Energy storage is a key enabling technology to the larger area of sustainable energy. For transport applications, the storage needs to mirror the energy density of petrol and diesel, and because sustainable energy-sources such as solar and wind are variable in nature, the buffering capacity of storage is also of importance. The main energy-storage technologies are batteries, capacitors, pumped storage, hydrogen, flywheels, and compressed (or liquefied) gasses. Each has its technological and logistical challenges, with Chaps. 7 and 8 examining the two major areas of lithium — ion batteries and hydrogen storage, respectively.

The central theme in both battery and hydrogen-storage materials is increasing energy density gravimetrically and volumetrically, in a way that enables the energy to be stored and retrieved in a time and energy-efficient way. It is difficult for either battery or hydrogen storage materials to compete with the energy density of petrol and diesel. Neutron techniques of analysis here are used to understand the inter­actions that hold the energy-carrier in place, but can then release it when it is required for use. In these chapters the role of in situ studies is particularly important and this is an area in which the penetration of neutrons excels. It is sufficiently important that neutron-scattering centres provide custom made gas-handling and potentiostat/galvanostat equipment that can be used by several different groups.

Chapter 7 shows a large volume of study has been undertaken to determine the crystal structure of electrode materials to determine the position and environment of lithium in complex and multiphase electrode materials, even as the battery is charged and discharged. However, disordered and amorphous materials often have the desired materials properties, and it is illustrated that neutron scattering is also relevant to these by using the total scattering technique and spectroscopy. Although lithium is better suited to study by neutron scattering than many other techniques, it is nevertheless challenging, particularly when compared to hydrogen (Chap. 8). First, the absorption cross-section of one of the naturally-occurring lithium isotopes is high, and whilst this can be overcome, and even exploited by isotopic selectivity, this is expensive. Secondly, not only would we like fast diffusion for better bat­teries, but it would make the study of diffusion by neutron scattering easier. Current

neutron instrumentation can access lithium diffusion when used at high resolution, but this limits the neutron counting-rate, making the experiments more difficult, and in some cases impractical. Fortunately, in some examples it is reasonably straightforward to derive the dynamics of the lithium by studying the response (or driving dynamics) of the environment, in this case it is the environment that has a strong neutron signal.

Hydrogen storage (Chap. 8) has probably been the most important area of sustainable-energy research for neutron scattering due to the special interaction between neutron and hydrogen and its isotope deuterium. In common with battery materials, there has been a large volume of work using diffraction to determine the position and environment to be derived of the H-atoms (or molecules), much of which existed prior to hydrogen-storage applications. The interplay between atomistic modelling and neutron scattering pervades sustainable-energy research, but is particularly important in hydrogen storage due to the ease with which the dynamics of the model can be compared with neutron-spectroscopy results. This enables very detailed information about how hydrogen interacts with all the com­ponents of its environment, and whilst this is also possible for the battery materials above, the experimental difficulties associated with lithium make it much more challenging. Hydrogen is a light element and quantum effects in its dynamics are pronounced. Because of the way in which the neutron spin interacts with the nuclear spin of hydrogen, neutrons provide a very sensitive measure of these quantum dynamics, which is in turn very sensitive to its environment. These dynamics are not only of interest for tuning the interaction of hydrogen with its storage material, but are also of fundamental interest in their own right and much of the theory for understanding the measured neutron-scattering spectra had already been established because of this. There is a considerable body of analogous work for methane, which whilst of considerable technological importance, is not covered in this book. The analogy is strongest for storage in metal-organic framework and clathrate materials, and in most cases the extension of the work presented here for hydrogen to the methane case is straightforward.