Nanocellulose-laden composite polymer electrolytes for high performing lithium–sulphur batteries

Author’s Accepted Manuscript Nanocellulose-laden Composite Polymer Electrolytes for High Performing Lithium-Sulphur Batteries Jijeesh R. Nair, Federico Bella, Natarajan Angulakshmi, Arul Manuel Stephan, Claudio Gerbaldi www.elsevier.com/locate/ensm

PII: DOI: Reference:

S2405-8297(15)30094-5 http://dx.doi.org/10.1016/j.ensm.2016.01.008 ENSM39

To appear in: Energy Storage Materials Received date: 29 November 2015 Revised date: 27 January 2016 Accepted date: 27 January 2016 Cite this article as: Jijeesh R. Nair, Federico Bella, Natarajan Angulakshmi, Arul Manuel Stephan and Claudio Gerbaldi, Nanocellulose-laden Composite Polymer Electrolytes for High Performing Lithium-Sulphur Batteries, Energy Storage Materials, http://dx.doi.org/10.1016/j.ensm.2016.01.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Nanocellulose-laden Composite Polymer Electrolytes for High Performing Lithium-Sulphur Batteries

Jijeesh R. Nair*,a, Federico Bellaa, Natarajan Angulakshmia, Arul Manuel Stephan*,b, Claudio Gerbaldi*,a

a

GAME Lab, CHENERGY Group, Department of Applied Science and Technology – DISAT, POLITECNICO DI TORINO, C.so Duca degli Abruzzi 24, 10129 Torino – Italy;

b

Electrochemical Power Systems Division, Central Electrochemical Research Institute (CSIRCECRI), 630006 Karaikudi - India.

Corresponding

Authors:

C.

Gerbaldi

([email protected])

and

J.

R.

Nair

([email protected]), phone: +39 011-0904643; A. M. Stephan ([email protected]), phone: +91-4565241426

Abstract In the endless search for superior and green power sources, lithium sulphur (Li-S) batteries held the promise of opening up a new era of long lasting and high energy batteries for variety of applications. They might envisage remarkable benefits in utilizing polymer electrolytes instead of liquids in terms of safety, low-cost and gravimetric/volumetric energy densities. In this work, for the first time, nanoscale microfibrillated cellulose-laden polymer systems are prepared using

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a thermally induced polymerisation process and tested as electrolyte separator in a Li-S rechargeable battery that contains sulphur-carbon composite based cathode. The polymer electrolyte demonstrates excellent ionic conductivity, thermal stability and most importantly stable interface towards lithium metal. While comparing our earlier report with non-aqueous liquid electrolyte, the present cell based on the abundant truly-natural cellulose-based polymer electrolyte as separator exhibits better cycling stability, higher specific capacity, superior coulombic efficiency and rate capability at ambient conditions.

Keywords: green energy; polymer electrolyte; natural cellulose; lithium sulphur battery; cycling stability; sustainable energy.

1. Introduction Thundering advances are made in the Li-ion battery (LiB) technology, which has revolutionized the battery powered electronic gadgets such as laptop computers, smart mobile phones, digital camera, etc. for the past two decades [1,2,3]. Nevertheless, the commonly used cathodes (transition metal oxides, phosphates) based on intercalation chemistry exhibit limited practical specific capacity in the range of 130-160 mAh g–1 due to the involvement of one single electron during their redox reactions [4,5,6]. The energy density limitations, safety concerns, high cost and environmental hazards are partially or completely impeding the effective large-scale application of this leading energy storage technology into large-scale scenarios such as electric vehicles, plug-in hybrid electric vehicles and smart power grid community [3,7]. In the endless search for the design and fabrication of niche power sources, Li-S batteries held the promise of opening up a new era of long lasting and high energy batteries for a variety of applications, including green transportation, stationary load-levelling in intermittent power

