Safety

12Jun 2015

Kevin Gibb of TechInsights published recently an article in EE Times that shows a teardown of the lithium-ion battery used inside the iPhone 6 Plus. While the teardown and the article seemed motivated by determining the cost of this battery — somewhere near $4.00 — it contained some very nice cross sectional photographs taken using optical and electron microscopy of the various layers that make the iPhone 6 Plus.  Anything that carries an Apple logo seems to attract a lot of attention, but the battery inside the iPhone 6 Plus is similar in performance and structure to many other Li+ (lithium-ion) polymer batteries used in mobile devices.  For example, the battery capacity of the iPhone 6 Plus is rated at 2.915 Ah, within a rounding error of the capacity of batteries used in the Sony Xperia Z3 and Z3+, the LG G3 and G4. Let’s use this very nice teardown report of the iPhone 6 Plus battery to shed more light onto the inner structure of a lithium-ion battery and its workings especially in view of fast charging.

I described in an earlier post the various shapes of a lithium-ion battery. A 18650 cell is encased in a metallic cylinder, whereas a polymer one is a thin and flat pancake-like without any external metallic protection. Yet, the insides are nearly identical, all consisting of a set of electrodes called anodes opposing another set of electrodes called the cathodes with both sets of electrodes separated by a porous membrane called a — hold your breath — “separator.” The first picture below shows a cross section of the polymer battery inside the iPhone 6 Plus viewed through an optical microscope. For reference purposes, the iPhone 6 Plus battery is approximately 3 mm thick.

In mobile devices, the vast majority of batteries use a metal oxide called lithium-cobalt-oxide (LCO) deposited on an aluminum backplate to act as the cathode (the positive electrode during charging). You can see the bright white aluminum back layer in the photo above, but it is difficult to see the LCO layer at this magnification. The anode is nearly always made of a thin carbon graphite layer deposited on top of a copper backplate. There is a very thin separator layer that sits between each set of anode/cathode layers. During charging, the ions, yes, the lithium ions, travel from the cathode through the porous separator to the anode, and embed themselves inside the graphite. As every skilled engineer should know, charge balance means that there is an opposing current made of electrons that goes through the external circuit between the anode and the cathode. This means that maintaining a low electrical external path resistance is essential to the operation of the battery — one of the reasons why aluminum and copper conductors are used.

The photo above shows a stack of alternating layers of anodes and cathodes. There are 11 anode/cathode layer pairs, which means the pitch is approximately 275 microns. This particular construction is unique to LG Chem with a stack of parallel layers. Other battery manufacturers use what is known as a jelly-roll, with the layers of anodes and cathodes rolled together like a cigar. This mechanical structure, while seemingly immaterial to the novice, plays a big role in the distribution of electrical current inside the battery, and consequently the governing degradation mechanisms. Let’s zoom in a little more.

The second photograph shows a scanning electron micrograph (SEM) of two sets of anode/cathode layers. Now we can see the individual structural materials. The separator is typically near 10 to 20 microns in thickness. The graphite and LCO layers are often around 50 microns but can vary depending on battery capacity and current rating. This SEM now shows that the LCO layer is granular in nature. The graphite layer is granular too.  The grains, varying in size from a few to several microns in diameter, consist of crystalline layers — a lattice-like — where the lithium ions can embed themselves. In charging, they embed themselves in the graphite lattice, and in discharge, in the LCO lattice. The graphite lattice is pictured next using a transmission electron micrograph (TEM). The lattice is made of atomic layers that are a mere 0.34 nm apart — think of it as atomic Swiss cheese.

The LCO and graphite have a limited capacity of how many lithium ions they can “hold” inside their lattice. This determines the amount of LCO and graphite material that is needed for a battery of a given capacity, i.e., of a given mAh. This in turn determines the energy density. Well, sort of, because there is another kink in the design of the battery, and that is the size of the grains (both LCO and graphite) and how tightly packed they are in the electrode layers. If the grains are too tightly packed, then the lithium ions will find it difficult to travel through all the grains; in other words, the maximum current capability of the battery is impaired. So you are hopefully getting a little taste of the various compromises a battery designer needs to go through….and we haven’t even yet gotten to charging.

