Sodium ion battery cathode material development
In recent years, with the rapid development of portable electronic devices, electric vehicles and hybrid vehicles, research resources, energy-efficient and environmentally friendly energy storage materials have become an international research hotspot. In order to meet the huge market demand, it is not enough to rely on the electrical performance of the battery to measure the battery material. The safety of the battery, the manufacturing cost, the energy consumption and whether it pollutes the environment have become an important indicator for evaluating the battery material. At present, the development prospects of lithium-ion batteries are relatively clear, but with the excessive demand for lithium resources, it is bound to face a shortage.
The development of sodium-ion batteries is mainly to solve the contradiction between the huge demand for power batteries and the scarce energy of lithium. As we all know, the explosion of automobile production capacity has caused the price of lithium resources to skyrocket, from 30,000 yuan/t in 2014 to a maximum of nearly 200,000 yuan/t. In addition to lithium, lithium batteries use another rare metal, cobalt (Co). According to a survey conducted by the NTT Facilities Research Institute, the production of a pure electric vehicle (EV) using the current technology requires approximately 20 kg of lithium and approximately 40 kg of CoCo. Even if global production is supplied to EVs, the annual output of lithium is only enough for 7 million vehicles, and cobalt is only enough for 1 million vehicles. According to the national new energy automobile industry plan, there will be at least 30 million new energy vehicles in China in the future in 2030. From the current 300,000 to 30 million, the scarce energy such as lithium and cobalt will inevitably face resource depletion and price. Soaring. Sodium, the second light metal element second only to lithium, has an abundance of 2.3% to 2.8%, which is 4 to 5 orders of magnitude higher than lithium. In the future, once lithium resources are depleted, sodium-ion batteries will hopefully replace them.
2. Research status of sodium ion battery
As early as the 1970s and 1980s, sodium-ion batteries with the name "post-lithium battery" have been proposed, starting with lithium-ion batteries almost simultaneously, but with the successful commercialization of lithium-ion batteries, sodium-ion battery research gradually Was diluted. In addition, at the time, the researchers simply applied the electrode materials successfully applied on lithium-ion batteries to sodium-ion batteries, and did not consider the difference in the lattice structure requirements of sodium-ion batteries and lithium-ion batteries, resulting in almost all attempts. The failure ended. In recent years, on the one hand, researchers have realized that lithium resources are brought about by the large-scale application of lithium-ion batteries. On the other hand, researchers have also designed electrode materials from the specificity of sodium-ion batteries, and have obtained many good results. The sodium ion battery has become a research hotspot again.
After the development competition in recent years, the storage energy of sodium-ion batteries reaches 90% of lithium, which can be side by side, and a few companies have begun preliminary research and development and application. For example, at the international level, researchers at the National Center for Scientific Research in France have developed a prototype of a 18650 battery that is comparable in capacity and time to some lithium-ion batteries. In 2013, Sumitomo Electric Industries developed a sodium-ion battery that works even when the internal temperature of the battery is low. Because there is no need for heat dissipation, the volume has been successfully reduced to below the lithium battery. The goal is to apply to residential batteries and pure electric vehicles. Toyota Battery Research Division also announced the development of new materials for the positive electrode of sodium ion batteries at the Battery Technology Committee of the Japan Electrochemical Society held in May 2015. Mitsubishi Chemical has also been conducting research on sodium-ion batteries with Tokyo University of Science. Domestically, Maike Lithium (Jiangsu) Co., Ltd. has made breakthroughs in the production of sodium ion battery materials and platform construction. Shenzhen BAK Battery Co., Ltd. also announced that the development of sodium ion battery has entered the pilot stage, and the technical improvement of the low energy density of sodium ion battery will be continuously improved.
III. Characteristics of sodium ion battery
Sodium and lithium belong to the same main family, and many physical and chemical properties are similar, which also determines the possibility of sodium ion battery research and development. Compared with lithium-ion batteries, sodium-ion batteries have two major advantages: First, the cost of raw materials is low, and high-priced rare metals such as lithium and cobalt are not used. The biggest advantage of sodium is that it is rich in resources such as seawater, and it is “inexhaustible”. The second element is that the existing production process can be used. The working mechanism of the sodium ion battery is the same as that of the lithium battery. The existing production equipment of the battery enterprise can be directly used to produce the sodium ion battery, because the equipment investment is basically unnecessary, so each It is easy for companies to produce them as replacement batteries. The biggest problem facing sodium ion batteries to date is the low energy density and power density, which is the biggest problem that limits their future commercial application.
4. Structure and properties of cathode materials for sodium ion batteries
For sodium-ion batteries, research on cathode materials can be described as a hundred schools of thought. The positive electrode material is not only a battlefield for improving the performance of sodium ion batteries, but also a major bottleneck limiting the cost of sodium ion batteries. At present, there are many reports on the layered cathode materials of sodium ion batteries, but most of them contain transition metal nickel (Ni) or Co elements, while Ni and Co are widely used elements in lithium ion battery cathode materials, and are used in sodium ion batteries. The cost reduction space is limited, so Ni and Co are not the preferred elements for sodium ion battery cathode materials; and these materials are unstable in air, easily absorb water or chemically react with water-oxygen (carbon dioxide), which will undoubtedly increase the production of materials. , transportation and storage costs, and will have an impact on battery performance. Therefore, in order to realize the practical application of the sodium ion battery, it is necessary to develop an active element which can replace Ni or Co and a stable new electrode material.
