Electrospun cellulose polymer nanofiber membrane with flame resistance properties for lithium-ion batteries

Journal Pre-proof Electrospun Cellulose Polymer Nanofiber Membrane with Flame Resistance Properties for Lithium-Ion Batteries Yue Chen, Linlin Qiu, Xiangyu Ma, Lika Dong, Zhengfei Jin, Guangbo Xia, Pingfan Du, Jie Xiong

PII:

S0144-8617(20)30081-3

DOI:

https://doi.org/10.1016/j.carbpol.2020.115907

Reference:

CARP 115907

To appear in:

Carbohydrate Polymers

Received Date:

1 September 2019

Revised Date:

20 January 2020

Accepted Date:

20 January 2020

Please cite this article as: Chen Y, Qiu L, Ma X, Dong L, Jin Z, Xia G, Du P, Xiong J, Electrospun Cellulose Polymer Nanofiber Membrane with Flame Resistance Properties for Lithium-Ion Batteries, Carbohydrate Polymers (2020), doi: https://doi.org/10.1016/j.carbpol.2020.115907

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Electrospun Cellulose Polymer Nanofiber Membrane with Flame Resistance Properties for Lithium-Ion Batteries

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Yue Chen, Linlin Qiu, Xiangyu Ma, Lika Dong, Zhengfei Jin, Guangbo Xia, Pingfan Du1,2,3,*, Jie Xiong1,2,* 1 College of Textile Science and Engineering, Zhejiang Sci-Tech University, Hangzhou 310018, PR China 2 Key Laboratory of Advanced Textile Materials and Manufacturing Technology (Ministry of Education), Zhejiang Sci-Tech University, Hangzhou 310018, PR China 3 Zhejiang Provincial Key Laboratory of Fiber Materials and Manufacturing Technology, Zhejiang Sci-Tech University, Hangzhou 310018, PR China

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*Corresponding authors. Tel.: +86 571 86849370; fax: +86 571 86843603. E-mail addresses: [email protected] (P. Du); [email protected] (J. Xiong).

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Graphical abstract

Highlights 

PVDF/TPP/CA composite membranes are prepared by one-step electrospinning

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PVDF/TPP/CA composite membranes show flame resistance properties

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PVDF/TPP/CA membranes have improved electrolyte wettability and thermal stability

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PVDF/TPP/CA-based batteries have high capacities and better rate performances 1

Abstract Current developments of lithium-ion batteries (LIBs) are mainly focused on improving security and cycle performance. Herein, a novel polyvinylidene fluoride (PVDF) / triphenyl phosphate (TPP) / cellulose acetate (CA) nanofiber membrane was fabricated by one-step electrospinning and used as separator in lithium-ion batteries. Compared to traditional polyethylene membrane, the obtained composite showed

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higher porosity, elevated thermal stability, superior electrolyte wettability, and

improved flame resistance. In addition, batteries assembled with PVDF/TPP/CA membrane exhibited excellent electrochemical properties and cycle stability. The

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enhanced performances were attributed to the porous structure and presence of CA and

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TPP. Overall, the proposed hybrid organic cellulose-based composite polymer

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membranes look promising as separators for advanced LIBs. Keywords: Polyvinylidene fluoride; Cellulose acetate; Flame resistance; Lithium-ion batteries

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1. Introduction

Lithium-ion batteries (LIBs) are promising secondary power sources due to their

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excellent qualities, such as high energy density, portability, stable cycling performance,

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and no memory effect (Liu et al., 2017; Li et al., 2018; Shen et al., 2018). As a result, LIBs have been successfully applied in smart electronic devices, hybrid electric vehicles, and other fields (Kim, Kwon, Yim, & Choi, 2019). However, LIBs still suffer from some drawbacks and require improvement in terms of energy density, prolonged life, and cost (Susai et al., 2018). In addition, explosion accidents related to laptops and cellphones due to battery failure have raised safety concerns that require solutions (Zhai 2

et al., 2014). One reason behind the safety hazards of LIBs is linked to failure of the membranes under certain conditions, such as high temperature exposure, crash, or penetration (Zhong et al., 2019). Therefore, the membrane placed between the cathode and anode to prevent internal short circuits requires high temperature resistance, appropriate mechanical strength, and even flame resistance properties (Wang, Wu, Chiu, & Chou, 2019). On the other hand, high porosity and superior electrolyte wettability