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generation of renewable energies such as solar/wind mills as well as space and military applications [8,9,10,11]. Presently, Li-S is overwhelming other technologies due to the high theoretical capacity of sulphur (i.e., 1672 mAh g‒1, one order of magnitude higher than conventional solid cathode materials, with a corresponding specific energy density of about 2600 Wh kg‒1 assuming a complete reaction to Li2S) [12,13,14], and improved safety and reliability due to the moderately low operating potential (multi-electron reactions at below 2.5 V vs. Li+/Li) [15,16,17]. Other credentials such as the abundance, non-toxicity and low cost of sulphur (one of the ancient non-metal on the Earth’s crust and also a by-product of oil refinery) qualifies Li-S battery systems as the most appropriate “green sources” of power for meeting the rising demand of energy in the modern carbon-constrained society [3,18,19]. In its typical configuration, the Li-S rechargeable battery comprises of three major components: a lithium metal anode, a sulphur composite cathode separated by a porous polymeric membrane supporting an organic liquid electrolyte [20]. Generally speaking, the large molecule cyclo-S8 present in the cathode undergoes multi-step open-ring reduction reaction with lithium, through breaking of the S‒S bonds, resulting in long-chain lithium polysulphides, which then turn into Li2S2 and finally into Li2S upon further discharge. However, the detailed operating mechanism is still controversial even after more than 50 years of research (indeed, Herbert and Ulam first proposed the Sulphur cathode concept in 1962) [21]. However, the commercialization of Li-S cells is still hampered due to the technical concerns such as poor electronic conductivity of sulphur (5×10‒30 S cm‒1 at 25 °C), formation and shuttling of polysulphides and poor interfacial properties of lithium metal anode with electrolytes [22]. In particular, the poor cyclability and low Coulombic efficiency of Li-S cells arise from the long-chain lithium polysulphides produced in the electrolyte solution during cycling. Indeed, these intermediates,

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which are highly soluble in commonly used organic liquid electrolytes, shuttle between the electrodes besides taking part in reactions such as reduction at the anode and oxidation at the cathode. While the charge/discharge processes are repeated, some of these soluble polysulphides are reduced to insoluble polysulphide, and are deposited onto the anode and cathode surfaces [23,24]. This phenomenon increases the cell resistance, degrades the electrodes structure and often results in blocking these active sulphur mass from further reactions. Attempts have been made to overcome this problem by different means [25,26]; however, desired results are far from being achieved. In summary, these effects reduce the Coulombic efficiency and deteriorate the cell performance, thus causing rapid capacity fading during repeated cycling [27]. Numerous attempts have been made to enhance the electronic conductivity of elemental sulphur by wrapping/incorporating sulphur in a carbonaceous matrix [28,29], but discussion on this subject, which is comprehensively detailed in several reviews [3,7], is beyond the scope of this article. In a parallel effort devoted to hinder the shuttling of polysulphides, solid additives have also been incorporated in the cathode materials [30,31,32]. Replacing the conventional liquid electrolytes by fully solid or gel polymer electrolytes has been recently envisaged as an effective way to prevent the shuttling of polysulphides and reduce the loss of sulphur by dissolution [33,34,35]. Polymer electrolytes are generally referred to as membranes having transport properties similar to common liquid electrolyte solutions along with intrinsic mechanical characteristics and safety features as that of solid networks [36]. Several polymer matrices have been experimented as solid/gel electrolytes. Among them PEO-based electrolytes have been well exploited as host materials for lithium batteries and other electrochemical devices [37,38]. Attempts have been made to configure all-solid-state Li-S cells with composite solid polymer electrolytes comprising PEO+ZrO2+LiCF3SO3 complexes [39].

