Poly(dimethylsiloxane) Thin Film as a Stable Interfacial Layer for High-Performance Lithium-Metal Battery Anodes

Transcription

1 Poly(dimethylsiloxane) Thin Film as a Stable Interfacial Layer for High-Performance Lithium-Metal Battery Anodes Bin Zhu, Yan Jin, Xiaozhen Hu, Qinghui Zheng, Su Zhang, Qianjin Wang, and Jia Zhu* Lithium metal has been intensively pursued as a promising anode material for rechargeable Li batteries since the 1970s because of its high theoretical specific capacity (3860 ma h g 1 ), low density (0.59 g cm 3 ), and the lowest negative electrochemical potential ( V vs the standard hydrogen electrode). [1,2] However, there are some intractable barriers limiting its large-scale applications. [3 11] The two most critical problems are the infinite volumetric change and the uncontrolled solid electrolyte interface (SEI) during cycling, which together lead to dangerous lithium dendrites, low Coulombic efficiency (CE), and short cycle life (Figure 1a). [12 16] In the past decades, considerable efforts have been carried out to address these problems. For example, the modifications of the components of electrolytes have been demonstrated to suppress the formation of Li dendrites. [17 20] Various mechanical protective layers have also been proposed to block the formation of Li dendrites during cycling. [21 25] It is widely regarded [2,24] that the ideal interfacial layer needs to be chemically stable in a highly reducing environment, mechanically strong, and also flexible to accommodate the volumetric expansion of Li deposition. For example, recently Arie et al. proposed the coating of a diamond-like carbon film on the lithium-metal surface to accommodate the Li-dendrite formation. [23] Zheng et al. designed an interconnected hollow carbon nanosphere to suppress the growth of the Li dendrites. [24] Improved electrochemical performances were demonstrated with those protective layers, but the complicated process in most of previous studies (such as template synthesis, flash evaporation, thermal treatment, etc.) largely restrains the scalability. Here, we demonstrate that a modified poly(dimethylsiloxane) (PDMS) film with nanopores, fabricated by convenient spin-coating and a hydrofluoric (HF) acid etching process, can be employed to improve the cycle stability of Li-metal batteries. The nanopores of the PDMS film allow efficient lithium-ion transport, while the PDMS film is mechanically and chemically stable in the electrochemical cycling to suppress Li-dendrite formation, and therefore compatible with different electrolytes. For example, a stable cycling over 200 cycles with an averaged Coulombic efficiency of 94.5% is demonstrated in conventional carbonate electrolyte at a B. Zhu, Y. Jin, Dr. X. Hu, Q. Zheng, S. Zhang, Dr. Q. Wang, Prof. J. Zhu National Laboratory of Solid State Microstructures College of Engineering and Applied Sciences and Collaborative Innovation Center of Advanced Microstructures Nanjing University Nanjing , China jiazhu@nju.edu.cn DOI: /adma current density of 0.5 ma cm 2 with the modified PDMS film as a protective layer, a significant improvement over the performance of unmodified lithium metal. The scalable and low-cost process, compatibility with different electrolytes, together with much improved electrochemical performance, provides a complementary approach to the development of new electrolytes for next-generation lithium-metal anodes. PDMS is widely used in microfluidic and other fields because of its process convenience and chemical inertness. [26 29] It should be noted that the regular PDMS film is not a lithiumion conductor. Figure S1a (Supporting Information) presents the electrochemical performance of the electrode coated with PDMS film (without nanopores). It is clear that Li could not deposit on Cu foil and the electrochemical reaction could not proceed. To provide pathways for Li + transport, acid treatment is employed to intentionally create nanopores in the PDMS film. Pores with different sizes can be obtained by controlling the etching time. Figure S1b d (Supporting Information) show the scanning electron microscopy (SEM) images and electrochemical performances of PDMS film after HF treatment for 5, 15, and 30 min, respectively. After 5 min HF treatment, we can obtain nanopores with the size around nm, and the CE is stable during 100 cycles at 0.5 C (Figure S1b, Supporting Information). Comparatively, if the time of HF treatment increases (Figure S1c, Supporting Information), more and more pores on the PDMS film appear and the size of pores become bigger ( 500 nm). Very long time HF treatment can lead to a large direct contact area between Li and the electrolyte. The Li dendrite would grow out through the bigger nanopores more easily, so the CE gradually decayed during cycling and maintained only 80% after 100 cycles. Figure S1d (Supporting Information) shows the performance of the modified electrode with PDMS after 30 min HF treatment. After a longtime etching, a larger area of Cu foil was exposed, resulting in an unstable CE during cycling. Thus, in the study we chose the PDMS film with 5 min HF treatment for further study. As illustrated in Figure 1b, Li can be deposited on Cu foil through the nanopores of the thin PDMS film, while the flexible and highmodulus PDMS layer can accommodate the volumetric change of the Li deposition without fracture during cycling. Therefore, the modified PDMS film can protect the surface of the deposited Li from the electrolyte, which leads to much suppressed dendrite formation and improved stability. The entire process is highly scalable and low cost. Through the modified spin-coating process, PDMS film around 500 nm can be obtained (Figure 1c, see the Experimental Section for more details). After 5 min HF acid treatment, nanopores can be clearly observed on the PDMS film, as shown in the SEM image (Figure 1d). The size of the nanopores obtained is about nm (Figure S2, Supporting Information). This modified PDMS-based protective (1 of 6) wileyonlinelibrary.com 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

