Surface Functionalization of a Conventional Polypropylene Separator

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Surface Functionalization of a Conventional Polypropylene Separator with an Aluminum Nitride Layer toward Ultrastable and High-Rate Lithium Metal Anodes Patrick Joo Hyun Kim and Vilas G. Pol* Davidson School of Chemical Engineering, Purdue University, West lafayette, Indiana 47907, United States

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S Supporting Information *

ABSTRACT: Lithium (Li) metal as a next-generation anode has received great interest from industry and academic institutes due to its attractive benefits of a high theoretical capacity (3860 mAh g−1) and the lowest negative potential (−3.04 V vs SHE) among the anode candidates. However, major issues associated with dendritic Li growth, infinite volume expansion of Li, and low Coulombic efficiency cause severely degraded cycle stabilities and fatal safety issues (such as short-circuit). Herein, we first designed a functional membrane, comprising an aluminum nitride (AlN) layer and a polypropylene (PP) separator, in order to curb the sharp Li dendrite growth, restrain the propagation of dendritic Li toward the PP separator, and consequently improve the electrochemical stabilities of Li metal batteries. When the designed membrane was introduced in either the Li/Cu half-cell or the Li/LCO full-cell, Li dendrite growth was significantly suppressed and side reactions associated with electrode degradation was effectively prevented by the material benefits of the AlN layer, thus leading to the significantly enhanced cycle performances. Low temperature stability tests further demonstrated the optimiztic potentiality of the designed membrane for enabling the stable operation of Li metal batteries under harsh conditions. Our approach of adopting a metal nitride layer to the PP separator can be a compelling strategy to improve the long-term electrochemical stability of the Li metal electrode. KEYWORDS: AlN layer, lithium metal batteries, functional membrane, Li dendrite growth suppression, LCO cathode

1. INTRODUCTION

mentioned challenges, several strategies have been attempted by: (a) introducing a functional layer on top of the Li metal surface, (b) engineering an electrolyte with functional additives, and (c) designing a 3D current collector with high surface area.6−11 These approaches have presented outstanding improvements in extending the lifetime of Li metal electrode as well as improving the efficiency of Li plating/stripping. Apart from these approaches, an alteration of a polypropylene (PP) separator with multifunctional materials (SiO2 nanosheet, graphene oxide, etc.) has been intensively explored due to its methodological benefits and effectiveness.12−16 It is known that AlN has an outstanding thermal conductivity (∼320 W (m K)−1) and low cathodic limit

As state-of-art electronic applications extending from portable devices to large-scaled devices (such as electric vehicle, smart grid, etc.) have become an indispensable part of our lives, the necessity for establishing energy storage systems with high energy density and long-term stability has kept increasing.1,2 Among many of next-generation anode candidates, lithium (Li) metal electrode has obtained a significant interest from various engineering disciplines and industries, owing to the high theoretical energy capacity (3860 mAh g−1), the most negative potential (−3.04 V vs SHE) and its impressive potential to other next-generation applications including Li−S and Li−air systems.3,4 However, major issues, mainly associated with dendritic Li growth, infinite volume expansion of Li, and poor Coulombic efficiency, hamper the use of Li anode in global battery markets and cause safety hazards (e.g., short-circuit).3−5 With the purpose of addressing the above© XXXX American Chemical Society

Received: October 24, 2018 Accepted: January 3, 2019 Published: January 4, 2019 A

DOI: 10.1021/acsami.8b18660 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Characterization. (a) Photo images of a fabricated AlN-separator, a bent AlN-separator, and a twisted AlN-separator. (b) Photo images of a punched PP separator and a punched AlN-separator; top SEM images of (c) a pristine separator and (d,e) an AlN-separator; (f) cross-section SEM image of the AlN-separator; (g) XRD patterns, (h) Raman spectra, and (i) BET analysis of bulk AlN and milled AlN.

