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DOI: 10.1039/C3RA40732B (Review Article) RSC Adv., 2013, 3, 11404-11417

Battery separators based on vinylidene fluoride (VDF) polymers and copolymers for lithium ion battery applications

Carlos M. Costa a, Maria M. Silva b and S. Lanceros-Méndez *ac
aCentro/Departamento de Física, Universidade do Minho, 4710-057 Braga, Portugal. E-mail: lanceros@fisica.uminho.pt; Fax: +351 253 604 061; Tel: + 351 253 604 073
bCentro/Departamento de Química, Universidade do Minho, 4710-057 Braga, Portugal
cINL—International Iberian Nanotechnology Laboratory, 4715-330 Braga, Portugal. E-mail: senentxu.lanceros.us@inl.int

Received 11th February 2013 , Accepted 3rd April 2013

First published on 3rd April 2013

Abstract

Poly(vinylidene fluoride), PVDF, and its copolymers exhibit interesting properties for use as separator membranes in lithium-ion battery applications. This review presents the developments and summarizes the main characteristics of these materials for battery separator membranes. The review is divided into three categories regarding the composition of the polymer membranes: single polymers, polymer composites and polymer blends. For each category, the main characteristics for battery separator membranes and the ion transport behaviour is presented. Finally, guidelines for further investigations are outlined.

Carlos M. Costa

Carlos M. Costa

Carlos Miguel Costa graduated in Physics in 2005 and obtained his Master Degree in Materials Engineering in 2007. Currently, is a 3rd year PhD student in the Department of Physics at the University of Minho and is member of the Research Group in Electroactive Smart Materials at same university. His recent research focuses on the development of polymer based porous membranes for energy storage applications.

Maria M. Silva

Maria M. Silva

Maria Manuela Silva obtained a Research Assistant position in the Department of Chemistry of the University of Minho (Braga, Portugal) in 1991. She received her PhD in Chemistry from the University of Minho in 1999 and was promoted to Assistant Professor at the same University. Her research activities focus on the synthesis and characterisation of solid polymer electrolytes (SPE) and their application in the domains of solid state electrochemistry (solid state batteries and electrochromic devices). She is also interested in the chemistry (synthesis via sol–gel) of silica-based organic-inorganic hybrid materials.

S. Lanceros-Méndez

S. Lanceros-Méndez

S. Lanceros-Mendez graduated in physics at the University of the Basque Country, Leioa, Spain, in 1991. He obtained his Ph.D. degree in 1996 at the Institute of Physics of the Julius-Maximilians-Universität Würzburg, Germany. He was Research Scholar at Montana State University, Bozeman, MT, from 1996 to 1998 and visiting scientist at the A. F. Ioffe Physico-Technical Institute, St. Petersburg, Russia (1995), Pennsylvania State University, USA (2007) and University of Potsdam (2008). Since September 1998 he has been at the Physics Department of the University of Minho, Portugal, where he is associate professor and since 2012 he is also associate researcher at the INL-International Iberian Nanotechnology Laboratory. His work is focused in polymer based smart materials for sensors and actuators, energy and biomedical applications.

1 Introduction

After Pike Research Consulting, the market of portable batteries will reach $30.5 billion dollars in 2015 with an annual growth rate of 8.5%.1 The most used type of portable batteries are lithium-ion batteries as they are light, cheap, show high energy density, low charge lost, no memory effect, prolonged service-life and high number of charge/discharge cycles. The market for lithium-ion (Li-ion) cells is mainly focused in portable electronic devices such as notebook computers and mobile phones. The first Li-ion batteries were commercialized 1991.2,3 This commercialization was preceded by several scientific achievements, including the pioneering work of Yazami4 regarding the use of lithium-graphite as a negative electrode. A Li-ion battery is an electrochemical cell that converts chemical energy into electrical energy.5,6 The basic constituents of an electrochemical cell are the anode, cathode and the separator, as illustrated in Fig. 1.
Schematic representation of the main components of a lithium-ion battery.
Fig. 1 Schematic representation of the main components of a lithium-ion battery.

The separator membrane separates the anode and the cathode and it is essential in all electrochemical devices.7,8 The role of the separators is to the serve as the medium for the transfer of the lithium ions between both electrodes and to control the number of lithium ions and their mobility.9 The separator is constituted by a polymer matrix soaked by the electrolyte solution, i.e., a liquid electrolyte where salts are dissolved in a solvent, water or organic molecules. Most commonly, the liquid electrolyte solution is composed by a lithium salt in a mixture of one or more solvents. The solvents present in the electrolyte solution must meet a combination of requirements for battery applications, which are, in some cases, not easy to achieve, as for example, high fluidity vs. high dielectric constant.10 The characteristics of an ideal solvent are high dielectric constant, for dissolving high salt concentrations; low viscosity, for improving ion transportation; to be inert to all cell components and to be in the liquid state in a wide temperature range. The nonaqueous solvents most used in electrolyte solutions belong to organic esters and ethers classes.11 In both classes, the most used solvents are ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC). Other possibility for the fabrication of polymer electrolyte separators is by incorporating the lithium salts directly into the polymer matrix.12

A large diversity of requirements determine the performance of separator membranes for battery applications, such as low ionic strength, mechanical and dimensional stability, physical strength to allow easy handling, resistance to thermal and chemical degradation by electrolyte impurities and chemical reagents, to be easily wetted by liquid electrolytes and to show uniform thickness.9,12,13Table 1 summarizes the typical values and the relevance of the main requirements of lithium-ion battery separators, adapted from.12,13

Table 1 Ideal value and relevance of the typical parameters for lithium-ion battery separators
Parameter Ideal Value Relevance
Thickness (μm) <25 Determines the mechanical strength of the membrane and the risk of inner battery electrical shorting.
Electrical resistance (MacMullin no.) <8 Describes the relative contribution of a separator to cell resistance.
Gurley (s) ∼25/mil Expresses the time necessary for a specific amount of air to pass through a specific area of the separator with a specific pressure.
Porosity (%)/Pore Size (μm) 40/<1 Determines the permeability required for battery separators.
Shrinkage (%) <5% in both MD and TD Dimensional stability. The separator should not shrink when exposed to the electrolyte solution.
Tensile strength (%) <2% offset at 1000 psi The separator should stand mechanical stress between the electrodes.
Shutdown T (°C) 130 The temperature safety range of the battery that is provided by the separator.
High-temperature melt integrity >150 Separators with good mechanical properties at high temperatures may provide a larger safety margin for batteries
Skew (mm m−1) <0.2 When a separator is laid out, the separator should be straight and not bowed or skewed.