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Despite their advantages such as high solvating power for lithium salts, wide electrochemical stability and compatibility with the lithium metal anode, PEO-based fully solid electrolytes have not been employed for practical applications due to their low ionic conductivity (10‒8 S cm‒1 at 25 °C). A few attempts have also been made on poly(vinylidene fluoride-hexafluoropropylene) PVdF-HFP-based electrolytes. However, the elemental sulphur and polysulphide may replace fluorine from PVdF-HFP, thus resulting in formation of thiols [40] and vulcanization of unsaturated polymers [41]. Zhang and Tran compared the cycling and storage performance of composite gel polymer electrolytes (CGPE) composed of PEO:SiO2 in the 50:50 wt% ratio with poly(propylene) membranes [42]. The authors concluded that CGPE significantly enhanced the wettability, offering better adhesion between the electrolyte and the electrodes. Wang and coworkers employed a thin sheet of non-aqueous gel electrolyte based on PVdF with a sulphur– carbon nano-composite cathode for Li-S cells [43]. Although the cell demonstrated good cycling stability in its initial cycles, the reversible capacity was found to be rather low (440 mAh g‒1), which was attributed to the low loading level of sulphur in the cathode. Very recently, Huang et al. [9] proposed multi-functional separator systems with ion selective/electrical conductive polymer, metal oxide modified separator, micro/mesoporous carbon and other conductive interlayers along with biomass-derived materials for advanced Li-S batteries. Manthiram and Chung have introduced a novel carbonaceous polysulfide inhibitor derived from natural leaf [44]. In a similar study, the authors have also employed eggshell membranes as polysulfide reservoir for lithium sulphur batteries [45]. Learning from the above reported results, in the present study, we prepared novel green composite polymer electrolytes based on methacrylic oligomers and reinforced with raw nanoscale cellulose fibres, where the crosslinked polymer matrix acts as a cage to trap effectively

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the liquid electrolyte solution. The non-aqueous liquid electrolyte used for membrane activation comprised 0.75 M solution of LiTFSI in a 1:1 (v/v) mixture of tetraethylene glycol dimethylether (TEGDME), having high solubility for polysulphides, and 1,3-dioxolane, to reduce the electrolyte viscosity and also to prevent corrosion of the lithium metal electrode [46]. Lithium nitrate (LiNO3, 0.5 M) was also added into the liquid electrolyte in order to improve the sulphur utilisation, and cyclability, as it forms a protective solid electrolyte interface (SEI) film on the lithium surface by inhibiting the LixS precipitation during the discharge process [47,48]. Truly natural, abundant nanoscale microfibrillated cellulose was specifically embedded in the polymer matrix during the preparation step. It is also envisaged to improve the amorphicity of the crosslinked network, ensure shape retention during organic liquid electrolyte uptake, increase the pore structure of the electrolyte system, and possibly hinder the polysulphide shuttle through the polymer matrix. The sum of these features would result in improved transport characteristics due to the increased liquid electrolyte retention as well as greatly enhanced mechanical robustness of the composites. Overall, nanoscale bio-sourced materials, being readily available, cheap and easily recyclable, represent a concrete step forward for the next generation of high performing, safe and durable polymeric Li-S batteries to be successful in this respect [49].

2. Experimental 2.1 Materials and polymer electrolyte preparation The polymer electrolyte preparation started by formulating a pre-polymer reactive mixture that contains a series of components to impart the necessary characteristics of an ideal separator. The pre-polymer contains a dimethacrylate based soft crosslinker (bisphenol A ethoxylate (15 EO/phenol) dimethacrylate, BEA, Mn = 1700 Da) [50], an amine based mono-functional methacrylate (2-diethylamino ethyl methacrylate, DEAM), a source of lithium ions, bis

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(trifluoromethylsulfonylimide) lithium salt (LiTFSI, battery grade, Solvionics), an appropriate temperature

sensitive

free

radical

initiator

(i.e.

2,2′-azobis(2-methylpropionamidine)

