j ourna l ho me page: www.elsev ier .com/ locate /e lec tac ta
lectrospun Trilayer Polymeric Membranes as Separator forithium–ion Batteries
. Angulakshmi, A. Manuel Stephan ∗
SIR-Network Institutes of Solar Energy(CSIR-NISE), Electrochemical Power Systems Division, Central Electrochemical Research Institute (CSIR-CECRI),araikudi 630 006, India
r t i c l e i n f o
rticle history:eceived 4 November 2013eceived in revised form 21 January 2014ccepted 28 January 2014vailable online 12 February 2014
a b s t r a c t
Poly(vinylidene fluoride- hexafluoropropylene) (PVdF-HFP)/poly (vinyl chloride) (PVC)/(PVdF-HFP)based- trilayer porous polymeric membrane (PM) was prepared by electrospinning for lithium batter-ies. The formation of beads was significantly reduced by increasing the concentration and by reducingthe surface tension of the polymer solutions. Although, single layer PVdF-HFP membrane exhibited highporosity and uptake of electrolyte, its mechanical integrity was found to be poor (not free-standing).
On the other hand, electrospinning of PVC over PVdF-HFP enhanced the mechanical integrity of themembrane. The prepared membranes were subjected to SEM, ionic conductivity, electrolyte uptake andshrinkage analyses. A 2032-type coin cell composed of Li/PM/LiFePO4 has been assembled and its cyclingprofile was examined at different C-rates. The (PVdF-HFP)/PVC/(PVdF-HFP) trilayer membrane can be astrong contender for lithium battery applications.
harge-discharge studies.
. Introduction
Lithium batteries are identified as the ultimate choice of powero energize portable electronic devices such as laptop computers,igital cameras, cellular phones etc.,[1]. They are the technologyf choice for future hybrid electric vehicles, which are urgentlyeeded for addressing energy and environmental issues [2]. Thetate-of-art lithium-ion battery comprises a graphitic electrodeanode) and a positive electrode (cathode) obtained from lay-red/olivine lithium transition metal oxides separated by a polyolifine) separator soaked in a non- aqueous liquid electrolyte3,4].The key role of a separator is to prevent electrical short cir-uits between the electrodes with a rapid admission of ionic chargearriers [5].
The ionic conductivity of the porous membranes mainlyepends on the conductivity of liquid electrolyte, membrane’sorosity, tortuosity of the pores, thickness and its wettability
6,7]. Microporous polyolefin membranes which are made up ofoly ethylene (PE) or poly (propylene) (PP) are commonly usedor lithium-ion battery applications. Although, these membranesrovide excellent chemical and mechanical properties, the low
porosity (about 40%) and poor wettability, remain a problem area[8]. Consequently, these factors restrict the performance of thebatteries [9,10]. Therefore, in order to circumvent these problemsnumerous attempts are being made to develop porous polymericseparators for lithium-ion batteries.
The commercially available Celgard (2325) membrane iscomposed of poly (propylene) (PP)/poly(ethylene) (PE)/poly(propylene)(PP) trilayer structure. The low melting point of PEenables its use as a thermal fuse. When the temperature approaches(due to unexpected chemical reactions in a battery system) themelting point of the polymer 135 ◦C, for PE and 165 ◦C for PP, theshutdown process takes by losing its porosity [5]. However, thesemembranes possess only 50% porosity and the wettability of themembranes is also low.
Very recently, the electrospinning method has drawn attentiondue to its versatility and simple preparative methods [11,12]. Theseare made up of thin fibres from micron to submicron diameters.Another advantage is the inter-laying fibres generate large porosity(>90%) with fully interconnected pore structure and large surfacearea to volume ratio facilitating high electrolyte uptake and easytransport of ions [13]. PVdF-HFP has slightly lower critical sur-face tension value ‘�c’ (25 mN m−2) than commercially availablepoly (propylene) separator (29 mNm−2) which facilitates for bet-
ter wettability of the non-aqueous electrolytes [14]. On the otherhand, PVC is mechanically robust, inexpensive and compatible witha large number of carbonate plasticizers, and has been reported forlithium-ion battery applications [15,16].
168 N. Angulakshmi, A.M. Stephan / Electrochimica Acta 127 (2014) 167–172
embra
lticatC
2
asf2umTmoHPnapmuswwbabt(qo
Fig. 1. SEM image of PVdF-HFP (a) membranes with beads (b) m
In the present work, an attempt has been made to prepare a tri-ayer polymeric membrane by electrospinning in order to enhancehe uptake of electrolyte solution and to improve the mechanicalntegrity and thermal stability of the polymeric membrane. Theycling performance of the trilayer membrane was analysed byssembling a 2032-type coin cell with Li/LiFePO4 configuration andhe obtained results are compared with the commercially availableelgard membrane.
