With the consideration of the low melting point of Zn, a PECVD route[22] was devised to in-situ grow conformal NOC layer directly on commercial Zn foil at a temperature range of 300 − 400°C. As depicted in Fig. 1a, pyridine was employed as the precursor to ensure the generation of N-containing carbon species under plasma treatment (Figure S1a). The growth rate of NOC layer can be accurately regulated by adjusting the flow rate of pyridine (i.e., the partial pressure). The carrier gas Ar (containing a trace amount of O2) can aid the transport of carbon species from upstream to Zn metal surface.[23,24] Impressively, the color of Zn foil rapidly turns to light brown upon reaction for 10 min, implying the successful formation of carbon coating (Figure S1b). A macroscopically sized sample with a lateral dimension of 20 cm × 12 cm can be obtained by employing a 4-inch furnace (Figure S1c), holding promise for the scalable production of NOC-skinned Zn metals in an economic fashion. Impressively, directly-grown NOC skin affording excellent adhesion and robustness remains intact over Zn foil upon bending/twisting, which is essential for heavy-duty operations (Figure S2). In contrast, self-assembled graphene oxide coating shows obvious disconnection off the Zn surface after the identical deformation treatment, losing their ability to protect Zn anode (Figure S3). Raman spectra of NOC layers produced at different reaction temperatures were collected (Figure S4), 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] Note that the carbon deposition is always accompanied by the Zn sublimation under a low pressure. Sublimation and melting of Zn become extremely rampant at temperatures above 400°C, resulting in uncontrollable CVD process (Figure S5). Hence, 380°C was selected as an optimum growth temperature in our procedure.
High-resolution aberration-corrected transmission electron microscopy (HRTEM) was used to unravel the exact atomic structure of NOC. The TEM image at low magnification corroborates the lamellar feature of NOC layer (Fig. 1b; Figure S6a). Energy dispersive X-ray spectroscopy (EDS) mapping under high-angle annular dark field-scanning TEM (HAADF-STEM) mode showcases the homogeneous distribution of C, N, and O elements (Fig. 1c), implying the successful incorporation of nitrogen and oxygen atoms.[26,27] It was found that the growth rate of NOC on Zn foil exerted a profound effect on the growth quality. Because of the sublimation phenomenon, Zn foil is prone to break if the carbon deposition rate is too slow. Although fast-growing NOC can quickly cover the Zn surface to buffer the Zn sublimation, the resulting NOC is otherwise of poor quality. As shown in HRTEM images (Fig. 1d; Figure S6b), the fast-growing NOC layer comprises turbostratic domains consisting of randomly distributed carbon atoms, indicative of fully amorphous nature.[28] As for the slow-growing NOC, the honeycomb nanocrystalline graphene domain emerges as a mosaic phase in NOC matrix (Fig. 1e; Figure S6c-d; Figure S7), suggesing elevated crystallinity. Diffraction ring could be recognized in the selected area electron diffraction (SAED) pattern (Fig. 1e inset), verifying the formation of graphitic nanodomains.[29] In further contexts, the electrical conductivity of NOC layers obtained under different growth rates were measured (Table S1). The conductivity value of fast-growing NOC layer is lower than 5.0 × 10− 7 S cm− 1, showing an electrically insulative state. In contrast, slow-growing NOC material displays a boosted electrical conductivity.
