A Single Li-Ion Conductor Based on Cellulose

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A Single Li-Ion Conductor Based on Cellulose Christian Hänsel, Erlantz Lizundia, and Dipan Kundu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00821 • Publication Date (Web): 09 Jul 2019 Downloaded from pubs.acs.org on July 18, 2019

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A Single Li-Ion Conductor Based on Cellulose Christian Hänsela, Erlantz Lizundiab, Dipan Kundu*a a Laboratory

for Multifunctional Materials, Department of Materials, ETH Zurich, Vladimir-

Prelog-Weg 5, 8093 Zurich, Switzerland b

Macromolecular Chemistry Research Group (LABQUIMAC), Dept. of Physical Chemistry,

Faculty of Science and Technology, University of the Basque Country (UPV/EHU), Leioa, Bizkaia 48940, Spain Email: [email protected] ABSTRACT Polymeric single-ion conductors - also known as ionomers - are a potential alternative to binary liquid electrolytes, promising better energy and power densities and stable dendrite free metal deposition in alkali metal batteries. While typically Li+ ionomers are designed from complex synthetic polymers, here we introduce one based on the most abundant natural polymer – cellulose. Evaporation-induced self-assembly of aqueous cellulose nanocrystal (CNC) suspension results in free-standing porous CNC membranes, which are turned into Li+ conductor through lithiation of the -OSO3H groups on the surface of the CNCs. Infused with organic solvent, the CNC membrane shows a good Li-ion conductivity of 0.2x10-4 S cm-1 at room temperature, a near unity Li+ transference number (𝑡Li+ = 0.93), and a wide operational window (≥ 4.5 V) against Li. These properties of the CNC ionomer electrolyte enable smooth Li metal deposition and stable cycling of Li-LiFePO4 cells. Keywords: Single Li-ion conductor, biopolymer based Li-ionomer, porous cellulose nanocrystal membrane, smooth lithium deposition, Li-metal battery. TOC Graphic

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INTRODUCTION The electrolyte is a vital cog in the pursuit of high energy, high power, and safe batteries required for next generation of electric vehicles and other advance applications. Conventional Li-ion battery (LIB) electrolytes, consisting of a lithium salt and a liquid organic solvent, which have been a key to LIB’s impressive success, pose significant challenges to their further improvements.1-3 Moreover, high energy Li metal anode, deemed critical for the development of Li-S (lithium-sulfur) and Li-O2 (lithium-oxygen) batteries, is not stable enough in binary liquid electrolytes.4-7 Competitive transport of Li+ and counter anions in binary electrolytes promotes dendritic Li metal deposition that can eventually lead to short-circuit and safety hazards.8-9 Mobility of both cations and anions is further known to generate a concentration overpotential, which limits the power and energy density of LIBs.10 To alleviate these issues, a variety of high Li+ transference number (𝑡Li+) electrolytes are being explored,11-16 of which polymeric single Li+ conductors (𝑡Li+~ 1) - alias ionomers - are an appealing candidate.17-18 While immobilization of anions in the polymer backbone renders prevailing Li+ transport, Li+ conductivity is often boosted via infusion of liquid solvents into porous ionomer membranes to facilitate Li+ motion through the mobile liquid phase. A great variety of complex polymeric backbones have been designed either to improve solvent infiltration or to impart directional Li+ transport,19-25 and highly fluorinated moieties in the polymeric chain and/or anionic group have even shown to result in Li+ conductivities approaching that of binary liquid electrolytes.26-27 However, use of an abundant biopolymer in the design of a Li+ ionomer electrolyte is hitherto unknown. Here, we demonstrate the development of a Li+ ionomer membrane electrolyte using the most naturally abundant and inexpensive polymer – cellulose. Biopolymers like lignin, chitin, chitosan and cellulose based separators28-33 and gel polymer electrolytes34-37 have previously been explored in Li-ion28-30, 32, 34-37, Li-S31 and Li-O232 batteries. However, we have taken that a step further, designing cellulose nanocrystal (CNC) based Li+ single-ion conducting ionomer membranes, which upon infusion with liquid organic carbonate (solvent, not electrolyte) displays a Li+ conductivity of 0.2x10-4 S cm-1 at room temperature (RT ~ 23˚C) and a very high 𝑡Li+ of 0.93. Possessing a wide operational window, the CNC ionomer electrolyte enables smooth Li metal deposition and stable operation of Li-LiFePO4 cells with high Coulombic efficiency (~100%) and good rate capability. 2 ACS Paragon Plus Environment

