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Electrolyte solutions based on fluorinated solvents were studied in high-voltage Li-ion cells using lithium as the anode and Li 1.2 Mn 0.56 Co 0.08 Ni 0.16 O 2 as the cathode. Excellent performance was achieved by replacing the conventional alkyl carbonate solvents in the electrolyte solutions by fluorinated cosolvents. Replacement of EC by DEC and by their fluorinated counterparts FEC, 2FEC, and fluorinated ether (F-EPE) considerably improved the cycling behavior of the cells charged up to 4.8 V. The improvement achieved is attributed to formation of a stable and protective surface films on the cathode particles due to unique surface reactions that are enabled by the nature of the fluorinated solvent molecules. The surface films formed on the lithiated transition metal oxide cathodes isolate the active mass, which is highly reactive toward the electrophilic alkyl carbonates, from continuous detrimental reactions with solution species. The positive effect of fluorinated electrolyte solutions on the performance of high-voltage cathodes was exploited in experiments with full graphite−Li 1.2 Mn 0.56 Co 0.08 Ni 0.16 O 2 cells. Excellent cycling performance was recorded (1000 cycles) with solutions containing 2FEC, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (F-EPE) and 1% TMSP, tris(trimethylsilyl)-phosphate (TMSP), which also provided very good results with Li−Li 1.2 Mn 0.56 Co 0.08 Ni 0.16 O 2 cells in shorter experiments. The extraordinary electrochemical stability of this electrolyte solution makes it a suitable candidate for other high-voltage cathode materials. KEYWORDS: lithium-ion batteries, Li 1.2 Mn 0.56 Co 0.08 Ni 0.16 O 2 /graphite cells, fluorinated solvents, fluoroethylene carbonate (FEC), difluoroethylene carbonate (2FEC), 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (F-EPE), tris(trimethylsilyl)-phosphate (TMSP), Mn dissolution

Combined CV studies and UV-Vis-NIR spectroelectrochemical investigations revealed an unusual stability of the p- and n-doped PMOThOD in the wide potential window of 4 V. The n-doping process occurs in this polymer down to -2.7 V (vs. Ag/Ag+) in a non-destructive way with the characteristic development of the omega3 transition as a function of the doping level. In situ electronic transport studies revealed a high conductivity of the n-doped polymer which implies high mobility of the negatively charged carriers in the freshly doped PMOThOD film electrodes. An increase in the cathodic polarization, long-term cycling of the film electrodes, especially of higher thickness, results in a growing contribution of the negatively charged carriers trapping to the redox properties of the PMOThOD. The trapping of the charged carriers reduces gradually the electronic conductance of the PMOThOD film, but its effect on the redox-capacity of the film (in a typical scan rates range up to 50 mV s(-1)) is only minor.

Structure−reactivity relationship (SRR) studies to understand the effect of N-α substitution toward electroactive poly(pyrrole) film formation have carefully been performed. To conduct these SRR studies, a selected chemical library of 21 different N-substituted pyrrole derivatives has been developed using the key well-known Clauson−Kass synthetic reaction from corresponding amine precursors. While investigating electropolymerization features of these novel N-substituted pyrroles, we observed that the steric factor due to α-substitution, one among other factors, plays the most significant role in preventing electropolymerization of these oxidizable chemical species. Interestingly, even a sterically small chemical group like a CH 3 one present at α-position is large enough to prevent monomer electropolymerization. Density functional theory calculations were carried out to analyze the polymerization of substituted pyrrole molecules and to understand the effect of N-substituents on electropolymerization toward the formation of functional conductive film. This systematic study paves the way to effectively design the right N-substitution for the obtainment of corresponding modified functional electrodes for further related applications.

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