Computational Analysis of Solvent Effects on Electron Transfer in Organic Radical Battery Cathode Materials

Organic polymers and molecules containing stable radical groups are currently of interest in a variety of electronic and energy storage applications. However, in many of these systems very little is understood about the fundamental mechanisms by which charge is transported. This includes electron transport, ionic migration as well as molecular rearrangements and solvent reorganization.  This work focuses on utilizing modeling, specifically large scale molecular dynamics simulations and electronic structure calculations to understand charge transport at a molecular level for the prototypical nitroxide radical containing polymer poly(2,2,6,6-tetramethyl-1-piperidinyloxy-4-yl methacrylate) (PTMA). Transport in PTMA films occurs homogeneously between the radical-containing groups, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO). Considering the Marcus formalism, that the rate of transfer depends on several properties of the molecular system: the energy of an electron at each TEMPO site; the electronic coupling between pairs of TEMPO sites; and the molecular reorganization energy associated with the homogeneous electron transfer reaction.  Furthermore we performed simulations to evaluate the electron transfer process in a pure PTMA film and a PTMA film swelled by acetonitrile, similar to what would happen in solution. This was accomplished by quantum mechanical calculations to find the site energy of each TEMPO group in the film and then compute the electronic density of states and the related distribution of charge-transfer reaction enthalpies. In addition, pairs of TEMPO groups were extracted and the inter- and intra-chain electronic coupling was also calculated. Utilizing this data, we found that based on the inter-TEMPO coupling values and the distances between sites the electronic-coupling-weighted radial distribution function to predict the effective electron transfer lengths as a function of solvent concentration. Finally, the implications of these results for how transport occurs within other organic radical polymer materials will be discussed in greater detail.