A New Emerging Technology: Na‐Ion Batteries

Abstract

Increasing awareness of environmental pollution and requirement of reliable energy are driving to develop various energy storage systems. Among them, rechargeable batteries play a key role in our modern society. “Building better batteries” remains an ongoing process to cater diverse energy demands starting from small-scale consumer electronics to large-scale automobiles and grid storage. Constantly promoting the development of battery technologies towards better, cheaper and safer properties is the persistent mission of scientific researchers. Due to the similar working principle to Li-ion batteries, abundant resource advantages, and mature fabrication technologies, Na-ion batteries (NIBs) have become a rising star and aroused a great deal of interest particularly for the application in the grid electrical energy storage. To realize the goal, the scientific community is making unremitting endeavors in the search for more efficient, reliable and economical materials and technologies, where the new designing and discovering, the endless improvements and innovations, as well as the proposed solutions and strategies are the best proof. This special issue presents a body of work discussing the important topics in NIB research: i) the synthetic condition- and composition-dependent layered oxides, ii) the multielectron reactions of polyanionic materials, iii) the application of Prussian blue analogs in organic or aqueous electrolytes, iv) the synthesis of anode materials and understanding of storage mechanism, and v) the effect of salt concentration on electrolyte and electrode property. This issue begins with a research news of sodium-oxygen batteries to review the cell chemistry and reaction mechanism, and consists of three communications, five reviews and nine research papers to introduce the above-mentioned topics, discuss them in detail, and summarize recent advances. As one of the main components of NIBs, cathode materials play a key role in overall performance, and consequently capture considerable interest. Among them, layered metal oxides, polyanionic compounds and Prussian blue analogs have been intensively examined due to their respective advantages. Four papers cover sodium layered oxides (investigating the synthetic condition, element doping and structure design for the material modification), six papers involve polyanionic cathodes (with regard to the versatile skeleton, multielectron reaction and tunable redox potential to realize high power or high energy density batteries), and two papers concern Prussian blue analogs (possessing rigid open framework with large interstitial sites and capable of applying in both organic and aqueous electrolytes). Compared with cathode materials, the selection of anode materials suffer from limited options. Two reviews summarize the state-of-the-art achievements of carbonaceous anodes: one aims to help readers comprehensively learn how to fabricate one-element doped and several-elements co-doped carbons to tune the anode performance, and the other intends to demonstrate the sodium storage ability of graphitic carbons with strategies of solvent co-intercalation, graphite expanding and active material compositing. One research paper on the cobalt/chalcogenides in B, N co-doped graphene for sodium storage is a live example combining the main content of the two reviews. Besides, another research paper reports evidence for reversible silicon redox activity by investigating the sodium storage mechanism in silicon oxycarbide. Electrolytes are also crucial to influence the whole performance of battery system. Organic electrolytes containing solvents and salts are still the mostly used type, where the salt-solvent coordination greatly impacts the ionic conductivity, viscosity, electrochemical stability window etc. One research paper systematically investigates the salt concentration on the properties of organic carbonate-based electrolyte and finds that viscosity plays a major role in influencing ionic conductivity rather than ions and molecules’ coordination. The best electrolyte should make a good compromise between physicochemical and electrochemical properties. This special issue in Small Methods attempts to offer highlights on the new developments of electrode material design, electrolyte composition optimization, sodium storage mechanism interpretation, and electrochemical performance improvement for NIBs in a broad range. Even there are still many challenges to be resolved, given the facts that important achievements have been realized and over 10 companies in the world start to commercialize this technology, in particular, HiNa Battery Company has successfully installed a Na-ion battery system with a capacity of 30 kW/100 kWh for the energy storage in Liyang city of China (Figure 1). We are sure that NIBs will have promising applications in the future. It has been our pleasure to edit this special issue, and we would like to thank Small Methods editors, particularly, Dr. Guangchen Xu, for proposing the special issue and handling all the papers. We would also express our earnest thanks to all the authors for their commitment and great work, and appreciate all the reviewers who have made excellent contributions to guarantee this high-quality issue for the community.