Elucidating the Impact of Cobalt Doping on the Lithium Storage Mechanism in Conversion/Alloying‐Type Zinc Oxide Anodes

Despite great technological interest in nanostructured zinc oxide (ZnO) for a large variety of applications, such as lightemitting diodes, gas sensors, or dye-sensitized solar cells, their investigation as lithium-ion anode materials has been reported rather scarcely to date. In fact, although a theoretical specific capacity of about 988 mAhg@1 (if the reaction ZnO+ 3Li+3e$LiZn+Li2O is considered to be fully reversible) certainly arouses interest, apart from a few outstanding exceptions most electrochemical studies revealed poor performance and rapid capacity fading. To explain the origin of this inferior electrochemical performance of ZnO-based electrodes, Pelliccione and co-workers very recently performed an in situ X-ray absorption fine structure (XAFS) spectroscopy study. It was found that this substantial capacity decay mainly originates from the formation of relatively large metallic zinc particles upon continuous de-/lithiation. Once the size of these particles exceeds a certain limit, the formation of ZnO accompanied by the degradation of Li2O, that is, the conversion reaction, shows no further reversibility and only the alloying reaction of zinc with lithium takes place reversibly, which, in turn, is associated with the characteristic issues of alloying materials. In addition, the large volume changes upon alloying–dealloying lead to pulverization of the active material, ongoing electrolyte decomposition, and finally the loss of electrical contact within the composite electrode. Additionally, the decrease in electronic conductivity within the electrode owing to the insulating nature of Li2O presumably contributes to the observed rapid capacity fading. We have recently reported that these challenges and, in particular, the reversibility of the conversion reaction can be substantially enhanced by introducing a transition-metal dopant into the metal oxide structure. This new concept follows the general approach of introducing a metallic element that does not alloy with lithium once reduced to the metallic state, which thus ensures a sufficient electron supply throughout the initial particles to enable the degradation of Li2O. However, little is known so far about the detailed reaction mechanism of this new class of conversion/alloying materials. Herein, we present an in-depth structural and electrochemical characterization of pure ZnO and Co-doped ZnO nanopartiHerein, an in-depth investigation of the influence of transitionmetal doping on the structural and electrochemical characteristics of a hybrid conversion/alloying-type lithium-ion anode material is presented. Therefore, pure zinc oxide (ZnO) and cobalt-doped ZnO (Zn0.9Co0.1O) were investigated comparatively. Characterization by using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) confirmed the successful incorporation of the cobalt (Co) dopant into the wurtzite ZnO structure, which led to a decreased particle size for the doped compound. The in situ electrochemical XRD analysis of the first de-/lithiation of ZnO and Zn0.9Co0.1O revealed the highly beneficial impact of the transition-metal dopant on the reversible degradation of lithium oxide (Li2O) and suppression of zinc crystallite growth upon lithiation; both effects are essential for greatly improved electrochemical performance. As a result, Co doping leads to a substantially increased specific capacity from 326 mAhg@1 for pure ZnO to 789 mAhg@1 for Zn0.9Co0.1O after 75 full charge–discharge cycles.