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.