Stable Li metal anode by a polyvinyl alcohol protection layer via modifying solid-electrolyte interphase layer

Stable Li metal anode by a polyvinyl alcohol protection layer via modifying solid-electrolyte interphase layer

Accepted Manuscript Stable Li metal anode by a polyvinyl alcohol protection layer via modifying solidelectrolyte interphase layer Yuming Zhao, Daiwei ...

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Accepted Manuscript Stable Li metal anode by a polyvinyl alcohol protection layer via modifying solidelectrolyte interphase layer Yuming Zhao, Daiwei Wang, Yue Gao, Tianhang Chen, Qingquan Huang, Donghai Wang PII:

S2211-2855(19)30600-7

DOI:

https://doi.org/10.1016/j.nanoen.2019.103893

Article Number: 103893 Reference:

NANOEN 103893

To appear in:

Nano Energy

Received Date: 3 June 2019 Revised Date:

7 July 2019

Accepted Date: 9 July 2019

Please cite this article as: Y. Zhao, D. Wang, Y. Gao, T. Chen, Q. Huang, D. Wang, Stable Li metal anode by a polyvinyl alcohol protection layer via modifying solid-electrolyte interphase layer, Nano Energy (2019), doi: https://doi.org/10.1016/j.nanoen.2019.103893. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical abstract

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TOC

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The low cost and eco-friendly polyvinyl alcohol (PVA) polymer can be applied as an effective protection layer for Li metal anode in both ether- and carbonate-based electrolytes. Through

participation in formation of a superior polymeric composite

solid electrolyte interphase (SEI) layer for Li metal anode, dendrite-free Li deposition

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and alleviated electrolyte consumption can be achieved by the PVA protection layer, enabling a stable cycling of Li metal anode in both half-cells and full-cells (Li-Sulfur,

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Li-LiFePO4 and Li- LiNi0.6Co0.2Mn0.2O2). This research provides a low-cost and

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practical approach for Li metal anode protection.

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Stable Li Metal Anode by a Polyvinyl Alcohol Protection Layer

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via Modifying Solid-Electrolyte Interphase Layer Yuming Zhao, Daiwei Wang, Yue Gao, Tianhang Chen, Qingquan Huang, Donghai Wang* Department of Mechanical Engineering, The Pennsylvania State University, University Park,

Corresponding Author

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*[email protected] (Donghai Wang)

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Pennsylvania, USA

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ABSTRACT: Lithium (Li) metal is considered an ideal anode for next-generation high energy Li metal batteries (LMBs) due to its high theoretical specific capacity and low electrochemical potential. However, stable cycling of LMBs has long been restrained by the extremely unstable

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interfaces between the Li metal anode and liquid electrolyte. Here, we found that low-cost polyvinyl alcohol (PVA) polymer can be applied as an effective protection layer for Li metal anodes in both ether- and carbonate-based electrolytes. The PVA protection layer will participate

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in and facilitate formation of a superior PVA-modified solid electrolyte interphase (SEI) layer on Li metal, leading to uniform deposition of Li and alleviated consumption of electrolyte. Li||Cu

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half-cells with the PVA protection layers show dendrite-free Li deposition and greatly extended stable cycling with high Coulombic efficiency (CE) (e.g. average CE of 98.3% for over 630 cycles). Li-Sulfur, Li-LiFePO4 and Li-LiNi0.6Co0.2Mn0.2O2 full-cells using the PVA-protected Li metal anodes also show significantly improved electrochemical performance with better capacity

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retention and higher CE, even under lean electrolyte (7.5 µL mAh-1) condition.

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Keywords: Lithium metal anode, solid electrolyte interphase, polyvinyl alcohol, protective layer,

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solid polymer electrolyte, Lithium metal battery.

