A look inside the world of batteries

07Nov 2016

For the average reader, electrochemical impedance spectroscopy, often abbreviated as EIS, is more than a mouthful. Understanding its utility can be relegated to the category of unresolved mysteries. Today’s post will shed some light and a little intuitive thinking on this powerful method.

The reader’s first question might be “why are you talking about EIS in a battery blog?” The answer is simple. EIS is the foremost standard  tool in laboratories around the world to measure electrochemical processes and reactions. Electrochemistry, one of the most extensive branches in chemistry, is the study of chemical reactions that have an inherent relationship to electricity, i.e. they can either generate electricity or can be influenced by electricity. Yes, you guessed right, batteries are a prime example of electrochemistry. Another practical example of electrochemistry put to good use: the gold plating on your necklace or bracelet.

What does the name EIS imply? Electrochemical impedance is scientific jargon that refers to the electrical resistance of the device under study, in this case, the lithium-ion battery. In its most elemental form, impedance is voltage divided by current. For electrical engineers, it represents components such as resistors or capacitors. For other scientists, it represents the resistance the device exhibits against the flow of electricity.

Spectroscopy is the branch of science that deals with how a property changes with frequency. Hence, EIS is the methodology and science that seek to understand how impedance measurements change with frequency, and more particularly, how these changes are intimately tied to the underlying chemical reactions.

Why frequency? Frequency adds a lot more information about the nature of the chemical process that is taking place. In science, frequency plays a very important role. Take for example the difference between blue and red light. They are both made of photons, but differ in frequency. Medical MRI imaging depends on the frequency of the oscillation of the hydrogen atoms in our bodies. Distinguishing between different broadcast stations on the radio dial operates on similar principles. In other words, we use frequency to uniquely identify chemical or physical processes.

With this long introduction, let’s dive a little deeper into EIS as related to a lithium-ion battery. If you were to measure the impedance of a standard electrical resistor component — the kind of components you may find inside your smartphone — you will find that you will measure exactly the same impedance value whether you apply a low voltage or a high voltage, or whether you measure at low frequency or high frequency. In other words, for this resistor component, the value is independent of voltage (also known as bias) and frequency. Resistors are consequently easy components to understand.

That is NOT the case for a battery.  Change the voltage or frequency and you will get a different value. In other words, the battery can look like a resistor in some circumstances, or like a capacitor in others, or some complex combinations of both. When we change the voltage of the battery, it now operates at a different “state of charge,” in other words, it will have a different amount of electrical charge stored in it. As I described in this earlier post on fuel-gauges, the terminal voltage of the battery is a direct proxy of the amount of electrical charge stored in the battery, which is the state of charge (or the percentage of battery remaining).

In contrast, changing the frequency relates to different electrochemical processes that occur inside the battery. Such electrochemical processes could relate to the diffusion of the electrical charge (in this case, the lithium ions) from one electrode to the other. One can imagine that the ions have to travel a certain distance and insert themselves in the “Swiss-cheese” matrix of the material. So intuitively, this feels like a slow process, and it is. It takes several seconds to even minutes for the lithium ion to go through this diffusion process — meaning that diffusion of ions is characterized by a low-frequency signature. A distinctly different electrochemical process is how lithium ions and electrons interact right at the surface of the electrode. This interaction involves electrons and ions over very short distances. Intuitively, one can see that this can be a very fast reaction, usually on the order of microseconds. Hence its signature contains high frequency signals.

reactions

All of this goes to say that the impedance value at a particular frequency is a “unique signature” for the underlying electrochemical process of interest to our study. And that is what makes EIS such a powerful tool. To the trained scientist, he or she can read the EIS measurement as a map of the various electrochemical processes and reactions that are taking place inside the battery without cutting it open or damaging it. It also provides tremendous insight into what can also go wrong inside the battery. Not all electrochemical processes are desirable. For example, the underlying process that causes lithium metal plating is highly undesirable and can be readily measured using its unique EIS signature.

