Mesoporous Cellulose Nanocrystal Membranes as Battery Separators

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Cite This: ACS Appl. Energy Mater. 2019, 2, 3749−3761

Mesoporous Cellulose Nanocrystal Membranes as Battery Separators for Environmentally Safer Lithium-Ion Batteries Renato Gonçalves,†,‡,○ Erlantz Lizundia,*,§,∥,⊥,○ Maria Manuela Silva,# Carlos M. Costa,*,†,# and Senentxu Lanceros-Meń dez⊥,∇ †

Centro de Física, Universidade do Minho, 4710-057 Braga, Portugal Institute of Science and Innovation for Bio-Sustainability (IB-S), University of Minho, 4710-057 Braga, Portugal § Department of Graphic Design and Engineering Projects, Bilbao Faculty of Engineering, University of the Basque Country (UPV/EHU), Bilbao 48013, Spain ∥ Laboratory for Multifunctional Materials, Department of Materials, ETH Zürich, Vladimir-Prelog-Weg 5, 8093 Zürich, Switzerland ⊥ BCMaterials, Basque Center for Materials, Applications and Nanostructures, UPV/EHU Science Park, 48940 Leioa, Spain # Centro de Química, Universidade do Minho, 4710-057 Braga, Portugal ∇ IKERBASQUE, Basque Foundation for Science, 48013 Bilbao, Spain

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S Supporting Information *

ABSTRACT: Cellulose emerges as a promising sustainable alternative to traditional polyolefin-based lithium-ion battery (LIB) separators because of its good mechanical properties and inherent hydrophilic character. Therefore, in this work we fabricate high specific surface area mesoporous cellulose nanocrystal (MCNC) membranes with different pore morphology as novel three-dimensional porous separators. N2 adsorption−desorption isotherms and scanning electron microscopy (SEM) analyses reveal that membranes are composed of loosely packed cellulose nanocrystals (CNCs) with specific surface areas up to 172 m2· g−1 and porosities up to 75.3%. The prepared mesoporous separators show low contact angle values when evaluated against conventional electrolyte and ionic liquid, suggesting a pivotal role of the porous 3D structure on the resulting wettability, where mesopores provide efficient paths for Li+ ion migration through the membrane. Furthermore, the membranes show ionic conductivities of 2.7 mS·cm−1, wide electrochemical stability, and good interfacial compatibility with the lithium electrode, delivering a specific capacity of 122 mAh·g−1 and 85 mAh·g−1 at a C/2 and 2C rate, respectively. On the basis of the displayed high discharge capacity, small overpotential, wide electrochemical window, and good cycling stability, the use of ionic liquids (ILs) as suitable electrolytes together with MCNC membranes for the development of environmentally friendly separatorelectrolyte systems was also evaluated. KEYWORDS: cellulose nanocrystals, mesoporous materials, environmental-safe, lithium-ion battery (LIB), separator

1. INTRODUCTION

time that enables a good ionic transport between such electrodes.5,6 Although separators are one of the key components in LIBs, the study of the separator function on the resulting performance of LIBs has been somewhat limited. Typically, separators are based on a porous polyolefin membrane where increased porosities allow an easier transport

Our society is facing an increasing demand for efficient energy storage systems as a result of the higher share of electricity generation from environmentally friendly sources, the rapid development of hybrid/electric cars, and the widespread use of portable electronic devices.1 Since they were invented in the early 1990s, lithium-ion batteries (LIBs) achieved a predominant role in energy storage systems because of their large energy density and superior cycling stability.2−4 Among the different battery components, the separator in LIBs avoids any physical contact between both cathode and anode at the same © 2019 American Chemical Society

Received: March 3, 2019 Accepted: May 8, 2019 Published: May 8, 2019 3749

DOI: 10.1021/acsaem.9b00458 ACS Appl. Energy Mater. 2019, 2, 3749−3761

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ACS Applied Energy Materials

spacing. The small pore size of the separators (few nanometers) will provide an effective physical barrier to Li dendrite growth, as Li dendrites typically grow more easily when in the presence of membranes with micrometer-sized pores. The suitability of the fabricated membranes as lithium-ion battery separators is investigated in terms of electrolyte uptake, electrolyte contact angle, mechanical properties, ionic conductivity, electrochemical stability, interfacial compatibility, and cycling stability performance using 1 M LiPF6 in ethylene carbonate−dimethyl carbonate (EC-DMC) as a model electrolyte. Owing to displayed high discharge capacity, small overpotential, wide electrochemical window, cycling stability, and renewable character, we also evaluated the use of ionic liquids (ILs) as sustainable electrolytes together with MCNC membranes for the development of environmentally friendly separator−electrolyte systems. Overall, the main contribution in this work is to prove for the first time the ability of CNCs, which have been classically used as reinforcing elements in polymer composites, to form mesoporous membranes which efficiently work as separators for LIBs, not only using the conventional LiPF6 in EC-DMC as electrolyte solution but also using ionic liquids. Thus, the present work will open new avenues on the development of novel energy storage devices using separator−electrolyte systems based on renewable resources and ionic liquids.

