Cellulose Aerogel Membranes with a Tunable Nanoporous Network

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Cellulose Aerogel Membranes with a Tunable Nanoporous Network as a Matrix of Gel Polymer Electrolytes for Safer Lithium-Ion Batteries Jiqiang Wan,†,‡ Jinming Zhang,† Jian Yu,*,† and Jun Zhang*,†,‡ †

CAS Key Laboratory of Engineering Plastics, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: Cellulose aerogel membranes (CAMs) are proposed as a matrix for gel polymer electrolyte to the fabrication of lithium-ion batteries (LIBs) with superior thermal stability. The CAMs are obtained from a celluloseionic liquid solution via a dissolution−regeneration−supercritical drying route. The presence of high porosity, the nanoporous network structure, and numerous polar hydroxyl groups benefits the quick absorption of liquid electrolytes for gelation of the CAMs and improves the ionic conductivity of the gelled CAMs. LIBs assembled with the gelled CAMs display excellent electrochemical performance at room temperature, and more importantly, the intrinsic thermal resistance of cellulose allows the LIBs to run stably for at least 30 min at working temperatures as high as 120 °C. The CAMs, with their excellent thermal stability, are promising for the development of highly safe, cost-effective, and high-performance LIBs. KEYWORDS: cellulose aerogel membrane, ionic liquid, nanoporous network, safer, lithium-ion battery

1. INTRODUCTION Lithium-ion batteries (LIBs), due to their high energy density, high power density, and long cycle life, have become the standard power source for portable electronic devices, wearable electronics, and electric tools in recent years. Furthermore, large-capacity LIB systems are promising power sources for hybrid electric vehicles, plug-in hybrid electric vehicles, and fully electric vehicles as well as for the storage of wind, solar, and tidal energy in smart grids.1−6 However, practical applications of LIBs have been impeded by their potential safety issues, which originate from the poor thermal stability of commercial polyolefin separators and the inherent dangers of leakage of volatile and flammable organic liquid electrolytes.7−9 Polymer electrolytes, including solid polymer electrolytes and gel polymer electrolytes (GPEs), provide a promising choice to eliminate the drawbacks of liquid electrolytes.10−13 Although solid polymer electrolytes overcome the leakage problem of liquid electrolytes, they are quite far from meeting the demands of LIBs due to their low ionic conductivity at room temperature, poor mechanical strength, and high sensitivity to humidity.14,15 In comparison, GPEs, in which a stable gel is © 2017 American Chemical Society

formed by the incorporation of liquid electrolytes with a polymer membrane matrix, combine the advantages of the high ionic conductivity of liquid electrolytes with the good safety of solid polymer electrolytes. In addition, GPEs also show unique characteristics, such as a wide electrochemical window, good thermal stability, good compatibility with electrodes, and good processability.16,17 Over the past decades, GPEs based on polyacrylamide, poly(methyl methacrylate), poly(ethylene oxide), poly(vinylidene fluoride), and the copolymer poly(vinylidene fluoride cohexafluoropropylene) (PVDF-HFP) have been widely studied.18−29 However, the application of GPEs in large-scale systems is still limited because of their high cost, poor mechanical strength, and complicated manufacturing process.30,31 Therefore, further effort is required to identify a suitable polymer matrix to meet the application demands of GPEs in LIBs. Received: May 4, 2017 Accepted: June 27, 2017 Published: June 27, 2017 24591

DOI: 10.1021/acsami.7b06271 ACS Appl. Mater. Interfaces 2017, 9, 24591−24599

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ACS Applied Materials & Interfaces

Figure 1. Schematic diagram of the preparation process of cellulose aerogel membranes.

and the results have been compared with those obtained from a commercial Celgrad 2400 separator.