The materials used as separators for batteries are mainly polymers or polymer composites with dispersed fillers of various types. The most used polymers are poly(ethylene) (PE),14,15 poly(propylene) (PP),16 poly(ethylene oxide) (PEO).17–19 poly(acrylonitrile) (PAN)20–22 and poly(vinylidene fluoride) (PVDF) and its copolymers.23–27 The most used fillers incorporated into the polymer hosts are inert oxide ceramics (Al2O3, SiO2, TiO2), molecular sieves (zeolites), ferroelectric materials (BaTiO3) and carbonaceous fillers, among others, with the main function of increasing the mechanical stability and/or ionic conductivity of the separator.28

Fig. 2 illustrates the increasing number of published scientific articles related to lithium ions battery separators and polymer electrolytes.


Research articles published on battery separators and polymer electrolytes for lithium ion battery applications. Search performed in Scopus database with the keywords “battery separators” and “polymer electrolytes”.
Fig. 2 Research articles published on battery separators and polymer electrolytes for lithium ion battery applications. Search performed in Scopus database with the keywords “battery separators” and “polymer electrolytes”.

The strong growth of work in this field in the past decade results from the development of new materials and processing techniques, which allows rapid and efficient technology transfer of the novel developed materials.

PVDF is semi-crystalline polymer in which the amorphous chains are embedded between the lamellar crystalline structures with a degree of crystallinity ranging from 40% to 60%. It exhibits four polymorphs called α, β, γ, δ.29,30 The most common and important polymorphs of PVDF are the α- and β-phases. The α-phase is non-polar, it is the phase thermodynamically more stable when the material is obtained from the melt and when the solvent is evaporated at temperatures above 80 °C.31 The β-phase is the most interesting phase for technological applications due to its electroactive properties: piezoelectric, pyroelectric and ferroelectric.32 The β-phase is obtained with a porous microstructure directly by solution at crystallization temperatures below 70 °C33 or by mechanical stretching of the α-phase at temperatures between 70 °C and 100 °C.34 The dielectric constant of the β-phase ranges between 10 at 13 and the conformational repeating unit (planar zigzag, all-trans) has a dipolar moment of 7 × 10−30 Cm.35

The semi-crystalline copolymer poly(vinylidene fluoride-co-trifluoroethylene), P(VDF-TrFE), shows, for specific molar ratios of VDF and TrFE, a polar ferroelectric transplanar chain conformation similar to the one of the β-phase of PVDF.36 P(VDF-TrFE) exhibits the ferroelectric (FE)-paraelectric (PE) phase transition at a Curie temperature, Tc, below the melting temperature, Tm. Both temperatures depend on the crystallization conditions and molar ratio of VDF and TrFE.37–39 The copolymer poly(vinylidene fluoride-co-hexafluoropropene), P(VDF-HFP), is also a semi-crystalline polymer with a degree of crystallinity significantly reduced due to the addition of hexafluoropropylene (HFP).40 Therefore, it shows high flexibility as compared to PVDF41 and a dielectric constant of 8.4.

In the copolymer poly(vinylidene fluoride-co-chlorotrifluoroethylene), P(VDF-CTFE), the amount of chlorotrifluoroethylene, CTFE is essential for determining properties and applications.42 For 25–70% mol of VDF, the P(VDF-CTFE) is amorphous43 being for the remaining concentrations a semicrystalline copolymer with a hexagonal structure.44 The dielectric constant of P(VDF-CTFE) is 1345 and shows high electromechanical response for 9 and 12 mol% CTFE content.46

PVDF and its copolymers poly(vinylidene fluoride-co-trifluoroethylene), PVDF-TrFE, poly(vinylidene fluoride-co-hexafluoropropylene), PVDF-HFP, and poly(vinylidene fluoride-co-chlorotrifluoroethylene), PVDF-CTFE show strong advantages for their use as separator membranes in comparison to polyolefins47 and other used materials due to their strong polarity (high dipolar moment) and high dielectric constant for a polymer material, which can assist ionization of lithium salts. It is also possible to control the porosity of the materials through binary and ternary polymer/solvent systems. Further, they are wetted by organic solvents, chemically inert, show good contact between electrode and electrolyte and are stable in cathodic environment.48–56 Different processing techniques, such as solvent casting, electrospinning and hot-press have been used for the development for battery separators from these materials.57–62

This review focused on battery separators and polymer electrolytes based on PVDF and its copolymers, PVDF-HFP, PVDF-TrFE and PVDF-CTFE, for lithium-ion battery application due to the recent advances and their large potential for energy storage applications. A summary of the obtained results will allow establishing the maturity of these materials for the intended purpose as well as to reflect on the future steps to be taken both in research and technology transfer.

This review is structured in three sections devoted to the state of the art in single polymers, composites and polymer blends, respectively. For each section, the materials and electrolyte solutions will be presented as well as the main characteristics of the materials, such as porosity, ionic conductivity and related properties. Some remarks on the electrodes of batteries based on the aforementioned separators will be provided and, finally, the main problems and future directions will be presented.

