This entry is the latest in a Worldwatch blog series on innovations in the climate and energy world.
The Nissan Leaf proudly advertises that it can go 100 miles on a single charge. Chevrolet, Toyota, and other car companies have promoted their plug-in gas-electric hybrids as the more rational alternative, since you can switch to the gasoline option when you need extra range. But what if charging your electric car were as easy as filling your gas tank?
For electric vehicles to become the dominant mode of personal transportation, the charging process will have to evolve: it will need to be either much faster, or far less frequent. In a recent article in Nature Nanotechnology, scientist Paul Braun and his research team at the University of Illinois at Urbana-Champaign describe their blueprint for a new battery with a greatly reduced charging time. Their most successful lithium-ion prototype reaches a 90 percent charge in just two minutes.
All batteries have the same basic structure. If the battery is connected to a computer, cell phone, or other “load,” chemical reactions produce electrons that flow from the negative end of the battery through a wire, discharging their energy into the load and then returning to the positive terminal to form a complete loop. The process is triggered by two metal electrodes (the anode and cathode) connected by an electrically conductive “electrolyte,” usually a liquid. The electrons flow from the anode through the wire to the load and back into the cathode; meanwhile, positive ions flow from the cathode through the electrolyte to the anode to compensate for the movement of electrons. Recharging a battery requires forcing this circuit to run in reverse.
Recharging a battery obviously requires energy, but currently it requires significant time as well. In many cases, cell phones and cameras still must be recharged overnight to reach full capacity. And Nissan estimates the charging time for the Leaf at seven hours on a 220/240 volt charging station (most standard U.S. outlets are 120 volts). This is clearly an area ripe for improvement, and many researchers have attempted to tackle the problem.
Options for speeding the charging process include increasing how quickly the ions migrate, and decreasing the distance that the ions and electrons need to travel. The latter is more effective, as the charging time is proportional to the square of the average distance traveled. Decreasing the average distance that ions have to travel by thinning the battery structure, however, limits the overall volume of the electrodes and therefore limits the scale of the charge. Braun’s team has proposed a solution to this problem, a new architecture for the cathode that creates large areas of contact between the cathode and the electrolyte (enabling ions anywhere in the cathode to reach the electrolyte quickly) while not losing much cathode volume compared to existing batteries.
They started by self-assembling spheres of polystyrene, a ubiquitous and inexpensive plastic, into a tight lattice structure with small but entirely connected spaces. They then filled these spaces with nickel through a process known as electrodeposition, putting a negative charge on the polystyrene and submerging it in a nickel-based salt solution. When the positively charged ions from the salt come into contact with the polystyrene, they receive electrons and are reduced to nickel.
The material is then melted to remove the polystyrene, leaving only the nickel surrounding spherical voids. Very small passages between the voids remain at the points where the polystyrene spheres once touched. The cathode (a metal compound that varies by battery type) is then plated onto the nickel, thickening the metal structure but still leaving smaller voids to allow for a continuous ion pathway through the electrolyte that will eventually flood the structure. Without this pathway, the battery’s overall charge will be reduced and the charging time increased, since the electrolyte will not be able to permeate the entire structure. To avoid plugging the gaps, Braun’s team used a process known as electropolishing to remove the top layer of nickel and widen the gaps so the cathode can be deposited.
The chemistry of the cathode depends on the type of battery. Braun’s team created prototypes for both nickel metal hydride (NiMH) and lithium-ion batteries, using cathodes of nickel oxyhydroxide (NiOOH) and lithiated manganese dioxide (MnO2), respectively. An electrolyte then fills the remaining holes. This design provides large areas of contact between the nickel, cathode, and electrode without sacrificing much cathode volume.
What does it bring to the table?
Potentially changing the way we think about batteries.
Braun says that these batteries can charge 10 to 100 times faster than today’s commercial batteries. At such rates, small electronic devices could potentially charge in seconds, and larger devices, even cars, in minutes. An improvement of this nature would fundamentally alter the way we interact with electronics. Laptops and tablets would become almost endlessly portable, and the maximum range of an electric vehicle would become no more important than the size of another car’s gas tank. Braun is now looking to see if a similar strategy can be used to improve the anode. If so, charging rates could climb further still.
The blueprint that Braun has created is also flexible enough to work with many different battery chemistries. The short diffusion lengths, Braun hypothesizes, may even allow for reconsideration of anode and cathode materials that are currently impractical due to their poor conductivity.
How close is it to commercialization?
Not as far as you might think.
In talking to The Economist, Braun estimates that the increase in production cost above current batteries, once the process reaches commercial scale, would be 20–30 percent. This technology may already be sought after for certain applications at such a premium, and if costs fall any farther, other applications may also become viable. For electric cars, the benefit of reduced charging time is especially valuable, but costs are already high compared to other vehicles. The use of this kind of battery might make electric vehicles even more of a luxury product, at least for a time.
How scalable is it?
If these batteries take off, they could eventually find their way into everything from cell phones to public buses, and become as large a part of our daily lives as other batteries are currently.
What is the biggest obstacle to success?
Aside from the cost premium, the major obstacle to uptake of these batteries is the larger current necessary to charge the batteries so quickly. Electric vehicles, in particular, would need to be adapted to properly handle the swift movement of so many electrons.
The final word(s):
A potential game-changer.
Widespread uptake of electric vehicles, if paired with low-carbon electricity production, would go a long way toward reducing greenhouse gas emissions to the point necessary to avert the worst effects of climate change. A rapid-charge battery of this sort certainly isn’t sufficient for achieving such a goal, but it is absolutely necessary.