Cellulose, the most abundant natural biopolymer on the earth, is renewable, biodegradable, and affordable. The good thermal stability and chemical stability of cellulose, due to its strong intra- and intermolecular hydrogen bonds, are favorable characteristics to ensure the safety of LIBs.32,33 Recently, Cui and co-workers34,35 fabricated a series of cellulose-based composite membranes as heat-resistant LIB separators via a wet paper making process. Lee and colleagues36,37 prepared cellulose nanofiber paper-derived separators with a nanoporous structure that showed electrochemical safety, negligible thermal shrinkage, and a strong affinity for liquid electrolyte. However, these methods require the mechanical treatment of natural cellulose, making fabrication a high-energy-consumption and low-efficiency process. On the other hand, GPEs have been prepared from microporous membranes of cellulose derivatives with good affinity toward liquid electrolytes, such as methyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, and cyanoethylated cellulose.38−41 Although the preparation processes were simple, these cellulose-based membranes had the disadvantage of low porosity or large pore diameter, which would adversely affect the physical properties and electrochemical performance of the corresponding GPEs. Low porosity makes it difficult to absorb a large amount of liquid electrolyte to obtain high ionic conductivity, which is an important factor to improve the electrochemical performance of LIBs. With a large pore size, LIBs will have the risk of internal short circuiting and self-discharging due to the mass transport of electrode particles. Additionally, lithium dendrites are easy to form and grow during long-term cycling, leading to the failure of the battery.42,43 Hence, cellulose membranes with both high porosity and a nanoporous structure, prepared by a simple and effective method, hold great potential for the fabrication of high-safety and high-performance LIBs. In this work, cellulose aerogel membranes (CAMs) with high porosity and a nanoporous network structure were developed to act as a matrix for gel polymer electrolyte in LIBs. The CAMs were obtained by a dissolution−regeneration−supercritical CO2 drying method by utilizing cotton pulp as the raw material and the room-temperature ionic liquid 1-allyl-3methylimidazolium chloride (AmimCl) as a green solvent. In recent years, dissolution-regeneration technology using new direct solvents has been widely used for processing the cellulose materials.44−48 By the selection of the appropriate conditions for dissolution, regeneration, and drying, the nanoporous structure of the regenerated CAM can be conveniently controlled.49−51 The morphology and physical properties of the CAMs were examined as a function of the initial concentration of the cellulose solution. The electrochemical performances of cells assembled with GPEs composed of the CAMs saturated with LiPF6 liquid electrolyte were investigated,

2. EXPERIMENTAL SECTION 2.1. Materials. Cotton pulp with an average degree of polymerization of 650 was kindly supplied by Henglian New Materials Co., Ltd. (Shandong, China). The room-temperature ionic liquid AmimCl was synthesized following the method described in our previous work.47 Additionally, 1 M lithium hexafluorophosphate (LiPF6) solution in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) (1:1:1, w/w/w) was purchased from CAPCHEM Technology Co., Ltd. (Shenzhen, China). The lithium foil, acetylene black, PVDF binder and LiFePO4 were purchased from KEJING STAR Technology Co., Ltd. (Shenzhen, China). A Celgard 2400 separator purchased from Celgard Company (USA) was used as a reference. All other chemicals were of analytical quality and used as received. 2.2. Preparation of the Cellulose Aerogel Membranes. The preparation process of the CAMs is shown in Figure 1. First, 2 and 4 wt % cellulose−AmimCl solutions were prepared by mechanically stirring AmimCl and dried cotton pulp in a 100 mL flask at 80 °C for 1 h, and the 7 and 10 wt % cellulose−AmimCl solutions were prepared via a small vacuum kneader at 100 °C with a dissolution time of 1.5 h. The obtained homogeneous and clear solutions were cast onto glass plates with a doctor blade to obtain 300 μm thick layers, which were then regenerated by immediate immersion in a 60 wt % aqueous AmimCl solution as a coagulation bath, as reported previously in the literature.49 The obtained cellulose gel membranes were washed several times with absolute ethanol (12 h each wash) until no chloride ions were detected when tested by aqueous AgNO3. Then, the cellulose gels were dried in supercritical CO2 at a pressure of 12 MPa and temperature of 40 °C, followed by the slow release of CO2 at 40 °C to extract ethanol. Finally, the obtained CAMs were dried at 50 °C for 48 h under a vacuum to remove residual solvent. The thickness of the final CAMs was 85−100 μm. The CAMs prepared by different initial cellulose concentrations are denoted by CAM-2, CAM-4, CAM7, and CAM-10, respectively. 2.3. Membrane Characterization. The surface and crosssectional morphologies were observed using a JEOL JSM-6700F scanning electron microscope (SEM) at an accelerating voltage of 5 kV. The brittle fractured sample was sputter-coated with gold before observation. Nitrogen adsorption−desorption isotherms were measured at 77 K with a Micromeritics Tristar II 3020 system. The light transmittance of the CAMs was measured with a TU-1901 UV−vis spectrophotometer (Beijing Purkinje General Instrument Co., Ltd., China) over a wavelength range of 300 to 800 nm. Dynamic mechanical analysis (DMA) was performed on a DMA Q800 (TA Instruments) in tensile mode. The data was recorded from 50 to 150 °C at a heating rate of 3 °C/min and a frequency of 1 Hz. Differential scanning calorimetry (DSC) measurements were carried out on a Q2000 (TA Instruments) under a nitrogen atmosphere. The sample was heated at a rate of 20 °C/min from 50 to 300 °C, then quenched to 50 °C, and finally heated again to 300 °C at a rate of 20 °C/min. The data was obtained from the second heating run to minimize the thermal history effect. Tensile tests were conducted using a universal 24592


