The manuscript reports the synthesis of Fe/Zn co-doped δ-MnO2 nanoflowers via a hydrothermal method and investigates their performance as cathodes for aqueous zinc-ion batteries (AZIBs). The authors claim that the synergistic effect of dual-metal doping expands the interlayer spacing, introduces oxygen vacancies, and enhances conductivity, leading to improved capacity and cycling stability (432.5 mAh g−1 at 0.2 A g−1). While the electrochemical performance is decent and the characterization is relatively complete, the novelty is somewhat incremental as doping strategies for MnO2 are well-established. Furthermore, there are critical concerns regarding the electrochemical contribution of the electrolyte additive and the rigor of the kinetic analysis that must be addressed before publication.
1. The authors used an electrolyte containing 0.1 M MnSO4. It is well-known in the field of Mn-based AZIBs that Mn2+ additives can oxidize and deposit as MnO2 on the cathode during charging, contributing significantly to the "apparent" capacity and cycling stability. The authors must provide evidence that the high capacity (432.5 mAh g−1) and stability are derived from the intrinsic FZMO material and not from the continuous electrodeposition of Mn2+ from the electrolyte. Control experiments without Mn2+ or determining the mass change of the electrode after cycling are necessary.
2. While the paper claims a "synergistic effect" of Fe and Zn, the specific distinct roles of each ion are not clearly deconvoluted. The manuscript states that both ions induce defects and expand spacing. Why are two metals needed if they perform similar functions? The authors should provide a more detailed comparison or theoretical support (e.g., DFT calculations) to explain why the Fe/Zn combination is superior to an optimized concentration of single Fe or Zn doping.
3. The ex-situ XRD (Figure 5) shows the formation and disappearance of ZSH during discharge/charge. This indicates a pH-driven conversion mechanism involving H+ intercalation. However, the text emphasizes Zn2+ intercalation facilitated by expanded interlayers. The authors need to clarify the contribution ratio between H+ intercalation (leading to ZSH) and Zn2+ intercalation. The current discussion blurs the line between these two distinct storage mechanisms.
4. In Figure 2f (O 1s spectrum), the peak at ~530.4 eV is assigned to oxygen vacancies. However, this binding energy region in transition metal oxides often overlaps with surface hydroxyl groups (Mn-OH) or defect-oxide species. Given that this is a hydrothermally synthesized material with high surface area, surface -OH groups are expected. The authors should provide stronger evidence for "oxygen vacancies" (e.g., EPR spectroscopy) rather than relying solely on curve-fitted O 1s XPS data.

Accept