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1. Introduction Lithium (Li) metal with ultrahigh theoretical specific capacity (3860 mAh g−1) and the lowest electrochemical potential (−3.04 V vs standard hydrogen electrode) is the most promising metal

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anode candidate to pair with a cathode material (e.g. metal oxide, sulfur or oxygen) to achieve high energy density in metal batteries [1, 2]. However, to realize practical application of Li metal batteries (LMBs), several intractable issues must be solved. Among them, one of the most

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challenging problems is how to achieve stable interface/interphase between the highly reactive Li metal anode and liquid electrolyte [3-5]. The conventional solid-electrolyte interphase (SEI)

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layer formed by direct contact of Li metal anode with liquid electrolyte is inorganic dominated and too fragile to withstand the enormous volume fluctuations during Li plating and stripping cycling [6]. In particular, the repeated cracks/regenerations of SEI layer will lead to severe consumption of Li metal and electrolyte, low Coulombic efficiency (CE), fast increase of

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internal resistances and short cycle life. Moreover, the needle-like dendrite growth during Li deposition will cause internal short circuits and safety issues in LMBs [7]. Many strategies have been adopted to stabilize Li metal anodes, such as modification of

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electrolytes using additives to regulate the formation of SEI layer [8-15], application of a protective layer as an artificial SEI layer [16-20], use of solid state electrolyte as a physical

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barrier to suppress dendrite propagation [21, 22], and use of 3D host materials to manipulate Li ion flux and control Li deposition [23-26]. Among them, constructing a protective layer on the Li metal anode is one of the most effective practical approaches to stabilize Li metal anode interfaces, due to facile control of composition, morphology, mechanical strength, and flexibility. Inorganic protective layers, such as Li3PO3 [27], LiN3 [28], tetraethoxysilane [29] or Al2O3 [30], have been applied on Li metal anodes, however, these layers tend to break and wear off due to

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their poor flexibility and integrity, especially during long term cycling or at high deposition capacity. Therefore, polymers with superior electronically insulating properties and flexibility to accommodate volume changes have been considered as suitable protective layers [31]. Various

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types of polymers such as polyimide [32], styrene butadiene rubber [16], poly(dimethylsiloxane) [19], adaptive polymers [18, 33], poly(vinylidene difluoride) [34], poly(vinylidene-cohexafluoropropylene) [35], and Li polyacrylic acid [36], have demonstrated anode protection.

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However, it is still challenging to develop a low-cost but effective polymer protective layer that

alleviate consumption of electrolyte.

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can help Li metal anodes form a superior stable SEI layer, suppress Li dendrite growth, and

Recently, our group developed reactive polymers as protection layers for the Li metal anode that can protect the anode and enable excellent stable cycling of LMBs [37-39]. The reactive polymers can participate in the formation of SEI layer by chemical/electrochemical interfacial

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reaction with Li metal. By using this strategy, we can elaborately regulate composition and mechanical properties of SEI layer through well-designed reactive polymers. Meanwhile, it is also our pursuit to find a commercially available, low-cost and eco-friendly polymer that can

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similarly regulate the SEI layer and provide effective anode protection. In this work, we applied polyvinyl alcohol (PVA) as the potential protection layer for Li metal anodes. It is found that

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PVA can interact with a variety of Li salts to become an ionically conductive solid polymer electrolyte, due to a high content of hydroxyl polar groups in PVA [40, 41]. Therefore, the PVA layer on the surface of the Li metal anode may form a solid Li-ion conductive layer and thus participate in formation of the SEI layer. In addition, PVA is a low cost and widely used synthetic polymer with excellent thermal, mechanical, chemical stabilities and good filmforming property. We found that the PVA protection layer can indeed participate in the

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formation of a robust SEI layer for Li metal anodes, suppressing Li dendrite growth and leading to a uniform deposition of Li during cycling (as schemed in Fig. 1A). This PVA-modified SEI layer can enable stable cycling of Li metal anode in both ether- and carbonate-based electrolytes.

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The Li plating/stripping cycling tests show extended cycle life and high CEs of Li metal anode (e.g. average CE of 98.3% for over 630 cycles at 2 mA cm-2 and 1mAh cm-2 and average CE of 98.5% for over 220 cycles at 2 mA cm-2 and 3mAh cm-2) by using the PVA protection layer.