of the electrolyte across the material, leading to improved battery performance.7 With the increasing environmental concerns raised, among others, by the global warming and the exhaustion of combustible fossils, there is a need for a widespread strategy for sustainability that includes the development of energy storage devices based on renewable resources. In this framework, battery separators fabricated from biopolymers emerge as a novel alternative for traditional fossil-based polymer separators. Accordingly, separators based on biomass-based cellulose and its derivatives have been proven so far to be an efficient alternative to traditional polyolefin-based separators such as polyethylene (PE) and polypropylene (PP).8 Cellulose is widely available, presents an hydrophilic character essential for an easy impregnation of liquids,9 displays a fairly good thermal resistance up to temperatures above 200 °C,10 and has a good ionic conductivity.11 Accordingly, cellulose has been recently used as separator material in electrolytic capacitors12 and alkaline batteries13 and has been used as a gel polymer electrolyte.14 In LIBs, cellulose has already been used as a separator membrane in the form of bacterial cellulose15 and electrospun nanofibers12 and as nonwoven mats.16 Cellulose-derived materials also meet the requirements for environmentally friendly and biocompatible products.17 In this burgeoning trend of synthesizing novel battery separators, there is a need for improvement by providing novel highly porous architectures based on cellulose. Together with the separator material itself, its structure and more precisely its porosity and pore morphology markedly influence the performance of the LIB. Among all cellulosic derivatives, nanocellulose is raising an increasing scientific and technological interest due to its outstanding mechanical properties, low cost, anisotropic shape, and renewable character.18,19 Despite the increasing interest in nanocellulose, nanocellulose separators are based on cellulose nanofibers (CNFs).20 To the best of our knowledge no previous work has been reported on the use of mesoporous films based on cellulose nanocrystals (CNCs) for LIBs. In this sense, here we introduce mesoporosity into CNC separator as a strategy to improve the performance of the LIB. Accordingly, mesoporous cellulose nanocrystal (MCNC) structures with pores in the 2−50 nm range emerge as an attractive threedimensional porous separator as it will allow electrolyte adsorption because of their highly porous hydrophilic character. Such structures can be obtained upon the concomitant assembly of a silica precursor such as tetramethyl orthosilicate (TMOS, SiC4H12O4) with cellulose nanocrystals (CNCs) and subsequent silica removal under a controlled alkali treatment.21−23 Moreover, the inherent strength of CNCs provides mesoporous cellulosic films a high mechanical stability with a good resistance against pinhole formation, essential for safe LIBs.24 Although these materials have shown a large potential for optical,25 sensing,21,25 and catalytic applications,23 to date, scarce works have been focused on their application as battery separators. In order to increase mechanical and thermal stability, cellulose has been coated by polydopamine for separator applications.26 Also, separators based on cellulose fibers are prepared by filtration dewatering,27 and recently, cellulose has been used as a biodegradable membrane in LIB.28 In this work, porous cellulosic structures have been synthesized with large surface areas and mesoscale pore

2. EXPERIMENTAL SECTION 2.1. Starting Materials. Microcrystalline cellulose with a particle size of 20 μm (310697-500G), sulfuric acid, tetramethyl orthosilicate (TMOS, 1.023 g/mL, 341436-25G), and glucose (D-(+)-glucose, G5767-500G) were supplied by Sigma-Aldrich. The solvent N,N′dimethylpropyleneurea (DMPU) was purchased from LaborSpirit. The conventional electrolyte 1 M LiPF6 in ethylene carbonate− dimethyl carbonate (EC-DMC, 1:1 vol) and the IL, 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide, [C2 mim][NTf2], were purchased from LaborSpirit, Solvionic, and Iolitec, respectively. 2.2. Mesoporous Cellulose Nanocrystal (MCNC) Film Fabrication. Nonporous cellulose nanocrystal (CNC) films were obtained by evaporation induced self-assembly (EISA) method by directly casting water-dispersed CNCs onto hexagonal polystyrene weighing dishes,9 while mesoporous cellulose nanocrystal (MCNC) films were obtained upon the coassembly of CNCs together with TMOS and subsequent silica removal.29 First, CNCs were synthesized through sulfuric acid hydrolysis of microcrystalline cellulose using a 64% (w/w) sulfuric acid solution at 45 °C for 30 min, as previously described.9 An aqueous dispersion of CNCs at a 1.3 wt % with a pH of 2.4 was then obtained. Two mesoporous films, termed as MCNC0.5 and MCNC-1, were fabricated by using a 1:0.5 and 1:1 ratios of CNC/TMOS (mg to μL). An amount of 100 mg of water-dispersed CNCs was mixed with 100 mg of glucose, and the mixture was ultrasonicated for 60 min. The required amount of TMOS (50 and 100 μL for MCNC-0.5 and MCNC-1, respectively) was then added and the solution was vigorously stirred for 1 h. The suspension was cast onto a hexagonal polystyrene weighing dish, and it was allowed to dry for 72 h under ambient conditions. Once fully dried, mesoporous films were obtained after the removal of silica and glucose with 20 mL of 2 M NaOH for 48 h followed by supercritical carbon dioxide (sCO2) drying. From the obtained films, 50 mm diameter (see Figure S1) and ∼150 μm thick crack-free separator membranes were prepared. 2.3. MCNC Separator Characterization. Individual CNCs were observed under transmission electron microscopy (TEM) after dropping a 0.1% (w/w) suspension of CNCs onto a carbon-coated grids and further negatively staining it with 1% uranyl acetate. A Philips CM120 Biofilter apparatus with STEM module at an acceleration voltage of 120 kV was used. X-ray powder diffraction 3750


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Figure 1. (a) TEM image, (b) FTIR spectra, and (c) XRD patterns of synthesized CNCs. (d) N2 adsorption−desorption isotherms of nonporous and mesoporous CNC films. where m0 is the weight of the dry membrane and mi is the weight of the membrane after immersion in the electrolyte solution or ionic liquid. Impedance spectroscopy measurements were carried out in MCNCs soaked in electrolyte solution or ionic liquid after immersion during 15 min. The applied frequency range was between 500 mHz and 65 kHz in an AutolabPGSTAT-12 (Eco Chemie) equipment in the temperature range from 20 to 60 °C using a constant volume support equipped with gold blocking electrodes located within a Büchi TO 50 oven. The ionic conductivity (σi) of the MCNC separators was calculated with the following equation:

