Nanocellulose Structured Paper-Based Lithium Metal Batteries

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Cite This: ACS Appl. Energy Mater. 2018, 1, 4341−4350

Nanocellulose Structured Paper-Based Lithium Metal Batteries Zhaohui Wang,*,† Ruijun Pan,† Rui Sun,‡ Kristina Edström,† Maria Strømme,‡ and Leif Nyholm*,† †

Department of Chemistry-Ångström, The Ångström Laboratory, Uppsala University, Box 538, SE-751 21 Uppsala, Sweden Nanotechnology and Functional Materials, Department of Engineering Sciences, The Ångström Laboratory, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden

‡

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S Supporting Information *

ABSTRACT: We report for the first time, a lithium metal battery (LMB) design based on low-cost, renewable, and mechanically flexible nanocellulose fibers (NCFs) as the separator as well as substrate materials for both the positive and negative electrodes. Combined with carbon nanofibers, the NCFs yield 3D porous conducting cellulose paper (CCP) current collectors with large surface areas, enabling a low effective current density. The porous structure yields a dendrite-free deposition of lithium (Li), faciliates the mass transport within the electrodes, and also compensates for the volume changes during the cycling. Stable Li electrodes are obtained by electrodepositing Li on CCP substrates while positive electrodes are realized by embedding LiFePO4 (LFP) particles within the flexible CCP matrix. The mesoporous NCF separator features a homogeneous pore distribution which provides uniform current distributions at the electrodes. This effect, which yields a more homogeneous Li deposition on the negative electrode as well as improves the safety, lifespan, and sustainability of the LMB. As a result, the present all-nanocellulose-based LMB demonstrates excellent cycling stability for a Li metal battery obtained to date, with 91% capacity retention after 800 cycles and 85% capacity retention after 1000 cycles at a rate of 2 C (i.e., 1.27 mA cm−2). KEYWORDS: nanocellulose, current collector, flexible electrode, cellulose separator, conductive cellulose paper, Li deposition, paper batteries, lithium metal batteries

1. INTRODUCTION The development of high-energy density Li-ion batteries has paved the way for the unprecedented advancement of portable electronic systems during the past two decades.1 However, to increase the energy density further new electrode materials need to be employed. Li metal is generally regarded as the most promising negative electrode material for high-energy density batteries due to its very high theoretical specific capacity (i.e., 3860 mAh g−1) and low standard potential (i.e., −3.04 V vs a standard hydrogen electrode).2,3 Unlike intercalation materials such as graphite, Li electrodes operate via repetitive Li deposition and oxidation processes upon charge and discharge, resulting in significant volumetric and morphological changes of the electrode.4−8 As a result of this, the spontaneously formed solid electrolyte interface (SEI) layer on the Li anode cracks, exposing fresh Li metal to the electrolyte.9,10 This continuous reaction between the Li and electrolyte decreases the Coulombic efficiency (CE) and is also assumed to accelerate the inhomogeneous and uncontrolled Li deposition giving rise to dendritic or mossy Li.11−14 As the latter can lead to internal short circuits and safety concerns, the use of Li negative electrodes is presently associated with formidable challenges which has impeded practical applications of Li metal batteries (including Li-sulfur and Li−air batteries) during the past four decades. © 2018 American Chemical Society

Significant efforts have so far been made to stabilize Li metal electrodes by attempting to suppress or manipulate the Li dendrite formation and/or to design a more favorable SEI layer.7,15−19 In these attempts various electrolyte additives,13,20,21 such as HF,22 LiNO3 and Li2S8,23 Cs+ and Rb+ ions,11,24 have been employed and it has also been shown that the use of artificial protective layers, e.g. a thin carbon layer,5,25 Li3PO4 layer,26 or a polydimethylsiloxane layer,27 can decrease the Li dendrite growth rate, in addition to solid and gel electrolytes with high shear moduli.28,29 Modifications of the surface and structure of the separator to homogenize the Li+ flux over the Li metal surface constitutes another promising strategy to slow down the growth of Li dendrites.19,30−33 It has also been demonstrated that a decreased current density as a result of the use of a porous nanostructured matrix with a large surface area can prolong the lifetime of Li electrodes by decreasing the Li dendrite growth rate and accommodating the large volume changes.34−39 Different types of 3D current collectors (3D Cu,35,40,41 graphitized carbon paper42), reduced graphene oxide,43 and porous carbon44,45 have consequently been proposed for this purpose. While the latter approach is Received: June 13, 2018 Accepted: July 31, 2018 Published: July 31, 2018 4341

DOI: 10.1021/acsaem.8b00961 ACS Appl. Energy Mater. 2018, 1, 4341−4350

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ACS Applied Energy Materials

delithiation of the positive electrode material (i.e., LFP), thereby improving the cycling stability of the LMB full cell. It is also demonstrated that the employed mesoporous nanocellulose separator exhibits a homogeneous porous structure enabling the attainment of a homogeneous current density at the electrode surfaces. As this yields a more uniform Li nucleation as well as a compact Li deposit structure, the allnanocellulose-structured Li/LFP battery exhibits a significantly improved cycling stability, i.e., a capacity retention of 85% after 1000 cycles at a rate of 2 C (using a current density of 1.27 mA cm−2). The present work therefore provides new possibilities for the realization of LMBs with high cycling stabilities.

