Chemistry

22Jan 2016

I described in the earlier post how adaptive systems turned smartphones into great cameras. Let’s now talk about how adaptivity and adaptive charging can make a battery perform far better.

Let’s start briefly with the basic operation of a lithium ion battery. The early posts of this blog describe the operation of the lithium-ion battery in more detail.  I will briefly recap here the basic operation and explain where its performance is limited. For the reader who wants to learn more, select “The Basics” category tag and feel free to review these earlier posts.

The figure below illustrates the basic structure of a lithium-ion battery. On the left hand side, one sees an electron microscope image of a battery showing the anode, the cathode and the separator, essentially the three basic materials that constitute the battery. On the right hand side, one sees a sketch illustrating the function of these materials during the charging process: The lithium ions, “stored” inside the individual grains of the cathode, move through the separator and insert themselves inside the grain of the graphite anode. If you are an engineer or physicist, you are asking, “where are the electrons?”  A neutral lithium atom becomes an ion in the solution, travels through the separator to the anode. The electron travels in the opposite direction through the external circuitry from the Aluminum collector to the Copper collector, where then it is captured by a lithium ion to form a molecular lithium-carbon bond.

Structure of the lithium ion battery

This seems simple enough, so what can go wrong? lots! I will focus here on a handful of mechanisms that become critical as the battery’s storage capacity and energy density increase. Looking at the diagram above, it is hopefully obvious that increasing energy density means to the reader packing more and more ions into this little sketched volume. It means reducing the dimensions of the anode, the cathode, the separator, and trying to saturate the capabilities of the anode grains to absorb ions. It’s like when you try to put as much water as possible inside a sponge. Now, in this process, small variations in manufacturing become really detrimental to performance. Look at the left photograph and observe the coarseness of the grain size for both electrodes. That means the uniformity of the ionic current is poor. As the energy density rises, a large number of ions are all rushing from the cathode to the anode. But this lack of uniformity creates stress points, both electrical and mechanical, that ultimately lead to failure:  gradual loss of material, gradual loss of lithium ions, and gradual mechanical cracking, all leading in time to a gradual loss of capacity and ultimate failure.

I will jump to two key observations. First, it should be apparent that when energy density is low, these effects are benign, but when energy density is high, there are so many ions involved in the process that small manufacturing variations become detrimental. Second, it should be apparent too that faster charging results in the same effect, i.e., more ions are trying to participate in the process.

Clearly, battery manufacturers are trying to improve their manufacturing processes and improve their materials — but let’s face it, this is becoming an incredibly expensive process. Smartphone and PC manufacturers are not willing to pay for more expensive batteries. This is very similar to the earlier post about camera lenses. Make great lenses but they become very expensive, or shift the burden to computation and correct the errors dynamically and adaptively.

That’s precisely what adaptive charging does: Be able to measure the impact of the manufacturing variations, embedded defects, non-uniformity of material properties and what have you in real time, assess what these errors are and how they may be progressing in time, then adjust the voltage and current of the charging current in such a way to mitigate these “errors”….then keep doing it as long as the battery is in operation. This makes each battery unique in its manufacturing history, material properties, and performance, and lets the charging process get tailored in an intelligent but automated fashion to the uniqueness of the battery.

It’s a marriage of chemistry, control systems and software, that shifts the burden from expensive manufacturing to less expensive computation. But what is clear is that it does not make battery manufacturing any less important, and it does not replace battery manufacturing — it is complementary. It is no different that how adaptive algorithms in the camera are complementary to the lens, not replacing it. This is cool innovation!

16Jan 2016

Our new website presents our suite of products called Adaptive Charging Software. It is fair to say that everyone understands and recognizes the meaning of “Charging” and “Software”…but “Adaptive”? What does it really mean? The purpose of this post is to give the reader an intuitive feeling of the meaning of (and consequently the need for) “adaptive” as it relates to technology.

Let’s first start with the classical definition of adaptive:

a•dapt•ive (ə-dăpˈtĭv)  adj. Relating to or exhibiting adaptation.

Ok, it relates to adaptation, but adapting to what? and why? For that, let’s illustrate with an example at how adaptive algorithms and software became instrumental to modern photography.

Let’s look at two photographs of Liberty Cap from a recent trip I took to Yosemite National Park. Can you tell by looking at the photographs what camera(s) were used in taking the shots? I doubt it. They both offer plenty of resolution, richness of color and great image quality (you may click on each photo to enlarge it).

                         D7K_1310_1200px

 IMG_2484_1200px

The top photograph was taken by a Nikon D7000 DSLR with a 24-mm f/2.8 prime lens. Total weight: 1,042 g (2.3 lb). Total cost when new: about $1,500.

