Impedance Studies of the Thin Film LiMn[sub 2]O[sub 4]/Electrolyte Interface

Impedance measurements at thin-film LiMn 2 O 4 electrodes in LiPF 6 ­ EC ­ DMC have been used to identify the spontaneous room temperature formation of LiMn 2 O 4 at the interface at voltages of 3.7 and higher. The impedance of the LiMn 2 O 4 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 Mn 3 + 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 LiMn 2 O 4 into Li 2 Mn 2 O 4 and a lithium-deficient spinel. With extended constant current cycling part of the Li 2 Mn 2 O 4 degrades to the Mn 2 O 3 , and the process is no longer reversible.