. Experimental setup
The electrospinning equipment (Plastomek, India) consists of high voltage supplier (25 kV), and a syringe pump with a plasticyringe equipped with a 22 gauge stainless steel needle. Aluminumoil was used to collect the membrane. PVdF-HFP (88:12) (Kynar801, Alf Chem, Japan), poly (vinyl chloride) (Aldrich, USA) weresed as received. The distance between the orifice and the alu-inum collector was 10 cm and the applied voltage was 12 kV.
he solution feeding speed was fixed as 1 ml/h at 25 ◦C. The poly-er solution composed of PVdF-HFP and acetone was electrospun
n an aluminum collector. Then PVC was electrospun over PVdF-FP. The same procedure was adopted to coat PVdF-HFP overVC in order to get a trilayer configuration. The overall thick-ess of the membrane was 70 microns. In order to maintain thedhesiveness among the membranes, the coating process was com-leted within 30mins. Morphological examination of the films wasade by a scanning electron microscope (FE-SEM, S-4700, Hitachi)
nder a vacuum condition (10−1 Pa) after sputtering gold on oneide of the films. The histogram of the electrospun membranesere generated from the SEM images using the Image J soft-are. TG measurements were performed at a rate of 10 ◦C min−1
etween temperature ranges from 20 to 300 ◦C in a nitrogentmosphere. The ionic conductivity of the membranes sandwichedetween two stainless steel blocking electrodes (1 cm2 diame-
er) was measured using an electrochemical impedance analyzerIM6-Bio Analytical Systems) between 50 mHz and 100 kHz fre-uency range at ambient temperature. The mechanical strengthf the electrospun membrane was determined using a tensile
nes without beads (c) Histogram of the electrospun membrane.
machine (Tinius Olsen, Germany) according to ASTM D882-09standards with a constant cross-head speed of 10 mm min−1.The stretching of Celgard membrane was in machine direction(MD). The composite cathode was prepared by blending LiFePO4as active material with acetylene black carbon as electronic con-ductor and poly(vinylidene fluoride) as binder in the 70:20:10 wt.%ratio respectively as reported earlier[17,18].The electrolyte was 1 MLiPF6 in ethylene carbonate (EC)/dimethyl carbonate(DMC) with a1:2 volume ratio (Merk, Germany). The lithium metal foil (Aldrich,USA) was used as anode.
3. Results and discussions
Fig. 1 shows the SEM image of PVdF-HFP electrospun single layermembrane. The SEM image appears with lot of beads (Fig. 1a) withan average fibre diameter of less than 200 nm. The formation ofbeads along with the nanofibers is an undesirable property. Theexact reason for the formation of beads is yet to be understood.According to Shui and James[19] the bead formation is a com-plex process which competes with solidification. The formation ofbeads can be avoided by increasing the viscosity of the solution(higher polymer concentration) and increasing the charge densityor reducing the surface tension. In the present study, the formationof beads was avoided by increasing the polymer concentration andalso reducing the surface tension by adding dimethyl formamide asan additional solvent (Fig. 1b). Fong et al. [20] eliminated the forma-tion of beads in poly (ethylene oxide), PEO system by reducing itssurface tension with the addition of ethanol in water. Similarly, Linet al. [21] formed a beads-free poly (styrene) membrane by adding asmall amount of carbon surfactants during electrospinning. It is alsoobserved from the histogram of the electrospun membrane (Fig. 1c)that the average fibre diameter varies from 600 nm to 1.6 microns.
Fig. 2 (a) depicts the surface morphology of PVdF-HFP/PVC/PVdF-HFP trilayer membrane. The cross-sectional
SEM image of the PVdF-HFP/PVC/PVdF-HFP trilayer membrane isshown in Fig. 2(b) which implies a hairy rod structure with lotof pores which may facilitate to entrap huge amount of liquidelectrolyte and thereby lithium ion conduction. The histogram
N. Angulakshmi, A.M. Stephan / Electrochimica Acta 127 (2014) 167–172 169
F ross-s
ommo
phbCloaawo1a[pelawaot1
pHai
membrane may induce physical cross linking and eventually dete-riorate the surface morphology. Therefore, in the present study, allthe electrospun membranes were dried at 50 ◦C in a vacuum ovenfor 24 h before characterization in order to remove the solvents.
