Application of molecular modeling to the study of polymer electrolytes

SOLID STATE

Sohd State Iomcs 60 ( 1993 ) 19-28 North-Holland

IONICS Application of molecular modeling to the study of polymer electrolytes L. Xie and G.C. F a r n n g t o n Department of Matertals Science and Engmeermg, Unzverstty of Pennsylvanta, 3231 Walnut Street, Phtladelphza, PA 19104, USA

Molecular mechamcs and molecular dynamics slmulat]ons were apphed to the study of ~on-lon and ran-polymer mteracUons m PEO-based polymer electrolytes containing L1Br and ZnBr2 LiBr pmrs, as well as unpa]red L1+ and Br- ions, were seen m the electrolytes containing LIBr, and ZnBr + ion pmrs, ZnBr2 neutral species, and Zn2Br~ clusters were found m the PEO/ZnBr2 electrolytes In both the PEO/LIBr and PEO/ZnBr2 systems, ion assocmtlon increased wnh increasing temperature and salt concentratmn Th~s study clearly demonstrates that molecular s]mulatmns can prov]de useful ]ns]ght into the ion-ion and 1on-polymer mteract]ons and the ~on conduction mechanisms m polymer electrolytes

1. Introduction Molecular mechanics and dynamics simulation techniques are powerful tools for learnmg about local structure and dynamtcs in long-chain orgamc molecules One example is in the study of metal 1on association with protein molecules [ 1,2 ], work that suggested the potential apphcat~on of molecular mechanics and dynamtcs to the study of ton-ion and 1on-polymer interactions m complex polymeric systems such as polymer electrolytes. It is well known that certain polymers such as poly(ethylene oxide) (PEO) can solvate a wide variety of morganlc salts to form a special class of solid electrolytes [3,4] These materials are solid m form but resemble liquids In their electrochemical propertles and local structural characteristics As a result, the various dlffractton techmques normally used to charactertze and understand the relationships between structure and iomc conductlwty in crystalline sohd electrolytes are of only hmlted use with amorphous polymer electrolytes. Whde a variety of spectroscopic and electrochemical techniques have been used to obtam information about the nature of the charge carriers and ion association processes in these materials, much remains unknown [ 5 ] The focus of the work described in this paper has been on the apphcatlon of molecular mechamcs and molecular dynamics simulations to help understand

the ion-ion and ion-polymer interactions In PEO/ ZnBr2 and PEO/L1Br electrolytes The reason PEO has an extraordinary solvatmg ability was also explored. The results provide rather lnterestmg insight into the types of interactions that control the charactensttcs of polymer electrolytes and demonstrate the power of these simulation techniques m helpmg to budd useful models of the local structures responsible for their electrochemical properties

2. Simulation techniques POLYGRAFTM, a molecular simulation program [ 6 ] for molecular mechanics and molecular dynamtcs, was used in this study Readers interested in a more detatled discussion of molecular mechanics and dynamtcs methodology are referred to a comprehensive review of molecular simulation presented by Brook et al [7 ] At the heart of any molecular mechanics or dynamics modeling is the so-called "force field", a potential energy function contaming adjustable parameters, which describes the structure and relative energy of the system. Among the basic contributions to the force field are the potential energy functions that describe the bonding characteristics of each element. In modeling a polymer, those atoms making up the polymer chain are constramed to be bonded

