Spray drying has been applied for many years for production of fine powders of high quality for various types of batteries. Over the years, the demand to energy density and performance of batteries from new applications has led to the development of new generations of batteries, including secondary lithium-ion batteries. These batteries are foreseen to play an important role in the electric vehicle market, which is predicted to grow significantly in the coming years. For this reason, there is an intense race among leading manufacturers worldwide for developing proprietary Li-ion batteries with superior performance characteristics. This includes development and proper manufacture of cathode materials, such as lithium iron phosphate (LiFePO4), and spray drying will play a leading role in realizing the full industrial potential of these materials.
Powder engineering
There is a large variation between different manufacturers with respect to types and characteristics of lithium material powders applied or being developed for proprietary Li-ion batteries. Consequently, the design of the spray dryer must be tailored to the specific application and material in order to provide the customized and very precise powder specifications required. A standard off-the-shelf spray dryer design is simply insufficient if the target is to maximize the powder quality and battery performance. The tailor-making of the spray dryer design takes place in close collaboration with an experienced and competent spray-drying-technology supplier and involves pilot plant testing both for early stage feasibility studies of different nano-feed formulations for spray drying and larger scale pilot plant tests for optimization of the process design. This allows for an optimum spray dryer performance with production of powders in a consistent superior quality in the most energy- and cost-efficient way. Large know-how and experience in spray drying will also be required for continuously supporting manufacturers in optimizing existing spray dryers to the rapidly changing market demands.
Several types of atomization are employed in spray drying systems, including rotary atomization with wheels utilizing centrifugal forces, pressure nozzle and pneumatic atomization. For industrial-size spray dryers for lithium materials for Li-ion batteries, rotary atomization (Figure 1) is preferred due to its greater flexibility and ease of operation. Two-fluid pneumatic atomization is normally considered feasible only for small capacity plants due to the higher energy consumption. Two-fluid nozzles are also giving a wider particle-size distribution and potentially more oversize material. In comparison, the rotary atomizer has a number of distinct advantages, including the following:
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| Figure 1. Atomization by a rotary atomizer |
• It handles high feedrates without need for atomizer duplication
• It handles abrasive feed stocks without wear problems
• It has negligible blockage or clogging tendencies due to the large flow ports in the atomizer wheel
• Droplet size control is through simple adjustment to wheel speed
• Particle size is not affected by feedrate fluctuations
For the subject type of materials, a spray dryer with high-speed rotary atomization can produce a powder with nice spherical particles with low residual moisture content, a narrow particle size distribution with no oversize material and a typical mean particle size of 5–20 µm (Figure 2). These are some of the critical characteristics to be engineered into the powders in order to make them ideal for further processing.
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| Figure 2. A SEM (scanning electron microscope) photo of lithium iron phosphate particles spray dried by rotary atomization |
Lithium iron phosphate (LiFePO4) and its precursors, for example LiH2PO4/Fe2O3, lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4) and other cathode materials applied in Li-ion batteries are generally abrasive, which calls for specialized solutions to avoid rapid wear of surfaces exposed to very high feedstream velocities. For handling of such materials, special abrasive-resistant rotary atomizer wheels have been developed (Figures 3 and 4). They are of the bushing wheel type and when equipped with exchangeable bushings made of very hard materials, like silicon carbide or alumina, the wheels resist the hard wear they are exposed to when atomizing abrasive feed materials down to the very small particle sizes needed. In this way, one can avoid frequent maintenance interruptions caused by rapid wear of atomizer nozzles or a rotary atomizer wheel and the associated and undesirable, but inevitable, gradual decrease in powder quality. Instead, the manufacturer can obtain prolonged periods of operation of the spray dryer with production of powder of a consistent high quality despite the abrasive nature of the materials being processed. Another prerequisite for such long service intervals is that the rotary atomizer technology has long proven itself as a stable and reliable technology. This is also true for advanced rotary atomization technology, which involves very high peripheral velocities of the rotary atomizer wheel as required for making very fine powders.
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| Figure 3. This patented rotary atomizer wheel (Type AX) is abrasion resistant |
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| Figure 4. A high-capacity rotary atomizer with Type AX atomizer wheel is being installed in the workshop |
Aqueous feed materials
Spray drying plants with rotary atomization and co-current spray-air flow configuration are applied for the subject materials in either open- or closed-cycle process layouts. An open-cycle configuration is applied for aqueous feed materials. The open-cycle layout (Figure 5) involves intake of drying air from the atmosphere and discharge of the exhaust air to the atmosphere. The drying air is heated by means of a direct-fired air heater and flows co-currently with the atomized feed material in the drying chamber where the fine droplets are quickly dried to individual powder particles. Separation of the dried product from the drying air and exhaust air cleaning typically takes place in a bag filter. The design of the gas disperser in the top of the spray-drying chamber is crucial for ensuring a proper functioning of the spray dryer.
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| Figure 5. An open-cycle spray dryer process configuration with single-point powder discharge for aqueous feed material |
Non-aqueous feed materials
A closed-cycle layout is used for non-aqueous (organic solvent) feeds. Nitrogen is used as inert drying medium to eliminate any risk of fire or explosion due to the presence of solvent vapor in the dryer. The solvent evaporated from the spray during the drying process is fully recovered. The process is carried out as a normal spray drying operation (Figure 6). The outlet drying gas from the drying chamber is conveyed to a highly efficient bag filter for collection of the produced powder. The exhaust gas containing the evaporated solvent is recycled to a scrubber/condenser system where the solvent is condensed out of the gas flow and recovered as a continuous purge from the system. The recycled gas is then reheated via an indirect heating system prior to recycling to the drying chamber. Closed-cycle plants operate at a slight overpressure to prevent possible inward leakage of air. The control of the process also involves a continuous monitoring and control of the oxygen level in the system.
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| Figure 6. A closed-cycle, spray-dryer process layout with single-point powder discharge for non-aqueous feed materials |
Conclusion
Spray dryers based on rotary atomization offer a number of advantages to manufacturers of lithium material powders for Li-ion battery applications. Tailor-made spray dryer layouts must be developed for each specific application in close collaboration with knowledgeable spray drying technology suppliers having the experience to realize the full industrial potential of the lithium material powders. This involves pilot plant testing also for optimization of customized feed formulations as well as selecting the optimal dryer concept and design. The field of spray drying is constantly developing, and only those continuously increasing their level and accuracy of knowledge about the spray drying process and its dynamics will be able to provide proper support, design and technology for cost-efficient production of superior lithium material powders for Li-ion battery applications — a field which in itself is in continuous development and with constantly changing demands.♦
Author
Anders Bo Jensen is sales manager by GEA Niro (Gladsaxevej 305, DK-2860 Soeborg, Denmark; Phone: +45 3954 5454; Email: [email protected]). He is responsible for customization and technical specifications of industrial spray drying and related drying plants for the chemical industry. Anders Bo Jensen has more than 15 years of experience with process design and technology development in the chemical process industry. He holds a B.Sc. degree in chemical engineering from the Technical University of Denmark, Odense and a M.Sc. degree in chemical engineering from the University of Manchester Institute of Science and Technology (UMIST).