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Furthermore, Li-sulfur (Li-S) cells using the PVA-protected Li metal anode in ether-based electrolyte show apparently improved electrochemical performance (~86.9% capacity retention

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after 800 cycles with an average CE of ~99.3%). Li-LiFePO4 (Li-LFP) and LiLiNi0.6Co0.2Mn0.2O2 (Li-NCM) cells using the PVA-protected Li metal anode in carbonate-based electrolyte also show highly improved cycling performance (~87.3% capacity retention after 400 cycles with an average CE of ~99.8% for the Li-LFP cell, and ~74.5% capacity retention after

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400 cycles with an average CE of ~99.7% for the Li-NCM cell). Even under lean electrolyte (7.5 µL mAh-1) condition, Li-NCM cells with PVA-protected Li metal anode show very stable cycling (~74.5% capacity retention after 120 cycles and an average CE of ~99.3%). Considering

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the simplicity of this protective technique and the low-cost of material, this PVA protection layer

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would be valuable for practical application in LMBs. 2. Material and methods

2.1 Preparation of PVA-protected Cu and Li foils PVA (Sigma Aldrich, Mw = ~31000 g/mol with a hydrolysis of 88%) was dissolved in distilled water at room temperature with a mass concentration of 1, 2.5 or 5 wt%. For PVA (Mw = ~47000 g/mol) with a hydrolysis of 98%, the aqueous solution at the same mass concentration was prepared at 80 °C under stirring. To fabricate a uniform PVA layer on Cu foil, the doctor

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blade technique was used to coat the aqueous PVA solution onto Cu foil with a controlled gap of 50 µm. Then, the coated Cu foil was dried at 80 °C under vacuum for 24 h. The PVA-protected Cu foil was punched into 15 mm diameter discs for coin cell fabrication and tests.

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To prepare PVA-protected Li foil, the PVA (88% hydrolyzed) was dissolved in anhydrous NMethyl-2-pyrrolidone (NMP) with a concentration of 0.5 wt% at room temperature. Then, 50 µL of the PVA solution was dropped onto the top surface of Li foil and dried in a vacuum chamber

2.2 Electrochemical measurements

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layer and used for coin cell fabrication and tests.

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at room temperature for 24 h. After drying, the Li foil was covered with uniform PVA protection

All coin cell (CR2016) assembly was conducted in an argon-filled glove box with H2O and O2 level below 0.1 ppm. Li deposition/dissolution studies were performed on a Lanhe battery tester at room temperature under galvanostatic charging/discharging conditions. An ether-based

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electrolyte with 1 M lithium bis (trifluoromethanesulfonyl) imide (LiTFSI) and 4 wt% LiNO3 in a mixture of 1, 3-dioxolane (DOL) and 1, 2-dimethoxyethane (DME) (1:1 v/v) was employed. For cycling tests, CEs versus cycle number at different current density and areal capacity was

capacity.

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recorded, in which CE was calculated by the ratio between stripped Li capacity and deposited Li

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To evaluate in Li-S full-cells, sulfur cathodes were prepared through a conventional blade casting method. Firstly, carbon/sulfur (C/S) composite with 70 wt% sulfur was prepared by heat treatment of the mixture of sulfur and Ketjen black EC-600JD (KB) (in a mass ratio of 3:7) in a sealed vial at 160 °C for 10 h. Then the as-obtained C/S composite was mixed with conductive agent Super C and poly (vinylidene fluoride) (PVDF) binder in a mass ratio of 80:10:10 and dispersed in NMP to make a slurry. To fabricate the sulfur cathodes, the slurry was cast onto a

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carbon-coated aluminum (Al) foil and dried at 55 °C overnight under vacuum. The sulfurloading of the cathode was controlled at 1.0-2.0 mg cm-2. The coin cells were assembled using C/S composite as cathode, Celgard 2325 as the separator, and Li foil (400 µm in thickness) with

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or without the protection as anode. For cell tests, 1 M LiTFSI in DOL/DME with 4 wt% LiNO3 was used as electrolyte (40 µL for each cell) and the cells were cycled between 1.7 to 2.8 V at charging/discharging rate of 1C.

