Highly reversible Zn electrodeposition enabled by glutathione-protected copper nanoclusters for aqueous Zn-ion batteries

Abstract

Dendrite growth and side reactions occurring in Zn anodes hinder the development and practical application of aqueous Zn-ion batteries. Herein, for the first time, we demonstrated the use of ligand-protected Cu nanoclusters as Zn plating/stripping mediators to construct dendrite-free Zn anodes. The abundant polar functional groups (–COOH, –NH₂, and –CO–NH–) on the peripheral ligands modulated the solvation structure and regulated the transport of Zn²⁺, whereas the zincophilic Cu cores functioned as nucleating agents to guide the uniform nucleation and plating of Zn. Using such bifunctional CuNCs, the asymmetric cell achieves a coulombic efficiency of 99.40% for over 1000 cycles, and the symmetric cell exhibits low voltage hysteresis and superior cycling over 400 h with a high depth of discharge of 40%. These findings may contribute to the design of multifunctional mediators at the atomic level.

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Introduction

Aqueous Zn-ion batteries (AZIBs) have garnered extensive attention in the energy-storage field owing to their distinct benefits, including low cost, high theoretical capacity (820 mA h g⁻¹, 5855 mA h cm⁻³), low redox potential (−0.76 V vs. the standard hydrogen electrode (SHE)), and inherent safety.^1–5 Nevertheless, the uncontrollable growth of Zn dendrites, which widens the electrode–electrolyte contact area and exacerbates detrimental side reactions on the Zn anode surface, remains a thorny issue.^6,7 Various approaches, including surface modification,^8–13 use of electrolyte additives,^14–17 separator optimization,^18,19 and host design,^20–25 have been reported as solutions to these issues. Among these approaches, building three-dimensional current collectors is a feasible strategy to efficiently restrain the growth of Zn dendrites compared to conventional planar Zn anodes. This method reduces the local current density and homogenizes the electrical field on the anode surface, while mitigating volume variations during Zn plating/stripping, thereby enhancing electrode stability.²⁶

Owing to their high specific surface area, ultralight design, and unique vertical–horizontal network structure, carbon cloth (CC) current collector offers inherent advantages for future advancements in Zn metal anodes and flexible energy-storage applications. However, the absence of effective zincophilic sites results in a high nucleation barrier for Zn²⁺ ions on naked CC.²⁷ Our group doped CC with heteroatoms to increase the number of zincophilic sites, which can induce exposure of the Zn (002) crystal plane and significantly enhance the cell performance.²⁸ Liang et al.²⁹ prepared an amine-functionalized CC host. The introduced amine groups enhanced the binding energy of Zn on the CC surface and directed the nucleation and growth of Zn. Dong et al.³⁰ revealed that a layer of Cu nanosheets on CC can considerably reduce the Zn nucleation overpotential and provide a large number of evenly spaced Zn deposition sites. Most CC modification strategies focus on the modulation of their zincophilic ability while neglecting the optimization of the solvated structure of hydrated Zn²⁺ ions transported on their surface. Notably, the desolvation of Zn(H₂O)₆²⁺ has a high energy barrier, resulting in the inhomogeneous deposition of metallic Zn. Adjusting the coordination environment of Zn²⁺ ions can effectively regulate the solvation structure and chemical properties of hydrated Zn²⁺ ions, thereby inhibiting dendrites growth and side reactions.³¹ Thus, the construction of bifunctional modulators with high zincophilic ability and regulation of the solvated structure of Zn²⁺ ions on the CC surface is an effective method to further enhance the performance of current collectors.

Metal nanoclusters (NCs) are a type of nanomaterial sized 1–3 nm with a metal core and a surface modified by different organic ligands. They have a monodisperse nature and precise atomic structure, facilitating the analysis of the action site and mechanism during application. In addition, ligand-protected NCs have good stability.^32–35 By selecting different metals and organic ligands with characteristic functional groups to regulate the synthesis, the target clusters can be obtained more accurately and quickly.³⁶ Consequently, NCs are widely used in electrochemistry owing to their unique advantages. For example, using CuNCs as precursors, we have synthesized zinc-metal-free anode materials for high-performance AZIBs by precisely controlling the pyrolysis process of the NCs.³⁷ Beyond that, Wang et al. first proposed the improved performance of organic-covered metal NCs as active catalysts than those with partially or completely removed surface ligands.³⁸ With reasonable ligand selection and synthesis regulation, even intact metal NCs can be directly applied to electrochemistry to achieve unexpected results through the synergistic effect of ligands and cores.