Now let’s talk about the headaches that come with degradation of this structure especially with fast charging. High capacity and/or faster charging means a lot of ions need to zip in and out of the anode layers — since the anode is primarily responsible for storing the ions during charging. Think of cars on a highway at peak rush hours….it’s not easy; every pothole in the road now contributes to traffic flow. For example, small perturbances in the uniformity of grains means more ions will flow into one grain vs. another, thus creating differences in current density, and excessive stress on some grains (ultimately causing mechanical fracturing of the graphite lattice and loss of capacity). Small disturbances in the voltage distribution across the layers means some portions of the stack may see a potential difference between the anode and cathode that will promote the metallic plating of lithium — a very detrimental failure mode especially present with faster charging. These are only but two examples of the degradation mechanisms. There are several more that are becoming prevalent in modern batteries with high energy density and faster charging. The task is to tame these degradation mechanisms to extract maximum performance, and that is now falling onto the next frontier of clever charging algorithms — and that is what we do at Qnovo.

Fast charging a battery clearly involves a high degree of optimization in order to manage the large flow of ions. Historically, battery vendors did it while sacrificing grain size, or packing density of grain; in other words sacrificing energy density and overall battery capacity. This compromise is no longer acceptable.

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24Apr 2015

Ok, ok, I should not be joking about the name. Safety is a serious matter. But let’s talk honestly today about dishonesty.

 

Ultrafire is a China-based vendor of 18650 lithium-ion cells.  One can buy their 18650 cells from eBay, Amazon and lots of other web-based stores. I was intrigued last week when I saw on eBay Ultrafire 18650 cells advertised with a charge capacity of 5,000 mAh. Wow, 5Ah! Is this real? The most I had seen ever were 3,400 mAh from Panasonic (what’s inside the Tesla) and 4,000 mAh was the next promised land. But 5,000 mAh? I really wanted one.

 

You gotta love e-commerce. Within less than 48 hours, I was the proud owner of 4 Ultrafire 18650 cells each labeled 5,000 mAh — exactly as shown in the photograph above. Now to put things in perspective, an 18650 cell has a volume of about 17 cc. These cells are rated at a maximum voltage of 4.2 V and an average voltage of 3.7 V. A quick calculation reveals that they would have an energy density in excess of 1,000 Wh/l. Now, that’s either serious lying or some serious innovation. Let’s find out.

My team was generous enough with their time to run a capacity test on these Ultrafire cells, and compare them relative to the Panasonic 3.4 Ah cells. For you tech geeks wanting the specifics, the Panasonic cells were charged and discharged at 1.7 A (0.5 C rate) whereas the Ultrafire cells had a  much smaller charge/discharge current of 500 mA.  The cells were all charged from a minimum of 3.0 V up to a maximum of 4.2 V with a termination current of C/20, then discharged back to 3.0 V. Temperature was maintained at 25 ºC. What did we measure?

The graph shows the standard discharge curves for both Ultrafire and Panasonic cells. Panasonic provides a capacity of 3,000 mAh vs. the advertised 3,400 mAh. In actuality, one would probably get very very close to 3,400 mAh had we gone down to 2.7 V and charged at a much slower rate of 0.2 C (instead of 0.5 C). The Panasonic cell is made with an NCA cathode which provides additional energy down to 2.7 V. So Panasonic seems quite honest with their capacity claim.

But Ultrafire is not even close…shame on you, Ultrafire! The advertised 5,000 mAh has a charge capacity of barely above 1,000 mAh. I have known that crooks are everywhere attempting everything under the sun, but for some naive reason, I had thought that lithium-ion batteries might be, just might be immune to this degree of cheating. But making a claim that is 5X reality, well, I’d better reset my expectations.

The lesson du jour: if you see Ultrafire cells, run, and run fast!

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14Apr 2015

We have covered in prior blogs the operation of batteries in smartphones. The vast majority of such devices use single-cell batteries. In other words, there is one physical cell that is the battery. As such, it has a given charge capacity measured in mAh or Coulombs, and it has a voltage range that is between 3.0 and 4.35V. If we stack multiple cells in an electrical configuration, then in principle, one can obtain a multi-cell battery configuration, called a pack, that can deliver more charge capacity.

The electrical configuration of such cells defines the nomenclature – see figure below. If the cells are electrical tied in series, then the pack is called s-configuration. If they are tied in parallel, then they are in a p-configuration. The former serves to raise the maximum voltage of the pack in multiples of 4.35V, whereas the latter serves to increase the maximum current through the pack without increasing the voltage.

Now let’s examine what happens if the cells in a multi-cell pack are not identical. For example, they could be slightly different from the onset, or perhaps aged at different rates. In a parallel configuration, the voltage is always equal for both cells. Any difference in charge capacity between the cells will manifest itself as a difference in current in the two branches. In particular, this parallel configuration always guarantees that the cells do not exceed their maximum safe voltage, often 4.35V.

