Investigation on Relationship between the Amount of Ni4+ Ion and Thermal Stability in Ni-Rich Layered Cathode Materials
Authors: Lee Eunkang , Kim Taewhan , Yun Soyeong , Lee Sangyoon , Yoon Won-Sub 
Published: February 2020

Recently, the lithium ion batteries (LIBs) have applied to electric vehicle (EV), energy storage system (ESS) as a part of smart grids, and various electronic devices. However, LIBs was faced with deficiency of cyclability and demand for improved energy and power density due to the development of advanced applications. The electrochemical performance of LIBs has been enhanced through the Ni-rich layered cathode materials, such as LiMO2 (M = Ni, Co, Mn, and Al, etc.) [1]. However, these cathode materials are exposed to the risk of combustion and performance degradation in a little thermally harsher environment, due to the inherent thermal instability [2]. To develop thermally enhanced Ni-rich cathode material, therefore, it is the significant challenge to explore how the primary factor determines the thermal stability of Ni-rich cathode materials for the safety and credibility of LIBs. The primary reason for the thermal instability in Ni-rich cathode materials was often noted as the reduction of Ni4+ ion, which occurs in the course of the exothermic reaction [3,4]. However, there was no deep discussion on how the reduction of Ni ions influences the thermal stability of cathode materials due to the ambiguity of interpretation and the uncertainty of equipment.
In this study, the relationship between the amount of Ni4+ ion and thermal stability of LixNi0.5+yCo0.2Mn0.3-yO2 (x= 0.33, 1, y= 0, 0.1, 0.2) was investigated through synchrotron-based X-ray techniques; Time-resolved X-ray diffraction (TR-XRD), high-resolution powder diffraction (HRPD), and X-ray absorption spectroscopy (XAS) combined with differential scanning calorimeter (DSC). We observed that delithiated Ni-rich cathode material had a high degree of thermal expansion and a high possibility in formation of oxygen vacancy during heating process. They were identified through the comparison of lattice parameter and the k-weight (k = 1, 2, 3) dependent fitting using Ni K-edge EXAFS spectrum. Also, we specified the effect of amount of Ni4+ ion on the thermal expansion and formation of oxygen vacancy. Based on the results and well-known phase transition mechanism [5], we presented a complemented mechanism of 1st phase transition during heating process. Through the discussion, we will deliver how the primary factor determines the thermal stability of Ni-rich cathode materials. More details will be discussed in the meeting.

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