Uniform Lithium Deposition Induced by ZnFx(OH)y for High-Performance Sulfurized Polyacrylonitrile-Based Lithium-Sulfur Batteries

Lithium metal batteries are emerging as the next generation of high-density electrochemical energy storage systems because of the ultra-high specific capacity and ultra-low electrochemical potential of the Li metal anode. However, the uneven Li deposition on commercial Cu current collectors result in low Coulombic efficiencies (CEs) and poor cycle life. In this research, we proposed the modification of ZnFx(OH)y on Cu foils to expand the lifespan. As-generated ZnLi alloy and LiF could promote uniform Li nucleation and deposition, thus resulting in an improved Li plating/stripping CE and extended cycle life. The Li-S battery with sulfurized polyacrylonitrile cathode and Li-ZnFx(OH)y@Cu anode (N/P ratio of 1.5:1) maintains 95% capacity after 60 cycles, proving the feasibility of ZnFx(OH)y@Cu for practical applications.


Introduction
Lithium-ion batteries (LIBs) have been dominating the electrochemical energy storage market due to their light weight, high energy density, and long service life [1,2]. However, the current commercial LIB cathode and anode materials are approaching their capacity limit and it is quite difficult to achieve further improvement. With the rapid development of electric vehicles and portable electronics, the large-scale development of LIBs is constrained, so the demand for high-density energy storage systems is increasingly urgent [3,4]. Lithium metal anodes (LMAs) have attracted extensive research attention because of their ultrahigh specific capacity (3860 mAh g −1 ) and ultra-low electrochemical potential (−3.04 V vs. standard hydrogen electrode) [5,6]. Thus, as-fabricated lithium metal batteries (LMBs) are regarded as one of the most promising next-generation battery technologies.
The practical application of LMBs still has great challenges. LMAs are highly reactive with conventional liquid electrolytes and undergo continuous side reactions with liquid electrolytes to generate solid electrolyte interfaces (SEIs). During the lithium plating/stripping process, LMAs bear a large volume change, resulting in continuous formation and destruction of SEIs [7,8]. Moreover, the uneven deposition of Li is prone to the formation of Li dendrites, which can penetrate the separator and cause internal short circuits in the battery cell [9,10]. Therefore, low Coulombic efficiency (CE) and safety issues hinder
In addition, the ZnF x (OH) y -modified layer can also improve the interfacial stability and inhibit the side reaction between LMAs and the electrolyte, thereby improving the CE of lithium plating/stripping and prolonging the cycle life [29]. Lithium metal was electrodeposited on ZnF x (OH) y @Cu current collector, and the formed Li/ZnF x (OH) y @Cu anode was assembled with pPAN/SeS 2 cathode to form a lithium-sulfur (Li-S) battery. Under the condition of high cathode mass loading (~7 mg cm −2 ), lean electrolyte (20 µL) and limited lithium source (N/P = 1.5), the as-fabricated Li-S cell shows excellent cycling stability. It maintains as high as 95% of the original specific capacity after 60 cycles, thus proving the great potential of ZnF x (OH) y @Cu for advancing practical applications of Li-S batteries. , lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), 1,3 dioxolane (DOL) and 1,2-dimethoxyethane (DME) were all purchased from TCI. Selenium disulfide (SeS 2 ) and polyacrylonitrile (PAN) were both purchased from Sigma-Aldrich. Lithium foil (Li), aluminum foil (Al), copper foil (Cu), separator (Celgard 2400), carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), and carbon black SUPER C45 (SP) were purchased from Hefei Kejing Material Technology Co., Ltd.

Preparation of ZnF x (OH) y @Cu
First, a plastic beaker was filled with 200 mL deionized water, and 3.04 g of ZnF 2 ·4H 2 O particles were taken and dispersed in the above deionized water by ultrasonic stirring until the solution became clear. Then, 5 * 5cm copper foil was washed with 50 mL of 1 mol/L dilute hydrochloric acid to remove surface impurities, and added to the above ZnF 2 ·4H 2 O aqueous solution, which was mechanically stirred at room temperature for 3 h. Finally, the modified copper foil was washed with deionized water and ethanol 3-5 times to remove the alkaline salt on the modified copper foil, and the product obtained was named as ZnF x (OH) y @Cu. After the experiment, some lime water was prepared. The hydrolyzed HF was neutralized with lime water to produce non-toxic calcium fluoride precipitation.

