Gas Evolution and Self-Discharge in Li/MnO2 Primary Batteries

Abstract : A survey is presented of the available literature on the problem of gas evolution in the Li/MnO2 primary battery system during storage. This gassing is due to the breakdown of the solvents, primarily propylene carbonate (PC) and dimethoxyethane (DME). Of these, the major contributor to gassing during storage is PC, which undergoes a hydrolysis reaction to form CO2. This reaction is a function of the amount of absorbed and structural water. Techniques to help eliminate gassing during storage include predischarging and the use of water scavengers, cathode additives, reactive dehydration, properly heat-treated MnO2, weak oxidizing salts, and oxidation-resistant solvents. Although not a major factor in gas evolution in stored batteries, DME has been observed to form methane (CH4) during discharging, probably via an oxidation reaction that is ultimately a function of the absorbed water content. The report also briefly reviews the self-discharge characteristics of Li/MnO2. Three factors determine the self-discharge of Li/MnO2: (1) The oxidation state of the manganese ion. For the types of MnO2 used, it is expected that the self-discharge rate would be low. (2) The nature of the electrolyte salt and amount of absorbed water. Self-discharge is higher for LiClO4 and LiCF3SO3 electrolytes than for fluorinated salt electrolytes. The nature of the solvent. Self-discharge is higher for ethers (e.g., DME) than for carbonates (e.g., PC).