Synergetic Contributions from the Components of Flexible 3D Structured C/Ag/ZnO/CC Anode Materials for Lithium‐Ion Batteries

Low electronic conductivity and large volume changes during the (de)lithiation process are the two main challenges for ZnO anode materials used for lithium‐ion batteries (LIB). Here, a free‐standing, flexible, and binder‐free LIB electrode composed of ZnO nanorods and carbon cloth (CC) is fabricated. This is then decorated with Ag nanoparticles and finally coated by an amorphous carbon layer to form the hybrid electrode: (C@(Ag&ZnO)). The voids among the nanorods are sufficient to accommodate the volume expansion of the ZnO while the flexible CC, which acts as the current collector, relieves the volume change‐induced stress. The Ag nanoparticles are effective in improving the conductivity. This composite electrode shows excellent LIB performance with a stable long cycling life over 500 cycles with a reversible capacity of 1093 mAh g−1 at a current density of 200 mA g−1. It also shows good rate performance with reversible capacity of 517 mAh g−1 under a high‐current density of 5000 mA g−1. In situ Raman spectroscopy is conducted to investigate the contributions of the amorphous carbon layer to the capacity of the whole electrode and the synergy between the CC and ZnO nanorods.


Introduction
[3] Transition metal oxide anode materials have attracted broad attention due to their highenergy density, high-power density, and prominent cycling stability.[6] However, ZnO has huge volume changes, during charging and discharging, resulting in electrical contact loss with the current collector.The irreversible lithium ion consumption caused by repeated exposure to the electrolyte decreases the initial Coulombic efficiency and degrades the cycling performance of the battery.Designs incorporating nanostructures, such as nanoflower, [7,8] quantum dots, [9] nanorods, [10,11] porous nanosheets, [12,13] and nanoparticles, [14,15] are considered effective in alleviating the volume variations during lithiation/delithiation.Developing suitable composite materials are desirable for maintaining the structural integrity of the electrode. [16,17]Poor conductivity is another challenge for the ZnO anode.Metal decoration has been shown to be effective for improving the electrode conductivity.Huang et al. [18] fabricated an Ag decorated ZnO microspheres anode which had capacities of 439 and 196 mAh g −1 at 100 and 500 mA g −1 , respectively, higher than that of a pure ZnO microspheres anode (404 and 79 mAh g −1 at 100 and 500 mA g −1 , respectively).At the higher current densities, the capacity difference between the two electrode types increases, indicating the significant role Ag plays in improving the rate performance of ZnO electrodes.Park et al. [19] used ZnO nanorods decorated with Ni nanoparticles as an anode material which exhibited a reversible capacity of 595 mAh g −1 at 0.5 C after 100 cycles, 1.5 times higher than the pure ZnO nanorods anode (406 mAh g −1 ).The Ni nanoparticles improve the conductivity and promote the electrode reaction kinetics, resulting in higher specific capacity.
In addition to metal decoration, carbon coating is also a very common surface modification method.Carbon material has a unique mechanism of storing lithium ions during the electrochemical reaction process, and the volume change caused by the insertion and removal of lithium ions among the interlayer gaps is small.Consequently, it is widely used for coating anode materials which have large volume changes during the (de)lithiation process. [20]There are many research reports on carbon as a coating role for different ZnO anode materials.Eliana et al. [21] synthesized a binder-free graphite-coated zinc oxide nanosheet material on a stainless steel disk which gave a superior capacity of ∼600 mAh g −1 with minimal capacity fading upon 100 cycles.They attributed its good performance to the porous structure of zinc Low electronic conductivity and large volume changes during the (de) lithiation process are the two main challenges for ZnO anode materials used for lithium-ion batteries (LIB).Here, a free-standing, flexible, and binder-free LIB electrode composed of ZnO nanorods and carbon cloth (CC) is fabricated.This is then decorated with Ag nanoparticles and finally coated by an amorphous carbon layer to form the hybrid electrode: (C@(Ag&ZnO)).The voids among the nanorods are sufficient to accommodate the volume expansion of the ZnO while the flexible CC, which acts as the current collector, relieves the volume change-induced stress.The Ag nanoparticles are effective in improving the conductivity.This composite electrode shows excellent LIB performance with a stable long cycling life over 500 cycles with a reversible capacity of 1093 mAh g −1 at a current density of 200 mA g −1 .It also shows good rate performance with reversible capacity of 517 mAh g −1 under a high-current density of 5000 mA g −1 .In situ Raman spectroscopy is conducted to investigate the contributions of the amorphous carbon layer to the capacity of the whole electrode and the synergy between the CC and ZnO nanorods.
