Layering the charge: Layered metal oxides such as lithium cobalt oxide have attracted great attention for rechargeable batteries. In lithium cells, only the octahedral structure forms, but in sodium cells, trigonal prismatic structures are also possible. However, there is a lack of understanding about how to predict and control the formation of each structure. Zhao et al. used the simple properties of ions, namely their charge and their radius appropriately weighted by stoichiometry, to determine whether sodium in the interlayers between the transition metal or other ion-oxide layers remain octahedral rather than switching over to trigonal prismatic coordination. Science , this issue p. 708

Twinborn TiO ₂ –TiN heterostructures enable smooth trapping–diffusion–conversion of polysulfides and produce ultralong life lithium–sulfur batteries.

Flexible and safe batteries, coupled with high performance and low cost, constitute a radical advance in portable and wearable electronics, especially considering the fact that these flexible devices are likely to experience more mechanical impacts and potential damage than well-protected rigid batteries. However, flexible lithium ion batteries (LIBs) are vastly limited by their intrinsic safety and cost issues. Here we introduce an extremely safe and wearable solid-state zinc ion battery (ZIB) comprising a novel gelatin and PAM based hierarchical polymer electrolyte (HPE) and an α-MnO₂ nanorod/carbon nanotube (CNT) cathode. Benefiting from the well-designed electrolyte and electrodes, the flexible solid-state ZIB delivers a high areal energy density and power density (6.18 mW h cm⁻² and 148.2 mW cm⁻², respectively), high specific capacity (306 mA h g⁻¹) and excellent cycling stability (97% capacity retention after 1000 cycles at 2772 mA g⁻¹). More importantly, the solid-state ZIB offers a high wearability and an extreme safety performance over conventional flexible LIBs, and performs very well under various severe conditions, such as being greatly cut, bent, hammered, punctured, sewed, washed in water or even put on fire. In addition, flexible ZIBs were integrated in series to power a commercial smart watch, a wearable pulse sensor, and a smart insole, which has been achieved to the best of our knowledge for the first time. These results demonstrate the promising potential of ZIBs in many practical wearable applications and offer a new platform for flexible and wearable energy storage technologies.

High-temperature sodium–sulfur batteries operating at 300–350 °C have been commercially applied for large-scale energy storage and conversion. However, the safety concerns greatly inhibit their widespread adoption. Herein, we report a room-temperature sodium–sulfur battery with high electrochemical performances and enhanced safety by employing a “cocktail optimized” electrolyte system, containing propylene carbonate and fluoroethylene carbonate as co-solvents, highly concentrated sodium salt, and indium triiodide as an additive. As verified by first-principle calculation and experimental characterization, the fluoroethylene carbonate solvent and high salt concentration not only dramatically reduce the solubility of sodium polysulfides, but also construct a robust solid-electrolyte interface on the sodium anode upon cycling. Indium triiodide as redox mediator simultaneously increases the kinetic transformation of sodium sulfide on the cathode and forms a passivating indium layer on the anode to prevent it from polysulfide corrosion. The as-developed sodium–sulfur batteries deliver high capacity and long cycling stability.