When the charging voltage is higher than 4.3V, the reversible capacity of LiMO2 (M=Co, Ni, Mn) layered oxide will decrease significantly. This is caused by the dissolution of transition metals and the ion exchange between lithium ions and transition metal ions. The release of lithium in LixMO2 caused surface damage and structural collapse, causing safety problems. Due to the dissolution of manganese and the Jahn-Teller effect, the capacity of cubic spinel LiMn2O4 will also decrease during charge and discharge cycles. LiFePO4 has structural stability and chemical stability, but its electronic conductivity and ion conductivity are low.
Various surface modification methods are used to solve these problems, and different methods and materials can be used for different purposes. For example, in order to increase the reversible capacity of LiCoO2, metal oxides, metal fluorides or mixed metal oxides are used as coating materials to enhance the high-pressure stability of the material. This method suppresses the dissolution of transition metals or improves the surface stability of the material, so that the material can be charged and discharged under high voltage, thereby obtaining a larger capacity. In LiMn2O4, the dissolution of metal ions is controlled by embedding two different elements or surface modification, thereby improving structural stability. Surface modification can also improve cycle efficiency and thermal stability during high-rate discharge, increase capacity, increase power and extend life. Compared with other oxides, LiFePO4 dissolves few metal ions and has good high voltage stability. Surface modification is an effective improvement method for its low electronic conductance and ion conductance. Therefore, conductive materials such as carbon or nano metal particles are used as surface modification materials for LiFePO4.
However, the problems caused by surface modification must be solved. For example, the addition of a new material may reduce the specific capacity: the use of low ionic conductivity materials will hinder the movement of lithium ions during charging and discharging: the reduction of the surface area of lithium ions inserted and released by the material will reduce the high rate performance of the material. At the same time, from the perspective of manufacturing cost, the precursor surface coating process increases the cost.
①Layered structure compound
The surface of LiCoO2 is coated with a chemically stable material, which can significantly improve the charge-discharge cycle performance. During the charging and discharging process, lithium is extracted from the layered LixCoO2. As the product structure changes, the internal energy of LixCoO2 increases. In the overcharged state, in the unstable LixCoO2, the cobalt ions react with the electrolyte to cause the cobalt to dissolve. Ran noodles. The surface-modified LiCoO2 can inhibit the elution of cobalt even at a high voltage above 4.3V and a high temperature above 60°C.
Figure 1 shows the charge and discharge characteristics of various oxide-coated LiCoO2 at 2.75~4.4 V. It can be seen that when using ZrO2 surface modification, the reversible capacity changes little. According to the different coating materials, the degree of cycle performance improvement is increased in the following order: B2O3<TiO2<Al2O3<ZrO2, which is closely related to the robustness of these materials. Generally, the coating layer can only cover a part of the surface of the positive electrode material instead of the entire surface, and the leaching of transition metal can be prevented by adjusting the surface energy to be lower than or equivalent to the coating layer.
For LixNiO2 (x<0.5) in the delithium state, the instability of the surface structure caused by overcharging will cause the battery temperature to rise and cause safety problems. Similar to LiCoO2, Li2O2B2O3, MgO, AIPO4, SiO2, TiO2, ZrO2 and other materials can be used for surface coating to improve electrochemical performance. Surface modification can improve the structural stability of the material, increase the activation energy, and inhibit phase transition. From the electrochemical performance curve of ZrO2 coated LiNiO2 shown in Figure 2, it can be seen that the initial charge-discharge curve of the material is stable and the cycle performance is improved.
Similar results can be obtained by similar surface modification of LiNi 0.8Co0.2O2, which is mainly nickel. Surface coating with chemically and thermally safe materials reduces the reaction area and increases the interface stability, thereby improving the thermal stability of the material. This result is easily confirmed by thermal analysis methods such as DSC (differential scanning calorimetry). Figure 3 shows the thermal analysis results of CeO2-coated LiNi0.8Co0.2O2 when it is charged to a high voltage. As the starting position of the exothermic peak increases, its intensity decreases significantly.
For ternary cathode materials, related research is underway to obtain high capacity and stable deep delithiation state at the same time through surface modification. With AIF3 coating, good cycle characteristics and battery stability under high voltage can be obtained. Figure 4 shows the corresponding 2 ~ 3 nm uniform and dense coating.
② Spinel compound
Spinel LiMn2O4 has the advantages of environmental protection, low price, stable structure and good rate performance. Surface modification is used to improve its shortcomings of poor electrochemical performance at high temperatures. Although the charge, discharge and cycle performance of LiMn2O4 are excellent at room temperature, Mn ions dissolve from the electrode, the JahnTellerer effect of Mn3+, and the oxidation reaction between the particle surface and the electrolyte will significantly reduce the performance of LiMn2O4. Due to these questions
The problem is related to the surface electrochemical reaction and the valence of Mn, so the surface modification can be used to inhibit the reaction between the material and the electrolyte.
In order to reduce the Mn3+ concentration on the LiMn2O4 spinel surface, AI, Mg and Li elements with different valences can be used at the same time. Surface modification is particularly effective in preventing performance deterioration caused by the electrochemical reaction between the electrolyte and the LiMn2O4 particles at high temperatures. For example, the valence of Mn in LiMn2O4 increases with the progress of charging. Even if the active material remains unchanged, the unstable Mn3+ and the decomposition of the electrolyte will rapidly deteriorate battery performance. Figure 5 shows the cycle performance of LiMn2O4 coated with various oxides. LiCo1/2Ni1/2O2 coated LiMn2O4 can maintain up to 97.2% (110 mAh/g) reversible capacity after 100 cycles at 60°C.
Other measures to suppress surface reaction while improving rate performance have also been developed, such as the use of high-conductivity materials such as ITO (Indium Tin Oxide). However, the increase in the average valence of Mn caused by doping or the use of surface coating will reduce the mass specific capacity of LiMn2O4.
③Olivine type compound
The surface modification of LiFePO4 is mainly to improve the electrical conductivity of the material. The carbon precursor and the active material particles are mixed, and then heat-treated, a thin carbon layer with a thickness of tens to hundreds of nanometers can be formed on the surface of the particles. The carbon layer can increase the conductivity of LiFePO4, thereby improving its rate characteristics. However, the increase in the surface conductivity of the particles does not improve the internal conductivity and lithium ion conductivity of the particles. Therefore, in order to shorten the lithium ion diffusion path, the particle size needs to be minimized. The small particle size can reduce the diffusion distance of lithium in the particles and increase the reaction specific surface area. As shown in Figure 6, the use of small particles of carbon coating can obtain a capacity of more than 95% of the theoretical capacity. Substituting silver for carbon can achieve a similar effect, but at a higher cost. The use of nanoparticles and carbon coating has become a practical application technology for the commercialization of olivine-type compounds.