Surface electron reconfiguration of ceric dioxide artificial interface layer by cationic doping for dendrite-free zinc anode

Abstract

Aqueous zinc metal batteries (ZMBs) are regarded as strong contenders in secondary battery systems due to their high safety and abundant resources. However, the cycling performance of the Zn anode and the overall performance of the cells have often been hindered by the formation of Zn dendrites and the occurrence of parasitic side reactions. In this paper, a surface electron reconfiguration strategy is proposed to optimize the adsorption energy and migration energy of Zn²⁺ for a better Zn²⁺ deposition/stripping process by adjusting the electronic structure of ceric dioxide (CeO₂) artificial interface layer with copper atoms (Cu) doped. Both experimental results and theoretical calculations demonstrate that the Cu₂Ce₇O_x interface facilitates rapid transport of Zn²⁺ due to the optimized electronic structure and appropriate electron density, leading to a highly reversible and stable Zn anode. Consequently, the Cu₂Ce₇O_x@Zn symmetric cell exhibits an overpotential of only 24 mV after stably cycling for over 1600 h at a current density of 1 mA/cm² and a capacity of 1 mAh/cm². Additionally, the cycle life of Cu/Zn asymmetric cells exceeds 2500 h, with an average Coulombic efficiency of 99.9%. This paper provides a novel approach to the artificial interface layer strategy, offering new insights for improving the performance of ZMBs.

Graphical abstract

Keywords

Zn metal batteries / interface layer / solvation structure / electronic structure / Cu₂Ce₇O_x

1 Introduction

In the context of global energy structure transformation, the utilization and storage of renewable energy have become key scientific and technological issues [1–3]. Among various energy storage technologies, aqueous Zn metal batteries (ZMBs) have garnered significant attention due to their advantages, including safety, environmental friendliness, low cost, and abundant resources [4–7]. However, further application of ZMBs is hindered by several issues, such as the formation of Zn dendrite [8–12], side-reactions, and corrosion of Zn anode [13–15], which are primarily caused by the sluggish kinetics of Zn²⁺ desolvation [16,17]. Therefore, optimizing the solvation shell of [Zn(H₂O)₆]²⁺ to accelerate the Zn²⁺ desolvation process is crucial for addressing the challenges faced by ZMBs [18–21].

Several strategies have been explored to accelerate the Zn²⁺ desolvation process: 1) Electrolyte optimization: Certain electrolytes can provide more hydrogen bond donors and acceptors [22–24], or have a higher affinity for Zn²⁺. These electrolytes interact with Zn²⁺ preferentially and enter its solvation shell, altering the solvation structure environment. For example, DMSO can occupy more positions in the solvation structure of Zn²⁺ and reduce direct contact between H₂O molecules and Zn²⁺, due to the high number of hydrogen bond donors of dimethyl sulfoxide [25] (DMSO) compared to H₂O. However, electrolyte additives may spontaneously precipitate during cell operation [26,27], thereby potentially blocking ion transport channels. 2) Structural design: The design of three-dimensional (3D) anodes [28,29] with abundant pores can provide more contact sites for solvent molecules and Zn²⁺, facilitating better Zn²⁺ diffusion. For example, fibrous Zn anodes [30] prepared using techniques such as electrospinning are intertwined with each other to form a 3D network structure. This structure [31] can shorten the diffusion path for Zn²⁺ and provide additional sites for solvation. However, the uneven distribution of current density in such structures, and the uneven deposition of desolvated Zn²⁺ may negatively affect the performance of the cell. 3) Surface modification: Interface layers [32–34] that strongly interact with Zn²⁺ can facilitate the shedding of surrounding solvent molecules, reducing the nucleation energy barrier and accelerating the transmission of Zn²⁺ [35,36]. For example, Cao’s group [37] used a BaTiO₃/P(VDF-TrFE) (BTO/PVT) interface layer to reduce the nucleation overpotential and charge transfer resistance (R_ct). Zhou’s group [32] proposed the use of in situ-grown polyaniline to improve the redox kinetics of Zn²⁺ and redox kinetics. A zincophilic interface layer with strong chemical adsorption Zn²⁺ [38–41], can improve the electrochemical performance of Zn anodes. Although enhancing the adsorption capacity for Zn²⁺ can reduce the nucleation overpotential and guide the uniform deposition of Zn²⁺ [42], excessive adsorption can hinder the migration of Zn²⁺ near the interface, the transfer of Zn²⁺ on the anode surface, negatively impacting the overall performance of the cell. The weak adsorption effect of the non-zincophilic interface layer on Zn²⁺ is often accompanied by a lower migration energy barrier, which can achieve rapid transfer of Zn²⁺ and a lower overpotential. However, it also leads to Zn²⁺ being too easy to move on the surface, hindering the uniform deposition of Zn²⁺. Therefore, the interlayer is equally important for the adsorption and migration of Zn²⁺ [43–45].

