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The investigation of the lithiation−delithiation kinetics of anodes comprising carbon-coated ZnFe 2 O 4 nanoparticles is reported in here. The study confirmed that, as occurring with other conversion electrodes, lithiation of ZnFe 2 O 4 nanoparticles is a multistep process involving the presence of intermediate Li−Zn−Fe−O phases as precursors for the formation of amorphous Li 2 O. A detailed knowledge on the reaction kinetics of the involved electrochemical mechanisms has been achieved by using impedance spectroscopy. It has been observed that lithiation reactions introduce a long delay that limits the electrode charging, not related to diffusion mechanisms. The sloping curve following the conversion plateau of the galvanostatic discharge is connected to a retardation effect in the reaction kinetics. This limitation is seen as an additional resistive process originated by the specific lithiation microscopic features. It is concluded that capacitance spectra allow distinguishing two separate processes: formation of kinetically favored intermediate Li−Zn−Fe−O phases and subsequent reaction to produce highly dispersed LiZn and Fe 0 in an amorphous Li 2 O matrix. A detailed electrical model is provided accounting for the overall electrode lithiation process. ■ INTRODUCTIONLi-ion batteries have become core devices for the consumer electronics industry. Materials for commercial battery electrodes are mostly chosen from a set of intercalation compounds that reversibly accommodate lithium ions in host sites in the lattice without severely distorting the structure. In most transition metal compounds such as LiCoO 2 , LiNi 1−y−z Mn y Co z O 2 , LiFePO 4 , and Li 4 Ti 5 O 12, redox activity is restricted to a few exchanged electrons. Therefore, intercalation materials exhibit intrinsic limitations that make them unviable when high capacity requirements have to be fulfilled as in the case of large scale or automotive applications.1−3 During the past decades a new family of electrode materials operating under the so-called conversion reaction has been intensely studied.4 For these compounds lithiation occurs through the reaction that involves a complete metal reduction as M a X b + (bn)Li ↔ aM + bLi n X, where M = transition metal, X = anion (O, S, N, P, and F), and n = anion formal oxidation state. Conversion reaction is able to accommodate larger amount of Li atoms into the lithium binary compound Li n X, which explains specific capacities exceeding 1000 mAh g −1 as reported for many compounds. Interestingly, conversion materials usually show good reversibility because of the formation of a nanostructured matrix that comprises metallic nanoparticles surrounded by amorphous Li n X phases. Intimate phase contact facilitates reactivity as evidenced by the observation of remaining metallic nanoparticles after extended oxidation/ reduction cycling. 4 Despite their potentialities, conversion compounds present a series of performance limitations that hinders their straightforward application in commercial devices. There...

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