Nanofibrillated Cellulose (NFC) as a Pore Size Mediator in the

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Nano-fibrillated cellulose (NFC) as a pore size mediator in the preparation of thermal resistant separators for lithium ion batteries Hao Zhang, Jing Liu, Min Guan, Zhen Shang, Yiwei Sun, Zonghong Lu, Hailong Li, Xingye An, and Hongbin Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04203 • Publication Date (Web): 25 Feb 2018 Downloaded from http://pubs.acs.org on February 25, 2018

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Nano-fibrillated cellulose (NFC) as a pore size mediator in the preparation of

2

thermal resistant separators for lithium ion batteries

3

Hao Zhang1, Jing Liu1, Min Guan1, Zhen Shang1, Yiwei Sun1, Zonghong Lu1, Hailong Li1,

4

Xingye An1*, Hongbin Liu1*

5

1

6

29, 13th Street, TEDA, Tianjin, P.R.China, 300457

7

E-mail: [email protected]

8

Graphical Abstract:

Tianjin Key Laboratory of Pulp and Paper, Tianjin University of Science and Technology, No.

NFC

10 NBSK

8%NFC

NFC acts as a pore size mediator

PSA

Percent (%)

8 6 4 4%NFC

2 Celgard 2350

0

0.3

NBSK/PSA/NFC composite membrane

Sustainable product

0.6

NBSK/PSA/NFC composite membranes Celgard 2350 (vertical direction) Celgard 2350 (horizontal direction)

80 60 40 20 0

Stable thermal resistance 120

0%NFC

Narrow pore size distribution

140 160 Temperature (℃ )

180

0.9 1.2 Pore size (µm) Capillary absorption height (mm)

Papermaking progress

Shrinkages (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

50

1.5

1.8

NBSK/PSA/NFC composite membrane Celgard 2350

40 30 20 10 0

0

10

20

30

40

50

60

Time (min)

9 10 11

The sustainable LIB separator was prepared by adding NFC to control and optimize the pore size through a wet-laid process.

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT

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Battery separators play a vital role in the safety, sustainability, and electrochemical

14

performance of lithium ion batteries (LIBs). In this work, thermal resistant composite membranes

15

were fabricated by a wet-laid process using northern bleached softwood kraft (NBSK) fibers,

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polysulfonamide (PSA) fibers and nano-fibrillated cellulose (NFC), for lithium ion battery

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applications. NFC functioned as a mediator to control and optimize pore size in the composite

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membranes, while the PSA fibers provided superior heat resistance to the membranes. The

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as-prepared composite separator membranes have more uniform micro pores, superior thermal

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stability and electrolyte absorption in comparison with a commercial separator membrane

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(Celgard 2350).

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Keywords: Nano-fibrillated cellulose (NFC); Lithium ion battery separator; Pore size mediator;

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Thermal resistant; Electrolyte absorption

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INTRODUCTION

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Lithium ion battery (LIB) has been widely used in powering many portable electronic

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devices, including laptop, digital camera, cellphone, and some other devices due to its high

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specific energy, efficiency and energy density, long cycle life, fast recharge rate and low

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environmental pollution.1-3 As a key element, LIB separator has crucial functions of physically

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separating the anode and cathode while permitting free flow of lithium ions, which can directly

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affect discharge rate, recycle times and safety characteristics.4 Polyolefin (polyethylene or

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polypropylene) based membranes have been widely used as the separators for LIBs because of

32

their good electrochemical stability, proper thickness, and mechanical strength.5

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However, there are some limitations, for example, polyolefin based separators have poor


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electrolyte absorption and sharp dimensional shrinkage at elevated operating temperature.6 Poor

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electrolyte absorption would lower charge-discharge performance of battery due to the limitation

36

of ionic transportation.7 Severe dimensional instability may cause internal short-circuiting or lead

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to thermal runaway especially for the batteries at high charge/discharge current.8 In addition,

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polyolefin are petroleum based products, which are not renewable or biodegradable.

