By Professor Robert A. Huggins
Batteries are certainly in the news these days, especially as the result of several recent safety-related incidents, but also because of their employment in a number of important current applications, ranging from electric vehicles to the integration of wind and solar sources with the electric grid. The requirements for these various applications can be quite diverse. For vehicles, energy and power per unit weight and size are of primary importance, and cost and safety are also of concern, while for stationary applications the major issues are cost and safety, of course, but also calendar lifetime, cycle life, and high charge and discharge rates. Weight and size are secondary considerations.
In Silicon Valley, one is accustomed to rapid change. However, progress in the energy storage area has not been nearly as rapid as it has been in other areas. One fundamental reason for this is that electrical charge is stored by making use of phenomena involving atoms and ions, which have fixed values of size and weight. Thus the storage of more charge means the involvement of more ions, leading to larger dimensions and weight, or of more electrons per ion, which produces a large step in the output voltage.
It is interesting that progress in energy storage has not been a linear development of a single technology, but the emergence of several, with different characteristics and applications. Some important changes have come as surprises. It seems worthwhile to look at how this happened in some of the current battery systems.
Cd/Ni (or NiCad) cells were developed long ago in Europe, and used in German aircraft during World War II, but this technology was not introduced into the US until after that war. These batteries are now out of favor because of environmental concerns, and are gradually being replaced by metal hydride/Ni batteries, which were introduced in the 1980s by Ovonic Battery Co. in the US and a number of firms in Japan. The Japanese government established the Government–Industrial Research Institute (GIRIO) in Osaka. Japanese firms sent young workers there temporarily. Such a strong government-industrial relationship allowed know-how in this area to propagate rapidly, and this technology soon came to be dominated worldwide by Japanese firms. These batteries can be very reliable, with long cycle lives. They are currently used in the Toyota Prius, as well as in many other applications.
It is interesting that the metal hydride materials used in this type of batteries were discovered by accident in the Philips laboratories in the Netherlands. They were investigating the magnetic properties of rare earth alloys, and left them overnight in a hydrogen-atmosphere furnace for annealing. In the morning they found that the alloys had reacted with the hydrogen, and turned into fine powder. It was quickly recognized that this phenomenon could lead to a new type of battery with a hydrogen-storage electrode, but the company did not pursue its commercialization.
A major innovation has been the use of solid electrolytes, not just for all-solid systems, but also for liquid/solid/liquid systems. The Na/S high temperature battery was invented at Ford Motor Co. in 1967 as the result of the accidental discovery of the very high ionic conductivity of the beta phase of aluminum oxide. It was soon recognized that this material could be used to make an entirely new type of battery that might be used to propel electric vehicles. This work was soon terminated by the company management, however, for Ford did not want electric automobiles. The Director of Research was fired, and the key scientists were told to work on catalysts. The result was that further work in this direction, and its commercialization took place in Europe and Japan, not in the US.
Another innovation, the high temperature Na/NiCl2 “Zebra” battery, was developed in secret in South Africa. It first became known in the US in 1986, after a change in cold-war politics. It also involves the use of a solid electrolyte at elevated temperatures. Both of these high temperature batteries were considered for use in electric autos until testing showed them to be unsafe. However, work continued in Japan, and was revived recently by General Electric in the US, for their use in stationary configurations to support solar and wind power.
What about Li-ion cells? Work in this area started in our labs in Stanford in 1972. It was later pursued with considerable success in the EXXON research labs, and resulted in the development of a rechargeable Li/TiS2 battery. But the EXXON management decided that it was not interested in batteries, and that successful project was terminated.
Work in the Inorganic Chemistry lab in Oxford in 1980 showed that it is possible to rapidly remove lithium from two layer-structure oxides, LiCoO2 and LiNiO2. In the following year experiments in Grenoble, France and Bell Labs in the US showed that lithium can be inserted into the layer structure of graphite. But not much attention was given to these two independent research results. Then in 1990 SONY came up with a technological surprise. By putting these two results together SONY produced the first high voltage rechargeable lithium batteries, and used them in camcorders.
There is currently a lot of work on the possibility of improved Li-ion cells. One direction, followed in some laboratories, has been to replace the LiCoO2 positive electrode by less expensive and higher voltage alternatives. Another involves the replacement of carbon negative electrodes with materials with greater capacities, such as silicon or tin, which was first studied in our Stanford lab in 1980. The possibility of using silicon nanowires, which was demonstrated by Candace Chan here in 2008, has recently received a lot of attention.
But safety is a very important matter, as shown by the recent lithium battery fires in commercial aircraft. Work in our laboratory at Stanford showed the thermodynamic basis for the inherent safety problems in such systems, for the equilibrium internal oxygen pressure in the positive electrodes of lithium battery systems increases exponentially with the voltage. But, of course, the manufacturers and users of lithium battery technology always push for more energy storage, and this means higher voltages and reduced inherent safety.
In addition to applications in which weight and volume are important, the coupling of storage to intermittent sources such as wind and solar, and to the grid is also now getting a lot of attention. This is leading to new approaches to aqueous electrolyte batteries. One of these, which also originated in our Stanford lab recently, involves the use of ferrocyanide materials, instead of oxides, in its electrodes. We have been able to demonstrate extremely high rates and unusually long cycle lives using some very low cost and safe materials.
In summary, research on the storage of energy in batteries has been going on for a long time. A number of the innovations have originated in the United States, but they have often been followed up primarily in other countries. Interest in this area is now very high, with many both current and future applications. However, they often require different values of the critical parameters, leading to continued research on a range of different storage technologies.
Robert A. Huggins has been a Professor of Materials Science at Stanford University since 1954, and is the Chief Scientist at the Center for Solar Energy and Hydrogen Research at the University of Ulm. He has worked in the energy storage field for several decades, and has written two books on the subject: Advanced Batteries: Materials Science Aspects (2008), and Energy Storage (2010).