In this work, Cu atoms were successfully doped into Ce atoms to form Cu₂Ce₇O_x through a simple high-temperature calcination process [46]. Compared to ceric dioxide (CeO₂), the electron density near the Ce atoms in Cu₂Ce₇O_x decreases, and electrons become more dispersed between Cu and O atoms (Fig.1(a)). Moreover, Cu₂Ce₇O_x generates an unusual s-p-d orbital hybridization, which triggers electron rearrangement. This alters the electrostatic force due to the alteration of the electron cloud distribution, enhancing the interaction between the O atoms in Ce-O and Zn, thereby improving the chemical affinity for Zn²⁺. Since Cu₂Ce₇O_x, as the Zn anode interface layer, has appropriate adsorption energy and migration energy, it not only improves the cycling ability of the Zn anode but also reduces the nucleation overpotential during the deposition and dissolution processes.

The Cu₂Ce₇O_x@Zn/Cu₂Ce₇O_x@Zn symmetric cell exhibits a low overpotential of 24 mV, with stable cycling for over 1600 h at a current density of 1 mA/cm² and an electrical capacity of 1 mAh/cm², which is more than 10 times that of a bare Zn anode. The Cu/Cu₂Ce₇O_x@Zn/Cu asymmetric cells show a long cycle life of over 2500 h, with an average Coulombic efficiency of 99.9%. Furthermore, when assembled into a Cu₂Ce₇O_x@Zn/MnO₂ full cell, the battery maintains an excellent capacity retention of 88.9% over the first 800 cycles at 1 A/g. This work offers a novel strategy for constructing stable ZMBs, providing a significant step forward in their development for energy storage applications.

2 Results and discussion

The zincophilic interface layer constructed at the electrode/electrolyte interface of a Zn anode plays a significant role in promoting the desolubilization of [Zn(H₂O)₆]²⁺ and ensuring the even deposition of Zn²⁺ ions [47,48]. To explore the mechanism of CeO₂ and Cu₂Ce₇O_x on interfacial solvation, the electron distribution in these materials was analyzed using theoretical calculation. The electron localization function (ELF) was used to analyze the distribution of electrons around atoms. As shown in Fig.1(b)–Fig.1(e), doping Cu atoms to replace a portion of the Ce atoms in CeO₂ results in an unusual s-p-d orbital hybridization, which induces electron rearrangement, thereby enhancing the Ce-O bond. This doping of Cu atoms causes the electrons around Ce and O atoms to disperse. The decrease in electron density near O atoms weakens the chemical interaction between O atoms and Zn²⁺, which reduces the chemical affinity of O atoms for Zn²⁺. The behavior of Zn²⁺ at the electrode/electrolyte interface was verified by calculating the adsorption energy, migration energy for Zn²⁺ and the activation energy.

To further investigate the behavior of Zn²⁺ at the electrode/electrolyte interface, the adsorption energy, migration energy, and activation energy were calculated. The adsorption energies of CeO₂ and Cu₂Ce₇O_x for Zn²⁺ were calculated to evaluate the chemical affinity of the interface layer to Zn²⁺. The adsorption energy of the CeO₂ interface layer for Zn²⁺ was found to be −3.21 eV, which is significantly higher than that of bare Zn (−0.36 eV), suggesting a reduction in the dissolution energy barrier (Fig.1(f)). However, excessive adsorption energy can hinder the subsequent Zn²⁺ migration and nucleation and may cause deactivation of the CeO₂ interface layer, which is harmful to the construction of highly stable ZMBs. In contrast, Cu₂Ce₇O_x exhibits a moderate adsorption energy of −1.88 eV, which is ideal for reducing the dissolution energy barrier while ensuring the long-term ZMBs operation.