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Cellulose is a natural polymer with the most widely distribution and the largest reserves on

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the earth. Compared to synthetic polymers, cellulose has many advantages, like fully

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biodegradability, non-toxic, non-pollution, easy modification, good biocompatibility and

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renewable resource.9,10 It has become one of the main raw materials for energy and chemical

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industries. In addition to high yield and easy processing, cellulose has some unique properties:

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inter- and intra- molecular hydrogen bonds, good heat resistance, electrolyte absorption,

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resistance to chemical solvents, and electrochemical stability. Cellulose membranes prepared by

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the low-cost papermaking process have a great potential to replace conventional polyolefin

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materials for lithium ion battery applications. It was reported that mesoporous cladophora

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cellulose membranes are good LIB separators.11 The cellulose separators with a thickness of 35

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mm and an average pore size of 20 nm were readily wetted by electrolyte LP40, and thermally

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stable at 150 °C. Cells with these separators showed stable cycling and high coulombic

51

efficiencies.

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As a new-type of renewable material, nano-fibrillated cellulose (NFC) has been used in

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many applications in a diverse range of fields, owing to its large and active surface area, good

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biodegradability, low thermal expansion and shrinkage, high strength and good chemical

55

durability.12,13 A NFC separator14 comprised of polyethylene terephthalate (PET) layer and



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cellulose nanofibers layer exhibited higher porosity (70%), and it could be wetted in a few

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seconds, which showed superior electrolyte absorption compared to polypropylene (PP)

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separators. A nano-fiber reinforced microporous polyvinylidene fluoride (PVDF) composite

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separator15 had a high tensile strength exceeding 30 MPa due to the extensive entanglement of the

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tough cellulose fibrils and showed an improved thermal stability as the fact that it maintained its

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original size when the temperature was increased up to 250 °C. Likewise, with tunable

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nanoporous network channels, a cellulose nanofiber paper-derived (CNP) separator16 showed a

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lower air permeability of 487 s, reflecting a high ionic conductivity of 0.77×10−3 S·cm−1, which

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delivered the excellent discharge rate capability.

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Polysulfonamide (PSA) fibers are widely used for insulation paper in the electrical

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machines.17 Xu Q et al (2014) explored to fabricate cellulose/PSA composite membrane via a

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facile papermaking process as LIB separators.18 The electrolyte-soaked cellulose/PSA composite

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membrane delivered superior ion conductivity (1.2×10−3 S·cm−1) and maintained the original

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dimension after 200°C for 0.5 h. Yue L et al (2014) developed a heat resistant and flame-retardant

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PSA/ PP composite nonwoven separator with better inflame retarding properties.19 The discharge

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capacity retention after 30 cycles was found to be 90% at an elevated temperature of 120°C.

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The objective of the present study was to fabricate thermal resistant NBSK/PSA/NFC

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composite membranes for lithium ion battery applications, via a papermaking process due to the

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excellent and special merits of PSA (thermal stable) and cellulose (sustainable), especially NFC

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(favorable length to width ratio and nano dimension). It is desirable to have a uniform pore size

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of the battery separator, and the function the NFC in the preparation of NBSK/PSA/NFC

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composite membrane is to fill the large pore size of the NBSK/PSA network as a mediator, so that


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the resultant membrane has a decreased pore size and more importantly, much improved pore size

79

distribution. In addition, the PSA fibers provided superior heat resistance to the membranes. The

80

effect of NFC content on the micro structure and pore size distribution was investigated. The

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resultant NBSK/PSA/NFC composite separators were characterized in terms of pore size

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distribution, tensile strength, electrolyte absorption, thermal stability, ionic conductivity and cycle

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performance.

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EXPERIMENTAL

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Materials

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NFC used in the experiments, as shown in Figure 1, was supplied by Tianjin Wood Elf

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Biological Technology Co., LTD. (Tianjin, China). It can be distinctly observed from Figure 1

88

that the NFC sample shows homogeneous dispersion as well as uniform dimensions. NFC sample

89

shown in the TEM image (Figure 1) has length of several micrometers and width of 20-30

90

nanometers.

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NBSK fibers were provided by a paper mill in Jiangsu province, China. PSA fibers were

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purchased from Shanghai Jufeng Insulation Material Co., Ltd., China. 1 M lithium

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hexafluorophosphate (LiPF6) in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1/1, v/v)

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(Guotai-huarong New Chemical Materials Co., Ltd., China), n-butyl alcohol (Sinopharm

95

Chemical Reagent Co.,Ltd., China), and a commercial separator (Celgard 2350) were purchased

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from the respective suppliers. The materials were used without further purification.



0.5µm 97 98 99

Figure 1 TEM image of NFC used in the experiments Preparation of the NBSK/PSA/NFC composites

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The experimental procedure for preparing the NBSK/PSA/NFC composite membranes by

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following the papermaking process. The softwood pulp was refined by a PFI beater at a

102

concentration of 10% (w/w) to a range of refining degrees from 60°SR to 92°SR. The refined

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softwood pulp and PSA fibers were mixed with the proportion of 1/0, 4/1, 3/1, 2/1, 1/1 (w/w).