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Figure 2. Photograph of CAM-4 sample (a) and optical transmittance curves for various CAMs and Celgard 2400 membrane (b). testing machine (Chengde Jinjian Instrument Co., Ltd., China.), according to the ASTM D-882 standard, at a cross-head speed of 5 mm/min. For each sample, at least five specimens were tested, and the average value is reported. The porosity of the CAM (P) was calculated by eq 1:

⎛ ρ⎞ Ρ (%) = ⎜⎜1 − a ⎟⎟ × 100% ρc ⎠ ⎝

A 2032 coin-type half-cell was assembled with a lithium foil anode, LiFePO4 cathode (LiFePO4/acetylene black/PVDF = 80:10:10, w/w/ w), and gelled CAM or Celgard 2400 soaked with liquid electrolyte. The cells were cycled at a fixed charge/discharge rate of 0.5 C/0.5 C under a voltage range between 2.5 and 4.2 V. The rate capability of the cells was tested at the charge/discharge rates of 0.2 C/0.2 C, 0.5 C/0.5 C, 1 C/1 C, 2 C/2 C, 4 C/4 C, 8 C/8 C, and then 0.5 C/0.5 C, with five cycles for each rate. Cycling performance and rate capability tests were conducted using a LAND battery testing system at 25 °C. Open-circuit voltage (OCV) experiments of the fully charged cells assembled with 1 M LiPF6 liquid electrolyte and 0.5 M lithium bis(oxalate)borate (LiBOB) in propylene carbonate (PC) and EC (1:1, w/w) liquid electrolyte, respectively, were performed at 120 °C to evaluate the safety of the LIBs under harsh environments.

(1) −1

where ρc is the density of bulk cellulose (1.528 g cm ) and ρa is the density of the CAM, calculated from its weight (g) and volume (cm3). The thermal shrinkage of the membrane was determined using eq 2:

shrinkage (%) =

S0 − S × 100% S0

(2)

