All you wanted to know about batteries but were afraid to ask: August 2015

Friday, August 21, 2015

73. LITHIUM PLATING IS IMMENSELY HAZARDOUS

One of the serious consequences of fast charging lithium-ion batteries is the formation of lithium metal on the surface of the anode (the negative electrode when the battery is being charged). While the battery industry has invested significant effort to ensure the mechanical integrity of the battery and avoid unintended fires in case of mechanical damage, the formation of lithium plating during fast charging is a new challenge to battery vendors. Some battery manufacturers take it very seriously, whereas others tend to me more lax if not somewhat cavalier about its risks.

Let's be clear about...Lithium metal plating inside the battery creates extremely hazardous conditions that may lead to fires or even exploding batteries. Lithium plating leads to the formation of lithium metal deposits on the surface of the graphite (carbon) anode. These islands tend to grow over tend, both across the surface area and in the thickness forming dendritic-like structures. If they pierce (and they can) the separator -- the porous plastic layer between the two electrodes -- then an electrical short-circuit occurs leading to excessive heating and potential fires (in battery parlance, it is politely known as thermal runaway).

For the most part of the history of the lithium-ion battery, lithium plating was not a major concern. Well designed batteries ensured that they stayed away from the precursor conditions to lithium plating. Some battery manufacturers implemented additional safety measures -- such as special surface coatings -- that are intended to reduce the risk of a dendritic short-circuit. But with advent of high energy density cells and the rapid deployment of fast charging, the batteries are often operating near dangerous conditions. And some battery manufacturers seem to intentionally skirt the problem as it is not visible during daily operation -- that is until a fire occurs and the damage is done.

The next photograph shows the anode surface of a dissected polymer lithium-ion cell -- in fact, two identical cells, one charged at a slow charge rate (left side), and at a higher charge rate (right side). The cells were cycled 100 times before cut open and observed.

On the left side, the surface of the graphite anode is pristine. On the right side, bright stripes of lithium metal are apparent on the edges. That's where lithium metal tends to start forming -- the current density on the edges tend to be higher (concentrated electric field lines) thus presenting favorable conditions for the formation of lithium metal. Additionally, manufacturing defects are more likely to be present on the edges, also presenting "seeds" for plating. As the cell is further cycled (and aged), the lithium plating propagates and covers more of the anode surface, creating increased risks of a catastrophic failure.

Friday, August 14, 2015

72. LET'S TALK ABOUT THE WAIST

Not yours, of course....the smartphone's waist. We see a race among the smartphone makers to go thin. The iPhone 6 Plus is 6.9 mm thick and it is already been outflanked by some new devices coming from China. In particular, the Oppo R5 boasts a thickness of only 4.85 mm, and the Vivo X5 Max is an even thinner 4.75 mm. So what determines how thin one can go?

Naturally, the mechanics of the device are clearly one limiting factor...nobody wants their smartphone to "bend." For the most part, manufacturers are now using hardened aluminum cases for added resistance to bending. With the exception of some early complaints about the iPhone 6 Plus, there have been no credible reports of additional bending failures. Another limiting factor is the touch screen. There have been some great innovation here, most of it related to fusing the touch glass with the display, thereby reducing the touchscreen thickness. For example, the AMOLED screen on the Vivo X5 Max is only 1.35 mm thick.

So that leaves the battery as the last frontier...why am I not surprised? The battery seems to consistently win the title of bottleneck, and this is the topic of today's discussion. Why can't we make batteries ultra thin?

The answer is actually "yes, we can." Batteries can be made really thin, I mean thinner than you might imagine, sub 1 mm. But naturally, there are tradeoffs. The first tradeoff is that thinner batteries cannot boast the same energy density than their thicker counterparts -- there is just too much "electrical overhead" (e.g., connectors, plates) that they become dominant when the battery is too thin. See this earlier post that shows the impact of thickness on energy density. For a smartphone device, somewhere around 3 mm is the lower limit of battery thickness. Some smartphone makers instead choose to go thick just to provide more battery capacity -- the most recent example is the Moto X whose thickness is a whopping 11 mm, more than double Oppo's thickness !!!! So the first tradeoff is battery capacity vs. stylishness. Judging from the market trends, stylishness seems to be winning for now.

