Effects of Different Manufacturing Processes on TEMPO-Oxidized

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Effects of Different Manufacturing Processes on TEMPO-oxidized CNF Performance as binder for Flexible Lithium-ion Batteries Huiran Lu, Valentina Guccini, Hyeyun Kim, German Salarzar Alvarez, Göran Lindbergh, and Ann Cornell ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10307 • Publication Date (Web): 03 Oct 2017 Downloaded from http://pubs.acs.org on October 6, 2017

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ACS Applied Materials & Interfaces

Effects of Different Manufacturing Processes on TEMPO-oxidized CNF Performance as binder for Flexible Lithium-ion Batteries Huiran Lu,† Valentina Guccini, §,‡ Hyeyun Kim, †,‡ Germán Salazar-Alvarez, §,‡ Göran Lindbergh,† and Ann Cornell*,†,‡ †

Applied Electrochemistry, Department of Chemical Engineering, KTH Royal Institute of

Technology, SE-100 44 Stockholm, Sweden. ‡

Wallenberg Wood Science Center, KTH Royal Institute of Technology, SE-100 44 Stockholm,

Sweden. §

Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm

University, SE-106 91 Stockholm, Sweden.

KEYWORDS: CNF, binder, charge density, degree of homogenization, flexible Li-ion batteries

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ABSTRACT: Carboxylated cellulose nanofibers (CNF) prepared using the TEMPO-route are good binders of electrode components in flexible lithium-ion batteries (LIB). However, the different parameters employed for the defibrillation of CNF, such as charge density and degree of homogenization, affect its properties when used as binder. This work presents a systematic study of CNF prepared with different surface charge densities and varying degrees of homogenization and their performance as binder for flexible LiFePO4 electrodes. The results show that the CNF with high charge density had shorter fiber lengths compared with the CNF with low charge density, as observed with atomic force microscope (AFM). Also, CNF processed with a large number of passes in the homogenizer showed a better fiber dispersibility, as observed from rheological measurements. The electrodes fabricated with highly charged CNF exhibited the best mechanical and electrochemical properties. The CNF at the highest charge density (1550 µmol g1

) and lowest degree of homogenization (3+3 passes in the homogenizer) achieved the overall

best performance, including a high Young’s modulus of approximately 311 MPa and a good rate capability with a stable specific capacity of 116 mAh g-1 even up to 1C. This work allows a better understanding of the influence of the processing parameters of CNF on their performance as binder for flexible electrodes. The results can also contribute to the understanding of the optimal processing parameters of CNF to fabricate other materials, e.g., membranes or separators.


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INTRODUCTION The next generation electronic devices, flexible electronic devices, are rapidly developing in order to meet the strong consumer market demand for various applications, such as flexible lightemitting diode (LED) displays 1, 2, wearable health monitoring 3, flexible sensors 4, and flexible radio frequency identification (RFID) tags 5. The main power source for ubiquitous electronic devices is lithium-ion batteries (LIB), which possess high specific energy, high efficiency and long life 6. Flexible LIB have gained considerable attention and are urgently required to match practical requirements for flexible electronic devices. Research on flexible LIB has mainly focused on the development of flexible components, such as flexible electrodes, electrolytes, separators and current collectors 7-12. Cellulose is the most abundant biopolymer present primarily in wood biomass, which can reinforce the plant living bodies. Owing to this feature, cellulose-based materials, such as cellulose fibers 13-17, micro-fibrillated cellulose (MFC) 18-20 and cellulose nanofibers (CNF) 10, 2123

, have been widely investigated as reinforcement components in the preparation of paper-based

electrodes and polymer electrolytes for flexible LIB. In previous works, CNF has been demonstrated as binder material for flexible electrodes 22-25, using a filtration process similar to conventional paper making. Compared to standard industrial LIB electrode production the filtration has advantages of being aqueous based with no need for toxic solvents as N-Methyl-2pyrrolidone (NMP). CNF is a biomaterial, not from fossil sources, which is the case for conventional binders as polyvinylidene fluoride (PVDF). Another advantage is the possibility to produce papers with several layers (compare multiply paper), where the individual layers can be current collectors 25, electrodes or separator 22-23 integrated in a single paper sheet. The binder is an inactive material, which can increase the polarization of the electrodes. Previous work shows 3 ACS Paragon Plus Environment


