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Flexible Paper Electrodes for Li-ion Batteries Using Low Amount of TEMPO-oxidized Cellulose Nanofibrils as Binder Huiran Lu, Marten Behm, Simon Leijonmarck, Göran Lindbergh, and Ann Cornell ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05016 • Publication Date (Web): 30 Jun 2016 Downloaded from http://pubs.acs.org on July 5, 2016

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ACS Applied Materials & Interfaces

Flexible Paper Electrodes for Li-ion Batteries Using Low Amount of TEMPO-oxidized Cellulose Nanofibrils as Binder Huiran Lu,† Mårten Behm,† Simon Leijonmarck,†,‡ Göran Lindbergh†, Ann Cornell*,† †

Applied Electrochemistry, Department of Chemical Engineering and Techology, KTH Royal

Institute of Technology, SE-100 44 Stockholm, Sweden. ‡

Swerea KIMAB AB, Isafjordsgatan 28 A, SE-164 40 Kista, Sweden.

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ABSTRACT: Flexible Li-ion batteries attract increasing interest for applications in bendable and wearable electronic devices. TEMPO-oxidized cellulose nanofibrils (TOCNF), a renewable material, is a promising candidate as binder for flexible Li-ion batteries with good mechanical properties. Paper batteries can be produced using a water-based paper making process, avoiding the use of toxic solvents. In this work, finely dispersed TOCNF was used and showed good binding properties at concentrations as low as 4 wt%. The TOCNF was characterized using atomic force microscopy and found to be well dispersed with fibrils of average widths of about 2.7 nm and lengths of approximately 0.1-1 µm. Traces of moisture, trapped in the hygroscopic cellulose, is a concern when the material is used in Li-ion batteries. The low amount of binder reduces possible moisture and also increases the capacity of the electrodes, based on total weight. Effects of moisture on electrochemical battery performance were studied on electrodes dried at 110 oC in vacuum for varying periods. It was found that increased drying time slightly increased the specific capacities of the LiFePO4 electrodes, whereas the capacities of the graphite electrodes decreased. The coulombic efficiencies of the electrodes were not much affected by the varying drying times. Drying the electrodes for 1h was enough to achieve good electrochemical performance. Addition of vinylene carbonate to the electrolyte had a positive effect on cycling for both graphite and LiFePO4. A failure mechanism observed at high TOCNF concentrations is the formation of compact films in the electrodes.

KEYWORDS: TEMPO-oxidized cellulose nanofibrils, binder, flexible paper electrodes, moisture, Li-ion batteries


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INTRODUCTION Li-ion batteries are the most important power source for portable electronics because of their high energy density, large power density and long-term stability.1 Flexible Li-ion batteries have attracted increasing interest as a key component to achieve future flexible electronic devices2, 3, such as flexible light-emitting diodes (LEDs),4, 5 wearable and implantable electronic devices,5-7 Radio Frequency Identification (RFID) flexible tags,8, 9 and bendable electronic devices.10 Typically electrodes in Li-ion batteries consist of active materials, a conductive material, a binder and current collectors. The binder holds the active materials and conductive materials together, plays an important role in the fabrication of the electrodes and also affects the electrochemical performance of the batteries. Polyvinylidene fluoride (PVDF) and carboxymethylcellulose (CMC) are conventional binder materials for Li-ion batteries. However, PVDF is relatively expensive and requires dissolving in toxic and volatile organic solvents, typically N-methyl-2-pyrrolidone (NMP). Electrode films made of PVDF also have poor mechanical properties,11 which limits its application for flexible electronic devices. CMC is an extremely stiff and brittle polymer and can form a continuous matrix around the active particles, limiting flexibility and conductivity as well.12, 13 New types of cellulose-based materials, such as cellulose fibers and cellulose nanofibrils (CNF), have received considerable attention as component materials for flexible power sources, such as Li-ion batteries,14-21 supercapacitors,22-25 and Na-ion batteries,26 as a solution to meet the demands of these applications. CNF consist of approximately 3-4 nm wide and micrometer long cellulose fibrils, suitable as a reinforcement component for composite materials.27, 28 However, the fibrils have surface hydroxyl groups and are aggregated by hydrogen bonds. In order to separate individual fibrils and decrease the aggregation, a catalytic oxidation using 2,2,6,63 ACS Paragon Plus Environment


