Samsung finally released details early this week on the root cause of the Galaxy Note 7 fires and explosions that started to show up in August 2016. You can find details on the story here, but the gist of the matter is – failures and explosions started to show up with batteries from Manufacturer A, soon after the phone started to sell. Samsung provided replacement phones with batteries from Manufacturer B, but the failures and explosions continued (if you are in the battery world you would use the phrase “spontaneous disassembly”). The company had to halt production and eventually scrap the entire product.
The root cause came down to a bad design and manufacturing defects. (Check out this blog article for great battery failure analysis pictures and clearly explained root cause):
a) Root cause of cell failures from Manufacturer A was due to deformation and bending of the negative electrodes at the top corner of the polymer pouch constrained by allotted space for the battery in the phone (not having enough space to accommodate cell expansion during cycling). Internal shorts could now be created in one or more ways – a thinner separator in some regions; separator defects; or, other damage during manufacturing.
b) Root cause of battery failures from Manufacturer B (which were used in the replacement Note 7 phones) was due to burrs created during the ultrasonic welding of the positive tab to the positive electrode. These protrusions created burrs, resulting in an internal short between the electrodes. What is even more shocking is that some of the batteries were found to be missing insulating tape on the positive electrode tabs (separator was still intact). Clearly consistency in the manufacturing process was lacking.
People in the battery industry must surely be scratching their heads, trying to figure out how two suppliers of such a large corporation with mature manufacturing processes have two different battery failures related to manufacturing/design issues, both occurring within a span of a few weeks. Some news reports suggest that batteries in the replacement phones were manufactured under a time crunch, and the manufacturers may have taken some shortcuts. Whatever the real reasons, there are a few things we can all take away from this story:
1) A sound manufacturing process is the first and most important line of defense in battery safety and reliability. You can have batteries with latent manufacturing defects that pass quality control checks and essentially masquerade as good batteries. As you age or cycle batteries, the manufacturing defects get exacerbated due to expansion and contraction effects during cycling, expansion of electrodes from high temperature aging, etc. One example is a battery with marginal electrode adhesion on the anode side. As the cell is cycled, the graphite anode can start to delaminate from the copper current collector. When this happens a portion of the anode is removed from the reaction, and now the lithium ions have fewer “cubbyhole spaces” to intercalate into during the charging process. With nowhere to go, the ions plate onto the surface as lithium plating, increasing the risk of internal shorting and also initiating self-sustaining reactions between the electrolyte and the anode.
It is therefore of utmost importance to ensure that you have a sound manufacturing and assembly process with adequate quality checks at every single stage going back all the way to the individual components in the stack up and the electrode fabrication.
2) Lithium ion batteries have a very narrow window of voltage and temperature in which they operate well and safely. The job of the battery safety management system in electronics is to make sure that the battery is not overcharged or over-discharged and does not operate below or above specified temperatures. If you go outside this window, you can start to see a snowballing effect. Battery users often use low-cost spare chargers in a vehicle or at home, and many of these work improperly. As users we should unplug chargers when batteries are charged, make sure they are not exposed to excessively high temperatures and only use approved chargers for our devices.
3) Safety systems don’t always protect in case of internal shorts. There are internal safety protection systems in lithium ion batteries as well as external ones that prevent the battery from over-charge or over-discharge. The problem is that temperature is a lagging indicator of thermal runaway, not a leading indicator. In a thermal runaway situation resulting from an internal short circuit, the safety systems are not able to respond fast enough to prevent them. The only preventive action is to ensure strict process control on the manufacturing process.
In summary, it all starts with the battery manufacturing process and there are simply no shortcuts. We sometimes forget that lithium ion batteries have immensely high energy density, and carry flammable organics inside. Clearly something with such potential danger deserves respect. If we want to be safe, we as users need to respect their operating boundaries, and manufacturers need to ensure utmost quality in the assembly process.
For other helpful articles and webinars related to root cause analysis, failure mechanisms or product reliability please visit our resources page or watch our recently recorded webinar, Battery Management Systems and Their Role in Safety and Reliability by simply clicking below.