Stabilizing Li metal anodes through a novel self-healing strategy

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Stabilizing Li metal anodes through a novel self-healing strategy Ximing Cui, Ying Chu, Liming Qin, and Qinmin Pan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b02564 • Publication Date (Web): 05 Jul 2018 Downloaded from http://pubs.acs.org on July 6, 2018

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ACS Sustainable Chemistry & Engineering

Stabilizing Li metal anodes through a novel self-healing strategy

Ximing Cui, Ying Chu, Liming Qin, Qinmin Pan* (School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, P. R. China)

Corresponding author: Qinmin Pan Harbin Institute of Technology 92 West Dazhi Street, Nangang District, Harbin 150001, P. R. China E-mail: [email protected]

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ABSTRACT Poor stability is a long-standing problem preventing the practical application of Li metal anodes, which is fundamentally attributed to their fragile solid electrolyte interphase (SEI) layers that are intrinsically neither adaptable to the dynamic volume change nor self-healable after breakage. Here a Li metal anode is effectively stabilized by in-situ integrating its SEI layer into a self-healable polydimethylsiloxane (PDMS) network cross-linked via imine bonding. The self-healing network enables the integrated SEI layer to readily accommodate the volume change but also to repair itself after breaking. Consequently, the resulting anode exhibits excellent cycling stability and a dendrite-free morphology. In a Li/LiFePO4 full cell, this strategy leads to capacity retention up to 99% and a Coulombic efficiency >99.5% after 300 cycles. Our investigation provides a novel self-healing strategy for developing stable Li-metal anodes aiming at high energy-density batteries.

KEYWORDS: Li metal anodes, SEI layer, in-situ integration, self-healable PDMS network, dendrite free, cycling stability

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INTRODUCTION Li metal is considered as an optimal anode for high energy-density batteries because of high theoretical specific capacity (3860 mAh g–1) and the lowest negative potential.1–4 However, the practical application of a Li metal anode is fundamentally impeded by its high reactivity with the electrolyte, as well as intrinsically inhomogeneous Li deposition/stripping behavior.5–8 The former will produce electrochemical irreversible compounds on the Li anode, mainly in the form of brittle solid electrolyte interphase (SEI), and thus reducing the Coulombic efficiency of Li deposition/stripping.9–11 The later will cause local regions of the anode to experience highly relative volumetric change, which leads to straining and even breaking of the fragile SEI layer. The breakage further accelerates the inhomogeneous process, giving rise to the growth of lithium dendrites from the cracks.12 The dendrite growth results in internal short-circuit of a Li metal-based battery, which dramatically deteriorates its reliability, safety and service life. Recently, considerable studies had attempted to address the above-mentioned challenges. Among various efforts, chemically passivating Li metal anodes with additive-containing liquid electrolytes was found to improve the stability of resulting SEI layers.13–20 Nevertheless, since the SEI layers are intrinsically brittle and cannot accommodate the volume change of the anodes, crack formation and dendrite growth is still unavoidable during the Li deposition/stripping process. Another solution to the problems is the use of solid-state electrolytes that can mechanically depress the dendrite growth.16,21–27 However, besides low ionic conductivity, most of these electrolytes are apt to delaminate from the Li anodes during the dynamic volume change process, resulting in interfacial contact problems and dendrite growth. Therefore, controlling the reactivity with the electrolyte, especially forming an intrinsically robust and flexible SEI layer is crucial for the stability of Li metal anodes. These require that the layer effectively eliminates side reactions between the anodes and the electrolyte, but also readily accommodates the morphological and volumetric change during the Li deposition/stripping process.26,30 More importantly, the layer can heal itself 3



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without external stimuli once it is broken by the volume expansion. However, to our knowledge, such an adaptable and self-healable SEI layer had rarely been realized for a Li metal anode. Here a Li metal anode is effectively stabilized by in-situ integrating its SEI layer into a self-healing PDMS network cross-linked by imine bonding. Owing to its extendibility and self-healability, the network enables the integrated SEI layer to readily adapt the volume change in the cycling process but also to fast repair itself after breaking (Figure 1). As a result, detrimental electrolyte decomposition and nonuniform Li deposition is significantly depressed for the resulting Li anode, leading to excellent cycling stability and a dendrite-free morphology. The present finding offers a new insight into the stability of Li metal anodes through a self-healing strategy. Given its simplicity and effectiveness, the strategy is extendable for other metal anodes. (a)