2 Polymer electrolytes based on poly(vinylidene fluoride) and its copolymers

2.1 Single polymer and copolymers

Fluorinated polymers such as PVDF and its copolymer show advantages when compared to commercial polyolefine separators (PE) due to their high polarity and dielectric permittivity, which provides larger affinity with polar liquid electrolytes. The characteristics of the developed PVDF and copolymer membranes are summarized in Table 2 as achieved in chronological order.
Table 2 Developed polymer electrolytes based on PVDF and co-polymers, presented in chronological order, and their main properties
Material Electrolyte solution/lithium ions Porosity (%)/Fiber Diameter* (electrospun) (nm) Uptake (%) σ i/(S cm−1) at 25 °C Ref.
PVDF-HFP 1 M LiPF6 in EC/PC — 60 0.8 × 10−3 68
PVDF 1 M LiPF6 + PC/EC/3DMC 70 65 3.7 × 10−3 63
PVDF 1 M LiTFSI in EC/DEC (2/3 in volume ratio) — — 6.7 × 10−3 27
PVDF 10% LiBF4 in EC/PC (1[thin space (1/6-em)]:[thin space (1/6-em)]1) — — 3.4 × 10−4 69
PVDF 10% LiPF6 in EC/PC (1[thin space (1/6-em)]:[thin space (1/6-em)]1) — — 4.7 × 10−4 69
PVDF 10% LiAsF6 in EC/PC (1[thin space (1/6-em)]:[thin space (1/6-em)]1) — — 6.6 × 10−4 69
PVDF 1 M LiTFSI in EC/DEC 0 20 5.6 × 10−8 72
PVDF 1 M LiTFSI in EC/DEC 23 32 2.7 × 10−6 72
PVDF 1 M LiTFSI in EC/DEC 30 41 1.0 × 10−6 72
PVDF 1 M LiTFSI in EC/DEC 70 60 9.8 × 10−5 72
PVDF 1 M LiTFSI in EC/DEC 75 65 1.3 × 10−4 72
PVDF-HFP 1 M LiPF6 in 1/1 w/w (EC/DEC) — — 1.5–2.0 × 10−3 73
PVDF 1 M LiPF6 in 1/1 w/w (EC/DEC) 23 33 2.2 × 10−5 74,75
PVDF 1 M LiPF6 in 1/1 w/w (EC/DEC) 30 39 2.4 × 10−5 74,75
PVDF 1 M LiPF6 in 1/1 w/w (EC/DEC) 38 45 1.5 × 10−4 74,75
PVDF 1 M LiPF6 in 1/1 w/w (EC/DEC) 71 77 1.0 × 10−3 74,75
PVDF-HFP 1 M LiClO4–EC/PC (1[thin space (1/6-em)]:[thin space (1/6-em)]1) 83 220 1.5 × 10−3 76
PVDF 1 M LiPF6–EC/PC — — 2.0 × 10−3 77
PVDF 1 M LiPF6–EC/DMC/DEC (2/2/1) 70 142 5.0 × 10−2 78
PVDF-HFP 1 M LiPF6–EC/DMC (1/1) 23 76.4 0.3 × 10−3 79
PVDF 1 M LiTFSI in distilled water 100–800* 50–73 1.6–2.0 × 10−3 64
PVDF-HFP 1 M LiBF4 in 1/3 w/w (EC/GBL) — 120 3.4 × 10−3 80
PVDF 1 M LiPF6–EC/DMC/DEC (2/2/1) — — 3.5 × 10−3 81
PVDF 20 wt% LiClO4 — — 8.7 × 10−4 82
PVDF 1 M LiPF6–EC/DMC/DEC (2/2/1) 70 — 3.1 × 10−3 83
PVDF-HFP 1 M LiPF6–EC/DEC (1/1) 70–90 — 1.2 × 10−3 84
PVDF LiBF4–PC:EC — — 1.0 × 10−3 85
PVDF EC/PC/LiPF6 = 43/43/7 (in wt%) — — 1.0 × 10−3 86
PVDF 15 wt% of LiFePO4 — — 6.7 × 10−6 87
PVDF 1 M LiPF6-EC/DMC/DEC (1/1/1). 750–1630* 300–400 6.7 × 10−2 88
PVDF-HFP 0.5 M LiTFSI in BMITFSI <1000* 750 2.3 × 10−3 65
PVDF-HFP 0.5 M LiBF4 in BMIBF4 <1000* 600 2.3 × 10−3 65
PVDF-HFP 1 M LiPF6 in EC/DMC 59 165 9.1 × 10−2 89
PVDF-HFP 1 M LiCF3SO3 in TEGDME 59 210 1.8 × 10−2 89
PVDF 1 M LiPF6-EC/DMC/EMC (1/1/1) 70 230 1.4 × 10−3 50
PVDF 1 M LiCF3SO3 in TEGDME/DIOX (1/1) — 250 0.6 × 10−3 90
PVDF-HFP 1 M LiPF6-EC/DMC/EMC (1/1/1) — — 1.8 × 10−3 91
PVDF-HFP 1 M LiPF6-EC/DMC (1/1) 78 321 3.36 × 10−4 92
PVDF-HFP 1 M LiPF6-EC/DMC/DEC (1/1/1) 70 — 1.4 × 10−3 66
PVDF 1 M LiPF6-EC/DMC/EMC (1/1/1) — 230 4.8 × 10−3 67
PVDF-HFP 0.3 M Mg(CF3SO3)2 in EMITf — — 4.8 × 10−3 93
PVDF 1 M LiPF6-EC/DMC (1/1) 77 — 1.9 × 10−3 56
PVDF-CTFE 1 M LiPF6-EC/DMC (1/1) 230* 800 2.0 × 10−3 94
PVDF 50 wt% LiTFS — — 1.7 × 10−2 95
PVDF-HFP 40 wt% LiTf — — 7.8 × 10−5 96
PVDF-HFP 0.8 M LiTFSI in 1 g 13TFSI — 670 3.2 × 10−4 97
PVDF-HFP LiTFSI-PC (0.15/0.3 wt%) — — 1.0 × 10−5 98
PVDF 1 M TEABF4 in AN 80 117 1.8 × 10−3 53
PVDF 1 M LiPF6-EC/DEC (4/6) 48 142 — 99
PVDF-TrFE 1 M LiClO4-PC 72 255 2.7 × 10−7 23,100
PVDF-TrFE LiClO4·3H2O (n = 1.5) 67 — 3.5 × 10−11 101

The porous battery separators of fluorinated polymers are most commonly obtained by phase inversion processes such as thermal induced phase separation (TIPS), using solvent and non-solvent system and electrospinning.27,63–65

The achieved porosity of the battery separators ranges between 0 to 90% and the pore size from 0.5 μm to 16 μm.23,63 Porous membranes with controlled porosity and pore sizes of 2 μm66 and 1 μm67 were also obtained by adding urea and salicylic acid, respectively, as foaming agents for PVDF or PVDF-HFP.

In 1996, Tarascon et al. produced the first Li-ion battery with a fluorinated polymer (PVDF-HFP) as battery separator.68 The performance of such a battery compares favourably in terms of gravimetric or volumetric energy density, life cycle, power rate and self-discharge with its liquid counterparts, while having enhanced safety characteristics, larger shape flexibility and scale ability. One of the main advantages of fluorinated polymers is their ability to be tailored in different geometries, including very thin cells.

Kataoka et al.27 showed that the ionic conductivity depends on the immersion time of the polymer membrane in the electrolyte solution and on the aging time after removal from the solution. PVDF for polymer electrolytes is optimized with 1[thin space (1/6-em)]:[thin space (1/6-em)]1 EC[thin space (1/6-em)]:[thin space (1/6-em)]PC plasticizer in salts such as LiAsF6 (lithium hexafluoroarsenate), LiPF6 (Lithium hexafluorophosphate) and LiBF4 (Lithium tetrafluoroborate). Nevertheless, LiAsF6 gives better results for ionic conductivity than LiBF4 and LiPF6, irrespective of the nature of the polymer and the amount of plasticizer.69 Salts with a polarizing cation and a large anion with a well delocalized charge, and therefore with low lattice energy, are the most suitable for polymer electrolytes.70

Modifications of the properties of PVDF have been achieved by radiation grafting for improving adhesion to electrodes, leading to good rate performance and stable cycle life.71

From Table 2 it is observed that PVDF and PVDF-HFP with LiPF6 and LiCF3SO3 in different organic solvents lead to the best values of ionic conductivity (1.8 − 5 × 10−2 S cm−1).

