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Pushing to the very edge of safe Li-ion charging

I recently had the opportunity to speak with Nadim Maluf, CEO of Qnovo, and Robert Nalesnik, Qnovo’s VP of Marketing, part of the visionary team seeking to re-imagine and improve the way batteries are charged. Qnovo developed the concept of adaptive battery charging to augment battery performance.

 

Maluf and Nalesnik are keenly aware of what’s happening in the battery market, so we sat down to talk about current events involving rechargeable Li-ion batteries and glean insight into what electrical engineers need to know to design a long-lasting, safe, portable product.

 

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Power Electronics News (PEN): What do engineers need to know about battery fires in mobile phones?

 

Maluf: We learned a lot from the laptop fires that occurred about 15 years ago. For example, if you open up a battery, there is a chip at the two terminals of the battery — the battery protection circuit module (PCM) chip, [which is] used for overvoltage/overcurrent protection. They were developed after researching what went wrong with the laptops. These chips are in place to make sure the battery does not exceed its maximum voltage or current.

 

Nalesnik: So, for example, if you shorted the battery’s terminals, then the PCM chip would shut off current so you wouldn’t get into a dangerous situation.

 

Maluf: However, I pointed out in a recent blog that, although this is necessary, it isn’t sufficient anymore. The reason is because the battery chemists and vendors are being asked to do much more than they had to do in the past. In particular, they have to put a lot more capacity in the batteries and with more energy density. They have to charge these batteries at a much faster rate. And that is really straining the chemistry in the battery and forcing the battery chemists to find ways to reduce the error margins. Electrical engineers know that when you design something, you build in a margin for error in your design. Those margins for error are being lowered dramatically.

 

So, what do electrical engineers need to know about the fires in batteries for mobile phones? They need to understand more about the battery. Typically, EEs don’t really understand the chemistry of batteries. They look at the battery as an external device that has specs for maximum current/voltage and how it’s charged.

 

Nalesnik: Basically, the EE trusts that if they stay within the specs for the battery, then everything will be fine. Unfortunately, that’s not true anymore.

 

Maluf: There are mechanisms in place today for qualification. For example, let’s say your favorite chip is a processor. If you are Apple and you buy a processor from Intel, you don’t just look at a spec sheet; you audit and you test everything you can on that chip. That’s not the case for the battery. If I’m the battery vendor, I tell you what battery you need for your requirements. Most companies don’t have the means to test the batteries the way they need to be tested. And the EEs don’t have the knowledge needed about batteries, and the knowledge they do have is dismal.

 

Battery Basics

Let’s take a side-step for a second and look at some key points provided by Maluf about today’s batteries and how they are manufactured. 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 and then rolled together like a cigar before they are packaged into a protective sleeve. This is a gross simplification, but it 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 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!

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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 be deposited as lithium metal, and that is called lithium plating.

 

In practical terms, the anode is wider than the cathode, and only by a few percent. 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.

 

Now, let’s examine one particular manufacturing defect in which a slight misalignment between the anode and cathode occurs during the assembly process. The same structure as above now has the anode layer shifted slightly to the right. At the misaligned edge, the requisite overlap of the anode relative to the cathode is now diminished or even possibly eliminated. The anode/cathode ratio at this spot drops below the requisite limit for ensuring safety. The result 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! There is a catastrophic failure.

Qnovo_li-ionBattery02_sep2016

To prevent the failure, Samsung released new software that limits the maximum charge in the faulty Galaxy Note 7 to only 60% of maximum. Reducing the maximum charge in the battery also reduces the risk of lithium metal plating.

 

The tolerance requirements in the manufacturing of lithium-ion batteries have risen sharply with increasing energy density. Short of using new materials (which 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. They are still needed for other functions and safety, but they do not contribute to storing electrical charge. Therefore, battery designers keep reducing this overhead and, in the process, make the manufacturing tolerances even tighter. 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.

 

Nalesnik: Additionally, there is no accelerated test methodology for batteries. For example, in chips, you can test 80 chips and run high-temperature operating life testing and forecast the failure-in-time (FIT) rate in ppm or ppb and very quickly get the results. However, there is no accelerated way to test batteries.

 

Maluf: Today, EEs don’t have the tools to really understand when a battery begins to flake out. They need more intelligence; they need a tool and diagnostics that allow them to peer into the battery much like they did when the industry had fires in PCs and the industry implemented the PCM chip to prevent it. Now these new demands are being put into place to help you gauge what’s going on inside the battery before you get to a fire. There will always be defects in batteries, and it is being compounded by a very slim margin of error. Engineers need to know when that break will happen so they can avoid it.

 

PEN: Is there anything that can help with this problem?

 

Nalesnik: Fundamentally, even though you have a power control module that can handle overvoltage/overcurrent, if a fire occurs, the PCM never sees it because it goes into a runaway mode before it gets to overvoltage/overcurrent. So the charging processes are blind to what’s going on inside the battery because it’s an open-loop process. Essentially, you can get a safer process by turning the charging into a closed-loop process with an adaptive control system. That means you are looking at the battery operation and what the expected behavior is on very small time increments on the order of seconds. It lets you pose what-if scenarios such as: If I put this current in, then I should get this certain response. One by-product is that you get an extra layer of safety. If after a series of queries you don’t get the expected response, then you can provide information to the system to shut down the battery. So, when you put a control loop into the charging process, you are adding a layer of safety.

