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Polymer electrolyte structure and its implications
Electrochimica Acta 45 (2000) 1417 – 1423 www.elsevier.nl/locate/electacta
Polymer electrolyte structure and its implications Yuri G. Andreev, Peter G. Bruce * Department of Chemistry, Centre for Electronic and Material Science, School of Chemistry, Uni6ersity of St. Andrews, Fife KY16 9ST, UK Received 30 November 1998
Abstract There remains intense interest in developing solid polymer electrolytes, free from low molecular weight plasticiser and with a sufficiently high ionic conductivity for application in all-solid-state rechargeable lithium batteries. For such applications, conductivities above the present maximum of 10 − 4 S cm − 1 are required. Innovative designs of polymers and salts which suppress crystallinity and yield amorphous polymer electrolytes with a low Tg have led to substantial improvements in the level of ionic conductivity compared with early systems, but the above mentioned barrier remains. We promote the view that it is important now to change our thinking concerning how to optimise ionic conductivity. We emphasise the importance of understanding the structure of polymer electrolytes in order to better understand the ion transport mechanism. Such studies indicate the importance of organising polymer chains while preserving chain dynamics. Recent evidence is presented from the work of others, supporting the view that more structured polymer electrolytes can lead to enhanced ionic conductivity. En route to this view, we present the crystal structures of several polymer salt complexes including the first structure of a 6:1 complex PEO6:LiAsF6. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Polymer electrolyte; Lithium batteries; Ionic conductivity; Structure-conductivity relationships
1. Introduction Gel electrolytes, in which a liquid electrolyte is entrapped in a polymer matrix, possess levels of ionic conductivity that are sufficient for application in lithium batteries [1]. These materials will lead to the first commercialisation of solid polymer batteries [2]. However such electrolytes are not without their problems. Many of the disadvantages associated with liquid electrolytes are retained in the gel. There remains a need for a polymer electrolyte, i.e. a material consisting of a salt dissolved in a high molecular weight polymer solvent, with a sufficiently high level of ionic conductivity for use in lithium batteries. * Corresponding author. Tel.: + 44-1334-463825; fax: +441334-463808. E-mail address: [email protected] (P.G. Bruce)
Great progress has been made over the last 20 years in increasing the level of ionic conductivity exhibited by polymer electrolytes [3,4]. The realisation that conductivity is confined largely to amorphous polymer electrolytes above their Tg and that this is related to a unique conduction mechanism involving the creation of free volume, arising from the dynamics of the polymer chains, led to design strategies for new polymer electrolytes in which crystallinity was suppressed and segmental motion maximised [3,5,6]. This in turn, led to increases in room temperature ionic conductivity of several orders of magnitude compared with the original materials. However in recent years, despite innovative designs of flexible polymers and the synthesis of salts containing asymmetric anions capable of suppressing crystallinity, levels of ionic conductivity are persistently limited to a ceiling of around 10 − 4 S cm − 1 at room temperature. Such a level is insufficient for many lithium battery applications.
0013-4686/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 9 9 ) 0 0 3 5 3 - 9
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When such barriers are reached in science it is time to change the way we think. Our vision is that we must direct our attention to the structure of polymer electrolytes. The very success of the credo that dictates we should design amorphous polymer electrolytes with low Tg, has led to a view in some quarters that ion transport occurs in a structureless continuum of fluctuating free volume. In fact, polymer electrolytes have considerable structure. It is vital to understand this structure, in particular the coordination around the mobile ions, in order to better understand the mechanism of ion transport, leading in turn to levels of ionic conductivity above the present maximum. Our approach to understanding polymer electrolyte structure is different from previous efforts. We have focussed on establishing the structures of the crystalline polymer–salt complexes because crystallographic techniques are capable of yielding structural information to a high level of detail [7]. The crystal structures are then used as the basis of interpreting spectroscopic studies of analogous amorphous phases, in order to achieve a deeper understanding of the structural chemistry of polymer electrolytes in the amorphous state. In this paper we summarise the polymer electrolyte crystal structures including the first structure of a 6:1 complex, PEO6:LiAsF6. This is followed by a description of how the structure in the crystalline state is largely retained in the analogous amorphous phase. Finally the implications that the polymer electrolyte structures have for ionic conductivity are presented. 2. Crystal structures Crystalline polymer–salt complexes are available at only a few discreet compositions. As a result, determining their structure, has a major influence on understanding the structural chemistry of polymer electrolytes in general.
