Silicon/Lithium Iron Phosphate Safe and High-Energy Density Lithium-Ion Cells: From Material Research to Demonstrator

Abstract

Under the German-Greek bilateral R&D cooperation initiative 2013-2015, the Safe High Energy Lithium-ion Cells for Electric Vehicles” – SHELION project aimed to investigate both the anode and the cathode materials with a clear objective to manufacture cells with a specific energy density of more than 150 Wh/kg with adequate cycling charging/discharging performance, complying with industrial manufacturing practices, safety standards and environmental regulations. Towards this direction, this project is focused on the lithium-ion technology and most precisely with the Silicon (Si)-Lithium Iron Phosphate (LFP) electrochemical system that due to silicon should provide excellent energy density and due to LFP excellent safety properties and cost advantage [1]. Silicon as anode in Li-ion technology is considered as a promising material that will eventually replace graphite mainly thanks to its high specific capacity that may attain more 4200 mAh/g (theoretical value) compared to graphite’s specific capacity (372 mAh/g, theoretical value) [2,3]. However, silicon undergoes mechanical stress that is induced during lithiation and delithiation, which leads to poor life cycle [4]. During this project, silicon deposited by DC sputtering was investigated and after multiple substrates used, anodes with high specific capacity of more than 2000 mAh/g and 2.5 mAh/cm2 were manufactured [5,6]. Silicon’s capacity was found to be stable for more than 50 galvanostatic cycles. Concerning the cathode, slurry based LFP was developed and achieved specific capacity of more than 3.5 mAh/cm2 and 180 mAh/g. The electrochemical characterization of these materials and their combinations with various electrolytes was performed in small cells. Based on these results, 820 mAh pouch cells were designed, developed and manufactured (demonstrators). Galvanostatic cycling were performed and an overcharge safety test according to IEC 62660-2. The demonstrator cell with a capacity of nominal 820 mAh was put into a sealed stainless steel can, equipped with an internal camera to observe the cell during the test. Additionally the can was linked to a mass spectroscope, a gas chromatograph and a FTIR device to measure released gas online. Temperature sensors on the demonstrator pouch cell enabled a monitoring of the cell temperature during the test. The demonstrator was charged with C/2 until the state of charge (SOC) was measured to be 200% and therefore fulfilled the guidelines of IEC 62660-2 successfully. A thermal runaway was enforced when the demonstrator was charged with 4C and had a SOC of 300%. The temperature at thermal runaway was measured to be ~340 °C. This contribution will show a distribution of released gas components during the overcharge safety test and provide information about the harmfulness of these substances. To our knowledge, this is the first time that safety tests were conducted to Si/LFP cells. This work was supported by the European Regional Development Fund and National Funds/German-Greek Bilateral R&D Cooperation Initiative/2013-2015.