A look inside the world of batteries

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.

19Aug 2016

As I pondered over the past couple of weeks what might be a befitting topic for this 100th post, a group at MIT announced that they discovered how to make batteries with double the energy. Of course, the operative word in the press release was “first-prototype” which means that it might be a long while before, that is if, we see commercial deployment. However this announcement was the catalyst to focus this post on the state of the lithium-ion battery: In other words, if we ignored future inventions, what is the best that we can expect from the lithium-ion battery today across a number of applications.

For the vast majority of modern applications, the lithium-ion battery is capable of delivering the requisite performance. So if you are wondering why is it that most users complain about the battery, I will use an analogy of a jigsaw puzzle. The solution to storing electrical energy involves many pieces, like the pieces of a jigsaw puzzle. These pieces include the battery materials, chemistry and design, which is often provided by one party: the battery manufacturer. But also a critical piece includes the power management of the device and system, in particular the electronics and software needed to monitor how the energy is efficiently used, say by the apps on your smartphone. An equally critical piece is the battery management intelligence, which is what we do here at Qnovo, that is responsible for the integrity and efficient operation of the battery. If water use is to represent energy use, then the battery is the reservoir; power management is akin to water conservation, something we, Californians, are familiar with; and battery management is ensuring the integrity of the reservoir and its contents, making it large and free of toxins.

Separately and individually, each piece of the jigsaw puzzle is today at an exceptional state of the art for consumer electronics, energy storage, and electric vehicles. For example, energy density of batteries in commercial deployment is already near or at 700 Wh/l. This energy density is sufficient to power smartphones comfortably for a full day of use, or power an electric vehicle for 300 miles. Power management has become quite sophisticated, especially in consumer electronics where now the operating systems, e.g., Android OS and iOS, are asserting clever decisions on how apps may use power. Battery management intelligence has also become quite sophisticated, peering deep into the battery in real time and ensuring its continued health and integrity under extreme operating conditions.

But now imagine for a moment trying to build a jigsaw puzzle where multiple players share in putting the puzzle together, or worse yet, each player owns a subset of the puzzle pieces, but not all the pieces. Now you can imagine that putting the puzzle together can get quite complicated. You see, battery vendors know how to build the battery itself but tend to be quite novices in power management and battery management intelligence. System integrators and OEMs tend to have plenty of experience in power management but their knowledge of the battery chemistry tends to be limited. As to battery management, both battery vendors and OEMs have historically under-estimated its need and are playing catch up.

This, for example, begins to explain why Tesla Motors wants to own all these three pieces of the puzzle, beginning with their widely discussed Gigafactory but also their less-advertised efforts in power management and battery management. Apple is also in the same league. While Apple does not manufacture their own batteries, it is widely known that Apple does design their own batteries as well as having growing expertise in both power and battery management. But these tend to be early adopters who have recognized that they need to lead in owning and putting the pieces of the puzzle together. There are other giants who are in need of playing catch up, and they include the likes of Google, Microsoft, Facebook…as well as industrial players who are eyeing batteries for stationary energy storage and electric vehicles. Gradually, they are all beginning to make power and battery management integral to their long-term strategies.

Historically, system integrators and OEMs treated the battery as yet another component they source from suppliers, like the display or other electrical or mechanical components. But this model of outsourcing the battery expertise is beginning to fray. First, battery vendors are hitting the limits of materials and are struggling to meet the increasing demands of their customers without the use of intelligent power and battery management algorithms. Second, there is a growing discomfort, and I might dare say mistrust, between the OEMs and system integrators on one side, and the battery vendors on the other. Third, with the advent of cheap, meaning both inexpensive and lower quality, batteries form China, the business model of the traditional battery vendors, in particular Sony Energy, Samsung SDI, LG Chem is under pressure. A quick evaluation of their financials is sufficient to show they are not healthy. Sony recently announced the sale of their battery business to Murata Manufacturing. These shifting dynamics complicate the necessary tasks required to put together this battery puzzle and are forcing participating companies to seek different alliances.  For example, see the growing alliance between Tesla and Panasonic as well as between GM and LG Chem. In China, witness the growing influence of BYD in making batteries and making electric vehicles. The result is that the optimal battery system that incorporates the right battery, right power management intelligence, and right battery management intelligence is accessible to limited few organizations that either have the means to be vertically integrated or put together the necessary alliances.

As the reader can gather, the challenges in offering a great battery experience is really not technical in nature, but rather have economic and organizational origins. For consumer applications, the technology already exists to build elegant smartphones with battery capacities in excess of 3,000 mAh, charging very fast at 1 to 1.5C, and lasting 800 to 1,000 cycles. These specifications give the average consumer an excellent overall battery experience. For energy storage, the challenge is not the battery chemistry but rather hitting the right price points and building out comfort in the specifications from extensive testing. For electric vehicles, the cost of the battery is rapidly dropping. The Chevy Bolt and the promised Tesla Model 3 are prime examples of vehicles targeting the broader population at an increasingly affordable price. That is not to say that engineering innovations and continued disciplined product improvements are not necessary; they are important. But the perception that the battery industry is in dire need of a large breakthrough in technology and materials is not well founded. Instead, there is a bigger need for all the players around the battery jigsaw to learn to work together and leverage each other’s expertise and technologies. This is happening; in the process, we will continue to see a race to build up intellectual property, patent ownership, expertise, and skills by the various participants.