(−0.004 V vs Li/Li+).17−20 In addition, metal nitrides have a better electrochemical and chemical stability against the Li metal than other metal oxides or sulfides.20 We assume that these features render the AlN as an ideal layer material for alleviating unexpected side reactions happened on the Li metal interface. There are a few reports that used a thermally conductive material (e.g., boron nitride, i.e., BN) for suppressing the sharp Li dendritic growth.21−23 It showed remarkable results in controlling the size and diameter of dendritic Li and enhancing the cycle retention of Li-metal batteries. Inspired by the previous reports, herein we first designed a functional membrane (AlN-separator) comprising an AlN layer and a PP separator (Celgard 2500) to restrain the propagation of dendritic Li toward the PP separator, inhibit the growth of long and sharp Li dendrites, and consequently improve the overall performances of Li metal batteries. The membrane was prepared by laminating AlN nanopowders onto the PP separator through tape-casting. To give an intimate contact between the AlN layer and the Li anode and, eventually, reduce the overall interfacial impedance of the cell, AlN powder (with size of ∼100 nm) was softly milled in Ar atmosphere to break agglomerated particles into AlN nanopowder (Supporting Information, Figure S1). When the AlNseparator was introduced in a Li/Cu half-cell, electrochemical performances and stabilities were dramatically improved over

100 cycles without noticeable change of Coulombic efficiency. These are ascribed to two key benefits of the fabricated membrane: (a) an effective stabilization of Li metal surface and (b) a physical isolation of propagating Li dendrite from touching a counter electrode. When it was coupled with conventional cathode electrodes (LiCoO2 and LiFePO4), these systems also exhibited excellent results in achieving highly stable cycle performances, along with high discharge/charge capacity. Throughout this approach, we successfully demonstrated the optimistic potential of the AlN-separator for reducing the growth of sharp and long Li metal dendrites and accomplishing the highly stable cycle performance of Li metal electrodes.

2. EXPERIMENTAL SECTION 2.1. Preparation of AlN Nanopowder and AlN-Separator (or Al2O3-Separator). Before the milling process, 1000 mg of AlN powder (Sigma) was put into a container with a few plastic balls in an Ar-filled glovebox. Once the container was loaded in a milling station, it was homogenized for 15 min. As to the preparation of the AlNseparator, 80 mg of milled AlN nanopowder and 20 mg of polyvinylidene fluoride (PVdF) were dispersed in N-methyl-2pyrrolidone (nMP) via the mixing process to make the uniform slurry. The prepared slurry was casted onto the one side of a PP separator via tape-casting, followed by drying the membrane in the vacuum oven more than 24 h. Once the prepared membrane was completely dried, it was cut into a membrane with a diameter of 15 mm and used it for cell fabrication (2032 type). The preparation of a B


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Figure 2. Electrochemical performance of Li/Cu half-cells. (a) Schematic illustration to depict the function of each separator on the formation of Li metal dendrites, (b) cycle performance of each cell, (c) voltage profiles, and (d) Nyquist curves of the Li/Cu half-cell with a pristine separator at different cycle numbers. (e,f) SEM images of the cycled Cu foil tested with a pristine separator, (g) voltage profiles, and (h) Nyquist curves of the Li/Cu half-cell with an AlN-separator at different cycle numbers. (i,j) SEM images of the cycled Cu foil with an AlN-separator. Al2O3-separator followed the exact preparation step as the AlNseparator. 2.2. Assembly of Li/Cu Cells and Other Full Cells (LCO and LFP). To evaluate each PP separator and AlN-separator in Li/Cu cells, pure 12 mm Li disc and 12 mm Cu electrode were employed for battery assemblies. The tested electrolyte is 1.0 M LiPF6 ethylene carbonate/diethyl carbonate (EC/DEC = 1:1, Sigma-Aldrich). The Li plating (discharge) capacity was 1 mAh cm−2, and the upper cutoff voltage was 1.0 V. In regard to the cathode preparation, 80 wt % active materials (LiCoO2 or LiFePO4), 10 wt % PVdF binder and 10 wt % conducting agent were homogenized in nMP, and then the prepared ink was laminated onto an Al foil. Once it was fully dried, the electrodes were cut and used for fabricating full cells. The same amount of electrolyte was utilized for full-cell studies. 2.3. Characterizations. The morphological images of a pristine PP separator and an AlN-separator were analyzed by NanoSEM 450. The specific surface areas of each bulk AlN and milled AlN were estimated by BET (Micromeritics Tristar 3000). Raman spectra of bulk AlN and milled AlN were analyzed by Raman microscopy (ThermoScientific). The XRD measurement (Rigaku SmartLab Xray

