Exploration of alternative energy storage systems has been more than necessary in view of the supply risks haunting lithium-ion batteries. Among various alternative electrochemical energy storage devices, sodium-ion battery outstands with advantages of cost-effectiveness and comparable energy density with lithium-ion batteries. Thanks to the similar electrochemical mechanism, the research and development of lithium-ion batteries have forged a solid foundation for sodium-ion battery explorations. Advancements in sodium-ion batteries have been witnessed in terms of superior electrochemical performance and broader application scenarios. Here, the strategies adopted to optimize the battery components (cathode, anode, electrolyte, separator, binder, current collector, etc.) and the cost, safety, and commercialization issues in sodium-ion batteries are summarized and discussed. Based on these optimization strategies, assembly of functional (flexible, stretchable, self-healable, and self-chargeable) and integrated sodium-ion batteries (−actuators, −sensors, electrochromic, etc.) have been realized. Despite these achievements, challenges including energy density, scalability, trade-off between energy density and functionality, cost, etc. are to be addressed for sodium-ion battery commercialization. This review aims at providing an overview of the up-to-date achievements in sodium-ion batteries and serves to inspire more efforts in designing upgraded sodium-ion batteries.

Though flexible electrochromic devices have shown huge potential application in the fields of safety warning, display and smart windows, limited attention was paid on preparing flexible electrochromic fiber because of the difficulty in fabricating the multilayer electrochromic device structure in one-dimensional form. In this study, a flexible electrochromic nylon fiber based on Ag nanowires (NWs)/PEDOT:PSS/WO₃ nanoparticles (NPs) (PEDOT:PSS = poly(3,4-ethylenedioxythiophene):polystyrenesulfonic acid) is successfully fabricated, delivering rapid color switching (2.5 and 9 s for bleaching and coloration) and high optical modulation (65.5% at 633 nm), and sustainable to repeated mechanical deformations. Ag NWs, PEDOT:PSS and WO₃ NPs were dip-coated on the nylon fiber, resulting in an electrochromic fiber electrode with stable fiber resistance of 50–100 Ω/10 cm, which can withstand mechanical deformation against 300 times of bending cycles with bending radius of 0.5 cm, and sustain 30 times of tape-peeling. During the galvanostatic tests, the capacitance of the electrochromic electrode can maintain 70% of the initial value even after 5,000 times of charge–discharge cycles. Even in knotted shape, the fiber still shows excellent color contrast. This study provides a novel method to construct flexible electrochromic fiber and pave the way for the development of flexible optoelectronic devices, such as flexible and wearable displays.

The next generation of sensors should be self-powered, maintenance-free, precise, and have wide-ranging sensing abilities. Despite extensive research and development in the field of pressure sensors, the sensitivity of most pressure sensors declines significantly at higher pressures, such that they are not able to detect a wide range of pressures with a uniformly high sensitivity. In this work, we demonstrate a single-electrode triboelectric pressure sensor, which can detect a wide range of pressures from 0.05 to 600 kPa with a high degree of sensitivity across the entire range by utilizing the synergistic effects of the piezoelectric polarization and triboelectric surface charges of self-polarized polyvinyldifluoride-trifluoroethylene (P(VDF-TrFE)) sponge. Taking into account both this wide pressure range and the sensitivity, this device exhibits the best performance relative to that of previously reported self-powered pressure sensors. This achievement facilitates wide-range pressure detection for a broad spectrum of applications, ranging from simple human touch, sensor networks, smart robotics, and sports applications, thus paving the way forward for the realization of next-generation sensing devices. Moreover, this work addresses the critical issue of saturation pressure in triboelectric nanogenerators and provides insights into the role of the surface charge on a piezoelectric polymer when used in a triboelectric nanogenerator.