oxide nanosheets, which can provide more active sites for lithium ions.Also the good structural stability of graphite can alleviate the internal stress caused by the volume changes during the charge and discharge process of the ZnO nanosheets.Zhang et al. [22] designed a core shellstructured ZnO@C@CN anode which gives a discharge capacity of 546.2 mAh g −1 at 1 A g −1 after 1000 cycles with a capacity retention of 69.1%.The capacity of the bare ZnO electrode was 340 mAh g −1 after 200 cycles, showing the N-doped carbon coating improves the cycle stability of the electrode.Hao et al. [23] reported a pea pod-like ZnO@C structure providing internal voids between the ZnO and the carbon layer.This showed a capacity of 565.1 mAh g −1 at 200 mA g −1 after 200 cycles with a capacity retention of 99% (compared with the 2 nd cycle).The plain ZnO only achieved 87.9 mAh g −1 after 50 cycles.The designed internal void space and outer carbon shell can alleviate the ZnO volume change-induced stresses during the electrochemistry, resulting in enhanced cycling stability and rate performance.Guo et al. [24] reported a ZnO@C:N composite anode with a capacity of 608 mAh g −1 at 0.1 A g −1 over 500 cycles and capacity retention of 55.3%.While the commercial ZnO could only obtain a capacity of 20 mAh g −1 after 10 cycles (from an initial capacity of 972 mAh g −1 ).The excellent cycle performance of the ZnO@C:N anode is attributed to the protective effect of the carbon layer and the more active sites from the N-doping.Li et al. [25] successfully prepared ZnO in a 3D N-doped carbon nanosheet framework (ZnO-NCNF-700) as the anode material.This had a specific capacity of 770 mAh g −1 at 500 mA g −1 for 1000 cycles, much higher than the pure ZnO anode (18 mAh g −1 ).Song et al. [26] fabricated a porous M-ZnO/C anode for LIB, displaying a discharge capacity of 1044 mAh g −1 at 0.1 C after 100 cycles, while the bulk ZnO can only achieve 51 mAh g −1 .The better performance was attributed to the porous structure and the protective mechanism of the carbon matrix.Almost all the reports on ZnO and carbon composite materials focused on the roles carbon played in protecting the structural stability of zinc oxide during the charging and discharging process and its contributions to the construction of the conductive network for the entire electrode.However, there is little discussion in the literature about the contribution of carbon to the specific capacity of the entire LIB and the synergies between the ZnO and carbon materials.These are systematically investigated and reported in this work.
Here, we have designed a composite carbon fiber-ZnO nanorods anode material for LIB (Scheme 1).The gap among the nanorods provides suitable space for the volume expansion.The good flexibility of carbon fiber alleviates the stresses caused by volume changes during the charging and discharging process.Ag nanoparticles were deposited on the surface of ZnO nanorods to enhance the performance under highcurrent density/rate.An amorphous carbon layer was coated onto the Ag/ZnO composite to stabilize the structure of ZnO nanorods and to improve the integrity of the SEI during cycling. [27]In situ Raman spectroscopy was used to systematically study the roles of carbon layers, and to offer direct evidence for the contribution of amorphous carbon to the electrochemical behavior of the ZnO electrode during the (de) lithiation process.

Structure and Morphology
All of the XRD patterns of ZnO&CC, Ag&ZnO, and C@(Ag&ZnO) display the diffraction peaks of wurtzite type hexagonal ZnO as shown in Figure 1a.The three diffraction peaks at 31.7°, 34.4°, and 36.2°correspond to the (100), (002), and (101) planes of ZnO, respectively.The broad diffraction peaks at around 26°and 44°come from the CC current collector.The diffraction peak at 38.1°corresponding to the (111) Scheme 1. Schematic illustration of the preparation of the C@(Ag&ZnO) on carbon cloth composite.
Energy Environ.Mater.2023, 6, e12537 plane of Ag nanoparticles can be found in the Ag&ZnO and C@ (Ag&ZnO) samples.The XRD results indicate that the crystallinity and crystal phase of the zinc oxide was not changed during the silver nanoparticles deposition and amorphous carbon layer coating.
