Microwave Enhanced Solid State Synthesis of Battery Materials Dr. Kuruvilla (Karl) Cherian Spheric Technologies Inc. Phoenix, Ariz. Microwaveassisted processing offers a rapid, efficient solid-state chemical synthesis approach for lithium (Li)-ion battery electrode materials. New battery and energy storage materials are key factors in the new energy economy. Electrode materials have a significant influence on the electrochemical performance of these new batteries. Therefore, specific ceramic phases and the technologies required to rapidly synthesize them are key to successful commercial production of more efficient, cost-effective batteries. Olivine-structured LiFePO 4, first reported by Goodenough, et.al, is a promising cathode material for lithium (Li)-ion batteries. Factors such as low cost, low toxicity, environmental compatibility, high theoretical capacity, and perfect thermal cycling stability have led to increasing use of LiFePO 4 as an electrode material in small and large power applications. Li-ion batteries are now used in portable electronic devices, and larger Li-ion batteries are being developed as power sources for next-generation electric vehicles. Lithium-titanium spinel (Li 4 Ti 5 O 12 ) is a significant anode material because it is a zero strain lithium insertion host material due to its extremely small variations of lattice parameters during the charge and discharge processes as reported by Ohzuku, et.al. Significant improvements in capacity have been achieved by reducing the synthesized material particle size and coating particles with a conducting material. These two materials are representative of anode and cathode materials in this discussion on developing microwave-enhanced solid state synthesis technologies for battery materials. Several synthesis routes have been explored at laboratories for various battery electrode materials. There is still a need for rapid, commercially viable processing technologies to produce these and similar materials at low cost. This article deals with the possibility of developing faster, less expensive, and greener electrode materials-synthesis technologies using advanced microwave processing equipment available in the U.S., and process scale up from lab/pilot scale microwave batch furnace to a continuous microwave furnace that offers greater throughput to meet commercial level production. LiFePO 4 and Li 4 Ti 5 O 12 and other electrode materials can be produced at commercial levels via microwave processing with the necessary set-up and process modifications. 300 250 Intensity, counts 200 150 100 50 0 LiFePO4 Fe2O4 20 30 40 50 60 70 80 90 100 Two theta, deg Fig. 1 XRD results showing near single-phase product LiFePO 4 (plus traces of Fe 3 O 4 ). ADVANCED MATERIALS & PROCESSES JANUARY 2011 23
Counts/s 150 100 50 0 20 30 40 50 60 70 80 Position[2 theta](copper, Cu) Fig. 2 XRD results showing presence of Li 4 Ti 5 O 12 and TiO 2. ; lithium titanium oxide Microwave-enhanced synthesis drivers Conventional solid state synthesis routes often are time and energy intensive. Application of microwaves to materials processing (including synthesis) offers advantages over conventional heating methods, including improved product yield, unique microstructures and properties, energy savings and reduction in manufacturing cost, and efficient synthesis of new materials often with stabilization of unique phases. In the case of LiFePO 4, conventional synthesis routes include solid-state reactions (several chemical systems), solgel, co-precipitation in aqueous medium, and hydrothermal. Notable non-microwave enhanced commercial LiFePO 4 synthesis processes that have sought IP protection include: Chiang, et.al. (A123, US Patent Appl. 20070190418) and Saidi, et.al. (Valence Technology Inc., US Patent #7060238). In the case of Li 4 Ti 5 O 12, conventional synthesis routes include solid-state reactions involving several chemical systems. Notable, non-microwave enhanced commercial scaleup processes that have sought IP protection include: Gorshkov, et.al. (US Patent Appl. 20070238023) and Zhang, et.al. (US Patent Appl. 20070281211). A common factor that needs to be improved in conventional methods is long processing times (up to 15+ hours). Shorter processing times could yield economical and material-property advantages. This raises the question of whether shorter processing time rapid synthesis is possible. The answer is yes with the use of microwave-enhanced materials processing. Several research groups have indicated positive