Doping of LiCoO2

Doping of LiCoO2

Due to the lack of cobalt resources and the high price, the application and development of lithium cobalt oxide, a cathode material for lithium-ion batteries, are limited due to high cost and other factors. In addition, studies have shown that pure LiCoO2 , when the amount of lithium ion deintercalation exceeds 50%, its electrochemical performance will be degraded a lot. In order to further improve the performance of lithium cobalt oxide materials, many researchers have done a lot of work on the doping of materials and achieved good results. Commonly doped elements are Li, K, B, Mg, Al, Cr, Ti, Cd, Ni, Mn, Cu, Sn, Zn and rare earth elements.

The excess of lithium can also be called doping. Due to the excess of lithium, in order to maintain electrical neutrality, LixCO2 contains oxygen defects. Treatment with high pressure oxygen can effectively reduce the oxygen defect structure. The reversible capacity is obviously related to the lithium content. Many studies have been conducted on excessive lithium doping. When Li/Co=1~1.1, the electrochemical performance of lithium cobalt oxide is improved; when Li/Co>1.1, the reversible capacity decreases due to the decrease of Co content. Excessive Li﹢ does not reduce Co3+, but produces oxygen ions in a new valence state, with high binding energy and low surrounding electron density, and the hole structure is evenly distributed in the Co layer and O layer, which improves the Co-O bond合 Strength. In addition, there are reports showing that introducing K element in an appropriate amount can increase the reversible capacity of the material.

The doping of magnesium ions has little effect on the reversible intercalation capacity of lithium, but improves the cycle stability of lithium cobalt oxide. The reason is that the doped Mg2+ forms a solid solution instead of a multiphase structure. Reports have shown that doping a small amount of Mg2+ in LiCoO2 can increase its electronic conductivity from 10-3S·cm-1 to 0.5S·cm-1, without changing the crystal structure of the material, and at the same time during the charge and discharge cycle. The middle material has a single-phase structure. This is because the doped Mg2﹢ occupies the position of Co in the LiCoO2 lattice, thereby generating Co4+, that is, holes according to the equilibrium mechanism, so the conductivity of LiCoO2 can be greatly improved after M2﹢ doping.

Al3+ (0.535A, coordination number of 6) and Co3+ (0.545A, coordination number of 6, low spin) have basically the same ionic radius, which can form a solid solution LiCol-xAlxO2 in a larger range, which can be stable after doping Structure, increase rate capacity and improve cycle performance. Reports have shown that when x≤0.5, the material presents a single phase; when 0.6≤x≤0.9, the material presents a two-phase LiCol-xAlxO2 and LiAlO2 coexisting state; when x=1, the material presents a single phase, which is LiAlO2 phase. The upper limit of the material median, that is, the maximum solid solubility of Al is about 0.5. In the single-phase region (x≤0.5), with the increase of Al doping, the lattice structure parameters of the material change, the a-axis is shortened, the c-axis becomes longer, c/a basically increases linearly, and the layered properties of the material are more obvious . In addition, A13+ has no 3d orbital hybridization with oxygen orbitals, which in turn causes the lithium ion deintercalation potential of LiCol-xAlxO2 to rise, which improves the voltage platform of the material. There is also heat treatment after the introduction of Ca2+-containing compounds, because Ca2+ in the product has one more positive charge than Li+, resulting in electrical positiveness, and this easily causes O2-to move, thereby improving the conductivity of lithium cobalt oxide, which is conducive to rapid charging. Discharge.

Appropriate amount of Cr3+, Ti4+ and V5+ doping can improve the electrochemical performance of LiCO2. Studies have shown that in LiCo1-xCrxO2 (0≤x≤0.2), with the increase of x, since the ionic radius of Cr3+ is larger than Cr3+, the crystal parameters a and c Increase. For Ti-doped LiCol-xTixO2 (0≤x≤0.5), a single-phase structure can be obtained when the titanium doping amount is less than 10%. Gopukumar reported that the first charge/discharge capacity of LiCo0.99Ti0.01O2 reached 157mA.hg﹣1 and 148mA.hg﹣1 under 0.2C rate, respectively, and it can still maintain 90% reversible capacity after 10 cycles, while commercial LiCoO2 Under the same conditions, the first charge/discharge capacity of the cycle is only 137mA.hg﹣1 and 134mA.hg﹣1. The introduction of vanadium makes the internal structure of LiCoO2 change, so that its crystal form is not easy to change during the charge and discharge process, and the cycle performance is improved.