Additionally, the migration energy of Zn²⁺ was assessed. As shown in Fig.1(g), on the bare Zn surface, the migration energy of Zn²⁺ is 0.35 eV, which is beneficial to two-dimensional (2D) planar diffusion of Zn²⁺. Moreover, on the Cu₂Ce₇O_x interface layer, the migration energy increases to 0.91 eV, which can inhibit excessive 2D planar diffusion, promoting more uniform Zn²⁺ deposition. However, the migration energy on the CeO₂ interface layer is 1.83 eV, likely due to its high adsorption energy, which is not conducive to Zn²⁺ migration and nucleation. Moreover, the activation energy for the reaction kinetics was calculated by measuring the charge transfer resistance (R_ct) to reaction kinetics (Fig. S1). The activation energy of the Cu₂Ce₇O_x@Zn electrode was found to be 27.85 kJ/mol, which is lower than that of the bare Zn electrode (38.83 kJ/mol), indicating fast reaction kinetics on the Cu₂Ce₇O_x@Zn surface (Fig.1(h)).

These findings suggest that the Cu₂Ce₇O_x interface layer optimally balances adsorption and migration energy for Zn²⁺, improving the performance of Zn metal anodes in aqueous zinc metal batteries (ZMBs) by enhancing both the dissolution and deposition processes.

To confirm the successful doping of Cu atoms into CeO₂, a series of characterizations were conducted on the synthesized materials. As shown in Fig.2(a) with X-ray diffraction (XRD analysis), the diffraction peaks at 28.6°, 33.1°, 47.5°, and 56.3° correspond to the (111), (200), (220), and (311) crystal planes of CeO₂, respectively, based on the JCPDS-43-1002 reference. When compared to pure CeO₂, due to the change of the lattice of CeO₂ by Cu doping, the XRD characteristic peak for Cu₂Ce₇O_x shift toward lower energy levels, showing that Cu doping has altered the lattice structure of CeO₂. These XRD results suggest that Cu atoms are incorporated into the crystal structure of CeO₂, replacing some Ce atoms.

SEM and high-resolution TEM images show that Cu₂Ce₇O_x is in a nanosheet-like morphology (Fig.2(b)). Moreover, elemental analysis based on the mass ratio of Cu and Ce indicates the presence of 8.34% Cu and 27.97% Ce, respectively, which approximates a 2:7 ratio. Energy dispersive X-ray spectroscopy (EDS) (Fig. S2) further confirms that Cu, Ce, and O are uniformly distributed throughout Cu₂Ce₇O_x, providing additional evidence that Cu atoms have been successfully doped into the CeO₂ matrix.

In the TEM analysis, the (111) crystal plane of CeO₂ was detected and measured, with a lattice spacing of 0.322 nm for Cu₂Ce₇O_x (Fig.2(c)). This value is larger than that of pure CeO₂ (Fig. S3), suggesting that Cu atom doping has expanded the lattice structure of CeO₂. According to the Bragg’s equation, the shift in the XRD peaks toward lower angles is consistent with an increase in lattice spacing, which further confirms that the doping of Cu atoms into CeO₂ causes a lattice distortion. Notably, the lattice spacing corresponding to Cu atoms was not detected in the TEM, indicating that Cu atoms are fully integrated into the CeO₂ substrate.

X-ray photoelectron spectroscopy (XPS) was also employed to examine the chemical state of CeO₂ and Cu₂Ce₇O_x [49]. The Ce 3d spectrum revealed two distinct peaks for Ce³⁺ and Ce⁴⁺ in both CeO₂ and Cu₂Ce₇O_x. In the case of CeO₂, the binding energies of Ce³⁺ and Ce⁴⁺ for the Cu₂Ce₇O_x are higher than those of CeO₂. Additionally, the intensity of the Ce⁴⁺ peak at 901.4 eV in the Cu₂Ce₇O_x is higher than in CeO₂, while the intensity of the nearby Ce³⁺ peak is diminished (Fig.2(d) and Fig.2(f)). This observation shows that the doped Cu atoms reduce the electron density around Ce atoms.

The O 1s XPS spectrum of CeO₂ and Cu₂Ce₇O_x shows three distinct oxygen peaks: O_α, O_β, and O_γ at 528.7, 530.9, and 532.2 eV, corresponding to oxygen atoms in the lattice structure, oxygen vacancy in the materials, and adsorbed oxygen species, respectively. It is worth noting that the proportion of oxygen vacancies (O_β) in Cu₂Ce₇O_x has increased compared to CeO₂, which further modifies the electronic structure of Cu₂Ce₇O_x, promoting Cu₂Ce₇O_x surface electron rearrangement (Fig.2(e) and Fig.2(g)).

The Cu₂Ce₇O_x material was then uniformly coated on a Zn foil using a simple doctor blade interface layer process, forming a 300 nm thick interface layer (Figs. S5 and S6). SEM images reveal that the nanosheet-like structure of Cu₂Ce₇O_x is still retained (Fig.2(h)), and the Cu₂Ce₇O_x interface layer remains smooth and dense, which is beneficial in reducing material shedding due to volume changes on the surface during cycling.

