During the charging and discharging process, the negative electrode material composed of metal oxide has two different behaviors when it reacts with lithium. One is to perform lithium intercalation/deintercalation while maintaining the crystal structure, and the other is the decomposition of oxides during the lithium reaction. Some typical anode materials belonging to the latter are transition metal oxides with a rock salt structure, such as CoO, NiO, FeO, and TiO2. These oxides have a large irreversible capacity and a high average discharge voltage. The voltage is 0.8~2.0v, depending on their own structure. TiO2 oxides have multiple phases, such as anatase, rutile, and TiO2 rhombic manganese. Ore and TiO2﹣B.
Titanium dioxide has a body-centered cubic (I41/amd) structure, with a lattice constant of a=3.782Å, c=9.502Å, and a density of 3.904 g/ml. LixTiO2 can be obtained through an electrochemical reaction, and x changes reversibly in the range of 0.0 to 0.5. The chemical equation is as follows:
TiO2 +xLi＋ +xe﹣ = LixTiO2, (x=0~0.5)
For anatase TiO2, the insertion and extraction reactions occur in the plateau area, but there is no electrolyte decomposition in the reaction between TiO2 and lithium, so the charge and discharge are performed in the curved area. In Li0.05TiO2 with tetragonal and orthorhombic crystal structure, such intercalation and deintercalation reactions are two-phase equilibrium reactions, and the electrochemical potential at this point is 1.8 v (Li/Li+). Although the Li0.5TiO2 material has a specific capacity as high as 200 mAh/g, the average voltage of the intercalation reaction between TiO2 and lithium on the particle surface is 1.8V, resulting in a higher irreversible capacity, so the material is still in application. restricted. Unlike carbon-based materials, Li0.05TiO2 materials have low electronic conductivity, so there is a significant potential difference between the charge and discharge curves. It can be observed from Figure 1 that there is a large polarization resistance, so this material is very useful in areas that require low voltage and low current density.
The rock salt structure of rutile TiO2 is the same as that of LiTiO2, and it has electrochemical activity. However, it reacts slowly, and the volume expansion during charging is 4.5%. The average working voltage of rutile TiO2 is similar to that of anatase TiO2, but it has a sloping potential curve. Figure 1 shows the charge and discharge curves of anatase and rutile TiO2. In the early stage of charging, the initial open circuit voltage of TiO2 is 3.0 V, but during charging, TiO2 reacts with lithium ions and then rapidly drops to 1.8 V. Because the voltage of lithium intercalation oxide is higher than the decomposition voltage of the electrolyte, SEI will not be formed. membrane. Unlike carbon materials, the lack of coating on the surface of the particles allows TiO2 materials to have better output characteristics. Nano-scale particles can obtain high-rate discharge performance by increasing the specific surface area. Despite this, the electronic conductivity and lithium ion diffusivity of the material itself are still quite low.
Transition metal MO (M:Co, Ni, Fe, etc.) has a rock salt structure and reacts with lithium to form Li2O and nano-metals through oxidation and decomposition. The formed nano metal is dispersed in Li2O. For CoO, the reaction equation is as follows:
The reversible reaction to form CoO from the continuous phase of Li2O is based on nano-metals with high surface energy and good dispersion. If the transition metal oxide is of nanometer size, these reversible reactions may be realized. As shown in Figure 2, the lithium reaction potential in the above-mentioned reversible reaction is higher than 0.8V, and the voltage difference between the charge and discharge curves is large. This is because the continuous phase of Li2O acts as an insulator, resulting in a rapid decline in electronic conductivity. Even if the nano metal in the Li2O continuous phase is electrochemically active, the current is still interrupted by the insulator, resulting in a larger polarization resistance, and the voltage difference between the charge and discharge curves may cause overheating in the battery. . However, these metal oxides can achieve high energy density, so it is necessary to study some technologies to solve the problem of poor electronic conductivity.
