Novel lithium salt and polymer electrolytes for polymer lithium batteries.

摘要

Synthesis and characterization of a novel lithium salt that operates on the principle of steric occlusion was conducted. Lithium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (LiTMPB) incorporating a bulky anion with electronic delocalization was synthesized.;Ethylene oxide (EO) comb polymer with poly(trimethylene oxide) backbones and -O-(CH2CH2O)m-CH3 (m=7) side chains, and trimethylene oxide (TMO) comb polymer with poly(trimethylene oxide) backbones and -O-(CH2CH2CH2O)n-CH 3 (n=4) side chains were prepared and complexed with lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and LiTMPB salts to form homogeneous electrolytes. All these electrolytes were amorphous and thermally stable up to 150°C. The glass transition temperatures (Tgs) of the pure EO and TMO comb polymers were -68 and -82°C, respectively. The Tgs of the TMO electrolytes were at least 10°C lower and rose more slowly with lithium salt concentration than EO counterparts. Their conductivities rose with the increase of salt concentration to a maximum and then dropped. The highest conductivities for the EO comb polymer/LiTFSI and EO comb polymer/LiTMPB electrolytes at ambient temperature and 100 KHz were 3.19*10-5 S/cm at 25:1 [O]/[Li] and 4.65*10-6 S/cm at 50:1 [O]/[Li], respectively. At 100°C and 100 KHz, they rose to 9.0*10-4 and 2.0*10-4 S/cm, respectively. For their TMO counterparts, the highest conductivities at ambient temperature and 100 KHz were 4.62*10-5 S/cm at 10:1 [O]/[Li] and 6.34*10-6 S/cm at 30:1 [O]/[Li], respectively. At 100°C and 100 KHz, they rose to 1.1*10-3 and 2.6*10 -4 S/cm, respectively. At given [O]/[Li] ratios from 70/1 to 10/1, TMO comb polymer electrolytes had comparable or even higher conductivities than their EO counterparts. The conductivity/temperature dependence of these electrolytes obeyed the VTF equation instead of the Arrhenius equation, which confirmed their amorphous characteristics.;Conductivity measurements at low frequencies were used as an approximation for cell polarization. It showed that the LiTMPB electrolytes had less cell polarization than their LiTFSI analogues.;An equivalent circuit model composed of three parallel circuits, each with a capacitance and resistance in series, was proposed for electrical behavior simulation and could accurately simulate the ethylene oxide electrolytes' capacitance and resistance at different frequencies and temperatures.