Electron microscopy for understanding swift heavy ion irradiation effects on Electroactive polymers

Transcription

1 A. Méndez-Vilas and J. Díaz (Eds.) Electron microscopy for understanding swift heavy ion irradiation effects on Electroactive polymers A. Kumar *, Somik Banerjee and M. Deka Materials Research Laboratory, Department of Physics, Tezpur University, Tezpur , Assam, India * Corresponding id: ask@tezu.ernet.in There is a growing interest in developing new smart materials or altering the properties of existing materials, which respond to external stimuli by changing their shape, size and/or properties. Conventionally polymers have been used as passive structural materials, however with the advent of electroactive polymers (EAPs) they are being used as active device components e.g. semiconducting and conducting polymers, ion conducting polymer electrolytes, sensors, actuators, light-emitters, active drug delivery devices etc. Swift heavy ion (SHI) irradiation is a novel technique for modification of materials at the molecular and electronic level. SHI interactions with EAPs bring about remarkable structural, conformational and morphological changes, which result in enhancement in their performance and properties such as conductivity, electrochemical stability, sensitivity, crystallinity, solubility, porosity, density etc. In this chapter, we present a comprehensive study of the SHI irradiation effects on polyaniline (PAni) nanofibers and PAni nanofiber reinforced gel polymer electrolytes using electron microscopy. Many interesting aspects of SHI interactions with conducting PAni nanofibers and PAni nanofiber reinforced gel polymer electrolytes are revealed. Electron microscopy has been used for explaining the variations in physico-chemical properties viz., crystallinity, phaseseparation, electronic and ionic conductivity, porosity and morphology of these materials upon SHI irradiation. HCl and CSA doped PAni nanofibers and de-doped nanofiber reinforced P(VdF HFP) based gel polymer electrolytes have been irradiated with 90 MeV O 7+ ions at different fluences. SHI irradiation causes fragmentation of the PAni nanofibers with increasing fluence as observed by transmission electron microscopy. XRD and micro-raman (µr) studies reveal amorphization of the PAni nanofibers which is corroborated by the TEM results. µr studies also reveal a benzenoid to quinoid transformation in the PAni nanofiber backbone chain. In the case of P(VdF HFP) based gel polymer electrolytes it has been observed that upon SHI irradiation the ionic conductivity of the electrolyte films increases at lower fluences ( ions/cm 2 ), whereas a decreasing trend in conductivity is observed at higher fluences ( ions/cm 2 ) as compared to the unirradiated film. Maximum ambient ionic conductivity of S/cm has been achieved after irradiation with a fluence of ions/cm 2. The increase in ionic conductivity at lower fluences could be attributed to the scissioning of polymer chains, which leads to larger segmental motion of polymer backbone resulting in faster ionic transport through the polymer matrix. However at higher fluences ( ions/cm 2 ), the PAni nanofibers get phase separated out from P(VdF HFP) (PC+DEC) LiClO 4 leading to a decrease in ionic conductivity. The phase separation has been observed by scanning electron microscopy and corroborated by DSC and dielectric loss spectra studies. Keywords: Electron microscopy, conducting polymers, polymer silicate nanocomposite electrolytes, swift heavy ion irradiation, micro-raman spectroscopy, dielectric loss spectra 1. Introduction A truly smart material possesses built-in sensing, processing, actuating, energy conversion and storage functions and is a dynamic material with chemical and physical properties that can be readily manipulated. Most of intelligent biosystems used by nature in the development of smart systems and structures are polymers. Antibodies and enzymes, which provide the molecular recognition capabilities; muscles, the most useful actuator systems; biosensors that allow us to smell, see, taste, touch and hear by converting physical and chemical stimuli into electric impulses that transmit along the nervous system, are all polymeric structures. Conversely, it is the effect of electrical stimuli on our macromolecular bio-systems that enables us to activate appropriate mechanical and other responses [1]. The field of conjugated, electrically conducting and electroactive polymers continues to grow, since the path breaking discovery of high conductivity in polyacetylene in 1977 [2]. In conventional polymers such as polyethylenes, the valence electrons are bound in sp 3 hybridized covalent bonds. Such "sigma-bonding electrons" have low mobility and do not contribute to the electrical conductivity of the material. Conducting polymers, on the other hand, have backbones of contiguous sp 2 hybridized carbon atoms. One valence electron on each carbon atom resides in a p z orbital, which is orthogonal to the other three sigma-bonds. The electrons in these delocalized orbitals have high mobility, when the material is "doped" by oxidation, which removes some of these delocalized electrons. Thus the p-orbitals form a band, and the electrons within this band become mobile when it is partially emptied. In principle, these same materials can be doped by reduction, which adds electrons to an otherwise unfilled band. However, as a practice most organic conductors are doped oxidatively to give p-type materials. Polymer electrolytes have attracted the attention of researchers worldwide for application in various electrochemical devices particularly in solid-state rechargeable lithium batteries mainly because of their flexibility and shape versatility [3]. The most commonly studied polymer electrolytes are the complexes of Li salts with high molecular weight 1755

