What's Wrong With Lithium-Ion Batteries?
The announcement last month that 46 million Nokia-branded lithium-ion (Li-ion) batteries made by Matsushita Battery Industrial could potentially short circuit and overheat was just the latest in a spate of product advisories and recalls of the technology over the past two years.
But it’s not as if Li-ion batteries are at the early point in their life cycle when you would expect these sorts of problems to crop up. Sony invented the technology back in 1990. So why is it failing now?
The theories behind the technology’s recent spotty performance are complex and varied, which makes fixing the problem a perplexing engineering challenge.
A Constantly Evolving Technology
“You can’t really say that for the first ten years the battery makers got it right and now they’re screwing it up,” says Jim Miller, Manager of Argonne National Lab’s Electrochemical Technology Program. Funded by the U.S. Department of Energy, his group’s research is directed at developing new materials for Li-ion batteries and addressing some of the major issues in scaling up the technology.
Miller points out that Li-ion battery technology is not just a single design or composition, but rather it’s an entire family of chemistries that is constantly evolving. “When Sony invented it in 1990, it was lithium cobalt oxide. But cobalt is expensive and so engineers started replacing it with nickel, which costs less. And then as time went on engineers found that they could substitute cheaper nickel manganese alloys for the nickel.”
Cost reduction isn’t the only driving force behind the evolutionary march of Li-ion batteries. The desire to extend battery life, achieve higher energy densities and faster charging times, and improve reliability has led to a constant tinkering of the technology. Energy densities are double what they were five years ago, for example, and new surface coatings are being applied to make the batteries more stable and reduce their reactivity rates.
Ever-Increasing Demands, More Trade-offs
The trade-offs inherent in these often mutually exclusive goals make for a diabolical design challenge: You can make a Li-ion battery that has high performance, for example, but the trade-off is a shorter life. And as every design engineer knows, making the right trade-offs and getting everything right takes time, experience, and a bit of finesse.
“A problem doesn’t necessarily pop up during the first generation of cells,” says Miller. “Things may look fine in the lab and then when you go to production you find that the technology behaves in a slightly different way, which means things can and do go wrong.”
Something certainly went wrong at Sony last year, resulting in the recall of millions of its Li-ion laptop batteries. As for what exactly led to the short-circuiting problem that posed a risk of fire and in one case caused a Dell notebook to burst into flames, Sony Spokesperson Rick Clancy says that there were different conclusions at different levels.
“When you produce lithium ion batteries, the objective is to either have zero metal contaminants or at least as few of them as possible and surround them by a protective shell or layer so that they cannot penetrate the separator,” explains Clancy. The separator in a Li-ion battery keeps the anodes and cathodes from touching each other and causing a short circuit.
Clancy says that Sony engineers discovered that there was a greater frequency of these metal particles escaping from one part of the cell and entering the other part. They’ve addressed the issue at a product level by designing in a stronger lining, he notes.
But there were other findings at a systems level, specifically relating to variances in configurations and specifications for battery packs from the PC makers, says Clancy. “They are doing the most they can to optimize their products and make them as competitive as possible, which is putting more demands on the power supply as it relates to the battery.”
He adds that some manufacturers’ charging systems are more aggressive than others, which could have had the effect of either vibrating or shaking the batteries more aggressively, a phenomenon that may have played a role in the short circuiting problem. He says engineering teams from Sony and the PC manufacturers are working closely together to better understand and more effectively manage these systems issues.
A representative of Matsushita Battery Industrial (MBI), the company that manufactured the 46 million Li-ion batteries named in the recent Nokia product advisory told Electronics Weekly in a phone interview that the company is still investigating the cause of 100 incidences of batteries overheating.
But he too pointed to the ever-increasing demands on Li-ion batteries. “Generally speaking the batteries are getting smaller and smaller, and at the same time they are being required to deliver more power and capacity. The engineering challenge for us is to maintain the same degree of reliability throughout,” he says.
Although he ruled out any possibility of process-related problems, the manufacturing landscape has widened since the Japanese developed the technology in the 1990s.
The cells for the batteries implicated in the Nokia advisory, for example, were manufactured by MBI in Japan and shipped to a factory in China where they were assembled into battery packs.
The batteries involved in the Sony recall carried labels marked “Made in Japan,” “Made in China,” or “Battery Cell Made in Japan Assembled in China.” Sony produces Li-ion cells in plants in Japan and China, assembles some battery packs at a Sony plant in China, and in some cases sells Li-ion cells to third party makers of battery packs. Sony says that off-shoring was not a factor in last year’s recall.
But Don Sadoway, a professor of Materials Chemistry at MIT who is an expert in advanced battery technologies, worries about off-shoring of a chemistry he asserts “needs to be treated with respect.”
“I have 100% confidence in the Japanese battery manufacturers,” he says. “And my guess is that they never had the problems they’re seeing now when the same batteries were manufactured from start to finish in Japan.”
He notes that one of the challenges with Li-ion batteries in particular is that it is very difficult to verify that the manufacturing and assembly is being performed according to specifications. That’s because once it’s assembled into a battery pack, the device cannot be inspected from the outside nor can it be easily tested.
Sadoway points to the separator material between the electrodes as an example. Acting like a kind of fuse, it is designed to soften and collapse at a specific temperature, causing the battery to essentially go into an open circuit condition and die.
In fact, he wonders why that didn’t happen in the case of the Dell laptop that burst into flames last year.
“You could think you are specifying a porous polypropylene material for the separator, but once the thing is packaged up you would have no way of knowing what you actually got. Even under the best of circumstances, you can get screwed by your own job shop. What if the workers took a short cut and substituted the original material with cardboard?”
In at least one case, it’s suspected that battery manufacturers were supplied with counterfeit raw materials.
Argonne’s Miller agrees that is very difficult and expensive to test and verify Li-ion batteries, adding to the cost that manufacturers presumably hoped to reduce by off-shoring assembly in the first place. But he says that quality assurance can be engineered into the battery design, and that he believes suppliers of cell phones and laptops are tightening up the process. “They are getting more precise in the materials they are using and in their cell designs,” he says.
Still, Sadowy believes that much more rigorous oversight and stringent quality assurance is required, especially as Li-ion batteries scale up. The technology is expected to hit the road in the next few years in electric vehicles under development such as the GM Volt.
“If your MP3 player fries, it’s not a big deal, you don’t get to listen to your favourite tunes,” says Sadoway. “I have real worries when we try to build a large format Li-ion battery with 100X the capacity and put it out there on the highway.”