The results of first-principles calculations show that there are two lattice factors that increase the diffusion of Li﹢: one is to increase the distance between Li layers, which mainly refers to the increase of the c-axis distance; the other is to introduce low-valence ions. Ceder pointed out that the Li layer spacing fluctuates within the range of 2.64 (±4%) A, but a 4% fluctuation will result in a 200% change in activation energy. For M3+ to replace Co3+, late transition metals (such as Ni, Fe, etc.) have a higher oxygen electron cloud density and a low barrier similar to Co, so they are better than former transition metals (such as V, Cr, Ti, etc.) Easy to apply to layered oxide electrode materials. Lower valence cations (such as Cu2﹢) help reduce the Li migration barrier, thereby improving the Li ion diffusion performance of the material. Since cobalt and nickel are adjacent elements in the same period, they have similar external electron arrangement, and LiCoO2 and LiNiO2 are both a-NaFeO2 type compounds, so cobalt and nickel can be mixed in any ratio and maintain the layered shape of the product Structure, the prepared LiCol-xNixO2 has the advantages of both Co-based and Ni-based materials. In addition, the ion radius of rare earth elements (RE) is generally relatively large.

The crystal form of the cathode material LiCo0.99RF0.01 (RE=Y, La, Tm, Gd, Ho) doped with rare earth elements has not changed, and it is still hexagonal, but the a-axis and b-axis of the resulting product Compared with the pure phase LiCoO2 , there are different degrees of shrinkage, the c-axis has a relatively large elongation, the volume of the unit cell is larger than that of the pure phase LiCoO2 , and the increase rate is about 0.7%. This shows that the doped rare earth element partially replaces the Co element in the original unit cell, and the increase of the c value indicates that the layer spacing of the obtained cathode material becomes larger, which means that the product has faster Li + insertion and migration capabilities, Better charge and discharge stability, which has better electrochemical performance. Liao Chunfa and others also used XRD research to find that no matter what kind of rare earth doped, the peak value of the XRD spectrum of LiRExCo1-xO2 is higher than that of pure LiCoO2 . Evenly. Deng Bin et al. prepared LiCo1-xRExO2 as a cathode material for lithium-ion batteries doped with rare earth elements by high-temperature solid-phase synthesis. The cathode materials for lithium-ion batteries doped with trace amounts of Y, La and other rare earth elements can greatly improve acidity. The specific capacity of the lithium cathode material. However, due to the relatively large atomic weight of most rare earth elements, as the content of rare earth elements doped in LiCoO2 increases, the charge-discharge mass specific capacity of the obtained cathode material gradually decreases. The greater the proportion of doped elements, the decrease of charge-discharge capacity The greater the amplitude. Based on the synthesis of LiCoO2 , Liao Chunfa et al. synthesized LiRExCo1-xO2 by doping rare earth La, Ce, Lu, Y, etc. by co-precipitation method; the results showed that the synthesized LiRExCo1-xO2 has a LiCoO2 structure, when the amount of RE added is less than At 0.05, the rare earth can completely form a single LiRExCo1-xO2 phase; the incorporation of rare earth can promote the crystallization of LiCoO2 , and at the same time increase the relative diffraction intensity of the (104) plane; the first discharge capacity of LiRExCo1-xO2 reaches 147.4mA.hg-l, and the cycle is stable Sex has also improved. The doping of Nd does not significantly change the structure of LiCoO2, which still belongs to the hexagonal crystal system. With the difference of doping amount, the unit cell parameters change slightly, but the difference is not big. Secondly, the doping element Nd does not significantly improve the initial discharge capacity of the material; in addition, doping a small amount of Nd will make the discharge platform of the material more stable.

Reports have shown that anion doping can also improve the electrochemical performance of LiCoO2 , and B doping can reduce polarization, reduce electrolyte decomposition, and improve cycle performance. The introduction of P can significantly change the structure of LiCoO2 , thereby improving the rapid charge-discharge capability and cycle performance of the material. The introduction of amorphous substances in LiCoO2 , such as boric acid, silicon dioxide, tin compounds, etc., will cause the structure of LiCoO2 to change from a hexagonal crystal system to an amorphous structure. This doped LiCoO2 material has good stability during charge and discharge cycles.