In this paper, for the first time, we present a new strategy to construct stable dendrite-free Zn anodes using a CC substrate modified with glutathione (GSH)-protected CuNCs (GSH-CuNCs) (Fig. 1a). The polar functional groups (–COOH, –NH₂, and –CO–NH–) on the peripheral GSH ligand have high binding energy with Zn²⁺ to promote the desolvation of Zn(H₂O)₆²⁺ and accelerate the Zn²⁺ transport kinetics. In addition, the Cu cores can be regarded as Zn-deposited seeds that guide the homogeneous nucleation and growth of Zn on the CC substrate. The asymmetric cell employing CuNCs achieved a coulombic efficiency (CE) of 99.40% after 1000 stable plating/stripping cycles at 1.0 mA cm⁻². Meanwhile, the lifetime of the symmetric cell with CuNCs-modified current collector was improved by more than 500 h, which is 12 times higher than that of pristine CC (PCC) (Fig. 1b; Fig. S1, ESI†). This initial application of NCs in a dendrite-free Zn anode design provides a reference for high-performance electrode modification and promotes the development of NCs for energy-storage devices.

Fig. 1 (a) Preparation process diagram of NCC and NCC@CuNCs. (b) The comparison histogram of cycle times of symmetric cells based on three different scaffolds at a current density 1 mA cm⁻² and an areal capacity of 1 mA h cm⁻².

Experimental section

Experimental materials

CC was purchased from Keshenghe (Suzhou, China). L-Glutathione (L-GSH, 99%) was purchased from J&K Scientific. Sodium hydroxide (≥96.0%) was purchased from Tianjin Fengchuan chemical reagent technology Co., Ltd. Zinc sulfate heptahydrate (AR) was purchased from Aladdin. Acetone (≥99.5%), ammonia solution (25%–28%), manganese sulfate monohydrate (99.99%), and potassium permanganate (≥99.5%) were purchased from Luoyang chemical reagent plant. Copper sulfate pentahydrate (AR, 99%) was purchased from Macklin. The above experimental materials are used directly for experiments without further purification.

Preparation of NCC@CuNCs

GSH-CuNCs were synthesized according to a previous report.³⁹ PCC was placed in ammonia solution by adopting solvothermal method at 150 °C for 24 h for N-doped treatment, and then rinsed with ultra-pure water to PH = 7 and dried at 60 °C for 12 h to obtain nitrogen-doped carbon cloth (NCC). The prepared NCC was immersed in CuNCs solution for 12 h at 4 °C, then washed with ultra-pure water and dried to obtain NCC anchored with GSH-CuNCs (NCC@CuNCs).

Preparation of NCC@Zn and NCC@CuNCs@Zn anodes

The NCC@Zn and NCC@CuNCs@Zn were prepared by electrochemical deposition method on a LAND-CT2001A battery testing system. The CR2032 coin-type battery was assembled with 2 M ZnSO₄ as electrolyte, zinc foil (diameter 12 mm and thickness 100 μm) and NCC/NCC@CuNCs of the same size were employed as counter and working electrodes, respectively. After that, metallic zinc was electrodeposited on CC at current density of 5 mA cm⁻² for 1 h with a deposition capacity of 5 mA h cm⁻². Finally, the working electrodes were removed from the coin-type cells as well as cleaned with ultra-pure water and dried at 60 °C to obtain NCC@Zn and NCC@CuNCs@Zn electrodes.

Preparation of MnO₂ cathode

0.1 M KMnO₄ solution and 0.6 M MnSO₄ solution were prepared and mixed in equal volumes, stirred magnetically for 10 minutes, then transferred to the polytetrafluoroethylene (PTFE) lining of stainless steel reactor and reacted in an oven at 140 °C for 12 h. Next, the reaction liquid was centrifuged, and washed with ultrapure water repeatedly. Finally, it was dried at 60 °C to obtain β-MnO₂. β-MnO₂, acetylene black and polyvinylidene fluoride were mixed according to the mass ratio of 7 : 2 : 1, and the appropriate amount of 1-methyl-2-pyrrolidinone was added to fully grind, then uniformly coated on the stainless steel mesh by doctor blade method, and the MnO₂ cathode was prepared by vacuum drying at 60 °C for 12 h. Finally, the mass loading of active material was about 1.5 mg cm⁻².