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RESULTS AND DISCUSSION In nature, cellulose is packed into so-called microfibrils, which consist of ordered regions, where cellulose chains are tightly packed together in crystallites, and disordered regions that link the crystalline regions. These crystalline regions are isolated from the cellulose microfibrils by a controlled acid hydrolysis in sulfuric acid.38 While the crystalline regions have a higher resistance to hydrolysis and remain intact,39 the non-crystalline regions are preferentially hydrolyzed, leading to aqueous suspension of rod-like cellulose nano-crystals (CNC) (Figure S1). Simultaneously, the sulfuric acid chemically reacts with the surface hydroxyl groups of the CNCs to yield negatively charged sulfate groups,40-41 which promote their dispersion in water via electrostatic repulsion.42-43 Upon slow evaporation of the water, the CNCs undergo an evaporation-induced self-assembly (EISA) and form films with layered structures.44-45 Two different types of CNC membranes have been developed in this work and their preparation methods are schematically shown in Figure 1. In the first process, dense CNC membranes have been obtained by casting 100 mg of water-dispersed CNCs (1.23 w/w %) in weighing dishes and EISA of this solution results in 40 μm thick free-standing membranes referred to as dCNC membranes. (Figure 1 and Figure S2) A second type of CNC membranes has been prepared from a mixture of water-dispersed CNCs (1.23 w/w %, 100 mg) with tetramethyl orthosilicate (TMOS) and D-Glucose (CNC:TMOS:D-Glucose 1:0.5:1 w/w %). The aqueous CNC-solution (pH 2-3) hydrolyzes the TMOS, which then interacts with the CNC surface and prevents dense packing of the CNCs during EISA, as schematically shown in Figure 1. The added glucose prevents cracking of the dried CNC films during EISA by reducing the pressure gradients formed during water evaporation.46 Washing of the dried CNCsilica composite films in 2M LiOH solution to dissolve the silica and glucose results in a highly porous structure (Figure S2 and Figure S3). Interestingly, the LiOH solution does not only remove the silica and glucose but also exchanges the proton of the -OSO3H groups on the CNC surface with Li+ (Figure S4). These highly porous, silica-free and lithiated CNC films have been washed in ethanol and after supercritical CO2 drying 75 μm thick free-standing mesomacroporous CNC membranes referred to as mCNC have been obtained (Figure 1 and Figure S2).44 BET analysis, presented in Figure 2a, reveal that the porous structure of the mCNC membrane results in a large surface area of 194 m2 g-1 and a porosity of 87% compared to the dCNC membrane with a surface area of 11 m2 g-1 and a porosity of 56%. DFT pore size-volume analysis on the desorption branch of the BET isotherm, as shown in Figure 2b, further divulge the presence of distributed meso-macropores in the range of 5-70 nm in the mCNC membranes, 3 ACS Paragon Plus Environment

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which account for its large surface area. These small pores form an interconnected porous network with larger macropores (> 100 nm) that are visible in the SEM micrographs (Figure S2). The interconnected porous structure throughout the whole membrane enables an uptake of up to 60 wt% (~10 μL for a 12 mm wide and 75μm thick mCNC membrane) of the organic solvent mixture EC-DEC (1:1 v/v). It is important to note that although the large macropores (> 100 nm), observed under SEM, have insignificant contribution to the total pore volume in the mCNC membranes, they are crucial for efficient solvent infusion. The dCNC membranes, which consist of only a small population of very small ~5 nm pores, are only able to soak up to 20 wt% of the organic solvent mixture.

Figure 1. Schematic representation of the fabrication processes for the dCNC and mCNC membranes. The chemical interaction of the CNCs with TMOS under acidic conditions as well as the silica removal under basic conditions are illustrated. The different approaches result in membranes with distinct optical appearance and morphology as revealed by the optical images and SEM micrographs.