X-ray photoelectron spectroscopy (XPS) measurement was carried out to probe the chemical composition of as-grown NOC film. The survey spectrum bears out the existence of Zn, C, N, and O elements (Figure S8). The C 1s spectrum encompasses four peaks at 284.4, 285.2, 286.1, and 287.8 eV (Fig. 1f), which can be respectively assigned to C–C/C = C, C–N, C = O/C = N, and –COOH bonding.[26] As shown in Fig. 1g, the high-resolution 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),[22,30] disclosing the presence of N dopants in NOC layer. Meanwhile, the high-resolution O 1s spectrum was also analyzed (Fig. 1h), where the two signals at 530.6 and 531.8 eV are associated with C = O and C–OH bonding. The thickness of NOC skin could be adjusted by the growth duration, which can be simply reflected by the varied color of coated Zn surface (Figure S9). To obtain detailed thickness information, NOC layer was first grown on mica and then transferred on SiO2/Si substrate to allow atomic force microscopy (AFM) characterization. The thickness of NOC layer can be fine-tuned, reaching ca. 20 nm upon a 10 min PECVD synthesis (Fig. 1i; Figure S10). Furthermore, upon the introduction of heteroatom dopants in NOC layer, the aqueous electrolyte contact angle decreases from 87° to 61° (Figure S11). It is anticipated that the enhanced wettability is beneficial to facilitating the ion transport and hence reversible Zn plating/stripping.[31]
Representative scanning electron microscopy (SEM) observation of fast-growing NOC@Zn demonstrates a film-like morphology with a wealth of randomly distributed cracks (Fig. 1j; Figure S12a), which might originate from the uneven thermal expansion during the PECVD reaction. Apart from the cracks, the remaining regions are covered with a smooth carbon film, which is not conducive to Zn2+ transport (Figure S12b). In turn, the Zn deposition behavior on NOC@Zn was evaluated by electroplating 0.015 mAh Zn. Impressively, Zn would merely deposit along the cracks (Fig. 1k; Figure S12c-d). This is mainly because the relatively dense and insulating NOC layer derived via fast growth can transport neither electrons nor Zn ions. Nevertheless, Zn foil would also show damage at a too slow growth rate (Figure S13a and b). In our work, an optimized partial pressure of 8 Pa was selected for the conductive NOC growth (Figure S13c). The produced surface is featured by the presence of nanopores (Fig. 1l; Figure S13d), which is beneficial to facilitating the transport of Zn2+. More encouragingly, the electro-deposited Zn possesses a regularly hexagonal stacking structure (Fig. 1m; Figure S13e and f). EDS mapping results further confirmed that Zn was deposited on the upper surface of NOC layer (Figure S14),[20] suggesting that an electrical conductivity of 2.0×10− 5 S cm− 1 is sufficient for electron transport to ensure Zn-ion reduction. Furthermore, side-view SEM observation indicates the thickness of Zn foil decreases by ~ 2 µm (from 100 to 98 µm) upon PECVD owing to the Zn sublimation (Figure S15). Collectively, these results verify the successful preparation of conductive NOC layer affording N/O co-doping over Zn foil by PECVD. Such an ultrathin carbon skin, which lacks long-range order but presents mosaic nanocrystalline graphene, is anticipated to play a crucial role in guiding Zn electrodeposition.
Cyclic voltammetry (CV) curves of Ti–Zn half cells were collected to probe the Zn nucleation behavior in the presence of NOC skin. As shown in Fig. 2a, the apparently lower Zn plating 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; Figure S16), 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; Figure S17), which is superior to its counterpart. The voltage hysteresis of NOC@Ti–Zn cell is far lower than that of bare Ti–Zn cell. When elevating the discharge capacity to 2.0 and 5.0 mAh cm–2, the NOC@Ti–Zn cell can still maintain a longevous cycling (Figure S18).
Symmetric cell measurements were carried out to further elucidate the reversibility of Zn plating/stripping under the regulation of NOC skin. As shown in Fig. 2d, the voltage profile of bare Zn cell gradually fluctuates and eventually short circuits after 45 h. In comparison, the NOC@Zn symmetric cell acquires a stable cycling performance over 3040 h at 1.0 mA cm− 2/1.0 mAh cm− 2, whose cyclic life is approximately 66 times longer than bare Zn congener. As such, the voltage hysteresis of NOC@Zn can maintain stable at ~ 30 mV upon cycling, indicative of the effective inhibition of dendrite growth and by-product formation.[32] When elevating the current density to 10 mA cm− 2 (Figure S19a), the NOC@Zn symmetric cell still displays a highly stable voltage profile that sustains more than 1550 h. Meanwhile, the NOC@Zn symmetric cell harvests durable cycling performances at high capacities of 5.0 and 10.0 mAh cm− 2 (Figure S19b and c). Figure 2e and Table S2 draw the performance comparisons between this work and previous reports,[11,18,19,22,33–42] demonstrating the superior performance of NOC AIL to other carbon materials. Since the prevailing Zn metal anodes are typically compromised by shallow DOD,[13] it is meaningful to augment the Zn utilization to evaluate the protective effect of NOC. 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 (Figure S20a and b). The corresponding DOD of ~ 63% is far higher as compared to most reported Zn anodes (Table S3). Remarkably, it can also sustain a stable cycling operation over 136 h under 30.0 mA cm− 2/30.0 mAh cm− 2 (Fig. 2f; Figure S20c), outperforming most of the state-of-the-art Zn electrodes under such a stringent condition. Furthermore, symmetric cell performances with different NOC thicknesses were compared. An optimized NOC thickness of 20 nm could be gained in response to enabling a prolonged cycle life (Figure S21).
