Harvesting heat from the environment into electricity has the potential to power Internet-of-things (IoT) sensors, freeing them from cables or batteries and thus making them especially useful for wearable devices. We demonstrate a giant positive thermopower of 17.0 millivolts per degree Kelvin in a flexible, quasi-solid-state, ionic thermoelectric material using synergistic thermodiffusion and thermogalvanic effects. The ionic thermoelectric material is a gelatin matrix modulated with ion providers (KCl, NaCl, and KNO3) for thermodiffusion effect and a redox couple [Fe(CN)64–/Fe(CN)63–] for thermogalvanic effect. A proof-of-concept wearable device consisting of 25 unipolar elements generated more than 2 volts and a peak power of 5 microwatts using body heat. This ionic gelatin shows promise for environmental heat-to-electric energy conversion using ions as energy carriers.

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Ionic thermoelectric (i‐TE) cells, using ions as energy carriers, have the advantage of achieving a high voltage of 1−5 V at approximately ambient temperature, showing a promise as a technology for powering Internet‐of‐Things (IoT) sensors. However, the low output power of i‐TE cells restricts their applications. Here, a 3D hierarchical structure electrode is designed to enlarge the electroactive surface area, significantly increasing the thermogalvanic reaction sites and decreasing the interface charge transfer resistance. The quasi‐solid‐state gelatin‐KCl‐FeCN4–/3– i‐TE cells achieve a record instantaneous output power density (8.9 mW m–2 K–2) and an ultrahigh 2 h output energy density (E2h) (80 J m–2) under an optimal temperature range. An average E2h value of 59.4 J m–2 is obtained over the course of a week of operation. A wearable device consisting of 24 i‐TE cells can generate a high voltage of 2.8 V and an instantaneous output power of 68 µW by harvesting body heat. A simple and easy‐to‐operate electrode optimization strategy is provided here to increase the long‐term output power performance of i‐TE cells. This work represents a promising approach to develop reliable and green power sources for IoT sensors near room temperature.

Ionic thermoelectric (i-TE) materials, using ions as the energy carrier, can generate a voltage under a temperature difference, bearing similarities to the Seebeck effect of electrons and holes in solid-state materials. Recent experiments have demonstrated large thermopower of quasi-solid-state i-TE materials, which are attractive for harvesting ambient heat as large enough voltage can be generated under a small temperature difference to match the voltage input needs of sensors for internet-of-things applications. In this perspective article, we discuss similarities and differences of i-TE materials from electronic-based thermoelectric materials and also different i-TE thermoelectric effects including the thermodiffusion (Soret) effect and the thermogalvanic effect, in which the latter includes redox reaction entropy and the Soret effect. Strategies to improve performances of materials and devices are elaborated, together with needs for future research in understanding microscopic origins of different effects.

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The surface of spinel LiMn2O4 is modified with different quantities of a Mn 4+ -rich phase prepared by a facile sol-gel method to improve electrochemical properties at elevated temperatures. Impurityfree and uniform morphologies for the LiMn2O4 particles are demonstrated from the X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The Mn 4+ -rich phase modified on the surface of the LiMn2O4 alleviates the dissolution of manganese in the electrolyte, thus improving the cycling performance and rate capability relative to the bare LiMn2O4. 1 wt.%modified LiMn2O4 delivers a capacity retention of 92.7% and a discharge capacity of 113.5 mAhg -1 after 200 cycles at 1 C and 25 °C, compared with that of 83.1%, and 100.8 mAhg -1 for the bare LiMn2O4. In addition, after 100 cycles, a capacity retention of 88.6% at 1 C is achieved for 1 wt.%modified LiMn2O4 at 55 °C, which is higher than the 76.0% for the bare LiMn2O4. Furthermore, this sample shows the best rate capability among all samples. The Mn 4+ -rich phase is an appropriate candidate for modifying surfaces to suppress dissolution of manganese, thereby improving the electrochemical properties of LiMn2O4.

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