Material characterization

The morphology and microstructure of the samples were characterized by scanning electron microscope (SEM) (Zeiss Sigma 500, German) equipped with an energy-dispersive X-ray spectroscopy (EDS) detector. X-ray diffraction patterns (XRD) were recorded with Rigaku MiniFlex600 diffractometer equipment using Cu K_α radiation (k = 1.5418 Å) at 30 kV to analyze the crystal structure of the products. The surface composition and content of the electrodes were characterized by X-ray photoelectron spectroscopy (XPS) using Thermo Scientific K-Alpha photoelectron spectrometer with monochromatic Al Kα (1486.6 eV) radiation. The hydrophilicity difference of electrodes was measured by Chengde Dingsheng JY-82C video contact Angle tester. The surface defect degree of CC was characterized by Thermo Scientific DXR 3Xi Raman spectrometer. At room temperature, Fourier Transform Infrared Spectroscopy (FT-IR) was recorded by ALPHA II infrared spectrometer in the range of 4000–500 cm⁻¹. Transmission electron microscopy (TEM) images were captured by Japan-JEOL-JEM 2100 F. The ultraviolet-visible (UV-vis) spectra were collected on UV-2700.

Electrochemical characterization

The cycle reversibility and stability of different scaffolds were studied by means of symmetric cells, which were assembled with NCC/NCC@CuNCs as the working electrode and Zn foil as the counter electrode. In order to evaluate the electroplating/stripping efficiency of the anodes, asymmetric batteries were assembled with NCC/NCC@CuNCs as the working electrode and Zn foil as the counter electrode. The full cells were assembled using an as-prepared β-MnO₂ cathode and NCC@Zn/NCC@CuNCs@Zn anode, respectively. All the above components were packaged in the CR2032 coin-type battery case, using glass fiber as the diaphragm, 2 M ZnSO₄ as the electrolyte for symmetric and asymmetric batteries, and 2 M ZnSO₄ + 0.1 M MnSO₄ as the electrolyte for full batteries. Using the LAND-CT2001A battery test system, the batteries were tested at 30 °C, including long cycle performance, nucleation overpotential, rate, CE and discharge specific capacity.

At room temperature, the electrochemical performance tests were recorded through electrochemical workstation (CHI760E). Electrochemical impedance spectroscopy (EIS) was measured at frequencies ranging from 0.01 Hz to 100 kHz. Cyclic voltammetry curves (CV) were collected at a scan rate of 0.1 mV s⁻¹. The diffusion curves were tests via i–t curves at −150 mV overpotential. The hydrogen evolution reaction (HER) performance was measured by conducting linear sweep voltammetry (LSV) curves, where NCC@Zn or NCC@CuNCs@Zn was used as the working electrode and the Pt foil and Ag/AgCl electrode were used as the counterpart and the reference electrode, respectively. The corrosion property was also measured by means of LSV, the difference was that a two-electrode system was used, where NCC@Zn or NCC@CuNCs@Zn as the working electrode and Zn foil as the counter electrode.

Density functional theory (DFT) calculations

The binding energy of between Zn²⁺ and H₂O molecule or GSH ligand respectively were calculated with DMol3 module by Materials Studio. The exchange–correlation functional was performed by generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE). The force convergence criteria, energy criteria for geometry optimization and energy criteria for SCF convergence were 0.002 Ha Å⁻¹, 1.0 × 10⁻⁵ Ha and 1.0 × 10⁻⁶ Ha, respectively. The wave function was fitted by DNP basis set. The core electrons were processed by Effective Core Potential (ECP) method. The orbittal cutoff radius was 4.4 Å. The binding energy (E_b) were calculated as the following equation:

E_binding = E_AB − E_A − E_B

where E_AB represents the total energy of H₂O or Zn²⁺ combined with GSH, E_A is the energy of H₂O or Zn²⁺, and E_B is the energy of GSH.

Results and discussion

Preparation and characterization of GSH-CuNCs and different hosts

GSH-CuNCs were synthesized using a facile one-pot method. The UV-vis absorption spectrum of the obtained CuNCs solution exhibits an absorption peak at 250 nm (Fig. 2a), indicating the successful formation of CuNCs.³⁹ The solution appeared light green under natural light and bright blue under UV light (inset in Fig. 2a). The dispersion state of the GSH-CuNCs in the aqueous solution was observed by TEM. The prepared GSH-CuNCs are uniformly distributed in a spherical shape (Fig. 2b).

Fig. 2 (a) UV-vis absorption spectrum of CuNCs (inset: digital photos of the CuNCs solution under natural (the left) and UV light (the right)). (b) TEM image of CuNCs. (c) XPS high-resolution N 1s spectra of PCC and NCC. (d) XPS high-resolution Cu 2p spectrum of NCC@CuNCs. (e) TEM image of NCC@CuNCs. (f) UV-vis image of solution of NCC@CuNCs.