To gain insight into the specific roles of nitrogen and oxygen co-doping in anode protection, the carbon film doped solely with oxygen on Zn foil (OC@Zn) was also prepared by using ethanol precursor (Figure S22). It can be observed that OC@Zn has the smallest electrolyte contact angle, suggesting the positive effect of oxygen doping in improving the electrolyte wettability. Although the cyclic performance of OC@Zn has a relative improvement over the bare Zn, it is still inferior to that of NOC@Zn, indicating the leading role of nitrogen doping in improving the cyclic reversibility. In addition, the NOC@Zn symmetric cell harvests a lower voltage hysteresis than bare Zn counterpart at each current density varying from 1.0 to 40.0 mA cm–2 (Figure S23), implying facile charge transfer kinetics under the management of NOC skin. The drastically reduced nucleation overpotential of symmetric cell can be harvested under various current densities upon the introduction of NOC (Figure S24), which is beneficial to uniform deposition of Zn. To acquire a comprehensive understanding of electrochemical performance, the calculated cumulative capacity of NOC@Zn symmetric cell can be substantially elevated to 600, 1520, 2040, 2200, and even 7750 mAh cm− 2 under different working conditions (Fig. 2g), which is superior to that of bare Zn symmetric cell. Obviously, the NOC skin can boost homogeneous Zn nucleation and growth toward highly reversible dendrite-free Zn anode.
To shed light on the protective mechanism of such ultrathin NOC skin, exhaustive characterizations in combination with electrochemical measurements were carried out. In-situ optical microscopy was first employed to visualize Zn plating behavior with a current density of 5.0 mA cm–2 (Figure S25). After electrodeposition for 10 min, a multitude of protrusions randomly appear on the surface of bare Zn, which become increasingly obvious during electrodeposition (Fig. 3a; Video S1). NOC@Zn electrode otherwise maintains a smooth surface morphology without discernible dendrites (Fig. 3b; Video S2). The detailed morphologies of Zn plating were disclosed by SEM to further elucidate the impact of NOC conformal coating on mitigating the dendrite. As depicted in Fig. 3c and Figure S26a, 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; Figure S26b). 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; Figure S26c). In stark contrast, the NOC@Zn electrode enables a dense and flat Zn deposition morphology (Fig. 3f; Figure S26d). Accordingly, the stripped bare Zn electrode (Figure S26e) is full of ravines owing to the uneven stripping process, while the NOC@Zn (Figure S26f) displays a uniform morphology. 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 (Figure S27). Therefore, the NOC skin can facilitate the homogeneous Zn plating/stripping and suppress the dendrite growth even under high current densities (Figure S28).
X-ray diffraction (XRD) was employed to examine 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 skin 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 namely Zn4SO4(OH)6·4H2O.[6] Nonetheless, this diffraction peak can rarely be observed for NOC@Zn electrode, suggesting that side reactions can be effectively mitigated by the NOC. In addition, the NOC@Zn symmetric cell possesses a higher transference number[43] than bare Zn (Fig. 3j; Figure S29), implying that the NOC skin 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 (Figure S30). 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.[20]
The planar Zn deposition morphology is of paramount significance to the construction of highly durable NOC@Zn anode. The slow-growing NOC layer possesses an optimized electrical conductivity of ~ 2.0×10–5 S cm–1, which supports the Zn-ion reduction and subsequent Zn deposition on its forefront surface. As reported,[44] the slight mismatch (7%) of hexagonal lattice between graphene and Zn (002) plane could ensure a heteroepitaxial deposition behavior when graphene was used as current collector. In our case, as a conductive AIL on Zn, the NOC skin affording mosaic nanocrystalline graphene can also induce orientational deposition, which might stem from two aspects: i) guided deposition on graphene domains and ii) heteroatom doping effect. In response, density functional theory (DFT) calculations were performed to reveal the adsorption mechanism of N and O atoms on different crystal planes including Zn (002), Zn (100) and Zn (101) (Fig. 4a and b; Figure S31). This is anticipated to further help exploring the role of NOC skin 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 S4), which might be caused by the higher stability of Zn (002) facet.[45,46] 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), which would cause the erosion of Zn(100)/Zn(101).[47] 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. Meanwhile, 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). Collectively, the NOC skin promotes Zn (002)-dominated nucleation and growth behaviors, thereby mitigating the dendrite formation and ensuring the excellent stability of Zn plating/stripping process.