A good wettability is necessary since a dry CNC membrane itself shows no ionic conductivity and behaves like an ionically blocking membrane (Figure S5). However, an appreciably high Li-ionic conductivity (0.2x10-4 S cm-1 at RT) is observed for the mCNC membrane infused with EC-DEC (1:1 v/v) (Figure 2c and S5b), which is comparable to the ionic conductivity of lithium exchanged Nafion infused with EC-DEC.21 The liquid medium boosts the Li-ion conductivity by promoting dissociation and rapid transport of the Li ions through the mCNC membranes. The CNC surface decorated with -OSO3Li groups provides the mobile Li+ and the infused liquid acts as the transport medium, as schematically shown in Figure 2d. While anionic -OSO3- groups are covalently tethered to the CNC surface, the Li+ ions are weakly associated with the polymer via ionic interaction, and thus easily dissociate by solvation and move rapidly in the organic solvent medium. A very high surface area of the 4 ACS Paragon Plus Environment

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mCNC membrane increases the number of -OSO3Li groups in contact with the liquid transport medium and therefore increases the number of charge carriers. In comparison to the mCNC membrane, dCNC ones display one order of magnitude lower ionic conductivity (0.8x10-5 S cm-1 at RT; Figures 2c and S5b). This is not surprising as the dense membranes infuse the organic solvent poorly and the lower surface area compared to the mCNC membranes limits the fraction of CNCs (sulfate groups) available for ionic conduction throughout the solvent medium. Figure 2c depicts the variations of ionic conductivities of the EC-DEC (1:1 v/v) soaked mCNC and dCNC membranes as a function of temperature, in the range 25-90°C. Although both follow Arrhenius type behavior, the mCNC has a smaller activation energy (mCNC: 0.13 eV and dCNC: 0.22 eV) for ionic conduction (Figure S5c and d), which also facilitates its ionic conductivity. Shorter travel distance/time of the ions between abundant OSO3Li sites and greater number of transitions per unit time most likely contributes to better Li+ transport in the mCNC membranes. Given the low activation barrier, increasing the number of anionic (OSO3-) groups on CNCs, and thus the number of charge carriers (Li+), would be a viable route to improve the ionic conductivity of the mCNC membranes.

Figure 2. (a) Nitrogen adsorption-desorption isotherms for the dense (dCNC) and meso-macroporous (mCNC) membranes. (b) DFT pore size analysis on the desorption branch of the BET isotherm for the dCNC and mCNC membranes. (c) The temperature dependent Li+ conductivities of the dCNC and mCNC membranes infused with EC-DEC (1:1 v/v). (d) SEM micrograph of a mCNC membrane and a

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schematic illustration of the metal ion conduction in a porous ionomer membrane where the pores are filled with pure organic solvent.

Good electrochemical stability is a basic requirement for an electrolyte. Therefore, the electrochemical stability window of the EC-DEC (1:1 v/v) soaked mCNC membrane has been investigated by voltammetry for which the soaked membrane was sandwiched between a stainless-steel working electrode and a metallic lithium counter (reference) electrode. Figure 3a shows the cyclic voltammogram in the potential range -0.3 – 1.8 V combined with the linear sweep voltammogram in the potential range 1.8 – 5.5 V. Clearly, the soaked membrane is highly stable in the 0 - 4.5 V (versus Li/Li+) window. The increase in current above 4.5 V can be explained by oxidation of the organic carbonates (EC-DEC).47 The smooth positive and negative peaks around 0 V indicate a highly reversible plating and stripping of lithium onto the stainless-steel working electrode. These results highlight the sufficient electrochemical stability of the EC-DEC soaked mCNC membrane against most state of the art cathode materials like LiFePO4 or LiCoO2 and most importantly metallic lithium anode. To confirm the single-ion conductivity feature of the lithiated mCNC membrane the Li+ transference number (𝑡Li+) was determined by using the Bruce-Vincent method.48 The mCNC membrane soaked in EC-DEC (1:1 v/v) was sandwiched between two Li-metal discs and the current-time response was measured under a constant DC bias potential of 20 mV (Figure 3b). The measured initial current I0 reflects the movement of both cations and anions, while the final steady-state current Iss results exclusively from the motion of cations. The slight increase in the resistance (Rintial = 312 Ω, Rafter = 334 Ω) upon polarization can be explained by the interaction between the organic solvent and the lithium metal.1, 6 Based on these results, the 𝑡Li+ was calculated to be 0.93 (Table S2). Such a high value confirms the single-ion conductivity characteristic of the mCNC/EC-DEC electrolyte system. The small deviation

