A new type of rechargeable battery, the molecular cluster battery (MCB), has been developed by utilizing molecular clusters such as Mn12 and polyoxometalate (POM) clusters, as cathode active materials and a lithium metal as an anode. It was found that MCBs can exhibit higher battery capacities than conventional lithium ion batteries. In operando X-ray absorption fine structure analyses on MCBs of Mn12 and POMs revealed that they exhibit 8- and 24-electrons reduction during discharge, respectively. Such electron sponge behavior realized only in solid-state electrochemistry resulted in a high battery capacity. To improve the battery performances furthermore, nanohybrid materials of single-walled carbon nanotubes and POMs were prepared, and both charging/discharging rate and battery capacity of their MCBs were significantly better than those of the non-nanohybrid MCBs. Finally, in situ magnetic measurements under solid-state electrochemistry on a mixed-valent chromium Prussian blue analogue ferrimagnet revealed continuous changes in magnetization, transition temperature and coercive force. It is demonstrated that solid-state electrochemistry is a useful technique to explore various physical properties from fundamental sciences to applications.