diffractometer) was performed to check the crystalline phase change of the bulk AlN before and after milling. The electrochemical performances of cells were evaluated by a battery cycler (MTI). The electrochemical impedance spectroscopies (EIS) measurement of each cell with a PP separator and an AlN-separator were tested by an potentiostat/galvanostat in the frequency ranging from 100 kHz to 0.1 Hz. The XPS analysis was performed to ascertain the change of Li metal anode in binding energy before and after cycle (Supporting Information, Figure S2). To depict the stability of the AlN-separator over repetitive electrochemical tests, the cycled separator was analyzed by SEM measurement (Supporting Information, Figure S3).

3. RESULTS AND DISCUSSION Figure 1a displays the photo images of a fabricated AlNseparator, a bent AlN-separator, and a twisted AlN-separator. The size of the membrane is 10 cm long and 4 cm wide. The membrane can be bent and twisted without structural deformation due to the excellent adhesion property between C


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Figure 3. The effect of AlN layer on the interfacial reaction and rate capability. (a) Nyquist plots of Li/Cu half-cells with a pristine separator and an AlN-separator. Voltage profiles of (b) the Li/Cu half-cell with a pristine separator and (c) the Li/Cu half-cell with an AlN-separator at different current densities. Cycle performances of (d) the Li/Cu half-cell with a pristine separator and (e) the Li/Cu half-cell with an AlN-separator at high current densities.

with the bulk AlN powder to prepare the milled AlN. After coating the milled AlN onto the pristine PP membrane via tape-casting, SEM images show well-distributed AlN nanopowders over the PP separator (Figure 1d,e). The thickness of the AlN layer is around 10 μm (Figure 1f). To confirm how the milling process affects the crystallinity of AlN powder, XRD measurement was performed with the bulk AlN powder and the milled AlN, respectively. Interestingly, there was no noticeable change of XRD pattern and intensity before and after milling, indicating that this process hardly affects the crystalline phase of AlN (Figure 1g). Raman spectrum of the

the PP separator and the AlN layer. Figure 1b presents the pictures of a punched PP separator and a punched AlNseparator, indicating both separators can be cut into any shape and used for the cell evaluation. To characterize the morphologies of each separator, scanning electron microscopy (SEM) characterizations were conducted. Figure 1c shows plenty of gaps and pores of the PP separator all over the area, which facilitates the efficient permeation of organic liquid electrolyte.15 To reduce the granule size and give an intimate contact between the Li metal and the AlN layer for the subsequent study, the milling process was softly carried out D


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Figure 4. Li/LCO full-cell evaluation and postdiagnostic study with cycled Li metals and a cycled AlN-separator. (a) Cycle performance of each full-cell. Voltage profiles of each full-cell at (b) the 1st cycle and (c) the 100th cycle. (d) Photo image and (e) morphology of the cycled Li metal disassembled from the Li/LCO full-cell with a pristine separator. (f) Photo image and (g) morphology of the cycled Li metal disassembled from the Li/LCO full-cell with an AlN-separator. SEM and EDS mapping images of (h,i) the AlN side and (j,k) the PP side of the cycled AlN-separator.

