Galaxy Note 7 Battery Autopsy and Analysis Background and the Public Picture Samsung Galaxy Note 7 phones have caught fire, often while charging. This caused a recall of the phones,

and the FAA to issue a ban for carrying or using the phones on airplanes in the USA.

While we don't have the full picture, we do have some clues. The Verge says "the separators were flawed

and let the two electrodes to touch.” In a statement, Samsung said,

"Based on our investigation, we learned that there was an issue with the battery cell. An overheating of

the battery cell occurred when the anode-to-cathode came into contact which is a very rare

manufacturing process error. We are working with multiple suppliers to ensure that a rigorous inspection

process is conducted to ensure the quality of our replacement units and we do not anticipate any further

battery issues."

Let's put this in context. A battery consists of

four basic components: anode, cathode,

separator and electrolyte. Basically, the anode

and cathode hold the charge. During charge and

discharge, the ions flow through the electrolyte

from one electrode to the other, while the

electrons flow through the circuit or device (in

this case, the phone). The separator is a porous

membrane which holds the electrolyte,

allowing the ions to flow back and forth, but

keeping the anode and cathode electrically

isolated from each other.

It is important to note that the electrolytes are organic solvents and burn similar to lighter fluid or gasoline.

So they must be kept cool and can in no case be exposed to a spark or high heat.

Over time, the technology used for the separator has evolved. Originally, polypropylene or polyethylene

film was made to have pores in them, and this film was used. Those bare films are first generation

separators.

But those films shrink and melt at relatively low temperatures, so the industry has adopted coating them

with a thin layer of highly pure ceramic nanoparticles. This coating reinforces the separator, covers any

flaws, and improves slightly the thermal stability. These coated films are second generation separators

and have been adopted by much of the industry. However, they also will shrink when brought close to or

above the melting point of the underlying film.

Samsung SDI and ATL The batteries made for these phones were made by Samsung SDI and by ATL. If not identical, the batteries

were of very similar design. Both Samsung SDI and ATL are among the very best battery companies in the

world, and produce batteries of the highest quality.

In this study, there was no indication of a “very rare

manufacturing process error,” but it is possible.

The First Cell and Autopsy Results Two Samsung Galaxy Note 7 phones were obtained

from individuals. With the first, we discharged the

battery by running the phone until it stopped. The

phone was opened using a heat gun and running a

blade between the bottom glass and the battery

until all the adhesive had been broken. The bottom

glass was removed, then several screws removed

and a few pieces, to reveal the battery. See the

picture. It was clear that the battery was adhered

into the phone, but we did not want to use a heat

gun. (A smarter person, at this point, would have

realized that the cell still contained energy and

taken the precautions that we took with the second

cell.)

Using a flat head screwdriver, I gently tried to lift

the cell out of the case. I heard a pop, then saw a

bit of smoke coming from a hole along the edge of

the cell. I picked up the phone with a hot glove and

took it out to the back porch, where it swelled and

clearly caught fire, though did not give a high level

of external sparks or flame. The second picture is of

the phone after the event had occurred.

The cell was then autopsied. There was no smell of

electrolyte, indicating that it had all burned off. No

discernable separator could be identified. In fact,

the cell appeared to be only a roll of copper and

black powder, which is the remains of the cathode,

separator, anode and aluminum current collector.

The fact that there was no discernible aluminum left

in the cell indicates that the temperatures got very

hot indeed, but not hotter than the copper melting

temperature. Aluminum will oxidize at room

temperature, and very rapidly at 800 C, while

copper melts at 1085 C. The last two pictures below

show the burned cell after removal from the phone,

and after disassembly. The black powdery material

is all there was except the copper.

The Second Cell A second phone was obtained, and the phone opened like before.

This time, a few other pieces were removed gently to reveal the tabs

for the cell. The tabs were then connected through a high-power

resistor, discharging the cell until the voltage across the cell read

zero volts. At this point, a flat screwdriver was again used to gently

lift the cell from the base where it was adhered. “Gently,” here is a

misnomer, because the cell was adhered quite well, and some force

had to be used to remove the cell. Once removed, to be sure, the

cell was again discharged through the high-power resistor.

To the right is a picture of the removed cell, and below is a picture

of the jelly roll. Notice that the form was of a cell wound on a

mandrill and then flattened. After disassembly, the

dimensions were obtained of the various parts and

listed in the table below. The copper, electrode

materials, and separator were all standard

components. More details are given below.

