Lithium insertion in sputtered amorphous molybdenum thin films

Abstract

Molybdenum oxide thin films are interesting materials for solid state electrochemical devices such as microbatteries and electrochromic systems. In this work, molybdenum oxide thin films were produced by r.f. reactive sputtering of a metallic molybdenum target in an O 2-Ar atmosphere. Thin films with distinct stoichiometries were obtained by varying the oxygen flux and maintaining all other deposition parameters constant. X-ray diffraction indicated an amorphous structure for all films. Lithium intercalation was promoted by electrochemical means from an aprotic, Li + containing electrolyte. Galvanostatic, potentiostatic and potentiodynamic experiments were performed. Monochromatic transmittance and stress measurements were performed in situ. The as-grown samples changes from transparent coloration (high oxygen fluxes, probable stoichiometry MoO 3) to blue coloration (low fluxes, substoichiometric MoO 3). Substoichiometric samples presented high contrast ( ΔT = 40%) and fast response time (5 s on bleaching). Stoichiometric samples showed low contrast. All samples showed deep changes for some limited cathodic potential, (far away from expected lithium deposition), with formation of grey/brown colored irreversible phases. All samples were able to intercalate lithium at high degrees, as indicated by the stress measurements. The relationship between optical, electrochemical and mechanical behaviour of the above mentioned films is reported and discussed in this work.