(XRD) patterns were recorded using a PANalytical Empyrean powder diffractometer in reflection mode using Cu Kα radiation and operating at 45 kV and 40 mA. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) measurements were performed on a Bruker Alpha FT-IR spectrometer equipped with diamond ATR optics. Nitrogen sorption experiments were carried out on a Quantochrome Autosorb-iQ-C-XR at 77 K, with nitrogen (99.999%) and helium (99.999%) provided by PanGas AG, Switzerland. Before the measurement, the samples were degassed in vacuum at 60 °C for 24 h. The surface area was determined via the Brunauer−Emmet−Teller (BET) method. Film morphology was analyzed in a Hitachi S-4800 field emission scanning electron microscope (FE-SEM) at an acceleration voltage of 5 kV. Before analysis, surfaces were copper-coated in a Quorum Q150T ES turbopumped sputter coater (5 nm thick coating). The mechanical behavior of separators was studied under uniaxial tensile experiments using an Autograph AGS-J from Shimadzu at a deformation rate of 1 mm·min−1. Before testing, samples were conditioned at 22 °C and 51% relative humidity overnight. Reported data represent the mean and standard deviation over three samples. 2.4. Contact Angle, Uptake Value, Ionic Conductivity, and Cyclic Voltammetry of the MCNC Separator. Both conventional electrolyte 1 M LiPF6 in ethylene carbonate−dimethyl carbonate (EC-DMC, 1:1 vol) and IL, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C2mim][NTf2] have been used as probe liquid for the determination of the wettability at the separator surface. Measurements were carried out by sessile drop method (3 μL per drop at a 2 μL/s rate) at room temperature. Reported values represent the mean and standard deviation over four measurements. The uptake value was obtained by immersing the MCNC membranes into the electrolyte solution and ionic liquid and applying the following equation:

uptake =

mi − m0 × 100 m0

σi =

d Rb × A

(2)

where Rb is the bulk resistance, d is the thickness, and A is the area of the sample. The electrochemical stability of the system was evaluated in the two-electrode cell configuration with a gold microelectrode (diameter of 25 μm) as working electrode and a lithium disk (Aldrich, 99.9%; 19 mm diameter, 0.75 mm thick) as counter electrode through cyclic voltammetry within a dry argon-filled glovebox using an Autolab PGSTAT-12 (Eco Chemie) equipment at a scan rate of 5 mV s−1 inside a Faraday cage. 2.5. Electrode Preparation, Battery Assembly, and Evaluation. 2.5.1. Cathode Preparation. The cathode was prepared using 80 wt % C-LiFePO4, 10 wt % carbon black, and 10 wt % poly(vinylidene fluoride), PVDF, in 2.25 mL of DMPU for 1 g of solid material. More details on electrode preparation are reported in ref 30. The resulting slurry was then casted on aluminum foil by doctor-blade technique and dried at 100 °C for 2 h. The electrode and active mass loading were ∼2.5 and 2 mg·cm−2, respectively. 2.5.2. Battery Preparation, Electrical Properties, and Cycling Performance. It should be taken into account that the inherent hygroscopic behavior of cellulose may limit its use as separator due to the fact that water contents below 20−50 ppm are required to avoid LiPF6 hydrolysis and HF formation in LIBs.31 Accordingly, the obtained mesoporous films were dried at 60 °C under vacuum for 24 h before they were quickly transferred into an argon filled glovebox,

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ACS Applied Energy Materials where half-cells were mounted. Swagelok type Li/C-LiFePO4 halfcells were assembled in a homemade argon-filled glovebox and prepared using the MCNCs membrane as separator (10 mm diameter) soaked in electrolyte solution or ionic liquid. Metallic lithium (8 mm diameter) was used as anode and the C-LiFePO4 based electrode as cathode (8 mm diameter). The electrical properties of the Li/C-LiFePO4 half-cells with electrolyte solution or ionic liquid were measured by electrochemical impedance spectroscopy (EIS) with an Autolab PGSTAT12 instrument, in the frequency range from 10 mHz to 1 MHz at open-circuit potential (OCP), with an amplitude of 10 mV ac voltage signal. Charge−discharge tests were obtained at room temperature in the voltage range of 2.5−4.2 V at current rates from C/5 to 2C (C = 170 mA·g−1) using a Landt CT2001A instrument. A minimum of five half cells were fabricated with the MCNC separators to test their cycling behavior.

carried out to quantify the surface area (SBET) of the obtained materials as well as to confirm the presence of mesoporosity within both the MCNC-0.5 and MCNC-1 films (Figure 1d). A classical type IV isotherm with type H2 hysteresis characteristic of mesoporosity is observed for both MCNC-0.5 and MCNC-1 films,38 while the nonporous CNC film obtained after the direct casting of aqueous CNC dispersion onto hexagonal polystyrene weighing dish presents a rather flat shape with no hysteresis, suggesting the presence of mesopores within the synthesized MCNC films.39 A Brunauer−Emmett− Teller (BET) surface area (SBET) of 7.3, 172.0, and 169.7 m2· g−1 is obtained for the CNC, MCNC-0.5, and MCNC-1 films, respectively. Similarly, a pore volume of 1.550 and 1.188 cc3· g−1 is obtained for MCNC-0.5 and MCNC-1, respectively, while CNC solid films present a pore volume as low as 0.017 cc3·g−1. The Barret, Joyner, and Halenda (BJH) pore size distribution and cumulative pore volumes are shown in Figure S2. A sharp peak in the wide range of the mesopore region (3− 10 nm) is observed, strongly suggesting the presence of abundant mesopores in the MCNC structure. Both MCNC-0.5 and MCNC-1 films show a pore size distribution with a peak centered at 4.78 nm. Although it is generally accepted that the specific surface area (SSA) and pore volumes can be increased upon further addition of silica precursor, larger TMOS loadings do not increase the SSA as the presence of a large silica phase leads to thick silica walls, preventing the formation of mesopores.40 Both mesoporous films show an apparent density (ρ0) of 0.3946 g·cm−3 and a porosity of about 75% as obtained from eqs 5 and 6: mass ρ0 = apparent volume (5)