The crystal structures of polymer electrolytes are seldom amenable to study by single crystal techniques because of inadequate data quality. However it is possible, with care, to prepare polymer electrolytes which yield excellent powder diffraction patterns (Fig. 1). Methods of refining the details of a structure provided a good starting model is available, are already well established in powder crystallography. For example the approach developed by Rietveld involves fitting the calculated powder diffraction pattern, based on a trial structural model, to the observed pattern by non-linearleast-squares [8]. However, so little is known about the structures of polymer electrolytes that refinement is not always an option in establishing the structure and one is soon faced with having to implement an ab initio approach, which requires no prior knowledge of the structure. Recently we have developed such a method which is particularly powerful in the context of solving crystal structures of flexible molecular solids. The method involves the generation of random structural models and the location of the global minimum in the goodness-of-fit between observed and calculated powder patterns using simulated annealing. It has applications far beyond polymer electrolytes but has been vital in revealing the crystal structures of several polymer electrolyte complexes. The method is described in detail elsewhere [9 – 11]. The structure of a 3:1 complex, PEO3:LiCF3SO3 is shown in Fig. 2 [12]. The PEO chains adopt a helical conformation with all CO bonds trans and CC bonds either gauche or gauche minus. Three ethylene oxide units are involved in the basic repeating sequence which is ttgttgttg¯. A cation is located in each turn of the helix and is coordinated by three ether oxygens in the case of Li+ salts, or four, in the case of Na+ salts [13]. Each cation is also coordinated by two anions and each anion bridges between two neighbouring cations along the chain. This basic structure is retained for a variety
Fig. 1. Fibre diffraction pattern of PEO4:KSCN (left) and powder pattern (right) of PEO3:NaClO4 complex.
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Fig. 2. The structure of PEO3:LiCF3SO3. (Left) A single PEO chain with associated ions. (Right) View of the structure along the fibre axis. Thick green lines represent carbon in the PEO chain and thick red lines represent oxygen. Blue spheres, lithium; green, carbon; magenta, fluorine; yellow, sulphur; red, oxygen. Thin lines indicate coordination around the cation. Hydrogen atoms are not shown. Fig. 3. The structure of PEO4:KSCN. (Left) A single PEO chain with associated ions. (Right) View of the structure along the fibre axis. Thick green lines represent carbon in the PEO chain and thick red lines represent oxygen. Violet spheres, potassium; blue, nitrogen; green, carbon; yellow, sulphur. Thin lines indicate coordination around the cation. Hydrogen atoms are not shown.
of anion sizes and shapes, ranging from Ito [N(SO2CF3)2]− [14,15]. Each PEO chain is associated with a dedicated set of cations and anions that do not coordinate to any other chain. In other words there is
no ionic cross-linking between chains, only weak van der Waals interactions. Conventional wisdom in the field dictated that cations larger than Na+ could not be accommodated
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within a PEO helix but must instead be located between the PEO chains [16]. Our structural studies have shown that this is completely erroneous. Cations with ionic radii up to and including that of Rb+ can be located within a PEO helix [17]. However the higher coordination number demanded by the larger cations means that the composition of the crystalline complexes changes from 3:1 to 4:1. Despite considerable similarity between the 3:1 and 4:1 structures there are important differences (see PEO4:KSCN in Fig. 3) [18]. Although the PEO chain is helical the conformation changes to provide a fatter helix capable of accommodating the larger cations. The conformation now involves four EO units in the repeating sequence; ttgttgttg¯ttg¯. The cations, located in each turn of the helix, are coordinated by five ether oxygens and two anions, the latter again bridge neighbouring cations along the chain. As is the case for the 3:1 complexes there is no ionic cross-linking between chains. Changing the polymer–salt ratio from 3:1 to 1:1 has a profound influence on the structure (see PEO:KCF3SO3 in Fig. 4) [19]. The polymer chain conformation changes from helical to a stretched zig– zag arrangement and the cations are coordinated by only two ether oxygens and four anions. The anions coordinate simultaneously cations which are themselves associated with different PEO chains. As a result there is extensive interchain cross-linking in the 1:1 complexes. Increasing the polymer:salt ratio from 3:1 to 6:1 has an equally profound influence on the crystal structure (see PEO6:LiAsF6 in Fig. 5) [20]. The cations are arranged in rows with each row being located within a cylindrical tunnel formed by two PEO chains. The chains are not helical and indeed the conformation is different from any previously studied PEO system. Six EO units are required to define the repeat sequence which, starting from a CO bond may be described as ctgg¯tgcg¯tcttgtg¯cgt. Each chain forms the surface of a half cylinder and the two chains interlock on each side to complete the cylindrical arrangement. The ether oxygens point inwards and five such oxygens coordinate each Li+ ion, three from one chain and two from the other, with the sixth oxygen remaining uncoordinated to a cation. Each of the five ether oxygens is coordinated to only one Li+ ion. The LiO distances in A, are, for chain one 2.07 (5), 2.26 (4), 2.28 (4) and for chain two 2.05 (5), 2.14 (6). The LiLi distance along the row is 5.4 (1) A, . The anions do not coordinate the cations but are instead located outside the dimensions of the PEO cylinder in the interchain space. The anions are also arranged in rows.