The presence of Zn, Ag, O, and C elements are confirmed by XPS in the Ag&ZnO and C@(Ag&ZnO) samples as shown in the full XPS spectra plotted in Figure 1b. Figure 1c shows the Zn 2p XPS spectra of the two samples, the two high-resolution peaks around 1021.3 eV and 1044.4 eV correspond to Zn 2p 3/2 and Zn 2p 1/2 , respectively.The difference between the two peaks is about 23.1 eV in both of the samples, which agrees well with the standard value of 22.97 eV, indicating the existence of Zn 2+ instead of Zn 0 in the composites.This indicates that negligible other side reactions occurred during the silver planting process and the carbon deposition procedure. [27]owever, the Zn 2p 3/2 and Zn 2p 1/2 peaks in the C@(Ag&ZnO) are shifted to higher binding energy in Figure 1c.Similar shifts were also observed in the Ag 3d XPS spectra in Figure 1d.In the C 1 s spectra in Figure 1e, the three fitted peaks at around 284.8, 286.7, and 288.8 eV correspond to C-C, C=O, and O-C=O bonds respectively. [28]A new peak at around 285.7 eV, which is generally believed to be related to the formation of C-O-Zn bonds, [29] is also found in Figure 1e.This indicated that the C-O-Zn bonds are formed on the surface of the zinc oxide nanorods.For the O 1 s spectra displayed in Figure 1f, there are two fitted peaks centered at 531.1 and 532.6 eV.The peak at 531.1 eV is usually identified as the characteristic of metal oxides.The peak at 532.6 eV in the O 1 s XPS spectra of Ag&ZnO is assigned to oxygen defects in the metal oxide matrix. [30]The O 1 s spectra in the C@(Ag&ZnO) sample shifted to higher energy comparing to that in Ag&ZnO.The blue shift of the Zn 2p, Ag 3d, and O 1 s may be related to the change in the electronic state of the zinc oxide and Ag caused by the coating of the amorphous carbon layer.This is beneficial for enhancing the conductivity of the electrode materials and for improving the properties of high-performance electrode materials for energy storage. [31]canning electron microscopy (SEM) was used to evaluate the morphology of the ZnO&CC, Ag&ZnO, and C@(Ag&ZnO) samples.The surface of the CC current collector and one with the seed layer are shown in Figure 2a,b.The seeds of zinc oxide are very evenly distributed on the surface of each carbon fiber.This is beneficial for the subsequent hydrothermal growth.SEM images in Figure 2c display the ZnO nanorods which grow uniformly on the carbon fiber.The magnified image, inset in Fig- ure 2c, presents a typical regular hexagonal structure.The C@(ZnO&CC) shows similar morphology to that of ZnO&CC (Figure 2d), but the diameter of the nanorods is larger after the carbon deposition.The TEM image of the C@(Ag&ZnO) in Figure 2e shows that the Ag nanoparticles adhere well to the surface of zinc oxide nanorods.A thin layer of amorphous carbon, about 5 nm thick, is uniformly coated on the surface of Ag nanoparticles and ZnO nanorods as shown in Figure 2f,g.The amorphous carbon layer can help the nanorods to avoid contact with electrolyte, impeding the excessive formation of SEI film.As seen in Figure 3f, the high-resolution transmission electron microscopy (HRTEM) image shows that the lattice spacing of 0.26 and 0.24 nm corresponds to the (002) plane of ZnO and the (111) plane of cubic Ag, respectively.The EDS elemental mapping images (Figure 2h-l) prove the uniform distributions of Ag nanoparticles on the surface of ZnO nanorods.The SEM and TEM confirm that the composite structure of C@(Ag&ZnO) was successfully fabricated.