0167-2738/93/$ 06 00 © 1993 Elsevier Science Pubhshers B V All rights reserved

20

L Xw, G C Farrmgton/ Apphcatlon oJ molecularmodehng

together, and their potentials revolve a considerat~on o f b o n d lengths, b o n d angles, n o n - b o n d e d interactions, and torsion Other atoms, such as those o f a dissolved salt in a polymer, may be relatively unconstrained and free to Interact w~th each other as well as with atoms on the p o l y m e r chain via nonb o n d e d interactions, the principal being electrostatic interactions described by C o u l o m b potentials In addition to the Coulomb~c mteractmns, potentials that represent van der Waals interactions can also be apphed to all n o n - b o n d e d atoms In this study, the b o n d e d p a r a m e t e r s describing the p o l y m e r chains were taken from the D r e l d m g force field [ 11 ] The van der Waals parameters for n o n - b o n d e d interactions were also taken from the D r e l d m g force field except those for h t h l u m runs, which were based on data in the literature [ 8 ] Force constants and some g e o m e t n c constants were selected from the hterature [ 9,10 ] The calculations o f partial charge for several polyethers were based on the m e t h o d of R a p p e and G o d d a r d [ 12 ] In general, the s~mulated systems consisted of four PEO chains, each containing 25 ethylene oxide monomers, and v a n o u s populations o f dlssocmted L1Br and ZnBr2 molecules Some s~mulat~ons were carn e d out with rune PEO chains to compare the results with those using four In order to m o n i t o r the Interaction process between ions and the PEO chains, the chains were mxtmlly arranged parallel to each other The cations and anions were separated as far as possible and distributed randomly among the PEO chains The length of the time step for the molecular dynamics simulations was 0 001 ps, and all simulations were carried out for a total o f 20 ps simulated time An Ardent c o m p u t e r was used m th~s study, typically about 20 C P U hours o f computing time was required for each s~mulatlon S~mulatlons were also carried out starting w~th an amorphous arrangement of PEO chains The method o f Karasawa and G o d d a r d [ 13 ] was used to build the initial PEO a m o r p h o u s structures by the use of a weighted r a n d o m selection o f the b o n d angle as each a d d m o n a l carbon or oxygen a t o m was a d d e d to the backbone It was interesting to find that the s~mulatlon results were the same regardless whether the starting state was crystalline or a m o r p h o u s A representative starting configuration is shown in fig 1 Clearly the arrangement of PEO chains and

ions is totally unphysical and far from any energy m i n i m u m If a molecular dynamics simulation were begun w~th this configuration, the m o v e m e n t s of the ions and chains would be so d r a m a t i c that the program would stop functioning Therefore, the modehng process begins with a molecular mechanics calculation m which the starting coordinates are adjusted so that the total system energy was minimized This calculation brings the system into a much more realistic configuration It is i m p o r t a n t to emphasize that the structure that results is still not at a global energy m i n i m u m and more hkely only at a local m i n i m u m , but it is far closer to physical reahty than at the start and thus serves as a satisfactory starting point for subsequent molecular dynamics simulation To begin the molecular dynamics portion o f the simulation, kinetic energy is a d d e d to the system, a step which corresponds to bringing the ensemble o f ions and chains to a simulated t e m p e r a t u r e The ions and chains are allowed to move freely within the constraints imposed by their b o n d e d (lntra-chaln) and n o n - b o n d e d ( i o n - i o n , i o n - c h a i n , and c h a i n chain) interactions During the molecular dynamics period, the potential energy of the system generally decreases as the atoms gradually find positions closer to those of a global energy m l m m u m The molecular dynamics s~mulat~on ~s continued until the statistical properties of the system, such as t e m p e r a t u r e and total energy fluctuations, become i n d e p e n d e n t o f t~me At that point, the system is quenched by subtracting the kinetic energy that was a d d e d earher Since the system configuration that results may not be at quite the lowest potential energy configuration, a subsequent molecular mechamcs s~mulatlon is used, an effect, to "fine-tune" it to its lowest potentml energy state, from which information about i o n - i o n and m n - P E O chain interactions can be extracted

3. Results and discussion

3 1 Solvatmg abthty of simple polyethers F o r a polymer to form an electrolyte it first must have the ability to dissolve ionic salts The factors influencing the energetlcs o f solvatlon include the

L Xw, G C Farrmgton/Apphcatton of molecular modehng

006t~

-044

007

•

a[

w 008

0 O8

(a)

0,.08

21

(c) 0 05

° 1° ~N~.,~10 -0 16~L....._

%/

/~00s ..,/

-0 49

(b)

(d)

