Structural colors of 2D gratings are iridescent, color-tunable, and never fade, which renders them appealing for anti-counterfeiting applications. However, for advanced security, it still remains a challenge to completely hide the encrypted color patterns and reveal them on demand. In this work, a water-responsive photonic grating consisting of a micropillar array and a hydrogel overcoat with a similar refractive index, termed “hydrocipher”, is presented. The joint effect of stimuli-reversible refractive-index (mis)match and reconfigurable grating-based diffraction coloration enables a complete encryption of the structural color and rapid decryption. The photonic structure shows a strong iridescence due to the angle-dependent diffraction when the hydrogel overcoat is swollen from water. Upon drying, the micropillars bend and the refractive index contrast disappears, which dramatically lessens the diffraction intensity and renders the surface highly transparent. The dehydrated-to-hydrated state transition can occur within 1 s, enabling fast decryption. The color switching is highly reversible over a prolonged hydration/dehydration cycle, and the dehydrated hydrogel layer protects the delicate micropillar array from external mechanical stress. The creative combination of hydrogel material and the 2D grating structure offers a new and simple strategy for realizing reversible, durable, and fast-response cryptography with potentially broad impact on the anti-counterfeiting technology market.

The pulverization of lithium metal electrodes during cycling recently has been suppressed through various techniques, but the issue of irreversible consumption of the electrolyte remains a critical challenge, hindering the progress of energy-dense lithium metal batteries. Here, we design a single-ion-conductor-based composite layer on the lithium metal electrode, which significantly reduces the liquid electrolyte loss via adjusting the solvation environment of moving Li+ in the layer. A Li||Ni0.5Mn0.3Co0.2O2 pouch cell with a thin lithium metal (N/P of 2.15), high loading cathode (21.5 mg cm-2), and carbonate electrolyte achieves 400 cycles at the electrolyte to capacity ratio of 2.15 g Ah-1 (2.44 g Ah-1 including mass of composite layer) or 100 cycles at 1.28 g Ah-1 (1.57 g Ah-1 including mass of composite layer) under a stack pressure of 280 kPa (0.2 C charge with a constant voltage charge at 4.3 V to 0.05 C and 1.0 C discharge within a voltage window of 4.3 V to 3.0 V). The rational design of the single-ion-conductor-based composite layer demonstrated in this work provides a way forward for constructing energy-dense rechargeable lithium metal batteries with minimal electrolyte content.