(1Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084)

　　Abstract: This article reviews the development history of lithium-ion battery cathode material production and preparation technology, and analyzes the development direction of lithium-ion battery cathode materials. At the end of the last century, from the perspective of the processing performance of lithium-ion battery cathode materials and battery performance, a research team from Tsinghua University proposed a technology to control crystallization to prepare high-density spherical precursors. Combined with the subsequent solid-phase sintering process, they proposed a method for preparing lithium-containing electrode materials. industrial technology. Among them, the controlled crystallization method to prepare precursors can regulate and optimize the properties of the material at four levels: unit cell structure, primary particle composition and morphology, secondary particle size and morphology, and particle surface chemistry. The materials produced using this technology have the characteristics of easy control of particle size and morphology, good uniformity, batch consistency and stability, and can simultaneously meet the comprehensive requirements of the battery for the electrochemical performance and processing performance of the material. Due to the high packing density of the material, it is especially suitable for high specific energy batteries. This technology is suitable for a variety of cathode materials and is suitable for mass production. Over time, it has been gradually proven to be the best production technology for cathode materials for lithium-ion batteries, and has been generally accepted and recognized by the industry today. This is also one of the important contributions made by Chinese scientists to the international lithium-ion battery industry.

　　Keywords: lithium-ion battery; cathode material; production technology; controlled crystallization; development direction

　　Energy Storage Science and Technology, 2018, 7(5): 888-896

　　Lithium-ion batteries have the advantages of high specific energy, high energy storage efficiency and long life. In recent years, they have gradually occupied the main market share of electric vehicles, energy storage systems and mobile electronic devices. Since the Japanese company Sony took the lead in commercializing lithium-ion batteries in 1990, the negative electrode material has always been carbon-based materials, while the positive electrode material has made great progress and is the most critical material to promote the performance improvement of lithium-ion batteries.

　　The research and development of cathode materials for lithium-ion batteries is mainly carried out in three aspects: 1) The basic science level, mainly the discovery of new materials, or the calculation, design and synthesis of material composition, crystal structure and defect structure, with a view to discovering electrode materials. New cathode materials with excellent chemical properties; 2) At the material chemistry level, the synthesis technology is mainly discussed in order to optimize the material structure factors such as material crystal structure, orientation, particle morphology, interface, etc., and obtain electrochemical performance, processing performance and battery performance. Best match, the purpose is to develop material structures and synthesis methods that can optimize the comprehensive performance of cathode materials; 3) Material engineering technology level, mainly to develop large-scale, low-cost, stable equipment and processes, in order to develop reasonable Engineering technology to meet market needs.

　　In order for lithium-ion battery cathode materials to exert the best performance in the full battery, it is necessary to further optimize the crystal structure, particle structure and morphology, particle surface chemistry, material packing density and compaction density of the material on the premise of optimizing the material composition. physical and chemical properties, and it is also necessary to strictly prevent the introduction of trace metal impurities during the process. Of course, stable, high-quality mass production is an important guarantee for the stable performance of materials in battery manufacturing. As lithium battery technology improves and the lithium battery market matures, the application fields of different cathode materials are gradually divided, that is, lithium-ion batteries have different performance requirements for various cathode materials. Therefore, the mainstream synthesis technologies and processes of cathode materials have also experienced different development paths.

　　This article reviews the industrial application history of the main cathode materials for lithium-ion batteries, focuses on the development of materials' industrial technology, and looks forward to the development direction of cathode material manufacturing technology.

　　1. Performance requirements for cathode materials in lithium-ion batteries

　　(1) Industry performance requirements for lithium-ion batteries

　　To understand the technical indicators of cathode materials, we need to start with the technical indicators of the battery. In the early days of the lithium-ion battery industry, it mainly served the development of mobile electronic products, such as laptops, tablets, mobile smart terminals (mobile phones), etc. In recent years, the new energy industry and the electric vehicle industry have risen rapidly, and the demand for lithium-ion batteries has grown rapidly, stimulating the lithium battery industry to accelerate its development. Therefore, lithium-ion batteries need to meet many technical performance indicators in order to be recognized by the industry and achieve further development. Among these technical indicators, the most basic include specific energy, cycle stability, specific power, cost, safety and reliability, durability, manufacturing efficiency, sustainability, etc. The indicators are interrelated, and different application fields have important Priorities for lithium-ion battery metrics are different. Compared with lithium-ion batteries in portable electronic products, the biggest difference between energy storage and lithium-ion batteries used in the electric vehicle industry is that the capacity of a single battery has increased by ten or even dozens of times. At the same time, the function and structure of the battery module And the complexity of applications has increased significantly, which has put forward higher requirements for the consistency and reliability of lithium-ion batteries.