promising, the manufacturing of many of the so far designed nanostructured scaffolds require precise synthetic control which would generate considerable economic and technical problems during the up-scaling process required for practical applications. There is consequently a need for new approaches for straightforward manufacturing of inexpensive nanostructured materials suitable for use in conjunction with Li electrodes. From the literature,30,46 it is likewise evident that the development of stable Li electrodes should be facilitated by the use of separators with homogeneous pore densities as well as improved thermal stabilities and electrolyte wettabilities. It can hence be expected that the combined use of an inexpensive 3D conducting substrate and an optimized separator could further enhance the performance (e.g., the safety and stability) of Li metal batteries. Reports based on such a combined strategy are, however, still scarce. While most research efforts on LMBs have been concentrated on the Li metal electrode, the progress regarding Li metal full cells has been relatively slow. This can be ascribed to the two main challenges in the design of the Li metal full cells,3,47 namely the significant volume variations at the electrode and battery levels and the detrimental transport of species from the positive electrode to the Li electrode. These issues, which depend on the choice of electrolyte and separators as well as the stability of the SEI, result in poor cycling performances for LMBs. Furthermore, the LMB electrodes are generally fabricated by casting slurries containing the active materials (such as a carbon host for a Li electrode), conducting agents (such as carbon black) and polymeric binders onto metallic current collectors. The additives and the metallic current collectors give rise to extra manufacturing costs as well as a decreased mass fraction of the active materials in the electrodes.48,49 Due to the volume fluctuations occurring in the cell during the cycling, there is also a need for flexible and porous electrodes that can handle the internal stress and strain buildup. Otherwise there is hence a risk of loss of performance and the generation of safety issues which give rise to significant engineering challenges with respect to the battery packaging.50 It is therefore evident that considerable efforts are needed to design high-capacity electrodes with minimal volume fluctuations for stable LMBs. Herein, we describe for the first time a novel allnanocellulose-based stable LMB full cell with low-cost, naturally abundant, biocompatible, renewable, and mechanically flexible nanocellulose fibers NCFs as the separator as well as substrate materials for both the positive and negative electrodes. In the nanocellulose based LMBs, (which do not contain any metallic current collectors or polymer binders), 3D porous conducting cellulose paper (CCP) current collectors composed of nanocellulose fibers (NCFs) and carbon nanofibers (CNFs) are used. The high surface area NCFs serve as a dispersion agent and as flexible building blocks, enabling the attainment of a porous carbon fiber containing 3D electrode structure.48 A stable Li electrode is obtained by electrodepositing Li on a CCP current collector while a positive electrode is realized by imbedding Li iron phosphate (LFP) particles within the 3D CCP matrix. The latter interconnected architecture provides a large specific surface area, enabling a low effective current density, while the porous structure provides ample space for the deposited Li. As a result of this, dendrite formation is not observed for these Li-CCP electrodes during cycling. The mechanically flexible NCFs also reduce the internal stress caused by the repeated lithiation/

2. RESULTS AND DISCUSSION 2.1. Conductive Cellulose Paper for Li Metal Electrodes. Prior to discussing the results, it should be pointed out that two different carbon materials were used as conducting additives in conjunction with the NCFs. CNFs with a diameter of 200 nm were thus used to prepare the CCP current collector for the Li anode application whereas carbon nanotubes (CNTs) with a diameter of 10 nm were employed to prepare the flexible paper LFP cathode. As is illustrated in Figure 1,

Figure 1. Schematic illustration of the fabrication processes used to manufacture the nanocellulose based electrodes and separator.

describing the electrode manufacturing process, a suspension containing NCFs and CNFs was first sonicated to ensure a good mixing, after which the obtained dispersion was vacuumfiltered through a nylon filter membrane until no water was visible on the top of the membrane. The paper-like nanostructured conducting NCFs/CNFs film was finally peeled off the filter membrane (see the Experimental section in the Supporting Information) and used as Li metal substrate as is further described below. The freestanding CCP used as a 3D current collector for the Li metal electrode, affords the following advantages: (i) the NCFs act as binder, dispersion agent and structural platform,48,51−53 yielding cost-efficient and flexible electrodes with tunable structures; (ii) a CCP with a large surface area and high porosity can be realized to provide plenty of space for the electrochemical deposition of Li and the ion mass transport; (iii) the interpenetrating flexible NCFs/ CNFs network can provide the mechanical flexibility needed to 4342