Fig 1 Charge distributions of (a) poly(methyleneoxlde), (b) poly(ethyleneoxlde), (c) poly(propylene oxide) and (d) poly(tnmethylene oxide )

chemical nature of the ton-coordmatmg groups and spacing between them, stertc constraints imposed by the polymer chain, and the nature of the caUon and anton comprising the salt It has been observed experimentally that the solvatmg abihty of a simple polyether is closely related to its - C - O - sequence, the &stance between sequential oxygens, as shown in table 1. PEO, w~th a monomer umt o f - C H 2 - C H 2 O-, appears to have the optimum - C - O - sequence In contrast poly(methylene oxide) (PMO) and poly (trlmethylene oxide ) (PTMO) are much poorer solvents for lomc salts and do not readdy form polymer electrolytes. Poly(propylene oxide) (PPO), despite having the same spacmg between ether oxygens

as PEO, has a much lower solvatmg power than PEO The remarkable differences in the solvatlng powers of these polyethers can be largely explained by geometric factors, m particular the dtffering ablhties of the chains to wrap around a cation so that the ether oxygens form a chelation shell shielding tt from strong association wtth amons The most efficient chelation interactions occur when the chains and the cartons form five-membered rings Larger or smaller rings are less effective at stablhzmg cations However, tt also is mterestlng to consider the influence of the charge &strlbutton along a chain on its ability to solvate salts. PEO and PMO are good contrasts, the former dissolves salts and the latter does not

Table 1 Monomer umt and some physical properties of polyether Polyether

Monomer umt

Ablhty to form polymer electrolytes

PMO PEO PTMO PPO

-(-CH2-O-)-(-CH2-CH2-O-)- (-CH2-CH2-CH2-O-)-(-CH2-CH (CH3)-O-)-

poor excellent poor hm~ted

22

L Xte, G C Farrmgton/Apphcatlon oj molecular modehng

Calculations of the partial charge distributions in pure PMO and PEO were carried out and the results are shown in fig 2 For PEO, the calculated partial charges are - 0 49 for O, + 0 10 for C and 0 07 for H The partial charge of the oxygen in P M O is about - 0 44, shghtly lower than that in PEO However, the partial charges of the neighboring C ( ~ + 0 20) and H ( ~ + 0 12) m PMO are significantly higher than those in PEO The total positive charge of one C and two H's is + 0 44 for PMO and + 0 24 for PEO, thus it is energetically more favorable for the ether oxygens in PEO to solvate cations than for those in PMO It would appear that the poor solvatlng abihty of the ether oxygens In P M O is in part the result of the high partial charges on the neighboring carbons and hydrogens For PPO and P T M O , the total partlal charges on the neighboring C and H atoms are similar to those in PEO Compared with PEO, the extra CH3 group in PPO and the CH 2 group in P T M O contribute nearly zero partial charges to the structures, but sterically may hinder the cations from interacting with multiple ether oxygens to achieve effective solvation 3 2 lon-lon and ton-polymer mteractlons 3 2 1 PEO/LIBr electrolytes Simulations were performed on a range of P E O / L1Br composltmns Shown in fig 3 are the local structures that resulted for LI + and B r - ions m L1Br(PEO)m As expected, L1 + ions are more polarizing than B r - ions and thus more strongly as-

Free La÷

L1Brpmr . . ~

_

..

Free Br" " ~ ~ Br ~ La ~ O

OO(coordmatedvathLa) O C OH

Fig 2 The ion assocmtmns and in-polymer mteractmn in LIBr(PEO) io

.~.--p

(a)

(b) Fig 3 The Br-Ll mn interactions in LIBr(PEO)m at different temperatures (a) at 300 K and (b) at 400 K p=L1Br SOClated with the ether oxygens of the PEO chains Some h t h l u m ions were found to be relatively free of bromides and associated with four ether oxygens, others occur in LIBr contact ion pairs While it may be tempting to think of the cations in polymer electrolytes as either being solvated completely by the polymer chains or existing in 1on pairs and clusters that are completely unassociated with the chains, the s~mulatlons suggest that reality is not quite so simple In fact, m the case of the LIBr contact ion pairs, the h t h m m in the pair is still associated with the polymer, showing that formation of a pair does not completely hberate lithium ions from association with the polymer Other lithium ions mteract with B r - as next nearest-neighbors in whxch the polymer chain is interposed between the a m o n and cation Those ions closest to being truly free are a fraction of the bromide ions, but even these are weakly b o n d e d to the protons on the polymer chains It ~s important to emphasize that the relative populatlons of the various ion types in the simulation results are not reahstlc The sample size in the simulation is simply too small and the other constraints too significant to expect the populations to be quantitatively correct Nevertheless, it is reasonable to look for trends in the ion populations as, for ex-

