The Paris Agreement is a landmark representing a common decision of all nations to make ambitious efforts to combat the global climate change. At the General Debate of the 75th Session of The United Nations General Assembly, Xi Jinping President of the People's Republic of China expounded China's principled position in alignment with the Paris Agreement to foster the climate resilience with low greenhouse gas emission development. As a long‐term goal of development, China will scale up its Intended Nationally Determined Contributions by adopting more vigorous policies and measures. We aim to have CO₂ emissions peak before 2030 and achieve carbon neutrality before 2060.

To achieve the above goals, it is essential to replace fossil fuels with renewable clean energy. However, the temporal and spatial differences in the distribution of typical renewable energies such as solar and wind represent the major challenge for practical applications. Electrochemical energy storage devices are thus particularly important to regulate the energy output of above intermittent energies. At present, lithium‐ion batteries (LIBs) are dominating this market by virtue of their long calendar life and high energy density. Over the coming years, the battery manufacturing is expected to grow with the increasing decarbonizing demand. In view of the accelerated expansion of LIBs, concerns have been raised about its short supply given the limited reserve of the global lithium resource. Consequently, a surge in the battery price is almost foreseen, which threatens the large‐scale energy storage industry.

Compared with LIBs, sodium‐ion batteries (SIBs) are superior with the abundant Na reserves, low cost, and potentially long cycle life. Developing SIB technology would be of great significance for the large‐scale energy storage applications. However, because of the larger ionic radius of sodium ions, SIBs normally suffer from slower diffusion kinetics than LIBs, which poses difficulty in seeking suitable electrode materials and electrolytes for SIBs. Through the extensive research over the past 10 years, a diversity of high‐performance and promising SIB systems has been developed by employing electrodes such as transition metal oxides, phosphates, and carbon‐based materials. In view of the prosperous research progress, Professor Yan Yu, a pioneer in the field of sodium‐ion energy storage, wrote the book “Sodium‐Ion Batteries” to overview the current research status of SIBs inspired by the research works from her own group. Specifically, this book systematically introduces the materials applied as the anodes, cathodes, electrolytes, and binders for SIBs and discusses their key scientific challenges, modification strategies, and future outlook. This book aims to provide both academic and industry community with more comprehensive and cutting‐edge knowledge about SIBs.

Finally, I wish that this book will promote the vigorous development of SIB technology for the large‐scale energy storage, with more fruitful achievements to facilitate the early accomplishment of CO₂ emissions peak and carbon neutrality.

13 August 2021 Tianjin, China
	Professor Jun Chen
	Academician of Chinese
	Academy of Sciences
	Nankai University

Excessive exploitation and use of fossil fuels have caused rapid reduction of the natural resources and a series of environmental issues such as extreme weather and greenhouse effect. With the awakened public awareness of the finiteness of the earth's resources, the renewable energy sources (e.g. solar, wind, tide, etc.) are developed as alternatives. While the intermittent character of these energy sources calls for more efficient energy storage systems. Over the past 30 years, lithium‐ion batteries (LIBs) technology has advanced with unparalleled progress and is now dominating the market in portable electronic devices and electric vehicles. While with dwindled reserves of the raw materials, a rise in LIB price is foreseen.

Sodium‐ion batteries (SIBs) share similar electrochemistry and fabrication technologies to LIBs while featuring with low cost and better safety due to earth‐abundant sodium resources, rendering them a promising alternative to LIBs in large‐scale energy storage. Over the past years, a wealth of research works has been dedicated to the development and commercialization of SIBs. There has been gratifying progress achieved and on other hand, urged greater efforts are still needed to address the unresolved troubling issues.

This book intends to provide researchers and graduate/undergraduate students in related fields of materials science and electrochemistry with a snapshot of current efforts in improving the SIB technology, as well as pave the way for the development of a new generation of higher‐energy density, rechargeable SIBs through comprehensive optimization in electrode materials, electrolytes, and other key components.

Chapters 1 and 2 provide a basic introduction (including history, operation principle, key materials, etc.) to SIBs, as well as outline some design principles in terms of energy density, power density, cycling life, and safety to build high‐performance SIBs.

Chapters 3 to 6 systematically introduce the cathode materials of transition metal oxides, polyanionic compounds, Prussian blue analogues, and organic compounds for SIBs, in terms of structural design, electrochemical properties, and the corresponding reaction mechanisms.

From Chapter 7 to Chapter 10, intercalation‐type anode materials (carbon‐based materials, titanium‐based materials, etc.), alloy‐ and conversion‐type anode materials (metals/metal alloys, metal oxides/metal chalcogenides, phosphorous/phosphides, etc.), and Na metal anodes are described.

Electrolytes including organic liquid electrolytes, ionic liquid electrolytes, and solid electrolytes are mentioned in Chapters 11 to 13. Besides, binder is also an important part of SIBs, and its progress is summarized in Chapter 14.

For practical applications, three types of sodium‐ion full batteries (SIFBs) including non‐aqueous, aqueous, and all‐solid‐state SIFBs are discussed in Chapter 15. And, the last Chapter 16 gives perspectives on the opportunities and potential benefits of SIBs for large‐scale energy storage.

Finally, I gratefully acknowledge the substantial contribution from Prof. Yanglong Hou, Prof. Xiaogang Zhang, Prof. Xianhong Rui, Prof. Hongfa Xiang, Dr. Lina Zhao, Prof. Yuan‐Li Ding, Prof. Changbao Zhu, Prof. Laifa Shen, Prof. Jianmin Ma, Prof. Feixiang Wu, Prof. Xuyong Feng, Dr. Dan Yang, Dr. Xianghua Zhang, and Dr. Wei Luo to this book.