Tuning Discharge Behavior of Hollandite α-MnO<sub>2</sub> in Hydrated Zinc Ion Battery by Transition Metal Substitution
Thanh Duc Le, Esther S. Takeuchi, Kenneth J. Takeuchi, Amy C. Marschilok, Ping Liu
Abstract
The tunnel-type hollandite α-MnO 2 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-MnO 2 by substituting the Mn ions on the tunnel wall with a transition metal (V or Cr) during the H + -/Zn 2+ -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 Cr 4+ comes from its unique atomic and electronic structures. Upon discharge, it can be reduced to Cr 3+ more readily than Mn 4+ and thereby limits the formation of unstable Mn 3+ or Mn 2+ centers; the formed Cr 3+ is more stable than Mn 3+ and Mn 2+ from the reduction of Mn 4+; and Cr 3+ 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-MnO 2 as a cathode in hydrated ZIBs.