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Ultrastrong Polyoxyzole Nanofiber Membranes for Dendrite-Proof and Heat-Resistant Battery Separators Xiaoming Hao,† Jian Zhu,*,‡ Xiong Jiang,† Haitao Wu,† Jinshuo Qiao,† Wang Sun,† Zhenhua Wang,*,†,§ and Kening Sun*,†,§ †

Beijing Key Laboratory for Chemical Power Source and Green Catalysis, School of Chemical Engineering and Environment, Collaborative Innovation Center of Electric Vehicles in Beijing, Beijing Institute of Technology, No. 5 Zhongguancun South Avenue, Haidian District, Beijng, 100081, P. R. China ‡ Department of Materials Science and Engineering, Northwestern University, 2200 Campus Drive, Evanston, Illinois 60208, United States §

S Supporting Information *

ABSTRACT: Polymeric nanomaterials emerge as key building blocks for engineering materials in a variety of applications. In particular, the high modulus polymeric nanofibers are suitable to prepare flexible yet strong membrane separators to prevent the growth and penetration of lithium dendrites for safe and reliable high energy lithium metal-based batteries. High ionic conductance, scalability, and low cost are other required attributes of the separator important for practical implementations. Available materials so far are difficult to comply with such stringent criteria. Here, we demonstrate a high-yield exfoliation of ultrastrong poly(p-phenylene benzobisoxazole) nanofibers from the Zylon microfibers. A highly scalable blade casting process is used to assemble these nanofibers into nanoporous membranes. These membranes possess ultimate strengths of 525 MPa, Young’s moduli of 20 GPa, thermal stability up to 600 °C, and impressively low ionic resistance, enabling their use as dendrite-suppressing membrane separators in electrochemical cells. With such high-performance separators, reliable lithium−metal based batteries operated at 150 °C are also demonstrated. Those polyoxyzole nanofibers would enrich the existing library of strong nanomaterials and serve as a promising material for large-scale and cost-effective safe energy storage. KEYWORDS: Lithium metal-based battery, poly(p-phenylene benzobisoxazole), nanofiber, ultrastrong material, lithium dendrite, nanoporous membrane It is crucial to find high modulus yet flexible battery separators to meet the stringent demands in the future lithium battery technologies. These separators also capably address additional safety concerns caused by external mechanical abuse, abrupt change of environmental temperatures, or accidental high charge rates. Moreover, the properly engineered separators can be easily adapted for other battery types.16 Conventional microporous or nonwoven polymeric separators are flexible but are usually mechanically weak.17,18 Strong, stiff, and tough polymers are attractive separator materials, but they are typically difficult to process into porous networks for effective ion transport. Recently, membranes consisting of entangled high modulus dielectric nanofibers (NFs), such as cellulose NFs or aramid NFs, have been investigated for high performance separators.7,19 Cellulose NFs have yet to be useful in stunting the dendrite growth, primarily owing to the difficult combination of high porosity and mechanical strength.20,21 Aramid NFs have been combined with poly(ethylene oxide) to

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echargeable lithium metal-based batteries hold great promise for future energy storage, since lithium metal anode has a theoretical density almost ten times that of graphite anode in conventional lithium ion batteries.1−3 Nevertheless, their development has been hindered by inhomogeneous lithium deposition and dendrite formation, causing safety issues and unreliable electrochemical performances.2,4−6 In particular, lithium dendrites might penetrate through separators to shortcircuit cells, inducing thermal runaway or catastrophic cell failures.7−11 Suppressing the lithium dendrite growth is therefore indispensable to capitalize the benefits of these cells. The addition of halogenated or polysulfide and nitrate salts in carbonate or ether electrolytes partially alleviate the problem by reducing the lithium nucleating sites for a stable solid electrolyte interface (SEI).2,4 Alternatively, high modulus separators, such as ceramic solid lithium electrolytes or nanoporous ceramics and their composites, are capable of physically stemming the flow of mobile lithium atoms during the deposition.12−15 However, cracks might form in these brittle ceramic separators during battery fabrications, thereby providing undesirable sites for dendrites to grow and proliferate.7,15 © 2016 American Chemical Society

Received: December 16, 2015 Revised: April 1, 2016 Published: April 22, 2016 2981

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Figure 1. Preparation of PBO nanofibers (PBO-NFs). (a) Diagram of the PBO molecular structure. PBO consists of three ring moieties: benzene (B), oxazole (O), and phenylene (P) groups. Digital photos of (b) PBO yarn and (c) PBO-NF dispersion. SEM images of (d) a pristine PBO microfiber and (e) a PBO-NF network.