Electrochemical Performance
To explore the influence of the amorphous carbon layer and silver nanoparticles on the electrochemical performance of the ZnO anodes, CR2016 coin-type half-cells with Li foil as counter electrodes were assembled and tested under different conditions.Cyclic voltammogram (CV) was conducted at a scan rate of 0.1 mV s −1 with the potential range between 0.01 and 3.0 V to explore the (de)lithiation behavior of the four anodes (ZnO&CC, C@ZnO, Ag&ZnO, C@ (Ag&ZnO)).Figure 3a displays the CV curves of the ZnO&CC sample during the initial three cycles.The large broad reduction peak at around 0.26 V in the first cycle corresponds to the formation of Li-Zn alloy.The small peak at 0.8 V is due to the reduction of ZnO to Zn and the formation of the solid electrolyte interphase (SEI) layer resulting from the decomposition of the electrolyte during the electrochemical reactions. [25,32,33]Since the voltage ranges corresponding to these reactions are relatively close, a larger broad peak will be formed in the first cathodic scans.The cathodic peak at about 0.01 V comes from the lithium intercalation process of CC.The response of the lithium intercalation peak related to carbon in C@ZnO and C@ (Ag&ZnO) is stronger than that in ZnO&CC and Ag&ZnO due to the coating of the amorphous carbon layer.In the subsequent cathodic scans, the large broad peak splits into two small peaks at around 0.7 and 0.1 V which correspond to the reduction of ZnO to Zn and the alloy formation of lithium and zinc, respectively. [34]In the anodic scan, there are three oxidation peaks below 1 V, one broad peak at 0.23 V and two small peaks at 0.53 and 0.68 V.The peak at 0.23 V could be related to the CC current collector and the one step LiZn→Zn dealloying process, the anodic peaks at 0.53 and 0.68 V could be ascribed to the following multistep de-alloying process, LiZn→Li 2 Zn 3 →LiZn 2 →Li 2 Zn 5 →Zn.The broad peak at 1.35 V is assigned to the reforming of ZnO. [28]It is worth noting that the peak current of the oxidation peak, corresponding to this potential, decreases as the number of cycles increases.This indicates that not all zinc crystals can be re-oxidized to zinc oxide. [25]According to previous research, after the zinc crystals reform, only the smaller ones can be re-oxidized to zinc oxide. [35]A strong oxidation peak appears at around 2.6 V, related to the reversible decomposition of the SEI film formed during the first discharge process. [36]Comparing with the first cycle, the intensity of the oxidation peak at around 2.6 V decreased in the second and the third cycle.Figure 3b shows the charge-discharge curves of the initial three cycles for the ZnO&CC under a current density of 200 mA g −1 at 0.01-3.0V.In the first discharge curve, three voltage plateaus can be clearly observed.The first voltage plateau around 0.7 V corresponds to the reduction of ZnO to Zn (0) and the formation of Li 2 O. [37,38] The other two plateaus near 0.25 and 0.1 V can be related to the alloying reaction of Zn (0) with Li + ions and the lithiation of the CC.In the second and the third cycle, voltage curves with similar slopes were observed, which is in good agreement with the CV result.
In situ Raman spectroscopy was used to further detect the (de)lithiation behavior of ZnO NRs.The schematic diagram of the device for the in situ Raman spectroscopy test is shown in Figure 3c.The ZnO&CC composite was used as the anode material and Li metal was used as the counter electrode.The charge and discharge of the device was carried out by directly connecting the device to the electrochemical working station from the Raman test chamber.The Raman laser directly hits the anode through the quartz window on top of the device and the Raman signal was collected in situ.The ZnO material has a hexagonal wurtzite structure with one Zn ion surrounded tetrahedrally by four oxygen ions.The zone-center optical phonons can be classified into polar and Raman modes as ) are nonpolar and Raman active only, B 1 modes are silent. [39]The E 2 modes can be used as the fingerprint Raman signal for ZnO. Figure 3d shows the changes in the peak at 437 cm −1 which corresponding to the E 2 high mode of ZnO.At the beginning of the discharge process (indicated by the red arrow in Figure 3d), the intensity of the peak was the strongest at 3.0 V.As the discharge reaction progressed, the peak intensity became weaker and weaker and accompanied by a slight blue shift.When the voltage dropped to 0.3 V, the peak disappeared completely.In the charging process as indicated by the blue arrow in Figure 3d, the peak of 437 cm −1 appeared at 0.9 V.With the increase of charging voltage, the peak slightly shifted to a smaller wavenumber.The slight blue shift in discharging process and the red shift in the charging process may be related to the internal stress changes caused by the lithium-ion insertion and extraction from the ZnO nanorods. [40]The in situ Raman characterization of the cell confirms the reduction and oxidation of ZnO in the LIB.