L Xte, G C Farrmgton/Apphcatwn of molecular modehng ample, salt concentration a n d temperature are changed One such simulation "experiment" was c a m e d out by varying the concentration of L1Br in PEO The results for LIBr (PEO)1o, an electrolyte with a rather high concentration of dissolved salt, have already been s u m m a r i z e d At lower salt concentrations, the extent of ion pairing would be expected to decrease Unfortunately, the only way the modeling process could simulate a truly low salt concentration would be by greatly increasing the n u m b e r of polymer chains m each simulation, which cannot be done because it would require far too much computer time However, ~t was possible to simulate a salt concentration that is somewhat lower, in this case, LiBr(PEO)5o Even at this lower L1Br concentration, LIBr contact ~on pairs were observed, despite the fact that the individual L1+ a n d B r - ions in the starting configuration were widely separated and given every chance to remain i n d e p e n d e n t of each other as the simulation progressed. Qualitatively, it was found that the degree of ion association m P E O / L I B r electrolytes increased with increasing L1Br concentration in PEO, a result consistent with logic and experimental observation Modeling was also used to explore the effect of mcreasing temperature on the degree of ion pair form a t i o n (fig. 4 a - b ) . The results of sImulaUons at two temperatures, 300 K and 400 K, for LIBr(PEO)Io are shown in fig 4a. Initially, ten L1+ ions and ten B r - ions were distributed over four PEO chains, each consisting of 25 ethylene oxide mers In the figure, the polymer chains have been removed to highlight the ionic species, which include L1 + b o n d e d to PEO, free B r - ions, a n d LIBr pairs When LIBr(PEO)~o was equilibrated at 400 K, more L1Br pairs formed than at 300 K, which is conslstent with reports by Kaklhana et al [ 14,15 ] who studied ion association In PEO c o n t a i n i n g NaCF3SO3 They observed the presence of both dissociated ions a n d ion pmrs and found ion association to increase at higher temperatures. An increase in lon-xon association w~th temperature was also reported for several poly (propylene oxides )-salt complexes [ 16,17 ] Dale and Frech [ 18 ] have also shown that, m poly (propylene o x l d e ) - N a S C N electrolytes, NaSCN precipitates out above 165°C, an effect which represents the limit of i o n - i o n association.

23

(a)

Co)

ZnBr2 cluster (n)

ZnBr + pazr (p)

Zn2Br3 + cluster (c)

(c) Fig 4 The Br-Zn ion mteracUonsm ZnBr2(PEO)ll at different temperatures (a) at 300 K and (b) at 400 K The lager spheres represent the Br and the smaller spheres correspond to the Zn p=ZnBr +, n=ZnBr2 and c=Zn2Br3+ Schemauc representation of Zn and Br ion associations in ZnBr2(PEO) t~ The increase of ion association with temperature is logical in light of the thermodynamics of polymer electrolytes, albeit not for solutions of salts in small molecule solvents in which salt dissocmtlon is favored by higher temperatures In a PEO electrolyte