Figure 2. Spectroscopic analyses of PBO-NFs. (a) XRD patterns of a PBO microfiber and PBO-NFs with Gaussian peak fittings (b) Raman spectra and (c, d) FTIR spectra of a PBO microfiber and PBO-NFs. The stretching and bending/deforming modes of different functional groups are indicated by γ and δ respectively. Abbreviations: ring groups including benzene (B), oxazole (O), phenylene (P) and other atoms including carbon (C), nitrogen (N), oxygen (O), and hydrogen (H). Refer to Supporting Information for detailed assignments of different peaks.

produce dendrite-suppressing membranes, but a laborious and iterative assembly process is required.7 Here, we describe the fabrication of a thin nanoporous network from exfoliated poly(p-phenylene benzobisoxazole (PBO) nanofibers through a simple blade casting process. The obtained nanoporous membranes (NMs) have superior mechanical properties with ultimate strengths of 525 MPa and Young’s moduli of 20 GPa. The NMs are further demonstrated

as dendrite-proof rechargeable lithium battery separators with persistent electrochemical stability and thermal stability up to 600 °C. In addition, lithium−metal based batteries with these NM separator incorporated can be reliably operated at 150 °C. Such an easily scalable approach to prepare a mechanically strong nanoporous separator will facilitate practical implementation of rechargeable lithium metal batteries, or other metal 2982

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Figure 3. Fabrication of a PBO nanoporous membrane (PBO-NM). (a) Schematic drawing of the PBO-NM preparation using blade casting method. Photographs of flexible PBO-NM (b) lying on the table and (c) after rolling. (d) Cross-sectional view of PBO-NM by SEM (e) Stress−strain curve of PBO-NM. (f) TGA of PBO microfiber, PBO-NM, and Celgard 2400.

wavelength, β is the full-width at half-maximum of the peak in radians, and θ is the Bragg angle of the corresponding peak, the size of crystalline domain in PBO NFs can be calculated to be ca. 4.0 nm based on the broadening of (200) reflection.27 The crystallite size matches well with the morphological analyses in the SEM and TEM images (Figure 1d and S2). Raman and FTIR spectroscopy are used as complementary methods to probe the chemical information on PBO. In the both spectra (Figure 2b−d), the characteristic vibrational peaks23 in harvested PBO-NFs from IPA show no shift as compared to those in PBO microfiber, demonstrating the integrity of PBO molecules after the exfoliation process (see Supporting Information, Tables S1 and S2 for detailed peak assignments). The peak intensity of PBO-NFs in Raman spectra is much stronger under the same measurement condition (Figure 2b), despite PBO microfiber has more materials in the laser spot area. The peak intensity might be enhanced by the improved light transmission and light scattering from the PBO-NF network surfaces. In the FTIR spectra, the changes of relative peak intensities are also observed, which might be attributed to the different chemical environments experienced by PBO molecules in the aligned microfiber structure and the random nanofiber networks. In order to produce a nanoporous membrane from the PBONF dispersion in the mixed MSA/TFA acids, a blade-casted thin liquid film (Figure 3a) was immersed into an IPA coagulation bath and was gradually transformed into a solid film (Figure S3). A flexible PBO nanoporous membrane (PBONM) with an area of 4 cm × 30 cm was prepared via this scalable method (Figure 3b and c). The phase segregation occurs when the attached protons are stripped off from the solvated PBO-NFs in the acids. The IPA serves as a Brønsted base to accept protons in the coagulation process. After complete removal of the acids and thorough drying at room temperature, a 3.1 μm thick membrane with high uniformity is obtained (Figure 3d). The film consists of interwoven PBONFs with apparent pores ranging from 5 to 25 nm on the

