On The Synthesis And Properties Of Lithium Perfluoroalkyl Sulfonyl Imides-Polyphosphazene Solid Polymer Electrolytes

Solid polymer electrolytes have emerged as an exciting class of novel electrolyte membrane materials with potential applications in solid-state batteries and other devices. Although poly(ethylene oxide) (PEO) was the first solid polymeric electrolyte discovered, poor ambient ionic conductivity was obtained due to its crystallinity. The limitation of this polymer has prompted a search for alternatives, polyphosphazenes bearing oligoethyleneoxy side groups are typical examples.A new series of lithium superacid salts have been mixed with poly(oligoethylene oxide)phosphazenes by dissolving-melting method to prepare the SPEs and its conductivity, electrochemical stability and thermal stability behaviors of these SPEs have been studied. The results showed that the behavior of these SPEs had typical characterizations of the amorphous SPE systems.A branched ester-type lithium imide-----lithium bis[(1,1,1,3,3,3-hexafluoro -2-propoxy) sulfonyl] imide (LiN[SO2OCH(CF3)2]2, LiHFPSI) was used as the salt for methoxy ether-substituted poly(organophosphazenes) {NP[(OCH2CH2)XOCH3]2}n (where x = 2~4, MExP) based polymer electrolytes. The maximum conductivity at 25℃ (which varied with the lithium salt concentration) increased as the lengths of the side chains increased, which was in good agreement with the activate energy (Ea) calculated from the VTF equation. In comparison with MExP-LiTFSI (Lithium bis(trifluoromethyl sulfonyl) imide) system, MExP-LiHFPSI had similar conductivities but better electrochemical stabilities, and the bigger size and special structure of the anion would also help to improve the cationic transference number. What's more, the electrochemical stability window of the SPEs increased along with the increasing number of fluorine-containing groups, which is in accordance with the change of their HOMO energy calculated by PM3 semi-empirical method. The results of the thermal analysis indicated that the glass transition temperature (Tg) of the SPEs prepared in our experiments were very low, and no melting temperature had been found in the temperature range from –100~100℃. As indicated by the TGA result, these materials remains stable up to 200℃ without losing any weight. The optimum system for MExP-LiHFPSI complexes is obtained when x=4 and the molar ratio of Li+ to repeat unit is 3:8 with the highest conductivity of 4.48×10-4 S/cm, which also possessed an electrochemical stability window of more than 6.0V and an excellent thermal stability.A series of SPEs based on lithium poly(polyfluoroalkoxy) sulfonyl imide) were prepared in our experiment. The MEEP-2# salt (lithium poly(4,4'-(hexafluoro isopropylidene) diphenoxy) sulfonyl imide) (mol Li/rpt unit=0.125) system had the best performance, with an conductivity of 2.42×10-5S/cm at 25℃ and an electrochemical stability window of 5.5 V. It also had good thermal stability and good dimensional stability as well. In summary, the polymer electrolytes based on methoxy ether-substituted poly (organophosphazenes) and a novel series of lithium superacid salts would have tremendous potential in the applications of secondary lithium battery and other electrical devices due to their satisfactory ionic conductivities, outstanding electrochemical stabilities and thermal stabilities.