In Figure 3e,f, we compared the CV results and the charge-discharge curves of ZnO&CC, C@ZnO, Ag&ZnO, and C@(Ag&ZnO) in the first cycle.It can be found that the current of the peak at 0.26 V, which corresponds to the formation of Li-Zn alloy in the first cathodic scan, becomes smaller and the voltages of the plateaus around 0.26 V reduce after the deposition of the amorphous carbon (C@ZnO).The change was even bigger with Ag nanoparticle (Ag&ZnO) decoration.This indicates that the coating of amorphous carbon and the addition of silver nanoparticles significantly improve the electrochemical reaction kinetics of the anode and reduces the electrode polarization.The electrode polarization of the C@ (Ag&ZnO) is the smallest due to the co-contribution of Ag and amorphous carbon.This is further verified in the charge and discharge measurement shown in Figure 3f.
Figure 4a,b shows the cycle performance of the ZnO&CC and Ag&ZnO under different current densities.When the current density is small (100 mA g −1 ), there is almost no significant difference in the performance of ZnO&CC and Ag&ZnO as shown in Figure 4a.When the current density increased to 200 mA g −1 , the Ag&ZnO shows a higher specific capacity than that of ZnO&CC after 100 cycles.Figure 4c shows the rate performance of the four samples under current densities of 100, 200, 500, 1000, 2000, 5000, and 100 mA g −1 .The C@(Ag&ZnO) has the highest specific capacity under the different current densities among the four samples.At 100 and 200 mA g −1 , the C@ZnO sample has higher capacity than the Ag&ZnO sample.As the current density increased to 500 mA g −1 , the Ag&ZnO shows a capacity higher than the C@ZnO sample.The results suggested that Ag deposition has a more visible effect on enhancing the high-current performance of the cell.In the design of a modified ZnO-based LIB the function of Ag is expected to increase the conductivity of the anode material and the C layer was intended to improve the volumetric variation during de/lithiation.From Figure 4c we can conclude that eliminating the volumetric variation is the main consideration at small currents.However, in high-current and fast charging/discharging applications the conductivity of the anode material plays the more important role than volumetric variation.Figure 4d shows the EIS results of ZnO&CC, C@ZnO, Ag&ZnO, and C@ (Ag&ZnO).From the Nyquist plot of the four samples, we can see that all of them consist of a semicircle at the medium-high frequency and a straight line at low frequency.The inset graph indicates the equivalent circuit of the four electrodes, where R s denotes Ohmic resistance which is usually related to the resistance of lithium ions transported in the electrolyte, R ct, and CPE represent the charge transfer resistance at the interface between the electrode and electrolyte, and the corresponding capacitance.The straight line can be fitted with a Warburg impedance (W o ) due to ion diffusion inside the solid electrode. [41,42]The R s and R ct of the four samples can be calculated from the diameter of the circle and the slope of the straight line.Figure 4e shows the calculated R s and R ct of the four samples.The R s of the four samples are very close since the resistance value of R s is mainly related to the electrolyte.The four samples used the same electrolyte, the small R s indicates good infiltration of electrolyte with the anode materials.R ct is much larger than R s .The C@(Ag&ZnO) electrode has the smallest R ct (28.5 Ω) and the ZnO&CC has the largest R ct (68 Ω), the R ct of Ag&ZnO (34.5 Ω) is smaller than the C@ZnO (65.9 Ω) sample.The relative values of R ct are consistent with the electrochemical performance of the cells.The results indicate that the charge transfer resistance at the interface between the electrode and electrolyte is the determining factor for the performance of the cell.The diffusion coefficient of Li ions can be obtained from the following two equations: where σ is the Warburg impedance coefficient, ω is the angular frequency, R is the molar gas constant, T is the temperature, A is the surface area of the electrode, n is the number of electrons transferred in redox reactions, F is the Faraday constant, and C 0 is the concentration of Li ions in bulk phase of electrode materials.As described in Figure 4f, the Warburg impedance coefficient can be obtained from the relationship between Z Re and ω −1/2 in the low-frequency region according to Equation (1).The σ of C@(Ag&ZnO), Ag&ZnO, C@ZnO, and ZnO&CC is 27.8, 68.2, 101.4,and 110.4 respectively.Considering that Ag has no capacity contribution to the electrode materials c) The rate capability of C@(Ag&ZnO), C@ZnO, Ag&ZnO, and ZnO&CC at current densities from 100 to 5000 mA g −1 .d) Electrochemical impedance spectroscopies of C@(Ag&ZnO), C@ZnO, Ag&ZnO, and ZnO&CC.e) The comparison of the resistances, R S and R CT , after fitting with the Zview software.f) Relationship between Z Re and ω −1/2 in the low-frequency region.g) Long-term cycling stability of C@(Ag&ZnO), C@ZnO, Ag&ZnO, and ZnO&CC at 200 mA g −1 for 500 cycles.