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it is, unfortunately, difficult to observe the NCFs as their diameters were less than 20 nm, whereas the CNFs had a typical diameter of 200 nm. The porous structure of the composites was characterized by nitrogen sorption analysis and the Brunauer−Emmett−Teller (BET) surface area of the CCP was found to be 29 m2 g−1 while the porosity and the pore volume were 74% and 7.7 × 10−2 cm3 g−1, respectively, (see Figure S1). This porous structure can be expected to readily accommodate the deposited Li and hence also the significant volume changes associated with the Li deposition and oxidation reactions.42,54 To investigate the properties of the 3D porous CCP current collector when used as a Li substrate, the deposition and oxidation of Li metal on the CCP were studied in twoelectrode pouch cells containing Solupor separators and Li metal foils as combined reference and counter electrodes. The electrolyte used in these experiments was composed of 1.0 M lithium bis(trifluoromethane)sulfonimide (LiTFSI) in 1,3dioxolane (DOL)/1,2-dimethoxyethane (DME) (1:1 by volume) and 2 wt % LiNO3. A precycling of the cells between 0 and 1.0 V (vs Li+/Li) was carried out during three cycles using a current density of 0.125 mA cm−2 since a significant irreversible capacity was seen on the first cycle (Figure S2). The latter was mainly due to the reduction of oxygen containing surface groups present on the CNFs but there should also have been a small contribution from the formation of an SEI layer.42,45,48,55 Li was then deposited on the CCP using a current density of 0.5 mA cm−2 and a fixed deposition capacity, followed by Li oxidation (at 0.5 mA cm−2) to a cutoff voltage of 1.0 V. In agreement with previous results,42 the CNFs initially appeared to undergo the proposed Li+ insertion reaction during the reduction step (i.e., the discharge) after which elemental Li was deposited as the voltage dropped below 0 V (Figure 3a). The reduction seen at potentials below about 0.5 V is, however, more likely due to Li deposition on

accommodate the volume changes associated with the Li deposition and oxidation reactions. In this way it should thus be possible to produce a 3D Li electrode with a high areal capacity using a scalable and facile paper-making approach. As will be described below, the same concept can also be used to manufacture nanocellulose paper based LMBs. As can be seen in Figure 2a and b, the manufactured flexible NCFs/CNFs films featured a glossy black paper-like

Figure 2. SEM images depicting the morphologies of the pristine CCP (a and b) and the CCP current collector after the deposition of lithium using a charge of 4 mAh cm−2 (c and d), and 8 mAh cm−2 (e and f), respectively. (g−i) Cross-section SEM images for CCP electrode after deposition of 8 mAh cm−2 Li, where h and i were adapted from the selected area marked in g. The inset in a shows an optical image of the pristine CCP.

appearance while the SEM images indicate the presence of an open mesoporous structure with CNFs randomly oriented within the porous NCFs/CNFs structure. In the SEM images

Figure 3. (a) First and second cycle galvanostatic Li deposition and oxidation curves obtained in 1.0 M LiTFSI in DOL/DME (v/v = 1:1) also containing 2 wt % LiNO3 with the CCP electrode using a current density of 0.5 mA cm−2 and a charge of 8 mAh cm−2. (b) Coulombic efficiency as a function of the cycle number for Li deposition and oxidation on Cu and CCP electrodes, respectively. (c) Galvanostatic Li deposition and oxidation curves obtained with Cu−Li|Li and Li-CCP|Li cells using a current density of 4 mA cm−2 and a fixed deposition charge of 2 mAh cm−2. 4343


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oxidation charge of 8 mA h cm−2 and a current density of 0.5 mA cm−2) are shown as a function of the cycle number. For the Cu current collector, the CE value rapidly became unstable and the average CE also dropped below 80% after 11 cycles (i.e., around 340 h). A more stable behavior was seen for the CCP electrode exhibiting a CE value of about 97% for nearly 36 cycles (i.e., more than 1150 h). These results indicate that use of the porous CCP electrode gave rise to an improved long-term cycling performance. This can be explained by the decreased current density resulting from the larger surface area of the CCP electrode and the fact that some of the deposited Li diffuse into the copper electrode.56 Plots of the cell voltage as a function of time for Li deposition on Cu and CCP electrodes versus the Li counter/reference electrode are shown in Figure S5. The difference between the sharp tip voltage and the voltage plateau is defined as the overvoltage,60 which reflects the nucleation barrier for the Li deposition. The lower overvoltage for the CCP electrode probably stemmed from its the larger surface area resulting in a lower current density. Cu−Li|Li and Li-CCP|Li cells were also used to compare the long-term cycling performance of the two electrode types (see the Experimental Section) using an initial Li deposition charge of 6 mAh cm−2, Li deposition and oxidation charges of 2 mAh cm−2 and a current density of 4 mA cm−2. As seen from the galvanostatic cycling curves in Figure 3c, a significant increase in the overpotential was seen after about 40 cycles for the Cu− Li|Li cell. This effect was most likely due a decreased capacity due to the diffusion of Li into the Cu electrodes56 although the effect previously has been attributed to a continuous SEI formation process.2,54 As demonstrated recently,56 the latter explanation is unlikely due to the small charge that should be associated with the SEI process. By contrast, the Li-CCP|Li cell maintained a stable behavior with low overvoltages for about 125 cycles (i.e., more than 120 h). Similar tests, carried out with a lower current density, i.e., 2 mA cm−2, also showed (see Figure S6) that a more stable behavior (i.e., lower overvoltages and longer lifetime) was obtained with the Li-CCP|Li cell. The latter cell thus showed a flat plateau and little variation in the shape of the voltage profiles and overpotential upon cycling. The Cu−Li|Li cell, on the other hand, exhibited a dramatically increased cell voltage most clearly seen after about 10 cycles, and the cell also failed after about 40 h. As indicated above, the latter was most likely due to a loss of Li associated with the diffusion of Li into the Cu electrode. It can hence be concluded that Li deposition on the CCP electrodes resulted in Li electrodes with capacities up to 823 mAh g−1 after normalization using the weight of the CCP current collector and that these electrodes performed significantly better than the Li electrodes obtained by depositing Li on copper foils. The Li-CCP electrodes also exhibited a better cycling stability most likely due to their larger surface area yielding a lower current density and the problems caused by Li diffusion into the Cu foils. The Li-CCP electrodes will therefore be tested in Li metal full cells comprising nanocellulose separators and flexible cellulose based LFP cathodes after a brief discussion of the latter two components. 2.2. Nanocellulose Separators with Uniform Pore Structures. It has already been demonstrated that NCF separators are well-suited for use in Li based batteries due to their excellent electrolyte wettability, high thermal stability, controllable pore structure, and straightforward manufacturing.61,62 The NCF separators exhibit a uniform pore

