Operando Mossbauer Spectroscopy Investigation Of The Electrochemical Reaction With Lithium In Bronze-Type Fef3 Center Dot 0.33h(2)O

The electrochemical reaction of bronze-type FeF3 center dot 0.33H(2)O, synthesized via a simple room-temperature solution route, with lithium was investigated by operando Mossbauer spectroscopy and X-ray diffraction. The two techniques revealed a complex electrochemical mechanism where the pristine crystalline compound is gradually transformed into an amorphous material containing nanodomains of a FeF2-like rutile structure mixed to iron nanoparticles. Upon charge, two steps are dominating the electrochemical behavior: partial reformation of the initial bronze structure and oxidation to Fe(III). This reaction mechanism, however, is not constant, and noticeable variation can be observed during galvanostatic cycling (up to 55 cycles) until eventually an amorphous material containing rutile nanodomains composes the final active electrode. The material performance, under the form of a fluoride/graphene oxide composite, is also assessed with respect to the long-term effect of the depth of first discharge.