Safety

22Nov 2016

Qualcomm announced this week their 4th generation Quick Charge™ technology to be available in their upcoming Snapdragon 835 chipset. Quick Charge™ 4 continues to build on making fast charging an integral part of modern smartphones and consumer devices. In this latest generation, Qualcomm adds a number of key features, in particular, higher efficiency in delivering the power from the wall socket to the device, more power available for charging faster, and better thermal management. I applaud the continued evolution of Qualcomm’s QC technology.

As fast charging becomes an entrenched technology in the mobile landscape, the emphasis on battery safety itself during fast charging begins to take priority. As I highlighted in this earlier post, fast charging done improperly causes irreparable damage to the battery causing a loss of capacity (mAh) or worse yet, battery safety problems. Combining fast charging with high-energy density cells, especially the new generation that is operating at 4.4V, is a recipe for potential disasters. This post is about what can go wrong when we mix fast charging with high-energy density batteries, but neglect to implement the necessary charging intelligence and the necessary controls around the battery.

First, let me clarify a few things.

  • Fast charging includes the realm of charging the battery at rates near or above 1C . At 1C, the battery charges to half-full from empty (0 to 50%) in 30 minutes. QC 4.0 is capable to going at twice that rate, or 2C. That is very fast.
  • High-energy density batteries are those with energy densities in excess of 600 Wh/l, with the most recent ones at or near 700 Wh/l. The newest generation of these batteries are almost universally operating at 4.4V. This earlier post explains the risks and perils of operating at this voltage.
  • The last point I want to clarify is that the common charging approaches, namely CCCV and step charging do NOT provide any intelligence or controls around charging. They are open-loop methods with no mechanism to gauge the state or health of the battery in order to make the proper adjustments and avoid the risks that I will highlight below.

The mix of fast charging and high-energy batteries makes a very volatile situation. This reminds me of fancy car commercials with the fine print warning at the bottom of the screen: “Professional drivers on a closed course. Do not attempt.” Fast charging high-energy batteries is rapidly approaching this realm of cautionary warnings. The consequences of neglecting such advice can be dire especially as smartphone fires are fresh in our collective memories.

So what can go wrong?

To begin with, lithium metal plating is a huge risk when one attempts to fast charge a 4.4V cell. We see lithium plating on most if not all cells from reputable battery suppliers when charged using CCCV or step charging. This is a serious problem if not mitigated with the proper battery intelligence. Left unchecked, lithium metal plating can lead to safety hazards and potential fires. What makes lithium metal plating even more hazardous is that it is not easy to detect its presence inside your smartphone. By the time it develops into a potential electrical short inside the battery, it is often too late. Therefore it is imperative that the intelligence in the battery management seeks to avoid its forming from the very beginning of the battery’s life in your smartphone.

A second serious hazard is excess swelling of the battery. Yes, the battery will physically grow thicker as it is repeatedly charged. It is nearly impossible to measure the thickness of the battery once it is embedded inside your smartphone. Clever estimates of the thickness without physically touching the battery belong to the category of advanced intelligent algorithms that are becoming increasingly necessary. You might say: so what, let the battery swell! Excessive swelling will most certainly break your display screen.

A third hazard relates to the battery’s behavior at high temperature. The electronics inside your device consume power and cause the smartphone to get hot.  Those of you who have fast charging on your devices will attest to this fact. One misconception is that the battery itself heats up because of fast charging. That is not correct. The battery gets hot because of the heat generated by the electronics inside the smartphone. These temperatures can rise inside the smartphone to 40 °C, and in some many cases approaching 45 °C. These elevated temperatures accelerate the degradation of materials inside the battery especially at the elevated voltages. This leads to a rapid loss of charge capacity (your mAh drop very quickly) accompanied with excessive swelling of the battery. If you are an Uber driver with your smartphone fast charging on your dashboard on a hot summer day, this does not bode well for you.

These are only three examples of potential battery safety hazards associated with fast charging high-energy density cells using traditional charging methods…each one of them can lead to serious battery safety problems. That’s a good time to heed the warning in the car commercials. If you are not a professional, please do not attempt.

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.

17May 2016

A young woman, Anna Crail, was flying on 19 March of this year on an Alaska Airlines flight from Seattle to Honolulu. About 90 minutes prior to landing, her iPhone 6 suddenly broke out in flames causing panic in mid flight. The fire was rapidly extinguished by the flight attendants, but not without leaving the airline and the FAA searching for answers. This is one of several safety-related battery incidents that are becoming increasingly common. There are countless reports on hoverboards that are catching fire. While safety-related incidents involving Apple iPhones appear to be sparse in the media, there are increasing reports of Android-type mobile devices posing serious safety hazards, especially in Asian geographies. This post provides first insights on the factors that impact the safety of the lithium-ion battery in mobile devices.

First, let’s start with some background material. Three key design factors heavily influence the lithium-ion battery in a smartphone: i) Higher charge capacity, i.e., more mAh; ii) Faster charging ; and lastly iii) thin profiles.