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that a low amount of well-dispersed CNF can be used as binder for the flexible electrodes while still maintaining good mechanical and electrochemical properties 24. In order to achieve good fibril dispersion, individualization of cellulose fibrils (nanofibers) can be performed by chemical modification of the fibril surface using a regioselective oxidation of the hydroxyl group in the C6 position mediated by the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical. The oxidation is carried out using TEMPO/NaBr/NaClO at basic conditions or TEMPO/NaClO/NaClO2 under weakly acidic or neutral conditions 26-28. The TEMPO-mediated oxidation process is used to help dispersing plant cellulose quantitatively in water as individual fibrils by loosening the adhesion between cellulose fibrils through the introduction of electrostatic repulsion which prevents the formation of strong interfibril hydrogen bonds 29. The carboxyl content (charge density) of the fibril surface can be controlled by adjusting the conditions of the TEMPO-mediated oxidation process 28, 29. The dispersion of fibrils is completed by shear-induced defibrillation (also called mechanical homogenization) of the carboxylated cellulose into nanofibers. Both the surface charge density (oxidation) and the degree of fibrillation (mechanical homogenization) are important processes that influence the colloidal properties of CNF (e.g., dispersibility or gel formation 30, 31), as both may reduce the crystallinity and size of the fibers 30. To our knowledge this is the first systematic study of the effect of processing parameters, i.e., charge density and degree of homogenization, on the performance of CNF as binder for flexible LIB. In this work, carboxylated CNF was prepared with three different charge densities (up to 1550 µmol g-1) and defibrillated with varying number of passes through a microfluidizer, and then studied as binder for flexible electrodes. The morphology and colloidal behavior of the different CNF were investigated using atomic force microscope (AFM) and rheology, respectively. The morphology and mechanical properties of the fabricated electrodes containing CNF as binder 4 ACS Paragon Plus Environment

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were evaluated by scanning electron microscopy (SEM) and tensile tests, respectively. Lastly, the electrochemical performance of the CNF-containing electrodes was evaluated using electrochemical techniques.



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EXPERIMENTAL Materials. 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO, free radical, 98 %) and sodium hypochlorite (NaClO) were supplied from Alfa Aesar and Sigma-Aldrich, respectively. Carbon-coated lithium iron phosphate (LiFePO4, LFP) of the type Life Power P2 and Super-P carbon were provided by Phostech Lithium and Imerys Graphite & Carbon, respectively. Ethanol 96 % and 99.5 % were purchased by Solveco. Dried acetone >99.9 % and pentane 99.9 % were supplied by Merck KGaA and Prolabo, respectively. All of these solvents were used during solvent exchange as described below. The electrolyte used in the cells was composed of 1 M LiPF6 salt in ethylene carbonate (EC): diethyl carbonate (DEC) 1:1 by weight and was obtained from Merck KGaA. All water used was deionized (Milli-Q). Aluminium foil of 25 µm thickness was used as current collector and was supplied by Advent Research Materials. TEMPO mediated oxidation of cellulose. Never dried cellulose pulp was supplied by Domsjö Fabriker AB (Domsjö, Sweden). The pulp was oxidized following the protocol of Saito et al 29. Firstly, the suspension was washed with a solution of HCl at pH 2. Successively, 40 g (dry content) of pulp was suspended in 2 L of deionized water and mixed together with the TEMPO catalyst (4 µmol) and sodium bromide (4 µmol). The pH of the suspension was adjusted to 10 and kept constant during the reaction by addition of 0.5 M sodium hydroxide solution. The different amounts of surface charge were achieved by varying the amount of sodium hypochlorite. Specifically, 37.5 µmol, 80 µmol and 240 µmol sodium hypochlorite were slowly added to the suspension to obtain respectively low, medium and highly charged CNF, see Table1.