based batteries for cost-effective and safe electrochemical energy storage. PBO is a rod-like aromatic heterocyclic polymer consisting of repeating units of benzobisoxazole and phenylene functional groups (Figure 1a). It is commercially available as Zylon yarns (Figure 1b), in which PBO molecules are aligned in a microfiber through a typical wet spinning process. It is worth noting that Zylon yarns typically have higher strengths and moduli than Kevlar yarns.22−24 To attain PBO nanofibers (PBO-NFs), PBO yarns were dissolved in a mixed acid of methanesulfonic acid (MSA) and trifluoroacetic acid (TFA) at room temperature, forming a stable dispersion (Figure 1c).25 Those strong acids gradually protonate the nitrogen or oxygen atoms on the polymer backbone to reduce intermolecular attractions and chain stiffness.26 Eventually, the 10 μm wide PBO microfiber (Figure 1d) is exfoliated into PBO-NFs with diameters of 2−10 nm according to scanning electron microscopy (SEM) analyses (Figure 1e and S1). The exfoliation is further confirmed by transmission electron microscopy (TEM), demonstrating a similar diameter distribution of 2−7 nm (Figure S2). Similar extraction of NFs from microfibers was observed for aramid NFs from Kevlar using a deprotonation strategy, in which KOH saturated dimethyl sulfoxide was employed.27,28 Likewise, PBO-NFs are incapable of further disintegration into individual molecules by the acids, possibly due to the strong energy penalty of surface energy increase and durable cohesive interchain attractions. Actually, those NFs are elementary constituents in the hierarchical structure of a PBO microfiber, as observed during the fiber extrusion process in early studies.29,30 The diminishing crystallite size in the PBO-NFs harvested from an isopropanol (IPA) coagulation bath is confirmed by Xray powder diffraction (XRD) spectra, where the peaks characteristic of PBO crystalline reflections of (200), (210̅ ), and (010) are broadened in comparison to those for a PBO microfiber (Figure 2a).31 On the basis of the Scherrer equation, D = 0.89λ/β cos θ, where D is the crystallite size, λ is the X-ray 2983

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Figure 4. Evaluation of PBO-NM as a battery separator. (a) Contact angle of liquid electrolytes on PBO-NM. (b) Nyquist plots of PBO-NM and Celgard 2400 as separators in symmetric lithium coin cells. Voltage profiles for lithium plating/stripping experiments as a function of time at a fixed current density of 0.38 mA/cm2 using (c) Celgard 2400 and (d) PBO-NM. The current direction is changed every 3 h. SEM images of lithium electrode in contact with (e) Celgard 2400 after 240 h cycling and (f) PBO-NM after 700 h cycling.

surface (Figure S4). Brunauer−Emmett−Teller (BET) analysis of the PBO-NM shows a similar pore diameter distribution (Figure S5), a surface area of 47 m2 g−1, and a porosity of 15%. Other solvents, such as water, acetone and ethyl acetate, also induce the coagulation of PBO-NFs, but the generated solid films appear denser with less conspicuous nanopores or NFs (Figure S6). Those solvents tend to precipitate the dispersed PBO-NFs more easily than IPA due to their stronger ability to accept protons, thus causing the coalescence of the PBO-NFs in the converted solid films. In addition, the drying conditions would significantly alter the pore sizes and porosity (Figure S7). For example, when the film is freeze-dried, an aerogel structure with micron-scale pores is obtained (Figure S7). These observations may serve as guidelines for future optimization of pore sizes and distributions. PBO-NMs show impressive mechanical properties with ultimate strengths of 525 ± 20 MPa, and Young’s moduli of 20 ± 3 GPa (Figure 3e). In comparison to other NF films, the ultimate strength and Young’s modulus of PBO-NM are 2.4 and 1.5 times those of a cellulose NF film,32 4.4 and 2.9 times those of an aramid NF film,33 and 53 and 25 times those of a carbon nanotube film.34 Such extraordinary mechanical properties not only stem from high intrinsic strength in PBO-NFs, but strong van der Waal interactions between NFs as well. The cross-section of PBO-NM after a mechanical failure shows NFs protruding from edges (Figure S8), indicative of efficient stress transfer. In addition, the nacre-like hierarchically layered structure owing to drying induced gradual collapse of PBONFs might make the membrane more defect-tolerant as well.35−38