cycle number and the morphology of the Li dendrite, electrochemical impedance spectroscopy (EIS) measurements and postdiagnostic studies were performed. As shown in Figure 2d, the charge transfer resistance, estimated by a diameter of semicircle, of the Li/Cu half-cell with a pristine PP separator, exhibited 103.1 Ω at first cycle and increased to 230.6 Ω at 100th cycle. It is most likely associated with the harsh growth of sharp and long Li metal dendrites caused by the electrolyte depletion; this dramatically increased the interfacial impedance between the Li electrode and the PP separator.12 In addition, the SEM image of the Cu foil evaluated with a PP separator clearly reveals the morphologies of Li dendrites after 100 cycles, showing long and sharp Li dendrites over the entire surface of the Cu foil (Figure 2e,f). Interestingly, the Li/Cu half-cell with an AlN-separator displayed a small change in charge transfer resistances (from 105.4 to 144.5 Ω) even after 100 cycles due to the improved interfacial reactions and the suppressed growth of long and sharp Li dendrites (Figure 2h).21−23 As provided in the SEM images, the morphology and size of Li dendrite became blunt and larger (Figure 2i,j), which agrees well with previous works using a thermally conductive BN layer.3,5,23 It is known that a thermally conductive layer enables a homogeneous thermal distribution during discharge/ charge, then it significantly reduces the Li nuclei and enlarges the size of Li dendrites accordingly. This phenomenon leads to a less consumption of liquid electrolyte and eventually minimizes the chances of short-circuiting.21,22 Extrapolating from the above results, it can be concluded that the AlN layer formed on the top of the PP separator significantly affects the dendrite morphologies associated with electrochemical reactions and kinetics and enables the stable operation of Li plating/stripping in the Li/Cu half-cell.

milled AlN showed an analogous pattern shape with the bulk AlN powder, which additionally supported the result of XRD measurement (Figure 1h). After milling process of the bulk AlN powder (13.5 m2 g−1), the surface area of milled AlN increased to 116 m2 g−1 (Figure 1i). Figure 2a depicts the functions of each separator on the interface between a Li metal and a separator and shows how these affect the morphologies of Li dendrites on the current collector (the Cu foil). The PP separator has no capability to control the localized heat through the interface between the Li metal and the pristine separator, leading to the growth of long and sharp dendritic Li. We assume that an AlN layer on top of the AlN-separator disperses the intensely localized heat and stabilizes the Li metal surface, which results in the formation of mossy Li dendrites which have larger diameter and thus reduces the possibility of a short-circuit. As presented in Figure 2b, the initial Coulombic efficiency of each electrode exhibited an analogous value around 92%. However, the efficiency of the Li/Cu half-cell with a pristine PP separator began to go up and down at the 45th cycle due to the fast electrolyte consumption and the unstable Li metal interface.12,21 The voltage curves of the cell with a pristine PP separator show deteriorated electrochemical reactions and behaviors at the 50th and 100th cycle, which are attributed by the drastic Li dendrite formation (Figure 2c). By contrast, the Li/Cu half-cell with an AlN-separator stably maintained its Coulombic efficiency around 92 % over 100 cycles without noticeable decay, which is supplemented by the voltage profile tendency (Figure 2g). The morphologies of Li metal surface evaluated with a PP separator and an AlN-separator were examined and provided in Supporting Information, Figure S4. To ascertain how the interfacial impedance is affected by the E


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Figure 5. Electrochemical stability test at low temperatures in Li/Cu cells. (a) Initial potential curves and (b) cycle performances of Li/Cu halfcells at 10 °C. (c) Initial potential curves and (d) cycle performances of Li/Cu half-cells at 0 °C. Each cell was evaluated at 0.5 mA cm−2 and Li stripping capacity was 1 mAh cm−2.

contrast, the cell with an AlN separator stably retained its Coulombic efficiency around 80 % even under high current conditions (Figure 3e). To practically demonstrate the potentiality of the AlNseparator in conventional battery systems, full cell tests were carried out by coupling a Li metal film (as an anode) with a conventional cathode electrode (LiCoO2). The LCO electrode (∼14.8 mg cm−2) was used to apply a high areal current to fullcells and see how the high areal current density affects the electrochemical performances and the formation of Li dendrites in full cells. Electrochemical performances of full cells are presented in Figure 4. Figure 4a displays the cycling results of each cell with and without an AlN-separator at 1 C. To clearly ascertain the influence of the AlN layer on stabilizing the electrochemical performances of LCO electrode, each cell was tested with the high upper cutoff potential (at 4.4 V). In general, LCO electrode experiences a crystal lattice deterioration when the cell is operated at high cathode potentials (>4.2); it aggravates the overall electrochemical performances of full cells.25,26 After 100 cycles, the Li/LCO cell with a PP separator delivered a poor capacity of 47.4 mAh g−1 (28.5 % of initial capacity). On the contrary, the Li/LCO cell with an AlN-separator showed a much higher discharge capacity of 122.5 mAh g−1 (with higher cycle retention of 73.6 %) at the 100th cycle. The voltage profiles of each electrode before and after 100 cycles clearly shows a tendency of capacity fading. The initial voltage profiles display the analogous curve shape (Figure 4b). The Li/LCO cell with a pristine separator

