Liquid electrolytes for lithium and lithium-ion batteries

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.