2 Figure 1. Schematic diagrams of Li deposition. a) Li deposition on bare Cu foil, mossy Li, and dendrite growth appear after many cycles. b) Li deposition on the Cu foil coated with PDMS thin film, with suppressed dendrite growth after many cycles. c,d) Cross-section SEM image of the PDMS film (c), and top view SEM image of PDMS film after HF acid treatment (red circles: nanopores) (d). Figure 2. SEM of Li deposition on bare Cu foil and modified Cu foil with PDMS film at 0.5 ma cm 2 current density for 1 ma h cm 2 Li deposition. a,b) Cross-sectional and top-view SEM of first Li deposition on bare Cu foil. c) Top-view SEM image of bare Cu foil after Li stripping. d,e) Cross-sectional (d) and top-view (e) SEM of first Li deposition on modified electrode with protective PDMS film. White arrow: PDMS film. f) Top-view SEM image of modified electrode after Li stripping WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com (2 of 6)

3 surface offers several advantages as listed below: i) the PDMS film is chemically and mechanically stable in the electrochemical cycling to suppress dendrite formation, and at the same time flexible to accommodate the volumetric change of Li deposition, ii) nanopores can enable efficient lithium-ion transport, and iii) the entire process is highly scalable, and, as PDMS is chemically inert, this approach is compatible with other strategies such as the development of electrolytes. As an example, under the protection of the modified PDMS film, the cell demonstrated a stable CE of 95% over 200 cycles in a conventional carbonate electrolyte, making it a feasible approach complementary to other existing efforts. The morphology changes of the Li deposition on the current collectors were carefully examined under SEM. Figure 2a,b show the morphology of deposited Li on bare Cu foils. After the first Li deposition at a current rate of 0.5 ma cm 2, from the cross-section view (Figure 2a) and top view (Figure 2b) we can see a clear dendrite-like Li morphology which is in agreement with previous reports. [19,24] In contrast, with the protective layer of modified PDMS, the plating of Li metal shows up as a distinct morphology, a smooth and flat surface after 1 ma h cm 2 Li deposition (Figure 2e). From Figure 2d, we can clearly observe a thin layer of Li metal after deposition without any obvious dendrites under the PDMS film. It confirms that, with the protection of PDMS, a uniform layer of Li metal can be deposited on Cu foil. Some other SEM images of deposited Li under PDMS film are presented in Figure S3 (Supporting Information). Figure 2c,f present the surface changes of the bare Cu foil and modified electrode after the first Li plating/stripping, respectively. After the first cycle, the bare Cu foil maintains some dead Li and SEI with a rough surface (Figure 2c). However, the modified electrode still presents a rather smooth surface with PDMS on Cu foil (Figure 2e), which indicates that Li can strip away effectively from the current collector through the nanopores on the PDMS film. Therefore, it is confirmed that Li can deposit on Cu foil through the nanopores of a thin PDMS film, while the PDMS layer protects the surface of the deposited Li from the electrolyte. X-ray photoelectron spectroscopy (XPS) was performed to further examine the surface components of electrodes with and without a protective layer after Li deposition and dissolution. Figure 3a presents the XPS result of the bare Cu foil without PDMS after the first cycle, with Li1s, C1s, O1s, and F1s peaks clearly appearing. Figure S4 (Supporting Information) shows the Li1s and C1s spectra fitting results. It is clear that the existing elements are derived from Li 2 CO 3, ROCO 2 Li, and LiF, which are the main components of the SEI generated between the Li and carbonate electrolyte. Therefore, all the main peaks appearing in Figure 3a correspond to the SEI layer, and resulted from the side reaction of Li and carbonate electrolyte and residual Li. [30] Figure 3b shows the XPS results of the modified electrode before and after one cycle. The gray line shows the XPS result of modified electrode before cycling with the Si2p, Si2s, C1s, and O1s peaks corresponding to standard PDMS. [31] From the modified electrode after one cycle (black line), we can see that the binding energies of the main peaks were 102, 153.2, 284.6, and ev, which correspond to the initial Si2p, Si2s, C1s, and O1s peaks, and no other peaks appeared. This further confirms that the PDMS film can withstand Li Figure 3. XPS characterization of the bare Cu foil and Cu foil coated with PDMS film. a) The result of bare Cu foil after first cycle. b) The spectra of modified electrode before and after first cycle. expansion without fracture and remain unchanged during Li plating and stripping. Additionally, there is no obvious Li peak that can be observed, which indicates that the formation of the SEI is suppressed, and lithium is deposited beneath the PDMS film rather than on top of it. To study the electrochemical performances of the modified electrodes, we conducted a galvanostatic cycling test. Even though various electrolytes have been developed recently for improving electrochemical performance, [17 20] first, 1 m LiPF 6 in ethylene carbonate/diethyl carbonate as the most common electrolyte was chosen. Figure 4 shows the specific electrochemical performance of the cells with and without the PDMS film coating. CE is considered to be an important parameter for a long cycle life, representing the ratio of the amount of stripped Li versus that of plated Li in each cycle. A stable value of CE typically represents a stable surface between the electrode and the electrolyte. From Figure 4a, it is clear that the modified electrode shows a more stable CE and longer cycle life under various current densities. The CE is maintained well above (3 of 6) wileyonlinelibrary.com 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

