Chemistry

16Jan 2015

Imagine a world where your device, whatever it may be, has a source of energy — may be a battery, may be not — that lasts, I mean lasts to the point you don’t think about the period. It lasts in its operation and lasts in its longevity. If you ever find yourself with a low energy level, and I mean ever, then it’s only minutes before your device is recharged. And yes, the energy source is so tiny that you can’t even point at it. I know, this is in a distant universe, but why is it so distant? After all, a drop of gasoline has enough energy to power a mobile device for days or weeks, and one can replenish this source of energy in no time. And you don’t have to worry about it going bad. So will we ever get there? Yes, with time, patience and lots of effort, in other words, dollars, sweat and time. You have got to believe that StarTrek will happen in this distant future, and we will have dilithium fuel to power everything.

But who’s participating in this effort? Is there hope? This blog sheds some light on the numerous companies, many of which are innovative startups, that are trying to change the world on this front. Two types of energy storage seem to garner interest and development effort: Fuel cells and batteries. I will focus here on batteries, especially lithium-ion batteries that are applicable to mobile devices.  Innovation and development are loosely broken into five categories as shown in the figure below.

Developing new anode materials is one of the prime focus areas for several startup companies. This is aimed at raising the energy density beyond the wall that is now towering over the industry.  Companies such as Amprius, Enevate, SiNANOde are examples of startups focused on silicon-based anodes. Quantumscape, a spinout from Stanford University, is focused on a entirely new structure and material system with high dielectric constants. Envia seeks to develop higher voltage cathodes — by raising the voltage, one also raises the energy density. Lab demonstrations have been very promising at many laboratories but scaling to real-world production remains a tall task for these companies.

Electrolytes is another fertile area of investigation. Plenty of effort is aimed at replacing the liquid or gel-like electrolyte medium by a solid-state material with conductive properties for the ions. SEEO, Sakti3 and several other companies are working on launching batteries with solid-state electrolyte. It’s a promising area but serious limits still remain including scaling to cost-effective volume production.

Improving the design and manufacturing of batteries is also another area of great interest. Enovix is combining newer materials that can be deposited and manufactured using semiconductor-like processes. Imprint Energy seeks to screen print batteries in what should be a cost-effective and repeatable process.We, at Qnovo, are focused on clever control systems and software to open up the operating envelope of batteries.

There are tens of startups working on new battery technologies. Inevitably, I have missed in my brief list several others that are also leaving their mark on this field. Collectively, the contributions of all these companies will ultimately elevate energy storage to an entirely new dimension. It will take time but the goal is achievable.

14Jan 2015

What does the new iPhone 6, Samsung Galaxy A7, Huawei Ascend P6 have in common? First, they are amongst the thinnest mobile smartphones ever made. The iPhone is 6.9 mm. The A7 is a mere 6.3 mm and the P6 comes in at a paper-thin 6.2 mm. Thickness, or lack thereof, does make these devices quite elegant.

The second thing they share: They have amongst the smallest battery capacities, and consequently struggle in delivering a long battery life. In comparison, the smartphones with the best battery lives, for example, the Sony Xperia Z3, clocks in at 7.3 mm, a whole millimeter thicker than the Samsung device.

Naturally, a thicker phone allows the use of a thicker battery. That extra millimeter may not sound a lot, but it adds an extra 15-20% of capacity to the battery. That translates to about 350 – 500 mAh more in capacity, or 2 to 4 hours of additional use time.

But the impact is more meaningful than that. A thin battery suffers from a dramatic drop in energy density. Let’s look at the chart below that shows a survey of several batteries from one of the leading manufacturer. It shows how steeply the energy density drops with every millimeter of thickness that is removed.

Let’s take the Sony Xperia Z3 as an example. It is 7.3 mm thick. Its battery is about 4.5 mm thick. Its energy density is nearly 600 Wh/l. Shaving one millimeter from the battery thickness yields a serious loss of energy density, down to about 500 – 525 Wh/l, or about 15%. This is in addition to the volumetric loss of ~20% that I mentioned earlier. In other words, the total loss of capacity is now approaching 35%, or more than 750 mAh, corresponding to several hours of use time that the end consumer no longer gets to have. Put a little differently, the cost of shaving one, just one, millimeter of thickness from your elegant smartphone device is equal to 5 or more hours of lost use time to you. So, next time you go shopping for a smartphone, ask yourself a simple question: Elegance vs. Practicality? If you favor elegance, then go thin, but now you can’t complain about your battery.