Similar to the PBO yarns, PBO-NM demonstrates excellent thermal stability. Thermogravimetric analysis (TGA) on PBONM indicates that no obvious weight loss occurs below 600 °C in N2 atmosphere (Figure 3f), and the differential scanning calorimetry analysis on PBO-NM confirms no phase change until 480 °C (Figure S9). This superior thermal tolerance could effectively prevent internal short-circuit at evaluated temperatures. In comparison, the industrial staple, Celgard 2400 separator, decomposes sharply at 450 °C (Figure 3f) and exhibits an endothermic peak at 122 °C corresponding to its melting point (Figure S9). The time-lapse photographs of those two separators on a 190 °C hot plate further demonstrate a clear difference (Figure S10). The PBO-NM exhibits a negligible dimension change, whereas the Celgard 2400 separator immediately melts with its area shrunk by 80%. These results elucidate that PBO-NM is a suitable candidate for electrochemical devices operated at high temperatures. PBO-NMs are then evaluated as battery separators. It is known that the wettability of liquid electrolytes on a separator directly affects the electrochemical performance.39 For such information, contact angle measurements were performed using liquid electrolytes containing 1 M LiPF6 in a mixed solvent of ethylene carbonate (EC), diethyl carbonate (DEC), and ethylmethyl carbonate (EMC) with a volume ratio of 1:1:1 on the PBO-NM and Celgard 2400 (Figure 4a). The electrolyte contact angles are 20° and 45° on PBO-NM and Celgard 2400, respectively, implying that PBO-NM is more wettable than Celgard 2400 separator. We also observe that the PBO-NM readily absorbs the liquid electrolytes in the experiments. Such 2984

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Figure 5. Battery performance of coin cells using PBO-NMs as separators. (a) Charge−discharge curve for a CR2032 button cell consisting of a LiCoO2 cathode, a PBO-NM separator wetted with 1 M LiPF6 in EC/DEC/EMC, and a lithium metal anode at 0.1C (0.23 mA cm−2). (b) Cycling stability of the cells with Celgard 2400 and PBO-NM at room temperature. (c) Open circuit voltages of the cells with Celgard 2400 and PBO-NM in response to the increasing environmental temperature. (d) Cycling stability of the cells with PBO-NM at 150 °C at 0.25C (0.58 mA cm−2).

slightly larger ionic conductivity than aramid NFs/PEO composites,7 and 32% of the ionic conductivity in an alumina separator with 20 nm straight pores.13 Thanks to the robust mechanical properties, PBO-NM can be processed into an ultrathin film without losing its integrity. The overall internal resistance of PBO-NM can thus be comparable or smaller than what is needed for batteries (Figure S13). Such a combination of high moduli and low ionic resistances is rare in the materials used for separators (Figure S13). Strong solvent resistance of PBO makes the mechanical properties of PBO-NM almost unchanged in the liquid electrolytes. It was theoretically predicted that separators with high shear moduli larger than 7 GPa could be used for dendrite suppression.12,43 The shear modulus (G) of the PBO-NM is estimated from Young’s modulus (E) using E = 2G(1 + ν), where ν is the material and structure related Possion ratio. The Possion ratio typically ranges from −1 ≤ ν ≤ 0.5; thus G of PBO-NM has a lower bound of 6.7 GPa. In addition, it was also experimentally demonstrated that cross-linked block polymers with G of 0.1 GPa showed high resistance to lithium dendrite growth.43 Therefore, the shear modulus of PBO-NM is large enough for dendrite-proof battery applications. Galvanostatic lithium plate/strip electrochemical cycling measurements were performed in symmetric lithium/separator/lithium cells to test our analysis. The cells were subjected to 0.38 mA/cm2 current density, and the current direction was reversed every 3 h.4 The voltage profile of the control cell with Celgard 2400 shows more erratic shifts than PBO-NM in the first few cycles (Figure 4c versus d) in which SEI gradually forms. The higher overpotential of plating or stripping for Celgard 2400 relates with its larger interfacial resistance.6 In addition, the voltage plateau in Celgard 2400 cells decreases from 0.031 V between 50 and 100 h to 0.024 V for over 200 h (Figure 4c). Such phenomenon is due to the “soft short” when the separator is slowly penetrated by lithium dendrites.7 As for the PBO-NM cell, the steady voltage profile is maintained at