To investigate the interaction between Cu₂Ce₇O_x and the electrolyte, the contact angle of electrolyte on bare Zn and Cu₂Ce₇O_x@Zn in a 2 mol/L ZnSO₄ solution was measured. The contact angles for bare Zn and Cu₂Ce₇O_x@Zn were found to be 92.8° and 109.6°, respectively (Fig.2(i)). The higher contact angle of Cu₂Ce₇O_x@Zn demonstrate better hydrophobicity compared to bare Zn, which can reduce the access of H₂O molecules to the electrode/electrolyte interface, thereby improving the overall performance of the Zn anode in zinc metal batteries.

In summary, these characterizations confirm that Cu atoms have been successfully doped into CeO₂, and the resulting Cu₂Ce₇O_x material exhibits significant changes in its crystal structure, electronic properties, and interaction with the electrolyte, all of which contribute to enhanced performance in zinc metal batteries.

The electrode/electrolyte interface plays a crucial role in determining the stability and performance of zinc metal batteries (ZMBs). In aqueous ZMBs, the stability of the Zn anode is even more crucial. Therefore, it is essential to conduct a series of analyses of the electrode/electrolyte interface to understand the impact of the interface on cell performance. One key factor influencing this is the solvation structure of Zn²⁺ ions, which can exist in several ion pair forms [50]: solvent-separated ion pair (SSIP), contact ion pair (CIP), and aggregate ion pair (AGG). Raman spectroscopy can effectively detect SSIP and CIP, and it can be used to evaluate the interfacial desolvation kinetics of Zn²⁺ ions based on the relative ratio of these ion pairs (Fig.3(a)).

Raman spectroscopy [51,52] was used to investigate the interfacial solvation structure on bare Zn/electrolyte, CeO₂@Zn/electrolyte, and Cu₂Ce₇O_x@Zn/electrolyte interfaces. The results reveal that the proportion of CIP on Cu₂Ce₇O_x@Zn is 58.4%, which is higher than that on CeO₂@Zn (48.9%) and bare Zn (42.4%). This indicates that more SO₄^2– participate in the solvation structure and bound H₂O content is reduced. A higher proportion of CIP can promote the desolvation of [Zn(H₂O)₆]²⁺ and reduce the adsorption and decomposition of H₂O molecules at the interface, improving the overall performance (Fig.3(b)).

To further explore the solvation structure of the ZnSO₄ electrolyte at the electrode/electrolyte interface, Fourier transform infrared spectroscopy (FTIR) was used to analyze the interactions between three kinds of Zn electrode [53,54]. In the FTIR spectra, characteristic peaks at 1025–1210, 1600–1800, and 3200–3700 cm⁻¹ corresponded to the interactions of Zn²⁺ with SO₄^2–, and the O–H bond in H₂O molecules, and hydrogen bonds respectively (Fig.3(c)–Fig.3(e) and S7). Compared to the blank electrolyte, the characteristic peak of SO₄^2– in ZnSO₄ electrolyte on the Cu₂Ce₇O_x surface exhibits a redshift, indicating that SO₄^2– participates more actively in the solvation structure. This increases the proportion of CIP and decreases the desolvation energy barrier. Additionally, the O–H bond of H₂O molecules in ZnSO₄ electrolyte on the Cu₂Ce₇O_x surface undergoes a blueshift, indicating that Cu₂Ce₇O_x weakens the interaction between H₂O molecules, thereby increasing the proportion of free H₂O. The peak for hydrogen bonding undergoes a redshift, indicating that Cu₂Ce₇O_x enhances the hydrogen bond network [55] and increases the stability of the electrolyte.

To quantify the influence of artificial interface on the solvation structure, the desolvation energy required for the transition from [Zn(H₂O)₆]²⁺ to Zn²⁺ was calculated (Fig.3(f) and S9). The results show that Cu₂Ce₇O_x@Zn/electrolyte interface has lower desolvation energy due to the enhanced hydrogen bond network, which reduces the binding energy between H₂O molecules and Zn²⁺ in [Zn(H₂O)₆]²⁺, thereby accelerating the redox reaction kinetics.