Energy Environ.Mater.2023, 6, e12537 Equation ( 2) can be simplified as D Li + /1/σ 2 , then the following relationship can be obtained: D Li+,C@(Ag&ZnO) > D Li+,C@ZnO and D Li+, Ag&ZnO > D Li+,ZnO&CC .The modification of Ag nanoparticles can increase the diffusion efficiency of Li ions.According to the Arrhenius equation: where ΔG is the energy barrier, k B is the Boltzmann constant, and D 0 is the pre-factor estimated empirically.Ag nanoparticles can reduce the Li ions diffusion energy barrier, accelerate the electrode reaction kinetics and reduce the reaction polarization. [43,44]In summary, the decoration of Ag nanoparticles on the surface of ZnO can significantly reduce the interfacial resistance, and reduce the Li ions diffusion energy barrier, so as to realize the rapid transfer of electrons and ions to improve the rate performance of the electrode.Figure 4g the cycling stabilities of the four samples at a current density of 200 mA g −1 .The C@(Ag&ZnO) composite shows good cycling performance with a reversible capacity of 1093 mAh g −1 , much higher than that of C@ZnO (831 mAh g −1 ), Ag&ZnO (566 mAh g −1 ) and ZnO&CC (487 mAh g −1 ) after 500 cycles.It is worth noting that the samples containing the amorphous carbon layer (C@(Ag&ZnO), C@ZnO) show better cycling stability and better reversible specific capacity than (Ag&ZnO, ZnO&CC) without the amorphous carbon layer.This confirms the contribution of the amorphous carbon layer to the stability of the electrode.The morphologies of the ZnO&CC and C@ZnO electrodes after 100 cycles at a current density of 200 mA g −1 are further characterized.The diameter of the ZnO nanorods in ZnO&CC electrode increased from 400 to 500 nm after 100 cycles.The nanorods huddled together, and the regular hexagonal structure was no longer visible (Figure 5a,b).The diameter of the ZnO nanorods in the C@ZnO electrode remains 300 to 400 nm, and there are still a large number of voids between the nanorods (Figure 5c,d), which indicates that the amorphous carbon layer can effectively alleviate the serious volume changes of ZnO nanorods during the charging and discharging process, again improving the electrode cycling stability.The lithium storage performance of the C@(Ag&ZnO) electrode is compared with the reported ZnO-based anode materials, the results are summarized in Table 1.It can be found that the C@(Ag&ZnO) electrode has better performance than most of the reported results in literatures.
Figure 6a,b shows the 1st and 2nd discharge profiles of ZnO&CC and C@ZnO.In the first discharge cycle, the C@ZnO sample has a higher capacity than that of the ZnO&CC sample, the difference in capacity (ΔC) was 147 mAh g −1 .In the second cycle, the difference in capacity increased to 209 mAh g −1 .These results show that the amorphous carbon layer has contributed to the increase in capacity.The amorphous carbon layer participates in the electrochemical reaction with its large surface area and good conductivity.In order to further study the improved lithium storage performance of C@ZnO, in situ Raman spectroscopy was used to characterize the structural change of carbon in the electrode during the discharge process.From Figure 6c,d we could see the changes in the D and G peaks of the ZnO&CC and C@ZnO during the discharge cycle.At the beginning of discharging with the voltage of 3.0 V in sample ZnO&CC, as shown in Figure 6c, the ratio of D and G peaks is 1.06%.The small ratio of D and G peaks in the ZnO&CC confirms the good crystallization of C which came from the CC current collector.The ratio of the D and G peaks in C@ZnO (1.325%) was higher than for ZnO&CC as shown in Fig- ure 6d.It implies that the amorphous carbon layer offers more active sites and vacancies for lithium storage.In addition, there is a broad peak at around 1320 cm −1 in the sample C@ZnO, which comes from the amorphous layer.The ratio of the D and G peaks in the two samples did not change much in the voltage range of 3.0 V to 0.8 V.As the voltage decreased to 0.8 V, the intensity of the D and G peaks in both Table 1.Comparison of various ZnO-based electrodes with the C@ (Ag&ZnO) electrode in this work.