Higher capacity is driving an increase in energy density at the rate of about 5 to 7% per year. The energy density of lithium-ion batteries when the first iPhone was launched in 2007 was near 400 Wh/l. Today’s state-of-the-art mobile batteries are in the range of 600 to 700 Wh/l. These higher energy densities are associated with two important physical parameters: higher terminal voltage (near 4.4V, up from 4.2V ten years ago) and significantly higher current flux inside the batteries (a lot more ions are making the journey between the electrodes) elevating the risk of damage within the battery.

Fast charging is now an expected feature in mobile devices, at least those on the higher end of the spectrum.  A charge rate of 1C corresponds to a 50% charge in 30 minutes…1C or faster is becoming the norm. Faster charging also means a lot more ions are making the journey within the battery between the two opposite electrodes. Again, faster charging = higher risk of battery damage.

Lastly, thin profiles of smartphones are pushing the battery to ever thinner dimensions. The next generation of smartphones are employing batteries that are a mere 3 to 4 mm thick, with this figure being pushed down even further where possible. Thin batteries create a slew of headaches for engineers…their performance tends to be inconsistent; manufacturing non-uniformities are amplified; and the current flux (and corresponding ion density) within the battery is also pushed to higher levels.

So what do these really mean? they mean that the perfect safety storm is brewing if proper care, battery intelligence, and diagnostics are not implemented.  One of the first consequences of higher energy density (especially higher operating voltages near 4.4V) is the increased risk of formation of lithium metal on the carbon anode (the negative electrode in the battery during charging). This is called lithium plating. Combine high energy density with fast charging and thin batteries, and the risk of lithium plating becomes dangerously significant. But lithium metal (i.e, in its molecular form, not in its ionic form) is highly flammable especially in the presence of oxidants. Additionally, spurs of lithium metal can cause electrical shorts within the battery…both of these mechanisms have seriously hazardous consequences.

The next figure illustrates the safety risk and its relationship to charge rate and energy density. Right around 1C and 600 Wh/l, the battery may become a safety hazard, especially in the absence of proper and diligent designs. Some battery manufacturers are better than others, with batteries made by Chinese manufacturers being the most prone to increased safety risks. Some device manufacturers (OEM) choose to sacrifice some battery specifications to gain a little safety margin. For example, some OEMs reduce the operating voltage of the battery as it ages, for example, from 4.4V down to 4.35V or less. This means that you will be robbed of mAh without being told. Your smartphone may have a great battery (say for example, 3000 mAh) at the beginning of its life (and when it operated at 4.4V), but a few months in its operation, the maximum voltage is intentionally reduced to 4.35V thereby reducing the capacity by 150 – 200 mAh; in other words, your battery is now only about 2800 mAh. Ouch! That’s not good, especially when you are not aware of it.

Safety_box

The safety risk also depends on temperature, rising rapidly with lower temperatures…and by low temperatures, I really don’t mean sub-freezing temperatures. The vast majority of battery safety tests are conducted at room temperature, usually near 25 °C (77 °F). What is considered as “low temperature” for a battery is 10 °C (50 °F) or lower. Right around this temperature range, the probability of plating of lithium metal soars creating serious hazards.

For the time being, batteries catching fire have been mostly limited in frequency and consequences. But with rising energy density and charge rates, the safety hazard is slated to become a lot more serious in the near future. Look for smartphone OEMs that are investing in the proper solutions to give you an excellent battery experience AND a safe one too.

02Apr 2016

For a seemingly simple device with only two electrical connections to it, a battery is deceivingly misunderstood by the broad population, especially as batteries are now a common fixture in our technology-laden daily lives. I will highlight in this post five common misconceptions about the lithium-ion battery:

1. STAND ON ONE LEG, EXTEND YOUR ARM, THEN PLUG YOUR CHARGER INTO YOUR DEVICE:

Well, not literally, but the acrobatic move captures the perceived hypersensitivity of the average consumer about past or secret special recipes that can help your battery. One of the silliest one I ever heard was to store the battery in the freezer to extend its life. PLEASE, DO NOT EVER DO THIS! Another silly is to charge the battery once it drops below 50%, or 40% or 30%….Let me be clear, you can use your phone down to zero and recharge it, and it will be just fine.

It is also now common to find apps that will “optimize” your battery. The reality is they do nada! Don’t bother.  Don’t also bother with task managers; no they don’t extend your battery life. Both Android and iOS are fairly sophisticated about managing apps in the background.

Turning off WiFi, GPS and Bluetooth will not extend your battery life, at least not meaningfully. These radios use such little power that turning them off will not give you any noticeable advantage. The fact is that your cellular radio signal (e.g., LTE) and your display (specifically when the screen is on) are the two primary consumers of battery life — and turning these off render your mobile device somewhat useless.