In order to improve the hydrophilicity and enhance the adsorption capacity of the CC substrate for water-soluble GSH-CuNCs, the PCC was hydrothermally treated with ammonia solution to obtain NCC.^40–42

As shown in Fig. S2a (ESI†), both PCC and NCC own smooth surfaces, indicating that the morphology of the CC remains essentially unchanged after N doping. The corresponding XRD spectra also confirm this finding (Fig. S2b, ESI†). The elemental mapping images depict the uniform distribution of C and N elements in the NCC (Fig. S3, ESI†), proving the successful N doping into the CC substrate. Meanwhile, the peak areas of the N 1s XPS spectra of CC were compared, as shown in Fig. 2c. NCC has a large N 1s peak area, illustrating its significantly higher N content (approximately 2.17 at%) than that of PCC. The high-resolution N 1s spectra of PCC and NCC can be deconvoluted into three main peaks at 397.9, 399.9, and 402.5 eV, corresponding to pyridinic N, pyrrole N, and graphite N, respectively (Fig. S4, ESI†).⁴³ The pyrrole N content of NCC is determined to be 67.1% (Table S1, ESI†), elucidating the abundant defects in CC after N doping. This was also verified by Raman spectroscopy (Fig. S5, ESI†), where the D peak (∼1350 cm⁻¹) represents the defect degree of disordered C materials and the G peak (∼1580 cm⁻¹) is the main characteristic peak of graphene owing to the in-plane vibration of C atoms; thus, the peak strength ratio (I_D/I_G) reflects the degree of structural disorder of C materials.⁴⁴ The I_D/I_G ratios of PCC and NCC are calculated to be 1.04 and 1.14, respectively, demonstrating the increased disorder and number of defects in CC after N doping, which is conducive to anchoring CuNCs.

After that, NCC was immersed in CuNCs solution to obtain NCC@CuNCs. XPS was performed to explain its successful construction and analyze the surface composition. The XPS spectra of NCC@CuNCs exhibit constituent element peaks of CuNCs (Fig. 2d; Fig. S6, ESI†). In particular, the satellite peak at approximately 942 eV is absent in the Cu 2p XPS spectrum, which is consistent with the XPS results of previously reported CuNCs.³⁹ The EDS elemental mapping analysis shows homogeneous distribution of C, N, S, O, and Cu elements on the NCC host (Fig. S7, ESI†), illustrating that there is no obvious agglomeration of the GSH-CuNCs adsorbed on NCC. In order to further illustrate the successful anchoring of CuNCs to the NCC substrate, TEM analysis of NCC@CuNCs was conducted. As expected, small spotted CuNCs are uniformly distributed on the NCC fibers (Fig. 2e), and conspicuous lattice fringes are observed in the high-resolution TEM images. The lattice fringes with a lattice plane distance of 2.07 Å corresponds to the Cu (111) crystal plane (JCPDS 89-2838; Fig. S8, ESI†).⁴⁵ Subsequently, NCC@CuNCs was ultrasonically treated in ultrapure water. The UV-vis image of the solution exhibits CuNCs absorption peak (Fig. 2f). These characterizations indicate the successful preparation of the NCC and NCC@CuNCs substrates. What's more, the properties of the CuNCs anchored to the NCC are still maintained.

Zn plating/stripping performance of different substrates

The degree of wetting at the interface of the electrolyte and electrode indicates the uniformity of the charge distribution, which directly affects the Zn deposition on the anode surface.⁴⁶ Therefore, we separately investigated the contact angles of 2 M ZnSO₄ on the surfaces of the PCC, NCC, and NCC@CuNCs substrates (Fig. S9, ESI†). NCC@CuNCs displays a contact angle of 112.13°, which is smaller than those of NCC (115.42°) and PCC (125.07°) in the initial state. Although the contact angles of three samples all mitigate after standing for 5 and 10 min respectively, NCC@CuNCs remains the smallest. This can be interpreted as the presence of CuNCs to improve electrode hydrophilicity and electrolyte accessibility, which facilitates a uniform Zn²⁺ flux and decreases the interfacial free energy between NCC@CuNCs and the electrolyte, thereby promoting the rapid diffusion and uniform nucleation of Zn²⁺ on the CC surface.⁴⁷ In order to verify the stability of CuNCs on the substrate during the charge–discharge cycle, 5 mA h cm⁻² Zn was first deposited on NCC@CuNCs substrate and then stripped at equal capacity, then the UV image of the solution after ultrasound maintains the characteristic absorption peak of CuNCs (Fig. S10, ESI†), suggesting the stable CuNCs loading on the CC during the charge–discharge process.

