ELECTROCHEMICAL INTERCALATION OF LITHIUM INTO GRAPHITE - INFLUENCE OFTHE SOLVENT COMPOSITION AND OF THE NATURE OF THE LITHIUM SALT

The electrochemical intercalation of lithium into graphite was studied in various electrolytes containing chloroethylene carbonate (ClEC), e thylene carbonate (EC) and propylene carbonate and their mixtures as s olvents and LiClO4, LiBF4 and LiAsF6 as lithium salts. The reduction o f ClEC and EC, catalyzed on the surface of the graphite electrode, sta rts at potentials equal to 1.7 and 0.9 V, respectively, and leads to t he formation of reduction products which passivate the carbon surface. The effect of the nature of the lithium salts on the properties of th e interfacial layer is also examined. With LiBF4, the first reduction of the carbon electrode results in a massive reduction of the electrol yte, especially LiBF4, that limits the reversible lithium intercalatio n. With the electrolytes containing ClEC, the reversible intercalation is observed with formation of LiC6. Moreover, a slight excess of capa city compared with LiC6 is observed after the first cycle. It could be attributed to an extra fixation of lithium in the interphase layer ge nerated by C-Li(ads.)-Cl bonds. (C) 1998 Elsevier Science Ltd. All rig hts reserved.