 

Maluf: The key point is that this is predictive. The old PCM idea is that they react to a situation that is already past the danger point. The algorithm software is predictive and can predict that a battery will fail in days or even weeks.

 

Our diagnostic tools are peeking into the battery in real time and we are looking at the chemistry. What you really want to know is what the chemical reactions are doing at that moment of time — not what the Rs and Cs are doing. We also can provide information about what batteries from what vendors are prone to problems. Our software knows if you are using a certain battery that is prone to certain problems, how it is controlled, and then red-flag it, if necessary.

 

Nalesnik: It really means that you can’t look at charging as an open-loop process anymore. Even though the failure rate is measured in ppm, if you happen to be that person who gets the one battery that isn’t safe, then it makes it a big deal. You can’t just depend on the chemistry and an open-loop process anymore.

 

It’s not only Samsung that is having this problem. If you look for recalls, you will see that it is becoming a more prevalent problem as more devices are run by batteries. And as billions of batteries are out there, then ppm/ppb starts to become significant.

 

PEN: Maybe this isn’t about the ppm; maybe it’s about the safety of the consumer.

 

Nalesnik: Yes, companies need to have zero failures. You can’t afford to have safety problems. But if the charging is open loop, then you can’t get to zero failures.

 

Maluf: Lithium plating is developed inside the battery and is not visible to the user from the outside. But lithium plating is ultimately what creates the massive short. It could happen because of a bad design; it could be defects and many other reasons. These problems can’t be seen by the EE unless you force the battery manufacturer to run thousands of tests before they ship the battery. There is no means of testing for it today, but the Qnovo software is helping to solve this.  When  the engineer commits to a battery design and has done the initial qualifications, then you can ship the finished product with this battery. Qnovo has frequently seen that the battery vendors will change their initial recipe. And it could vary ever so slightly. They change from the prototype fab where they are doing the initial battery design and then they ship it to China to do the mass production. In the semiconductor industry, for example, if a semiconductor company makes one minor change to its process, they better tell their customer. That’s not true for battery designs. In other words, what you’re shipping in volume could be a little different than what you thought you had.

 

Engineers have things they can do, but it’s more of a Band-Aid solution. They can drop the voltage of the battery. In other words, today’s batteries, individual cells, are 4.35 V max, and the newest crop of batteries are at 4.4 V max. That means battery manufacturers are pushing the limits of materials to the precipice. If you run a 4.4-V battery at 4.35 V, you are sacrificing basically 5% of the capacity. Engineers often sacrifice capacity by reducing the voltage. That means you have to tell your customers that they will lose an hour or more of use time. That’s something that engineers do as a Band-Aid safety fix, but it doesn’t make consumers happy. It’s essentially backing off the edge of the cliff. Another trick you will see some vendors doing is to start at full voltage at the beginning and then drop the voltage. So you think you have 3,000 mAh, but unknown to you, the energy drops during operation to 2,800 or 2,900 mAh, and vendors don’t tell you that. And there is no standard or control in place to prevent that from happening.

 

PEN: Are we reaching the end of Li-ion batteries?

 

Nalesnik: I think we are reaching the end of open-loop charging of Li-ion batteries.

 

Maluf: It’s a great question — and the answer is, it depends on what you mean by reaching the end. An analogy of airplanes from the last hundred years, the main metric early on was speed. But since the 1970s, the planes haven’t really gotten faster. It’s no longer the main metric for comparing jets. Cost (dollars/mile per passenger flown), safety, efficiency. So for Li-ion batteries, if the metric is energy density and wondering if it will keep growing the way it did for the last 10 years, then yes, we are reaching the limits for Li-ion. We are somewhere between 600 Wh/l and 700 Wh/l and, barring some new materials that are yet to be discovered on the commercial scale, I don’t think the energy density will keep growing. It’s like the speed of jets — it will begin to level off. But, as Robert [Nalesnik] said, safety, efficiency, cost, [and] system integration are all needed together to use this limited resource. Think of the charge in a battery as a very precious resource, like water. How do I use the battery in a really efficient way? That is what we need to know. The adaptive algorithm and the other concepts we are putting together are asking: How do I efficiently use what is a very precious resource? And the answer is that we are just beginning, we are not even close to the end of Li-ion batteries. There is a huge amount of innovation that can go into the battery for this metric, but not in terms of putting more mA in the battery.

 

By Paul O’Shea, Editor, Power Electronics News

 

Many thanks to Nadim Maluf, CEO of Qnovo, and Robert Nalesnik, VP of Marketing for Qnovo for their participation in this interview.

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Nadim Maluf, CEO, Qnovo                   Robert Nalesnik, VP Marketing, Qnovo

 

 

Read the other articles in this Special Report Beyond the Exploding Battery.

 

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