Jun 19, 2011
2011 is a remarkable year for electric vehicles. GM is finally rolling out the long‑anticipated, plug‑in hybrid Chevrolet Volt, and Nissan is releasing their full‑electric Leaf—both bringing the first electric vehicles to the mass market. At price tags of $40,000 and $33,000 (retail) respectively, both Volt and Leaf offer about ~50 miles of all‑electric range (70 miles cruising for Leaf). While most of us do not drive above 50 miles a day on a regular basis (the average American drives just 36 miles a day), what do we do when we need to drive to a ski resort, take a foliage trip, or just generally travel anywhere farther than 50 miles away from home? Sure, the Volt has a built‑in gasoline engine, but that certainly does not look like a long‑term solution, especially when considering the mounting environmental, economic and political challenges associated with using gasoline. A truly sustainable solution would be a CO2‑free, 300-mile range electric vehicle – the billion-dollar question. But first we have to answer the million-dollar question: how do we even go beyond 50 miles?
First, we do not lack an energy storage technology that would enable a 300-mile range in a car. For example, we can simply increase the size of the battery to store enough energy to drive 300 miles. The problem is that nobody wants to lug around, let alone pay for, a battery that could drive that far, at least not with today’s technology. Using a rule of thumb of 1 kWh of energy required per 3 miles of range (100 mpg equivalent) for a sedan, a 100 kWh Li‑ion battery would be required to reach a 300-mile range. This would cost at least $60,000 just for the Li‑ion battery alone and also add ~1000 kg to the car weight, comprising almost half the weight of the total vehicle (a regular sedan weighs about ~1500 kg). If we would like to continue using Li‑ion and hope to make a 300-mile range car out of it, we will need to increase the gravimetric energy density by at least a factor of 3 while substantially decreasing the price. Luckily, a number of technologies, in various stages of development, hold some promise for achieving these goals.
Examples of those technologies are development of new materials for next‑generation Li‑ion storage. Next-generation Li-ion batteries hold some potential for increasing the energy density of automotive battery packs, but they’re still in the research stage. One option is to replace the widely used graphite anode with silicon, another low-cost material that can (at least, theoretically) store more than 10 times more lithium charge per gram of material, corresponding to a ~20-fold increase in energy density (silicon also has a slightly higher discharge voltage than graphite). However, this advantage is diminished when assembling a full battery with a metal oxide or phosphate cathode, which is the main limiter of the performance of the entire cell in terms of charge stored per weight. While silicon can get 4,200 mAh/g and graphite 372 mAh/g, cathode materials such as lithium cobalt oxide can store only ~150 mAh/g. When you assemble a battery, the capacities on the anode and cathode have to roughly match, which means you’re already using a lot less anode material compared to the cathode. An improvement in anode energy density helps, but the overall improvement at the cell level is only a fraction of that at the electrode level. When we consider further that silicon undergoes a 400% volume expansion during charging, and that its reported cycle life is therefore typically only 10-20 cycles (an electric vehicle battery’s target cycle life is 3000-5000 cycles), it’s clear that silicon is no easy answer.
To make a more energy-dense battery, one company, Pellion Technologies, is tackling the issue via a divalent battery concept. By incorporating the magnesium ion (Mg++), which carries two positive charges (hence “divalent”) to replace the lithium ion (Li+), which carries only one positive charge (“monovalent”), a higher gravimetric energy density is achievable in principle. However, because of a lack of knowledge about these divalent battery materials, they are a bit of a futuristic technology at this point. Using a computational approach, Pellion is hoping to accelerate their understanding of these batteries and design novel materials to enable an unprecedented gravimetric energy density intercalation technology.