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To evaluate in Li-LFP or Li-NCM full-cells, LFP or NCM cathode was prepared by mixing LFP (or NCM), PVDF, and Super C in a weight ratio of 90:5:5 using NMP as the solvent to

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make a slurry and cast onto an Al foil and dried at 120 °C overnight under vacuum. The areal mass loading of LFP or NCM is around 10 mg cm-2. The coin cells were assembled in an argonfilled glove box with LFP (or NCM) as cathode, Celgard 2325 as the separator, and Li foil (400 µm in thickness) with or without the protection as anode. For the cell tests, 1.0 M lithium

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hexafluorophosphate (LiPF6) in ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (3:7, v/v) with addition of 10 wt% of fluoroethylene carbonate (FEC) was used as electrolyte (40 µL for each cell, and 15µL for the lean electrolyte testing). Li-LFP cells were cycled between 2.4 to

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4.0 V at a charging/discharging rate of 0.5C. Li-NCM cells were cycled between 2.7 to 4.2 V at a charging/discharging rate of 0.3C. The Li||Li symmetric cells using Li foils with or without the

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protection were tested in the same carbonate electrolyte (1.0 M LiPF6 in EC/EMC with addition of 10 wt% of FEC).

2.3 Characterization

Scanning electron microscopy (SEM) images were acquired on a Nova NanoSEM 630 instrument. X-ray photoelectron spectroscopy (XPS) analysis was carried out on a PHI VersaProbe II Scanning XPS Microprobe. For SEI characterization, all the batteries were cycled

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at the same conditions for 10 cycles, and then Li metal was stripped leaving the SEI layer on the Cu foil for analysis. Electrochemical impedance spectroscopy (EIS) was collected at open circuit potential (OCP) using a Solartron SI1287 electrochemical interface analyzer in a frequency range

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from 100 kHz to 0.1 Hz with an amplitude of 10 mV. Fourier transform infrared (FTIR)

mode with a Spectra Tech Collector II accessory. 3. Results and discussion

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3.1 Preparation of PVA-protected Cu or Li foils

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characterizations were performed on a Bruker Vertex V70 spectrometer in diffuse reflection

The PVA-protected Cu foils were prepared by a doctor blade coating method. Since the higher concentration (e.g. 2.5 and 5 wt%) of PVA aqueous solution and 98% hydrolyzed PVA will lead to a deteriorated electrochemical performance of Li||Cu half-cells (Fig. S1-S3), we chose to use the 88% hydrolyzed PVA aqueous solution with a concentration of 1 wt% coated

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Cu foils for the following electrochemical tests and characterization. Fig. 1B shows the optical images of bare and PVA coated Cu foils. After coating and drying, the color of Cu foil becomes more reddish brown. From the SEM images of bare and PVA coated Cu foils, wrinkled surface

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with fine grain and tiny protrusions is observed for the bare Cu foil (Fig. 1D), and more smooth

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surface is observed for the PVA coated foil (Fig. 1E). The thickness of the PVA protection layer on Cu foil is around 0.33 µm (inset in Fig. 1E). The surface PVA protection layer on Cu foil was also characterized by XPS analysis, as shown in Fig. S4. To prepare the PVA-protected Li metal foils, a drop coating method was employed that quantitative PVA solution was dropped on the surface of Li metal foils. After drying, there is no obvious color change between the bare and PVA coated Li foils (Fig. 1B). From the SEM images of bare and PVA coated Li foils, uneven surface morphology is observed for the bare Li foil (Fig. 1F), and more smooth and covered

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surface was observed for the PVA coated Li foil (Fig. 1G). The thickness of the PVA protection layer on Li foil is around 2.0 µm (Fig. S5). In addition, we compared the stability of bare and PVA-protected Li foils when exposed to air. As shown in Fig. 1C, the bare Li foil becomes

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partially black immediately (less than 1 min) after exposing to air and totally black after standing for 15 min, while the PVA-protected Li foil remains silvery after exposure to air for 15 min. This result indicates that the uniform PVA protection layer can improve the stability of Li

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metal foil in the air atmosphere.

