Today’s lithium ion battery technology as it exists today is scarcely an ideal energy storage device. It’s relatively expensive. The chemical formulation that optimizes for energy storage has safety issues and is limited in how fast it can charge and discharge as well as in the number of charge/discharge cycles. The formation that’s optimized for charge/discharge time (some usually some form of LiFePO4 and referred to as “lithium iron phosphate”), has a higher power capacity and is much safer, but is quite poor at energy storage.

IBattery chemistriesmperfect though they are, as things stand now lithium ion batteries are also a necessary link in the attempt to build a new energy industry that includes alternate forms of renewable but intermittent energy sources, like solar and wind, that require some form of energy storage. Batteries play an even more important role in the quest for a practical plug-in electric hybrid vehicle (PHEV) (aka extended-range vehicle or ERV) that requires a battery that can enable at least 40 miles of driving before the car’s small internal combustion engine kicks in for longer distances. Improvements in battery technology are necessary before renewable energy and petroleum-free transportation can be a reality.

This week MIT researchers Gerbrand Ceder and Byoungwoo Kang published an article in Nature magazine that reports on their research at MIT into a new lithium ion-based battery technology that can perform a complete discharge in under 10 seconds. You can read a summary of the paper here on ArsTechnica; The Nature article itself is behind a paid reg wall. Here’s an even terser summary: The new battery technology, which is based on the power-centric LiFePO4 chemistry, makes for rapid charge and discharge times, but does not improve energy storage density. The research is being greeted with descriptions such as “revolutionary” and “game changing”

The charge and discharge figures are impressive: "... the authors tweaked the cathode to allow higher currents to be run into the cell. Increasing the rate by a factor of 100 dropped the total capacity down to about 110mAh/g, but increased the power rate by two orders of magnitude ... compared to traditional lithium batteries. Amazingly, under these conditions, the charge capacity of the battery actually increased as it underwent more charge/discharge cycles. Doubling the charge transport from there cut the capacity in half, but again doubled the power rate. At this top rate, the entire battery would discharge in as little as nine seconds. That sort of performance had previously only been achieved using supercapacitors. "

Some examples of what this performance might mean for common battery applications: “A 1Wh cell phone battery could charge in 10 seconds, but would pull a hefty 360W in the process. A battery that's sufficient to run an electric vehicle could be fully charged in five minutes—which would make electric vehicles incredibly practical—but doing so would pull 180kW, which is most certainly not practical.”

Leaving aside the problems of whether such rapid charging is practical (a concern that’s independent of battery technology), what conclusions can we draw from what’s known so far about this research? After all, not all university research results in a viable product. What questions should we be asking about this new formulation?

I went to three notables in the battery industry: Dr. Yevgen Barsukov, senior applications developer at Texas Instruments and TI’s go-to guy for batteries, Dr Robin Tichy, Technical Marketing Manager, Micro Power Electronics, a battery pack d esign and manufacturing house, and Dr. Christina Lampe-Onnerud, founder and CEO of Boston-Power, a manufacturer of the Sonata line of rapid-charge batteries currently being sold for laptops.

Dr. Barsukov of TI agrees that the discovery is important, but perhaps not as earth-shaking as it might initially appear. I’ll give his full, detailed response here (and you should read the article linked to above to understand all of his references): “This appears to be an important discovery that will help reduce diffusion limitations in LiFePO4 and possibly other materials.

“However, it is important to understand that that diffusion is not the only limiting factor in battery kinetics. Other important factors are ohmic conductivity of bulk active material and ionic conductivity of the electrolyte in the pores and separator, as well as electron transfer resistance on the surface. In fact these factors contribute 30-60% of total cell resistance depending on the design and will limit charging rate regardless of diffusion.

“The relative contribution of these factors vs diffusion depends on the thickness of active material. Typical cells have layer of material which is 20-50um thick. So even if you drastically improve diffusion inside particles it will change only the small portion of overall impedance.

“ It appears that the cell they tested in the lab and reported charge rates with has extremely thin active layer, which hides this effect of bulk material resistance. Any modern LiFePO4 is already prepared in form of nano-particles which makes diffusion contribution quite low already because of short diffusion path in nano-particles. So if say A123 sys would make a cell with similar thickness of active material it could also be charged extremely quickly.

“But (!) it can’t be done for real batteries because it will mean a lot of current collectors that only contribute weight but no capacity and tiny active material, resulting in very bad energy density not acceptable to most applications.

“To summarize: This is some useful discovery to improve diffusion behavior and might find widespread use.

“However the claims on magnitude of improvement are exaggerated. Even if diffusion impedance would be reduced to zero, it would only reduce overall cell impedance by 40% for a typical cell. That is the actual improvement to charge rate that can be expected in normal commercial cells.”

Next, here are the just-as-blunt comments of Dr. Tichy of MicroPower, the battery pack manufacturer: “I am struggling to see how the technology could change the market landscape.

“Historically, Li-ion batteries were designed to maximize capacity and traditional materials served this purpose very well. More recently, materials like iron phosphate and manganese spinel have been designed for high c-rate applications (power tools, for example). In the case of the high rate cells, the capacity of the material is lower, but the cell capacity is limited by the size requirements of the current collectors and battery pack components. The charge time is limited by the design constraints on the power supply. The article makes reference to these limitations.

“So, this material change doesn't seem to compete with traditional high capacity cells, and it would have the same limitations as the high rate cells.”

And finally, Dr Lampe-Onnerud of Boston Power. I have interviewed Dr Lampe-Onnerud several times over the years as she has worked to bring a new battery formulation to market, including raising capital, perfecting the technology, developing manufacturing facilities, and establishing a reliable, repeatable process for an affordable pbattery product line. Her comments are both measured and tactful:

“Lithium-ion has earned its place as the battery technology of choice for rechargeable, efficient energy storage. Materials science research such as what is being done by Gerbrand Ceder and Byoungwoo Kang promises to be another strong step along the way to adding even more value.

“Many in the industry have believed for some time now that lithium-ion batteries could be made to charge much more quickly, and this research appears to reinforce that belief. We’ll all watch closely as the process enters the stage of moving beyond a laboratory environment and into something that can be mass manufactured and delivered to the market. Batteries are pretty complicated devices, so that’s not a simple task. It will require a great deal of effort and we support their hard work….” (She also put in a strong plug for Boston-Power’s battery technology, but I cut that.)

Revolutionary? Game-changing? Maybe not. But important? Yes – as is every step forward we take in improving our understanding of energy storage.


Source:
http://www.edn.com/blog/1470000147/post/1550041955.html?nid=3351&rid=2920794