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tetramethylpiperidine-1-oxyl radical (TEMPO) has been proposed.29, 30 TEMPO-mediated oxidation converts primary hydroxyl groups into carboxylate groups when using the TEMPO/NaClO/NaClO2 system under weakly acidic or neutral conditions. This TEMPOoxidized CNF (TOCNF) is a promising binder material for flexible Li-ion batteries with good mechanical properties.31 The flexible electrodes made from TOCNF can be produced using a water-based paper making process, avoiding the use of toxic solvents. In contrast to PVDF, CNF is an environmentally friendly and renewable material. Table 1. Characteristics of the flexible electrodes in recent works Binder in the electrode

Ratio of binder (wt %)

Active material

Microfibrillated cellulose14

10

graphite

Cellulose fibers32

20

graphite

15

25

graphite

Cellulose fibers16

10

graphite

Cellulose fibers

33

20

graphite

Cellulose fibers

17

16

LiFePO4

282±33

1.4±0.6

Cellulose nanofiber34

10~33

Titanate

35

1100~1500

Nano-fibrillated cellulose31

11

LiFePO4

120

3

10

Titanate

125

Cellulose fibers

PVDF

4

Young Modulus (MPa)

Strain at break (%)

100

~300

~1.5

101±8

~500

~300

126±7

~162

101±19

Thickness Conductivity (µm)

(S/m)

85±10

30

~100 90±9

~300

4.1

A concern with CNF is that water trapped in the hygroscopic cellulose structure may negatively influence the Li-ion battery performance.31, 35, 36 A goal of the present study has been to better understand the effects of moisture, by investigating electrodes dried in vacuum for varying time periods, and also to lower the amount of cellulose binder in the electrodes. The latter leads to less moisture in the battery and also to a higher capacity based on the total weight of the battery. To our knowledge there are no studies of the effect of drying time of nanocellulose 4 ACS Paragon Plus Environment

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for Li-ion batteries in the literature. Flexible electrodes for Li-ion batteries in literature contain about 10-33 wt% binder, see Table 1. In the present work we have produced very finely dispersed TOCNF with excellent binding capacity in order to produce electrodes with as low as 4 wt% binder, still with relatively good mechanical properties. Both positive electrodes (LiFePO4) and negative electrodes (graphite) were synthesized and characterized. Additionally, the effect of vinylene carbonate (VC) was studied, as this is a well-known electrolyte additive to enhance the coulombic efficiency (CE).

EXPERIMENTAL SECTION Materials. The cellulose source was never-dried dissolving pulp (60% Norwegian spruce and 40% Scots pine) from Domsjö Fabriker AB, Örnsköldsvik, Sweden. NaClO, NaClO2 and TEMPO were purchased from Sigma-Aldrich. Carbon-coated lithium iron phosphate (LiFePO4) of the type Life Power® P2 was provided by Phostech Lithium. Graphite of the type Timrex SLP 30 AH-354 and Super-P carbon were kindly provided by Imerys Graphite & Carbon. The densities of these materials were: LiFePO4 – 3.6 g/cm3, graphite – 2.1 g/cm3, Super-P carbon – 2.0 g/cm3 and TOCNF – 1.5 g/cm3. Ethanol 96% and 99.5% were supplied by Solveco. Dried acetone >99.9% and pentane 99.9% were provided by Merck KGaA and Prolabo, respectively. All of these solvents were used during solvent exchange as described below. PVDF and NMP were purchased from Sigma-Aldrich Sweden AB. The electrolyte used in the cells composed of 1 M LiPF6 salt in ethylene carbonate (EC): diethyl carbonate (DEC) 1:1 by weight was provided from Merck KGaA. When so stated, the electrolyte also contained 2 wt % VC purchased from Sigma-Aldrich. The current collectors (aluminum foil with the thickness of 25 µm and copper foil with the thickness of 20 µm) were supplied by Advent Research Materials. Silicon wafers (boron-doped, p-type) with thickness of 610-640 µm and single side polished were purchased 5 ACS Paragon Plus Environment