It is important to note that all of the materials and

workmanship of the cell appeared to be of the

highest quality and excellent manufacturing. There

was absolutely no indication that any corners were

cut to reduce costs that might have contributed to

the problem.

The cell was only weighed after opening, so most of

the electrolyte was lost. The weight without

electrolyte was 44.5 g. If the electrolyte was

assumed to be 10.5 g (probably an overestimate), then the total weight was 55g for a 3.5 Ah, 14 Wh cell,

giving 250 mWh/g, a very high energy density. With the cell dimensions below, the total volume of the

cell was 0.02 liters, giving an energy density of 700 Wh/l, also a very high energy density.

Part Length Width Thickness

Full Cell 105 +/- 5 mm 38 +/- 2 mm 4.9 +/- 0.5 mm Copper 595 +/- 10 mm 91 mm 6.5 m +/- 1 m Anode Coating 515 +/- 10 mm 75 m +/- 5 m Aluminum 650 +/- 10 mm 91 mm 10 m +/- 1 m Cathode Coating 570 +/- 10 mm 70 m +/- 5 m Separator 6.6m +/- 0.1 m

Components

Separator The separator had total thickness 6.6 microns, with ~1 micron of ceramic on one surface. The ceramic

coating had particle sizes that ranged from 500 nm to 2 microns, so in most places the coating was only a

few particles thick. The separator was wet processed polyethylene. See the scanning electron

micrographs (SEMs) below, clockwise from top left:

Cross section of separator showing thickness and ceramic coating.

Exposed area of wet processed polyethylene film

Ceramic coating at low magnification

Ceramic coating at higher magnification

Anode The anode was graphite on a copper current collector. The copper was 6 microns thick, though there may

have been an adhesive coating on one side of approximately 1 micron. The anode coating was 75 microns

thick, coated on both sides. Energy dispersive x-ray spectroscopy (EDS) revealed the anode to be a

combination of carbon and graphite with a fluorine-based binder. See the SEMs below.

Cathode The cathode was identified by EDS to be LCO on an aluminum current collector with a fluorine-based

binder. The aluminum was 10 microns thick, and the cathode coating was 70 microns thick, coated on

both sides. The particle size of the cathode was large, up to 10 microns, and gave large variation in coating

thickness.

The SEMs below are, clockwise from the upper left:

Copper with thickness measured

Copper and anode coating with thickness measured

Aluminum and cathode coating with thickness measured

Aluminum and cathode coating with thickness measured (2nd image)

What Were the Risk Factors? By our assessment, there were four prominent risk factors that, combined, gave a much higher probability

of a cell failure than with a cell without these risk factors. Note that we are not assessing any rare

manufacturing defects which would also have compounded the problem.

Wound Cell vs. Stacked Cells The cell was wound on a mandrill and then

flattened. This process creates stresses at the

corners, where the electrode coating is

compressed on one side and expanded on the

other, and where the separator is made to fit

between these layers as they change dimension.

These stress points can also be a source of high

current density during charge and discharge as the

high curvature of the metal foils can create high

electric fields. Last, because of non-uniform

current flows, these areas can be more prone to

overcharge and undercharge, where plating and

dendrites are more likely to occur as well.

Fast Charge The Galaxy Note 7 was equipped with a fast charge, which would enable the cell to be charged to get four

hours of use in just 10 minutes. This fast charge will cause much higher current than normal a normal

charge, and also likely have a much less uniform charge, especially around the “pinch points” at the folds

formed when the jelly roll is compressed. In addition, the fast charge will heat the cell more and faster

than a slow charge, and this heat may be concentrated in areas of higher current.

Thin Separator The 6.6-micron thin separator, ceramic coated on a single side, was likely among the thinnest ever used

in a production lithium ion battery, and could have presented some issues. The base layer was

polyethylene, which has a melting point of only 135 C, meaning that any heat generated from other

sources would quickly melt the separator. If a hot spot is created due to a dendrite or other defect, the

separator may shrink away from the hot spot, creating a bigger short with higher current density, creating

more heat and more shrinkage and eventually ending in enough heat to bring the cell into thermal

runaway. While many separator manufacturers will talk about a “shutdown” feature in which the cell,

when uniformly heated, can show very high resistance when the uniformly heated cell reaches a

temperature somewhat below the melting temperature of the polymer. Shutdown, however, does not

protect against a local hot spot or against cells in which the temperature rises above the melting point of

the polymer film. This separator would normally be described as a shutdown separator, and clearly any

safety imparted by this feature was inadequate.