3. Amorphous polymer electrolyte structure In order to investigate the degree to which the structure in the crystalline state is retained on moving to the amorphous state, a sample of crystalline PEO3:LiCF3SO3 was subjected to infrared spectroscopic measurements as a function of temperature from 25°C, through the melting point at 179°C and into the amorphous state [21]. Part of the IR spectrum is shown in Fig. 6 highlighting the symmetric-SO3 stretch. Since the crystal structure of this complex had already been determined by us, it was possible to interpret the infrared spectrum of the crystalline complex and then, by following the temperature dependence into the amorphous state, to gain a much clearer insight into the structure of the amorphous polymer. By analysing the SO3 and CF3 vibrations as well as modes associated with the PEO chain, it was possible to conclude that the structure is largely retained on passing from the crystalline to the amorphous state with only a loss of register between the chains, leading to disruption of the long range order. In particular it appears that the PEO chain retains a helical conformation with the cations remaining inside the helices and associated with their anions. In other words the PEO chain with its dedicated set of cations and anions appears to remain intact in the amorphous state. These results are certainly consistent with the relatively low melting temperature of the crystalline complex, which is commensurate with a disruption of weak van der Waals forces rather than the stronger ion – polymer interactions.
4. Implications of the structure If we may assume, as has been demonstrated for the PEO3:LiCF3SO3, that the structure is largely maintained on moving from the crystalline to the amorphous state at a given composition, then a number of important implications flow from a knowledge of the crystal structures. In particular if the cation – polymer substructure is maintained then at both the 3:1 and 6:1 compositions, the cations remain within tunnels defined by the PEO chains. This, in turn, suggests that cation transport occurs preferentially along such tunnels, with the rate limiting step being transfer between tunnels. If this is the case, then a random arrangement of chains, such as might be envisaged in a simple amorphous polymer, is not conductive to ion transport. Instead, by organising chains in a more aligned fashion, transport along and between chains will be facilitated (Fig. 7). By combining this with sufficient local chain mobility, particularly involving dynamic modes propagating along the axis of the chains, levels of ionic conductivity higher than those available from a less organised amorphous solid should be possible. In other words the
Y.G. Andree6, P.G. Bruce / Electrochimica Acta 45 (2000) 1417–1423
structural studies suggest that organisation of chains should enhance ionic conductivity. It is well known that anions contribute significantly
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to the total ionic conductivity of polymer electrolytes. If, on passing from the crystalline to the amorphous state, not only the cation – polymer substructure is re-
Fig. 4. The structure of PEO:KCF3SO3. Thick green lines represent carbon in the PEO chain and thick red lines represent oxygen. Violet spheres, potassium; green, carbon; magenta, fluorine; yellow, sulphur; red, oxygen. Thin lines indicate coordination around the cation. Hydrogen atoms are not shown. Fig. 5. (Left) The structure of PEO6:LiAsF6 viewed along the polymer chains. (Right) View of the structure showing the relative position of the chains and their conformation. Blue spheres, lithium; white, arsenic; magenta, fluorine; light green, carbon in chain 1; dark green, oxygen in chain 1; orange, carbon in chain 2; red, oxygen in chain 2. Thin lines indicate coordination around the cation. Hydrogen atoms are not shown.
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In general, the ionic conductivity of polymer electrolytes increases substantially on increasing the polymer:salt ratio from 3:1 to 6:1. The structural studies reported here may give some insight into why this is so. Since, for the 3:1 electrolytes, cations are contained in each loop of the helix, there are no vacancies (except perhaps a small number that are thermally generated) available for cation transport. In contrast, the 6:1 structure involves coordination of the lithium ions by only five of the six ether oxygens. The sixth, free, ether oxygen could facilitate a conduction mechanism involving cooperative displacement of lithium ions along the cylindrical tunnels. Another important difference between the 3:1 and 6:1 structures is that in the former case the anions are bound to the cations whereas in the 6:1 complex there is no evidence of ion pairing, in this case, the cations are free to migrate independently. Indeed the 6:1 crystal structure suggests that the anions should exhibit a high level of transport. The significant difference in ion – ion interactions between the 3:1 and 6:1 complexes will make the measurement of ion transport numbers particularly important in probing further the relationship between structure and ionic conductivity. The specific implications that the crystal structures have for ionic conductivity must at this stage be regarded as speculative. However there can be no doubt that the crystallographic studies have led to an unprecedented understanding of polymer electrolyte structure. Fig. 6. Symmetric stretching mode, nS(SO3), as a function of temperature for PEO3:LiCF3SO3.