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from Addison Engineering, Inc.(San Jose, CA, USA) A blue LED lamp was purchased from Kjell & Company, Sweden. All of the water used was deionized water (Milli-Q). Preparation of TOCNF. TEMPO-oxidized cellulose was synthesized following the procedure reported elsewhere.29 The dissolving pulp was oxidized by TEMPO mediated treatment with NaClO and NaClO2. The charge density of the TEMPO-oxidized celluloses were determined to be 630 µeq/g by conductometric measurement.37 A homogenizer with a high-pressure fluidizer (M110EH, Microfluidics Corp, US) was used to mechanically disintegrate the fibers. It was equipped with two chambers of different sizes connected in series. 1 wt% TOCNF gel was achieved after 3 passes through the large chambers in series (400 and 200 µm, respectively) and 3 passes through the smaller chambers in series (200 and 100 µm, respectively). Preparation of flexible paper electrodes. The electrodes were prepared using a water-based paper-making type process, as shown in the schematic illustration in Figure 1. Suspensions with different weight ratios of LiFePO4 or Graphite, Super-P carbon and TOCNF were mixed with an Ultra Turrax D125 Basic disperser at 8000 rpm for 20 minutes. Then the suspensions were vacuum filtered through a Durapore membrane filter, type 0.22 µm GV, supplied by Millipore. When no longer any liquid was visible on the film, solvent exchange was done by adding 50 ml each of ethanol 96%, ethanol 99.5%, dried acetone and pentane in sequence until no solvent was visible on the filter films. The films were dried at 110 °C in vacuum for 1h, 27h, 71h and 169h, respectively, for positive electrodes and 1h, 23h, 100h and 287h, respectively, for negative electrodes. The weight of the electrodes corresponds well to the total weight of added components in the suspensions. Pouch cells with lithium metal as counter electrodes were built in a glove-box under argon atmosphere.


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Electrodes bound by PVDF were prepared and used as a reference material in the electrochemical evaluation. An NMP slurry composed of 84 wt% LiFePO4, 8 wt% Super-P carbon and 8 wt% PVDF for positive electrodes and an NMP slurry consisting of 90 wt% graphite, 1 wt% Super-P carbon and 9 wt% PVDF for negative electrodes were prepared and coated onto an aluminum and copper foil, respectively. Subsequently, NMP was evaporated in a vacuum oven at 110 oC overnight. The composition of the positive electrodes was varied to find an optimal component ratio, keeping the total mass of the electrode constant. An abbreviated notation is used to present the formulation of the composite electrodes, defining the wt% of active material, conductive particles (Super-P carbon) and binder, respectively. For instance, 887-5 indicates that the electrode consists of 88 wt % LiFePO4, 7 wt % Super-P carbon and 5 wt % TOCNF.

Figure 1. Schematic illustration of the water-based paper-making process for flexible electrodes. Geometry. Electrode thickness was measured using a micrometer (Mitutoyo) with a resolution of 1 µm. The loading was calculated as the weight of active material in the electrode divided by the area of the electrode. The porosity of the electrodes (ɛ) was estimated using the measured volume (V1) and the theoretical volume of the electrode (V2) as follows:

ɛ

(1)

∗ ∗

(2)



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+

+

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(3)

where is the electrode thickness, is the radius of electrode, , and

!" are

densities

of active materials (LiFePO4 or graphite), Super-P carbon and TOCNF, respectively, and # , # and #

!"

are the weights of active materials (LiFePO4 or graphite), Super-P carbon and

TOCNF, respectively, in the electrode. The properties of the electrodes are shown in Table 2. Table 2. Properties of the flexible electrodes Geometries Thickness (µm)

Positive electrodes

Negative electrodes

̴100

̴36

Loading (mg/cm )