Preparation of pPAN/SeS 2
PAN and SeS 2 powder (1:4 w/w) were sealed in glass containers and incubated at 380 • C under argon for 8 h. The samples were then placed in a porcelain boat, kept at 350 • C for 6 h under flowing nitrogen to remove excess sulfur and selenium, and then naturally cooled to obtain the pPAN/SeS 2 product.

Characterization
Scanning electron microscope (SEM, HITACHI-SU8220) was used to observe the microstructure and element distribution of ZnF x (OH) y @Cu. Elemental analysis was performed on an energy dispersive X-ray spectroscopy (EDX) spectrometer connected to a HITACHI-SU8220. The phase composition of ZnF x (OH) y @Cu was further determined by X-ray diffraction (XRD, SmartLab 9 kW). The test range was 10~80 • , the scanning rate was 5 • /min, and the Cu target was used in a continuous scanning mode. To observe the morphological changes of Li deposition after cycling of Cu and ZnF x (OH) y @Cu. The cells were disassembled and the lithium-coated anodes were rinsed with DME to remove electrolyte residues and LiTFSI salts on Li/Cu and Li/ZnF x (OH) y @Cu, and then dried for 24 h before characterization for testing. All procedures were performed in an argon-filled glove box.

Electrochemical Measurements
All electrochemical properties were measured using CR2032 coin cells, which were assembled in an argon-filled glove box with both O 2 and H 2 O below 0.1 ppm. To evaluate Li plating/stripping efficiency, Li/Cu and Li/ZnF x (OH) y @Cu half cells were assembled using Cu or ZnF x (OH) y @Cu as the working electrode (ϕ16 mm) and Li foil (ϕ15.5 mm) as the counter/reference electrode. The cells were first cycled five times at 50 µA in the voltage range of 0-1 V (vs. Li + /Li), followed by a long-term cycling test at a current density of 0.5 mA cm −2 and a lithium deposition capacity of 1 mAh cm −2 . For the full cell test, the cathode was composed of pPAN/SeS 2 , SP, and CMC/SBR mixed in a mass ratio of 8:1:1, and the mass loading of the cathode was~7 mg cm −2 . The Li-deposited ZnF x (OH) y @Cu was used as the anode, which was assembled with the high-load cathode to form a full cell. The applied current density during full cell cycling was 0.1 A g −1 , the working potential window was 1 to 3 V, and the utilized electrolyte volume was~20 µL. The electrolyte used in the cells was the solution of 1 M LiTFSI dissolved in DOL and DME (v/v = 1:1) with 3.7 wt% LiNO 3 as additive, and the separator was Celgard 2400.

Results and Discussion
The ZnF x (OH) y @Cu current collector was fabricated through in situ hydrolysis of ZnF 2 copper foil: ZnF 2 + H 2 O → ZnF x (OH) y + HF ( Figure 2a). The morphologies of the as-prepared ZnF x (OH) y @Cu current collectors were investigated by SEM. As shown in Figure 2b-d, after copper foil was soaked in ZnF 2 aqueous solution for 3h, polyhedral ZnF x (OH) y particles were uniformly coated over the entire copper surface. The bare copper foil and ZnF x (OH) y @Cu showed completely different colors; the surface of bare copper foil was bright orange-pink, while the surface of ZnF x (OH) y @Cu was covered with a dark red substance ( Figure S1). The morphologies of the as-prepared ZnF x (OH) y @Cu current collectors were investigated by SEM, and the results are presented in Figure 2. As shown in Figure 2b, after copper foil was soaked in ZnF 2 ·4H 2 O aqueous solution for 3h, ZnF x (OH) y particles were uniformly coated on the whole copper surface. The locally enlarged topography in Figure 2c shows that the ZnF x (OH) y particles are not uniform in size, but are regular polyhedrons in shape. Similarly, in Figure 2d, it can be observed that the polyhedral ZnF x (OH) y particles effectively coat the copper foil and enhance the adhesion performance to the copper foil current collector without the formation of aggregates. Furthermore, the EDX mapping images clearly show that Zn and F elements are uniformly distributed on the ZnF x (OH) y @Cu current collector, demonstrating the homogeneity of the ZnF x (OH) y coating ( Figure S2). The phase composition of ZnF x (OH) y @Cu was further analyzed by XRD. There are no characteristic diffraction peaks of ZnF x (OH) y in the spectrum, indicating that the deposited ZnF x (OH) y films are amorphous ( Figure S3).
