The extra lithium can be integrated with the layered structure in the form of Li2MnO3 and LiMO2 (M=Cr, Co) solid solutions. The transition metal ions can also be nickel or manganese ions, including systems such as LiNi1-yCoyO2, Li2MnO3 that can be replaced by similar metals, such as Li2TiO3 and Li2ZrO3. Li2MnO3 can regenerate the lithium-rich layered compound Li[Ni1/3Mn2/3]O2. These solid solutions can therefore generate compounds with the molecular formula LiM1-y[Ni1/3Mn2/3]yO2, where M can be Cr, Mn, Fe, Co, Ni or their composites. Excessive lithium can increase the tendency of manganese to change from +3 valence to +4 valence, so the Jahn-Teller effect produced by Mn3+ can be reduced to a minimum.
Of particular interest is that when manganese is in the +4 oxidation state in the Li2MnO3 system, it will exhibit unexpected electrochemical activity during charging. This kind of "overcharge" is related to the following two aspects. One is the loss of oxygen during the delithiation process, thereby forming oxygen lattice defects; the other is that when the delithiation process occurs and dissolves in the electrolyte, it will increase the number of cations in the electrolyte, so that lithium ions can be exchanged with the material. Which aspect dominates depends on the temperature and the chemical composition of the oxide lattice. There is no change in manganese oxide in both. When a certain amount of hydrogen participates in the ion exchange, the MO2 layered structure will slide to form a prism shape, and the interlayer spacing will shrink by about 0.03nm, forming a hydrogen bond. When the oxide is heated to 150°C, these protons will disappear. Filtering Li2MnO3 with acid can also make delithiation exist, which is conducive to proton exchange. After filtering with acid, a compound with a certain stoichiometric ratio can be formed, such as LiNi0.4Mn0.4Co0.2O2, which can also remove lithium and perform a small amount of ion exchange.
The reduction of spinel Li[Li1/3 Mn5/3]O4 with hydrogen at 200°C confirmed that when lithium is excessive, oxygen is easily released from its tightly packed lattice. At the same time, studies have shown that the oxygen in it can be charged and released by electrochemical methods when the charging voltage is about 4.3V; then it shows the discharge characteristics of a typical spinel material, and the discharge voltage is 4V. The research process of this kind of reduced materials can also be analogized to the Li[Li1/3Mn5/3]O4-δ system.
For Li2MnO2-LiNiO2 solid solution system, it can be written as Li[NiyLi(1/3-2y/3)Mn(2/3-y/3)]O2=yLiNiO2+(1-y)Li[Li1/3Mn2/3]O2, when nickel is added, the lattice parameters a and c increase linearly, that is, when 0.08≤y≤0.5, the value of c/3a decreases linearly, indicating that the addition of nickel can reduce delamination as expected. These materials show an irreversible voltage plateau of about 4.5V after being assembled into a battery, which should be caused by the oxygen deficiency described above. Before this plateau, all the nickel in the compound is oxidized to Ni4+. After "overcharge", the cycle performance is very good in the voltage range of 2.0~4.6V at 30°C. When the value of y becomes larger, such as y=1/2, 5/12, 1/3, the capacity increases instead, specifically 160mA·h/g, 180mA·h/g, 200mA·h/g. When the cycle temperature rises to 55°C, the material capacity with y=1/3 increases to 220mA·h/g. Although it has a good charging effect, its thermal stability is very poor. Adding excess lithium to the 550 material improves the thermal stability of Li1+x(Ni0.5Mn0.5)1-xO2.
The Li2MnO3-LiNO2-LiMnO2 system has been studied, and the results show that it can be completely dissolved along the Li2MnO3-LiNO2 line; when the charging voltage is lower than 4.3V and the nickel content decreases, the capacity drops rapidly.
Li2TiO3 and LiNi0.5Mn0.5O2 can also form a solid solution, and titanium can enhance the insertion of the second lithium in the structure. Adding Li2MnO3 to the layered material is a manganese-rich material. In comparison, it will form a spinel phase and reduce stability. Li[Li0.2Ni0.2Mn0.6]O2 is charged and discharged at 0.1mA/cm2, the voltage is 2.0~4.6V, after 10 cycles, the capacity is a constant value of about 200mA·h/g. The stability and electrochemical properties at high rates have not been reported.
In summary, the excessive lithium content, considering the addition ratio of nickel, cobalt, and manganese, can design an ideal cathode material with appropriate composition. This material has a stable crystal lattice (Mn), which acts as an electrochemical active center (Ni) to order transition metals or improve rate performance and conductivity (Co), as well as increase battery capacity (Li). Each factor has its own role. Whether there are other elements that can play a key role remains to be further confirmed.