Figure 3. (a) Electrochemical stability window of a mCNC/EC-DEC membrane electrolyte as studied by combined cyclic voltammetry (-0.3 – 1.8 V) and linear sweep voltammetry (1.8 – 5.5 V) with SSworking and Li -counter electrodes at 1 mV s-1 scan rate. (b) The current-time profile and the impedance spectra (inset) taken before and after the voltage polarization (20 mV) for a symmetric Li/Li cell with a mCNC/EC-DEC electrolyte. (c) Voltage versus time profile for a Li/Li symmetric cell with a EC6 ACS Paragon Plus Environment

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DEC soaked mCNC membrane as electrolyte, for 2 h Li plating/stripping at a current density of 250 A cm-2.

from an ideal unity value might be due to the retention of some mobile ionic species in the membrane during fabrication or some anionic decomposition products formed from EC/DEC reduction. The reversible ion transport through the mCNC membrane and the Li electrodeposition behavior were investigated by galvanostatic cycling in a symmetric (Li/Li) cell configuration at room temperature. Figure 3c shows the reversible voltage response under a constant current density of ± 0.25 mA cm-2 for a lithium plating/stripping capacity of 0.5 mA h at room temperature. The overpotential, which starts out slightly high (60 mV) during the first few cycles (60 mV) - most likely stemming from poor and uneven electrode-electrolyte contact, drops later (to 25 mV), and the cell shows stable cycling for more than 100 cycles with no apparent change in the overpotential or degradation of the CNC membrane (Figure S6). It is also important to note the square wave shape of the polarization curves (Figure 3c, inset), a typical signature of a stable single ion conducting electrolyte in operation. To probe whether the single ion conductivity feature of the mCNC/EC-DEC electrolyte translates to a homogeneous and dendritic (sharp) structure free Li metal deposition, the Li electrode was investigated under SEM. Figure 4 shows the typical SEM images of a pristine Li foil surface and the Li foil surfaces after cycling in a symmetric Li-cell, either with the single ion conducting mCNC/EC-DEC membrane electrolyte or a Celgard® membrane soaked in 1M LiPF6/EC-DEC (1:1 v/v) electrolyte. The corresponding optical images are shown in the inset. Both cells were cycled for 10 plating/striping cycles under identical condition (Figure S7). Expectedly, a stable and homogenous metal deposition is obtained for the single-ion conducting membrane, as evidenced by the flat and smooth deposits (Figure 4b). Whereas, the conventional binary electrolyte (i.e., 1M LiPF6 in EC-DEC) results in rough and rod-shaped Li deposits, indicating to a inhomogeneous and unstable Li deposition that is known to trigger dendritic short circuit.8-9 However, since ionic conductivity of the electrolyte is also a crucial parameter in the inhibition of dendritic Li growth, improvement of mCNC/EC-DEC electrolyte system’s ionic conductivity would be necessary for higher current studies. Nevertheless, stable dendrite free Li deposition enabled by the mCNC/EC-DEC membrane electrolyte is the first essential step towards its implementation with metallic Li anode. A good ionic conductivity of the electrolyte and a stable metal stripping/plating behavior at room temperature are of critical importance for an optimal Li-metal battery.

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Therefore, the performance of the mCNC/EC-DEC membrane electrolyte was further investigated in a full cell configuration with a metallic lithium anode and a LiFePO4 cathode.

Figure 4. SEM micrographs of the Li metal surfaces: (a) a pristine surface before galvanostatic cycling; the surface after galvanostatic cycling using (b)mCNC/EC-DEC membrane and (c) a Celgard membrane soaked in 1M LiPF6 EC-DEC (1:1 v/v) as electrolyte. The inset in each shows the optical image of the corresponding Li metal foil.