104

The NFC after disintegration in the fiber deflaker, was added to the well-dispersed (also achieved

105

by using the fiber deflaker) fiber mixtures of refined softwood pulp and PSA fibers at the

106

percentage of 0%, 4%, 8%, 12% (w/w), respectively, and was kept stirring (in the stirrer) to

107

obtain homogeneous suspensions. Finally, the slurry was added into a lab sheet-former to form a

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wet NBSK/PSA/NFC composite membrane. The wet composite was subsequently pressed at 5

109

MPa for 5 min of each side. After that, the composite was dried under the vacuum at 95 °C for 10

110

min then, calendered at 120 °C under 2 MPa on a lab scale calender. The basis weight of the

111

obtained composite membrane is 40 g·m−2.


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Characterization

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The air permeability of the samples was determined with a Gurley-type densometer (4110N,

114

Gurley). It is typically characterized using the Gurley value, which was measured by the amount

115

of time to allow 100 mL of air pass through 6.4 sq. cm. (l sq. in.) circular area of the sample

116

under a pressure difference of 1.22 kPa according to TAPPI Test Method T 460.

117

The pore size distribution and the minimum, mean and maximum pore size were tested by

118

capillary flow pore size analyzer (Porolux 100, Porometer). The pore size was calculated by the

119

surface tension of a particular liquid (porefil, 16 N/m) and the corresponding pressure of the

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nitrogen blowing through the pores.

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The microstructures of samples were observed by JEOL JSM-IT300LV scanning electron

122

microscope (SEM). The surface of the sample was gold coated, and the accelerating voltage of

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SEM was 15 kV.

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The morphology of NFC was characterized by a JEM 2010F transmission electron

125

microscope (TEM) operated at an accelerating voltage of 200 keV. NFC was transferred to a

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carbon-coated copper gird and air-dried before testing.

127

The porosity of separator membranes was tested using the method of n-butyl alcohol

128

immersion: immersing the sample in n-butanol 1 h. The porosity was obtained using the

129

following equation:

130 131 132 133

porosity = (ma/ρa) / (ma/ρa + mb/ρb) × 100%

(1)

where ma and mb were the mass of n-butanol and the sample, and ρa and ρb were the density of n-butanol and the sample, respectively. Tensile strength was evaluated using a tensile strength tester (L&W, Sweden). The samples



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134

were cut to specimen trips of 100 mm in length and 15 mm in width and then measured in

135

accordance with TAPPI Test Method T 494.

136

The electrolyte uptake of membranes was determined with the weights of separators before

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and after soaking in the electrolyte (1 M LiPF6 in EC/DMC (1/1, v/v)) for 2 hours, in accordance

138

with the following equation:

139

Electrolyte uptake (%) = (Wb – Wa) / Wa × 100%

(2)

140

where Wa and Wb are the weights of sample before and after soaking in the electrolyte. The

141

capillary absorption height of electrolyte was determined through the rising distance of samples

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for specific time in the electrolyte (1 M LiPF6 in EC/DMC (1/1, v/v)).

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Thermal shrinkage of separators was evaluated by calculating the area of dimensional

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change after heat treatment in an oven for 1h, at temperature of 120 °C, 140 °C, 160 °C or 180 °C,

145

as the following equation:

146

Thermal shrinkage (%) = (Sa – Sb) / Sa × 100%

(3)

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where Sa and Sb are the area of sample before and after heat treatment.

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The ionic conductivity of the liquid electrolyte-absorbed separator between two

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stainless-steel plate electrodes was evaluated using the electrochemical impedance spectroscopy

150

(EIS) measurement by applying an AC voltage of 20 mV amplitude in the frequency range of 1 to

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1 × 106 Hz. A unit cell (2032-type coin) was assembled by sandwiching the sample between a

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LiCoO2 cathode and a natural graphite anode, and then activated by filling it with the liquid

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electrolyte of 1 M LiPF6/EC+DMC (1:1, v/v). All assembly of cells was carried out in an

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argon-filled glovebox. The discharge current densities were varied from 0.2 to 8.0 C under a

155

voltage range between 2.75 and 4.2 V. The cells were cycled at a fixed charge/discharge current


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density of 0.5 C/0.5 C for cycle performance testing.

