A comparison between the high temperature electrode/electrolyte reactions of LixCoO2 and LixMn2O4

Accelerating rate calorimetry (ARC) is used to compare the reactions between LixCoO2 (4.2 V) or LixMn2O4 (4.2 V) and an equal mass of 1 M LiPF6 (ethylene carbonate/diethyl carbonate) electrolyte. The ARC results show that under these conditions, the self-heating rates and hence thermal power are significantly larger for LixMn2O4 than for LixCoO2. This result is not consistent with the results of oven exposure tests on 18650 size cells using the same materials, where higher temperatures are needed to initiate thermal runaway for cells with LixMn2O4 cathodes. The amount of reaction heat generated by the reaction of LixCoO2 with 1 M electrolyte is independent of the electrode/electrolyte mass ratio (at least for the first reaction process), while that generated by the reaction of LixMn2O4 with electrolyte increases with electrolyte amount. ARC experiments using an approximate 1:6 electrolyte: to electrode mass ratio, which mirrors the conditions found in 18650 cells, demonstrate that LixCoO2 is much more reactive than LixMn2O4, as is observed in commercial cells. The work presented here suggests that the safety of Li-ion cells using LiMn2O4 cathodes can be improved by decreasing the positive electrode porosity. (C) 2001 The Electrochemical Society.