Giant and Durable Electrocaloric Effect in Polar High-Entropy Polymers

For a practical electrocaloric (EC) device, a ΔT &gt; 5 K is required for its solid-state working body. Although giant ECEs have been observed for more than 15 years, the large temperature change (ΔT &gt; 5 K) still requires large electric fields, <i>e.g.</i>, 5.5 K under 29 MV/m for PST ceramic MLC (Nature 575, 468, 2019) and 5.1 K under 83.3 MV/m for P(VDF-TrFE-CFE) polymeric MLC (Science 357, 1130, 2017). These two materials represent the best EC material in inorganics and organics, but the fields applied were too close to the breakdown field (E_b) of respective materials, raising issues in reliability and practical deployment of the technology. In this work, we developed a high-entropy EC polymer by incorporating C=C double bonds in the polymeric main chain to induce localized nano-scale structural modification. Compared to state-of-the-art polymers, this new polymer tripled the cooling effect under ultra-low electric field of polymer,<i> i.e.</i>, the ΔT was enhanced from below 2 K to over 7.5 K (3.75-fold) under ultra-low field of 50 MV/m, which is merely 14% of its breakdown field. For the first time, we show that the large ECE (ΔT &gt; 5 K) can operate with fatigue resistance for more than one million cycles.