2 A. Méndez-Vilas and J. Díaz (Eds.) polyethylene oxide (PEO) which has high solvating power for lithium salts and compatibility with lithium electrode [4, 5]. However, one of the major drawbacks of PEO based solid polymer electrolytes is their low ionic conductivity ( S/cm) at ambient temperature, which limits their practical applications [6-8]. On the other hand, plasticized or gel polymer electrolyte (GPE) can exhibit high ambient temperature ionic conductivity by immobilizing large amount of liquid electrolyte in the polymer host [9]. However, reasonable conductivity achieved of such plasticized film is offset by their poor mechanical properties at high plasticizer content [10]. An alternative strategy for creating polymer nanocomposites electrolytes with improved electrochemical and mechanical properties is to produce polymer silicate nanocomposites. These are a class of materials in which nano-scale clay particles are molecularly dispersed within a polymer matrix [11-13]. Clay mineral materials are inorganic fillers with intercalation properties. These fillers not only reduce the crystallinity of host polymers, resulting in higher ionic conductivity but also sustain the mechanical property of the polymers. Swift heavy ion (SHI) irradiation can modify the molecular structure in polymers in a controlled way leading to changes in their chemical, electronic, electrical, tribological and optical properties [14,15]. SHI (energy > 1 MeV/u) irradiation deposits the energy in the material in the near surface region mainly due to the electronic excitation [16]. Ion irradiation of polymers can induce irreversible changes in their macroscopic properties. Electronic excitation, ionization, chains scission and cross-links as well as mass losses are the events that give rise to the observed macroscopic changes. The impinging ions do not get implanted in the material due to their large range typically a few tens of µm. Ionization trail produced by SHI causes bond cleavages producing free radicals, which are responsible for most of the chemical transformations in polymers such as chain scission, cross linking, generation of active sites, double and triple bond formation, emission of atoms, molecules and molecular fragments [17, 18]. The structural modifications in polymers depend on electron energy loss S e and ion fluence. At low fluence, small value of S e produces decrease in free volume i.e. densification occurs. Beyond a threshold S e value SHI produces zone of reduced density along its trajectory (ion track). Electron microscopy has been found to be an efficient technique for understanding the structural and morphological modifications of materials upon SHI irradiation as function of type of ion, ion energy and ion fluence. Swift Heavy ion, while passing through a material, produces a narrow region of radiation affected material, known as a 'latent track'. The process of track formation upon SHI irradiation can be viewed as knocking out the orbital electrons of the target atoms lying along its trajectory producing cylindrical region full of positive ions, radicals and electrons creating a highly charged cylindrical strained-region due to Coulomb interactions. Such latent tracks having a diameter of a few hundred angstroms can only be seen with an electron microscope [19]. 2. Swift heavy ions interaction with electroactive polymers 2.1 Conducting polymers It is generally accepted that metals are electrical conductors and polymers are insulators, Conducting polymers are a special class of organic polymers that have alternate single-double bond conjugation and can conduct electricity when doped. They combine the mechanical properties such as flexibility, toughness, malleability, elasticity, etc. of plastics with high electrical conductivities, generally attributed to metals. However, the mobility of charge carries ( cm 2 /V.s) in conducting polymers is very low as compared to that in inorganic semiconductors ( cm 2 /V.s). This difference is diminishing with the invention of new polymers and the development of new processing techniques. The conjugated polymers in their undoped (pristine) state are semiconductors or insulators. The HOMO-LUMO (ππ*) energy gap is > 2 ev, which is too large for thermally activated conduction. Therefore, undoped conjugated polymers, such as polythiophenes, polyacetylenes have a low electrical conductivity of around to 10 8 S/cm. Even at a very low level of doping (< 1 %), electrical conductivity increases several orders of magnitude up to values of around 10 1 S/cm. Subsequent doping of the conducting polymers results in a saturation of the conductivity at values around S/cm. Highest values reported up to now are for the conductivity of stretch oriented polyacetylene with values of around S/cm [20]. The properties of conducting polymers are greatly affected by SHI irradiation. Conducting polymers show enhancement in electronic conductivity upon exposure to SHI irradiation. The increase is due to the electronic stopping mechanism, which produces large number of charged and active chemical species, cations, anions, radicals and electrons along the ion track. Coulombic interactions among these active charged species may give rise to either crosslinking or bond breaking depending upon the energy and fluence of the irradiating beam. Collective excitations (plasmons), which produce a large excited volume resulting in coercive interaction among the ions and radical pairs produced in the adjacent chains within the volume as a result of electron stopping mechanism, are responsible for crosslinking of the polymer chains. Inter-chain electron hopping required for conduction between two chains, which increases the resistance of the polymer, is facilitated due to the cross-linking of the polymer chains after irradiation. The increase of crystallinity of the polymer films upon SHI irradiation also contributes to the increase in conductivity. At high energy (~100 MeV) irradiation of polymer, cross linking/ recrystallization processes dominate over chain scission 1756

3 A. Méndez-Vilas and J. Díaz (Eds.) and bond breaking processes, which increase the conductivity by enhancing the carrier mobility through the polymer chains which otherwise occurred by the inter chain hopping mechanism required for conduction between the chains. Defect sites in the molecular structure of the polymer chain created by SHI irradiation also contributes to higher dc conductivity as charge accumulation takes place which produces charge carriers, the electrons. Electron microscopy has been found to be very useful in understanding the morphological changes in conducting polymers upon SHI irradiation. Conducting polymers are found to exhibit changes in their grain size upon irradiation with swift heavy ions. The surface morphological studies of the poly(3-methylthiophene) conducting polymer films pdoped LiCF3SO3 dopant carried out by taking SEM images after irradiation with 120 MeV Si9+ ion at different fluences of 5 x 1010, 5 x 1011 and 3 x 1012 ions/cm2 are presented in Fig 1(a-d) [21]. The changes observed in the morphology of the polymer films have been ascribed to the grain growth, agglomeration and displacement of the polymer molecules from hilly to valley regions making the film surface dense and smooth due to huge energy deposition in the conducting polymer film as a result of electronic energy loss mechanism. A decrease in porosity of the polymer films is observed after SHI irradiation due to densification owing to the cross-linking of polymer molecules. Fig 1: SEM micrographs of LiCF3SO3 doped poly (3methylthiophene) films (a) before, and after irradiation with fluence (b) 5 x 1010, (c) 5 x 1011 and (d) 3 x 1012 ions/cm2. Fig 2: SEM micrograph of LiCF3SO3 doped polypyrrole (a) before, and after irradiation with fluence (b) 5 x 1010, (c) 5 x 1011 and (d) 3 x 1012 ions/cm2.. However the surface morphological studies of the polypyrrole conducting polymer films doped with LiCF3SO3 carried out by taking SEM images before and after irradiation at different fluences of 5 x 1010, 5 x 1011 and 3 x 1012 ions cm-2 presented in Fig 2(a-d) show cauliflower like flaky surface morphology. As the fluence of SHI irradiation increases the grain size decreases and at the fluence of 3 x 1012 ions/cm2, the surface of the polymer films becomes smooth and dense [22]. 