PVDF polymer as battery separator was found to be effective in enhancing the lithium transport number due to selective interactions with the anion. The ionic conductivity of PVDF is associated to the total solution uptake, which depends on the gelation process related to porosity and pore size. The solution introduced in the polymer is stored in the pores and then penetrates into the polymer, swelling the polymer network.72 Other possibility for obtaining polymer electrolytes taking advantage of the properties of PVDF is by coating a microporous polyolefin membrane with a fluorinated polymer.73 The cells with these polymer electrolytes showed good electrochemical and rate performance during cycling. Ideal membranes for porous polymer electrolytes based on PVDF for battery applications should present high porosity and small pore diameters with a narrow distribution. Experimental results show that porosity should be >80% and pore diameter should be <1 μm.76 This porous structure has been also achieved with electrospun nanofiber webs.64

The effect of the liquid organic solvents in PVDF microporous membranes was studied by Saunier et al.81 It was observed that the affinity of PVDF for the liquid electrolyte may affect its mechanical strength and compromise battery safety. This indicates that the thermal and mechanical stability are affected when too much solvent is incorporated into the polymer. The reversible modifications can also affect the membrane properties, as the glass transition and melting temperature are lowered.81

The ionic conductivity of the PVDF microporous membranes is also affected by solvent/polymer and solvent/salt interactions, ionic dissociation and tortuosity value.83 It was proven also that interactions between PVDF and PC mainly occur in the surface area of the PVDF crystalline phase, whereas interactions between PC, PVDF, and lithium salt mainly occur in the amorphous area.49

It was also observed that ionic conductivity decreases in the order EC/DEC > EC/EMC > EC/DMC among the electrospun PVDF fiber-based polymer electrolytes with the same weight fraction of EC.88

For PVDF-HFP-based solid polymer electrolytes, lithium triflate salt effectively reduces the degree of crystallinity of the polymer and increases the ionic conductivity of the membrane.96 Costa et al. demonstrated which parameters-porosity, treatment of lithium ion and processing technique-influence at most the electrical performance of PVDF-TrFE membranes for battery separators. The membrane that exhibits the highest ionic conductivity is a porous membrane prepared by the uptake technique. The performance of the membrane for battery applications are, therefore, strongly influenced both by porosity and processing technique due to its influence in the specific surface available for lithium ion adsorption and trapping.102

The ionic conductivity depends not only on the characteristics of the electrolyte solution but also on the properties of the membrane—porosity and pore size—as shown in Fig. 3.


Porosity vs. uptake for various electrolyte solutions incorporated into PVDF membranes.
Fig. 3 Porosity vs. uptake for various electrolyte solutions incorporated into PVDF membranes.

In Fig. 3, it is observed that for the same porosity are obtained different uptake ratios and ionic conductivities (Table 2) due to the interactions with the cations and anions produced from the Li salts by the solvation process. The viscosity of the solvent also influences the transport and the transference numbers of the ions.103,104

Therefore, the main problem still to be optimized for battery separators persists: to obtain a combination of good ionic conductivity with high uptake ratio and excellent mechanical properties without deterioration of the ionic conductivity in the temperature range of the lithium-ion battery operation.

PVDF was proven to battery separator by Yamamoto et al. in a 4.4 V Li-ion polymer battery. The discharge capacity reached 520 Wh l−1 and the capacity retention ratio was 91.4% at 3C.86

2.2 Polymer and copolymer composites

To solve some of the problems existing in single polymer membranes, battery separators have been developed by the incorporation of suitable fillers into the host polymer for improving mechanical strength, thermal stability and ionic conductivity. Among these fillers are oxide ceramic, zeolites, ferroelectric ceramics, carbon, etc.28,105 These fillers can be divided into two groups: the fillers that participate in the ionic conduction process and the fillers that are not involved in the lithium transport process.28

The characteristics of PVDF and copolymer composites for separator membranes are summarized in Table 3 in chronological order.

