ZIF‐Derived Co3O4 Nanoflake Arrays Decorated Nickel Foams as Stable Hosts for Dendrite‐Free Li Metal Anodes

Developing high‐performance anode current collectors with three‐dimensional structure and lithiophilic layers is of great importance to further advance the application of lithium metal batteries. However, relatively few research has focused on the transition of substrate and the intrinsic structure stability after electrodeposition of lithium on substrates, which leads to an incomplete understanding of the behavior of lithium deposition. Herein, a lithium metal anodes host with a highly stable and 3D structure has been effectively produced through in situ development of nanoflake arrays embedded with Co3O4 obtained from ZIF on nickel foams (Co3O4‐NF). And the actual lithium deposition sites and lithium deposition process on Co3O4‐NF are elucidated via a combination of characterization techniques and electrochemical analytical methods. Consequently, the resulting Co3O4‐NF@Li anodes could effectively inhibit lithium dendrite formation and mitigate volume expansion, demonstrating a significantly extended and consistent lifespan of 800 cycles (1600 h) at 1 mA cm−2 with low overpotential and insignificant voltage fluctuation for the process of lithium stripping and plating in symmetric cells. Herein, it is aimed to examine the transitions of metal oxides as a lithiophilic site for the lithium metal anode. It offers novel perspectives and approaches for the design of dendrite‐free lithium metal anodes.


Introduction
The use of metallic lithium (Li) anodes has been widely considered as a highly promising alternative to the conventional graphite anodes in Li-ion batteries.This is primarily due to their significant theoretical capacities of 3860 mA h g À1 and their low negative electrochemical potential of À3.040 V compared to the standard hydrogen electrode. [1]The achievement of high energy density can be realized by the combination of lithium (Li) metal anodes with lithium-free sulfur (S) and oxygen (O 2 ) cathodes.This approach has promise in meeting the growing need for energy storage solutions.Nevertheless, the formation of Li dendrites, instability of the solid electrolyte interphase (SEI), and significant volume fluctuations resulting from repeated charge-discharge cycles remain significant challenges that impede the practical application of Li metal anodes. [2]The presence of inherent defects in Li metal batteries has a notable influence on their low discharging capacity, limited cycling life, and safety concerns. [3]umerous approaches have been suggested for solving these challenges, including the utilization of innovative electrolyte additives for SEI stabilization, the development of artificial interfacial layers, and the implementation of high-modulus solid/composite electrolytes to mitigate dendrite formation. [4]he absence of a host material in Li metal anodes, which leads to the fracture of the SEI during charge-discharge cycles, causes continuous volume changes in the anode.However, the treatments indicated earlier have not yet overcome this problem.
In this regard, the application of a 3D current collector or a nanostructured electrode has the ability to reduce the local Developing high-performance anode current collectors with three-dimensional structure and lithiophilic layers is of great importance to further advance the application of lithium metal batteries.However, relatively few research has focused on the transition of substrate and the intrinsic structure stability after electrodeposition of lithium on substrates, which leads to an incomplete understanding of the behavior of lithium deposition.Herein, a lithium metal anodes host with a highly stable and 3D structure has been effectively produced through in situ development of nanoflake arrays embedded with Co 3 O 4 obtained from ZIF on nickel foams (Co 3 O 4 -NF).And the actual lithium deposition sites and lithium deposition process on Co 3 O 4 -NF are elucidated via a combination of characterization techniques and electrochemical analytical methods.Consequently, the resulting Co 3 O 4 -NF@Li anodes could effectively inhibit lithium dendrite formation and mitigate volume expansion, demonstrating a significantly extended and consistent lifespan of 800 cycles (1600 h) at 1 mA cm À2 with low overpotential and insignificant voltage fluctuation for the process of lithium stripping and plating in symmetric cells.Herein, it is aimed to examine the transitions of metal oxides as a lithiophilic site for the lithium metal anode.It offers novel perspectives and approaches for the design of dendrite-free lithium metal anodes.current density and mitigate the volume fluctuations of lithium by virtue of the skeleton's elevated specific surface area.Therefore, this approach can effectively impede the initiation and progression of dendrite development. [5]Nickel foams (NFs) have been extensively utilized as current collectors in energy batteries owing to their exceptional electrochemical stability and mechanical strength.Numerous endeavors have been undertaken to fabricate three-dimensional nickel host materials possessing notable surface area and distinctive structure, aiming to mitigate the proliferation of lithium (Li) dendrites.Examples of such efforts include the utilization of NF as a host material [6] and the implementation of 3D Ni scaffolds on copper (Cu) foils. [7]owever, it has been shown that Ni exhibits lithiophobic properties and possesses a noticeable nucleation barrier during lithium nucleation. [8]Previous research has indicated that the utilization of nanomaterials possessing low overpotential, such as Ag, LiC 6 , and SnO 2 , as the foundational components for a "lithiophilic surface" has been documented as an effective means of achieving stable lithium deposition, hence manifesting the lithiophilic characteristics. [9]Nevertheless, the lithiophilic layers often exhibit a smooth and planar morphology, which limits the potential for further enhancement of the specific surface area in order to reduce the local current density.Hence, the implementation of straightforward techniques to fabricate a current collector with a substantial surface area and a diminished energy barrier for Li nucleation holds great potential in enhancing the electrochemical efficiency of the Li metal anode.Metal oxides [10] have been broadly reported as effective lithiophilic materials to improve the performance of lithium metal batteries.Some studies believe that the interaction of metal oxide and lithium includes two processes, taking Co 3 O 4 as an example, including electrochemical lithiation and chemical conversion.Both reactions are closely related to the electron transfer process and the diffusion rate of Li þ .Therefore, the development of lithiophilic oxides that can achieve fast electron/lithium transfer is the good choice for homogeneous lithium deposition. [11]owever, relatively few research has focused on the transition of substrate and the intrinsic structure stability after electrodeposition of lithium on substrates.As a result, the transition of the real structure and components leads to the actual lithium deposition site is not consistent with our initial material, which leads to an incomplete understanding of the behavior of lithium deposition.Without a doubt, there remains a pressing need to further investigate lithiophilic, conductive, and stable hosts, as well as to devise more straightforward synthetic methods that result in optimized hybrid structures.These efforts are crucial for enhancing the electrochemical capabilities of Li metal composite anodes.
