Abstract
Li-ion cell designs, component integrity, and manufacturing processes all have critical influence on the safety of Li-ion batteries. Any internal defective features that induce a short circuit, can trigger a thermal runaway: a cascade of reactions, leading to a device fire. As consumer device manufacturers push aggressively for increased battery energy, instances of field failure are increasingly reported. Notably, Samsung made a press release in 2017 following a total product recall of their Galaxy Note 7 mobile phone, confirming speculation that the events were attributable to the battery and its mode of manufacture. Recent incidences of battery swelling on the new iPhone 8 have been reported in the media, and the techniques and lessons reported herein may have future relevance. Here we look deeper into the key components of one of these cells and confirm evidence of cracking of electrode material in tightly folded areas, combined with a delamination of surface coating on the separator, which itself is an unusually thin monolayer. We report microstructural information about the electrodes, battery welding attributes, and thermal mapping of the battery whilst operational. The findings present a deeper insight into the battery’s component microstructures than previously disseminated. This points to the most probable combination of events and highlights the impact of design features, whilst providing structural considerations most likely to have led to the reported incidences relating to this phone.
Highlights
Lithium-ion batteries (LIBs) are statistically safe, with a 1 in 10 million failure rate established during the manufacturing and such failures are considered an exception and not a reliability problem [1]
The primary purpose of our activity was to develop and combine a suite of forensic investigation techniques to detect and characterise failure modes in LIBs
From the tomography scans around the edges of the battery it appears that the cell which we analysed was derived from Manufacturer A: Samsung SDI
Summary
Lithium-ion batteries (LIBs) are statistically safe, with a 1 in 10 million failure rate established during the manufacturing (rather than due to field ageing) and such failures are considered an exception and not a reliability problem [1]. Following a short circuit event the localised temperature inside the cell begins to rise and at of electrolyte solution on the surface of the electrodes. This is termed the ‘solid electrolyte interphase’. A series of exothermic reactions follow between SEI short circuit event the localised temperature inside the cell begins to rise and at 100 compounds and electrolyte such as ethylene carbonate (EC). °C the SEI film begins tosolvents decompose [11] Whencompounds this cascade reactions proceeds it causes rapid elevation of the internal andof electrolyte solvents suchuninterrupted, as ethylene carbonate (EC)aand diethyl carbonate temperature whereby series of reactions can generate large volumes of flammable. This is a very high priority and active area within the energy storage research community [20]
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