Federal authorities are investigating the safety and proper handling of lithium-ion automotive batteries after a Chevrolet Volt plug-in hybrid caught fire three weeks after a routine crash test.
The National Highway Traffic Safety Administration has asked General Motors, Nissan, Ford and others about the fire risk posed by li-ion batteries used in EVs and plug-in hybrids, …
The death of a battery: we’ve all seen it happen. In phones, laptops, cameras, and now electric cars, the process is painful and — if you’re lucky — slow. Over the course of years, the lithium-ion battery that once powered your machine for hours (days even!) will gradually lose its capacity to hold a charge. Eventually you’ll give in, maybe curse Steve Jobs, and then buy a new battery, if not a whole new gadget.
But why does this happen? What’s going on in the battery that makes it give up the ghost? The short answer is that damage from extended exposure to high temperatures and a lot of charging and discharging cycles eventually starts to break down the process of the lithium ions traveling back and forth between electrodes.
The longer answer, which will take us through a description of unwanted chemical reactions, corrosion, the threat of high temperatures, and other factors affecting performance, begins with an explanation of what happens in a rechargeable lithium-ion battery when everything’s working well.
Lithium-ion Battery 101
In a typical lithium-ion battery, we’ll find a cathode, or positive electrode, made out of a lithium-metal oxide, such as lithium cobalt oxide. We’ll also find an anode, or negative electrode, which today is generally graphite. A thin, porous separator keeps the two electrodes apart to prevent electrical shorting. And an electrolyte, made of organic solvents and lithium-based salts, allows for the transport of lithium ions within the cell.
During charging, electric current forces lithium ions to move from the cathode to the anode. During discharging (in other words, when you use the battery), ions move back to the cathode.
Daniel Abraham, a scientist at Argonne National Laboratory leading research into how lithium-ion cells degrade, compared this process to water in a hydropower system. Moving water uphill requires energy, but it flows downhill very easily. In fact, it delivers (kinetic) energy, said Abraham. Similarly, a lithium cobalt oxide cathode, “does not want to give up its lithium,” he said. Like moving water uphill, it requires energy to take lithium atoms out of the oxide and load them into the anode.
During charging, ions are forced between sheets of graphene that make up the anode. But as Abraham put it, “they don’t want to be there. When they get a chance, they’ll move back,” like water flowing downhill. That’s discharging. A long-lasting battery will survive several thousand of these charge-discharge cycles, according to Abraham.
When Is a Dead Battery Really Dead?
When we talk about “dead” batteries, it’s important to understand two performance metrics: energy and power. For some applications, the rate at which you can get energy out of the battery is very important. That’s power. In electric vehicles, high power enables rapid acceleration and also regenerative braking, in which the battery needs to accept a charge within a couple seconds.
In cell phones, on the other hand, high power is less important than capacity, or how much energy the battery can hold. Higher capacity batteries last longer on a single charge.
Over time, the battery degrades in a number of ways that can affect both power and capacity, until eventually it simply can’t perform its basic functions.
Think of it in terms of another water analogy: Charging a battery is like filling a bucket with water from a tap. The volume of the bucket represents the battery’s energy, or capacity. The rate at which you fill it—turning the tap on full blast or just a trickle—is the power. But time, high temperatures, extensive cycling and other factors end up creating a hole in the bucket (dear Liza, dear Liza…).
In the bucket analogy, water leaks out. In a battery, lithium ions are taken away, or “tied down,” said Abraham. Bottom line, they’re prevented from going back and forth between the electrodes. So after a few months, the cell phone that initially required a charge only once every couple of days now needs a charge every day. Then it’s twice a day. Eventually, after too many lithium ions have been tied down, the battery won’t hold enough of a charge to be useful. The bucket will stop holding water.
Why does this happen? Well, in addition to the chemical reactions that we want to happen in the battery, there are also side reactions. Barriers arise that impede the motion of lithium ions. So the electric car that went, say, zero to 60 in five seconds off the lot, will take eight seconds after a few years, and maybe 12 seconds after five years. “All the energy is still there, but it can’t be delivered fast enough,” said Abraham. The ions run into roadblocks.
What Breaks Down and Why
The active portion of the cathode (the battery’s source of lithium ions) is designed with a particular atomic structure, for stability and performance. When ions are removed, sent over to the anode, and then inserted back into the cathode, we ideally want them to return to the same spot, in order to preserve that nice stable crystal structure.
Problem is, the crystal structure can change with each charge and discharge. An ion from Apartment A doesn’t necessarily come home but could instead insert itself into Apartment B next door. So the ion from Apartment B finds her place occupied by this drifter and, not being one for confrontation, decides to take up residence down the hall. And so on.
Gradually, these “phase changes” in the material transform the cathode to a new crystal structure, with different electrochemical properties. The particular arrangement of atoms, which enabled the desired performance in the first place, has been altered.
In hybrid vehicle batteries, which only need to provide power during acceleration or braking, noted Abraham, these structural changes occur much more slowly than in electric vehicles, because only a small fraction of lithium ions in the system move back and forth in any given cycle. As a result, he said, it’s easier for them to return to their original locations.
Problem of Corrosion
Degradation can occur in other parts of the battery as well. Each electrode is paired with a current collector, which is basically a piece of metal (typically copper for the anode, aluminum for the cathode) that gathers electrons and moves them to an external circuit. So you have slurry made from an “active” material like lithium cobalt oxide (which is ceramic and not a very good conductor), plus a glue-like binder painted over this piece of metal.
If the binder fails, the coating can peel off the current collector. If the metal corrodes, it can’t move electrons as efficiently.
