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Combined in situ X-ray diffraction, in situ X-ray absorption spectroscopy, and selected area electron diffraction analyses have confirmed the occurrence of a reversible conversion reaction in the BiF 3 /C nanocomposite upon cycling, which leads to the formation of Bi 0 and LiF during lithiation and the reformation of BiF 3 during delithiation. It has been shown that only the high-pressure tysonite phase of BiF 3 reforms during the oxidation sweep and that no bismuth fluoride compound with an oxidation state of the bismuth lower than 3 is formed as intermediate during the lithiation or delithiation reactions. Finally, it has been demonstrated that the different plateaus or pseudo plateaus observed on the lithiation and delithiation voltage profiles stem from polarization changes brought about by the dramatic structural changes occurring in the nanocomposite upon cycling. A model, based on the variation of the electronic and ionic transport mechanisms as a function of the state of completion of the conversion and reconversion reactions, is proposed to explain those polarization changes.Conversion reactions are lithiation reactions in which the active material is fully reduced by lithium to the metal according to the following equationwhere M stands for a cation and X an anion. As all the oxidation states of the material are utilized, capacities much higher than in the intercalation reactions currently used in rechargeable Li-ion batteries can in theory be obtained, thereby leading to much higher energy densities.Conversion reactions have been known for a long time in primary battery applications with, for instance, the Li/CuO and Li/CF x systems that are still utilized today. 1 Due to the highly destructive nature of the reaction, which literally involves the breaking of chemical bonds and the formation of new ones, it has long been thought that such reactions were irreversible. To the best of our knowledge, Besenhard was the first to report the electrochemical reversibility of a conversion reaction in 1978 2 in a paper on bismuth chalcogenides. In this paper, it was shown that Bi 2 S 3 was reduced into Bi 0 and Li 2 S during lithiation and was reoxidized with 75% efficiency during the following delithiation; but this reversibility was observed for one cycle only. Selwyn et al. reported in 1987 that the conversion reactions occurring in Mo and W dichalcogenides exhibited some electrochemical reversibility but did not clearly identify the compound͑s͒ formed during the delithiation. 3 The possibility to have full electrochemical and structural reversibility in conversion reactions of dichalcogenides during charge has been proven for the first time only about 5 years ago by Tarascon's group. 4 They reported specific capacities as high as 700 mAh/g and full reversibility for several tenth of cycles when different transition metal oxides 4 and sulfides 5 were used as negative electrode or lowvoltage positive electrodes.As the output voltage of a conversion reaction scales with the ionicity of the M-X bond, metal nitride...