3. RESULTS AND DISCUSSION 3.1. Morphology and Physicochemical Properties of Materials and Separator Membranes. Transmission electron microscopy (TEM) observations were carried out to observe the morphology of individual CNCs. According to Figure 1a, CNCs show a needle-like appearance with lengths oscillating between 100 and 200 nm and widths of about 8 nm. Fourier transform infrared spectroscopy (FTIR) has been used to study the physicochemical conformation of obtained CNCs. For all the samples the characteristic features of cellulose are observed in the FTIR spectra in Figure 1b, with a broad band in the 3650−3200 cm−1 region (O−H stretching) and narrower bands in the 1800−800 cm−1 region (C−O−H bending, C−O−C bending, and C−O−C asymmetric stretching).32,33 X-ray diffraction analyses were performed to study the crystalline structure of mesoporous films. XRD patterns in Figure 1c display the crystalline phases corresponding to cellulose I for all the three films, with the main peaks at 2θ angles 14.9°, 16.5°, and 22.7° and a weaker one at 34.6°, corresponding to the (1−10), (110), (200), and (004) planes, respectively, indicating that the original crystallinity of CNCs was preserved during the alkaline treatment and subsequent sCO2 drying.34,35 The crystallinity index (Xc) of CNC and MCNC films has been calculated as22,36 Ac Xc (%) = × 100 Ac + A a (3)

ij ρ yz porosity (%) = jjjj1 − a zzzz × 100 ρc { k

where ρc is the bulk density of cellulose crystals taken as 1.6 g· cm−3.41 Such porosity is notably higher than the porosity of the widely used commercial polyolefin separators (about 40− 50%), which a priori should improve the Li+ ion permeability through the membrane.5 The morphology of mesoporous CNC films was assessed by scanning electron microscopy (SEM), and film thicknesses of about 25 and 150 μm are observed for CNC and MCNC films, respectively. A highly porous structure consisting of loosely packed CNCs with diameters of around 10 nm is observed for the MCNC-1 film in both the cross sections shown Figure 2 and the top views shown in Figure S3, while CNCs are closely packed into a layered “solid” structure with no noticeable cavities in the nonporous samples as previously reported.42,43 Such abundant connective pore morphology across the MCNC membrane would provide enough space form liquid electrolyte adsorption, which in turn would ensure a fast Li+ ion conduction throughout the separator. Interestingly, the small size of the pores present in MCNC films is beneficial toward the prevention of battery internal short-circuiting and selfdischarge as separator puncture-resistance is enhanced.44 In order to avoid internal short circuits, battery separators should withstand mechanical stresses associated with rough surface electrodes and lithium dendrite growth.8 Accordingly, as shown in Figure S4, the mechanical behavior of separators under axial stress has been evaluated through tensile testing.

where Ac and Aa account for the total crystalline area and total amorphous area of the deconvoluted PXRD pattern based on Ruland’s principles and Rietveld analysis. Moreover, the interplanar spacing (d) obtained from the main diffraction peak corresponding to the (200) plane was extracted following Bragg’s equation: nλ = 2dhkl sin θ

(6)

(4)

where λ represents the wavelength of X-ray (1.5418 Å), n is an integer, and θ is the angle between crystal planes. Neat CNC film show a Xc of 94% and a d(200) interplanar spacing of 3.94 Å, which is characteristic of cellulose I structure,37 while Xc drops to a value of 89% and 81% for the MCNC-0.5 and MCNC-1 separators, respectively. Although the crystalline polymorph of the original CNC is not modified, it is observed that silica removal by NaOH results in a decreased crystallinity induced by the disruption of hydrogen bonding within CNC crystallites (the higher is the presence of silica, the larger the Xc decrease results). As the aim of this work is to fabricate highly porous cellulosic structures, N2 adsorption−desorption isotherms were 3752


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MCNC-0.5 and MCNC-1 display a similar stress−strain response). These results highlight that both CNC and MCNC membranes present enough ductility to be easily handled during cell assembly. Interestingly, the elasticity of the cellulosic films could be slightly enhanced by the introduction of mesoporosity. Moreover, the E of the CNC film markedly increases up to 4170 ± 1410 MPa, while MCNC membranes have a Young modulus of 1860 ± 180 MPa. Such high E value for the CNC membranes is due to the strong bonding between cellulosic nanoparticles.45,46 The E of the mesoporous films does not drop much in spite of their large porosity, which may ensure a proper mechanical stability of the separators when submitted to stress (i.e., avoid lithium dendrite to puncture). These cellulosic separators here fabricated show a proper ductility for being handled during mounting while their high E ensures an enhanced safety of the LIB. Moreover, thermogravimetric analysis (TGA) results in Figure S5 show that the thermal stability of the separators is improved after the removal of the sulfate half-ester groups (−OSO3−) during NaOH washing step,22,47 where the peak degradation temperature (Tp) is delayed by 149 °C. Accordingly, such mesoporous films can be applied in energy storage devices that need to work at temperatures up to 150 °C (it is worth noting that the mechanical properties are kept up to such high temper-

Figure 2. SEM micrographs showing the cross section of nonporous CNC and MCNC-1 films. A higher magnification image provides further details on the nanoscale packing of CNCs.

