To understand the basic properties of lithium secondary batteries which consist of nonflammable and nonvolatile room-temperature ionic liquid electrolytes, we examined the ionic conductivity, electrolyte/electrode interfacial resistance, and charge-discharge rate characteristics by varying the lithium salt concentration in the room-temperature ionic liquid, lithium salt binary electrolytes. By using a modified imidazolium cation-based room-temperature ionic liquid as an electrolyte, the lithium secondary batteries achieved a stable charge-discharge operation of more than 100 cycles (cathode LiCoO2, anode lithium metal, voltage region 3.0 - 4.2 V, current density 1/8 C). Moreover, we found that an optimal lithium salt concentration exists for obtaining an excellent battery rate performance, which depends on delicate balances in several factors, such as ionic conductivity (viscosity), interfacial resistances at the LiCoO2 cathode/electrolyte interface, and the lithium metal anode/electrolyte interface. (c) 2007 The Electrochemical Society.