3. RESULTS AND DISCUSSION 3.1. Morphology and Physical Properties of the Cellulose Aerogel Membranes. The CAMs are obtained from a cellulose−ionic liquid solution though a dissolution− controlled regeneration and supercritical CO2 drying process. Figure 2 shows the photograph of a typical CAM sample and the light transmittance curves in the 300−800 nm range for the various CAMs. The results indicate that all CAMs exhibit high transparency and excellent flexibility. In comparison, the Celgard 2400 membrane is translucent, even with a lower thickness of 25 μm (Figure S1). As shown in Figure 2b, the optical transmittance of the CAM samples increases with wavelength, while that of Celgard 2400 remains very low at 1.5%. At 800 nm, the transmittance of the CAM samples is 90% for CAM-2, 84% for CAM-4, 83% for CAM-7, and 78% for CAM-10, indicating that all the CAMs have high transparency, especially the CAM prepared from a cellulose solution with a low initial concentration. To determine the reason for the transmittance difference between the CAMs and the commercial separator, their surface and cross-sectional morphologies were investigated, and the SEM images are shown in Figure 3. For all CAM samples, both the surface and cross-sectional morphology contain homogeneous interconnected 3D nanofibrillar networks with pore sizes in the nanometer range. It is well-accepted that the porous structure of cellulose aerogels is primarily formed in the regeneration step. In this study, the uniform nanoporous structure in the CAMs was formed by using a highconcentration aqueous AmimCl solution as a regeneration bath to control the gelation of the cellulose solution, as outlined in our earlier report.49 However, both the surface and crosssectional morphology of Celgard 2400 (panels i and j of Figure 3) are nonuniform and contain numerous slit-like pores of approximately 50 × 200 nm, which are formed via a uniaxial stretching process.42 Consequently, the high transparency of the CAM samples, compared to Celgard 2400, is largely due to

where S0 is the original area of the CAM and S is the area of the CAM after heating for 30 min at different temperatures ranging from 100 to 150 °C. The electrolyte uptake (η) of the membrane was calculated by eq 3:

η (%) =

Wt − W0 × 100% W0

(3)

where W0 is the initial weight of the sample and Wt is the weight of the sample after absorbing the electrolyte. 2.4. Electrochemical Evaluation. The CAMs were punched into circular pieces (d = 19 mm) and then immersed in 1 M LiPF6 liquid electrolyte to obtain gelled CAM samples, which were used simultaneously as the electrolyte and separator in further cell assembly. Coin-type (2032) test cells were assembled in an argon-filled glovebox with water and oxygen contents lower than 0.1 ppm. The electrochemical characteristics were measured in a CHI660E electrochemical workstation (Chenhua) at 25 °C without specification. For comparison, cells using the commercial Celgard 2400 separator saturated with 1 M LiPF6 liquid electrolyte were assembled and tested under the same conditions. The ionic conductivity (σ) was measured employing electrochemical impedance spectroscopy (EIS) route over a frequency range of 1 to 105 Hz with an amplitude of 10 mV. The GPE membrane was sandwiched between two stainless steel electrodes, and the ionic conductivity was calculated by eq 4:

σ=

d Rb × S

(4)

where d is the thickness of the separator, Rb is the bulk resistance, and S is the contact area between the separator and the stainless steel electrodes. The interfacial impedance between the separator and lithium foil electrode was also determined by EIS over a frequency range of 0.1 to 106 Hz with an amplitude of 10 mV using cells assembled with the separator sandwiched between two lithium foil electrodes. The electrochemical stability window was determined by linear sweep voltammetry. The measurement was performed between 0 and 6 V (versus Li+/Li) at a scan rate of 1 mV s−1 on a working electrode of stainless steel and a counter and reference electrode of lithium foil. 24593


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Table 1. Physical Properties of CAMs and Celgard 2400 sample CAM-2 CAM-4 CAM-7 CAM-10 Celgard 2400

porosity (%)

specific surface area (SBET; m2 g−1)

average pore sizeb (nm)

tensile strength (MPa)

94.7 79.4 45.6 37.2 41a

281.9 223.8 133.6 96.2 −

21.2 27.9 16.2 10.4 50 × 200c

0.99 10.7 41.1 67.6 12

a

Obtained from supplier. bObtained from desorption isotherms by Barrett−Joyner−Halenda method. cSlit-like pores; size obtained from the scanning electron microscopy images.