The solvation sheath of Zn2+ (Zn(H2O)62+) is also a key to regulating the interfacial redox reaction.[48,49] It is established that Zn2+ can be reduced and deposited on the electrode only after removing the solvation sheath.[41] To elucidate the effect of NOC layer on modulating the solvation structure in charge-transfer process, the adsorption energy between H2O and NOC was accordingly calculated. The optimized adsorption configurations of H2O with six adsorption sites are depicted (Fig. 4c; Figure S32). 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, the increased binding energy (–0.142 eV for C = O and − 0.34 eV for C–OH) indicates 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 skin with the main 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 skin. It is evident that the charge-transfer resistance (Rct, Table S5) is drastically reduced with the assistance of NOC layer (Fig. 4e; Figure S33). As illustrated in Fig. 4f, the quantitatively derived activation energy[6,50] (Ea) of NOC@Zn symmetric cell (21.3 kJ mol− 1) is considerably lower than that of bare Zn (46.3 kJ mol− 1). The high ionic conductivity of the NOC layer is beneficial to its Zn2+ transport (Figure S34). The Zn2+ diffusion dynamics tested by chronoamperometry imply the introduction of NOC layer can effectively inhibit the two-dimensional diffusion,[50] thereby promoting homogeneous deposition (Figure S35). 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 skin. Therefore, the NOC skin avoids the direct contact between Zn anode and electrolyte, which further inhibits the side reaction and corrosion effect.[20]
Taken together, the plating and cycling process on bare Zn and NOC@Zn anodes are schematically illustrated (Figs. 4h − j). 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 (Fig. 4h). As for the fast-growing NOC AIL, Zn only nucleates and deposits over crack sites because of the dense and insulative carbon layer, which still leads to dendrite formation and rapid short-circuit of the battery (Fig. 4i). In contrast, the slow-growing conductive NOC skin can mitigate dendrite growth and stabilize Zn anode via two mechanisms (Fig. 4j). For one thing, it can guide the oriented nucleation on crystalline graphene mosaic 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.
To envisage the potential usage of NOC@Zn anode in practical devices, AZIB full cells comprising a KV12O30-y·nH2O (KVOH)[6,51] cathode and a NOC@Zn anode were assembled. The NOC@Zn − KVOH cell delivers an initial capacity of 148.1 mAh g− 1 and maintains at 141.9 mAh g−1 with a capacity retention of 96% 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.[41,52] 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 (Figure S36).
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.[3,44] 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 by 2 µm, which stands for a capacity of 4.68 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 4.5. Furthermore, the average charging time of NOC@Zn-KVOH full cell is as short as 2.2 min at 5 A g–1, which is expected to meet the needs of pragmatic fast-charging scenarios (Figure S37). Figure 5d draws a comparison of current density and areal capacity based on Zn anode between this work and previous reports,[6,11,39,41,53–64] manifesting the superiority of NOC@Zn anode (Table S6).
More impressively, NOC@Zn anode harnessing mechanical robustness enables facile construction of bendable full cells in pursuit of flexible energy storage applications. As depicted in Fig. 5e, KVOH cathode and NOC@Zn anode are separated by glass fibre and encapsulated by polyimide tape.[65] 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 flexible full cell harvesting a low charge-transfer resistance delivers stable operation over 250 cycles under different bending states (Fig. 5h; Figure S38). Notably, the bendable full cell achieves a favorable capacity retention of 68% with a bending angle of 90°. Typically, the capacity of bendable full cells decays much faster than that of coin-type cells, this is because the lack of tight interfacial contact in bendable full cell might prevent the smooth carrier transportation during battery operation. Moreover, the local electric field intensity might become inhomogeneous during bending, which would induce massive Zn dendrite growth in flexible batteries.[66] Figure S39 further demonstrates the extended-cycling performance of NOC@Zn-KVOH soft-packing battery with an N/P ratio of 9. These evaluations collectively substantiate that the NOC skin 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 devices.