tained but some memory of the chain alignment also persists and then anions may be somewhat constrained to move in one dimension. Whereas the cation–polymer substructure of the crystalline state is likely to be retained to a high degree, this is less certain for the rest of the structure. Several authors have reported recently evidence supporting the view that organisation of chains can be important in enhancing conductivity. Golodnitsky and Peled [22], have shown that by stretching a PEO-based polymer electrolyte, ionic conductivity is enhanced. Wright and co-workers have synthesised liquid crystalline polymer electrolytes by introducing groups into the PEO chains which contain long aliphatic side chains. The side chains form hydrophobic blocks with the PEO chains constrained within layers at the interfaces between neighbouring blocks. Salts are dissolved within the PEO layers. These materials show enhanced ion transport, exhibiting levels of ionic conductivity at room temperature above those for the best conventional amorphous polymers such as PEO:LiN(SO3CF3)2 [23].
Fig. 7. Disorganised (a) and organised (b) models of an amorphous polymer electrolyte.
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[7] P.G. Bruce, Phil. Trans. R. Soc. Lond. A 354 (1996) 415. [8] H.M. Rietveld, J. Appl. Crystallogr. 2 (1969) 65. [9] Y.G. Andreev, P.G. Bruce, J. Chem. Soc. Dalton Trans. (1998) 4071. [10] Y.G. Andreev, P. Lightfoot, P.G. Bruce, J. Appl. Crystallogr. 18 (1997) 294. [11] P.G. Bruce, G.S. MacGlashan, Y.G. Andreev, in: J.A. Howard, F.H. Allen (Eds.), NATO Science Series E: Applied Sciences, Kluwer, Dordrecht, 1999. [12] P. Lightfoot, M.A. Mehta, P.G. Bruce, Science 262 (1993) 883. [13] P. Lightfoot, M.-A. Mehta, P.G. Bruce, J. Mater. Chem. 2 (1992) 379. [14] Y. Chatani, Y. Fujii, T. Takayanagi, A. Honma, Polymer 31 (1990) 2238. [15] Y Andreev, P. Lightfoot, P.G. Bruce, J. Chem. Soc. Chem. Comm. (1996) 2169. [16] T. Hibma, Solid State Ion. 9/10 (1983) 1101. [17] J.B. Thomson, P. Lightfoot, P.G. Bruce, Solid State Ion. 85 (1996) 203. [18] P. Lightfoot, J.L. Nowinski, P.G. Bruce, J. Am. Chem. Soc. 116 (1994) 7469. [19] G.S. MacGlashan, Y.G. Andreev, P.G. Bruce, J. Chem. Soc. Dalton Trans. (1998) 1073. [20] G. MacGlashan, Y.G. Andreev, P.G. Bruce, Nature 398 (1999) 792. [21] R. Frech, S. Chintapalli, P.G. Bruce, C.A. Vincent, J. Chem. Soc. Chem. Commun. (1997) 157. [22] D. Golodnitsky, E. Peled, Electrochim. Acta, in press. [23] F.B. Dias, et al., Electrochim. Acta 43 (1998) 1217.
Furthermore, they carry key messages for the conduction process and demonstrate how important it will be in the future to adopt a more structural view of polymer electrolytes.
Acknowledgements PGB is indebted to the EPSRC and the Leverhulme Trust for financial support.
References [1] B. Scrosati (Ed.), Application of Electroactive Polymers, Chapman and Hall, London, 1993. [2] J.-M. Tarascon, A.S. Gazdz, C. Schmitz, F. Shakoohi, P.C. Warren, Solid State Ion. 86–88 (1996) 49. [3] P.G. Bruce (Ed.), Solid State Electrochemistry, Cambridge University, Cambridge, 1995. [4] F.M. Gray, Polymer Electrolytes. RSC Materials Monographs, The Royal Society of Chemistry, Cambridge, 1997. [5] M. Ratner, in: J.R. MacCallum, C.A. Vincent (Eds.), Polymer Electrolyte Reviews, vol. 1, Elsevier, Amsterdam, 1987. [6] P.M. Blansky, D.F. Shriver, P.E. Austin, H.R. Allcock, J. Am. Chem. Soc. 106 (1984) 6854.
