Synthesis and properties of new carboxyborate lithium salts as electrolytes for lithium-ion batteries

Bis(carboxytrifluoroborate lithium) salts [R(CH2COOBF3Li)2] with oxyethylene groups R of oligomeric molar masses [R = O(CH2CH2O)n, where n = 3 or 11, BCB3 and BCB11, respectively] were synthesized via reaction of carboxylates salts with boron fluoride. The new salts were characterized by spectroscopic analysis. The physical properties of the salts were determined by oxyethylene chain length. For n = 3 the salt was crystalline with mp = 197 °C and for n = 11 it showed properties of an ionic liquid at ambient temperature. Their thermal stability was at least 250 °C. The values of lithium-ion transference numbers (T+) of the solutions in polar aprotic solvents, determined by a well established steady-state technique, were in the range of 0.2-0.6. Electrochemical impedance spectroscopy analysis of solid polymer electrolytes (SPEs) based on PEO and studied salts with different concentration (from 24 to 94 wt %) was carried out. The ionic conductivity of SPEs was in the order of 10−8-10−7 S cm−1 at room temperature and 10−4 S cm−1 at 80 °C. A distinguishing feature of SPEs with the studied new salts is the high immobilization of anions, which causes almost a monoconducting character of charge transport. Lithium transference numbers (T+) exceed 0.9.