Revisiting the Charging Mechanism of α-MnO2 in Mildly Acidic Aqueous Zinc Electrolytes.

In recent years, there have been extensive debates regarding the charging mechanism of MnO2 cathodes in aqueous Zn electrolytes. The discussion centered on several key aspects including the identity of the charge carriers contributing to the overall capacity, the nature of the electrochemical process, and the role of the zinc hydroxy films that are reversibly formed during the charging/discharging. Intense studies are also devoted to understanding the effect of the Mn2+ additive on the performance of the cathodes. Nevertheless, it seems that a consistent explanation of the α-MnO2 charging mechanism is still lacking. To address this, a step-by-step analysis of the MnO2 cathodes is conducted. Valuable information is obtained by using in situ electrochemical quartz crystal microbalance with dissipation (EQCM-D) monitoring, supplemented by solid-state nuclear magnetic resonance (NMR), X-ray diffraction (XRD) in Characterization of Materials, and pH measurements. The findings indicate that the charging mechanism is dominated by the insertion of H3O+ ions, while no evidence of Zn2+ intercalation is found. The role of the Mn2+ additive in promoting the generation of protons by forming MnOOH, enhancing the stability of Zn/α-MnO2 batteries is thoroughly investigated. This work provides a comprehensive overview on the electrochemical and the chemical reactions associated with the α-MnO2 electrodes, and will pave the way for further development of aqueous cathodes for Zn-ion batteries.

Prolonging the Cycle Stability of Anion Redox P3-Type Na0.6Li0.2Mn0.8O2 through Al2O3 Atomic Layer Deposition Surface Modification.

Sodium-ion batteries (SIBs) are becoming an alternative option for large-scale energy storage systems owing to their low cost and abundance. The lattice oxygen redox (LOR), which has the potential to increase the reversible capacity of materials, has promoted the development of high-energy cathode materials in SIBs. However, the utilization of oxygen anion redox reactions usually results in harmful lattice oxygen release, which hastens structural deformation and declines electrochemical performance, severely hindering their practical application. Herein, a ribbon-ordered superstructured P3-type Na0.6Li0.2Mn0.8O2 (NLMO) cathode with a uniform Al2O3 coating through atomic layer deposition (ALD) was synthesized. The cycling stability and rate capability of the materials were improved by a proper thickness of the Al2O3 layer. Differential electrochemical mass spectrometry (DEMS) results clearly suggest that the Al2O3 coating can inhibit the CO2 release caused by the highly active surface of the NLMO material. Moreover, the results of transmission electron microscopy (TEM) and etching X-ray photoelectron spectroscopy (XPS) show that the Al2O3 coating can effectively prevent electrolyte and electrode side reactions and the dissolution of Mn. This surface engineering strategy sheds light on the way to prolong the cycling stability of anionic redox cathode materials.