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RESULTS AND DISCUSSION

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Concept of preparing NBSK/PSA/NFC composite membranes with reduced pore size and

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uniform pore size distribution

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The main constituents of the composite membrane are cellulose and PSA fibers. The NBSK

161

fibers used are well refined, due to fibrillation, the hydroxyl and carboxyl groups on the NBSK

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fiber surfaces are ready to form hydrogen bonds, thus, imparting good strength properties and

163

surface attributes. In addition, the composite materials have good heat resistance, electrolyte

164

absorption, resistance to chemical solvents, and electrochemical stability. On the other hand, the

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NBSK/PSA fiber network has a large pore size, and a wide distribution of these pores, which

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would not be desirable for LIB separators.

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Our solution to the problem would be to use NFC as a mediator to decrease and optimize the

168

pore size. NFCs have favorable length to width ratio20, with the length of several micrometers

169

and the width in the range of 10~20 nm. They exhibit a particularly high specific surface area,

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excellent flexibility, and contain a high amount of hydroxyl groups.21,22 Thanks to these unique

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morphological features, NFCs are effective in filling in the large pores created by the networks of

172

NBSK/PSA fibers, as shown in Figure 2. Therefore, one function of these thin and flexible NFCs

173

would be as fillers for these pores. In the literature, Dai et al (2017) have found that NFCs can be

174

used as fillers to obtain guar gum/CNFs nano composite films with increased barrier properties

175

against gases, such as oxygen.23

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Another function of NFCs is to increase the strength properties of the NBSK/PSA/NFC

177

composite membrane. This is because NFCs can form tortuous and entangling network. In



178

addition, the abundant hydroxyl and carboxyl of the NFCs would form more hydrogen bounds in

179

the NBSK/PSA/NFC composites.

10

8% NFC 5µm

8

4% NFC

Percent (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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6 5µm

4

0% NFC 2

5µm

0 0.3

0.6

0.9

1.2

1.5

1.8

Pore size (µm) 180 181

Figure 2 Schematic of the NBSK/PSA/NFC composite membrane with reduced pore

182

size and narrow pore size distribution: NFCs are filling in the large pores created by the

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NBSK/ PSA fibers, thanks to their high length/ width ratio; consequently, the resultant

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NBSK/PSA/NFC composite membrane exhibit reduced pore size and narrow pore size

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distribution.


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Air permeability

300 Gurley value (s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


200

100 0

4

8

12

NFC content (%) 187 188 189

Figure 3 Air permeability of NBSK/PSA/NFC membranes at various NFC contents (the beating degree of NBSK was 80 °SR and the NBSK/PSA ratio was=3:1)

190

Air permeability of LIB separators is related to the porous structure of the membranes.16

191

Figure 3 shows that the addition of NFC increased the Gurley value significantly. NFC fibers

192

have very high aspect ratio and nano-scale diameter. These nano fibrils materials can fill the

193

voids/pores creased by the NBSK/PSA fiber networks (as illustrated in Figure 2), which would

194

lead to the reduced air permeability. When the beating degree of the softwood pulp was 80°SR

195

and the mass ratio of softwood pulp and PSA was 3:1, the air permeability decreased by 54.3%

196

with 8% of NFC, and it decreased by 64.7% with 12% NFC. When the NBSK beating degree was

197

80 SR, the NBSK/PSA ratio was 3:1 and the NFC content was 8%, the air permeability of the

198

NBSK/PSA/NFC composite membranes was 229s, which was similar to the commercial Celgard

199

2350 (236s) separator.

200

Pore size distribution

201

Determination of the pore size can be more intuitive to see the true size of the pores of



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202

separators.24 And the microstructure of the separator has a crucial influence on battery

203

performance and of the transport of lithium ions.16 Table 1 shows that the pore size of the

204

NBSK/PSA/NFC composite membranes decreased markedly with the increasing of NFC addition.

205

When the NFC dosage was 8%, the mean pore size of the composite membranes decreased to

206

0.323µm, which was smaller than that for a commercial separator (Celgard 2350) with a mean

207

pore size of 0.555µm. The NFC acted as a pore size “mediator” to control and optimize the pore

208

size and its distribution, due to its favorable length to width ratio, particularly its high specific

209

surface area, excellent flexibility, and rich hydroxyl groups. More uniform and smaller pore

210

created the more current transport. Furthermore, the porosity results of NBSK/PSA/NFC

211

composite membranes with the addition of NFC were shown in Table 1, which were higher than

212

that of Celgard 2350, supporting that NFC created more channels for lithium ion to transfer

213

charge.

