Electrochemical insertion of sodium ions into carbon using solid polymer electrolytes or organic liquid electrolytes is described. Cells with the configuration Na/P(EO)sNaCF3SOJCP(EO) = polyethylene oxide) or Na/liquid electrolyte/C were galvanostatically discharged, charged, and cycled. The extent of insertion into C (Le., x in Na§ was found to be a strong function of the type and particle size of the carbon used, and the reversibility of the process was highly dependent upon the type of electrolyte used. The possibility of designing a sodium ion rocking chair cell is discussed, and a first-generation example, using a petroleum coke anode, polymer electrolyte, and sodium cobalt bronze cathode is described. Rocking chair batteries, in which both the anode and cathode are intercalation materials, have recently been commercialized. Because the anodes are commonly inexpensive carbons such as petroleum coke or graphite, reductive intercalation of lithium into these materials is now the subject of intense scrutiny] Similar sodium insertion reactions into carbons have been observed, 2 but have not yet been exploited for use in batteries. We now describe a preliminary study of these insertion reactions and discuss the possibility of developing a sodium ion cell analogous to the wellknown lithium ion systems. Experimental Conoco petroleum coke, Shawinigan black, and JohnsonMatthey microcrystalline graphite were either ground in an attritor mill or used as received after heat-treatment. Polymer electrolytes of composition P(EO)sNaCF3SO3 (PEO = polyethylene oxide) and composite cathodes containing the carbon of interest, PEO, and NaCF3SO3 were made as described previously. 3 Electrodes for use in cells with liquid electrolytes consisted of carbon and ethylene propylene diene monomer (EPDM) binder (2% by weight) and were vacuum dried prior to use. Battery-grade solvents from Mitsubishi Petrochemical Company were stored in an inert atmosphere glove box (02 < 1 ppm) and used as supplied. Sodium was purified as described previously?