Materials
Current density Ref.
C@(Ag&ZnO) 200 500 1093 This work ZnO@C 200 100 1154 [10]   Ag-decorated ZnO 100 50 410 [18]   Ni-decorated ZnO 489 (0.5 C) 100 595 [19]   G 350 -ZnO NS 100 100 600 [21]   ZnO@C@CN 1000 1000 546.2 [22]   Peapod-like ZnO@C 200 200 565.1 [23]   ZnO@C:N 100 500 608 [24]   ZnO-NCNF 500 1000 770 [25]   M-ZnO 97.8 (0.1 C) 100 1044 [26]   Energy Environ.Mater.2023, 6, e12537 ZnO&CC and C@ZnO samples go through a sharp decline and then disappeared as lithium ions began to react with ZnO to form Zn(0) and Li 2 O. [25] The result suggested that the reaction between Li ions and ZnO played a leading role at this voltage level; the CC does not influence the Li ion intercalation into ZnO NRs.The two peaks continue to decrease with decrease in discharging voltage, suggesting that the reaction of Li and ZnO continues.The D and G peaks reappeared in the voltage range of 0.4 V to 0.01 V.The intercalation of Li ion into the CC usually takes place below 0.50 V, [30,45] the reappearance of the D and G peaks suggested that the active sites and vacancies in the CC remain in both the ZnO&CC and in the C@ZnO electrode during the discharging process.The ratio of the D and G peaks in C@ZnO was higher than that of ZnO&CC during the whole charge and discharge process, indicating that the structure of the amorphous carbon layer did not change.
The CC current collector provides a 3D conductive channel increasing the conductivity as well as increasing the flexibility of the whole electrode.The Ag nanoparticles deposited on the surface of ZnO nanorods improve the electrical conductivity of the nanorods, reduce the Li ions diffusion energy barrier, accelerate the electrode reaction kinetics, and reduce the reaction polarization.Given the lithiation potential of CC and amorphous carbon is in the same range as that of the Zn-Li alloying reaction, the capacity in this potential range is not solely provided by ZnO, but from contributions from all the three components.

Conclusion
In conclusion, we have designed a binder-free and free-standing ZnObased anode material for lithium ion batteries and investigated its performance over multiple charging and discharging cycles.The space between the nanorods can fully accommodate the volume expansion of ZnO.The flexibility of the carbon cloth which acts as the current collector relieves any stress that would otherwise be caused by the large volume changes.Due to the depositions of the Ag nanoparticles and amorphous carbon layer, the Li + diffusion transfer kinetics and structural integrity during the charge/discharge process are greatly enhanced.Electrochemical analyses and in situ Raman tests prove that the excellent performance of the composite structure is due to the synergy between the CC current collector and ZnO nanorods, the capacity contribution from the amorphous carbon layer reduces electrode polarization.Also the low-energy barrier caused by Ag nanoparticles allows faster transportation of Li ions into the electrode.Our results provide greater insight into the operation of ZnO-based anode materials and put forward feasible solutions to improve their practical application.

Experimental Section
Synthesis of ZnO seed layer: The carbon cloth current collector (CC, Guangdong Canrd New Energy Technology Co., Ltd.) was sonicated in acetone and ethanol sequentially for 20 min and dried at 70°C for 1 h.The cleaned CC was then put into the chamber for magnetron sputtering and the chamber was kept at a vacuum of 1 × 10 −4 Pa.The ZnO seed layer was sputtered onto the CC current collector with a working pressure of 2 Pa.The flow rate of Ar is 20 sccm and of O 2 is 10 sccm during the sputtering process.The working temperature is 300°C and the RF power is 75 W.The sputtering lasts for 1 h.Preparation of ZnO nanorods on CC (ZnO&CC): The ZnO&CC sample was synthesized through a one-step hydrothermal method.All the chemicals were obtained from commercial sources and used without further purification.A total of 0.05 M zinc acetate dihydrate (99%, National Medicine), 0.05 M hexamethylenetetramine (99%, National Medicine), and 0.01 M polyethyleneimine (99%, Sigma) were mixed to form the hydrothermal precursor.Then, the precursor solution was preheated