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The electrochemical performance of spinel LiMn2O4 at room temperature has tended to be industrialized. Its initial reversible capacity can reach 130mA·h/g. After 200 cycles of cycles, the capacity retention rate is above 80%, but it is more than 45℃. The reversible capacity attenuation is severe, hindering its application. To achieve its large-scale industrialization, such as being used as an energy source for electric vehicles, it must overcome its shortcomings of poor stability and fast capacity decay in high-temperature environments.
1. Dissolution and dissolution mechanism of manganese
The dissolution of manganese is the direct cause of the reversible capacity decay at high temperature. The team led by M.Oh Seung found that the dissolution of manganese caused the loss of the positive electrode active material, resulting in a decrease in the reversible capacity, and the amount of manganese dissolved was closely related to the temperature and the specific surface area of the material. At room temperature, the capacity loss caused by the dissolution of manganese accounted for 23% of the total capacity loss. With the increase of temperature, the capacity loss caused by the dissolution of manganese increased sharply at 55°C, accounting for 34% of the total capacity loss. Through the measurement of H+during the cycle, it is found that H+ is the direct cause of manganese dissolution. The relationship between the content of hydrogen ions and the temperature is basically the same as the relationship between the amount of manganese dissolved and the temperature. On this basis, the following dissolution mechanism is proposed, and the reaction formula is as follows:
2LiMn2O4+4H+→2Li++3λ-MnO2+MnO2+Mn2++2H2O
H+ is mainly derived from α-H in the solvent, and the reaction mechanism of hydrogen ion formation is proposed:
This mechanism can better explain the oxidation of ether solvents, but cannot analyze the oxidation process of ester solvents. It was discovered that another source of hydrogen ions is caused by electrolyte. Taking LiPF6 as an example, due to the presence of a trace amount of water in the electrolyte, the following reaction has occurred, resulting in a large amount of HF.
LiPF6+H2O→2HF+LiF+POF3
G.G.Amatucedi and J.M.Tarascon conducted a further study on the dissolution of manganese. They proposed that the dissolution of manganese caused by H+ resulted in the formation of protonated λ-MnO2 without electrochemical activity. The reaction formula is as follows:
LiMn2O4-yMn2++2yLi+→Li1+2yMn2-yO4
Li1+2yMn2-yO4 (in the presence of H+)→LixHzMn2-yO4+Li+
HzMn2-yO4( +Li+)→LixHz-xMn2-yO4
In summary, the capacity decay caused by the dissolution of manganese is mainly due to the oxidation of the electrolyte to generate trace hydrogen ions, which react with LiMn2O4 to cause the dissolution of manganese, and finally generate protonated λ-MnO2 without electrochemical activity.
2. Oxidation of electrolyte
The oxidation of the electrolyte mainly causes the reversible capacity attenuation of the material from two aspects. First of all, it is the main source of hydrogen ions. As mentioned earlier, the oxidation of the electrolyte generates free electrons, causing the oxidation of the electrolyte. The electrolyte is oxidized to generate hydrogen ions at a higher voltage. If lithium perchlorate is used as the electrolyte, the following reaction mainly occurs to generate a large amount of H+, which accelerates the dissolution of manganese.
ClO4→●ClO4+e
●ClO4+RH→HClO4+●R
On the other hand, the electrolyte directly reacts with the positive electrode material to produce organic compounds with no electrochemical activity. Some people have found that the oxidation of the electrolyte generates free electrons, which further react as follows:
LiyMn2O4+2δEI→LiyMn2O4-δ+δ(oxidation EI)2O
The reaction between the electrolyte and the material causes the loss of the positive electrode material and the electrolyte. At the same time, a passivation film is formed on the surface of the electrode, which prevents electron transmission and causes the material's reversible capacity attenuation.
3. Polarization of the electrode
M. Oh S. studied the relationship between electrode polarization and reversible capacity attenuation. By comparing the cycle performance of materials with different conductive agent content, and using AC impedance to measure the polarization of the electrode. Studies have shown that the dissolution of manganese and the concentration of hydrogen ions increase with the increase of the conductive agent content. Nevertheless, the samples with higher content showed better cycle performance. Impedance analysis shows that although the electrode with high conductive agent content causes a higher degree of oxidation of the solvent and the dissolution of manganese is greater, its polarization is smaller. The electrode with less content affects the contact between the conductive agent and the material due to the dissolution of the surface material , The polarization is serious. This indicates that the reversible capacity degradation is mainly caused by the polarization of the electrode.
At the end of the discharge, the average valence of manganese is close to 3.5, and the content of trivalent manganese ions increases, and the distortion of the crystal structure increases, which destroys the three-dimensional tunnel structure of the spinel and hinders the insertion of lithium ions, resulting in a reversible capacity loss.
During the cycle, the material structure undergoes irreversible changes. At a higher voltage platform, the shape of the charge-discharge curve changes from an L shape at room temperature to an S shape. The shape change of this curve shows that at high temperatures, the material changes from the coexistence of two states to a more stable state. structure. This more stable one-state structure is not conducive to the insertion and reversible extraction of lithium ions, and finally leads to a decline in capacity. Some people have studied one-phase and two-phase and believe that lithium-rich materials undergo charge-discharge cycles at room temperature, and the curve shows a one-phase structure, but it has good lithium ion insertion and extraction capabilities and good cycle performance; on the contrary, The samples with the two-state structure curve performed poorly. It can be seen that the change of one-phase-two-phase structure cannot explain whether the ability of lithium ion insertion and extraction at high temperature is good or bad. Therefore, although structural changes are considered to be one of the main reasons for the reversible capacity decay of materials at high temperatures, how the material structure affects the cycle performance of materials has not yet been determined.