Glass microfiber separators present a semiductile behavior characterized by an elongation at break (εb) of 5.8 ± 0.1% and a Young modulus (E) of 1470 ± 120 MPa. It is observed that the εb slightly drops up to values in the range of 3.4 ± 0.5% for CNC film and 4.4 ± 1.2% for the mesoporous films (both

Figure 3. (a) Representative images of conventional 1 M LiPF6 in EC-DMC electrolyte and ionic liquid drops at the surface of commercial, CNC, MCNC-0.5, and MCNC-1 separators. For the sake of comparison, mean contact angle values are shown. (b) Electrolyte uptake; (c) ac impedance spectra of half-cells having MCNC separators; (d) linear voltammograms of MCNC separators in the 0−5.0 V Li/Li+ range, and (e) Nyquist plots of MCNC separator membranes before cycling with equivalent circuit. 3753


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ACS Applied Energy Materials atures).48 This fact is also demonstrated through the optical images after thermal treatment (Figure S6). 3.2. Contact Angle, Electrolyte Uptake, Ionic Conductivity Value, and Electrochemical Window. The high porosity and large surface area of mesoporous CNC membranes together with the inherent hydrophilicity of cellulose are expected to facilitate the electrolyte wettability. Electrolyte wettability is of paramount relevance toward the development of efficient LIB separators as a high electrolyte affinity of the membrane ensures an efficient soaking with the liquid medium, which typically results in increased lithium ion mobility through the membrane. More precisely, a poor wettability of the separator toward the electrolyte yields nonfilled pores with the electrolyte, resulting in blocked paths for Li+ ion diffusion.49 Accordingly, electrolyte wettability of CNC, MCNC-0.5, and MCNC-1 has been evaluated by sessile drop method using both the conventional electrolyte and the IL (for comparison, the results corresponding to the commercial glass microfiber membrane are also shown). Figure 3a displays representative images of an electrolyte drop (either conventional 1 M LiPF6 in EC-DMC or the IL) on the surface of the separators together with the obtained contact angle values (images are taken at time zero). It is observed that the commercial glass microfiber membrane immediately absorbs the liquid, resulting in contact angles of nearly 0° for both electrolytes. On the contrary, the CNC films present a contact angle of 37 ± 1° and 54 ± 2° toward 1 M LiPF6 in EC-DMC and the ionic liquid, respectively. It is worthy to note that when using 1 M LiPF6 in EC-DMC as a liquid probe, the CNC separators present a markedly lower contact angle than commercial polyethylene (Celgard 2730) or polypropylene (Celgard 2500) separators, which have a contact angle of 56.1° and 84°, respectively.44,50 The poor wettability of commercial PE membranes with 1 M LiPF6 in EC-DMC arises from the mismatch between the nonpolar polyolefin separators and the polar organic liquid electrolytes,51 while the enhanced wettability of the CNC separators is due to the dominant hydrophilic character of cellulose impaired by the interaction between the oxygen-containing groups on the cellulose surface.9,52 It should be taken into account that a priori, the lower wettability of the CNC films in comparison with the commercial glass microfiber separator (although much higher than commercial polyolefin separators) may hinder Li+ ion transport through the separator, resulting in lower capacities. As denoted by the increase in contact angle, the wettability of CNC with the ionic liquid is lower when comparing with the conventional electrolyte. This behavior is explained in terms of the higher viscosity of the IL at room temperature, which gives rise to a higher surface tension and thus poorer separator wettability.50 In any case, the contact angle for CNC separators when using IL as a probe liquid remains below the values reported so far for commercial polyolefin separators,53 denoting a relatively good compatibility between CNCs and the IL. In this framework, we further evaluated the wettability of MCNC-0.5 and MCNC-1 separators and interestingly, it is found that the wettability was markedly improved as denoted by the low contact angle values of 12 ± 1° and ∼0° for the conventional electrolyte, respectively, and 17 ± 2° and 10 ± 1° for the ionic liquid, respectively. It is worthy to note that previous works have shown that NaOH modification increases the water contact angle from 45 °C for sulfated CNC dense film up to 73° after sodium hydroxide treatment.22 Therefore, the porous 3D

structure has a pivotal role on the enhancement of the electrolyte affinity of the membranes, where the mesopores provide efficient paths for Li+ ion migration through the membrane, enabling comparable performance of MCNC separators soaked with ILs or conventional carbonate electrolytes. Figure 3b shows the electrolyte uptake for both mesoporous MCNC-0.5 and MCNC-1 membranes. It is observed that mesoporous cellulosic membranes present uptake values of 240−280%, showing that they are able to absorb large amounts of electrolyte. The obtained uptake values remain markedly above those reported for commercial polyethylene (Celgard 2730) and polypropylene separators (Celgard 2500), where for the electrolyte system, uptake values of 103% and 125% were obtained, respectively.44,50 Such poor values are due to a combined effect of poor surface affinity and low porosity.54 Consequently, the prepared mesoporous cellulosic membranes may provide rapid ion transport pathways between electrodes, ensuring a good electrochemical performance of the LIB.55 Results indicate that despite the larger pore volume of MCNC-0.5 in comparison with MCNC-1 (1.550 vs 1.188 cc3· g−1), the electrolyte uptake of MCNC-0.5 remains slightly below that obtained for MCNC-1. Those results are ascribed to the fact that once a critical pore volume has been reached, a very large porous structure fails to provide a suitable enough structural framework for electrolyte absorption, yielding decreased electrolyte uptake values. Electrochemical impedance spectroscopy (EIS) behavior of the electrolyte-soaked MCNC membranes is shown in Figure 3c, characterized by straight lines corresponding to the typical behavior of electrode/electrolyte double layer capacitance. The bulk resistance of the membranes (and thus ionic conductivity) could be extracted from the high-frequency intercept of the Nyquist plots with Z′ (see the inset in Figure 3c). Accordingly, values of 2.7 and 2.1 mS·cm−1 are obtained for MCNC-0.5 and MCNC-1 membranes, respectively, suggesting that a larger pore volume supports Li+ transport through the wet MCNC separator. Such conductivities remain above those described for mesoporous Cladophora cellulose separators (0.4 mS·cm−1),56 nanofibrous cellulose nonwoven membranes (1.75 mS·cm−1),16 or electrospun mats of rough regenerated cellulose (1.32 mS·cm−1 ).12 The fact that such high conductivities are obtained for 150 μm thick separator membranes, in comparison with commercial 20−25 μm thick membranes,57 make MCNC films potential candidates for safer LIBs as the thicker is the separator, the better is the resistance against puncture, reducing the occurrence of associated internal short circuits. Therefore, these developed membranes show enhanced lithium ion transport in comparison with related previously thinner reported systems, which may lead to improved electrochemical performance of safer LIBs. From the practical point of view, the electrochemical stability of the electrolytes together with the battery separator within the operation voltage window should be as large as possible. The electrochemical stability of the separator was therefore evaluated in the 0.0−5.0 V Li/Li+ operation window. The linear sweep voltammogram (LSV) curves presented in Figure 3d denote no traces of electrolyte decomposition in the 2−4.5 V range, suggesting that MCNC membranes display good resistance against oxidative and reductive conditions during cycling. The stability of MCNC-1 separators remains lower compared to the stability of the MCNC-0.5 as a result of the lower crystallinity of the film as proven by XRD results. In 3754


