This issue contains assessments of battery performance involving complex, interrelated physical and chemical processes between electrode materials and electrolytes. Transformational changes in battery technologies are critically needed to enable the effective use of renewable energy sources such as solar and wind to allow for the expansion of hybrid electric vehicles (HEVs) to plug-in HEVs and pure-electric vehicles. For these applications, batteries must store more energy per unit volume and weight, and they must be capable of undergoing many thousands of charge-discharge cycles. The articles in this theme issue present details of several growing interest areas, including high-energy cathode and anode materials for rechargeable Li-ion batteries and challenges of Li metal as an anode material for Li batteries. They also address the recent progress in systems beyond Li ion, including Li-S and Li-air batteries, which represent possible next-generation batteries for electrical vehicles. One article reviews the recent understanding and new strategies and materials for rechargeable Mg batteries. The knowledge presented in these articles is anticipated to catalyze the design of new multifunctional materials that can be tailored to provide the optimal performance required for future electrical energy storage applications.

The commercialization of lithium-ion batteries has intimately changed our lives and enabled portable electronic devices, which has revolutionized communications, entertainment, medicine, and more. After three decades of commercial development, researchers around the world are now pursuing major advances that would allow this technology to power the next generation of light-duty, electric, and hybrid-electric vehicles. If this goal is to be met, concerted advances in safety and cost, as well as cycle-life and energy densities, must be realized through advances in the properties of the highly correlated, but separate, components of lithium-ion energy-storage systems.

Considering the need for designing better batteries to meet the rapidly growing demand for large-scale energy storage applications, an aspect of primary importance for battery materials is elemental abundance. To achieve sustainable energy development, we must reconsider the feasibility of a sustainable lithium supply, which is essential for lithium(-ion) batteries. Lithium is widely distributed in the Earth, but is not regarded as an abundant element. Therefore, widespread use of large-scale lithium batteries would be inevitably restricted. Sodium(-ion) batteries are thus promising candidates for large-scale applications because sodium is the most advantageous next to lithium considering its atomic weight, standard potential, and natural abundance. Rechargeable sodium-ion batteries consist of two different sodium insertion materials similar to Li-ion batteries. Sodium insertion materials, especially layered oxides, have been studied since the early 1980s, but not extensively for energy storage devices due to the expanded interest in lithium insertion materials in the 1990s. In recent years, materials researchers have again been extensively exploring new sodium insertion materials to enhance battery performance. This article reviews recent advancements and trends in layered sodium transition metal oxides as positive electrode materials for Na-ion batteries.

“Abundance” is an important keyword in materials development. This is particularly the case for the energy storage sector, where materials themselves function as a storage host. The amount of materials is directly linked to the amount of energy stored in the device. In lithium (Li)-ion batteries, transition metal elements are necessary to accommodate a large number of electrons/holes in a reversible redox reaction. Iron, as the fourth most abundant element in the earth’s crust, is an ideal redox center, but practical storage electrodes with Fe redox have long been the “holy grail” of the Li-ion battery since its commercialization in 1991. In this article, the history of replacing Co with Fe in Li-battery electrodes is briefly reviewed followed by recent technical achievements toward more sustainable batteries using Na+ as a guest ion, where the goal would be to discover a high voltage electrode material composed of Na and Fe without compromising the energy density.

The success of high capacity energy storage systems based on lithium (Li) batteries relies on the realization of the promise of Li-metal anodes. Li metal has many advantageous properties, including an extremely high theoretical specific capacity (3860 mAh g–1), the lowest electrochemical potential (–3.040 V versus standard hydrogen electrode), and low density (0.59 g cm–3), which, all together, make it a very desirable electrode for energy storage devices. However, while primary Li batteries are used for numerous commercial applications, rechargeable Li-metal batteries that utilize Li-metal anodes have not been as successful. This article discusses the properties of Li metal in the absence of surface stabilization, as well as three different approaches currently under investigation for stabilizing the surface of Li metal to control its reactivity within the electrochemical environment of a Li-based battery.

Markets for energy storage that go beyond portable electronics have emerged rapidly this decade, including powering electric vehicles and “leveling the grid” fed by renewable sources such as solar energy, which are intermittent in supply. These new demands require a significant step-up in energy density that will probably not be met by Li-ion batteries; estimates suggest they are starting to approach their theoretical limits. But in the world of “beyond Li-ion,” the options are limited. One of the most hopeful is the Li-S battery, for which greater energy storage can potentially be realized through phase-transformation chemistry using elemental sulfur as a positive electrode material, which converts to lithium sulfide. These future generation systems offer up to a five-fold increased specific energy and greatly reduced cost factors, but commercialization has been hindered owing to key challenges. Efforts over the last two years to better manipulate the cell chemistry and overcome these challenges are presented.

Lithium-air batteries have received extraordinary attention recently owing to their theoretical gravimetric energies being considerably higher than those of Li-ion batteries. There are, however, significant challenges to practical implementation, including low energy efficiency, cycle life, and power capability. These are due primarily to the lack of fundamental understanding of oxygen reduction and evolution reaction kinetics and parasitic reactions between oxygen redox intermediate species and nominally inactive battery components such as carbon in the oxygen electrode and electrolytes. In this article, we discuss recent advances in the mechanistic understanding of oxygen redox reactions in nonaqueous electrolytes and the search for electrolytes and electrode materials that are chemically stable in the oxygen electrode. In addition, methods to protect lithium metal against corrosion by water and dendrite formation in aqueous lithium-air batteries are discussed. Further materials innovations lie at the heart of research and development efforts that are needed to enable the development of lithium-oxygen batteries with enhanced round-trip efficiency and cycle life.

The high specific capacity, reactivity, and abundance of magnesium in the earth’s crust and the relatively good safety features of Mg metal, despite its being a reactive metal, drive intensive efforts to develop rechargeable Mg batteries as a follow-up to the success of Li-ion battery technology. However, Mg anodes cannot function in usual non-aqueous electrolyte solutions. Consequently, it is important to develop unique, complex solutions for reversible Mg metal anodes. Also, finding relevant cathode materials that can reversibly insert bivalent Mg ions is a great challenge. In this article, we review the efforts and success in the development of several families of electrolyte solutions for secondary Mg batteries, in which Mg anodes behave fully reversibly, but also exhibit the necessary wide electrochemical window. We also review attempts to develop positive electrodes for rechargeable Mg batteries. The first generation of secondary Mg batteries has already been demonstrated, but their specific energy density remains relatively low. The challenge now is to develop novel Mg battery prototypes that possess high energy density.

In recent years, the emerging areas of nanophotonics and, in particular, plasmonics and metamaterials, have seen an explosion of novel ideas. However, transforming revolutionary designs into practical devices requires a significant amount of effort. The constituent materials in plasmonic structures and metamaterials play a crucial role in realizing useful and efficient devices. Similar to the way silicon shaped the nanoelectronics field, finding the best set of materials for plasmonic and metamaterial devices could revolutionize the field of nanophotonics. As a potential solution, alternative plasmonic materials have recently gained significant attention. Metals, despite being essential components of plasmonic and metamaterial devices, pose many technological challenges toward the realization of practical devices—primarily due to their high optical loss, integration, and fabrication limitations. Hence, searching for an alternative is vital to the success of future nanophotonic devices. Several classes of materials, including doped semiconductor oxides and ceramics, are discussed as potential alternatives to metals that could lead to devices with drastically improved performance and new functionalities by providing low intrinsic loss, tunability, and compatibility with standard semiconductor fabrication processes.