To explore the electrochemical properties of ZnF x (OH) y @Cu, Li/ZnF x (OH) y @Cu half cells were assembled. Figure 3a demonstrates Coulombic efficiencies of Li plating/stripping on Cu and ZnF x (OH) y @Cu current collectors. At a current density of 0.5 mA cm −2 and areal Li deposition capacity of 1 mAh cm −2 , the first Coulombic efficiencies of the anodes based on bare Cu and ZnF x (OH) y @Cu are 97.1% and 98.5%, respectively. The lower initial Coulombic efficiency is related to the formation of a solid electrolyte interphase (SEI) layer during the first cycle. For the SEI layer on the anode of the lithium metal battery, it is generally believed that the extremely reactive metal Li reacts with the anion in the electrolyte, and the reaction products (mostly insoluble) are deposited on the surface of lithium metal, forming a passivation film thick enough to prevent electrons from passing through. The formation of the SEI layer will consume active Li in the battery, resulting in low Coulombic efficiency. During cycling, the Coulombic efficiency of bare copper began to fluctuate slightly after 60 cycles, and the average Coulombic efficiency was 98.4% over 100 cycles. The fluctuating Coulombic efficiency of Li/Cu half cells proves that the surface SEI film was continuously collapsing and regenerating, consuming a large amount of lithium, while the average Coulombic efficiency of ZnF x (OH) y @Cu can be stabilized at about 98.8% over 100 cycles, implying that nucleation sites (ZnF x (OH) y ) can promote uniform nucleation/deposition of Li and form a stable SEI layer. homogeneity of the ZnFx(OH)y coating ( Figure S2). The phase composition of ZnFx(OH)y@Cu was further analyzed by XRD. There are no characteristic diffraction peaks of ZnFx(OH)y in the spectrum, indicating that the deposited ZnFx(OH)y films are amorphous ( Figure S3). To explore the electrochemical properties of ZnFx(OH)y@Cu, Li/ZnFx(OH)y@Cu half cells were assembled. Figure 3a demonstrates Coulombic efficiencies of Li plating/stripping on Cu and ZnFx(OH)y@Cu current collectors. At a current density of 0.5 mA cm −2 and areal Li deposition capacity of 1 mAh cm −2 , the first Coulombic efficiencies of the anodes based on bare Cu and ZnFx(OH)y@Cu are 97.1% and 98.5%, respectively. The lower initial Coulombic efficiency is related to the formation of a solid electrolyte interphase (SEI) layer during the first cycle. For the SEI layer on the anode of the lithium metal battery, it is generally believed that the extremely reactive metal Li reacts with the anion in the electrolyte, and the reaction products (mostly insoluble) are deposited on the surface of lithium metal, forming a passivation film thick enough to prevent electrons from passing through. The formation of the SEI layer will consume active Li in the battery, resulting in low Coulombic efficiency. During cycling, the Coulombic efficiency of bare copper began to fluctuate slightly after 60 cycles, and the average Coulombic efficiency was 98.4% over 100 cycles. The fluctuating Coulombic efficiency of Li/Cu half cells proves that the surface SEI film was continuously collapsing and regenerating, consuming a large amount of lithium, while the average Coulombic efficiency of ZnFx(OH)y@Cu can be stabilized at about 98.8% over 100 cycles, implying that nucleation sites (ZnFx(OH)y) can promote uniform nucleation/deposition of Li and form a stable SEI layer.  The voltage profiles of Li plating/stripping during the first cycle of Cu and ZnFx(OH)y@Cu current collectors are shown in Figure 3b. Due to the lithiophobicity of Cu, the voltage drops to −124 mV (vs. Li + /Li) at the beginning of Li plating and then reaches a flat voltage plateau of −43 mV (vs. Li + /Li). The voltage difference between the bottom of the voltage dip and the plateau represents the overpotential barrier for heterogeneous Li nucleation deposition on the substrate [30]. The bare Cu foil exhibits a large nucleation overpotential of 54 mV. In contrast, the nucleation overpotential of ZnFx(OH)y@Cu