Abstract: Acyclic, asymmetric ethyl alkyl carbonates, particularly for use with a carbonaceous, e.g., graphite, anode, in electrolytes suitable for portable power sources, are disclosed. Asymmetric alkyl carbonates having the general structural formula EtO--CO.sub.2 R, where R is larger than ethyl, and most preferably equal to butyl, iso-butyl or sec-butyl, are particularly useful in causing the freezing point of the electrolytes in which they are used to decrease dramatically, thus providing the key to low temperature, high cycle life and high capacity for portable power sources.