Cell-phone batteries, solar cells, display screens, and other devices are typically centimeters in length, yet their performance—and improvements to their performance—often depends on what happens at the nanoscale. The second session of the 2013 AIP Industrial Physics Forum, held at the Long Beach convention center during the AVS annual meeting, looked at nanoscale processes in one crucial technology: energy storage.
Atomic layer deposition
The ability to paint surfaces with atom-thick layers of functional materials has long been a staple of surface science research. Ganesh Sundaram of Ultratech reviewed some of the industrial applications of one such painting method, atomic layer deposition (ALD).
The goal of ALD is straightforward: to coat a surface with atom-thick layers of a compound, such as titanium dioxide. One element or “precursor”—titanium, say—is wafted in gas form over a substrate under vacuum. Once a monolayer has been deposited, any excess precursor is purged from the reaction vessel using argon, nitrogen, or other inert gas. Next, the second precursor—oxygen, say—is wafted over the surface to react with the first to form a monolayer of their compound. Excess atoms of the second precursor are purged. The sequence—first precursor, purge, second precursor, purge—is repeated until the compound layer has reached the desired thickness.
As Sundaram explained, ALD has four significant advantages. First, ALD is a low-temperature process. Most layers are deposited below 300°C, which means that ALD can be used on plastic substrates. Second, ALD is self-limiting: Deposition of each compound layer stops when atoms of the second precursor can no longer find atoms of the first precursor to react with. Third, because the layers are built up via gas-phase surface reactions, ALD coats surfaces uniformly—even those surfaces that have overhangs, trenches, or other complex three-dimensional geometry. Fourth, the layers are so thin that it’s possible to mimic the effect of uniformly mixing in a dopant by sandwiching a film with thin layers of the dopant.
The ALD's principal drawback is its slowness. Although layers can be deposited in a few minutes, each layer is very thin. It can take an hour to coat a surface to the desired thickness. To speed up the process, Ultratech and other companies are developing so-called spatial systems in which components are conveyed like cars on an assembly line through a sequence of deposition chambers whose linear arrangement corresponds to the temporal sequence of the traditional ALD deposition cycle.
The ability of ALD to create reliably thin layers has applications in the electronics industry, and most notably in the manufacture of memory chips. Sundaram, however, outlined three other applications: lithium-ion batteries, solar cells, and organic light-emitting diodes (OLED).
For batteries, Sundaram cited the work of Se-Hee Lee of the University of Colorado and his collaborators. Previous studies had shown that coating a LiCoO2 cathode with 10–100 nm layers of metal oxide boosted performance. Lee discovered that the boost could be made even bigger by using ALD to deposit a much thinner layer of metal oxide, just 0.3 nm thick. After 120 charge cycles, the ALD-coated cathodes retained 89% of their capacity, whereas the cathodes with thicker coatings retained only 45%.
In the case of solar cells, ALD improves performance by reducing the thickness of the so-called buffer layer that separates the window from the absorber. OLED-based TV screens, as Sundaram explained, can already match or exceed the performance of LCD- or plasma-based screens while using much less power. But OLEDs are extremely susceptible to water damage. They can tolerate a water vapor transmission rate of no more than 10−6 g/m2/day. Ultrathin ALD layers can protect the OLEDs without impairing performance.
A solid-state, rechargeable battery
Amy Prieto of Colorado State University began her talk about lithium-ion batteries by showing a photo of a Tesla roadster. Powered by 6831 lithium-ion cells, the mid-engine, rear-wheel drive car can reach 60 mph from rest in 3.7 seconds and has a range of 244 miles. But the car’s lithium-ion batteries come with significant disadvantages. It takes up to 16 hours to fully charge the car. And the batteries are potentially hazardous. Some jurisdictions, Prieto said, advise first responders not to enter a crashed Tesla unless a HAZMAT team has first declared it safe.
Prieto has set herself the goal of building a safer, faster-charging battery. And with an eye to eventual commercialization, she restricted herself to technologies that do not require extreme temperatures, exotic or expensive materials, or the use of clean rooms.
