Synthesis and characterization of NOC overlayer
With the consideration of the low melting point of Zn (419.5 ℃), a PECVD route (20) was devised to in-situ grow ultrathin NOC overlayer directly on commercial Zn foil at a temperature range of 300 − 400 ℃. As depicted in Fig. 1A, pyridine was used as the precursor to ensure the generation of N-containing carbon species under plasma treatment. The carrier gas Ar (containing a trace amount of O2) can aid the transport of carbon species from upstream to Zn metal surface (21, 22). Impressively, the color of Zn foil rapidly turns to light brown upon reaction for 10 min, implying the successful formation of carbon coating (fig. S1). During the growth, nitrogen and oxygen atoms are simultaneously incorporated into the carbon skeleton (23, 24). The thickness of carbon overlayer could be adjusted by the growth time, which is simply reflected by the varied color of coated Zn surface (Fig. 1B). More impressively, a macroscopically sized sample with a lateral dimension of 20 cm × 12 cm can be obtained by employing a 4-inch furnace (Fig. 1C), holding promise for the scalable production of NOC@Zn materials in an economic fashion.
Raman spectra of NOC overlayers produced at different reaction temperatures were collected (fig. S2), where two peaks centering at 1345 and 1596 cm− 1 are attributed to the typical D band (defective feature) and G band (graphitic feature) signals of carbonaceous materials, respectively. The D/G intensity ratio (ID/IG) gradually decreases from 0.98 to 0.93 with the rise of growth temperature from 300 to 380°C, indicative of improved crystallinity (25). The disappearance of X-ray diffraction (XRD) signals of graphite might be owing to the poor crystallinity (fig. S3). X-ray photoelectron spectroscopy (XPS) measurement was carried out to probe the chemical composition of as-grown NOC film (fig. S4). The survey spectrum bears out the existence of Zn, C, N, and O elements. The C 1s spectrum encompasses four peaks at 284.4, 285.2, 286.1, and 287.8 eV, which can be respectively assigned to sp2 C = C, sp2 C = N, sp3 C–N, and C–OH bonding (23). In addition, N 1s spectrum can be deconvoluted into the signals of pyridinic-N (Npy, 398.5 eV), pyrrolic-N (Npr, 400.1 eV), and graphitic-N (Ngr, 401.7 eV) (20, 26), disclosing the abundant N dopants in NOC overlayer.
Representative scanning electron microscopy (SEM) observation of thus-produced NOC@Zn demonstrates a film-like morphology with a wealth of randomly distributed cracks (Fig. 1D and fig. S5A, B), which might originate from the uneven thermal distribution during the PECVD reaction. The thickness of Zn foil decreases by ~ 4 µm (from 10 to 6 µm) upon PECVD process because of the thermal sublimation effect (fig. S5C, D). Atomic force microscopy (AFM) inspections indicate that the thickness of NOC overlayer can be tailored at ca. 20 nm after 10 min synthesis (Inset of Fig. 1D and fig. S6). Transmission electron microscopy (TEM) characterizations further corroborate the lamellar feature of NOC overlayer (Fig. 1E and fig. S7A). The diffraction ring can be recognized in the selected area electron diffraction (SAED) patterns (Inset of Fig. 1E), implying the formation of graphitic nanodomains (27) in NOC overlayer. This is evidenced by the atomically resolved TEM imaging to show nano-sized honeycomb lattice of graphene (Fig. 1F and fig. S7B-D). Energy dispersive X-ray spectroscopy (EDS) mapping under high-angle annular dark field-scanning TEM (HAADF-STEM) mode in Fig. 1G showcases the homogeneous distribution of C, N, and O elements. These results collectively verify the successful preparation of NOC overlayer affording N/O co-doping over Zn foil by PECVD. Furthermore, upon the introduction of heteroatom dopants in NOC overlayer, the electrolyte contact angle decreases from 87° to 61° (fig. S8). It is anticipated that the enhanced wettability is beneficial to facilitating the ion transport and hence reversible Zn plating/stripping (28).