There is also a second and very important tradeoff, and that relates to swelling. I described in a very early post what happens to the battery as it ages...it bloats, and consequently becomes unsafe. This "swelling" phenomenon, through which the battery physically grows, has two components. They are shown in the next chart.

This chart shows the actual and measured thickness of a 3-Ah cell used in the LG G2 smartphone. It is a polymer cell and is embedded (i.e., non-removable) inside the mobile device. The thickness is measured over 60 cycles of charging and discharging. One readily observes two separate trends, almost like a yoyo on an escalator:

One trend is a fast variation in thickness with a known periodicity of one cycle (this is the yoyo effect). The thickness varies by about 0.15 mm, or approximately 3% of the cell's thickness but is a fully reversible effect. This is due to the physical expansion of the graphite anode. During charging, lithium ions intercalate (fancy language for "insert themselves") inside the carbon-graphite material (also known as matrix) thereby pushing the carbon atoms aside and causing physical growth. During discharge, the opposite happens and the anode returns to a thinner state.

The second trend is a slow, semi-linear growth in thickness (this is the escalator effect). This is related to irreversible damage to the graphite anode -- as the lithium ions go in and out of the anode, they leave just a tiny bit of damage that accumulates over time into this irreversible thickening of the anode (and consequently of the cell). As one can immediately observe, this second trend is significantly larger in magnitude than the first trend. For this cell made by LG Chem, the increase in thickness over 60 cycles is 0.15 mm, or 3% of the original thickness. Typically, over 500 cycles, this may reach 8 or even 10%.

As a result, manufacturers of smartphones need to make an allowance inside the device for the battery cell to grow in time -- this allowance is somewhere between 10 and 15% of the cell's thickness, or up to 0.7 mm; quite a significant number. Failing to provide this allowance risks placing large pressures on the touchscreen and cracking it.

Monday, August 10, 2015

71. IS THERE ENOUGH LITHIUM ?

Elon Musks' Gigafactory comes on line in 2017 to build some 35 GWh annually worth of batteries for Tesla. Thirty-five giga....that sounds like a big number. But wait, thirty-five gigabytes is a small capacity for your disk drive. So let's put things in perspective in this post.

Tesla's Gigafactory should be capable of producing by 2020 enough battery packs for some 500,000 electric vehicles. That corresponds to a capacity of about 75,000 kWh for each vehicle, with each pack including about 7,000 individual 18650 cells -- see this earlier post on the battery pack of a Tesla S. Quick math reveals that the annual output of the Gigafactory equals a little less than 4 billion 18650 cells annually. The annual worldwide production of lithium-ion batteries stood in 2014 just around 35 GWh, which means that Tesla intends to effectively double the global production -- and this is only for making 500,000 cars. So this begs the question, will there be enough lithium to make all, and I mean all, cars on the road electric? Let's explore.

First, let's estimate the total usage requirement of lithium. An individual state-of-the-art 3-Ah 18650 cell weighs about 48 g, and contains on average a mere 2 g of lithium. The lithium is in a salt form suspended in a solvent (the electrolyte) or embedded in the cathode material, often a composite metal oxide of lithium (such as lithium cobalt oxide). Therefore 35 GWh, or equivalently 4 billion 18650 cells, consume 7,900 metric tons of lithium.

But wait, lithium does not exist naturally in a metallic form -- remember, lithium metal is reactive. Lithium is mined in the form of lithium carbonate; by weight, it is 5.2x heavier than lithium. In other words, the demand in 2014 corresponds to 41,000 tons LCE (in the lingo of the mining industry, LCE stands for lithium carbonate equivalent to account for other forms of mined lithium salts such as lithium hydroxides). With a selling price of $8,000 per ton (and rising), one can readily see that lithium mining for batteries only is a $320m annual business growing at 7-8% per year. That's a pretty decent figure but it pales compared to the market size of other metal commodities such as copper. It's worth noting here that lithium is used for several other applications such as glass manufacturing. In 2014, the total worldwide demand for lithium exceeded 100,000 tons LCE. The vast majority of presently operating lithium mines are in South America, in particular Chile's Atacama salt flat and Bolivia. Increasingly, deposits in China and Canada, for example the Whabouchi field in northern Quebec, are being explored for further mining as demand and prices for lithium increase.