　　Based on more than 20 years of research and engineering practice experience, the team of this article believes that the most important technical indicators of lithium-ion batteries are specific energy and cycle performance, followed by performance indicators such as specific power, safety, reliability, cost and consistency. The higher the specific energy, the material cost per unit energy (Wh) decreases; the longer the cycle life, the lower the actual use cost of the battery. At present, lithium-ion batteries for mobile smart terminals need to meet the requirements of specific energy of more than 700 Wh/L and cycle performance of more than 200 times, while lithium-ion batteries for electric vehicles need to meet the specific energy of 140 Wh/kg (lithium iron phosphate or lithium manganate cathode material) or 200 Wh/kg (layered oxide cathode material) or more, and cycle performance of more than 1,500 times. Lithium-ion battery cathode materials must meet the above battery indicators before they can be accepted by the mainstream battery market. At present, the specific energy and cycle performance of lithium-ion batteries mainly depend on the cathode material [1-6]. Therefore, the main research and development goals of cathode materials for lithium-ion batteries are high specific energy and long cycle life.

　　For lithium-ion batteries used in laptops, tablets, and mobile smart terminals, the volume specific energy is the most important indicator. Of course, batteries with a high volume specific energy usually also have a high mass specific energy. Because customers want to put more battery energy into devices of a specific volume (such as mobile phones), currently the graphite | lithium cobalt oxide system lithium-ion battery is the most mature in industrialization and has the highest volume specific energy. Lithium in other material systems It is difficult for ion batteries to shake the dominance of lithium-ion batteries in this system in the mobile electronics industry. Safety, reliability and certain cycle performance are also important for this type of battery. Since it is mainly used in a single form, the consistency and cost of the battery are not so important.

　　For lithium-ion batteries used in electric vehicles, although their volume specific energy requirements are not as stringent as batteries for portable electronic products, after all, the space of passenger cars is limited, and the weight of the car body will affect the driving range of the electric vehicle, so the mass ratio of the battery Energy and Volume Ratio Energy is still very important. In addition, automotive lithium-ion batteries have almost stringent performance requirements for almost all other properties, which are much higher than the performance requirements of batteries for portable electronic products. There are three biggest differences from batteries for portable electronic products. First, the power supply of electric vehicles requires higher voltage and current, and requires a large number of single cells to be combined in series and parallel. This makes the actual specific energy that can be utilized by the battery pack not only depend on the specific energy of the single battery, but also the specific energy of the single battery. Consistency, especially dynamic consistency, the consistency of power batteries has gradually attracted people's attention in recent years [7]. Second, the scale of single cells has increased significantly, which makes the price of single cells higher and the harm caused by thermal runaway more serious. Therefore, the market is more sensitive to the safety and reliability of batteries. Third, since electric vehicles require a service life of 10-15 years, they have very high requirements for cycle performance, generally requiring more than 1,500 times. In addition, since electric vehicles need to start and accelerate, the power battery also has certain requirements for power.

　　With the rapid development of the electric vehicle industry, power lithium-ion batteries will become the mainstream products of the lithium battery industry together with batteries for portable electronic products in the future. Specific energy and cycle performance are the most important performance indicators that are always pursued in the development of lithium-ion battery technology. As safety, reliability, specific power and consistency receive increasing attention, technology in this area is expected to develop rapidly. It should be noted that as lithium-ion batteries gradually penetrate into various fields of the national economy, there will be more and more non-mainstream lithium-ion battery market segments, which have special requirements for battery performance indicators and are not discussed in this article. scope.

　　(2) Cathode materials that meet the needs of the mainstream lithium-ion battery industry

　　Currently, the cathode materials that meet the battery performance requirements of the mainstream lithium-ion battery market mainly include layered lithium cobalt oxide LiCoO2 material (LCO), spinel lithium manganate LiMn2O4 material (LMO), and olivine lithium iron phosphate LiFePO4 material (LFP) , olivine lithium iron manganese phosphate LiMn0.8Fe0.2PO4 material (LMFP), layered ternary material LiNi1/3Mn1/3Co1/3O2 material (NMC333), layered ternary material LiNi0.4Mn0.4Co0.2O2 (NMC442), LiNi0.5Mn0.3Co0.2O2 (NMC532), LiNi0.6Mn0.2Co0.2O2 (NMC622), LiNi0.7Mn0.2Co0.1O2 (NMC721), LiNi0.8Mn0.1Co0.1O2 (NMC811) and layered high-nickel material LiNi0 .8Co0.15Al0.05O2 (NCA), etc. From the perspective of industrial application, the above materials have different physical and chemical characteristics and are suitable for lithium-ion batteries in different application fields. Therefore, the key performance indicators of the material products are also different.