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are basic qualities required for membranes since they provide ionic channeling filled

with electrolytes (Boriboon, Vongsetskul, Limthongkul, Kobsiriphat, & Tammawat, 2018; Liu et al., 2018). Currently commercialized membranes are mainly made of

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polyethylene (PE), polypropylene (PP), and their combinations (Li et al., 2018). The

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membranes based on polyolefin polymers are advantageous in terms of high tensile, low cost, and advanced electrochemistry. However, low melting point and poor

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wettability limit their applications in large-size high-power batteries or elevated

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working temperatures (Dong et al., 2019; Li et al., 2019). Electrospinning is an efficient technology for producing membranes with high

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porosity and large specific surface area for upgraded electrolyte uptake and ionic conductivity (Yoon, Yang, Lee, & Yu, 2018; Zhao et al., 2019). Many types of high

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molecular polymers have so far been investigated as membrane materials, including poly-m-phenyleneisophthalamide (PMIA) (Kang et al., 2016), polyethersulfone (PES) (Liu, Ma, Hsiao, Chu, & Tsou, 2016), polysulfonamide (PSA) (Peng, Wang, & Ji, 2017), polyacrylonitrile (PAN) (Lee, Manuel, Choi, Park, & Ahn, 2015), and Polyimide (PI)(Lee, Lee, Park, & Kim, 2014). In particular, polyvinylidene fluoride (PVDF) 3

possesses numerous C-F chemical bonds, leading to good chemical stability, high dielectric constant, and low surface energy. Hence, it received increasing attention as a promising host for electrolyte in LIBs (Lee & Liu, 2017). Nevertheless, PVDF membrane suffers from several drawbacks like hydrophobic surface and high crystallinity, leading to poor electrolyte retention and low ionic conductivity (Asghar et al., 2018; Wang et al., 2019; Zuo et al., 2018).

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Cellulose is the most abundant resource on earth with biocompatibility properties, renewability, and low cost (Guo, Song, Jin, Sun, & Li, 2019; Pankonian, Ounaies, & Yang, 2011). Hence, cellulose and its derivatives like cellulose acetate (CA) have

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gained increasing attention as alternatives for LIBs membranes. Membranes made of

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cellulose have great electrolyte and electrolyte uptake capabilities. Their high initial decomposition temperature (above 270˚C) also leads to superior thermal stability,

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renewability, and elevated thermal-resistant (Weng, Xu, Alcoutlabi, Mao, & Lozano,

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2015; Zhang et al., 2013). Thus, the addition of CA may improve the ionic conductivity and thermal stability of pure PVDF membrane.

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Meanwhile, triphenyl phosphate (TPP) was introduced to produce membranes with flame retardant properties. TPP is a popular flame retardant for scavenging hydrogen

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radicals from flames, hence inhibiting chain reaction and flame propagation (Högström et al., 2014) PVDF/TPP/CA composite membrane was then prepared by one-step electrospinning (Fig. 1). The as-obtained membrane exhibited higher porosity, better electrolyte uptake and enhanced ionic conductivity than PE and pure PVDF membranes. Moreover, the cells assembled with this composite membrane exhibited stable 4

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electrochemical properties and cycle performances.