So as to investigate how the AlN layer influences the kinetics of electrochemical reactions, comparison studies were performed by using Li/Cu half-cells with a PP separator and with an AlN-separator. Figure 3a shows the Nyquist curves of each fresh cell. Each cell shows an analogous curve shape and similar impedance value even though an additional AlN layer was placed in between the Li metal and the PP separator. Throughout this result, it is confirmed that ∼10 μm AlN layer hardly affects the interfacial resistance between the electrode and the separator and metal nitride material, i.e., AlN, is highly stable against Li metal. Another cycle test of the Li/Cu half-cell with an Al2O3-separator further supports that metal nitrides have a better electrochemical and chemical stability against Li metal than metal oxides (Supporting Information, Figure S5). When each cell was tested at different current densities, it showed interesting electrochemical behaviors (Figure 3b,c). The cell tested with an AlN-separator exhibited a much less polarization than that with a PP-separator. It is likely that the efficient distribution of localized heats induced by harsh electrochemical conditions (e.g., high constant current) gives rise to the homogeneous nucleation and growth of Li dendrites associated with electrochemical reactions and kinetics and, accordingly, contributes to high rate capability of Li metal batteries.24 For further examining the cycle stabilities at high current densities, each cell was tested at 1 and 2 mA cm−2, respectively, and the Li plating capacity was set as 1 mAh cm−2. The cell with a PP separator rapidly decayed and reached a Coulombic efficiency of 20 % over 60 cycles (Figure 3d). In F


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unstable electrochemical reaction for the first few cycles and a rapid decay of efficiency after 20 cycles (Figure 5b). At 0 °C, each cell exhibited a larger polarization gap than the cell tested at 10 °C due to an increased interfacial impedance of each electrode and a poor Li ion diffusivity (Figure 5c). Despite the increased polarization, the Li/Cu half-cell with an AlNseparator still delivered an excellent cycle retention over 100 cycles as comparison of that with a pristine separator (Figure 5d). Extrapolating from these results, it is ascertained that the AlN layer can not only extend the lifetime of Li anode-based cells but also effectively stabilize the electrochemical reactions even under low-temperature environments.

