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

Accelerating rate calorimetry (ARC) is used to compare the reactions betwee n LixCoO2 (4.2 V) or LixMn2O4 (4.2 V) and an equal mass of 1 M LiPF6 (ethyl ene carbonate/diethyl carbonate) electrolyte. The ARC results show that und er these conditions, the self-heating rates and hence thermal power are sig nificantly larger for LixMn2O4 than for LixCoO2. This result is not consist ent with the results of oven exposure tests on 18650 size cells using the s ame materials, where higher temperatures are needed to initiate thermal run away for cells with LixMn2O4 cathodes. The amount of reaction heat generate d by the reaction of LixCoO2 with 1 M electrolyte is independent of the ele ctrode/electrolyte mass ratio (at least for the first reaction process), wh ile that generated by the reaction of LixMn2O4 with electrolyte increases w ith electrolyte amount. ARC experiments using an approximate 1:6 electrolyt e: to electrode mass ratio, which mirrors the conditions found in 18650 cel ls, demonstrate that LixCoO2 is much more reactive than LixMn2O4, as is obs erved in commercial cells. The work presented here suggests that the safety of Li-ion cells using LiMn2O4 cathodes can be improved by decreasing the p ositive electrode porosity. (C) 2001 The Electrochemical Society.