Herein, a lithium metal anodes host with a highly stable and 3D structured has been effectively produced through in situ development of nanoflake arrays embedded with Co 3 O 4 obtained from ZIF on NFs (Co 3 O 4 -NF).The enhanced lithiophilicity of the Co 3 O 4 -NF host can be substantially attributed to the predicted synergistic effects resulting from the 3D structure and the presence of embedded Co 3 O 4 nanoparticles, which have the ability to chemically react with Li.The experimental results and electrochemical analysis confirmed that the actual lithium deposition sites and lithium deposition process on Co 3 O 4 -NF.The resulting Co 3 O 4 -NF@Li anodes could effectively inhibit lithium dendrite formation and mitigate volume expansion, demonstrating a significantly extended and consistent lifespan of 800 cycles (1600 h) at 1 mA cm À2 with low overpotential and insignificant voltage fluctuation for the process of lithium stripping and plating in symmetric cells.Moreover, when considering full-cell designs utilising LiFePO 4 cathodes, it is seen that the Co 3 O 4 -NF@Li anodes exhibit significantly enhanced cycle stability and rate capability.This study examines the transformation of metal oxides into lithiophilic sites on the lithium metal anode, offering valuable insights for the development of dendrite-free lithium metal anodes in advanced lithium metal batteries.

Characterizations of Co 3 O 4 -NF
The Co 3 O 4 -embedded nanoflake arrays were synthesized by means of in situ decorating and conversion of ZIF-L nanoflakes on the NF.In this study, ZIF-L nanoflake arrays were synthesized using a simple solution approach on the NF substrate, which was treated with different roller pressures to change its pore size and select the appropriate pores for in situ growth.The difference of the Ni foam before and after roller pressures is shown in Figure S1, Supporting Information.It can be seen that the pore size of Ni foam has been reduced, which is beneficial for the deposition of lithium metal and inhibiting the formation of lithium dendrites.The synthesis involved mixing a certain quantity of Co(NO 3 ) 2 ⋅6H 2 O and 2-methylimidazole (2-Melm) in an aqueous solution at ambient temperature.This method enables the creation of ZIF-L nanoflake arrays on a large scale, as depicted in Figure 1a.And Figure 1b,c shows scanning electron microscopy (SEM) images illustrating the presence of smooth NF frameworks, which have been uniformly coated with 3D arrays of ZIF-L nanoflakes.It is noteworthy that a significant number of ZIF-L nanoflakes exhibited a vertical orientation on the surface of the NF skeleton, resembling the arrangement of leaves on branches (Figure S2, Supporting Information).The confirmation of the successful growth of ZIF-L arrays on NF was achieved through the identification of ZIF-L X-ray diffraction (XRD) peaks (Figure S3, Supporting Information).Following heat treatment in an air environment and subsequent exposure to air, the ZIF-L nanoflake arrays underwent a chemical transformation, resulting in the formation of Co 3 O 4 -embedded nanoflake arrays on NF (Co 3 O 4 -NF).The loading of Co 3 O 4 was calculated to be 1.387 mg cm À2 (Figure S4, Supporting Information).The Co 3 O 4 nanomaterials were obtained by in situ growth on NF, which will lead to a low loading of Co 3 O 4 on the Ni substrate.As a result, the low loading of Co 3 O 4 and the substrate Ni significantly limit the identification of Co 3 O 4 with the existence of strong Ni peak.Different scan rates (5°, 10°, and 15°min À1 ) were applied to collect the XRD patterns, and the results are shown in Figure S5, Supporting Information.It can be seen that the scan rate of 5°min À1 shows the highest quality.Moreover, the enlarged section (Figure 1d) indicates that the Co 3 O 4 sample exhibits characteristic diffraction peaks of Fd3m cubic Co 3 O 4 ((220) facet at 31.2°, (311) facet at 36.8°, and (440) facet at 65.2°).And no peaks assigned to other materials are observed.Therefore, it is suggested that Co 3 O 4 with a high purity has been loaded onto Ni substrate.Figure 1b,c and S6, Supporting Information demonstrate that the Co 3 O 4 -NF material effectively retains the branch-leaf-like shape and exceptional flexibility observed in the parent ZIF-NF structure.The Co 3 O 4 nanoflakes on the NF substrate exhibit a vertical height of roughly 4 μm and a thickness of around 200 nm.The results presented in Figure 1e demonstrate the presence of Co 3 O 4 nanoparticles, characterized by an average particle size of %5 nm, which were evenly distributed among the carbon nanoflakes.10a,11a] The SEM elemental mapping images presented in Figure S7, Supporting Information demonstrate the regional distributions of Co, O, C, and N, providing additional evidence to support the assertion that the Co 3 O 4 coating on the NF skeleton is uniformly distributed.