the CNFs as this reaction (unlike Li deposition on a Li electrode) should take place at potentials above 0 V vs Li+/Li. The latter can be explained based on the Nernst equation as the initial Li activity on the surfaces of the CNFs should be significantly lower than unity.56 An increasing Li activity on the surface of the CNFs would then explain the shift in the potential as a function of time seen prior to obtaining a bulklike Li layer on the CNFs. When increasing the Li deposition charge to 4 mAh cm−2, the thickness of the CNFs was found to increase (Figure 2c and d), and spherical particles of metallic Li also showed up on the surface of the CNFs. After using a Li deposition charge of 8 mAh cm−2, the diameters of the CNFs were found to be even larger and a marked decrease in the porosity of the electrode could be seen. Figure 2g−i and S3 shows the cross-sectional SEM image of the CCP electrode after deposition of 8 mAh cm−2 Li; it was found that the Li is uniformly coated on the CNFs and densely distributed within the electrode. As there were no signs of any dendritic or mossy Li (Figures 2e and f and S3), these results further support the formation of a homogeneous Li layer on the CNFs. Figure 3a shows typical galvanostatic discharge/charge profiles obtained with the CCP electrode in 1.0 M LiTFSI in DOL/DME (v/v = 1:1) containing 2 wt % LiNO3. The specific gravimetric capacity of the obtained Li metal electrode was found to be 823 mAh g−1 after normalization using the weight of the CCP whereas the volumetric capacity based on the total volume of CCP was around 41 mAh cm−3. Although the specific volumetric capacity clearly is important for a porous electrode such values are generally not reported and we are therefore not able to compare our value with literature data. However, since the volumetric capacity for Li is approximately 2060 mAh cm−3,57 it is immediately clear that the volumetric capacity of the present electrode was relatively low. As similar values likewise can be expected for other porous Li electrodes it can therefore be concluded the porous Li electrode approach would be less suitable for applications where the volume of the battery is an important factor. It should, however, be pointed out that the present volumetric capacity most likely can be increased using the macropore reduction approach previously employed in conjunction with another porous cellulose based composite material,58 without compromising the structure that must remain sufficiently porous after the Li deposition step. As no significant difference could be seen between the behavior on the first two deposition and oxidation cycles (see Figure 3a) it is reasonable to assume that all the deposited Li could be oxidized using the fixed oxidation charge of 8 mAh cm−2. This finding, which is supported by the CE data in Figure 3b, indicates that there was no significant loss of Li due to the formation of dead Li as typically is seen after the deposition of Li on conventional Li metal electrodes.54,59 Figure S4 also shows that the electrode remained porous after the Li oxidation step. These results thus show that the Li deposition and oxidation could be carried out reversibly and that the porosity of the CCP based electrode enabled the deposition and oxidation of a homogeneous layer of Li on the CNFs even when a Li deposition charge of 8 mAh cm−2 was used. To further investigate the performance of the Li coated CCP electrode, the efficiency and cycling stability of the Li deposition and oxidation were compared with those for Li deposition and oxidation on commercial Cu foil using Li counter electrodes. In Figure 3b the CE for Li deposition on the CCP and Cu foil electrodes (using a Li deposition and 4344


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ACS Applied Energy Materials distribution and a pore size distribution featuring a peak at 20 nm (Figure S7) with a porosity of about 40%. In a LMB, the uniform pore distribution is particularly important as it should facilitate homogeneous deposition of Li on the negative electrode via the attainment of a homogeneous current distribution.46 A more efficient use of the electrode area should also give rise to lower current densities and consequently also lower Li deposition overpotentials. To demonstrate the advantage of the mesoporous NCFs separator, Li|Li symmetric cells equipped with either NCF or commercial PE separators were assembled and their electrochemical performances were compared using an LP40 electrolyte. Figure 4a shows the galvanostatic cycling perform-

after cycling a NCF separator based Li|Li cell for 300 cycles (i.e., 600 h) at 0.5 mA cm−2, 1 mAh cm−2 in an LP40 electrolyte (Figure S9). 2.3. Flexible LFP Cathodes. Flexible and freestanding LFP positive electrodes were finally prepared using a straightforward and inexpensive paper-making process involving vacuum filtration of a dispersion containing the NCFs, CNTs, and LFP particles. This process is interesting as it can provide important up-scaling possibilities based on the industrial processes currently employed for paper-making.52,63−65 As is shown in the inset of Figure 5, depicting

Figure 4. (a) Voltage profiles obtained with Li−Li symmetric cells equipped with either a NCF or PE separator. SEM images of the cycled Li metal surfaces recovered after 50 cycles from (b) the PE and (c) the NCF separator containing cells, respectively. (b and c, insets) Magnifications of the surfaces. Electrolyte: LP40 (i.e., 1.0 M LiPF6 in EC/DEC (v/v = 1:1)).