Table 3 Polymer electrolytes from PVDF based composite materials and their properties, presented in chronological order
Material Fillers Electrolyte solution/lithium ions Porosity (%) Uptake (%) σ i/(S cm−1) at 25 °C for maximum amount Ref.
PVDF-HFP SiO2 1 M LiPF6 in EC/DMC (1[thin space (1/6-em)]:[thin space (1/6-em)]1) — 100–250 0.87–3.1 × 10−3 106
PVDF-HFP MgO 1 M LiPF6 in EC/DMC (1[thin space (1/6-em)]:[thin space (1/6-em)]1) — 40 4.0 × 10−4 107
PVDF SiO2 1 MLiPF6 in EC/PC (1/1) — — — 112
PVDF SiO2 1 M LiClO4 in EC-PC (1/1) — — — 112
PVDF SiO2 1 M LiPF6 in EC-PC (1/1) — — 3.5 × 10−2 113
PVDF-HFP MMT LiCF3SO3 in PC — — 1.0 × 10−3 108
PVDF-HFP SiO2 1 M LiTFSI in EC/DEC (1/1) 77 — 2.7 × 10−2 114
PVDF-HFP SBA-15 1 M LiPF6 in EC/DMC/EMC (1/1/1) 59 76 0.8 × 10−3 111
PVDF-HFP MCM-41 1 M LiPF6 in EC/DMC/EMC (1/1/1) 14 30 4.6 × 10−2 111
PVDF-HFP NaY 1 M LiPF6 in EC/DMC/EMC (1/1/1) 9 39 3.0 × 10−3 111
PVDF-HFP TiO2 DMBITFSI/LiPF6 — — 1.3 × 10−3 115
PVDF-HFP AlO[OH]n 5 wt% of LiN(CF3SO2)2 — — 1.1 × 10−2 116
PVDF-HFP TiO2 LiClO4 in EC/PC 26 110 4.1 × 10−2 117
PVDF-HFP MgO LiClO4 in EC/PC 27 62 3.7 × 10−2 117
PVDF-HFP ZnO LiClO4 in EC/PC 23 61 5.5 × 10−2 117
PVDF-HFP MCM-41 LiClO4 in EC/PC 42 93 6.1 × 10−2 117
PVDF-HFP SBA-41 LiClO4 in EC/PC 52 82 5.0 × 10−2 117
PVDF-HFP MMT 1 M LiPF6 in EC:DMC (1/1) — 40 2.5 × 10−3 118
PVDF-HFP LiAlO2 1 M LiClO4 in EC:DEC (1/1) 87 121 8.1 × 10−3 119
PVDF-HFP ZrO2 1 M LiClO4 in EC:DEC (1/1) 86 91 11 × 10−3 120
PVDF-HFP TiO2 1 M LiPF6 in EC/DMC/DEC (1/1/1) 67 — 0.9 × 10−3 121
PVDF-HFP SiO2 1 M LiClO4 in EC/PC (1[thin space (1/6-em)]:[thin space (1/6-em)]1) — — 4.3 × 10−3 122
PVDF-HFP SiO2 LiClO4 + PC + DEC — — 1.0 × 10−2 123
PVDF-HFP TiO2 1 M LiPF6 in EC/DMC (1/1) — 125 1.0 × 10−3 124
PVDF-HFP TiO2 1 M LiPF6 in EC/DMC (1/1) 60 359 1.7 × 10−3 92
PVDF-HFP MgO 1 M Mg(ClO4)2 in EC/PC (1/1) — — 8.0 × 10−3 125
PVDF-HFP DMOImPF6 0.5 M NH4PF6 — — 3.0 × 10−5 126
PVDF SiO2 — 136 — — 127
PVDF-HFP BaTiO3 LiBETI + EC + PC — — 0.8 × 10−3 128
PVDF-HFP Al2O3 1 M LiPF6 in EC/DEC (1/1) — — — 129
PVDF-HFP Effervescent disintegrant 1 M LiPF6 in DMC/EC/EMC (1/1/1) 55 — 1.2 × 10−3 130
PVDF-HFP α-MnO2 1 M LiTFSI-PMMITFSI — — 1.3 × 10−3 131
PVDF MCM−41 + SO42−/ZrO2 1 M LiPF6 in EC/DMC/DEC (1/1/1) 62 161 1.0 × 10−3 132
PVDF-HFP SiO2 1 M LiPF6 in EC/DEC (1/1) 68 — 0.61 133
PVDF Fe2O3, SnO2 and CoO 1 M LiPF6 in EC/DMC (2/1) — — — 134
PVDF Organic clays — 75 — — 135
PVDF MMT 1 M LiClO4 in PC/DEC (1/1) — 177 2.3 × 10−3 109
PVDF TiO2 1 M LiPF6 in EC/DMC (1/1) 65–79 — — 136
PVDF-HFP SiO2 1 M NaTf in EC/PC (1[thin space (1/6-em)]:[thin space (1/6-em)]1) — — 4.1 × 10−3 137
PVDF SiO2 1 M LiPF6 in EC/DMC (1/1) 75 — 1.4 × 10−3 138
PVDF-HFP SiO2 1 M LiPF6 in EC/DEC (1/1) 61 — 0.9 × 10−3 139
PVDF-HFP Cellulose 1 M LiTFSI in BMPyrTFSI 58 712 4.0 × 10−4 140
PVDF-TrFE MMT 1 M LiClO4·3H2O-PC 90 335 8.0 × 10−7 141
PVDF-TrFE NaY 1 M LiClO4·3H2O-PC 36 233 2.0 × 10−6 142

From Table 3 it is observed that separator membranes with the different fillers increase the ionic conductivity with respect to the pristine polymer matrix (Table 2), the characteristics/properties of fillers playing an important role in the conduction mechanism of separator membranes.

Du Pasquier et al. showed that the combination of a phase-inversion process and the presence of finely divided silica in the separator results in the formation of a stable porous structure, in which the pores are mechanically reinforced by the silica particles at their inner surface and the ionic conductivity of the PVDF-HFP membrane increases.106

The addition of MgO fillers increases the compatibility between separator and electrodes (anode and cathode) and batteries with these membranes exhibit high power density (at 3C rate was > 280 W kg−1).107

Some authors verified that the presence of Montmorillonite (MMT) fillers have an effect on the nano-scale microenvironment for composite materials and a positive increment of the charge carriers and its mobility, the membranes exhibiting high electrochemical characteristics for Li-ion battery applications.108 Further, this filler is adequate for battery separators as it enhances the uptake of liquid electrolyte due to the excellent affinity of clays towards electrolyte molecules.109

The effect of powder particle size on battery separator was studied by Takemura et al. It was observed that composites with 0.01 μm ceramic powders (Al2O3) showed excellent cycling properties.110

The addition of molecular sieves has expanded the electrochemical stability window of polymer electrolytes, enhanced the interfacial stability of polymer electrolyte with lithium electrodes, and inhibited the crystallization of the PVDF-HFP matrix.111

Stephan et al. verified that the incorporation of inert fillers reduces the crystallinity of the polymer host, acts as ‘solid plasticizer' capable of enhancing the transport properties and provides better interfacial properties towards lithium metal anodes.116

The uptake of electrolyte solution is not related directly to the surface area or dielectric constant of the oxides. It may be due to the affinity of the metal oxide toward the electrolyte solution.117 The incorporation of fillers such as SiO2 and Al2O3 in the PVDF membrane promotes amorphicity, explaining the conductivity enhancement in PVDF-based electrolytes.143

PVDF-HFP with SiO2 nanoparticles has been prepared for Na/S batteries with a first discharge capacity of 165 mA h g−1.137

Galvanostatic cycling experiments of PVDF membranes with SiO2 showed that these membranes have a behaviour similar to the corresponding liquid electrolyte, without significant differences in capacity.112

Miao et al. showed that TiO2 added to the composite electrolyte membranes helps to improve mechanical strength, electrolyte uptake, ionic conductivity, and the electrode/electrolyte interfacial stability.92

Composite polymer electrolytes containing ionic liquids have been found to be thermally stable up to 300 °C and show results adequate to be used as battery separators.126

The nature of the filler and the filler content play therefore a very delicate role in the ionic conductivity of the composite materials.128 The maximum amount of fillers found in the different works was 32 wt%. The ionic conductivity of the composite materials as battery separators depends on the nature of the fillers, the characteristics of the membrane (porosity) and the electrolyte solution type (lithium salts and solvent). For ionic conductivity improvement, the Lewis acid–base interactions between filler surface groups, polymer matrix and cations/anions play an essential role.

Different fillers also incorporate complementary characteristics to the separator membranes. The molecular sieves produce a specific conducting pathway on the membranes and improve mechanical strength.132,142 The MMT particles do not affect the morphology of the polymer matrix and increase of electrochemical behaviour of the battery separator.108,118,141 The inert oxide ceramics (Al2O3 TiO2 ZrO2) reduce the degree of crystallinity and promotes of Li+ transport at the boundaries of the filler particles.144 Ferroelectric ceramic fillers (BaTiO3) increase the polarity of the battery separator due of the high dielectric constant of the fillers and due to the charge separation.28 The interfacial stability between electrodes and battery separators as well as the ionic conductivity are improved with fillers based of carbon (CNT, CNF).145

Fig. 4 shows the best ionic conductivity of the composite materials obtained with the different fillers.


