Substituent effect on the reductive decomposition thermodynamics and kinetics of PFA ethylene carbonates for facilitating SEI formation of LIB

Redox trends of PFA Ethylene Carbonates with quantum mechanics • Among the additives of PFA-EC, PFO-EC has the highest reduction potential (0.78 V). • Hybrid DFT (B3PW91) provides moderate thermodynamic and kinetic data. • PFO-EC has the lowest Gibbs free energy change for forming the open radicals. • The electro-reduction paths of PFO-EC also have the lowest energy barrier. A variety of density functional theories (hybrid, dispersion-corrected and long-range corrected DFTs) were used to comprehensively investigate the electro-reductive decomposition of polyfluoroalkyl-substituted ethylene carbonates (PFA-EC), novel electrolyte additives of lithium ion batteries for improving SEI, in terms of frontier molecular orbitals, thermodynamics as well as kinetics. Their LUMOs decrease with the size of PFA side chain, while the HOMOs increase with the size of the substituents. The opposite trend of the frontier MOs enable PFO-EC with the longest side chain to sacrificially oxidize and reduce in EC-based electrolyte solution of lithium ion batteries. Among the investigated PFA-substituted ethylene carbonates PFO-EC has the highest reduction potential (φ a ∼ 0.78 V with SMD-B3PW91/6–311++G**) to form a reduction precursor, which agrees very well with the experimental one (∼0.75 V) and is higher than that of EC (∼0.5 V). Two pathways of the reductive decomposition for Li + (PFA-EC) were well identified, forming primary and secondary radicals respectively. The latter paths are thermodynamically (higher negative Δ G ) and kinetically (lower energy barrier, E a ) than the former. Among the investigated PFA-EC, FEC and EC, PFO-EC shows the highest negative Δ G and the lowest energy barrier. Thus, the reductive decomposition of PFO-EC is the most favorable thermodynamically and kinetically, which favors the formation of SEI components. Regarding the performance of different DFTs, hybrid DFT (B3PW91) provides moderate thermodynamic and kinetic data, while the results from dispersion corrected and long-range corrected DFTs are either too low or too high.