Figure 5. SEM images (a and b) as well as an optical image (a, inset) of the flexible LFP paper material used as the positive electrode material. (c) Specific capacity as a function of the cycle number and rate and (d) specific capacity as a function of the cycle number at a rate of 2C for a flexible LFP cathode cycled in a cell containing a metallic Li foil anode, a PE separator, and an LP40 electrolyte. The specific capacities were calculated using the weight of the LFP.

ances of these Li|Li cells for a fixed current density of 0.5 mA cm−2 and a Li deposition and oxidation charge of 1 mAh cm−2. It can be seen that the cell with the NCF separator exhibited a steady response for close to 600 h whereas larger and increasing overvoltages were seen for the cell equipped with the PE separator. This indicates that the NCF separator had a positive effect on the lifetime of the symmetric Li|Li cell most likely due to the attainment of a more homogeneous deposition of lithium. The latter hypothesis is supported by the SEM images in Figure 4b and c depicting the surface morphologies of the Li electrode that initially served as the negative electrode in the NCF and PE separator based cells. While the pristine Li metal surface was uniformly flat with small defects (Figure S8), the surfaces of the electrodes that had been cycled for 50 cycles were rougher and also featured irregular flake-shaped islands, particularly for the electrode from the cell with the PE separator (Figure 4b). In the latter case, numerous randomly positioned holes and several cracks could also be seen, all of which rendered the electrode surface very porous. As the surface of the Li electrode from the cell containing the NCFs separator was smoother with smaller flake-like islands and exhibited a more compact structure it is reasonable to assume that the more homogeneous pore distribution of the NCF separator gave rise to a more homogeneous Li deposition. This clearly shows that the structure of the separator affects the electrochemical performance of the Li electrode significantly. It should also be mentioned that the NCF separator was found to be unaffected

SEM images of the flexible LFP electrode, the latter had a glossy black paper-like appearance. Although it is difficult to distinguish between the CNTs and the NCFs in the SEM images as the diameters of the CNTs and NCFs were almost the same (i.e., ∼10 and 20−30 nm, respectively), the CNTs and NCFs appeared to be uniformly distributed within the material. The LFP particles, which according to Figure S10 were between 100 and 800 nm large, were likewise found to be approximately uniformly distributed within the porous NCFs/ CNTs structure yielding a LFP mass loading of about 4 mg cm−2. Since the NCFs/CNTs matrix served as a conductive 3D host for the LFP particles there was no need for a metallic current collector. Pieces of the LFP paper were hence studied as freestanding cathodes in pouch cells containing metallic Li foils as combined reference and counter electrodes, PE separators and LP40 as the electrolyte (this electrolyte is typically used together with LFP electrodes). As shown in Figures 5c and S11, the LFP cathode exhibited discharge capacities of 143, 141, 136, and 127 mAh g−1 (based on the LFP mass) at 0.2, 0.5, 1, and 2 C, respectively. Note that the CE of the first cycle of the LFP/Li cell is 99.1% at 0.2 C. These capacities are significantly higher than those (e.g., 100 mAh g−1 at 2 C) previously obtained with LFP cathodes prepared by the conventional slurry-coating approach.61,62 The latter can most likely be explained by the more porous structure of the present LFP electrodes which facilitates the mass transport within the electrode hence enabling a more 4345


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Figure 6. (a) Schematic illustration of the present all-nanocellulose-structured LMB concept. (b−e) Performances of the Li-CCP|NCF|LFP, Li| NCF|LFP, and Li|PE|LFP cells in 1.0 M LiTFSI in DOL:DME also containing 2 wt % LiNO3. (b) Specific capacity as a function of the cycle number for different cycling rates, (c) typical voltage profiles obtained at a rate of 2 C, (d) the specific capacity as a function of the cycle number for a rate of 2 C (the periodic fluctuations were caused by day and night temperature variations in the laboratory). (e) Capacity retention as a function of the cycle number for the present LMBs and different LFP based LMBs described in the literature: GCF−Li,42 3D Cu−Li,54 Li−GT,59 PVDF−Li,67 and FNC separator.19