24

L Xte, G ( Famngton /Apphcatton o! molecular modehn~

containing dissolved LaBr, at lower temperatures the ether oxygens of the chains compete effectively with B r - for association with La + simply on the basis of Coulomb~c attraction However, the strong assocaal i o n o f the cataons with the polymer chains decreases the entropy of the electrolyte through the formataon of hagher densaty, more structurally-ordered regions of the polymer chains As temperature as increased, the thermodynamac gain of forming aon-chaan complexes is ultamately exceeded by the entroplc cost, so i o n - i o n association increases with temperature and ulumately, m m a n y composatlons, culmanates with phase separation and salt precapatataon 3 2 2 PEO/ZnBr2 electrolytes It was anterestlng to observe the effect of increasing the cataon charge from the + 1 of L1 + to + 2 for Zn 2+ on the polymer electrolyte structure F~gs 5 and 6 show the aon-aon and aon-polymer associations that resulted from the simulation of ZnBr2 ( P E O ) ~ The simulation began with a r a n d o m distribution of zinc and bromade ions widely-separated among the PEO chaans which were arranged an a fashion similar to that in fig 1 The time step for this samulataon was 0 001 ps and the total calculation tame was 20 ps A close e x a m i n a t i o n of the surroundings of each Zn 2+ aon In the final samulated conformation reveals that the cations are no longer free and have formed a n u m b e r of rather specific complex species These are shown an fig 6a (300 K ) and 6b (400 K ) , which depact all of the Zn 2+ cations and B r - anions at the end of each samulatxon, the polymer chains having been removed for clarity The specafic complexes observed are detailed an fig 6c The varaety of ion associataon types seen an thas electrolyte as quate anterestang In addataon to simpler specaes such as B r - , ZnBr +, and ZnBr2, clusters of two Z n 2+ i o n s and three B r - aons were also found

N o Zn 2+ ions w i t h o u t at least one assocmted Br were observed Clearl), ion assoclauon m P E O / ZnBr2 electrolytes is richer and more varaed than in PEO/L1Br, m which only free B r - , Ll + and LIBr paars were observed One important issue that has not yet been discussed concerns the craterla for decadang when an 1on paar or cluster has formed The craterla used to interpret the results of the P E O / Z n B r 2 slmulataons are summarized m table 2, which shows the a m o n - c a t Ion distances of the ZnO and ZnBr clusters observed m the samulataons Table 2 also hsts the Z n - O d~slance m crystalline Z n O and the Z n - B r dastance m crystalline ZnBr2 Ions close enough to be considered pmrs or clusters were first selected vasually and then thear cauon-anaon distances were determined In fact, at was found that the bond dastances for the ZnBr, ZnBr2 and Zn2Br3 species all fall within a small range, clearly lnd~catlng strong c a t i o n - a n i o n correlation The values presented m table 2 are average d~stances for these complexes The Z n O and ZnBr dastances previously determaned for PEO/ZnBr2 electrolytes from EXAFS studies are also included m table 2 [19] Not surprisingly, the samulated c a U o n - a m o n d~stances are somewhat larger than an the crystalhne compounds, at least m the case of Z n - O As mentaoned earher, in contrast to the PEO/L~Br simulation an whach free L~+ ~ons associated only w~th ether oxygens but no a m o n s were observed, no free Zn 2+ ions were found in the samulated P E O / ZnBrz electrolyte No Zn 2+ aons were found to be coordinated solely w~th the polymer chatns, a Br~on was always present among thear first-nearestneaghbors It should be emphasazed that during the anataal eqmhbration phase of the s~mulatlon, free Zn 2+ ~ons were created artificially As the samulat~on unfolded, the Zn 2+ ions first assocaated with bromade ions via intervening PEO chains, and then

Table 2 Catlon-amon distance m ZnO, ZnBr2and PEO/ZnBr2 complexes ZnO Zn-O Zn-Br

ZnBr2

1 97-199 2 399-2 426

PEO/ZnBr2 ~2 32 (simulation) 2 23 (EXAFS) ~2 44 (simulation) 2 34 (EXAFS)

25

L Xte, G C Farrmgton/Apphcatton of molecular rnodehng

)

{

ZnBr ÷ pair

(a)

ZnBr2 cluster

(b) Br

OZn

~

O

@O(coordinatedwithZn)

