Reaction mechanism for the α-MnO₂cathode in aqueous Zn ion batteries revisited: elucidating the irreversible transformation of α-MnO₂into Zn–vernadite

Abstract

Rechargeable aqueous Zn/α-MnO2 batteries have attracted much attention due to the advantages in safety and high capacity. However, the reaction process of α-MnO2 electrode in long cycles is still unclear....

Traditional Electrochemical Zn2+ Intercalation/Extraction Mechanism Revisited: Unveiling Ion‐Exchange Mediated Irreversible Zn2+ Intercalation for the δ‐MnO2 Cathode in Aqueous Zn Ion Batteries

“…E-pH diagram for the Mn-H 2 O system reveals that MnO 2 is prone to dissolve at the voltage range and the solution pH during discharge. [28,51] As shown in Table S5 (Supporting Information), the mass of 𝛿-MnO 2 electrode significantly decreased after the 1 st discharge. ICP result shows that considerable Mn 2+ was released in electrolyte.…”

“…The morphology, elemental composition and the lattice spacings of 0.288, 0.267, 0.253, 0.247 and 0.153 nm observed in HRTEM image (Figure 6d) are same with the vernadite nanosheets formed on the 𝛼-MnO 2 electrode as reported in our previous work. [28] Vernadite formed by the electrochemical reaction between Mn 2+ dissolved in electrolyte and ZHS: [28] 2…”

“…[53] However, we revealed that the new peaks should be assigned to Zn 2+ -containing vernadite in our previous work. [28] Vernadite is existed as a form of natural mineral that has various interlayer spacing with different cations occupied the interlayer space because of the different formation environments. Unfortunately, there is no available standard JCPDS card for vernadite.…”

“…Recently, the reaction mechanism of 𝛼-MnO 2 electrode was elucidated that the H + intercalation/extraction and the electrodissolution-electrodeposition reaction of 𝛼-MnO 2 dominate the charge storage process. [28] However, the reaction mechanism of 𝛿-MnO 2 is still unclear. 𝛿-MnO 2 has typical layered structure with alkaline cations and water molecules occupied the interlayer space.…”

Aqueous MnO2/Mn2+ electrochemistry in batteries: progress, challenges, and perspectives

“…Some studies have pointed out that H + can reversibly insert into primary a-MnO 2 while its gradual dissolution, or react with the host b-MnO 2 forming MnOOH during MnO 2 dissolution/deposition. 45,46 However, Takeuchi et al found that there should be a ZHS-free region during charge and discharge (Fig. 3d), where the stable pH of the electrolyte could not allow the participation of H + , as evidenced by operando XRD characterization and in situ pH measurements.…”