CE is a crucial parameter for evaluating the reversibility of Zn anodes.^24,48 We assembled Zn//NCC and Zn//NCC@CuNCs asymmetric cells to probe the Zn plating/stripping behavior on different substrates. During the CE test, Zn was plated on the CC hosts by discharging at various current densities (1, 3, and 5 mA cm⁻²) for 1 h and then stripped to a cutoff voltage of 0.5 V at the same current density for each cycle. As shown in Fig. 3a, the Zn//NCC@CuNCs cell displays an average CE of ∼99.40% after 1000 cycles at 1 mA cm⁻² with an areal capacity of 1 mA h cm⁻². This cycling life is approximately 20 times longer than that of the Zn//NCC cell which undergoes short circuit after only 50 cycles. Besides, NCC@CuNCs exhibits a lower nucleation overpotential (NOP) of 128.2 mV at 2 mA cm⁻² compared to 358.5 mV of NCC (Fig. 3b), which can be attributed to the CuNCs that guarantee homogeneous Zn nucleation and deposition on the CC surface. Zn//NCC@CuNCs exhibits an initial CE of 89.50% as the current density is increased to 3 mA cm⁻², outperforming Zn//NCC (86.90%). In addition, the Zn//NCC@CuNCs cell (110 cycles) has a longer lifespan than Zn//NCC (37 cycles) even at a current density of 5 mA cm⁻² (Fig. 3c). The performance of the asymmetric cells is more competitive with the reported AZIB hosts (Table S2, ESI†).^{20,21,24,26,49–52} The PCC was immersed in CuNCs solution to obtain PCC@CuNCs for demonstrating the need for a defect-rich host to anchor CuNCs. The cycle life of the corresponding asymmetric cells at three current densities is longer than that of the NCC but less than that of NCC@CuNCs (Fig. S11, ESI†). This not only shows the advantages of CuNCs, but also emphasizes the importance of proper amount of anchoring to improve the performance of AZIBs.

Fig. 3 Asymmetric cells (a) CE plots of Zn plating/stripping on NCC and NCC@CuNCs at 1 mA cm⁻² and 1 mA h cm⁻². (b) Nucleation overpotentials of Zn deposition on different current collectors at a current density of 2 mA cm⁻² and areal capacity of 2 mA h cm⁻². (c) CE plots of Zn plating/stripping on NCC and NCC@CuNCs electrodes at different current densities. SEM morphologies of (d) NCC@CuNCs and (e) NCC after the first cycle under different current densities. (f) Schematic diagrams of Zn deposition on the two kinds of substrates.

Furthermore, we fetched the electrodes after the first cycle under different current densities and areal capacities to explore the surface morphology difference of the substrates using SEM. As illustrated in Fig. 3d, the smooth CC fibers of the NCC@CuNCs indicate the almost complete stripping of Zn at different current densities owing to substrates enhanced cyclic reversibility. Conversely, there are numerous irregular Zn nanosheets stacked and exhibited a vertical distribution, forming bumps on the electrode surface (Fig. 3e).

More impressively, the electric field at these bumps is unevenly distributed, resulting in a “tip effect”. Consequently, an increasing number of Zn are attracted to deposit where the current density is higher, aggravating Zn dendrite growth during subsequent deposition. Dendrite growth increases the contact area between the electrode and electrolyte, leading to serious side reactions that poses a low CE and eventual failure of Zn//NCC.⁵³

Under a constant voltage of −150 mV, the Zn nucleation behavior on the surface of different substrates was further studied by chronoamperometry (CA). The deposition current density of the NCC@Zn electrode exhibits a monotonically increasing trend within 400 s owing to the two-dimensional diffusion of Zn²⁺ that formed Zn dendrites on the electrode surface. Meanwhile, the surface current of the NCC@CuNCs@Zn electrode increases in the first 50 s of Zn deposition and then remains constant, reflecting the three-dimensional diffusion characteristic of Zn²⁺. This manifests the uniform Zn deposition on the electrode surface without obvious Zn dendrite formation (Fig. S12, ESI†).⁵⁴Fig. 3f presents the nucleation and growth behaviors of Zn on the surfaces of two types of substrates. Due to the lack of nucleation sites, Zn deposition on NCC surface is uneven and covers with irregular dendrites after cycling. On the contrary, the presence of CuNCs induces uniform and dense deposition of Zn on the surface of the CC, resulting in dendrite-free NCC@CuNCs@Zn electrode.