(b)

(c)

Figure 1. (a) Synthesis of self-healing PDMS-DFB network. Schematic illustration compares the (b) fragile SEI layer and (c) integrated SEI layer during the Li deposition/stripping process. The fragile SEI layer will break 4


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during the dynamic volume change, resulting in the dendrite growth in its cracks. In contrast, the integrated SEI layer can effectively accommodate the volume change but also repair itself after breaking.

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EXPERIMENTAL SECTION Chemical and materials Xiameter OFX-8040A fluid (PDMS-NH2) were provided by Dow Corning, 1,4-diformylbenzene (DFB), hexamethylene

diisocyanate

(HMDI),

lithium

bis(trifluoromethanesulphonyl)imide

(LiTFSI),

1,2-dimethoxyethane (DME), lithium nitrate (LiNO3) and 1,3-dioxolane (DOL) were provided by Sinopharm Chemical Reagent Co., Ltd (China). Copper foils with a thickness of ~18 µm were supplied by Luoyang Haotai Copper Co., Ltd (China). Metallic lithium foils with a thickness of 1.0 mm and a diameter of 20 mm were purchased from China Energy Lithium Co., Ltd. Synthesis of the self-healable PDMS-DFB elastomer The self-healable PDMS-DFB elastomer was synthesized according to a modified process reported in the literature.1 Briefly, 7-15 mg DFB was dissolved in 18 mL toluene under stirring, later 2 g PDMS-NH2 was added to the above solution. After stirring for 5 min, the resulting mixture was dried at ambient temperature to obtain a transparent PDMS-DFB elastomer (Figure S1). Preparation of the PDMS-DFB coated copper electrodes (Cu-PDMS) 7 mg DFB and 2 g PDMS-NH2 were dissolved in 18 mL toluene under stirring. The resulting solution was then coated onto a copper foil by spin coating. After the evaporation of toluene at 60 oC for 12 h, the as-prepared foil was punched into disks of 15.0 mm in diameter. The thickness of the self-healable PDMS-DFB coating (2-5 µm) was controlled by the amount of the solution used. Electrochemical measurements Two-electrode symmetric coin cells (CR2032) were assembled by using the Cu-PDMS electrodes, Li foils and porous polypropylene separators. The electrolyte was 1.0 mol L–1 LiTFSI dissolved in DOL/DME (1:1 volume ratio) with 1 wt% LiNO3 as additives. Each cell was infused with 60 µL of the electrolyte. The cell assembly was conducted in an argon-filled glove-box with water and oxygen content lower than 2 ppm. The assembled cells 6


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were tested in a galvanostatic mode on a Neware battery tester (China). In each test cycle, 1.0 mAh cm–2 of Li was first deposited onto the working electrodes, and then it was stripped up to the voltage of 1.2 V. Electrochemical impedance spectroscopy (EIS) measurements were carried out on a CHI660D electrochemical workstation. Self-healing of the PDMS-DFB elastomer A PDMS-DFB elastomer with the size of 40 mm length × 10 mm width × 2 mm thickness was first cut into several pieces by a razor blade. The resulting pieces were then brought in contact allowing for geometric matching. After contact for 3-5 min at room temperature (25 oC), the halves autonomously coalesced into a single one. The morphology of the healed region was observed by an optical microscope (PH50-1B43L-A/PL, China). Tensile experiments The self-healability of the PDMS-DFB elastomer was evaluated by tensile experiments. Typically, a piece of the original (or self-healed) PDMS-DFB elastomer was cut into a strip of 2 mm in diameter and 2 cm in length. The ends of each strip were separately fixed to the clips of a high sensitivity microelectromechanical balance (DCAT-21, DataPhysics). The strip was then stretched at a rate of 1 mm s–1 until it was broken. The stress-strain curves of the strip were recorded to measure its tensile strength and strain. Mechanical healing efficiency (η, %) was calculated by the formula η = Th/To × 100, where Th and To are the tensile strength of the healed and the original strips, respectively. Preparation of the covalently cross-linked PDMS coated Cu electrodes 2 mL PDMS-NH2 was added to 18 mL toluene under stirring, later the resulting solution was coated on a copper foil of 5 cm × 5 cm in size by spin coating. After drying at 25 oC for 10 min, the resulting foil was treated with the curing agent (5 mg HMDI in 2 mL toluene) for 30 min. The treated foil was washed with methanol and then dried at 60 oC in vacuum. The obtained foil was punched into disks of 1.5 cm in diameter. Assembly of Li/LiFePO4 full cells LiFePO4 powder, acetylene black and polyvinylidene fluoride (PVDF) with a mass ratio of 8:1:1 were mixed in 7



