A potential energy surface (PES) has been calculated for the PFSI anion, [N(C2F5SO2)(2)](-), (bis(pentafluoro-ethanesulfone)imide ion), using quantum mechanical ab initio Hartree-Fock SCF molecular orbital methods (HF/6-31G*). Two different minimum energy structures have been found, and the vibrational spectra have been calculated for the two conformers. The energy difference between the two conformers is small, 5.3-7.3 kJ mol(-1) and the energy barrier between them is low, similar to 2.3 KJ mol(-1), according to the PES. Thus, a large internal flexibility of the anion, a significant population of the two conformers, and rapid conversion between the two even at room temperature can be expected. Infrared vibrational spectra of the lithium salt of the anion, LiPFSI, dissolved in poly(ethylene oxide), PEG, have been recorded and compared with the calculated spectra, which suggest that no ion pairs of Li+-PFSI are present for ether O/Li ratios n > 6.5. Comparisons are also made with ab initio calculations on the corresponding superacid HPFSI and with previously reported calculations and spectra on the more commonly used TFSI anion. The electronic structure of the PFSI anion and its calculated thermodynamic stability are evaluated from the point of view of the anion＇s use as a highly noncoordinating anion in solid polymer electrolytes in novel thin-film lithium polymer batteries (LPB＇s). No significant features that allow us to prefer PFSI to TFSI for usage in LPB＇s were found.