Molecular dynamics (MD) simulations were employed to further investigate the solvation structure at the electrode/electrolyte interface. On the CeO₂@Zn interface, Zn²⁺ is strongly adsorbed to the interior of CeO₂, resulting in the loss of interaction between Zn²⁺ and SO₄^2– (Fig. S8). MD simulations also provided the proportions on SSIP, CIP, and AGG for the ZnSO₄ electrolyte on bare Zn, CeO₂@Zn, and Cu₂Ce₇O_x@Zn interfaces (Fig.3(g)). The proportion of CIP near the Cu₂Ce₇O_x@Zn interface is the highest, reaching 40%, which is higher than that of CeO₂@Zn (38%) and bare Zn (27%). This observation is consistent with the results from MD simulations, FTIR, and Raman spectroscopy.

Moreover, to investigate the stability of these interfaces, MD simulations were also used to calculate the content of interfacial free H₂O (Fig.3(h)). The content of free H₂O near the bare Zn interface is 84.6%, which is relatively low and could hinder the Zn²⁺ desolvation process. In contrast, the interfaces of Cu₂Ce₇O_x@Zn (86.3%) and CeO₂@Zn (86.5%) have a higher content of free H₂O, indicating that these interfaces enhance the desolvation process of Zn²⁺.

Finally, the radial distribution function (RDF) and coordination number of Zn²⁺ with H₂O molecules and SO₄^2– at these interfaces were also calculated using MD simulations (Fig.3(i) and Fig.3(j)). The coordination number [21] for Zn-O (SO₄^2–) of ZnSO₄ electrolyte at the Cu₂Ce₇O_x interface is 0.61, which is higher than that at the CeO₂ (0.23) and bare Zn interfaces (0.50). In particular, the coordination number of Zn-O (SO₄^2–) of ZnSO₄ electrolyte on CeO₂ interface is 0.23, which is lower than that on the Cu₂Ce₇O_x interface and the bare Zn interface due to the excessive adsorption of CeO₂ for Zn²⁺.

In conclusion, this analysis demonstrates that an appropriate adsorption capacity for Zn²⁺ is crucial for enhancing the desolvation process and improving the redox kinetics of Zn²⁺, ultimately contributing to the improved performance of ZMBs.

To demonstrate the protective effect of the Cu₂Ce₇O_x interlayer for Zn anode, as well as its role in inhibiting side reactions and promoting uniform deposition of Zn²⁺, the immersion experiment of the anode without or with the Cu₂Ce₇O_x@Zn interface was performed, respectively (Fig.4(a) and Fig.4(b)). After soaking for seven days, the bare Zn surface showed the formation of flake-like Zn₄SO₄(OH)₆·3H₂O (ZHS), a product of the corrosion process. Nevertheless, thanks to the protective effect of the Cu₂Ce₇O_x artificial interface layer, no significant formation of ZHS was observed on the Cu₂Ce₇O_x@Zn surface (Fig.4(c), Fig.4(d) and S10), indicating that the Cu₂Ce₇O_x interlayer has an excellent anti-corrosion ability. This protective layer prevents the anode interface from being eroded by H₂O molecule, hereby inhibiting the generation of ZHS.

To understand the impact of Cu₂Ce₇O_x on the Zn deposition behavior, in situ optical microscopic was performed on the electrode without or with Cu₂Ce₇O_x interface in ZnSO₄ electrolyte at a current density of 5 mA/cm² (Fig.4(e) and Fig.4(f)). After 10 min of deposition, protrusions began to form on the surface of the bare Zn electrode. As time increased to 30 min, these protrusions grow in size, causing the bare Zn surface to become very uneven. These protrusions would eventually continue to grow under the action of an electric field and finally form dendrites. In contrast, after 30 min of deposition under the same current conditions, the Cu₂Ce₇O_x@Zn surface remained flat with no protrusions, demonstrating the protective effect of the Cu₂Ce₇O_x interface layer on Zn electrode.

Further tests using a symmetric cell with bare Zn electrode or Cu₂Ce₇O_x@Zn electrode after several cycles were conducted and analyzed by SEM (Fig. S11). After 20 cycles, the bare Zn surface exhibited visible ZHS, indicating uneven Zn deposition. After 50 cycles, the bare Zn surface began to crack due to volume changes caused by uneven deposition and stripping, as well as dendrite growth. In contrast, no obvious ZHS were observed on the Cu₂Ce₇O_x@Zn surface, where Zn²⁺ was evenly deposited on the Cu₂Ce₇O_x@Zn electrode surface in a sheet-like form after 50 cycles, indicating stable and uniform deposition behavior on the Cu₂Ce₇O_x@Zn electrode.