Similar to the case of using metal oxides as the positive electrode, lithium titanium oxide, such as Li(Li1/3,Ti5/6)O4, has electrochemical activity to lithium, has a specific capacity of 175 mAh/g, and is as high as 1.5 V ( The potential of Li＋/Li). (Li1/3 Ti5/6) O4 is considered a zero-strain material, that is, there is no change in the crystal lattice before and after charging. In the general form of Li4Ti5O12, both Li and Ti are located in the 16d octahedral gap, while the remaining Li is located in the 8a tetrahedral gap. Li1(Li1/3 Ti5/6)O4 and Li2(Li1/3 Ti5/6)O4 have the same Fd-3m(227) space group, and their character constants are 8.3595Å and 8.3538Å, respectively. The reduction process is accompanied by volume reduction It is very weak, only 0.0682%. The electrochemical reaction is as follows:
Li1(Li1/3 Ti5/6)O4+ Li＋＋e﹣= Li2(Li1/3 Ti5/6)O4
Figure 3 shows the typical charge and discharge curves of Li4Ti5O12 as a negative electrode material. Because of the two-phase reaction, there is a plateau voltage region. The efficiency of the first charge and discharge is close to 100%, because the high working voltage prevents the formation of the negative SEI film caused by the decomposition of the electrolyte. This material can be used for high output characteristics.
It can be seen from the platform charge-discharge potential curve that nanoparticles should be used to minimize the migration path to overcome the problem of low lithium ion diffusion coefficient. It can be seen from the commercialized Li4Ti5O12 particles in Figure 4 that the nano-sized primary particles agglomerate to form secondary particles.
The preparation of the electrode slurry of nano-sized particles requires a large amount of solvent, which will reduce the yield of the electrode. In addition, nano-sized particles are very sensitive to moisture. If the particles are exposed to the air, they will absorb too much moisture. This not only hinders the manufacturing process of the electrode, but also deteriorates the performance of the battery. When the moisture content in the electrode increases, the hydrogen and oxygen in the battery will decompose to release gas, which affects the performance of the battery. This problem can be solved by introducing additional processes to control nano-scale particles, or by polymerizing primary nano-particles as electrode active materials, then this problem will also be solved.
The specific capacity of Li4Ti5O12 is relatively low, but the rate performance is good, so it has also been extensively studied for use in HEV batteries. Figure 5 shows the cycle life characteristics of a battery with Li4Ti5O12 as the anode and spinel LiMn2O4 as the anode under high-rate conditions. We can find that when the charge rate is 2C and the discharge rate is 10C and 20C, the cycle life characteristics of the battery are very stable. When Li4Ti5O12 is used as a negative electrode material, it will greatly help improve battery life characteristics. Since the decomposition voltage of the electrolyte is not within the operating voltage range of the battery, the formation of SEI film can be effectively avoided. In addition, by using nanoparticles with rate characteristics, the battery can achieve high rate performance.
②Nitride anode material
Li2 (Li1-xMx) N anode (M=Co, Ni or Cu) is a typical nitride anode material, which has a layered structure and high ion conductivity. Figure 6 shows the crystal structure of Li2.6Co0.4N with P6/mmm space group, with a lattice constant a=3.68Å, c=3.71Å, and a density of 2.12 g/ml, which is very similar to graphite.
Figure 7 compares the charging and discharging curves of Li2.6Co0.4N electrode and the current graphite electrode. The charging and discharging of Li2.6Co0.4N occurs in the potential range of 0~1.4 V, and shows a potential of up to 800 mAh/g. Channel capacity. The capacity of this material is more than twice that of graphite, and it also has superior cycle performance. In the half-cell, the discharge potential of Li2.6Co0.4N to lithium (0.7-0.8 V) is much higher than that of graphite. When lithium is released in the early stage, the crystal of the material will be transformed into an amorphous state, which will affect the charge and discharge characteristics. Due to their high sensitivity to moisture, nitride materials face many limitations in many applications. In a lithium secondary battery, the positive electrode will receive the lithium ions first released from the nitride negative electrode. Unlike traditional batteries, batteries using nitride anode materials do not require charging for the first time.
Figure 8 shows the rate characteristics (a) and cycle life characteristics (b) of a lithium secondary battery with Li2.6Co0.4N as the negative electrode, and LixCoO2 that has been sloughed out is used as the positive electrode. Compared with the rate of C/10 (30 mA), the battery capacity at 1C rate is 96%. It shows excellent cycle characteristics, even after 200 cycles, the battery capacity remains at 100%. However, it is still difficult to determine whether Li2.6Co0.4N can be used in commercial batteries from the results in Figure 8 b, because the rate characteristics of this material in Figure 8 b are relatively low. The Li2.6Co0.4N material should first undergo a discharge reaction to release lithium, because it has already stored lithium during the preparation process. If this material is mixed with a negative electrode material with high irreversible capacity, the capacity of the entire battery will likely be improved.