The bottom photograph was taken by an iPhone 6 Plus. Total weight:172 g (0.38 lb). Total cost when new: about $600 for the smartphone. The camera component is less than $15 and only a few grams.

So why are the two photographs so similar, and what is the purpose of using DSLRs over a smartphone if the differences are so minuscule if not inexistent?

From an optical standpoint, the camera optics of the iPhone 6 Plus are absolutely no match to the superb optics of the Nikon lens. The iPhone 6 Plus camera sensor is also no match to the one in the Nikon D7000 — though both are manufactured by Sony but to vastly different requirement standards.

Today’s cameras, both DSLRs and smartphone cameras included, incorporate very sophisticated computational electronics on board. The iPhone 6 Plus boasts a powerful Apple ARM processor, and the Nikon camera includes a sophisticated Expeed processor. Both of these processors perform corrections on the fly before, during and after the photograph is taken. For instance, they both incorporate algorithms that assess the nature of the scene (e.g., is it a landscape, or does it include faces?) to determine the exposure parameters. Same for the focusing. Additionally, they both make corrections on the fly for the optical errors coming from the optics…and this is just the beginning.

Now, the Nikon 24mm prime lens is a superb lens and has excellent optics. In contrast, the lens used in the iPhone 6 Plus is no match. It suffers from significant optical errors called aberrations. For instance, one of these errors is called distortion: the photograph, uncorrected, looks distorted. Another error is chromatic aberration: different colors have different focus points. Guess what? Both camera processors correct for all of these errors: this is what “adaptive” does. It adapts and corrects. In other words, there are algorithms (and intelligence) that measure and recognize errors in the system (here, the camera and the optics) that may vary depending on the device and circumstances, then make the proper corrections in real time such that the end product is nearly free of problems. The smartphone industry cleverly shifted the burden of camera performance from expensive and sophisticated lens manufacturing (what it used to be in the past decades) to inexpensive computation. Brilliant!

It becomes immediately obvious to the reader that the biggest beneficiary from this “adaptive” performance is the inexpensive plastic lens used in the iPhone 6 Plus. In other words, the benefit of shifting the burden to computation is the use of lower cost components, in this case, a lower cost sensor and lens, albeit with worse optical specifications. And I mean much lower cost: in this example here, it is about 100X less expensive.

Adaptive systems are not new by any stretch of the imagination. They were initially proposed and used in complex systems — for example, correcting the optical errors in large telescopes as a result of variations in the upper atmosphere. However, the rapid decline in the cost of computing over the past decade has made the implementation of “adaptivity” accessible across a broad range of applications.

So, now you can begin to imagine what adaptive solutions can do to improve the performance of batteries where materials and manufacturing can have significant variability and associated costs. This will be the topic of a future post.

19Nov 2015

For those of you old enough to remember the Sony Walkman portable radios in 1970s, they ushered a new era of consumer electronics, and one could argue, the first mobile battery-powered “devices,” the ancestral precursor to the Apple iPod three decades later. These early electronics were powered by replaceable batteries. Lithium-ion batteries didn’t exist back then. So how did they get invented?A large amount of research and development effort has gone and continues to pour into rechargeable batteries, but one could point to three seminal moments that transformed rechargeable batteries in general, and lithium-ion batteries in particular.

The first moment was in the early 1970s at Exxon. It was a time when large corporations such as GE, Exxon, IBM and others competed with AT&T’s famed Bell Labs for scientific supremacy….a time not much different than ours today with the likes of Google and Apple competing for new innovations. An English-born chemist at Exxon’s research laboratories, Stanley Wittingham, made an important scientific discovery; he found that ions can “intercalate” in between sheets (or layers) of titanium  sulfide, and effectively store electrical charge. By shuttling these ions back and forth between two electrodes with such layered materials, he could build a rechargeable battery. Exxon filed for its first battery patent in 1976, and was awarded a US patent 4,084,046 in 1978.

US4084046Wittingham2

But Exxon and Wittingham ran into several challenges: the batteries degraded fast and they were prone to explode. Exxon couldn’t capitalize on this discovery.

The second seminal moment came from John Goodenough, now professor emeritus at UT Austin, but at the time, he was a professor of Chemistry at Oxford University in the UK. After researching metal oxides and testing several varieties, he and his group discovered that lithium-cobalt-oxide (LCO) was a very effective cathode material. The results were published in 1980: the battery had a higher voltage than Wittingham’s cell (2.2 volts); its energy density was far better than anything on the market; it worked very well at room temperature. It was the missing link to making a rechargeable battery.

Someone had to turn these discoveries into a product; that role was exceptionally fulfilled by Sony in 1991. Sony combined Goodenough’s LCO cathode with a graphite/carbon anode to produce its first commercially available rechargeable lithium-ion battery. Sony put these new batteries into their camcorders and cameras…it was a commercial success. Sony went on to rule lithium-ion batteries for a decade or more. Sony continues to date to be one of the major producers of lithium-ion batteries, albeit several other companies have since emerged as even larger suppliers.