3.99-4.18

2.04

Porosities (%)

̴85

̴71

2

Characterization. The morphology of the finely dispersed TEMPO-oxidized nanofibrils was

scanned using atomic force microscope (AFM). Silicon wafers were oxidized at 1000 °C for 1 hour to obtain an oxide layer of approximately 30 nm. In order to get a hydrophilic surface, silicon wafers were treated with 10% NaOH, cleaned using water, ethanol and water and then dried with N2. Then plasma treatment (PCD002, Harrick Scientific Corp., Ossinging, NY, USA) was used to further clean the silicon wafers for 2 min. TOCNF gel was dispersed in water using the Ultra Turrax D125 Basic disperser at 12000 rpm for 10 min. A droplet of 0.001 w/w% solution of TOCNF was deposited on the chemically modified silicon wafers with an anchoring layer of polyvinyl amine (PVAm) (Lupamin 9095, BASF) by soaking in a 0.1 g/L PVAm solution at pH 7.5 for 2 min. The AFM was performed using a Multimode 8 Scanning Probe Microscope (Bruker AXS, Santa Barbara, USA) in PeakForce Quantitative Nanomechanical Mapping (QNM) mode using a scanasyst air cantilever with nominal spring constant of 0.4 N/m at a resonant frequency of 70 kHz. The image processing and widths of TOCNF were carried out using the NanoScope Analysis software (Version 1.5). 8 ACS Paragon Plus Environment

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Dynamic light scattering (DLS) (Zetasizer Zen3600, Malvern Instrument, UK) was employed to determine the hydrodynamic radius of spherical particles of the TOCNF at a temperature of 25 °C. The supernatants obtained after centrifugation of 1 g/L TOCNF suspensions were dispersed using the Ultra Turrax D125 Basic disperser at 8000 rpm for 20 min. DLS measurements were carried out on 1 mL samples at 0.1 g/L to remain below an overlap concentration. The results were obtained as an average of five replicate measurements. The surface morphology and fracture surface of the electrode materials were investigated using a Hitachi S-4800 field emission scanning electron microscope (SEM). The electrodes were broken for characterization of the cross-sections, by first immersing in liquid nitrogen for 20 min. Tensile tests of the electrode films were performed with an Instron 5944 mechanical testing system at 25 °C. Five strip samples with width of 7-10 mm and length of 20 mm were used for tensile testing with a rate of 10 % per minute under 500 N load. Values of stress and strain at break were obtained. Thermal gravimetric analysis (TGA) was performed using a Mettler Toledo TGA/DSC 1 STARe apparatus. The electrode was dried at 110 oC for 20 min before being heated at a rate of 10 oC/min up to 800 oC in N2 at a flow rate of 30 mL/min. Electrochemical Measurements. The electrochemical properties of the positive electrodes were measured using a Gamry PCI4 G750 potentiostat. In order to evaluate the stability of the negative electrodes, measurements of the specific capacity and CE were carried out using a setup with high precision voltage and current measurement, which is similar to the so called highprecision charger (HPC) described in several articles from the Dalhousie University, Canada, see an example in reference.38 The current is supplied with high precision by a Keithley 220 current source. A resistor with 0.01% precision is connected in series with the cell for current 9 ACS Paragon Plus Environment


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measurement in order to improve precision even further. A Keithley 2700 scanning digital multimeter (DMM) with a Keithley 7700 multiplexer module is used to measure voltage drop over the resistor and cell voltage. One scanning DMM serves up to five cells with current and voltage measurements by scanning a total of ten points within a time period of two seconds. Control and data collection is achieved using a PC with LabVIEW. The negative electrode cells were kept and cycled at 25 °C in a temperature controlled box. A schematic of the setup is shown in Figure 2. The charge and discharge characteristics were evaluated in the voltage range of 2.84.0 V vs. Li+/Li(s) for positive electrodes and in the range 0.002-1.5 V vs. Li+/Li(s) for negative electrodes, with and without 2 wt % VC in the electrolyte.