Subsequently, an analysis was conducted on the surface and electrical properties of the sample.10a,11] The primary peaks observed in the spectrum can be resolved into subpeaks at 796.7 and 781.5 eV, which can be attributed to the presence of Co 3þ .Additionally, subpeaks at 794.9 and 779.6 eV are observed, which can be attributed to the presence of Co 2þ .This confirms the coexistence of both Co 2þ and Co 3þ in the Co(II)Co 2 (III)O 4 compound.Regarding the O 1s spectrum presented in Figure 1j, the observed peaks at 532.0, 531.0, and 529.5 eV can be assigned to distinct oxygen species.Specifically, the peak at 532.0 eV corresponds to oxygen originating from adsorbed moisture (O in H 2 O), the peak at 531.0 eV corresponds to oxygen associated with surface hydroxyl groups or adsorbed oxygen-containing species (absorbed O), and the peak at 529.5 eV corresponds to lattice oxygen involved in metal-oxygen bonding (lattice O). [11a] Based on the aforementioned findings, the presence of Co 3 O 4 in Co 3 O 4 -NF is substantiated.

Li Deposition and Stripping Behavior
The morphologies of cycled Co 3 O 4 -NF@Li and NF@Li half-cells were examined under plating current densities of 0.5 mA cm À2 while gradually increasing the areal capacity.This experimental approach aimed to assess the Li plating behaviors of samples.When the areal capacity was established at 1 mA h cm À2 , the absence of lithophilic material on the NF current collectors resulted in the random deposition of lithium ions.The process of nucleation and growth of lithium are shown in Figure 2a,b.As a result of the uneven deposition process, a minor occurrence of dendritic lithium formation was also detected on the current collectors made of NF at a charge capacity of 1 mA h cm À2 .In contrast, no dendritic lithium was found on the current collectors composed of Co 3 O 4 -NF (Figure 2c,d).At a capacity of 3 mA h cm À2 , spherical and dendritic lithium was generated and deposited onto the surfaces of NF.In contrast, the surface of the Co 3 O 4 -NF substrate does not exhibit any production of Figure 2. The SEM images of NF and Co 3 O 4 -NF at fixed plating current densities of 0.5 mA cm À2 with increasing areal capacities (1, 3, 5, and 10 mA cm À2 ).Cross-sectional view and top-view SEM images of NF a,b) at the areal capacities of 1 mA h cm À2 , e,f ) 3 mA h cm À2 , i,j) 5 mA h cm À2 , and m,n) 10 mA h cm À2 .Cross-sectional view and top-view SEM images of Co 3 O 4 -NF c,d) at the areal capacities of 1 mA h cm À2 , g,h) 3 mA h cm À2 , k,l) 5 mA h cm À2 , and o,p) 10 mA h cm À2 .lithium dendrites, as depicted in Figure 2e-h.At an elevated areal capacity of 5 mA h cm À2 , the existence of several lithium metal layers that are unevenly distributed and have a loosely arranged structure on the NF current collector resulted in significant increase of the lithium volume.Additionally, dendritic lithium formations were seen within the substrate, as depicted in Figure 2i,j.In contrast, the Co 3 O 4 -NF substrate exhibited the formation of a homogeneous and compact lithium metal layer, with no observable presence of lithium dendrites in the cross-sectional view and top-view SEM images (Figure 2k,l).When increasing the capacity of 10 mA h cm À2 (Figure 2m-p), not only dendritic lithium was found on the pristine NF surface but also cracks and large volume expansion of the lithium layer were observed, and due to the loose structure, which worsened as the cycle progressed.In contrast, for Co 3 O 4 -NF substrate, under high areal capacity deposition, a smooth and dense lithium layer was still formed, and even though some mossy Li was formed, the lithium metal tends to grow inplane due to the limitation of the 3D structure, which successfully inhibits the formation of lithium dendrites and decelerates the expansion of volume.In conclusion, the Co 3 O 4 -NF material has superior lithium deposition behavior across various areal capacities.This material efficiently prevents the formation of lithium dendrites and enhances volume stability by facilitating the production of a uniform and compact lithium metal layer during cycling.In contrast, for NF substrates, lithium dendrites are formed at different areal capacity, and loose and cracked lithium metal layers were observed at high areal capacity.
The investigation also included an examination of the Li stripping behavior of the current collectors in the sample.In the initial step, the deposition of lithium metal onto the substrates was carried out with the areal capacity of 5 mA h cm À2 and a current density of 0.5 mA cm À2 .The data shown in Figure 3a-h illustrate that the surface of the collector exhibits a higher degree of lithium metal filling for capacities ranging from 1 to 5 mA h cm À2 .Lithium deposition on the surface of the Co 3 O 4 -NF substrate was seen at the areal capacity of 1 mA h cm À2 , resulting in the transformation of the nanoflake into a lithium rod (Figure 3d).The Co 3 O 4 -NF substrate facilitates the deposition of lithium in close proximity to the lithium rods, resulting in the formation of a lithium metal layer that is both dendrite-free and densely packed.This phenomenon is illustrated in Figure 3h, where a Li metal layer with a capacity of 5 mA h cm À2 can be observed.However, lithium dendrites were formed on NF (Figure 3f ).Next, lithium stripping was performed on the collector at a current density of 0.5 mA cm À2 from 5 to 1 mA h cm À2 .The morphology is shown in Figure 3i,j.For the NF collector, the lithium dendrites and "dead lithium clumps" left were observed on the NF surface.The NF collector was also partially exfoliated.In contrast, the morphology of the remaining lithium on Co 3 O 4 -NF remains relatively flat (Figure 3k), no Li dendrites were generated, and the same lithium rod structure as before cycling is observed in the top view (Figure 3l).This proved that Co 3 O 4 -NF has good stripping behavior and can effectively inhibit the formation of dendrites.As shown in the Figure S8, Supporting Information, it can be observed that the original structure remains stable after completely stripped, and the original structure can still be maintained after cycling.The maintained structure plays a key role in the process of lithium metal deposition, providing stable nucleation sites and inhibiting the formation of lithium dendrites.
