Theoretical Investigations On Structures And Properties Of Organic Disulfide Cathode Materials

Organic disulfide (polymers) is one of the most promising cathode materials for lithium battery. Based on energy conversion during the reversible process of disulfide bond formation and breaking,[-S-S-]+2e (?)2[-S]-, it can realize the storage of energy. Such cathode materials have the advantages of high energy density, low operating temperature, non-toxicity, low costs, designability of the active molecular structure, and etc. The S-S bond can capture an electron to form a two-center three-electron(2c-3e) bond, donated as [S∴S]-, which has relatively high stability."π-Backbone for electric conduction, side chain for energy storage" has become a new idea of developing conducting organic disulfide polymers. Quantum chemistry calculations are capable of characterizing the bonding features of [S∴S]-and structural modification effect, and provide a basis for the design and application of organic disulfide cathode materials.In this dissertation,4model systems of organic disulfides have been considered. Structural and electronic properties of redox intermediates of these molecules were investigated theoretically. The nature of [S∴S]-and structure modification effect were discussed. We also explored the reversible lithium intercalation-deintercalation process, and predicted the standard redox potentials of organic disulfides in DMSO and acetonitrile solution by the polarizable continuum model(PCM).The main results in the dissertation are summarized as follows:(1) Equilibrium geometries, electron affinities, ionization potentials, and effects of structural modification on their properties of selected organic disulfides have been investigated by B3LYP/AUG-cc-pVDZ calculations. The present results show that the S-S bond of these disulfides can accommodate the excess electron effectively via the σS-S*antibonding orbital and form a stable two-center three-electron (2c-3e)[S∴S]-bond. The presence of strong electron-withdrawing group-NO2can slightly change the energy order of frontier orbitals to some extent, but the σS-S*is still the lower-energy unoccupied frontier orbital for the storage of excess electrons. (2) Equilibrium geometries of optimized lithium-disulfide complexes and Gibbs free energies for the Li insertion reaction have been investigated by B3LYP/AUG-cc-pVDZ calculations. Lithium atoms insert into organic disulfides in the form of S-Li-S bridge bond. The present results show that the step at the even number of lithium atoms embedding reaction is irreversible, while the reversibility for the step with the odd number is good. Calculations show the cleavage of S-S bond from capture of two electrons is irreversible, while formation of [S∴S]-is reversible. Accordingly, we need to design new disulfide cathode materials which tend to form many radical anion moieties with [S∴S]-, achieving high charge-discharge cycle performance of lithium battery. In particular, each S-S bond is easily to accomodate a first electron (embedding a first lithium), and difficult to be further reduced (embedding a second lithium).(3) The standard redox potentials of organic disulfides in DMSO and acetonitrile solution have been calculated through the thermodynamic cycle by high precision method G2and SMD solvent model. Also, the empirical strategy of B3LYP density functional calculation plus0.28V correction was evaluated. Present results show that, this strategy is not suitable for our research system. Compared to acetonitrile, organic disulfides in DMSO are relatively easily reduced by the electrochemical process. Generally, standard reduction potential of X-/X (X denotes model compounds) are more negative than that of X2-/X-, but for the model compound which forms biradical anions, just the reverse.