is as low as 34 mV. These results indicate that the super-lithophilicity of ZnFx(OH)y@Cu helps to reduce the interface energy between Li and the deposition substrate and lower the heterogeneous nucleation barrier. Moreover, the ZnFx(OH)y particles react with Li to form an favorable SEI layer, which induces uniform growth of Li metal, thereby suppressing the formation of Li dendrites. The morphological evolution of Li plating/stripping on the bare copper and ZnFx(OH)y@Cu were investigated by SEM. Figure S4 shows the morphologies of bare copper and ZnFx(OH)y@Cu at a current density of 0.5 mA cm −2 and The voltage profiles of Li plating/stripping during the first cycle of Cu and ZnF x (OH) y @Cu current collectors are shown in Figure 3b. Due to the lithiophobicity of Cu, the voltage drops to −124 mV (vs. Li + /Li) at the beginning of Li plating and then reaches a flat voltage plateau of −43 mV (vs. Li + /Li). The voltage difference between the bottom of the voltage dip and the plateau represents the overpotential barrier for heterogeneous Li nucleation deposition on the substrate [30]. The bare Cu foil exhibits a large nucleation overpotential of 54 mV. In contrast, the nucleation overpotential of ZnF x (OH) y @Cu is as low as 34 mV. These results indicate that the super-lithophilicity of ZnF x (OH) y @Cu helps to reduce the interface energy between Li and the deposition Polymers 2022, 14, 4494 6 of 10 substrate and lower the heterogeneous nucleation barrier. Moreover, the ZnF x (OH) y particles react with Li to form an favorable SEI layer, which induces uniform growth of Li metal, thereby suppressing the formation of Li dendrites. The morphological evolution of Li plating/stripping on the bare copper and ZnF x (OH) y @Cu were investigated by SEM. Figure S4 shows the morphologies of bare copper and ZnF x (OH) y @Cu at a current density of 0.5 mA cm −2 and a plating capacity of 1 mAh cm −2 after 1 cycle of plating, showing the deposition of limited amounts of lithium. It can be observed that a very rough surface is displayed on the bare copper, and the Li deposition on bare copper exhibited highly porous and rough non-uniform surface morphology, on which Li particles, accompanied by a multitude of dead Li, were non-uniform in size. This moss-like Li morphology may lead to persistent side reactions with the electrolyte to accelerate Li and electrolyte consumption, whereas compared with uncontrolled growth of Li dendrites on the bare copper, the Li deposition shows uniform and dense morphology on the surface of ZnF x (OH) y @Cu, which is favorable for a good cycling stability. The improved Li deposition can be attributed to the fact that the lithiophilic ZnF x (OH) y coating can provide multiple and uniform nucleation sites, which can effectively induce smoother Li deposition and inhibit the growth of Li dendrites, thereby improving the Coulombic efficiency and cycle life of the battery. Both of surface morphology and the morphological evolution of Li plating/stripping demonstrate the better performance of the surface-modified ZnF x (OH) y @Cu, which is consistent with the electrochemical data. Furthermore, the voltage profiles of the 2, 5, 10, 50, and 100 cycles of ZnF x (OH) y @Cu show that the lithium cycling presents stable polarization at a current density of 0.5 mA cm −2 . The discharge-charge curves of Li/ZnF x (OH) y @Cu batteries almost overlap, again confirming the excellent cycling stability ( Figure S5). The above results indicate that the modified Li/ZnF x (OH) y @Cu cell has a more stable electrochemical cycling behavior. To confirm the low polarization and cycling stability of ZnF x (OH) y @Cu, we examined the resistance changes of Li/Cu and Li/ZnF x (OH) y @Cu after 5 cycles using electrochemical impedance spectroscopy (EIS), to analyze internal resistance and interface stability. The radius of the depression in the high frequency region usually reflects the value of the charge transfer resistance (R ct ). Compared with the bare copper current