cellulose molecular chains in the cellulose−AmimCl solutions, resulting in a dense morphology and reduced porosity. In CAM-10, the pores are hard to distinguish, even under a high magnification of 30 000×. Meanwhile, the porosity of CAM-10 significantly decreases to 37.2%. Therefore, the morphology and porosity of the CAMs can be tailored very efficiently by adjusting the concentration of the initial cellulose−AmimCl solutions. Gas sorption measurements were conducted to further examine the porous nature of various CAMs. Figure 4 shows the N2 adsorption−desorption isotherms and the pore-size distribution curves of different CAMs. According to IUPAC classification, the isotherms are of Type IV with hysteresis loops, indicating the presence of mesoporous structures.52 The pore-size distribution, calculated from the desorption data from the isotherm using the Barrett−Joyner−Halenda (BJH) model, shows that the CAMs contain pores in the size range from 2 to 100 nm, and most of the pores in the CAM-2 and CAM-4 samples are larger than those in the CAM-7 and CAM-10 samples. Therefore, with increasing cellulose concentration, the average pore size of the CAMs decreases with an increase in cellulose concentration from 21.2 nm of CAM-2 and 27.9 nm of CAM-4 to 16.2 nm for CAM-7 and 10.4 nm for CAM-10, which is consistent with the SEM results (Table 1). The specific surface area (SBET), calculated from the BET equation, was found to be as high as 281.9 m2 g−1 for CAM-2 (Table 1). Similarly, the SBET of the CAMs decreases steadily from 281.9 to 96.2 m2 g−1 with an increase in cellulose concentration as the morphology of the aerogel membranes becomes denser. Therefore, the CAMs possess a nanoporous network structure, which is different from the conventional Celgard 2400 membrane. When the CAMs are used to separate the electrodes in LIBs, the unique tortuous nanopore structure can effectively avoid self-discharge and internal short circuits in LIBs and suppress the formation and growth of lithium dendrites, which are favorable for improving the safety performance of LIBs.53−56 The tensile strength of the CAMs was also measured to comprehensively evaluate the physical properties of the CAMs, which are compiled in Table 1 and Figure S2. The mechanical strength of CAM-2 is only 0.99 MPa due to the loosely packed network structure. It is worth mentioning that the tensile strength of CAM-4 remarkably increases to 10.7 MPa, which is similar to that of Celgard 2400. Thus, CAM-4 has a high enough strength to fulfill requirements for assembly and use in LIBs, although it possesses a relatively high porosity. The tensile strength further increases to 41.1 MPa for CAM-7 and 67.6 MPa for CAM-10. The above results indicate that the physical properties of the CAMs also strongly rely on the

Figure 3. SEM images of surface morphology (a, c, e, g, i), and crosssection morphology (b, d, f, h, j) for various samples: CAM-2 (a, b); CAM-4 (c, d), CAM-7 (e, f), CAM-10 (g, h), and Celgard 2400 (i, j). Magnification: 30 000×.

the homogeneous size and dispersion of the nanoporous structure. Moreover, Figure 3a−h indicates that the initial cellulose concentration plays a significant role in modulating the morphology of the CAMs. At concentrations of 4 wt % and below, the nanofibrils in the obtained aerogel membrane are loosely packed to form a network structure with relatively large pores. In addition, CAM-2 and CAM-4 have high porosities of 94.7% and 79.4%, respectively, as shown in Table 1. With an increase in the cellulose concentration, the pore size of the aerogel membranes decreases and the pore distribution narrows due to the increased aggregation and entanglement of the 24594


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Figure 4. N2 adsorption−desorption isotherms (a) and BJH pore-size distribution (b) of various CAMs.

