Making the Case for Lithium-ion Batteries
According to Navigant Research, the market for electric car batteries could reach $24-billion by 2023 helped, in large part, by global emission and fuel economy regulations that force automakers to electrify a growing portion of their fleet. Stung once before by premature regulation—California’s abortive electric vehicle mandate of the 1990s—automakers don’t want to be sitting around with dealer lots full of unsold EVs with ridiculous rebates on the hood. Thus, their ultimate goal is to build vehicles that meet the regulations while providing driving ranges and recharge times that tempt drivers out of their internal combustion vehicles. And while the industry, battery suppliers, universities, and even companies like IBM are pushing hard to create the next great battery breakthrough (see sidebar), for the next decade or two, the future belongs to lithium-ion (Li-ion).
“I think that for the next 10 to 15 years, in terms of what you see on the road, it will be some flavor of lithium-ion,” says Dr. Prabhakar Patil, CEO, LG Chem Power (lgcpi.com). “The reason for this longevity goes back to the periodic table.” Lithium is the lightest metal and the third lightest element behind hydrogen and helium. Unlike helium, it is not inert. Hydrogen is not inert and has a good gravimetric density, but not much in the way of volumetric density. Also it suffers from large pumping losses in its gaseous form. “There are many fundamental challenges with using hydrogen [to power vehicles], and just as many fundamental reasons the industry keeps coming back to lithium,” says Dr. Patil. And while he encourages those investigating lithium-magnesium or lithium-air batteries to continue their work, Dr. Patil believes that the time it takes to go from the laboratory to the road gives Li-ion batteries a minimum 10- to 15-year period in which it is dominant.
This is all the more surprising given that Li-ion batteries first entered the market in 1991 in Sony consumer electronics. Their creation was driven by the desire to produce more powerful hand-held devices with a longer time between charges. Alkaline batteries, which had carried the day until then, were woefully underpowered; the number of batteries necessary to produce the same output as a Li-ion cell was more than triple. This negatively impacted the size and weight of the electronic devices, and led the industry to adopt production-ready nickel-cadmium (Ni-Cad) batteries for its early products. They were, however, just a stopgap. Compared to Li-ion, Ni-Cads have approximately half the volumetric and gravimetric efficiency, a theoretical limit far lower, and are more difficult to recycle. As volumes increased, prices for the then-exotic Li-ion batteries began to tumble, and continued development raised storage capacity and output even further. Electrification has brought this same roadmap to vehicle batteries, and Li-ion cell prices have begun to drop. How much they have dropped is open to debate.
While the cost of a Li-ion cell has dropped by a factor of 20 in terms of dollars per kilowatt-hour since Sony’s commercial introduction of the technology in 1991, it does not mean this pace will continue. “Though it is doing better than expected,” explains Dr. Patil, “some of that has to do with improving the characteristics of the battery.” The best example for this is the Chevrolet Volt. The first generation had a range of 40 miles, which required 8 kWh of usable energy. However, guaranteeing that it would have that energy at the end of its life—10 years—meant taking into account a 30% degradation over that time. This, in turn, meant you could count on using about 70% of the state of charge rather than using 100% of the nominal energy. Explains Dr. Patil: “If you put those two together, that’s a factor of two. Which means that, in order to deliver 8-kW hours usable energy at the end-of-life, you had to put in 16-kW hours of nominal energy at the beginning of life.” Changing the material characteristics to flatten out the degradation curve not only allowed use of a wider state-of-charge window, it reduced the installed capacity of the battery. (The 2016 Volt uses 96 fewer cells, while adding 13 miles to electric driving range.) And that, says Dr. Patil, “automatically brings down the cost. In addition, there are significant improvements in the cost of the materials, and some of that gets into economies of scale.” Of course, switching to some sort of standardized Li-ion battery for all electrified vehicles would drive the cost curve down faster, correct?
Unfortunately, this equation isn’t quite that simple to solve. “The materials that you use from a battery electric vehicle to a hybrid vehicle are completely different,” he says. “One way to characterize this is to look at the power-to-energy ratio. Going from a non-plug in hybrid, where the battery provides half the power and the IC engine provides the other half, to a plug-in hybrid, where the power requirement doubles because the battery has to provide all of the power, actually increases the energy capacity needed by as much as a factor of 20. That’s because it has an electric range of 20 to 40 miles versus one mile for the hybrid.” Furthermore, Patil says, the power-to-energy ratio drops by a factor of 10, “because the power went up by factor of two, but the energy necessary went up by a factor of 20 or more.” Moving to a full electric vehicle means keeping the power the same, but increasing energy by whatever factor is necessary to reach your mileage goal. As Dr. Patil explains, “If you go to 100 miles, that’s maybe another factor of two or three. If you go to 200, that’s another factor of 10. Which means you have gone to at least two orders of magnitude change in power-to-energy ratio, and that requires a very different type of material and other elements of the cell. Of course, that plays into what happens in terms of the battery’s cost.” This is why he is reticent to trumpet cost gains reported in the press. “You have to be careful what you use as the denominator in the cost because, while a full kilowatt hour is useful for an electric vehicle or even a plug-in hybrid, it is almost meaningless when you’re talking about a non-plug-in hybrid because the battery’s responsibility in that example is energy, not power.”
Although battery makers will continue to offer custom batteries, much of that customization will come in the form of fitting the battery pack to a particular vehicle. Tesla and other bespoke electric vehicle makers are able to gang together standardized battery cells, while major automakers adapt their internal combustion designs. However, a certain amount of standardization will take place in terms of battery chemistry for particular types of applications (hybrid, plug-in hybrid, EV), but even that has to evolve. Says Dr. Patil, “It has to change to allow an improvement, and it also has to change in terms of the external form factor to allow us to package better in the available space. There is some standardization and some customization but you keep having to have flexibility for the overall optimization of the vehicle.” Much of this change will come in terms of the cathode material, as most battery developers consider it to be the key to battery performance. As ions move from the anode through the electrolyte and across a separator to the cathode, they generate an electric current. Thus, the more ions the cathode can hold, the more energy the battery can store and release. Thus, according to Dr. Patil, “Lithium ion can also be a solid-state battery because it is only a matter of how the electrode gets constructed. Currently it is a slurry put on a film, much as you would have on the tape of a tape recorder. But you could do a solid-state deposition without fundamentally changing the underlying chemistry.”
What, then, will it take to get a majority of people to consider an electric vehicle as a rational alternative to their current car or truck? “Maybe the 200-mile vehicle will finally get people past the whole range anxiety problem,” says Dr. Patil. “From the battery perspective, the technology is at a point where you can recharge to 60 or 70% of the capacity in a 15-minute timeframe without doing anything bad to the battery, though that’s not something that you can do at home. A 200-mile range vehicle will typically need a battery pack with at least 60 kWh of energy, and that means you’re trying put 50 kWh back through recharging. Doing that in 15 minutes means you need a 200-kW charging station. If you have that kind of charging infrastructure, maybe that’s the tipping point that moves it to where electric vehicle adoption is driven by the customer, and not by regulation. With a vast enough charging infrastructure, you may even find that people begin taking their vehicles on long distance trips. That could be the tipping point.”