In conclusion, the Cu₂Ce₇O_x interlayer effectively promotes the smooth and uniform deposition of Zn²⁺ on the anode surface. The schematic diagram of the mechanism (Fig.4(g) and Fig.4(h)) shows that the Cu₂Ce₇O_x@Zn interlayer inhibits the formation of ZHS and inhibits the hydrogen evolution reaction (HER) [56,57], which enable Zn²⁺ to be uniformly deposited on the anode surface, inhibiting the appearance of Zn dendrites. This ultimately enhances the stability and lifespan of the Zn anode in zinc metal batteries.

To evaluate the impact of Cu₂Ce₇O_x on regulating the Zn deposition behavior and overall cell performance, symmetric cells with different electrode configurations, i.e., bare Zn/bare Zn, CeO₂@Zn/CeO₂@Zn, and Cu₂Ce₇O_x@Zn/Cu₂Ce₇O_x@Zn, were assembled and subjected to a series of electrochemical tests.

Nucleation behavior: At an overpotential of −150 mV, chronoamperometry was performed to analyze the respective nucleation modes for nucleation modes (Fig.5(a)). The current density of the bare Zn keeps increasing for up to 300 s, a behavior attributed to preferential nucleation occurring in local regions on the electrode surface. Zn²⁺ preferentially accumulates in these regions for nucleation, causing the local current density to increase continuously, indicative of an irregular 2D deposition nucleation mode. This irregular nucleation causes Zn²⁺ to easily move freely on the anode surface, when it reaches the Zn anode surface. Moreover, the Zn²⁺ will continuously deposit randomly in places with lower nucleation energy, eventually leading to a homogeneous surface of the Zn anode which is prone to dendrite formation. In contrast, the symmetric cells with CeO₂@Zn and Cu₂Ce₇O_x@Zn electrodes showed a different pattern. After 50 s, the current density remains basically stable. This is because CeO₂ and Cu₂Ce₇O_x provide nucleation sites for the electrodes, restricting the migration of Zn²⁺ on the electrode surface. As a result, Zn²⁺ can nucleate and grow uniformly under the action of the electric field. They show a three-dimensional (3D) progressive nucleation mode in the subsequent deposition process [58,59]. This nucleation mode can limit the movement of Zn²⁺ on the CeO₂@Zn and Cu₂Ce₇O_x@Zn electrode surface. Moreover, nucleation can occur simultaneously in multiple directions on the CeO₂@Zn and Cu₂Ce₇O_x@Zn electrode surface, which is conducive to increasing of nucleation sites and improving the deposition efficiency, thereby inhibiting the formation of dendrites. More importantly, because of the strong adsorption energy of CeO₂@Zn electrode for Zn²⁺, it is not conducive to the homogeneous deposition of Zn²⁺ in the subsequent deposition process on the CeO₂@Zn electrode surface.

Corrosion resistance and hydrogen evolution: To further investigate of the kinetic processes of the three kinds of Zn electrodes, Tafel curves [60,61] were obtained to characterize the protective effect of artificial layer on Zn anode (Fig.5(b)), and the corrosion potentials and corrosion currents of the electrodes were measured. The Cu₂Ce₇O_x@Zn electrode has the highest corrosion potential and the lowest corrosion current, indicating that it has superior corrosion resistance both thermodynamically and kinetically. Furthermore, LSV was used to examine the hydrogen evolution potentials of bare Zn/bare Zn, CeO₂@Zn/CeO₂@Zn, and Cu₂Ce₇O_x@Zn/Cu₂Ce₇O_x@Zn symmetric cells, respectively (Fig.5(c)). Obviously, the hydrogen evolution potential of Cu₂Ce₇O_x@Zn/Cu₂Ce₇O_x@Zn symmetric cell is more negative than others. Cu₂Ce₇O_x@Zn shows outstanding ability in inhibiting the corrosion resistance and hydrogen evolution potential, which is crucial for achieving uniform deposition of Zn²⁺.

Cyclic stability: To evaluate the long-term stability of the Zn anode, symmetric cells with bare Zn/bare Zn, CeO₂@Zn/CeO₂@Zn, and Cu₂Ce₇O_x@Zn/Cu₂Ce₇O_x@Zn electrodes were cycled for a prolonged period (Fig.5(d)). The bare Zn/bare Zn symmetric cell short-circuited and failed after approximately 90 hours while the CeO₂@Zn/CeO₂@Zn symmetric cell lasted for 700 h. Although CeO₂@Zn electrode can improve cell cycle performance, its overly strong adsorption effect on Zn²⁺ leads to unfavorable Zn desorption/adsorption during deposition. The bare Zn/bare Zn and CeO₂@Zn/CeO₂@Zn symmetric cell display a cyclic stability of 147 and 700 h and an overpotential of 53 and 36 mV. In contrast, the Cu₂Ce₇O_x@Zn/Cu₂Ce₇O_x@Zn symmetric cell has exceptional stability, cycling for more than 1600 h with an overpotential of only 24 mV. Moreover, the R_ct for the Cu₂Ce₇O_x@Zn/Cu₂Ce₇O_x@Zn symmetric cell is only 150 Ω at room temperature, which is lower than the R_ct of the bare Zn/bare Zn (332 Ω) and CeO₂@Zn/CeO₂@Zn (202 Ω) symmetric cells (Fig. S12). These results show that Cu atoms doping changes the physicochemical properties of the CeO₂ substrate, balancing the adsorption energy and migration energy of Zn²⁺, enhancing the performance of the ZMBs.