dihydrochloride, AAPH) and nanoscale microfibrillated cellulose (nMFC). nMFC particles were prepared as described elsewhere [51] from a sulphite bleached spruce pulp (supplied by Domsjo). A nMFC aqueous suspension (1 wt%) was used in this work. All the materials were mixed in appropriate proportions where BEA to DEAM ratio was 40:60, which constitutes the 77% of the total polymer electrolyte components. The rest 23% was comprised of 10 wt% of LiTFSI, 10 wt% nMFC and 3 wt% AAPH. All materials were mixed thoroughly and casted over a Teflon sheet, which was dried within 1 hour in a fume hood under vigorous airflow. This set up was transferred to an oven (kept at 80 °C) where the polymerization reaction was carried on for 1 h under nitrogen flux. The average thickness of the final composite polymer membrane was 100±10 µm and it was easily peeled off from the Teflon sheet after the polymerization. The selfstanding membranes were later dried under high vacuum (< 10‒5 mBar, Hind High Vacuum Technologies - HHV) at 100 °C for 24 h. Unless otherwise stated separately, materials were purchased from Sigma Aldrich. The polymer electrolyte membrane (MPE) was also activated (A-MPE) by rapid soaking in a non-aqueous liquid electrolyte. The liquid electrolyte comprised a 0.75 M solution of LiTFSI in a 1:1 (v/v) mixture of tetraethylene glycol dimethyl ether (TEGDME), 1,3-dioxolane and lithium nitrate (LiNO3, 0.5 M). 2.2. Characterization techniques and methods The insoluble fraction (gel content) study of the crosslinked polymer electrolyte was conducted as follows: an accurately weighed sample was enclosed in a stainless steel metal net, and soaked in a CHCl3 bath for 18 hours in order to extract the soluble (unreacted) content from the polymer

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electrolyte. The insoluble (crosslinked) fraction was then calculated by dividing the weight of the final dry sample left after the extraction by the original sample weight. The glass transition temperature (Tg) was evaluated by differential scanning calorimetry (DSC) using a DSC 204 F1 Phoenix® (Netzsch) instrument. The accurately weighed polymer samples were placed in aluminium crucibles. In a typical measurement, the polymer sample was cooled from room temperature to 85 °C, and heated to 80 °C at a rate of 10 °C min‒1 under continuous N2 flux. The Tg was calculated as the midpoint of the heat capacity change observed in the DSC profile during the transition from glassy state to rubbery state. The thermal stability of the samples was tested by thermogravimetric analysis (TGA) with a TG 209 F1 Libra ® instrument from Netzsch (25 °C  600 °C) under N2 flux at a heating rate of 10 °C min−1. Unless otherwise stated separately, all samples for analysis were prepared in an environmentally controlled dry room (10 m2, RH < 2 ± 1% at 20 °C, Soimar). The ionic conductivity of the activated composite polymer electrolytes, properly sandwiched between two stainless steel blocking electrodes (1 cm2 diameter), was measured using an electrochemical impedance analyser (IM6-Bio Analytical Systems) between 50 mHz and 100 kHz frequency range at ambient temperature [52,53]. The activation energy was calculated from conductivity values obtained at various temperature, and the resulting values are fitted with Vogel-Tamman-Fulcher (VTF) equation, which is typically used to describe relation between viscosity and temperature near to the Tg of polymer matrix. The equation used is given below:

where σ is the ionic conductivity, EaVFT is equivalent to the activation energy, R is the gas constant, T is the experimental temperature, and T0 is the temperature, which is 50 °C below the Tg. The compatibility of the composite polymer electrolyte was also performed by sandwiching 8

the activated sample between two lithium metal anodes and following the evolution of the diameter of the semicircle with time. The lithium transference number (tLi+) was determined by d.c. polarization method combined with impedance spectroscopy as proposed by Bruce and Evans [54]. It was calculated using the following equation:

A d.c. pulse of 10 mV polarized the Li/A-MPE/Li cell. Time evolution of the resulting current flow was then followed. The initial (I0) and steady state (Iss) values of current flowing through the cell during the polarization were measured. R0 and Rss represent the resistance values before and after the perturbation of the system. Impedance spectra were recorded before and after the pulse application in order to correct the changes. 2.3. Electrode preparation, cell assembly and electrochemical testing In the present study, sulphur-activated carbon (S-AC) composite was used as the active material as previously for charge-discharge studies: it was prepared in the form of a thin electrode film by casting a cathode slurry over an aluminium foil current collector. Briefly, high-quality sisal fibers were thoroughly washed and dried, which were obtained from the fields in Tamilnadu (India). Finally, white dried fibers were treated with concentrated solution of phosphoric acid (H3PO4) as porogen for 5 days at 35 °C. The fiber to porogen ratio was 1:10 by weight. Pyrolysis was carried out under nitrogen atmosphere at 800 °C at a heating ramp of 5 °C min−1 for a period of 3 h. The obtained activated carbon was washed several times with de-ionized water until it reached a pH value of 7. S-AC was prepared by in-situ deposition from an aqueous solution of sodium thiosulfate (Na2S2O3∙5H2O) with the addition of hydrochloric acid. In particular, 10 g of