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2. Experimental 2.1 Materials

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Fig. 1 The schematic illustration of the preparation of electrospun composite membrane and battery assembling

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Poly (vinylidene fluoride) (PVDF, Arkema) was dried under vacuum at 80˚C for 24h. Cellulose acetate (CA) and triphenyl phosphate (TPP) were purchased from

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Macklin reagent. N, N-dimethylformamide (DMF, AR, 99.5%), acetone (AR,99.5%) and n-butanol were all from Sigma-Aldrich and used as received without further

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treatment. Lithium hexafluorophosphate (LiPF6, 1M) in ethylene carbonate and ethyl methyl carbonate (EC + EMC + DEC, 1:1:1 by volume) was supplied by Ferro Corporation. Microporous polyethylene (PE) membranes were provided by Celgard and used for comparison. 2.2 Preparation of composite membranes 5

PVDF was first dissolved in DMF/acetone (7/3, wt/wt) at concentration of 15%. Certain amounts of CA and TPP were then added into the solution and stirred for 12h at 40˚C Next, the solution was electrospun at high voltage of 18 kV, distance of 20 cm, and flow rate of 0.6 ml/h. The obtained membranes were dried in a vacuum oven at 80˚C for 12h. For comparison, pure PVDF (15%) membrane and PVDF/TPP composite membrane with the same content as TPP were synthesized by the same method.

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2.3 Characterization

The microstructures and morphologies of the composite membranes were

characterized by field emission scanning electron microscopy (FE-SEM, HITACHI S-

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4100). Energy dispersive X-ray (EDS) connected to SEM was employed to investigate

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the elemental composition of the composite fibers. Nano-measurer software was employed to collect distribution of fiber diameter from the SEM images. Fourier

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transform-infrared spectrometry (FT-IR, Nicolet-5700 spectrometer) was utilized to

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collect the functional groups present on the membranes. Contact angles between membranes and liquid electrolyte were conducted by means of JY-82B video contact

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tester (Chengde Dingsheng Testing Machine Co., Ltd.). The mechanical strengths of the membranes were evaluated by tensile instrument

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(KES-G1, Kato, Japan). The porosity (P) of each electrospinning membrane was measured by immersing the membrane in n-butanol followed by calculation using Eq. (1): P=

𝑚𝑤 −𝑚𝑑 𝜌𝑏 𝑉

× 100%

(1)

where mw and md are respectively the weight of the membrane after and before 6

immersion in n-butanol, ρ b is density of n-butanol, and V is total volume of the membrane. Electrolyte uptake (EU) was calculated according to Eq. (2): EU =

𝑚−𝑚0 𝑚0

× 100%

(2)

where m and m0 are the weights of the membrane after and before soaking in electrolyte, respectively.

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The thermal dimensional stabilities of the membranes were tested after treating the membranes at different temperatures for 30 min.

The ionic conductivity (σ) of each membrane was evaluated in the frequency range

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from 0.01Hz to 100kHz by Electrochemical Impedance Spectroscopy (Zahner Zennium

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Electrochemical Analyzer). The symmetric cells with membranes sandwiched between two stainless steel electrodes were prepared, and conductivity was calculated using Eq.

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(3):

ℎ

(3)

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σ = 𝑅𝑆

where h, R and S are the thickness of the membrane, bulk resistance and contact area

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of stainless steel electrodes, respectively. The interfacial stabilities of the membranes and lithium anodes were measured by

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EIS method. Linear sweep voltammetry (LSV) in the voltage range of 3-5.5 V and potential scanning rate of 1mVs-1 was used to evaluate the electrochemical stability. Li electrode was employed as reference and counter electrode, and stainless steel as working electrode. The cyclic and rate performance of the membranes were tested in CR2025 coin cells with LiFePO4 as cathodes and lithium tablets as anodes. The 7

charge/discharge profiles of coin cells under different rates were obtained under room temperature using LAND system (Wuhan Blue CT-2001A battery test system). All cells were assembled in a glove box filled with argon gas. 3. Results and discussion Fig. 2.a-d shows the SEM images and fiber diameter distribution histograms of pure PVDF membrane and composite membrane. Three-dimensional networks were

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formed by interconnected fibers during electrospinning. In addition, the randomly oriented fibers formed many cavities and voids, yielding moderate pore size and prominent porosity. This structure should be convenient for absorption of more

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electrolyte and providing abundant channels for ion transfer. In Fig. 2.a, the mean fiber