Electrochemical performance of NOC@Zn
Cyclic voltammetry (CV) curves of Ti–Zn half cells were collected to probe the Zn electrodeposition behavior in the presence of NOC overlayer. As shown in Fig. 2A, the apparently lower Zn nucleation overpotential on NOC@Ti than that on bare Ti is beneficial to activating more nucleation sites, thereby promoting uniform Zn deposition. Note that the NOC@Ti–Zn cell maintains a steady cycling over 400 cycles under 2.0 mA cm− 2/0.5 mAh cm−2 (Fig. 2B and fig. S9), whereas the bare Ti–Zn cell can merely sustain up to 100 cycles with a drastic fluctuation of Coulombic efficiency (CE) values. More encouragingly, the NOC@Ti–Zn cell can obtain an average CE of 99.5% over 800 cycles under an elevated current density of 10.0 mA cm− 2 (Fig. 2C and fig. S10), which is superior to its counterpart. The voltage hysteresis of NOC@Ti–Zn cell is far lower than that of bare Ti–Zn cell.
Symmetric cell measurements were carried out to further elucidate the reversibility of Zn plating/stripping under the regulation of NOC overlayer. As shown in Fig. 2D, the NOC@Zn symmetric cell acquires a stable cycling performance over 1125 h with a low voltage hysteresis at 10.0 mA cm− 2/1.0 mAh cm− 2, whose cyclic life is approximately ten times longer than bare Zn symmetric cell. As such, the voltage hysteresis of bare Zn cell increases gradually and ultimately augments to an irreversible value, implying the rapid cell failure without the protection of NOC overlayer. In comparison, the voltage hysteresis of NOC@Zn can maintain at 60 mV upon cycling for 1125 h, indicative of the effective inhibition of dendrite growth and by-product formation (29). Note that an optimized NOC thickness of 20 nm could be gained in response to enabling a prolonged cycle life with a small polarization (fig. S11). The NOC@Zn symmetric cell further displays durable cycling performances with low voltage hysteresis at 5.0 mA cm− 2/5.0 mAh cm− 2 and 10.0 mA cm− 2/10.0 mAh cm− 2 (fig. S12). Remarkably, it can still sustain a stable cycling operation over 136 h under 30.0 mA cm− 2/30.0 mAh cm− 2 (Fig. 2E and fig. S13A), outperforming most of the state-of-the-art Zn electrodes under such a stringent condition.
Since the prevailing Zn metal anodes are typically compromised by shallow DOD (10), it is meaningful to augment the Zn utilization to evaluate the protective effect of NOC overlayer. In this sense, the NOC@Zn symmetric cell enables a stable cycling over 60 h under a plating/stripping capacity of 36.0 mAh cm− 2 (Fig. 2F and fig. S13B). Note that the corresponding DOD reaches ~ 64%, which is far higher as compared to most reported Zn anodes (table S1, 2). In addition, the NOC@Zn symmetric cell harvests a lower voltage hysteresis than bare Zn symmetric cell at each current density varying from 1.0 to 40.0 mA cm− 2 (fig. S13C), implying facile charge transfer kinetics under the management of NOC overlayer. To acquire a comprehensive understanding of electrochemical performance, the calculated cumulative capacity of NOC@Zn symmetric cell can be substantially elevated to 1080, 1800, 2200, and even 5625 mAh cm− 2 under different working conditions (Fig. 2G), which is superior to that of bare Zn symmetric cell. Obviously, the NOC overlayer can boost homogeneous Zn nucleation and growth toward highly reversible dendrite-free Zn anode.