.
Polymer electrolyte structure and its implications Yuri G. Andreev, Peter G. Bruce * Department of Chemistry, Centre for Electronic and Material Science, School of Chemistry, Uni6ersity of St. Andrews, Fife KY16 9ST, UK Received 30 November 1998
Abstract There remains intense interest in developing solid polymer electrolytes, free from low molecular weight plasticiser and with a sufficiently high ionic conductivity for application in all-solid-state rechargeable lithium batteries. For such applications, conductivities above the present maximum of 10 − 4 S cm − 1 are required. Innovative designs of polymers and salts which suppress crystallinity and yield amorphous polymer electrolytes with a low Tg have led to substantial improvements in the level of ionic conductivity compared with early systems, but the above mentioned barrier remains. We promote the view that it is important now to change our thinking concerning how to optimise ionic conductivity. We emphasise the importance of understanding the structure of polymer electrolytes in order to better understand the ion transport mechanism. Such studies indicate the importance of organising polymer chains while preserving chain dynamics. Recent evidence is presented from the work of others, supporting the view that more structured polymer electrolytes can lead to enhanced ionic conductivity. En route to this view, we present the crystal structures of several polymer salt complexes including the first structure of a 6:1 complex PEO6:LiAsF6. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Polymer electrolyte; Lithium batteries; Ionic conductivity; Structure-conductivity relationships
1. Introduction Gel electrolytes, in which a liquid electrolyte is entrapped in a polymer matrix, possess levels of ionic conductivity that are sufficient for application in lithium batteries [1]. These materials will lead to the first commercialisation of solid polymer batteries [2]. However such electrolytes are not without their problems. Many of the disadvantages associated with liquid electrolytes are retained in the gel. There remains a need for a polymer electrolyte, i.e. a material consisting of a salt dissolved in a high molecular weight polymer solvent, with a sufficiently high level of ionic conductivity for use in lithium batteries. * Corresponding author. Tel.: + 44-1334-463825; fax: +441334-463808. E-mail address: [email protected] (P.G. Bruce)
Great progress has been made over the last 20 years in increasing the level of ionic conductivity exhibited by polymer electrolytes [3,4]. The realisation that conductivity is confined largely to amorphous polymer electrolytes above their Tg and that this is related to a unique conduction mechanism involving the creation of free volume, arising from the dynamics of the polymer chains, led to design strategies for new polymer electrolytes in which crystallinity was suppressed and segmental motion maximised [3,5,6]. This in turn, led to increases in room temperature ionic conductivity of several orders of magnitude compared with the original materials. However in recent years, despite innovative designs of flexible polymers and the synthesis of salts containing asymmetric anions capable of suppressing crystallinity, levels of ionic conductivity are persistently limited to a ceiling of around 10 − 4 S cm − 1 at room temperature. Such a level is insufficient for many lithium battery applications.
0013-4686/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 9 9 ) 0 0 3 5 3 - 9
Y.G. Andree6, P.G. Bruce / Electrochimica Acta 45 (2000) 1417–1423
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When such barriers are reached in science it is time to change the way we think. Our vision is that we must direct our attention to the structure of polymer electrolytes. The very success of the credo that dictates we should design amorphous polymer electrolytes with low Tg, has led to a view in some quarters that ion transport occurs in a structureless continuum of fluctuating free volume. In fact, polymer electrolytes have considerable structure. It is vital to understand this structure, in particular the coordination around the mobile ions, in order to better understand the mechanism of ion transport, leading in turn to levels of ionic conductivity above the present maximum. Our approach to understanding polymer electrolyte structure is different from previous efforts. We have focussed on establishing the structures of the crystalline polymer–salt complexes because crystallographic techniques are capable of yielding structural information to a high level of detail [7]. The crystal structures are then used as the basis of interpreting spectroscopic studies of analogous amorphous phases, in order to achieve a deeper understanding of the structural chemistry of polymer electrolytes in the amorphous state. In this paper we summarise the polymer electrolyte crystal structures including the first structure of a 6:1 complex, PEO6:LiAsF6. This is followed by a description of how the structure in the crystalline state is largely retained in the analogous amorphous phase. Finally the implications that the polymer electrolyte structures have for ionic conductivity are presented. 2. Crystal structures Crystalline polymer–salt complexes are available at only a few discreet compositions. As a result, determining their structure, has a major influence on understanding the structural chemistry of polymer electrolytes in general.