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diameter of PVDF is 380 nm. Higher solution viscosity clearly induced larger fiber diameters, where viscosity was proportional to content of polymer dissolved in the

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solvent (Huang, Zhang, Kotaki, & Ramakrishna, 2003). With the addition of CA, the

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fiber diameter of PVDF/CA increased to 487 nm, while that of PVDF/TPP is only 257 nm. The addition of both CA and TPP resulted in increased fiber diameter to 430 nm

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(Fig. 2.d). Uniform fiber diameter ranging from 300 to 600 nm was noticed, indicating excellent compatibility between components. The chemical compositions of nanofibers

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were further characterized by EDS and the results are shown in Fig. 2s. The peaks assigned to C, O, F and P were recognized, confirming coexistence of PVDF and TPP in the nanofibers.

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Fig. 2 The SEM images and fiber diameter histograms of electrospun polymer membranes (a) pure PVDF, (b) PVDF/TPP/CA, (c) PVDF/TPP, (d) PVDF/CA

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The FT-IR profiles of pure PVDF, PVDF-based composite membranes and pure CA are gathered in Fig. 3.a. The peak at 1402 cm-1 was attributed to vibration of C-H

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in -CH2-, and peak at 1170 cm-1 was ascribed to the vibration bands of C-F. Both absorption peaks were characteristics of PVDF. The typical peaks of CA at 1231 cm-1

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and 1752 cm-1 could be assigned to deformation vibration of C-O stretching and

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vibration of C=O stretching, respectively. In PVDF/TPP/CA spectrum, the peaks at 1587 cm-1 and 1484 cm-1 were attributed to vibration of aromatic ring skeleton, belonging to TPP (Huang, Chang, & Li, 2017). All these data confirmed the successful introduction of TPP into the composite membranes. To thermal stabilities of pure PVDF, PVDF/TPP, PVDF/CA and PVDF/TPP/CA composite membranes were evaluated by DSC (Fig. 3.b). Compared to pure PVDF, two 9

endothermic peaks appeared at ~50˚C and ~164˚C, corresponding to melting temperature (Tm) values of TPP and PVDF, respectively. The crystallinity percentages of the membranes were calculated according to crystallinity (Xc) % formula and the results are compiled in Table 1s (Masoud, El-Bellihi, Bayoumy, & Mohamed, 2018). The Tm values of the membranes were slightly affected by the introduction of CA with low crystallinity and high melting point (Wang, Zhang, Shao, & Liu, 2019). The

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crystallinity decreased from 44.1 % to 38.4 % due to amorphous regions provided by CA. The decrease in crystallinity would further result in raised segmental motion of the polymeric chain, which may enhance the ionic conductivity (Bhute, & Kondawar,

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2019)

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The mechanical properties play crucial role in battery safety. The stress-strain curves of different membranes are shown in Fig. 3.c. The tensile stress of pure PVDF

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membrane was estimated to 6.9 Mpa with elongation rate of 87 %. The elongation rate

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of PVDF/CA membrane is 103% due to the flexible long-chain of CA (Li et al., 2017). The addition of TPP dramatically increased the elongation rate to 140 % and strength

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to 7.5 Mpa. The addition of both CA and TPP raised the strength of PVDF-based membrane to 8.5 Mpa and elongation rate to 155%. The benzene ring of TPP could

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form hydrogen bond with the polymer macromolecule, thereby increasing the strength of the composite membranes. The addition of small molecules also enhanced flexibility of the macromolecules long chain. Both CA and TPP improved the mechanical properties of composite membrane, which made it more suitable for requirement for ordinary lithium batteries. 10

Fig. 3 FTIR spectra of PVDF, CA and different membranes (a); DSC curves of different membranes (b); Stress-strain curves of PVDF and composite membranes (c)

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The highly porous structure would facilitate the adsorption and penetration of the organic electrolyte, thereby improving the cycle stability and rate performance of

batteries (Zhang et al., 2018). The results of porosity and electrolyte uptake are