Figure 5. Illustration of the gelation of CAM-4 by liquid electrolyte absorption (a), photograph of gelled CAM-4 (b), and electrolyte uptake of CAMs and ionic conductivity of the corresponding of GPEs (25 °C) (c).

attracted increasing attention in electric vehicles and energy storage systems. In addition, cellulose-based GPEs have the potential to be used for developing transparent and foldable electronics. The electrolyte uptake of the CAMs and the ionic conductivity of the corresponding GPEs are vital for the performance of LIBs. As seen in Figure 5c, the electrolyte uptake of the CAM-2 is as high as 615% due to the high porosity and the presence of favorable interactions between the CAM and the electrolyte, resulting in a high ionic conductivity of 5.76 mS cm−1 for gelled CAM-2. CAM-4, as a sample prepared from a moderate-concentration cellulose−AmimCl solution, has an electrolyte uptake of 325%, which is still obviously higher than that of 90% for Celgard 2400. Accordingly, gelled CAM-4 displays an ionic conductivity of 2.81 mS cm−1. The ionic conductivity abated to 0.43 mS cm−1 for gelled CAM-7 with an electrolyte uptake of 150% and to only 0.2 mS cm−1 for gelled CAM-10 with 95% electrolyte uptake as the topological structure of the CAMs became denser. Taking the above results into consideration, CAM-4 has wellbalanced performance in terms of mechanical strength and ionic conductivity. The tensile strength of CAM-4 is comparable to that of Celgard 2400, while the ionic conductivity of gelled CAM-4 is more than 5 times higher

concentration of the initial cellulose−AmimCl solutions and can be conveniently modulated over a wide range. The wettability by liquid electrolyte is an important property for separators in LIBs. Figure 5a shows that CAM-4 exhibits rapid liquid electrolyte absorption and instantaneous gelation behavior when organic electrolyte is dropped onto its surface. The results are similar for all CAMs due to the capillary effect of the nanoporous network structure and the strong affinity for polar liquid electrolytes because of the abundant hydroxyl groups in the cellulose chains.38 Gelled CAM-4 has a flexible and transparent appearance (Figure 5b) because the nanoporous network of the cellulose skeleton is retained during the rapid diffusion and homogeneous distribution of electrolyte into the membrane. In contrast, the electrolyte wettability of the commercial Celgard 2400 separator is poor because of its hydrophobic surface and low surface energy, as shown in Figure S3. Consequently, the challenging problem of wetting the entire separator rapidly and uniformly in the fabrication process of large-capacity LIBs can be solved by using CAMs.37 Moreover, the strong interaction between the liquid electrolyte and the hydrophilic cellulose in the gelled CAMs reduces the risk of electrolyte leakage under harsh conditions.57 Therefore, the rapid electrolyte absorption and gelation performance of CAMs is of great significance for improving the safety and production efficiency of large-capacity LIBs, which have 24595


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Figure 6. Electrochemical characteristics of cells assembled with gelled CAM-4 and Celgard 2400: linear-sweep voltammetry plots (a), Nyquist plots of Li//GPE//Li cells (b), cycling performance (c), and rate capability (d) of LiFePO4//GPE//Li half-cells.