Corrosion within the battery cell can result from an interaction between the electrolyte and electrodes. The graphite anode is highly “reducing,” which means it gives up electrons easily to the electrolyte. This can produce an unwanted coating on the graphite surface. The cathode, meanwhile, is highly “oxidizing,” which means it easily accepts electrons from the electrolyte, which in some cases can corrode the aluminum current collector or form a coating on the cathode particles, Abraham said.
Too Much of a Good Thing
Graphite — the material commonly used to make an anode — is thermodynamically unstable in an organic electrolyte. What that means is the very first time our battery is charged, the graphite reacts with the electrolyte. This forms a porous layer (called a solid electrolyte interphase, or SEI) that actually protects the anode from further attacks. This reaction also consumes a little lithium, however. So in an ideal world, we would have that reaction occur once to create the protective layer, and then be done with it.
In reality, however, the SEI is a sadly unstable defender. It does a good job protecting the graphite at room temperature, said Abraham, but at high temperatures or when the battery runs all the way down to zero charge (“deep cycling”) the SEI can partially dissolve into the electrolyte. (At high temperatures, electrolytes also tend to decompose and side reactions accelerate.)
When friendlier conditions return, another protective layer will form, but this will eat up more lithium, giving us the same problem we had with the leaky bucket. We’ll have to recharge our cell phone more often.
Now, as much as we need that SEI to protect the graphite anode, there can be too much of a good thing. If the layer thickens too much, it actually becomes a barrier to the lithium ions, which we want to flow freely back and forth. That affects power performance, which is, as Abraham emphasized, “extremely important” for electric vehicles.
Building Better Batteries
So, what can be done to make our batteries last longer? In the lab, researchers are looking for electrolyte additives to function like vitamins in our diet, enabling the battery to perform better and live longer by reducing harmful reactions between the electrodes and electrolyte, said Abraham. They’re also seeking new, more stable crystal structures for the electrodes, as well as more stable binders and electrolytes.
Engineers at battery and electric car companies, meanwhile, are working on the battery pack and thermal management systems to try and keep lithium-ion cells within a constant, healthy temperature range. The rest of us, as consumers, can avoid extreme temperatures and deep cycling, and for now keep grumbling about those batteries that always seem to die too soon.
Image courtesy of Argonne National Labs, felixtsao, warrenski, MitchClanky2008, bizmac.
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The next-generation of lithium ion batteries aren’t just here to power the first wave of electric cars and remake the power grid, they’ll be providing better energy storage for our gadgets and computers, too. On Monday, venture capital-backed lithium ion battery player Leyden Energy (formerly called Mobius Power) is launching a replacement lithium ion battery for laptops that won’t degrade (start losing its full charge) for at least three years, and will come with a three-year warranty.
Most standard laptop batteries start losing their ability to fully charge (providing fewer and fewer hours of battery life) after about a year and a half. Anyone who’s a laptop user knows how annoying it is to have a battery that all of a sudden won’t hold a charge for very long, pretty early into the life of the laptop itself. Leyden Energy says its battery has one of the highest energy densities and run times for a lithium ion laptop battery on the market, with 440 watt hours per liter and over 1,000 cycles, and the battery can operate at higher temperatures than traditional batteries.
Leyden Energy’s 3-year warranty battery will cost a premium over a standard 1-year battery, and while Leyden Energy hasn’t yet determined the exact price it will sell the battery for, Leyden Energy CEO and President Aakar Patel told me in an interview that a 3-year battery will be less than double the cost of a 1-year battery. Leyden Energy on Monday will also announce a deal to sell its battery through the Canadian battery retailer Dr. Battery, and interested customers will be able to buy the battery online in a couple weeks through the retailer.
Leyden Energy was founded in 2007 with a patent acquired from chemical giant Dupont, and a .5 million investment from investors at Walden International, Lightspeed Venture Partners and Sigma Partners. Leyden’s secret sauce is an innovation for the electrolyte part of the battery — a battery has a a positive and a negative plate and then an electrolyte in between, which is the substance through which electrons transfer back and forth while the battery charges and discharges.
While standard lithium ion batteries use a salt-based solvent within the electrolyte that starts degrading at a temperature of between 70 to 80 degrees Celsius, Leyden uses a salt-solvent in its electrolyte that doesn’t degrade up to temperatures of 300 degrees Celsius. Leyden Energy holds a patent for this innovation. As Patel explained it to me: when a battery charges and discharges, think of the electrons as rods that move across the electrolyte (between the anode and the cathode) and fill holes on the other side. After a certain point in time standard electrolytes, particularly at high temperatures, let the rods start to break down and the holes start to fill up, but Leyden’s battery can maintain the integrity of those rods and holes at higher temperatures for a longer period of time.
In the grand scheme of innovations, and with startups trying to change the game with designs for battery-powered cars with hundreds of miles of range, Leyden’s innovation is kind of baby steps. But if Leyden can manage to get an deal with a major laptop manufacturer to embed the battery directly in a laptop, or market the battery with a popular laptop, then the company could do well. In 2008, Boston Power launched its 3-year-lasting lithium ion battery with laptop maker HP, and is backed by Oak Investment Partners, Venrock, GGV Capital and Gabriel Venture Partners.
Like Boston Power, Leyden Energy has been eying the electric vehicle battery market, too, and is working with Brammo to supply the battery for its electric motorcycle the Empulse, a more powerful version of Brammo’s original e-scooter the Enertia (which I test drove here). Leyden and a vehicle maker partner were also awarded a .96 million grant from the California Energy Commission to produce ten electric vehicle batteries per month. Leyden seems like it’s focusing more on batteries for the laptop and consumer electronics markets, instead of electric vehicles, as it seems like the market for electric vehicles is moving slower than some have expected (see A123 Systems, and Ener1).
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