2.2 Gel polymer electrolytes The path breaking studies of Wright and Armand on ionically conducting polymers, called polymer electrolytes in the 1970s have opened an innovative area of materials research with potential applications in the power sources [23]. The main applications of the polymer electrolytes are in rechargeable lithium batteries as an alternative to liquid electrolytes [24]. The gel polymer electrolyte systems based on poly(methyl methacrylate) (PMMA) [25-29] have been proposed for lithium battery application, particularly because of their beneficial effects on stabilization of the lithium electrolyte interface [30]. However, reasonable conductivity achieved of such plasticized film is offset by poor mechanical properties at high plasticizer content. Rajendran et al. [25, 26] were able to improve the mechanical property of PMMA by blending with poly(vinyl alcohol) (PVC). However, a decrease of ionic conductivity was observed due to higher viscosity and lower dissociability of lithium salt. Recently poly(vinylidenefluoride) (PVdF) as a host has drawn the attention of many researchers due to its high anodic stability and high dielectric constant (ε = 8.4) which helps in greater ionization of lithium salts [31]. Unfortunately, PVdF-based polymer electrolytes suffer due to syneresis; a phenomenon by which the liquid component separates out from the host matrix in due course or upon application of pressure leading to battery leak and related safety problems. Recently, poly(vinylidenefluoride-co-hexafluoropropylene) {P(VdF-HFP)} based systems have drawn the attention of researchers because of their various appealing properties like high dielectric constant, low crystallinity and glass transition temperature [32]. P(VdF-HFP) has excellent chemical stability due to VdF unit and plasticity due to HFP unit. The addition of nanoparticles to P(VdF-HFP) leads to superior mechanical strength and enhances its electrochemical stability and interfacial stability with electrode [33]. Essentially because of this idea, nanocomposite polymer electrolytes, wherein nano-sized inert solid particles are added to the polymer electrolytes, are presently the focus of many studies, both practical as well as theoretical [34]. 1757

4 A. Méndez-Vilas and J. Díaz (Eds.) Permanent modifications in the molecular weight distribution and solubility [35], electrical [36], optical [37] and mechanical properties [38] of polymers and other materials have been detected after ion irradiation. Crystallinity is also affected by ion irradiation and most semi-crystalline polymers exhibit a decrease in crystallinity at high dose irradiation ( 1MGy ions cm 2) [39]. Ordering of the molecular chains after low fluence ( 200 kgy ions cm 2) light ion irradiation has also been reported [40]. The recrystallization phenomena in polymers as a consequence of constructive phase transition, i.e. transition from amorphous to crystalline phase characterizing the growth of new crystallites or even formation of new lamellar stacks upon irradiation, is a relatively new area of research offering many potentialities [41]. Crystallinity plays a crucial role in almost all polymer properties, such as mechanical, optical, electrical and even thermal properties. SHI on polymer electrolytes changes the surface morphology completely which can be clearly observed from microscopy experiments. Upon ion irradiation the porosity of the electrolytes membranes increases, which helps to trap more liquid electrolyte in the same volume of polymer resulting in higher ionic conductivity. SEM images of C5+ and Li3+ ions irradiated P(VDF HFP) (PC + DEC) LiClO4 gel polymer electrolyte systems are shown in Fig. 3 [42, 43]. Fig. 3: SEM images of (a) Unirradiated P(VDF HFP) (PC + DEC) LiClO4 gel polymer electrolyte, (b) C5+ irradiated P(VDF HFP) (PC + DEC) LiClO4 gel polymer electrolyte, and (c) Li3+ irradiated P(VDF HFP) (PC + DEC) LiClO4 gel electrolyte SHI irradiation of the conducting polymer nanofibers and the polymer electrolyte films was carried out under high vacuum ( Torr) in the Materials Science (MS) Chamber of Inter University Accelerator Centre, New Delhi (India) with 90 MeV O7+ at different fluences. For PAni nanofibers the fluence was varied from 3 x 1010 to 1 x 1012 ions cm-2, whereas for the polymer electrolyte films it was varied from 5x1010 to 1x1012 ions cm-2 at normal beam incidence. The samples were loaded on four sided ladder (six samples on each side), which could be rotated and moved up and down to bring a particular sample in front of the ion beam for irradiation. Structural changes of PAni nanofibers upon irradiation were investigated by a JEOL-JEM 100 CX-II transmission electron microscope. Surface morphology of the composite electrolytes was studied by using Scanning Electron Microscope (JEOL model JSM 6390 LV). X-ray diffractograms were studied by Rigaku miniflex X-ray diffractometer. Micro-Raman spectra were acquired using a Renishaw in-via Raman spectroscope (Ar ion laser; power: 0.5 mw; excitation: nm for 10 s). The ionic conductivity of the polymer electrolyte films was determined from the complex impedance plots obtained by using a Hioki LCR HiTester in the frequency range from 42 Hz to 5MHz keeping the electrolyte films between two stainless steel blocking electrodes. 3. SHI irradiation effects on PAni nanofibers Polyaniline (PAni) has been one of the most widely studied conjugated polymers due to its environmental stability and reversible acid base doping dedoping chemistry. In recent years, one-dimensional (1-D) nanostructures of polyaniline have attracted growing attention due to the potential advantage of having low-dimensionality. Such materials are most useful for applications that depend on ultra-small, high surface area features such as chemical sensors [44]. PAni nanofibers were synthesized using the interfacial polymerization method by using HCl and camphor-sulfonic acid (CSA) as dopants []. The synthesized nanofibers were purified and dispersed uniformly in a 2% PVA solution. The solution was then cast on glass plates (1cm x 1cm) for irradiation purpose. ). Swift heavy ion (SHI) irradiation has been used as an efficient tool for enhancing the physico-chemical properties of conducting polymers such as conductivity, electrochemical stability, sensing properties [14, 45, 46] etc. 3.1 Transmission electron microscopy Figs. 4(a-d) and 5(a-d) are the transmission electron micrographs of the pristine and 90 MeV O7+ ion irradiated PAni nanofibers doped with HCl and CSA, respectively. A continuous decrease in the size of the nanofibers with increasing ion fluence is revealed by the TEM micrographs. It is observed from the micrographs that with the increase in irradiation fluence the average diameter of the HCl doped PAni nanofiber decreases from 40 nm to about 10 nm. In case of the CSA doped PAni nanofibers, the average diameter decreases from 50 nm to 15 nm with increasing irradiation 1758