a charge−discharge rate of 0.5 C/0.5 C and an excellent cycling stability with negligible capacity loss even after 100 cycles. For the cell using the Celgard 2400 separator, the discharge capacity decreases slightly after 60 cycles and is maintained at 132.8 mA h g−1 after 100 cycles. The stable cycling performance and high capacity retention of the cellulose-based GPE are ascribed to the highly developed nanoporous network, which facilitates rapid ionic transport, as well as favorable interactions between the cellulose and the liquid electrolyte, which provide better electrolyte maintenance during cycling. Figure 6d shows the discharge capacities of the cells with gelled CAM-4 and Celgard 2400 separator at different charge/ discharge rates ranging from 0.2 C/0.2 C to 8 C/8 C. It is wellknown that the discharge capacities of the cells drop with increasing charge/discharge rate. At low charge/discharge rates of 0.2 C/0.2 C and 0.5 C/0.5 C, the rate capacities of the cell using gelled CAM-4 are comparable with those of the Celgard 2400 separator. With increasing charge/discharge rates above 2 C/2 C, the cells using gelled CAM-4 have higher discharge capacities than those of the commercial separator. At a very fast rate of 8 C, the specific rate capacity of the cell using gelled CAM-4 is as high as 80.5 mA h g−1, while the specific rate capacity is below 70 mA h g−1 for the cell with the Celgard 2400 separator. The excellent rate behavior of the cellulosebased GPEs is mainly due to the good interfacial compatibility and high ionic conductivity, which is favorable for reducing the internal impedance at high current density, even for membranes with higher thicknesses. The remarkable cycling stability and high rate capacity endow the LIBs assembled with cellulosebased GPEs with very promising cell performance. 3.3. Thermal Stability of the Cellulose Aerogel Membranes and Improved Cell Performance at High Temperatures. The development of a polymer separator with superior thermal resistance is a vital step to overcome the safety problems and for the long-term application of LIBs. Commercial LIBs typically contain flammable liquid organic electrolyte and meltable polyolefin separators. When the temperature exceeds the melting point of the polyolefin, the

than that of the commercial separator. Therefore, CAM-4 is chosen as a representative sample for further evaluation of the cell performance. 3.2. Electrochemical Characteristics of Lithium-Ion Batteries. The effects of the membrane characteristics of gelled CAM-4 on its electrochemical performance were investigated by examining the electrochemical stability window, the interfacial resistance between the GPE membrane and the electrode, the C-rate capability, and the cycling performance, and the results are presented in Figure 6. For comparison, cells using Celgard 2400 membranes saturated with 1 M LiPF6 liquid electrolyte were assembled and tested under the same conditions. Figure 6a shows the linear sweep voltammetry results of both separators, which were used to evaluate their electrochemical stability window. The cells using CAM-4 and Celgard 2400 were shown to have stable currents at voltages below 4.6 and 4.2 V versus Li/Li+, respectively. Therefore, the gelled CAM-4 shows no appreciable decomposition of any components over a wider voltage range than that of the commercial separator. The CAM samples show good electrochemical stability against the liquid electrolyte, providing a promising alternative for highvoltage LIBs. The interfacial compatibility between the separator and the electrodes was examined in cells assembled by sandwiching the separator with lithium foils. The Nyquist plots of the cells are shown in Figure 6b. The interfacial resistance of gelled CAM-4 (approximately 360 Ω) is lower than that of the Celgard 2400 separator (approximately 460 Ω), indicating that the cellulosebased GPEs have good compatibility with the electrode materials. The good interfacial compatibility satisfies the requirements for high-performance LIBs. Furthermore, cycling performance and rate capability tests were carried out to investigate the practical application of cellulose-based GPEs in a half-cell using LiFePO4 as the cathode and lithium metal as the counter and reference electrode. Figure 6c shows that the cell using gelled CAM-4 as the GPE displays a high discharge capacity of 138.1 mA h g−1 at 24596


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Figure 7. Thermal properties of CAM-4 and Celgard 2400: DSC curves (a), thermal shrinkage over a temperature range from 100 to 150 °C (b) (inset: the photographs of CAM-4 and Celgard 2400 after thermal treatment at 150 °C for 0.5 h), and DMA curves (c).