Inhibited hydrogen evolution and corrosion side reactions by NCC@CuNCs substrate

The depth of discharge (DOD) is an important metric reflecting the performance of AZIBs, representing the Zn utilization in symmetric cells.⁵⁵ A conventional thick Zn foil (thickness = 100 μm) is used to assemble a symmetric cell, which fails because of the short circuit after 80 h with the DOD of only 3.4% (eqn (1) and Fig. S13a, ESI†), thereby significantly increasing the manufacturing cost and reducing the actual energy density of the battery. We selected a reasonable DOD of 40% (eqn (2), ESI†) and used the predeposition method on NCC and NCC@CuNCs to prepare Zn metal anodes (the details are shown in the Experimental section), thus reducing the amount of excess Zn and improving the Zn utilization. The XRD patterns of NCC@Zn and NCC@CuNCs@Zn show obvious characteristic diffraction peaks of Zn respectively (Fig. S14, ESI†), indicating the successful Zn deposition on CC. The long-term galvanostatic cycling test of the symmetric cell was carried out under 2 mA cm⁻² and areal capacity of 2 mA h cm⁻². Short circuit is noted on Zn//NCC after 70 h, which can be interpreted as the uncontrolled growth of Zn dendrites that eventually punctured the diaphragm. In comparison, Zn//NCC@CuNCs has a longer cycle life of over 400 h (Fig. 4a; Fig. S13b and S15, ESI†). The polarization voltage of Zn//NCC fluctuates greatly in the initial cycle, immediately followed by a short circuit, while Zn//NCC@CuNCs maintains basically stable voltage profiles for nearly 300 h. Notably, the NCC@CuNCs@Zn anode exhibits better cycling performance than most previously reported anodes (Table S3, ESI†).^{20,22,24,26,28,29,49–52,56–60} These results reveal the excellent cyclic reversibility of NCC@CuNCs@Zn, effectively inhibiting the Zn dendrites growth and side reactions under the guidance of the zincophilic Cu cores and polar functional groups. The long cycle life of symmetric cells based on the PCC@CuNCs substrate at current densities of 1 and 2 mA cm⁻² is between that of NCC and NCC@CuNCs (Fig. S16, ESI†) owing to the limited hydrophilicity of the electrodes, again highlighting the positive role of CuNCs in improving electrode dynamics.

Fig. 4 (a) Cycling performance of symmetric cells Zn//NCC and Zn//NCC@CuNCs at 2 mA cm⁻². (b) Rate test of NCC@Zn//Zn and NCC@CuNCs@Zn//Zn at different current densities. (c) LSV curves in 1 M aqueous Na₂SO₄ electrolyte. (d) Linear polarization curves of NCC@Zn and NCC@CuNCs@Zn. (e) The DFT calculations for binding energies of Zn²⁺ to H₂O and GSH ligand. (f) Schematic diagram of Zn²⁺ transport and uniform deposition on the surface of NCC@CuNCs substrate.

The rate performance of a symmetric cell is the standard for measuring whether the uniform deposition of Zn²⁺ on an electrode during cycling. Therefore, symmetric cells were tested with a fixed areal capacity of 1 mA h cm⁻² to investigate the Zn²⁺ deposition of the NCC and NCC@CuNCs substrates at current densities 1, 2, 4, 5, and 10 mA cm⁻², respectively (Fig. 4b). According to the obtained voltage curves, NCC@CuNCs has a lower voltage hysteresis than NCC, which is more prominent at high current densities, indicating a lower Zn nucleation barrier on the NCC@CuNCs substrate and further revealing the improvement of the Zn electroplating/stripping kinetics. This conclusion is also confirmed by the smaller charge-transfer resistance (R_ct) of Zn//NCC@CuNCs in the Nyquist plots of the impedances of the symmetric cells (Fig. S17, ESI†).

In order to analyze the inhibitory effect of the NCC@Zn and NCC@CuNCs@Zn anodes on hydrogen evolution, LSV was performed at a scanning rate of 5 mV s⁻¹ in a 1 M Na₂SO₄ aqueous solution. As there is no interference from the Zn deposition, the obtained current signal can accurately reflect the HER intensity. As shown in Fig. 4c, NCC@CuNCs@Zn (−1.189 V) exhibits a lower negative overpotential than NCC@Zn (−1.169 V) at a fixed current density of 10 mA cm⁻², demonstrating the suppressed HER for NCC@CuNCs@Zn electrode.⁵³ Inhibition of HER can reduce the OH⁻ formation near the anode, restricting the generation of byproducts at the electrode–electrolyte interface. Moreover, the corrosion behaviors of the substrates with and without loaded CuNCs in 2 M ZnSO₄ electrolytes were detected using the Tafel curves at a sweep speed of 1 mV s⁻¹. NCC@Zn exhibits a corrosion current density of 0.3238 mA cm⁻², which is higher than that of NCC@CuNCs@Zn (0.3057 mA cm⁻², (Fig. 4d)). A lower corrosion current density suggests a preferable corrosion resistance of the electrode.¹¹ Hence, the relatively good corrosion resistance of NCC@CuNCs@Zn demonstrates the role of CuNCs introduction to inhibit the occurrence of side reactions.

