Impedance studies of the thin film LiMn2O4/electrolyte interface

Impedance measurements at thin-film LiMn2O4 electrodes in LiPF6-EC-DMC have been used to identify the spontaneous room temperature formation of LiMn2O4 at the interface at voltages of 3.7 and higher. The impedance of the LiMn2O4 films 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 open-circuit voltage and impedance tended to grow logarithmically with time, with lower rates for lower Mn3+ content in the film. The increased impedance was removed by oxidation of the film to 4.5 V vs. Li/Li+. The observations are consistent with a reversible disproportionation of part of the LiMn2O4 into Li2Mn2O4 and a lithium-deficient spinel. With extended constant current cycling part of the Li2Mn2O4 degrades to the Mn2O3, and the process is no longer reversible. (C) 2001 The Electrochemical Society.