Analysis on Reaction and Degradation Mechanism for Mixed LiMn2O4-LiNi1-x-YCoxMnyO2 Composite Cathode in Lithium Ion Battery

Abstract

A mixed LiMn2O4 (LMO)-LiNi1-x-yCoxMnyO2 (NCM) composite cathode has been used in batteries for Electric Vehicles (EVs) or Plug-in Hybrid Electric Vehicles (PHEVs) because of its safety, long-life and high capacity lengthening a cruising distance. However, there were few reports for the reaction of mixed composite cathodes and the detailed reaction and degradation mechanisms have not been clarified yet. In this study, the power fading mechanism of the LMO-NMC mixed composite cathode was studied with analysis using synchrotron radiation.The mixed composite cathodes were prepared by disassembling “initial” and “after degradation” (storage under 45° C and SOC100% for 900 days) lithium ion batteries for EVs. Laminate type cells were assembled with these cathodes, Li foil anode, and electrolyte (1M LiPF6 ethyl carbonate/dimethyl carbonate/ethyl methyl carbonate=3/3/4) for synchrotron radiation measurement. Time resolve X-ray diffraction (TRXRD) and quick X-ray absorption fine structure (QXAFS) measurements during discharge at low (0.83 mA cm-2) and high rate (7.50 mA cm-2) were performed at beamline 28XU of SPring-8 (Hyogo, Japan) in order to clarify the reaction and degradation (power fading) mechanisms for LMO and NCM. In addition, transmission electron microscope-electron diffraction (TEM-ED) and electron energy-loss spectroscopy (TEM-EELS) were performed to check the morphology of cathode particle.In low rate discharge experiment, the capacity for “initial” and “after degradation” electrodes was almost the same. However, in high rate discharge, the capacity remarkably faded for the “after degradation” electrode. In TRXRD and QXAFS results at low rate discharge, it was clarified that LMO discharged first and then NCM. In addition, the combination of the TRXRD and QXAFS measurements successfully allowed us to evaluate the reaction ratio of LMO to NCM in the mixed composite cathode. As a result, it was clarified that the reaction ratio of LMO was decreased at high rate discharge for the “after degradation” electrodes. In XRD profiles (Fig. 1), the LMO (531) diffraction peak was split into multiple peaks for the “after degradation” electrode at high rate discharge and the peak shift was observed continuously during discharge. These results suggested that the power fading in the mixed composite cathode was due to the decrease of reaction ratio of LMO to NCM by reaction distribution. Furthermore, as a result of TEM observation for the cross-section of an electrode, some amorphous layer on the surface of LMO particle was observed for the “after degradation” electrode. This suggested that Li intercalation/deintercalation at the surface of the LMO particle was disturbed by this amorphous layer and reaction distribution was generated in LMO.To summarize the results, the combination of the TRXRD and QXAFS measurements successfully allowed us to evaluate the reaction ratio of LMO to NCM in the mixed composite cathode. The power fading of the mixed composite cathode on storage under high temperature conditions was due to the decrease of reaction ratio of LMO to NCM caused by the reaction distribution.This work was supported by Research and Development Initiative for Scientific Innovation of New Generation Batteries (RISING) project from New Energy and Industrial Technology Development Organization (NEDO) in Japan.