Thermal stability of 18650 size Li-ion cells containing LiBOB electrolyte salt

Lithium bis(oxalato)borate (LiBOB) has been proposed recently as an electrolyte salt for Li-ion batteries, however safety testing of full Li-ion cells incorporating this salt has not been reported. Earlier accelerating rate calorimetry (ARC) work demonstrated that the thermal reactivity between LiBOB ethylene carbonate/diethyl carbonate (EC/DEC) electrolyte and Li0.81C6 was lower than that between LiPF6 EC/DEC electrolyte and the same negative electrode material, suggesting that LiBOB may be an attractive salt choice for safer Li-ion cells. Here, we report ARC studies of the reactions between LiBOB EC/DEC electrolyte and Li0.5CoO2 and compare them to the reactions between LiPF6 EC/DEC and the same positive electrode material. Unfortunately, the reactivity of LiBOB electrolyte with the charged positive electrode initiates at a substantial rate at about 40degreesC lower in temperature than for LiPF6 electrolyte. Oven exposure tests on charged (4.2 V) 18650 Li-ion cells made using the same electrolytes and electrode materials show that the heat caused by the reaction of the negative electrode with electrolyte is less for LiBOB electrolyte than for LiPF6 electrolyte but that the opposite is true for the heat caused by the reaction of the positive electrode with electrolyte, as expected based on the ARC measurements. Using ARC tests on the individual electrode reactivity and oven exposure testing on 18650 Li-ion cells, the usefulness of electrolytes with mixed LiBOB/LiPF6 salts is also explored. (C) 2004 The Electrochemical Society.