5 A. Méndez-Vilas and J. Díaz (Eds.) fluence. The length of the CSA doped PAni nanofibers decreases as the irradiation fluence increases but at a fluence of 1 x 1012 ions/cm2 the length of the PAni nanofibers is found to increase slightly, however, the diameter decreases. Fig. 4 Transmission electron micrographs of HCl doped polyaniline nanofibers (a) before and after irradiation with 90 MeV O7+ ions at fluences (b) 3 x 1010, (c) 3 x 1011 and (d) 1 x 1012 ions/cm2. Fig. 5 Transmission electron micrographs of CSA doped polyaniline nanofibers (a) before and after irradiation with 90 MeV O7+ ions at fluences (b) 3 x 1010, (c) 3 x 1011 and (d) 1 x 1012 ions/cm2 The interaction of ionizing radiation with polymers leads to irreversible modifications of the structure and the chemical composition [47]. Mainly two phenomenological models have been proposed to describe the formation of tracks and the conversion mechanism of the energy of the excited electrons into the kinetic energy of the target atoms in materials. The Coulomb-explosion model is based on the assumption that the intense ionization and excitation along the ion path leads to an unstable zone in which atoms are ejected into the non-excited part of the solid by Coulomb repulsion [48]. In the thermal-spike model, the energy deposited by the ion leads to a transient temperature increase and the cylindrical volume around the ion path melts due to electron-phonon coupling, which is subsequently quenched by thermal conduction [49]. Conjugated (conducting) polymers are organic materials that conduct when doped. Depending upon the degree of doping, the carrier concentration in conducting polymers is comparable to that of semiconductors/metals, the carrier mobility, however, is very low (~ cm2/v.s) as compared to that of semiconductors/metals (~ cm2/v.s). Keeping this in mind Coulomb explosion seems to be more likely. However, there is no evidence of Coulomb explosion occurring in materials except for alkali halides. Even if Coulomb explosion does occur in conducting polymer it will lead to a local temperature rise and will be followed by a thermal spike. Within the tracks thus generated, the PAni nanofibers get amorphized and fragmentation is observed. As the ion fluence increases, the tracks overlap and fragmentation increases leading to a reduction in the size of the PAni nanofibers. 3.2 Variations in domain length and strain X-ray diffraction patterns of the pristine and irradiated PAni nanofibers doped with CSA are shown in the Fig. 6. The xray spectra reveals that the peak due to (100) reflection centered at 2θ = 200, ascribed to the parallel periodicity of PAni, is the most prominent peak. The (110) reflection peak at attributed to the perpendicular periodicity of PAni is of very low intensity. Figure 7 presents the changes in the most intense (100) reflection peak of CSA doped PAni nanofibers as a function of increasing ion fluence. Full width at half maxima (FWHM) of the (100) peak increases as a function of irradiation fluence. Line broadening in the x-ray spectra may be attributed to two major factors viz., the size and strain contributions. The former depends on the size of coherently diffracting domains, not limited to only the grains but may also include the effects of stacking and twin faults and subgrain structures such as small-angle boundaries. The latter is caused by lattice imperfections viz., different point defects and dislocations [50]. 1759

6 A. Méndez-Vilas and J. Díaz (Eds.) Fig. 6 X-ray diffraction patterns of pristine and irradiated polyaniline nanofibers doped with CSA Fig. 7 X-ray diffraction patterns of the pristine and irradiated CSA doped PAni nanofibers showing the most intense (100) peak of PAni The x-ray diffraction patterns also indicate that the degree of crystallinity of the PAni nanofiber samples decreases as the most intense (100) peak shows broadening. Single line approximation technique employing Voigt function [51] has been used to separate the contribution of crystallite size (referred to as domain length or the range of order (L) in case of polymers) and strain towards line broadening which are given in Table 1. The table also includes the values of the dspacings deduced from the angular positions 2θ of the observed reflections using Bragg s formula. It is observed that as the irradiation fluence is increased the domain length (L) of both HCl and CSA doped PAni nanofibers decreases whereas the strain increases. Since the Cl/N ratio has been kept constant for all the samples, the decrease in the domain length can be interpreted as a result of the reduction in particle size upon SHI irradiation, which is corroborated by the TEM micrographs. Table 1: Domain length (L), strain and d-spacings of HCl (S1) and CSA (S2) doped PAni nanofibers Ion Fluence (ions/cm2) Pristine 3 x x x 1012 L (Å) S1a Strain (%) S2b S1a d (Å) S2b S1a S2b The increase in d-spacing with irradiation fluence suggests an increase in the tilt angle of the chains with respect to the (a, b) basal plane of polyaniline [52]. The x-ray patterns thus reveal that not only the crystalline domain length but also the conformation of the PAni back-bone chain changes with the increasing irradiation fluence. 