Mechanistic insight into the protection effect of NOC overlayer
To shed light on the protective mechanism of NOC overlayer, exhaustive characterizations in combination with electrochemical measurements were carried out. Operando optical microscopy was first employed to visualize Zn plating behavior with a current density of 5.0 mA cm− 2 (fig. S14). After electrodeposition for 10 min, a multitude of protrusions randomly appear on the surface of bare Zn, which becomes increasingly obvious during electrodeposition (Fig. 3A and video S1). NOC@Zn electrode otherwise maintains a smooth surface morphology without discernible dendrites (Fig. 3B and video S2). The detailed morphologies of Zn plating were disclosed by SEM to further elucidate the impact of NOC overlayer on mitigating the dendrite. As depicted in Fig. 3C and fig. S15A, Zn flakes with high dip angles are randomly deposited on bare Zn, whilst hexagonal Zn plates on NOC@Zn are parallel to the substrate and exhibit a highly ordered orientation (Fig. 3D and fig. S15B). Upon plating/stripping for 100 cycles at 10.0 mA cm− 2/1.0 mAh cm− 2, rampant dendrites and pits can be observed on the surface of bare Zn foil (Fig. 3E and fig. S15C). In stark contrast, the NOC@Zn electrode enables a dense and flat Zn deposition morphology (Fig. 3F and fig. S15D). Optical surface profilometry imaging further shows the NOC@Zn electrode displays a far smaller surface height difference (6 µm, Fig. 3H) as compared to the bare Zn (50 µm, Fig. 3G) after electrochemical cycling, in good agreement with side-view SEM observation (fig. S15E, F). Consequently, the NOC overlayer can facilitate the homogeneous Zn deposition and suppress the dendrite growth even under high current densities (fig. S16).
XRD was employed to investigate the crystal structure of electrodes after Zn deposition. The (002) intensity on NOC@Zn is greatly enhanced in comparison with bare Zn (Fig. 3I), implying a highly oriented deposition. The guided (002)-oriented Zn deposition via NOC overlayer contributes to a dendrite-free morphology of NOC@Zn anode. With respect to the bare Zn electrode, a noticeable peak at 8.6° can be recognized in the XRD pattern, which is ascribed to the formation of detrimental by-product (Zn4SO4(OH)6·4H2O) (4). Nonetheless, this diffraction peak can scarcely be observed for NOC@Zn electrode, suggesting that side reactions can be effectively mitigated by the NOC overlayer. In addition, the NOC@Zn symmetric cell possesses a higher transference number (30) than bare Zn (Fig. 3J and fig. S17), implying that the NOC overlayer can promote Zn2+ transport and hence facilitate homogenous Zn deposition.
The orientation of deposited Zn plates plays a vital role in influencing the electric field distribution of Zn anode. In this case, finite element simulation based on COMSOL Multiphysics was carried out to explore the electric field distributions (fig. S18). With respect to the bare Zn, the electrical field at the corner of vertical plates is drastically enhanced (Fig. 3K), which would accordingly trigger uneven Zn deposition. On the contrary, the horizontal Zn plates endow the NOC@Zn electrode with homogeneously distributed electrical field (Fig. 3L). These results uncover that the horizontal deposition of Zn plates can aid to homogenize the electric field distribution, thereby further contributing to a uniform ion flux and ultimately promoting the reversibility of Zn anode (16).
The planar Zn deposition morphology is of paramount significance to the construction of highly durable NOC@Zn anode. To demystify its origin, density functional theory (DFT) calculations were performed with the aim to reveal the adsorption mechanism of N and O atoms on different crystal planes including Zn (002), Zn (100) and Zn (101) (Fig. 4A, B, and fig. S19). This is anticipated to further help exploring the role of NOC overlayer in manipulating Zn nucleation. Note that the free N/O atoms and fully relaxed crystal surface were selected as initial sites for structural optimization. Our DFT calculation indicates that the adsorption energy of N/O atom on Zn (002) is slightly lower than that on Zn (100) and Zn (101) (table S3), which might originate from the higher stability of Zn (002) facet (18, 31). More intriguingly, the optimized adsorption configurations imply that the N/O atom is prone to bury into the Zn (100) or Zn (101) surface, in contrast to the adsorption scenario on Zn (002). Consequently, the moderate binding energy of N/O atom on Zn (002) is beneficial to guiding preferential nucleation along Zn (002) plane at the initial stage of electrodeposition. However, the unstable embedded configurations manifest that the Zn nuclei can barely grow along Zn (100) and Zn (101) during plating process. In other words, the incorporated N/O atoms in the carbon skeleton could induce the nucleation along Zn (002).
