  The tremendous amount of wasted heat from solar radiation and industry
dissipation has motivated the development of thermoelectric concepts that
directly convert heat into electricity. The main challenge in practical
applications for thermoelectrics is the high cost from both materials and
manufacturing. Recently, breakthrough progresses in ionic thermoelectrics open
up new possibilities to charge energy storage devices when submitted to a
temperature gradient. The charging voltage is internally from the ionic Seebeck
effect of the electrolyte between two electrodes. Hence electrolytes with high
thermoelectric figure of merit are classified as ionic thermoelectric
materials. Most ionic thermoelectric materials are composed of abundant
elements, and they can generate hundreds of times larger thermal voltage than
that of electronic materials. This emerging thermoelectric category brings new
hope to fabricate low cost and large area heat-to-energy conversion devices,
and triggers a renewed interest for ionic thermodiffusion. In this review, we
summarize the state of the art in the new field of ionic thermoelectrics, from
the driving force of the ionic thermodiffusion to material and application
developments. We present a general map of ionic thermoelectric materials,
discuss the unique characters of each type of the reported electrolytes, and
propose potential optimization and future topics of ionic thermoelectrics.