The crystal structures of polymer electrolytes are seldom amenable to study by single crystal techniques because of inadequate data quality. However it is possible, with care, to prepare polymer electrolytes which yield excellent powder diffraction patterns (Fig. 1). Methods of refining the details of a structure provided a good starting model is available, are already well established in powder crystallography. For example the approach developed by Rietveld involves fitting the calculated powder diffraction pattern, based on a trial structural model, to the observed pattern by non-linearleast-squares [8]. However, so little is known about the structures of polymer electrolytes that refinement is not always an option in establishing the structure and one is soon faced with having to implement an ab initio approach, which requires no prior knowledge of the structure. Recently we have developed such a method which is particularly powerful in the context of solving crystal structures of flexible molecular solids. The method involves the generation of random structural models and the location of the global minimum in the goodness-of-fit between observed and calculated powder patterns using simulated annealing. It has applications far beyond polymer electrolytes but has been vital in revealing the crystal structures of several polymer electrolyte complexes. The method is described in detail elsewhere [9 – 11]. The structure of a 3:1 complex, PEO3:LiCF3SO3 is shown in Fig. 2 [12]. The PEO chains adopt a helical conformation with all CO bonds trans and CC bonds either gauche or gauche minus. Three ethylene oxide units are involved in the basic repeating sequence which is ttgttgttg¯. A cation is located in each turn of the helix and is coordinated by three ether oxygens in the case of Li+ salts, or four, in the case of Na+ salts [13]. Each cation is also coordinated by two anions and each anion bridges between two neighbouring cations along the chain. This basic structure is retained for a variety
Fig. 1. Fibre diffraction pattern of PEO4:KSCN (left) and powder pattern (right) of PEO3:NaClO4 complex.
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Fig. 2. The structure of PEO3:LiCF3SO3. (Left) A single PEO chain with associated ions. (Right) View of the structure along the fibre axis. Thick green lines represent carbon in the PEO chain and thick red lines represent oxygen. Blue spheres, lithium; green, carbon; magenta, fluorine; yellow, sulphur; red, oxygen. Thin lines indicate coordination around the cation. Hydrogen atoms are not shown. Fig. 3. The structure of PEO4:KSCN. (Left) A single PEO chain with associated ions. (Right) View of the structure along the fibre axis. Thick green lines represent carbon in the PEO chain and thick red lines represent oxygen. Violet spheres, potassium; blue, nitrogen; green, carbon; yellow, sulphur. Thin lines indicate coordination around the cation. Hydrogen atoms are not shown.
of anion sizes and shapes, ranging from Ito [N(SO2CF3)2]− [14,15]. Each PEO chain is associated with a dedicated set of cations and anions that do not coordinate to any other chain. In other words there is
no ionic cross-linking between chains, only weak van der Waals interactions. Conventional wisdom in the field dictated that cations larger than Na+ could not be accommodated
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within a PEO helix but must instead be located between the PEO chains [16]. Our structural studies have shown that this is completely erroneous. Cations with ionic radii up to and including that of Rb+ can be located within a PEO helix [17]. However the higher coordination number demanded by the larger cations means that the composition of the crystalline complexes changes from 3:1 to 4:1. Despite considerable similarity between the 3:1 and 4:1 structures there are important differences (see PEO4:KSCN in Fig. 3) [18]. Although the PEO chain is helical the conformation changes to provide a fatter helix capable of accommodating the larger cations. The conformation now involves four EO units in the repeating sequence; ttgttgttg¯ttg¯. The cations, located in each turn of the helix, are coordinated by five ether oxygens and two anions, the latter again bridge neighbouring cations along the chain. As is the case for the 3:1 complexes there is no ionic cross-linking between chains. Changing the polymer–salt ratio from 3:1 to 1:1 has a profound influence on the structure (see PEO:KCF3SO3 in Fig. 4) [19]. The polymer chain conformation changes from helical to a stretched zig– zag arrangement and the cations are coordinated by only two ether oxygens and four anions. The anions coordinate simultaneously cations which are themselves associated with different PEO chains. As a result there is extensive interchain cross-linking in the 1:1 complexes. Increasing the polymer:salt ratio from 3:1 to 6:1 has an equally profound influence on the crystal structure (see PEO6:LiAsF6 in Fig. 5) [20]. The cations are arranged in rows with each row being located within a cylindrical tunnel formed by two PEO chains. The chains are not helical and indeed the conformation is different from any previously studied PEO system. Six EO units are required to define the repeat sequence which, starting from a CO bond may be described as ctgg¯tgcg¯tcttgtg¯cgt. Each chain forms the surface of a half cylinder and the two chains interlock on each side to complete the cylindrical arrangement. The ether oxygens point inwards and five such oxygens coordinate each Li+ ion, three from one chain and two from the other, with the sixth oxygen remaining uncoordinated to a cation. Each of the five ether oxygens is coordinated to only one Li+ ion. The LiO distances in A, are, for chain one 2.07 (5), 2.26 (4), 2.28 (4) and for chain two 2.05 (5), 2.14 (6). The LiLi distance along the row is 5.4 (1) A, . The anions do not coordinate the cations but are instead located outside the dimensions of the PEO cylinder in the interchain space. The anions are also arranged in rows.