Figure 2. A schematic diagram of the setup for high-precision cell cycling. The conductivity measurements were carried out using both a four-probe Van der Pauw setup with 4 graphite electrodes and a Swagelok cell with two stainless steel plates. For the four-probe Van der Pauw testing, current passes in the plane of the electrode. Both sides of the electrodes were measured. For the Swagelok cell, the current goes through the plane of the electrode. The


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electrodes were pressed between the stainless steel plates until obtaining a constant conductivity value. Specific capacity is defined as capacity per loading mass of active electrode material.

RESULTS AND DISCUSSION The first step in the present study was the optimization of component ratios for the positive paper electrodes. To obtain a high capacity, it is important to reduce the amount of inactive materials, such as carbon and a binder material, without losing much of conductivity and mechanical integrity. As shown below, we managed to considerably reduce the amount of TOCNF for the positive electrodes. The active material for the negative electrodes is graphite, which is already a conductive material. 2 wt% Super-P carbon and 4 wt% TOCNF were used for the negative electrodes. Table 3. Electrical properties of the flexible positive paper electrodes Components (wt %) (Described as: LiFePO4 - SuperP carbon - TOCNF)

four-probe Van der Pauw (S/m)

Swagelok cell (S/m)

80-9-11

0.0008

88-7-5

0.07

0.029

86-9-5

0.23

0.072

84-11-5

0.35

0.097

85-11-4

1.14

0.22

87-9-4

0.81

0.18

Table 3 shows the conductivity measured using a four-probe Van der Pauw and a Swagelok cell of positive paper electrodes of varying composition, dried for 24 hours. The conductivity of the 80-9-11 electrode, containing 11 wt% TOCNF, was much lower than those measured for the other electrodes and will be discussed more later. The 88-7-5 electrode showed poor conductivity due to the low carbon content. In order to improve the electrical conductivity between the active particles, the content of Super-P carbon was increased. Indeed, the conductivities of 86-9-5 and 11 ACS Paragon Plus Environment


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84-11-5 electrodes were found to increase with increased amount of Super-P carbon. TOCNF is a non-conducting and electrochemically inactive part in the electrode, and therefore the smaller amount of TOCNF, the higher the conductivity. The 85-11-4 and 87-9-4 electrodes exhibited higher conductivities than the electrodes with a higher amount of TOCNF for both methods, and still with good enough binding properties at only 4 wt % TOCNF. An 86-11-3 electrode was also prepared, however, the mechanical properties of the electrode were too poor. The 85-11-4 electrode shows the highest conductivity for both four- probe Van der Pauw and the Swagelok cell (1.14 S/m and 0.22 S/m respectively).

Figure 3. Galvanostatic charge/discharge voltage profiles of flexible positive paper electrodes (described as: LiFePO4 - Super-P carbon - TOCNF) for 4th cycle at C/10. Electrodes dried for 24 hours. Figure 3 shows the galvanostatic charge/discharge voltage profiles of positive electrodes with different component ratios in the electrolyte with 2 wt % VC. Figure 3 demonstrates that the 887-5 electrode shows high polarization as also indicated by the conductivities in Table 3. By increasing the carbon ratio, in the 86-9-5 and 84-11-5 electrodes, the polarization sequentially 12 ACS Paragon Plus Environment

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decreased, see Table 3. In order to load more active material, the amount of binder material (TOCNF) was decreased, resulting in 85-11-4 and 87-9-4 electrodes. Figure 3 shows that the 8511-4 electrode yields the highest specific capacity (defined as capacity divided by loading mass of active material in the electrodes) and lowest polarization, in good agreement with Table 3. This can probably be ascribed to the optimization of inter-particles contacting between LiFePO4 and Super-P carbon. It was not possible to cycle the electrode 80-9-11 due to its very high resistance and the batteries immediately reached the set voltage limits. Compared to previous studies of flexible electrodes given in Table 1, only 4 wt% TOCNF is a very low amount which decreases the materials cost, increases the energy density and maybe enhances the cycling stability of the electrodes. Two different magnification AFM height images of TOCNF are shown in Figure 4. It can be seen that every single nanofibril was disintegrated and the TOCNF are overall well dispersed with average widths of about 2.7 nm and lengths of approximately 0.1-1 µm. The shape of almost all of the nanofibrils has kinks, sharp bends between the straight segments, a similar morphology as Usov’s work.39 The hydrodynamic radius of TOCNF measured by DLS depends on the number of passes during the mechanical defibrillation process.40 A high hydrodynamic radius indicates a high degree of aggregation of TOCNF. In this work, a hydrodynamic radius of about 472±22 nm was obtained after 3 passes in each chamber of the homogenizer.



