Introduction. Analysis

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1 10/21/2017 Memorandum To: Mike Kozicki, CTO, Second Solar, Inc. From: Athena Combs-Hurtado and Marc Hensel Re: Using Lithium Iron Phosphate Batteries for Utility Scale Storage Applications Introduction This memorandum serves as a response to the question posed regarding promising battery storage technologies to be used in conjunction with solar. Specifically, this report discusses lithium iron phosphate, or lithium ferrophosphate (LFP) batteries and their potential in the solar + storage market. Because of current limitations in purchasing options at the utility scale, these batteries have not had much consideration for utility scale storage applications, but have flourished in residential applications due to their long lifespans, power density, and safety. This can be an opportunity to use LFP batteries in utility scale operations. Analysis Chemistry Composition In general, a lithium-ion battery (LIB) describes a battery with an anode, which is a source of positive lithium ions, a cathode, which is a sink for the lithium ions, and an electrolyte or electrolytic solution to transport the ions. In this way, all LIB technologies share similar chemistry. While charging, electrons and positive lithium ions flow to the anode, and electrons flow out at the cathode. In a charged state, the lithium ions are stored in the anode. Upon discharge, ions flow in the opposite direction through the electrolyte to the cathode, and electrons also flow in the opposite direction into the cathode and out of the anode. The anode in any LIB is typically graphite or graphene, with some silicon introduced to hold more lithium ions [1]. Silicon can hold more lithium because it forms Li4Si, while graphite forms LiC6. However, this also means silicon swells much more than graphite when charged, which can cause spatial and longevity problems. The major differences between lithium-ion technologies is the material used in the cathode. According to Padhi [2], the requirements for the cathode material in a lithium ion chemistry battery must contain a readily reducible/oxidizable ion, the material reacts with lithium in a reversible manner, the material reacts with lithium with a high free energy of reaction, the material reacts with lithium very rapidly both on insertion and removal, the material is a good electronic conductor, preferably a metal and the material is stable,... the material is low cost and the material is environmentally benign. Advantages and Disadvantages Traditional lithium ion batteries use LiCoO2 or LiMnO2 as the cathode, while LFP uses LiFePO4. Using iron phosphate means that the molecules in the cathode are physically larger due to the presence of four oxygen atoms instead of two, so ions can enter and exit more freely [3]. LiFePO4 cathodes, while less energy dense, are more power dense and have better longevity when compared to other LIB chemistries. 1

2 The typical LIB close-packed -O2 ions, when depleted of lithium, have a high partial pressure of oxygen to which the cobalt or manganese is not strongly bonded [5]. This leads to a flammability and explosively concern. Additionally, cobalt is relatively rare and expensive, driving the cost of batteries up. The figures below illustrate the advantages in power density expressed as charge time and cycle life for various battery technologies. Figure 1 Charge Time for Various Battery Technologies [4], [10] Figure 2 Cycle Life for Various Battery Technologies [4], [10] Manufacturing Techniques Many battery manufacturing companies such as Sonnen, Coda Energy, and JLM Energy, have adjusted their lithium ion battery manufacturing processes to incorporate lithium iron phosphate. Because LFP is manufactured in the same way as other lithium ion batteries, the most significant process change will only be in the mixture of the cathode. [6] The two most prevalent methods include solid state processing,s which uses high temperatures to create crystalline structures, and solution based processes, which creates a more complex carbon coated crystalline structure. [7] 2