The main goal of this study was to elucidate the behavior of the mCNC based electrolyte in a full cell and not the optimization of the positive electrode or improving the electrolyte– electrode contact. The composite cathode consisted of 70 wt% LiFePO4 (MTI, USA), 15 wt% carbon black (Super P) and 15 wt% lithiated Nafion (Li-Nafion). Besides serving as a binder, the lithiated Nafion ensures ionic conductivity within the cathode, which is soaked in the salt free solvent mixture EC-DEC (1:1 v/v). Solubility of the Li-Nafion in NMP (N-Methylpyrrolidon) allows it to be intimately mixed with the LiFePO4 and conductive carbon particles, creating unperturbed Li+ percolation pathway in the electrode. Practically any Li-ionomer with good ionic conductivity and decent solubility in common electrode slurry processing solvents can be used. Whereas, insolubility of Li-CNC in NMP (or in most organic solvents) makes it difficult to use it in the electrode fabrication process. The galvanostatic voltage profiles at a 0.5 C-rate (1C: 170 mA g-1), shown in Figure 5a, demonstrates a small voltage polarization between the charge/discharge curves (1st cycle = 120 mV; 50th cycle = 70 mV) which confirms the good ionic conductivity of the membrane electrolyte over many cycles. The corresponding cycling data is presented in Figure 5b, which depicts 93% retention of the starting capacity after 100 cycles. Furthermore, a good rate capability is also achieved, delivering 110 mAh g-1 of capacity at a 1C rate, and recovering the initial 0.1C capacity after 30 cycles at variable rates (1st cycle = 138 mA h g-1; 30th cycle = 137 mA h g-1) (Figure 5c). Interestingly, despite of having one order of magnitude lower ionic conductivity compared to some fluorinated Liionomers, the room temperature initial capacity and capacity retention of Li-LFP cells, as observed here with the mCNC/EC-DEC electrolyte, are on a par with that reported with those synthetic ionomer electrolytes.18 Additionally, not just LiFePO4, high voltage cathodes like 8 ACS Paragon Plus Environment

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LiCoO2 and LiNi1/3Mn1/3Co1/3O2 would be equally compatible with mCNC/EC-DEC electrolyte due to its good anodic stability (≥ 4.5 V vs. Li).

CONCLUSION To summarize, in this letter, we demonstrate for the first time the development of a Li+ single ionic conductor based on a naturally abundant polymer like cellulose. Prepared by a facile selfassembly process, the porous cellulose nanocrystal membrane shows an appreciable Li-ion conductivity (>10-5 S cm-1) at room temperature, a very high cation transference number (𝑡Li+ = 0.93), and a wide electrochemical stability window (≥ 4.5 V vs. Li). The stable and homogenous plating/stripping of the metallic-lithium electrode and noteworthy performance of the Li metal full cell underpin the great potential of this new material as an inexpensive electrolyte for environmentally friendly batteries, and might be a first step towards recyclable or even biodegradable batteries.

Figure 5. (a) Galvanostatic charge/discharge profiles for the first 50 cycles and (b) evolution of the discharge capacity (black) and Coulombic efficiency (red) for a cell using metallic lithium as anode, mCNC soaked in EC-DEC (1:1 v/v) as electrolyte and LiFePO4 as cathode. The cycling was performed at a 0.5 C rate. (c) Discharge capacities and corresponding Coulombic efficiencies of the Li /mCNC (EC-DEC)/LiFePO4 cell at different discharge rates. Rate of 1C is equivalent to a current density of 170 mA g-1).

ASSOCIATED CONTENT The supporting information contains: experimental details, XRD, FTIR, and TGA data for the cellulose nanocrystal; SEM micrographs for the dCNC/mCNC membrane surface and crosssection; surface area analysis for various mCNC membranes; EDX analysis of the mCNC membrane; 7Li NMR of the mCNC membrane; Nyquist impedance plots and Arrhenius fitting of the temperature dependent ionic conductivities for the mCNC and dCNC membranes. AUTHOR INFORMATION The authors declare no competing financial interest. ACKNOWLEDGEMENTS 9 ACS Paragon Plus Environment

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D.K. and C.H. acknowledges the Swiss National Science Foundation (SNSF) for the financial support for this work through their Ambizione grant. We also acknowledge Dr. René Verel for the 7Li NMR analysis. We thank Professor Markus Niederberger for hosting D.K. and providing all the research facilities in his lab. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

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