Crystalline Structure and Lithium-Ion Channel Formation in Self-Assembled Di-lithium Phthalocyanine:  Theory and Experiments

Abstract

The crystalline structure of di-lithium phthalocyanine (Li2Pc) is analyzed via a sequence of theoretical methods starting with ab-initio optimizations of a single molecule and dimers, followed by a series of classical molecular dynamics simulations that emulate four alternative crystalline structures. Calculated X-ray spectra are compared with those from experiments, and the results suggest that the features correspond to a dominant β-phase, although similarities in the calculated spectrum of alternative phases may imply the possible existence of polymorphism in this material. Since Li2Pc has been proposed as a solid electrolyte for lithium-ion batteries, the existence of ion-conducting channels is examined through the analyses of the simulated structures. Dynamical properties such as the lithium-ionic diffusion coefficient are determined through the velocity autocorrelation function and compared to experimental values.

Aerospace Power Scholarly Research Program. Delivery Order 0011: Single Ionic Conducting Polymer Electrolyte

“…These calculated results suggest that lithium ion transport in the crystalline phase can be very rapid approaching a specific conductivity of 10 -4 S/cm at 300 K. However, these calculated results reflect ion movement via selfdiffusion as determined by the field created by the self-assembly of Li 2 Pc molecules, while in the actual experiment, diffusion across the electrolyte/electrode interface is driven by a difference of chemical potential and by an electric field gradient established between the electrodes of an electrochemical cell. Thus, one may expect higher specific ionic conductivities in an actual experiment, as has been reported in reference (2). Our work also shows that one can fabricate full electrochemical cells with Li 2 Pc as the solidstate electrolyte and obtain cells with stable open circuit voltages(3).…”

Aerospace Power Scholarly Research Program. Delivery Order 0011: Modeling Lithium-Ion Conducting Channel

“…The separations between layers have been computed (from the last MD configuration of each case) as averages over measured distances between all the equivalent two adjacent layers within the simulation cell. 18 Comparison of these distances with those of (Li 2 Pc) 2 calculated by B3LYP/6-31G(d) (3.72 Å for the staggered form and 3.77 Å for the shifted form) indicates good agreement between MD and DFT regarding the staggered forms, whereas shorter distances are found by MD in the shifted configurations. We used as input the coordinates from the final molecular structure of the MD simulations in the primitive cells of each of the four crystalline forms, along with their respective primitive cell parameters, 18 to obtain simulated X-Ray powder diffractograms.…”

“…The cell parameters of the four configurations along with all simulation details have been reported. 18 Among these polymorphs, the α-, β-, and ε-form structures have monoclinic unit cells, and the χ-form has a tetragonal unit cell. The primitive unit cells for the α-, β-, and χ-forms have the same dimensions as the reported structures for the polymorphs of LiPc except that the shortest cell dimension was elongated to fit the experimental density of Li 2 Pc (1.40 g/cm 3 ).…”

“…We can visualize these conducting channels in α-form, β-form, and ε-form when combining the top (not shown) and side views of the unit cells, 18 or rotating the unit cell of χ-form. These chains of ions are oriented along the same direction across layers.…”

“…The velocity autocorrelation function of lithium ions shows oscillations in the time domain, indicating the rattling motion of lithium ions in the "cage" of their nearest neighbors. 18 The corresponding conductivity of lithium ions was calculated via the Nernst-Einstein equation,…”