Operando SQUID Magnetometry for the Characterization of Electrochemical Systems: Charge Transfer in Battery Materials and Electrochemical Dealloying Processes

Abstract

The combination of magnetism and electrochemistry has attracted considerable attention for application-relevant issues, like the electrodeposition of magnetic thin films or the electrochemically-induced switching of magnetic properties [1,2]. The fact that magnetism variations occurring during electrochemical charging can also serve as diagnostic tool for the corresponding electrochemical processes has been little exploited until now. In this talk, we will demonstrate the capability of operando SQUID magnetometry measurements to characterize electrochemical systems based on two examples: (i) the identification of the electroactive species in Li-ion and Na-ion battery cathodes, (ii) the distinction between different dissolution stages during electrochemical dealloying of Co-Pd alloy.Enabling such operando magnetic studies we have developed a three-electrode electrochemical cell for operation in a commercial state-of-the-art SQUID magnetometer [3].This new cell design allows a precise correlation between the detected variations of the magnetic moment and the electrochemical processes occurring in the investigated sample. Its capability was proven by performing the first in-situ electrodeposition experiments in a SQUID magnetometer [3,4].Applying this new operando technique to (i) Li-ion and Na-ion battery cathodes we could enable a continuous monitoring of the oxidation states of the transition metal ions and therefore an identification of the redox active ions. Our measurements on LiNi0.33Mn0.33Co0.33O2 revealed that first Ni acts as redox active ion and changes its oxidation state stepwise from Ni2+ to Ni3+ and then from Ni3+ to Ni4+. When more than 2/3 of the Li ions are extracted, a simultaneous oxidation of Co3+ and O2- ions takes place that is associated with irreversible capacity losses [5]. Operando magnetic studies on NaV2(PO4)3 cathodes have shown that the vanadium ion is the only ion undergoing oxidation and reduction upon battery operation. However, in the first charging cycle peculiarities in the magnetic susceptibility indicate the occurrence of parasitic side reactions [6].In the case of (ii) in situ dealloying of a Co75Pd25 in a SQUID magnetometer the observed variations of the magnetic moment allowed a distinction between the primary (dissolution of the initial alloy) and secondary (dissolution of residual atoms from porous structure) dealloying steps. The observed transition from alloy ferromagnetism to superparamagnetic behavior, can be ascribed to the formation of Co-rich clusters in the obtained nanoporous Pd framework [7]. Furthermore, the magnetic coupling between these clusters can be reversibly tuned by electrochemical hydrogen intercalation, leading to an On and Off switching of magnetism at room temperature [8].All in all, these two examples prove that operando SQUID magnetometry measurements can serve as a sensitive fingerprint for the processes taking place in various electrochemical systems.Financial support by the Austrian Science Fund (FWF): P30070-N36 is gratefully acknowledged. This work was performed in the framework of the inter-university cooperation of TU Graz and Uni Graz on natural sciences (NAWI Graz).[1] K. Leistner, Curr. Opin. Electrochem. 25 (2021) 100636[2] C. Navarro-Senent et al., APL Mater. 7 (2019) 030701[3] S. Topolovec et al., Rev. Sci. Instrum. 86 (2015) 063903[4] S. Topolovec et al., J. Magn. Magn. Mater. 397 (2016) 96[5] G. Klinser et al., Appl. Phys. Lett. 109 (2016) 213901[6] G. Klinser et al., Phys. Chem. Chem. Phys. 21 (2019) 20151[7] M. Gößler et al., J. Appl. Phys. 128 (2020) 093904[8] M. Gößler et al., Small 15 (2019) 1904523