The Sony commercialization was also a major catalyst for laboratories around the world to accelerate the material discovery. John Goodenough’s team at UT Austin went on to discover another category of cathode material, lithium-iron-phosphate (LFP), that was safer than LCO but at the expense of lower energy density.

This post is by no-means intended to give all the inventive credit to the three groups mentioned above. Hundreds if not thousands of innovators and organizations have greatly contributed to the evolution of the lithium-ion battery and continue to do so. Much like the semiconductor industry points to a handful discoveries that transformed electronics, one can trace similar inflection points in the history of the lithium-ion battery.

22Sep 2015

Pause for a second and wonder why electric vehicles have frustratingly limited driving ranges? or why your smartphone lasts only for a few limited hours instead of an entire month? Yet, a good ol’ combustion-engine car can go for hundreds of miles without a problem. This is the manifestation of energy density. Let’s talk about it in more detail and hopefully give the reader a bit of more intuitive sense on the importance of this metric.

Energy density, as the title implies, is a measure of how much energy is stored in a certain volume. A battery or a gas tank has a certain limited volume, therefore it is important to have a metric that relates to how much energy can be stored in that volume. Obviously, energy is what powers our smartphones or vehicles, therefore energy density is a metric that describes how far one can drive  or use a device given the limited amount of energy stored in the “tank.”

The following table compares a select number of energy-storing materials or mechanisms, all the way from the traditional lead-acid battery (the type that you will find under your hood) to much more sophisticated energy sources such as nuclear fission. So what is this table telling us?

EnergyDensity3

The first nine rows are all batteries, or devices that can store electrical energy. Batteries are either primary (i.e., non-rechargeable) or secondary (fancy term for rechargeable). The last three rows are widely used energy sources in our society today and are used here for comparison purposes: Ethanol and gasoline are examples of carbon-based fuels, and the last row, well, we all know what nuclear power can do, both the good and the evil.

The first observation: Even the best battery, the absolute best, has 10X lower energy density that carbon-based fuels. That means the same tank (or equivalently sized battery) will let you drive 10X more miles using carbon-based fuels. I cheated a little here — electric systems are more efficient than carbon-fuel systems, so the difference is more like 3X rather than 10X, but that will be left to another discussion.

The second observation: The difference between the best battery and worst battery (in terms of energy density) is substantial, also about a factor of 10X. Lead-acid batteries, discovered over a 150 years ago, don’t provide a lot of energy density. NiCd batteries also leave a lot to desire. Do you remember the bulky batteries in the early cell phones back in the 1990s? Or just google the GM EV1, the first electric vehicle from GM that used lead acid batteries.

But…there is always a but: While NiCd have for the most part disappeared, lead acids are incredibly inexpensive, and they survive. Until the day comes when the price point of alternative batteries drops radically, lead acids will continue to be the king of batteries in applications where energy density is not critical — i.e., where it is ok to occupy a larger volume, for example backup systems for cell phone towers.

The third observation: Energy density increased by a factor of 10X over 150 years! That’s not terribly promising unless the future brings forth some serious breakthroughs in materials. Is there anything on the horizon? There is a lot of promising good material research, but when one takes into account cost, cycle life, and other constraints such as manufacturing and capital, it is very hard to point to one particular technology that is likely to be commercialized in the next 5 years. So the wait and the hope continue.

The fourth observation: Lithium-ion technologies, first commercialized by Sony in 1991, encompass a wide range of energy densities depending on the particular choice of material for the electrodes. Lithium-ion batteries using nickel-cobalt-aluminum oxide (NCA) electrodes — the type used in the Tesla Model S — have over 3X the energy density of lithium-ion batteries with lithium iron-phosphate  (LFP) electrodes. So why is anyone considering LFP lithium-ion batteries: Cycle life! Welcome to the world of compromises.

So by now, you are probably disappointed about the future of batteries! It is true that the progress of batteries over the last 150 years has been slow, and it is true that batteries can’t yet compete with carbon-based fuels…but that does not mean that the incremental progress in batteries is insufficient to meet many needs of our society. Yes, they can be better, but present batteries boasting 700 Wh/l can and are sufficient to provide an electric vehicle with a range of 300 miles. In other words, don’t expect miracles in batteries, but do expect that incremental technologies from materials to algorithms and electronics will be sufficient to address a wide range of energy storage needs, including smartphones that can last an honest day to electric vehicles with a range of 200 – 300 miles.

There is plenty to look forward to here, just be careful about wild claims of amazing discoveries. If there are too good to be true, then there is a probably a good reason to be skeptical.

21Aug 2015

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.

plating2

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.