collector, the interfacial charge resistance value of the lithiated ZnF x (OH) y @Cu electrode is significantly lower, indicating that lithium ions form a stable and thin SEI layer on the surface of the ZnF x (OH) y @Cu electrode, and the lithium ions form a stable and thin SEI layer on the surface of the ZnF x (OH) y @Cu electrode. The kinetics of ions during cycling are improved and the transport in the SEI layer is faster. Therefore, the improved cycling stability and CE can be ascribed to more reaction sites provided by the lithiophilic ZnF x (OH) y to induce uniform Li deposition, and the ZnF x (OH) y @Cu electrode has higher lithium ion diffusivity than the bare copper electrode ( Figure S6). N/P ratio refers to the areal capacity of the Li anode by that of the sulfur cathode [31]. A Li-S cell with maximized energy density should operate at an N/P ratio of 1 [32]. However, excessive Li is normally required to offset the Li loss from electrolyte consumption and SEI formation [33]. Here, we report polyacrylonitrile (PAN)/selenium disulfide (pPAN/SeS 2 ) as a cathode material for Li-S batteries. The pPAN/SeS 2 was synthesized from a mixture of SeS 2 and PAN (4:1 by weight) under heat treatment at 380 • C. As shown in Figure 4a, SEM image shows the round morphology of pPAN/SeS 2 particles, and most of the particles agglomerate into large clusters. The phase structure of pPAN/SeS 2 was investigated by XRD. As shown in Figure 4b, the XRD pattern of PAN has a characteristic diffraction peak at 17 • , which corresponds to the (110) crystal plane of the PAN crystal structure. The XRD pattern of SeS 2 does not show a characteristic peak. In the XRD pattern of pPAN/SeS 2 , only a distinct broad peak appeared at 25 • , demonstrating the amorphous phase of carbon. No diffraction peaks corresponding to SeS 2 compounds were observed in the XRD pattern, indicating the formation of an amorphous structure in pPAN/SeS 2 [34]. In order to construct a Li-S full battery with a thin lithium anode, based on the capacity of the highly loaded pPAN/SeS 2 cathode, we calculated that when N/P = 1.5:1, the areal capacity of Cu or ZnF x (OH) y @Cu current collectors for lithium deposition is 6.0 mAh cm −2 .
Li/Cu and Li/ZnF x (OH) y @Cu cells were disassembled after deposition, and images of Cu or ZnF x (OH) y @Cu current collectors after Li deposition were compared ( Figure S7); the ZnF x (OH) y @Cu current collector was fully covered by the deposited lithium while the bare Cu current collector was not, indicating the much higher Li deposition uniformity on ZnF x (OH) y @Cu. In this research, Li-plated Cu and ZnFx(OH)y@Cu (the lithium deposition amount was 6.0 mAh cm −2 ) were fabricated as the anodes, and pPAN/SeS2 were used as the cathode. Se has better ionic/electronic conductivity than S, and can accelerate the conversion reaction kinetics, which has been demonstrated in previous research [35]. Li-S cells were constructed and tested under practical testing conditions of high cathode mass loading (~7 mg cm −2 ), lean electrolyte (E/S ratio of less than 3 μL mg −1 ), and low N/P ratio of 1.5:1. The cycling performance of both full cells were evaluated at a current density of 0.1 A g −1 . As shown in Figure 4c, the capacity of the Li/Cu|pPAN/SeS2 full cell decreased as the cycling progressed, resulting in an 88% capacity retention after 60 cycles. The capacity decline of the Li/Cu|pPAN/SeS2 full cell may be due to the uneven distribution of lithium on the bare copper foil, and as the cycling continues, lithium dendrites accumulate on the surface of the bare copper foil, resulting in the decay of the active capacity. In contrast, the Li/ZnFx(OH)y@Cu|pPAN/SeS2 full cell maintained a discharge capacity of 519 mAh g −1 after 60 cycles with a capacity retention rate of 95%, which was significantly better than that of the Li/Cu|pPAN/SeS2 full cell, with a CE of ~100%. It can be seen that a uniform and stable Li-deposited interfacial layer will contribute to the cycling stability of the battery.