due to the generation of a porous Li surface during the Li deposition and oxidation processes as such problems often are encountered for Li electrodes cycled in an LP40 electrolyte.42,59 As will be shown in the next section, these problems can be decreased when using the DOL/DME based electrolyte employed in conjunction with the CPP−Li electrode and NCF separator. The capacity retention seen for the freestanding LFP cathode in the LP40 electrolyte is still better than those previously reported for many conventional LFP cathodes used together with protected Li anodes, ether-based electrolytes and lower current densities (e.g., a graphitized carbon fiber/Li anode used with an LFP cathode42 (80% retention after 300 cycles at 0.5 C), a 3D Cu−Li anode employed with an LFP cathode54 (90% capacity retention after 300 cycles at 0.5 C), as well as a Li−Ni anode used together with an LFP cathode66 (90% capacity retention after 50 cycles at 0.2 C)). This makes the flexible LFP cathode a very promising cathode. Since the lithiation and delithiation of the LFP particles are associated with volume changes, it is reasonable to ascribe the better cycling stability of the flexible LFP cathodes to the porosity of the NCF/CNT network. The latter facilitates the mass transport within the electrode and therefore facilitates a more complete use of the LFP present in the electrode.

complete use of the electrode material. It should be pointed out that the thickness of the present LFP electrode, most likely, can be increased significantly without introducing any significant mass transport problems due to its porous nature. It can also be expected that the present LFP electrodes should exhibit better capacity retentions at high cycling rates than conventional slurry-based electrodes. The above-mentioned 0.2 C discharge capacity corresponds to a capacity of 86 mAh g−1 based on the full weight of the cathode (i.e., the weights of the LFP, NCFs and CNTs) and a volumetric capacity of 64 mAh cm−3 based on the volume of the flexible cathode. The volumetric capacity can most likely be improved further by an optimization of the pore size distribution of the porous LFP electrode as indicated by our previous findings.58 The cell containing the flexible LFP cathode also demonstrated good cycling performance with a capacity of 114 mAh g−1 and 90% capacity retention after 300 cycles (see Figure 5d) at a cycling rate of 2 C. The latter can be compared with the capacity of 125 mAh g−1 (based on the weight of the LFP) and 93% capacity retention after 50 cycles obtained with a conventional LFP electrode at a cycling rate of 0.5 C.61 The fast capacity decay seen in Figure 5d after 475 cycles at 2 C, was in fact most likely due to problems with the Li electrode 4346


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exhibited a capacity of 110 mAh g−1 (84% of the first cycle capacity) and a CE of 99.9% after 800 cycles whereas the LiCCP|NCF|LFP cell featured a capacity of 122 mAh g−1 (91% of the first cycle capacity) and a CE of 99.9% after 800 cycles. Although the capacity of the Li-CCP|NCF|LFP cell decreased to 112 mAh g−1 (85% of the first cycle capacity) after 1000 cycles, the long-time cycling performance of the all-NCFsstructured LMB (i.e., the Li-CCP|NCF|LFP cell) is still, to the best of our knowledge, the best reported to date (see Figure 6e). It should also be mentioned that the capacity of the LiCCP|NCF|LFP cell decreased to become equal to that of the Li|NCF|LFP cell after 1000 cycles most likely as a result of the lower capacity of the Li-CCP electrode (i.e., 8 mAh cm−2) compared to that of the Li anode, which had a capacity of 25.7 mAh cm−2 (based on a 125 μm thick Li foil). Nevertheless, the excellent long-term cycling performance of the Li|NCF|LFP cell (see Figure 6e) clearly demonstrates the advantages of the all NCF-based cell design in which the porous and flexible NCFs-based network provides the mass transport and mechanical robustness needed to accommodate the volume variations associated with the delithiation/lithiation of the LFP particles as well as the Li deposition/oxidation during the cycling.

