Atomistic Insights into the Crucial Role of Lithium-Ion Coordination on the Emergence of Different Chain-Length and Salt-Concentration Regimes in PEO–LiTFSI Electrolytes

High Resolution Image Download MS PowerPoint Slide We report extensive molecular dynamics simulations of common PEO–LiTFSI electrolytes (PEO = poly(ethylene oxide), LiTFSI = lithium bis(trifluoromethanesulfonyl)imide), covering a broad range of chain lengths ( n EO = 2 to 128 ether oxygens per chain) and salt concentrations ( r = 1/80 to 2/5 lithium ions per ether oxygen). The obtained systematic and consistent data set reveals the importance of the lithium-ion coordination for the emergence of different chain-length and salt-concentration regimes in lithium-ion transport. Regarding the effect of chain length on cation transport, three regimes are identified. (i) A low-molecular-weight regime ( n EO ≤ 6) with a highly chain-length-specific behavior caused by the great variety of lithium-ion coordination motives and with a sharp regime crossover at the preferred lithium–oxygen coordination number of six. (ii) An intermediate regime characterized by a monotonic decrease of the lithium-ion dynamics caused by the gradually decreasing center-of-mass motion of the polymer chains. (iii) A high-molecular-weight regime featuring chain-length independent ion dynamics after a smooth regime crossover at around n EO ≈ 50 to 100, caused by a transition in the leading transport mechanism away from the polymer’s center-of-mass motion to segmental dynamics and chain transfers. The chain-length dependence of the specific volume of polymer electrolytes is governed by chain-end effects like in ordinary polymer melts. Regarding the effect of salt concentration on the cation transport, two regimes are distinguished. (i) A salt-in-polymer regime ( r ≲ 1/8) with a strong coupling of the cation dynamics to the polymer dynamics due to an almost pure PEO coordination of the lithium ions. (ii) A polymer-in-salt regime ( r ≳ 1/8) characterized by a partial decoupling of the cation and polymer dynamics caused by a mixed PEO/TFSI coordination of the lithium ions. A clear crossover from a salt-in-solvent to a solvent-in-salt regime is only observable in electrolytes with a strong preferential binding of the cations to the solvent.