Ionic conductivity for different filler types.
Fig. 4 Ionic conductivity for different filler types.

Fig. 4 shows that the best ionic conductivities are achieved for MgO, ZnO and MCM-41 fillers. The MgO and ZnO are inert oxide ceramics that change the dynamics of the polymer chains and MCM-41 are molecular sieves with strong Lewis acid centers in their frameworks and increase the Li+ transference number

2.3 Poly(vinylidene fluoride) and copolymer based polymer blends

Another strategy for enhancing the ionic conductivity and other relevant properties of battery separator membranes such as mechanical and thermal properties is the fabrication of polymer blends. In the polymer blends for battery separators the strategy has been the following: one polymer should show a very good affinity with the liquid electrolyte and the other polymer must show excellent mechanical properties. The dimensional and electrochemical stability are also necessary requirements for polymer blends.

The developed PVDF and copolymers based polymer blend membranes are summarized in Table 4 in chronological order.

Table 4 Polymer electrolyte blends based on PVDF and copolymers and their properties, presented in chronological order
Material Blends Electrolyte solution/lithium ions Porosity (%) Uptake (%) σ i/(S cm−1) at 25 °C Ref.
PVDF-HFP PAN 1 M LiPF6 in EC/DMC (1/1) 76 82 1.9 × 10−3 146
PVDF-HFP PAN 1 M LiBF4 in EC/DMC (1/1) 76 80 1.2 × 10−3 146
PVDF-HFP PE 1 M LiClO4/PC + EC — — 0.2 × 10−3 147
PVDF PMMA 10 mol% LiClO4 — — 3.1 × 10−5 148
PVDF-HFP PVP 1 M LiBF4 in EC/DMC (1/1) — 62 0.4 × 10−3 152
PVDF PAN LiClO4–PC-EC — — — 153
PVDF-HFP PEG LiTFSI — — 1.0 × 10−5 154
PVDF-HFP PMAML 1 M LiBF4 in EC/DMC (1/1) 76 75 2.6 × 10−3 149
PVDF PMMA-PEGDA 1 M LiPF6 in EC/DMC/EMC (1/1/1) — 600 4.5 × 10−3 155
PVDF-HFP PEG-PEGDMA 1 M LiPF6 in EC/DEC (1/1) 15 98 1.0 × 10−3 156
PVDF-HFP PVK 1.5 M LiBF4 in EC — — 0.7 × 10−3 150
PVDF PEGDA-PMMA LiPF6/LiCF3SO3 in EC/DMC/EMC (1/1/1) — — 1.0 × 10−3 157
PVDF PE 1 M LiPF6 in EC/DEC/PC (35/60/5, w/w/w) 48 302 1.1 × 10−3 151
PVDF PE 1 M LiPF6 in EC/DEC/PC (35/60/5, w/w/w) 53 290 8.9 × 10−4 158
PVDF-HFP PEO 1 M LiTFSI in EC/PC — — — 159
PVDF-HFP PEO 1 M LiTFSI in EC/PC (1/1) — — — 160
PVDF PEO 1 M LiClO4 in PC 84 210 2.0 × 10−3 161
PVDF-HFP PAN 1 M LiClO4 in EC/DEC (1/1) — — 3.4 × 10−3 162
PVDF-HFP PVP-PEG 1 M LiPF6 in DMC/EMC/EC (1/1/1) 49 125 0.5 × 10−3 163
PVDF-HFP P(EO-EC) LiCF3SO3 65 61 3.7 × 10−5 164
PVDF PMMA 1 M LiPF6 in DMC/EMC/EC (1/1/1) — — — 165
PVDF-HFP PEG 1 M LiPF6 in DEC/EC (1/1) 90 100 1.0 × 10−4 166
PVDF PVC NaClO4 + PC — — 1.5 × 10−4 167
PVDF-HFP PVA 1 M LiClO4 in EC/DEC (1/1) 86 90 7.9 × 10−3 119
PVDF PVC LiClO4 + EC/PC — — 3.7 × 10−3 168
PVDF PAN 1 M LiClO4 in PC 85 300 7.8 × 10−3 20
PVDF-HFP PAN 1 M LiPF6 in EC:EMC (1[thin space (1/6-em)]:[thin space (1/6-em)]3) 83 — 6.7 × 10−3 169
PVDF PMMA 1 M LiPF6 in EC:DMC (1[thin space (1/6-em)]:[thin space (1/6-em)]1) — 260 7.9 × 10−3 55
PVDF PDPA 1 M LiClO4 in PC — 280 3.6 × 10−3 24
PVDF-HFP PEGDMA 1 M LiClO4 in EC/DEC — 125 3.8 × 10−4 170
PVDF PEGDA-PEO-PPO-PEO 1 M LiClO4 in EC/PC (1/1) 32 63 1.9 × 10−3 171
PVDF PMMA 1 M LiClO4 in EC/PC (1/1) — 292 1.9 × 10−3 172
PVDF-HFP SN LiClO4 — — 1.0 × 10−3 173
PVDF-HFP PE 1 M LiPF6 in EC/ DEC (1/1) — — 0.8–1.2 × 10−3 174
PVDF-HFP PMMA 1 M LiPF6 in EC:DMC (1[thin space (1/6-em)]:[thin space (1/6-em)]1) — 377 2.0 × 10−3 175
PVDF-HFP PET 1 M LiPF6 in EC:DMC (1[thin space (1/6-em)]:[thin space (1/6-em)]1) — — 0.8 × 10−3 176
PVDF-HFP PVA 8 wt% LiBF4 + 67 wt% EC — — 1.2 × 10−3 177
PVDF PDMS 1 M LiPF6 in EC:DMC:EMC (1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1) 55 250 1.2 × 10−3 178
PVDF-HFP PPG-PEG-PEG 1 M LiClO4 in EC/PC (1/1) — 259 1.3 × 10−2 179
PVDF-HFP PMMA 1 M LiClO4 in EC/DEC (1/1) 50 403 1.7 × 10−3 180
PVDF-TrFE PEO 1 M LiClO4·3H2O-PC 74 92 3.0 × 10−4 181

Table 4 shows that the polymer blends show high ionic conductivity and the polymers more used with PVDF and its copolymers are PMMA and PEO due to the increased adhesion of electrodes and battery separators as well as to the ability to solvate a wide variety of salts, respectively.