results in synthesizing small amounts of material on a lab scale using modified domestic ovens or small desktop laboratory furnaces. Two challenges of commercialization are obtaining microwave synthesized product of required phase and phase purity on a lab scale and scale up from lab scale to pilot scale to production scale for large volume production and commercialization. Microwave synthesis routes generally enable much shorter process times (minutes vs. hours) than conventional synthesis routes. Shorter process times result in smaller particles/grains in the synthesized product, which is another advantage of a microwave-assisted/enhanced synthesis. Microwave-enhanced solid state synthesis of LiFePO 4 LiFePO 4 synthesis through a closed pressure vessel microwaveenhanced liquid-phase (solvo thermal) synthesis route is reported by Murugan, et. al. (US Patent Appl. 20090117020). Results include shorter processing times and high crystallinity and controlled size of the product. However, obtaining similar results in large volume production requires a large volume pressure vessel that is microwave transparent and stable at the synthesis pressure and temperature. Further, this approach still is a batch process. It was questioned whether there might be alternative microwave-enhanced synthesis routes that did not require a closed pressure vessel and could be scaled up to a continuous production configuration. Spheric Technologies decided to investigate the possibility of achieving microwave-enhanced solid state chemical synthesis of LiFePO 4 and scale up for continuous throughput production. The previous success of producing small amounts of product involved using modified domestic microwave ovens with inert atmospheres and a simple microwave power/temperature ramp up and down processing strategy. It was realized that a programmed time-temperature profile processing strategy could ensure better, repeatable results using Spheric s advanced microwave furnaces. In initial LiFePO 4 synthesis trials, lithium carbonate, ammonium dihydrogen phosphate, and iron(ii) oxalate were weighed in the required stoichiometric ratio. Acetone was added to form a slurry, which was vigorously milled for 1 hour and dried. Pellets weighing 5 g each were pressed at ~5,000 psi, and the required number of pellets was used for each run in a programmable HAMiLab-V6 industrial microwave batch furnace. Processing trials were carried out with various programmed time-temperature and hold times. Initially, a multiphase product was obtained by holding for 15 minutes at 800 C. Subsequently, a near single phase prod- 24 ADVANCED MATERIALS & PROCESSES JANUARY 2011
(a) (b) H-field (c) E-field From top From side Fig. 3 Industrial microwave equipment for microwaveenhanced processing technology development: (a) Spheric/SynoTherm HAMiLab-V6 advanced microwave batch furnace, (c, d, and e) E and H field distribution analyses of the applicator volume (b) shows optimum processing zone. MW frequency and power 2,450 MHz, 0.6-6.0 kw variable Working (heat) zone ~8 in. diam. x ~ 8 in. high Maximum temperature Temperature measurement 1600 C+ Raytek IR pyrometer 450-2000 C 28 in. L x 31 in. W x 71 in. H + 50 in. L x 32 in. W x 64 in. H (e) (d) System footprint uct was obtained with process modifications (Fig. 1). Further modifications to precursor material composition and pre-treatment produced a phase pure product (details available from Spheric Technologies). Microwave-enhanced solid-state synthesis of Li 4 Ti 5 O 12 An innovative microwave-processing strategy was successfully used by Roy, et.al., at Penn State to synthesize barium titanate. This involved conversion of one of the reactants to a microwave coupler by a reduction step and microwave processing the mixture to yield ~99% conversion to a tetragonal BaTiO 3 phase through an anisothermal heating mechanism. It was reported that the microwave route could achieve the conversion in 5 minutes at 900 C, whereas the conventional approach requires 60 minutes at 1300 C, yielding Ba 2 TiO 4. Anisothermal microwave heating effects could therefore lead to very rapid synthesis of specific phases. This synthesis approach was adopted with necessary modifications at Spheric Technologies for Li 4 Ti 5 O 12, achieving a significant degree of success. In preliminary microwave-synthesis trials, precursor preparation included weighing lithium carbonate and reduced titania in the required stoichiometric ratio, adding acetone to form a slurry and vigorously milling for 1 