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N-methyl-pyrrolidinone (NMP) to form slurry. The slurry was coated onto an Al foil. After drying at 120 oC under vacuum, the resulting foil was punched into circular plates with a diameter of 1.5 cm and used as LiFePO4 cathodes. The loading of LiFePO4 was ~1.5 mg cm–2. Li metal anodes were prepared by depositing Li onto the Cu-PDMS electrodes with a capacity of 5 mAh cm–2. Li/LiFePO4 full cells were assembled by using the above cathodes and Li metal anodes. The electrolyte was 1.0 mol L–1 LiPF6 in EC/DME (1:1 in volume). Characterizations Scanning electron microscopy (SEM) observation was conducted on a Zeiss Supra 55. Optical microscopic images were recorded by a PH50-1B43L-A/PL (Phenix, China). Fourier transform infrared (FT-IR) spectra were recorded by a Nicoletis10. X-ray photoelectron spectroscopy (XPS) was performed on a PHI-5700ESCA.

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RESULTS AND DISCUSSION At first the self-healing PDMS network was synthesized through a Schiff base reaction (Figure 1a).31 By simply mixing 1,4-diformylbenzene (DFB) and Xiameter OFX-8040A Fluid (PDMS-NH2) in toluene, we obtained a transparent and flexible PDMS-DFB elastomer (Figure S1, Supporting Information). The composition of the elastomer was identified by Fourier transform infrared spectroscopy (FT-IR, Figure S2 of Supporting Information). Here DFB cross-linked linear PDMS-NH2 chains via reversible imine (−C=N−) bonds formed by the Schiff base reaction, while its electron-withdraw aromatic ring stabilized the negative charge on the imine nitrogen.32,33 Through the transamination between −C=N− and −NH2 (Figure 1a), the elastomer was self-healable after mechanical damage. Here two pieces of the elastomer were separately dyed orange and blue, and later they were cut into pieces by a blade. The resulting pieces were alternately placed in contact allowing for alignment. After contact for only 3 min, the pieces spontaneously merged into a single one that could support its own weight (Figure 2a). Optical microscopy observation showed that there were almost no scars or crevices at the healed region (Figure 2b), indicating full recovery of the configuration integrity. Importantly, the PDMS-DFB elastomer was also self-healable in the organic electrolyte like 1.0 mol L−1 LiTFSI/DOL-DME (Figure S3, Supporting Information).

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(c)

Figure 2. Self-healing of the PDMS-DFB elastomer under ambient conditions (25 oC), (a) optical images, (b) optical microscopic images, (c) stress-strain curves of the elastomer at different healing stages, inset is the mechanical healing efficiency calculated from the curves. (d) Mechanical healing efficiency of the elastomer after multiple cut/healing cycles. The self-healability of the PDMS-DFB elastomer was further evaluated by tensile experiments. Figure 2c records the stress-strain curves of the healed samples after healing for different durations. Compared with its original counterpart, the healed elastomer restores over 84.0% of tensile stress and strain within 1 min. It reaches a maximum mechanical healing efficiency of 99.53% after 3 min healing, suggesting fast restoration of mechanical 10


