Thermal Stability Analysis of NMCs with Various Co Concentrations
Authors: Oda Mami , Nakagawa Takeshi , Yahiro Jumpei , Tanikawa Chikashi , Tabuchi Mitsuharu 
Published: February 2020

INTRODUCTION

Lithium ion batteries(LIBs) are used widely ,expanding in application. LIB's safety is urgent problem to be solved before practical use of large scale battery for automobiles. Although there are many factors to trigger so-called "thermal-runway" at elevated temperature, positive electrode material is one of the most important factors because oxygen is released by the decomposition of the cathode active material's crystal structure. In addition to the safety problem, recent rise in cobalt prices has also become a big problem, because cobalt is commonly used in the commercial positive electrode materials.It is important to obtain the guidance for designing oxide cathode active materials to satisfy both of safety and cost down. NMC, cathode including -Ni, Mn and Co, becomes less stable at elevated temperatures, and the tendency becomes obvious with the increase of Ni concentration 1). The purpose of this study is to reveal how thermal stability changes with constant Ni content and decreasing Co concentrations in NMCs(532, 541, 550). In this study,we prepared three NMCs and its chemically delithiated compounds, to obtain the thermal behavior of cathode active material only.

EXPERIMENTAL

Three NMC samples were obtained by coprecipitation-calcination method. The calcination condition was fixed at 850ºC for 5h in air. Chemically delithiated Li0.3MO2(M=Ni, Co, Mn) are obtained through the reaction with oxidizer (NO2BF4)2) from NMCs in various Co concentrations(532, 541, 550). The chemical composition of the delithiated samples were determined by ICP-AES. TPD-MS was performed for these delithiated samples to analyze amount and behavior of oxygen release during heating process, and in situ XRD at elevated temperature was conducted to analyze crystal structure, and fine structure around transition metals were analyzed by FT-EXAFS.

RESULTS AND DISCUSSION

Oxygen release has detected roughly in two temperature regions(Fig.1), one is around 300ºC, denoted as region 1, and the other is around 450ºC, denoted as region 2. Total amount of oxygen release decreased with decreasing Co concentration (NMC532 > 541> 550) in cathode active materials, that is, the total amount of oxygen release is smallest in NMC550. It suggests NMCs with lower Co concentrations have lower oxygen evolution and higher thermal stability.In the region 1, peak top temperature was detected higher in order of lower Co concentrations(532<541<550), but on the other hand, in the region 2, peak top temperature was not in order of Co concentration, that is, the order of peak top temperature is 532,550 < 541.To reveal the reason why thermal stability is different due to the Co concentrations, we analyzed structural change using XRD and FT-EXAFS at elevated temperature.
XRD indicates that layered rock salt structure changes into spinel structure mainly in region 1 and spinel into rock salt structure in region 2, which is common behavior in all samples. Focusing on (003) diffraction peak, 550 and 541 have a clear shoulder peak at higher angle at 200ºC. These shoulders imply that there are 2 layered rock salt phases wtih the different Li occupancies (Li-rich and Li-poor). In the process of heating, these shoulders disappeared at 350ºC in 550 and 541. This shoulder was not clearly detected in 532 through heating. So, the appearance of shoulder peak differs due to the Co concentrations.
Ni K-edge FT-EXAFS spectra showed that layered rock salt structure changes into spinel structure mainly in region 1 and the spinel structure into rock salt structure in region 2, in all samples. From Co K-edge FT-EXAFS spectra show that the layered rock salt structure does not clearly change in region 1, but changes into Co3O4-type spinel in region 2, which is the same behavior for 532 and 541. Mn K-edge FT-EXAFS spectra show different behavior depending on the Co concentration. Over 300ºC, the peak shift around 4.5～5.5Å was detected in 532 and 541, which suggests that lithiation into layered rock salt. In contrast, over 300ºC, the peak intensity at around 2.5Å increased in 550 and 541, which is identified as LiMn2O4. Therefore, It is concluded that the difference of thermal stability was explained by reconstruction around Mn-rich domain, the generation of LiMn2O4, and decomposition around Co and Ni-rich domain, both of which results in the appearance of lithiated structure.
These structural change cause O2 generation depending on temperature region. So,we need to design cathode both in the view points of O2-release amount and generation temperature to maintain the balance of safety and cost down.

References

1)Xiqian Yu et al., Chin. Phys. B. Vol.25, No.1 (2016), 018205
2)S. Venkatraman and A. Manthiram, Chem. Mater. 2002, 14, 3907-3912

Figure 1