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displayed in Table 2s. The porosity of PVDF membrane was recorded as 73 % and

electrolyte uptake was 228 %, much higher than those of PE. The addition of TPP and

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CA induced a composite membrane with porosity of 90% and electrolyte uptake of

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301%. For further research, pictures of the climbing distance after membranes immersion in the liquid electrolyte for 1 h were taken and shown in Fig. 4.g. Pure PVDF

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and PVDF/TPP/CA membranes were wetter by the electrolyte and attributed to the porous structure of the membranes prepared by electrospinning due to similar polarity

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between PVDF and electrolyte and the hydrophilic nature of CA. Meanwhile, the

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wettability of each membrane could affect the battery resistance. Fig. 4.a-f shows the wetting behaviors of different membranes. The contact angle of PE was estimated to 46.0˚ while those of pure PVDF membrane and PVDF/TPP/CA composite membrane were 24.6˚ and 14.6˚, respectively. The wettability values of the membranes were further investigated by dropping 0.05 ml electrolyte on their surfaces. Compared to PE, the electrolyte wetted PVDF and composite membranes immediately spread out more 11

effectively.

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Fig. 4 Contact angles of different membranes : (a) PE, (b) PVDF, (c) PVDF/TPP/CA; Photographs showing liquid electrolyte wetting behavior of (d) PE, (e) PVDF, (f) PVDF/TPP/CA membranes, and (g) Contrast of liquid electrolyte immersion-height of PE, PVDF, PVDF/TPP, PVDF/TPP/CA membranes

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Fig. 5 Photographs of PE, PVDF, and PVDF/TPP/CA nanofiber membranes after

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exposed at 120˚C，130˚C，140˚C，150˚C，160˚C，170˚C for 30 minutes

The dimensional thermostabilities of the membranes were evaluated by treating PE,

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PVDF and PVDF/TPP/CA membranes in hot oven for 30 min at 120˚C 130˚C, 140˚C, 150˚C, 160˚C, and 170˚C. Fig. 5. shows photographs of the membranes before and after

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thermal treatment. The change in PE membrane dimension started at 120˚C, leading to transparency and severe shrinkage due to its low melting point. Pure PVDF membrane depicted changes at 160˚C while still maintaining its dimensional integrity at 170˚C. Thus, addition of CA could improve the thermal stability of the membranes. To evaluate the flame resistance of composite membranes upon thermal triggering, 12

the membranes were first completely wet by electrolyte and then ignited by direct lighter flame (Fig. 6.). The flame was clearly extinguished in less than one second,

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indicating the good flame retardancy of the membrane wet by electrolyte.

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Fig. 6 Digital photographs showing the flammability of PVDF/TPP/CA (a to d) and PE (e to h) wetted by the electrolyte

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The Nyquist plot is useful for illustrating the impedance performance (Fig. 7.a), in

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which bulk resistance presents the intercept with x-axis. The ionic conductivity was calculated using Eq. (3) and the data are compiled in Table 3s. The bulk resistance and

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ionic conductivity of the composite membrane were estimated to 0.7 Ω and 4.4 mS cm, respectively. Compared to PE and pure PVDF membranes, the smallest resistance

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and highest ionic conductivity of PVDF/TPP/CA membrane were associated with addition of CA that reduced the crystallinity and TPP, inducing changes in dynamics of the polymer chain. Both CA and TPP relatively promoted the migration of more lithium ions. The electrochemical stability is important for safe application of membranes in 13

batteries, and could be assessed by linear sweep voltammetry. Fig. 7.b shows the electrochemical stability windows of PE, PVDF, and composite membrane. The electrochemical oxidation limit of PVDF/TPP/CA was recorded as 4.9 V, higher than those of PE (4.4 V) and PVDF (4.6 V). Therefore, the composite membranes possessed

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sufficient electrochemical stabilities for normal use of batteries.