3.3 Micro-Raman analysis of the conformational changes after SHI irradiation Figure 8 shows the micro-raman (µr) spectra of the pristine and irradiated CSA doped PAni nanofibers. In the µr spectra of the pristine PAni nanofibers [Fig. 8a], the bands at cm 1, cm 1 and cm-1 correspond to the C H benzene deformation modes and C=N stretching mode of the quinoid units. The bands at cm 1 and cm 1 are the signatures of the delocalized polaronic charge carriers which indicate that the PAni nanofibers are in doped ES-I form. The band at cm-1 is characteristic of the semiquinone radical cation. The absorption peak at cm 1 corresponds to the N H bending deformation band of protonated amine. The C C deformation bands of the benzenoid ring positioned at cm 1 are characteristic of the semiquinone rings. It is observed that all the Raman active modes decrease as the irradiation fluence is increased which indicate amorphization of the material upon irradiation. The amorphization may be attributed to the reduction in size as is evident from the TEM micrographs. Figure 9 shows the µr spectra of the pristine and irradiated CSA doped PAni nanofibers within the range of cm-1. It is observed in the micro-raman spectra [Figs. 6 (a-d) and 7] that upon SHI irradiation, the symmetric C=C stretching peak around 1595 cm-1 can be deconvoluted into two sub-peaks. The main peak is assigned to the benzenoid structure whereas the shoulder peak is a signature of the presence of quinoid structure. As the fluence of SHI irradiation increases the main peak of the symmetric C=C stretching mode is up-shifted from to cm-1 [Fig. 9] and the intensity decreases from 1228 to The shoulder peak of the symmetric C=C stretching mode is also upshifted from to cm-1 but the intensity of the peak increases from to upon SHI irradiation. The FWHMs of both the main and shoulder peak of PAni nanofibers increases by 6.16 cm-1 and 7.29 cm-1, respectively upon SHI irradiation. This is indicative of the fact that the intercyclic rings of the PAni nanofibers become distorted; the 1760

7 A. Méndez-Vilas and J. Díaz (Eds.) proportion of the benzenoid structure decreases, while there is a corresponding increase in that of the quinoid structure upon SHI irradiation. The distortion of PAni influences the π-conjugation length of the PAni chains, which is associated with π-electron delocalization accounting for a reduction in the coplanarity of the aromatic ring and π-conjugation length [53]. The observed up-shifting of the symmetric C=C stretching mode can be attributed to the shortening of πconjugation length (i.e., π-electron delocalization) of PAni nanofibers upon SHI irradiation. Fig 8: Micro-Raman spectra of CSA doped polyaniline nanofibers (a) before, and after irradiation with 90 MeV O7+ ions at fluences (b) 3 x 1010, (c) 3 x 1011 and (d) 1 x 1012 ions/cm2 Fig 9: A comparison of the deconvoluted sub-peaks corresponding to the symmetrical C=C stretching modes of the pristine and 90 MeV O7+ ion irradiated PAni nanofibers at different fluences 4. SHI irradiation effects on nanofiber reinforced gel polymer electrolytes In this section the SHI irradiation effects on nanocomposite gel polymer electrolyte films comprising P(VdF-HFP) copolymer, (PC+DEC) as plasticizer, LiClO4 as salt and dedoped PAni nanofibers as insulating fillers will be discussed in view of our results obtained from electron microscopy, XRD, DSC and dielectric loss spectra. The host copolymer poly(vinylidenefluoride-co-hexafluoropropylene) {P(VdF-HFP)} (Mw 40,0000) and salt lithium perchlorate (LiClO4), received from Aldrich, USA, were heated at 50º C under vacuum before use to remove the moisture. Organic solvents propylene carbonate (PC) and diethyl carbonate (DEC) were used without further treatment as obtained from E-Merck. P(VdF-HFP)-(PC+DEC)-LiClO4- dedoped PAni nanofibers polymer electrolyte films were prepared by conventional solution casting technique. Predetermined amounts of P(VdF-HFP), LiClO4 and (PC+DEC) were dissolved in acetone in the ratio of 6:1.5:1.5:1 (by weight). Subsequently the solution was stirred at 50 C for 12 hours. Subsequently the dedoped polyaniline nanofibers were added in the gel polymer solution and stirred for another 12 hours. The viscous solution, thus obtained, was cast onto Petri dish and allowed to dry at room temperature to obtain free standing polymer electrolyte films of ~ µm. The weight ratio of PAni nanofibers were fixed at 6 wt. %, since at this concentration of PAni nanofibers highest ionic conductivity was obtained [54]. 4.1 Ionic conductivity and X-ray diffraction analysis Fig. 10 presents the temperature dependence of ionic conductivity of unirradiated and irradiated nanocomposite gel electrolytes with different fluences in the temperature range from 250C to 700C. It is observed that the ionic conductivity of the nanocomposite gel polymer electrolytes increases with increase in fluence and attains a maximum value of S/cm at the irradiation fluence of 1011 ions/cm2. Above that fluence a decreasing trend of ionic conductivity is observed as compared to that for the unirradiated samples. This could be attributed to the fact that at lower fluences, the bonds in the polymer chains are broken and chain scission process dominates, which leads to faster ion transport through the polymer matrix assisted by the increased segmental motion of the polymer backbone [42]. However at higher fluence (> 1011 ions/cm2) phase separation of PAni nanofibers takes place, which results in decrease in ionic conductivity. Phase separation occurs when PAni nanofibers-rich region in the electrolyte becomes more PAni nanofibers-rich and P(VdF-HFP)-rich phase becomes more P(VdF-HFP)-rich. This requires the mobility of the atoms or molecules in the electrolyte. The activation for the mobility is provided by the ion impact of SHI at higher fluences. After phase separation, it becomes easier for the polymer to shrink that increases its density, which in turn results in chain folding and cross-linking of polymers, causing the formation of new crystalline regions [55] leading to a decrease in ionic conductivity. It is worth mentioning that during irradiation, the energy deposited in the polymer causes chain scission or produce radicals which subsequently decay or cross-link with neighbouring radicals i.e. both chain scission 1761

8 A. Méndez-Vilas and J. Díaz (Eds.) and cross-linking occur during irradiation. At low fluence, chain scission predominates because of wide spatial separation of radicals on different chains, which cannot crosslink. However, as fluence increases radical concentration increases resulting in formation of closely spaced radicals along the ion track. As a result coercive interaction among the radical pairs increases, which eventually allows the adjacent polymer chains to cross-link [56]. As shown in Fig. 10 the temperature dependence of ionic conductivity follows the Vogel-Tamman-Fulcher (VTF) behaviour governed by the equation [41, 43, 57] σ = σ0 exp [-Ea /k(t-t0)] (1) where σ, σ0, Ea, k and T are the ionic conductivity, pre exponential factor, activation energy, Boltzmann constant and temperature in Kelvin, respectively. T0 is the idealized glass transition temperature of the polymer, which is K below the glass transition temperature of the polymer [41, 43]. As expected the increase in temperature leads to increase in ionic conductivity because as the temperature increases the polymer chains flex at increased rate to produce more free volume resulting in enhanced polymer segmental