at 95°C for 1 h.After that, the CC with ZnO seed layer was soaked in the precursor solution and submitted to hydrothermal treatment at 95°C for 15 h, with the ZnO seed layer facing down.The product was washed with ethanol and deionized water three times and dried at 70°C for 1 h.The average mass of ZnO nanorods is 1.5 mg.Ag decoration and amorphous carbon coating C@(Ag&ZnO): A chemical reduction method was used for Ag decoration.The prepared ZnO&CC was firstly soaked in the AgNO 3 (0.01 M) solution for 2 h.Then 0.01 M sodium borohydride solution was spin coated on the surface of the soaked sample to reduce silver into Ag nanoparticles (Ag&ZnO).An amorphous carbon layer (~0.2 mg) was deposited on the Ag&ZnO sample by magnetron sputtering to get the C@ (Ag&ZnO) sample.The gas flow rate of Ar during the sputtering process is 80 sccm, the working pressure is 1 Pa, the sputtering temperature is room temperature and the RF power is 200 W. Single depositions of Ag or amorphous carbon were carried out for comparison and the samples are labeled as Ag@ZnO and C@ZnO, both of them were grown on CC. Materials characterization: The morphology and structure of the obtained samples were characterized by field emission scanning electron microscopy (FESEM, Sigma 500) and transmission electron microscopy (FEI Tecnai G2 F30, 300 kV).Phase structure and chemical composition of the samples were determined by Xray diffraction with an X-ray diffractometer (Bruker, D8Advance) using Cu Kα (wavelength λ = 0.15418 nm) irradiation and the X-ray photoelectron spectroscopy (XPS, Escalab 250Xi Thermo Fisher Scientific).In situ Raman spectroscopy was carried out to study the (de)lithiation behavior of the samples using a LabRAM HR micro Raman spectrometer with an EMCCD detector at a laser excitation of 633 nm.Electrochemical measurements: The electrochemical properties of the final structures were tested by assembling CR2016 coin cells in an Argon filled glovebox where the concentration of O 2 and H 2 O were below 0.1 ppm.The composite structures were directly used as the working electrode without any binders and conductive additives.The metal lithium sheet and Celgard 2400 served as the counter electrode and separator, respectively.The electrolyte was composed of 1 M LiPF 6 in EC:DEC (1:1, v/v).The galvanostatic charge and discharge measurement of the electrodes were carried out on a battery test system (Neware CT-4008 T) with the cut-off potential range of 0.01-3.0V. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were carried out on an electrochemical workstation (Shanghai Chen Hua Chi660D), in which CV was performed in the range of 3.0-0.01V (versus Li + /Li) at a sweep rate of 0.1 mV s −1 and EIS was performed in the frequency range from 100 kHz to 0.01 Hz at an amplitude of 5 mV.All the EIS measurements were fitted with Zview software.

Figure 3 .
Figure 3. a) The CV curves of ZnO&CC at a scan rate of 0.1 mV s −1 .b) Charge-discharge profiles of the ZnO&CC for the 1st, 2nd, and 3rd cycles at a current density of 200 mA g −1 .c) Schematic representation of the in situ Raman test.d) The in situ Raman spectrum of ZnO&CC during the discharge and charge process.e) The CV curves of ZnO&CC, C@ZnO, Ag&ZnO, and C@(Ag&ZnO) during the first cycle and f) the corresponding charge-discharge profiles.

Figure 4 .
Figure 4. a, b) The cycling performance and Coulombic efficiencies of ZnO&CC and Ag&ZnO under current densities of 100 and 200 mA g −1 .c)The rate capability of C@(Ag&ZnO), C@ZnO, Ag&ZnO, and ZnO&CC at current densities from 100 to 5000 mA g −1 .d) Electrochemical impedance spectroscopies of C@(Ag&ZnO), C@ZnO, Ag&ZnO, and ZnO&CC.e) The comparison of the resistances, R S and R CT , after fitting with the Zview software.f) Relationship between Z Re and ω −1/2 in the low-frequency region.g) Long-term cycling stability of C@(Ag&ZnO), C@ZnO, Ag&ZnO, and ZnO&CC at 200 mA g −1 for 500 cycles.

Figure 6 .
Figure 6.a) The 1st and b) 2nd discharge profiles of ZnO&CC and C@ZnO electrodes between 0.1 and 0.01 V.The in situ Raman spectroscopy of c) ZnO&CC and d) C@ZnO at different voltages during the discharge process.
where A 1 and E 1 are both Raman and IR active polar modes, the E 2 modes (E 2 low and E 2 high