Small Cavity The cell was in a cavity that was not much thicker than the cell. When charged, the electrode materials

expand, making the cell a bit thicker than it was when discharged. As the cell is charged in a confined

space, it can undergo very high pressures. Here is what is going on with each component:

Potential stress point where coatings

are compressed & stretched, and

separator is compressed.

Ceramic electrode materials expand

Metal (Al and Cu) current collectors cannot compress, but remain rigid.

Separator compresses under the stress of the increased cell volume in the confined space.

This, especially with a separator as thin as 6.5 microns and along with the other risk factors listed above,

can lead to a higher risk of failure.

Discussion and Conclusions It is entirely likely that a single of the risk factors listed above may not have caused a problem at all, or

certainly not of the magnitude that has been experienced. In fact, the cell itself, outside of the confined

space of the cavity inside the phone, may not have ever had an issue and may have fast charged 100s or

1000s of times without issue. Likely the cells did during the qualification testing. Normal safety testing

of the cell may also have posed no issue, passing the same hot box, nail penetration, hard short and other

tests that are normally demanded of lithium ion cells.

However, when the cell, with its thin separator and tight corners caused by the compression of the

mandrill-wound jelly roll, was fast charged in a confined space, the risk of failure was much higher than if

those design choices had been made differently. Any one or two may have ended up without issue, but

all four together likely contributed to the high frequency of failure and eventual recall of the Galaxy Note

7. That the cell failures continued even in the replacement phones indicates that it was more of a system

problem than one of a “very rare manufacturing process defect,” as originally indicated.

A Potential Solution One thing is certain from the failed cells—that when a hot spot or spark occurred, the separator shrank,

allowing the electrodes to come into contact, which caused more current to flow, causing more shrinkage,

increasing the electrode contact surface area, increasing the current, increasing the heat, until thermal

runaway occurred. This is the vicious cycle which resulted in a fire. The “shutdown feature” did not

prevent this failure, and was clearly an inadequate protection that has nonetheless been widely adopted

by the industry.

As many of the readers of this note will know, Dreamweaver is a manufacturer of a separator that does

not melt and shrink under thermal stress, but rather undergoes a slow oxidation, which will only begin

after the cell has reached a very high temperature. But even when it oxidizes, the separator remains in

place, keeping the anode and cathode apart,

rather than allowing a short to grow. In the case

of cells with this type of separator, when a short

occurs, the separator remains intact until the next

weakest link degrades. The next weakest link is the

aluminum current collector, which will oxidize into

alumina, a resistor. Instead of a vicious cycle

where the worse it gets, the worse it gets, we have

a virtuous cycle, where the worse it gets, the better

it gets. The more heat that is generated, the more

the aluminum current collector will oxidize,

shutting off the current. In test after test, cells

Aluminum Oxide

Separator

with a thermally stable separator are not going into thermal runaway where those made with low-melt

separators, even ceramic coated and with shutdown feature, are allowing thermal runaway.

As a final note, see the last figure, which shows a nail penetration test in a cell made with the

Dreamweaver Gold separator. The aluminum current collector layer ends in a 100-micron thick layer of

alumina, breaking the short, and the separator can be seen intact all the way to the edge of the current

collector. Likely some oxidation had occurred, but the separator remained in place, and the electrodes

did not come into contact.

The Future So why don’t Samsung, and others, choose to use Dreamweaver separators in their cells? There are two

reasons.

Consumer Pressure They are under extreme pressure from the

consumers of their devices to deliver very

long lasting cells with high energy densities in

very small spaces. To achieve this, they

choose separators that are thinner than

Dreamweaver can currently manufacture.

The thinnest Dreamweaver separator for

lithium batteries currently available is 20

microns. Our target is to have 15 micron

separators available in 2017, and 12 microns

available in 2018. Looking at the chart to the

right, if one goes from a 6.6 micron separator

to a 12 micron separator, the total cost in

energy density is only 3%. In hindsight, this

might be a small price to pay for the

assurance of a much safer battery. But

Dreamweaver will only deliver this product in 2018.

DWI Product Availability Our first product, Dreamweaver Silver, has just reached full manufacturing capability in mid-2016, and is

only now being tested and qualified in cells. Our advanced safety separator, Dreamweaver Gold, will only

be available in 2017 at full manufacturing scale. Both products will start first at 20 and 15 microns, and

only be available at 10 – 12 microns in 2018. However, once available, we can expect that problems like

those experienced by Samsung will soon become a thing of the past.

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Energy Density mWh/g % change vs 6.6 microns