a of the V@Zn electrode is about 48.92 kJ mol^-1, which is much lower than the 64.84 kJ mol^-1 of the Zn electrode foil. This indicates that the V₂O_5-x protective layer can form a fast Zn²⁺ transport channel, which can accelerate the de-solvation process.

Figure 5. (A) Coulombic Efficiency comparison at 3 mA cm^-2; (B) Voltage profiles at the 50th cycle of Zn plating/stripping of V@Zn and bare Zn at 3 mA cm^-2; (C) Coulombic Efficiency comparison at 2 mA cm^-2; (D) The CV diagram tested at 0.1 mV s^-1; EIS of the Zn foil (E) and the V@Zn electrode (F) at different temperatures; (G) The calculated de-solvation activation energies of both electrodes by using the Arrhenius equation. CV: Cyclic voltammetry; EIS: Electrochemical impedance spectroscopy.

In addition, direct evidence of uniform Zn deposition of V₂O_5-x layers was obtained from in-situ optical visualization observations^[57,58]. An in-situ optical system operating at 5 mA cm^-2 was utilized to conduct Zn plating tests in order to investigate the effect of the V₂O_5-x-protective technique on dendrite formation. During the plating process, the surface of the V@Zn remains smooth at covered time [Figure 6A]. In contrast, some small protrusions appeared on the surface of bare Zn as early as 15 min, and gradually evolved into Zn dendrites. SEM was used to observe the bare Zn and V@Zn electrodes after 50 plating/stripping cycles. The Zn dendrites of the bare Zn electrode are clearly visible, and the V@Zn electrode is almost horizontal, making the dendrite undetectable [Figure 6B and C]. This indicates that the V₂O_5-x protective layer can promote the uniform deposition of Zn²⁺ and prevent dendrite formation. Hydrogen evolution curves were measured for both bare Zn and V@Zn anodes. As shown in Supplementary Figure 11, the V@Zn anode exhibits superior corrosion resistance compared to bare Zn. This improvement can be attributed to the V₂O_5-x protective layer coated on the Zn surface, which prevents direct contact between Zn and the electrolyte, thereby suppressing hydrogen evolution. XPS characterizes the changes in the V 2p and Zn 2p of V@Zn during Zn ion plating/stripping [Figure 6D, Supplementary Figure 12]. When Zn²⁺ is intercalated/detached, V³⁺ appears, and the low-valence V can make it easier for the oxygen anion in the crystal structure to bind to Zn²⁺ to form a Zn²⁺ fast transport channel. The position and shape of the Zn 2p peaks on the V@Zn electrode closely align with the peaks of bare Zn. This consistency further confirms the stability of the V₂O_5-x layer before and after cycling [Supplementary Figure 13].

Figure 6. (A) 100 μm scale bar represents light microscopic pictures of Zn deposition on bare Zn and V@Zn surfaces; The SEM pictures of (B) bare Zn and (C) V@Zn after 50 cycles at 1 mA cm^-2; (D) V 2p spectra of the V@Zn electrodes in pristine, discharged and charged states. SEM: Scanning electron microscopy.

The full cell (Zn//V₂O₅ and V@Zn//V₂O₅) was built for electrochemical performance testing to evaluate the effect of the V@Zn anode. The CV curve shows a pair of redox peaks, indicating the insertion/extraction reaction of Zn²⁺ or H⁺ [Figure 7A]. Furthermore, the gap (0.12 V) between the redox and A1/B1 peaks of V@Zn//V₂O₅ was less than that of the redox peaks of bare Zn//V₂O₅ (0.19 V), indicating that the V₂O_5-x interface layer on the Zn anode enables superior electrochemical activity. In addition, EIS testing is also used to verify the reaction kinetics effect of the V₂O_5-x layer on the cell. V@Zn//V₂O₅ has a small charge transfer resistance. In Zn//V₂O₅, it is shown that the V₂O_5-x coating improves the kinetics of the cell [Supplementary Figure 14]. The V@Zn//V₂O₅ battery definitely has a higher rate capability than Zn//V₂O₅ batteries [Figure 7B]. At a current density of 5 A g^-1, V@Zn//V₂O₅ and Zn//V₂O₅ cells were tested for long-term cycling. The capacity of Zn//V₂O₅ batteries is significantly lower than that of V@Zn//V₂O₅ batteries. In addition, Zn//V₂O₅ cells experienced significant capacity decay after 1,500 cycles [Figure 7C]. In contrast, V@Zn//V₂O₅ achieves stable long cycles of more than 2,500 cycles with almost no capacity degradation. In order to make the study more comprehensive, long-term cycling tests were carried out on V@Zn//V₂O₅ and Zn//V₂O₅ cells at a current density of 1 A g^-1. V@Zn//V₂O₅ cells maintain high capacity (~340 mA h g^-1) over 200 cycles. The capacity of Zn//V₂O₅ cells is significantly lower than that of V@Zn//V₂O₅ cells [Supplementary Figure 15]. The findings demonstrate that the V₂O_5-x coating can effectively prevent dendrite formation, increase corrosion resistance, guide uniform Zn deposition, and promote de-solvation effect, all of which greatly enhance electrochemical performance of zinc-ion batteries.

Figure 7. Electrochemical performance of full cells with bare Zn and V@Zn anodes is compared. (A) CV curves at 0.4 mV s^-1; (B) Rate performance; (C) Long-term cycling performance at 5 A g^-1. CV: Cyclic voltammetry.

CONCLUSIONS

A V₂O_5-x layer is in situ grown on the surface of Zn foil by a simple chemical reduction method to guide uniform Zn deposition during plating/stripping process thus preventing the growth of Zn dendrite. In addition, the interface layer can be coordinated with Zn²⁺, which promotes the de-solvation effect, and can be used as a physical barrier to avoid direct contact between the electrolyte and the Zn anode, reducing the occurrence of side reactions. Therefore, symmetrical cells based on V@Zn anodes exhibit low hysteresis voltage and long cycle life. In addition, the V@Zn//V₂O₅ full cell shows a capacity of 275.9 mA h g^-1 after 2,500 cycles, demonstrating excellent cycling stability. Therefore, this work is expected to open up new avenues for the development of high-performance zinc-ion batteries.