Comparison of LiNi0.5Mn1.5O4 Spinel having Two Space Groups; Fd_3m and P4332

Abstract

Recently, some research group have reported that transition metal-substituted spinel materials (LiMxMn2-xO4, M=Cr, Co, Fe, Ni, Cu) showed high voltage plateau at around 5 V [1-4]. The capacity and voltage plateau in Li/LiMxMn2-xO4 cells strongly depend on the kind of transition metals (M) and their content. Among those materials, LiNi0.5Mn1.5O4 received many attentions for the high discharge capacity and its dominant plateau at around 4.7 V while other materials exhibited two plateaus at 4.0 V and ca. 5 V [3]. Terada et al. [5] revealed that the plateau at around 4.7 V is attributed to the Ni2+/Ni4+ redox while the plateau at 4.3 V arises from Mn3+/Mn4+ redox couple by In situ XAFS analysis during the charge/discharge process of the Li1-xNi0.31Mn1.69O4 cell. For a stoichiometric LiNiII0.5MnIV1.5O4 composition, the oxidation state of Mn should be fixed to 4+ showing only Ni2+/Ni4+ redox couple during the charge/discharge process. During the synthesis of the LiNi0.5Mn1.5O4, however, high calcination temperature leads to the reduction of Mn oxidation state from 4+ to 3+ [2,6,7]. Accordingly, the plateau at around 4 V appears by the Mn3+/Mn4+ redox couple during the electrochemical cycling. Ohzuku et al. employed annealing process at 700oC after the calcination [7]. The resulting powders delivered flat voltage profiles at around 4.7 V with only a small plateau at around 4 V indicating that most of the manganese ions oxidized to 4+ on the annealing process. The neutron-diffraction results showed that stoichiometric LiNi0.5Mn1.5O4 have the cubic spinel structure with a space group of P4332 instead of Fd_3m [6]. In this work, we synthesized and compared the LiNi0.5Mn1.5O4 with two space groups of Fd_3m and P4332 in point of structure and electrochemistry. LiNi0.5Mn1.5O4 powder was synthesized at 900oC by molten salt method using LiCl-LiOH mixed salts. Some of the obtained powders were annealed again at 700oC for 48 h. Then, we compared LiNi0.5Mn1.5O4 without annealing process with annealed LiNi0.5Mn1.5O4 powders. Figure 1 show the XRD patterns of the LiNi0.5Mn1.5O4 powders. Rietveld refinement and TEM revealed that LiNi0.5Mn1.5O4 before annealing process have a space group of Fd_3m while annealed LiNi0.5Mn1.5O4 have a space group of P4332. Two kinds of Li/LiNi0.5Mn1.5O4 cells show quite different voltage shapes as shown in Fig. 2. Annealed Li/LiNi0.5Mn1.5O4 cell shows flat voltage plateau at around 4.7 V compared to the Li/LiNi0.5Mn1.5O4 cell without annealing. 4 V plateau due to the Mn3+/4+ redox was hardly observed for the annealed Li/LiNi0.5Mn1.5O4 cell. Two Li/LiNi0.5Mn1.5O4 cells have similar discharge capacities at a low current density of 20 mA/g (1/7C rate) in Fig. 3. However, at a higher current density of 325 mA/g (3C rate), annealed Li/LiNi0.5Mn1.5O4 cell delivered lower discharge capacity compared with the LiNi0.5Mn1.5O4 without annealing. Impedance measurement and XRD, TEM for the two LiNi0.5Mn1.5O4 electrodes after 50 cycles were performed for the comparison of LiNi0.5Mn1.5O4 with two space groups of Fd_3m and P4332.