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Figure 4. AFM height images of TOCNF: (a) (b) different magnifications.

Figure 5. Photograph of a flexible positive paper-based electrode (a) and a bended cell (b).


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Figure 6. SEM images of two different component ratios of positive electrodes at different magnifications: (a) (b) 80-9-11, (c) (d) 85-11-4, (e) (f) cross-sectional SEM images of 85-11-4 (total electrode thickness about 100 µm). Figure 5a shows a photograph to illustrate the flexibility of such a positive paper-based electrode produced by the water-based paper making process. A bendable cell powering a blue LED is shown in Figure 5b. SEM images of the positive electrodes with different component ratios are shown in Figure 6a-d. The components of the electrodes are active material particles 15 ACS Paragon Plus Environment


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(LiFePO4), conductive particles (Super-P carbon) and binder material (TOCNF). The electrode containing a large amount of TOCNF binder material is shown in Figure 6a and 6b. It is clearly seen that both LiFePO4 and Super-P carbon have aggregated and that TOCNF form a compact film, which limits the conductivity of the electrode. However, as shown in Figure 6c and 6d, the electrode with low amount of TOCNF is uniform, and no compact film is found. The LiFePO4 particles are relatively homogeneously dispersed with Super-P carbon. The TOCNF were welldispersed and clearly acts as a binder material with a visible network binding the particles together. The cross-section SEM images of the 85-11-4 electrode film was also investigated. As shown in the Figure 6e and 6f, the active materials, conductive particles and binder material are well dispersed also within the electrode bulk. It shows a morphology quite similar to that of the electrode surface (Figure 6c and 6d) and illustrates that the electrode materials are uniformly distributed.

Figure 7. SEM images of the fracture surface of the positive electrodes: (a) 80-9-11, (b) 85-11-4. Figure 7 shows the SEM images of the fracture surface of the electrodes after the tensile test. As shown in Figure 7a, the electrode is layer by layer separated by TOCNF compact, and probably insulating, films that may cause poor electrochemical performance. Figure 7b is clearly


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shown that the electrode with less TOCNF does not contain this kind of compact film and the particles are dispersed well. The SEM images of the 94-2-4 negative electrode are shown in Figure 8. Figure 8a and 8b show that the electrode is relatively uniform with graphite and Super-P carbon particles bound by TOCNF, which is similar to the electrode shown in Figures 6c and 6d.

Figure 8. SEM images of the 94-2-4 negative electrode at different magnifications: (a)×500, (b)×4000. Figure 9 shows the mechanical properties of the 80-9-11 and 85-11-4 positive electrodes dried for 1h at 110 oC. It is clearly shown that the modulus and strength increase with increasing amount of TOCNF in the electrode. However, the 85-11-4 electrode still has good mechanical properties with a Young’s Modulus of about 250 MPa, an ultimate strength of approximately 5 MPa and a strain at break of about 6 %.



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Figure 9. Tensile test plots of the 80-9-11, 85-11-4 positive electrodes. Figure 10 shows the TGA thermograms of the 85-11-4 electrode. As shown in Figure 10, the electrode started to lose weight at about 210 oC, below that temperature it shows that the electrode has a high thermal stability. The weight loss of about 4 wt% up to 350 oC is most likely related to decomposition of TOCNF.30

Figure 10. TGA thermograms of the 85-11-4 electrode.