30
40
50
60
70
80
90
100
Weig
ht
(%)
TG-Single layer
TG-Trilayer
TG-Celgard
ig. 2. (a) Surface morphology of PVdF-HFP/PVC/PVdF-HFP trilayer membrane (b) c
f the trilayer membrane Fig. 2 (c) shows that the electrospunembrane has an average fibre diameter between 800 nm to 1.9icrons. The maximum number of fibres has an average diameter
f 1.45 micron.In order to determine the thermal stability of electrospun
olymeric membranes TG-DTA analysis was made. Generally, theeating process brings a lot of changes in the polymeric mem-ranes and finally leaving behind inert residue. The TG traces ofelgard, PVdF-HFP(single layer) and PVdF-HFP/PVC/PVdF-HFP (tri-
ayer) polymeric membranes are displayed in Fig. 3(a). A weight lossf around 2% was observed for the PVdF-HFP single layer membraneround 50 ◦C and is attributed to the removal of residual solventsnd moisture absorbed at the time of loading the sample. Furthereight loss between 125-130 ◦C is attributed to the melting point
f PVdF-HFP [14,22].The irreversible decomposition starts around60 ◦C. For the trilayer membrane the onset decomposition startsround 230 ◦C and is attributed to the higher melting point of PVC15]. The electrospinning of PVC resulted in a strong barrier effectreventing the PVdF-HFP from the thermal degradation to a certainxtent and this observation is an indication of the fact that the tri-ayer membrane is stable up to a temperature of 230 ◦C in nitrogentmosphere [23,24]. On the other hand, the observed weight losshich corresponds to 8 wt.% at 135 ◦C for the Celgard membrane is
ttributed to the melting point of poly ethylene. Moreover it meetsut a weight loss of nearly 10% at 135 ◦C. The photographs of therilayer and Celgard membrane before and after heat treatment at50 ◦C are shown as Fig. 4.
In order to quantify the mechanical strength, the stress-strain
roperties have been measured and are shown in Fig. 5. The PVdF-FP (single layer) system shows a typical behaviour of flexiblemorphous thermoplastic material whose deformation character-stic is very similar to rubber [25]. Upon electrospinning a second
ectional SEM of tri-layer membrane (c) Histogram of the electrospun membrane.
layer of PVC on to the PVdF-HFP mat a bilayer configuration isobtained. The fine nanofibers of PVC fill the interstices of the PVdF-HFP mat [25,26]. Upon further electrospinning of a third layer ofPVdF-HFP on to a bilayer leads to the sandwich configuration. Itis also evident from the Fig. 5 that the trilayer membrane exhibitshigher tensile value than the single layer (PVdF-HFP) membrane.However, the tensile strength of the trilayer membrane is inferiorto the Celgard membrane [27,28].
According to Choi et al. [29] the existence of the solvents such aschloroform and acetone on the surface of the electrospun polymeric
55050045040035030025020015010050
Tempera ture (00C)
Fig. 3. TG–traces of single, trilayer and Celgard membranes.
170 N. Angulakshmi, A.M. Stephan / Electrochimica Acta 127 (2014) 167–172
Fig. 4. Shrinkage test on ceramic membrane. (i) Before heat treatment.(((
t[
t
rt
R
wc
T
tb
Fb
a) Trilayer membrane (b) Celgard membraneii) after heat treatment at 150 ◦Cc) Trilayer membrane (d) Celgard membrane
The prepared membranes were soaked in a non-aqueous elec-rolyte for 1 h and its porosity was determined using the equation30]:
P = Ma/�a
Ma/�a + Mp/�p(1)
Where Mp, Ma is the mass of the dry and electrolyte absorbed inhe membrane, respectively and �p is the density of the polymer.
The electrolyte leakage ratio of the solution “Rl” is defined as theatio of electrolyte absorbed by the membrane and retained afterhe test [31].
l = Raf − Rai
Rai(2)
Where Rai and Raf respectively denotes initial and final absorbedeight of the electrolyte. The tortuosity of the membrane was cal-
ulated from the ionic conductivity and porosity data [5,32];
= � x P1/2(3)
�0
Where �, �0 and P respectively represents the conductivity ofhe liquid electrolyte, polymer electrolyte and porosity of the mem-rane. The tortuosity of the membrane was measured as 0.8.
3002502001501005000.0
0.4
0.8
1.2
1.6
2.0
Str
ess
(M
Pa)
Strain (%)
Single layer
Tril ayer
ig. 5. The stress vs. strain behavior of electrospun membrane and Celgard mem-ranes.