superior wettability makes PBO-NM a suitable separator for electrochemical storage. For practical battery applications, it is important to investigate the electrochemical stability of the electrolytes within the operation voltage of a battery system. The electrochemical stability window of the PBO-NM was measured from the linear sweep voltammograms.40 PBO-NM saturated with the carbonate electrolytes possesses a decomposition voltage around 4.5 V vs Li+/Li (Figure S11), similar to that of the pure liquid electrolytes.41 In addition, no obvious side reactions occur in the low voltage region of the PBO-NM voltammetry curve in comparison to that of Celgard 2400, further proving the inertness of PBO-NM. Impedance spectroscopy is used to characterize the ionic conductance for PBO-NM impregnated with the liquid electrolytes of 1 M LiPF6 in the EC−DEC−EMC = 1:1:1 solvents (Figure 4b). The membranes were sandwiched between two lithium foils for the measurement. The impedance spectra depict a smaller semicircle for PBO-NM than Celgard 2400. The semicircle in the impedance spectra are usually related to the interfacial resistance of a separator,6 which are 181 Ω cm2 for Celgard 2400 and 124 Ω cm2 for PBO-NM. The lower interfacial resistance indicates that PBO-NM possesses better interfacial compatibility with lithium electrodes.42 In addition, PBO-NM exhibits lower ionic resistance with 1.4 Ω cm2 extracted from the impedance spectrum as compared to 2.2 Ω cm2 for Celgard 2400. Similar ionic resistances of 2.2 Ω cm2 and 3.3 Ω cm2 for PBO-NM and Celgard 2400 respectively are obtained when stainless steel blocking electrodes are used for impedance measurements (Figure S12). The ionic conductivity of PBO-NM is probed by taking its thickness of 3.1 μm into account. As expected, PBO-NM has a smaller ionic conductivity of 2.3 × 10−4 S cm−1 compared to 1.2 × 10−3 S cm−1 in 25 μm thick Celgard 2400 at room temperature, due to limited ion diffusion in the nanopores. In comparison to other nanoporous membranes, PBO-NM has 2985

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polymer49 or carbon nanotube50 fibers. In addition, alternative eco-friendly organic solvents might be chosen for the exfoliation of PBO nanofibers.51 Moreover, bottom-up synthesis of colloidal PBO nanofibers from molecular precursors also seems achievable, considering the huge progress in the recent decade on the synthesis of nanomaterials with various shapes and sizes.52 These strategies will be explored in future studies for the low-cost scalable production of PBO-NMs. In conclusion, a thin PBO-NM is assembled from top-down synthesized PBO-NFs using a simple blade casting and solvent exchange process. The flexible PBO-NMs have decent liquid electrolyte wettability, low ionic resistance, and superior thermal stability. In addition, the PBO-NMs possess high mechanical properties with Young’s moduli of 20 GPa and ultimate strengths of 525 MPa. Such a combination of great electrochemical, thermal, and mechanical properties makes PBO-NM a suitable separator for lithium dendrites suppression and safe and stable operation of rechargeable lithium batteries. Furthermore, PBO-NMs can be easily adopted for other electrochemical devices, such as lithium−sulfur batteries, sodium or aluminum batteries, and high temperature supercapacitors. Aside from energy applications, the polyoxyzole nanofibers are attractive reinforcing building blocks for high performance composites, hydro- or aero- gels, and dielectric thin films.