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Figure 4. (a) Galvanostatic charge/discharge curves of MCNC-1 separators at different charge/discharge current densities; (b) comparison of the galvanostatic charge/discharge curves of MCNC-0.5 and MCNC-1 separators; (c) rate capacity of cells using MCNC-0.5, MCNC-1, and glass microfibre separators, and (d) capacity retention (%) upon increasing cycling rate.

better interfacial compatibility between lithium metal and separator, improving battery performance. 3.3. Battery Performance. 3.3.1. Electrolyte Solution. Cell performance including charge−discharge capacities, discharge C-rate capability and cycling stability using MCNC membranes as separators are presented in Figure 4. Galvanostatic cycling tests were carried out at different rates in order to evaluate MCNC membranes as separators for LIBs. Stable charge and discharge plateaus are observed in Figure 4a, where the battery exhibits a capacity as high as 120 mAh·g−1 with a small overpotential (difference between charge and discharge plateaus) at a C/5 rate. It is also observed that specific capacity continuously drops as cycling rate increases, in particular above C (a cycling rate of 2C delivers a specific capacity of 84 mAh·g−1). For comparison, the galvanostatic charge−discharge curves at C/8 and 2C for MCNC-0.5 and MCMC-1 membranes are shown in Figure 4b. The difference between the charge and discharge voltages is due to the resistance polarization within the battery, which arises as a result of the ohmic resistance of lithium ions transferring through the soaked separator, the electronic conductivity between electrodes, and the interfacial electrode−separator resistance.60,61 Therefore, the better performance of the MCNC-1 membrane, especially at a high cycling rate of 2C, matches well with the reported lower interfacial resistance value of MCNC-1 films. Furthermore, an intriguing effect is here observed: while at slow cycling MCNC-0.5 membranes deliver higher specific capacity, their performance drastically drops at 2C in comparison with the specific capacity delivered

view of these results, mesoporous cellulosic membranes emerge as good candidates for the development of highpower LIBs which operate in the 2.5−4.2 V range, which agrees well with previously reported works.12,16 Ensuring a good interfacial compatibility between the lithium electrode and the separator is of paramount relevance in LIBs for practical applications. Accordingly, Figure 3d shows the Nyquist plots of a lithium metallic/electrolyte-soaked MCNC/C-LiFePO4 cathode cell, from which the interfacial resistance between the liquid and the electrolyte-soaked separator could be extracted. In such plots, the diameter of the semicircle should be as small as possible as its size represents the interfacial resistance between the lithium metal and the electrolyte-soaked MCNC membranes. The chargetransfer resistance is estimated from the equivalent circuit, insert of Figure 3e. The equivalent circuit consists of a resistor R1 (ohmic resistance of the electrolyte solution) connected in series to a resistor R2,ct (charge transference resistance) in parallel to a constant phase element (CPE, due to the interface between electrodes and membrane separator) and a Warburg impedance WS (mass transport component).58 Interfacial resistances of 757 and 470 Ω are obtained for MCNC-0.5 and MCNC-1 membranes, respectively, indicating that an increased pore volume does not necessarily reduce the interfacial resistance. In fact, a too large pore volume leads to lower mechanical stability of the separator, which may result in a loss of contact between the electrode and the separator, yielding increased interfacial resistivity.59 Therefore, an intermediate pore volume of 1.188 cc3·g−1 would endow 3755


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Figure 5. (a) Cycle performance of the cells using MCNC-0.5 and MCNC-1 separators and (b) Nyquist plots of MCNC separator membranes after cycling.

Table 1. Comparison of Cellulosic Separators and Corresponding LIB Performance polymer acetate cellulose with titania bacterial cellulose bacterial cellulose cladophora cellulose cellulose nanofibers cellulose nanofibers cellulose/PVDF MCNCs

pore size (μm)

porosity (%)

σi (mS·cm)

capacity (mAh·g−1)

ref

79@2C ∼110@C at 50 °C ∼100@2C ∼105@C ∼130@C ∼90@C

12 67 68 56 69 70 71 in this work

0.31

78

0.62 6.5

0.2 0.3

46 70

0.4

0.8−2

62−72 ∼75

0.75 2.53 2.1

84@2C

5 results in improved performance of MCNC-1 separators. This is particularly remarkable at a cycling rate of 2C, where MCNC-0.5 membranes display a discharge capacity of 26 mAh·g−1, while MCNC-1 ones deliver 84 mAh·g−1. Interestingly, when the cycling rate was switched back to C/8 after the rate test, the reversible capacity of the half-cells with mesoporous CNC separators returned to values very close to the original capacity. In addition, Figure 4c shows the discharge capacity value obtained with commercial glass microfibre separators. Whatman glass microfiber separators have been selected as a model commercial separator as it is commonly used in diverse energy storage devices,65,66 and its wettability with both electrolytes here used (conventional 1 M LiPF6 in EC-DMC and ionic liquid) is better than the one shown by commercial polyolefin separators.44,50 It is observed that at C-rates above C the discharge value of the MCNC films is higher than the one for the glass microfibre separator due to the mesoporosity provided by the cellulosic membrane. As summarized in Figure 4d, the capacity retention of MCNC-0.5 markedly drops upon increasing the scan rate. On the contrary, because of its increased electrolyte uptake, pore morphology, and reduced interfacial resistance, MCNC-1 separator shows a capacity retention above 90% up to C (discharge capacities above 110 mAh·g−1). The cycling stability of MCNC-0.5 and MCNC-1 membranes was evaluated at C and 2C. As shown in Figure 5a, a capacity decay is observed during the first 5−10 cycles from the initial delivered capacities of 116 and 132 mAh·g−1 for MCNC-0.5 and MCNC-1 membranes, respectively. In addition, the difference observed for MCNC-1 at 2C in Figures 4c and 5a for over 50 cycles may be caused by the loss of electrical contact between the active material and conductive additive particles originated by the mechanical strains in the

