- 1. transition metal oxide
- 2. Transition metal vanadate
- 3. Sulfides
- 4. Phosphate
1. transition metal oxide
According to the different mechanisms of material deintercalation of lithium, transition metal oxides can be divided into two categories. The first type of material is lithium intercalation oxide in the true sense. The insertion of lithium is only accompanied by a change in the material structure without the formation of lithium oxide. . Representatives of this type are TiO2, WO2, MoO2, Fe2O3, Nb2O5, etc. This type of oxide usually has good reversibility of lithium deintercalation, but its specific capacity is low and the lithium intercalation potential is high. The second type of transition metal oxide is represented by MO (M=Co, Ni, Cu, Fe). The lithium intercalation of the material is accompanied by the formation of Li2O, but this is different from the inactive Li2O formed by the aforementioned SnO intercalation. The electrochemically active Li2O can be delithiated during delithiation of similar materials, thereby forming metal oxides again. For example, it is reported in "Nature" that the nano-sized transition metal oxide MO (M=Co, Ni, Cu, Fe) as a negative electrode material for lithium-ion batteries has good electrochemical performance.
TiO2 is the earliest metal oxide anode material that has been studied. It has three different crystal types. Only the two structures of anatase and rutile can insert lithium. Layered rutile has a close-packed hexagonal structure and is composed of needle-like particles; while anatase is composed of spherical particles and has good reversible lithium absorption and release performance. However, due to the influence of polarization, the TiO2 of these two structures is There is a potential hysteresis in the process of absorbing and releasing lithium.
Fe2O3 is used as a negative electrode material to form Li6Fe2O3, with a theoretical capacity of 1000mA·h/g. The research results of the electrochemical performance of α-Fe2O3 with different morphologies show that the cycle performance of the amorphous structure is worse than that of the nanocrystalline. Due to its small particle size and large specific surface area, it is easier for the electrolyte to decompose on its surface to form an SEI film, which hinders the adsorption of lithium on the α-Fe2O3 electrode and the electrochemical reaction.
P.Polzot et al. synthesized a series of nano-sized transition metal oxides Co3O4, CoO, FeO, NiO, etc. The study found that the nano-scale CoO and Co3O4 electrodes have a specific capacity of 700~800mA·h/g, and the cycle is 100 After the second, its capacity remains at about 100%. However, this type of 3d transition metal oxide belongs to the rock salt phase structure, and there are no vacancies for the insertion of lithium ions. Therefore, the lithium storage mechanism can be considered as that when charging lithium and the oxygen in the transition metal oxide junction all form Li2O, and when discharging Li2O is reduced to lithium again, and the transition metal oxide is regenerated. Taking CoO as an example, the main reaction process is as follows:
Transition metal oxide anode materials (Co3O4, Co0, FeO, NiO) have a high specific capacity of 600 ~ 1000mA·h/g, and they are also dense and can withstand higher power charging and discharging. However, the main disadvantage of this type of material is its higher working potential. In practical applications, the voltage of a battery composed of a positive electrode material is relatively low. For example, a battery composed of CoO and LiMn2O4 has an average voltage of only 2.2V.
Since both Fe2O3 and MO (M=Cu, Ni, Co) can store lithium, they are prepared into composite ferrite oxides (MFe2O4, M=Cu, Ni, Co), such as NiFe2O4 prepared by the template method Thin-film electrodes and PPY/NiFe2O4 composite thin-film electrodes exhibit a long low-voltage discharge platform at 0.05V, but the low-voltage platform cannot be maintained. The mechanism for this is also under discussion. Someone has studied the deintercalation characteristics of MnV2O6 and MnMoO4 oxides. Their reversible capacity is as high as 800~1000mA·h/g, but the cycle performance needs to be improved. After the material is first intercalated with lithium, its structure changes to an amorphous form, and it is difficult to explain such a high capacity simply by the change of the metal valence state, so the specific mechanism needs to be further studied.