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Fig. 7 AC impedance spectra of PE, PVDF and PVDF/TPP/CA membranes (a); Electrochemical stability windows of PE, PVDF and PVDF/TPP/CA (b); AC impedance spectra of PE, PVDF and PVDF/TPP/CA membranes (c)

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The compatibility between lithium metal anode and electrolyte is very important for safe LIBs. Fig. 7.c illustrates the initial interface impedance spectra of cells

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containing PE, PVDF, and PVDF/TPP/CA membranes. The interface impedance

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corresponded to the X-axis intercept of the semi-arc. The initial interfacial resistances for PVDF and composite membrane were estimated to 320 Ω and 190 Ω, respectively.

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Both values were smaller than that for PE membrane (420 Ω). Moreover, the decrease in interface impedance suggested improvement in interface stability, conducive to

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facilitate the detaching speed of lithium ions from the electrode surface, suitable for maintaining the battery cycle stability. The cycling performances of the membranes were investigated at 0.2C and the results are displayed in Fig. 8.c-d. The initial discharge capacities of the cells with PE and PVDF/TPP/CA membranes reached ~139.7 mA h g 14

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and 141.4 mA h g-1,

respectively. After 100 cycles, the discharge capacity of the cell with PE decreased to ~100.8 mA h g-1 with capacity retention of 72.1%. However, the cell assembled with PVDF/TPP/CA membrane showed a discharge capacity of ~122.8 mA h g-1 with capacity retention of 86.9%. The charge/discharge curves of cells with PE and PVDF/TPP/CA at the 1st, 20th and 50th cycles are depicted Fig. 8.a-b. The composite membrane displayed better cycle stability, including better discharge capacity and

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coulombic efficiency than PE. This could be attributed to the excellent ability of composite membrane to retain more electrolyte than PE, preventing leakage of electrolyte during cycling and improving cycle performance.

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The rate performances at current densities of 0.1C, 0.2C, 0.5C, 1C and 2C are

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exhibited in Fig. 9. PVDF/TPP/CA membrane displayed better electrical properties than PE membrane. The increase in polarization and internal battery loss caused a decline in

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discharge capacity. As discharge rate increased, the discharge capacities of PE and

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composite membranes both decreased gradually. However, the discharge capacity of the cell with PVDF/TPP/CA membrane was always higher than that with PE membrane,

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indicating better rate capability of the cell with composite membrane.

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Fig. 8 Charge/discharge curves for cells in the potential range of 2.7V-4.2V at 0.2C rate under room temperature conditions. (a) PE, (b) Composite membrane; Cycling performance of the LiFePO4/membrane/Li cells at 0.2C rate. (c) PE, (d) Composite membrane

Fig. 9 Rate capabilities of LiFePO4/membrane/Li half cells using PE and PVDF/TPP/CA membranes

4. Conclusions Hybrid polymer PVDF/TPP/CA nanofiber membrane was successfully prepared by one-step electrospinning. This composite membrane showed high porosity, better 16

electrolyte uptake ability, and appropriate mechanical properties. The addition of CA improved the thermal stability of membrane and induced flame resistance after introduction of TPP. Meanwhile, the membrane exhibited excellent ionic conductivity and electrochemical stability window. CA and TPP both played great roles in improving Li

+

transfer efficiency. The cell with PVDF/TPP/CA membrane showed remarkable

performance in terms of cycle and rate capability. In sum, the proposed novel composite

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membranes look promising candidates for high-performance lithium-ion batteries.

Credit Author Statement

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Manuscript title: “Electrospun Cellulose Polymer Nanofiber Membrane

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with Flame Resistance Properties for Lithium-Ion Batteries” (CARBPOLD-19-03685).

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Yue Chen has made substantial contributions to the conception or design

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of the work; or the acquisition, analysis, or interpretation of data for the work; And I have drafted the work or revised it critically for important

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intellectual content.

Professor Du and Professor participated in and confirmed the design of the

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work. Both of them revised this manuscript and rechecked the content. All of the other co-authors helped to finish the experiment and test the sample.

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Acknowledgments This work was supported by Zhejiang Provincial Natural Science Foundation of China (LY18F050011) and the Applied Basic Research Project of China National Textile and

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Apparel Council (J201801)

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