But why is the effect so dramatic? If you recall from an earlier post, a lithium-ion battery is made of alternating layers of cathode-anodes separated with a “separator” material. The active material, i.e., the stuff that is actually responsible for storing the electrical charge constitutes only a fraction of this layered arrangement. In other words, there is an “overhead” of extra material that needs to be there, but does not really contribute to storing energy. When the battery becomes thin, the contribution of this “overhead” becomes much more prominent, thereby reducing the amount of active material available per unit volume, and hence reducing the energy density. It’s pure geometry.

Of course, one can try to increase the amount of active material in this small volume. For example, one can “compress” more of the graphite material in the anode. But these come at a cost: the charge rate and the cycle life of the battery drop very rapidly. So once again, this goes to illustrate how the battery manufacturers are hitting the wall!

22Dec 2014

You have got to be saying, “Wait!”, this is non sense. That perennial Periodic Table that we all took in high-school chemistry said that Lithium had an atomic number of 3, yes three, and Gold had an atomic number of 79. Gold is 28 times heavier than Lithium…are we preaching alchemy?

First, let me just say that this not a metaphor for lithium bringing financial rewards. Lithium is fairly abundant in the Earth’s crust in the form of lithium salt deposits, and second, I have yet to see a battery manufacturer that is reaping huge financial profits. Financial margins in battery manufacturing are dismally challenged, and the financial markets have not rewarded such performers. Dare I say A123, an old and no-more darling of Wall Street?

This is much simpler. You see, graphite turns into a pretty golden color when lithium diffuses and intercalates inside its carbon matrix. Think of the graphite as swiss cheese. Naturally, this is not evident to most of us since the battery is actually sealed and we don’t see what happens inside. But should you develop a transparent battery, then its colors shine. This is precisely what researchers from Michigan did, and while they were at it, they took some nice photographs of the graphite as it turned from black to gold.

Source: S. J. Harris et al., Chemical Physics Letters, 2010 (pages 265 – 274)

The first photograph in the sequence on the far left shows half of a small battery as we look through a transparent window. The battery is discharged, in other words, the graphite is empty of lithium ions. As the battery is slowly charged, lithium ions move from the cathode, the other electrode, with the direction of motion upwards in the screen. The lithium ions are physically intercalating themselves inside the carbon matrix. In the process, the graphite electrode swells to accommodate the physical presence of these lithium ions. This and the chemical bonds that the lithium forms with the carbon change the nature of the material, and consequently its optical properties. The graphite slowly turns from black to reddish and ultimately to gold color. Adding more lithium ions to the carbon ultimately results in the lithium depositing on the surface of the carbon as metallic lithium which has a silver-like appearance.

Now, take a deep look at the third photograph from the left. The bottom of the graphite electrode is red, indicating that it is beginning to fill up with lithium ions, yet the top of the electrode is still black.   This is called diffusion: Put a little red dye in a glass of water and see how it slowly diffuses into the surrounding water. The diffusion of lithium ions creates a steep gradient across the thin electrode (only 0.8 mm thick). Consequently, this puts enormous mechanical stresses across the graphite and is one of several causes of battery failure. This gradient, and consequent battery degradation, is the result of how you charge the battery, and in particular CCCV and its variants such as step charging.  You see, CCCV has no idea what may be the diffusion characteristics of the lithium ions. Charging at faster rates only makes this situation worse unless more clever charging algorithms are incorporated to mitigate this and other degradation effects.

17Dec 2014

I just returned from travel in China. The Chinese airport authorities take very seriously the transport of lithium ion batteries on board of commercial airliners. If a passenger is carrying an unknown or unlabeled or improperly marked lithium ion battery in any form, the authorities will confiscate the battery. I saw a disposal bin past the security check point at Beijing Airport that was full of confiscated battery packs.

Why are the authorities so seriously concerned about the safety of lithium ion batteries? I am not suggesting that all lithium ion batteries are unsafe but under some conditions, from both perspectives of battery design and battery operation, a lithium ion battery can become a fire hazard. That’s the topic of today’s blog.

1. Can the design of the lithium ion battery make it inherently unsafe?

Absolutely! There are countless stories of battery factories that have caught fire in the past decades. The fundamental reason is that lithium metal (not as ions, but as lithium metal in the form of Li2) is highly flammable in the presence of oxygen or water vapor, both abundantly present in air. Therefore, it comes down to assessing whether the design of the battery can allow the formation of lithium metal inside the battery. Unfortunately, as energy density increases, battery manufacturers are forced to pack more material into the electrodes and compress the battery into smaller volumes. One of  unintended consequences of this trend is increased risk of lithium plating.