2. Transition metal vanadate
Transition metal vanadate (M-V-O, M=Cd, Co, Zn, Ni, Cu, Mg) is used as a negative electrode material for lithium-ion batteries and exhibits high capacity at a low potential relative to lithium. When the voltage is lower than 0.2V, 7 Li can be inserted reversibly, reaching a capacity of 800~900mA·h/g, which is more than twice the capacity of the graphite electrode. In the first lithiation process, this electrode material will become amorphized, making the voltage composition curve of the first discharge (the voltage changes stepwise) and the second (the voltage is smooth and continuous) different . This is disadvantageous during actual battery use. Another type of vanadate RVO4 (R=In, Cr, Fe, Al, Y) is used as a negative electrode material for lithium-ion batteries, which can react with lithium at low voltages. Among them, InVO4 and FeVO4 have a reversible capacity as high as 900mA·h/g. Compared with crystalline materials, amorphous materials have better electrochemical performance. The current problem with this material is that the cycle performance still needs to be improved.
TiS2, MoS2 and other sulfides can also be used as anode materials for lithium-ion batteries, and can be matched with 4V-level cathode materials such as LiCoO2, LiNiO2 and LiMn2O4 to form batteries. This type of battery has a low voltage. For example, TiS2 is used as the negative electrode and LiCoO2 is used as the positive electrode to form a battery. The voltage is about 2V, and its cycle performance is better, which can reach 500 times.
Among the many nitrogen-containing materials, there are relatively many studies on the lithium storage performance of nitrides (to be described later) and antimonides. However, arsenic compounds are usually toxic, so there is no relevant report. Phosphorus is rich in resources and cheap. Phosphorus has a smaller atomic weight than antimony, and its lithium storage capacity is higher. In terms of phosphides, the phosphides studied include MnP4, CoP3, FeP2 and Li7MP4 (M=Ti, V, Mn), etc. system.
Mix the red phosphorus, metal manganese powder and tin powder in a certain ratio (Mn:P:Sn=1:10:6) in a glove box, transfer them into a quartz tube, and heat at 550~650℃ for two weeks under vacuum conditions. After 1+1 hydrochloric acid treatment, the MnP4 material was finally obtained. The study of the electrochemical performance of the material shows that the first lithium insertion curve of MnP4 presents a very flat voltage plateau around 0.62V, which corresponds to the insertion of 7 lithium. When the first delithiation reaches 1.7V, it is equivalent to the removal of 5 lithium, and the reversible delithiation capacity is about 700mA·h/g. After 50 cycles, the capacity stabilized at about 350mA·h/g. The process of deintercalating lithium is:
The process of releasing lithium is accompanied by the breaking and recombination of P-P bonds.
Some people seal red phosphorus and metallic cobalt in a stainless steel tube filled with argon in a certain ratio (Co:P=1:3), and heat it at 650°C for 24 hours to obtain CoP3 material. The charge and discharge curve of CoP3 is shown in Figure 1.
Figure 1 Charge and discharge curve of CoP3
For the first time lithium insertion has a voltage plateau around 0.35V, which corresponds to the insertion of 9 lithium. The first delithiation to 1.7V is equivalent to the removal of 6 lithium, the reversible delithiation capacity is greater than 1000mA·h/g, after 10 charge and discharge cycles, it decays to 600mA·h/g, and the final capacity stabilizes at 400mA·h/ g around. Studies have shown that the mechanism of CoP3 deintercalation of lithium is completely different from that of MnP4. The first intercalation of lithium is accompanied by the formation of metallic cobalt and lithium phosphide Li3P, while the subsequent deintercalation process is carried out between Li3P and LiP. The valence of cobalt The state has not changed.
The electrochemical performance of phosphides is usually that the first few times the lithium deintercalation capacity is relatively high, but the cycle performance and the first charge and discharge efficiency are relatively low. Their mechanisms for deintercalating lithium are different, and phosphides have one thing in common: the change in the valence state of phosphorus plays a major role in maintaining the charge balance of the system when deintercalating lithium.