In view of the large-scale application of lithium iron phosphate LiFePO4 in lithium ion batteries, sodium iron phosphate NaFePO4 is naturally a preferred cathode material for sodium ion batteries. The olivine-structured NaFePO4 has the largest theoretical specific capacity in all phosphate-based sodium ion battery cathode materials, 154 mAh/g, as shown in Table 1. In NaFePO4, Na+ occupies the Wychoff position of 4(c), and Fe2+ occupies the 4(a) position, similar to olivine-type LiFePO4, and its crystal structure is shown in FIG. Oh et al  found that the working voltage of Na/NaFePO4 half-cell is 2.7V. Under the charge-discharge rate of 0.05C and the charge-discharge rate of 0.5C, the specific capacity is stable at 125mAh/g and 85mAh/g, respectively. After 50 cycles, XRD results show that the olivine structure is still good, indicating that the material has excellent stability in the process of sodium disodium.
Compared with other sodium ion battery cathode materials, NaFePO4 has a high theoretical capacity, but the research of this material has not been sufficient so far, which is mainly limited by the difficulty of its synthesis method. NaFePO4 synthesized by common solid or liquid phase methods is a chemically inert pyrite structure, not an active olivine structure. Therefore, in the future, the research on NaFePO4 must make a breakthrough in the synthesis method, so that it can be applied to the large-scale application of sodium ion storage batteries.
2. NASICON structure Na3V2(PO4)3
The NASICON structure is a sodium ion superconductor structure with a large three-dimensional channel structure for rapid deintercalation of sodium ions. NASICON type phosphate materials have high working voltage, good structural thermal stability, and can improve their capacity and rate performance through carbon coating and doping. It is considered to be the most industrial stage of sodium ion development. Positive electrode materials for application prospects. At present, Na3V2(PO4)3 is used as a representative material, and the material belongs to a hexagonal crystal system, and the space group is R-3c. Figure 3 is a crystal structure diagram of Na3V2(PO4)3 , whose crystal structure is composed of each VO6 octahedron connected by three O2 tetraatoms through a common O atom, wherein Na+ has two occupied sites: Na1 and Na2. Among them, there is one Na+ at the Na1 position and two Na+ at the Na2 position, and two Na+ at the Na2 position are first deintercalated during charge and discharge.
At present, methods for synthesizing Na3V2(PO4)3 include solid phase method, sol-gel method, hydrothermal method, and carbothermal reduction method. The most common one is the high temperature solid phase method. Although the method is simple, the temperature control is troublesome. In addition, the preparation method has a long preparation period and cannot control the particle size of the material, and the agglomeration phenomenon of the prepared material is more obvious, and the performance of the material is greatly affected.
The sol-gel method enables mixing at the molecular level of the raw materials. The solution is formed by dispersing colloidal particles having a diameter of 1 to 100 nm in a solution. After forming a gel, the solution has a unique network structure in the precursor solution, so that the prepared product has a uniform particle size distribution, small particle size and uniform distribution. However, the preparation method has a long preparation cycle, complicated operation and many influencing factors, so it is difficult to realize industrial application.
Shen et al. improved the conductivity of Na3V2(PO4)3 by sol-gel method to achieve nitrogen-doped carbon coating and composite carbon nanotubes. The conductivity of the modified composites was significantly improved. According to its electrical performance test, as shown in Figure 4, the material has a similar charge-discharge curve to LiFePO4, and its voltage platform is 3.4V. When discharging at 0.2C and 70C, the specific capacity can reach 94mAh/g and 70mAh/g, the capacity retention rate can reach 86% after 300 weeks of 30C cycle.
In the positive electrode of sodium ion battery, although Na3V2(PO4)3 is relatively mature and has excellent structural stability, its theoretical specific capacity is low, only 118mAh/g, and it can only be applied to larger batteries in the future. At the same time, vanadium ions have certain toxicity and have certain limitations for future industrial production.
Sodium-ion batteries have a similar working principle as lithium-ion batteries, but the development of lithium-ion batteries is relatively mature. At present, the preparation of the corresponding sodium ion battery cathode material by using the relevant experience of the positive electrode of lithium ion battery has become a main research method, and shows a good battery performance to some extent. However, there are several key problems in the development of sodium-ion batteries. First, sodium-ion batteries are a battery system different from lithium-ion batteries. The positive electrode materials of lithium-ion batteries are used to develop positive electrodes for sodium-ion batteries. The material is a shortcut. The known sodium ion battery cathode material has more or less problems. Looking for a new sodium ion battery cathode material with high energy density and power density is the improvement of sodium ion battery performance. The important way is also the key to the early application of sodium ion batteries to large-scale energy storage. Second, by doping metal ions and conductive agents, controlling particle size, and developing simpler and more efficient synthesis methods, the electrochemical properties of the positive electrode materials are also significantly improved. Thirdly, the development of a negative electrode material, an electrolyte solution and a separator which are matched with a positive electrode material is also an urgent problem to be solved before the industrialization of the sodium ion battery.
There are many kinds of materials that can be used for the positive and negative electrodes of sodium ion batteries, and it is imperative to carry out the development of sodium ion batteries in an emergency. Precisely, in the near future, high energy density, high power density, high conductivity and cyclic electrode materials will continue to emerge. By then, it will really be possible to apply sodium-ion batteries to large-scale energy storage, adding a touch of color to the enduring topic of the “energy” of the entire human world!