What is the charge and discharge curves of LiCoO2 and LiNiO2?
The charge and discharge curves of LiCoO2 and LiNiO2 are shown in Figure 1. When the charge termination voltage is 4.3V, the curve of LiCoO2 is quite smooth under the same voltage platform, while the curve of LiNiO2 is more complicated, showing multiple voltage platforms. In the following charge and discharge process, the composition of the LiNiO2 compound will be expressed as Li1-χNiO2, and the complex curve is attributed to the structural transformation. Initially, Li1-χ NiO2 maintains the original structure, but transforms into a monoclinic phase when 0.22<χ<0.64; when χ>0.70, it is further delithiated to form NiO2. For LiCoO2, in the interval of χ<0.25, two crystal phases with R3m structure (starting LiCoO2 and Li0. 75 CoO2) are formed;If the monoclinic phase same as Li1-χNiO2 does not appear in a very narrow range near Li0.5 CoO2, the second rhombohedral phase will continue. But by increasing the temperature, the monoclinic crystal phase will be transformed into the rhombohedral crystal phase. However, lithium-rich LiCoO2 exhibits a different behavior from stoichiometric LiCoO2: when χ<0.25, there is no two-phase region; when χ is about 0.5, there is no monoclinic phase. This is easy to judge from its charge and discharge curve. The electrochemical reaction of lithium-rich LiCoO2 is carried out in the single-phase region. As the degree of delithiation increases, the c-axis gradually lengthens and the a-axis gradually shortens.
Regarding the formation range of monoclinic phase and NiO2 phase, researchers have their own opinions. This may be due to the easy formation of Li1-χNi1+χO2 (χ>0), resulting in differences in the composition of the studied samples. As the degree of delithiation increases, the length of the a-axis and the volume of the unit cell continue to shrink. These changes can be well explained by the following reason, that is, the ionic radius of the transition metal decreases with the increase in the degree of oxidation. As shown in Figure 2, although the length of the c-axis (interlayer spacing) increases with the increase in delithiation, when the work is greater than 0.6, there is a difference between the c-axis length changes of LiCoO2 and LiNiO2. For LiCoO2, the length of the c-axis gradually decreases after a maximum value; for LiNiO2, when> 0.6 or higher, the interlayer spacing becomes constant; when χ ≥ 0.7 or higher, the NiO2 phase appears; for LiCoO2 , CoO2 phase can be formed only when χ is close to 1. As mentioned earlier, during the charge and discharge process, the structure of layered LiCoO2 and LiNiO2 are quite different. However, when manganese is used to replace 20% of the nickel in LiNiO2, this behavior is the same as LiCoO2. In short, the lithium content and the type of transition metal ions have a very large impact on the structure of the layered material after charging.
The upper limit of the monoclinic crystal formation range of LiCoO2 (χ=0.75) is almost the same as the lower limit of the NiO2 formation range of LiNiO2. If there are stacking defects in the structure of Li1- χCoO2 within this range, when x>0.7, the stack of oxygen ions in LiCoO2 and LiNiO2will change; when x<0.7, the structure change is due to the atomic position. Slight deviation. In addition, χ>0.7 will cause changes in the stacking of the oxygen layer, which is the main reason for the poor cycle performance of LiCoO2 and LiNiO2 when the capacity is high.
The shape of the charge and discharge curve also depends on the stack of oxygen layers. O2-Li0.7MnO2 shows a complex charging curve with multiple voltage plateaus. When the Li+ dropout exceeds 0.5, the stacking of the oxygen layer changes from (ABCB)n to (ABCB-CABABCAC)n mode. This structural change causes the layer spacing to shrink.
This study shows that although the length of the a-axis shrinks intermittently when the charging capacity approaches 125mA • h/g (χ=0.45), when χ reaches χ=0.62 and χ=0.76, LiNi0.5Mn0.5O2 and LiCo1/3 Ni1/3Mm1/3O2 still maintain the R3m symmetric structure. If these compounds do not undergo the rearrangement of the oxygen layer stack, in the case of overcoming the decomposition of the electrolyte, higher capacity can be obtained by increasing the charging voltage, and the cycle performance will not be affected.
The 02-Li0.7Li1/18Mn17/18O2 prepared by ion exchange using P2-Na0.7Li1/18Mn17/18O2 can obtain a capacity of 15mA • h/g at a voltage of 4.0~4.5V, at a capacity of 3.0~3. 5V A capacity of 130mA • h/g can be obtained under voltage. When part of the manganese ions are replaced by transition metal ions M (Ni, Co, etc.), the capacity will be reduced under the voltage of 3.0-3. 5V, and the new voltage platform is now 2.5~3.0V . The structure of oxygen remains unchanged in all voltage ranges, and the material exhibits excellent cycle performance at 30°C, but the cycle performance at 55°C deteriorates.
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