mobility. Fig 10: Temperature dependence of ionic conductivity of P(VdF-HFP)-(PC+DEC)-LiClO4-6 wt. % dedoped PAni nanofibers composite gel polymer electrolyte (a) Unirradiated, and O 7+ ion irradiated with fluence (b) ions/cm2, (c) 1011 ions/cm2, (d) ions/cm2, (e) 1012 ions/cm2. Fig 11: XRD patterns of P(VdF-HFP)-(PC+DEC)LiClO4-6 wt. % dedoped PAni nanofibers composite gel polymer electrolyte (a) Unirradiated, and O 7+ ion irradiated with fluence (b) ions/cm2, (c) 1011 ions/cm2, (d) ions/cm2, (e) 1012 ions/cm2. XRD patterns of unirradiated and 90 MeV O7+ ions irradiated P(VdF-HFP)-(PC+DEC)-LiClO4- dedoped PAni nanofibers composite gel polymer electrolytes are shown in Fig 11. Literature reveals that the three peaks observed at 2θ=18.5º, 20 and 380 for pure P(VdF-HFP) correspond to (100), (020) and (021) reflections of P(VdF-HFP) [54]. In the present case, the unirradiated nanocomposite gel polymer electrolyte (Fig. 11a) exhibits a prominent peak only at 2θ=20º and other peaks are decreased in intensity implying that crystallinity decreases. Normally, for pure PVdF, an increase in crystallinity is observed at lower irradiation fluences (<1011 ions/cm2), whereas crystallinity decreases at higher fluences [58]. However, in P(VdF-HFP) the presence of HFP units helps in reducing the crystallinity of the polymer. Moreover, the addition of high aspect ratio (> 50) dedoped PAni nanofibers prevent polymer chain reorganization resulting in decreased crystallinity compared to pure P(VdF-HFP) [54]. As the fluence increases, the (020) peak at 2θ=20º gets broadened up to a fluence of 1011 ions/cm2 indicating that degree of crystallinity is decreased. Table 2: Degree of crystallinity, volume occupation, d-spacing, crystallite size (lc) and micro-strain (ε) for (020) reflection and ionic conductivity (σ) of P(VdF-HFP)-(PC+DEC)-LiClO4-6 wt. % PAni nanofibers at different irradiated fluences Fluence (ions/cm2) Degree of crystallinity (%) Volume occupation (Vp) d-spacing (Å) Crystallite size (lc) (nm) Microstrain (ε) Ionic conductivity (ms/cm) The degree of crystallinity is determined by a method described elsewhere [43] and the values of degree of crystallinity at different fluence are given in Table 2. Above 1011 ions/cm2 an additional peak appears at 2θ=23º, which can be assigned to dedoped PAni nanofibers, indicating the phase separation of dedoped PAni nanofibers from P(VdFHFP) matrix at higher irradiation fluence. As observed from the Fig. 11, the (020) reflection of P(VdF-HFP) changes 1762

9 A. Méndez-Vilas and J. Díaz (Eds.) significantly upon ion irradiation. This change has been analyzed quantitatively using single-line approximation method employing double Voigt function [51] and the d-spacing, crystallite size (lc) and strain (ε) have been calculated at different irradition fluences, which are presented in Table 2. This procedure involves the extraction and analysis of Gaussian (βg) and Lorentzian (βl) component of integral breadth of a single Bragg peak corrected for instrumental broadening. It is observed from the crystallite size and microstrain analysis of the irradiated polymer electrolyte films that the crystallite size and microstrain decrease with the increasing ion irradiation fluence up to 1011 ions/cm2 and increase at higher fluence (> 1011 ions/cm2). 4.2 Scanning electron microscopy Scanning electron micrographs of pristine P(VdF-HFP), unirradiated and 90 MeV O7+ irradiated P(VdF-HFP)(PC+DEC)-LiClO4- dedoped PAni nanofibers composite gel polymer electrolytes at fluences of 1011 and 1012 ions/cm2 are shown in Fig. 12 (a-d). It is observed that pure P(VdF-HFP) shows porous structure with uniform pore distribution (Fig. 12a). Addition of high aspect ratio dedoped PAni nanofibers resulted in improved morphology (Fig. 12b), since the nanofibers occupied the pores along with the plasticizers. Highly porous surface morphology of the polymer electrolytes (unirradiated) as compared to that of pure P(VdF-HFP) is due to the interaction of dispersed dedoped (insulating) nanofibers with polymer component as well as the affinity with solvent molecules [54]. (a) (b) (c) (d) Fig 12: SEM micrographs of (a) pure P(VdF-HFP), irradiated with fluence (b) 0, (c) 1011 ions/cm2 and (d) 1012 ions/cm2 Upon irradiation with lower fluence (1011 ions/cm2) the porous structure becomes much denser with well dispersed pores (Fig. 12c), which leads to better connectivity of the liquid electrolyte through the pores accounting for the increase in ionic conductivity. On the other hand, at higher fluence of 1012 ions/cm2, the porous structure is disrupted possibly due to recrystallization of the polymer (Fig. 12d). The two phase microstructure in the SEM image reflects the phase separation at higher fluence of 1012 ions/cm2. It is well known that porosity plays an important role in determining the ionic conductivity in gel polymer electrolytes [59]. Higher is the porosity of the polymer, larger is the entrapment of liquid electrolytes in the pores and eventually higher is the ionic conductivity. The change in porosity in the present nanocomposite gel polymer electrolytes upon SHI irradiation at different fluences have been determined by measuring the volume occupation (Vp) of the polymer electrolytes [60] given by Vp = ρapp / ρpoly (2) where ρapp is the apparent density of the polymer electrolyte and ρpoly is the calculated density of the polymer electrolyte from the volume fraction and density of individual components in the electrolyte. The volume occupation Vp of unirradiated and 90 MeV O7+ ion irradiated P(VdF-HFP) based nanocomposite gel polymer electrolyte is given in Table 2. It shows that porosity increases (i.e. Vp decreases) as a function of ion fluence up to 1011 ions/cm2, which results in more entrapment of liquid electrolytes in the pores giving rise to increase in ionic conductivity. At higher fluence (>1011 ions/cm2) porosity decreases probably due to crystallization of polymer [61] exuding the liquid electrolyte out of the pores resulting in the decrease in the ionic conductivity. 4.3 Dielectric loss spectra and DSC analysis The evidence of phase separation is further provided by the dielectric loss spectra. Dissipation of energy due to an alternating electric field is termed as dielectric loss and is generally consists of contributions from ionic transport and polarization of dipole [62]. Fig. 13 (a-e) presents dielectric loss as a function of frequency for unirradiated and irradiated P(VdF-HFP)-(PC+DEC)-LiClO4-6 wt. % dedoped PAni nanofibers composite gel polymer electrolytes. It is observed from the figures that for unirradiated (Fig. 13a) and irradiated samples at lower fluences (< 1011 ions/cm2 ) (Fig. 13b-c) the dielectric loss decreases with increasing frequency and becomes constant after a certain frequency. However, for samples irradiated with higher fluence (>1011 ions/cm2), a sudden rise of dielectric loss is observed giving rise to a polarization peak. The polarization peak appears when the stress given by the electric field cannot be dissipated by ionic transport. This can be attributed to the phase separation of PAni nanofibers from P(VdF-HFP matrix at higher irradiation fluence (> 1011 ions/cm2). After phase separation, two phases exist in the electrolyte, one is PAni-rich phase and other is plasticizer-rich P(VdF-HFP) phase. It is difficult for the ions at the interface along the PAni-rich phase to 1763

