A Study on Structural and Electrochemical Properties of Co Fixed LiNi0.5+XCo0.2Mn0.3-XO2 (x=0, 0.1, 0.2) Using Synchrotron Based X-Ray Techniques
In this study, the structural and electrochemical properties of Co fixed LiNi0.5+xCo0.2Mn0.3-xO2 (x=0, 0.1, 0.2) cathode material were investigated by using synchrotron based X-ray diffraction and absorption techniques to understand the effects of Ni and Mn components. The Rietveld refinement of diffraction data was performed by using R-3m model. Refinement results show that lattice parameters of LiNi0.5Co0.2Mn0.3O2 (x=0) are a=2.8697Å, c=14.2378Å, and by increasing the nickel content in LiNi0.5+xCo0.2Mn0.3-xO2 (x=0, 0.1, 0.2) lattice parameters “a” and “c” shrink as shown in Table 1. Generally, it is believed that the Ni2+ content in the Li layer increase when the total amount of Ni component increase in the three component system. However, in the Co fixed LiNi0.5+xCo0.2Mn0.3-xO2 cathode material system, interestingly cation mixing decreases by increasing the nickel content. The NCM721 (x=0.2) electrode has lower Ni2+ ions in the lithium layer compared to NCM622 (x=0.1) which has even lower quantity of Ni2+ ions than that of NCM523 (x=0). In pristine electrode material, the theoretical value of average Ni oxidation state is Ni2.4+, Ni2.67+, Ni2.86+ for x=0, 0.1, 0.2 respectively and this trend is confirmed by Ni K-edge XANES spectra.
The Ni K-edge XANES spectra show shift in energy position with change of nickel content in the electrode material. In NCM523, Ni K-edge position is close to that of Ni2+, while that of NCM721 is at higher energy position, indicating higher Ni valence (close to Ni3+) as shown in Figure 1. The increase of average Ni oxidation state in NCM721 pristine material suggests that Ni3+ component is predominant in its transition metal layer. Therefore, shrinking of lattice parameters by increasing the the nickel quantity can be explained on the basis of Ni3+ content in the transition metal layer. In the Co fixed system (rCo3+=0.545Å), the unit cell dimensions in a hexagonal setting shrink by increasing the Ni content because of the difference in size between divalent and trivalent nickel ions (rNi2+=0.69Å and rNi3+=0.56Å). When the Ni3+ increases, the Ni2+ and the Mn4+ (rMn4+ =0.53Å) content decrease, net result is decrease in lattice parameters because the increment is small (rNi3+-rMn4+=0.03Å) compare to the decrement (rNi3+-rNi2+=-0.13Å). For the Ni2+ content in Li layer, we can also say that there is low opportunity for Ni2+ to go into the Li layer because the total amount of Ni2+ content decreases with increase of the Ni content. Consequently, these effects of nickel content in the layered structure would affect electrochemical properties such as capacity, cyclability and rate capability; hence their study can assist in designing safe and high performing electrode materials.
The more detailed structural and electrochemical properties of Co fixed LiNi0.5+xCo0.2Mn0.3-xO2 cathode material will be presented at the time of meeting.
 M.S. Whittingham, Chem. Rev. 104 (2004) 4271-4301