"Hybrid This…Hybrid That" – Hybrids in Energy Storage

In recent years, the world has come to understand and live by the word hybrid. From simple gadgets like a thermos for your coffee on a cold morning to the more complex electric vehicle, “hybrid” has in of itself become a fad in our society. You can go to a shopping mall and find the next great thing made of a hybrid composite which has the capability of resisting damage from water exposure. Or perhaps you can improve your golf game by investing in hybrid technology which blends a wood with an iron. Then there’s the plug-in hybrid electric vehicle. If you don’t have range anxiety, you can improve your carbon footprint driving one of these vehicles. All of these aspects of hybrid materials have lured us into believing in their potential. But what’s the deal with hybrids and how can they potentially advance battery technology and energy storage applications?

We need to take a step back to understand how nanotechnology has advanced our understanding of their potential in electrochemical energy storage. While the energy storage umbrella is large, a ready example of this is the new advances in lithium ion batteries. In recent years, many researchers have strived to develop cathode and anode materials that could boost the energy density of lithium-ion batteries for enhanced performance in electric vehicles. Most of the focus has been towards the anode side of the battery given the greater potential to improve a batteries energy density. There have been numerous studies done that show that replacements for the state of the art graphite are very capable of improving the energy density of a battery. This includes silicon, germanium, and other carbon-based nanomaterials (e.g. carbon nanotubes). Specific capacities upwards of 2500 mAh/g clearly show that alternatives to graphite, with a specific capacity of 300 mAh/g, are plausible with the proper introduction in the market 1. However, these alternatives, by themselves, have severe pitfalls which limit their advancement to market.

...“what you put in is what you get out” will be the determining factor if hybrids are to make their mark on a critical technological market.

Silicon and germanium have volumetric expansion issues where material breakdown upon insertion and extraction leads to failure of the anode after a few cycles. Carbon nanotubes have a severe capacity fade attributed to the solid electrolyte interface which forms upon first cycling of lithium ions causing an irreversible capacity loss. Efforts through pre-lithiation have helped to address this major problem with carbon nanotubes; however, it is recognized much more work must be done to truly yield a viable carbon nanotube anode candidate.

Researchers have recognized that hybridization of the alternative anodes noted above not only could improve their mechanical stability but also their electrochemical performance. The union of silicon, germanium, and carbon nanotubes has yielded an advanced anode material with specific capacities greater than 1200 mAh/g and a greater than 40% improvement in electrochemical performance over previous native systems 2. This enhancement in electrochemical performance shows that the hybrid systems can play vital roles in advancing battery technology towards commercialization. Other research with hybrid electrode systems have validated these findings and projected new opportunities for advanced electrode systems for a variety of large form factor applications.

Returning to the 35,000-foot view, one must ask if, and when, hybrid electrochemical systems will be fully engaged in the respective markets of interest. There is no simple answer to such a question. A variety of roadblocks will have to be overcome before society can fully realize the benefits of these hybrid electrode systems. Such factors like scale-up and material viability must be affirmed before manufacturers would be willing to incorporate potentially expensive overhauls to their processes. Raw material costs would also have to stabilize to ensure high quality products exit a process at a reasonable cost for consumers. These are only a couple of the challenges we face in bringing hybrid electrochemical systems to market.

In summary, the prospect of hybrids in energy systems is at an all-time high. As researchers continue to develop efficient technologies for small scale and large scale applications, the need for examining their costs becomes critical to ensuring scale up from bench to manufacturing. Material integrity and stability will be a constant challenge to creating high quality hybrid energy storage systems. In the end, “what you put in is what you get out” will be the determining factor if hybrids are to make their mark on a critical technological market.

1. M.W. Forney, R.A. DiLeo, A. Raisanan, M.J. Ganter, J.W. Staub, R.E. Rogers, R.D. Ridgley, B.J. Landi, “High performance silicon free-standing anodes fabricated by low-pressure and plasma-enhanced chemical vapor deposition onto carbon nanotube electrodes”, Journal of Power Sources 228 (2013), 270-280; 2. R.A. DiLeo, M.J. Ganter, M.N. Thone, M.W. Forney, J.W. Staub, R.E. Rogers, B.J. Landi, “Balanced approach to safety of high capacity silicon–germanium–carbon nanotube free- standing lithium ion battery anodes”, Nano Energy (2013) 2, 268-275.

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