Rational optimization of substituted α-MnO₂ cathode for aqueous zinc-ion battery

Density functional theory (DFT) is utilized to explore the effects of increasing concentrations of vanadium (V) and chromium (Cr) substitution on the discharge of α-MnO₂ cathode in hydrated zinc-ion batteries (ZIBs). During H⁺-intercalation and Zn²⁺-intercalation, Cr-substitution proves to be more effective than V-substitution in promoting discharge behaviors. Transitioning from Mn_0.875Cr_0.125O₂, Mn_0.75Cr_0.25O₂ to Mn_0.625Cr_0.375O₂ is found to consistently enhance the discharge voltage, along with improved tunnel structure retention and volume expansion suppression. In comparison, the promoting effect of increasing V-substitution is relatively small at initial discharge stages and leads to degradation at later stages, primarily due to an increased concentration of unstable Mn²⁺ ion. The superior effect of Cr-substitution is attributed to the unique atomic and electronic structures of substituted Cr⁴⁺ and reduced Cr³⁺ ions during discharge. These ions serve as active electron acceptors to limit the formation of Mn³⁺ and Mn²⁺ ions, and as anchors to stabilize the α-MnO₂ framework and intercalated H⁺/Zn²⁺ ions, respectively. Our study highlights fine-tuning through substitution to enhance the performance of α-MnO₂-based cathode materials in ZIBs.