Furthermore, XPS spectra presented in Figure S9, Supporting Information were used to analyze the surface chemical composition of the Co 3 O 4 -NF following lithium stripping at a current density of 1 mA h cm À2 .Regarding the element Co, it is noteworthy that none of the samples exhibit discernible Co 2p peaks, as illustrated in Figure S9, Supporting Information.This absence could perhaps be attributed to the positioning of Co 3 O 4 behind the SEI layer, rendering it undetectable by XPS, a technique employed for surface analysis. [12]The observed increase in the O 1s and C 1s signals suggests the presence of SEI at the surface of the lithium plating.Additionally, the absence of cobalt signals confirms that Co 3 O 4 -NF primarily contributes to the initial nucleation stage of lithium plating and does not play a significant role in the subsequent growth process.Additionally, it is noteworthy that in the event of the rupture of the SEI layer and the subsequent appearance of cracks, Co 3 O 4 exhibits a preference for reacting with Li þ ions.This reaction leads to the formation of new SEI components that are rich in Li 2 O, effectively suppressing any further side reactions and inhibiting the growth of dendrites. [13]The remarkable reversible lithium stripping/plating behavior of Co 3 O 4 -NF can be related to the presence of a uniform coating layer of Co 3 O 4 and a stable SEI interface.These factors contribute to the uniform nucleation of lithium and the ability to reversibly plate and remove lithium ions. [14]Based on the aforementioned findings, it can be inferred that the application of a lithiophilic material coating significantly influences the efficacy of the current collector in mitigating the production of lithium dendrites in long-lasting lithium metal batteries.

Electrochemical Performances
In order to assess the electrochemical reversibility of the ongoing processes involving Li plating and stripping, an examination was conducted on the Coulombic efficiencies (CEs) of half-cells.Nevertheless, quantifying the CE of Li plating and stripping on Li substrates present challenges due the presence of extra lithium on a Li substrate, which can counterbalance the irreversible loss of Li throughout the cycle process. [15]Herein, to precisely calculate the CE of lithium cycling on the substrate, two CEs calculations were used to together verify the plating/stripping on the lithium substrates.
In most academic literature, Figure 4a presents the computation commonly employed to determine lithium consumption throughout the plating and stripping processes. [16]The cell underwent cycling through the application of a 0.2 mA h cm À2 lithium plating (Q T ) onto the substrates.This was followed by charging to 1 V to strip the lithium off (Q S ).Lithium foils can supplement lithium losses during lithium cycling processes, resulting in larger Q S enlarged values after some plating/ stripping.And the average CE on the Co 3 O 4 -NF substrate of Li plating/stripping after 1000 cycles was 99.59% (Figure 4b), which is much superior to the NF current collectors (98.11% in 1000 cycles).Due to unstabilized cycling, the current calculation approach exhibits a flaw in accurately assessing the irreversible loss of lithium from the substrate during the plating/ stripping process.As a result, there is a heightened influence of poorer initial conversion efficiency (ICE) on the average Coulombic efficiency when the number of cycles is inadequate.
The nucleation overpotentials seen during the process of lithium metal plating can provide additional insights into the lithiophilicity of the current collectors.The determination of the nucleation overpotential involves measuring the potential difference between the sharp tip of the negative voltage and the subsequent stable plating voltage plateau of a lithium plating profile. [17]According to the data presented in Figure 4c, the initial Li plating onto the substrate at a current density of 0.5 mA cm À2 reveals that the nucleation barriers of Li on the Co 3 O 4 -NF substrate are measured to be 44.9 mV.In contrast, the pristine NF current collector exhibits a significantly higher nucleation barrier of 91.4 mV.These findings provide empirical evidence supporting the beneficial impact of the lithiophilicity of the Co 3 O 4 coating.In general, an increased loading of lithiophilic materials has the potential to provide a greater number of nucleation sites and reduce the overpotentials associated with nucleation.Moreover, following the initial nucleation process, the voltage profile exhibits an increase to a stable level, indicating the subsequent growth stage after nucleation.This plateau allows for the determination of the minimal plating overpotential. [17]o mitigate the influence of comparatively poor ICE on the average CE measurement, an additional experimental procedure known as the Aurbach CE test [16a] was conducted.This was done to enhance the evaluation of lithium (Li) loss on the substrate used for Li plating (Figure 4d).Initially, a lithium plating/stripping cycle was conducted on collectors with a capacity of 4 mA h cm À2 .Subsequently, preexisting lithium with an initial capacity of 4 mA h cm À2 was electrodeposited onto current collectors to serve as a lithium reservoir (Q T ).In the end, the cells underwent 100 cycles with a constant capacity of 0.5 mA h cm À2 (Q C ) and were charged to 1 V (Q S ) for lithium completely stripped removing the Li in the last cycle.16a] CE Based on Aurbach CE test method,the Co 3 O 4 -NF current collector has a stable cycling performance over 100 cycles, while maintaining an average CE of 99.31%.This CE value is significantly greater compared to that seen on the NF current collectors, which exhibited an average CE of 98.37% over the same number of cycles.The aforementioned findings indicate that the utilization rate of lithium on the Co 3 O 4 -NF substrate surpasses that on the NF substrate.Moreover, the judicious choice of a more lithiophilic material can significantly enhance the lithium deposition behavior.Additionally, the formation of a compact lithium deposition layer can effectively suppress the growth of lithium dendrites and minimize the occurrence of dead lithium.
