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.

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.

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.

implementation of widespread approaches for renewable energy generation, [1] and mass-scale adoption of electric vehicles. [2] Such a green transition is only possible if efficient and environmentally friendly energy storage systems are developed. [1][2][3] As the most prominent and versatile energy storage system, batteries are anticipated to be the vital cog for the storage/ delivery of on-demand power in an environmentally and socioeconomically sustainable manner. [4] Ideally, a sustainable energy storage device should deliver large capacities, present good rate capability, have long operating lifespans, and, most importantly, rely on nontoxic and noncritical materials. [5][6][7] These stringent requirements displace lithium ion batteries (LIBs) as the preferred choice for truly green batteries. [5] Current LIBs use toxic and flammable chemicals in the electrolyte (lithium hexafluorophosphate, carbonate esters) and elements listed by the European Union as critical raw materials (CRMs), including cobalt, lithium, or graphite. [8,9] Besides their high supply risk with primary resources in Bolivia, Argentina, Chile, Australia, and the Democratic Republic of Congo, the disposal and subsequent marine/ landfill accumulation of CRMs seriously threaten animal and Transient batteries are expected to lessen the inherent environmental impact of traditional batteries that rely on toxic and critical raw materials. This work presents the bottom-up design of a fully transient Zn-ion battery (ZIB) made of nontoxic and earth-abundant elements, including a novel hydrogel electrolyte prepared by cross-linking agarose and carboxymethyl cellulose. Facilitated by a high ionic conductivity and a high positive zinc-ion species transference number, the optimized hydrogel electrolyte enables stable cycling of the Zn anode with a lifespan extending over 8500 h for 0.25 mA cm −2 -0.25 mAh cm −2 . On pairing with a biocompatible organic polydopamine-based cathode, the full cell ZIB delivers a capacity of 196 mAh g −1 after 1000 cycles at a current density of 0.5 A g −1 and a capacity of 110 mAh g −1 after 10 000 cycles at a current density of 1 A g −1 . A transient ZIB with a biodegradable agarose casing displays an open circuit voltage of 1.123 V and provides a specific capacity of 157 mAh g −1 after 200 cycles at a current density of 50 mA g −1 . After completing its service life, the battery can disintegrate under composting conditions.

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