Fig. 1. (A) Schematic illustration of Li plating/stripping behavior on Cu foils with and without

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PVA protection layer. (B) Optical images of bare Cu and Li metal foil, and PVA-protected Cu ([email protected]) and Li metal foil ([email protected]). (C) Optical images of bare (left) and PVA-protected

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(right) Li foils exposed to air at 22 °C with 65% humidity. SEM images of (D) bare and (E) PVA-protected Cu foils. SEM images of (F) bare and (G) PVA-protected Li metal foils. 3.2 Morphology of deposited Li on PVA-protected Cu foils The morphology of deposited Li on bare and PVA-protected Cu foil was studied by SEM. As shown in Fig. 2A and B, after 10 cycles at a current density of 2 mA cm-2 and capacity of 2mAh cm-2, the deposited Li on bare Cu foil shows a coarse and porous morphology and a fluffy and

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cracked cross section, indicating an accumulation of mossy and dendritic Li. The highly porous structure will increase the surface area and accelerate the reaction between Li and electrolyte, resulting in a low CE, fast consumption of electrolyte and short cycle life of Li metal anode. In

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contrast, after cycling at the same condition, the deposited Li on PVA-protected Cu foil shows very flat surface (Fig. 2C) and compact cross section (Fig. 2D), indicating a dendrite-free Li deposition. Even after 200 cycles, the deposited Li on PVA-protected Cu foil still shows a flat

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surface and a compact cross section (as shown in Fig. S6), further confirming the effective

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suppression of Li dendrite growth by the PVA protection layer.

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Fig. 2. Morphologies of the deposited Li on bare and PVA-protected Cu foils after 10 cycles at 2 mA cm-2 and 2mAh cm-2. (A) Top-view and (B) side-view SEM images of Li deposition on bare Cu foil. (C) Top-view and (D) side-view SEM images of Li deposition on PVA-protected Cu foil.

3.3 Li||Cu half-cell performance and SEI characterization Li||Cu half-cells were further used to investigate the effectiveness of PVA protection layer for Li deposition on Cu foil. 1 M LiTFSI in DOL/DME with 4 wt% LiNO3 was employed as the

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electrolyte. Fig. 3A-C shows a comparison of cycling stability of cells using bare and PVAprotected Cu foil at different cycling conditions. When the cells were cycled at a capacity of 1 mAh cm-2 and current density of 2 mA cm-2 (Fig. 3A), the CEs of the cell using bare Cu foil

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begin to drop rapidly after around 150 cycles. In contrast, the cells using PVA-protected Cu foil shows significantly improved performance, achieving very stable cycling for over 630 cycles with an average CE of 98.3%. Furthermore, when the cells using a PVA-protected Cu foil were

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cycled at higher deposition capacities of 2 mAh cm-2 (Fig. 3B) and 3 mAh cm-2 (Fig. 3C), average CEs of 98.4% for over 320 cycles and average CEs of 98.5% for over 220 cycles can be

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achieved, respectively. Fig. 3D shows the voltage profiles of cell with PVA-protected Cu foil cycled at 2 mA cm-2 and 2 mAh cm-2 at different cycle numbers. The voltage polarization of cells with the PVA-protected Cu foil was gradually decreased upon cycling and stable platingstripping voltage profiles with lower voltage polarization were observed after 10 cycles,

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indicating steady Li plating/stripping in Li||Cu half-cells. Similarly, as shown in Fig. 3E, the cells with PVA-protected Cu foil shows a relatively stable voltage hysteresis (between 40-30 mV) after the first 20 cycles. The cells using PVA-protected Cu foil were further investigated by