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Figure 11 shows the specific capacities and values of CE, with and without VC in the electrolyte, for electrodes dried for 1h at 110 °C. The influence of VC as an electrolyte additive for Li-ion batteries has been widely investigated.41-46 The studies show that the presence of VC in the electrolyte in many cases improves the cycling performance. VC may react on the surface of the electrodes and polymerize to poly alkyl Li- carbonate species, which prevent further reduction of the electrolyte.42 It is clearly seen from Figures 11a-d that VC improves the cycling performance in terms of both specific capacity and CE for both negative and positive electrodes. The latter surprising, since EI Ouatani’s work showed that the VC polymerization mechanism was not observed at the surface of the LiFePO4 electrode using PVDF as binder.47 The specific capacities of the positive and negative electrodes containing VC are relatively higher and more stable than for electrodes without VC. After 10 cycles, the discharge capacities for the positive and negative electrode with VC are approximately 152.5 mAh g-1 and 344 mAh g-1, respectively. For the positive electrodes, the CE of the initial cycle with VC is around 99% which is slightly higher than that without VC. After three cycles, the CE with VC reaches higher than 99.8% and remains stable in subsequent cycles, however, the CE starts to decrease dramatically after 4 cycles in the absence of VC (Figure 11b). The CE of the negative electrode with VC present is around 98.4% in the initial cycle and more than 99.4% after three cycles, which is overall higher than that of the VC-free electrolyte (Figure 11d). It indicates that VC as additive can obviously enhance the cycling stability and reduce the irreversible capacity of the flexible paper electrodes.



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Figure 11. Electrochemical performance for electrodes cycled in the electrolyte with VC and without VC at C/10: (a) Specific capacity for 85-11-4 electrodes, (b) CE for 85-11-4 electrodes, (c) Specific capacity for 94-2-4 negative electrodes, (d) CE for 94-2-4 negative electrodes. The electrochemical performances of the paper electrodes as a function of drying time are shown in Figure 12. Non-free standing electrodes containing PVDF binder with Al or Cu foils as current collectors were used as a standard to compare the performances of free-standing flexible electrodes containing TOCNF as binder. Specific capacity and CE were evaluated. The positive and negative electrodes using TOCNF as binder are the 85-11-4 and 94-2-4 electrodes, respectively. 20 ACS Paragon Plus Environment

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Figures 12a-c show the specific capacities of the positive and negative electrodes as a function of drying time. It is clearly seen that the drying time during preparation of the electrodes affects the specific capacities. The specific capacities of the 85-11-4 positive electrode slightly increased with increased drying time, as shown in Figures 12a and 12c, although the effect is relatively small. The capacities of all the 85-11-4 electrodes were found to be higher than 150 mAh g-1 at C/10, close to the theoretical capacity of lithium iron phosphate of 170 mAh g-1, and the cycling was stable over the 20 cycles performed. On the contrary the specific capacity for the negative electrodes decreased with increased drying time as shown in Figures 12b and 12c. Also, it decreased with increasing number of cycles for the electrodes dried for 100h and 287h, whereas a more stable cycling was observed for those dried for a shorter period. The specific capacity of the graphite electrodes was 345 mAh g-1 at C/10 for the sample dried for 1h, which is not far from the capacity of 350 mAh g-1 of the graphite electrode using PVDF binder. It could be ascribed to the longer drying time, as shown in Figure 12d, the negative electrodes have a higher resistance. The electronic conductivity of the negative electrodes as a function of drying time is shown in Figure 12e. When increasing the drying time, the conductivity of the negative electrodes is decreased and this may be explained by a more brittle of TOCNF, not enabling to accommodate the volume expansion of the graphite during cycling, and causing the graphite particles to loose contact. The CE of both positive and negative electrodes as a function of drying time are shown in Figures 12f and 12g. The drying time has no significant effect on CE neither for positive nor negative flexible electrodes. The initial CEs of all of the 85-11-4 positive electrodes were above 97% (Figure 12f). After the first cycle, the CE of the 85-11-4 electrodes rapidly increased to more than 99% and remains high throughout the subsequent cycles. Figure 12g shows no major 21 ACS Paragon Plus Environment


