Lawrence Berkeley National Laboratory

The impedance of pure dense Li1+yMn2-yO4 films in LiPF6-EC-DMC electrolyte exhibited two time constants: at about 14 kHz and 60 to 200 Hz. The high frequency loop is dependent on film morphology and was attributed to the substrate/oxide interface. The low frequency behavior was dependent on both state-of-charge (SOC) and time at a given SOC. At full charge the impedance in this electrolyte was stable at room temperature over several days. At high lithium contents, film OCV and impedance tended to grow logarithmically with time, with a rate three times higher at EOD than at a mid-SOC. This impedance rise was reversible by cycling the film between 4.5 and 3.7 V vs. Li/Li+. The observations are consistent with an ion exchange reaction between impurity protons in the electrolyte and Li+ in the oxide. A similar step involving the removal of Mn from the oxide has already been proposed in the literature.