In order to conduct a more comprehensive assessment of the cycling stability of the composite Li electrode, symmetrical cells were constructed and subjected to multiple Li plating and stripping processes at various current densities, and the cycling capacities of different capacities are shown in Figure S10, Supporting Information.The Co 3 O 4 -NF current collectors demonstrate reduced overpotentials and extended cycling lives.As anticipated, the stability performance of Co 3 O 4 -NF@Li||Co 3 O 4 -NF@Li is found to be superior, as evidenced by a significantly low overpotential.The composite Li electrodes used in this study were constructed into symmetric cells.These electrodes were created by plating Li onto current collectors, achieving a capacity of 1 mA h cm À2 .The investigation also encompassed an examination of the long-term cyclic stabilities of the symmetrical cells of the sample current collectors.The voltage profiles of the symmetrical cells with a current density of 1 mA cm À2 and a fixed areal capacity of 1 mA h cm À2 are depicted in Figure 5a.The matching terminal voltage profiles are presented in the insets of Figure 5a.The cell configuration Li||Li exhibits a progressive rise in overpotentials until reaching a duration of 400 h, at which point a short circuit occurs.The cell with the composition NF@Li||NF@Li exhibits a notable rise in overpotentials up to 1000 h, after which it experiences a decline leading to a value of 60 mV at 1250 h.Subsequently, the cell undergoes a short circuit.The observed limited cycle stability of the NF@Li||NF@Li and Li||Li configurations can be attributed to the inherent instability of the lithium-electrolyte interface and the accumulation of SEI layer.These factors contribute to the uncontrolled growth of dendritic lithium, which can penetrate the separator and result in a short circuit.
In contrast, the Co 3 O 4 -NF current collectors demonstrate reduced overpotentials and extended cycling lives.As anticipated, the stability performance of Co 3 O 4 -NF@Li||Co 3 O 4 -NF@Li is found to be superior, as evidenced by a significantly low overpotential of approximately 10 mV after 1600 h, better than most of the work reported so far (Table S1, Supporting Information).Symmetrical cells were constructed and subjected to multiple Li plating and stripping processes at various current densities, and the cycling capacities of different capacities are shown in Figure S10, Supporting Information.The Co 3 O 4 -NF current collectors demonstrate reduced overpotentials and extended cycling lives.As anticipated, the stability performance of Co 3 O 4 -NF@Li|| Co 3 O 4 -NF@Li is found to be superior, as evidenced by a significantly low overpotential.The electrochemical impedance analysis (EIS) of symmetric cells was conducted to investigate the fast charge transfer behavior of the Co 3 O 4 -NF@Li||Co 3 O 4 -NF@Li electrode.This electrode is known for its exceptional kinetic process.The EIS measurements were performed before the 1st cycle and after the 100th cycle, both at a current rate of 1 mA cm À2 and a capacity of 1 mA h cm À2 .The diameter of the semicircle observed in the high-frequency portion of the Nyquist plot is indicative of the charge-transfer resistance (Ret) at the electrodeelectrolyte interface. [18]According to the data presented in Figure 5b,c, it can be observed that the retention (Ret) of the Co 3 O 4 -NF@Li||Co 3 O 4 -NF@Li electrode is much lower compared to that of the NF@Li||NF@Li electrode both before the first cycle and after the 100th cycle.This finding provides more evidence supporting the notion that the Co 3 O 4 -NF@Li anodes exhibit enhanced kinetics for the lithium stripping/plating reaction.In Figure 5d, voltage profiles at different cycle numbers of Co 3 O 4 -NF@Li anodes were shown.
In order to showcase the potential application performance of the Co 3 O 4 -NF@Li anodes, a series of experiments were conducted.Specifically, the Co 3 O 4 -NF@Li anodes were combined with the LiFePO 4 (LFP) cathodes (Figure S11, Supporting Information) to produce full cells, denoted as Co 3 O 4 -NF@Li|| LFP.The Co 3 O 4 -NF@Li||LFP cell demonstrates a notable initial capacity of 147 mA h g À1 , and even after 300 cycles, it maintains a reversible capacity of 102 mA h g À1 (Figure 5e).Nonetheless, the initial specific capacity of the NF@Li||LFP cell is recorded as 132 mA h g À1 , but it experiences a quick decline to approximately 10 mA h g À1 after completing 100 cycles.Furthermore, it can be observed from the charge/discharge curves depicted in Figure 5f that the Co 3 O 4 -NF@Li||LFP cell exhibits a reduced voltage polarization compared to the NF@Li||LFP cell (Figure S12, Supporting Information).Furthermore, the Co 3 O 4 -NF@Li||LFP cells also demonstrate an enhanced rate capability.When subjected to different cycling rates (1C, 2C, 5C, 10C, and 20C) as shown in Figure 5g, the Co 3 O 4 -NF@Li||LFP cells exhibit significantly greater specific capacities compared to the NF@Li||LFP cell.Specifically, the Co 3 O 4 -NF@Li||LFP cells achieve specific capacities of 149, 141, 118, 85, and 51 mA h g À1 , respectively, whereas the NF@Li||LFP cell only achieves specific capacities of 139, 110, 78, 51, and 22 mA h g À1 , respectively.The enhanced capacity retention and impressive rate capability shown in the Co 3 O 4 -NF@Li||LFP cell provide additional evidence supporting the structural benefits of the dendrite-free Li composite anodes facilitated by the stable Co 3 O 4 -NF host.
