Li-ion batteries (LIBs), commercialized in 1991, have the highest energy density among practical secondary batteries and are widely utilized in electronics, electric vehicles, and even stationary energy storage systems. Along with the expansion of their demand and application, concern about the resources of Li and Co is growing. Therefore, secondary batteries composed of earth-abundant elements are desired to complement LIBs. In recent years, K-ion batteries (KIBs) have attracted significant attention as potential alternatives to LIBs. Previous studies have developed positive and negative electrode materials for KIBs and demonstrated several unique advantages of KIBs over LIBs and Na-ion batteries (NIBs). Thus, besides being free from any scarce/ toxic elements, the low standard electrode potentials of K/K + electrodes lead to high operation voltages competitive to those observed in LIBs. Moreover, K + ions exhibit faster ionic diffusion in electrolytes due to weaker interaction with solvents and anions than that of Li + ions; this is essential to realize high-power KIBs. This review comprehensively covers the studies on electrochemical materials for KIBs, including electrode and electrolyte materials and a discussion on recent achievements and remaining/emerging issues. The review also includes insights into electrode reactions and solid-state ionics and nonaqueous solution chemistry as well as perspectives on the research-based development of KIBs compared to those of LIBs and NIBs.

Li-ion battery commercialized by Sony in 1991 has the highest energy-density among practical rechargeable batteries and is widely used in electronic devices, electric vehicles, and stationary energy storage system in the world. Moreover, the battery market is rapidly growing in the world and further fast-growing is expected. With expansion of the demand and applications, price of lithium and cobalt resources is increasing. We are, therefore, motivated to study Na- and K-ion batteries for stationary energy storage system because of much abundant Na and K resources and the wide distribution in the world. In this account, we review developments of Na- and K-ion batteries with mainly introducing our previous and present researches in comparison to that of Li-ion battery.

The four volt K-ion battery is demonstrated as a possible alternative to 4 volt Li-ion battery.

Stable cycling of a 4 V-class potassium-ion battery is demonstrated with a highly concentrated potassium bis(fluorosulfonyl)amide 1,2-dimethoxyethane solution as an electrolyte. Not only graphite and K2Mn[Fe(CN)6] half cells but also graphite//K2Mn[Fe(CN)6] full cells filled with the electrolyte exhibit higher coulombic efficiency and better cyclability than those of KPF6/carbonate ester solutions.

Potassium metal electrochemical cells are widely utilized to examine potassium insertion materials for nonaqueous potassium-ion batteries. However, large polarization during K plating–stripping and unstable rest potential are found at the potassium electrodes, which leads to an underestimation of the electrochemical performance of insertion materials. In this study, the electrochemical behavior of K-metal electrodes is systematically investigated. Electrolyte salts, solvents, and additives influence the polarization of K metals. Although a highly concentrated electrolyte of 3.9 M KN­(SO2F)2/1,2-dimethoxyethane realizes the smallest polarization of 25 mV among all the electrolytes investigated in this study, the polarization of K metals is still larger than those of Li and Na metals. The issue of inaccurate rest potential is solved by pretreating the K electrodes with a plating–stripping process, which is essential in evaluating the intrinsic electrode performance of potassium insertion materials.