commercial separators may suffer from severe dimensional instability, which is negative to the safety of LIBs because the internal short circuiting and leakage current may lead to thermal runaway and explosion. In this work, CAMs are introduced to replace conventional separators to promote the safety of LIBs under harsh conditions. The thermal properties of CAM-4 and Celgard 2400 were first investigated using DSC, thermal shrinkage, and DMA measurements. In the DSC curves shown in Figure 7a, there is an endothermic peak at 165 °C for Celgard 2400, which is attributed to the melting of the polymer. In comparison, CAM-4 shows no thermal peak below 300 °C, which reflects the characteristics of cellulose. Furthermore, Figure 7b shows that the thermal shrinkage for Celgard 2400 is obviously larger than that for CAM-4 after thermal treatment over a temperature range of 100 to 150 °C, which increases with an increase in treatment temperature. The inset of Figure 7b clearly shows that CAM-4 (left) retains its original dimensions after treatment at 150 °C for 30 min, while Celgard 2400 (right) displays severe shrinkage of approximately 40%. In addition, the changes in mechanical properties with temperature are depicted in Figure 7c. CAM-4 has a smaller storage modulus than Celgard 2400 at room temperature due to its higher porosity. The storage modulus of CAM-4 decreases slightly with an increase in temperature. In contrast, the storage modulus of Celgard 2400 decreases remarkably with temperature and finally falls to zero when the temperature exceeds 130 °C. The above results suggest that the CAMs fabricated in this work have excellent thermal stability compared with the commercial polyolefin separators at elevated temperatures. Therefore, GPEs based on these thermally stable CAMs are promising candidates for assembling safer LIBs. To better investigate the performance of CAMs in practical battery applications, the OCV of fully charged cells was investigated at 120 °C. It can be seen in Figure 8 that the OCV of the cell based on the Celgard 2400 separator rapidly drops to 0 V in 5 min. This indicates that the cell loses stability because of internal short circuit mostly due to the shrinkage of the Celgard 2400 separator. However, the cell assembled with gelled CAM-4 runs well for at least 30 min, resulting from the high thermal stability of the CAMs. However, the unavoidable evaporation of the low-boiling-point EC/EMC/DMC electrolytes leads to a slight drop in the OCV. Therefore, a highly polar EC/PC-based liquid electrolyte (0.5 M LiBOB in EC/PC = 1:1, w/w), which has low volatility and good thermal/ electrochemical stability,58,59 was used to eliminate solvent evaporation. The highly polar electrolyte also exhibits good wettability to CAM-4 because of the favorable interactions. As shown in Figure 8, no OCV drop occurs for the cell employing CAM-4 and the EC/PC electrolyte over the entire measurement period. However, the Celgard 2400 separator is not wetted well by the highly polar EC/PC-based liquid electrolyte

Figure 8. OCV curves of cells employing CAM-4 and Celgard 2400 separator at 120 °C.

(Figure S4); hence, the cell cannot work at all. The superior mechanical and dimensional stability of the CAMs at elevated temperature and their affinity for heat-resistant electrolytes endow them great potential for the fabrication of highly safe lithium-ion batteries.

4. CONCLUSIONS In this study, cellulose aerogel membranes are prepared by a green route from cellulose solutions in room-temperature ionic liquid using the most abundant natural resource as a raw material. In addition to robust flexibility and excellent transparency, the CAMs reported here have high porosity, a uniform nanoporous structure, and excellent tensile strength. The CAMs show fast absorption and gelation behavior when exposed to liquid electrolyte, and the gelled CAMs have high ionic conductivity in comparison to commercial separators. Cells assembled with the gelled CAMs show excellent electrochemical stability and battery performance. More importantly, the thermally stable CAMs endow the cells superior thermal resistance, which could run well even at a high temperature of 120 °C. Consequently, the CAMs have promising applications in the highly safe and large-capacity lithium-ion batteries.

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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.7b06271. Images showing stress−strain curves, photographs of the samples, and an illustration of the wetting behavior. (PDF) 24597


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AUTHOR INFORMATION

Corresponding Authors

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

Jiqiang Wan: 0000-0003-1853-2200 Jinming Zhang: 0000-0003-3404-4506 Jian Yu: 0000-0003-0591-0524 Jun Zhang: 0000-0003-4824-092X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (grant nos. 51425307, 51573196, 21374126, and 51273206) and the Program of Taishan Industry Leading Talents (Shandong Province).

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Cellulose Aerogel Membranes with a Tunable Nanoporous Network

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