Abstract
The introduction of transition metal dopants such as Fe and Co in zinc oxide enables substantially enhanced reversible capacities and greater reversibility of the de-/lithiation reactions occurring. Herein, we report a comprehensive analysis of the electrochemical processes taking place in Mn-doped ZnO (Zn0.9Mn0.1O) and carbon-coated Zn0.9Mn0.1O upon de-/lithiation. The results shed light on the impact of the dopant chemistry and, especially, its coordination in the crystal structure. When manganese does not replace zinc in the wurtzite structure, only a moderate improvement in electrochemical performance is observed. However, when applying the carbonaceous coating, a partial reduction of manganese and its reallocation in the crystal structure occur, leading to a substantial improvement in the material’s specific capacity. These results provide important insights into the impact of the lattice position of transition metal dopants—a field that has received very little, essentially no attention, so far.
Highlights
Effect of Applying a Carbon Coating on the Crystal Structure and De/Lithiation Mechanism of Mn-Doped ZnO Lithium-Ion Anodes
An important role in this regard plays the use of graphite as the state-of-the-art active material for the negative electrode due to its high theoretical specific capacity of 372 mAh g−1 and its low de-/lithiation potential, allowing for very high energy densities at the full-cell level.[3]
Major differences between the Cyclic voltammetry (CV) profiles of Zn0.9Mn0.1O and Zn0.9Mn0.1O-C, especially after the first cyclic sweep, are: (i) a much broader current peak A/A*, which has its maximum at lower potentials, in the case of Zn0.9Mn0.1O-C; (ii) a more reversible de-/alloying reaction thanks to the carbon coating, as revealed by the greater reversibility of the current response for the features B and C; and (iii) a much more reversible re-conversion reaction in the case of Zn0.9Mn0.1O-C, as indicated by the substantially enhanced reversibility of the current response for feature D. These differences are in line with the results reported earlier for Co-doped ZnO,[19] where cobalt occupies the same position as zinc in the crystal structure.[39]
Summary
Effect of Applying a Carbon Coating on the Crystal Structure and De/Lithiation Mechanism of Mn-Doped ZnO Lithium-Ion Anodes. Both kinds of electrodes display four distinct regions after the initial electrolyte decomposition, labeled A (and A* after the first cyclic sweep), B, C, and D and representing the conversion, the alloying, the de-alloying, as well as the re-conversion reaction, respectively.[19] Generally, the CV data show essentially the same features as those reported earlier for pure ZnO,[19] indicating that the overall reaction mechanism is the same when introducing manganese into the crystal structure.
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