Some researchers are looking farther beyond lithium-ion to two concept batteries that, if enabled, offer the highest energy densities of any battery: lithium-air and lithium-sulfur batteries. Lithium-air batteries, which represent a conceptual mix between a battery (closed system) and a fuel cell (open system), operate by reacting lithium ions with molecular oxygen from the air to form a solid discharge product that fills the pores of the cathode during discharge. This process can in principle be reversed, although the charging voltage (~3.8 V or higher) is much higher than the voltage you get out during discharge (~2.8 V). So what’s the attraction? Lithium-air batteries, which use a lithium metal anode and a pure carbon or catalyzed carbon cathode, have the potential to increase the cell-level energy density of batteries by up to 5-fold. Unfortunately, the best known catalysts for improving the efficiency of lithium-air batteries are precious metals such as platinum or gold. What’s more, the cycle life is still incredibly poor (less than 50 cycles with ~50% capacity fade). But for those who can remember the early days when lithium-ion batteries could barely cycle more than 50 cycles, there’s hope that this can be improved.
A third possibility for increasing vehicle range lies with lithium-sulfur batteries. Owing to the fairly low weight of sulfur coupled with super-light lithium metal, the projected energy improvement is comparable to that of lithium-air batteries – around three to five times. However, sulfur is insulating, making it a less than ideal electrode material, and some polysulfide byproducts that form as a result of discharging the cell can migrate inside the battery, depositing in unwanted locations where they can block electrode surfaces from cycling. Recent work by Linda Nazar at the University of Waterloo has reinvigorated interest in this area by showing how electrode design using templated carbon can help address some of the cycling and stability issues. One company in particular, Sion Power, is trying to commercialize lithium-sulfur batteries and is banking that if these challenges can be overcome (and they say they can), then lithium-sulfur is one good answer to the range problem.
At this point, it is tempting to ask: why not consider H2 fuel cells? In principle, H2 fuel offers up to 33 kWh/kg (~100x the battery energy density). So it is possible that H2 might one day power a full-range electric vehicle. In practice, however, a heavy tank to contain compressed H2 often reduces its advantage of high gravimetric energy density (realistically, to a value about 10 times greater than battery energy density.) Nonetheless, a 300‑mile range, fully electric SUV based on a H2 fuel cell has already been demonstrated. The prospects of H2 fuel cells for a full‑range electric vehicle is therefore a very realistic possibility if one can manage to bypass the Pt requirement in the cathode, which currently represents a large fraction of the cost (and is also a rare element). Italian‑based Acta and Israeli‑based Cellera are two startups that are attempting to replace Pt with transition metal composite catalysts. Cellera has reported 70% cost saving from a traditional H2 fuel cell using their anion‑exchange membrane technology. Another possibility is to construct a battery‑fuel cell hybrid, where a battery would provide short energy bursts during acceleration and be recharged during cruising by a fuel cell. This hybrid would effectively reduce the power requirement on the fuel cell, which is currently the reason behind the high Pt requirement.
There also exists a company seeking to extend electric vehicle range by improving the physics of the car, allowing it to travel farther for a given unit of energy. Aptera aims to advance automotive design to boost efficiency by lowering weight and improving aerodynamics. Aptera’s engineers are working toward a 200 mpg vehicle, which would translate to approximately 6 miles per kWh (twice as high as just putting Li-ion batteries into existing sedans). If they can achieve this efficiency, a car would only need half the battery that we expect today, which would ultimately cut both cost and weight by more than a factor of 2. While this alone will not be enough (the battery would still cost $30,000 and weigh 500 kg), having this efficiency would certainly give the battery and fuel cell scientists a much easier target to reach.
While battery and fuel cell researchers search for technological solutions to the range problem, it’s important to stand back and look at the big picture: do we even need a car that can go 300 miles on a single charge? We’re trained to think so based on expectations from driving gasoline-powered vehicles. However, some people would argue that the range issue is really an infrastructure problem: just build battery swapping stations alongside gas stations on the highway, and allow drivers to change out batteries every 100 miles or so when they are taking a long driving trip. It’s an interesting model that’s being explored by Better Place, a Palo Alto-based startup founded by Shai Agassi. We’ll examine their model more thoroughly in an upcoming article
 Assuming a battery energy density of 100 Wh/kg.
 Y. C. Lu, Z. C. Xu, H. A. Gasteiger, S. Chen, K. Hamad-Schifferli and Y. Shao-Horn, J. Am. Chem. Soc., 2010, 132, 12170-12171.
 X. Ju, K. T. Lee and L.F. Nazar, Nature Materials, 2009, 8, 500-506.