So how is the measurement made? In the laboratory, the oft-expensive and bulky instrument applies a small electrical current at a well defined frequency to the battery, then measures the voltage. Divide the voltage by the current and you now have the impedance at this frequency. For example, apply 1 mA of current at a frequency of 100 Hz, you might measure 0.5 mV. Hence the impedance is 0.5mV/1mA = 0.5 ohms at 100 Hz. This, of course, does not take into account the complex value of the impedance but it is a simple illustration of the concept. “Complex” numbers are mathematical tools to show values that have both real and imaginary components. Don’t worry if you don’t understand them fully —the key thing is that an impedance measurement has two values to represent it.

eis

A full EIS chart shows by convention the imaginary component of the impedance (vertical axis) vs. its real value (horizontal axis). The far left of the chart shows the measurements made at high frequencies, in particular highlighting what happens in the metal conductors inside the battery as well as what occurs at the surfaces of the electrodes. As we follow the purple dots and move towards the right, the frequency of the signature gradually decreases highlighting now a different set of electrochemical processes, in particular what happens at the insulating interface between the electrode and the electrolyte (also known as SEI layer). Ultimately, to the far right of the chart, the frequency is low and is unique to the diffusion effects of the lithium ions.

An EIS tool is present in every electrochemistry laboratory around the world. Young graduates in this discipline spend countless hours operating this tool. It is not a small instrument…it fits on a desk, may weigh several pounds, and costs several thousands of dollars. Now imagine how the world would look like if an EIS tool can somehow fit inside each and every smartphone!

18Oct 2016

State-of-the-art lithium-ion batteries, whether used in smartphones or electric vehicles, all rely on the same fundamental cell structure: two opposing electrodes with an intermediate insulating separator layer, with lithium ions shuffling between the two electrodes.

The positive electrode during charging, usually called the cathode, consists of a multi-metal oxide alloy material. Lithium-cobalt-oxide, or LCO, is by far the most common for consumer electronic applications. NCM, short for lithium nickel-cobalt-manganese oxide, also known as NMC, is gradually replacing other materials in energy storage and electric vehicle applications. LCO and NCM have a great property of storing lithium ions within their material matrix. Think of a porous swiss cheese: the lithium ions insert themselves between the atomic layers.

In contrast, the anode, or negative electrode during charging, is almost universally made of carbon graphite. Carbon historically was and continues to be the material of choice. It has a large capacity to store lithium ions within its crystalline matrix, much like the metal oxide cathode.

So how do manufacturers increase energy density? In some respects, the math is simple. In practice, it gets tricky.

Energy density equals total energy stored divided by volume. The total stored energy is dictated by the amount of active material, i.e., the available amount of metal oxide alloy as well as graphite that can physically store the lithium ions (i.e., the electric charge). So battery manufacturers resort to all types of design tricks to reduce the volume of inactive material, for example, reducing the thickness of the separator and metal connectors. Of course, there are limits with safety topping the list. To a large extent, this is what battery manufacturers did for the past 20 years — amounting largely to about a 5% increase annually in energy density.

But once this extra volume of inactive material is reduced to its bare minimum, increasing energy density gets tricky and challenging. This is the difficult wall that the battery industry is facing now. So what is next?

There are two potential paths forward:

1.  Find a way to pack more ions (i.e., more electric charge) within the electrodes. This is the topic of much research to develop new materials capable of such feat. But any such breakthrough is still several years away from commercial deployment, leaving the second option to….

2.  Increase the voltage. Since energy equals charge multiplied by voltage, increasing the voltage also raises the amount of energy (remember that energy and charge are related but are not commutable). This is the object of today’s post.

The battery industry raised the voltage a few years back from a maximum of 4.2 V to the present-day value of 4.35 V. This was responsible for adding approximately 4 to 5% to the energy density. A new crop of batteries is now beginning to operate at 4.4 V, adding an additional 4 to 5% to the energy density. But that does not come without some serious challenges. What are they?