3. Amorphous polymer electrolyte structure In order to investigate the degree to which the structure in the crystalline state is retained on moving to the amorphous state, a sample of crystalline PEO3:LiCF3SO3 was subjected to infrared spectroscopic measurements as a function of temperature from 25°C, through the melting point at 179°C and into the amorphous state [21]. Part of the IR spectrum is shown in Fig. 6 highlighting the symmetric-SO3 stretch. Since the crystal structure of this complex had already been determined by us, it was possible to interpret the infrared spectrum of the crystalline complex and then, by following the temperature dependence into the amorphous state, to gain a much clearer insight into the structure of the amorphous polymer. By analysing the SO3 and CF3 vibrations as well as modes associated with the PEO chain, it was possible to conclude that the structure is largely retained on passing from the crystalline to the amorphous state with only a loss of register between the chains, leading to disruption of the long range order. In particular it appears that the PEO chain retains a helical conformation with the cations remaining inside the helices and associated with their anions. In other words the PEO chain with its dedicated set of cations and anions appears to remain intact in the amorphous state. These results are certainly consistent with the relatively low melting temperature of the crystalline complex, which is commensurate with a disruption of weak van der Waals forces rather than the stronger ion – polymer interactions.
4. Implications of the structure If we may assume, as has been demonstrated for the PEO3:LiCF3SO3, that the structure is largely maintained on moving from the crystalline to the amorphous state at a given composition, then a number of important implications flow from a knowledge of the crystal structures. In particular if the cation – polymer substructure is maintained then at both the 3:1 and 6:1 compositions, the cations remain within tunnels defined by the PEO chains. This, in turn, suggests that cation transport occurs preferentially along such tunnels, with the rate limiting step being transfer between tunnels. If this is the case, then a random arrangement of chains, such as might be envisaged in a simple amorphous polymer, is not conductive to ion transport. Instead, by organising chains in a more aligned fashion, transport along and between chains will be facilitated (Fig. 7). By combining this with sufficient local chain mobility, particularly involving dynamic modes propagating along the axis of the chains, levels of ionic conductivity higher than those available from a less organised amorphous solid should be possible. In other words the
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structural studies suggest that organisation of chains should enhance ionic conductivity. It is well known that anions contribute significantly
1421
to the total ionic conductivity of polymer electrolytes. If, on passing from the crystalline to the amorphous state, not only the cation – polymer substructure is re-
Fig. 4. The structure of PEO:KCF3SO3. Thick green lines represent carbon in the PEO chain and thick red lines represent oxygen. Violet spheres, potassium; green, carbon; magenta, fluorine; yellow, sulphur; red, oxygen. Thin lines indicate coordination around the cation. Hydrogen atoms are not shown. Fig. 5. (Left) The structure of PEO6:LiAsF6 viewed along the polymer chains. (Right) View of the structure showing the relative position of the chains and their conformation. Blue spheres, lithium; white, arsenic; magenta, fluorine; light green, carbon in chain 1; dark green, oxygen in chain 1; orange, carbon in chain 2; red, oxygen in chain 2. Thin lines indicate coordination around the cation. Hydrogen atoms are not shown.
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In general, the ionic conductivity of polymer electrolytes increases substantially on increasing the polymer:salt ratio from 3:1 to 6:1. The structural studies reported here may give some insight into why this is so. Since, for the 3:1 electrolytes, cations are contained in each loop of the helix, there are no vacancies (except perhaps a small number that are thermally generated) available for cation transport. In contrast, the 6:1 structure involves coordination of the lithium ions by only five of the six ether oxygens. The sixth, free, ether oxygen could facilitate a conduction mechanism involving cooperative displacement of lithium ions along the cylindrical tunnels. Another important difference between the 3:1 and 6:1 structures is that in the former case the anions are bound to the cations whereas in the 6:1 complex there is no evidence of ion pairing, in this case, the cations are free to migrate independently. Indeed the 6:1 crystal structure suggests that the anions should exhibit a high level of transport. The significant difference in ion – ion interactions between the 3:1 and 6:1 complexes will make the measurement of ion transport numbers particularly important in probing further the relationship between structure and ionic conductivity. The specific implications that the crystal structures have for ionic conductivity must at this stage be regarded as speculative. However there can be no doubt that the crystallographic studies have led to an unprecedented understanding of polymer electrolyte structure. Fig. 6. Symmetric stretching mode, nS(SO3), as a function of temperature for PEO3:LiCF3SO3.