2.4. All-NCFs-Based Li Metal Batteries. Based on the results presented above, it is reasonable to assume that an allNCF-based LMB (Figure 6a), comprising a NCF-based Li anode, NCFs separator, and an NCF-based flexible LFP cathode, would exhibit better cycling performance than a conventional LMB. To test this hypothesis three cells were assembled comprising (i) a Li-CCP anode (8 mAh cm−2), an NCF separator, and a flexible LFP cathode (denoted the LiCCP|NCF|LFP cell); (ii) a metallic Li foil anode, an NCF separator, and a flexible LFP cathode (denoted the Li|NCF| LFP cell); and (iii) a metallic Li foil anode, a PE separator, and a flexible LFP cathode (denoted the Li |PE|LFP cell). All three cells thus contained flexible LFP cathodes with very similar capacities, as well as the same electrolyte, i.e. 1.0 M LiTFSI in DOL:DME also containing 2 wt % LiNO3. In Figure 6b, which shows the specific capacities (normalized with respect to the LFP mass) as a function of the cycle number for different cycling rates, it can be seen that while the specific capacities were almost the same at the lowest rate (i.e., 0.2 C), a significant capacity difference could be seen for the higher cycling rates (e.g., 2 C). It is clearly seen that the cells containing a NCF separator combined with either a Li-CCP or Li foil anode exhibited the highest capacities (i.e., 136 and 133 mAh g−1, respectively) at a rate of 2 C whereas the corresponding capacity for the Li|PE|LFP cell was 126 mAh g−1. The better rate capability for the cells equipped with a NCFs separator was mostly likely due to the fact that the ionic conductivity of the electrolyte filled NCF separator was higher than that for the PE separator.61 This effect can likewise explain the larger overvoltages seen for the Li|PE|LFP cell in Figures 6c and S12. The very similar capacities and cycling curves seen for the Li-CCP|NCF|LFP and Li|NCF|LFP cells are not surprising as the capacity of the cells should have been limited by the capacities of the flexible LFP electrodes. The difference between the capacities for the latter two cells can therefore most likely be explained by small differences between the capacities of the LFP electrodes employed in these cells. As shown in Figures 6c and S12 the Li-CCP|NCF|LFP cell electrode exhibited the lowest voltage hysteresis values at all rates. It is tempting to ascribe this to a better performance of the porous Li−CCP anode compared to that of the metallic Li foil anode particularly as similar voltage hysteresis effects previously have been reported for Li−3D carbon-based anodes.42,44 However, as the LFP electrodes should have been capacity limiting in the cells it is more likely that the voltage hysteresis differences were related to iR drop effects and a somewhat smaller LFP mass loading in the Li/NCF/LFP cell as discussed above. This hypothesis is also supported by the larger overvoltage values seen for the Li|PE|LFP cell. Figure 6d shows the long-term cycling performance of the LMBs for a current density of 1.27 mA cm−2 corresponding to a rate of 2 C. The Li|PE|LFP cell exhibited a discharge capacity of about 113 mAh g−1 and a capacity retention of 90% after 500 cycles as well as a drop in the specific capacity to less than 10 mA h g−1 after 730 cycles. This capacity retention is significantly better than that obtained with the LP40 electrolyte (compare Figures 5d and 6d) which indicates that the long-time performance depended on the choice of the electrolyte. As the LFP redox reaction (i.e., FePO4 + xLi+ + xe− = LixFePO4) should not be directly influenced by the choice of electrolyte, the longtime cycling results most likely reflect the changes in the properties of the Li electrode as discussed in conjunction with Figure 5d. The Li|NCF|LFP cell

3. CONCLUSIONS An all-NCFs-structured LMBs, based on inexpensive, renewable, and mechanically flexible NCFs, has been manufactured using a straightforward approach that facilitates the development of advanced and sustainable LMBs with improved lifetimes. It has been shown that the flexible NCFs can act as a dispersion agent and flexible building blocks to yield freestanding 3D porous CCPs when combined with CNFs or CNTs. The multifuctional NCF matrix or CCP scaffold provides a large surface area and ample space to accommodate the volume changes associated with the Li deposition and oxidation reactions taking place during the cycling. The asobtained Li−CCP symmetric cell exhibits stable cycling over 120 h at a high current density of 4 mA cm−2, whereas the flexible LFP cathode demonstrated a stable cyling performance during 475 cycles at a rate of 2 C in a LP40 electrolyte. It is also shown that a NCFs-based membrane can be empolyed as a mesoporous separator featuring a homogeneous pore distribution and high conductivity that facilites the attainment of a homogeneous Li deposit and gives rise to Li electrodes with increased lifetimes. All-NCFs-structured LMBs consisting of NCFs-based Li anodes, NCFs separators, and flexible NCFsbased LFP cathodes were found to exhibit the best cycling stability for LMBs so far reported with 91% capacity retention after 800 cycles at 2 C (for a current density of 1.27 mA cm−2) in an ether-based electrolyte. These results pave the way for further LMB inventions based on the use of abundant, low-cost nanofiber materials. As the all-NCFs-structured cell architecture readily can be combined with a variety of electrode materials, the approach provides a straightforward and versatile route for the development of highly efficient rechargeable metal battery systems. 4. EXPERIMENTAL SECTION 4.1. Materials. The Cladophora NCFs were obtained from FMC Biopolymers (USA), while the multiwalled CNTs (10 nm diameter) and CNFs (200 nm diameter) were purchased from Sigma-Aldrich and used without further purification. Solupor separators (E-8P01E, DSM, 12 μm thick, referred to as PE separators in the text), LP40 4347


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(diameter = 15 mm), 60 μL of LP 40 as the electrolyte, and NCFs or PE separators (diameter = 20 mm). The LFP cathodes were evaluated in coffee-bag cells also comprising Li metal anodes, PE separators, and LP 40 electrolyte. In the Li metal full cell experiments, Li−CCP anodes (obtained using a Li deposition charge of 8 mAh cm−2) were obtained from preprocessed cells. Pouch cells containing a Li metal or a Li−CCP (8 mAh cm−2 Li deposition charge) electrode (diameter = 15 mm), a flexible LFP cathode (diameter = 13 mm), and either a PE or NCF separator (diameter = 20 mm) were then assembled. The electrolyte was 60 μL of 1.0 M LiTFSI in DOL/DME (v/v = 1:1) also containing 2 wt % LiNO3. The cells were cycled at various rates ranging from 0.2 to 2 C in a voltage window between 2.5 and 4.2 V to study the rate capabilities while the cycling stability tests were conducted at a rate of 2 C using a current density of 1.27 mA cm−2.