First, there is the electrolyte. It is a gel-like solvent that imbibes the inside of the battery. Short of a better analogy, if ions are like fish, then the electrolyte is like water. It is the medium within which the lithium ions can travel between the two electrodes. As the voltage rises, it subjects the electrolyte to increasingly higher electric fields causing its early degradation and breakdown. So we are now seeing a new generation of electrolytes that can in principle withstand the higher voltage — albeit, we see in our lab testing that some of these electrolyte formulations are responsible for worse cycle life performance. This is a first example of the compromises that battery designers are battling.

Second, there is the structural integrity of the cathode. Let’s take LCO as an example. If we peer a little closer into the cathode material (see the figure below), we find a crystal structure with layers made of cobalt and oxygen atoms. When the battery is fully discharged, the lithium ions occupy the vacant space between these ordered layers. In fact, there is a proportion of lithium ions to cobalt and oxygen atoms: there is one lithium ion for every one cobalt and two oxygen atoms.

lco

courtesy of visualization for electronic and structural analysis (VESTA)

As the battery is charged, the lithium ions leave the cathode to the anode vacating some of the space between the ordered layers of the LCO cathode. But not all the lithium ions can leave; if too many of them leave, then the crystal structure of the cathode collapses and the material changes its properties. This is not good. So only about half of the lithium ions are “permitted” to leave during charging. This “permission” is determined by, you guessed it, the voltage. Right about 4.5 V, the LCO crystal structure begins to deteriorate, so one can easily see that at 4.4 V, the battery is already getting too close to the cliff.

Lastly, there is lithium plating. High energy-density cells push the limit of the design and tolerances in order to reduce the amount of material that is not participating in the storage. One of the unintended consequences is an “imbalance” between the amount of cathode and anode materials. This creates an “excess” of lithium ions that then deposit as lithium metal, hence plating.

These three challenges illustrate the increasing difficulties that battery manufacturing must overcome to continue pushing the limits of energy density. As they make progress, however, compromises become the norm. Cycle life is often shortened. Long gone are the days of 1,000+ cycles without intelligent adaptive controls. Fast charging becomes questionable. In some cases, safety may be in doubt. And the underlying R&D effort costs a lot of money with expenses that are stretching the financial limits of battery manufacturers without the promise of immediate financial returns in a market that is demanding performance at a the lowest possible price.

It is great to be a battery scientist with plenty of great problems to work on…but then again, may be not.

15Sep 2016

A recent article published by The Verge attempted to explain the science behind the exploding Samsung Note 7 batteries. The article touches on several important aspects of battery safety but the handwaving did not really talk about much science. So this post will address a failure mode of lithium-ion batteries and how defects can form during manufacturing with catastrophic results.

One of my earlier posts described the inner structure of a lithium battery. In a nutshell, there are alternating material layers that form the basic structure of the battery: a sandwich of two electrodes, called the anode and the cathode, with an insulating separator between them. During manufacturing, these layers are assembled then rolled together like a cigar before they are packaged into a protective sleeve. This is a gross simplification but highlights the basic structure and assembly of the lithium-ion battery. With some minor exceptions, the manufacturing is primarily an assembly process, and does not resemble in any form the manufacturing processes used in semiconductor devices.

The first figure below shows a rudimentary drawing of the basic structure of the lithium-ion cell. The graphite anode, shown in black, sits counter to the cathode, shown in green. The separator, shown in blue, is sandwiched between the two electrodes and acts as an insulator, in other words, its primary function is to prevent internal electric shorts between the two conductive electrodes. We all know that electric shorts are not good!

One of the basic requirements in the design of the battery is for the graphite anode to physically extend beyond the edges of the cathode. In other words, the anode is wider than the cathode at every point, especially the long edges of the sheets. This is needed to maintain safety within the cell and prevent the formation of lithium metal. Intuitively, there has to be more anode material than cathode material to absorb all the lithium ions. When the anode is not properly sized, the excess lithium ions will deposit as lithium metal, and that is called lithium plating. If you would like to dig a little deeper into lithium plating, this earlier post will shed some additional insight.

