Lithium batteries operate by exchange of lithium ions between an anode and a cathode, through an electrolyte that comprises a lithium salt in solution in a liquid solvent or in a polymer solvent.  In one particular embodiment, the electrolyte is a solid solution of a salt in a solvent, and the various components of the battery are in the form of films.  The film forming the positive electrode is applied to a current collector, the film forming the electrolyte is between the two films that respectively form the positive electrode and the negative electrode.  A polymer may be used as a solvent for the salt if it is capable of solvating the salt cations.  Polymers mainly composed of ethylene oxide units (PEO) have been widely used as solvents for the salt. 
However, the mechanical strength conferred by a PEO on the electrolyte film is weak, especially in the temperature range in which the battery operates.  Moreover, during successive operating cycles of the battery, the lithium has a tendency to form dendrites, which greatly reduces the lifetime of the battery.  EP-1 139 128 describes a battery whose configuration aims to suppress the problem created by the formation of dendrites.  Said battery comprises, as an electrolyte, an ion-conductive polymer film placed between the anode and the cathode.  The ion-conductive polymer film is composed of a matrix of said polymer that holds a nonaqueous solution containing a lithium salt, said matrix being formed by two layers formed one on the anode and the other on the cathode.  The lithium salt concentration is higher in the polymer matrix on the cathode side than on the anode side.  For example, the conductive polymer layer on the anode side is composed of a mixture of trifunctional PEO bearing three terminal acrylate groups and a solution of LiPF6 in an EC/EMC mixture dissolved in DPMA and cast on the anode then crosslinked by irradiation, and the conductive polymer layer on the cathode side is composed of a mixture of the same trifunctional PEO and of a solution of LiPF6 in an EC/γ-butyrolactone mixture dissolved in DPMA and cast on the cathode, then crosslinked by irradiation.  However, the presence of nonaqueous liquid in the conductive polymer tends to form foams, which deactivates some of the lithium, and therefore makes it necessary to use an excess of lithium. 
U.S. Pat.  No.  5,925,483 describes an electrolyte in the form of a film composed of two layers, and its use in a battery of which one electrode is an alkali metal.  The first layer is of the ion-conductive polymer type, and it is in contact with the anode.  The second layer (also of the ion-conductive polymer type) is in contact with the first layer and with the cathode, said second layer containing at least one aprotic liquid and at least one alkali metal salt, which diffuse into the first layer to activate it.  In all the examples, the second layer effectively contains an aprotic liquid.  However, the presence of the aprotic liquid that diffuses into the first layer causes said liquid to be brought into contact with the alkali metal anode, which has the result of causing the formation of an alkali metal foam, to the detriment of the lifetime of the battery which contains the bilayer as an electrolyte.