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

Temperature (℃ ). NBSK/PSA/NFC composite membranes. Celgard 2350 (vertical direction). Celgard 2350 (horizontal direction). 9. The sustainable LIB s...
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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

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thermal resistant separators for lithium ion batteries

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

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Xingye An1*, Hongbin Liu1*

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1

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

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E-mail: [email protected]

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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

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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.

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ABSTRACT

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

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

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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

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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

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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

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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

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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

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

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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

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

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

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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

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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.

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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

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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).

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

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

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

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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

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

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

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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,

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

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

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

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

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

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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

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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

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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.

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

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

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following equation:

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

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accordance with TAPPI Test Method T 494.

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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

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with the following equation:

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

(2)

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

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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,

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as the following equation:

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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

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(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

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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

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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

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

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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

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

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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

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

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

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

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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

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

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

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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

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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

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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)

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

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

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

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

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

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

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

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

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

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2350 (236s) separator.

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Pore size distribution

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

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

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

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

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

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

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

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

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

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

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

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

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charge.

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

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

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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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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%)

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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

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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

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-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

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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

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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

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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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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

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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

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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.

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