Monday, December 22, 2014

45. LITHIUM IS GOLD                                                                        

You have got to be saying, "Wait!", this is non sense. That perennial Periodic Table that we all took in high-school chemistry said that Lithium had an atomic number of 3, yes three, and Gold had an atomic number of 79. Gold is 28 times heavier than Lithium...are we preaching alchemy?

First, let me just say that this not a metaphor for lithium bringing financial rewards. Lithium is fairly abundant in the Earth's crust in the form of lithium salt deposits, and second, I have yet to see a battery manufacturer that is reaping huge financial profits. Financial margins in battery manufacturing are dismally challenged, and the financial markets have not rewarded such performers. Dare I say A123, an old and no-more darling of Wall Street?

This is much simpler. You see, graphite turns into a pretty golden color when lithium diffuses and intercalates inside its carbon matrix. Think of the graphite as swiss cheese. Naturally, this is not evident to most of us since the battery is actually sealed and we don't see what happens inside. But should you develop a transparent battery, then its colors shine. This is precisely what researchers from Michigan did, and while they were at it, they took some nice photographs of the graphite as it turned from black to gold.

Source: S. J. Harris et al., Chemical Physics Letters, 2010 (pages 265 - 274)
The first photograph in the sequence on the far left shows half of a small battery as we look through a transparent window. The battery is discharged, in other words, the graphite is empty of lithium ions. As the battery is slowly charged, lithium ions move from the cathode, the other electrode, with the direction of motion upwards in the screen. The lithium ions are physically intercalating themselves inside the carbon matrix. In the process, the graphite electrode swells to accommodate the physical presence of these lithium ions. This and the chemical bonds that the lithium forms with the carbon change the nature of the material, and consequently its optical properties. The graphite slowly turns from black to reddish and ultimately to gold color. Adding more lithium ions to the carbon ultimately results in the lithium depositing on the surface of the carbon as metallic lithium which has a silver-like appearance.

Now, take a deep look at the third photograph from the left. The bottom of the graphite electrode is red, indicating that it is beginning to fill up with lithium ions, yet the top of the electrode is still black.   This is called diffusion: Put a little red dye in a glass of water and see how it slowly diffuses into the surrounding water. The diffusion of lithium ions creates a steep gradient across the thin electrode (only 0.8 mm thick). Consequently, this puts enormous mechanical stresses across the graphite and is one of several causes of battery failure. This gradient, and consequent battery degradation, is the result of how you charge the battery, and in particular CCCV and its variants such as step charging.  You see, CCCV has no idea what may be the diffusion characteristics of the lithium ions. Charging at faster rates only makes this situation worse unless more clever charging algorithms are incorporated to mitigate this and other degradation effects.

© Qnovo, Inc. 2014 / @QNOVOcorp @nadimmaluf #QNOVOCorp

Saturday, December 20, 2014

44. YOUR BATTERY IS FINE, YOUR APPS ARE NOT!                               

It is nearly guaranteed that every consumer has looked at least once at their mobile device in total bewilderment, wondering how their battery came to be depleted. If it happens more than once, they begin to suspect that they battery is defective or has gone bad. In reality, before suspecting that the battery is defective, they should inspect their power usage, examine whether an app has gone rogue and is responsible for the battery drain. Naturally if you battery is old, say one or two years, then you should suspect that the battery may need replacement. So let's do a couple of debugging checks.

1. Has your battery gone bad? 

A few Android phones are capable of reporting the capacity of the battery. For example, dialing the following sequence *#*#7378423#*#* on your Sony Xperia Z device gives you access to a host of tests including reading the actual battery capacity in mAh. But if your mobile device does not have this feature, then one simple trick is to measure the charge time of your mobile device and compare it to the charge time when it was new. Yes, I know, it is not ideal but it gives a good indication of lost capacity. Make sure your screen is off, all of your apps are shut down, and you use the same AC adapter rating. If your measured charge time is 20% shorter than what you measured for a new phone, you can suspect a bad battery.

2. Do you have a rogue app?

Unfortunately, some apps especially in the Android universe are improperly designed and can be suspect of excessive power usage, and consequently faster battery drain. Additionally, the consumer behavior has a big impact on battery drain. If your app is using the radio or GPS (location services) continuously, your battery will drain fast. The good news is you can now detect this in both Android as well as iOS 8.1.

In Android, go to Settings>Battery. In iOS, go to Settings>General>Usage>Battery Usage. You will see screens like the ones shown below.

Both screens will list the approximate usage pattern as well as the apps or processes that are consuming power. I say approximate because each operating system will add its own interpretation on top of inexact measured battery numbers, but the screens do offer a reasonable guidance on what is draining your battery charge. In the case of Android, you will notice that "Screen" seems to top the charts. That's because it does take a lot of power to maintain the lighting of the LCD screen. Scroll through your list and identify what is potentially a rogue app. Then either delete it and reinstall it, or if you can live without it, then do so. In some cases, your rogue app may be operating constantly in the background, so be sure to turn off background services.

© Qnovo, Inc. 2014 / @QNOVOcorp @nadimmaluf #QNOVOCorp

Wednesday, December 17, 2014

43. ARE LITHIUM ION BATTERIES BECOMING UNSAFE?                             

I just returned from travel in China. The Chinese airport authorities take very seriously the transport of lithium ion batteries on board of commercial airliners. If a passenger is carrying an unknown or unlabeled or improperly marked lithium ion battery in any form, the authorities will confiscate the battery. I saw a disposal bin past the security check point at Beijing Airport that was full of confiscated battery packs.

Why are the authorities so seriously concerned about the safety of lithium ion batteries? I am not suggesting that all lithium ion batteries are unsafe but under some conditions, from both perspectives of battery design and battery operation, a lithium ion battery can become a fire hazard. That's the topic of today's blog.

1. Can the design of the lithium ion battery make it inherently unsafe?

Absolutely! There are countless stories of battery factories that have caught fire in the past decades. The fundamental reason is that lithium metal (not as ions, but as lithium metal in the form of Li2) is highly flammable in the presence of oxygen or water vapor, both abundantly present in air. Therefore, it comes down to assessing whether the design of the battery can allow the formation of lithium metal inside the battery. Unfortunately, as energy density increases, battery manufacturers are forced to pack more material into the electrodes and compress the battery into smaller volumes. One of  unintended consequences of this trend is increased risk of lithium plating. 

For readers who are technically inclined, lithium plating occurs when the voltage of the carbon anode relative to a fictitious lithium reference electrode approaches zero. I explained in an earlier blog the potential contribution of each electrode. Let's re-examine this graph once more. The voltage of the anode is shown in red. Lithium plating happens to the right side of the chart when the graphite is getting filled with lithium ions. Inherently robust designs adjust the geometry of the cathode relative to the anode so that full battery capacity never coincides with an x = 1.0. In other words, the battery is full of charge (i.e., 100% of charge) but the graphite anode is actually at x < 1.0, thereby ensuring that the lithium plating threshold is never reached. The trick, from a battery design standpoint, is to also not sacrifice energy density. This dilemma, avoiding lithium plating vs. increasing energy density, is where battery designs tend to trip and become sensitive to lithium plating.

2. Can one operate the battery unsafely and cause the battery to catch fire?

Absolutely! Even a well-designed battery, in other words, one that is designed to be safe within some given parameters, can be operated in an unsafe manner. Three examples of bad operation come to mind:

i) Charging the battery to voltages above its rated maximum, often 4.35 V: When this happens, the cathode voltage increases above 4.35V and the anode voltage drops below zero, thereby causing lithium plating.
ii) Charging at high charge rates using CCCV or some derivative of CCCV: The high charging current, if not applied properly with the right control algorithms, can also cause the anode voltage to dip below zero and result in lithium plating.
iii) Charging at low temperatures: As the battery temperature drops below 10 °C, the electrolyte becomes viscous, think of gummed up, and consequently, the ions have difficulty in making their journey from the cathode to the anode. This also creates the conditions necessary for lithium plating. 

Fortunately, modern battery protection systems are there to ensure that these unsafe operations are not allowed -- that is if they are well-designed; hence I suspect the origin of the caution by the Chinese airport authorities.

Lastly, one might ask: If the lithium plating happens inside the sealed battery and is never exposed to air, why is it a hazard? The answer is quite simple. Lithium metal plating will grow in time as the battery is used. Once this metal deposit or dendrite grows sufficiently long, it will form an electrical short between the anode and the cathode....and boom, catastrophic failure ensues.

© Qnovo, Inc. 2014 / @QNOVOcorp @nadimmaluf #QNOVOCorp

Monday, December 15, 2014

42.  WHY STEP CHARGING IS MISLEADING                                              

You are now asking, what is "step charging?" The standard charging methodology for lithium ion batteries has been something called CCCV, which stands for constant-current-constant-voltage. As the name implies, the charging starts with a phase of constant charging current into the battery, until the battery terminal voltage reaches 4.35 V, at which point the charging circuitry switch to maintaining a constant voltage at the battery terminals. This ensures that no excessive and unsafe voltages are applied. As I indicated in an earlier blog, CCCV has proven to be a major source of headaches for the battery. Over the past 20 years, a variation on CCCV called step charging, continues to come in and out of consideration.  The figure below illustrates how step charging introduces an additional charging step where the constant current value is reduced so that the charging is gentle on the battery.

But step charging is very misleading and does not solve the problem in fast charging. Instead, it forces the manufacturers to create more compromises that are covered up with marketing gimmicks. Here are a couple of reasons.

 Step charging includes an initial phase of increased charging current, then drops into a second phase of much reduced charging current to be gentle on the battery. Therefore the fast charging is only limited to the first phase of charging. But this phase tends to be relatively short, typically taking the battery to only about 30 or 40% of charge. The overall charge time for the battery remains very long.  In other words, once the battery reaches 30 or 40%, the charge time slows down and your anxiety takes off. In fact, the overall charge time for step charging ends up being worse than that of CCCV. And users notice this very quickly. Browsing the comment sections on the Moto X (one of very few phones that use step charging) illustrate how users notice that the fast charging is really limited. Considering that very few consumers will wait to charge when the battery is below 20%, the benefit of step charging becomes very small.

One of the hidden secrets of step charging is that its charge time grows with battery age. If we look in greater detail at the current profile of step charging in the figure above, the duration of "fast charging" is directly related to the duration of the first constant current phase, i.e., when the charging current is high. The onset of the "step" is when the current drops in value, at which point the charging really slows down. Well, the hidden secret is that as the battery ages, the onset of this step moves to the left. In other words, the duration of the fast charging phase shrinks as the battery ages. So instead of reaching 40%, now this step happens near 20%. So it is a double whammy: as the battery ages and its amount of charge dwindles, and the user loses battery life, it now takes far longer to charge the battery. How much longer? The figure below shows after 500 cycles, the charge time from 0 to 50% of the battery, CCCV and Qnovo charging is about 32 minutes, whereas step charging has grown to 50 minutes! This is not acceptable!

So how can you tell whether your mobile device is using step charging. For now, the only devices on the North American market that are using step charging are made by Motorola. In particular, the Moto X makes an additional and very serious compromise: it reduces the capacity of the battery in order to use step charging. The Moto X has a battery capacity of 2,300 mAh, greatly insufficient for a full day of use, especially for this 5.2-in giant.  In other words, step charging creates more reasons for more compromising. This is not innovation!

© Qnovo, Inc. 2014 / @QNOVOcorp @nadimmaluf #QNOVOCorp

Saturday, December 6, 2014

41. LCO, LFP, NMC...CRYPTIC LIVES OF THE CATHODE                              

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.

© Qnovo, Inc. 2014 / @QNOVOcorp @nadimmaluf #QNOVOCorp

Thursday, December 4, 2014

40. WHAT ARE THE REAL  0% AND 100% POINTS?                                                      

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.

© Qnovo, Inc. 2014 / @QNOVOcorp @nadimmaluf #QNOVOCorp

Tuesday, December 2, 2014

39. THE BATTERY SCANDAL OF THE BOEING 787                            

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.

© Qnovo, Inc. 2014 / @QNOVOcorp @nadimmaluf #QNOVOCorp