Multiscale Synchrotron Characterization of Electrode-Electrolyte Interfaces in Multivalent Metal-Ion Batteries

The development of multivalent metal-ion battery chemistries (Zn 2+ , Mg 2+ , Al 3+ , etc.) is still impeded by fundamental scientific questions and engineering technical challenges. Particularly, the debate persists on the charge storage mechanisms of the cathode and their implications for achievable battery performance. The charge density of multivalent metal ions offers advantages in multielectron redox processes but is accompanied by strong Coulombic interactions within the bulk electrolyte, at electrode–electrolyte interfaces, and during solid-state diffusion in the electrode(1). Herein, we have designed operando electrochemical batteries and developed operando and ex situ synchrotron X-ray techniques based on beamlines in Shanghai Synchrotron Radiation Facility (SSRF) for multiscale characterization of multivalent metal-ion batteries. In this work, we employed multiscale synchrotron X-ray characterization to probe the electrode-electrolyte interface in aqueous zinc batteries and rechargeable magnesium batteries, respectively; and further in conjunction with electrochemical measurement to reveal their charge storage mechanisms. In aqueous zinc batteries, the addition of MnSO 4 to the ZnSO 4 electrolyte commonly exhibit an initial-cycle increase in capacity. To better understand the role of Mn 2+ additive in the electrolytes, operando synchrotron X-ray diffraction, ex situ X-ray absorption spectroscopy and electrochemical analyses were carried out to elucidate the reaction mechanism and pathway. A reversible Mn 2+ /MnO 2 deposition/dissolution reaction occurred at the surface of cathode, in which the change in the electrolyte environment enabled Zn 2+ /Zn 4 SO 4 (OH) 6 ∙5H 2 O deposition/dissolution. The chemical reactions of Zn 2+ /Zn 4 SO 4 (OH) 6 ∙5H 2 O contributed no capacity and hindered the diffusion kinetics of the Mn 2+ /MnO 2 reaction, which prevented operation of the ZIBs at high currents(2). In rechargeable magnesium batteries, the utilization of copper current collector presented a remarkable capacity and improved cycling performance. To understand the role of the copper current collector in battery performance, we conducted multiscale operando X-ray characterization, coupled with electrochemical analyses, to investigate the interfacial and bulk reactions between the electrolyte and electrode. Operando synchrotron X-ray diffraction was employed during battery operation to track the real-time structural evolution of cathode and copper current collector at the atomic scale. Additionally, operando synchrotron X-ray imaging was used to observe the morphological changes in the cathode, current collector, and anode at the microscale. The results and discussion reveal that the rechargeable magnesium batteries were driven by a redox chemistry of copper, in which the characteristic charge and discharge plateau represent redox reactions between copper and copper ions. Nevertheless, battery failure was also caused by depletion of copper current collector, which is attributed to an irreversible process whereby copper ions migrate towards the magnesium anode and are reduced to copper deposits. Reference M. A. Schroeder, L. Ma, G. Pastel and K. Xu, Current Opinion in Electrochemistry , 29 , 100819 (2021). Z. Li, Y. Li, X. Ren, Y. Zhao, Z. Ren, Z. Yao, W. Zhang, H. Xu, Z. Wang, N. Zhang, Y. Gu, X. Li, D. Zhu and J. Zou, Small , 19 , 2301770 (2023). Figure 1