Jan 6, 2011
When it comes to energy storage technology, many researchers have set their sights on new and disruptive discoveries: novel chemistries for battery electrodes that can improve energy density, or cheaper catalysts for fuel cells that can compete with prohibitively expensive platinum. However exciting, the discoveries or designs of new, suitable chemistries will take several decades to evolve from laboratory-scale successes to commercialized products. Where, then, is there hope in the shorter term? The answer might be found by taking a new perspective and “thinking small”, by reconsidering known materials and their performance at the nanoscale.
“Nanotechnology” has been touted as the “next big thing” for well over two decades now, but the truth is that no one knows whether it will save us from the current energy crisis. However, a few companies are betting on it, and a flurry of recent exciting findings in the laboratory might yet spark the nanotechnology revolution we’ve been promised. We took a look at the hype and the real potential of nanotechnology for batteries and fuel cells, to see why we’re not there yet – and more importantly, to figure out where “there” really is.
Why are materials interesting at the nanoscale? First, it’s about volume (or mass) vs. surface area — the smaller the diameter of the electrode or catalyst particles, the higher the total surface area for a fixed volume of material. In other words, a higher fraction of atoms lie on the surface of tiny particles (“nano” usually refers to dimensions below 100 nanometers) rather than in the interior. This matters because the performance-determining reactions occur at the surface. In fuel cells, the consumption of fuel to generate electrical energy occurs at the surface of the platinum catalyst; any platinum inside the catalyst surface is unused. In batteries, the lithium ions transfer from the electrolyte into the solid electrode particles, where they are stored within the atomic structure. A larger surface area provides more sites for lithium ions to enter, avoiding traffic jams and facilitating charging at higher currents. What’s more, once lithium ions cross through the surface and enter the particle, they have to diffuse to sites where they can be stored; the speed of this diffusion process can be the limiting factor in charging at high powers. As the particle size shrinks, the diffusion distance decreases, effectively reducing diffusion losses and improving power performance.
Higher surface area and shorter diffusion distance are not the only advantages to nanoscale materials. As scientists drive the size of materials smaller, some strange and interesting properties begin to emerge. One example is the high catalytic reactivity of gold nanoparticles. Gold, the noblest metal, is in fact so noble at the macroscale that it hardly reacts at all in most chemical reactions. Nanoscale gold, however, has surprisingly high reactivity. This property is bad news for those who plan to use gold-based electrical connections in nanoscale devices, but nanoscale gold is also an enabling technology in certain applications. A research group from Brookhaven National Laboratory has found that adding these gold “clusters” could stabilize platinum nanoparticles during fuel cell operation. The unique properties of nanoscale gold, once patched onto platinum nanoparticles, modifies the platinum’s chemical properties to prevent platinum dissolution, one of the common failure modes in the cathodes of fuel cells.
In addition to fuel cells, batteries receive performance benefits from the properties of nanomaterials. The very small dimensions of nanoparticles, nanotubes and nanowires make batteries more structurally resilient during cycling. Micron-sized particles (what you’ll find in the electrodes of your laptop and cell phone batteries) undergo cyclic mechanical strains as lithium ions are stored and removed, and these strains ultimately cause the particles to fracture. These fractured pieces can detach from the electrode and become electrically isolated or float away altogether, leading to a steady decline in battery capacity. Research shows that materials at the nanoscale, however, can better accommodate the strain related to lithium insertion and removal. This resilience opens up exciting new possibilities, especially for cheap, high-capacity anode materials such as silicon and aluminum, which alloy with lithium but degrade rapidly at the macroscale. Yi Cui’s research group at Stanford was the first to report cycling of anodes utilizing silicon nanowires, showing the world that silicon is viable as a rechargeable electrode material and kicking off a widespread effort, to develop novel nano-silicon-containing composite materials. Another advantage of incorporating nanoscale materials into batteries is the higher electronic conductivity associated with crystalline nanostructures such as nanotubes, nanowires and 2D sheets of carbon (graphene), which, when added to electrodes, can enable super-high-speed electronic “wiring” of the electrode.
While optimists would say that these phenomena give scientists more knobs to turn when designing a new material, the drawback of these unique nanoscale properties is that they no longer behave as we expect them to. Indeed, despite the enthusiasm that is driving prodigious research efforts, “nano” has not yet revolutionized the energy storage market. This disconnect can be attributed to several unique challenges associated with understanding nanomaterials, optimizing their performance, and integrating them into practical cells. The first of these challenges stems from the high surface area: while advantageous for high power delivery, increased surface areas also support the parasitic side reactions (such as self-discharge in batteries and platinum dissolution in fuel cells) that take place at the electrode-electrolyte interface. Because the particles are so small, most nanomaterials in batteries also require large amounts of binder (the glue that holds an electrode together) to ensure that particles remain in good electrical contact and don’t detach during cycling. This adds unwanted weight to the electrode and can block parts of the surface that would otherwise enable higher charge and discharge rates. Finally, small particles with very high curvature pack less efficiently into a given volume than larger particles, making low volumetric energy densities a key concern, especially for automotive applications.
Despite these challenges, several companies have made great strides towards commercializing nanotechnology. A salient example is A123 Systems, which makes batteries with a nanoscale lithium iron phosphate cathode developed by Yet Ming-Chiang of MIT. The iron phosphate chemistry was long considered attractive for battery cathodes because it is safer, cheaper and more abuse-tolerant than state-of-the-art oxide materials, but its electronic and ionic conductivity were too low at the micron scale. By nano-sizing the material and applying special coatings, A123 achieved high-power performance and good cycle life, all within a battery that is safer than conventional cells. Another battery company, Altairnano, specializes in nanostructured lithium titanate anode materials that could replace graphite, the current industry standard. The main selling points of lithium titanate are its low cost, its chemical stability compared to graphite and its “zero-strain” property, which means that it is mechanically robust over thousands of cycles. Bringing lithium titanate to the nanoscale was key to realizing this material’s intrinsic high-power performance that makes it suitable for electric vehicles and electric grid applications. NanoEner Technologies, a subsidiary of Indiana-based Li-ion battery maker Enerdel, is pursuing advanced physical and chemical vapor deposition methods to make thin-film, high-power electrodes for anodes and cathodes that are free of the conductive carbon additives and binder. That means that the electrode weight consists entirely of active material for storing lithium ions, which can ultimately lead to lighter cells. In the less active fuel cell sector, Acta Spa, an Italian‑based startup, is utilizing the high surface area of transition metal‑based nanocomposites to replace platinum catalysts.
Because most of the companies that are breaking into the nanotech area are specializing in the chemistry of a single cell component, their core technology will generally reflect expertise in synthesizing and processing novel electrode materials. Mastering the right electrode ‘recipe’ and identifying the best coatings, heat treatments and other post-processing techniques could be the real value-adding steps. However, achieving these steps will not alone lead to market penetration. Integration of electrode materials into cells and packs is a crucial step in demonstrating the technology and can be done through partnership with larger, better-established companies who already have the know-how and equipment. Besides providing a proof of concept, the better‑established companies can also provide a market opportunity for the start‑up: OEMs can use new nanotech materials as drop-in replacements to their existing electrode materials. Identifying a strategic partnership will be a key to success for the nano‑based startup. Consumers might not hear as much about these companies as they do the larger battery makers, but the technology will steadily make its way into laptops, cell phones and electric vehicles. We’ve grown accustomed to seeing “Intel Inside” on our laptops, and while we don’t expect to see “Nano Inside” on any car labels anytime soon, it may well be there, silently powering a new generation of clean energy technologies.