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properties. The elastomer was self-healable for at least 50 cycles with an average healing efficiency over 95.0% (Figure 2d). It was revealed that the DFB content had an important role in the self-healability of the PDMS-DFB elastomer. Figure S4 is the dependence of mechanical healing efficiency on the mass ratio of DFB to PDMS-NH2. The mechanical healing efficiency decreases with the increase of the DFB/PDMS-NH2 mass ratio. However, further increasing the DFB content significantly deteriorates the self-healing capability. The mechanical healing efficiency dramatically drops to 54.5% as the DFB/PDMS-NH2 ratio is 7.5 wt‰. Moreover, the elastomer is also stretched >200% strain without breaking (Figure S5, Supporting Information). Therefore, our elastomer combines high extensibility and excellent self-healability, which enables it readily adapt the volumetric variation but also spontaneously restore the configuration and mechanical properties after breaking. Then a self-healing PDMS-DFB layer with a thickness of 2-5 µm was coated onto copper electrodes (Figure 3a). The electrochemical property of the resulting Cu-PDMS electrodes was evaluated by galvanostatic charge/discharge (GCD) in coin cells using Li metal foils as the counter electrodes. The electrolyte was 1.0 mol L– 1

LiTFSI dissolved in DOL/DME (1:1 volume ratio) with 1 wt% LiNO3 as additives. For comparison, a bare Cu

electrode was also tested by the GCD mode (Table S1, Supporting Information). During each GCD cycle, Li was deposited onto the working electrodes to an areal capacity of 1 mAh cm–2 from the Li foils. After complete stripping of the Li from the working electrodes, the Coulombic efficiency (CE) was determined by the ratio of the stripped amount to the deposited capacity. Figure 3b records the CE of a typical Cu-PDMS electrode in the cycling process. It exhibits good cycling stability since its average CE is >95.7% over 120 cycles at 0.5 mA cm–2. In contrast, the CE of the bare Cu electrode is only 95.0% after 65 cycles (Figure 3b). As the current density is increased to 1.0 mA cm–2, the Cu-PDMS electrode remains its average CE at 96.8% after 50 cycles, whereas the bare Cu electrode reduces the value to 85.0% for only 15 cycles (Figure 3c). Even at a high current density of 3.0 mA cm–2, the Cu-PDMS electrode keeps an average CE above 92.4% after 30 cycles, while the bare electrode exhibits poor cycling stability (Figure S6, Supporting Information). The Cu-PDMS electrode also showed better 11



cycling stability than the bare Cu electrode at high deposition capacity of 2-5 mAh cm–2 (Figure S7, Supporting Information). The results demonstrate that the self-healable PDMS-DFB coating effectively improves the cycling stability of the Li metal anode without hindering fast charge transfer at the electrode/electrolyte interface. (a)

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Figure 3. (a) Cross-sectional (left) and top (right) SEM images of the Cu-PDMS electrode, (b-c) Coulombic efficiency of Li deposition/stripping (1.0 mAh cm–2) on the bare Cu and Cu-PDMS electrodes at current densities of (b) 0.5 mA cm–2 and (c) 1.0 mA cm–2. (d-e) Voltage profiles of the (d) bare Cu and (e) Cu-PDMS electrodes at the 10th (black) and 80th (red) cycles. (f-g) Nyquist plots of the (f) bare Cu and (g) Cu-PDMS electrodes after different deposition/stripping cycles. Figure 3d-e records the voltage profiles of both electrodes after 10 and 80 deposition/stripping cycles. Compared with its bare counterpart, the Cu-PDMS electrode is much stable in voltage profiles during cycling. Additionally, the Li deposition potential of the Cu-PDMS electrode decreases from -12.7 to -9.6 mV, while that of the bare electrode is increased by 12.7% from -11.8 to -13.3 mV after 80 cycles. The increased deposition potential is due to continuous rupture, reconstruction and thickening of the SEI layer during cycling.24 The reduced polarization of the Cu-PDMS electrode indicates that such a thickening process is largely alleviated by the self-healable PDMS-DFB coating. Furthermore, the voltage hysteresis, which is the voltage difference between Li deposition and Li stripping, decreases and then keep stable during the cycling for the Cu-PDMS electrode. In contrast, the bare electrode continuously increases its voltage hysteresis to ~300 mV after test for 80 cycles (Figure S8, Supporting Information). The discrepancy in the voltage hysteresis demonstrates that the self-healable PDMS-DFB coating effectively mediates the interface between the electrode and the electrolyte, and thus forms a stable SEI layer with high ionic conduction. 13