But a series configuration creates a different and more challenging situation. The current is shared and equal to both cells, and hence, each cell will manifest a different voltage. Let’s first examine the charging of two cells in series. If the two cells are truly identical, then they will reach their maximum capacity and their maximum voltage at the same moment. But if there is a difference in capacity between them, then the cell with a smaller capacity will reach 4.35V before the other cell does. At this point time, one cell is 100% full while the other one is not. If the charging is not disconnected immediately, one cell will certainly get overcharged and cause a hazard.

To remedy this situation, electrical circuits called cell balancing are used. In principle they are simple They add a little switch and a small resistor across each cell in series. This added circuitry provides the ability to “bleed off” additional charge from the “strong” cell, so that its voltage stays about equal to that of the weak sister. This type is called “passive balancing.” Naturally, this is not a very energy-efficient nor cost-efficient method, but at least it guarantees that the weak cell will not be overcharged. As we covered in prior blogs, lithium-ion cells, unlike lead-acid batteries, risk catching fire or exploding when they are overcharged above their maximum voltage, typically 4.35V for one individual cell.

Let’s now examine discharging two cells in series. The figure below shows the voltage vs. charge curve for two similar but slightly different cells. They are both nominally 7,000 Coulombs (or about 1,900 mAh) but in reality, one cell is 7,200 Coulombs and the other one is 6,800 Coulombs. This is about 5% difference in capacity, and can readily happen in a pack without the proper precaution.

Let’s now assume that both cells are charged to an identical voltage. For the blue cell, this will correspond to a stored electrical charge of 3,600 Coulombs, or about 100 Coulombs more than its sister cell. Let’s now start discharging the cells in series; in other words, the exact same discharge current flows through both of them for exactly the same duration of time. This means that both cells will lose the same amount of charge; for the purpose of this discussion, we assume it is 3,000 Coulombs. We notice from the figure above that the blue cell will have a terminal voltage across its cells that is higher than the red cell (the more aged cell). Any further discharge will cause the red cell to drop precipitously and cause it further degradation, effectively over discharging the cell. This is not an unsafe event but it is a phenomenon where the weak cell (the red cell) will actually degrade at a faster rate in a series configuration. This is why it is always said that a “pack is only as good as its weakest cell.” In other words, without the use of clever algorithms and balancing, the cycle life of the entire pack will be equal to the cycle life of its weakest cell.

Battery-pack manufacturers try to minimize this problem by matching the cells in a pack as much as possible. It is very common for pack manufacturers (including makers of electric vehicles) to measure the capacity of each and every cell in a pack, and matching the cells to within less than 1% in charge capacity. But as one will immediately observe, this gets very expensive especially for large packs as the yield of useable cells can be quite low.

In some extreme cases, some packs can utilize “active balancing.” This includes more sophisticated electronic circuits that will actually shuffle charge from the strong cell to the weak cell. The effect is to increase the cycle life of the pack by shoring up this weak cell and ensuring that it does not get overcharged nor over-discharged.

It is important to close here by saying that the vast majority of mobile devices use single cell configuration, and hence do not implement cell balancing. Most laptop computers and some tablets use 2S configurations (two cells in series). They often implement rudimentary passive balancing. For example, the Apple MacBook series of products often use the bq20zxx family of fuel gauges with integrated cell balancing from Texas Instruments — such consumer-grade fuel gauges can handle cell balancing for small packs up to 4 cells in series.

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26Jan 2015

On 24 September 2009, A123 Systems become a public company trading under the ticker symbol of AONE on the NASDAQ. Its shares soared on the first day of trading closing the day at $20.29 per share, making the company valued at nearly $1.2 Billion. It had posted revenues of $36 million during the first six months of 2009, mostly in service revenues. Its marquee investors included names like GE, Qualcomm, and others who, along with the US Department of Energy, had collectively invested over several years in excess of $500 million into the company.

Nearly 3 years later, on 16 October 2012, the company filed for bankruptcy after missing a $2.7 million dollar in interest payment on its outstanding debt. In December of that same year, a bankruptcy judge approved its sale to Wanxiang Group, China’s largest auto parts company, for $257 million. Why would a darling company of the CleanTech industry and Wall Street fall so fast and so hard, and what lessons should the industry heed?

Let’s start with a quick recap of the company’s history. It was formed in 2001 as a spin out from MIT to commercialize a new material system, called nano phosphate, upon which lithium-ion batteries could be built. The background to this new material lied with the safety and reliability issues that plagued the lithium-ion battery industry in the previous decade. The too-frequent fires at battery factories in Asia and product recalls on laptop batteries made the lithium-cobalt-oxide (LCO) material system unsafe at least by reputation, and certainly unsuited for the envisioned electric vehicles of the future.  By 2006, the company had collaboration agreements with the US Advanced Battery Consortium (USABC), an automotive consortium bringing together Detroit’s Big Three, along with the US Department of Energy, and was already building battery packs using its proprietary lithium-iron-phoshpate as its primary cathode material. The new batteries were supposedly safer, had very long cycle life (upwards of 2,000 cycles) that was suitable for automotive warranties, and were capable of handling large current spikes — in battery parlance, it is known as power capabilities. But as time would prove, LFP, as this new nano phosphate material system was known, suffered from lower energy densities compared to the material system it was trying to displace. 