Lastly is the question of “should I charge the battery to 100%?” Well, yes! but you don’t have to if you don’t want to or can’t. In other words, stop thinking about it. The battery is fine whether you charge it to 100% or to 80% or anything else. Sure, for those of you who are battery geeks, yes, you will get more cycle life if the battery is not charged to 100%. But to the average population, you can do whatever you like — your usage is not wrong. These are design specifications that the device manufacturer is thinking about on your behalf.

2. LITHIUM BATTERIES HAVE LONG MEMORIES:

Yes, as long as the memory of a 95-year old suffering from Alzheimers!! Sarcasm aside, lithium ion batteries have zero memory effects. Now, if you are a techie intent on confusing your smartphone or mobile device, here’s a little trick. Keep your device’s battery between 30% and 70% always….this will confuse the “fuel gauge,” that little battery monitor that tells you how much juice you have left. The battery will be just fine but the fuel gauge will not report accurately. Every so often, the fuel gauge needs to hit close to zero and 100% to know what these levels truly are, otherwise the fuel gauge will not accurately report the amount of battery percentage. This is like your gas gauge in your car going kaput…it does not mean that the battery has memory or other deficiencies. Should you suspect that your fuel gauge is confused, charge your phone to 100% and discharge it down to 10% a few times. That is sufficient to recalibrate the gauge.

3. WE NEED NEW BATTERY CHEMISTRIES — THE PRESENT ONES ARE NO GOOD:

This one garners a lot of media interest. Every time a research lab makes a new discovery, it is headline news and makes prime time TV. The reality is that the path from discovery in the lab to commercial deployment is extremely rocky. There have been dozens such discoveries in the past 5 – 10 years, yet virtually none have made it into wide commercial deployment. History tells us it takes over $1 billion and about 10 years for a new material to begin its slow commercial adoption cycle….and for now, the pipeline is rather thin. Additionally, present lithium ion batteries continue to improve. Granted, it is not very fast progress, but there is progress that is sufficient to make great products….just think that current battery technology is powering some great electric vehicles.

Let me be more specific. Present-day lithium ion batteries are achieving over 600 Wh/l in energy density — that is nearly 10x what lead acid batteries can deliver. This is enough to put 3,000 mAh in your smartphone (sufficient for a full day of use), and 60  kWh in your electrical car (enough for 200 – 250 miles of driving range). With the proper control systems and intelligence, a mobile device battery can last 2 years or more, and an electric vehicle battery can last 10 years. Does it mean we stop here? of course not, but this sense of urgency to develop new materials or chemistries is rather misplaced. Instead, we need to keep optimizing the present batteries materials and chemistries. Just reflect on how silicon as a semiconductor material was challenged by other candidate materials in the 1980s and 1990s (do you remember Gallium Arsenide), only for it to continue its steady progress and become an amazing material platform for modern computation and communication.

4. LITHIUM-ION BATTERIES ARE EXPENSIVE:

What made silicon the king of semiconductor materials is its amazing cost curve, i.e., decreasing cost per performance, aka Moore’s law. Now, lithium ion batteries don’t have an equivalent to Moore’s law, but, the cost of making lithium ion batteries is dropping fast to the point they are rapidly becoming commoditized. A battery for a smartphone costs the device OEM somewhere between $1.50 and $3.00, hardly a limiting factor for making great mobile devices. GM and Tesla Motors have widely advertised that their battery manufacturing costs are approaching $100 /kWh. In other words, a battery with sufficient capacity to drive 200 miles (i.e., 50 to 60 kWh) has a manufacturing cost of $5,000 to $6,000 (excluding the electronics)…with continued room for further cost reduction. It’s not yet ready to compete with inexpensive cars with gas engines, but it sure is very competitive with mid-range luxury vehicles. If you are in the market for a BMW 3-series or equivalent, I bet you are keeping an eye on the new Tesla Model 3. Tesla Motors pre-sold nearly 200,000 Model 3 electric vehicles in the 24 hours after its announcement.  This performance at a competitive price is what makes the present lithium ion batteries (with their present materials) attractive and dominant especially vis-a-vis potentially promising or threatening new chemistries or new materials.

5. LITHIUM-ION BATTERIES ARE UNSAFE:

Why do we not worry about the immense flammability of gasoline in our vehicles? Isn’t combustion the most essential mechanism of gas-driven cars? Yet, we feel very safe in these cars. Car fires are seldom headline news. That’s because the safety of traditional combustion engine cars has evolved immensely in the past decades. For example, gas tanks are insulated and protected in the event of a car crash.

Yes, lithium is flammable under certain but well known conditions. But the safety of lithium ion batteries can be observed as religiously as car makers observe the safety of combustion engines. It is quite likely that some isolated accidents or battery recalls may occur in the future as lithium ion batteries are deployed even wider than they are today. In mobile devices, the track record on safety has been very good, certainly since the battery industry had to manage the safety recalls at the turn of the century. Is there room for progress and can we achieve an exceptional safety record with lithium ion batteries? Absolutely yes. There are no inherent reasons why it cannot be achieved, albeit it will take time, just like the automotive and airline industries have continuously improved the safety of their products.