Structural Transition Behavior of LMO-NMC Composite Used for Cathode of Li Ion Battery at High Voltage Operation

LiMn₂O₄ (LMO)-LiNi_1-x-yCo_xMn_yO₂ (NCM) composite has been considered as one of the best cathode materials for Electric Vehicles (EVs) or Plug-in Hybrid Electric Vehicles (PHEVs) because of its long-life, high capacity and so on.  However, its degradation mechanism has not been understood enough and researchers have attempted to clarify the mechanism.  We clarified that degradation stored under a high-temperature condition forms some crystalline layer attributable to rock-salt type structure on the particle surface of active cathode material and this layer relates to the output fading.¹  In addition, it is considered that this layer influences on the reaction between active cathode material and electrolyte because this layer exits on the surface of active cathode material.  Therefore, in this study, we investigated the influence at high voltage operation to accelerate the reaction.

 For sample preparation, laminate type cells were assembled with a Li foil anode, an electrolyte (1 M LiPF₆ ethyl carbonate/dimethyl carbonate/ethyl methyl carbonate=3/3/4) and the LMO-NCM cathode sampled from commercial LiB cell of EVs before and after degradation test, called “initial sample” and “after degradation sample”, respectively hereafter, (storage under 45° C and SOC100% for 900 days).  While the laminate type cells were charged up to 6 V by CC-CV (CV time: 5 hr.), in-situ time-resolved X-ray diffraction measurement was carried out at synchrotron radiation facility SPring-8 (BL28XU, Hyogo, Japan) to observe electrochemically induced change of their crystal structure. 

 The relationship between SOC and lattice constants of LMO and NCM is shown in Figure 1.  For LMO, the lattice constant change was almost same in “initial sample” and “after degradation sample”.  On the other hand, for NCM, the lattice constant change was different in “initial sample” and “after degradation sample”.  That is to say, the lattice constant change of “initial sample” was larger than that of “after degradation sample” for NCM in more than SOC 600% particularly.  However, in comparison with each current integration, the lattice constant of NCM remain nearly unchanged before and after the degradation.  These results propose that the lattice constant of NCM has strong dependency on the current integration.  In addition, gas chromatograph-mass spectrometer analysis of the gas and electrolyte revealed that decomposition of the used electrolyte occurred at 6 V.

 In order to understand the difference of reaction between “initial sample” and “after degradation sample”, the surface state of NCM particle and LMO-NCM composite electrode were characterized by transmission electron microscope and soft X-ray absorption spectroscopy.  Interestingly, some crystalline layer attributable to rock-salt type structure was observed on the surface of the NCM particle of “after degradation sample” before and after charge to 6 V.  Based on these experimental evidences, we can conclude that the charge current at high voltage operation is mainly consumed by the decomposition of the electrolyte and the structure change of NCM.  In addition, the rock-salt type layer on the surface of NCM particle formed by degradation disturbs the reaction with electrolyte.

 This work was supported by Research and Development Initiative for Scientific Innovation of New Generation Batteries project from New Energy and Industrial Technology Development Organization in Japan.

(1) T. Fujimoto, K. Kitada et al, Transactions of Society of Automotive Engineers of Japan, 45, 297 (2014).