But any shortcoming on energy density was not sufficient to detract the company from focusing on electric vehicles (EV). By 2008, it had signed agreements with TH!NK to supply batteries to this Norwegian electric-vehicle maker. The next year, it had inked deals with Chrysler, Shanghai Automotive Industry Corp., and Fisker. The future was bright and the potential was enormous. It was time for an IPO.

Underlying this exuberant optimism, especially in 2009 when gas prices hit nearly $5 per gallon and electrification of cars was the future, were some weak fundamentals. Yet, they were either unknown or ignored by many…ultimately, these weak fundamentals led to the demise of the company and its sale. Leaving execution out, these weak fundamentals boil down to choice of technology, product and market.

First, there was and still is a mismatch between the technology of choice, LFP, and the requirements of the EV market. Electric vehicles required a long driving range, which in turn dictated a high-energy density battery technology. LFP has a substantially lower energy density that the LCO material system, and it was doubtful that LFP would improve in time to shrink this gap. In other words, LFP was not suitable to build batteries capable of reaching a 200-mile driving range. For comparison purposes, the energy density of the A123 material system was nearly ⅓ that of the batteries used in the Tesla Roadster — the first model of Tesla Motors. A123, and its list of partner EV manufacturers, were willing to compromise driving range for better reliability and safety. Tesla in contrast, made driving range a key priority for its cars, and elected to improve the safety of the battery through clever engineering designs of its battery pack, i..e, in the mechanical design as well as how the electronic systems safely manage the lithium-ion cells. Nearly a decade later, experience shows that driving range is of paramount importance to drivers of electric cars, and that LCO-based battery packs can be made very safe.

Comparison of select battery properties used in electric vehicles.

Second, A123 Systems was fundamentally a battery materials company. That’s where its innovation lied. As such, it focused primarily on improving the design and manufacturing of its battery materials. Yet, the battery pack in an electric vehicle was a complex integrated system that brought together both the battery and its materials along with a sophisticated battery management system (BMS), i.e., the electronics and software that control the battery’s performance and reliability. A123 Systems largely left the design of the BMS to its customers. That meant that the overall battery pack system could not be fully optimized as long as its key ingredient subsystems were designed by different parties. In contrast, Tesla elected to design and build the entire battery pack themselves, using a battery cell design (18650) that had been around for at least a decade. In other words, in the complex balance of battery materials vs. system design, where does a company put its emphasis? History now shows us that the system-emphasis proved to be more optimal. Materials in general take a long time — upwards of a decade — and large investment capital to reach commercial maturity. Systems development tend to reach maturity at a faster pace.

Third, or perhaps it should have been first, is cost. The 18650 used in the Tesla models was already being used in millions of laptop computers. It was relatively inexpensive to manufacture. It made sense for, at the time, a nascent electric vehicle company to leverage the scale that the PC industry brought to batteries. In contrast, A123 Systems and all the supporters of LFP had to start from scratch: Build a manufacturing infrastructure, develop an efficient supply chain, establish scalability and ensure reliability. None of these are easy tasks, and they tend to take time and a lot more money. Ultimately, these delays and investments show up as losses in the company’s financial statements.

Lastly, it was about the initial choice by A123 and its partner customers of targeting electric vehicles for the mass market, which meant pushing for affordable car pricing thus introducing serious cost pressures on the supply chain. The electric vehicle market is still in its infancy, even after several years of government incentives and increasing regulation. Therefore targeting electric vehicles for the mass market was a tall order, especially when performance and overall cost were not matching those for traditional vehicles with an internal combustion engine. It greatly increased the challenges that the customers of A123 Systems had to overcome. Over the past several years, we saw both TH!NK and Fisker go bankrupt (Fisker was too acquired by Wanxiang).  GM and Ford ultimately chose batteries from LG Chem, a large industrial giant that was willing to underwrite the necessary capital to penetrate Detroit…a luxury that a comparatively small company like A123 could not undertake. Once again, Tesla made a different choice of targeting niche markets, first with an expensive sports car, then going after the high-end luxury market. Both of these choices relaxed the cost constraint and allowed the design and manufacture of an electric vehicle with few if any compromises compared to their combustion-engine counterparts.

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17Dec 2014

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.

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