Tuning Discharge Behavior of Hollandite α-MnO₂ in Hydrated Zinc Ion Battery by Transition Metal Substitution

The tunnel-type hollandite α-MnO₂ is a promising cathode material for rechargeable aqueous zinc-ion batteries (ZIBs) due to its low cost in synthesis and high energy density. However, irreversible structural degradation upon continuous cycling prevents the cathode from being utilized commercially. Herein, density functional theory (DFT) was utilized to conduct a systematic study on tuning the behavior of α-MnO₂ by substituting the Mn ions on the tunnel wall with a transition metal (V or Cr) during the H⁺-/Zn²⁺-intercalation in hydrated ZIB. Our study revealed that both substituents aid cyclability and capacity retention with Cr outperforming V. In term of discharge voltage, only the Cr-substitution displays clear promotion at the early stage of discharge. The superior performance of substituted Cr⁴⁺ comes from its unique atomic and electronic structures. Upon discharge, it can be reduced to Cr³⁺ more readily than Mn⁴⁺ and thereby limits the formation of unstable Mn³⁺ or Mn²⁺ centers; the formed Cr³⁺ is more stable than Mn³⁺ and Mn²⁺ from the reduction of Mn⁴⁺; and Cr³⁺ can also greatly stabilize the neighboring Mn ions. This study highlights the significant tuning effect of transition metal substitution on the electrochemical and physical performance of α-MnO₂ as a cathode in hydrated ZIBs.