A Study of the Electrochemical Reaction Mechanism on LiNi0.5Mn1.5O4-δ using Synchrotron X-Ray Techniques
Authors: Kim Donghwi , Yoon Jeongbae , Muhammad Shoaib , Oh Woong , Yoon Won-Sub 
Published: February 2020

Recently, as development of cathode materials proceeds, it provides comparatively high energy density at fast charge/discharge rates that meets the demands for high-power application such as hybrid electric vehicles(HEV) and plug-in hybrid electric vehicles(PHEV) etc. Among possible candidates, LiNi0.5Mn1.5O4 is one of the most promising candidate material due to its high power density, high capacity and good cyclic performance without Jahn-Teller distortion related to the presence of Mn3+ and good thermal stability[1]. Two types of phase structure, ordered P4332 and disordered Fd-3m, have been reported for LiNi0.5Mn1.5O4. The disordered Fd-3m spinel was found to have superior rate capability as compared to the ordered spinel P4332. It was also experimentally shown that Ni/Mn site disorder in spinel structure increases the specific capacity and conductivity of LiNi0.5Mn1.5O4 [2]. Degree of disorder in transition metals of this electrode plays important role in defining its electrochemical properties. Still, there is a lack of understanding of nickel and manganese site disorder on the octahedral sites and its effects on the elementary electrochemical reactions. In this study, we tried to systematically investigate the site disorder in high voltage spinel and its role in improving the electrochemical performance of this electrode by in-situ X-ray Diffraction (XRD) and X-ray Absorption Spectroscopy (XAS). Disordered LiNi0.5Mn1.5O4 powders namelyLiNi0.4Mn1.6O4
and LiNi0.44Mn1.56O4 were obtained from the GSEM (Korea) and these powder were annealed in air at 900°C to tune the Mn3+ contents in spinel structure. The working electrodes were prepared by using 86% active material, 10% Super P as conductor material, and 4% poly-vinylidene fluoride (PVDF) binder with N-methyl pyrrolidone (NMP) solvent. The cathodes were packed in the in-situ cell with Li foil as counter electrode, Celgard separator and 1.3M LiPF6 in a 3:7 volume mixture of EC(ethylene carbonate): DEC(diethyl carbonate) as electrolyte in the argon-filled glove box. Synchrotron X-ray based experiments were performed at Pohang Light Source (PLS-II) South Korea. Figure 1 (a) shows in-situ XRD patterns of highly disordered phase (LiNi0.4Mn1.6O4) during first charge. Structural changes in this XRD pattern are easy to track during the charge as compared to the LiNi0.44Mn1.56O4
because of a pseudo one phase behavior. Furthermore, this pattern shows that the solid solution range was wider than that of slightly disordered spinel(LiNi0.44Mn1.56O4) during first charge. These results interpret that physical strains and stresses in the LiNi0.4Mn1.6O4 resulting from these phase transitions are less severe in the LiNi0.4Mn1.6O4
compared to that of LiNi0.44Mn1.56O4 which results in improved Li+ transport because of the reduced number of phase boundaries. This phenomenon is in good accordance with the electrochemical cycling and rate capability data. To study the in-depth mechanism of these electrodes,
ex-situ samples at different state of charge were prepared and Mn local structure was investigated by XAS experiment performed on beam line 8C at PLS-II. Figure 1(b) shows normalized Mn K-edge XANES spectra for highly disordered spinel (LiNi0.4Mn1.6O4) at 3.5~4.4V region. The capacity in this voltage range originates from the 4V plateau which attributes the reversible transition of Mn3+
to Mn4+. Supposedly, As the Ni contents decreases, the average Mn oxidation state also decreases and increases the Mn3+content. More detailed discussion on the charge/discharge mechanism of LiNi0.5Mn1.5O4-

δ

will be presented at the time of meeting.
[1] Patoux S et al. ElectrochimicaActa(2008) 53, 4137
[2] J Xiao et al. Advanced Materials(2012) 24, 2109