Structural Characterization of Layered Li_xNi_0.5Mn_0.5O₂ (0 < x ≤ 2) Oxide Electrodes for Li Batteries

Abstract

X-ray diffraction and X-ray absorption spectroscopy experiments were used to study chemical and electrochemical Li insertion and extraction reactions of LiNi0.5Mn0.5O2. These results, along with galvanostatic cycling data, suggest that LiNi0.5Mn0.5O2 layered electrodes in lithium batteries operate predominantly off two-electron redox couples, Ni4+/Ni2+, between approximately 4.5 and 1.25 V and Mn4+/Mn2+ between 1.25 and 1.0 V versus metallic Li, respectively. The retention of a stable layered framework structure and the apparent absence of Jahn−Teller ions Ni3+ and Mn3+ in the high- or low-voltage region is believed to be responsible for the excellent structural and electrochemical stability of these electrodes. The LiNi0.5Mn0.5O2 layered oxide reversibly reacts chemically or electrochemically with Li to form an air-sensitive, dilithium compound, Li2Ni0.5Mn0.5O2, with a hexagonal structure analogous to Li2MnO2. The cycling behavior of Li/LiNi0.5Mn0.5O2 cells over a large voltage window (4.6−1.0 V) and with very slow rates shows that rechargeable capacities >500 mA·h/g can be obtained.

Investigating the first-cycle irreversibility of lithium metal oxide cathodes for Li batteries

“…8a and b), which may be an indicator of two-phase coexistence along the low-voltage plateau. However, the overlithiated ''Li 2 MO 2 '' phase, which has a space group of P-3m1 and shows a (001) peak at *17°2h CuKa [11][12][13], was not detected in the diffraction patterns obtained in the low-voltage region (not shown), probably because the volumetric fraction of the ''Li 2 MO 2 '' phase on the particle surface was below the XRD detection limit.…”

“…Due to the slow rate of lithium-ion diffusion in the layered structure compared to the lithium insertion rate during discharge, the lithium concentration increases significantly near the particle surface, which eventually induces formation of an overlithiated ''Li 2 MO 2 '' (Li 2 MO 2 or Li 2 MO 2 -like) phase on the oxide particle surface [5,8]. The Li 2 MO 2 phase has been observed for M = Ni [11] and M = Ni 0.5 Mn 0.5 [12] from heavily overlithiated LiNiO 2 and LiNi 0.5 Mn 0.5 O 2 , respectively, in lithium cells and for M = Mn [13] by chemical lithiation of LiMn 2 O 4 with n-butyl lithium. Therefore, the sudden voltage drop near the end of discharge (starting at *3.6 V in Fig.…”

Materials Challenges Facing Electrical Energy Storage

“…One way to increase the storage capacity in lithium batteries is to get more than one electron per transition metal redox center. Several materials, including VSe 2 , 13 VOPO 4 , 14 and Li x Ni 0.5 Mn 0.5 O 2 , 15 can achieve close to two electrons, as shown in Figure 4. However, in each of these cases, there is a large difference between the potential of the first reduction and that of the second reduction, which will prevent both steps from being used in a practical battery.…”

Aging Analysis of Graphite/LiNi1/3Mn1/3Co1/3O2Cells Using XRD, PGAA, and AC Impedance

“…Therefore, if the latter hypothesis were correct, it should be possible to recover the full capacity of NMC, if the discharge is not terminated by a voltage limit but continued at constant voltage to compensate for the slow lithium ion diffusion. The results of such an approach are shown in Figure 6: after an initial charge and discharge cycle at 0.1C in the in situ XRD half-cell, the potential was first held at 3.0 V for 12 h, then at 2.0 V for 10 h, and finally at 1.6 V for 8 h. Note that the potential is kept well above 1.5 V to prevent the formation of the overlithiated "Li 2 MO 2 " on the particle surface which would result in a two-phase coexistence and two different Li diffusion processes; 38,39 its absence is evidenced in Figure 6c, as the associated additional peak at ∼8• 2θ Mo,Kα 41,42 left to the (0 0 3) peak is not appearing. Holding the potential at 3.0 V for 12 hours results in a ∼50% recuperation of the ICL (marked by the gray area in Figure 6a), concomitant with a close approach of the c/a value toward its initial value (second-to the left black vs. red symbol in Figure 6b).…”

“…• 2θ Mo,Kα 41,42 left to the (0 0 3) peak is not appearing. Holding the potential at 3.0 V for 12 hours results in a ∼50% recuperation of the ICL (marked by the gray area in Figure 6a), concomitant with a close approach of the c/a value toward its initial value (second-to the left black vs. red symbol in Figure 6b).…”