Solid-state batteries tend to be safer than batteries with a liquid component, yet the diffusion of lithium ions—the key process during charging and discharging—is faster through a liquid than it is through a solid. Prieto’s design seeks to retain the safety of solid state, while reducing diffusion times through an innovative structural design.
Her starting point is copper foam, a commercially available material whose volume comprises 10% copper threads and 90% empty space. The battery’s anode is made by electroplating the copper foam with the anode material, copper antimonide, in a citric acid solution at room temperature. Like the anode, the electrolyte, which the lithium ions pass through to reach the cathode, is another thin solid layer. Consisting of a proprietary polymer, the electrolyte is deposited on top of the anode. The cathode is made of carbon and forms the final layer.
Prieto chose Cu2Sb because the intermetallic’s open, silicon-like crystal structure readily accommodates lithium ions. The electrolyte, polyacrylonitrile, works well.
With her students, Prieto has formed a company to develop and market the battery. They have demonstrated that the technology is scalable. Using a set of 20-liter tanks for each chemical step and humans to effect the transfer between the steps, she estimates she can make up to 1000 cells a year.
Why batteries fail to meet expectations
Continuing the theme of nanoscale optimization of macroscale devices, Stephen Harris of Lawrence Berkeley National Laboratory (LBNL) summarized the investigations of his colleagues and collaborators into the microscopic origin of the various ways in which lithium-ion batteries degrade.
But at the start of his talk, Harris identified four areas in which a 25% rise in performance is conceivable: capacity, operating voltage, energy density, and lifetime. His talk focused on improving lifetime, which is not just a matter of durability or longevity. Because of degradation, only about 75% of a battery’s capacity is typically used. Understanding, and then mitigating, degradation would release that unused capacity.
In the 1970s John Newman of LBNL developed a theory of porous, liquid-immersed electrodes for simulating the performance of batteries. The theory has proved successful at predicting battery performance. Indeed, Harris pointed out that the theory has enabled the capacity of lithium-ion batteries to be doubled without the need for new materials or new chemical processes.
But Newman’s theory assumes that electrodes and electrolytes are homogeneous. If batteries degrade because they fail to live up to Newman’s model, it could be because of inhomogeneities. Or it might be because the movement of lithium ions is somehow impeded. In either case, the path to a solution entails looking at the anatomy and physiology of batteries on the mesoscale.
One approach Harris takes is to examine the anodes and cathodes using neutron scattering, which, unlike x-ray scattering, can readily reveal the disposition of lithium and other light elements. After charging and recharging a battery until it lost 40% of its capacity, Harris and his collaborators examined the electrodes. Whereas the bulk structure appeared unchanged, the edges were evidently “dead.” Better packaging might provide a route to preserving the capacity of a battery's peripheral regions.
Newman’s theory defines an effective diffusion coefficient in terms of the uniform diffusion coefficient, the porosity, and “tortuosity,” a term that characterizes how the electrode’s internal structure impedes the passage of ions. Working with Edwin Garcia of Purdue University, Harris has found that tortuosity can vary by as much as a factor of 10. Whereas patches of excess tortuosity retard charging and discharging, patches of reduced tortuosity cause dangerous overcharging.
Some cathodes consist of micron-scale particles of LiCoO2 or other materials. The rate at which lithium ions enter the particles can be modeled by assuming that the particles are spherical. Harris teamed up with Scott Burnett of Northwestern University to use focused ion beams to determine the particles’ shapes. The shapes they found were highly irregular, with crevices and lumps. The presence of crevices is significant because lithium ions can exploit them get close to a particle’s core while remaining in the liquid state. Because liquid diffusion is faster than solid-state diffusion, the result is non-uniform “lithiation”—a failure mode.
To avoid the problems caused by inhomogeneities, Harris suggested that electrodes could be constructed via the self-assembly of uniform particles.
Charles Day is Physics Today's online editor.