The discharge-charge curves of the Li/ZnFx(OH)y@Cu|pPAN/SeS2 full cell at 0.1 A g −1 are shown in Figure 4d. The Li/ZnFx(OH)y@Cu|pPAN/SeS2 full cell exhibits a very stable voltage profile during successive discharge-charge cycles. At the 50th cycle, the Li/ZnFx(OH)y@Cu|pPAN/SeS2 full cell showed a more stable voltage plateau and lower voltage polarization than the Li/Cu|pPAN/SeS2 full cell, indicating a significant kinetics improving ( Figure S8). The above results indicate that the uniform Li layer in the full cell reduces the subsequent consumption of limited Li and electrolyte due to the stable interface and low Li nucleation barrier of ZnFx(OH)y@Cu. Therefore, the modified Li/ZnFx(OH)y@Cu|pPAN/SeS2 full cell can maintain high capacity retention, which is In this research, Li-plated Cu and ZnF x (OH) y @Cu (the lithium deposition amount was 6.0 mAh cm −2 ) were fabricated as the anodes, and pPAN/SeS 2 were used as the cathode. Se has better ionic/electronic conductivity than S, and can accelerate the conversion reaction kinetics, which has been demonstrated in previous research [35]. Li-S cells were constructed and tested under practical testing conditions of high cathode mass loading (~7 mg cm −2 ), lean electrolyte (E/S ratio of less than 3 µL mg −1 ), and low N/P ratio of 1.5:1. The cycling performance of both full cells were evaluated at a current density of 0.1 A g −1 . As shown in Figure 4c, the capacity of the Li/Cu|pPAN/SeS 2 full cell decreased as the cycling progressed, resulting in an 88% capacity retention after 60 cycles. The capacity decline of the Li/Cu|pPAN/SeS 2 full cell may be due to the uneven distribution of lithium on the bare copper foil, and as the cycling continues, lithium dendrites accumulate on the surface of the bare copper foil, resulting in the decay of the active capacity. In contrast, the Li/ZnF x (OH) y @Cu|pPAN/SeS 2 full cell maintained a discharge capacity of 519 mAh g −1 after 60 cycles with a capacity retention rate of 95%, which was significantly better than that of the Li/Cu|pPAN/SeS 2 full cell, with a CE of~100%. It can be seen that a uniform and stable Li-deposited interfacial layer will contribute to the cycling stability of the battery.
The discharge-charge curves of the Li/ZnF x (OH) y @Cu|pPAN/SeS 2 full cell at 0.1 A g −1 are shown in Figure 4d. The Li/ZnF x (OH) y @Cu|pPAN/SeS 2 full cell exhibits a very stable voltage profile during successive discharge-charge cycles. At the 50th cycle, the Li/ZnF x (OH) y @Cu|pPAN/SeS 2 full cell showed a more stable voltage plateau and lower voltage polarization than the Li/Cu|pPAN/SeS 2 full cell, indicating a significant kinet- ics improving ( Figure S8). The above results indicate that the uniform Li layer in the full cell reduces the subsequent consumption of limited Li and electrolyte due to the stable interface and low Li nucleation barrier of ZnF x (OH) y @Cu. Therefore, the modified Li/ZnF x (OH) y @Cu|pPAN/SeS 2 full cell can maintain high capacity retention, which is beneficial to prolong the cycle life of Li-S batteries.

Conclusions
In summary, uniform and dense ZnF x (OH) y layers have been coated on Cu current collectors through in situ hydrolysis. ZnF x (OH) y reacts with lithium to form Li-Zn alloy and LiF, which reduces the Li nucleation energy barrier and facilitates rapid Li + diffusion at the surface of LMAs. As a result, the average Coulombic efficiency of ZnF x (OH) y @Cu can be improved to 98.8% over 100 cycles. Under practical testing conditions of high cathode mass loading (~7 mg cm −2 ), lean electrolyte (E/S ratio of less than 3 µL mg −1 ), and low N/P ratio of 1.5:1, the Li/ZnF x (OH) y @Cu|pPAN/SeS 2 full cell shows a superior cycling stability, with a capacity retention ratio as high as 95% after 60 cycles. This work sheds light on the great potential of ZnF x (OH) y modification on Cu foil for advancing practical applications of Li-S batteries.
Supplementary Materials: The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/polym14214494/s1, Figure S1. The Optical photographs of (a) bare Cu foil and (b) ZnF x (OH) y @Cu. Figure S2. (a) SEM image of ZnF x (OH) y @Cu. (b-d) EDS elemental mapping images of ZnF x (OH) y @Cu (fluorine, zinc, copper). Figure S3. Comparison of XRD patterns of copper-based current collectors before and after the reaction. Figure S4. SEM image after lithium deposition. (a) Bare Cu (b) ZnF x (OH) y @Cu (current density: 0.5 mA cm −2 ; deposition capacity: 1.0 mAh cm −2 ). Figure S5. Electrochemical Li plating/stripping curves of the ZnF x (OH) y @Cu at 0.5 mA cm −2 with a specific capacity of 1 mAh cm −2 . Figure S6. The EIS plots of the ZnF x (OH) y @Cu and bare Cu electrodes after the 5th cycles. Figure S7. Photo images of the bare Cu foil and the ZnF x (OH) y @Cu foil after Li deposition at 6.0 mAh cm −2 . Figure S8. The discharge-charge curves of Li/Cu|pPAN/SeS 2 full-cell at 0.1 A g −1 .