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EIS, as shown in Fig. 3F. Consistently with their stable voltage profiles, a large semicircle of Nyquist plot was observed after the first cycle and became relatively stable after 25 cycles,

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implying decreased and stable interfacial and charge-transfer resistance of the cell. In contrast, for cells with bare Cu foil, both voltage hysteresis (Fig. 3E) and interfacial and charge-transfer resistance in EIS (Fig. S7) keep increasing with proceeding of the cycling, indicating a continuous growth of SEI layer and the resultant increased impedance without the PVA protection.

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Fig. 3. Electrochemical performance of Li||Cu half-cells. Cycling performance of cells using bare (black) and PVA-protected (red) Cu foils at (A) 2 mA cm-2 and 1 mAh cm-2, (B) 2 mA cm-2 and

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2 mAh cm-2 and (C) 2 mA cm-2 and 3 mAh cm-2. (D) Voltage profiles of cells using PVAprotected Cu foil cycled at different cycle number at 2 mA cm-2 and 2 mAh cm-2. (E) Voltage

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hysteresis changes of cells using bare (black) and PVA-protected (red) Cu foil cycled at 2 mA cm-2 and 2 mAh cm-2. (F) The EIS results of cells using PVA-protected Cu foil at different cycle numbers cycled at 2 mA cm-2 and 2 mAh cm-2.

The stable cycling of Li||Cu half-cells using PVA-protected Cu foils can be attributed hypothetically to their more stable SEI layer induced by the PVA protection layer. To verify this, the composition of SEI layer was investigated through XPS characterization. First, an interfacial

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chemical reaction by mere direct contact of PVA film and Li metal was confirmed through the XPS characterization, as shown in Fig. S8. The new peak at ~55 eV in Li 1s spectrum can be ascribed to the C-O-Li bonds formed by the chemical reaction between the surface OH groups in

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PVA and Li metal. Fig. 4 shows the C 1s, F 1s and S 2p XPS spectra of SEI formed on bare Cu foil (Control SEI) and PVA-protected Cu foil (PVA-modified SEI) after 20 cycles. From the C 1s spectra (Fig. 4A and D), an obviously higher ratio of C-O bond at 286.3 eV, which is

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consistent with the C-O bond in PVA (Fig. S4B), can be observed for the PVA-modified SEI. These results indicate that the PVA protection layer can participate in formation of SEI layer for

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Li metal anodes through the interfacial chemical and electrochemical reaction. Moreover, it is reported that the hydroxyl groups in PVA polymer can interact with Li salts to form salt complexes and become highly ionic conductive [40, 41]. The interaction between PVA and LiTFSI salt can be verified based on the narrower OH stretching peak appeared at higher wave

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number in FTIR spectra (Fig. S9). The PVA-Li salt complex with high Li-ion conductivity would be favorable for the smooth Li ion transport through the SEI layer. Furthermore, a relatively decreased content of LiF (684.4 eV) over -CF3 (688 eV) in F 1s spectra is observed in

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PVA-modified SEI compared with control SEI, as shown in Fig. 4B and E. Since LiF is a reduction product of LiTFSI salt [42], the higher content of LiF in control SEI manifests the

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decomposition of higher amount of electrolyte salt. In addition, from S 2p spectra (Fig. 4C and F), the peak at 164.6 eV can be ascribed to Li2SO3, which is also a decomposition product of LiTFSI salt [42]. The apparently higher content of Li2SO3 peak in S species in control SEI further demonstrates its severe decomposition of electrolyte salt. The results indicate that the PVA protection layer can facilitate formation of stable SEI and thus alleviate the decomposition of LiTFSI salt, which is important for the long-term stable cycling of the Li metal anode. As a

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benefit of this PVA-modified composite SEI layer, prolonged stable Li plating/stripping cycling

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was achieved, as shown in Fig. 3A-C.

