Detrimental MnPO4F and MnF2 formation on LiMn2O4 in the 3 V region

Abstract

In this work, we studied the electrochemistry of LMO using thin-film electrodes in LiClO₄ and LiPF₆ based electrolyte solutions. The thin-film system allows the removal of all passive components, leaving only the LMO with a single well-defined electrode/electrolyte interface, making it an ideal system to study interactions at this interface. We observed a peculiar decomposition reaction when using a LiPF₆ salt based electrolyte solution, as soon as the LMO electrode was discharged below 3 V. This decomposition reaction led to the formation of a highly ionically resistive interphase layer, greatly affecting the insertion kinetics into the active material. This was in stark contrast to the LiClO₄ based electrolyte solutions, which showed ideal insertion behaviour. Through the combination of elastic recoil detection analysis (ERDA) and X-ray photoelectron spectroscopy (XPS) we determined that the reaction stems from a direct interaction between the LiPF₆ salt and tetragonal Li₂Mn₂O₄.