4 Figure 4. Cycling performances of bare Cu foil and modified electrode with thin PDMS film. a) Comparison of the Coulombic efficiency of bare Cu foil and modified electrode at different current densities for total 1 ma h cm 2 Li deposition. b) Voltage profiles for the bare Cu foil electrode at different cycle with a current density of 0.5 ma cm 2. c) Voltage profiles for the modified electrode with PDMS film at different cycles with the current density of 0.5 ma cm 2. d) Comparison of voltage hysteresis of the Li plating/stripping process for different electrodes with current density of 0.5 ma cm 2. e) The Coulombic efficiency change of modified electrode with PDMS in long cycle time (the inset is the voltage time curve of this cell). 90% for more than 100 cycles at 0.25, 0.5, and 1 ma cm 2, respectively (Figure 4a). In comparison, electrodes without the PDMS protective layer present a quick decay in CE and unstable cycling. After about only 60 cycles, the CE drops to less than 70% at 0.25 and 0.5 ma cm 2. Moreover, the CE of electrodes without PDMS decreases to 20% rapidly after just 30 cycles when cycling at 1 ma cm 2 current density. It should be noted that the average CE with the PDMS coating reaches 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com (4 of 6)

5 % during 100 cycles in this study, while, typically, Li-metal cycling in a carbonate electrolyte only has a CE of 85%. [22] Figure S5 (Supporting Information) presents the CE change of the modified electrode at different current densities. It is clear that even for cycling at a current density as high as 2 ma cm 2 (Figure S6, Supporting Information), the CE of modified electrode with PDMS is stable in the 60 cycles reaching 93%. In comparison, the electrode without the PDMS demonstrates a very unstable CE and the CE decreases below 60% after only 50 cycles. This indicates that the PDMS protective layer was stable during cycling, largely improving the electrochemical stability of the electrodes. Significant differences in the voltage profiles of the cells with/without PDMS layer were also observed. Figure 4b,c present the voltage profiles at different cycles of bare Cu foil and modified electrode cell at the same current density of 0.5 ma cm 2, respectively. For the bare Cu foil without a PDMS layer, as the cycle number increases, less and less Li can be stripped from the Cu foil, which corresponds to the change of CE in Figure 4a. This indicates that much of the Li deposited on the Cu foil reacted with electrolyte and could not be dissoluted from the Cu electrode. Meanwhile, the voltage profiles vary significantly in different cycle numbers, and the sum of the potential for Li plating and stripping increased to higher than 2 mv after 100 cycles. In comparison, Figure 4c shows the highly stable voltage profiles of the modified electrode. We see little change of the voltage profiles as the cycle number increases, which should be attributed to the stable interface between the Cu foil and the electrolyte under the protection of PDMS film. Figure 4d plots the voltage hysteresis versus cycle number. Here, the voltage hysteresis is defined as the sum of the over potential for Li deposition and Li dissolution. The bare Cu/Li cell without a PDMS layer presents a large voltage hysteresis, which reaches above 200 mv after 100 cycles. This indicates that the internal resistance increases after long cycles, which mainly results from the large change of deposited Li morphology and the unstable SEI, while the cell with PDMS protection demonstrates a steady voltage hysteresis, which maintains just 80 mv even after 100 cycles. It should be attributed to the stable interface with the help of the PDMS layer, which leads to a lower internal resistance. We also tested the impedance of Cu foil with/without PDMS film before/after the first Li deposition. From the result in Figure S7 (Supporting Information), we can see the impedance of the modified Cu foil with the PDMS film becomes larger than that of the bare Cu foil, which is mainly attributed to the insulation property of the PDMS. However, after the first Li deposition, the impedance of the modified Cu foil decreased to around 180 Ω, which is close to that of the bare Cu foil (130 Ω). Figure 4e demonstrates the CE changes and voltage time curve of the modified electrode with PDMS film during 200 cycles. It is clear that, with the protection of the PDMS film, the average CE can reach 94.5% for over 200 cycles at 0.5 ma cm 2. The voltage time curve inset in Figure 4e also corresponds to this 200 cycles. From the two amplified