In order to assemble the components of an electrochemical generator using lithium, which are in the form of thin films and comprise a lithium- or lithium alloy-based negative electrode and a solid polymer electrolyte with conduction using the lithium ion, an intermediate assembly is used which consists of a thin film of lithium supported by a film of inert plastic with controlled adhesion to the lithium, as a precursor of the negative electrode. This intermediate assembly can be produced, inter alia, by rolling a sheet of lithium between two films of inert plastic.