9

Na2S2O3∙5H2O were dissolved in 100 mL of de-ionized water and magnetically stirred for more than 12 h along with the activated carbon. During this process, HCl (0.5 M) was slowly added in the solution. According to the following reaction: S2O32‒(aq) → S(s) +SO32‒(aq) The sulphur was deposited in and/or on the surface of the activated carbon matrices. The solution was filtered and the obtained S-AC was dried and kept at 80 °C in a ventilated oven for 12 h. After the procedure, an overall sulphur loading of 60 wt% was observed, and the rest was sisal fiber components. The cathode slurry was based on a mixture of 70 wt% of the S-AC active material, 10 wt% of poly(vinylidene fluoride) (PVdF) binder and 20 wt% of Super P carbon dispersed in Nmethyl-2-pyrrolydone. The very well blended slurry was deposited using a doctor blade over the Al foil, followed by drying at 100 °C in an air oven. The dried electrodes were roll pressed and cut into circular disks. These disks were vacuum treated at 120 °C for 3 h and then transferred into an Ar-filled dry glove box (M Braun) for cell assembly. Thus, the overall sulphur loading in the final S-AC electrode film was 42 wt% with respect to the total electrode materials. This means that the final electrode film has a sulphur loading of 0.8 ± 0.05 mg cm−2. A 2032-type coin cell was assembled by sandwiching the activated cellulose-laden composite polymer electrolyte (A-MPE) between the S-AC based cathode and the lithium metal foil anode (Foote Minerals). The average weight of a typical MPE having an average thickness of 100 µm is 12 ± 1 mg cm−2. After an uptake of 180% of liquid electrolyte, the resulting polymer electrolyte membrane (A-MPE) weights 34 ± 2 mg cm−2. This means, the amount of liquid electrolyte per 1 cm diameter membrane is 22 ± 2 mg cm−2. In addition, we can say that the amount of liquid electrolyte is lesser in the polymeric cell than in the corresponding liquid

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electrolyte cell. The liquid cell was rich in electrolyte, indeed worked with excess amount of liquid electrolyte. Constant current charge/discharge cycling was performed between 3.0 and 1.5 V vs. Li+/Li [55] by a computer-controlled battery testing unit (Maccor Model 4200 multifunction automated test system). The electrochemical impedance spectroscopy (EIS) measurements were carried out on an Autolab PGSTAT302N potentiostat/galvanostat/frequency response analyser (Metrohm Autolab) before, after cycling, and for different depth of discharge (DOD) of the Li-S cell.

3. Results and Discussions 3.1. Characterization of the composite polymer electrolyte The precursor mixture comprising BEA, DEAM, nMFC, LiTFSI and the thermal initiator AAPH, was deposited over a substrate and dried for 1 h. This set up was further thermally cured to obtain an opaque, self-standing and non-tacky polymer electrolyte membrane. It is shown in Figure 1. BEA acts as a soft crosslinker, DEAM acts as a crosslinking controller, nMFC mainly acts as a reinforcing agent and LiTFSI acts as the source of lithium ions. AAPH is the initiator, which decomposes under temperature to form free radicals. These free radicals react with the methacrylate groups to form a three dimentional network in which nMFC acts a an interpenetrated network like matrix which can improve the mechanical integrity of the overall membrane. Before arriving to the reported formulation, several tests were performed to understand the fundamental aspects of polymer electrolytes to decide the quantity and type of photoinitiator as well as the solubility of salt, ionic mobility in terms of [EO]/[Li] ratio, and mechanical integrity.

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The succesful polymerisation was evaluated by gel content analysis, which confirmed that the monomers were effectively converted into polymers. The insoluble content exceeded 90%, which confirms that only 10% of materials remained unreacted.

Figure 1. Sketched representation of materials and processes leading to the crosslinked natural cellulose-laden composite polymer electrolyte membrane (MPE).