10 A. Méndez-Vilas and J. Díaz (Eds.) conduct giving rise to decreased ionic conductivity and increased dielectric loss. The appearance of polarization peaks at higher irradiation fluence (>1011 ions/cm2) is a strong evidence of accumulation of ions at the interface between PAni-rich phase and plasticizer-rich phase. On the other hand polarization peak is not observed in the samples irradiated with lower fluences (< 1011 ions/cm2) suggesting that homogenous mixture is retained at lower fluences. Fig. 13: Dielectric loss spectra of P(VdF-HFP)(PC+DEC)-LiClO4-6 wt. % dedoped PAni nanofibers composite gel polymer electrolytes: (a) Unirradiated, and 90 MeV O7+ ion irradiated with fluence (b) ions/cm2, (c) 1011 ions/cm2, (d) ions/cm2, (e) 1012 ions/cm2 Fig. 14: DSC thermograms of (a) pure P(VdFHFP), (b) pure dedoped PAni nanofibers and P(VdF-HFP)-(PC+DEC)-LiClO4-6 wt % PAni nanofibers composite gel polymer electrolytes irradiated with fluence (c) 0, (d) 1011 ions/cm2 and (e) 1012 ions/cm2. DSC thermograms, shown in Fig. 14, reveal that the melting temperature of P(VdF-HFP) appearing at 148 oc (Fig. 14a) is reduced and broadened upon low irradiation fluence of 1011 ions/cm2 (Fig. 14d) suggesting that the degree of crystallinity is reduced. However, at the fluence of 1012 ions/cm2 (Fig. 14f), peak around 250 oc, which is due to the melting peak of PAni nanofibers (Fig. 14e) reappears, which is indicative of occurrence of phase separation. 5. SHI irradiation effects on polymer-clay based nanocomposite electrolytes Recently, the synthesis of nanocomposites based on intercalation of polymers into layered inorganic host lattices has shown a very promising future due to their enhanced thermal stability, mechanical performance, molecular barrier, flame retardance, corrosion protection and electrorheology properties even with small loadings of layered inorganic compounds [63]. These layered materials also serve as model systems for understanding the effect of confinement of the polymers on properties in relation to those in bulk [64]. Among the most commonly used inorganic layered host, montmorillonite (MMT) is a favoured choice in view of its special features of high aspect ratio (~ 1000), high cationexchange capacity (CEC ~ 80 meq/100 g), large specific surface area (~ m2 g-1), appropriate interlayer charge (~ 0.55) and length scale (clay channel width = 16 Å). Intercalating polymer in the layered host can produce a polymer nanocomposite electrolyte with huge interfacial area. A higher interfacial area not only enhances the mechanical properties of PVDF-based gel polymer electrolytes but also increases the solubility of lithium salts due to higher dielectric property of the layered host. In this section an attempt will be made to understand the effect of MMT clay addition on ionic transport and morphological change of Poly(vinyledene flouride) based gel electrolytes. PVdF (Sigma Aldrich, Mw = 2,75,000), propylene carbonate (PC, E-Merck, Germany), diethyl carbonate (DEC, E-Merck, Germany), lithium perchlorate (LiClO4, Sigma Aldrich) were vacuum dried prior to use wt. % octadecylamine modified montmorillonite clay (MMT) (Sigma Aldrich) was used as nanoclay. Tetrahydrofuran (THF, E-Merck, Germany) and other analytical grade reagents were used without purification. Different quantities of modified MMT (1, 2.5 and 4 wt. %) were dispersed in 5 ml of THF by ultrasonication. The dispersed MMT solution was then mixed with PVdF solution (15% solid content (w/v) in THF) by mechanical stirring at 50 0C followed by ultrasonication for half an hour. The nanocomposite films were obtained by solution casting method and dried under vacuum at room temperature. The dried films were denoted as PVdF1, PVdF2.5 and PVdF4 corresponding to nanoclay loading of 1, 2.5 and 4 wt. %, respectively. 5.1 X-Ray diffraction analysis Fig. 15 shows the XRD patterns of organically modified MMT (Fig. 15a), pure PVdF (Fig. 15b) and PVdF-clay nanocomposites (Fig. 15c-e) with different wt. % of MMT loading. Pure MMT exhibits (001) diffraction peak at 2θ = (Fig. 15a) corresponding to the interlayer spacing of 20 Ǻ. This peak is shifted to 2θ = 2.550, 2.40 and corresponding to the interlayer spacing of 34.6 Ǻ, 36.5 Ǻ and 37.5 Ǻ for 1, 2.5 and 4 wt. % clay loading in the nanocomposites, respectively as shown in Fig 15 (c-e). The gallery spacing increases by more than 1 nm in the nacomposites as compared to that for pure MMT exhibiting intercalation of polymer chains inside the galleries of the 1764

11 A. Méndez-Vilas and J. Díaz (Eds.) layered silicate. At low clay content of 1 wt. %, the intercalation of the polymer leads to the disordering of the layered clay structure leading to a decrease in the XRD scattering intensity, as shown in Fig. 15b and demonstrates both the exfoliation and intercalation properties. With the further addition of clay, the intensity of the d001 peak increases, indicating intercalation of the polymer in the interlayers without disruption of the ordered structure [65]. However the intensity of the (001) diffraction peaks for 2.5 and 4 wt. % clay loading is low as compared to that for pure MMT suggesting that the clay layers are partially exfoliated. Fig. 15: XRD patterns (a) pure organophilic MMT, (b) pure PVdF, (c) PVdF1, (d) PVdF2.5 and (e) PVdF4 Fig. 16: XRD patterns of (a) PVdF4 and O7+ ion irradiated PVdF4 with fluence (b) ions/cm2, (c) 1011 ions/cm2, (d) ions/cm2 and (e) 1012 ions/cm2 For irradiation purpose, PVdF4 sample has been selected as it shows the highest intercalation among all the compositions. The XRD results of 90 MeV O7+ ion irradited PVdF4 with four different fluences viz ions/cm2, 1011 ions/cm2, ions/cm2 and 1012 ions/cm2 are presented in Fig. 16 (a-e). Upon irradition with lower fluence (< 1011 ions/cm2) the (001) diffraction peak shifts more towards lower 2θ indicating higher gallery spacing of 3.96 nm and 4.1 nm than that of the pristine PVdF4 for ions/cm2 and 1011 ions/cm2, respectively (Fig. 16b-c). However, at higher fluence (.