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Fig. 4. SEI characterization by XPS. C 1s, F 1s and S 2p XPS spectra of (A-C) control SEI and (D-F) PVA-modified SEI layer.

3.4 Full-cell performance by the PVA-protected Li metal anode

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The effectiveness of the PVA protection layer for Li metal anode was further investigated in full-cells using S, LFP or NCM as cathode and the PVA-protected Li metal as anode. For Li-S

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cell, ether-based electrolyte (1 M LiTFSI in DOL/DME with 4 wt% LiNO3) was employed. As shown in Fig. 5A, Li-S cell with bare Li metal anode shows very fast capacity fading with only ~167.2 mAh g-1 capacity remaining after 800 cycles and dramatically increased voltage polarization (Fig. S10A) due to the poor interfacial stability between Li metal anode and electrolyte. In contrast, the Li-S cell with PVA-protected Li metal anode shows stable cycling with higher capacity retention of ~86.9% after 800 cycles and more stable voltage profiles (Fig.

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S10B). This result indicates that the PVA layer can provide effective protection for Li metal anodes and enable an improved cycling performance of Li-S batteries. It is known that the carbonate-based electrolyte is more corrosive to Li metal anode than ether-based electrolyte [43].

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Therefore, we applied the PVA-protected Li metal anode in the carbonate-based electrolyte, as demonstrated by the Li||Li symmetric cells (Fig. S11) and Li-LFP or Li-NCM (Fig. 5B and C) full-cells. 1.0 M LiPF6 in EC/EMC with addition of 10 wt% of FEC was employed as the

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carbonate-based electrolyte. As shown in Fig. S11, Li||Li symmetric cell using the PVAprotected Li metal foils shows very stable cycling for over 900 h with voltage overpotential less

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than 60mV, in contrast to dramatically increased voltage overpotential and fluctuation when using bare Li metal foils after around 190 h. Further, Li-LFP full-cell with bare Li metal anode exhibits fast capacity drop (less than 80% capacity retention after only 100 cycles, as shown in Fig. 5B) and dramatically increased voltage polarization (Fig. S12A). For cell with PVA-

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protected Li metal anode, significantly improved cycling performance with a capacity retention of ~87.3% even after 400 cycles and an average CE of ~99.8% can be achieved. Meanwhile, it shows a very stable charge-discharge voltage profile with very limited increase of voltage

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polarization, as shown in Fig. S12B. Similarly, Li-NCM cell with PVA-protected Li metal anode also shows greatly improved cycling performance (~74.5% capacity retention after 400 cycles

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and an average CE of ~99.7%, as shown in Fig. 5C), and stable charge-discharge voltage profile (as shown in Fig. S13B) than the Li-NCM cell with bare Li metal anode (Fig. 5C and Fig. S13A). Even under lean electrolyte (7.5 µL mAh-1) condition, Li-NCM cells with PVA-protected Li metal anode show very stable cycling performance (~74.5% capacity retention after 120 cycles and an average CE of ~99.3%, as shown in Fig. 5D red), compared with Li-NCM cell with bare Li metal anode (Fig. 5D black). These results fully demonstrate that the PVA layer can

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provide effective protection for Li metal anodes in both ether- and carbonate-based electrolyte,

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and can enable excellent cycling performance of LMBs.

Fig. 5. Electrochemical performances of Li-S, Li-LFP and Li-NCM full-cells. (A) Cycling performance of Li-S cells using bare (black) and PVA-protected (red) Li metal anodes cycled at

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1C in ether-based electrolyte. (B) Cycling performance of Li-LFP cells using bare (black) and PVA-protected (red) Li metal anodes cycled at 0.5C in carbonate-based electrolyte. (C) Cycling performance of Li-NCM cells using bare (black) and PVA-protected (red) Li metal anodes

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cycled at 0.3C in carbonate-based electrolyte. (D) Cycling performance of Li-NCM cells using bare (black) and PVA-protected (red) Li metal anodes cycled at 0.3C under lean electrolyte (7.5 µL mAh-1).