The glass transition temperature was calculated from the DSC traces (Figure 2A) and it was found to be –35.1 °C. It was clear the absence of any crystallinity in the polymer matrix, which was confirmed from the linearity and the neatness of the DSC trace. The thermal stability was tested by means of TGA analysis between 25 and 600 °C. The initial weight loss observed around 70 °C was attributed to the removal of moisture and water absorbed at the time of loading the sample (sample preparation for TGA analysis was carried out in a fume hood under ambient atmosphere) [56]. Around 240 °C, a 10% weight loss was observed, which was referred as T10 value. The next main weight loss was observed at around 340 °C, which corresponds to the decomposition of DEAM, while the following decomposition at 410 °C corresponds to the BEA material decomposition. Finally, above 500 °C, only less than 10 wt% of residue remained,

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which was also referred mainly to the LiTFSI and nMFC residual content. This reveals that the polymer electrolyte is stable up to 200 °C, which is much higher than the normal operating temperature envisaged for practical operation of lithium-sulphur batteries.

Figure 2 A) Differential scanning calorimetry (DSC) trace of the composite polymer electrolyte membrane and B) thermogravimetric analysis (TGA) along with related differentiation curves (dotted lines in blue colour code). Taking into account the experimental errors, the weight loss pattern is consistent with the MPE composition.

3.2 Ionic conductivity and compatibility studies We had decided to go with only one composition of MPE for the complete article, as it was superior in terms of performance if compared to the other formulations we tested. Polymer electrolytes composed of –EO– moieties and Li salts typically use [EO]/[Li] mole ratio as a parameter to increase the mobile Li cation content; in our selected formulation, an [EO] to [Li] ratio of 16:1 was considered as optimal. Indeed, the –EO– moieties, which are taking part in the crosslinking reaction, are coming from the BEA molecule; thus, their mobility is somewhat restricted. However, the further activation step (soaking of the polymer membrane in liquid

13

electrolyte for 2 hours) will surely change this [EO]/[Li] ratio. Other results are not included in the present article as the intention is to demonstrate the use of a nanoscale microfibrillated cellulose-laden polymer electrolyte as separator for lithium-sulphur batteries. Noteworthy, the specifically selected MPE can function as an all solid state polymer electrolyte without any kind of activation, as it already contains lithium salt (LiTFSI) in its matrix. However, those studies or attempts are completely avoided being out of the scope of the present work, and only the activated sample is considered for electrochemical characterisation. Here, the idea is to demonstrate the functioning of MPE at ambient conditions. In order to achieve acceptable conductivity, the natural cellulose laden composite polymer electrolyte membrane was subjected to an activation step in a liquid electrolyte [57] as described in the experimental section, and from here on the activated MPE is called as A-MPE. The membrane swelled very well with an electrolyte uptake of 180%. At this point, it is worth mentioning that the nMFC helped to retain the size of the A-MPE membrane even if it contains huge amount of liquid inside its matrix. The volume change after the activation was only 42%. The membrane without nMFC inside typically broke into several pieces after activation. Thus, the A-MPE, being a truly quasi-solid electrolyte system, was used for further electrochemical characterizations. Moreover, after activation by soaking in the liquid electrolyte solution, the newly elaborated A-MPE can easily retain its physical integrity even upon bending test. Indeed, it can still be easily rolled up on the cylinders used for the test withstanding the entire bending radius until 3 mm. On the contrary, a membrane without microfibrils was difficult to handle and it failed if bended on a cylinder with a 10 mm radius [58,59]. The ionic conductivity of the activated-MPE was analysed in the temperature interval between 0 and 80 °C. It is clear from the resulting Arrhenius plot, which is shown in Figure 3A,

