Role of Solution Structure in Solid Electrolyte Interphase Formation on Graphite with LiPF₆ in Propylene Carbonate

An investigation of the interrelationship of cycling performance, solution structure, and electrode surface film structure has been conducted for electrolytes composed of different concentrations of LiPF₆ in propylene carbonate (PC) with a binder-free (BF) graphite electrode. Varying the concentration of LiPF₆ changes the solution structure, altering the predominant mechanism of electrolyte reduction at the electrode interface. The change in mechanism results in a change in the structure of the solid electrolyte interface (SEI) and the reversible cycling of the cell. At low concentrations of LiPF₆ in PC (1.2 M), electrochemical cycling and cyclic voltammetry (CV) of BF graphite electrodes reveal continuous electrolyte reduction and no lithiation/delithiation of the graphite. The solution structure is dominated by solvent-separated ion pairs (Li⁺(PC)₄//PF₆^–), and the primary reduction product of the electrolyte is lithium propylene dicarbonate (LPDC). At high concentrations of LiPF₆ in PC (3.0–3.5 M), electrochemical cycling and CV reveal reversible lithiation/delithiation of the graphite electrode. The solution structure is dominated by contact ion pairs (Li⁺(PC)₃PF₆^–), and the primary reduction product of the electrolyte is LiF

Solvate Structures and Computational/Spectroscopic Characterization of LiBF₄ Electrolytes

Crystal structures have been determined for both LiBF₄ and HBF₄ solvates: (acetonitrile)₂:LiBF₄, (ethylene glycol diethyl ether)₁:LiBF₄, (diethylene glycol diethyl ether)₁:LiBF₄, (tetrahydrofuran)₁:LiBF₄, (methyl methoxyacetate)₁:LiBF₄, (succinonitrile)₁:LiBF₄, (<i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>″,<i>N</i>″-pentamethyldiethylenetriamine)₁:HBF₄, (<i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetramethylethylenediamine)_3/2:HBF₄, and (phenanthroline)₂:HBF₄. These, as well as other known LiBF₄ solvate structures, have been characterized by Raman vibrational spectroscopy to unambiguously assign the anion Raman band positions to specific forms of BF₄^–···Li⁺ cation coordination. In addition, complementary DFT calculations of BF₄^–···Li⁺ cation complexes have provided additional insight into the challenges associated with accurately interpreting the anion interactions from experimental Raman spectra. This information provides a crucial tool for the characterization of the ionic association interactions within electrolytes

Structural Interactions within Lithium Salt Solvates: Cyclic Carbonates and Esters

Only limited information is available regarding the manner in which cyclic carbonate and ester solvents coordinate Li⁺ cations in electrolyte solutions for lithium batteries. One approach to gleaning significant insight into these interactions is to examine crystalline solvate structures. To this end, eight new solvate structures are reported with ethylene carbonate, γ-butyrolactone, and γ-valerolactone: (EC)₃:LiClO₄, (EC)₂:LiClO₄, (EC)₂:LiBF₄, (GBL)₄:LiPF₆, (GBL)₁:LiClO₄, (GVL)₁:LiClO₄, (GBL)₁:LiBF₄, and (GBL)₁:LiCF₃SO₃. The crystal structure of (EC)₁:LiCF₃SO₃ is also re-reported for comparison. These structures enable the factors that govern the manner in which the ions are coordinated and the ion/solvent packingin the solid-stateto be scrutinized in detail