Due to the high requirements of the electric vehicle market for lithium-ion power batteries for vehicles in terms of energy density, cycle life, output power and safety performance, the energy density, life and safety of batteries have become the main research and development directions of lithium-ion power battery technology. In practical applications, it is difficult to take into account the energy density index and safety. At present, there is still a competition between lithium manganate (including ternary materials) and lithium iron phosphate materials in the field of power batteries. Theoretically, manganese-based batteries have higher energy density, but slightly worse cycle life and safety, while lithium iron phosphate materials have good safety but low energy density. From the perspective of industrial application and promotion, lithium manganate system materials occupy a certain advantage at this stage, but it cannot be ignored that China's lithium iron phosphate battery industry is very large, and greatly exceeds the production capacity of lithium manganate system batteries.
Lithium-ion battery technologies using lithium manganate, ternary materials and lithium iron phosphate as cathode materials are all relatively mature and have achieved large-scale applications. The U.S. Department of Energy invested US$5 million in research on lithium-sulfur batteries in 2010, and continues to increase investment, planning to develop lithium-sulfur battery products with an energy density of 500Wh/kg. Japan also regards lithium-sulfur batteries as one of the main research directions of its own vehicle power battery technology, and plans to develop products with an energy density of 500Whkg by 2020, and the energy density target of lithium-sulfur batteries defined by German research is to reach 1000Wh/kg. At present, under the influence of the governments of the United States, Japan and Germany, China advocates vigorously developing lithium-sulfur batteries.
Sulfur is ubiquitous in nature and is an underutilized natural resource. Sulfur in nature mainly exists in the form of elemental sulfur (S) which is thermodynamically stable at room temperature. Elemental sulfur has the characteristics of low toxicity, low price, large stock and low density. In particular, sulfur-lithium material has high theoretical energy density and is currently known as the cathode material with the highest specific capacity. The higher specific capacity and energy density mean that lithium-sulfur batteries can achieve smaller cell mass and volume while having strong discharge capacity. But at the same time, lithium-sulfur batteries have encountered some problems in the research and development, mainly including the following aspects.
(1) The lithium metal of the negative electrode reacts with the sulfur dissolved in the electrolyte. Elemental sulfur gradually shrinks and forms polysulfides in the positive electrode area. The polysulfides are peeled off from the positive electrode and soaked in the electrolyte, and then react with metallic lithium. The positive electrode active material is depleted and eroded, and finally the positive electrode area collapses.
(2) During the discharge process of the lithium-sulfur battery, after the formed polysulfides enter the electrolyte, the highly enriched polysulfides increase the viscosity of the electrolyte, resulting in a decrease in the conductivity of the electrolyte, and thus the battery performance is significantly reduced.
(3) Since elemental sulfur is non-conductive at room temperature and cannot be used alone as a positive electrode material, it is usually mixed with a certain amount of conductive material in the preparation of lithium-sulfur batteries to improve the conductivity of the positive electrode region. However, if the conductive materials are mixed excessively, the specific energy of the lithium-sulfur battery will be significantly reduced.
The above problems will eventually lead to the occurrence of low utilization rate of active materials and rapid capacity fading in lithium-sulfur batteries. Effectively improving and solving these problems has become the key to promoting the application and development of lithium-sulfur batteries and their key materials.
At present, the research and development directions of lithium-sulfur batteries mainly include the design and manufacture of diversified sulfur-based cathode materials, the improvement of metal lithium anodes, and the reduction of the working temperature of the battery and the prevention of the dissolution of discharge products in the cathode area. There are porous carbons in the research of cathode materials for lithium-sulfur batteries, such as large mesoporous carbons, activated carbons, carbon gels, etc, as well as carbon nanotubes, nanostructured conductive polymer materials, such as multi-wall carbon nanotubes (MWCNT), conductive polymers are polypyrrole (Ppy), polyaniline (PAN); in addition, there are also many research results in polyacrylonitrile (PAN) and solid electrolyte. At present, the representative manufacturers of lithium-sulfur battery research in the world include Sion Power in the United States, Polypus, Oxis in Moltech and Samsung in South Korea. The research and development of lithium-sulfur batteries in China is still concentrated in research institutions, represented by Tianjin Electronics 18, Institute of Chemical Defense, Tsinghua University, Shanghai Jiaotong University, National University of Defense Technology, Wuhan University, Beijing Institute of Technology and other research institutes. Compared with the research and development, there are very few companies involved.
Judging from the demand and research and development capabilities of China's secondary battery market, lithium-ion batteries will still be the mainstream technology in the next few years, and there is still a lot of room for technological progress in mature lithium-ion batteries. Although lithium-sulfur batteries have a high theoretical energy density (the specific capacity of elemental sulfur is as high as 1675mAh/g, and the mass specific energy is as high as 2600Wh/kg), however, lithium-sulfur batteries have great difficulties in technological development. In addition, the energy density of lithium-ion batteries was long thought to be around 250 Wh/kg in the past. However, with the successful development of new material technologies such as lithium-rich manganese-based materials (also known as solid solution cathode materials) and solid-state electrolyte materials, the energy density of lithium-ion batteries has achieved breakthroughs, which can reach 500Wh/kg or even 700Wh/kg. Judging from the current technological progress, lithium-rich secondary battery products with an energy density of 500Wh/kg may be commercialized and applied earlier than lithium-sulfur battery products with the same energy density.
Toyota Motor Corporation has developed all-solid-state lithium-ion battery technology that can be mass-produced, and General Electric Co. is also a supplier of all-solid-state lithium-ion battery technology and lithium-ion battery technology based on lithium-manganese-rich materials. Nissan, the most active in the development of pure electric vehicles, is also focusing on developing lithium-ion battery technology using lithium-rich manganese-based materials. Other world-renowned manufacturers of lithium-ion batteries such as Panasonic (Panasonic), Sony (SONY), LG Chem (LG Chem), Samsung SDl (SAMSUNG SDI) and other companies also hold the same attitude. On the one hand, these companies have been investing huge sums of money to expand the production capacity of lithium-ion batteries for several years. Therefore, for enterprises, the development risk of lithium-sulfur batteries is relatively high, and the probability of success of R&D applications is not high at least at this stage. In comparison, batteries with more mature technology using lithium oxide as the cathode material are more likely to achieve technological breakthroughs.