The Basics

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.

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.

14Aug 2015

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.

12Jun 2015

Kevin Gibb of TechInsights published recently an article in EE Times that shows a teardown of the lithium-ion battery used inside the iPhone 6 Plus. While the teardown and the article seemed motivated by determining the cost of this battery — somewhere near $4.00 — it contained some very nice cross sectional photographs taken using optical and electron microscopy of the various layers that make the iPhone 6 Plus.  Anything that carries an Apple logo seems to attract a lot of attention, but the battery inside the iPhone 6 Plus is similar in performance and structure to many other Li+ (lithium-ion) polymer batteries used in mobile devices.  For example, the battery capacity of the iPhone 6 Plus is rated at 2.915 Ah, within a rounding error of the capacity of batteries used in the Sony Xperia Z3 and Z3+, the LG G3 and G4. Let’s use this very nice teardown report of the iPhone 6 Plus battery to shed more light onto the inner structure of a lithium-ion battery and its workings especially in view of fast charging.

I described in an earlier post the various shapes of a lithium-ion battery. A 18650 cell is encased in a metallic cylinder, whereas a polymer one is a thin and flat pancake-like without any external metallic protection. Yet, the insides are nearly identical, all consisting of a set of electrodes called anodes opposing another set of electrodes called the cathodes with both sets of electrodes separated by a porous membrane called a — hold your breath — “separator.” The first picture below shows a cross section of the polymer battery inside the iPhone 6 Plus viewed through an optical microscope. For reference purposes, the iPhone 6 Plus battery is approximately 3 mm thick.

In mobile devices, the vast majority of batteries use a metal oxide called lithium-cobalt-oxide (LCO) deposited on an aluminum backplate to act as the cathode (the positive electrode during charging). You can see the bright white aluminum back layer in the photo above, but it is difficult to see the LCO layer at this magnification. The anode is nearly always made of a thin carbon graphite layer deposited on top of a copper backplate. There is a very thin separator layer that sits between each set of anode/cathode layers. During charging, the ions, yes, the lithium ions, travel from the cathode through the porous separator to the anode, and embed themselves inside the graphite. As every skilled engineer should know, charge balance means that there is an opposing current made of electrons that goes through the external circuit between the anode and the cathode. This means that maintaining a low electrical external path resistance is essential to the operation of the battery — one of the reasons why aluminum and copper conductors are used.

The photo above shows a stack of alternating layers of anodes and cathodes. There are 11 anode/cathode layer pairs, which means the pitch is approximately 275 microns. This particular construction is unique to LG Chem with a stack of parallel layers. Other battery manufacturers use what is known as a jelly-roll, with the layers of anodes and cathodes rolled together like a cigar. This mechanical structure, while seemingly immaterial to the novice, plays a big role in the distribution of electrical current inside the battery, and consequently the governing degradation mechanisms. Let’s zoom in a little more.

The second photograph shows a scanning electron micrograph (SEM) of two sets of anode/cathode layers. Now we can see the individual structural materials. The separator is typically near 10 to 20 microns in thickness. The graphite and LCO layers are often around 50 microns but can vary depending on battery capacity and current rating. This SEM now shows that the LCO layer is granular in nature. The graphite layer is granular too.  The grains, varying in size from a few to several microns in diameter, consist of crystalline layers — a lattice-like — where the lithium ions can embed themselves. In charging, they embed themselves in the graphite lattice, and in discharge, in the LCO lattice. The graphite lattice is pictured next using a transmission electron micrograph (TEM). The lattice is made of atomic layers that are a mere 0.34 nm apart — think of it as atomic Swiss cheese.

The LCO and graphite have a limited capacity of how many lithium ions they can “hold” inside their lattice. This determines the amount of LCO and graphite material that is needed for a battery of a given capacity, i.e., of a given mAh. This in turn determines the energy density. Well, sort of, because there is another kink in the design of the battery, and that is the size of the grains (both LCO and graphite) and how tightly packed they are in the electrode layers. If the grains are too tightly packed, then the lithium ions will find it difficult to travel through all the grains; in other words, the maximum current capability of the battery is impaired. So you are hopefully getting a little taste of the various compromises a battery designer needs to go through….and we haven’t even yet gotten to charging.

Now let’s talk about the headaches that come with degradation of this structure especially with fast charging. High capacity and/or faster charging means a lot of ions need to zip in and out of the anode layers — since the anode is primarily responsible for storing the ions during charging. Think of cars on a highway at peak rush hours….it’s not easy; every pothole in the road now contributes to traffic flow. For example, small perturbances in the uniformity of grains means more ions will flow into one grain vs. another, thus creating differences in current density, and excessive stress on some grains (ultimately causing mechanical fracturing of the graphite lattice and loss of capacity). Small disturbances in the voltage distribution across the layers means some portions of the stack may see a potential difference between the anode and cathode that will promote the metallic plating of lithium — a very detrimental failure mode especially present with faster charging. These are only but two examples of the degradation mechanisms. There are several more that are becoming prevalent in modern batteries with high energy density and faster charging. The task is to tame these degradation mechanisms to extract maximum performance, and that is now falling onto the next frontier of clever charging algorithms — and that is what we do at Qnovo.

Fast charging a battery clearly involves a high degree of optimization in order to manage the large flow of ions. Historically, battery vendors did it while sacrificing grain size, or packing density of grain; in other words sacrificing energy density and overall battery capacity. This compromise is no longer acceptable.