What new technologies support the high performance of lithium batteries?

　　In addition to increasing the diameter of lithium-ion batteries, another effective way to increase the specific energy of lithium-ion batteries is to reduce the thickness of the separator. The thickness of the currently common PP-PE-PP three-layer composite separator generally reaches more than 30um, reaching the positive and negative electrodes. About 20% of the thickness of the film, which also results in serious waste of space. In order to reduce the space occupied by the separator, currently the majority of lithium-ion battery manufacturers generally use thin separators with coatings. The thickness of these separators can reach less than 20um. On the premise of ensuring the safety of lithium-ion batteries, it can significantly reduce the volume proportion of the separator, increase the proportion of active materials, and increase the specific energy of lithium-ion batteries.

　　Another way to increase the proportion of active materials is to start from the perspective of the battery production process. First, increase the proportion of active materials in the electrode. Generally, the electrode of a lithium-ion battery is mainly composed of four parts, active material, conductive agent, binder and current collector. In order to increase the proportion of active materials, it is necessary to reduce the proportion of other parts. By using new conductive agents and binders Thereby reducing the proportion of conductive agent and binder, and using thinner current collectors to reduce the proportion of inactive substances. Secondly, it is necessary to increase the coating amount of the positive and negative electrodes, but there is also a problem when increasing the coating amount of the electrodes: when the electrode is too thick, the Li+ diffusion kinetic conditions of the electrode will become worse, affecting the rate and cycle of the lithium-ion battery. Performance, in order to solve this problem, BorisBitsch et al. [1] of Karlsruhe Institute of Technology in Germany used capillary suspension and multi-layer electrode processes to prepare high-performance thick electrodes with gradient porosity. In the lower layer close to the copper foil, BorisBitsch and others used ordinary slurry, which makes it have lower porosity and good conductivity. On the surface layer far away from the copper foil, BorisBitsch used capillary suspension slurry and applied it to the lower layer. The addition of 1-octanol significantly increases the porosity and improves the kinetic conditions of the electrode. As a result, the porosity of the electrode gradually increases from bottom to top, significantly improving the kinetics of thick electrodes. Conditions improve the electrochemical performance of thick electrodes, thereby achieving an increase in the weight and volume specific energy of the battery without reducing the cycle performance of the battery.

　　Another important method to improve the specific energy of lithium-ion batteries is to control the amount of electrolyte. Reducing the amount of electrolyte can effectively increase the energy density of lithium-ion batteries. The electrolyte plays a role as a medium inside the lithium-ion battery. Li+ in the positive and negative electrodes diffuses through the electrolyte. Therefore, the electrolyte is theoretically a "non-consumable product". As long as there is a small amount of electrolyte, the Li+ in the positive and negative electrodes can be ensured. Free diffusion between the negative electrodes is enough, but in fact due to the formation of the SEI film during the formation process, the electrolyte decomposes, and the electrolyte decomposition caused by SEI film damage and positive electrode oxidation during the cycle causes the electrolyte to actually It is continuously consumed, so the electrolyte in the battery is generally excessive, which is also an important reason for the low specific energy of lithium-ion batteries. In order to reduce the amount of electrolyte and ensure the performance of the battery, we need to control the electrolyte The solvent system and electrolyte additive system are improved to improve the stability of the electrolyte. In order to improve the stability of the electrolyte in the ternary material NMC battery, Yunxian Qian et al. [2] from the University of Münster in Germany added a small amount of electrolyte to the electrolyte using traditional EC and EMC (weight ratio 3:7) as solvents. FEC additive, it is found that FEC additive can effectively reduce the decomposition of electrolyte, improve the first Coulombic efficiency of NMC battery, and significantly improve the cycle stability of the battery.

