Liquid electrolytes for lithium and lithium-ion batteries George E. Blomgren Blomgren Consulting Services Ltd., 1554 Clarence Avenue, Lakewood, OH 44107, USA Abstract A number of advances in electrolytes have occurred in the past 4 years, which have contributed to increased safety, wider temperature range of operation, better cycling and other enhancements to lithium-ion batteries. The changes to basic electrolyte solutions that have occurred to accomplish these advances are discussed in detail. The solvent components that have led to better low-temperature operation are also considered. Also, additives that have resulted in better structure of the solid electrolyte interphase (SEI) are presented as well as proposed methods of operation of these additives. Other additives that have lessened the flammability of the electrolyte when exposed to air and also caused lowering of the heat of reaction with the oxidized positive electrode are discussed. Finally, additives that act to open current-interrupter devices by releasing a gas under overcharge conditions and those that act to cycle between electrodes to alleviate overcharging are presented. As a class, these new electrolytes are often called ‘‘functional electrolytes’’. Possibilities for further progress in this most important area are presented. Another area of active work in the recent past has been the reemergence of ambient-temperature molten salt electrolytes applied to alkali metal and lithium-ion batteries. This revival of an older field is due to the discovery of new salt types that have a higher voltage window (particularly to positive potentials) and also have greatly increased hydrolytic stability compared to previous ionic liquids. While practical batteries have not yet emerged from these studies, the increase in the number of active researchers and publications in the area demonstrates the interest and potentialities of the field. Progress in the field is briefly reviewed. Finally, recent results on the mechanisms for capacity loss on shelf and cycling in lithium-ion cells are reviewed. Progress towards further market penetration by lithium-ion cells hinges on improved understanding of the failure mechanisms of the cells, so that crucial problems can be addressed. # 2003 Published by Elsevier Science B.V. Keywords: Electrolytes; Lithium; Battery 1. Introduction The author reviewed the progress in electrolytes for lithium and lithium-ion batteries at the 9th International Meeting on Lithium Batteries [1]. Since that time, a number of new approaches and advances have occurred that have led to important improvements particularly in lithium-ion bat- teries. In this paper, improvements in extended temperature operation, and the effect of additives in improving storage stability, lowering flammability and activating current inter- rupter devices during overcharge are all considered. The effects of tailored SEI layers for lithium metal systems and the relation to the electrolyte phase is also discussed as well as work done on lithium alloy systems. An indication of the increased interest in ionic liquids for lithium and lithium-ion (as well as sodium) batteries is the greatly increased level of publication using these electrolytes [2]. Perspectives in this area are given. Finally, the losses and failure mechanisms in lithium-ion cells and the cycle life and calendar life issues resulting are discussed. The view is a personal one and no effort has been made to be comprehensive in coverage, but only to highlight what the author feels are the most impor- tant advances in the last 4 years. 2. Extended temperature operation for lithium-ion battery electrolytes Much electrolyte work of the past several years has been devoted to extend the temperature range of battery operation. In particular, the limitations of commercial cells to low temperature operation have hampered the application to aero- space and military uses for these batteries. Many of the recent studies for low temperature aerospace uses have come from the US Jet Propulsion Lab. A recent paper highlights the state of the art for these electrolytes [3]. An electrolyte with 1.0 M LiPF 6 salt in 1:1:1 ethylene carbonate (EC); diethyl carbonate (DEC): dimethyl carbonate (DMC) gives good performance at 20 8C as well as good stability and performance at room tem- perature. Attempts to improve the performance to 30 and Journal of Power Sources 119–121 (2003) 326–329 0378-7753/03/$ – see front matter # 2003 Published by Elsevier Science B.V. doi:10.1016/S0378-7753(03)00147-2