A molecular dynamics simulation study of LiFePO4/electrolyte interfaces: structure and Li+ transport in carbonate and ionic liquid electrolytes

Abstract

We have performed atomistic molecular dynamics (MD) simulations of the (010) surface of LiFePO(4) in contact with an organic liquid electrolyte (OLE), ethylene carbonate : dimethyl carbonate (3 : 7) with approximately 1 mol kg(-1) LiPF(6), and an ionic liquid-based electrolyte (ILE), 1-ethyl 3-methyl-imidazolium: bis(fluorosulfonyl)imide (EMIM(+) : FSI(-)) with approximately 1 mol kg(-1) LiFSI. Surface-induced structure that extends about 1 nm from the LiFePO(4) surface was observed in both electrolytes. The electrostatic potential at the LiFePO(4) surface was found to be negative relative to the bulk electrolyte reflecting an excess of negative charge from the electrolyte coordinating surface Li(+). In the ILE system negative surface charge is partially offset by a high density of EMIM(+) cations coordinating surface oxygen. The electrostatic potential exhibits a (positive) maximum about 3 A from the LiFePO(4) surface which, when combined with the reduced ability of the highly structured electrolytes to solvate Li(+) cations, results in a free energy barrier of almost 4 kcal mol(-1) for penetration of the interfacial electrolyte layer by Li(+). The resistance for bringing Li(+) from the bulk electrolyte to the LiFePO(4) surface through this interfacial barrier was found to be small for both the OLE and ILE. However, we find that the ability of EMIM(+) cations to donate positive charge to LiFePO(4)/electrolyte interface may result in a significant decrease in the concentration of Li(+) at the surface and a corresponding increase in impedance to Li(+) intercalation into LiFePO(4), particularly at lower temperatures.

Molecular Modeling of Electrolytes

“…In polarizable simulations of pyr 13 TFSI, pyr 14 TFSI, pyr 14 TFSI, and emimFSI doped with LiTFSI or LiFSI salts [53,[85][86][87][88], the Li + cation was found to be coordinated, on average, by roughly four oxygen atoms. Both monodentate (one oxygen of TFSI bound to a Li + ) and bidendate (two oxygens of TFSI bound to a Li + ) coordinations were observed with a preference for the monodentate configuration in contrast to the conclusions of infrared and Raman studies of alkyl-substituted imidazolium-TFSIbased IL doped with LiTFSI [94].…”

“…[53,[85][86][87][88] and nonpolarizable [89,90] force fields were used in these simulations. MD simulations employing polarizable force fields predicted Li + transport in pyr 14 TFSI doped with LiTFSI below experimental values [86], while MD using nonpolarizable force fields predicted significantly slower ion transport.…”

A Combined Theoretical and Experimental Study of the Influence of Different Anion Ratios on Lithium Ion Dynamics in Ionic Liquids

“…Due to strong polarization of anions by a small Li + cation, it is important to include many-body polarizable terms in the intermolecular potential (force field) used in MD simulations. Previous simulations using atomic polarizable force field for liquids, electrolytes and polymer (APPLE&P) have accurately predicted both structural and transport properties of pure ILs and IL doped with Li + salts [9][10][11][12][13].…”

Electrolyte Solvation and Ionic Association

“…Recent papers, however, indicate that battery electrolytes with LiFSI have very promising properties-i.e., a relatively high thermal and hydrolytic stability (relative to LiPF 6 ), a high conductivity (comparable to electrolytes with LiPF 6 ) and a wide liquidus range. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] One key concern originated from reports which indicated that the use of the LiFSI salt in electrolytes results in severe corrosion of the battery Al current collector at high potentials. 3,15 It has recently been demonstrated, however, that this was likely due to chloride impurities in the salt (from the synthesis procedures) rather than the LiFSI salt itself.…”