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Figure 12. Electrochemical performances of electrodes using TOCNF and PVDF as binder cycled at C/10 as a function of drying time: (a) Specific capacity for positive electrodes, (b) Specific capacity for negative electrodes, (c) Specific capacity after 20 cycles for both positive and negative electrodes, (d) Charge/discharge voltage profiles for negative electrodes, (e) electric conductivity of negative electrodes, (f) CE for positive electrodes, (g) CE for negative electrodes. Electrodes with PVDF binder were dried for 48 hours. effect of drying time on CE, and electrodes dried for 1h and 287h, respectively, exhibited quite similar CE.

Figure 13. Rate capabilities for the flexible electrodes versus a Li metal electrode: (a) 85-11-4 positive electrode dried for 48h; (b) 94-2-4 negative electrode dried for 1h. Figure 13 shows rate capabilities for both positive and negative electrodes. At low currents, C/10, the specific capacities of positive and negative electrodes approach are 150 mAh g-1 and 350 mAh g-1, respectively. After 6 cycles at C/10, the positive electrode was tested for a further 10 cycles at a high current rate, 1C, obtaining a relatively steady specific capacity of about 110 mAh g-1. When increasing the rate from C/10 to C/4 for the negative electrodes, the specific capacity slightly decreased from 350 mAh g-1 to 300 mAh g-1 due to high polarization at



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increased current density. The negative electrode obtained a capacity of 240 mAh g-1 at C/2, corresponding to about 70% of the capacity at C/10. At high rate 1C, the negative electrode still showed an acceptable capacity of 120 mAh g-1. The specific capacity of both positive and negative electrodes remained stable compared to the first 5 cycles at C/10 when decreasing the testing current rate from 1C back to C/10.

Figure 14. Cycling performance of the electrodes dried for 1h at C/4: (a) 85-11-4 positive electrode, (b) 94-2-4 negative electrode. Figure 14 shows the cycling stability of both positive and negative electrodes dried for 1h at C/4. Very good cycling stability was obtained for the positive electrode as shown in Figure 14a. The discharge capacity was about 146 mAh g-1 overall of the 70 cycles with CE more than 99.9% after first cycle. Figure 14b shows that the cycling performance of the negative electrode. The discharge capacity was approximately 270 mAh g-1 after 50 cycles, which corresponds to approximately 90% retention of the first discharge capacity (300 mAh g-1). It keeps relatively stable in the subsequently cycles. After 3 cycles, the CE of the negative electrode rapidly increases to more than 99% and remains high throughout the subsequent cycles. These results illustrate that a short drying time of 1h is enough for obtaining stable cycling performance. A 24 ACS Paragon Plus Environment

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short drying time is advantageous in an industrial production, making the process of producing the electrode simpler.

CONCLUSIONS Flexible positive and negative electrodes for Li-ion batteries using low amount of TEMPOoxidized CNFs as binder can be successfully produced through a water-based paper making process. Only 4 wt% finely dispersed TOCNF was used to prepare the electrodes, still with good mechanical properties. A failure mechanism for electrodes containing a high amount of TOCNF is the formation of compact cellulose films, lowering the electronic conductivity of the electrodes. Adding VC to the electrolyte has a positive effect on specific capacities and CE for both LiFePO4 and graphite electrodes. Increased drying time during preparation of the electrodes slightly improves the electrochemical performance of the LiFePO4 electrodes, but has a clear negative effect on the graphite electrodes. No obvious influence on CE is observed by the varying drying time. Drying the electrodes for 1h in vacuum at 110 oC is enough to obtain a good cycling stability for Li-ion batteries.

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

ACKNOWLEDGMENTS 25 ACS Paragon Plus Environment


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The China Scholarship Council (CSC) is greatly appreciated for financial support. Nicholas Tchang Cervin is acknowledged for providing TEMPO-oxidized CNF and Huanhuan Xu, Tobias Benselfelt, Malin Nordenström and Yuanyuan Li for professional help with SEM, AFM, DLS and TGA measurements.

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