Thermal and Electrolytic Behavior of Lithium Chelatoborates and Application to Lithium Batteries

Abstract

The use of a series of multifunctional compounds as chelate ligands enables us to control charge delocalization around a central atom in the chelate anions and to investigate various properties systematically. We have investigated the substituent effect on the thermal stability and electrolytic properties of lithium bis [5-bromosalicylato (2-)] borate (5-BLBSB) and lithium bis [5-chlorosalicylato (2-)] borate (5-CLBSB), which we have newly synthesized, as well as lithium bis[salicylato(2-)]borate (LBSB), lithium bis[3-methylsalicylato(2-)]borate (3-MLBSB), lithium bis [3,5-dichlorosalicylato(2-)]borate (DCLBSB), and lithium bis[3,5,6-trichlorosalicylato(2-)]borate (TCLBSB). The thermal decomposition temperature decreases in approximately the following descending order: 5-BLBSB ≈ 3-MLBSB (330°C) >DCLBSB (310°C) >5-CLBSB (300°C) >LBSB (290°C) >TCLBSB (260°C:). The stability to oxidative decomposition decreases in the sequence TCLBSB ≈ DCLBSB >5-CLBSB >5-BLBSB >LBSB >3-MLBSB, and the reductive stability increases in the opposite order except for 5-CLBSB. Furthermore, we have applied the lithium chelatoborates to lithium batteries and have investigated cycling efficiency of a lithium anode, discharge characteristics of Li/V2O5 prototype cells, and performance of Li/LiCoO2 and C/LiCoO 2 coin cells with respect to retention of discharge capacity and specific energy, using EC-DEC (mole ratio 1:1) binary solutions containing LiPF6. The addition of a small amount of the lithium chelatoborate improves the cell performance.