To further validate the potential application performance of the Co 3 O 4 -NF@Li anodes, the Co 3 O 4 -NF@Li anodes were combined with the LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM(811)) and LiCoO 2 (LCO) cathodes for the full cell, as shown in Figure S13, Supporting Information, the Co 3 O 4 -NF@Li||NCM cell demonstrates a notable initial capacity of 165 mA h g À1 , and after 60 cycles, it maintains a reversible capacity of 105 mA h g À1 Nonetheless, the initial specific capacity of the NF@Li||NCM cell is recorded as 111 mA h g À1 .The Co 3 O 4 -NF@Li||LCO cell demonstrates a notable initial capacity of 188 mA h g À1 , and after 120 cycles, it maintains a reversible capacity of 140 mA h g À1 .Nonetheless, the initial specific capacity of the NF@Li||NCM cell is recorded as 174 mA h g À1 .The cycling performances of the full cells certainly improved, which means that the Co 3 O 4 -NF@Li in this work can perform better after cycling.The enhanced capacity retention capability shown in the Co 3 O 4 -NF@Li cell.It exhibited significant improvement in full cell cycle life in LFP, NCM(811) and LCO cathode systems.

Analysis of the Lithium Deposition Sites
Previous studies considered Co 3 O 4 as a lithophilic site and calculated its binding energy with lithium, [19] ignoring the transformation process that occurs in the transition metal oxides during the electrodeposition process, which resulted in the actual deposited structure not being the same as the synthesized material, while the actual lithium-deposited sites were also changed.To investigate the lithium deposition sites during the electrochemical processes of Co 3 O 4 -NF@Li anodes, electrochemical tests and characterizations were performed.
The cyclic voltammetry (CV) test was conducted on Co 3 O 4 -NF, as shown in Figure 6a.A conversion reaction with high reversibility is observed, following the Equation ( 2) and (3) [20] Co The cyclic voltammetry curve exhibits two sets of reversible redox processes.The reduction peaks R 1 and R 2 are associated with the chemical Formula (2) and (3).The oxidation peaks labeled as O 1 and O 2 are indicative of the reversible reaction associated with the reduction peaks.The CV curve demonstrates the favorable reversibility and stability of the Co 3 O 4 -NF and Co-Li 2 O matrix's mutual conversion, hence enabling the following deposition of lithium metal.Upon discharge to 0 V, the compound Co 3 O 4 undergoes a full conversion into Li 2 O and Co.Following the prelithiation process, the Li 2 O-rich SEI film formed was generated.Subsequently, the deposition of lithium metal persists onto the Li 2 O, resulting in the formation of a layer composed of lithium metal. [21]The discharging and charging curves of the Co 3 O 4 -NF@Li current collector are depicted in Figure 6b.Following the completion of three discharge and charge cycles within the voltage range of 0.01-3 V, the collector underwent cycling within the voltage range of 0-1 V. Subsequently, the deposition of lithium metal onto the collector is performed, followed by the execution of a galvanostatic discharge-charge test at a capacity of 1 mA h cm À2 .