214

Table 1 The porosity and maximum, mean and minimum pore size of NBSK/PSA/NFC

215

composite membranes at various NFC contents (For the composite membrane, the beating

216

degree of NBSK was 80 °SR and the NBSK/PSA ratio was=3:1) NBSK/PSA/NFC composite membranes

Celgard

Samples 0%

4%

8%

2350

Maximum pore size (µm)

5.18

3.588

1.76

3.17

Minimum pore size (µm)

0.219

0.200

250°C).35 PSA fibers also have high melting point and thermal degradation

285

temperature, which let them keep morphologic and physical characteristics at a temperature over

286

300 °C.36 (a)

NBSK/PSA/NFC composite membranes

25℃ ℃ 120℃ ℃ 140℃ ℃

(b)

Celgard 2350 In vertical direction

℃ 160℃ ℃ 180℃

(c)

Celgard 2350 In horizontal direction

℃ 160℃ ℃ 25℃ ℃ 120℃ ℃ 140℃ ℃ 180℃ 25℃ ℃

120℃ ℃

140℃ ℃

160℃ ℃

180℃ ℃

287 288

Figure 6 Images of (a) the NBSK/PSA/NFC composite membranes and Celgard 2350


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289

separator in (b) vertical direction and (c) horizontal direction heated at different

290

temperature for 1h (For the composite membrane, the beating degree of NBSK was 80 °SR,

291

the NBSK/PSA ratio was=3:1, and the NFC content was 8%)

292

Figure 6 shows the appearance of the NMSK/PSA/NFC composite membranes and the

293

Celgard 2350 separator after 1-hour heat treatment at various temperatures (25, 120, 140, 160 and

294

180 °C). It can be seen that the NBSK/PSA/NFC composite membranes had negligible dimension

295

change after the heat treatment, while the Celgard 2350 separator shrunk markedly and became

296

transparent when the heat treatment temperature was 140°C or higher. Moreover, it exists

297

potential dangerous to the differentiated shrinkage at the vertical and horizontal directions of

298

Celgard 2350 (Table 2, Figure 6(b) and (c)), probably due to unequal stretches in machine and

299

cross direction in the production process.37 The intrinsic heat resistance of cellulose materials38

300

and PSA fibers, as well as the uniform pores and fiber array enable the composite membranes to

301

retain properties and dimensional stability at elevated temperature.

302

Ionic conductivity Celgard 2350 NBSK/PSA/NFC composite membrane Linear Fit of Celgard 2350 Linear Fit of NBSK/PSA/NFC composite membrane

-2.2 -2.4

logσ (S·cm-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-2.6 -2.8 -3.0 -3.2 -3.4 2.6

2.8

3.0

3.2

3.4

-1

1000/T (K ) 303 304

Figure 7 Arrhenius plots of NBSK/PSA/NFC composite membrane and Celgard 2350

305

saturated with electrolyte (For the composite membrane, the beating degree of NBSK was



80 °SR, the NBSK/PSA ratio was=3:1, and the NFC content was 8%)

306 307

The Arrhenius plot in Figure 7 describes NBSK/PSA/NFC composite membrane and

308

Celgard 2350, reflecting the ionic conductivity of separators.38 The ionic conductivity at 20 °C

309

was 0.68 and 1.58 mS cm−1 for NBSK/PSA/NFC composite membrane and Celgard 2350,

310

respectively. The superior ionic conductivity of NBSK/PSA/NFC composite membrane is

311

significant compared with that of Celgard 2350 from 20℃ to 100℃. It is attributed to the

312

addition of NFC, which was beneficial to improve the ion passing efficiency and lower the

313

resistance of separator, thanks to the favorable results associated with permeability, porosity, pore

314

size distribution and electrolyte absorption of NBSK/PSA/NFC composite membrane, as

315

discussed in earlier sections.