For readers who are technically inclined, lithium plating occurs when the voltage of the carbon anode relative to a fictitious lithium reference electrode approaches zero. I explained in an earlier blog the potential contribution of each electrode. Let’s re-examine this graph once more. The voltage of the anode is shown in red. Lithium plating happens to the right side of the chart when the graphite is getting filled with lithium ions. Inherently robust designs adjust the geometry of the cathode relative to the anode so that full battery capacity never coincides with an x = 1.0. In other words, the battery is full of charge (i.e., 100% of charge) but the graphite anode is actually at x < 1.0, thereby ensuring that the lithium plating threshold is never reached. The trick, from a battery design standpoint, is to also not sacrifice energy density. This dilemma, avoiding lithium plating vs. increasing energy density, is where battery designs tend to trip and become sensitive to lithium plating.

2. Can one operate the battery unsafely and cause the battery to catch fire?

Absolutely! Even a well-designed battery, in other words, one that is designed to be safe within some given parameters, can be operated in an unsafe manner. Three examples of bad operation come to mind:

i) Charging the battery to voltages above its rated maximum, often 4.35 V: When this happens, the cathode voltage increases above 4.35V and the anode voltage drops below zero, thereby causing lithium plating.

ii) Charging at high charge rates using CCCV or some derivative of CCCV: The high charging current, if not applied properly with the right control algorithms, can also cause the anode voltage to dip below zero and result in lithium plating.

iii) Charging at low temperatures: As the battery temperature drops below 10 °C, the electrolyte becomes viscous, think of gummed up, and consequently, the ions have difficulty in making their journey from the cathode to the anode. This also creates the conditions necessary for lithium plating.

Fortunately, modern battery protection systems are there to ensure that these unsafe operations are not allowed — that is if they are well-designed; hence I suspect the origin of the caution by the Chinese airport authorities.

Lastly, one might ask: If the lithium plating happens inside the sealed battery and is never exposed to air, why is it a hazard? The answer is quite simple. Lithium metal plating will grow in time as the battery is used. Once this metal deposit or dendrite grows sufficiently long, it will form an electrical short between the anode and the cathode….and boom, catastrophic failure ensues.

06Dec 2014

What flavors does the lithium ion battery come with? The answer is: quite a few. They represent the various combinations of materials that are used for the two electrodes, the anode and the cathode.

It turns out the available choices for the anode are quite limited. Different forms of carbon, in particular graphite, are the common choice in all commercial applications. Silicon and silicon-carbon composites as well as tin are candidates for future anode materials but they are not presently in wide commercial use.

But the cathode has enjoyed a longer list of candidate materials, typically known by their acronyms.  All of these materials are lithium metal composite alloys, with many them using heavy metals such as cobalt and nickel.Each material seems to fit a particular application. Lithium-cobalt-oxide (LCO) is most commonly used in applications where high energy density and high capacity are needed, in particular mobile devices. Lithium-iron-phosphate (LFP) is of particular interest to the automotive industry especially in China. Its safety, long cycle life and low cost make it attractive to electric vehicles. Nickel and manganese composites have been rather limited in their utilization primarily due to cost considerations. For example, the Tesla Model S uses 18650 cells made by Panasonic with nickel-cobalt-aluminum (NCA) alloy for the cathode material.

Another salient difference between these materials is their open circuit voltage; in other words, how their terminal voltage varies with the amount of charge stored in the cell (the state of charge or SOC). The graph below shows that dependence during charging. As more charge is added to the battery, its terminal voltage rises.

One will quickly notice that LFP has the lowest average voltage, near 3.2V, considerably lower than the voltage for NMC or LCO, both hovering near 3.7V. In fact, modern cells made with LCO have a maximum voltage of 4.35V, up from 4.2V, thus raising the average voltage to 3.8V. This higher voltage, and consequently higher energy, makes LCO an attractive material for use in mobile devices.

Additionally, the voltage behavior near empty (below 15% SOC) plays a big role in the utility of the material. LCO can sustain a useful voltage above 3.6V down to about 5% remaining charge, whereas NMC drops below 3.5V when the SOC is at 10%. This is yet another reason why the mobile industry continues to choose LCO.