The morphology and surface chemical composition of the interfacial layer of Co 3 O 4 -NF were verified through the utilization of transmission electron microscopy (TEM) and XPS.Upon being charged to a voltage of 1.0 V, the Co 3 O 4 nanoparticles underwent an in situ transformation, resulting in the formation of nanoflakes that were uniformly dispersed throughout the material.This phenomenon is illustrated in Figure 6c and S14, Supporting Information.Figure S15a, Supporting Information illustrates the elemental distribution of Co, O, C, and N at a charging potential of 1.0 V, as determined by mapping.In addition, the nanoflakes formed mixed nanoparticles (Figure 6d and S15b, Supporting Information) composed by Co, O, C, and N element (Figure S15b, Supporting Information) after Li depositing to 0 V (the triangle mark of Figure 6b).The presence of Co and O elements is observed in the mixed matrix due to the uniform production of the SEI film on its surface.However, the detection of C and N elements is limited to trace levels. [22]The stability of the SEI is crucial for achieving high-performance lithium metal batteries as it is directly influenced by its composition.XPS was utilized to provide additional verification of the surface chemical constituents of the electrodes at a specific stage.Figure 6e-h and S16,S17, Supporting Information illustrate the disparities observed in XPS spectra when comparing the application of 1 and 0 V to Co 3 O 4 -NF.The survey spectra of the samples are depicted in Figure S16a and S17a, Supporting Information illustrating the presence of three prominent peaks at about 284, 532, and 689 eV, as well as three minor peaks at 55, 400, and 781 eV.15b,19] The Co 2p peaks, as depicted in Figure 6e and S17b, Supporting Information, exhibit a central energy of 797.0 and 781.3 eV.Additionally, two satellite peaks are found at energies of 802.5 and 786.4 eV.In comparison to the signal observed in Figure 1i, the intensity of the Co signal is notably diminished.This decrease in signal strength can be attributed to the redox process, which leads to the conversion of Co 3 O 4 to CoO.Despite the Li deposit being reduced to 0 V, the CoO compound remains.15b] The peak at 533.0 eV is assigned to C─O, and the peak at 531.8 eV is assigned to C═O.The peak at 530.0 eV is assigned to Li 2 O due to the low fraction of O belong to Li 2 O, it is speculated that the fraction of Li 2 O is much lower than that of ROCO 2 Li.Furthermore, a recent study has provided evidence indicating that the presence of a C─O group tends to facilitate the production of ROCO 2 Li, while the presence of a C═O group seems to encourage the formation of Li 2 CO 3 . [11]The compound Li 2 CO 3 has greater efficacy compared to ROCO 2 Li in the development of robust, structurally sound, and highly conductive SEIs.When transitioning from a voltage of 1 V to 0 V, the proportion of C═O to C─O increases, which can be considered a useful parameter for assessing the stability of the SEI.A comparable pattern in the Li 2 CO 3 versus ROCO 2 Li ratio is evident in the C 1s spectra, as depicted in Figure S16b and S17f, Supporting Information. [23]The high-resolution XPS spectra of F 1s exhibit the presence of two distinct peaks, as depicted in Figure 6g and S17d, Supporting Information.The observed peak at 685 eV can be attributed to the presence of LiF, while the peak at 689 eV is indicative of the C─F bond.The presence of LiF in the produced SEI is apparent, indicating its favorable role in promoting homogeneous Li plating.As shown in Figure 6h, the XPS pattern can be divided into LiF (56.9 eV) and Li 2 O (55.3 eV).Notably, as the peaks of Li 2 CO 3 and ROCO 2 Li overlap with each other, it is difficult to distinguish the assignment of the peak at 54.8 eV to Li 2 CO 3 or ROCO 2 Li. [24]The observed peak intensity of LiF exhibits a relatively low value, which is in line with the high-resolution F 1s spectra as depicted in Figure 6g.Based on previous research, it has been observed that LiF is predominantly found in the inner layer of the SEI.Conversely, the outer layer of the SEI primarily consists of inorganic Li 2 CO 3 and organic ROCO 2 Li.The latter components exhibit a higher concentration, while the former is present in lower quantities. [25]Figure 6i is the schematic diagram of the oxide changes during lithium deposition.Co 3 O 4 was not the actual lithium deposition sites, which was inconsistent with most of the results reported in the literatures.The transition of the real structure and components leads to the actual lithium deposition site is not consistent with our initial material.Without a doubt, there remains a pressing need to further investigate lithiophilic, conductive, and stable hosts, as well as to devise more straightforward synthetic methods that result in optimized hybrid structures.

Conclusion
In summary, this work demonstrated a prelithiation strategy to prepare the 3D current collector with Co 3 O 4 -embedded nanoflake arrays.The enhanced lithiophilicity of the Co 3 O 4 -NF host can be substantially attributed to the predicted synergistic effects resulting from the 3D structure and the presence of embedded Co 3 O 4 nanoparticles, which have the ability to chemically react with Li.The resulting Co 3 O 4 -NF@Li anodes could effectively inhibit lithium dendrite formation and mitigate volume expansion, demonstrating a significantly extended and consistent lifespan of 800 cycles (1600 h) at 1 mA cm À2 with low overpotential and insignificant voltage fluctuation for the process of lithium stripping and plating in symmetric cells.Moreover, when considering full-cell designs utilizing LiFePO 4 cathodes, it is seen that the Co 3 O 4 -NF@Li anodes exhibit significantly enhanced cycle stability and rate capability.The experimental results and electrochemical analysis confirmed that the actual lithium deposition sites and lithium deposition process on Co 3 O 4 -NF.This work investigates the transition of metal oxides as a lithiophilic site of the lithium metal anode, opens a new pathway to achieve stable and safe Li metal anodes for high-energy-density batteries, and offers valuable insights for the development of dendrite-free lithium metal anodes in advanced lithium metal batteries.

Experimental Section
Materials Synthesis: The NF was trimmed to dimensions of 10 Â 8 cm and subjected to multiple cleaning cycles using acetone, ethanol, diluted hydrochloric acid, and deionized water.This process aimed to eliminate any surface contaminants and oxide layers.Subsequently, the NF was submerged in a solution consisting of 40 mL of 2-methylimidazole (0.4 M) and 40 mL of Co(NO 3 ) 2 (0.05 M) for a duration of 8 h.Subsequently, the NF was extracted, subjected to a thorough rinsing with deionized water, and subsequently subjected to vacuum drying at a temperature of 60°Celsius.In order to acquire Co 3 O 4 -NF, the initially prepared ZIF-L/NF material was subjected to annealing in an air environment at a temperature of 350 °C for a duration of 2 h, with a gradual increase in temperature at a rate of 5 °C min À1 .
Materials Characterization: XRD patterns were measured by a Bruker D8 X-ray diffractometer with Cu Kα radiation (λ = 0.15406 nm).XPS data were collected using a K-Alpha1063 spectrometer with Al Kα radiation at 6 mA and 12 kV.To observe the morphology and investigate the structure, scanning electron microscopy (SEM) and TEM were carried out on ZEISS Sigma 300 and JEOL JEM-2100F, respectively.