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4. Conclusion

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In this work, we demonstrated that commercially available, low-cost PVA polymer can be applied as an effective protection layer for Li metal anodes. The PVA protection layer can participate in formation of a robust SEI layer for the Li metal anode, with reduced Li ion transport resistance and alleviated decomposition of electrolyte against Li metal, to enable dendrite-free Li deposition. Thanks to the PVA-modified SEI, this PVA protection layer enables

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a long-term stable cycling of Li metal anodes with improved CEs in both ether- and carbonatebased electrolytes. Greatly improved cycling performance with superior capacity retention and CEs in Li-S, Li-LFP or Li-NCM full-cells under flooded or lean electrolyte condition were also

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demonstrated. This work provides a low-cost and practical approach for Li metal anode

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protection potentially used in next-generation high energy LMBs. Acknowledgment

This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the US Department of Energy, through the Advanced Battery Materials Research (BMR) Program (Battery500 Consortium) award no. DEEE0008198.

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Conflict of Interest The authors declare no competing financial interest.

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Research Highlights: Polyvinyl alcohol (PVA) can be an effective protection layer for Li metal anode

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PVA participated in formation of a stable composite SEI layer for Li metal anode Dendrite-free Li deposition and alleviated electrolyte consumption by the PVA layer

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Dr. Yuming Zhao is currently a postdoctoral researcher in Donghai Wang’s Energy Nanostructure (E-Nano) lab at The Pennsylvania State University. He received his M.S. (2013) degree in Material Science & Engineering from Beijing University of Chemical Technology and Ph. D. (2016) degree in Polymer Chemistry and Physics from Institute of Chemistry, Chinese Academy of Sciences. His research interests include synthesis of functional polymeric materials and developing materials for energy storage and conversion application.

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Daiwei Wang received his B.S. (2015) in Chemical Engineering from Tsinghua University, China. He is currently a Ph.D. candidate in Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, working in Dr. Donghai Wang’s Energy Nanostructure (E-Nano) lab. His research focuses on synthesis of materials for energy storage and energy conversion.

Dr. Yue Gao is a postdoctoral researcher in Dr. Donghai Wang’s group. He received his B.S. degrees in 2012 from Department of Chemistry, Lanzhou University and Ph.D. degree in 2018 from Department of Chemistry, the Pennsylvania State University. His research interest is the electrochemical interface in rechargeable batteries.

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Tianhang Chen received his master degree (2015) in chemistry from University of Edinburgh, UK. He is currently a Ph.D candidate in Department of Materials Science and Engineering in the Pennsylvania State University, working in Dr. Donghai Wang’s Energy Nanostructure (E-nano) lab. His research focuses on stabilizing material surface for energy storage material.

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Dr. Qingquan Huang received his B.S. (2012) degrees in Department of Chemistry from Tsinghua University and Ph.D. (2018) degree in Department of Materials Science and Engineering from the Pennsylvania State University, under the direction of Dr. Donghai Wang. Now he is a postdoctoral researcher in Dr. Donghai Wang’s Energy Nanostructure (E-Nano) lab. His research interests are on materials synthesis and application for high-energy Li-ion batteries.

Dr. Donghai Wang is currently Professor at Department of Mechanical Engineering and Department of Chemical Engineering at The Pennsylvania State University. Before joining Penn State in 2009, he was a postdoc and subsequently became a staff scientist at Pacific Northwest National Laboratories where he developed functional materials for catalysis and energy storage

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techniques. He received a B.S. and Ph.D. degree in Chemical Engineering from Tsinghua University and Tulane University in 1997 and 2006, respectively. Dr. Donghai Wang's research interests have been related to design and synthesis of materials for a variety of applications. His recent research is focused on material development for energy conversion and storage technologies such as batteries, supercapacitors, fuel cells and solar fuels. Professor Wang has authored and co-authored over 100 peer reviewed publications, more than 15 patents and patent applications, and 4 book chapters.