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Table 1. Different CNF studied in this work. homogenizer charge density (µmol g-1) large chamber small chamber 350 650 1550 (400 and 200 µm) (200 and 100 µm) 3 3 350-3-3 650-3-3 1550-3-3 3 6 350-3-6 650-3-6 1550-3-6 3 9 350-3-9 650-3-9 1550-3-9

Mechanical fibrillation of oxidized celluloses. A microfluidizer (M-110EH, Microfluidics Corp, US) was used for mechanical homogenization of the oxidized celluloses. It uses two large chambers connected in series (400 and 200 µm) at 925 bar and two small chambers (200 and 100 µm) at 1600 bar. The cellulose was first run 3 consecutive passes in the large chambers at a fiber concentration of about 1 wt % and then 3, 6 or 9 passes in the small chambers, see Table 1. Preparation of the flexible electrodes. A water-based paper making process was used to prepare the electrodes, as described elsewhere 23, 24. Briefly, suspensions with 84 wt % LiFePO4, 11 wt % Super-P carbon and the various 5 wt % CNF samples were mixed with an Ultra Turrax D125 Basic disperser at 8000 rpm for 20 minutes. The electrodes were obtained by vacuum filtering the suspensions through a Durapore membrane filter, type 0.22 µm GV, provided by Millipore. A sequential solvent exchange was performed using 50 mL of ethanol 96 %, ethanol 99.5 %, dried acetone and pentane until no liquid was visible on the surface. The electrodes were dried at 110 °C in vacuum for 1 hour and stored in a glove box. The loading of active material in the obtained electrodes was approximately 4 mg cm-2. Pouch cells of the electrodes versus lithium metal were assembled with glass fiber Whatman paper (260 µm) as separator in a glove box under argon atmosphere (< 1 ppm O2 and H2O). Characterization. The surface charge was determined by conductimetric titration of the oxidized pulp, following an established protocol 32. The length and height of the CNF were determined from AFM images obtained under air using a Dimension 3100 SPM (Veeco, USA) in 7 ACS Paragon Plus Environment


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tapping mode. The samples were prepared by depositing a droplet of a 0.01 wt% dispersion on a mica substrate modified with 3-aminopropyl triethoxysilane (Sigma Aldrich, 99%). Rheological measurements of CNF suspensions of 0.95 ± 0.05 wt% were carried out using a Physica MCR 301 rheometer (Anton Paar) with a cylindrical cone-and-plate geometry (CP25-2SN7617, cone angle 2 °, diameter 25 mm, gap height 50 µm). All the samples were measured at a constant temperature of 23 °C. Oscillatory frequency sweep tests were performed at angular frequency from 1 to 100 rad s-1 with constant 2 % strain. Oscillatory strain sweep tests were performed at a deformation ratio of 1 to 1000% at 10 rad/s of constant angular frequency. A Hitachi S-4800 field emission SEM was used to investigate the morphologies of the electrodes. Tensile tests of the electrodes were carried out with an Instron 5944 mechanical testing system at 25 °C under 500 N load with a rate of 10 %/min. Each electrode was cut into five strip samples with a width of 7-10 mm and a length of 20 mm. Thermo-gravimetric analysis (TGA) was performed to determine the concentration of the CNF using a Mettler Toledo TGA/DSC 1 STARe apparatus at a rate of 10 °C min-1 in N2 at a flow rate of 30 mL min-1. The TGA started with a drying period of 20 min at 110 °C. Electrochemical Measurements. The electrodes were cycled in the voltage range 2.8-4 V vs. Li+/Li0 using a Gamry PCI4 G750 potentiostat and a BioLogic VMP-300 multipotentiostat. The conductivities of the electrodes were measured using a four-probe Van der Pauw setup where the current passes in the plane of the electrodes.

RESULTS AND DISCUSSION Figure 1 shows the AFM pictures of the CNF samples having different charge (350, 650, 1550 µmol/g) and homogenization treatment (3-3, 3-6, 3-9 passes). For all the samples it is possible to 8 ACS Paragon Plus Environment

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see the prevalence of individual nanofibers, characterized by a certain degree of flexibility as can be seen by the presence of kinks along the nanofibers length. The CNF samples having 1550 µmol g-1 surface charge have a lower amount of aggregates and bundles of nanofibres compared to the 650 µmol g-1 samples and particularly to the 350 µmol g-1 samples. From the AFM pictures no clear differences can be seen from the variations in the mechanical treatment. Overall the CNF samples with the high and medium charge look more homogeneous and better dispersed compared to the low charge samples. Our observations agree with the literature in which higher charge leads to the production of better dispersed CNF gel and suspension 26.