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The stability of the SEI layers on the Cu-PDMS electrode was also supported by electrochemical impedance spectroscopy recorded over cycling. Figure 3f-g is the Nyquist plots of the two electrodes after different cycles of Li deposition/stripping. Each plot consists of a semicircle at the high-frequency region and a declined line at the middle-frequency region. The semicircle is ascribed to the migration of Li ions through the SEI layer. As shown in Figure 3g, the Cu-PDMS electrode continuously shrinks its semicircle without shape change in the initial 40 cycles of deposition/stripping. Then the electrode keeps a stable semicircle in the subsequent cycles, indicating that the ionic conduction and morphology of its SEI layer almost remains unchanged in the process. Although the bare Cu electrode exhibits a similar tendency in the initial 40 cycles, it appears a second semicircle at medium-frequency region after the 50th deposition/stripping cycle (Figure 3f), implying the formation of a porous SEI layer with reduced Li ion conductivity. The result confirms that the self-healable PDMS-DFB coating can efficiently stabilize the SEI layer and thus allows for a steady charge migration process.

(a)

LiTFSI solution Li+ PDMS-DFB network Penetration

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SEI formation

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Cu

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Figure 4. (a) Schematics illustrate in-situ integration of the SEI layer into the self-healing PDMS-DFB network. SEM images of the Li deposits on the (b-c) bare Cu and (d-e) Cu-PDMS electrodes after 50 cycles of deposition/stripping with 1 mAh cm–2 at 0.5 mA cm–2. Cycling performance of Li/LiFePO4 full cells comprising the (f) PDMS-DFB coated Li metal anode and (g) bare Li metal anode. The anode was pre-deposited 5 mAh cm–2 Li metal and tested at 0.2 C for the first cycle and 1.0 C for the later cycles. The insets are the charge/discharge profiles of the corresponding cells. X-ray photoelectron spectroscopy was conducted to understand the reason for the stable SEI layer on the Cu-PDMS electrode. The results demonstrate that the self-healable PDMS-DFB network integrates with the main 15



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components of the SEI such as LiF and LiyC2Fx (Figure S9, Supporting Information). The integration is due to unique formation mechanism of the SEI layer (Figure 4a). At first the electrolyte solution penetrated into the PDMS-DFB coating through the swelling of the polymer network, forming an organogel layer on the Cu electrode. Later Li metal was deposited on the electrode at a negative potential. Meanwhile, the deposited Li metal reacted with the electrolyte trapped in the organogel, leading to in-situ formation of the SEI layer whose main components were embedded in the self-healing PDMS-DFB network. SEM observation and element mapping also confirmed a homogenously SEI layer on the Cu-PDMS electrode (Figure S10, Supporting Information). Clearly the integration greatly improves the stability of the SEI layer during the dynamic volume change. To confirm the dendrite inhibition of the integrated SEI layer, we observed the morphology of the underlying Li metal on the electrodes after 50 cycles of deposition/stripping. The electrodes were thoroughly washed with DOL to remove the PDMS-DFB coating and the SEI layers before scanning electron microscopy (SEM) observation. The bare Cu electrode shows a porous appearance comprising discrete voids and a plenty of filamentary protrusions (Figure 4b-c). The Li filaments arise from the breaking of the fragile SEI layer during the volume change and thereafter dendrite growth in the resulting cracks. These protrusions increase the contact area with the electrolyte and further accelerate its detrimental decomposition. On the contrary, the Cu-PDMS electrode shows a relatively compact and smooth morphology without filamentary protrusions (Figure 4d-e). The improved morphology homogeneity originates from the integration of the SEI layer in the PDMS-DFB network. Owing to the high extendibility of the network, the integrated SEI layer can readily accommodate the volume change in the deposition/stripping process. Once the SEI layer is broken by the volume expansion, it will fast repair itself through the reconstruction of the network via reversible imine bonding in the subsequent stripping process (Figure 1c). These features effectively prevent the crack formation in the SEI layer, which significantly suppresses detrimental electrolyte decomposition and dendrite growth in cracks. To further confirm the importance of self-healing on the dendrite suppression, we coated a Cu electrode with a 16


