We evaluated (1) thermal and electrochemical stability and (2) ion-dissociation ability of PF6−n (CF3) n − anions by computational method. The thermal stability order by ΔΔE (anion) is PF4(CF3)2 −>PF5(CF3)−>PF3(CF3)3 −>PF6 −. The ion-dissociation ability order by ΔΔE (Li salts) is LiPF3(CF3)3>LiPF4(CF3)2>LiPF5(CF3)>LiPF6. The conductivity of electrolyte solution with LiPF4(CF3)2 (3.9mS/cm) was a little lower than that of LiPF6 (4.4mS/cm) in 0.1mol/l Li salt/PC:DME electrolyte, while the oxidation potential of LiPF4(CF3)2 in PC was higher than that of LiPF6. The LiPF4(CF3)2-cell showed better cycle characteristics than LiPF6-cell.