The DFT calculation are shown in Fig. 4e, the binding energies of GSH ligand at five functional active sites (Fig. S18, ESI†) with Zn²⁺ are −11.99, −12.04, −13.75, −13.02 and −14.41 eV respectively, which are much higher than that of Zn²⁺–H₂O (−4.67 eV), implying that the peripheral GSH ligand of CuNCs can regulate the solvation structure of Zn²⁺ to promote the removal of the solvation sheath and increase Zn²⁺ flux.⁶¹ Consequently, the transport and deposition principles of Zn²⁺ on the NCC@CuNCs surface are legitimately inferred combined with the above CE and symmetric cell test results (Fig. 4f). In AZIBs, Zn²⁺ tends to solvate six water molecules to form Zn(H₂O)₆²⁺ as well as first needs to be desolvated during the electrochemical reaction; therefore, the solvation structure is unfavorable for the charge and discharge behavior. Moreover, the water molecules existing in Zn(H₂O)₆²⁺, which have higher electrochemical activity than the free water molecules, are preferentially decomposed into H⁺ and OH⁻ at the electrode surface, resulting in the HER and formation of an irreversible Zn₄SO₄(OH)₆·xH₂O by-product.⁶² Obviously, the removal of the solvated sheath is needed to improve the battery performance. On the one hand, the peripheral protective ligands of the CuNCs anchored on NCC have several polar functional groups, such as –COOH, –NH₂, and –CO–NH–, which have a strong affinity for Zn²⁺, thus reducing the desolvation energy barrier of Zn²⁺. On the other hand, the rich hydrogen bonding network in the ligands can keep detrimental water out (Fig. S19, ESI†), thus ultimately mitigating the interface polarization and significantly inhibiting the occurrence of anode dendrites and side reactions.^63–65 Besides, the coulombic attraction force between the −COOH and Zn²⁺ and the complexation between Zn²⁺ and –CO–NH– groups, which accelerate its transfer to the zincophilic Cu cores of the CuNCs, guarantee fast Zn²⁺ transport kinetics and finally achieve homogenous and dense Zn deposition on the CC surface through the effective induction of the zincophilic copper cores.^57,66

Verification of the performance improvement of AZIBs by CuNCs-based bifunctional anode

NCC was immersed in GSH and CuSO₄ aqueous solutions to obtain NCC@GSH and NCC@CuSO₄ for the symmetric and asymmetric cell tests, respectively, to fully demonstrate the advantages of CuNCs in improving the performance of Zn anodes. The XPS spectra of the corresponding elements demonstrate the successful loading of GSH (Fig. 5a; Fig. S20, ESI†) and Cu²⁺ (Fig. 5b; Fig. S21, ESI†) onto the NCC substrate. The symmetric cells based on NCC@GSH (150 h) and NCC@CuSO₄ (220 h) at a current density of 1 mA cm⁻² and areal capacity of 1 mA h cm⁻² show obvious voltage fluctuation during the cycle (Fig. 5c), which is caused by the unstable chemical environment on the electrode surface. The cycle life of both is better than that of NCC but less than that of NCC@CuNCs. This finding is consistent with the results obtained at a current density of 2 mA cm⁻² (areal capacity of 2 mA h cm⁻²; Fig. 5d). Unsurprisingly, the performance of the Zn//NCC@GSH and Zn//NCC@CuSO₄ asymmetric cells under the three current densities also basically verifies the above conclusions (Fig. 5e and f; Fig. S22, ESI†). Hence, although a single organic functional group or zincophilic active site can improve the performance, the extent of improvement is still limited and only the collaboration of both can better get rid of the dilemma faced by Zn anode. The NCC scaffold loaded with CuNCs possessed the aforementioned dual-modification advantages, revealing its tremendous application potential for AZIBs.

Fig. 5 XPS high-resolution (a) S 2p spectrum of NCC@GSH, (b) Cu 2p spectrum of NCC@CuSO₄. Cycling performance of Zn//NCC@GSH and Zn//NCC@CuSO₄ symmetric cells at (c) 1 mA cm⁻², (d) 2 mA cm⁻². CE diagrams of (e) Zn//NCC@GSH, (f) Zn//NCC@CuSO₄ at 1 mA cm⁻² and an areal capacity of 1 mA h cm⁻².