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covalently cross-linked PDMS layer that did not exhibit self-healability. The resulting control electrode was tested by the same GCD mode (Figure S11, Supporting Information). It quickly decreased the CE to 78.0% after only deposition/stripping for 35 cycles, exhibiting much poor cycling stability compared with its Cu-PDMS counterpart (Figure 3c). SEM observations further revealed that many voids and Li filaments existed on the control electrode after the cycling (Figure S12, Supporting Information). The poor performance of the control electrode is because its covalently cross-linked PDMS coating cannot repair itself after breaking by the volumetric or morphologic change, and thus the dendrite growth is still unavoidable. Our control experiment demonstrates that self-healing plays a crucial role in dendrite inhibition. As an example of application, we assembled a full cell using a LiFePO4 (LFP) cathode and the PDSM-DFB coated Li anode. The cell delivered a specific capacity of 116.7 mAh g–1 after 300 cycles with a retention rate of 99%, as well as kept a high average CE of 99.5% over the cycling process (Figure 4f). In contrast, a control cell using the bare Li metal anode quickly decreased its specific capacity to 81.8 mAh g–1 after 50 cycles. The result implies that our self-healable PDMS-DFB coating is applicable for a practical battery system.

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CONCLUSIONS In summary, the cycling stability and morphological homogeneity of a Li metal anode were significantly improved by integrating its SEI layer in a self-healing PDMS-DFB network. The improvements were attributed to the fact that the integrated SEI layer could readily accommodate the dynamic volume change during cycling, but also repaired itself via reversible imine bonding after breaking. As a result, detrimental electrolyte decomposition and dendrite growth were effectively suppressed in the repeating charge/discharge process. Application of the strategy was demonstrated by a Li/LiFePO4 full battery that exhibited capacity retention up to 99% and a Coulombic efficiency >99.5% after 300 cycles. The present finding offers a new self-healing strategy to address the stability issue of metal anodes aiming at high energy-density batteries.

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ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: Photographs of the PDMS-DFB elastomer and PDMS solution; FT-IR spectrum of the PDMS-DFB elastomer; effect of the mass ratio of DFB to PDMS-NH2 on the stress-strain curves and mechanical healing efficiency of the resulting PDMS-DFB elastomers; optical images show the extensibility of the PDMS-DFB elastomer, coulombic efficiency of Li deposition/stripping on the bare Cu and Cu-PDMS electrodes; hysteresis of Li deposition/stripping on the bare Cu (red) and Cu-PDMS (black) electrodes; X-ray photoelectron spectra of the SEI layer formed on the Cu-PDMS electrode; coulombic efficiency of Li deposition/stripping on the covalently cross-linked PDMS coated Cu electrode and SEM images of the Li deposits on the covalently cross-linked PDMS coated Cu electrode

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AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Qinmin Pan: 0000-0002-5263-2101 Notes

The authors declare no competing financial interest. ACKNOWLEDGMENTS

This study is financially supported by National Natural Science Foundation of China (51473041) and a self-planned task of the State Key Laboratory of Robotics and System of Harbin Institute of Technology (SKLRS201604C) and Foundation for Innovative Research Groups of the National Natural Science Foundation of China (51521003). Supporting Information Available: [Description of material.] This material is available free of charge via the Internet at http://pubs.acs.org.

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TOC

The stability of a Li-metal anode was greatly improved by integrating its SEI layer into a self-healing PDMS network.

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Stabilizing Li metal anodes through a novel self-healing strategy

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