(1) Reduce the surface area of the material
The most direct way to reduce the dissolution of manganese is to reduce the contact between the cathode material and the electrolyte. Someone has compared the high temperature cycle performance of materials with different specific surface areas, and the results show that the capacity of the material after grinding is seriously attenuated, because the electrolyte is catalyzed by the electrode surface at a higher voltage. Research by Masaki Yoshio et al. believes that the specific surface area of the fixed material after 100 cycles at 50°C has a capacity decay rate of only 12% and a reversible capacity of 105mA·h/g. M.Oh Seung et al. compared the surface and attenuation rate and found that when the surface area was changed from 3.64m2/g to 21.2m2/g, after 60 cycles, the initial capacity retention rate dropped from 83% to 50%. When the chromium-doped lithium manganese oxide particles are smaller than 354m, their reversible capacity rapidly decays from 127.5mA·h/g to 97.5mA·h/g after 20 cycles. It can be seen that controlling the specific surface area of the material is an effective way to improve the cycle performance of the material at high temperatures. However, the surface area of the material is too small, it will also affect the diffusion of lithium ions, destroy the full contact between the conductive agent and the material, reduce part of the reversible capacity, and affect the cycle performance of the material.
The synthesis of LiMxMn2-xO4 by adding other ions to increase the average valence of manganese and reduce the capacity attenuation at high temperatures has always been a hot spot and method used by people. The citric acid complexation method was used to add appropriate amounts of Ni3+ and Co3+, and it was found that the cycle curve of the compound after the introduction of cations changed, from the original two-state structure to the one-state structure. When the doping amount is 0.04, the material exhibits ideal cycle performance, and its initial capacity is about 130mA·h/g. After 50 cycles, the reversible capacity is still about 115mA·h/g. R.E.White et al. also studied the performance of lithium manganese oxide compounds added with cobalt ions, and found that adding cobalt ions can increase the exchange current density and reduce the impedance, and the specific surface of the material can be reduced by nearly 50%. Investigating the influence of different addition amounts, it is found that when the addition amount is 0.16, the material exhibits the best cycle performance, and its reversible capacity is still 100mA·h/g after 85 cycles. Someone synthesized a series of nickel-added compounds by the sol-gel method, and measured their unit cell parameters by XRD. It was found that as the nickel content increased, the unit cell parameters became smaller. When its content is 0.5, the material has good cycle performance, with an initial capacity of about 118mA·h/g, and after 30 cycles, there is almost no capacity attenuation. Robertson et al. studied the addition of different cations and found that the samples with Cr3+ and Ga3+ added have better stability and can effectively prevent the capacity attenuation. When the addition amount is 0.02, it shows better cycle performance at 55°C, and after 100 cycles, the reversible capacity is 110mA·h/g. Wakihara et al. introduced magnesium to lithium manganese oxide compounds. After 100 cycles of this material, its capacity is still about 100 mA·h/g. By simultaneously introducing fluorine and aluminum into the lithium manganese oxide compound, the first reversible capacity of up to 140mA·h/g can be obtained. After 300 cycles at 55°C, the capacity decay rate is 15%. Part of the lithium is substituted for manganese to synthesize an anisotropic compound. After the material is cycled 300 times, its reversible capacity is still 105mA·h/g, and the capacity decay rate is 12.5%. Sulfur and aluminum are introduced into the lithium manganese oxide compound, and it is found that the addition of sulfur can effectively suppress the Jahn-Teller effect. When the discharge cut-off voltage is 2.4V, the structure can still be maintained. The total reversible capacity of the 3V and 4V platforms is as high as 196mA·h/g, and there is almost no capacity degradation after 30 cycles. It has been reported that the lithium manganese oxide compound with magnesium ions was synthesized by the ultrasonic dispersion pyrolysis method. The material exhibits excellent stability during the cycle. When the added amount is 0.015, the first reversible capacity is 135mA·h/g, and after 100 cycles, it still remains at about 130mA·h/g. A lithium-rich spinel lithium manganese oxide compound was synthesized by adding a small amount of lithium ions, and satisfactory results were obtained. The reversible capacity of the material was still more than 100 mA·h/g after 100 cycles at 55°C. Good results were also obtained by introducing a small amount of fluorine into the spinel lithium manganese oxide compound. After 60 cycles, the reversible capacity hardly decayed and remained at about 110 mA·h/g.
(3) Surface modification
On the surface of the spinel, manganese has unpaired single electrons, and there are a large number of catalytic active centers, which can catalyze the oxidation of the electrolyte, cause the generation of hydrogen ions, accelerate the dissolution of manganese, and cause the material's reversible capacity attenuation. Wrapping a certain conductive material on the surface of the material can reduce its catalytic activity, reduce the specific surface area of the material, and effectively reduce the capacity attenuation of the material. If polypyridine is used for surface coating, after the material is cycled at 55°C for 120 times, its reversible capacity is still about 95mA·h/g. Some people use acetylacetone and borate to modify the surface of the material, and its specific surface area and catalytic activity are reduced. The reversible capacity is about 95mA·h/g after being cycled at 55°C for 150 times. The surface modification of the material with polyaniline greatly improves the cycle stability of the material at 50°C. After 20 cycles, the reversible capacity is still 110mA·h/g.
(4) Modified electrolyte
The trace water and trace acid in the electrolyte are the sources of hydrogen ions. Therefore, removing the trace water and trace acid in the electrolyte is an effective method to reduce the dissolution of manganese. Hamamoto et al. used molecular sieves to absorb trace amounts of water and HF in the electrolyte. The water and HF content in the electrolyte after treatment was less than 30 mg/kg, and the capacity retention rate was as high as 90% after 100 cycles of treatment. In addition, adding an appropriate amount of Lewis base is an effective way to destroy the activity of hydrogen ions. By adding a certain amount of Lewis base to the electrolyte, the high temperature cycle stability of LiMn2O4 is greatly improved. After 100 cycles at 55°C, the reversible capacity remains above 100mA·h/g.
In summary, the main reasons for the capacity degradation of LiMn2O4 at high temperatures can be summarized as: dissolution of manganese; electrolyte oxidation; electrode polarization; Jahn-Teller effect; structural changes. In view of the above reasons, researchers have adopted the methods of adding different ions, optimizing the content of conductive agent, reducing the specific surface area of the material, coating the surface of the material, and purifying the electrolyte to suppress the capacity decay of LiMn2O4 at high temperatures, and good progress has been made.