3 In the interest of safety, many manufacturing companies have decided to incorporate lithium iron phosphate into their energy storage products. These products are also appealing because of the integration of battery management systems, however this also rises the cost of these batteries. Lithium ion batteries must be used in conjunction with a battery management system in order to prevent thermal issues that are prevalent in the chemistry in lithium ion batteries. [8] Cost Although there have been vast improvements in scaling up LFP batteries, very few companies have been able to incorporate utility scale solar + storage systems. This may be due to the high initial cost of LiFePO4 batteries. Despite iron being an abundant element, any lithium ion batteries have a higher initial cost than lead acid batteries because of the incorporation of battery management systems as well as the use of high quality cells. The cost of lithium ion batteries are justified when examining the lifetime compared to the lead acid battery. The lead acid battery will need to be replaced more often than a lithium ion battery, which has more than twice the lifetime of a lead acid battery. [4] Table 1 Cost of Various Batteries [9] Cell chemistry Cost ($/kwh) Lead-acid $56 $145 Lithium cobalt oxide $356 Lithium iron phosphate $300 Lithium manganese oxide $356 Zinc-carbon $316 Conclusion Since energy density is not of great concern for stationary applications like residential and utility energy storage in conjunction with solar, LFP could be a good design solution. For residential use, LFP is a safe alternative to other lithium ion batteries because of its safety features and it longevity. There are also many companies that are manufacturing LFP batteries, which allows for diversity in the market. In utility or large commercial applications, LFP fits well because of its high power density. For these applications, it can absorb the large amount of power produced by a large array, and also meet the power demands placed on it. Despite the large cost of LFP batteries, there has been a moderate growth in utility scale projects that incorporate LFP batteries, which can lead to more opportunities in utility scale operations. [11, 12] 3

4 Appendices Figure 3 Lithium Iron Phosphate Two Part Reaction Figure 4 Lithium Iron Phosphate Electron Flow Diagram Figure 5 Lithium Ion Manufacturing Process 4

5 Figure 6 Manufacturing Process of Lithium Iron Phosphate Cathode Solid State [7] Figure 7 Manufacturing Process of Lithium Iron Phosphate Cathode Solution Based [7] 5

6 References 1. Song, Xuefeng; Wang, Xiaobing; Sun, Zhuang; Zhang, Peng; and Gao, Lian. Recent Developments in Silicon Anode Materials for High Performance Lithium-Ion Batteries. Retrieved Path: recent-developments-in-silicon-anodematerials.html. 2. "LiFePO4: A Novel Cathode Material for Rechargeable Batteries", A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, Electrochemical Society Meeting Abstracts, 96-1, May, 1996, pp Armand, Michel; Goodenough, John B.; Padhi, Akshaya K.; Nanjundaswam, Kirakodu S.; Masquelier, Christian (Feb 4, 2003), Cathode materials for secondary (rechargeable) lithium batteries. Retrieved BU-205 Types of Lithium-Ion Retrieved Path: 5. "Harding Energy Lithium Ion batteries Lithium Polymer Lithium Iron Phosphate". Harding Energy. Retrieved Hanisch, Christian; Diekmann, Jan; Stieger, Alexander; Haselrieder, Wolfgang; Kwade, Arno (2015). "27". In Yan, Jinyue; Cabeza, Luisa F.; Sioshansi, Ramteen. Handbook of Clean Energy Systems Recycling of Lithium-Ion Batteries (5 Energy Storage ed.). John Wiley & Sons, Ltd. pp ISBN doi: / hces Torrey Hills Technologies, LLC. Furnace Temperature and Atmosphere Influences on Producing Lithium Iron Phosphate(LiFePO4) Powders for Lithium Ion Batteries. Retrieved October 18, Path: 8. Zipp, Kathie. "What are the components of a solar energy storage system?." Solar Power World, Solar Power World, 13 Aug Retrieved 18 Oct Path: 9. Comparison of commercial battery types (2017, September 10). In Wikipedia. Retrieved October 18, Path: Messenger, Roger, and Amir Abtahi. Photovoltaic Systems Engineering. fourth ed., Baca Raton, CRC Press, 2017, Ch Scott, P. B., & Simon, M. (2015). Utility Scale Energy Storage Grid Saver Fast Energy Storage System. Poway, CA: Transportation Power Inc. Retrieved from CODA ENERGY BEGINS OPERATION OF THE LARGEST BEHIND-THE-METER ENERGY STORAGE SYSTEM IN THE LOS ANGELES BASIN (2014, December 19). In CodaEnergy Exergonix. Retrieved from (n.d.). In Amita Technology Inc.. Retrieved October 17, 2017, from 6