　　2. Selection of positive and negative active materials

　　As our requirements for the energy density of lithium-ion batteries continue to increase, traditional LiCoO2 materials have physically met the needs of high-specific-energy lithium-ion batteries. In order to further improve the energy density of lithium-ion batteries, we have two general directions to choose from: 1) Increase the working voltage of lithium-ion batteries; 2) Increase the capacity of positive and negative electrode materials. First, let’s analyze the feasibility of improving the energy density of lithium-ion batteries from aspect 1). The operating voltage of lithium-ion batteries depends on the voltage difference between the positive and negative electrode materials. At present, the voltage of the negative electrode material graphite is already very low (about 0.1 VvsLi+/Li), there is not much room for further reduction. Improving the voltage of lithium-ion batteries mainly starts from the cathode material. Currently, the high-voltage materials available for selection mainly include the following categories:

　　(1) The high-voltage spinel lithium nickel manganate LiNi0.5Mn1.5O4 material has an operating voltage of up to 5.0V, a voltage platform of around 4.7V, a theoretical capacity of 147mAh/g, and an actual capacity of more than 138mAh/g. . (2) Olivine-based high-voltage materials, such as LiMnPO4 and LiCoPO4 materials, among which the voltage platform of LiMnPO4 material can reach about 4.1V, and that of LiCoPO4 material can reach about 4.8V. (3) Lithium-rich materials. The theoretical capacity of lithium-rich materials can reach more than 200mAh/g, or even 300mAh/g. However, in order to take advantage of the high capacity of lithium-rich materials, its working voltage needs to be increased, even to about 4.8V. .

　　In addition to solving their own problems, these high-voltage materials also face the same problem: the instability of the electrolyte at high voltage. Currently, commercial lithium-ion battery electrolytes are generally based on carbonate-based organic electrolytes. The solvent salt is LiPF6, which also causes the electrolyte to be easily oxidized and decomposed at high potentials, resulting in serious degradation of battery performance and even safety issues. In order to overcome this problem, we can proceed from two aspects. The first is the electrolyte solvent system. In order to improve the electrochemical stability window of the electrolyte, more stable ionic liquid electrolytes and new electrolyte salts can be used. On the other hand, in order to reduce the oxidation of the electrolyte by the high-voltage material, the surface of the high-voltage material can be coated to isolate the electrolyte and the active material. DongruiChen et al. [3] of South China Normal University used Li3PO4 to surface-coat lithium-rich layered materials. Li3PO4 coating significantly improved the cycle performance of lithium-rich materials, reduced the dissolution of transition metal elements, and inhibited the layering process. transformation from a spinel-like structure to a spinel structure. Another important aspect of improving the specific energy of lithium-ion batteries is to increase the specific capacity of positive and negative active materials, which requires both positive and negative electrode materials. In terms of cathode materials, the high-capacity cathode materials that we can choose from mainly fall into the following two categories: 1) ternary materials NCM and NCA; 2) lithium-rich materials.

　　Ternary materials are currently the most mature high-capacity cathode materials. As the Ni content increases, the specific capacity of ternary materials will also increase accordingly. For example, the high-nickel NCM811 material has a specific capacity of about 200mAh/g. The specific capacity of high-nickel NCA materials can also reach about 190mAh/g, which is much higher than that of LiCoO2 materials. Lithium-rich materials are newly developed high-capacity cathode materials in recent years. Their specific capacity can reach more than 200mAh/g, or even 300mAh/g. However, lithium-rich materials are currently relatively rare in the market. The main reasons are as follows. Points: 1. High irreversible capacity; 2. Voltage decay; 3. Poor cycle performance. Improving its performance requires aspects such as element doping and surface coating, as well as material structure design.

　　In terms of high-capacity anode materials, we mainly have the following options: 1) silicon-based materials; 2) N-doped graphite materials; 3) transition metal Side materials; 4) metallic lithium anodes. Needless to say, silicon anode material is the most mature and reliable high-capacity anode material on the market. The specific capacity of crystalline Si can reach more than 4200mAh/g, but it has large expansion and poor cycle performance. Although SiOX has a slightly lower capacity (1500mAh/g) g), but the cycle performance is excellent, but the disadvantage is low first efficiency. N-doped graphite materials have been a research hotspot for high-capacity anode materials in recent years. The electronegativity of N atoms is about 3.5. After incorporating N elements into graphite, the specific capacity of the graphite anode can be significantly improved. According to research from Wuhan University KaifuHuo et al. [4] used the template method to prepare N-doped mesoporous carbon hollow sphere materials. At a current density of 0.1A/g, the specific capacity can reach 931mAh/g. At a current density of 0.5A/g, the cycle time is 1100. Therefore, the specific capacity of 485.7mAh/g can still be maintained.

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