hour, and drying. Pellets were pressed at ~5,000 psi and microwave processing trials were carried out using various programmed profiles (time-temperature and hold times) in the microwave batch furnace. A sample was obtained using a processing (a) MW frequency, power, 2,450 MHz, 0.9-12.0 kw variable, 1550 C and maximum temperature Control system Programmable, 8 in. digital input touch pad, temperature, time, power inputs Data output Calculated power, real power, temperature, time to data port Temperature measurement, Raytek XR pyrometer, 250-1650 C, atmosphere control 2 channel gas flow meter air, N 2, Ar mix System type, speed and Pusher, adjustable15-600 mm/h; boats: crucible size 150 mm x 100 mm x 60 mm Footprint 4 m L x 1 m W x 1.5 m H (13 x 3 x 5 ft) Output 50-100 kg/day powder production; component size dependent Fig. 4 Industrial microwave equipment for microwave-enhanced processing technology development. Spheric/SynoTherm AMPS advanced microwave continuous pusher furnace: (a) furnace and control cabinets, (b) AMPS furnace from the sample input side, and (c) view through the exit throat showing part of the hot zone and an exiting boat. profile of heating to 825 C in 20 minutes and immediate cool down with zero hold time at temperature. XRD analysis of the sample (Fig. 2) indicated the formation of Li 4 Ti 5 O 12 together with TiO 2. It was expected that better yield and de- (b) (c) ADVANCED MATERIALS & PROCESSES JANUARY 2011 25
Fig. 5 Schematic of a Spheric/ SynoTherm AMPS- 9 continuous microwave furnace with temp-time processing profile from HAMiLab-V6 batch microwave processing above, and its conversion to temp-distance curve for AMPS continuous microwave processing, below. From: Temperature variation in time (HAMiLab-V6) AMPS Continuous MW furnace (schematic) To: Temperature variation in space (AMPS) sired phase purity should be attainable with further process modifications. These results showed that obtaining microwave synthesized electrode materials of required phase purity could be achieved using the advanced microwave batch furnace having time-temperature and microwave power programming and control capabilities. These advanced capabilities together with larger hot-processing volumes would help obtain the best processing profile for optimum volumes of precursor materials for various electrode materials synthesis. The processing profiles obtained in a microwave batch furnace were helpful in scaling up the process for larger throughput through a microwave continuous furnace. Advanced microwave systems Limitations in controlling process parameters together with limited process volumes have, until now, placed severe constraints in scaling up the microwave assisted/enhanced battery material synthesis to pilot scale and to commercially viable production levels. The availability of advanced larger scale batch and continuous 2.45-GHz advanced microwave furnace systems (AMPS) removes those limitations, and opens up the possibilities in the field of commercial-scale solid-state chemical synthesis in general and battery-materials synthesis in particular. Figures 3 and 4 show examples of advanced batch and continuous microwave furnace systems (currently in operation in the U.S.) for process development work. The batch system (HamiLab-V6) offers an effective uniform heating volume of ~200 mm ID 200 mm HT within an applicator with dimensions of ~ 500 mm ID 500 mm HT. The continuous system (AMPS-9) is a pusher type using rectangular crucibles ~150 mm long 100 mm B 60 mm high and lateral system speed of 15-600 mm/h. These systems, with a microwave power output of 0.6 6 kw for the batch system and 0.9-9 kw (or higher depending on customer needs and specifications) for the continuous system, have built-in computer capabilities for temperature programming and control in a temperature range of 450 to 2250 C range. Various temperature ramp-up rates to predetermined temperature hold stages (up to ~1550 C) and hold times are possible. These provide the systems with the versatility required to develop synthesis and processing regimes and profiles of relevance to various materials; these are of specific relevance to microwave-enhanced solid-state chemical synthesis of inorganic materials and battery electrode materials. Scale up from batch to continuous processing The processing scale-up strategy is to first develop the best processing profile for the required product in the batch system and adapt this knowledge to develop the continuous processing routine for larger scale