> 1011 ions/cm2) there is hardly any diffraction peak in the range 2θ = indicating the possibility of having exfoliated silicate nanolayers of organophilic clay dispersed in PVdF matrix (Fig. 16d-e). Thus irradiation of PVdF4 with lower fluence (< 1011 ions/cm2) leads to mainly intercalated nanocomposites whereas higher fluence irradiation (> 1011 ions/cm2) results in the formation of exfoliated nanocomposites. During irradiation, SHI creates a cylindrical molten zone of a few nanometers, transiently along its path, during which the temperature of the sample is quite high and the viscous polymer can easily diffuse into the gallery to cause more intercalation [66]. Larger intercalation of polymer chains causes more strain inside the gallery, which ultimately leads to the exfoliation of MMT clay at higher fluence (> 1011 ions/cm2). 5.2 Ionic conductivity and TEM analysis Intercalated and exfoliated as well as fragmented structures can be directly observed in TEM micrograph shown Fig. 17. The TEM micrographs show that PVdF-MMT has a mixed nanostructure. The clay layers are exfoliated and distributed randomly at small MMT clay loading of 1 wt. %. The extent of MMT exfoliation is more than that of intercalation in case of PVdF1 as observed in Fig. 17a. However, PVdF4 sample with higher MMT loading of 4 wt. % exhibits mostly intercalated morphology (Fig. 17a) with well diffused polymer chains into the clay layers. The TEM results corroborate the XRD results. For ionic conductivity measurement the nanocomposite gel polymer electrolytes were prepared by immersing the above synthesized films into a liquid electrolyte solution containing 1M LiClO4 in PC:DEC = 1:1 (v/v) for a period of 10 h. The percentage of electrolyte uptake by the polymer electrolyte films was calculated by the relation: Uptake (%) = (Wt W0)/W0 100 (3) where Wt and W0 are the weights of the wet and dry films, respectively. It was found that the weight of both PVDF and PVDF-clay nanocomposites increases with the increase of soaking period up to 6 h, but the increase of % of swelling for PVdF-clay nanocomposites was more than that for PVDF for the same soaking period. With the increase of electrolyte absorption as a function of clay loading, the room temperature ionic conductivity increases and attains a maximum value of S/cm at 4 wt. % clay content as shown in Fig. 18. It is to be noted here that above 4 wt. % of clay loading the polymer nanocomposite films become brittle and lose their mechanical integrity and cannot be used in battery fabrication, so the ratio of organophilic MMT to PVDF should be controlled up to 4 wt. %. The highest ionic conductivity at 4 wt. % clay loading is attributed to the highest electrolyte uptake, which results in increase in the number of charge carriers for the same volume. 1765

12 A. Méndez-Vilas and J. Díaz (Eds.) (b) (a) Fig. 17: TEM micrographs of (a) PVdF1 and (b) PVdF5 Fig. 18: Temperature dependence of ionic conductivity of (a) gel PVdF, (b) gel PVdF1, (c) gel PVdF2.5 and (d) gel PVdF4 Another factor that contributes to the increase in ionic conductivity with clay loading is the increased dielectric constant of the system in presence of negatively charged silicate platelets [11], which in turn dissolves more electrolyte salt (LiClO4) resulting in an increase in ionic conductivity. An interesting effect occurs in high temperature range wherein at about 70 oc the conductivity of the clay-free gel polymer electrolyte dramatically decays as observed in Fig 18a. This is ascribed to the fact that at this temperature the polymer softens and the film loses its mechanical consistency [67]. On the other hand, the polymer-mmt clay nanocomposite electrolytes are thermally more stable, whose conductivity goes on increasing up to 90 oc (Fig. 18b-d). This is attributed to the fact that the flow of the polymer chains is restricted by the presence of MMT fillers increasing their thermal stability and the mechanical strength of the polymer electrolytes is increased making them suitable materials for high temperature applications [67]. 6. Conclusions Swift heavy ion irradiation is a very effective technique for the modification in the physico-chemical properties of electroactive polymers. Electron microscopy has been found to be an inevitable probe for the investigation of the SHI induced structural and morphological modifications in electroactive polymers. In this chapter, we have thrown light on the application of electron microscopy for the study of the SHI irradiation effects on PAni nanofibers, PAni nanofiber reinforced gel polymer electrolytes and PVdF-clay nanocomposite polymer electrolytes. Transmission electron micrographs of PAni nanofibers reveal downsizing with increasing irradiation fluence from an average diameter of about 50 nm for the pristine structure to about 15 nm when irradiated with a fluence of ions/cm2 for the CSA doped PAni nanofibers while for the HCl doped nanofibers the diameter decreases from 40 nm to about 10 nm upon irradiation which is also accompanied by conformational changes as observed from XRD and µr analysis. 90 MeV O7+ ion irradiation effects on P(VdF-HFP)-(PC+DEC)-LiClO4-dedoped PAni nanofibers composite gel polymer electrolytes show that the ionic conductivity of the nanocomposite gel polymer electrolytes increases with increasing ion fluence up to 1011 ions/cm2. This has been attributed to the chain scissioning of the polymer, which leads to faster ionic transport through the polymer matrix assisted by larger segmental motion of the polymer backbone. At higher fluence (> 1011 ions/cm2) PAni nanofibers get phase separated out from the polymer electrolytes as revealed by XRD and DSC analyses. SEM results show increase in porosity with uniformly dispersed pores, resulting in better connectivity of the liquid electrolytes through the polymer giving rise to higher ionic conductivity. In PVdF-clay systems the insertion of PVdF chains into the galleries of MMT increases the d-spacing from 2 nm to 3.75 nm for 4 wt. % clay loading as revealed by XRD and TEM results. Higher intercalaton of PVdF into the galleries of MMT enhances the electrolyte uptake thereby increasing the ionic conductivity. SHI irradiation on PVdF-clay enhances the gallery spacing at low irradiation fluence (< 1011 ions/cm2), whereas higher irradiation fluence leads to the formation of exfoliated nanostructure as revealed by XRD analysis. 7. References [1] [2] [3] 1766 Wallace GG, Spinks GM, Kane-Maguire LAP and Teasdale PR. Conductive Electroactive Polymers Intelligent Materials System 2nd edn (Boca Raton, FL: CRC Press LLC) chapter 1, 2000 NW Corporate Blvd. Shirakawa H, Louis EJ, MacDiarmid AG, Chiang CK, Heeger AJ, Synthesis of electrically conducting organic polymers: halogen derivatives of polyacetylene, (CH)x.J. Chem. Soc., Chem. Comm. 1977: Tarascon JM, Armand M. Issues and challenges facing rechargeable lithium batteries Nature (London) 2001; 414: