Safety

06Dec 2014

What flavors does the lithium ion battery come with? The answer is: quite a few. They represent the various combinations of materials that are used for the two electrodes, the anode and the cathode.

It turns out the available choices for the anode are quite limited. Different forms of carbon, in particular graphite, are the common choice in all commercial applications. Silicon and silicon-carbon composites as well as tin are candidates for future anode materials but they are not presently in wide commercial use.

But the cathode has enjoyed a longer list of candidate materials, typically known by their acronyms.  All of these materials are lithium metal composite alloys, with many them using heavy metals such as cobalt and nickel.Each material seems to fit a particular application. Lithium-cobalt-oxide (LCO) is most commonly used in applications where high energy density and high capacity are needed, in particular mobile devices. Lithium-iron-phosphate (LFP) is of particular interest to the automotive industry especially in China. Its safety, long cycle life and low cost make it attractive to electric vehicles. Nickel and manganese composites have been rather limited in their utilization primarily due to cost considerations. For example, the Tesla Model S uses 18650 cells made by Panasonic with nickel-cobalt-aluminum (NCA) alloy for the cathode material.

Another salient difference between these materials is their open circuit voltage; in other words, how their terminal voltage varies with the amount of charge stored in the cell (the state of charge or SOC). The graph below shows that dependence during charging. As more charge is added to the battery, its terminal voltage rises.

One will quickly notice that LFP has the lowest average voltage, near 3.2V, considerably lower than the voltage for NMC or LCO, both hovering near 3.7V. In fact, modern cells made with LCO have a maximum voltage of 4.35V, up from 4.2V, thus raising the average voltage to 3.8V. This higher voltage, and consequently higher energy, makes LCO an attractive material for use in mobile devices.

Additionally, the voltage behavior near empty (below 15% SOC) plays a big role in the utility of the material. LCO can sustain a useful voltage above 3.6V down to about 5% remaining charge, whereas NMC drops below 3.5V when the SOC is at 10%. This is yet another reason why the mobile industry continues to choose LCO.
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04Dec 2014

Your fuel gauge indicator, whether it is in your smartphone, tablet or even electric vehicle, gives an indication of remaining charge in your battery. That indicator is universally given in percentage. It is assumed that at 100%, the battery is full, and at 0%, the battery is empty. But what is the definition of full and what is the definition of empty. That’s the topic of today’s post.

In an earlier post, I showed how the voltage across the terminals of an individual battery cell is really the composite voltage contributions of both electrodes, the anode as well as the cathode. The voltage contribution for each electrode varies with the fraction of lithium ions that are embedded inside the electrode. A user can only measure the composite voltage sum of the two electrodes when he or she measures the terminal voltage of the cell.

The first point to understand is the relationship between the fraction of lithium ions inside the electrode material vs. the notion of empty or full. When the graphite or carbon anode is completely devoid of lithium ions, the cell is truly empty. In other words, there are no available lithium ions and consequently, no “stored” charge. From that earlier post, one can see that the composite cell voltage can be very low, somewhere near 1V or even less. Cells never operate near that low voltage point. A truly empty battery cell has most likely incurred serious damage to its internal structure. If any of your lithium ion cells measure less than 2V, it is time to discard them. As a result, most battery cells consider a safe lowest operating voltage to be between 2.5V and 3.0V. This is the definition that one may get from the battery manufacturer. In practice, however, a smartphone will display 0% when the cell voltage is near 3.3V. This is because several electronics components, most notably the power amplifier for the radio, will not operate efficiently below 3.3V. Consequently, your mobile device shuts off at that low voltage threshold. This is the definition of zero as made by the mobile device maker. In either case, you will observe that the definition of empty is usually related to a low operating voltage threshold, and less so about being “empty.”

The chart below illustrates the dependence of voltage on the amount of charge taken out of the cell during discharge. When the battery is fully charged — to the far left of the chart — the voltage is at its maximum. As charge is slowly removed from the cell, the voltage declines. At some point near 3.6V, it begins to drop precipitously; in other words, one needs to take out only a small amount of charge before the voltage drops rapidly. The rate at which the voltage declines depends on the choice of material. The chart below shows the voltage dependence for a battery that is made with a carbon anode and a lithium-cobalt-oxide (LCO) alloy cathode. The battery nominally stores 3,000 mAh.

If you think about this chart for a brief moment, you will quickly realize that the area under the curve is the amount of energy stored in the battery — after all, energy is the product of charge and voltage.  So let’s compare this to NiMH batteries that have a nominal cell voltage of 1.2V. Which one has a higher energy density? Naturally, lithium-ion: by at least a factor of 3X.

What about the definition of the 100% point? Is the battery full and hence cannot accept more charge? Not really. The definition of 100% is simple: the terminal voltage of the battery has reached 4.35V (sometimes, it is 4.2V but more often in consumer devices, it is 4.35V). This voltage threshold is strictly due to safety. Above 4.35V, three unsafe mechanisms begin to take place. First, lithium plating occurs in lithium ion batteries that use a carbon-based anode. These lithium metal deposits can short the cell and cause a fire. Second, the electrolyte, being liquid or gel-based, deteriorates rapidly and decomposes. The electrolyte is the medium through which the lithium ions can travel from one electrode to the other. And finally, the structure of the cathode itself begins to change its material phase and it becomes unstable.

So there you have it, empty is really not empty, and full is really not full. Both limits are defined primarily on the basis of safety and practical utilization of the battery.

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02Dec 2014

The National Transportation Safety Board (NTSB) released today its incident report on the battery fire that led to the grounding of the Boeing 787 fleet for many months. It is a thorough report, nearly 100 pages long, that examines in great detail all aspects of the lithium ion battery and its systems. The result is shared responsibilities for the battery fire, with blame targeting GS Yuasa, the Japanese manufacturer of the lithium ion individual battery cells, and Thales, the French system integrator responsible for the power conversion subsystem, as well as Boeing. Even the FAA shared some of the blame in how it managed the certification process.

The report reserved special blame on the poor design, test and manufacturing practices at GS Yuasa. In essence, the NTSB reports that defects in the assembly and manufacturing of the cells led to internal shorts in the battery. Inadequate testing during certification throughout the chain did not catch these problems. Specifically, these defects led to shorts and consequently thermal runaway in cells 5 and 6. The battery contained a stack of 8 cells connected in series for a nominal voltage of 29.6 Volts.  CT scans and disassembly of defective batteries showed the failure points — defects in the form of protrusions or wrinkling in the cathode-separator-anode winding that effectively created minute electric shorts between the cell’s terminals.

The first figure below shows a schematic from the NTSB report of the design of an individual cell. It consists of a sandwich of the lithium cobalt oxide (LCO) cathode layer, the separator layer, and the carbon anode layer. Aluminum and copper foils act as the electrical conducting planes for each of the cathode and anode, respectively. This material construction is nearly identical to the ones used in the consumer industry. These layers are then folded together to form the cell windings.

The next photograph shows how the cell windings are flattened before insertion into the metal packaging can. The NTSB pointed to this winding formation and flattening process as one of the culprits in the origination of the defects and shorts. This manufacturing process created wrinklings; this is a highly technical term referring to the buckling of the electrode foils, thus compromising their integrity right during the cell manufacturing steps. Additionally, the NTSB found that the electrolyte filling process was inconsistent with the practices used in the industry and possibly led to the incomplete formation of the SEI layer (it is a thin layer that forms between the anode and the electrolyte). In other words, the NTSB pointed a clear finger at the poor cell manufacturing at GS Yuasa.

So why weren’t these defects caught during the test and certification phases? Well, apparently the battery that made it on the actual B787 airplanes was different in design and properties from the batteries that underwent the tests. It seems that lots of heads at GS Yuasa and up the food chain to Boeing will be rolling — if they have not already. Shame on the engineers and the managers who supervised and led this process.

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21Nov 2014

I described earlier the two pieces of the fast charging puzzle. There is first the power delivery from the wall socket to your mobile device, and then there is managing the charging power into the battery so it does not get damaged.

The first piece of the puzzle is all about power circuitry, something electrical engineers enjoy and revel in. But the second piece of this puzzle is about managing the chemistry in the battery. Here, I have some bad news: electrical engineers, even the most successful ones, are likely to have scored a C- or a D+ on their chemistry college classes; in other words, they are not comfortable with chemistry. Like all of us humans, we drift towards our comfort zones, so chemists drift to chemistry, and electrical engineers drift to building electronics. Getting the two disciplines to work together takes hard work and lots, really lots, of discipline. As a result, it is rare to find interdisciplinary development in the mobile world, especially between battery chemistry and electronics. We, at Qnovo, happen to be one of these rare exceptions.

Well, enough self-promotion; let’s get back to real life. What happens if overzealous electrical engineers decide to charge a battery as fast as they can. After all, they are electrical engineers and know how to design circuits that can pump a lot of energy in the battery — some even give it great names like Quick Charge. The result is simple: They will destroy the battery (if you use CCCV charging). The data in the next graph illustrates the extent of the damage.

The graph is a standard capacity fade curve for a battery. In this case, it is a 2,400 mAh lithium-ion polymer battery from one of the leading manufacturers. The vertical axis shows the remaining capacity of the battery (in Ah) as the battery undergoes cycling (shown on the horizontal axis).  As the battery is repeatedly charged and discharged, it loses its ability to hold charge, hence the degrading curve. This graph shows the capacity degradation for four distinct charge rates, varying from a slow 0.5C up to a superfast 1.5C, all using CCCV. At 0.5C, the battery charges in about 3 hours, and at 1.5C, it charges in a little over an hour.

So what is the graph telling these overzealous engineers? It says that at the slow charge rate, the user will comfortably obtain over 600 cycles of operation. That’s plenty to cover a year and more. But the superfast charge rate of 1.5C does so much damage to the battery that the battery will barely last a couple of months. In other words, fast charging with CCCV is bad! Isn’t time to use new methodologies that give us fast charging without damaging the battery.

The moral of this post: Don’t trust everything your electrical engineers tell you…well, that is unless they really scored A’s on their chemistry college courses!

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14Nov 2014

You have heard it before from me: battery performance is getting seriously challenged. The battery vendors are having a difficult time, and I am being kind, in meeting the expectations of the mobile device manufacturers. Specifically, it is getting extremely difficult for the battery vendors to simultaneously deliver high capacity, fast charging, and long cycle life. But why is that? 

Let me digress briefly here and examine the average cruising speed of commercial airplanes over the past century. One will rapidly notice that airplanes have not gotten much faster in the last 50 years. They are more fuel efficient; they travel longer distances in greater comfort (well, if you pay for it); and they are far safer than they have ever been…but they are surely not faster. That’s because it gets very expensive to travel faster than the speed of sound (667 knots). The Concorde is a stark reminder of this economical limit. Chuck Yeager broke the physical limit of the sound barrier decades ago, but we can’t really change the economics around this limit.

Lithium-ion batteries are rapidly approaching a similar limit for energy density. Short of a major breakthrough in a new material system, we are staring at a difficult barrier somewhere between 600 and 700 Wh/l. With that, I mean achieving large-scale manufacturing with affordable economics that match the requirements of the mobile device industry. A key limiting factor is now the carbon anode material. It is possible that new carbon-silicon composite anodes can change this equation, but for the foreseeable future, these new composites will continue to suffer from poor cycle life and high manufacturing costs. Until then, the economics of rising energy densities will be severely disadvantaged.

Is there a scientific origin for this limit? There are plenty of reasons, but it is best to illustrate one: the impact of battery voltage on energy density. In a very simplistic description, higher energy density comes from packing more lithium ions inside the battery, as well as raising the voltage at the terminals (if you recall, energy is the product of charge, or ions, and voltage).

The voltage at the terminals is the difference between two voltages; that of the cathode voltage in reference to a fictitious lithium contact, minus the voltage of the anode, also in reference to that same fictitious lithium contact. This is illustrated in the graph below.

On the vertical axis, I show the voltages of both the cathode vs. lithium (top) and the anode vs. lithium (bottom). On the far left of the chart, i.e., when x is approaching zero, the graphite is void of lithium ions and cobalt-oxide is totally full of lithium ions. This is when the battery is “empty.” On the far right, when x is approaching one, the opposite is true; the battery is “full.” I specifically refrain from saying x = 0 or x = 1. At these extremes, bad things happen. When x = 0, the physical structure of the cobalt-oxide alloy is greatly damaged. This limits the low end of the battery voltage to about 3.0 V. In other words, never discharge your lithium-ion battery below 3.0 V; the risk of irreversible damage is great. Most smartphones actually cut off near 3.3 V (this is really when your phone reads zero percent).

At the opposite end when x = 1, lithium ions combine to deposit (or plate) as a metal.  The anode structure is also under immense mechanical stress. Additionally, when the cathode voltage rises past 4.2 V, the electrolyte begins to oxidize (and ultimately decompose). This effectively limits present-day lithium-ion batteries to a maximum voltage of 4.35 V with the understanding that the “bad stuff” begins to occur past 4.0 V, and becomes unsafe past 4.35 V.

To raise the energy density in the carbon/cobalt-oxide material system, one needs to raise the voltage and/or pack the electrodes as close as possible to each other. Well, raising the voltage past 4.35 V is getting very difficult. Finding electrolytes that can handle such voltages is no small feat. Additionally, the battery is now awfully close to the x = 1 point; in other words, the risk of lithium metal deposition  at the anode is dangerously large at high energy densities. Life is getting tough, and there is very little room left for the battery vendors to maneuver.

These are just a few physical insights behind the challenges that the battery designers and manufacturers are facing. Finding solutions to these challenges via brute-force material development is not the answer. If you find yourself stuck with these limits, talk to us at Qnovo!

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