Poly(ethylene oxide) (PEO)-based composite solid electrolytes are of interest for their applications in solid-state electrochemical devices. In this paper we study both thermal and electrical stability of a PEO-Li2O:3B(2)O(3)(glass)-LiClO4 solid electrolyte vs a month-long ageing performed at room temperature under dry atmosphere. A comparison is performed with homologous films prepared from different solvents without the lithium triborate. We show that the composite electrolyte is the most amorphous one immediately, after preparation and better resists crystallization. The conductivity after three months ageing is well above that of the reference films. Some anomalies have been found which are attributed to the glassy part of the composite.