Note further that the NOC layer possesses a poor electrical conductivity of ~ 3.0×10− 6 S cm−1 (fig. S20), which prevents the reduction and deposition of Zn on its forefront surface. In this sense, Zn2+ need to pass through the NOC layer to reach the surface of Zn metal, at which they will be reduced to Zn0. Fortunately, the NOC layer harnessing local interplanar spacing of 3.4 Å and abundant vacancies/defects can offer feasible diffusion paths for Zn2+ with a radius of 0.75 Å. As reported (32), the small mismatch (7%) of hexagonal lattice between graphene and Zn (002) plane can guarantee a heteroepitaxial deposition behavior. Therefore, the graphitic nanocrystal domain affording hexagonal lattice decorated in the NOC layer can effectively cultivate the preferential growth along Zn (002), which is in line with reported ZnSe overlayer (4). Collectively, the NOC overlayer promotes Zn (002)-dominated nucleation and growth behaviors, thereby mitigating the dendrite formation and ensuring the excellent stability of Zn plating/stripping process.
Apart from mass transfer, the solvation sheath of Zn2+ (Zn(H2O)62+) is also a key to regulating the interfacial redox reaction (33, 34). It is well established that Zn2+ can be reduced and deposited on the electrode only after removing the solvation sheath (35). To elucidate the effect of NOC layer on modulating the solvation structure in charge-transfer process, the adsorption energy between H2O and NOC layer was accordingly calculated. Note that the optimized adsorption configurations of H2O with five adsorption sites are depicted (Fig. 4C and fig. S21). The relatively low binding energy (–0.14 eV) between pristine graphene and H2O molecule would give rise to a weak adsorption effect (Fig. 4D). With the incorporation of O doping (–OH), the binding energy increases to − 0.34 eV, indicative of an enhanced capture capability for H2O. With respect to the three types of N doping, the pyrrolic N harvests the highest binding energy of − 0.39 eV. These simulation results suggest that Zn(H2O)62+ is prone to interact with NOC overlayer with the aid of pyrrolic N and –OH sites, thereby facilitating the desolvation process.
The electrochemical impedance spectra (EIS) at varied temperatures were collected to unravel the desolvation effect of NOC overlayer. It is evident that the charge-transfer resistance (Rct, table S4) is drastically reduced with the assistance of NOC layer (Fig. 4E and fig. S22). As illustrated in Fig. 4F, the quantitatively derived activation energy (4, 36) (Ea) of NOC@Zn symmetric cell (21.3 kJ mol− 1) is considerably lower than that of bare Zn (46.3 kJ mol− 1), manifesting that the NOC overlayer can promote redox kinetics at the electrode/electrolyte interface by facilitating desolvation process. The high ionic conductivity of the NOC layer is beneficial to its Zn2+ transport (fig. S23). The Zn2+ diffusion dynamics tested by chronoamperometry imply the introduction of NOC layer can effectively inhibit the two-dimensional diffusion (36), thereby promoting homogeneous deposition (fig. S24). The linear polarization test was carried out to explore the corrosion of Zn anode (Fig. 4G). In comparison with the bare Zn anode, the corrosion potential of NOC@Zn obtains a positive shift (from − 0.956 V to − 0.950 V). In addition, the declined corrosion current by ~ 302.3 µA cm− 2 also suggests the effective restriction of corrosion effect with the presence of NOC overlayer. Therefore, the NOC overlayer avoids the direct contact between Zn anode and electrolyte, which further inhibits the side reaction and corrosion effect (16).
