Recent advances in emerging nonaqueous K-ion batteries: from mechanistic insights to practical applications

K-ion battery (KIB), as an alternative to traditional Li-ion battery (LIB), has drawn ever-growing research interests because of its low cost, high voltage and similar working principle to that of LIB. Emerging understanding of energy storage mechanisms of KIBs reveals, however, significant differences between KIBs and other alkaline-ion batteries, which accounts for poor electrochemical performance when the electrodes are directly adopted from other alkaline ion batteries for KIBs. It is therefore of importance to gain deep mechanistic insights into how K ions are transported and stored in KIBs. This paper is dedicated to identifying current challenges to KIBs and critically assessing recent advances in fundamental understanding of K ion storage mechanisms, including intercalation, conversion, phase transition and alloying reactions during the charge/discharge cycles. Various approaches taken to design and develop new materials for KIB components, such as anodes, cathodes and electrolytes, are compiled and their relative merits and weaknesses are evaluated from a mechanistic perspective in an effort to establish a holistic picture of how to build cost-effective and high-performance KIBs. Finally, the challenges and outlooks on further optimizing the electrochemical performance of KIBs and their potentials of real-world applications are discussed.