316

Cycle performance in assembled battery

(b) 120 100 80 60 40 Celgard 2350 NBSK/PSA/NFC composite membrane

20 0

0

20

40

60

80

100

Discharge Capcity (mAhg-1)

(a) Discharge Capcity (mAhg-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 26

140 0.2C 0.5C

120

1C

2C

100

4C

80

8C

60 40

Celgard 2350 NBSK/PSA/NFC composite membrane

20 0

0

Cycle Number

5

10

15

20

25

30

Cycle Number

317 318

Figure 8 (a) Cycle performance and (b) rate capability of the cells using

319

NBSK/PSA/NFC composite membrane and Celgard 2350 (For the composite membrane,

320

the beating degree of NBSK was 80 °SR, the NBSK/PSA ratio was=3:1, and the NFC

321

content was 8%)

322

Cycling performance of the cells with electrolyte-absorbed NBSK/PSA/NFC composite

323

membrane and Celgard 2350 at charge/discharge rate of 0.5 C/0.5 C was depicted in Figure 8(a).


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324

The obtained discharge capacity of the cell with the NBSK/PSA/NFC composite membrane after

325

100 cycles was 93.2 mAh·g-1 compared with the original discharge capacity (118.2 mAh·g-1),

326

indicative of capacity retention at 78.8%, which is compared to 71.5% of Celgard 2350.

327

Obviously, the cycling performance of the cell with NBSK/PSA/NFC composite membrane was

328

better than that of Celgard 2350. The above results would be attributed to better liquid electrolyte

329

adsorption and pore size distribution.

330

Figure 8(b) showed the rate capability of the cells with electrolyte-absorbed

331

NBSK/PSA/NFC composite membrane and Celgard 2350. The cells with the NBSK/PSA/NFC

332

composite membrane exhibited much better rate capability as compared to the Celgard 2350 at

333

various rates. For example, the NBSK/PSA/NFC composite membrane kept a specific capacity of

334

118.2 mAh·g-1 at 0.5 C, whereas the specific capacity of the Celgard 2350 was 114.3 mAh·g-1.

335

The specific capacity of the cells using the NBSK/PSA/NFC composite membrane and Celgard

336

2350 at other various rates was 108.7 and 101.1 mAh·g-1 at 1.0 C, 101.7 and 94.7 mAh·g-1 at 2.0

337

C, 87.9 and 81.0 mAh·g-1 at 4.0 C, 65.5 and 53.5 mAh·g-1 at 8.0 C, respectively. The enhanced

338

rate capability was ascribed to lower interfacial resistance and higher ionic conductivity of the

339

electrolyte-absorbed NBSK/PSA/NFC composite membrane.

340

CONCLUSIONS

341

In this work, porous composite membranes were successfully fabricated using NBSK fibers,

342

PSA fibers and NFC in a papermaking process, for potential applications as the separators for

343

lithium ion batteries. The composite membranes exhibited a higher electrolyte uptake (294%) and

344

a higher electrolyte wetting rate, compared with a commercial lithium ion battery separator

345

(Celgard 2350). More importantly, it was found that the NFC addition decreased the pore size and



346

narrowed the pore size distribution markedly, which are highly desirable for lithium ion battery

347

application. In addition, the NBSK/PSA/NFC composite membranes had superior thermal

348

resistance. No dimensional change was observed after heat treatment at 180 °C for an hour. The

349

outstanding ionic conductivity and cycle performance in cells of the composite membranes is

350

desirable for lithium ion battery applications32.

351

ACKNOLEDGEMENTS

352

The authors would like to acknowledge the financial support from the National Key

353

Research and Development Plan (Grant 2017YFB0307902), the National Natural Science

354

Foundation of China (Grant 31670589).

355

SUPPORTING INFORMATION

356

Capillary absorption height of electrolyte in the NBSK/PSA/NFC composite membrane and

357

Celgard 2350 separator after 60 min

358

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NFC

10 NBSK

8%NFC

NFC acts as a pore size mediator

PSA

Percent (%)

8 6 4 4%NFC

2 Celgard 2350

0

0.3

NBSK/PSA/NFC composite membrane

Sustainable product

0.6

NBSK/PSA/NFC composite membranes Celgard 2350 (vertical direction) Celgard 2350 (horizontal direction)

80 60 40 20 0

Stable thermal resistance 120

0%NFC

Narrow pore size distribution

140 160 Temperature (℃)

180

0.9 1.2 Pore size (μm) Capillary absorption height (mm)

Papermaking progress

Shrinkages (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 26

50

1.5

1.8

NBSK/PSA/NFC composite membrane Celgard 2350

40 30 20 10 0

0

10

20

30

40

50

60

Time (min)

The sustainable LIB separator was prepared by adding NFC to control and optimize the pore size through a wet-laid process.


Nanofibrillated Cellulose (NFC) as a Pore Size Mediator in the

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