OC

oH

Fig 5 The ~on associations and ion-polymer mteracnon m ZnBr2 (PEO) ~~ (a) ZnBr+ pair and (b) ZnBr2 neutral cluster

eventually formed ZnBr + pairs and ZnBr2 clusters In the P E O / Z n B r 2 system, the Z n B r + ion pa~rs and ZnBr2 clusters were the p r e d o m i n a n t species present As shown in fig. 5a-b, the ZnBr ÷ ion paxrs appear to interact w~th more ether oxygens m the PEO chains than the ZnBr2 species In PEO/ZnBr2, the Z n z+ ions from the Z n B r + pairs are coordinated with five ox-

ygens and those associated with the ZnBr2 clusters with only two oxygens. Thus, the coordination n u m ber for the Zn 2+ associated with the ZnBr + pair is six (five oxygens a n d one B r - ion), and the coordination n u m b e r of the Z n 2+ from the ZnBr2 is four (sometimes five) with two (or three) oxygens and two B r - ions This behavior is quite logical consid-

26

L Xte, G C Farrmgton / 4pph~atton o/mole~ ular modehng

t -p

@ (a)

@

,g_f' (b)

.~--n

(c) Fig 6 Zn and Br ~on interaction increases with ZnBrz concentration increase m PEO (a) ZnBr2(PEO)5o, (b) ZnBrz(PEO)25 and (c) ZnBr2(PEO)t7 As shown, p ln&cates a ZnBr + pair and n means a ZnBr2 neutral cluster

enng that a ZnBr + pmr bears one positive charge and a ZnBr2 cluster is neutral However, considering ZnBr + and ZnBr2 as monopositive and neutral species, respectively, oversimphfies what appears to be a m o r e subtle structural Issue m the simulated electrolytes In fact, the bromide ions in these complex ions are not only associated with the Zn 1+ ions but also w~th hydrogens on the P EO chains Thus, each b r o m i d e ion provides

less than a full negative charge to Zn 2+ In addition, the ether oxygens on the PEO chains contribute only partial negative charge to the Zn 2+ complex ~ons, since, as m e n t i o n e d previously, the partial charges m pure PEO are about - 0 49 for oxygen, + 0 10 for carbon, and 0 07 for hydrogen Therefore, despite the natural preference of Zn 2+ for four-coordination, as seen in ZnBr2 and ZnO, the Zn 2+ ion in ZnBr + is associated with five ether oxygens, for a coordination n u m b e r o f six When Zn 2+ ~s associated with two Br ions, ItS interaction with PEO should be far weaker, considering that the interaction between B r and the P EO chains is very weak and, thus, the two positive charges of Zn 2+ ion are mostly compensated by two B r - ions Therefore, the coordination n u m b e r of Zn 2+ decreases from 6 to 4 (or 5) It is logical that, as the total salt concentratmn increases, the free b r o m i d e concentration should mcrease along with the cluster and pair concentrations, as shown in fig 5 for several concentrations o f ZnBr2 in PEO The formation of ZnBr pairs and more complicated clusters is even more pronounced m ZnBr2(PEO)I~ in which Zn2Br~ clusters are observed With increasing temperature (fig 4 a - b ) , the tendency to form clusters also increases, a result consistent with the behavior observed for the P E O / L I B r simulations In all of the simulated P E O / Z n B r 2 structures, the free Br ions were found to be located in the polymer regions o f lower density and associated with the positively charged hydrogen atoms surrounding the polymer backbone As a result o f the small partial charge on the hydrogen atoms and the large size o f B r - ions, the interactions between the B r - ions and the PEO chains are relatively weak Thus, the simulations indicate that Br ions should be the principal and most mobile charge carriers in the P E O / ZnBr2 electrolytes All of these conclusions are drawn from static pictures of the simulated electrolytes Information about dynamics should be even more interesting, although more difficult to extract One simple, albeit rather qualitative approach as to observe the motions o f the various ionic species on the c o m p u t e r screen Watchlng Zn 2+ shows that the correlation between the zmc and b r o m i d e ions is very strong indeed It seems that once Z n - B r pmrs form they stay formed and do not dissociate easily, which reinforces a transport model

L Xw, G C Farrmgton/Apphcatton of molecular modehng

in which Zn 2+ moves w a the diffusion o f complex ions In fact, no evidence that Z n 2+ i o n s m o v e as ind e p e n d e n t species free o f a m o n s was seen In the simulations