Figure 1. AFM height images of the CNF samples having different charge (350, 650, 1550 µmol/g) and homogenization treatment (3-3, 3-6, 3-9 passes). Figure 2a from top to bottom shows the fitted histograms of the length distribution of the CNF samples. The range in which the length of the nanofibers varies mainly depends on the surface charge and less on the number of passes in the microfluidizer. The high charge samples have a 9 ACS Paragon Plus Environment


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narrower length distribution, which is also centered towards smaller values, while the 350 samples are characterized by a broader distribution. For the high charge samples all the nanofibers analyzed are below 1000 nm in length, for the medium charged samples some of the nanofibers of the 3-3 batch are just above 1250 nm and for the 350 samples all the batches show a percentage of nanofibers with a length close to 1500 nm. No general trends can be seen regarding the effect on the length distribution, probably polydispersivity of the CNF. To some extent the homogenization passes lead to a disintegration and mechanical damage of the CNF, in agreement with the fact that the histogram was fitted by a logNormal function, characteristic of mechanical disintegration processes 33. CNF with high surface charge has more electrostatic repulsion between the nanofibers due to the charges, which lead to a more efficient homogenization treatment and ultimately to a narrow distribution. Our observations agree with the literature in which the TEMPO mediated oxidation for a high surface charge results in shorter 34 and better dispersed CNF gel and suspension 26. Figure 2b shows the height distribution of the CNF samples. The height of all the nanofibers is approximately 2.5 nm, which indicates high aspect ratio nanofibers. No significant variation is observed neither as a function of the surface charge nor as a function of the number of passes. The surface charge and the homogenization treatment affects not only the dimension of the CNF but also the efficiency in which the CNF can build an interconnected network.


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Figure 2. The distribution curves of the length and width of different CNF: (a) length distribution, (b) height distribution.

Figure 3. Effect of varying charge density and degree of homogenization of a 0.95 wt% of CNF suspension: (a) homogenization by 3 passages in the large chambers, followed by 3 passages in 11 ACS Paragon Plus Environment


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the small chambers, (b) 3 and 6 passages times in large and small chambers respectively, (c) 3 and 9 passages times in large and small chambers, respectively. The network strength of CNF is strongly influenced by the pretreatment process, chemical modification, degree of fibrillation, the amount of surface charge, concentration, aspect ratio of fibrils etc. Oscillatory shear measurements are usually performed to study the viscoelastic properties of the structures. Figure 3 displays the viscoelastic behavior of CNF suspensions with different charge densities and treated by different degree of homogenization, as a function of storage modulus (G’). As shown in Figure 3a, the high storage modulus at the milder fiberdisintegrating condition, particularly 350-3-3, 650-3-3, means that CNF fibers were less defibrillated compared with the other samples. It is known that an increase in the amount of carboxylic groups on the surface of cellulose fibers reduces the storage modulus G’ because of increased electrostatic repulsive forces between CNF. This results in good dispersion 35. Our rheology data in Figure 3 agrees this agreement with our AFM results in Figure 2. There was no significant difference between the samples treated by 3-3, 3-6 and 3-9 passages at the low charge density of 350 µmol g-1. At the charge densities of 650 and 1550 µmol g-1, the storage modulus decreased when increasing the degrees of homogenization due to reducing interfibrillar interaction, as shown in Figure 3. This is in agreement with Saarinen et al. works 36. Figure 4a illustrates the flexibility of the produced LFP electrode. A bendable pouch cell, contaning by the flexible electrode, powered a LED is shown in Figure 4b. Figure 4c-d show the SEM images of the electrodes using CNF at a high charge density of 1550 µmol g-1 as binder. As shown in Figure 4c, the well-dispersed CNF 1550-3-3 relatively homogeneously binds the LFP and Super-P carbon particles with a visible web whereas Figure 4d and Figure 4e show that both CNF 1550-3-6 and CNF 1550-3-9 prefer to form denser films. These dense films are more


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insulating, which may influence the electrochemical properties of the electrodes as described in the previous work 24. This result illustrates that the homogenization process affects the morphology of the electrodes. The less homogenization of highly charged CNF, the better the dispersion in the electrodes. The cross section image (Figure 4f) shows that the thickness of the electrode is about 90 µm.



