The two-phase battery concept: a new strategy for high performance lithium polymer batteries Pier Paolo Prosini * , Maria Carewska, Fabrizio Alessandrini, Stefano Passerini ENEA, Advanced Energy Technologies, C.R. Casaccia, Rome 00060, Italy Received 22 June 2000; accepted 02 January 2001 Abstract A new concept is proposed to realize high performance lithium polymer batteries. The two-phase battery concept is based on the use of two polyethers of different molecular weight. A low molecular weight polyether is used in the composite cathode to assure high ionic conductivity while a high molecular weight polyether is used in the electrolyte formulation. In this work, a composite cathode based on a solid, low molecular weight polyethylenglycol) dimethyl ether PEG-DME, M.W. 2000), was coupled with a solid polymer electrolyte SPE) based on polyethylenoxide) PEO, M.W. 4  10 6 ). The electrochemical stability window, evaluated by a slow sweep voltammetry, showed that the system has an anodic breakdown voltage higher than 4.0 V versus Li. The feasibility of two-phase batteries was evaluated by cycling tests. The results indicate that two-phase batteries have enhanced performance with respect to PEO based batteries. # 2001 Elsevier Science B.V. All rights reserved. Keywords: PEG; Composite cathode; Polymer electrolyte; Lithium battery 1. Introduction Electric vehicle EV) application requires batteries with high energy density, high pulse power and extremely long life at deep depth of discharge. Lithium metal, with a theoretical capacity exceeding 3.8 Ah/g, represents the ulti- mate frontier in the realization of high energy density storage devices. Chemical and electrochemical stabilities of SPE in contact with lithium metal were shown to be superior to those of liquid electrolytes [1]. However, although under development for over than 20 years, SPE technology has yet to demonstrate to be able to achieve the critical performance issues requested for EV applications. The failure of the SPE technology to deliver the above mentioned performance is related to  Inadequate ionic transport properties  Insufficient chemical and electrochemical stabilities. PEO based electrolytes have received great attention as SPE's. Due to their particular structure, the conductivity of PEO-based SPE's becomes appreciable at temperatures above the ambient. The operative temperature of the system is of fundamental importance and it must be carefully controlled. If the temperature is kept too low, the electrolyte transport properties and the kinetics at the electrode/elec- trolyte interface diminish. On the other hand, if the tem- perature is raised above a critical value, the electrolyte tends to decompose. At high discharge rate, current can ¯ow through the electrolyte at values higher than the limiting diffusion current. This situation cannot be sustained forever. Even- tually, the over-voltage related to the establishment of a concentration gradient would suddenly increase, as pre- dicted by the Sand equation [2]. The concentration polarization affects ®rst the composite cathode, where the cross-section of the polymer electrolyte is lower than in the separator [3]. In this paper, we propose a new approach to realize high energy density, high power lithium polymer batteries. The approach is based on the two-phase battery concept [4]. It is well known that the conductivity of polyether-based elec- trolytes decreases by increasing the molecular weight of the polymer [5]. Low molecular weight polyethylenglycol)'s PEG's) show high ion conductivity, even at ambient tem- perature, but their mechanical properties are not quite good. On the other hand, solid polymer electrolyte batteries suffer of low conductivity especially in the cathode compartment where strong mechanical stability is not requested. Journal of Power Sources 97±98 2001) 786±789 * Corresponding author. Tel.: 39-6-3048-6768; fax: 39-6-3048-6357. E-mail address: prosini@casaccia.enea.it P.P. Prosini). 0378-7753/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0378-775301)00653-X