Electrochemical Measurements: All the coin-type cells were assembled in an Ar-filled glove box and Celgard membranes K2045 were as the separator.The half cells (CR2025) were used to investigate the lithium plating/ stripping performance and composed of Li foils as the counter/reference electrode and as-prepared current collectors as the working electrode.
The electrolyte was containing 1 M bis (trifluoromethane) sulfonamide lithium salt (LiTFSI) in a mixture of 1,2-dimethoxyethane (DME) and 1,3dioxolane (DOL) (v/v, 1:1) with 5 wt% LiNO 3 .Galvanostatic charge/ discharge cycles performance was performed on a Neware CT-3008 W battery testing system.The batteries were first cycled between 0.01 and 1 V at 0.25 mA cm À2 for three times to stabilize the SEI and remove surface contaminations.For the cycling stability measurement, lithium was plated on the substrate at 0.4 mA cm À2 for 0.5 h and subsequently stripped at the same current density until the cutoff potential reached 1 V (versus Li þ / Li) in each cycle, respectively.CV measurements were tested on a CHI660D electrochemical workstation between 0.01 and 0.3 V. EIS was collected by a Princeton Parstat 2273 workstation from 10 5 to 10 À2 Hz.Symmetric cells (CR2025) were employed to evaluate the cycling stability and cycle life of the Li anodes on different current collectors.For the cycling stability measurement, lithium was plated on the substrate at 0.4 mA cm À2 for 0.5 h and subsequently stripped at the same current density until the cutoff potential reached 1 V (versus Li þ /Li) in each cycle, respectively.Symmetrical cells were constructed and subjected to multiple Li plating and stripping processes, lithium was plated on the substrate at 0.5 mA cm À2 for 2, 6, 10, and 20 h, respectively, and then the lithium plated electrodes were extracted from the half cells and the symmetrical cell was reassembled with two identical electrodes.The symmetric cells were cycled at 1 mA cm À2 for 1 h.
For the full cell (CR2025) test, the lithium anodes obtained from the half-cell were subjected to a purification process involving the use of diethyl carbonate followed by drying under the controlled environment of a glovebox chamber.The fabrication process of the LiFePO 4 cathode involved the combination of LiFePO 4 , acetylene black, and polyvinylidene difluoride (in a weight-to-weight ratio of 8:1:1) using N-methyl-2-pyrrolidone as the solvent.The cells underwent charging and discharging cycles within the voltage range of 2.5-4.0V, while varying the current densities.

Figure 1 .
Figure 1.a) Schematic illustration of fabrication process of Co 3 O 4 -coated nickel foam.SEM images of b,c) Co 3 O 4 -NF.d) XRD patterns of Co 3 O 4 -NF and e) TEM and f ) HRTEM images of Co 3 O 4 -NF.g) Raman spectra of Co 3 O 4 -NF and survey XPS spectra of Co 3 O 4 -NF and h) high-resolution XPS spectra of i) Co 2p and j) O 1s.

Figure 3 .
Figure 3. Cross-sectional view and top-view SEM images of sample Li plating at capacity of 1 mA h cm À2 and current density of 0.5 mA cm À2 : a,b) NF and c,d) Co 3 O 4 -NF.e-h) SEM images of sample at current density of 0.5 mA cm À2 and at increasing plating capacity of 5 mA h cm À2 .Cross-sectional view and top-view SEM images of sample current collectors after Li stripping from capacity of 5 to 1 mA h cm À2 at current density of 0.5 mA cm À2 : i,j) NF and k,l) Co 3 O 4 -NF.

Figure 4 .
Figure 4. a) Voltage versus time profile of Co 3 O 4 -NF||Li cell and corresponding illustration and b) CEs of Li cycling on a Co 3 O 4 -NF substrate.c) Initial Li plating voltage profile of sample current collectors at 1 mA cm À2 with a capacity of 6 mAh cm À2 .d) Voltage versus time profile of Co 3 O 4 -NF||Li cell via Aurbach CE evolutions and corresponding illustration with lithium loss on Co 3 O 4 -NF.

Figure 5 .
Figure 5. a) Galvanostatic discharge/charge voltage profiles of different samples in symmetric coin cells at 1 mA cm À2 .Nyquist plots of different symmetric cells b) before the 1st cycle and c) after 100 cycles.d) Voltage profiles at different cycle numbers.Typical charge/discharge curves of the Co 3 O 4 -NF@Li||LFP and NF@Li||LFP cells at 1C. e) Cycling performances of Co 3 O 4 -NF@Li||LFP and NF@Li||LFP cells and f ) voltage profiles of Co 3 O 4 -NF@Li|| LFP cells with different cycles.g) Rate performances of Co 3 O 4 -NF@Li||LFP and NF@Li||LFP cells.

Figure 6 .
Figure 6.a) The CV curve of Co 3 O 4 -NF current collector.b) Voltage-time curves of Co 3 O 4 -NF current collector.Charging to 1 V at the pentagram mark of Co 3 O 4 -NF current collector.c) TEM of charge to 1 V of Co 3 O 4 -NF current collector.d) TEM of Depositing Li at the triangle mark of Co 3 O 4 -NF current collector.e-h) The XPS curves of Co 2p, O 1s, F 1s, and Li 1s.i) A Schematic diagram of the oxide changes during lithium deposition.