In order for spinel to have excellent cycle performance, it is very important to maintain a single-phase reaction in the entire 4V region. The charging and discharging mechanism can be described as follows: There are two types of electrochemical reactions of lithium in spinel, one is single-phase reaction The opposite reaction, that is, the single cell shrinks or expands slightly during the charging and discharging process; the other is the two-phase reaction, that is, two different crystal phases coexist, and the ratio of the two crystal phases changes. The difference in the two electrochemical reaction processes can be observed by voltage curve (OCV) and X-ray diffraction. The voltage curve in a biphasic reaction should be flat, however in the non-equilibrium case it may be S-shaped and may be erroneously identified as a monophasic reaction. Furthermore, the XRD patterns formed using CuKa lines (irradiated at two wavelengths of Ka1 and Ka2) show that two diffraction lines of the spinel in the initial state and the resulting spinel with close-packed lattice parameters occur in the two-phase reaction. Coinciding, they may be seen as a broad diffraction line, which is mistaken for a single-phase reaction. Due to the high intensity of single-frequency X-rays, high-precision XRD analysis is possible using a synchrotron light source. The researchers confirmed for the first time that the charge-discharge mechanism of oxygen-depleted spinel at low voltage is a two-phase reaction.
The changes in cubic lattice parameters of three typical spinel compounds during charging are shown in Figure 1. The reactions of LiMn2O4 include a single-phase reaction under a low-voltage platform (lattice parameters continue to decrease) and a two-phase reaction under a high-voltage platform (the existence of two stereophases with different lattice parameters). In metal ion-doped oxidative stoichiometric spinels or lithium-rich spinels, the electrochemical reactions at high and low voltage platforms are single-phase reactions accompanied by continuous changes in lattice parameters. On the other hand, in oxygen-depleted spinels, two crystalline phases with different lattice parameters exist in the low-voltage plateau and the high-voltage plateau. That is to say, the cubic phase I with a lattice parameter of 0.825 nm and the cubic phase II with a lattice parameter of 0.817 nm exist at the same time under the low voltage platform. When x=0.5, only the cubic phase II exists. In addition, the cubic phase III with a thickness of 0.806 nm was formed during the delithiation process, and the cubic phases II and III coexisted. The ratio of cubic phase III increases with the progress of delithiation.
It has been reported that 5V cathode materials such as LiNi0.5Mn1.5O4 have similar lattice changes to spinel shown in Figure 1 (a), (c). Its electrochemical reaction is the same as that of oxygen-depleted spinel (c) at both low and high voltage platforms, both of which are two-phase reactions, suggesting that the reported sample may be oxygen-depleted.
During the charging and discharging process, LiFePO4 with orthorhombic olivine structure forms two crystalline phases coexisting with FePO4, so it has a smooth discharge curve. FePO4 with an orthorhombic crystal structure formed during charging can be precisely measured using the Rietveld method. Replacing iron with manganese changes the shape of the charge-discharge curve, and the capacity at the low-voltage plateau of 3.5V and the high-voltage plateau of 4.0V is related to the content of iron and manganese. Therefore, manganese substitution can effectively increase the energy density, however, it reduces the electrical conductivity, resulting in poor rate performance.