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Figure 4. (a) a bendable LFP electrode, (b) a rolled pouch cell, SEM images of the LFP electrode using CNF 1550 as binder with different degree of homogenization: (c) CNF 1550-3-3, (d) CNF 1550-3-6, (e) CNF 1550-3-9, (f) cross section. 14 ACS Paragon Plus Environment

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The thermal behavior of the electrode using CNF 1550-3-3 as binder is shown in Figure 5a. The electrode was thermally stable below a temperature of about 210 °C and started to lose weight above 210 °C caused by the decomposition of CNF 37. About 5 wt % was lost up to 350 °C, corresponding to the concentration of CNF in the electrode.

Figure 5. Properties of the electrodes: (a) TGA thermograms of the LFP electrode using CNF 1550-3-3 as binder, (b) (c) mechanical properties of the electrodes with different CNFs: tensile plots and Young's modulus, (d) conductivities of the electrodes using different CNFs as binder.



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Figure 5b and c show the mechanical properties of the electrodes using different CNF as binder. It can clearly be seen that the charge density does influence the tensile stress of the electrodes, as shown in the tensile plots in Figure 5b. The electrodes using CNF with a high charge density of 1550 µmol g-1 obtained a lower tensile strength than that of the electrodes using CNF with a charge density of 650 and 350 µmol g-1, which is likely caused by the shorter length of the CNF at a high charge density. The tensile strength of the CNF 1550-3-3 electrode was about 4 MPa, which is slightly higher than that of the CNF 1550-3-6 and 1500-3-9 electrodes. This may be attributed to the condensed films of CNF in the CNF 1550-3-6 and 1550-3-9 electrodes seen in the SEM images, making the electrodes non-uniform and resulting in poor mechanical properties. By increasing the charge density, compare the CNF 350-3-3, CNF 650-3-3 and CNF 1550-3-3 electrodes, the Young’s Modulus increased, see Figure 5c. The Young’s Modulus also increased with an increasing degree of homogenization for the electrodes using CNF at the charge density of 350 and 650 µmol g-1. However, it showed much lower values for the CNF 1550 based electrodes with a higher degree of homogenization. Figure 5c shows that the CNF 1550-3-3 electrodes yields the highest Young’s Modulus. The conductivities of the electrodes with different CNF are shown in Figure 5d. The electrodes using a low charge density CNF of 350 µmol g-1 show lower electrical conductivities compared to the electrodes using a higher charge density. The degree of homogenization had no clear effect on the conductivities of the CNF 350 and CNF 650 based electrodes. For the CNF 1550 based electrodes, the conductivities decreased with an increase of the degree of defibrillation and the 1550-3-3 electrodes achieved the highest conductivity. These results demonstrate that CNF with a high charge density and a low degree of homogenization is beneficial for a high conductivity of the electrodes.


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Figure 6 presents the potential profiles of the electrodes using high charge density CNF of 1550 µmol g-1 with different degrees of homogenization cycled at 0.1C. All of the curves show a typical flat potential plateau, caused by extraction of lithium with oxidation of Fe at about 3.5 V versus Li/Li+, and insertion of lithium with reduction of Fe below 3.4 V 38. The electrodes, 1550-3-3, 1550-3-6 and 1550-3-9, delivered specific capacities of about 158, 152 and 149 mAh g1

, respectively. A lower polarization was observed for the electrode using CNF with less passages

(1550-3-3) compared to the other two electrodes using CNF with more passages (1550-3-6 and 1550-3-9). This may be ascribed to the condensed and probably insulating CNF films in the CNF 1550-3-6 and 1550-3-9 electrodes, as shown in Figure 4. This demonstrates that a low degree of homogenization is beneficial to obtain good electrochemical properties for the electrode using CNF with the high charge density. This agrees well with the conductivity results in Figure 5dError! Reference source not found..

Figure 6. Galvanostatic charge/discharge curves of the LFP electrodes using high charge density (1550 µmol g-1) CNF with different degrees of homogenization as binder cycled at 0.1C.



