Many elements with different properties can be doped into the α-NaFeO2 structure, which has an impact on the layered structure, the stability of the structure when lithium ions are deintercalated, and the maintenance of the cycle capacity. Someone has studied the specific configuration and physical properties of the LiNi1-yCoyO2 system, and believes that its structure will become more regular as the cobalt concentration increases. At the same time, it is found that as y increases from 0 to 0.4, the value of c/3a also increases from 1.643 to 1.652, and when y≥0.3, the nickel at the position of lithium ion no longer exists. Therefore, the incorporation of cobalt restricts the migration of nickel to lithium ion sites in the mixed lithium nickel cobalt compound; similar behavior is also observed in the lithium nickel manganese cobalt compound. It has been reported that the presence of cobalt is beneficial to the oxidation of iron atoms in the structure, and other ions, such as iron, do not have this effect of cobalt. Therefore, in the LiNi1-yFeyO2 compound, as the concentration of iron ions increases, the capacity continues to decrease, and iron ions have no positive effect on the layered structure. For example, in the composite compound material prepared at 750°C, when y is equal to 0.10, 0.20, and 0.30, the content of 3d metal in the lithium ion layer is 6.1%, 8.4%, and 7.4%, respectively. Although LiFeO2 compound is an ideal low-cost battery material, it cannot easily deintercalate lithium ions under the normal voltage of a lithium battery. This phenomenon can be explained as the reduction of the shrinking force of FeO6 octahedrons, making Fe3+ difficult to be reduced.
Electronic conductivity is a common problem of all layered oxides. Compared with the lithium component or nickel substitution component, the electronic conductivity is different. When cobalt is substituted in LiNiO2 to produce LiNi0.8Co0.2O2, its electronic conductivity decreases. In addition, with the deintercalation of lithium ions in the structure of LixNi0.1Co0.9O2 or LixCoO2 (x increased from 1 to 6), it was found that the electronic conductivity increased by 6 orders of magnitude, reaching 1S/cm.
Studies have shown that when cobalt replaces nickel oxide, its structure is more stable, and compared with pure nickel oxide, it rarely loses oxygen. Other elements with poor redox activity, such as LiNi1-yMnyO2 in magnesium-substituted compounds, can reduce the capacity attenuation during cycling. Magnesium can prevent the complete removal of lithium ions, so that possible structural deformation is reduced.
At 25°C, a certain high oxygen partial pressure is reached (the equilibrium oxygen partial pressure exceeds 1atm), and the NiO2 in it is thermodynamically unstable.
The EPR results show that Al in LiCo1-yAlyO2 (y<0.7) exists in the form of octahedral coordination and tetrahedron, that is, a part of Al is in the interstitial position of oxygen, which hinders the diffusion of lithium ions. The electronic conductivity of LiCoO2 doped with Mg is greatly improved, which is nearly 2 orders of magnitude higher than that of undoped LiCoO2 at room temperature. This is because Mg enters the CoO2 layer octahedron to increase the electronic conductivity, thereby improving the cycle performance of the material, and the Mg entering the LiO2 octahedron does not cause capacity degradation.
LiCoO2-Li2MnO3 solid solution Li(Lix/3 Mn2x/3Col-x)O2 with a layered structure was synthesized at 900~1000℃ in the presence of excess lithium salt. With the increase of x, the unit cell parameters a and c both linearly increase, and its charge-discharge working curve is similar to that of LiCoO2, and the cycle stability is very good, but its discharge capacity is less than 160mA·h/g, and with the increase of Li2MnO3 content, the capacity decreases significantly. The ion exchange method can be used to synthesize LiCo1-xFexO2 material with a layered structure. With the increase of x, the values of unit cell parameters a and c increase, the charging platform increases, and the discharge capacity decreases significantly, indicating that the doping of Fe deteriorates the performance of LiCoO2; compared with the solid phase synthesis product, the LiCoO2 obtained by the ion exchange method has poorer cycle performance due to lower crystallinity. Excessive lithium source can make the Fe and Ni in the products LiFe0.1Mn0.9O2, LiFe0.2Mn0.8O2 and LiFe0.2Co0.6Ni0.2O2 are distributed in the octahedral position, and there is no Fe in the 3a position. This shows that excessive lithium can prevent Fe from entering the 3a position and help Fe enter the 3b position.
Doped and substituted composite nickel oxides, such as LiNi1-y-zCoyAlzO2, are currently used as the main alternatives for lithium battery cathode materials in HEV (hybirdelectric vehicle) large-scale systems. When charging, the nickel in the composite oxide is first oxidized to Ni4+, and then the cobalt in the composite oxide is oxidized to Co4+. SAFT company has produced this kind of battery that replaces nickel oxide, which is cycled 1000 times and has an energy density of 120~130w·h/kg, reaching 80% of the maximum discharge capacity.
