Characterization of Cathode/Electrolyte Interfacial Processes By Scanning Electrochemical Microscopy

Lithium transition metal oxides (LiMn2O4 and LiMO2 where M=Ni, Mn, Co, etc.) are widely applied in lithium ion batteries due to their considerable capacity and energy density. Multiple processes occurring at the cathode/electrolyte interface lead to performance degradation, one of which is the dissolution of transition metals. While transition metal dissolution has been known to occur for some time, open questions remain about mechanistic details of these processes. This work uses the scanning electrochemical microscope (SECM) to observe active cathode degradation processes in situ and immediately at the cathode/electrolyte interface. These results are also paired with complementary analysis employing inductively coupled plasma (ICP) and electron paramagnetic resonance (EPR) spectroscopies to yield insight into the properties of cathode degradation products and their reactivity. This work focuses on studies of LiMn2O4 thin films as model cathodes and characterizes products of the Mn dissolution process. The degradation process of binder-free LiMn2O4 thin films prepared by polymer assisted deposition was investigated in varied electrolytes. The properties of Mn dissolution products from cathode degradation is compared to Mn salt solutions introduced into complementary electrolytes. Our results appear to indicate substantial variations between the properties of dissolved Mn salts and actual cathode degradation products. Moreover, electrolyte composition appears to affect the electrochemical behavior of metal dissolution products significantly. This likely indicates that the LiMn2O4 degradation mechanism in differing electrolytes undergo different degradation reaction pathways.