five-cycle curve at the cycling time of 180 and 550 h, we can find the stable platform of Li platting/stripping, which further indicates the stabilizing effect of this modified PDMS film. PDMS is widely known for its extremely chemically stable property, therefore it can work with different new electrolytes. Thus, we also used a new electrolyte (1,3-dioxolane (DOL)/1,2- dimethoxyethane (DME) = 1/1(V/V) 1 m LiTFSI + 1% LiNO 3 ) to test the modified electrodes performance. For a current density of 1 ma cm 2 (Figure 5a), the average Coulombic efficiency of the modified Cu foil with PDMS in 100 cycles reaches 98.2%. This further illustrates that our design has compatibility with on-going developments of new electrolytes. In addition, to test the electrochemical performance in a practical configuration, we also assembled full cells with Li metal with modified Cu foil as the anode and LiFePO 4 as the cathode (Li@Cu LiFePO 4 ). First, 1 ma h cm 2 Li was deposited onto the modified Cu foil with the PDMS film (Li@Cu), then the cell was disassembled and the Li@Cu electrode was reassembled with LiFePO 4. Figure 5b shows the cycling performance of the full cell at the current density of 0.5 C. After 100 cycles, the capacity of the LiFePO 4 maintains 140 ma h g 1 with the stable CE of 99.8%. This indicates that our design still presents a stable electrochemical performance even in a practical Li-metal battery configuration. In summary, we have demonstrated a PDMS protective film for the Li-metal anode with much improved electrochemical performance. With the protection of the thin and porous Figure 5. a) The CE of the Cu foil with PDMS film at a current density of 1 ma cm 2 in LiTFSI electrolyte. b) Full-cell cycling performance with LiFePO 4 as cathode at 0.5 C (5 of 6) wileyonlinelibrary.com 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

6 PDMS film, the cycling Coulombic efficiency can be stable at 95% for more than 200 cycles with the conventional carbonate electrolyte. It is expected that the performance can be further improved, combining the PDMS protective film here with other on-going efforts, such as the development of new electrolytes. Thus, we anticipate that this feasible design achieved by convenient processes provides a complementary approach for the development of high-performance anodes of Li-metal batteries, such as lithium sulfur and lithium oxygen batteries. Experimental Section First, PDMS elastomer was mixed with crosslinker (10:1, wt/wt) and the bubbles in it were removed through vacuum treatment. Then it was diluted with methylbenzene (wt/wt, 10:1 methylbenzene to PDMS mixture) and stirred for 60 min. 80 µl of the diluted solution was transferred onto a copper foil. Prior to use, the copper foil should be washed with 1 m HCl. The solution was spin-coated at a low speed (100 rpm) to enable uniform coating on the copper foil at first, then the speed was increased to 9000 rpm for 2 min (an appropriate thickness cannot be obtained at lower speed). The obtained copper foil with solution was transferred to a vacuum oven at 70 C for 5 h to volatilize the methylbenzene and solidify the PDMS. Additionally, some HF solution (5%, wt%) was dropped on the surface of the PDMS for 5 min, then the HF was washed out with deionized water three times. Figure 1d presents the SEM image of the PDMS film after HF-acid treatment and some nanopores can be seen clearly on the PDMS film. The modified Cu foil was taken with PDMS to assemble cells after drying it in a vacuum oven. The 2032 coin-type half cells were assembled in an Ar-filled glovebox with the bare Cu foil and modified Cu foil as the substrates for Li-metal deposition and lithium foil as the counter/ reference electrode. The electrolyte used in this study was 1 m LiPF 6 in ethylene carbonate/diethyl carbonate (1:1 vol/vol, Guotai Huarong) with 2% (wt%) vinylene carbonate (VC) addictive. The effective area of the Cu foil for Li deposition was 1.13 cm 2 (diameter: 1.2 cm). During each cycle 1 ma h cm 2 Li was deposited on substrates at various current densities and the Li was stripped until the potential reached 1 V versus Li/Li +. Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements This work was jointly supported by State Key Program for Basic Research of China Grant 2015CB659300, National Natural Science Foundation of China Grants and , Natural Science Foundation of Jiangsu Province Grant BK , the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the Fundamental Research Funds for the Central Universities. 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