Hierarchically Anisotropic Networks to Decouple Mechanical and Ionic Properties for High-Performance Quasi-Solid Thermocells.

The rapid growth of wearable systems demands sustainable, mechanically adaptable, and eco-friendly energy-harvesting devices. Quasi-solid ionic thermocells have demonstrated the capability of continuously converting low-grade heat into electricity to power wearable electronics. However, a trade-off between ion conductivity and mechanical properties is one of the most challenging obstacles for developing high-performance quasi-solid thermocells. Herein, the trade-off is overcome by designing anisotropic polymer networks to produce aligned channels for ion-conducting and hierarchically assembled crystalline nanofibrils for crack blunting. The ionic conductivity of the anisotropic thermocell has a more than 400% increase, and the power density is comparable to the record of state-of-the-art quasi-solid thermocells. Moreover, compared with the existing quasi-solid thermocells with the optimal mechanical performance, this material realizes biomimetic strain-stiffening and shows more than 1100% and 300% increases in toughness and strength, respectively. We believe this work provides a general method for developing high-performance, cost-effective, and durable thermocells and also expands the applicability of thermocells in wearable systems.