In practical terms, the anode is wider than the cathode ever so slightly, only a few percents. Any extra width of the anode does not participate in energy storage. In other words, the extra width of the anode is required for safety reasons, but does not contribute to charge storage. So battery designers go to extremes to optimize the extra width of the anode for the requisite safety.

As energy density increases, these battery designers have limited choices, one of them is to reduce the width margin of the anode. This means that the additional width of the anode relative to the cathode is now at its bare minimum. Any errors in manufacturing that jeopardize this extra overlap may have dire consequences.

battery safety figure 1

So now let’s examine one particular manufacturing defect where a slight misalignment between the anode and cathode occurs during the assembly process. The figure below shows the same structure as above but now the anode layer is shifted ever so slightly to the right.

battery safety figure 2

At the misaligned edge, the requisite overlap of the anode relative to the cathode is now diminished or even possibly vanished. The A/C ratio at this locale drops below the requisite limit for ensuring safety. The result, as you expected, is the onset of lithium metal at this edge. The lithium metal forms on the anode edge. As the lithium metal grows in size and thickness, it ultimately punctures the separator and causes an electrical short between the anode and cathode. Boom! we now have a catastrophic failure.

battery safety figure 3

So this begs the question: why did Samsung release new software that limits the maximum charge in the faulty Galaxy Note 7 to only 60% of maximum? It is because the risk of lithium metal plating heavily depends on the voltage and the maximum charge in the battery. This is evident in the voltage chart of this earlier post: the higher the voltage, i.e., the higher maximum allowed charge, the higher the risk of lithium metal plating.

I will close by reiterating one final thought. The tolerance requirements in the manufacturing of lithium ion batteries have risen sharply with increasing energy density. Short of using new materials (that still do not exist in commercial deployment), increasing the energy density means reducing all the extra space inside the battery that is not made of anode and cathode materials. These are the only two materials that store energy. Everything else is just overhead…i.e., dead weight. They are still needed for other functions and safety, but they do not contribute to storing electrical charge. So battery designers keep reducing this overhead and in the process, make the manufacturing tolerances every so tight….and that is a recipe for many disasters to come unless we start adding a lot more intelligence to the battery to avoid and mitigate these undesired situations.

02Sep 2016

The images of melted Samsung Note 7 smartphones are all over the internet. News of Samsung’s massive recall are headline news. It is embarrassing to Samsung Mobile, its marketing and engineering teams, and most certainly its executives. Consumers are wondering how could Samsung ship units with defective batteries that can catch fire.

It is easy but not right to pick on Samsung or be critical of the company at this moment. Why? because this could happen to anyone…that’s right, anyone. If you are an OEM of smartphone devices or consumer devices with lithium-ion batteries, this is the time for you to pay attention to your products because you could be next.

While this sounds ominous, the intent here is to raise safety awareness in the entire ecosystem that depends on batteries. Samsung happened to be the first unlucky company to exhibit the strains that have been accumulating now for several years. I have covered in several past posts how the battery industry has been hitting the wall. Battery materials are reaching their limits. Battery economics are not favorable. Yet, the performance demands on batteries continue to rise. All of these factors are and continue to be precursors to the situation that Samsung finds itself in.

As is often the case in life, we tend to remain complacent until a crisis hits. The crisis is here, and now. Samsung is first to feel the pain, but each and every company in this ecosystem, from consumer devices to energy storage and electric vehicles, should acknowledge the severity of the situation and participate in its solution. Again, why?

This perfect storm has been brewing for a while, in particular, the drive to increase energy density along with faster charging while making less expensive batteries. Increased energy density and faster charging operate the battery near its physical limits. In other words, the margins for error at these elevated performance levels are really thin. For example, the newest lithium-ion cells now operate at a terminal voltage of 4.4 Volts, up from 4.2 Volts a few years back. This increase in voltage is one of the underlying physical tenants of increased energy density, yet it moves the battery every so close to the edge of the safety abyss. Another example relates to charging speed: it is widely accepted now that charge rates are approaching if not exceeding 1C. Electric vehicle makers are actively exploring very fast charging for EVs. Tesla is deploying their superchargers at a fast pace. These superchargers can charge a Tesla model at up to 1.5C, i.e., put in half a tank in about 20 minutes. Fast charging wreaks havoc inside the cell if not properly managed.