0.025 V from very beginning to 700 h (Figure 4d), indicating effective dendrite suppression. Similar observations are made for the symmetric lithium electrode cells operated at an elevated environmental temperature of 40 °C (Figure S14a,b). The temporal evolution of impedance spectra of the symmetric cells also shows diminishing interfacial resistances for Celgard 2400 within 200 h (Figure S14c,e), while stable impedance spectra for PBO-NM are maintained for more than 700 h (Figure S14d,e). This fact further demonstrates the dendriteproof capability of PBO-NMs. The lithium dendrite inhibition is also observed by SEM on lithium electrodes harvested from coin cells in an argon-filled glovebox. The electrodes were washed with PC to remove any electrolytes or salt residues and the electrolyte solvent was removed under vacuum in the glovebox. The mossy or dendritic lithium is visible on the electrode surface after 230 h of cycling with Celgard 2400, while smooth and flat lithium surface is observed after 700 h of cycling with PBO-NMs (Figure 4e and f). The direct evidence further proves PBONMs are able to depress lithium dendrites. We then evaluated the practicality of PBO-NM as the separator in a battery cell using a lithium metal anode and a LiCoO2 cathode and benchmarked its long-term energy storage performance at 0.1C (0.23 mA cm−2) against a cell with the commercial Celgard 2400. The cell with PBO-NM shows a typical charge−discharge curve with an initial discharge capacity of ca. 140 mAh g−1 (Figure 5a). Both the Celgard 2400 and the PBO-NM cells show capacity fading, which are likely contributed by the structural instability of LiCoO2 and side reactions during charge/discharge (Figure 5b).44−46 Typically, PBO-NM cells demonstrate higher long-term stability (Figure 5b) and Coloumbic efficiency (Figure S15a) in comparison to Celgard 2400 cells. This enhancement can be attributed to a more stable SEI due to the prohibition of dendrite formation.5,47 PBO-NM cells also demonstrate better performance than Celgard 2400 cells at higher charge/discharge rates (Figure S15b). It should be noted that these cells contain excess amount of lithium metal anode (∼1.5% lithium metal is involved in the charge/discharge process) so that both Celgard 2400 and PBO-NM cells can maintain columbic efficiency over 98% (Figure S15).47 The dendrite-proof PBO-NM is expected to play a more substantial role in stabilizing SEI and inhibiting capacity fading in practical lithium−metal batteries where the energy capacities of lithium metal anodes and cathode materials are matched.5,6 To demonstrate safe operation of PBO-NMs in extreme conditions, we incorporated them in high-temperature lithium metal-based batteries and made a comparison with batteries using Celgard 2400. The fabrication methods of these batteries are detailed in the Supporting Information. The open circuit voltages (OCVs) of the cells in response to the temperature are monitored (Figure 5c). The Celgard 2400 cell fails at ∼125 °C, corresponding to the melting temperature of Celgard 2400 (Figure S9), while PBO-NM cell continues to operate until a much higher temperature of 185 °C, at which the commercial electrolytes decompose. A stable cycling performance of the PBO-NM cell at 150 °C is shown in Figure 5d. The discharge capacity of the PBO-NM cell is maintained at ∼120 mAh g−1 after 65 cycles, despite more significant capacity fading of LiCoO2 at higher temperatures.48 Although strong acids are involved in the production of PBO-NMs, it is still feasible to mass-produce them in an industrial scale, as is demonstrated for high-performance

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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/acs.nanolett.5b05133. Methods and additional experimental data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Y. Gao and X. Meng for the measurement of pore volume and pore size distribution in this study. We also acknowledge X. Wang and Y. Yang for their assistance in various experiments. This work was supported by Natural Science Foundation of China (Grant No. 21376001, 21576028 and 21506012),Beijing Higher EducationYoung Elite Teacher Project (YETP1205), and Graduate Students Science & Technology Innovation Activities Grant (2014YR1001) funded by Beijing Institute of Technology.

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DOI: 10.1021/acs.nanolett.5b05133 Nano Lett. 2016, 16, 2981−2987