Electrochemical performance of the full cell

To evaluate the practical application value of the anodes, the NCC@CuNCs@Zn anodes were coupled with β-MnO₂ and 2 M ZnSO₄ + 0.1 M MnSO₄ as the electrolyte to assemble full cells. MnSO₄ was added to prevent the disproportionation of the MnO₂ cathode.⁶⁷ The XRD pattern shows typical β-MnO₂ peaks (PDF#24-0735; Fig. 6a), and the nanorod-like morphology of β-MnO₂ is verified by SEM (Fig. S23, ESI†). The XRD and SEM images of the anodes after 100 cycles at 1 A g⁻¹ were compared to further demonstrate the ability of the NCC@CuNCs@Zn anode to inhibit side reactions during cycling. As shown in Fig. 6b, the Zn₄SO₄(OH)₆·5H₂O by-product on the electrode surface of NCC@Zn has a higher diffraction peak, as well as the massive irregular particles on the electrode surface can be clearly seen in the SEM image. In contrast, the NCC@CuNCs@Zn anode shows a weak by-product peak in the XRD spectra and flat anode surface after cycling, which adequately indicates that the NCC@CuNCs@Zn anode can inhibit the HER and the generation of by-products.

Fig. 6 (a) XRD pattern of β-MnO₂. (b) XRD and SEM images (NCC@CuNCs@Zn: upper right, NCC@Zn: lower right) of different electrodes after 100 cycles of the full cells. The EIS comparison diagrams of (c) NCC@Zn//MnO₂, (d) NCC@CuNCs@Zn//MnO₂ full cells before and after 60 cycles. (e) Cycling performance at 1 A g⁻¹ of the full cells. (f) Rate performance test of the full cells.

The CV measurements exhibit typical and identical β-MnO₂ redox peaks (Fig. S24a, ESI†), indicating that the existence of CuNCs does not affect the Zn//MnO₂ redox reaction kinetics. Meanwhile, the charge/discharge curves also show similar charge and discharge platforms (Fig. S24b, ESI†), in agreement with the CV curves.⁶⁸ The R_ct of the full cell was compared before and after 60 cycles. The R_ct for NCC@Zn//MnO₂ increases significantly after 60 cycles (Fig. 6c), whereas that of NCC@CuNCs@Zn//MnO₂ increases from ∼50 Ω to ∼150 Ω after cycling (Fig. 6d). According to these results, it is not difficult to find that the NCC@CuNCs@Zn anode constructed based on CuNCs exhibits better Zn electroplating/stripping kinetics and electrochemical stability.⁶⁹

The cycling performance of the full cells is shown in Fig. 6e. NCC@CuNCs@Zn//MnO₂ possesses a higher specific capacity (61.4 mA h g⁻¹) than NCC@Zn//MnO₂ (13.9 mA h g⁻¹) after approximately 700 cycles, and the capacity of NCC@Zn//MnO₂ rapidly declines after 481 cycles. Fig. 6f displays the rate performance of the full cells. NCC@CuNCs@Zn//MnO₂ delivers higher average specific capacities of 338.1, 367.0, 279.0, 186.0, 110.6, and 56.4 mA h g⁻¹ at 0.1, 0.2, 0.5, 1.0, 2.0, and 5.0 A g⁻¹, respectively. When the current density is reverted to 0.1 A g⁻¹, NCC@CuNCs@Zn//MnO₂ maintains a high specific capacity of 367.1 mA h g⁻¹, whereas that of NCC@Zn//MnO₂ sharply attenuates to 60.81 mA h g⁻¹. Therefore, the GSH-CuNCs introduced during the modification of Zn-free anodes inhibit dendrite growth and side reactions, greatly improving the overall electrochemical performance of the full cells.

Conclusions

In conclusion, a novel three-dimensional dendrite-free Zn metal anode is constructed by anchoring CuNCs protected by organic ligands onto a hydrophilic CC substrate through simple immersion to effectively address the problems of hydrogen evolution and corrosion due to Zn dendrite growth. The polar functional groups (–COOH, –NH₂, and –CO–NH–) on the protective ligands of the outer layer of the CuNCs are conducive to the dissolution of Zn(H₂O)₆²⁺ and provide good zincophilic sites to accelerate the transport of Zn²⁺ to the cores of the CuNCs through coulombic attraction. Simultaneously, the zincophilic Cu cores can guide uniform Zn nucleation and deposition on the CC surface, thereby inhibiting dendrite growth. Compared with untreated NCC, NCC with CuNCs exhibits impressive cycling stability in the symmetric cell test and a CE of up to 99.40% in the asymmetric cell after 1000 cycles at 1 mA cm⁻². In addition, the NCC@CuNCs@Zn//MnO₂ full cell shows higher discharge capacity. Therefore, this study provides a new approach for modifying Zn anodes and confirms the development potential of metal NCs in AZIBs design.

Data availability

The data that supports the findings of this study are available in the article and ESI†.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 22209154), and Zhengzhou University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi02605e

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