Abstract
Since a laptop caught fire in 2006 at the latest, Li-ion cells were considered as more dangerous than other accumulators [1]. Recent incidents, such as the one involving a BYD e6 electric taxi [2] or the Boeing Dreamliner [3], give rise to questions concerning the safety of L#i-ion cells. This is a crucial point, since Li-ion cells are increasingly integrated in all kinds of (electric) vehicles. Therefore the economic success of hybrid electric vehicles (HEV) and battery electric vehicles (BEV) depends significantly on the safety of Li-ion cells. Lithium nickel manganese cobalt oxide (NMC) and lithium nickel cobalt aluminium oxide (NCA) are two standard Li-ion cathode chemistries, which are often used for today’s HEVs and BEVs Li-ion batteries. Cells with this two cathode technologies are investigated in detail and compared to cells with the alleged save lithium iron phosphate (LFP) technology. Furthermore only commercially available and mass produced Li-ion cells were tested, in order to get as close to real end-user applications as possible. To ensure comparability, cells with the most common 18650 casing have been used. Furthermore all cells had no built-in resistor with positive temperature coefficient (PTC-device). For each abuse test at least 2 cells have been tested to get to know the statistical dispersion. The spread was in all tests for all measured values of each cell type lower than 11 %. Consequently it can be supposed, that mass produced cells show equal behaviour also in abusive test. The performed electrical safety tests on these cells, involve overcharge, overdischarge and short circuit tests. These tests represent real abuse scenarios and are geared to established standards [15], [16], [17], [18]. To complete these measurements an accelerated rate calorimetry (ARC) test has been carried out, to determine the thermal stability of the cells. As in the literature discussed, the investigated LFP/C cells show a higher thermal stability and are therefore safer, although they do not have any overcharge buffer as the investigated NCA/C and NMC/C cells.
Highlights
Because of their high energy density principally Li-ion batteries are chosen for electric vehicles
Since a laptop caught fire in 2006 at the latest, Li-ion cells were considered as more dangerous than other accumulators [1]. Recent incidents, such as the one involving a BYD e6 electric taxi [2] or the Boeing Dreamliner [3], give rise to questions concerning the safety of L#i-ion cells
In this paper the safety behaviour of the nickel manganese cobalt oxide (NMC)/C and nickel cobalt aluminium oxide (NCA)/C technologies, which are often used in electric vehicles, are investigated and compared to the alleged safe LFP/C technology
Summary
Because of their high energy density principally Li-ion batteries are chosen for electric vehicles. The various Li-ion technologies show different properties in terms of safety behaviour, energy density, electrical loading capacity and voltage level. In this paper the safety behaviour of the NMC/C and NCA/C technologies, which are often used in electric vehicles, are investigated and compared to the alleged safe LFP/C technology. All of the cells had no built-in resistor with positive temperature coefficient (PTC). Their electrical characteristics are listed in Tab. 1. The lower and flatter voltage level compared to NMC/C and NCA/C is inherent for LFP/C cells. The standardized safety tests for Li-ion cells and batteries can be classified in mechanical, environmental and electrical abuse scenarios. The presented investigations focus on the electrical abuse behaviour of the named 3 Li-ion technologies. Since the thermal stability accomplishes the comprehensive investigations, each cell has been characterized by accelerated rate calorimetry (ARC)
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