High current performance: Even at a high current density of 5 mA/cm², the Cu₂Ce₇O_x@Zn/Cu₂Ce₇O_x@Zn symmetric cell can still cycle for nearly 1600 h, indicating excellent stability under demanding conditions (Fig. S13). Furthermore, rate performance tests reveals that the overpotential for Cu₂Ce₇O_x@Zn is the smallest, indicating its excellent interface stability (Fig.5(e)).

Coulombic efficiency: In the long-cycle test using Cu/Zn asymmetric cells at a current density of 2 mA/cm² and a capacity of 1 mAh/cm² (Fig. S15), the Cu₂Ce₇O_x@Zn electrode can significantly optimize the Zn deposition process and improve the cycle life, showing a high average Coulombic efficiency [62] of 99.9%. The potential capacity curves demonstrate that Cu₂Ce₇O_x@Zn/Cu asymmetric cells have a better reversibility and the lowest nucleation overpotential (Fig. S14).

Impedance analysis: Finally, in situ impedance measurements were conducted on symmetric cells during charge/discharge process, at a current density of 0.5 mA/cm² and a capacity of 0.5 mAh/cm² (Fig.5 (f) and 5(g)). Cu₂Ce₇O_x@Zn/Cu₂Ce₇O_x@Zn symmetric cell has a smaller R_ct during cycling compared to the bare Zn/bare Zn symmetric cell, indicating that the Cu₂Ce₇O_x@Zn interface remains more stable throughout the cycle and significantly improves the performance of the Zn anode.

In summary, the incorporation of Cu₂Ce₇O_x into the Zn anode interface significantly improves the electrochemical performance, stability, and lifespan of the ZMB. The Cu₂Ce₇O_x@Zn electrode provides superior corrosion resistance, enhances Zn deposition uniformity, reduces dendrite formation, and promotes stable cycling performance even under high current densities. These findings demonstrate that Cu₂Ce₇O_x is a promising material for improving the performance of Zn-based batteries, providing better interface stability and a more efficient Zn deposition process.

The study of full cells [63] with MnO₂ cathodes, synthesized with manganese dioxide (MnO₂) cathode, aimed to explore the improvement of the overall performance of the cell by constructing the Cu₂Ce₇O_x interlayer.

Cyclic voltammetry (CV) analysis: The CV curves of the bare Zn/MnO₂, CeO₂@Zn/MnO₂, and Cu₂Ce₇O_x@Zn/MnO₂ full cells in the third cycle, at a scanning rate of 0.5 mV/s (Fig. S16) show that the Cu₂Ce₇O_x@Zn/MnO₂ full cell has faster redox kinetics and lower polarization overpotential, compared to the other two configurations. The Cu₂Ce₇O_x interlayer helps balance the adsorption/desorption process and migration of Zn²⁺, thus improving the overall performance of the cell.

Rate performance: To simulate the overall stability of the cells during operation, the rate performance of the three full cells was tested. The Cu₂Ce₇O_x@Zn/MnO₂ full cell operates stably with a capacity of 234.5 mAh/g at a current density of 0.2 A/g. Even at a large current density of 2 A/g, the Cu₂Ce₇O_x@Zn/MnO₂ full cell still maintained a capacity of 138.9 mAh/g (Fig.6(a)). This demonstrates the ability of the Cu₂Ce₇O_x interlayer to maintain stable performance at high current densities, ensuring long-term stability.

Galvanostatic charge-discharge (GCD) curves: The GCD curves (Fig.6(b) and S17) of the Cu₂Ce₇O_x@Zn/MnO₂ full cell shows the highest capacity with the smallest electrode polarization, which is consistent with the CV results. These results further indicate that the Cu₂Ce₇O_x@Zn interface effectively reduces polarization and enhances the efficiency of the Zn deposition process.

