Thermoregulation has substantial implications for energy consumption and human comfort and health. However, cooling technology has remained largely unchanged for more than a century and still relies on cooling the entire space regardless of the number of occupants. Personalized thermoregulation by thermoelectric devices (TEDs) can markedly reduce the cooling volume and meet individual cooling needs but has yet to be realized because of the lack of flexible TEDs with sustainable high cooling performance. Here, we demonstrate a wearable TED that can deliver more than 10°C cooling effect with a high coefficient of performance (COP > 1.5). Our TED is the first to achieve long-term active cooling with high flexibility, due to a novel design of double elastomer layers and high-ZT rigid TE pillars. Thermoregulation based on these devices may enable a shift from centralized cooling toward personalized cooling with the benefits of substantially lower energy consumption and improved human comfort.

This work provides insight regarding the fundamental lithiation and delithiation mechanism of the popular lithium ion battery anode material, Li4Ti5O12 (LTO). Our results quantify the extent of reaction between Li4Ti5O12 and Li7Ti5O12 at the nanoscale, during the first cycle. Lithium titanate's discharge (lithiation) and charge (delithiation) reactions are notoriously difficult to characterize due to the zero-strain transition occurring between the end members Li4Ti5O12 and Li7Ti5O12. Interestingly, however, the latter compound is electronically conductive, while the former is an insulator. We take advantage of this critical property difference by using conductive atomic force microscopy (c-AFM) to locally monitor the phase transition between the two structures at various states of charge. To do so, we perform ex situ characterization on electrochemically cycled LTO thin-films that are never exposed to air. We provide direct confirmation of the manner in which the reaction occurs, which proceeds via percolation channels within single grains. We complement scanning probe analyses with an X-ray photoelectron spectroscopy (XPS) study that identifies and explains changes in the LTO surface structure and composition. In addition, we provide a computational analysis to describe the unique electronic differences between LTO and its lithiated form.

The demand for the large-scale storage system has gained much interest. Among all the criteria for the large-scale electrical energy storage systems (EESSs), low cost ($ k Wh −1) is the focus where MnO 2-based electrochemistry can be a competitive candidate. It is notable that MnO 2 is one of the few materials that can be employed in various fields of EESSs: alkaline battery, supercapacitor, aqueous rechargeable lithium-ion battery, and metal-air battery. Yet, the technology still has bottlenecks and is short of commercialisation. Discovering key parameters impacting the energy storage and developing systematic characterisation methods for the MnO 2 systems can benefit a wide spectrum of energy requirements. In this review, history, mechanism, bottlenecks, and solutions for using MnO 2 in the four EESSs are summarised and future directions involving more in-depth mechanism studies are suggested.

Sodium transition metal oxides with layered structures are attractive cathode materials for sodium-ion batteries due to their large theoretical specific capacities. However, these layered oxides suffer from poor cyclability and low rate performance because of structural instability and sluggish electrode kinetics. In the present work, we show the sodiation reaction of Mn3O4 to yield crystal water free NaMnO2−y−δ(OH)2y, a monoclinic polymorph of sodium birnessite bearing Na/Mn(OH)8 hexahedra and Na/MnO6 octahedra. With the new polymorph, NaMnO2−y−δ(OH)2y exhibits an enlarged interlayer distance of about 7 Å, which is in favor of fast sodium ion migration and good structural stability. In combination of the favorable nanosheet morphology, NaMn2−y−δ(OH)2y cathode delivers large specific capacity up to 211.9 mAh g–1, excellent cycle performance (94.6% capacity retention after 1000 cycles), and outstanding rate capability (156.0 mAh g–1 at 50 C). This study demonstrates an effective approach in tailoring the structural and electrochemical properties of birnessite towards superior cathode performance in sodium-ion batteries.

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