production in the continuous microwave furnace. In other words, transfer the processing profile in time to processing profile in space. This is discussed with reference to the schematic diagram shown in Fig. 5. The schematic of the Spheric/SynoTherm AMPS-9 continuous microwave furnace is shown in the middle. There are a total of 28 pusher boats from inlet to exit of the processing area at any time during its continuous operation. The furnace is divided into three sections: preheat zone, sinter-heat zone, and cooling zone. Four temperature-monitoring points (T1, T2, T3 and T4) are shown. Of these, the first three are active; they provide feedback for microwave power regulation, and, therefore, temperature at their locations. T1 can reach a max of 700 C, whereas T2 and T3 can reach ~1550 C. Each of these can be used to program predetermined set temperatures in their respective locations. Above the AMPS schematic is the best time-temperature processing profile (ramp rates, holding temperature, holding time, and cool down rates) obtained during Stage 1 trials in the Spheric/SynoTherm HAMiLab-V6 microwave batch furnace, say for synthesis of a specific phase A. The AMPS system is then programmed for suitable set temperatures at T1, T2, and T3 to achieve a profile in the AMPS-9 system similar to that shown below the AMPS schematic. To implement the hold/soak time at the maximum temperature in the HAMiLab-V6 microwave batch processing profile, a boat push-through rate is chosen to correspond to the hold time in the HAMiLab-V6 processing profile. This would enable each pusher boat to remain at the maximum temperature 26 ADVANCED MATERIALS & PROCESSES JANUARY 2011
point for a period equal to the AMPS push-though rate and corresponding to the hold/soak period in the HAMiLab-V6 system profile. If very long hold times are necessary for any particular process, instead of having very long-push through rates, one can program T2 and T3 so there could be a maximum temperature plateau in the T2-T3 section; in such a case, the push-through rate would be selected such that the time required for a boat with samples to traverse the T2-T3 region would correspond to the long hold time needed. Thus, a preliminary temperature variation with time profile can be used to obtain a corresponding temperature variation with space (distance) profile for AMPS continuous microwave processing. We have implemented such a strategy successfully for processing certain types of ceramic materials in an AMPS continuous microwave furnace in the U.S. The above results show that the broad major challenge of scale up from lab scale to pilot scale to production scale can be met using the advanced microwave continuous furnace (AMPS) that has programming and control capabilities for time-temperature, lateral material movement rate, and microwave power input. These advanced capabilities together with the system s continuous feed-through operation mode would help obtain pilot scale and commercial level production of various electrode materials. The processing profiles obtained earlier in a microwave batch furnace would be a very helpful step in subsequent scaling up of the process for larger throughput through the microwave continuous furnace, as discussed above. Conclusions Microwave-assisted processing offers a rapid, efficient solid-state chemical synthesis route for Li-ion battery electrode materials such as LiFePO 4 and Li 4 Ti 5 O 12. Processing profile-programming capabilities of the advanced batch and continuous microwave furnace systems offer advantages in controlling the reactions and obtaining the specific product composition of interest. Advanced batch and continuous microwave furnace systems with larger processing volumes/throughput are now available, offering the opportunity to improve and scale up laboratory scale experimental results to pilot scale and commercially viable production levels. The best processing profile obtained in the microwave batch processing system can be very helpful for faster development and implementation of processing profiles in a continuous microwave processing system for commercial scale production. For more information: Dr. Kuruvilla (Karl) Cherian is Director of Applied Research, Spheric Technologies Inc., 4708 E. Van Buren St., Phoenix, AZ 85008; tel: 602/218-9292; email: kcherian@ spherictech.com; Website: www.spherictechnologies.com. denise.sirochman@ asminternational.org www.asminternational.org/ onlinedatabases ADVANCED MATERIALS & PROCESSES JANUARY 2011 27