Taken together, the plating and cycling process on bare Zn and NOC@Zn anodes are schematically illustrated (Fig. 4H, I). In the case of bare Zn anode, a wealth of randomly distributed Zn plates, pits and H2 bubbles aggregate on the surface after repeated plating/stripping, thereby resulting in a curtailed lifespan. In contrast, the NOC overlayer can mitigate dendrite growth and stabilize Zn anode via two mechanisms. For one thing, it can guide the oriented nucleation of hexagonal Zn plates and enable the in-parallel deposition with respect to the substrate. For another, it can significantly boost the interfacial redox kinetics by decreasing desolvation energy, thus ultimately achieving a prolonged cycle life.
AZIB full cell performance
To envisage the potential usage of NOC@Zn anode in practical devices, AZIB full cells comprising a KV12O30 − y·nH2O (KVOH) (4, 37) cathode and a NOC@Zn anode were assembled. The NOC@Zn − KVOH cell delivers an initial capacity of 149 mAh g− 1 and maintains at 137 mAh g− 1 after 4000 cycles at 10.0 A g− 1 (Fig. 5A). In stark contrast, the bare Zn − KVOH cell undergoes a rapid capacity decay due to the rampant dendrite growth and side reactions (35, 38). Rate performance measurements indicate that NOC@Zn − KVOH cell harvests an advanced capacity at each current density as compared to bare Zn − KVOH, especially at the high rates (Fig. 5B). Such an excellent rate capability could be attributed to the robust anode/electrolyte interface. Nyquist plots reveal that the NOC@Zn − KVOH cell exhibits lower charge-transfer resistance and faster ion diffusion kinetics in comparison with the bare Zn–KVOH counterpart (fig. S25).
As a matter of fact, low loading cathodes (~ 1.0 mg cm− 2) and thick Zn anodes (≥ 100 µm) were ubiquitously employed in previously reported AZIB systems (3, 32). Consequently, the N/P ratio is typically higher than 50, which greatly hinders the improvement of energy density. It is imperative and meaningful to increase the mass loading of cathodes and reduce the thickness of Zn anodes. To corroborate the outstanding electrochemical performance of the as-prepared NOC@Zn anodes in AZIB full cells, high-loading cathodes (5.59 mg cm− 2) and thin Zn foils (10 µm) were employed. As aforementioned, the inevitable sublimation of Zn occurs in the PECVD process and hence the thickness of Zn foil can be decreased from 10 µm to 6 µm, which stands for a capacity of 3.51 mAh cm− 2. Shown in Fig. 5C, the cyclic durability degrades in response to the reduction of N/P ratio. Encouragingly, the NOC@Zn − KVOH cell can sustain a stable operation over 400 cycles at 5.0 A g− 1 at a low N/P ratio of 3.5. Figure 5D draws a comparison of current density and areal capacity based on Zn anode between this work and previous reports (table S5) (4, 8, 35, 39–46), manifesting the superiority of NOC@Zn anode in potential fast-charging scenarios.
More impressively, NOC@Zn anode harnessing mechanical robustness enables facile construction of bendable full cells in pursuit of flexible energy storage applications (fig. S26). As depicted in Fig. 5E, KVOH cathode and NOC@Zn anode are separated by glass fiber and encapsulated by polyimide tape (47). Two full cells in series can easily power a light-emitting diode (LED) indicator (Fig. 5F) or an electronic timer (Fig. 5G), showcasing the promising application potentials. Benefiting from its mechanical robustness, such a full cell harvesting a low charge-transfer resistance (fig. S27) delivers stable operation over 250 cycles under different bending states (Fig. 5H). Notably, the bendable full cell achieves a high capacity retention of 68% with a bending angle of 90°. These evaluations collectively substantiate that the NOC overlayer can inhibit parasitic reactions at the anode/electrolyte interface and guide uniform Zn deposition, ultimately in favor of elevating the stability and durability of practical AZIB.