PVDF-HFP/PAN polymer blend membranes were prepared by Kim et al. and high ionic conductivity and good mechanical properties were observed for the gel polymer electrolytes.146

PVDF-HFP/PE blend membranes show that PE particles dispersed in PVDF-HFP form a continuous film with 23 wt% of PE. The continuous PE film exhibits the ability to cut off the ion diffusion between cathode and anode and induces high ionic conductivity and good mechanical strength.147

Rajendran et al. determined that the resulting ionic conductivity of the blend membranes is determined by the overall mobility of ion and polymer, which depends on the free volume around the polymer chain.148 In the PMMA/PVDF (25–75) polymer blend with LiClO4 an ionic conductivity of 3.14 × 10−5 S cm−1 was obtained at room temperature.

PMAML/PVDF-HFP is a promising electrolyte candidate for rechargeable lithium ion polymer batteries as it shows high ionic conductivity (2.6 mS cm−1 at room temperature and electrochemical window around 4.6 V) and good electrochemical stability.149

Michael et al. demonstrated that PVdF-HFP/PVK with LiBF4 offers the room temperature ionic conductivity of 0.72 mS cm−1 with an ionic transference number of 0.49.150

A new type of separator was introduced by Lee et al.151 by coating poly(vinyl alcohol) (PVAc) on the surface of a PVDF/PE non-woven matrix. The coated separator exhibits smoother surface morphology and better adhesion properties toward electrodes.

Sannier et al., produced a polymer blend of PVDF-HFP/PEO and also highlighted the role of the macroscopic blend interfaces toward dendrite in bi-layered separators.160

For PVDF/PEO or PVDF-HFP/PEG blends, the addition of PEO or PEG in the PVDF matrix improves the pore configuration (connectivity) of the PVDF microporous membranes and increases ionic conductivity.161,166

Electrospun membranes based on PVDF were prepared and modified via pre-irradiation grafting with PMMA. PMMA possesses good affinity for the liquid electrolyte and gelled PMMA could substitute nonconductive PVDF for being in contact with the electrodes.55

Sohn et al. prepared a PVDF-HFP/PEGDMA coated PE separator for lithium ion battery applications by electron beam irradiation (EB). The EB treatment of the blend membranes containing PEGDMA was found to strongly improve the thermal shrinkage of the separators by the formation of crosslinked networks, enhancing also electrolyte uptake and ionic conductivity.170

The ionic conductivity of the polymer blends for battery separators depends on the affinity between polymers and the characteristics of the membrane (e.g. porosity, crystallinity, etc.), which also depends on the processing technique such as thermal induced phase separation (TIPS). Fig. 5 shows the best ionic conductivity for each developed polymer blend type.


Best ionic conductivity for the different polymer blends.
Fig. 5 Best ionic conductivity for the different polymer blends.

The common element for the polymer blends with the best ionic conductivity is the presence of PVDF-HFP (Fig. 5) due to the lower degree of crystallinity, its dielectric constant, ε = 8.4, and strong electron withdrawing functional groups (–C–F–).

3. Anode and cathode electrodes used with PVDF based separators

The two different types of electrodes, anode and cathode, immersed in the electrolyte solution create the electrical potential, i.e. the electrochemical cell.

During charging process, electrons move from the cathode to the anode (Fig. 6, left) and during discharge the electrons move from the anode to the cathode (Fig. 6, right).182


Representation of the charge and discharge modes of the electrochemical cell.
Fig. 6 Representation of the charge and discharge modes of the electrochemical cell.

The anode is the negative active material. It is commonly based on carbonaceous materials and non carbon alloys where reversion reaction occurs.183 Examples of carbonaceous materials used as anode materials are graphites, carbon nanotubes (CNT), carbon nanofibres (CNF) and lithium titanium oxides (Li4Ti5O12). Carbonaceous materials show the largest potential for improving the lithium ion cells and versatile, strong and highly conductive electrodes have been obtained to be used as anodes in batteries systems.184

The cathode is the positive active material. It is based on transition metal oxides and it is the main responsible for the cell capacity and cycle life.

Lithium cobalt oxide (LiCoO2), lithium manganese dioxide (LiMnO2), Lithium nickel oxide (LiNiO2) and lithium iron phosphate (LiFePO4) are some examples of materials used as cathodes.

For batteries with separator membranes based on PVDF and copolymers, the most used materials for the anode electrodes are Sn nanoparticles within a carbon matrix (Sn–C), graphite and lithium foil and for the cathode electrode are LiFePO4, LiCoO2 and lithium nickel manganese oxide (LiNi0.5Mn0.5O4).67,94,97

The above-mentioned electrodes are of general use for different separator membranes and some work still remains to be developed in this area in order to optimize electrodes for PVDF based separators. Further, it is to notice that electrodes are typically formed by an active material, additives and a polymer binder. The polymer binder used both as anode and cathode for lithium-ion batteries can be also based on PVDF polymer due to its electrochemical, thermal and chemical stability as well as its easy processing.

4 Conclusions and further work

This review presents an overview on PVDF and its copolymers for separator membranes in lithium-ion batteries applications. The battery separator is a critical element for improving lithium-ion battery performance. The review reports the research and developments in this field in the last decade. The correlation between properties and the fabrication methods is fundamental in order to achieve adequate battery separators for applications. It is essential the knowledge and control of their structure, stability and ionic conductivity in order to increase performance of the materials as battery separators. The review is divided into three different categories due of its characteristics, i.e., single polymers, composites and polymeric blends. Each category presents advantages and disadvantages:

For single polymers, PVDF and PVDF-HFP with LiPF6 and LiCF3SO3 in different organic solvents provide the best values of the ionic conductivity (1.8 − 5 × 10−2 S cm−1), and a large range of porosities and pore sizes can be obtained allowing tailoring membrane properties for the specific battery application. The ideal membrane for porous polymer electrolytes based on PVDF for battery applications should have high porosity and small pore diameters with narrow distribution. The experiments show that porosity should be >80% and pore diameter should be <1 μm. These characteristics are also obtained with up-scalable methods such as electrospinning.

The thermal and mechanical stability of single polymer membranes can be improved both by including specific fillers and by the development of polymer blends.

Fillers have been used that contribute to the ionic conduction process and fillers that are not involved in the lithium transport process. Particularly interesting are the fillers, such as MgO, that increase the compatibility between the separator and the electrodes. Also interesting are the molecular sieves that have proven to extend the electrochemical stability window of polymer electrolytes, enhanced the interfacial stability of the polymer electrolyte with the lithium electrode and inhibited the crystallization of the polymer matrix.

With respect to the polymer blends, one of the components should show a very good affinity to the liquid electrolyte and the other polymer must show excellent mechanical properties. The best blend membranes have been obtained with PVDF-HFP. Polymer blends also improve thermal shrinkage of the separators by the formation of cross linked networks and enhance electrolyte uptake and ionic conductivity.

