Pushing to the Limit: Samsung Galaxy Note 7 and Fitbit Flex 2
Authors: Tan Chaou Choak , Loveridge Melanie J , Dixon Shaun , Remy Guillaume , Kourra Nadia , Genieser Ronny , Barai Anup , Lain Michael J , Guo Yue , Amor-Segan Mark , Williams Mark A , Amietszajew Tazdin , Ellis Mark , Bhagat Rohit , Greenwood David 
Published: February 2020

The ongoing demand for smaller and lighter devices, with batteries having longer usage times, has put increasing operational pressure on the batteries. The use of higher capacity cathodes, increasing areal capacity and thinner separators are some of the approaches used to achieve higher energy densities. Here we report two case studies on batteries from the Samsung Galaxy Note 7 phone and fitness tracker wristband, Fitbit Flex 2, highlighting where the device manufacturers were pushing the boundaries to achieve this higher energy density.
In September 2016, several reports were published of the Samsung Galaxy Note 7 phone batteries overheating, exploding or catching fire, which led to a total product recall of the Samsung Galaxy Note 7 phone. “The launch of what was supposed to be its flagship model has turned into a fiasco for the world's largest smartphone maker”, the BBC reported1. We carried out a more in-depth investigation, “Looking Deeper into the Galaxy (Note 7)2”, inspecting each of the battery components. The study incorporated X-ray computed tomography, thermal imaging and scanning electron microscopy (with energy dispersive spectroscopy), to identify the most likely root causes of the battery failure. The findings reveal that thermal properties of the battery is not thought to be a contributing factor to battery failure but are from battery design/cell manufacturing defects: (1) Unintended folding of the anode coating at the corner near to the anode tab resulted from insufficient space to safely accommodate the electrodes; (2) No Insulating tape on the cathode tab, and (3) un-bonded welding sections with protrusion features observed at the cathode tab which may penetrate the separator. Such flaws can lead to internal short circuit between the cathode and anode; (4) A much thinner separator (≤ 5 µm, with <1 µm Al2O3 coating on both sides) used in Galaxy Note 7 also increased the likelihood of separator damage which can lead to short circuit. The ceramic coating layer detached itself quite easily when the cell was unravelled, indicating a lack of interfacial integrity.
We also report another forensic investigation on a miniature lithium ion rechargeable cylindrical cell with a diameter of 3.65 mm and length in 20 mm, and nominal capacity of 15 mAh, which is fitted in Fitbit Flex 2 fitness tracker. This miniature cylindrical cell shows an impressive volumetric energy density of 503 Wh/L despite having a limited space. The unravelling of the jelly roll shows severe anode coating delamination from the copper current collector, transferring unto the separator. The high bending stress exert on the cell components when rolled up to a 3 mm diameter jelly roll has resulted severe delamination of the anode coating.
1. http://www.bbc.co.uk/news/technology-37615496
2. Melanie J. Loveridge, Guillaume Remy, Nadia Kourra, Ronny Genieser, Anup Barai, Mike J. Lain, Yue Guo, Mark Amor-Segan, Mark A. Williams, Tazdin Amietszajew, Mark Ellis, Rohit Bhagat and David Greenwood, Looking Deeper into the Galaxy (Note 7). Batteries
2018, 4 (1), 3.

Figure 1