Ultrathin and Air-Stable Lithium Metal Anodes with Superlong Cycling Life in Ether/Ester-Based Electrolytes

Ultrathin and air-stable Li metal anodes hold great promise toward high-energy and high-safety Li metal batteries (LMBs). However, the application of LMBs is technically impeded by existing Li metal anodes with large thickness, high reactivity, and poor performance. Here, we developed a novel and scalable approach for the construction of a 10-l m-thick ﬂ exible and air-stable Li metal anode by conformally encapsulating Li within a multifunctional VN ﬁ lm. Speci ﬁ cally, the highly lithiophilic VN layer guides a uniform deposition of Li, while abundant and multilevel pores arising from assembly of ultrathin nanosheets enable a spatially con ﬁ ned immersion of metallic Li, thus ensuring an ultrathin and sandwiched Li anode. More impressively, the strong hydrophobicity of VN surface can effectively improve the stability of anode to humid air, whereas the highly conductive framework greatly boosts charge transfer dynamics and enhances Li utilization and high-rate capability. Bene ﬁ ting from such fascinating features, the constructed Li-VN anode exhibits ultrastable cycling stability in both ether (2500 h) and carbonate (900 h) electrolytes, respectively. Moreover, even exposed to ambient air for 12 h, the anode still can retain ~ 78% capacity, demonstrating excellent air-defendable capability. This work affords a promising strategy for fabricating high-performance, high-safety, and low-cost LMBs.

Ultrathin and air-stable Li metal anodes hold great promise toward highenergy and high-safety Li metal batteries (LMBs). However, the application of LMBs is technically impeded by existing Li metal anodes with large thickness, high reactivity, and poor performance. Here, we developed a novel and scalable approach for the construction of a 10-lm-thick flexible and airstable Li metal anode by conformally encapsulating Li within a multifunctional VN film. Specifically, the highly lithiophilic VN layer guides a uniform deposition of Li, while abundant and multilevel pores arising from assembly of ultrathin nanosheets enable a spatially confined immersion of metallic Li, thus ensuring an ultrathin and sandwiched Li anode. More impressively, the strong hydrophobicity of VN surface can effectively improve the stability of anode to humid air, whereas the highly conductive framework greatly boosts charge transfer dynamics and enhances Li utilization and highrate capability. Benefiting from such fascinating features, the constructed Li-VN anode exhibits ultrastable cycling stability in both ether (2500 h) and carbonate (900 h) electrolytes, respectively. Moreover, even exposed to ambient air for 12 h, the anode still can retain~78% capacity, demonstrating excellent air-defendable capability. This work affords a promising strategy for fabricating high-performance, high-safety, and low-cost LMBs.
interphase consisting of C 60 and magnesium metal bilayers is fabricated to provide an air-stable and dendrite-free Li metal anode. [33] Liu and coworkers designed an air/water proof graphene "house" consisting of an array of vertically aligned sheets and a roof of sloping-aligned hydrophobic sheets for water-defendable lithium metal anodes. [34] Very recently, Guo's group introduced a promising method to protect Li anode from air corrosion via silane coupling agent modification. [35] From the perspective of practical application, it will exhibit great potential to endow LMBs with good safety, high-energy and high utilization of Li resource by simultaneously integrating the advantages of the ultrathin thickness and air-stable feature with Li metal anodes. Unfortunately, to the best of our knowledge, no research of ultrathin and airstable lithium metal anodes has been reported so far.
In this contribution, we proposed a scalable and versatile strategy to fabricate ultrathin and air-stable lithium metal anodes for ultrastable Li metal batteries. Specifically, employing a tunable calendaring process, followed by a topological nitridation strategy, a free-standing, ultrathin and lithiophilic VN skeleton (10 lm) was fabricated. Afterwards, the ultrathin and air-stable Li metal anode was obtained via edge-contacting molten Li to the VN skeleton. Profiting from its highly lithiophilic nature, interconnected and porous architecture of the VN skeleton, molten Li preferentially infuses into the internal channels between the VN nanosheets and form the conformally encapsulated, ultrathin and robust anode. Impressively, the designed Li-VN anode exhibits a lowvoltage hysteresis, high coulombic efficiency, and extraordinary cycling stability for over 2500 h in the ether electrolyte and 900 h in the carbonate electrolyte at a current density of 1 mA cm À2 , markedly outperforming the stateof-the-art results. In addition, even after exposure to humid air (33% relative humidity) for 12 h, the Li-VN anode still can deliver~78% reversible capacity, revealing a superior air stability. The developed dendrite-free and airstable lithium metal anode accompanied with ultralong cycling stability in ether/ester-based electrolytes may hold potential for future highenergy LMBs with low cost and high safety.