did not retain its capacity at the 100th cycle, while the Li/LCO cell with an AlN separator exhibited a much stable electrochemical operation even after repetitive cycles (Figure 4c). These are attributed to two main effects of the AlN layer: (a) an effective stabilization of the Li metal surface and (b) a prevention of undesirable side reaction and/or dissolution of cathode electrode, which can be colligated with the previous ALD study.25 To unravel the capacity fade mechanism of the cells, each cell was disassembled and SEM measurements were performed. As shown in Figure 4d, the Li metal surface disassembled from the Li/LCO cell with a pristine separator shows fully covered black spots on the Li metal. The morphological shape of the black spots shows large and sharp Li dendrites, which corresponds with the obtained result (Figure 4e). As to the Li metal film disassembled from the Li/ LCO cell with an AlN-separator, it relatively has few black spots over the Li metal surface compared to the cycled cell with a PP separator (Figure 4f). In addition, the microstructure of bright black spots shows an agglomerated Li chunk with low surface area, which is also consistent with the obtained half-cell result (Figure 4g). Throughout this result, it is further confirmed that the AlN layer can stabilize the electrochemical reactions at the interface of Li anode and consequently affect the formation of Li dendrites even in the Li/LCO full-cell system. So as to observe the morphology and surface composition of the AlN-separator and investigate how the membrane changed after cycles, SEM measurements and energy dispersive spectroscopy (EDS) mappings were conducted with the cycled AlN-separator. Figure 4h presents the surface morphology of the AlN layer after cycling, showing uniformly distributed AlN particles over the PP separator. Further EDS mapping clearly distinguishes elements presented on the surface of AlN layer. Interestingly Co element was detected throughout the AlN side of the AlN-separator, which was attributed by the dissolution of Co element from the LCO structure (Figure 4i). On the contrary, a much less amount of Co element was found on the PP side of the AlN-separator (Figure 4j,k). Throughout these results, it was directly ascertained that the AlN layer not only inhibits the sharp Li dendrite growth but also prohibits the migration of Co element toward the Li metal, leading to enhanced electrochemical performances of full cells. In addition to the Li/LCO study, LiFePO4 (LFP) was tested as another potential cathode. It also delivered excellent cycle stabilities over 500 cycles without dramatic capacity fade, which further demonstrated the optimiztic potentials of the designed functional separator for practical cathode electrodes (Supporting Information, Figure S6). It is previously reported that Li dendrites are prone to be grown at low temperatures owing to the poor kinetics of Li diffusion and the high impedance of surface reaction on the electrode.27,28 To understand how the thermally conducting layer, i.e., the AlN layer, affects the cycle stability of Li metal batteries under harsh conditions (at low temperatures), electrochemical stability tests were carried out. Environmental chamber was used to evaluate the cell at different temperatures of 0 and 10 °C. Each cell tested at 10 °C shows an analogous polarization gap, which is also comparable to the polarization gap of cells tested at room temperature (Figure 5a). The Li/ Cu half-cell with an AlN-separator delivered a highly stable cycle stability without significant fade over 100 cycles, while the Li/Cu half-cell with a pristine PP separator exhibited an

4. CONCLUSION Herein, we successfully designed a functional membrane composed of an AlN layer and a PP separator in order to achieve the excellent cycle stability of Li metal anode, along with high Coulombic efficiency. The functional membrane was designed by forming an AlN layer on top of the conventional PP separator. When the designed separator was introduced in Li/Cu half-cells, the retention of Coulombic efficiency over 100 cycles was dramatically improved in comparison to the Li/ Cu half-cell tested with a PP separator. It also showed excellent cycle stabilities when paired with conventional cathode electrodes (e.g., LCO and LFP). Interestingly, the Li/Cu half-cell with an AlN-separator delivered reasonably stable electrochemical performances even in extremely low temperatures of 0 °C. These results are mainly contributed by the positive benefits of the AlN-separator: (a) the electrochemical and chemical stability of AlN layer against Li metal and (b) the effective suppression of sharp and long Li dendrite growth. Different from the previous reports using inorganic compounds and polymeric materials, the approach of employing thermal conductive nitride material, i.e., the AlN layer, offers an effective strategy to inhibit the growth of the sharp Li pillar and improve the electrochemical stability under harsh conditions (high areal current density and low temperature). It will open a new way of effectively addressing systemic and practical challenges underlying the use of Li anode and can be extended to other metal nitride materials which have excellent stabilities against Li metal.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b18660.

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Nyquist plots of the Li/Cu half-cell tested with a bulk AlN-separator and a milled AlN-separator, XPS characterization of the Li metal before and after cycle, SEM image of the cycled AlN-separator, SEM images of Li metal surface tested with a PP separator and an AlNseparator after cycle, comparison of cycle performance of each Li/Cu half-cell with a PP separator and an Al2O3separator, cycle performance of Li/LFP full-cell with a PP separator and an AlN-separator (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Vilas G. Pol: 0000-0002-4866-117X G


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ACS Applied Materials & Interfaces Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Prof. Pol acknowledges the financial support from Office of Naval Research, grant numbers N00014-15-1-2833 and N00014-18-1-2397.

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Surface Functionalization of a Conventional Polypropylene Separator

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