With respect to the future trends, single polymers and co-polymers have to be achieved with similar large degrees of porosity (∼80%) but with hierarchical pore size structures down to pore sizes below 1 μm in an up-scalable way. This will allow to improve uptake without compromising mechanical properties and to obtain larger batch productions.

The incorporation of ionic liquids in the single polymer membranes is a promising field for more environmental friendly battery separators with high ionic conductivity at room temperature and wider electrochemical windows.

In composite materials, some work is needed, e.g. through surface functionalization, to improve compatibility between fillers and polymer matrix and therefore to improve the stability of the membrane.

Polymer blends are one of the most promising ways to improve PVDF based separator membranes. The progress in this category involves the fabrication of multilayers or hierarchical pore structures to enhance the thermal, electrical, mechanical and electrochemical properties of the battery separators and to improve compatibility with electrodes.

In conclusion, PVDF based polymer electrolytes offer broad engineering possibilities for membranes preparation with tailored microstructure and properties, showing therefore large potential for a new generation of more efficient battery separator membranes.

List of symbols and abbreviations

13TFSI N-methyl-N-propylpiperidinium Bis(trifluoromethanesulfonyl) Amide
Al2O3Aluminum Oxide
AlO[OH]nAluminum Oxyhydroxide
ANAcetonitrile
BaTiO3Barium Titanate
BMIBF41-Butyl-3-Methylimidazolium Tetrafluoroborate
BMITFSI1-butyl-3-Methylimidazolium bis(trifluoromethanesulfonyl)imide
BMPyrTFSI1-Butyl-3-Methypyrrolidinium Bis (trifluoromethanesulfonyl)imide
CNFCarbon Nanofibres
CNTCarbon Nanotubes
CoOCobalt Oxide
CTFEChlorotrifluoroethylene
DECDiethyl Carbonate
DIOX1,3-Dioxolane
DMBITFSI1,2-dimethyl-3-n-butylimidazolium-bis-trifluoromethanesulfonylimide
DMCDimethyl Carbonate
DMOImPF62,3-Dimethyl-1-octylimidazolium Hexafluorophosphate
ECEthylene Carbonate
EMCEthyl Methyl Carbonate
EMITf1-ethyl-3-methylimidazolium trifluoromethanesulfonate
Fe2O3Iron Oxide
GBLγ-butyrolactone
HFPHexafluoropropene
ILsIonic Liquid
LiAlO2Lithium Aluminate
LiAsF6Lithium Hexafluoroarsenate
LiBF4Lithium Tetrafluoroborate)
LiBETILithium Bis(perfluoroethanesulfonyl)imide
LiCF3SO3Lithium Trifluoromethanesulfonate
LiClO4Lithium Perchlorate
LiClO4·3H2OLithium Perchlorate Trihydrat
LiCoO2Lithium Cobalt Oxide
LiFePO4Lithium Iron Phosphate
LiPF6Lithium Hexafluorophosphate
LiMnO2Lithium Manganese Dioxide
LiNiO2Lithium Nickel Oxide
LiNi0.5Mn0.5O4Lithium Nickel Manganese Oxide
LiTFSILithium Bis(Trifluoromethanesulfonyl)Imide
Li4Ti5O12Lithium Titanium Oxides
MCM-41Molecular Sieves
Mg(CF3SO3)2Magnesium Triflate
Mg(ClO4)2Magnesium Perchlorate
MgOMagnesium Oxide
NH4PF6Ammonium Hexafluorophosphate
MMTMontmorillonite
MnO2Manganese Dioxide
NaClO4Sodium Salt
NaTfSodium Triflate
NaYMolecular Sieves
PANPoly(acrylonitrile)
PCPropylene Carbonate
PDPAPolydiphenylamine
PDMSPoly(dimethylsiloxane)
PEPoly(ethylene)
PEGPoly(ethylene glycol)
PEGDAPoly(ethylene glycol diacrylate)
PEGDMAPoly(ethylene glycol dimethacrylate)
PEOPoly(ethylene oxide)
P(EO-EC)Poly(ethylene oxide-co-ethylene carbonate)
PEO-PPO-PEOPolyethylene oxide-co-polypropylene oxide-co-polyethylene oxide
PETPoly(ethylene terephthalate)
PMAMLPoly(methyl methacrylate-co-acrylonitrile-co-lithium methacrylate)
PMMAPoly(methyl methacrylate)
PMMITFSI1,2-dimethyl-3-propylimidazolium bis(trifluoromethanesufonyl)imide
PPPoly(propylene)
PPG-PEG-PPGPoly(propylene glycol)-co-poly(ethylene glycol)-co-poly(propylene glycol)
PVAPoly(vinyl alcohol)
PVCPoly(vinyl chloride)
PVDFPoly(vinylidene fluoride)
P(VDF-CTFE)Poly(vinylidene fluoride-co-chlorotrifluoroethylene
P(VDF-HFP)Poly(vinylidene fluoride-co-hexafluoropropene
P(VDF-TrFE)Poly(vinylidene fluoride-co-trifluoroethylene)
PVKPoly(N-vinylcarbazole)
PVPPoly(vinyl pyrrolidone)
SBA-15Molecular Sieves
SiO2Silicon Dioxide
SNSuccinonitrile
Sn–CSn nanoparticles within a carbon matrix
SnO2Tin Dioxide
TEABF4Tetraethylammonium Tetrafluoroborate
TEGDMATetraethylene Glycol Dimethyl Ether
TEGDMETetra(ethylene glycol) Dimethyl Ether
TiO2Titanium Dioxide
TrFETrifluoroethylene
VDFVinylidene Fluoride
ZnOZinc Oxide
ZrO2Zirconium Dioxide

Acknowledgements

This work is funded by FEDER funds through the “Programa Operacional Factores de Competitividade-COMPETE” and by national funds from FCT-Fundação para a Ciência e a Tecnologia, in the framework of the strategic project Strategic Project PEST-C/FIS/UI607/2011and the projects PTDC/CTM/69316/2006, project no. F-COMP-01-0124-FEDER-022716 (refª FCT PEst-C/QUI/UI0686/2011) and NANO/NMed-SD/0156/2007, and grant SFRH/BD/68499/2010 (C.M.C.) The authors thank Celgard LLC, Timcal, Solvay, Arkema and Clariant for kindly supplying their high quality membranes and excellent materials, respectively. The authors also thank support from the COST Action MP1003, “European Scientific Network for Artificial Muscles” and COST action MP0902, “Composites of Inorganic Nanotubes and Polymers (COINAPO)”.

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