Self-discharge performance: To assess the self-discharge characteristics of the cells, the CeO₂@Zn/MnO₂ and Cu₂Ce₇O_x@Zn/MnO₂ were left standing for 24 h to measure the impact of self-discharge on these full cells after one complete cycle (Fig.6(c)). The Cu₂Ce₇O_x@Zn/MnO₂ full cell retained 98.9% of its initial capacity, which is significantly higher than that of the CeO₂@Zn/MnO₂ full cell (90.4%). This highlights the Cu₂Ce₇O_x@Zn/MnO₂ full cell’s superior ability to maintain capacity during resting periods, likely due to its enhanced interface stability.

Cycle performance: The cycle performance of these full cells was evaluated at current densities of 0.5 and 1 A/g, respectively (Fig.6(d) and Fig.6(e)). The CeO₂@Zn/MnO₂ and Cu₂Ce₇O_x@Zn/MnO₂ show an upward curve for a certain period, which is the electrochemical activation phenomenon of the cells in the initial stage. The Cu₂Ce₇O_x/MnO₂ full cell exhibits an initial capacity of 190.1 mAh/g at 0.5 A/g, and retains 145 mAh/g after more than 500 cycles. At a higher current density of 1 A/g, the Cu₂Ce₇O_x/MnO₂ full cell displays an initial capacity of 180.9 mAh/g and a remaining specific capacity of 160.9 mAh/g after 800 cycles. These results indicate that the Cu₂Ce₇O_x artificial interface layer enhances the long-term cycling stability of the Zn-MnO₂ full cells by improving Zn²⁺ migration and deposition, reducing dendrite formation, and maintaining uniform Zn deposition.

Low-temperature performance: To assess the performance of the Cu₂Ce₇O_x interlayer at low temperatures, the cyclic performance of the full cells with bare Zn/MnO₂ and Cu₂Ce₇O_x@Zn/MnO₂ was tested at 0 °C. Impressively, the Cu₂Ce₇O_x/MnO₂ full cell has a maximum capacity of 138.2 mAh/g at a current density of 0.2 A/g, and the reversible capacity is 92.9 mAh/g after 500 cycles, showcasing its outstanding initial capacity even at subzero temperatures (Fig.6(f)), which highlights the great potential of the Cu₂Ce₇O_x interlayer to improve the low-temperature stability of ZMBs.

In summary, the Cu₂Ce₇O_x interlayer significantly enhances the performance of Zn/MnO₂ full cells by improving the redox kinetics, reducing polarization, maintaining stable Zn²⁺ migration, and enhancing cycle life. The Cu₂Ce₇O_x@Zn/MnO₂ full cell outperforms the bare Zn/MnO₂ and CeO₂@Zn/MnO₂ cells in terms of rate performance, self-discharge retention, and long-term cycling stability. Furthermore, it shows excellent low-temperature performance, demonstrating its great potential for use in practical applications, such as in high-power, long-cycle life, and low-temperature environments. The incorporation of Cu into the CeO₂ matrix effectively balances Zn²⁺ adsorption and migration, making Cu₂Ce₇O_x a promising material for improving the overall performance of Zn-based batteries.

3 Conclusions

In conclusion, this study presents a novel strategy of adjusting the d-band center of CeO₂ and changing the electronic structure by doping Cu metal elements to balance the kinetics of the adsorption/desorption and migration processes of Zn by CeO₂ as interlayers. The findings indicate that Cu₂Ce₇O_x as the interlayer for Zn anode, provides appropriate adsorption energy and migration energy for Zn²⁺, which benefits from the regulation of the d-band center.

The Cu₂Ce₇O_x interlayer significantly improves the stability and lifespan of Zn anodes by reducing the overpotential of the Zn²⁺ deposition/stripping process. Electrochemical testing shows that the Cu₂Ce₇O_x@Zn electrode maintains a high cycle life of over 1600 h with excellent reversibility and a high average Coulombic efficiency of 99.9% during the deposition/stripping process. Additionally, the Cu₂Ce₇O_x@Zn electrode has the smallest initial nucleation overpotential at a current density of 2 mA/cm² and a capacity of 1 mAh/cm². The Cu₂Ce₇O_x@Zn/MnO₂ full cell exhibits remarkable cycling performance, with a capacity retention rate of 92.2% after 800 cycles at a current density 1 A/g, further highlighting the benefits of the Cu₂Ce₇O_x interlayer in enhancing the stability and efficiency of Zn-based batteries.

Overall, this work offers a promising approach to improving Zn anode protection and provides valuable insights for developing advanced strategies to enhance the performance of Zn-MnO₂ full cells and other Zn-based batteries.

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