14

that the ionic conductivity of the A-MPE increases with an increase in temperature. At 0 °C the conductivity value is equal to 0.62 mS cm‒1 and at 20 °C it reaches 1.2 mS cm‒1. This is a practically important value if one considers the application of the A-MPE under study in real cell battery configurations. The swelled electrolyte was also subjected to DSC analysis and found that the A-MPE has a Tg equal to ‒63.4 °C. The mechanism of ionic conductivity in polymer electrolytes can be elucidated from activation energy (Ea) calculations. Ea was calculated from the ionic conductivity values obtained from impedance spectroscopy analysis and by fitting with VTF equation [60]. The resulting plots (Figure 3B) of ln vs 1/(T‒T0) was used to determine the Ea of the A-MPE. In general, the VTF equation is believed to explain the conduction behavior of highly concentrated liquid electrolytes and molten salts [61]. Indeed, in the present case, the fitting was calculated using the VTF equation and the Ea was found to be 6.02 kJ mol‒1, between 0 and 80 °C. This is comparatively low value and can be appreciated for a polymer electrolyte system that contains nanoscale microfibrillated cellulose in quantities equal to 10 wt%. In the present case, the T0 value was derived from the Tg value of the A-MPE, which was ‒63.4 °C.

15

Figure 3 A) Arrhenius plot showing the ionic conductivity vs. temperature for the A-MPE and B) Fitting of the plot using the VTF equation.

The performance of Li-S batteries is based on conversional chemistry in terms of anode and electrolyte, i.e. between lithium metal foil and organic electrolyte. Lithium metal foil is very reactive towards organic components, which readily decompose at the metal surface and form a passivating layer [62]. It is noteworthy that several strategies have been adopted to solve cathode related problems, but reports on issues associated with anodes are very scanty [63]. In Li-S batteries, lithium metal anode reacts with both electrolyte solution and soluble polysulphides. This passivating layer should be permeable to Li+ ions and should allow the migration of Li+ ions under applied electric field.

Figure 4. Nyquist plot representing the evolution of the interfacial resistance with time for the activated MPE sample, using the Li/A-MPE/Li cell configuration.

16

In order to understand the interfacial properties of the activated MPE with the lithium metal anode (compatibility studies), electrochemical impedance spectroscopy (EIS) was used (as described in Section 2.2). Figure 4 depicts the variation of the interfacial resistance Ri as a function of time (days). The interfacial resistance value increased with time, which was 185 ohms for a fresh cell, and stabilized at around 415 ohms after several days, then remaining stable at that value for at least 30 days. The increase in the values Ri and stabilization after the initial days of storage is attributed to the formation of a porous and thin-slightly-thick solid electrolyte interface (SEI), along with its partial dissolution and consolidation of polysulphides [64]. Figure 5 demonstrates the Chrono-amperometric curve of a Li/A-MPE/Li cell at 10 mV d.c. pulse. The inset shows the impedance response of a lithium symmetric cell assembled with the A-MPE before and after the perturbation. Apparently, both curves (before and after perturbation) overlap, indicating that there is no much difference between the initial R0 and the final resistance Rss of the two Li interfaces, which further confirms the stability of the lithium electrode in contact with the A-MPE [65,66]. The value of transference number was calculated as 0.34 using the method suggested by Evans and Bruce [34]. As an extra shoulder was observed in chrono-amperometry study, we have calculated the average of steady state current and used it for transport number calculation. The rather limited value of tLi+ could be attributed to the formation of ionic couples/aggregates resulting from the excess amount of mobile ions formed by the addition of LiNO3 [46].

17

Figure 5. Chrono-amperometry curves obtained for Li/A-MPE/Li cell, tested at 10 mV polarization until obtaining a steady state current under open circuit potential conditions. Inset: Nyquist plot of the cell before and after perturbation.

3.3 Constant current charge/discharge studies of polymeric Li-S cell Figure 6A shows the constant current charge/discharge cycles of a Li/S-AC cell with the A-MPE separating electrolyte at different current regimes. The cell delivered an initial discharge capacity of 1280 mAh g‒1 with a charging capacity of 1300 mAh g‒1 based on the weight of sulphur in the cathode material. Such a high initial discharge capacity is likely attributed to the high ionic conductivity offered by the composite polymer electrolyte, which offers adequate amount of lithium for electrochemical reactions with sulphur that contributes in the enhanced storage capability of the cell [67]. Moreover, we hypothesise that the dissolution of sulphur may occur very rapidly in a liquid electrolyte-based cell even in the first cycles as the electrolyte can impregnate through the whole electrode thickness reaching out to every active material particles. On the other hand, the liquid electrolyte content in the polymer membrane is encapsulated in the

