Monitoring Interfacial Dynamics of a Zinc-Ion Battery Cathode Using In Situ Grazing Incidence X-Ray Absorption Spectroscopy: A Case Study of Manganese Dioxide.

Despite such assets as intrinsic safety and low cost, the performance of zinc-ion batteries (ZIBs) is hindered by interfacial processes that occur at the cathode. Because of capacity fading and poor rate capability, manganese dioxide (MnO) cathodes are also negatively affected. Here, the novel application of in situ grazing incidence x-ray absorption spectroscopy (GI-XAS) to investigate the cathode-electrolyte interfacial dynamics in a MnO cathode ZIB is demonstrated. By using a low-incidence angle X-ray beam to selectively probe the cathode surface, in situ changes are captured in the MnO oxidation state and local structure during charge-discharge. Results reveal that MnO undergoes a dissolution-redeposition mechanism at the interface. During discharge, Mn is reduced to Mn and further to Mn species that migrate into the electrolyte. Whilst charging, these Mn species form a transient Mn-rich layer on the cathode surface. This surface layer impedes Zn transport and causes increased overpotential, correlating with capacity decay. Such interfacial transformations are fully reversible in the bulk of the cathode but only partially reversible at the surface, thus leading to residual Mn/ Mn species after recharge. The findings provide direct evidence of cathode surface reconstruction as a potent contributor to performance degradation.