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

Abstract

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.