I still haven't answered the key question: is there enough of it? Lithium is a very light element and is fairly abundant in the Earth crust -- light elements formed early during the evolution of the Sun. The USGS estimates its abundance at 500 atoms per million atoms of Silicon (the most abundant element in the upper Earth crust), or about 0.05%. By comparison, it is as abundant as zinc and copper, and a million times more abundant than gold. We never questioned whether there is enough gold in the world, did we?

There have been no accurate studies to estimate the world reserves of lithium. SQM, the world's largest lithium mining company, estimates that the world reserves stand near 100 to 300 million tons LCE, or equivalent to 2,400x the demand that the Gigafactory is expected to generate.

So the brief answer is: don't worry, there is plenty of lithium. If you are a speculative investor, it may be a good time to buy land in Chile or Bolivia, at least what SQM and other mining companies haven't bought yet. You see, 007 didn't get it right in Quantum of Solace. The coup d'etat should have been to corner Bolivia's lithium salt mines, not its water supply.

Sunday, August 2, 2015

70. REVIVING A DEAD BATTERY, REALLY?

A statement of this devilish nature during the middle ages would have earned its author a burning at the stake. The mere notion of a "battery" would have probably been in the realm of druids and witches, and reviving anything dead would have been...well, enough said, it is the 21st century.

I will digress today a little and talk about your average primary (i.e., non-rechargeable) battery, the type that Energizer, Duracell, and their competitors sell billions of every year, all of which wind up in the trash bin or recycling centers -- excepting the batteries that sit on one's desk for ages as if somehow they will disappear on their own. How do these batteries die, and when they do, are they really dead? Let's explore.

For the purpose of this discussion, let's focus on Alkaline batteries. The typical ones, like AA or AAA, are nominally rated at 1.5 V. In other words, when they are fresh and unused, one would measure 1.5 V at the battery terminals. As the battery is inserted into a gadget and gets used, the voltage at the terminals drops, and fast it does. As the voltage drops, it reaches a point where it is no longer sufficient to power the electronics in the gadget. Often, these gadgets, such as toys, employ inexpensive electronics. This means that these electronics do not employ the most modern electronics circuitry. This is parlance for electronics that are not low-voltage and low-power. In other words, these inexpensive electronics draw more current than they need to, and they operate at higher voltages than they should -- all in the spirit of saving costs. But these operating requirements place a bigger burden on the battery, the result of which the battery's voltage drains rapidly and meets an early death.

It should be apparent to the reader that the end of the battery -- its "death" -- is now defined as the time at which its terminal voltage is no longer able to power the electronics. This is somewhat subjective because that clearly depends on the quality and sophistication of the electronics in your gadget. Usually, many inexpensive electronics begin to stop operating somewhere between 1.2 V and 1.35 V. Very rarely, one may see electronics get lower in operating voltages but such gadgets would most likely be associated with higher price points, and could very well just use an embedded lithium-ion battery to project an image of a "good" product.

Looking at the Energizer E91 specification sheet, one immediately can observe that this battery has a life of less than 2 hours to hit 1.3 V, and 3 hours to hit 1.2 V (assuming a discharge current of 250 mA). At this point, the electronics begin to stop operating; the cheap display on your child's toy begins to fade, and voila, you pronounce the battery dead and discard it.

But wait! Is it really true that the battery is dead? Again, it is a matter of definition. For a helpless parent trying to appease a screaming child, the battery is DEAD. But to some engineers and entrepreneurs, they will be quick to observe that this battery continues to hold a lot of charge and energy. Looking at the voltage chart above for the E91, the area under the red curve is the amount of "energy" that the battery holds. So it becomes immediately clear that if the battery is declared dead at 1.2 V, it continues to hold about 75% of its original energy. This is a lot!

So the magic question becomes how to access this extra energy well? and how to do so in a cost-effective and reliable manner? This is where I will put a plug for the company Batteroo Inc.. The team figured out an elegant solution to put a very thin reusable sleeve around the presumed dead battery with low-power electronics that will "boost and regulate" the raw terminal voltage of the battery back up to a higher voltage, say 1.5 V, sufficient now to operate a gadget. This has an effect of reviving this "dead" battery and substantially extending its life. I love clever and simple ideas! Batteroo's challenge is now to fight off the battery vendors who will not be pleased with selling fewer batteries.