(i.e., 1.0 M Li hexafluorophosphate (LiPF6) in ethylene carbonate (EC)/diethyl carbonate (DEC) (1/1, v/v), BASF), 1.0 M Li bis(trifluoromethane)sulfonimide (LiTFSI), 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME), LiNO3, the nylon filter membrane (90 mm diameter, 0.45 μm pore size; Magna), and the 125 μm thick Li foil (Cyprus Foote Minerals) were purchased and used as received. The other materials (e.g., LiFePO4, ethanol, Al foil, and Cu foil) were likewise bought from commercial manufacturers and used as received. Deionized water (DI) was used in the different manufacturing processes described below. 4.2. Preparation of the CCP Electrode. A 80 mg portion of NCFs and 320 mg CNFs were dispersed in 120 mL DI by sonication. The as-obtained dispersion was then vacuum-filtered through a nylon membrane filter (0.45 μm pore size) to obtain a NCFs/CNFs layer, which was subsequently dried and peeled off to serve as a freestanding CCP current collector with a thickness of about 205 μm. 4.3. Preparation of the NCFs Separator. A 50 mg portion of NCFs was dispersed in 80 mL DI water by sonication, and the dispersion was then vacuum-filtered through a nylon membrane filter to generate a wet NCFs membrane on the filter. The latter membrane was subsequently dried and peeled off and used as NCFs separators with a thickness of approximately 12 μm. 4.4. Preparation of the Flexible LFP Cathode. A dispersion comprising LFP particles, NCFs, and CNTs was vacuum-filtered through a nylon membrane filter (0.45 μm pore size) to form a LFP/ NCF/CNT layer, which then was dried and peeled off to serve as a flexible LFP paper cathode. The LFP, NCF, and CNT weight ratios in the cathodes were 60%, 20%, and 20%, respectively, and the LFP mass loading was about 4 mg cm−2. 4.5. Material Characterizations. SEM images for all samples were obtained with a Leo Gemini 1550 FEG SEM instrument (Zeiss, Germany) operated at 1 kV, employing an in-lens secondary electron detector. The samples were mounted on aluminum stubs with doublesided adhesive carbon tape and no sputtering was used prior to the imaging. The specific surface area was calculated according to the BET method using N2 adsorption data, whereas the pore size distribution was determined using the BJH method. 4.6. Electrochemical Characterizations. The CCP paper, NCF separator, and LFP paper were cut into discs which were dried under vacuum at 80 °C for 5 h prior to use either as freestanding electrodes or separators in Li based batteries. The electrochemical tests were carried out with polymer coated aluminum pouch (i.e., “coffee-bag”) cells. LiTFSI (1.0 M) in DOL/DME (v/v = 1:1) also containing 2 wt % LiNO3 or LP40 (i.e., 1.0 M LiPF6 in EC/DEC (v/v = 1:1, Merck) was used as the electrolyte (see below). The cells were assembled in an argon-filled glovebox (O2 < 2 ppm, H2O < 1 ppm), and the charge/discharge tests were performed using a Digatron BTS-600 system at room temperature. The electrochemical performance of the CCP electrodes was studied using pouch cells composed of the CCP working electrodes (diameter = 13 mm), PE separators (diameter = 20 mm), Li foil electrodes (diameter = 15 mm) (as reference/counter electrodes), and 60 μL of 1.0 M LiTFSI in DOL/DME (v/v = 1:1) also containing 2 wt % LiNO3 as the electrolyte. The batteries were first subjected to three chronopotentiometric cycles using a current density of 0.125 mA cm−2 and a cycling window of 0.01 to 1.0 V in order to pretreat the surfaces of the CCP electrodes. After the pretreatment, 8 mAh cm−2 of Li was deposited on the CCP electrode using a current density of 0.5 mA cm−2 after which the battery was charged to 1.0 V to oxidize the deposited Li. The CE was calculated as the ratio between the Li oxidation and deposition capacities. In the LiCCP/Li cell test, Li metal was first preplated on the CCP electrode using a charge of 6 mAh cm−2 and current density of 0.5 mA cm−2 to obtain the Li−CCP electrode. Experiments with cells containing one Li−CCP electrode and one Li electrode were then performed at various current densities employing a fixed Li deposition and oxidation charge. Analogous experiments were also performed with Cu-based Li electrodes. To compare the performances of the NCF and PE separators, pouch-type symmetric Li|Li cells were made using two Li discs

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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.8b00961. Pore size distributions of pristine CCP and nanocellulose separator, electrochemical pretreatment of CCP electrode, electrochemical and morphological study of Li deposition into CCP, rate performance of different LFP cells, and SEM image of pristine Li metal (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.W.). *E-mail: [email protected] (L.N.). ORCID

Zhaohui Wang: 0000-0001-6118-0226 Ruijun Pan: 0000-0002-6301-4771 Kristina Edström: 0000-0003-4440-2952 Leif Nyholm: 0000-0001-9292-016X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Z.W. and R.P. contributed equally to this work. The Swedish Energy Agency (Project TriLi), The Swedish Foundation for Strategic Research (SSF) (grant RMA-110012), the Swedish Energy Agency (project SwedGrids), StandUp for Energy, the Carl Trygger Foundation, and the Bo Rydin Foundation are gratefully acknowledged for financial support.

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REFERENCES

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Nanocellulose Structured Paper-Based Lithium Metal Batteries

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