　　Lithium cobalt oxide LiCoO2 (LCO) material is currently the cathode material with the highest compaction density. Therefore, the prepared lithium-ion battery has the highest volume specific energy and has become the main cathode material for lithium-ion batteries used in tablet computers and mobile smart terminals. Its shortcomings are mainly limited cobalt resources and high cost, which limits its wide application in the field of electric vehicles. The structure and reaction characteristics of this material are that as the charging voltage gradually increases, the amount of lithium extracted gradually increases, and the available capacity of LCO gradually increases. However, when the amount of lithium extracted exceeds 55% (that is, relative to the charging potential of metallic lithium) 4.25V (compared to the charging voltage of graphite | LCO full battery is 4.2V), the structural stability of the material decreases rapidly, and the lifespan and safety deteriorate rapidly. Therefore, LCO cathode materials that can withstand higher charging voltages and have chemical stability that meets battery application requirements are the main development direction of current material preparation technology. LCO has a stable structure and is relatively easy to synthesize. Its preparation technology is simple and relatively mature. Before 2000, LCO was mainly produced through solid-phase sintering technology of cobalt oxide/lithium carbonate mixture. With people's ultimate pursuit of product packing density, specific modification, etc., the method of controlling crystallization to prepare lithium cobalt oxide precursor has changed. It has the advantage of material morphology control and has gradually become a major industrial preparation technology [8-11].

　　The main advantages of the spinel lithium manganate LiMn2O4 (LMO) material are abundant raw material resources, low cost, and good battery safety; its main recognized disadvantages are low battery specific energy and poor cycle stability. Since the 1990s, attracted by its low raw material and process costs and good safety, people have explored the application of LMO in electric buses, passenger cars, special vehicles, power tools and other fields. Traditional solid-phase sintering preparation technology cannot control the material structure. In order to improve its cycle stability and the tap density of the material, the author's team introduced a liquid-phase process to prepare the precursor in 2004 [12-14], and further used surface coating to Technologies such as coating, lattice doping, and surface gradient are used to improve material performance [15-22]. However, due to the high solubility of the material, the cycle stability of the battery has not been well satisfied. Only by further combining the electrolyte can the battery life be able to meet the demand. At present, although LMO is rarely used in vehicle power batteries, it has been widely used in small power battery industries such as electric bicycles that are more cost-sensitive. In addition, as people pay attention to the safety of large power batteries for vehicles, blending with ternary materials has become one of the main uses of LMO materials.

　　The main advantages of the olivine lithium iron phosphate LiFePO4 (LFP) material are abundant raw material resources, low cost, battery safety and good cycle performance. Its main disadvantage is the low specific energy of the battery. This material has been widely used not only in the electric bicycles, electric buses, electric buses, and special vehicle industries, but also in the large-scale energy storage industry. Since lithium ions in this material are transported along one-dimensional channels, the material has significant anisotropy and is extremely sensitive to defect structures. The preparation process needs to ensure a high degree of uniformity of the synthesis reaction and a precise Fe:P ratio to achieve better results. capacity and rate performance. Based on the complexity of the material structure and synthesis reaction, there are two main problems in the preparation of this material: First, the process requires a reducing atmosphere. The reaction raw materials have different requirements for the reducing atmosphere due to different types and particle sizes. The local reducing properties are too high or too high. If it is too low, impurities will remain in the product; second, the material needs to be surface carbon coated or compounded with other types of conductive agents, which makes it difficult to control the impurities and compaction density of the material. In 2005, the author's research group proposed to use controlled crystallization technology to prepare high-performance iron phosphate precursor (FP), and then prepare LFP through carbothermal reduction with lithium source and carbon source [11]. The above process route has been further improved and has become the current mainstream lithium iron phosphate material preparation technology [23-29]. In order to meet people's continuous pursuit of LFP battery performance, high uniformity and high batch stability have become the most concerned product indicators of LFP cathode materials. On the one hand, traditional solid-phase sintering technology is difficult to achieve efficient consistency in principle. Control, on the other hand, consistency control can lead to a significant increase in process costs. Compared with solid-phase processes, precursors prepared based on liquid-phase processes or cathode materials prepared based on hydrothermal/solvothermal processes have better structural adjustability and controllability [30], while batch stability and reaction Good uniformity. Similar to large chemical plants, continuous solvothermal processes can easily achieve ultra-large-scale production. Therefore, liquid phase technology has gradually become the development trend of the next generation of high-quality LFP cathode material preparation technology [31-37].

　　Olivine lithium iron manganese phosphate LiMn0.8Fe0.2PO4 (LMFP) material is an upgraded version of LFP material, with a specific energy 10% higher than LFP; due to the difference in the reaction kinetics of Mn and Fe raw materials and the requirements for reducing atmosphere, this material The main disadvantage is the difficulty of preparation. At present, the industrial preparation process based on the solid-phase method is not yet mature and has not yet been applied on a large scale. If the liquid phase preparation technology of LFP is industrially applied

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