by the MCNC-1 membranes. It seems that the higher ionic conductivity of the MCNC-0.5 membranes is enough to lower the cell resistance and provide good performance at low cycling rates. On the contrary, at high cycling rates, despite their lower ionic conductivity, MCNC-1 films perform better as their lower interfacial resistance and pore structure provide efficient pathways for the ion transport between the anode and the cathode. Figure 4c reports obtained discharge capacities form both MCNC-0.5 and MCNC-1 membranes. The main reason for this behavior is due to the larger porous structure of MCNC0.5 where the pore structure within the membrane will reduce the distance for random walk migration of species, leading to reduced mobility.62 Despite the fact that both MCNC-0.5 and MCNC-1 are composed by the same material and show a similar specific surface area, their different electrochemical stability may be explained in terms of the initial amount of TMOS used for their preparation. The amount of silica in MCNC-1 was 2 times larger than that for MCNC-0.5. This leads to thicker silica walls, which after their removal results in larger pores. Although the ionic conductivity is lower, these larger pores in MCNC-1 may facilitate lithiation/delithiation processes in comparison with the MCNC-0.5 membrane, whose small pores markedly increase the tortuosity of the separator. For MCNC-0.5, the discharge capacity value at C/8 increases over the first three cycles due to the cathode thickness and the contribution of aluminum current collector oxidation parasitic reactions due to the degradation of the LiPF6 salt that occurs during half-cell charging at a relative high potential,63 yielding a proton arising from the organic solvent oxidation.64 At low current densities MCNC-0.5 delivers larger discharge capacities, while increasing the cycling rate above C/ 3756


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Figure 6. (a) Nyquist plot of the MCNC-1 membranes soaked with [C2 mim][NTf2] IL, (b) electrochemical stability of the MCNC-1 membranes soaked with [C2 mim][NTf2] IL, (c) charge capacity value at C/8 as a function of the cycle number, and (d) impedance spectroscopy of cathodic half-cells before and after cycling for the MCNC-1 membranes with [C2 mim][NTf2] IL as electrolyte and the corresponding fitting with the equivalent circuit shown in Figure 3e.

cathode layer and/or by the fact that the electrolyte can penetrate into the detached areas of the current collector giving rise to capacity loss. After this initial capacity decay, good capacity retention is achieved for both membranes, with a Coulombic efficiency of nearly 100%. This is in marked contrast with the low specific capacities of 40 and ∼0 mAh·g−1 obtained for 20 and 40 μm thick Cladophora cellulose separators, respectively,7 indicating that the highly interconnected porous structure of MCNC separators provides good battery performance at rather high thicknesses, which in turn may reduce the risk of short circuits between the electrodes. In order to analyze the formation of solid electrolyte interphase (SEI) layer during cycling test, Nyquist plots of the EIS measurements for cathodic half-cells incorporating the MCNCs after cycling are shown in Figure 5b. The shape of the impedance curve after cycling is similar to the one before cycling, the values for the diameter of the semicircle increasing after cycling (the interfacial resistance is 3516 Ω for MCNC-05 and 1506 Ω for MCNC-1), indicating the formation of SEI layer for both samples through increase of the interfacial resistance. Considering the excellent charge−discharge results presented in Figures 4 and 5, Table 1 compares the electrochemical properties of the MCNC separator membranes with other cellulosic separator materials reported in the literature for the same electrode. It is observed that the electrochemical results obtained in the present work are similar or even higher than the ones reported in the literature for other cellulosic separator materials. Thus, the developed mesoporous cellulose membranes used in this work as battery separator are

promising for the development of environmental friendlier lithium-ion batteries. In order to evaluate the stability of the interface between the lithium metal film and MCNC films, lithium symmetric cells were assembled and the impedance response was analyzed in an open-circuit voltage configuration at different times (see Figure S7). This configuration can be used to get further insights on the formation and thickness of a solid electrolyte interface (SEI)72 layer that affects the cycling performance of the batteries. The obtained response represented in Figure S7 is described by semicircles due to a combination of resistance and capacitance associated with the passivation film on the lithium metal.73 It is observed that the extent of the semicircle increases over time as a result of the degradation of the salt and organic compounds present in EC and DMC solvents upon cycling.74 3.3.2. Ionic Liquid Electrolyte. The high discharge capacity, the relatively small overpotential, wide electrochemical window, cycling stability, and renewable character show mesoporous CNC films as suitable candidates to develop green batteries. In this context, the use of ionic liquids (ILs) as feasible electrolytes for the development of environmentally friendly separator−electrolyte systems has been evaluated. The ionic conductivity of the MCNC-1 membranes soaked with ionic liquid was measured by electrochemical impedance spectroscopy, EIS, as presented in Figure 6a. The Nyquist plot (Figure 6a) is characterized by a straight line for all frequency range due to the high ionic conductivity of 1.8 mS·cm−1 at 25 °C.60 Cyclic voltammetry was used for evaluating the electrochemical stability, essential for proper battery performance. Figure 6b shows a typical cyclic voltammogram 3757


