Why does the Samsung Note 7 battery explode?

Recently, there have been frequent reports of Samsung Note 7 explosions, and even the Chinese version has been "hit" (one blue, one gold, both from JD.com). Previously, the General Administration of Quality Supervision, Inspection and Quarantine has issued a notice stating that Samsung (China) Investment Co., Ltd. will recall some Galaxy Note 7 manufactured between July 20, 2016 and August 5, 2016, starting from September 14, 2016, with 1,858 units in mainland China.

It is understood that these products were provided as test experience machines through channels such as the Galaxy Community on Samsung's official website through the trade-in method before the official sale on September 1, 2016.

The General Administration of Quality Supervision, Inspection and Quarantine stated that the Note 7 batteries covered by this recall had short circuits when the anode and cathode separation membranes were partially thinned and the insulating tape did not completely cover the plate coating, causing the batteries to heat up abnormally and, in extreme cases, could catch fire, posing a safety hazard.

For digital mobile phones within the recall scope, Samsung (China) Investment Co., Ltd. will replace the customer with a new digital mobile phone of the same model that meets the relevant requirements free of charge to eliminate safety hazards.

It can be seen that the reason for the explosion of Note 7 was that the battery's anode and cathode separation membrane became locally thinner and the insulating tape did not completely cover the plate coating, resulting in a short circuit.

Why does the battery separator become thinner? What safety risks does it cause? Chen Liwei, a researcher at the Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, explains this question.

He said that the lithium-ion battery used in mobile phones realizes the charging and discharging function of the battery through the back and forth movement of lithium ions between the positive and negative electrodes. When charging, the current of the charger drives the lithium ions in the positive electrode out, and these lithium ions "swim" to the negative electrode through the electrolyte between the positive and negative electrodes; and when discharging, these lithium ions "swim" back to the positive electrode from the negative electrode through the electrolyte, providing power for the mobile phone. In this process, the positive and negative electrodes must not be in direct contact, otherwise a short circuit will occur, causing abnormal heating of the battery, and even fire and explosion.

How to ensure that the positive and negative electrodes inside the battery do not touch each other and there is enough electrolyte between them for lithium ions to "swim" back and forth? This is achieved through a key material in lithium-ion batteries - the diaphragm.

So, is the thinning of the Note 7 diaphragm a production design error or is there some other hidden reason?

Chen Liwei speculated that this might be because mobile phone manufacturers used thinner diaphragm materials in order to increase the volume energy density of the battery and extend the battery life of mobile phones. However, due to lax quality control of the diaphragm material or process defects, the diaphragm became locally thinner and could not effectively isolate the positive and negative electrodes, thus causing battery safety problems.

Chen Liwei said that mobile phone manufacturers are very concerned about the volume energy density of batteries. "Almost all manufacturers are trying to pack more power into a limited volume to extend battery life."

However, the industry consensus is that it will take a long time to improve the energy density of batteries. Chen Liwei explained to reporters: "The development laws of the energy storage industry and the electronics industry are very different: the latter is the well-known Moore's Law, the number of transistors that can be accommodated in integrated circuit chips increases exponentially, and it can double about every 18 months; while the battery performance improvement process is step-shaped. Based on the existing battery materials, an annual increase of 2%-3% in energy density is already quite good." Therefore, there is a scissors gap between the performance improvement of electronic products and the energy density improvement of batteries, and it continues to expand over time.

Chen Liwei admitted that the energy density of mobile phone batteries is currently approaching its "limit". In order to provide more energy in a smaller volume, manufacturers can only find ways to squeeze the space occupied by auxiliary materials and "slim down" the batteries.

However, it is difficult to achieve both "slimming" and "safety" in mobile phone lithium batteries.

"For the same battery, if the separator becomes thinner and the volume of this part is reduced, a little more positive and negative electrode materials can be loaded, which can improve the battery energy density and the battery life of the mobile phone. However, the thinner the separator, the more stringent the requirements for the process. Slight quality defects or battery process errors may cause separator defects, which in turn lead to battery short circuits." Chen Liwei pointed out the risks.

Finally, Chen Liwei called for that battery is an energy-containing device and its safety is always the top priority. Therefore, while pursuing higher energy density batteries, battery safety must not be ignored.

As a winner of Toutiao's Qingyun Plan and Baijiahao's Bai+ Plan, the 2019 Baidu Digital Author of the Year, the Baijiahao's Most Popular Author in the Technology Field, the 2019 Sogou Technology and Culture Author, and the 2021 Baijiahao Quarterly Influential Creator, he has won many awards, including the 2013 Sohu Best Industry Media Person, the 2015 China New Media Entrepreneurship Competition Beijing Third Place, the 2015 Guangmang Experience Award, the 2015 China New Media Entrepreneurship Competition Finals Third Place, and the 2018 Baidu Dynamic Annual Powerful Celebrity.