4. Conclusions Slmulattons using molecular mechanics coupled with molecular d y n a m i c s have proven to be powerful tools for exploring and understanding the likely i o n - p o l y m e r and i o n - i o n mteractlons in PEO/L1Br and P E O / Z n B r 2 electrolytes In P E O / L I B r , L1 ÷ b o n d e d to the PEO chains, relatively free B r - ions, and D B r patrs were observed In P E O / Z n B r 2 electrolytes, no free Zn z+ ions were observed, rather ZnBr +, ZnBr2, and at higher concentrations, Zn2Br~ clusters Ion assocmtlon was temperature and concentration dependent, higher salt concentrations and higher temperatures favored the formation o f more complex ionic species W h a t was most interesting IS just how reasonable the simulated results are. First they show that both LiBr and ZnBr2 dissolve m PEO, as they are known to do Then, it is clear that the strongest i o n - p o l y m e r interactions are those involving the cations and the ether oxygens, again as expected The s i m u l a u o n s indicate that the ions able to m o v e most freely are the antons, m a n y studies have found that PEO-type electrolytes have high anion transport numbers Also, as salt concentratton, cation charge, and t e m p e r a t u r e increase, m o r e complex Ion clusters form, again a result consistent with e x p e r i m e n t The strong c a t i o n chain b o n d m g suggests that the presence o f dissolved salt should increase the viscosity o f short chain PEOtype ohgomers, as has been reported by M e n d o h a et al [20] Even the c a t i o n - o x y g e n and c a t i o n - a n i o n distances are quite reasonable based on studtes o f real compounds Naturally, the slmulatmn m e t h o d has m a n y llmttanons, the most serious arises from the difficulty o f obtaining, determining, or calculating accurate potentmls describing the b o n d i n g characteristics o f the ions More accurate potentmls will be needed as well as more efficient computing algorithms a n d / o r faster computers before simulations o f p o l y m e r electrolytes can even hope to p r o w d e accurate estimates o f such i m p o r t a n t macroscopic properties as con-

27

ductlwty, tonic transport n u m b e r and crystal structure Nevertheless, the i o n - i o n and i o n - p o l y m e r complexes and associations observed through simulation provide a great deal o f insight into the nature o f the lnteracttons and spectes that m a y be present in real electrolytes In this way, the simulated results help to select among various possibihtles dertved from interpreting experimental results and themselves suggest worthwhile experiments to carry out It IS clear that simulations o f the sort discussed in this p a p e r can do much to help imagine the atomic level interactions which ultimately d e t e r m i n e macroscoptc properttes such as ionic conductivity and to provide a framework for sorting among various lnterpretattons of experimental d a t a Simulation and modeling may also be useful in designmg new p o l y m e r electrolyte hosts and m predicting which o f a selection o f potential p o l y m e r solvents is likely to yield the most conductive electrolytes The successful use o f c o m p u t e r m o d e h n g as an aid in the rattonal destgn o f p o l y m e r electrolytes would be a particularly exciting and valuable contribution to the field o f polymer electrochemistry

Acknowledgements This work was supported by a grant from the Defense A d v a n c e d Research Projects Agency through a contract m o n t t o r e d by the Office o f Naval Research. A d d i t i o n a l support from the N S F - M R L program under grant No DMR91-20668 is gratefully acknowledged

References [ 1 ] B B Martin, m Metal Ions in Biological Systems, ed H Slgel (Marcel Dekker, New York, 1984) [ 2] H Emspahr and C E Bugg, in Metal Ions m Biological Systems, ed H Slgel (Marcel Dekker, New York, 1984) [3] M Armand, m Polymer Electrolytes Reviews-l, eds J R MacCallum and C A Vincent (Elsevier, London, 1987) p 230 [4] G C Farrmgton and R G Lmford, in Polymer Electrolytes Reviews-2, eds J R MacCallum and C A Vincent (Elsevier, London, 1989) p 255 [ 5 ] M Kaklhana, S Schantz and L M Torell, J Chem Phys 92 (1990) 6171

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L Xte, G C Farrmgton /Apph~atton o/molecular modehng

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