Preface

There are many different forms of energy in nature: chemical energy in fuels, solar energy in sunshine, and kinetic energy in wind or running/falling water; in our daily lives we usually use them in the form of electricity. However, the reserves of fossil fuels are very limited and combustion of fossil fuels leads to significant global warming, which makes the greater utilization of alternative clean/renewable energy (solar energy, wind energy etc.) urgent. Importantly, these forms of clean energy are not freely accessible anytime and anywhere they are needed. Thus, it is essential to have highly efficient devices to convert, store and transport this clean energy. The term “energy storage devices” in the title of this book covers a wide range of electrochemical energy conversion and storage devices that can convert between electrical energy and chemical energy, including rechargeable batteries, fuel cells and super capacitors.

In pursuit of excellent performance, i.e. higher energy density, lower cost, longer lifespan and greater safety, significant research efforts have been made into the development of different energy storage devices in the past decades. As an example of secondary lithium-ion batteries, seeking new electrode materials or further optimizing available electrode materials that can work at high cell voltage and high specific capacity, and developing next-generation batteries with new chemical reactions are the major themes in the field. One of the most challenging issues that battery researchers encounter is how to obtain a fundamental understanding of the working mechanism and/or controlling factors of a new material/battery. Hence, having novel characterization/diagnosis techniques with adequate time resolution (in situ, operando) and spatial resolution (imaging) for active batteries is of great importance. In pursuit of this, a variety of in situ experimental techniques have been developed in the past few years to investigate the composition/structure evolution of the electrode/electrolytes on different time/spatial scales. In general, macroscopic and mesoscopic morphology/structure changes are probed by optical and electron microscopy. Long- and medium-range atomic ordering is probed by diffraction methods (X-ray, neutron or electron diffraction). Local structures on atomic scale can be investigated by X-ray absorption (EXAFS, XANES), by vibrational spectroscopies (infrared, ultraviolet-visible, Raman) or by NMR techniques.

Among numerous electrochemical ex situ and in situ characterization techniques, magnetic resonance techniques (NMR and MRI) are unique in terms of providing structural information at the atomic level, real-time phase and morphology evolution, and measurement of ionic motion at various timescales. The experimental design and data interpretation of NMR spectroscopy on electrochemical energy storage materials requires extensive knowledge of multiple disciplines, including NMR theory, crystallography, solid state chemistry/physics and electrochemistry etc., which significantly hinders a wider realization and application of NMR/MRI techniques. The purpose of this book is to act as an introductory textbook for battery/electrochemical researchers who want to apply NMR spectroscopy in their research, and for NMR experts who wish to enter the field of electrochemical energy storage. Our contributing authors come from major global groups actively working on NMR/MRI studies of electrochemical energy storage materials and devices and have varied backgrounds and expertize in NMR/MRI, electrochemistry, catalysis and theoretical chemistry fields. Thus, this book provides fundamental information about NMR/MRI and timely advances in the electrochemical energy systems with these powerful techniques, including their general theory and specific experimental designs/details for electrochemical applications. In addition, various working mechanisms of major electrochemical storage materials and devices are introduced and overviewed, and this book can therefore be considered to be a resource for the correlation of NMR data and electrochemical mechanisms.

This book comprises two parts: Part I Physical backgrounds and experimental methodology (Chapters 1–4) and Part II Case studies of electrochemical energy materials and devices (Chapters 5–17).

In Part I (theory and methodology), the theory required to interpret the NMR spectra is first presented in Chapter 1, with an emphasis on the theory of paramagnetic NMR and the related first principles calculation method to interpret and extract chemical information from transition metal containing materials, followed by general considerations and methodologies of electrochemical in situ NMR and MRI (Chapter 2). Specific methods dealing with quadrupolar nuclei and a newly developed signal enhancement method Dynamic Nuclear Polarization (DNP) can be found in Chapters 3 and 4, respectively. These methods are very helpful to battery/electrochemical researchers to aid them in gaining basic NMR knowledge and timely experimental design/details from our experts.

In Part II (case studies), illustrative examples of NMR studies on different materials and devices are reviewed. This part does not require a detailed understanding of NMR theory (beyond that discussed in Part I) and should be accessible to non-NMR readers. NMR studies on the key components of lithium/sodium ion batteries are first reviewed, including the layered cathodes (Chapter 5), polyanion-type cathodes (Chapter 6), anode materials (Chapter 7), electrolytes evolution and solid electrolyte interphase (Chapter 8). NMR/MRI investigations provide insightful results to understand (local) structure–properties relationships in these important electrode materials. Chapters 9–11 are dedicated to different types of solid electrolytes for all solid state batteries, including oxides (Chapter 9), polymer electrolytes and polymer–ceramic composites (Chapter 10) and sulfides (Chapter 11). NMR is adopted for kinetic studies on the Li⁺ migration rate and the Li⁺ transportation pathway in these materials. These NMR/MRI results are very important to understand which steps control and/or affect the ionic diffusion and origins of the doping/interfacial effects on the ionic conductivity in solid electrolytes. In addition to lithium/sodium (ion) batteries, NMR studies of other energy storage devices are covered in Chapters 12–15, including super-capacitors and pseudo-capacitance (Chapters 12 and 15), next generation Li-air batteries (Chapter 13) and fuel cells with an emphasis on electro-catalyst and electrode reactions (Chapter 14). Chapters 16 and 17 are devoted to examples of in situ NMR and MRI of LIBs.

Finally, the editors would like to thank all the authors for their great contribution to this book and many thanks to the Royal Society of Chemistry for their support during the course of preparing this book.

Yong Yang, Riqiang Fu and Hua Huo