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Related Articles:
  1. J. Phys. Chem. B (2004) 108, 2016 [DOI:10.1021/jp036673w]
  2. J. Phys. Chem. B (2004) 108, 17992 [DOI:10.1021/jp046979i]






DESCRIPTION

Battery technology has achieved spectacular progress in recent years. The most successful product is the rechargeable Lithium Ion Battery (LIB) that has reached an established commercial status with a production rate of several millions of units per month. The technology of LIBs is still in progress and important steps forward have been achieved in the development of battery systems using lithium metal as the anode. Ultimately, the lithium salt electrolyte is not held in an organic solvent like in the past models, but in a solid polymer gel electrolyte such as polyacrylonitrile, polyvinylidene fluoride, polyethilene oxide etc. There are many advantages of this design over the classic lithium ion design. The solid polymer electrolyte is not flammable, like the organic solvent that the Li-Ion cell uses. Thus these batteries are less hazardous if mistreated.

The vast majority of the electrolytes are electrolytic solution-types that consist of salts (also called ''electrolyte solutes'') dissolved in solvents (also called plasticizers), either water (aqueous) or organic molecules (nonaqueous), and are in a liquid state in the service-temperature range. The most used solvents are ethylene carbonate, tetrahydrofuran, propylene carbonate, and gamma-butyrolactone.

The electrolyte is in close interaction with both electrodes and serves as a medium of transport for the ions involved in the charging/discharging cycle. Conceptually, it should undergo no net chemical changes during the operation of the battery, and all Faradaic processes are expected to occur within the electrodes. Therefore, in an oversimplified expression, an electrolyte could be viewed as the inert component in the battery, and it must demonstrate stability against both cathode and anode surfaces. Experimentally it has been found that a mixture of two or more plasticizers is more convenient, as it allows us to optimize the balance between different features (such as dielectric constant, viscosity, ionic diffusion, salt dissociation, and chemical stability) and thus to enhance the battery performace and cyclability.

Even if the functioning scheme described above is quite simple, it should be stressed that many phenomena take place inside the battery which are complex and difficult to interpret through experiment. Understanding the molecular mechanisms by which these phenomena take place is of great interest. Recently some theoretical work has been appearing to explain degradation processes (like electrolyte decomposition, passive film formation on the electrode) and surface chemistry on the electrode. Some other works focus their attention on the plasticizer and on its interaction with lithium ions.

Following this line, we studied two plasticizers: ethylene carbonate (EC) and gamma-butyrolactone (GBL). EC and GBL (and their mixtures with other solvents) are quite widespread in LIBs. Moreover, it seems that they are good candidates to be used in next generation magnesium ion batteries. Few computational studies on these two molecules and on their solutions with lithium ions exist. The parameters used to model both the intra- and inter-molecular interactions (force field) were quite generic and could not fairly reproduce quantities of experimental interest. For example huge errors were found in the simulated vibrational spectrum of the two molecules. Moreover the diffusion coefficient both of the ion and of the molecules were underestimated, with respect to NMR measurements. We proposed ourselves to develop a new model for the interactions, which could reproduce the cited experimental results. To achieve this result we approached the problem with a new philosophy. We developed a new force field to study the characteristics of the two plasticizers both in the liquid and gas phases. In particular we focused our attention on:

1) simulating the vibrational spectrum of the molecules in both phases: the results were compared with experiments and an assignment of the vibrational modes were given;

2) studying how the strong influence of the lithium ion affects the surrounding solvent molecules, particularly in their structure and in their vibrational spectrum.