Molecular dynamics simulation of lithium ion mobility in a PEO surface
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.
References (11)
- et al.
Comput. Theor. Polym. Sci.
(1997) - et al.
J. Chem. Soc., Faraday Trans.
(1993) - et al.
J. Chem. Phys.
(1994)
Cited by (24)
Atomistic investigation of the nanoparticle size and shape effects on ionic conductivity of solid polymer electrolytes
2014, Solid State IonicsCitation Excerpt :Other works have targeted the same system but focused on ion pairing and lithium ion hopping process [33,34]. PEO hosts doped with different salts have also been investigated [35–49]. MD simulations have also been used to analyze polymer nanocomposite electrolytes.
Molecular Dynamics modelling a small-molecule crystalline electrolyte: LiBF<inf>4</inf>(CH<inf>3</inf>O(CH<inf>2</inf>CH<inf>2</inf>O) <inf>4</inf>CH<inf>3</inf>)<inf>0.5</inf>
2013, Electrochimica ActaCitation Excerpt :Therefore, Li ions can be transported along the glyme hemi-helical direction (z) or along the anion structures (y), or both. Since MD has previously shown to provide insights into ionic transport in different PEO-based materials [17,18], including the systems discussed above, here we use this computational technique to probe the conductivity mechanisms in LiBF4(CH3O(CH2CH2O)4CH3)0.5 (LiBF4:G40.5). To this end, we have applied a series of external electric fields over both the y- and z-directions of the crystal structure in order to distinguish the anisotropy of the ionic conduction processes.
Polymer geometry and Li<sup>+</sup> conduction in poly(ethylene oxide)
2008, Journal of Computational PhysicsCitation Excerpt :The units of CH2 are at a distance rCH = 0.1 nm from the x-axis, which is the axis of a linear segment of the narrow helix, and the units of O are at a distance rO = 0.04 nm from the x-axis. Typical charge distribution values are +0.245 for a unit of CH2 and −0.406 for a unit of O [13–15]. The spatial structure of the PEO is that of a random helix formed by the narrow helix – see Fig. 3.
Mesoscale models of conductivity in polymeric electrolytes-A comparative study
2007, Electrochimica ActaModeling and simulation of Li-ion conduction in poly(ethylene oxide)
2007, Journal of Computational PhysicsCitation Excerpt :Typical charge distribution values are +0.245 for a unit of CH2 and −0.406 for a unit of O [15–17].