Results and Discussion
The typical synthesis procedure of the ultrathin and air-stable Li-VN anode is schematically depicted in the Figure 1a. Firstly, the flexible V 2 O 5 with micrometer-scale thickness is produced by a controlled calendaring procedure (I). Subsequently, the as-prepared ultrathin and lithiophobic V 2 O 5 film was transformed into a highly conductive and lithiophilic VN film as the lithium skeleton through a topological nitridation strategy (II). Finally, the ultrathin and air-stable Li-VN electrode was obtained via a facile edge-contacting infusion approach (III). As shown in Figure 1b, the fluffy V 2 O 5 precursor was converted to the ultrathin V 2 O 5 film (~10 lm thickness) after a controllable calendaring process. Interestingly, despite the compression and nitridation process, abundant internal channels still reserve in the ultrathin VN film (Figure 1c), which will allow for feasible infiltration of the molten Li and conformally integrate within the interlayer of the VN framework. Furthermore, the ultrathin VN skeleton can well retain the smooth surface of the pristine V 2 O 5 during the phase transition procedure (Figure 1d,e), illustrating the good structural stability of the constructed VN film.
The SEM image in Figure 2a reveals the successful synthesis of the ultrathin VN nanosheet. The obtained VN nanosheet has a lateral size of several micrometers with smooth and well-defined layer-like structure. The TEM image clearly indicates a highly porous architecture for the obtained VN nanosheet (Figure 2b, as shown by red circles). Abundant mesopores generated inside the VN nanosheet are also confirmed by Brunauer-Emmett-Teller surface area analysis ( Figure S1, Supporting Information), and the pore size distribution is mostly ranged from 10 to 20 nm, which is advantageous for molten Li spontaneously penetrating the interlayer and forming the robust sandwiched anode.
The high-resolution TEM image in Figure 2c explicitly displays the interplanar spacing of 2.4 A, which corresponds to the (200) crystal facet of the VN. The existence and uniform distribution of N and V elements in the VN nanosheet were clearly revealed by energy-dispersive X-ray spectroscopy (EDS) (Figure 2d-f). Furthermore, the XRD analysis  (Figure 2g). [36] The surface composition and chemical state of the VN sample were further investigated by X-ray photoelectron spectroscopy. The characteristic peaks of the V-N bond can be seen from the V-2p spectrum in Figure 2h, demonstrating the formation of VN after the topological nitridation process. In addition, the observed V-O and V-N-O peaks in the spectrum are caused by the surface oxidization of VN when it is handled for the XPS test. Figure 2i shows the typical peak for the N-V bond at 396.8 eV. These results demonstrate that the topological nitridation treatment induced a completely phase transition from the V 2 O 5 nanosheet to the VN nanosheet.