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based on cellulose nanocrystals. The highly porous structure consisting of loosely packed CNCs allows an easy electrolyte adsorption, endowing ionic conductivities to the electrolyte soaked separators as high as 2.7 mS·cm−1 and low interfacial resistances with the lithium metal. When evaluated using LiFePO4 as the cathode and Li metal as the counter and reference electrodes, MCNC separators show a discharge capacity of specific capacity of 122 mAh·g−1 and 85 mAh·g−1 at a C/2 and 2C rates, respectively, with good cycling performance. A good capacity retention after 60 cycles at C and 2C was obtained, suggesting a long-term stability of such membranes. Further, it is proposed the combination of ionic liquids together with MCNC membranes, showing a specific capacity of 91 mAh·g−1 after 10 cycles at C/8 rate, for the development of environmentally friendlier separator−electrolyte systems. Because of their good electrochemical stability and thickness of 150 μm, MCNC membranes are suitable candidates for safer, environmentally friendlier, and high-power LIBs. Finally, due to their environmentally friendlier characteristics, mesoporous cellulosic films soaked into an ionic liquid electrolyte demonstrated the suitability of MCNC membranes for the development of novel energy storage devices.

characteristic curve for the membranes soaked with ionic liquid in the potential range from 0.0 to 5.0 V (vs Li/Li+) at a scan rate of 5 mV·s−1 at 25 °C on a gold micro working electrode. The electrochemical stability window is between ∼2.5 and 4.5 V (vs Li/Li+) with no electrochemical decomposition of the IL in this range. Thus, considering the voltage range of cathodes, C-LiFePO4 system is adequate for [C2 mim][NTf2] ionic liquid for lithium ion battery applications due to the high ionic conductivity and electrochemical stability. Taking into account, the high ionic conductivity and the excellent electrochemical window of the MCNC-1 membranes soaked with ionic liquid, Figure 6c shows the cycle stability at C/8 for 10 cycles for these samples as a proof of concept. For the C/8 rate, it is observed a good stability with high capacity retention over 10 cycles for IL in both charge and discharge processes. It is also observed in the voltage vs capacity value of the fifth cycle (insert in Figure 6c) that the MCNC-1 membrane with IL shows a low polarization. In addition, the capacity value of the MCNC-1 membranes with IL is somewhat small when compared to the conventional electrolyte, which is attributed to the high viscosity value of the IL, the diffusion being lower for higher viscosities.75 In order to better understand the electrochemical performance of the cathodic half-cells assembled with MCNC-1 soaked with IL, the EIS spectra of these half-cells were recorded before and after cycling, as shown in Figure 6d through the Nyquist plots with fitting curves obtained with the equivalent circuit presented in Figure 3e. The Nyquist plots obtained before and after cycling are characterized by a semicircle (overall resistance, which is the sum of the ohmic resistance that represents the contact film resistance and resistance contributions from the charge-transfer reaction resistance) in the high and medium frequency regions and a straight line that is associated with the Li+ diffusion process in the low frequency regions. Figure 6d shows that the overall resistance, as obtained by the fitting with the equivalent circuit presented in Figure 3e, before and after cycling is 550 Ω and 13651 Ω, respectively. The increase of the overall resistance is attributed to the formation of SEI layer during cycling, related to lithium deposition and plating.76,77 It is stressed that the performance of MCNC for environmentally friendlier LIBs is better than the works reported so far using the same IL as electrolyte. In this sense, porous poly(vinylidene fluoride) separators soaked in [C2 mim][NTf2] ionic liquid show a discharge capacity value of 45 mAh·g−1 at C/5 rate and discharge capacity retention as low as 60% after 10 cycles. Similarly, the resistance shown before and after cycling was 2100 Ω and 38 000 Ω.,78 which is 3−4 times larger than the resistances obtained using mesoporous cellulose separators. Furthermore, it was also shown that both commercial Celgard 2400 and poly(vinylidene fluoride-cohexafluoropropene) separators present a specific capacity of about 87 mAh·g−1 after 10 cycles when tested in half-cell configuration (Li metal as counter and reference electrodes and LiCoO2 as a working electrode).79 Overall, the present work demonstrates an approach for the preparation of environmentally friendlier separator membranes based on mesoporous cellulose for safer LIB applications using both conventional electrolytes and ILs as electrolytes without lithium salts and further additives.

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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/acsaem.9b00458.

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Photographs of samples, BET analysis results, SEM micrographs, stress−strain curves, thermogravimetric traces, impedance curves, and resistance values (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E.L.: e-mail, [email protected]. *C.M.C.: e-mail, cmscosta@fisica.uminho.pt. ORCID

Renato Gonçalves: 0000-0001-9763-7371 Maria Manuela Silva: 0000-0002-5230-639X Carlos M. Costa: 0000-0001-9266-3669 Author Contributions ○

R.G. and E.L. contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the FCT (Fundaçaõ para a Ciência e Tecnologia) for financial support under the Framework of Strategic Funding Grants UID/FIS/04650/2013, UID/EEA/ 04436/2013, and UID/QUI/0686/2016 and Project PTDC/ FIS-MAC/28157/2017. The authors also thank the FCT for financial support under Grants SFRH/BPD/112547/2015 (C.M.C.) and Investigator FCT Contract CEECIND/00833/ 2017 (R.G.). Financial support from the Basque Government under the ELKARTEK, HAZITEK, and PIBA (PIBA-2018-06) programs is also acknowledged. Technical and human support provided by SGIker (UPV/EHU, MICINN, GV/EJ, EGEF, and ESF) is gratefully acknowledged. The authors gratefully thank ETH Zurich for the financial support. E.L. thanks The

4. CONCLUSIONS The present work reports on the fabrication of novel highspecific surface area LIB separator with mesoscale pore spacing 3758


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ACS Applied Energy Materials Spanish Ministry of Education, Culture and Sport for the “José Castillejo” mobility grant.

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Mesoporous Cellulose Nanocrystal Membranes as Battery Separators

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