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Figure 7 depicts charge and discharge curves of the electrodes with different charge density and degree of homogenization. The CNF 350-3-3, 650-3-3 and 1550-3-3 electrodes exhibited similar potential profiles cycled at 0.1C, see Figure 7a, c and e. The CNF 350-3-3 electrode started to show sloping curves from 0.5C, whereas the CNF 1550-3-3 and 650-3-3 electrodes still show flat plateaus, even at 1C. The CNF 650-3-3 electrode showed higher polarization than that of the CNF 1550-3-3 electrode, when the current was higher than 0.2C. The CNF 350-3-9 showed more stable cycling curves than that of CNF 350-3-3. The CNF 650-3-9 exhibited lower polarization at a high current rate of 0.5C and 1C than that of CNF 650-3-9. These results show that a high charge density combined with a low degree of homogenization of the CNF is beneficial for the battery cycling properties.


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Figure 7. Charge/discharge potential profiles at varying current rates of the electrodes with CNF of varying charge density (1550, 650 or 350 µmol g-1) and homogenization (3-3, 3-6 or 3-9 passes) as binder: (a) 1550-3-3, (b) 1550-3-9, (c) 650-3-3, (d) 650-3-9, (e) 350-3-3, (f) 350-3-9.



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The rate capacities of the electrodes using different binders were studied and the results are shown in Figure 8. Evidently, the CNF 1550-3-3 electrode yields much better rate capacities than the other electrodes. The electrode delivers a considerably high and stable capability of 158, 156, 144 and 116 mAh g-1, respectively, when the current increased from 0.1C to 0.5C, 0.2C and 1C. After 20 cycles at different current rates, the electrode was tested at the rate of 0.1C again and obtained a stable specific capacity of 154 mAh g-1, which is similar to the first 5 cycles at 0.1C. The CNF 1550-3-9 electrode exhibited an unstable capacity cycled at 1C, the capacity dropped dramatically from 58 to 24 mAh g-1 within 5 cycles. This may be ascribed to the condensed CNF films formed in the electrodes, limiting the lithium ion diffusion at high current rates. The charge density do influence the rate capacities of the electrodes, see Figure 8a, c and e. The rate capabilities increased with an increase of the charge density. When increasing the degree of homogenization for CNF with the charge density of 650 µmol g-1, the electrodes achieved a slightly more stable cycling performance at 1C. The CNF 350-3-9 also exhibited a better cycling stability cycled at 0.2C compared to that of the CNF 350-3-3 electrode. These results illustrate that when the CNF has a low charge density, a higher degree of homogenization results in better cycling performance, and when the CNF has a high charge density, a lower degree of homogenization is positive to obtain good electrochemical properties. The CNF 1550-3-3 electrode showed the best electrochemical properties among the electrodes tested.


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Figure 8. Rate capabilities of the electrodes with different CNF of varying charge density (1550, 650 or 350 µmol g-1) and homogenization (3-3, 3-6 or 3-9 passes) as binder: (a) 1550-3-3, (b) 1550-3-9, (c) 650-3-3, (d) 650-3-9, (e) 350-3-3, (f) 350-3-9.



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CONCLUSIONS A systematic study of how different parameters in the production of CNF, including charge density and degree of homogenization, influence the properties of CNF-based electrodes is reported in this work. Flexible electrodes with varying CNF were successfully prepared using a water-based paper filtration process. A high charge density is beneficial for the mechanical properties (Young’s modulus) and for the electrochemical performance. Increasing the degree of the homogenization has a positive effect on the mechanical and electrochemical properties of the electrodes for the CNF with a low charge density. However, it has a negative effect for the electrodes based on high charge density of CNF. The CNF 1550-3-3 exhibited the highest conductivity and yielded the best cycling performance and rate capability.

AUTHOR INFORMATION Corresponding Authors *A. Cornell, Email: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financed by the China Scholarship Council (CSC) and the Wallenberg Wood Science Centre (WWSC). Hurian Lu thanks CSC for economic support. Valentina Guccini and Germán Salazar-Alvarez thank the WWSC for economic support and the KAW Foundation for the microscope facilities at Stockholm University. Mr. Yingxin Liu


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in Stockholm University is acknowledged for the technical support of the rheological measurements.

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Effects of Different Manufacturing Processes on TEMPO-Oxidized

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