The desired Li-VN composite anode is constructed by edgecontacting an ultrathin VN film with molten lithium, which promotes a fast capillary absorption of molten lithium into the interior voids of the VN skeleton ( Figure S2, Supporting Information). The fast infusion behavior of molten Li benefits from the high lithiophilicity and capillary effect ensured by abundant and multilevel pores in the VN film assembled by ultrathin layers. In contrast, the infusion procedure is not achievable with the lithiophobic V 2 O 5 film. Such edge-contact Li absorption process can effectively avoid an inordinate immersion of the ultrathin VN film with excessive molten lithium and store lithium only within the internal channels of the VN interlayer, instead of on the outer surface ( Figure 3a and Figure S2, Supporting Information). Hence, the Li-VN film can easily reserve its micrometer-scale thickness with controllable capacities, compared with thick commercialized Li metal film (with a thickness of 300 lm, Tianjin Zhongneng Lithium Industry Co., Ltd, Figure 3b). With our developed approach, an ultrathin (10 lm) and free-standing Li-VN film can be reliably constructed, demonstrating a promising strategy for designing ultrathin Li anodes for the lithium metal battery. More importantly, during the calendaring procedure, the thickness of the used V 2 O 5 film can be succinctly regulated by setting the distance between the rollers, consequently, the porous VN film with a thickness ranging from 10 to 40 lm can be easily achieved. With edge-contacting immersion of molten Li into the ultrathin film, free-standing Li-VN electrodes with controllable thicknesses (10,20,30, and 40 lm, respectively) are easily produced accordingly. Such novel and feasible strategy enables the rational design and controllable preparation of ultrathin Li anodes with desired thickness and capacity. When infused lithium is stripped completely, these micrometer-thick Li-VN electrodes enable tunable area capacities: 1.761, 3.722, 6.221, and 8.212 mAh cm À2 (corresponding to the 10-, 20-, 30-, and 40-lm-thick Li-VN films, respectively; Figure 3c). The electrochemical characteristics and voltage-capacity profiles of the Li-VN electrodes are identical to those of bare Li metal electrode, indicating that battery operation voltage is not compromised.
As shown in Figure 3d, the ultrathin VN film retains a much larger water contact angle (101.6°) than that of a commercial Cu foil (Figure S3, Supporting Information), suggesting that VN has a highly hydrophobic surface. Such superior hydrophobicity of VN layer can effectively enhance the resistance of the Li anode toward humidity. To confirm the stability in harsh condition, the crystal structure of the Li-VN electrodes with different exposure time to humid air (33% relative humidity) was investigated by XRD, and the obtained results are shown in Figure 3e. Interestingly, the Li-VN after exposure for 12 h in humid air shows much weaker diffraction peaks of LiOH compared with that of the bare Li foils ( Figure S4, Supporting Information), certifying that the VN layer can efficiently separate Li from the humid air to some extent.
To further prove the viability of the shielding effect of the VN skeleton toward harsh condition, the stripping voltage-specific capacity curves of the Li-VN electrodes after exposed to humid air for various hours are shown in Figure 3f. In the lithium stripping process, the specific capacity retention was as high as 78% after exposed to air for 12 h, demonstrating its good air-resistant capability. Symmetric cells assembled with air-exposed electrodes were employed to evaluate the effect of air exposure on their electrochemical performance. Figure S5a, Supporting Information, shows that the Li-VN electrode still has good charge-discharge performance after being exposed to air for 2 h. After exposure for 6 h, the overpotential of Li metal deposition and stripping will gradually increase, which is due to the increased electrode oxidation. In contrast, the bare Li foil electrode has completely failed after being exposed to air for 2 h ( Figure S5b, Supporting Information). These results indicate that the Li-VN electrode possesses good air stability.
In order to understand the lithiophilic mechanism of the VN for Li atom anchoring, density functional theory (DFT) calculations were performed to reveal the adsorption of Li atoms on the surface of the VN and Cu configurations. The calculated adsorption energies of Li on the VN and Cu surfaces are 1.98, 0.71 eV, respectively (Figure 4a,b). Unsurprisingly, VN host shows much higher value for adsorbing Li atom, implying the highly lithiophilic nature of the VN layer, which is favorable to guide the homogeneous deposition and dendrite-free growth of Li metal.
Electrochemical plating and stripping behaviors of the ultrathin VN host were investigated systematically under various current densities and area capacities using CR2032-type coin cells. Comparable tests were also performed under the same conditions using planar Cu electrodes. As shown in Figure 4c, the lithium nucleation behavior on the ultrathin VN film was investigated at 1 mA cm À2 for the initial deposition process. The nucleation overpotential (Dg = g n -g p , inset of Figure 4c) is a key parameter to evaluate the nucleation and deposition barriers. [37,38] The VN host presents an overpotential of 23 mV, which is substantially lower than that of the V 2 O 5 (68 mV) and bare Cu (188 mV), suggesting that the VN host possesses superior lithiophilicity for fostering uniform Li nucleation. The result is in qualitative agreement with the theoretical analysis in the Figure 4a,b. Figure 4d shows that the lithium metal is conformally encapsulated within the internal channels of the VN layer during the electrochemical deposition process. Unsurprisingly, no metallic lithium was observed on the outer surface of the VN layer after the plating process (Figure 4e). These results manifestly suggest that the lithium ions are preferentially attracted by the highly lithiophilic VN nanosheets and completely permeate the internal cavities between the porous VN layers. In this sense, the highly porous VN layers provide an ideal platform for the spatially confined storage of metallic lithium metal and act as the robust protective layer.
