Transient technology seeks the development of materials, devices, or systems that undergo controlled degradation processes after a stable operation period, leaving behind harmless residues. To enable externally powered fully transient devices operating for longer periods compared to passive devices, transient batteries are needed. Albeit transient batteries are initially intended for biomedical applications, they represent an effective solution to circumvent the current contaminant leakage into the environment. Transient technology enables a more efficient recycling as it enhances material retrieval rates, limiting both human and environmental exposures to the hazardous pollutants present in conventional batteries. Little efforts are focused to catalog and understand the degradation characteristics of transient batteries. As the energy field is a property‐driven science, not only electrochemical performance but also their degradation behavior plays a pivotal role in defining the specific end‐use applications. The state‐of‐the‐art transient batteries are critically reviewed with special emphasis on the degradation mechanisms, transiency time, and biocompatibility of the released degradation products. The potential of transient batteries to change the current paradigm that considers batteries as harmful waste is highlighted. Overall, transient batteries are ready for takeoff and hold a promising future to be a frontrunner in the uptake of circular economy concepts.

Sodium ion batteries (NIBs) based on earth‐abundant materials offer efficient, safe, and environmentally sustainable solutions for a decarbonized society. However, to compete with mature energy storage technologies such as lithium ion batteries, further progress is needed, particularly regarding the energy density and operational lifetime. Considering these aspects as well as a circular economy perspective, the authors use biodegradable cellulose nanoparticles for the preparation of a gel polymer electrolyte that offers a high liquid electrolyte uptake of 2985%, an ionic conductivity of 2.32 mS cm−1, and a Na+ transference number of 0.637. A balanced ratio of mechanically rigid cellulose nanocrystals and flexible cellulose nanofibers results in a mesoporous hierarchical structure that ensures close contact with metallic Na. This architecture offers stable Na plating/stripping at current densities up to ±500 µA cm−2, outperforming conventional fossil‐based NIBs containing separator–liquid electrolytes. Paired with an environmentally sustainable and economically attractive Na2Fe2(SO4)3 cathode, the battery reaches an energy density of 240 Wh kg−1, delivering 69.7 mAh g−1 after 50 cycles at a rate of 1C. In comparison, Celgard in liquid electrolyte delivers only 0.6 mAh g−1 at C/4. Such gel polymer electrolytes may open up new opportunities for sustainable energy storage systems beyond lithium ion batteries.

Transient batteries play a pivotal role in the development of fully autonomous transient devices, which are designed to degrade after a period of stable operation. Here, a new transient separator‐electrolyte pair is introduced for lithium ion batteries. Cellulose nanocrystals (CNCs) are selectively located onto the nanopores of polyvinyl alcohol membranes, providing mobile ions to interact with the liquid electrolyte. After lithiation of CNCs, membranes with electrolyte uptake of 510 wt%, ionic conductivities of 3.077 mS·cm–1, electrochemical stability of 5.5 V versus Li/Li+, and high Li+ transport numbers are achieved. Using an organic electrolyte, the separators enable stable Li metal deposition with no dendrite growth, delivering 94 mAh·g–1 in Li/LiFePO4 cells at 100 mA·g–1 after 200 cycles. To make the separator‐electrolyte pair transient and non‐toxic, the organic electrolyte is replaced by a biocompatible ionic liquid. As a proof of concept, a fully transient Li/V2O5 cell is assembled, delivering 55 mAh·g–1 after 200 cycles at 100 mA·g–1. Thanks to the reversible Li plating/stripping, dendrite growth suppression, capacity retention, and degradability, these materials hold a bright future in the uptake of circular economy concepts applied to the energy storage field.

The primary task of a battery is to store energy and to power electronic devices. This has hardly changed over the years despite all the progress made in improving their electrochemical performance. In comparison to batteries, electronic devices are continuously equipped with new functions, and they also change their physical appearance, becoming flexible, rollable, stretchable, or maybe transparent or even transient or degradable. Mechanical flexibility makes them attractive for wearable electronics or for electronic paper; transparency is desired for transparent screens or smart windows, and degradability or transient properties have the potential to reduce electronic waste. For fully integrated and self-sufficient systems, these devices have to be powered by batteries with similar physical characteristics. To make the currently used rigid and heavy batteries flexible, transparent, and degradable, the whole battery architecture including active materials, current collectors, electrolyte/separator, and packaging has to be redesigned. This requires a fundamental paradigm change in battery research, moving away from exclusively addressing the electrochemical aspects toward an interdisciplinary approach involving chemists, materials scientists, and engineers. This Outlook provides an overview of the different activities in the field of flexible, transient, and transparent batteries with a focus on the challenges that have to be faced toward the development of such multifunctional energy storage devices.

Developing efficient energy storage technologies is at the core of current strategies toward a decarbonized society. Energy storage systems based on renewable, nontoxic, and degradable materials represent a circular economy approach to address the environmental pollution issues associated with conventional batteries, that is, resource depletion and inadequate disposal. Here we tap into that prospect using a marine biopolymer together with a water-soluble polymer to develop sodium ion battery (NIB) separators. Mesoporous membranes comprising agarose, an algae-derived polysaccharide, and poly(vinyl alcohol) are synthesized via nonsolvent-induced phase separation. Obtained membranes outperform conventional nondegradable NIB separators in terms of thermal stability, electrolyte wettability, and Na + conductivity. Thanks to the good interfacial adhesion with metallic Na promoted by the hydroxyl and ether functional groups of agarose, the separators enable a stable and homogeneous Na deposition with limited dendrite growth. As a result, membranes can operate at 200 μA cm –2 , in contrast with Celgard and glass microfiber, which short circuit at 50 and 100 μA cm –2 , respectively. When evaluated in Na 3 V 2 (PO 4 ) 3 /Na half-cells, agarose-based separators deliver 108 mA h g –1 after 50 cycles at C/10, together with a remarkable rate capability. This work opens up new possibilities for the use of water-degradable separators, reducing the environmental burdens arising from the uncontrolled accumulation of electronic waste in marine or land environments.