tained but some memory of the chain alignment also persists and then anions may be somewhat constrained to move in one dimension. Whereas the cation–polymer substructure of the crystalline state is likely to be retained to a high degree, this is less certain for the rest of the structure. Several authors have reported recently evidence supporting the view that organisation of chains can be important in enhancing conductivity. Golodnitsky and Peled [22], have shown that by stretching a PEO-based polymer electrolyte, ionic conductivity is enhanced. Wright and co-workers have synthesised liquid crystalline polymer electrolytes by introducing groups into the PEO chains which contain long aliphatic side chains. The side chains form hydrophobic blocks with the PEO chains constrained within layers at the interfaces between neighbouring blocks. Salts are dissolved within the PEO layers. These materials show enhanced ion transport, exhibiting levels of ionic conductivity at room temperature above those for the best conventional amorphous polymers such as PEO:LiN(SO3CF3)2 [23].
Fig. 7. Disorganised (a) and organised (b) models of an amorphous polymer electrolyte.
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[7] P.G. Bruce, Phil. Trans. R. Soc. Lond. A 354 (1996) 415. [8] H.M. Rietveld, J. Appl. Crystallogr. 2 (1969) 65. [9] Y.G. Andreev, P.G. Bruce, J. Chem. Soc. Dalton Trans. (1998) 4071. [10] Y.G. Andreev, P. Lightfoot, P.G. Bruce, J. Appl. Crystallogr. 18 (1997) 294. [11] P.G. Bruce, G.S. MacGlashan, Y.G. Andreev, in: J.A. Howard, F.H. Allen (Eds.), NATO Science Series E: Applied Sciences, Kluwer, Dordrecht, 1999. [12] P. Lightfoot, M.A. Mehta, P.G. Bruce, Science 262 (1993) 883. [13] P. Lightfoot, M.-A. Mehta, P.G. Bruce, J. Mater. Chem. 2 (1992) 379. [14] Y. Chatani, Y. Fujii, T. Takayanagi, A. Honma, Polymer 31 (1990) 2238. [15] Y Andreev, P. Lightfoot, P.G. Bruce, J. Chem. Soc. Chem. Comm. (1996) 2169. [16] T. Hibma, Solid State Ion. 9/10 (1983) 1101. [17] J.B. Thomson, P. Lightfoot, P.G. Bruce, Solid State Ion. 85 (1996) 203. [18] P. Lightfoot, J.L. Nowinski, P.G. Bruce, J. Am. Chem. Soc. 116 (1994) 7469. [19] G.S. MacGlashan, Y.G. Andreev, P.G. Bruce, J. Chem. Soc. Dalton Trans. (1998) 1073. [20] G. MacGlashan, Y.G. Andreev, P.G. Bruce, Nature 398 (1999) 792. [21] R. Frech, S. Chintapalli, P.G. Bruce, C.A. Vincent, J. Chem. Soc. Chem. Commun. (1997) 157. [22] D. Golodnitsky, E. Peled, Electrochim. Acta, in press. [23] F.B. Dias, et al., Electrochim. Acta 43 (1998) 1217.
Furthermore, they carry key messages for the conduction process and demonstrate how important it will be in the future to adopt a more structural view of polymer electrolytes.
Acknowledgements PGB is indebted to the EPSRC and the Leverhulme Trust for financial support.
References [1] B. Scrosati (Ed.), Application of Electroactive Polymers, Chapman and Hall, London, 1993. [2] J.-M. Tarascon, A.S. Gazdz, C. Schmitz, F. Shakoohi, P.C. Warren, Solid State Ion. 86–88 (1996) 49. [3] P.G. Bruce (Ed.), Solid State Electrochemistry, Cambridge University, Cambridge, 1995. [4] F.M. Gray, Polymer Electrolytes. RSC Materials Monographs, The Royal Society of Chemistry, Cambridge, 1997. [5] M. Ratner, in: J.R. MacCallum, C.A. Vincent (Eds.), Polymer Electrolyte Reviews, vol. 1, Elsevier, Amsterdam, 1987. [6] P.M. Blansky, D.F. Shriver, P.E. Austin, H.R. Allcock, J. Am. Chem. Soc. 106 (1984) 6854.
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