The coulombic efficiency (CE), calculated by the ratio of totally stripped Li during charging to the plated Li during discharging in each cycle, is one of the essential metrics for evaluating the long-term viability and practicality of an electrode for LMBs. We explore the CE for the ultrathin VN electrode, V 2 O 5 electrode, and the control planar Cu electrode at various current densities and area capacities. As shown in Figure 4f, after a gradual growth for the initial period, the CE of the VN at 0.5 mA cm À2 finally stabilizes at above 98% for over 300 cycles. Furthermore, when the VN electrode is cycled at a higher current density of 1 mA cm À2 with a larger Li areal capacity of 1 mAh cm À2 , it still demonstrates stable plating and stripping behaviors. In contrast, the CE of the planar Cu electrode suffers from fast decay after 170 and 50 cycles, respectively, which may be a consequence of electrochemical inactivation of the deposited lithium. In addition, the CE of V 2 O 5 electrode fluctuates intensely from the beginning, so it cannot be practically used. The stable CE indicates that the VN skeleton can offer uniformly dispersed lithophilic sites as well as structural integrity to tolerate the volume dilation of the Li anode. The voltage-capacity curves of the Li¦¦VN and Li¦¦Cu half-cells at 1 mA cm À2 are shown in Figure 4g,h. Benefiting from its highly lithiophilic feature, the VN electrode only shows a 32 mV voltage polarization, which is much smaller than that of the lithiophobic Cu electrode (80 mV) and V 2 O 5 electrode (118 mV, Figure S6, Supporting Information), certificating a faster Li deposition/stripping kinetics. Further investigation of the electrochemical performance was carried out by using the Li-VN and bare Li anodes to assemble symmetric cells. The voltage versus time curve for symmetric cells in ether electrolyte with an areal capacity of 1 mAh cm À2 at 1 mA cm À2 is shown in Figure 5a. It is interesting to note that the Li-VN electrode shows a lower voltage hysteresis and excellent cyclability for over 2500 h without obvious voltage oscillations (Figure 5b). However, the bare Li counterpart electrode and Li-V 2 O 5 electrode exhibit a significantly larger voltage hysteresis from the beginning, which could be attributed to the inhomogeneous Li deposition on the uneven surface of bare Li foil or lithiophobic V 2 O 5 film. The insets in Figure 5a show the enlarged galvanostatic plating/stripping profiles of the 200th, 600th, and 1000th cycles for a detailed comparison. The Li-VN anode exhibits a flat voltage plateau and a low-voltage hysteresis during the electrochemical plating/stripping procedures. More importantly, the voltage profile still can remain a stable voltage plateau even after 1000 cycles. This cycling stability is superior to those of previously reported symmetric cells with lithium metal anodes. Furthermore, the symmetric cell of the Li-VN electrode shows a smaller voltage hysteresis (~17 mV) for more than 1200 cycles (Figure 5b), while the bare lithium metal electrode displays an increased voltage hysteresis during the plating/stripping processes. Since the voltage hysteresis reflects the voltage difference between a plating voltage and a stripping voltage, smaller voltage hysteresis normally indicates better plating/stripping behavior and lower resistance of charge transfer. Consequently, the large and fluctuant voltage hysteresis on the bare Li electrode was because of the unstable interface between the bare Li anode and electrolyte. The rate capability of the Li-VN and bare Li electrodes are also evaluated in Figure 5c. Impressively, the Li-VN anode still works stably even at a high current density of 10 mA cm À2 with a voltage hysteresis of only 43.7 mV, demonstrating its outstanding high-rate capability. We also compare electrochemical properties of our developed Li-VN electrode in this work with the state-of-the-art results on Li metal anodes with a thickness less than 100 lm (Figure 5d and Table S1, Supporting Information). [26,27,[39][40][41][42][43][44][45][46][47][48][49][50] Notably, the long-term cycling stability and thin thickness of our proposed Li-VN electrode is significantly superior to those of most reported Li metal anodes.
Considering the inferior thermal stability, low anodic stability, and high cost of ether-based electrolytes, which severely restricted their wide applications, appraising its cycling stability of LMAs in the esterbased electrolytes is highly needed. The long-cycle stability of the Li-VN anode in the corrosive ester-based electrolyte was also examined using symmetric cells (Figure 5e,f). More interestingly, the symmetric cell of Li-VN electrode exhibits distinguished long-term cyclability (over 900 h) with an overpotential of only~22 mV. In contrast, the bare Li electrode shows sharply increased voltage polarizations with random voltage oscillations even from the initial cycle, which is attributed to unstable interface and severe side reactions of the electrode/electrolyte in the carbonate electrolyte. In the inset of Figure 5e, the magnified voltage profiles of the Li-VN in selected cycles demonstrate that the Li-VN anode cycles with a relatively low overpotential and a steady voltage plateau. Furthermore, the VN host still can exhibit a small overpotential (32 mV) and stable coulombic efficiency in the carbonate electrolyte ( Figure S7, Supporting Information) as well. Conversely, the planar Cu electrode has a much higher overpotential (210 mV), large polarizations, and random voltages ( Figure S7, Supporting Information), which is attributed to the poor Li/electrolyte interface and electrical disconnection caused by the repetitive accumulation of dead lithium. Correspondingly, the interfacial stability between Li anode and ester-based electrolyte was explored and analyzed by the in-situ electrochemical impedance spectroscopy (in-situ EIS) study (Figures S8-S10, Supporting Information) and the morphological evaluation after lithium deposition in the ester-based electrolyte (Figures S11 and S12, Supporting Information). The results demonstrate the superiority of the unique VN film for the conformal protective of the Li metal during cycling in the ester-based electrolytes.
The Li-VN film and bare Li foil anodes were also paired with commercial LiFePO 4 (LFP) cathode (mass loading: 2.0 mg cm À2 ) to assemble a full battery and further investigate these extraordinary features and the performance of the ultrathin Li-VN foils for practical applications. In comparison with the bare Li¦¦LFP battery, the Li-VN¦¦LFP cell retains a reversible capacity of 150 mAh g À1 after 100 cycles at 1 C. (Figure 6a,b). Moreover, Figure 6c,d shows the rate capability of the Li-VN¦¦LFP full cell and the control Li¦¦LFP full cell. The Li-VN¦¦LFP battery has substantially higher discharge capacities than that of the bare Li¦¦LFP battery, with 155, 140, 121, 104, and 151 mAh g À1 at 0.5, 1, 2, 3, and 0.5 C, respectively, suggesting the superior high-rate performance of the Li-VN¦¦LFP cell.

Conclusion
In summary, a 10-lm-thick flexible and air-stable Li metal anode is fabricated through a scalable and tunable calendaring process combined with a topological nitridation strategy. The distinctive characteristics of the designed VN skeleton endow outstanding properties for the lithium metal anode. Particularly, the remarkable lithiophilicity and abundant nucleation sites guide even Li stripping/plating and dendrite-free growth of Li metal, whereas the highly interconnected and porous architecture are favorable for stimulating a spatially confined immersion of metallic Li within the interlayer of the VN nanosheets, and creating an ultrathin and sandwiched Li anode with robust structure. More importantly, the intrinsically hydrophobic nature of the VN surface enables a superior air resistance. In addition, the highly conductive network will greatly promote the charge transfer kinetics and enhance the Li utilization and high-rate capability. As a result, the constructed anode demonstrates excellent electrochemical properties in both ether and carbonate electrolytes. In addition, the anode also exhibits outstanding stability when exposed to humid air for 12 h. Notably, the assembled full cell shows superior performance as well. The proposed Li metal anode featuring with ultrathin thickness, good air stability and high performance simultaneously may provide a proof-of-concept design for constructing high-energy, high-safety and low-cost Li metal batteries.

Experimental Section
Detailed information related to the synthesis of active electrodes, physicochemical characterization, and electrochemical evaluation of bifunctional electrodes towards UOR and supercapacitor application is provided in Supporting Information.