Ionics 5 (1999) 59 Electrochemical Characterization of a Composite Polymer Electrolyte with Improved Lithium Metal Electrode Interfacial Properties G.B. Appetecchi l, F. Croce 1, F. Ronci I , B. Scrosati 1, F. Alessandrini 2, M. Carewska 2 and P.P. Prosini 2 1Department of Chemistry, University "La Sapienza", 1-00185 Rome, Italy 2ENEA C.R. Casaccia, Dipartimento di Energia, Divisione Tecnologie Energetiche Avanzate, 1-00060 Rome, Italy Abstract. In the development of rechargeable lithium polymer batteries it is of paramount importance to control the passivation phenomena occurring at the lithium electrode interface. It is well estabilished that the type and the growth of the lithium passivation layer is unpredictably influenced by the presence of liquid components and/or impurities in the electrolyte. Therefore, one approach to improve the stability of the lithium interface is the use of liquid-free, highly pure electrolytes. The electrochemical properties of a composite polymer electrolyte obtained by hot pressing a mixture of polyethylene oxide (PEO), a lithium salt (lithium tetrafluoroborate, LiBF4) and a powdered ceramic additive (y-LiA102), will be presented and discussed. The electrochemical characterization included the determination of the ionic conductivity, the anodic break-down voltage and, most importantly, the stability of the lithium metal electrode interface and the lithium stripping-plating process efficiency. The main feature of this dry, true solid-state electrolyte is a very good compatibility with the lithium metal electrode, demonstrated by a very high lithium cycling efficiency, which approaches a value of 99%. 1. Introduction In previous work [1] we have reported the characteristics of a new family of poly(ethylene oxide), PEO-based com- posite polymer electrolytes. In particular, we have shown that by assuring a true solid (i.e., liquid-free) state to the electrolyte membranes, a consistent improvement in the stability of the lithium metal electrode interface could be achieved. Such an important property results in a renewed interest in these composite polymer electrolytes of for the development of rechargeable lithium batteries. Indeed, the results [2] confirm this expectation. Considering this relevant technological prospect, we have extended the study of the family of these electrolytes with the aim of further improving the stability of the lithium interface in order to achieve longer battery cyclability. In this work we report the characteristics and the property of a new member obtained by hot pressing a mixture of poly(ethylene oxide), PEO, a lithium salt (LiBF4) and a micrometric particle size ceramic powder additive (7- LiA102). 2. Experimental Particular care was taken to assure a high purity to the composite polymer electrolyte samples. Poly(ethylene oxide) PEO (Aldrich Chemical Co., 4,000,000 average molecular weight) was dried at 50 ~ under vacuum for 48 hours and sieved in order to use only the fraction having particle size less than 125 ~tm. Lithium tetrafluoborate, LiBF4 (Merck "Selectipur", battery grade) was used as re- ceived. Low particle size 7-LiA102 (Cyprus Foote Mineral Company, HSA-10) was dried at 300 ~ under vacuum for 24 hours. The three powder components were