Based on the above experiments the porosity of the single layerwas measured as 70% and showed an electrolyte uptake of 247%with a solution leakage of 0.75%. The membrane exhibited an ionicconductivity of the order of 3.2 × 10−3 S cm−1 at 25 ◦C. On the otherhand, upon coating PVC over PVdF-HFP, the porosity of the trilayermembrane has been reduced significantly to 62% with a remarkablereduction in the uptake (230%) of electrolyte solution. The solu-tion leakage and ionic conductivity were calculated as 0.5% and1.58 × 10−3 S cm−1 respectively. The reduction in the porosity anduptake of the electrolyte may be attributed to higher fibre diameterof trilayer membrane (Fig. 1c above 800 nm). Table 1 compares thephysical properties of single, trilayer and Celgard membranes.
Fig. 6(a) illustrates the typical charge and discharge (potentialvs. time) profiles obtained at 25 ◦C between 2.5 and 4.0 V vs. Li/Li+
at different current regimes. The cell showed a highly reproducibleand well defined flat potential charge/discharge plateaus, aroundat 3.45 V vs. Li/Li+, which is a typical characteristic LiFePO4-basedcomposite electrode [23,33–35].
The discharge capacity as a function of cycle number is alsodepicted in Fig. 6(b).First, five formation cycles were performedat low C- rate (0.1 C). At its first cycle, a specific discharge capac-ity of about 125mAh g−1 was observed, for the cell with trilayermembrane while the cell with the Celgard delivered 120 mAh g−1.Upon cycling at C/5-rate the cell delivered a discharge capacity of123 mAh g−1
. An abrupt decrease in capacity was observed at 2 Cand 5C- rates. The reduction in the discharge capacity at highercurrent regime is a typical characteristic of LiFePO4 materialwhich is attributed to its low electronic conductivity and limiteddiffusion of Li+- ion into its structure that causes electrodepolarization [17,18]. Further, the declining discharge capacityat higher C-rates may be due to the solid electrolyte interface(SEI) film formation with electrolyte decomposition [36]. Recentstudy also revealed that, the increase in interfacial resistancevalue which originates from parameters related to the elec-trode design such as thickness and density can cause capacityfading at higher rates [37]. In the present study, irrespective ofthe C-rates, the Celgard membranes delivered a lower dischargecapacity when compared to trilayer membrane. While compar-ing the performance of trilayer membrane with the reductionin the discharge capacity of Celgard membrane is attributed tothe lower uptake of electrolyte solution which arises due to the
lower porosity of the polymeric membrane. In a similar way, Kimet al.,[38,39] reported better cycling performance for SiO2/Al2O3 -coated PVdF-HFP membranes than poly propylene separator withLi [Ni1/3Co1/3Mn1/3]O2 as cathode.
N. Angulakshmi, A.M. Stephan / Electrochimica Acta 127 (2014) 167–172 171
Table 1Physical properties of (PVdF-HFP), (PVdF-HFP)/PVC/(PVdF-HFP) and Celgard membranes.
Type of membrane Porosity (%) Electrolyte uptake(%) Ionic conductivity S cm−1 Solution leakage (%) Shrinkage (%)
Fig. 6. (a.) The cycling profile of LiFePO4/polymeric membrane (trilayer)/Li cell attheir. 11th cycle.FL
fieFahoc
4
HbC
[
[
[
[
[
[
[
[
[
[
[
[
[ganathan, Characterization of poly(vinylidene fluoride–hexafluoropropylene)
ig. 6 (b.) The discharge capacity as a function of cycle number for the cell.i/PM/LiFePO4 at different C-rates.
The cell operated with the expected voltage vs. capacity pro-les, and thus accounting for a good interfacial contact betweenlectrodes and trilayer membrane. It is also quite obvious from theigure that the cell retained its original capacity when cycled againt 1 C rate after 45 complete charge/discharge cycles. The resultsere discussed, although preliminary, demonstrate the feasibilityf employing trilayer polymeric membranes with better electro-hemical properties.
. Conclusions
A trilayer polymeric membrane composed of PVdF-
FP/PVC/PVdF-HFP was electrospun successfully and it exhibitedetter electrochemical properties than the commercially availableelgard membrane. The better performance of the electrospun
[
1.01 × 10−3 0.3 Rolled off
membrane has been attributed to higher porosity and uptake ofthe electrolytes over commercially available Celgard membrane.
Acknowledgements
The authors gratefully acknowledge Council of Scientific andIndustrial Research (CSIR, India) for financial support through TAP-SUN program.
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