So now add the push for making less expensive batteries. Battery manufacturing, unlike semiconductors, does not scale. There is no equivalent of Moore’s law. In other words, as energy density increases, the cost per Wh (per energy unit) does not decrease…au contraire, it tends to increase because manufacturing tolerances get tighter. As a result, capital expenditures go up. Combine that with low-cost, low-quality batteries coming out of China and at a fundamental level, you can see how the financials of battery companies do not look pretty. This invariably leads to changes in manufacturing processes as companies seek more efficient ways to manufacture. But when the design margin of error is so thin, it does not take much before small variations in manufacturing lead to disastrous consequences. Remember, all it took in the case of Samsung was 35 failing devices out of a total of 2,500,000 shipped to cause a recall. This is a failure rate of 14 ppm (parts per million). It is a small number but, clearly, not small enough.

This is not to say that battery manufacturing and battery technology are doomed. There are countless examples in history where engineers built far more complex systems and structures safely and economically…but usually these include a change in paradigm. For example, pause for a second and compare the first commercial airplanes with the most recent jetliners. The newest Boeing and Airbus commercial airliners are marvels in computation and software. Fly-by-wire and automated systems with redundancy are the norm today, yet these new airplanes are scantily faster than their predecessors. In other words, the industry added so much more intelligence and shifted the burden to computation. The result is that modern planes are vastly safer than ever and far more economical to operate.

This is precisely the opportunity in front of the battery manufacturers and their customers, the OEMs, to think deep and hard on how they are going to implement a lot more intelligence to manage their batteries. Kudos to Sony for recognizing this….the batteries in their smartphones carry a great deal of intelligence, perform incredibly well and are safe. I am biased here…a lot of that intelligence is from Qnovo, but that should not diminish from the importance of the point of needing intelligence to manage the vanishing margins of error that battery designers have to cope with.

23Aug 2016

Tesla Motors announced today upgraded versions of the Model S and X boasting 100 kWh battery packs, up from 90 kWh used in their earlier top-of-the-line electric vehicles. One hundred kilowatt-hours sounds like a lot, and it is, but I bet that many readers don’t have an intuitive sense of this amount of energy. This is what this post is for.

First, a kilowatt-hour is a unit of energy, not power, and is most commonly used in electricity. To put it in perspective, an average home in California consumes about 20 kWh of electrical energy per day, so this 100-kWh fully-charged Tesla battery would cover this home’s needs for about 5 days.  Now that’s great if you like to go off-grid.

A Nissan Leaf has a battery with a capacity of 30 kWh and has a driving range of approximately 107 miles (172 km). If the Nissan Leaf were to have its battery upgraded to 100 kWh, then its range would increase to 350 miles, or about what you get from your average gasoline-engine car. That would be real nice!

100kWh is also equal to 341,000 Btu, that is if you like to use the British system of units. At about 10,000 Btu to run a home-sized air conditioning unit, this battery will provide you 34 hours of uninterrupted cool air. It it also equal to 3.4 US Therms (each Therm is equal to 100 cubic feet of natural gas), sufficient to heat a California home in the winter for about 4 days.

Now let’s get a little more creative in this comparison exercise. This high amount of energy can be quite explosive if not designed and operated properly and safely; 100 kWh is the same amount of energy delivered in 86 kg (190 lbs) of TNT….enough to level an entire building.

On a more cheerful note, this battery packs the equivalent energy of 86,000 kilocalories, or what an average human consumes in food over 43 days!

Yet as big as this figure sounds, and it is big, only 3 gallons of gasoline (11 liters) pack the same amount of energy. Whereas the Tesla battery weighs about 1300 lbs (590 kg), 3 gallons of gasoline weigh a mere 18 lbs (8 kg). This illustrates the concept of energy density: a lithium-ion battery is 74X less dense than gasoline.