Elsevier

Solid State Ionics

Volume 143, Issue 1, 1 June 2001, Pages 83-87
Solid State Ionics

Molecular dynamics simulation of lithium ion mobility in a PEO surface

https://doi.org/10.1016/S0167-2738(01)00836-0Get rights and content

Abstract

A model for a poly(ethylene oxide) (PEO) host polymer surface, which was developed earlier, is exploited to probe the ionic distribution for Li+ and Cl ions in the PEO surface for an effective composition LiCl·(PEO)213 at 400 K. The local structural situation around the Li+ ions was analyzed specifically. Two general situations are observed: Li+ ions lying deeper into the bulk tend to be associated with one Cl and two oxygens; nearer the surface, they coordinate five ether oxygens belonging to the same PEO chain. The ratio between the two cases (Cl+2O:5O) moves smoothly from ca. 30:70 in the bulk to ca. 45:55 in the surface region.

Introduction

Poly(ethylene oxide) (PEO)-based polymers have attracted considerable attention as potential polymer electrolytes in modern electrochemical devices, especially in high energy-density lithium-ion polymer batteries [1], [2]. The physical properties of solid polymer electrolytes directly reflect the structure of the host polymer and its interaction with incorporated salt ions. In the absence of definitive experimental observations, molecular dynamics (MD) simulation is a powerful tool to provide structural insights at the atomic level into the processes involved [3]. Mechanisms relating to ionic transport in a polymer surface have a special relevance to the situation in a Li-ion polymer battery, and yet are very poorly understood. Of prime interest in this context is the electrochemically active polymer–electrode interface, especially since much of what is stated in this connection has a distinctly weak experimental basis. Potentials developed earlier to model crystalline and amorphous PEO bulk situations [4], [5] were later used for the simulation of a ‘crystalline’ PEO surface [6]. This model is used here to describe the PEO host surface for the introduction of a low concentration of Li+ and Cl ions. An ether oxygen:Li ratio of ca. 200:1 is used to probe the ion–polymer interaction.

Section snippets

The calculations

The PEO surface model used here is almost identical with that used in our earlier PEO simulations [4], [5]. It is based upon the unit cell of crystalline PEO: monoclinic, P21/a, containing two right- and two left-handed helical chains running in the z-direction. A 2×2×8 unit cell simulation box has been used with dimensions: a=16.10 Å, b=26.08 Å, c=155.84 Å, β=125.4° [4], [5]. A surface is realized through the creation of a highly asymmetric simulation box comprising 122 Å-thick ‘crystalline’

Results and discussion

The structural and dynamical information is extracted from the recorded sequence of atomic position and velocity coordinates obtained through the MD procedure described above. In maintaining an isotropic pressure tensor in the NpT ensemble, the MD cell expands by roughly 8% during the NpT simulation. Crude 3D diffusion coefficients (averaged over 200–500 ps) were calculated to a value of 1.0×10−7 m2/s for Li+ and 1.0×10−9 m2/s for Cl ions. It must be stated, however, that despite relatively

Conclusions

We have shown that our earlier derived crystalline bulk model can be adapted appropriately to provide a simplistic model for a PEO surface host into which Li–salt ions can be introduced. The behaviour of a low concentration of Li+ and Cl ions has been investigated. A smooth depth-dependent transition between two local structural situations could be extracted from the simulations, with Li+ ions associated either with one Cl and two oxygens deeper into the bulk, or with five oxygen in the

Acknowledgements

Grants are gratefully acknowledged from the Swedish Natural Science Research Council (NFR), the Swedish Board for Technical Development (NUTEK), and from The Parallel Computer Centre (PDC) of the Royal Institute of Technology (KTH) in Stockholm.

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