  The widely use of lithium-ion (Li-ion) batteries in various fields, from
portable products to large-scale energy storage systems, has revolutionized our
daily life. The 2019 Nobel Prize in Chemistry has been awarded to John B.
Goodenough, M. Stanley Whittingham, and Akira Yoshino for their contributions
in developing Li-ion batteries. Although Li-ion batteries are currently
on-growing research topics, lithium availability is still a problem for mass
production. In contrast to lithium, sodium resources are almost unlimited on
Earth, and sodium is one of the most abundant elements in the Earth's crust.
Hence, sodium-ion (Na-ion) batteries as a counterpart of Li-ion batteries have
the potential to serve as the next-generation batteries. In this work, a brief
history and recent development of Na-ion batteries are described. The
fundamental physical and electronic properties, such as geometric structures,
band structure, density of states, and spatial charge distributions, of
Na-intercalation compounds are discussed. The outlook of Na-ion batteries is
given at the last.