18

cellulose fibres and polymer matrix, besides being present at the electrode/electrolyte interface. This will allow a proper contact of the liquid electrolyte with the active sulphur particles, while the membrane efficiently behaves as separator useful to prevent polysulfide diffusion between cell electrodes. In addition, it is worth mentioning that a classic sulphur electrode (sulphur, binder, carbon black) delivered [68,69] an initial capacity of 1238 mAh g−1 in the first discharging step and in the second discharge cycle, it showed only 422 mAh g−1. Even if these data represent our experimental findings, it must be stated that for Li-S batteries the development and optimization of a rationale reference system is crucial. Indeed, other research groups reported that simple mixtures of a porous carbon black with sulfur [70,71,72] achieved equally good or even better capacity retention if compared to many of the "tailor-made" nanomaterials proposed in the literature. The specific capacity vs. cycle number of the Li/A-MPE/S-AC cell is showed in Figure 6B. It can be clearly observed that the specific capacity of the polymeric Li-S cell decreases in its initial cycles at low current and a stable cycling at around 730 mAh g‒1 is achieved at 1C rate. Noteworthy, this is higher than the results obtained with non-aqueous liquid electrolyte of same sulphurized activated carbon (S-AC) electrode which delivered less than 500 mAh g‒1 (see Figure S1 in the Supporting Information). The initial specific capacity decrease of the polymeric Li-S cell with S-AC electrode at lower current rates (0.1 and 0.2C) is attributed to the dissolution of polysulphides in the electrolyte and their diffusion and reaction with the lithium metal anode. We have observed a strange plateau at around 1.5 V vs. Li+/Li in the potential vs. time profiles for cells that cycled at 0.1C and 0.2C, which might be ascribed to the reduction of LiNO3 salt. The studies carried out by Zhang et al. support this hypothesis [73].

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Figure 6. A) Representative constant current charge/discharge profiles of the polymeric Li-S cell assembled with the configuration of Li/A-MPE/S-AC, which was cycled at 20 °C and at different current rates of 0.1C, 0.2C and 1C; B) Graph illustrating the specific capacity vs. number of cycles along with Coulombic efficiency.

Interestingly, at 1C rate (with short charging and discharging time), the capacity fading was significantly reduced and a stable cycling was achieved with >99% Coulombic efficiency. Good performance at high current rate may be ascribed to the efficient ionic conduction in the activated polymer electrolyte separator and the favourable interfacial charge transport between the electrodes and the electrolyte in the cell. The improved cyclability with outstanding Coulombic efficiency is attributed to the slower diffusion of polysulphides to the anode than the total electrochemical reaction time, due to the optimum characteristics of the nano-cellulose laden composite polymer electrolyte [74,75]. 4. Conclusions A green polymer electrolyte composed of a methacrylate based polymer matrix and naturally abundant, raw nanoscale microfibrillated cellulose was here prepared by thermal polymerisation

20

(curing) process. For the first time it was successfully demonstrated as quasi-solid separating electrolyte for high-energy lithium sulphur batteries conceived for ambient temperature application. The overall electrolyte preparation process is water-based and facile to scale up. The polymer electrolyte showed excellent thermal stability (>200 °C), and comparatively low Tg. The activated membrane in liquid electrolyte showed excellent conductivity (>1.2 mS cm‒1) and stable interface towards lithium metal. The final cell with a Li/A-MPE/S-AC cell showed excellent cycling stability at above 700 mAh g‒1 even at rather high 1C rate, which is noticeably higher than the specific capacity obtained with the corresponding system cycled in liquid electrolyte. Finally, the cell showed a Columbic efficiency of >99% at ambient temperature. The stable cycling profiles is attributed to significant reduction of the migration of polysulphide towards anode by the entrapment of microfibrillated cellulose in the polymer matrix, which is an encouraging way forward the realisation of green and reliable next-generation of high energy LiS batteries. Acknowledgement Dr. Davide Beneventi from Grenoble INP-Pagora & LGP2 is gratefully acknowledged for nMFC supply.

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Graphical abstract

References

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